Review—Surface Coatings for Cathodes in Lithium Ion Batteries: From Crystal Structures to Electrochemical Performance

The primary functions of LIBs are depicted in Fig. 1. LIBs consist of an anode, cathode, electrolyte, and porous separator. During charging of the battery, lithium ions are extracted from the cathode and pass through the electrolyte and separator before they are intercalated within the anode. At the same time, an equivalent number of electrons flow into the anode through an external circuit. During the reverse process (i.e., the discharge process), Li⁺ ions are extracted from the anode and move through the separator towards the cathode where these ions re-intercalate within the cathode materials. After charging, the potential of the anode relative to the cathode indicates the storage of electric energy that can be released during the discharge process. The process of discharging is thermodynamically favored and, hence, will be determined by enthalpically (ΔH) and entropically (ΔS) driven forces. ^4–6 Some solid-state mass diffusion will also occur during Li⁺ ion transport across the porous membranes. In an ideal situation, the total concentration of Li⁺ ion is maintained at a relatively constant level within the electrolyte, although the mass balance of Li species will shift from the cathode to the anode when charging the battery as Li-ions migrate through the separator. The mass balance subsequently shifts from the anode to the cathode during the discharge of the battery. The storage of Li species in the batteries can take place through three mechanisms: (i) chemical intercalation; (ii) chemical transformation; and (iii) formation of alloys. Reactions associated with chemical transformations have a limited reversibility, whereas the formation of alloys can result in relatively large volume expansion although alloys can offer a larger specific capacity. The intercalation mechanism requires the host electrode to exhibit enough space to accommodate the Li species and either the transportation of or the transformation of a charged counter ion species for maintaining charge neutrality within the system. Lithium vacancies are created by lithium ion deintercalation, which can lead to oxygen release at high states of charge (SoCs). This furthermore enables the migration of transition metal ions and, concurrently, the formation of new phases like the cation disordered phases. Intercalation reactions have been exploited to a wide extent as they have been thought to yield the required parameters for enhancing the output of LIBs.

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Two imperative parameters for LIBs are power density and energy density, which are ultimately dependent on the cell potential. The overall cell voltage is given by ^24,25

$$\begin{eqnarray}&&V=\displaystyle \frac{{\mu }_{A}-{\mu }_{C}}{e}\end{eqnarray} \tag{ 1 }$$

where ${\mu }_{A}$ and ${\mu }_{C}$ are the chemical potentials of the anode and cathode, respectively, and e is the electronic charge. Equation 1 implies that the energy of the anode should be as high as possible and cathode energy should be as low as possible to maximize the stored energy. In other words, anodes would require stabilization of lower oxidation states (at a higher energy band) and cathodes should be stabilized for higher oxidation states (at a lower energy band). Goodenough expanded upon this insight when he proposed that the top of O²⁻: 2p bands lie lower than the S²⁻: 3p bands. ²¹ For a cathode material, the chemical potential can be significantly reduced by accessing lower energy bands through the use of higher oxidation state species such as the transformation between Co³⁺/Co⁴⁺ that leads to an increase in the overall cell voltage up to 4 V (Fig. 2a). Therefore, the working voltage of the cell is limited by the electrochemical window or range of the electrochemical stability of the components (including the electrolyte). Figure 2b illustrates that the electrochemical window is a limiting factor for solid-state batteries.

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The energy gap (E_g) of a cell is represented by the difference between the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of the electrolyte. Two important points to take into consideration are to ensure that: (i) ${\mu }_{C}$ of the cathode is higher than the HOMO of the electrolyte; and (ii) ${\mu }_{A}$ of the anode is lower than the LUMO of the electrolyte. ^15,26,27 If the anode and cathode do not satisfy this condition, the electrolyte would be oxidized at the cathode and reduced at the anode. This degradation will otherwise result in the formation of a passivating solid electrolyte interphase (SEI). A space charge layer (SCL) is formed when two materials with different chemical potentials are brought into close contact with one another, which leads to a lack of local charge neutrality due to a limited migration of ions and/or electrons. ^28,29 These space charge layers can have beneficial effects such as increasing ionic diffusion especially within solid-solid dispersions, but can have detrimental effects such as increasing the internal resistance of the cell. ^30,31 The electronic structure of the electrodes will be modified after coming into contact with the electrolyte (Fig. 2c). The electrolyte initially possesses a higher chemical potential for Li⁺, which results in a migration of Li⁺ ions into the electrolyte and towards the cathode. This process aligns the electrochemical potentials of the system and results in the formation of a heterojunction. ¹⁵ Consequently, the Fermi level of the system will be readjusted and the energy level of each component changes due to band bending and formation of inner electric fields. Both electrodes also have current collectors that are essential for electron transport to and from the anode and cathode. The current collectors for the anode and cathode are copper and aluminum, respectively. These materials are largely considered to be non-reactive and highly conductive materials for some systems, but their stability towards the chemical and electrochemical changes in the system are important considerations.

LIBs should be operated within a reliable and safe potential window (≈2 to 4 V). Their operating conditions are limited by their voltage and temperature as demonstrated in Fig. 3. Generally, the operating voltage of LIBs is between 1.5 to 4.2 V (e.g., for C/LiNi_xMn_yCo_zO₂, C/LiNi_0.8Co_0.15Al_0.05O₂, C/LiCoO₂, C/LiFePO₄) and the charging and discharging temperatures are between 0 to 45 °C and −20 to 55 °C, respectively. ⁵ Some electrolyte systems start to decompose around 70 °C, and the SEI decomposes between 90 to 120 °C. ^32–35 Above 120 °C, the SEI films are unable to protect the anode from further side reactions with the surrounding electrolyte, which could result in the production of flammable gases. The cell also stops functioning properly when the separator starts to melt above 130 °C. At temperatures above 150 °C, the cathode materials start to decompose. For instance, LiCoO₂ breakdowns at 150 °C, LiNi_0.8Co_0.15Al_0.05O₂ breakdowns at 160 °C, and LiMn₂O₄ and LiNi_xMn_yCo_zO₂ decompose at 265 °C and 210 °C, respectively. Decomposition of LiFePO₄ occurs around 310 °C. Oxygen can also be produced during the decomposition of these cathode materials.⁵ If the heat that is produced accumulates within the cell (e.g., from side reactions of the electrodes with the electrolytes), thermal runaway can occur. Thermal runaway can result in a self-heating rate of 10 °C min⁻¹ or higher as a result of a large scale short-circuit within the cell, which leads to overheating of the system. ³⁶

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The cell might explode because of the release of gases following the decomposition of active components and combustion of the gaseous by-products. It is important to fully recognize the safety issues with LIBs. Potential solutions that improve their safety include the selection of cell materials, the fabrication processes, and introducing physical barriers and protective layers on the electrodes. There are many challenges to the design and safe operation of LIBs as depicted in Fig. 4. Adverse interactions can include graphite exfoliation, structural disordering, particle cracking, internal short-circuiting, binder decomposition, dendrite formation, and transition metal dissolution are some prominent causes of a loss in capacity. Cathode materials of LIBs tend to react with the electrolytes and, hence, undergo surface modifications accompanied by degradation. These side-reactions can be detrimental as they can result in an erosion of battery performance, formation of thick SEI layers and fading of capacity, thereby, causing a reduction of both battery life and power capacity. These are concerns that continue to be addressed by the scientific community. ³⁷

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It is difficult to address all the potential issues with regards to the safe operation of LIBs in one report. Hence, the present review will focus on coating materials being sought after for the cathode. The reason for selecting cathode materials as a focus is due to these materials being a limiting factor in terms of cell performance and cost. This review will cover details of the coating materials being sought for cathodes, the techniques to prepare these coatings, and cell performance behavior prior to and after applying these coatings. Before reviewing these areas in detail, it is important to gain further insight into the crystal structures and design of the microstructure within cathode materials, and to review their electrochemical performance.

Layered oxides are the most commonly used cathode materials in LIBs. These materials have a general formula of LiMO₂ where M can be Co, Ni, or Mn. The first layered oxide for Li-ion intercalation was LiCoO₂ (LCO), which was invented by Goodenough, Mizoshima and colleagues in 1976. The t_2g bands of Co³⁺ are redox active. These bands are completely filled and overlap with the O²⁻:2p bands. ^7,38,39 The Co³⁺ t_2g bands lose electron density as the Fermi level shifts toward the top of the O²⁻:2p band during charging of the cathode. As a result, Co³⁺ is oxidized to Co⁴⁺ and almost half of the electrons in its t_2g orbitals can be removed without hampering the structural integrity of these materials. This process yields a reversible capacity of 140 mA·h·g⁻¹ vs Li/Li⁺ at around 4 V. In the LiMO₂ structure, Li and M occupy the 3a and 3b sites, respectively, within the lattice. The Li and M form an ABC stacking sequence, occupying alternate octahedral sites between the (111) planes within rock-salt structures that yield a cubic-close packing of oxide ions (space group R$\bar{3}$m). The unit cell for this layered structure consists of sheets of CoO₆ octahedra that are separated by interstitial layers of Li⁺ ions. This crystal structure corresponds to an O₃ phase (Fig. 5a). ⁴⁰ The Li⁺ ions migrate during charging and discharging from one octahedral void to another by passing through a neighboring tetrahedral void because this pathway offers a lower energy barrier to Li⁺ transport.

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From the perspective of orbital overlap and bonding (Fig. 5b), the (n + 1)s, (n + 1)p and nd orbitals of the transition metal (TM) overlap with the s orbitals of O ligands to yield a_1g ^* , t_1u ^* , e_g ^* antibonding and a_1g , t_1u, e_g bonding states. ⁴¹ The t_2g and t_2g ^* are completely filled bonding states and partially filled antibonding states, respectively, which form when p orbitals from the O ligands interact with the t_2g orbitals of transition metal. The electrochemistry of the metal oxides is controlled by the (M–O)* band formed by the e_g ^* and t_2g ^*. A loss of electrons from the (M–O)* band or the d-band contributes to Li⁺ insertion during electrochemical charging because the 2p orbitals of the O ligands participate in the M–O band formation. ^7,42,43 During charging, the oxidation state of the Co changes (e.g., Co³⁺ to Co⁴⁺ during charging), which results in charge compensation during Li⁺ ion removal and formation of non-stoichiometric Li_1−x CoO₂ compounds. Though the theoretical capacity for Li_xCoO₂ is 280 mA·h·g⁻¹, degradation of its performance might occur at elevated temperatures. Deep cycling or a steady draw of power from the battery for an extended period of time, can result in thermal or structural instabilities that occur at high voltages.

A structural transformation from hexagonal to monoclinic will occur for LiCoO₂ if almost half of the lithium is extracted from the lattice during the discharge cycle. As a result, the practical capacity for LCO is 140 mA·h·g⁻¹ when charging at a voltage of 4.2 V (vs Li/Li⁺). ^2,7 The Li_xCoO₂ materials offer good electrical conductivity, which is attributed to the ordering of its cations and the direct Co–Co interactions within the planes containing the cobalt species. In addition to this, holes are incorporated into the low spin Co^3+/4+: t_2g bands during the charging process, endowing the Li_xCoO₂ with a metallic character. A significant contribution to the decay in performance of LiCoO₂ is structural degradation within its lattice that occurs at high voltages. These changes possibly result from an uneven lithiation and delithiation process within LMO₂ materials. For example, LiNiO₂ can attain a capacity of 240 mA·h·g⁻¹ and can be prepared at a lower cost in comparison to LiCoO₂. ^44–47 But LiNiO₂ undergoes structural transformations from rhombohedral to monoclinic to a mixed phase. These transformations compromise the material's thermal stability during cycling and results in a decomposition of its structure, such as a release of oxygen from its lattice. Nickel cations can also occupy sites within the lithium layers due to cation mixing with Li⁺, which obstructs Li⁺ migration during the charging and discharging processes. Another layered oxide of potential interest is LiCo_1−xNi_xO₂, which can be considered as an amalgam of LiCoO₂ and LiNiO₂. The LiCo_1−xNi_xO₂ has a higher thermal stability and structural integrity than LiNiO₂ for 0.1 ≤ x ≤ 0.3 at cutoff voltages from 4.2 to 4.3 V (vs Li/Li⁺). It is believed that Mn⁴⁺ can provide structural stability during the electrochemical process of charging and discharging. LiNi_0.5Mn_0.5O₂ has also been synthesized and exhibits a charge/discharge window of 3.6 to 4.3V associated with the transformation between the Ni²⁺/Ni⁴⁺ redox states. Due to the relatively small size of Li⁺ (0.71 Å) and Ni²⁺ (0.69 Å), cation exchange can occur and the Li⁺ can form flower-like structures where it is surrounded by consecutive rings of LiMn₆ and LiNi₆. An irreversible capacity of 200 mA·h·g⁻¹ can be obtained for LiNi_0.5Mn_0.5O₂ with a relatively small amount of capacity fade during the cycle. In contrast, although LiMnO₂ is relatively structurally stable as the e_g band of high-spin Mn³⁺ lies above the O²⁻: 2p levels, the Mn³⁺ is octahedrally coordinated and prone to Jahn-Teller distortions. The Jahn-Teller effect is regarded as a geometric distortion that reduces symmetry and energy of a non-linear molecular system, such as for octahedral complexes since their axial and equatorial bonds are unequal. ^1–3 These properties result in a transformation of LiMnO₂ to a spinel structure upon repeated electrochemical cycling. ^48–50

In 1999, Liu et al. developed a new series of cathode materials containing LiNi_xMn_1−x−yCo_yO₂ (NMC), which can be visualized as a solid solution of LiNiO₂/LiMnO₂/LiCoO₂. ⁵¹ Within an ideal NMC structure, the anions occupy 6c sites to form a close-packed geometry with the 3a and 3b sites occupied by Li⁺ and transition metals, respectively. A variety of NMC compositions have been reported in the literature. Prominent examples include LiNi_1/3 Mn_1/3Co_1/3O₂ (NMC 111 or 333), LiNi_0.5 Mn_0.3Co_0.2O₃ (NMC 532), LiNi_0.6Mn_0.2Co_0.2O₂ (NMC 622), LiNi_0.4Mn_0.4Co_0.2O₂ (NMC 442) and LiNi_0.8Mn_0.1Co_0.1O₂ (NMC 811). For NMC materials, the Mn⁴⁺ is Jahn-Teller inactive and the Ni²⁺ is energetically favored for the electrochemical transformations because the single high spin Mn³⁺ e_g electron is transferred to Ni³⁺. The Ni³⁺ has an octahedral site stabilization energy (OSSE) of −1.35 Δ_o, which is between that of Mn³⁺ (−0.42 Δ₀) and Co³⁺ (−2.13 Δ_o), yielding a competitive structural stability. ¹ The NMC 111 has exhibited promising properties as a cathode material with a specific capacity of 200 mA·h·g⁻¹ for a voltage range from 2.8 to 4.6 V (vs Li/Li⁺) and a reversible capacity of 160 mA·h·g⁻¹ [Fig. 5c].

