Li-doped transition metal high-entropy oxides (Li/TM-HEOs) as Li-Ion batteries cathodes: a first report on capacity fading and cycling stability

To produce the different HEO cathodes, inks were made using powdered active materials (i.e. 16Li/RS-HEO, 25Li/RS-HEO, 33Li/RS-HEO, 41Li/RS-HEO and 16Li/SP-HEO). The inks had a composition of 80% active material, 10% carbon C65, and 10% polyvinylidene fluoride (PVDF) in a 2% weight solution of N-Methyl-2-pyrrolidone (NMP). Firstly, PVDF was dissolved in NMP with overnight stirring. Once dissolved, C65 was added to the corresponding quantity of 2 wt% PVDF-NMP and mixed in an IKA stirrer for 5 min at a speed of 16000 rpm. The cathode active material was then added to the mixture and stirred for an additional 5 min at 16000 rpm. The prepared ink was allowed to rest for 1 h in a ball miller to remove as many bubbles as possible before being spread on aluminium foil using a Doctor Blade (RK control coater) with a blade thickness of 500 microns. The laminates were then dried at room temperature in air for 2 h and subsequently at 80 °C for 12 h in a vacuum oven. Finally, the designed HEO cathodes were obtained approximately 120 microns thick. Active material loadings of different samples were the following:

• Li16/RS-HEO: 8.45 mg;
• Li25/RS-HEO: 8.06 mg;
• Li33/RS-HEO: 8.06 mg;
• Li41/RS-HEO: 11.67 mg;
• Li16/SP-HEO: 8.45 mg.

Given the area of the tested cells being 1,13 cm², active material's loading in cathode is (in average) 7,5 mg /cm ². Figure 1 shows an example of the as-prepared 16Li/SP-HEO cathode).

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For the manufacture of cells, cathodes of 12 mm diameter were cut. All assembly is carried out inside a glove box, with an inert atmosphere. The lithium disk used as a counter electrode is 14 mm, and both are separated by the separator (fibreglass) soaked in a few drops of LiPF₆ electrolyte.

The LiPF₆-based electrolyte employed in our study is a 1 M solution (sourced from Merck) of lithium hexafluorophosphate (LiPF₆) dissolved in a solvent mixture of ethylene carbonate (EC) and diethyl carbonate (DC), with the composition being an equal volume ratio of 50/50 (1 M LiPF6 in EC/DEC 50/50 v/v).

The entire system is then hermetically sealed. Figure 2 exemplifies such a cell assembly procedure.

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The cell was connected to a 'Maccor series 4000 battery tester (Maccor Inc., USA)' and tested upon assembly. During the tests, a current flow was induced between the working and counter electrodes, and the applied potential was controlled by manipulating the polarization of the counter electrode. The potential is measured between the working electrode and the known stable potential of the reference electrode. Each tested cell performed several deeply charge–discharge cycles to evaluate its electrochemical response, by using the following current values: 50 mA/g, 100 mA/g, 200 mA/g, 500 mA/g, 750 mA g⁻¹, and 1000 mA/g.

Configurational entropy, i.e. S_conf, is one of the most important features of an HEO. In the case of the Li0/RS-HEO, the proof of high entropy was reported in the pioneereristic work of Rost et al [3]; therefore, the addition of lithium in the rocksalt structure, in octahedral position [38] likely leads to a further increase of entropy. Indeed, the calculation of S_conf in the ideal model by the well-known expression $R\sum {x}_{i}\mathrm{ln}{\rm{}}({x}_{i})$ (see [3], for example) leads to:

• 0Li/RS-HEO: 1.61 R;
• 16Li/RS-HEO: 1.76 R;
• 25Li/RS-HEO: 1.77 R;
• 33Li/RS-HEO: 1.72 R;
• 41Li/RS-HEO: 1.63 R

Thus, our calculations show that for each Li-doped RS-HEO sample, ΔS_conf is well greater than the threshold proposed in the literature to define an HEO (i.e. S_conf ≥ 1.5 R).

The determination of S_conf is much more complex in the case of spinel, as the presence of two cationic sites requires the knowledge of cationic distribution among them. Based on such cationic distribution, Sarkar [39] calculated S_conf for a system similar to one of the present study but without lithium. To estimate the cationic distribution between octahedral and tetrahedral sites, we have used the results obtained by Sarkar [39]. Indeed, he found that Co is all in the tetrahedral sites, Mn and Ni are all in the octahedral sites, whereas Fe is partly in the octahedral (1/3) and partly in the tetrahedral site (2/3); we have used such results whereas for Li we have assumed that it occupies only octahedral sites (on the base of our previous result [38]) and finally the distribution of Zn is derived from the ratio of tetrahedral and octahedral sites occupation in spinel and inverse spinel structure (i.e. 1:2), leading to this chemical formula:

(Co_0.5Fe_0.333Zn_0.167)(Li_0.25Mn_0.25Ni_0.25Fe_0.084Zn_0.167)₂(O_0.831)₄

The Oxygen coefficient has been estimated by using again the results of Sarkar [39] for the oxidation state of Fe (+ 3), Co (+ 2.4), Mn (+ 2.9) and Ni (+ 2), whereas we have assumed for Li and Zn, as oxidation states, +1 and +2 respectively. Under these hypotheses and applying the procedure reported in [39] (equations 1–5) we have obtained:

$$\begin{eqnarray*}&&{S}_{{\rm{con}}f}=1.57R\end{eqnarray*}$$

This value, greater than 1.5 R, ensures that, under the indicated hypotheses, the synthesized spinel is actually an HEO.

Figure 3(a) shows the XRD pattern of the as-synthesized SP-HEO, exhibiting a combination of a spinel-like phase (Fe₃O₄, ICDD Card n. 01-075-6109) and several different secondary phases attributable to ZnO (ICDD Card n. 01-070-2551), MnO (ICDD Card n. 01-075-0625), and cobalt hydroxide (ICDD Card n. 01-089-8616). Nevertheless, the presence of NiO cannot be excluded since it shares the same rocksalt structure as MnO, and its main peaks could overlap with those associated with MnO (being very broad and still partially overlapping with the main Fe₃O₄ reflections).

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Indeed, the XRD pattern of SP-HEO calcined (and quenched) at 1100 °C, shown in figure 3(b), mainly exhibits a single-phase spinel-like structure (ICDD Card n. 01-089-3854).

Conversely, the as-synthesized RS-HEO (shown in figure 3(c)) only exhibits some very broad peaks, indicating its nanocrystalline nature along with an amorphous phase. Again, after calcination (and quenching) at 1100 °C for 1 h, RS-HEO (with no impregnated Li) owns an entropy-stabilized rocksalt structure (ICDD Card n. 01-075-0418), as evident in its diffraction pattern of figure 3(d).

Therefore, once single-phase HEO was obtained for both rocksalt and spinel structure materials, the detailed study of phase evolution as a function of the calcination temperature was not carried out in the present work. Indeed, on the one hand, in the case of RS-HEO, that study is already present in the literature [3, 33] even when doped with Li⁺ [38]. On the other hand, the main focus of the present research was the electrochemical characterization of different HEOs doped with Li⁺ as possible electrodes for LIBs and not unravelling the intricate transformations leading to the entropy-stabilized single phase.

The lattice parameter of calcined/quenched RS-HEO (figure 3(d)) is 0.41981 nm, whereas the lattice parameter of calcined/quenched SP-HEO (figure 3(b)) is 0.83185 nm. These values have been obtained by the least square procedure adopted in the software UnitCell [40].

The morphology of the samples RS-HEO and SP-HEO prior to Li impregnation and the corresponding elemental distribution were analyzed by SEM/EDS.

The micrographs in figure 4 reveal, for both samples, powders characterized by heterogeneous particle size and irregular shapes, with the presence of large agglomerates, whose size is in the order of tenths of μm. The presence of hard agglomerates is indeed evident in the micrographs at higher magnification (see figures 3(b), (c), (f) and (g). This is an expected result considering the synthesis method and similar powder morphology is reported in the literature on both analogous and different chemical systems [41, 42], characterized by the presence of such hard agglomerates impairing their sinterability. However, being both RS-HEO and SP-HEO are intended as porous cathodes, it is believed that their final electrochemical behavior will benefit from such microstructural features.

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Figures 4(d) and (h) shows the atomic mapping of RS-HEO and SP-HEO, revealing a highly homogeneous distribution of all cations, being an indirect confirmation of their entropy-stabilized single-phase structures (either rocksalt or spinel-like).

Prior to the cathode fabrication, both RS-HEO and SP-HEO were loaded with different Li contents (16 mol%, 25 mol%, 33 mol% and 41 mol% for the RS-HEO system, and 16 mol% for the SP-HEO system) by using the largely-used and well-known wet impregnation method [29, 30].

Research evidence has shown that approximately 30% of introduced lithium is lost during the 'wet impregnation' [38]. Therefore, for all the prepared cathode active materials (16Li/RS-HEO, 25Li/RS-HEO, 33Li/RS-HEO, 41Li/RS-HEO), the nominal quantity of lithium nitrate has been increased by 30%.

Figure 5 shows the XRD patterns of both 16Li/RS-HEO (exemplary for the Li/RS-HEO series) and Li/SP-HEO systems calcined (adopting the thermal cycle described in the Materials and Methods section, designed to ease a homogeneous lithium nitrate decomposition) and quenched at 1100 °C after 1.5 h, highlighting how the Li-impregnation did not alter their crystal structures.