In 1980, Michael Thackeray reported the formation of LiMn₂O₄, the first spinel oxide for use as a cathode material in LIBs. ^52,53 This material has a relatively low cost to prepare and possesses relatively fast rates of intercalation and deintercalation of Li⁺ ions. The spinel structure contains a face-centered cubic packing created by its anions that occupy Wyckoff position 32e, which includes seven binary and four ternary anion substructures. The tetragonal 8a sites are occupied by lithium ions, and manganese cations occupy 16d sites, whereas the octahedral 16c sites remain empty. These empty 16c sites along with the 8a sites occupied by lithium serve as a three-dimensional (3-D) pathway for the migration of Li⁺ ions. These pathways offer the lowest energy barrier to Li⁺ transport. The drawback of these pathways is that they suffer from severe capacity fading upon electrochemical cycling at elevated temperatures. If the Mn³⁺ concentration is increased, Jahn-Teller distortions will occur upon changes to the crystal structure from a cubic to a tetragonal structure. Another critical issue associated with capacity fading in LiMn₂O₄ cathode materials is the dissolution of Mn from the lattice to the electrolyte (within acidic media), which has been attributed to result from a disproportionation reaction. ^54,55 During the disproportionation reaction, the Mn³⁺ ions within the lattice are both reduced and oxidized to Mn²⁺ and Mn⁴⁺, respectively, in the presence of H⁺. The Mn²⁺ is leached into the electrolyte and Mn⁴⁺ remains within the solid as follows: ⁵⁶

$$\begin{eqnarray}&&2M{n}^{3+}\to M{n}^{4+}+M{n}^{2+}\end{eqnarray} \tag{ 2 }$$

The dissolution of Mn²⁺ can also cause anode poisoning, thereby limiting the lifetime of the cell. Due to Mn dissolution, Li₂MnO₃ could be obtained from the spinel material as governed by the following reaction at a voltage of 4 V (vs Li/Li⁺): ^57,58

$$\begin{eqnarray}&&L{i}_{2}M{n}_{2}{O}_{4}\to L{i}_{2}Mn{O}_{4}+MnO\end{eqnarray} \tag{ 3 }$$

MnO dissolution into the electrolyte results in an increase in the Li/Mn ratio in the residual structure along with concomitant oxidation of Mn^{3 +} to Mn⁴⁺. Various methodologies have been implemented to suppress Mn dissolution and to enhance cell performance. Dopants such as Mg, Al, Zn, Ti, Ni, Cu and Li have been believed to disrupt Mn–Mn interactions and to result in an improved reversibility of the charging and discharging of these cathode materials. ^13–18 Another approach is to decrease the surface area of the cathode material accessible to the electrolyte by increasing the size of the LiMn₂O₄ particles. Although larger particles increase the electrode density and suppress Mn dissolution, it also results in an overall degradation of cell performance. Coating materials such as Al₂O₃, TiO₂, and B₂O₃ have been reported to improve cell characteristics because of the ability of the coating to prevent direct contact between the surfaces of the active cathode materials and the electrolyte. ^14–17 A promising spinel composition for use as a cathode material is LiNi_0.5Mn_1.5O₄, which is also referred to as high voltage spinel or HVS. ^1–5 Jahn-Teller distortion of Mn³⁺ can be eliminated if LiMn₂O₄ is doped with nickel. The inclusion of Ni³⁺ assists in promoting the oxidation of Mn³⁺ to Mn⁴⁺ during potential cycling. Depending on the conditions for its synthesis, LiNi_0.5Mn_1.5O₄ crystallizes in one of two forms: (i) an ordered lattice with a stoichiometric composition of the space group P4₃32; and (ii) a disordered lattice with a non-stoichiometric composition of the space group F$\bar{d}$3m. ⁷ For the ordered structure, empty sites occupy both the 4a and 12d positions, whereas the Li, Mn, and Ni occupy 8c, 12d and 4b sites, respectively. The Li⁺ ions can take two pathways for diffusion: (i) following an 8c to 12d to 8c through the partially occupied 12d sites; or (ii) following a 8c to 4a to 8c pathway through the empty 4a sites. For materials with a disordered lattice, the 16c positions are vacant and Li⁺ ions occupy 8a sites, whereas the Ni and Mn occupy 16d octahedral sites but with a random distribution. ^59,60 The resulting diffusion pathway in this disordered lattice is 8a to 16c to 8a for lithium migration. The operating window for LiNi_0.5Mn_1.5O₄ is increased to 4.7 V (vs Li/Li⁺), in comparison to ∼4 V (vs Li/Li⁺) for LiMn₂O₄, due to the formation of the Ni²⁺/Ni⁴⁺ and Mn³⁺/Mn⁴⁺ redox couples. The experimentally demonstrated capacity for LiNi_0.5Mn_1.5O₄ is 140 mA·h·g⁻¹. Although its voltage is one of the highest achievable for cathode materials, this voltage is outside of the stability window of the electrolyte. This results in the formation of the SEI and, therefore, affects the reversibility of the cell. It is also difficult to synthesize and to stabilize the high oxidation state of the M^3+/4+ species in a LiM₂O₄ spinel.

Polyanion compounds are regarded as a relatively new class of cathode materials, which were initially discovered by Goodenough and Manthiram. ^21,24,26,39 Large tetrahedral polyanion groups (XO₄)³⁻ (X = Ar, W, P, Si, Mo) stabilize the structure, and increases the redox potential of these materials by occupying lattice positions. The general formula for the polyanions is LiM (XO₄)³⁻ (M=Fe, Ni, Co, and Mn). ¹¹ Polyanions are relatively safe, environmentally friendly, cost effective to prepare, and stable to electrochemical cycling. Polyanion units share strong covalent bonds with the MO_x polyhedra. Isolated TM-oxygen polyhedral groups are often the determining factor for the electronic structure of polyanions. As a result of this, polyanion groups are primarily responsible for inducing a separation of the TM valence electrons resulting in a relatively low electrical conductivity. The most extensively investigated polyanion is an ordered olivine structure of LiMPO₄ (M=Mn, Co, Ni, and Fe) [Figs. 6a, 6b)]. These phosphates have a slightly distorted hexagonal close packed (hcp) lattice that results in the formation of materials of the D_2h space group Pmnb. ^61,62

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A representative phosphate based polyanion is LiFePO₄ (LFP) with an olivine structure, which can achieve high powers and can exhibit structural and thermal stability [Fig. 6a]. The P atoms are in tetrahedral sites of the lattice, and the Li/Fe²⁺ occupy octahedral sites. LiFePO₄ is an attractive cathode material for its superior properties such as its thermal and electrochemical stability at higher operating voltages, its relatively low cost to prepare, its excellent stability to electrochemical cycling, a high theoretical capacity of 170 mA·h·g⁻¹, and a stable redox potential of 3.5 V (vs Li⁺/Li). The LFP crystals possess anisotropic features, which suggests that their electrochemical performance can depend on their microstructure. ^61–63 The anisotropic structure of LFP is attributed to the one-dimensional channels of lithium ions that form along the [010] direction surrounded by the tetrahedral PO₄. The corners of the unit cell in LFP share FeO₆ and the edges share LiO₄, which propagate along the b-axis in a direction parallel to the c-axis. The FeO₆ octahedra are separated by PO₄ tetrahedra, which interrupt electron conduction through the FeO₆ scaffolding. This structure results in a relatively low electrical conductivity of LFP (∼10⁻⁹ S cm⁻¹ at room temperature or RT). This low conductivity is one of the main hindrances to a more wide-spread utilization of LFP as a cathode material. ^64,65

To improve the conductivity of these materials three approaches have been used: (i) doping Fe with other conducting elements; (ii) enhancing the available surface area through reducing the particle size; and (iii) applying a conductive coating on the LFP particles. LiMnPO₄ can achieve higher operating potentials and energy densities than LiFePO₄ [i.e., LMP achieving 4.1 V (vs Li/Li⁺) and 700 W·h·kg⁻¹]. ^64–66 Olivine LiMnPO₄ and LFP are isostructural with an almost identical electrochemical capacity. The Mn³⁺ ions can cause significant lattice distortions due to the Jahn-Teller effect, and these materials can also suffer from a low electrical conductivity (∼10⁻¹⁴ S cm⁻¹). In comparison to the volume change between LiFePO₄ and FePO₄, the LiMnPO₄ tends to exhibit a higher volume change of 9.5% between the LiMnPO₄ and MnPO₄ phases. The LiCoPO₄ and LiNiPO₄ are also attractive alternatives as cathode materials as they are capable of achieving potentials of 4.8 and 5.1 V (vs Li/Li⁺), respectively. ^62–66 At these high voltages the electrolyte will decompose and compromise the overall electrochemical performance.

Orthosilicates (Li₂MSiO₄, M = Fe, Co, Ni, Mn) is another class of polyanion cathode material that has attracted interest due to their properties. These properties include their improved safety, lower cost, and enhanced electrochemical performance. Layers of silicates and TMs within orthosilicates share edges, within a Pmn2 structure. ^11,67–69 Orthosilicates possesses a framework that improves the reversibility of changes to the oxidation state of the TM [Fig. 6b]. Theoretically the SiO₄ ⁴⁻ groups enable the valence of the 3d metals to change between +2 and +4 to accommodate the de-intercalation of two lithium ions per formula unit. For Li₂MSiO₄, the theoretical capacity is ∼333 mA·h·g⁻¹ if two Li⁺ ions are fully extracted per formula unit. ^67,68 Generally, Li₂MSiO₄ compounds exhibit several polymorphs categorized as β and γ phases, which depend upon the distribution of cations within the two available tetrahedral sites. For Li₂FeSiO₄, the theoretical capacity is reported to be 166 mA·h·g⁻¹ because the Fe³⁺/Fe²⁺ redox couple is relatively difficult to access: ^70,71

$$\begin{eqnarray}&&L{i}_{2}FeSi{O}_{4}\to LiFeSi{O}_{4}+L{i}^{+}+{e}^{-}\end{eqnarray} \tag{ 4 }$$

The Li₂MSiO₄ (M=Mn, Mn_0.5Fe_0.5, Fe, Co) materials follow the Curie-Weiss law for their temperature dependence and long-range M–O–Li–O–M interactions give rise to antiferromagnetic ordering at low temperatures. ^70–72 The Li₂CoSiO₄ polymorphs (Pmn2, Pbn2, Pmnb, and P2₁ln) have relatively poor electrochemical performance and their structural differences do not significantly alter the potentials for lithium insertion and removal. The Li₂MnSiO₄ possess a higher specific capacity (i.e., ∼333 mA·h·g⁻¹) and a higher redox potential than Li₂FeSiO₄ because two Li⁺ ions can participate in the electrochemical transformation. The redox couples for Li₂MnSiO₄ are Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺ with transformations at 4.1 and 4.5 V (vs Li/Li⁺), respectively. The Li₂MnSiO₄ does, however, suffer from capacity fade and degradation due to its relatively low electrical conductivity (∼5 × 10⁻¹⁶ S cm⁻¹ at room temperature). ⁷² It has been proposed that the instability of the Co⁴⁺ and Mn⁴⁺ species occupying the tetrahedral sites can result in a poor performance for the corresponding Li₂MSiO₄ (M=Mn, Co) materials because of the tendency of these metal ions to prefer octahedral coordination. ^64–66

Changes to the electronic structure (orbital filling) of orthovanadate polyanions have been simulated using density function theory (DFT) by substituting SiO₄ ⁴⁻ with VO₄ ³⁻ polyanions. ¹¹ These DFT calculations have indicated that substituting 12.5% of VO₄ ³⁻ into these sites can significantly enhance their capacity. Fluorophosphates, fluorosulphates, and borates have also been evaluated as potential cathode materials. ^73,74 Fluorophosphates (e.g., PO₄F) is assumed to be an amalgam with contributions from the inductive effect of the phosphate and the relatively high electronegativity of the F⁻ anion. The LiVPO₄F cathode materials can achieve a potential of ∼4.2V (vs Li/Li⁺) based on the reversibility of the V³⁺/V⁴⁺ redox couple. Preliminary investigations have revealed an excellent thermal stability for this material and a capacity of ∼155 mA·h·g⁻¹. ^74–76 In addition, Li₂CoPO₄F and Li₂NiPO₄F have also been proposed as suitable candidates for cathode materials. ^77–79 Although the theoretical capacity is estimated to be around 310 mA·h·g⁻¹, the stability to potential cycling (i.e., charge and discharge cycles) and their specific capacity still need to be validated due to the absence of an electrolyte with an appropriate potential window for these materials. In addition, fluorosulphates such as LiMgSO₄F can achieve Li⁺ ion conduction with a reversible capacity of ∼140 mA·h·g⁻¹ around 3.6 V (vs Li⁺/Li) at a rate of C/10 (C-rates are the determining factor for the charging and discharging capability of the secondary batteries). ^80,81 The LiFeSO₄F have a relatively high ionic conductivity (∼4 × 10⁻⁶ S cm⁻¹) in contrast to that of LiFePO₄, which eliminates the need of applying a coating material to the LiFeSO₄F. ⁸¹ Borate-based polyanion compounds such as LiMBO₃ (M=Mn, Fe, Co) can achieve a reversible capacity of ∼200 mA·h·g⁻¹ around 3 V (vs Li/Li⁺). Initial calculations have indicated that LiFeBO₃ cathode materials can suffer from surface poisoning, which will decrease its achievable potential and performance. ^82,83

Alumina (Al₂O₃) coatings

Alumina (Al₂O₃) reacts with LiOH and Li₂CO₃ in Ni-rich cathode materials at elevated temperatures. The Al₂O₃ exhibits a relatively high hardness and a resistance towards chemical attack by acidic and alkaline species. Coatings of Al₂O₃ also improve the chemical stability of the interface along with providing pathways for Li⁺ diffusion. Reports indicate that 1.5 wt% Al₂O₃ coated onto over lithiated Li_1.17Ni_0.135Mn_0.56Co_0.135O₂ cathode material has yielded a higher Li⁺ diffusion and a higher specific exchange current at lower temperatures than for the uncoated cathode materials. ⁸⁴ The uncoated Li_1.17Ni_0.135Mn_0.56Co_0.135O₂ exhibit a much steeper increase in charge transfer resistance (R_ct) as a function of increasing temperature. The coated Li_1.17Ni_0.135Mn_0.56Co_0.135O₂ achieved a specific capacity >300 mA·h·g⁻¹ when charged to 4.5 V (vs Li/Li⁺, C/20). In contrast, the Al₂O₃ created a close interconnectivity between the NMC 532 cathode particles at high sintering temperatures, and indicated a relatively homogeneous lithium doping of the Ni-rich cathode and its coating. ^17,85 As the coating content decreases from 2% to 1% and 0.5%, the initial capacity increases from 168.5 to 176.4 and 179.4 mA·h·g⁻¹ (3 to 4.5 V at C/10),respectively. These results indicate that a thinner coating could mitigate the capacity loss. In comparison, an Al₂O₃ coated lithiated LCO/Li yielded a specific capacity of ∼190 mA·h·g⁻¹ when charged to 4.5 V at C/9 (vs Li/Li⁺). ^16,85,86 A separate study investigated the effects of Al₂O₃ and LiAlO₂ coatings applied to NMC 622 particles. ⁸⁷ This study found that ultrathin coatings of LiAlO₂ on the NMC 622 cathode particles maintained a reversible capacity up to 149 mA·h·g⁻¹ after 350 cycles. The Al₂O₃ coated NMC 622 exhibited rate capacities of 196.9 mA·h·g⁻¹ and 131.9 mA·h·g⁻¹ at 0.2C and 3C charging rates, respectively.