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To reveal phase transformations leading to the formation of rock-salt single phase a DTA-TG run of sample 33Li/RS-HEO after the impregnation step was carried out. In figure 6 the corresponding thermograph is shown.

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The thermal behavior appears quite complex with the appearance of several endothermic events: the first one (α − 156 °C) represents the evolution of adsorbed water (with associated about 20% weigh loss), the second one (β − 257 °C) represents the melting of LiNO₃ [38]. Besides, there is a progressive and continuous loss of mass, started at 200 °C and ended at about 450 °C, providing an additional 15% weight loss, associated to the evolution of chemically-bonded water from the hydroxides or hydrous oxides formed during the coprecipitation [33]; for this decomposition, occurring in a wide range of temperature, there is no clear endo peak in the thermogram even though a shoulder around 300 °C is present in the DTA signal. Two additional endo events, γ − 490 °C and δ − 667 °C, are due to the thermal decomposition of LiNO₃, globally giving rise to an additional 15% of weight loss; indeed γ is the thermal decomposition of molten LiNO₃ to LiNO₂ (and O₂) whereas δ is the decomposition of LiNO₂ to Li₂O and a mixture of N₂, O₂, NO and NO₂ [43]. Eventually, there is a last, sharp endo event, λ − 950 °C, which can be associated to the transition multiphase-system to rock-salt single phase, according to Spiridigliozzi [38] as no weight loss is associated to this endo event. Indeed, it has been proved [38] that: (i) Li₂O formed by thermal decomposition of LiNO₃ firstly reacts with Co₃O₄ (obtained by natural oxidation of Co-based compound formed during coprecipitation) giving rise to LiCoO₂; (ii) at higher temperature, LiCoO₂ dissolves in the rocksalt structure, when the single phase is formed, according to the following reaction:

$$\begin{eqnarray*}&&{LiCo}{O}_{2}\mathop{\to }\limits^{{MO}}{{Li}}_{M}^{{\prime} }+{{Co}}_{M}^{\bullet }+2{O}_{O}^{x}\end{eqnarray*}$$

Thus Li⁺ occupies octahedral sites so balancing Co³⁺ and Ni³⁺ cations also present in the high entropy rock-salt structure so that no weight loss is associated to the transition multiphase system → single rock-salt phase, unlike occurring in RS HEO without doping with Lithium where a clear weight loss occurs in correspondence of such a transition.

A series of Nuclear Magnetic Resonance (NMR) tests have been developed to characterize the structure of the differently Li-loaded RS-HEO systems. The NMR transition frequencies are strictly related to the electron distribution around the nuclei which can shield them from the applied magnetic field. Thus, 7Li MAS NMR measurements have been used to characterize the Li environments of the Li/RS-HEO series. All the spectra of the RS-HEO doped with different amounts of lithium present one resonance located at about δ = −0,4 [ppm] with a wide spinning sideband pattern. The 1D MAS spectrum indicates the presence of a rocksalt-structured single phase. The recorded spectra, shown in figures 7(a)–(d), indicate that the samples with different Li loadings have quite different lineshapes, meaning that they have similar lattice parameters but slightly different lithium occupancy.

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Moreover, by comparing the Li/RS-HEO series spectra, it is possible to note an irregular shape in the range (−500; 500) [ppm] with increasing peaks per each one around 0 [ppm], meaning that exists a high disorder within the lattice, as expected for high-entropy materials. Carefully analyzing the same range, it is possible to note that the visible peaks vary according to the different Li loadings; in particular, increasing the percentage of Li in the RS-HEO series leads to a reduction of the number of peaks centered around 0 [ppm], that being an indirect confirmation of a reduction of the overall lattice disorder (the more Li is added to the system, the more the other cations are reduced relatively, with the equimolarity being the condition of maximum lattice disorder, i.e. 16Li/RS-HEO).

The maximum resonance peak for each analyzed sample is centered in δ = −0, 4 [ppm], being it is relatively sharp and symmetric, indicating that Li occupies octahedral sites of the structure. Besides, it is interesting to note that for samples 33Li/RS-HEO and 41Li/RS-HEO, the left sideband pattern is overlapped with another very small sideband pattern in the region 600-200 ppm (see figures 7(c) and (d), suggesting that a different environment surrounds a small quantity of the whole Li present within the system.

Finally, the RS-HEO series has been referenced to the LiCoO₂ spectrum (figure 7(e)), assessing that all the prepared systems are comparable with it, with the only difference between LiCoO₂ main peak (centered at δ = −0, 4 [ppm]) is given by a significantly higher degree of lattice disorder.