The effect of applying two distinct types of coatings has also been studied using alumina-based materials. For example, NMC 622 particles have been coated with both Al₂O₃ and a conductive copolymer poly(3,4-ethylenedioxythiophene)-co-poly(ethylene glycol) (or PEDOT-co-PEG). ⁸⁸ The cycle stability of these cathode materials were significantly enhanced under harsh testing conditions and the coated electrode retained 94% of its initial capacity even after 100 cycles. Another study prepared an NMC 532 cathode material with a coating containing both Al₂O₃ and AlPO₄, which were deposited using a wet chemical method outlined in Fig. 8. ⁸⁹ The NMC 532 particles with a Al₂O₃/AlPO₄ coating exhibited a lower polarization resistance (R_p) and a lower contact transfer resistance (R_ct) than the uncoated particles or those coated with a single layer, which resulted in improved electrochemical properties of the coated NMC materials.

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It is important that the coating sufficiently covers all surfaces of the active cathode materials to realize the benefits and limitations of the coating in contrast to the properties of the pristine materials. In the case of the doubly coated NMC materials with a coating that contained both Al₂O₃ and AlPO₄ its thickness was ∼20 nm, whereas the singly coated NMC materials had a coating of either Al₂O₃ or AlPO₄ with a thickness of ∼30 to 40 nm (Fig. 9). Although the double coating produced a thinner film over the NMC particles, this coating was also denser than the single coatings. These results suggested that the reactants of the second layer (applied using a solution-based method depicted in Fig. 8) strongly interacted with and potentially penetrated into the first layer. The double coating process was not merely one coating on top of another coating, but instead the combination of the two chemistries altering the properties of the resulting film. The resulting Al₂O₃/NMC, AlPO₄/NMC and Al₂O₃–AlPO₄/NMC particles exhibited discharge capacities of 179 mA·h·g⁻¹, 174 mA·h·g⁻¹ and 180 mA·h·g⁻¹, respectively.

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In contrast, a discharge capacity of 174 mA·h·g⁻¹ was achieved for the core NMC cathode material in the absence of any coating. The doubly coated materials exhibited a slight improvement in its capacity, which was attributed to combining the properties of both types of coatings. Double layer coatings provide better protection and less reactivity towards the electrolyte hence stabilizing the interface. However, the thickness of the coating material and relative ratios of the components therein should be carefully tailored to minimize the obtain least contact resistance and to enhanced Li⁺ transport.

In a recent study, Al₂O₃ coatings were applied to NMC 532 particles using two different methodologies: (i) a solution-based coating method; and (ii) atomic layer deposition (ALD). ⁹⁰ When the samples were prepared by solution-based processes, the particles were annealed at 800 °C for 8 h to strengthen the interactions between the Al₂O₃ coating and the core NMC 532 particles. This heat treatment likely resulted in the formation of LiAlO₂ species at the interface between the cathode particles and their coating. When the alumina loading is decreased from 0.5 wt% to 0.2 wt%, the capacity retention of the samples coated by solution-based techniques exhibited a considerable decrease, which was attributed to a decreased coverage of alumina over the surfaces of the particles. In comparison, the alumina coatings prepared using ALD processes exhibited less influence from the inclusion of a post-coating annealing step. A separate study found that low temperature ALD of Al₂O₃ onto NMC 622 prevented the leaching of transition metals and preserved the bulk lattice structure after potential cycling for 1,400 cycles. ⁹¹ Figs. 10a and 10b depict the data from potential cycling that was averaged over at least 4 independent experiments to assess the significance of the results of the ALD-based coating in comparison to the pristine NMC particles. The values of 4 and 10 indicate the number of ALD growth cycles used to prepare the respective coatings.

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The ALD-10@NMC-622 and ALD-4@NMC-622 each outperformed the pristine NMC 622 particles with specific discharge capacities of 127.2 ± 0.6 mA·h·g⁻¹, 126.5 ± 0.4 mA·h·g⁻¹ and 123.3 ± 0 mA·h·g⁻¹, respectively. The Coulombic efficiency increased by up to 99.8% within the first 20 cycles for the coated particles, which suggested that the coating increased the energy barrier for the ionic charge transfer. The thin alumina coating was sufficient to suppress unwanted side reactions at the interface with the electrolyte at high voltages. ⁹² Increasing the C-rate from 0.1C to 2C resulted in a higher polarization and a decrease in the capacity of the Al₂O₃ coated LNMO materials. Some of the initial capacity was recovered after a prolonged process of charging and discharging at 4C. This process likely results in a severe degradation of the core LNMO particles during the charge-discharge cycling [Figs. 10c, 10d].

Additional studies have also indicated that the retention of capacity for cathode materials could be increased with the inclusion of an alumina coating. For example, it has been reported that Al₂O₃ coated NMC 622 exhibited an increase in retention of capacity (relative to pristine NMC 622 cathodes) from 91% to 93% after 100 cycles (at a 0.5C rate). ⁸⁸ A series of Al₂O₃ coated NMC 532 cathode particles were also prepared by varying the annealing temperature, as well as the selection of precursors, solvents and concentrations of the precursors (e.g., 0.2 to 2 wt%). ⁹³ These precursors included aluminum nitrate, aluminum acetate, aluminum isopropoxide, and aluminum chloride. The solvents evaluated in this study were methanol (MeOH), ethanol (EtOH), xylene and water. Annealing at 800 °C formed additional LiAlO₂ phases and resulted in a crystallization of the interface between the coating and the particles regardless of the procedure used to prepare the coatings. This annealing process improved the reversibility of the potential cycling for the 0.2 wt% Al₂O₃ coated NMC 532 when compared to the performance of pristine cathode materials. A related study investigated the effects of changing the composition of transition metals within LiNi_xMn_yCo_1−x−yO₂ cathode materials along with applying an alumina coating. ⁹⁴ The alumina coatings diffused into the bulk cathode material after annealing at high temperatures, such as those cathodes prepared from LiNi_0.5Mn_0.3Co_0.2O₂ (NMC 532), LiNi_0.6Mn_0.2Co_0.2O₂ (NMC 622), and LiNi_0.8Mn_0.1Co_0.1O₂ (NMC 811) (Fig. 11). At an Al₂O₃ loading of 0.5 wt%, relatively smooth interfaces were obtained for each of these coated NMC particles after annealing at 800 °C. Upon increasing the loadings to 5 wt%, small particles could be seen on the surfaces of the NMC 532 and NMC 622, possibly due to aggregation of excess Al₂O₃. For NMC 811, the Al₂O₃ coating might have diffused into the bulk particles after annealing such that there were no Al₂O₃ particles observed on their surfaces even at higher loadings up to 5 wt% (Fig. 11). The fundamental understandings on interphase chemistry depict that a careful selection of the preparation method, solvents, precursors, temperature, annealing conditions, coating nature and loadings has a vital impact on the cathode performance.

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Zirconium dioxide (ZrO₂) coatings

A zirconium dioxide (ZrO₂) coating can act as a scavenger of HF and to assist in mitigating the dissolution of Mn, which hinders detrimental phase transformations at the interface between the cathode particle and the electrolyte. Increasing the operating temperature affects the cell performance via two mechanisms: (i) improved lithium ion and electron transport at elevated temperatures, which increases their electrochemical performance; and (ii) a faster Mn dissolution and electrolyte decomposition, which deteriorates their electrochemical performance. Ultrathin films of crystalline ZrO₂ have been deposited onto spinel LiMn₂O₄ cathode materials using ALD at 120 °C. ⁹⁵ The thickness of these layers could be adjusted up to 2.9 Å nm. Over this range of thicknesses, the thickness that maximized the electrochemical performance of these materials was found to be 1.74 nm. The improved performance of the 1.74-nm thick ZrO₂ coating on the cathode particles was attributed to a more uniform transport of both electrons and lithium ions through these surfaces, which led to a lower polarization resistance (transfer resistance between electrodes and electrolyte, R_p). In addition, strong oxygen bonds formed between the ZrO₂ coating and the LiMn₂O₄ cathode particles could reduce the Mn³⁺/Mn⁴⁺ redox shift at high potentials. It has also been found that ZrO₂ coated NMC 111 cathode materials exhibited an enhanced stability towards potential cycling and a high rate capability at a high cut-off voltage of 4.5 V (vs Li/Li⁺). ⁹⁶ The ZrO₂ coated cathode particles had a cell resistance that increased from 70 to 210 Ω between the 2nd and 40th cycle, which are lower values than those of the bare cathode particles. These results supported the observation that thickness of the ZrO₂ coatings affects the performance and can improve the diffusion of lithium during the charging and discharging processes.

A series of reports suggest that Zr can also yield improved performance of cathode materials through doping. For example, the Zr doping of NMC 532 can improve the stability of these cathode materials to potential cycling. ⁹⁷ The use of Zr doping instead of a coating can increase the number of cycles and capacity achievable for cathode materials like Li_1.2Ni_0.13Mn_0.54Co_0.13O₂, LCO, and NMC 111, which has been attributed to the high bond energy of the Zr–O bonds. ^98–100 For example, Li(Ni_0.5Mn_0.3Co_0.2)_0.99Zr_0.01O₂ retained 84% of its capacity after 1C for 100 cycles at 4.6 V in comparison to a retention of 69% of the initial capacity for pristine cathode materials under similar cycling conditions. Discharge values up to 100 mA·h·g⁻¹ (at 8C) have been reported for Zr doped cathode materials due to a decrease of R_p. In addition, significant damage was observed in the undoped particles after 100 cycles up to 4.6 V (vs Li/Li⁺). The relative stability of the Zr doped materials was attributed to the more negative value (compared to Ni and Mn) for ${{\rm{\Delta }}}_{f}{G}_{{{\rm{ZrO}}}_{2}}$ = −1042.8 kJ mol⁻¹. The lithium diffusion coefficient (D_Li+) for the Zr doped materials was ∼1.14 × 10⁻¹⁰ cm² s⁻¹, which was higher than that for the undoped cathode materials (i.e., 1.15 × 10⁻¹¹ cm² s⁻¹) after the first cycle. The improved electrochemical performance of the Zr doped materials could be attributed to a lower degree of cation mixing, better kinetics for lithium transport and a lower overall impedance.

The benefits of Zr doping could extend to thin films applied to cathode materials. The benefits of the Zr incorporation depends upon the techniques used to apply these coatings and the uniformity of these films. A ZrO₂ coating on LiNi_0.8Co_0.2O₂ was investigated as a function of its thickness (including the uniformity of the distribution of the coating material) on the cathode material. ¹⁰¹ The pristine LiNi_0.8Co_0.2O₂ yielded an initial capacity of 181.1 mA·h·g⁻¹ vs 170 mA·h·g⁻¹ for the ZrO₂ coated cathode. Capacity retention did, however, improve through incorporation of the coating as the capacity decreased by ∼3% for the coated cathode in comparison to a decrease of ∼25% in capacity for the uncoated cathode after 50 cycles. The use of a ZrO₂ film also enhanced the structural stability of Li_1−xCoO₂ (0 < x < 0.7) cathode materials by suppressing the c-axis expansion or phase transition, which indicated that the delithiation process did not alter the hexagonal symmetry of this cathode material. Bare Li_1−xCoO₂ retained ∼30% of its original capacity after 30 cycles, whereas the coated sample did not yield an observable decrease in its capacity even after 70 cycles. ^102–104 An NMC 532 cathode material coated with ZrO₂ using ALD techniques (e.g., 5 cycles) exhibited a high Coulombic efficiency, an improved stability to electrochemical cycling, and an improved range from 2.5 to 4.5 V (vs Li/Li⁺). ¹⁰⁵ During ALD the metal amide precursor initially interacts with the terminal OH groups on the surfaces of the cathode particles via a chemical absorption process. During this step, the metal nitrogen bond is broken, which results in the formation of a new metal bond along with the release of volatile dialkylamine by-products. In the second step, metal hydroxyl bonds and additional dialkylamines are generated when H₂O reacts with the surface bound metal amide species. Unreacted water and the dialkylamine by-products are purged from the system under an N₂ atmosphere. This process is repeated as necessary to prepare films of a different thickness. The resulting ZrO₂ coated NMC 532 particles had initial discharge capacities of 198.5 mA·h·g⁻¹, 216.5 mA·h·g⁻¹, and 206.1 mA·h·g⁻¹ for ZrO₂ coatings prepared from 2, 5, and 8 ALD cycles, respectively. In comparison, the discharge capacity of the pristine cathode materials was 190.1 mA·h·g⁻¹. The ZrO₂ coating prepared by 5 ALD cycles retained ∼89% of its discharge capacity (i.e., 155.3 mA·h·g⁻¹) after 50 cycles, in comparison to a decrease to 101.7 mA·h·g⁻¹ for the pristine, uncoated cathode particles. The Li⁺ diffusion constants for these materials can be calculated from their Nyquist plots [Figs. 12a, 12b] using the following equations:

$$\begin{eqnarray}&&{D}_{{{\rm{Li}}}^{+}}=\displaystyle \frac{0.5{R}^{2}{T}^{2}}{{n}^{4}{F}^{4}{\sigma }_{\omega }^{2}{A}^{2}{C}^{2}}\end{eqnarray} \tag{ 7 }$$

where R, A, F, T, n and C are the molar gas constant, surface area from BET calculations, Faraday's constant, absolute temperature, number of electrons/molecules in the reaction, and the lithium ion concentration, respectively. The value ${\sigma }_{\omega }$ is the Warburg parameter, which is calculated as follows:

$$\begin{eqnarray}&&{Z}_{{\rm{img}}}=k-{\sigma }_{\omega }{w}^{-1/2}\end{eqnarray} \tag{ 8 }$$

Z_img = imaginary impedance

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w = angular frequency, and k is a constant

R_f and R_ct correspond to Li⁺ ion diffusion on the surface and interface layer, respectively. R_f is attributed to the insulating solid electrolyte interface (SEI) layer formed at the surface (in the high frequency region). R_f is also formed due to electrolyte decomposition or organic compound breakdown on the electrode surface. R_ct is obtained from the semi-circle in the low-frequency region and corresponds to the resistance to electron transfer processes from one phase to another, such as from liquid to solid or vice versa.

The values for D_Li+ were found to be 7.38 × 10⁻⁹ cm² s⁻¹, 9.63 × 10⁻⁹ cm² s⁻¹, 2.11 × 10⁻⁸ cm² s⁻¹ and 1.69 × 10⁻⁸ cm² s⁻¹ for the pristine NMC, and the NMC coated with ZrO₂ prepared by 2 ALD cycles (2-ZrO₂), 5 ALD cycle (5-ZrO₂), and 8 ALD cycles (8-ZrO₂), respectively. It has been proposed that ultrathin coatings prepared from an amorphous phase can suppress the increases to the cell impedance and favor lithium diffusion due to the short-range order within these structures.

Alternatively, D_Li ⁺ can also be calculated from the relationship between peak current (I_p ) and san rate from the cyclic voltammetry curves as follows:

$$\begin{eqnarray}&&{I}_{p}=\left(2.69\times {10}^{5}\right){\rm{A}}\,{n}^{3}/2{\left({{\rm{D}}}_{{{\rm{Li}}}^{+}}\right)}^{1/2}{\rm{C}}\,{{\rm{V}}}^{1/2}\end{eqnarray} \tag{ 9 }$$

where I_p is peak current (mA), A is the area of the electrode (154 mm²), n is the number of electrons involved in electronic transfer reaction, C is the lithium concentration, and V is scan rate (mV s⁻¹). The epitaxial formation of ZrO₂ coatings on spinel LiMn₂O₄ particles using ALD techniques can yield a high degree of conformity and control over film thickness. ¹⁰⁶ LiMn₂O₄ particles coated with ZrO₂ using 6 ALD cycles exhibited an initial discharge capacity of 136.0 mA·h·g⁻¹ at 1C (55 °C), which was higher than the capacity of 124.1 mA·h·g⁻¹ observed for the pristine cathode particles. A more distinct effect of the ZrO₂ coating when prepared by ALD techniques was observed for the retention of capacity at high rates of charge and discharge, as well as at elevated temperatures. For example, at a current density of 600 mA·h·g⁻¹ (∼5C at 55 °C), the ZrO₂ coatings prepared by 2 ALD cycles exhibited a high initial discharge capacity of 123.4 mA·h·g⁻¹ in comparison to the discharge capacities of 112.7 mA·h·g⁻¹, 88.5 mA·h·g⁻¹, and 114.6 mA·h·g⁻¹ for the ZrO₂ from 6 ALD cycles, 10 ALD cycles, and the pristine (or uncoated) cathode particles, respectively. The application of a ZrO₂ coating to cathode particles can significantly alter their electrochemical stability and the properties of lithium diffusion.

Magnesium oxide (MgO) coatings

A coating of MgO has been found to have a beneficial impact on battery performance. Studies have shown that MgO coated LCO cathodes can be cycled between 2.5 and 4.7 V (vs Li⁺/Li) yielding a high capacity of 210 mA·h·g⁻¹ and without material degradation. ¹⁰⁷ The MgO coating protected the active, core material from the acidic electrolyte, which prevented Co dissolution from the LCO particles. An Li₂CO₃ insulating phase can form at the LCO/electrolyte interface through a series of electrochemical reactions, but its formation was prevented up to 4.4 V by inclusion of the MgO coating. ¹⁰⁸ Additionally, the MgO/LCO interface on the LCO particles also protects against the electrochemical reactivity of adsorbed ethylene carbonate at low voltages.

A coating of MgO prepared at a loading of <1 wt% by ALD methods on NMC 532 cathode materials provides better Li⁺ ion diffusion pathways as indicated by a relatively high lithium diffusion coefficient (D_Li ⁺) at a low overpotential. ¹⁰⁹ Other studies involved the use of magnesium ethoxide [Mg(OEt)₂] as a source of Mg and hydrous ethanol as a solvent to coat NMC 811 cathode particles through solution-based methods, which were followed by heat treatment to form an MgO coating. This route resulted in a change in the crystal structure of the surface states at the interface with the coating and a change in cycling performance. ¹¹⁰ Initial charge and discharge capacities of the MgO coated NMC 811 particles were 225.6 mA·h·g⁻¹ and 190.1 mA·h·g⁻¹, respectively, and the Coulombic efficiency was 84.1%. In contrast, the charge and discharge capacities for the uncoated NMC 811 particles were 235.2 mA·h·g⁻¹ and 194.2 mA·h·g⁻¹, but the Coulombic efficiency was ∼83%. Due to the electrochemical inactive nature of the Mg²⁺ ions, the initial charge and discharge capacities for the coated cathode particles were lower than that of the pristine cathode particles. The enhanced Coulombic efficiency of the MgO coated NMC 811 was attributed to a decrease in the place exchange or mixing of Ni and Li within the NMC lattice. Potential cycling of the MgO coated and uncoated samples [Figs. 13a, 13b)] revealed three pairs of anodic and cathodic peaks around 3.8, 4.0, and 4.2 V (vs Li⁺/Li), corresponding to the phase transition from hexagonal to monoclinic (H₁ → M), monoclinic to hexagonal (M → H₂), and hexagonal to hexagonal (H₂ → H₃), respectively. The electrochemical voltage difference, ΔV, between the anodic and cathodic peaks for the first cycle were 0.162 V for the coated particles and 0.173 V for the pristine particles. These results indicated that the MgO coated cathode particles possess a smaller electrochemical polarization (and a higher degree of reversibility) than the pristine NMC 811 particles.

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For layered LCO cathode particles, it was determined that the effects of surface modifications with MgO and doping with Mg did not alter the electrochemical potential window between 2.9 to 4.3 V (vs Li/Li⁺). ¹¹¹ The LCO particles were modified by solution-phase processing with Mg(CH₃COO), followed by a heat treatment of the coated particles at 600 °C. After the 30^th cycle, the cell capacities for the pristine and MgO modified LCO particles were 113 mA·h·g⁻¹ and 135 mA·h·g⁻¹, respectively. The capacity retention for the MgO coated, Mg doped, and pristine LCO particles were 86%, 90%, and 10%, respectively. These results indicate that the replacement of Co by Mg in the LCO lattice has a positive impact on the capacity retention, although Mg did not improve the stability to potential cycling. Thin films of MgO coated on LCO were also prepared by pulsed laser deposition (PLD), which revealed that the MgO modification suppressed an increase in resistance following repetitive Li⁺ insertion and extraction up to 4.2 V (vs Li/Li⁺). ¹¹² The charge transfer resistances (R_ct) for bare and coated LCO films were found to be 250 Ω and 750 Ω, respectively. A probable reason for the increase in charge transfer resistance for the coated LCO films could be due to the formation of an electrochemically inactive layer. Additional reports in the literature evaluated cobalt dissolution at potentials <4.2 V (vs Li/Li⁺), and found that the separation in peak potentials was not due to Co⁴⁺ dissolution. ¹¹³ Another explanation for the enhanced electrochemical properties of the MgO coated cathode is an increased structural stability due to the comparable ionic radii of Mg²⁺ (86 pm) and Li⁺ (90 pm). Hence, Mg²⁺ could migrate into the LiO₂ layers and could increase the attraction of adjacent CoO₂ layers and lead to the stabilization of these layers. Nano-crystalline MgO coatings deposited via sol-gel techniques onto layered LCO yielded an improved stability to potential cycling. ¹¹⁴ After the 40th cycle, the pristine LCO had a 13 mA·h·g⁻¹ discharge capacity in comparison to a discharge capacity of 120 mA·h·g⁻¹ for the MgO coated LCO. Pristine LCO exhibited a faster degradation in comparison to the MgO coated LCO possibly due to a mechanical failure of the uncoated electrodes due to the formation of fractures induced by cumulative stresses in these materials. Preparing MgO coatings with >1 mol% of the Mg precursor can, however, compromise the stability of the cathode materials with potential cycling along with an increase in the degradation of the discharge capacity. There was no direct evidence of Mg²⁺ substitution within the LCO materials. Future studies could include a detailed analysis of changes to the structure of the LCO and MgO/LCO interface as a function of annealing temperature for these sol-gel derived coatings.

Additional studies reported the effects of various thermal treatments to MgO coated LCO on their electrochemical performance. For example, MgO coated LCO annealed at 810 °C exhibited a high initial capacity and an ability to retain capacity over the voltage range from 3 to 4.35 V (vs Li/Li⁺) at 0.2C. ¹¹⁵ No phase transformations were observed at the interface between the MgO coating and the LCO cathode during the charging and discharging cycles. A separate study provided further insight into the effects of the MgO coating in comparison to an Al₂O₃ coating on layered LCO materials. ¹¹⁶ These materials were evaluated in 18650 li⁺ ion cells (18 mm diameter x 65 mm height cylindrical cell). The active cathode material was coated with either a Mg oxide or Al oxide using alkoxide-based solutions followed by their heat treatment between 300 and 500 °C. The ability of these cathode materials to retain their capacity with cycling improved for both coatings, but the Al₂O₃ coating was slightly more effective. In another study, a MgO coating on a cathode composed of a composite of 0.5 li₂MnO₃–0.5 liNi_0.5Mn_0.5O₂ minimizes subsequent reactions between the electrolyte and electrode and also stabilizes the structure of these cathode materials. ¹¹⁷ Their Coulombic efficiency increases from 73 to 75% with the application of the coating. The MgO coated cathode exhibited a discharge capacity of 143 mA·h·g⁻¹ after the 25th cycle, in comparison to 223 mA·h·g⁻¹ for the uncoated cathode material after 20 cycles. This decreased capacity for the MgO coated cathodes was attributed to the lower electronic and ionic conductivities of this coating. These results reveal that the cathode surface modification by MgO is effective to improve the lithium-ion transfer reaction kinetics electrode—electrolyte interface.

Titanium dioxide (TiO₂) coatings

Zinc oxide (ZnO) coatings

The ZnO coatings have been pursued for their potential to suppress cation dissolution from cathode materials by scavenging HF. Coatings on LiMn₂O₄ prepared from amphoteric oxides (capable of functioning as either an acid or base), such as ZnO, Al₂O₃, SnO₂, or ZrO₂, have been investigated for their ability to stabilize these cathode materials. ¹²⁶ Coatings from amphoteric oxides could maintain 97% of initial capacity of the cathode after 50 cycles when compared to retaining 75% capacity in the uncoated electrode after the same electrochemical conditions. The ZnO coating was found to be most effective at scavenging HF followed by Al₂O₃, ZrO₂, and SnO₂. Plasma-enhanced chemical vapor deposition (PECVD) techniques were used to prepare ZnO coatings on LCO with a ZnO loading in the range of 0.08 to 0.49 wt%. The coatings up to 0.21 wt% yielded uniform films of ZnO nanoparticles covering the surfaces of the LCO without any change in the ZnO particle size. ¹²⁷ The ZnO coatings also proved to be effective against Co dissolution. Extensive agglomeration of the ZnO particles was, however, observed on the surfaces of the cathode materials when the coating content was increased up to 0.49 wt%. The agglomerated ZnO particles could hinder lithium intercalation into the cathode and transport of Li⁺ ions to and from the electrolyte. The ZnO coated LCO retained ∼65% of its capacity in contrast to a retention of ∼37% of the capacity by the bare cathode materials after 30 cycles. A separate study investigated the influence of the ZnO coating (also prepared by PECVD) on LCO when used in a cell that was operated at various temperatures and C rates. ¹²⁸ At low operating temperatures (e.g., 0 °C), the electrochemical performance of the cathode was not significantly influenced by the thickness of the coating, which was attributed to the slower kinetics of charge transfer at lower temperatures. Increases in the thickness of the coating could suppress the degree of phase transition (and changes in the crystal structure) of the LCO. The ZnO coated LCO prepared with a 0.38 wt% ZnO loading exhibited a relatively poor reversibility at both 25 °C and 50 °C in comparison to the ZnO coated LCO prepared from a 0.20 wt% loading of ZnO. The pristine LCO had a capacity retention of ∼0.2% and ∼15% following potential cycling at 0 °C and 50 °C, respectively. In contrast, the 0.20 wt% loading of ZnO on LCO retained 44% and 53%, respectively, when operated under the same conditions. On the other hand, a 0.38 wt% loading of ZnO on LCO exhibited a poor performance in comparison to all the other type of electrodes, which was attributed to these thicker layers hindering the processes of inserting and extracting lithium. This effect was more pronounced under the harsh operating conditions of high charge and discharge rates and reduced or elevated temperatures.

An ultrathin layer of amorphous ZnO was deposited onto NMC 532 cathode materials at elevated temperatures using ALD technology. ¹²⁹ At a low rate of charge and discharge at 0.2C, both the coated and uncoated samples exhibited a reversibility of nearly 100% after 20 cycles. At elevated temperatures the uncoated electrode exhibited a decrease in its discharge capacity with retaining only 87% of its initial capacity after 60 cycles at 2C, whereas the ZnO coated sample retained up to 92% of its capacity under the same electrochemical conditions. Additionally, the ZnO coated NMC 532 yielded a discharge capacity of 256.7 mA·h·g⁻¹ at 1C, which was 30% higher than that achieved by the uncoated sample. In a separate study, a ZnO coating was prepared by co-precipitation on a composite of 0.5 li₂MnO₂–0.5 liNi_0.5Mn_0.5O₂. ¹³⁰ These coated materials exhibited a high charge and discharge capacity, a faster activation, and an improved rate performance than the uncoated materials. No significant change in the discharge capacity was observed when the C rate was increased from 0.05C to 0.1C for both the ZnO coated and uncoated cathode materials. At a rate of 0.2C the discharge capacities of the ZnO coated cathode and pristine cathode were 206 mA·h·g⁻¹ and 185 mA·h·g⁻¹, respectively. A separate study used reactive magnetron sputtering to prepare a ZnO coating on 0.3 li₂MnO_3−0.7 liNi_5/21Mn_11/21Co_5/21O₂ (LMO-NMC) cathode materials. ¹³¹ This sputtering technique achieved ZnO coatings that were ultrathin and dense, and that fully covered the electrodes. The RMS technique achieved low substrate temperatures during the coating process. These coatings were relatively uniform in thickness and exhibited a good adhesion to the substrate. A two-minute process used to coat the LMO-NMC electrodes with a thin layer of ZnO achieved a discharge capacity of 316 mA·h·g⁻¹ at 1.0C and an initial Coulombic efficiency of 89.1%. Additional studies of ZnO coatings prepared on LCO using radio frequency (RF) assisted magnetron sputtering suggested that the ZnO could fully cover the electrode and could also diffuse into the electrode materials due to the intrinsic properties of the electrode. ¹³² It was determined that these ZnO coatings had an optimal thickness around 17 nm, which retained 81% of its capacity after 200 cycles and exhibited an initial discharge capacity of 191 mA·h·g⁻¹ at 0.2C. These coated LCO materials exhibited a relatively high rate performance with a retained capacity of 106 mA·h·g⁻¹ at 10C. Magnetron sputtered ZnO films were also coated onto Li₄Ti₅O₁₂ composites. ¹³³ Additional studies demonstrated that lithium and ZnO can reversibly react: ^134,135

$$\begin{eqnarray}&&{\rm{ZnO}}+2\,Li\leftrightarrow {{\rm{Li}}}_{2}{\rm{O}}+{\rm{Zn}}\end{eqnarray} \tag{ 10 }$$

$$\begin{eqnarray}&&{\rm{Zn}}+{\rm{Li}}\to \mathop{\underbrace{{\rm{ZnLi}}}}\limits_{{\rm{Zn}}-{\rm{Li}}\,{\rm{alloy}}}\end{eqnarray} \tag{ 11 }$$

Both the Zn and the ZnLi alloy have good electrical conductivities. Structural and chemical stabilization of the ZnLi is provided by the Li₂O. The theoretical capacity for this two-step process is 978 mA·h·g⁻¹, which is very high. This alloy serves as a conductive network for the transport of both ions and electrons. This material is of interest for both energy storage and as a coating on cathode materials. The highest rate performance for the magnetron sputtered ZnO films was achieved using a coating that had a thickness of 47.5 nm. This coating was obtained after a 5 min sputtering process and exhibited a relatively good reversibility to potential cycling at rates up to 10C within the potential range from 1 to 3 V (vs Li/Li⁺).

Silicon dioxide (SiO₂) coatings

Silica (SiO₂) have thermal properties that are expected to correlate well with their thermal stability as a coating. This material is also readily available and can serve as an HF scavenger to suppress cathode degradation. The matrix of SiO₂ is created by covalent Si-O bonds, which can be organized in a crystalline or amorphous lattice. ^8–10 During its reaction with HF, a barrier layer of SiO_2-xF_2x species are expected to form based on the following reaction:

$$\begin{eqnarray}&&4{\rm{HF}}\,+{{\rm{SiO}}}_{2}\to {{\rm{SiF}}}_{4}+2{{\rm{H}}}_{2}{\rm{O}}\end{eqnarray} \tag{ 12 }$$

A method of reacting sodium silicate with carbonic acid has been used to deposit a thin layer of SiO₂ onto the surfaces of LiNi_0.8Mn_0.1Co_0.1O₂ (NMC 811). ¹³⁶ The initial discharge capacities were the same for the coated and uncoated NMC 811 cathodes, implying that the SiO₂ layer did not hinder the properties of the bulk cathode. After 300 cycles, the SiO₂ coated sample exhibited a loss in capacity of only 14.6%, in comparison to a loss of 30.2% for the pristine cathode materials. The enhanced cycling performance of the SiO₂ coated NMC 811 cathode is possibly due to the structure of this coating and its ability to serve as a barrier to protect against unwanted interactions with the electrolyte. During potential cycling the anodic peaks for the coated samples appeared at 3.758, 4.02 and 4.218 V (vs Li/Li⁺), whereas the cathodic peaks were present at 3.72, 3.98 and 4.156 V. For Ni-based cathode materials, a primary electrochemical transformation within these materials is between Ni³⁺ and Ni⁴⁺ during Li⁺ intercalation and de-intercalation. The observed peaks were attributed to: (i) an oxidative transformation from Ni²⁺ to Ni³⁺ and from Ni³⁺ to Ni⁴⁺ at 3.785 and 4.02 V, respectively; and (ii) an oxidative transformation from Co³⁺ to Co⁴⁺ at 4.218 V. In a separate study, it was found that the electrochemical performance during potential cycling of SiO₂ coated NMC 622 (prepared from a solution-phase process) could be greatly enhanced when the coating process was performed at an elevated temperature of 60 °C. ¹³⁷ Initially, the pristine NMC 622 and the coated NMC 622 both exhibited a similar discharge capacity of ∼175 mA·h·g⁻¹. The pristine sample had a capacity retention of 68% in comparison to 92% for the silica coated sample after 48 cycles. The influence of the silica coating can also be observed in the additional thermal stabilization of these materials. For example, the exothermic peak for thermal decomposition of these materials occurs at 275 °C for the pristine NMC 622 material (1882 J g⁻¹ of heat generated from this decomposition) and for the silica coated sample this decomposition shifted to 288 °C (releasing 1217 J g⁻¹ of heat).

Modifying the surfaces of NMC 811 with different amounts of SiO₂ (e.g., 0.25 to 1 wt%) reveals that a coating of 0.25 wt% of SiO₂ on NMC 811 delivers an initial capacity of 200.7 mA·h·g⁻¹, and retains 87% of its capacity after 100 cycles at 0.5C. ¹³⁸ Thin layers of SiO₂ were favorable for creating a reversible transport of lithium ions and electrons including at elevated temperatures (e.g., 55 °C). Contact resistance at the interface of the active materials and current collector (R_e ) was almost the same for the 0.25 wt% SiO₂ coated NMC 811 sample and the uncoated sample, which indicated the ease with which Li⁺ and electrons passed through these coatings. For the pristine NMC 811, the R_e values increased from 31 Ω to 39 Ω due to the formation of an SEI and structural distortion of the materials, which was attributed to Ni dissolution and vacancy formation within the lattice of the cathode particles. Silica provides a sacrificial protection by scavenging HF, and offers the benefit of being an economical material. These attributes make silica a potential coating of interest for the LIBs.

Mixed oxides and composite coatings

Fluorides have better ionic and electronic conductivities and an improved resistance to HF attack than oxides. The use of fluorides stems from their intrinsic stability and their ability to yield high energy density as electrodes. ¹¹³ Most fluorides are inert and cannot be easily reduced or oxidized under the operating conditions of a battery. It has been found that the addition of the F⁻ species can improve the rate performance and material cyclability, and also decreases the charge transfer resistance of LIB cathodes. ^13,176–178 The high electronegativity of F⁻ drives the formation of LiF, which can improve the interfacial stability of cathode materials. Lower Gibbs energies relative to their respective oxide counterparts drive the preferential formation of the metal fluorides in the presence of the oxides, which also implies a higher stability of the fluorides over the oxides. Enriching the SEI with lithium fluoride (LiF) has recently gained popularity to improve Li cyclability. ^8–11 For example, AlF₃ layer can suppress the decomposition of LiPF₆ salts and also prevent Mn dissolution. ^1,5 An LNMO coated with a 1 wt% AlF₃ can achieve a capacity retention of up to ∼94% after 50 cycles at 0.5C (over a range of 3.5 to 4.9 V vs Li/Li⁺), which was significantly higher than that achieved for the uncoated spinel cathode (i.e., ∼78% after the same potential cycling). ¹⁷⁹ Additionally, the 1 wt% AlF₃ coated LNMO exhibited a slow change in its impedance (both real and imaginary components), whereas the bare LNMO spinel exhibited a sharp increase in its impedance upon potential cycling. This change in the impedance of the uncoated LNMO might result from more resistive materials/phases being deposited onto its surfaces. In another study, AlF₃ coated LCO retained ∼91% of its capacity after 500 cycles at voltages up to 4.4 V (vs Li/Li⁺). ¹⁸⁰ At higher current densities (e.g., 320 mA g⁻¹), the pristine LCO retained only 35% of its capacity, whereas the AlF₃ coated LCO retained up to 85% of its capacity for a potential range of 3.3 to 4.4 V (vs Li/Li⁺). When preparing these or other fluoride-based coatings, it is important to keep in mind that the formation of LiF should be minimized as it is highly resistive to Li ion transport. If LiF is formed in excess, it would hinder the transport characteristics of the cell.

Magnesium difluoride, MgF₂, has a good thermal stability, a refractive index of 1.37, a relatively high hardness, and a relatively wide bandgap. The effects of a MgF₂ coating on LCO were studied using MgF₂ films prepared using a chemical deposition technique. ¹⁸¹ The cycling performance of these materials deteriorated as the MgF₂ content increased from 1 wt% to 3 wt%, which indicated that excessive MgF₂ hinders Li⁺ ion transport due to the chemical and electrochemical inactivity of the MgF₂ layers. The most effective performance for the coated LCO materials was obtained from the 1 wt% MgF₂ coating with a retention of 80% of the initial capacity and achieving a discharge capacity of 141 mA·h·g⁻¹ in comparison to the performance of coatings prepared from the 0.5 and 3 wt% loadings of MgF₂. In another study, a solution-phase process was used to prepare a coating of Al₂O₃/TiO₂/MgF₂ on LCO. These coated LCO materials were able to retain a capacity to a similar degree as the pristine LCO. In contrast, the TiO₂ coated LCO samples prepared by the same solution-phase method exhibited a capacity of 183 mA·h·g⁻¹, followed by MgF₂ and Al₂O₃ coated LCO. ^182,183 In another study, MgF₂ was coated onto LCO by spin casting from a 0.1 M or 0.05 M solution of precursors. ¹⁸⁴ The 0.05 M solution of MgF₂ precursors cast onto LCO retained 61.3% of its original capacity in comparison to a retention of 71.0% of the capacity being retained for the LCO coated with MgF₂ that was prepared from a 0.1 M solution of precursors. In contrast, ∼80% of the initial capacity was retained by the pristine LCO. Each of these studies were performed at a current density of 0.6 mA cm⁻² with a rate of 3C for a voltage range of 3.0 to 4.25 V (vs Li/Li⁺). These results indicated that thin coatings of MgF₂ were insufficient to enhance the cell performance. In contrast, a separate study indicated that a coating of MgF₂ on a LiMn₂O₄ spinel decreased the discharge capacity but enhanced the cyclability of this cathode material when the coating content was 3 wt% MgF₂ (when prepared by a precipitation method). ¹⁸⁵ At a rate of charge of 1C at 55 °C, the LiMn₂O₄ cathode material coated with 3 wt% MgF₂ retained 77% of its initial capacity, which was significantly higher than the retained capacity of the uncoated spinel. In another study, a binary coating was prepared from MgF₂ and C on a cathode of Li₃V₂(PO₄)₃ (or LVP), which resulted in an enhancement to the electrochemical properties of the LVP materials. ¹⁸⁶ These improvements were attributed to the synergistic effects of the two components within these coatings to prevent dissolution of the vanadium and to improve the conductivity of the coatings, respectively.

Zirconium tetrafluoride, ZrF₄, a heat and corrosion resistant material coated Li(Li_0.2Ni_0.17Mn_0.56Co_0.07]O₂ cathodes at a loading of 0.5, 1, 2, or 3 wt% ZrF₄ (each prepared by wet coating method) were observed to not significantly impact the performance of these cathode materials. ¹⁸⁷ The coatings prepared from a 1 wt% ZrF₄ exhibited the most optimal discharge capacity (i.e., 194 mA·h·g⁻¹) of this series of materials after 100 cycles at 4.8 V (vs Li/Li⁺). Due to the electrochemical inactivity of the ZrF₄, this layer can suppress the growth of the SEI and can also maintain a consistent charge transfer resistance upon potential cycling. Excess ZrF₄ can, however, reduce the electrochemical performance of the cathode materials by hindering the Li⁺ ion transport. In a separate study, a zirconium oxyfluoride (ZrO_xF_y) was coated onto LCO. ^152,168,188 At voltages below 4.6 V (vs Li/Li⁺) the discharge capacities for the coated and pristine LCO cathodes were 195.6 mA·h·g⁻¹ and 188.4 mA·h·g⁻¹, respectively, at a rate of 1C. A severe capacity loss was observed when the cells were charged to 4.6 V; the coated and pristine samples exhibited a capacity retention of 33% and ∼11%, respectively, after charging to 4.6 V at 1C.

Iron trifluoride, FeF₃, is a relatively stable compound that can also react with Li through the following transformation: ¹⁸⁹

$$\begin{eqnarray}&&{{\rm{FeF}}}_{3}+3\,{\rm{Li}}\to {\rm{Fe}}+3\,{\rm{LiF}}\end{eqnarray} \tag{ 13 }$$

The FeF₃ has a theoretical capacity of 237 mA·h·g⁻¹ between 2.0 and 4.5 V (vs Li/Li⁺) through the following intercalation reaction:

$$\begin{eqnarray}&&{{\rm{FeF}}}_{3}+{\rm{Li}}\to {{\rm{LiFeF}}}_{3}\end{eqnarray} \tag{ 14 }$$

A composite of FeF₃ and reduced-graphene oxide exhibited an enhanced stability to electrochemical cycling between 1.5 and 4.5 V (vs Li/Li⁺) and a discharge capacity of up to 202 mA·h·g⁻¹ at 0.1C after 50 cycles from 2.0 to 4.5 V. ¹⁸⁹ A FeF₃ coated Li(Li_0.2Ni_0.13Mn_0.54Co_0.13)O₂ prepared by solution-phase methods resulted in the formation of a 5-nm to 15-nm thick coating. ¹⁹⁰ This FeF₃ coated cathode retained 95% of its capacity (i.e., 190 mA·h·g⁻¹) after 100 cycles at 0.5C when cycled from 2.0 to 4.8 V (vs Li/Li⁺). The FeF₃ coated cathode exhibited a longer lifetime, a greater stability to potential cycling, and a better rate capability than the pristine cathode. Coatings of FeF₃ of different thickness, prepared using 0.25, 0.5 and 1.0 wt% solutions, on NMC 111 cathode materials suggested that the coating prepared from 1.0 wt% FeF₃ on NMC 111 offered the best protection against electrolyte-based degradation of these transition metals within the NMC cathodes. ¹⁹¹ The 1.0 wt% FeF₃ coated on NMC 111 retained 84% of the initial capacity whereas the 0.25 wt% FeF₃ coated on NMC 111 retained only 50% of its initial capacity after 50 cycles for the potential range from 3.0 to 4.6 V (vs Li/Li⁺).

In another study, a process was demonstrated that used an aqueous solution to epitaxially grow layers of LiF/FeF₃ on Li(Li_0.2Ni_0.2Mn_0.6)O₂ cathode materials. ¹⁹² The coin cell testing of these coated cathodes revealed an improved rate performance (e.g., 129.9 mA·h·g⁻¹ at 20C) and a complete retention of its initial capacity over the voltage range from 2.0 to 4.8 V (vs Li/Li⁺). The coatings of LiF/FeF₃ on the cathode were compared at both a relatively high loading (5 wt%) and a low loading (1 wt%) of LiF/FeF₃ to performance of the uncoated or pristine cathodes (Fig. 14).

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Epitaxial coatings of LiF/FeF₃ were sought to establish a relatively high capacity of 712 mA·h·g⁻¹ at an average potential of ∼2.7 V (vs Li/Li⁺) to enable the transformation outlined in Eq. 12. The LiF and Fe products were sought to serve as protective barriers against electrolyte decomposition. Another fluoride-based coating, also derived from a solution-phase process, was CaF₂ coated onto NMC 111. This coated cathode was able to retain ∼94% of its capacity when prepared from a 1.0 wt% loading of CaF₂ on the NMC 111 in comparison to a retention of 68% of the initial capacity for the pristine cathode material. ¹⁹³ Fluoride based coatings have also been proven to be protective against water absorption. An example is the protection of olivine phosphate when exposed to ambient levels of humidity. For example, SiF₄ coated LiMn_0.80F_0.20PO₄/carbon (or LMFP) were prepared to generate hydrophobic coatings that could protect the cathodes from moisture attack. ¹⁹⁴ The SiF₄ treated LMFP retained 89% of its capacity after 450 cycles from 2.5 to 4.25 V (vs Li/Li⁺) at a rate of C/10. In comparison, the uncoated LMFP retained 87% of its initial capacity. Each of these materials did, however, exhibit a similar performance after 450 cycles with a retention of 84% of their capacity.

Another fluoride derived from lanthanum (i.e., LaF₃) was coated onto LCO. This LaF₃ coating was reported to exhibit an initial discharge capacity of 177.4 mA·h·g⁻¹ between 2.75 and 4.5 V (vs Li/Li⁺) at a rate of 0.2C, and retained 62% of its capacity after 50 cycles. ¹⁹⁵ The tolerance to overcharging of the LaF₃ coated LCO was higher than that of the pristine LCO. A more recent study performed a systematic comparison of ten (10) fluoride coatings on NMC 532 cathodes: (i) BaF₂; (ii) ZnF₂; (iii) NiF₂; (iv) CeF₂; (v) ZrF₄; (vi) YF₃; (vii) BiF₃; (viii) PrF₃; (ix) SmF₃; and (x) AlF₃. ¹⁹⁶ Of these coatings, the AlF₃ coated NMC 532 exhibited less stability to potential cycling, which could be due to the electrochemical activity of the Ni²⁺. It was hypothesized that for metal fluorides derived from metals of a +3-oxidation state that the pH of the fluoride suspension and the ionic radius of the metal cations were prominent factors for determining the electrochemical activity of the coatings and the coated NMC 532 cathodes. For example, a high pH might not sufficiently dissolve all the Li species present in the solution (e.g., LiOH, Li₂CO₃) and those species dissolved in solution could react with acidic LiPF₆. In contrast, use of a low pH solution could degrade the surfaces of the NMC 532 particle. It can be concluded that the interface stability is dependent on the properties of the metal fluorides. A protective fluoride coating could be achieved by tailoring the pH of a fluoride containing suspension and using relatively small metal cations in the creation of the metal fluoride.

Phosphate based compounds often have strong P=O bonds that can enhance the thermal stability polyanions and inhibit the direct contact of the cathode materials with electrolytes. The inclusion of phosphate groups can inhibit oxygen release from the cathode lattices even at elevated temperatures, which is attributed to the strong affinity of phosphorus towards oxygen. For example, Mn₃(PO₄)₂ coatings prepared by a solution-phase sol-gel process on NMC 622 exhibited discharge capacities of 149 mA·h·g⁻¹ after 50 cycles at 25 °C, and 160 mA·h·g⁻¹ after 50 cycles at 60 °C. ¹⁹⁷ In comparison, the pristine or uncoated cathode achieved discharge capacities of 153 mA·h·g⁻¹ after 50 cycles at 25 °C and 142 mA·h·g⁻¹ after 50 cycles at 60 °C. Additionally, the thermal decomposition of these cathode materials shifted from 275 °C to 292 °C upon coating the cathode with Mn₃(PO₄)₂. The corresponding heats of decomposition were 1882 J g⁻¹ and 1170 J g⁻¹, respectively, which further indicated an improved thermal stability of the coated NMC 622 materials. In a separate study, an NMC 442 coated with MnPO₄ prepared from a 2 wt% solution also improved the capacity retention (i.e., 85%) of these cathodes, which were found to be independent of the discharge rates even at temperatures up to 60 °C and were also independent of the loading of active material. ¹⁹⁸ The MnPO₄ coating on the NMC 422 cathodes also extended their operational potentials up to 4.5 V (vs Li/Li⁺). A separate study investigated MnPO₄ coated NMC 622 and found that a coating prepared from a solution of 1 wt% MnPO₄ could substantially enhance the electrochemical performance of these Ni-rich cathodes with an excellent cycle stability. ¹⁹⁹ The MnPO₄ coated NMC 622 could retain up to 93%, ∼95%, and 98% of their capacity following 100 cycles at 0.1C, 2C, and 10C, respectively (Fig. 15). At 60 °C the coated NMC 622 retained 83% of its capacity after 100 cycles (10C), in contrast to a retention of 69% capacity for the uncoated, pristine NMC 622. At 10C and 60 °C, no capacity was retained for the uncoated NMC 622, but with the MnPO₄ coated cathode retained 71% of its initial capacity.

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Lithium phosphate coatings have also been pursued for stabilizing cathode materials. ^3,19,42 The Li⁺ ion conductivity in Li₃PO₄ (e.g., 10⁻⁶ S m⁻¹) may facilitate charge transfer reactions across the electrode/electrolyte interface. ²⁰⁰ In one example, the surfaces of Ni-rich NMC 622 were coated with Li₃PO₄ derived from a H₃PO₃ precursor. ²⁰⁰ The reduction of lithium compounds such as LiOH or Li₂CO₃ in response to the oxidation of phosphorous acid could result in an increased capacity for the Li₃PO₄ coated NMC 622 cathodes. In addition, better rate capabilities and a slower change in impedance of the coated NMC 622 were attributed to the absorption of water by the Li₃PO₄ coating. This water management could lead to less HF production upon LiPF₆ decomposition, which could protect the active material from HF attack during the charge and discharge processes. In addition, a Li₃PO₄ coated Li_1.18Ni_0.15Mn_0.52Co_0.15O₂ cathode can be prepared by a precipitation method (Fig. 16) that produces surfaces free of Li₂O₃ impurities. ²⁰¹

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At a 1 wt% coating of Li₃PO₄ on the cathode, the discharge capacity was 272.5 mA·h·g⁻¹ at 0.4C and 12.1 mA·h·g⁻¹ at 10C in comparison to 248.6 mA·h·g⁻¹ at 0.4C and 70.5 mA·h·g⁻¹ at 10C for the pristine cathode materials. Increasing the coating thickness by loading the cathode materials with 3 wt% Li₃PO₄ increases the energy loss and exhibits a decay in performance at lower voltages, which indicates that the larger amount of Li₃PO₄ can hinder electron and Li⁺ ion transport. The TEM images in Fig. 16 indicate a corresponding formation of an amorphous Li₂CO₃ impurity layer on the pristine cathodes with a thickness around 3 nm. As the Li₃PO₄ content during the coating procedure increased, the coating layer also increased in its thickness. The resulting coating was, however, non-uniform and resulted in a partial encapsulation of the cathode particles.

A composite containing layers of a lithium rich oxide (Li_1.2Ni_0.13Mn_0.54Co_0.13O₂) was coated with Li₃PO₄ through a synthetic processing using polydopamine as a template. ²⁰² This composite achieved a capacity of 118 mA·h·g⁻¹ at 5C, in comparison to 45 mA·h·g⁻¹ for the oxide without the Li₃PO₄ coating. A coating of Li₃PO₄ on NMC 622 was also derived using a citric acid assisted sol-gel method, which was observed to suppress the formation of Li₃PO₄ in solution (as opposed to on the surfaces of the NMC particles). ²⁰³ These Li₃PO₄ coated NMC 622 exhibited a capacity retention of 80% after 100 cycles with a cut-off voltage of 4.7 V (vs Li/Li⁺), in comparison to 64% for the bare NMC 622. Additionally, a smaller R_ct was also observed for the Li₃PO₄ coated NMC 622 particles. In another example, Li₃PO₄ coated onto NMC 811 through a solution-phase process increased the capacity retention of these cathodes; the coated NMC 811 retained up to ∼93% of its capacity at 25 °C and 1C after 100 cycles in comparison to 86.1% for the bare NMC 811. ²⁰⁴ While increasing the cycle number from 1 to 100 the R_ct of the pristine NMC 811 increased from 50.6 Ω to 511.1 Ω, while that for the coated cathode increased from 36.6 to 290.7 Ω. The Li⁺ ion diffusion coefficients were measured to be 8.16 × 10⁻¹² cm² s⁻¹ and 1.68 × 10⁻¹¹ cm² s⁻¹ for the pristine NMC 811 and the Li₃PO₄ coated NMC 811, respectively, indicating an enhanced electrochemical performance for the coated cathode material.

Iron phosphates have also been prepared as coatings on cathode materials for LIBs in the search for improved stability and enhanced performance. For example, NMC 111 coated with a nanotextured layer of 2 wt% FePO₄ using a co-precipitation method exhibited a capacity retention of up to 88% after 100 cycles for a voltage range of 2.8 to 4.5 V (vs Li/Li⁺). ²⁰⁵ The discharge capacities for the FePO₄ coated NMC 111 (prepared from a 2 wt% solution) after heat treatment at 400 °C was 143.5 mA·h·g⁻¹, which was considerably higher than the 103.2 mA·h·g⁻¹ capacity attained for the pristine NMC 111 after 100 cycles (from 2.8 to 4.5 V at 1C). A heat treatment of 600 °C had a negative influence on the performance of the FePO₄ coated NMC 111 cathode materials. It was hypothesized that the coating layer might diffuse into the cathode and affect its crystalline lattice at these temperatures, which would negatively impact the Li⁺ ion migration between the electrolyte and the electrode. In contrast, at lower process temperatures the resulting contact between the cathode and this coating might not be very strong. A heat treatment at 400 °C was determined to be near optimal based on the electrochemical performance of these materials. Modifying the surfaces of LCO with FePO₄ using a co-precipitation method has also been pursued to stabilize these cathode materials. ²⁰⁶ A FePO₄ coating of LCO, prepared from a 3 wt% solution of the iron phosphate precursor, delivered an initial discharge capacity of 146 mA·h·g⁻¹ at 4.3 V (vs Li/Li⁺) and 155 mA·h·g⁻¹ at 4.4 V while retaining 89% and 83%, respectively, of their capacity after 400 cycles. As an alternative approach, microwave-based processing has been used to prepare FePO₄ coated LCO. ²⁰⁷ These coatings stabilized the LCO up to 50 cycles at higher voltages (e.g., 4.7 V vs Li/Li⁺). These FePO₄ coated LCO cathodes retained 92% of their capacity after 50 cycles in comparison to 40% retention of capacity by the bare cathode materials treated using the same electrochemical conditions. Pristine LCO can become polarized after 50 cycles due to an increased impedance (due to an increase in both the capacitance and resistance terms) that results in an overall loss of capacity. The FePO₄ coating on LCO helped to suppress this degradation in performance. An ALD process has also been demonstrated for preparing FePO₄ coatings on LNMO samples. ²⁰⁸ This sample was able to improve the capacity retention as a function of the number of cycles. Coating the LNMO with 40 ALD cycles could enhance the stability of the LNMO but also decreased its capacity, possibly due to the low ionic conductivity of FePO₄. Conclusively, FePO₄ coating prevents the side-reaction at the electrode/electrolyte interface and suppresses an increase in charge transfer resistance increase upon charge discharge cycling. Additionally, it stabilizes the cathode surfaces due to the strong interactions between the PO₄ polyanions and Fe³⁺ ions. ^205–207 It also decreases the activation energy of the charge transfer process thereby enhancing the charge transfer processes at the electrode/electrolyte interface.

Another distinct phosphate-based coating that has been sought are aluminum phosphates (AlPO₄). The surfaces of Li[Li_0.2Fe_0.1Ni_0.15Mn_0.55]O₂ cathode materials have been modified with a 3, 5, and 7 wt% AlPO₄ through a co-precipitation technique. ²⁰⁹ The 5 wt% AlPO₄ coating on this cathode yielded the best cycling performance of this series of materials with a capacity retention of 74% at 220.4 mA·h·g⁻¹ after 50 cycles at 0.1C. This sample also exhibited a good rate capability with a capacity of 120.2 mA·h·g⁻¹ after 100 cycles at 10C. The PO₄ ³⁻ polyanion paired with Al³⁺ provided a protective layer that resisted interfacial reactions between the electrolyte and the electrode. A separate study investigated the influences of temperature (e.g., 400 °C and 700 °C) on AlPO₄ coated NMC 111. ²¹⁰ At 400 °C, the AlPO₄ reacted with NMC 111 to form Li₃PO₄. When heated at 700 °C, the spinel phases of CoAl₂O₄, Co₂NiO₄, and CoMnAlO₄ are expected to form at the interface between the coating and the cathode material (M΄₁M₂O₄, M=Ni/Co/Mn and M΄=Al). The surface resistances of the uncoated NMC 111 and the AlPO₄ coated NMC 111 that were heat treated to 400 °C both increased nearly 2 times after 50 cycles (at 4.6 V vs Li/Li⁺), whereas the resistance of the AlPO₄ coated NMC 111 that was heat treated to 700 °C remained relatively stable after the same potential cycling. Each of these samples exhibited a similar morphology, depicting little influence from the coatings and heat treatment on the particle's appearance and surface roughness.

Cobalt phosphates have also been pursued as Li-reactive coatings. For example, Co₃(PO₄)₂ coatings have been applied to an LiNi_0.8Co_0.16Al_0.04O₂ cathode. ²¹¹ After annealing at 700 °C, a Li_xCoPO₄ coating formed upon these cathode materials. The Co₃(PO₄)₂ can react with surface impurities, which can be used to minimize unwanted side reactions. After an extended period of charging the cathode at 4.3 V (vs Li/Li⁺) and 90 °C, the pristine cathode transformed into a spinel structure, whereas the coated cathode could maintain its layered phase. This result indicated that the coating was effective in preventing Ni⁴⁺ ions from dissolving into the electrolytes. A coating of Co₃(PO₄)₂ was also applied to Li(Li_0.2Ni_0.15Mn_0.55Co_0.1)O₂ through a combustion method. ²¹² It was determined from this study that a 3 wt% Co₃(PO₄)₂ annealed at 800 °C yielded a cathode material with the best electrochemical performance. It was also hypothesized that these processing conditions also yielded the most homogeneous distribution of phosphorus species on the surfaces of these materials. The Ni and Co species had almost the same distribution within these surfaces after creating the Co₃(PO₄)₂ coating and applying this heat treatment. Possibly the Li_xCoPO₄ diffused into the bulk material during the heat treatment and/or the interdiffusion of species was promoted due to an incomplete coating (e.g., forming an initially less-dense coating). Coating Co₃(PO₄)₂ onto LiV₃O₈ cathode materials by solution-phase processes demonstrated the ability to stabilize these cathode materials during prolonged potential cycling possibly due to reducing the phase transition of LiV₃O₈ and side-reactions. ²¹³ A separate study compared the influence of Al/Fe/Co derived phosphate coatings on LiNi_0.8Mn_0.05Co_0.15O₂ cathodes. ²¹⁴ The electrochemical performance of these coated cathodes was correlated to the metal used to prepare the coating and the ratio of metal cation to PO₄ ³⁻. For example, the ratio of Al³⁺ to PO₄ ³⁻ could be tuned to prepare a coating on NMC of an optimal performance, whereas tuning the ratio of Fe³⁺ to PO₄ ³⁻ to prepare coatings on NMC had little effect on altering the performance of these materials. A coating derived from a 1:1 mole ratio of Co³⁺ to PO₄ ³⁻ was the most effective surface modification for the NMC cathodes as it increased the capacity retention by up to 3% after 50 cycles at 1C.

A series of other phosphate-based coatings have also been explored for coating cathode materials. A nickel phosphate or Ni₃(PO₄)₂ coated NMC 111 retained 99% of its capacity following potential cycling between 2.7 and 4.6 V (vs Li/Li⁺) after 10 cycles at 5C. ²¹⁵ The specific capacities of pristine NMC 111 and Ni₃(PO₄)₂ coated NMC 111 were 80.5 mA·h·g⁻¹ and 91.3 mA·h·g⁻¹ at 5C. Lanthanum phosphate, LaPO₄, has also been applied as a coating to NMC 811 cathodes. ²¹⁶ These LaPO₄ coated NMC 811 cathode materials achieved a diffusion coefficient of 142 × 10⁻¹¹ cm² s⁻¹ in comparison to 3.84 × 10⁻¹² cm² s⁻¹ for the pristine NMC 811 cathode. ²¹⁷ In addition, the LaPO₄ coated samples retained a higher capacity (i.e., 91%) in comparison to a retention of 76% for the uncoated cathode at 1C after 100 cycles from 3.0 to 4.3 V (vs Li/Li⁺). A yttrium phosphate, YPO₄, has also been used as an additive along with Al₂O₃ to modify the surfaces of a layered LCO. ²¹⁸ The addition of Al₂O₃ and the YPO₄ yield a similar thermal performance, when added individually. An enhanced thermal stability of these coated materials was attributed to the formation of a solid solution of Li_xM_yCo_1−yO₂ (M=Al/Y) covering the surfaces of the layered LCO.

Fast ion conductors offer superior electrochemical performance over inert coatings such as oxides and fluorides. Fast ion conductors can be used directly as cathode materials. These materials have a relatively large space within their crystal lattice and a disordered lithium sub-lattice, which leads to relatively high rates of ion migration. Materials such as Li₂ZrO₃, Li₂TiO₃, LiAlO₂, LiNbO₃ and Li₂SiO₃ have been widely investigated as the coating materials and fast ion conductors. ^3–6,12,14

Lithium aluminate (LiAlO₂) coatings on NMC 811 have been prepared by a process of hydrolysis through a hydrothermal treatment. ²¹⁹ A lithium aluminate coating layer stabilizes the interface between the cathode and the electrolyte, and also significantly improves the rate capability of the cathode materials. The improved rate capabilities were attributed to the excellent Li⁺ conducting nature of LiAlO₂. ^87,219,220 The NMC particles were almost entirely covered by the LiAlO₂ coating with a thickness of 8 to 12 nm. These LiAlO₂ coated NMC 811 materials retained ∼94% of their capacity after 100 cycles. Their capacity changed from 181.2 mA·h·g⁻¹ after the first cycle to 169.5 mA·h·g⁻¹ after the 100th cycle (1C from 2.7 to 4.3 V vs Li/Li⁺). The pristine sample could retain ∼82% of its capacity after 100 cycles and the discharge specific capacity decreased from 181.8 mA·h·g⁻¹ for the first cycle to 148.3 mA·h·g⁻¹ for the 100th cycle. In another study, it was concluded that LiAlO₂ coated NMC 622 exhibited a significant improvement in performance when compared to Al₂O₃ coated NMC 622 at high cut-off voltages of 4.5 and 4.7 V (vs Li/Li⁺). ⁸⁷ These LiAlO₂ coated NMC 622 retained a capacity of 142 mA·h·g⁻¹ at 3C and 206.8 mA·h·g⁻¹ at 0.2C, in comparison to the Al₂O₃ coated NMC 622 with a capacity of 131.9 mA·h·g⁻¹ at 3C and 196.9 mA·h·g⁻¹ at 0.2C. Additionally, the LiAlO₂ coated NMC 622 materials can maintain a reversible capacity of 149 mA·h·g⁻¹ after 350 cycles with a decay of 0.08% per cycle. In another study, LiAlO₂ coated Li_1.2Ni_0.2Mn_0.6O₂ layered oxide exhibited a better performance and stability to potential cycling at room temperature and elevated temperatures in comparison to pristine Li_1.2Ni_0.2Mn_0.6O₂. ²²¹ At 55 °C the LiAlO₂ coated sample had an initial discharge capacity of 201.2 mA·h·g⁻¹ in comparison to 190.2 mA·h·g⁻¹ for the uncoated sample. After 80 cycles these capacities reduced to 184.1 mA·h·g⁻¹ for the coated cathode and 119.8 mA·h·g⁻¹ for the pristine, uncoated electrode. The LiAlO₂ coating applied to NMC 811 through a solution-phase method exhibited a decrease in the electrode resistance relative to the pristine NMC 811 materials through suppression of undesirable side reactions between the electrolyte and electrode, which also hindered formation of the SEI. ²²² A series of samples were also prepared with different coating thicknesses (0, 0.5, 1.0 and 2.0 wt%). From this series of samples, the 1 wt% coating of LiAlO₂ on NMC 811 exhibited the highest performance.

Each of these coatings of LiAlO₂ on NMC 811 exhibited a similar morphology and an average particle size of 11 μm (Fig. 17). These coated particles had a rough surface that changes with a higher loading of the coating material—as the thickness increased it becomes harder to distinguish between the granular features of the particles. It was determined that a small amount of Al³⁺ diffuses into these cathode particles during their calcination. The 1 wt% loading of LiAlO₂ on the NMC 811 cathode particles possessed the highest discharge capacity, achieving a capacity of 179.7 mA·h·g⁻¹ at 5.0C, retaining 86% of its original capacity. The charge transfer resistance of the LiAlO₂ coated cathode was also suppressed after 50 cycles, which suggested that the migration of Al³⁺ into the lattice of the NMC particles may have increased the structural stability of these materials.

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Lithium titanate, Li₂TiO₃, has also been sought to stabilize cathode materials including a coating on NMC 811 cathode materials. Monoclinic Li₂TiO₃ is isostructural to Li₂MnO₃ with a strong Ti–O bond relative to the Mn–O bond, and Li₂TiO₃ is electrochemically stable over a wide range of voltages exhibiting excellent structural stability in organic electrolytes. Nanosized monoclinic Li₂TiO₃ has a high AC conductivity of 10⁻³ S cm⁻¹, much higher than that (i.e., 10⁻⁶ S cm⁻¹) of the layered Mn-based oxides containing an excess of Li. ^223–226 The lithium titanate based coating on NMC 811 was able to retain 98% of the initial capacity in comparison with the uncoated NMC cathode that retained only ∼81% of its capacity after 70 cycles at 1C. ²²³ The surface film resistance (R_sf ) increased upon cycling from 121.1Ω (10th cycle) to 146.5Ω (100th cycle) for the pristine NMC 811, but the Li₂TiO₃ coated NMC 811 didn't exhibit much change in R_sf (81.25 Ω at the 10th cycle, to 85 Ω at the 100th cycle). This result suggested that the CEI (cathode electrolyte interface) for the coated NMC particles was stabilized during the potential cycling. Another study prepared a series of Li₄Ti₅O₁₂ (LTO) coatings on NMC 811 particles with a range of thicknesses. The Li₄Ti₅O₁₂ has also been extensively investigated as an anode material as their spinel structure provides ample channels for Li⁺ ion transport and stability for voltages up to 5.5 V (vs Li/Li⁺) without the formation of an SEI. ²²⁴ The Li₄Ti₅O₁₂ coated samples exhibited a better rate capability and stability to potential cycling than the uncoated materials. The Li⁺ diffusion coefficients were 8.77 × 10⁻¹⁵ cm² s⁻¹, 8.37 × 10⁻¹⁵ cm² s⁻¹, and 4.21 × 10⁻¹⁵ cm² s⁻¹ for a 2 wt% LTO loading on NMC 811, 1 wt% LTO loading on NMC 811, and the pristine NMC 811, respectively. These results indicated a bit enhanced mobility of Li⁺ ions through this coating. A strategy of both coating with Li₂TiO₃ and doping with Zr of Li_1.2Ni_0.2M_0.6O₂ cathode materials has also been investigated. ²²⁶ It was observed that the metal-to-oxygen bond length increased and the energy barrier to Li⁺ ion diffusion decreased as a result of the reduced repulsive interactions between the Li⁺ ions and the metal ions within the lattice of these coatings. These changes facilitated Li⁺ ion diffusion within these materials. Pristine Li_1.2Ni_0.2M_0.6O₂ suffered from a more severe fade in capacity than the coated cathode particles. The bare Li_1.2Ni_0.2M_0.6O₂ had an initial discharge capacity of 133.5 mA·h·g⁻¹, which decreased to 92.2 mA·h·g⁻¹ after 200 cycles for a retention of 69% of the initial capacity. In contrast, the retention capability of the Li₂TiO₃ coated and Zr-doped Li_1.2Ni_0.2M_0.6O₂ was ∼185% greater than that achieved by the bare Li_1.2Ni_0.2M_0.6O₂. In another study, a 3 wt% loading of Li₂TiO₃ onto the surfaces of Li(Li_0.2Ni_0.19Mn_0.51Co_0.1)O₂ revealed a capacity retention of 169.9 mA·h·g⁻¹ at 2C, and 149.1 mA·h·g⁻¹ at 5C. ²²⁵ This coated cathode also achieved a discharge capacity of 207.1 mA·h·g⁻¹ at 0.5C after 100 cycles (from 2.0 to 4.8 V vs Li/Li⁺), in comparison to a discharge capacity of 190.3 mA·h·g⁻¹ for the uncoated sample.

Lithium zirconate or Li₂ZrO₃ has been sought as a cathode coating as this material is stable in non-aqueous electrolytes and has a structure that promotes Li⁺ ion diffusion. For example, Li₂ZrO₃ coated NMC 442 has exhibited an improvement in the rate performance, discharge capacity, and capacity retention particularly at an elevated temperature of 50 °C. ²²⁷ When the calcination temperature was increased to 750 °C, the coating was not as apparent as observed by electron microscopy techniques and its surfaces appeared to be relatively smooth. The results of this heat treatment suggested that the coating layer might diffuse into the crystalline lattice of the NMC 442, forming a solid solution at elevated temperatures. In another study, a 1wt% loading of Li₂ZrO₃ on NMC 532 was prepared that exhibited a capacity retention of 87% after 100 cycles at 1C in comparison to retaining 73% for the pristine cathode. ²²⁸ The initial discharge capacity of the pristine sample was 187.7 mA·h·g⁻¹, which decreased to 136.6 mA·h·g⁻¹ after 100 cycles. The 1 wt% loading of Li₂ZrO₃ on the cathode materials, on the other hand, decreased from 193.6 mA·h·g⁻¹ to 168.6 mA·h·g⁻¹ after 100 cycles. The thermal stability also improved for the coated materials. For example, exothermic transformations for the pristine sample were observed at 283.2 °C (765 J g⁻¹) but similar transformations for the coated NMC 532 shifted to 294.7 °C (484 J g⁻¹). A different approach was pursued in one example of coating Li₂ZrO₃ onto NMC 811 cathodes. ²²⁹ The coating process was carried out simultaneous to the preparation of the cathode particles, rather than being applied as a coating on the product. A loading of 2 wt% Li₂ZrO₃ on these NMC 811 cathode materials achieved a capacity retention of ∼95% after 200 cycles at 1C, and a retention of ∼83% of their specific capacity under same cycling conditions, whereas the pristine NMC maintained only 71% of its initial capacity. The specific capacity for the base cathode material was 107.4 mA·h·g⁻¹ at 10C (from 3.0 to 4.3 V vs Li/Li⁺), which was lower than the specific capacity of 144.3 mA·h·g⁻¹ for the Li₂ZiO₃ coated NMC 811 under the same operating conditions.

Lithium silicates have also been sought after for their use as fast ion conductors and coatings on cathode materials. One example was Li₂SiO₃ that contained staggered chains of SiO₄ tetrahedra that were held together by Li⁺ ions. The Li⁺ ions diffuse through the (001) and (010) planes of the Li₂SiO₃ lattice. In one study, it was determined that Li₂SiO₃ coated Li_1.13Ni_0.30Mn_0.57O₂ prepared by a co-precipitation method improved the rates of Li⁺ ion diffusion, as well as the cycle stability, rate capability, and polarization relative to the uncoated samples. ²³⁰ The Li₂SiO₃ coated Li_1.13Ni_0.30Mn_0.57O₂ achieved a discharge capacity of 152 mA·h·g⁻¹, whereas the bare cathode attained a capacity of 79 mA·h·g⁻¹ after 100 cycles (from 2.0 to 4.8 V vs Li/Li⁺) at a current density of 100 mA g⁻¹. In a separate study, an NMC 811 cathode was coated with Li₂SiO₃, which retained 78% of its capacity after 50 cycles with a cut-off voltage of 4.6 V in comparison to 57% of retained capacity for the pristine sample under the same operating conditions. ²³¹ The Li₂SiO₃ coating effectively minimized the electrolyte decomposition and side reactions, along with enhancing the structural stability of the Ni-rich electrodes upon removal of the Li⁺ ions at high voltages. In another example, Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ was coated with Li₂SiO₃ using an isochronous lithium source and this method demonstrated no significant changes in structure or morphology of the samples with the preparation of the coating. ²³² Coating the electrode with a 3 wt% loading maintained a discharge capacity of 142 mA·h·g⁻¹ and exhibited a capacity retention of 85%, in comparison to 121.9 mA·h·g⁻¹ and ∼78%, respectively, for the pristine cathode after 100 cycles at 1C. The diffusion coefficient for Li⁺ ions within the cathode loaded with 3 wt% Li₂SiO₃ was 8.57 × 10⁻¹² cm² s⁻¹, which was higher than the 1 wt% and 4.5 wt% coated samples.

Lithium tungstate (Li₂WO₄) has been pursued as the coating material for cathode materials due to its fast ion conductivity. In one example, Li₂WO₄ was coated onto NMC 811 through a one-step synthesis followed by a high temperature calcination. This coated NMC 811 cathode could maintain a discharge capacity of 146 mA·h·g⁻¹ after 100 cycles at 1C after being stored in air for 60 d, which was a 36% higher capacity than the bare electrode materials handled and tested under the same conditions. ²³³ The diffusion coefficient for Li⁺ ions was 2.637 × 10⁻¹⁰ cm² s⁻¹ and 1.479 × 10⁻¹⁰ cm² s⁻¹ for the tungstate coated and bare cathode materials, respectively. These results indicated that the tungstate coating had a higher but comparable capacity for Li⁺ ion diffusion as the NMC 811 cathode. The Li₂WO₄ has also been used as a dopant to increase cathode performance. ²³⁴ A 5 wt% addition of Li₂WO₄ within an NMC 622 electrode could achieve a reversible capacity of 199.2 mA·h·g⁻¹, and could retain 73% of its capacity after 200 cycles at 1C.

Lithium niobate or LiNbO₃ is a conventional electrolyte with an excellent thermal stability and ionic conductivity. ^235,236 It has the potential to considerably enhance the interfacial stability of cathode materials. In one example, LiNbO₃ was applied as a coating to NMC 811 cathodes. ²³⁶ It was determined that the highly crystalline LiNbO₃ can effectively suppress structural changes to these cathode materials by providing strain relaxation due to the promotion of Li⁺ intercalation and de-intercalation to and from the lattice.

Glasses have also been investigated as coating materials due to their relative inertness to electrochemical and chemical processes, their flexibility in terms of their composition, and their cost effectiveness relative to other coating materials. Glasses contain an open random network, which is advantageous for Li⁺ ion diffusion. ^237–239 The relatively open structure within a glass is attributed to non-bridging oxygen atoms within its network due to its unique structural properties and stability. Lithium and boron containing compounds such as Li₂O₃-B₂O₃ (LBO) have been widely used as coating materials because lithium and boron effectively reduce electrolyte decomposition in comparison to metal oxide-based coatings. ²⁴⁰ It is expected that the LBO glass would evenly coat the surfaces of cathode materials due to the relatively low viscosity of its precursors and their good wetting properties. ^237,240 A layered LiNi_0.8Co_0.2O₂ cathode coated with a Li₂O–2B₂O₃ glass reduced the self-discharge of this cathode, and increased its reversible capacity and stabilized its cycling performance. ^240,241 Different loadings of Li₂O–2B₂O₃ glass on NMC 811 were prepared using a heat treatment method. It was determined that a 0.3 wt% of LBO coated on NMC 811 exhibited a retention of 82% of its capacity in contrast to the pristine NMC 811 that retained 51% of its initial capacity. ^242,243 All of the LBO coated NMC 811 exhibited better Coulombic efficiencies and discharge capacities than the pristine samples (Fig. 18). The surfaces of the NMC 811 were severely damaged upon cycling to 4.5 V, but the LBO coated NMC 811 exhibited a greater stability at these potentials.

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Another study compared coatings of Al₂O₃, ZrO₂, and Li₂O–2B₂O₃ on NMC 811 each prepared using solution-phase methods. ¹⁵³ The results of this study indicated that cycle performance and stability of the resistance of the LBO coated NMC 811 were slightly better than those properties observed for the Al₂O₃ and ZrO₂ coated cathodes. The retained capacity and an increase in cathode resistance observed for the LBO coated, ZrO₂ coated, and Al₂O₃ coated cathodes and the pristine cathode were 99.9%, 99.8%, 99.2%, and 99.5%, respectively, and 12.2 mΩ, 13.6 mΩ, 14.0 mΩ, and 19.6 mΩ, respectively. In another study, a 3 wt% loading of LBO on Li(Li_0.2Ni_0.13Mn_0.54Co_0.13)O₂ retained 92% of its capacity after 100 cycles (at 1C from 2.5 to 4.6 V vs Li/Li⁺), which was much better than observed for its pristine counterpart (i.e., only 70% retention of capacity under the same operating conditions). ²⁴⁴ The NMC 811 cathode coated with a 1 wt% loading of Li₂O–B₂O₃ yielded an improved electrochemical performance in comparison to the 0.5 and 2 wt% loadings on NMC 811, and relative to the base NMC 811 cathode material. ²⁴⁵ The bare NMC 811 cathode had a specific capacity of ∼182 mA·h·g⁻¹, whereas the NMC 811 coated with 0.5, 1, and 2.0 wt% Li₂O–B₂O₃ yielded specific capacities of ∼150, 198 and 159 mA·h·g⁻¹ after 20 cycles at 0.2C over the voltage range from 3.0 to 4.3 V (vs Li/Li⁺).

A glassy lithium phosphate or GLP has also been sought as a coating, such as on LFP prepared using magnetron sputtering techniques. ²⁴⁶ The cycle performance and rate capability of the GLP coated LFP gradually improved with an increase in the duration of sputter deposition time. These properties did not improve beyond 20 min of deposition, suggesting that deposition time (and likely both film thickness and its uniformity) plays an important role in optimization of these coatings. Initial discharge capacities of 152.6 mA·h·g⁻¹ and 165.8 mA·h·g⁻¹ at 1C were achieved for the bare cathode and coated LFP, respectively. After 100 cycles, the capacity and Coulombic efficiency for the GLP coated LFP and the bare LFP were 159.1 mA·h·g⁻¹ and 99.7%, and 135.7 mA·h·g⁻¹ and 99.1%, respectively. The amorphous nature of GLP might have reduced the anisotropy in the surface properties of the LFP cathode, which could enhance the Li⁺ ion migration through the bulk material. Additionally, GLP could serve to cross-link the networks between particles and to reduce the resistance to electron and ion transfer between LFP particles. The GLP could, therefore, create more conductive paths for both electrons and Li⁺ ions and enhance the electro-active regions within these coated cathodes.

A series of glasses based on LVSB or 20 li₂O − 30 V₂O₅ − (50 − x) SiO₂ − xB₂O₃ (x = 10, 20, 30, 40 mol%) have also been studied as potential cathode materials for LIBs. From one series of samples, the material with x = 10 mol% exhibited the best cycling capacity, which was attributed to the high ratio of V⁴⁺ yielding more polaron hopping, and hence an enhanced conductivity. ²⁴⁷ During 1^st cycle, all samples show a low charge capacity less than 50 mA h·g⁻¹. The discharge capacities of the LVSB10 (x = 10 mol%), LVSB20, LVSB30 and LVSB40 samples were 123.7 mA h·g⁻¹, 51.5 mA h·g⁻¹, 37.9 mA h·g⁻¹ and 19.5 mA h·g⁻¹, respectively (15 to 4.2 V). Ball milling was used to reduce the size of these particles. Impedance spectroscopy results suggested that the ball-milled samples effectively decreased the charge transfer resistance of these materials.

Many other types of materials have also been pursued as coating materials on cathodes for LIBs, such as polymers, lithium rich materials, metals, reduced carbon, and additional composites. A PPy (polypyrrole) polymer coated LiMn₂O₄ spinel achieved a high capacity. ²⁴⁸ In another example, poly (3, 4-ethylenediozythiophene) or PEDOT coated LiMn₂O₄ exhibited a capacity fade of 22% to 24% over 100 cycles at 1C while held at 32 °C, which is slightly lower than that of conventional LiMn₂O₄ cathode. ²⁴⁹ It has been proposed that PEDOT could have a beneficial influence on the properties and utility of metal oxide cathodes, but its energy barriers are still insufficient for the life-cycle targets of LIBs. Poly(diallyldimethyl-ammonium chloride) or polyDDA coated LiMn₂O₄ spinel were prepared in a separate study using an adsorption process to inhibit the surface degradation of the spinel and Mn²⁺ dissolution. ²⁵⁰ Performance of these coated cathode materials was dependent on the processing temperature during synthesis and the amount of adsorbed polymer. Although minor changes to the surface topography were observed because of potential cycling at room temperature, it was observed that the stability and cell performance improved for the polyDDA coated cathodes. Many of the cathode materials lack strong bonding interactions with the polymer coatings. Although these coatings are non-specific and are relatively loosely adhered to the cathodes, they can still exhibit beneficial properties. For example, a mixture of polyaniline (PANI) and polyvinyl pyrrolidone (PVP) coated NMC 811 have an initial discharge capacity of 200 mA·h·g⁻¹ and retain 89% of this capacity after 100 cycles, as well as a capacity of 152 mA·h·g⁻¹ at 1000 mA·g⁻¹ when cycled from 2.8 to 4.3 V (vs Li/Li⁺). ²⁵¹ In other reports, it is revealed that polypyrole coated LiMn₂O₄ exhibited an initial capacity of 165.5 mA·h·g⁻¹ at 0.2C between 2.7 and 4.5 V (vs Li/Li⁺). ^252,253 Compared to the inorganic coating materials, conducting polymers [e.g., polyaniline (PANI)] possess advantages of low cost and high electronic conductivity, which can enhance cyclic stability and improve the rate capability of cathode materials. ^251,252 However, coating a homogeneous conducting polymer layer on the cathode surface due to lack of bonding effects is still a topic of consideration.

A templating method was used to form a composite containing a lithium rich oxide (Li_1.2Ni_0.13Mn_0.54Co_0.13O₂) and Li₃PO₄. ²⁰² The composite achieved a capacity of 118 mA·h·g⁻¹ at 5C, compared to 45 mA·h·g⁻¹ for the oxide without the Li₃PO₄ coating. A 1 wt% ZnAl₂O₄ coating on a lithium rich Li_1.2Ni_0.2Mn_0.6O₂ exhibited an initial discharge capacity of 254.3 mA·h·g⁻¹, and a capacity of 84 mA·h·g⁻¹ following a high rate of discharge at 10C. This coated cathode material retained ∼99% of this capacity after 50 cycles at 10C when cycled from 2.0 to 4.8 V (vs Li/Li⁺), demonstrating the spinel was a suitable coating material. ¹⁶¹ Metals have also been coated onto cathode materials to improve their electron conduction. Silver nanoparticles coated onto LFP cathode materials achieved an increase in initial discharge capacity from 128 mA·h·g⁻¹ (for the uncoated LFP) to 156 mA·h·g⁻¹ at 0.1C over the potential range from 2.0 to 4.5 V (vs Li/Li⁺). ²⁵⁴ An analysis of a gold coated LNMO revealed that the method used for preparing the coating plays a dominant factor in determining the battery performance. ²⁵⁵ This noble metal is stable toward reactive compounds such as HF and could protect cathode particles against degradation while also improving the electronic conductivity of the electrode. At low rates of charge and discharge, such as C/6 and C/4, the capacity increased by 20% with the gold coating relative to the base cathode material, but at high rates of charge and discharge (e.g., 2C and 4C) the electrolyte decomposition was significant and limited the rate capability of these materials.

Carbon-based coatings of cathode materials have included carbon, graphene oxide, reduced graphene oxide (RGO), graphite, and other forms of carbon materials. ^256–262 For example, a study on LNMO spinel coated with carbon species determined that a 1 wt% loading of sucrose (mixed with cathode in a solvent mixture of ethanol and water) on this cathode material exhibited the best behavior. ²⁵⁶ After 100 cycles between 3 and 4.95 V (vs Li/Li⁺), a capacity of 130 mA·h·g⁻¹ was achieved and 92% of the capacity could be retained at a discharge rate of 1C. A super P-carbon (SPB) coated NMC 811 revealed that this type of carbon coating did not alter the morphology of the cathode. And a 0.5 wt% loading of SPB on NMC 811 retained ∼87% of its initial capacity at 2C, and also retained 88% of its capacity after 80 cycles for a voltage range from 3 to 4.3 V (vs Li/Li⁺). ²⁵⁷ A composite of Li₂FeSiO₄ and C (e.g., derived from sucrose or L-ascorbic acid using acid catalyzed hydrolysis) yielded an initial discharge capacity of 135.3 mA·h·g⁻¹ at a rate of C/16. ²⁵⁸ The cathodes with a composite coating derived from L-ascorbic acid exhibited a higher electronic conductivity and an improved Li⁺ ion diffusion coefficient than the pristine one, whereas the composite coatings derived from sucrose yielded a better stability to potential cycling and overall cell performance. In another study, an NMC 622 was coated with a composite of C and Al₂O₃, which exhibited a high degree of structural stability. ²⁶⁰ The C and Al₂O₃ composite improved the electrical conductivity of these materials and decreased its charge transfer resistance. The Al₂O₃ itself provided structural flexibility that favored Li⁺ ion diffusion though its network. It is worthwhile to note that the improved electrochemical performances and structural stability could be due to the synergistic effect of the conductive coatings (e.g., carbon, conductive polymers) and amorphous oxides (e.g., Al₂O₃, ZrO₂). The characteristics of cathode materials for LIBs are highly dependent on the composition and structure of both their core material and the coatings on their surfaces. Coatings on the cathode materials can significantly influence the electrochemical performance and stability of the encapsulated material. In addition to the properties of the coating material itself, the methods used to prepare the coating also influential the observed performance of the coated cathode materials. ^{12–15,261,262} The thickness of the coating is a vital parameter that influences Li⁺ ion diffusion. Thickness of the coating must be optimized to provide a protective layer against material degradation while also promoting Li⁺ ion transport. Hence, the precursors, methodology, selection of the coating material and variables like pressure-temperature should be carefully chosen when developing high performance cathode materials.