Figure 8 shows the Cyclic voltammetric (CV) profile of the 16Li/SP-HEO sample, including the first five cathodic scans at 50 mA/g. The first cycle (black line) only exhibits a negligible peak in correspondence of 1.75 V. The shape of the first cycle has been expected, as it represents the background analysis. Starting from the third cycle, the performed scans are more representative of the electrochemical behavior of the cathode itself, with the registered oxidation reactions (charge phase, i.e. delithiation of the cathode) occurring in the range 1.6–1.8 V. Such broad peaks very likely identify the oxidation of Fe, Mn and Co within the spinel-like lattice compensating Li⁺ losses. Conversely, upon discharge (i.e. lithiation of the cathode) starting from the 3rd cycle, the registered reduction peaks are not symmetrical with the corresponding oxidation ones, occurring at the following potential values: 0.453 V–3rd cycle, 0.399 V–4th cycle, 0.3737 V—5th cycle. Such asymmetry of RedOx peaks of the 16Li/SP-HEO sample indicates a strong irreversibility of the lithiation/delithiation process, as also evidenced in the following.

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The sharp oxidation peaks observable in the range 0.7 V − 0.75 V, very likely coupled with the reduction peaks at ∼0 V, can be attributed to the generation of a solid in the electrolyte interface (SEI) layer and Li₂O [44, 45].

In fact, even though the 16Li/SP-HEO specific capacity reaches a very good value (≈ 2000 mAh g⁻¹), due to the asymmetry of the lithiation/delithiation process, there is a rapid degradation of the cathode within the whole cell after few charge/discharge cycles, thus leading to a fast capacity fading.

Figure 9(a) shows 16Li/SP-HEO specific capacity (charging profile) at 50 mA g⁻¹, while figures 9(b)–(d) shows 16Li/SP-HEO specific capacity (charging profile) at 100 mA/g, 200 mA/g and 1000 mA g⁻¹, both after five complete cycles.

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As the number of complete cycles increases, 16Li/SP-HEO exhibits a large instability, particularly at low specific current conditions (50 mA/g in primis), and, consequently, a pronounced capacity fading. In other words, as soon as the sample is tested for the first time, unstable conditions involving very fast Li migration from the cathode up to the anode (probably due to the proneness of many cations of the SP-HEO system to be oxidized) are achieved, as also indicated by the very high capacity registered upon the first 2 cycles (≈1500 mAh g⁻¹). However, since the lithiation reverse process is not symmetric (figure 8(a)), not all the migrated Li is incorporated back into the cathode, thus causing a strongly evident capacity fading of the whole cell. Moreover, such irreversibility of the lithiation/delithiation mechanism is also very likely due to the formation of a solid electrolyte interphase (SEI) layer between the cathodes and the liquid electrolyte, induced by the large amount of Li⁺ ions migrating during the very first cycles.

By carefully analyzing the plots in figure 9, it is worth noting that the specific capacity of 16Li/SP-HEO decreases with the increasing of the current density. In particular, at 200 mA g⁻¹, the specific capacity of 16Li/SP-HEO is almost one order of magnitude lower than the one registered at lower current density (i.e. 50 mA g⁻¹), being ≈ 300 mAh g⁻¹. Finally, increasing the current density to 1000 mA/g, 16Li/SP-HEO specific capacity dramatically drops of one order of magnitude more, achieving ≈ 25–30 mAh g⁻¹ independently from the considered cycle. In other words, at high current densities 16Li/SP-HEO lithiation/delithiation mechanism becomes highly stable but to the detriment of its overall specific capacity, not being of practical interest as a substitute cathode for Li-ion batteries.

A possible explanation of this behavior is strictly related to the RedOx reactions occurring upon 16Li/SP-HEO lithiation/delithiation, as higher current densities (i.e. faster cell charge/discharge cycles) globally inhibit such reactions in favor of moving in absolute terms very low quantities of Li⁺ ions, related to the very low registered capacities. Conversely, at lower current densities, the highly disordered structure of 16Li/SP-HEO, as the proneness of the different cations to be oxidized upon cathode's delithiation, contribute to move globally high quantities of Li⁺ ions but, in turn, induce a strong irreversibility of the overall process, possibly involving a structural collapse during the first discharge cycles, leading to an unacceptable capacity fading upon very few charge/discharge cycles.

In order to investigate the Li loading effect on the reactions and kinetics of RS-HEO cathodes, cyclic voltammetric tests have been carried out on the Li/RS-HEO series samples. Figure 10 shows the Cyclic voltammetric (CV) profiles of the Li/RS-HEO series, i.e. 16Li/RS-HEO, 25Li/RS-HEO, 33Li/RS-HEO, and 41Li/RS-HEO samples, including the 3rd, 4th and 5th cathodic scans at 50 mA g⁻¹, as the 1st and 2nd cycles exhibited the same features of 16Li/SP-HEO, thus not representing a stable testing condition.

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All the registered profiles in figure 10 display clear oxidation and reduction peaks, even though with several interesting differences between the various samples. In particular, for each tested cathode, it is always possible to identify a sharp oxidation peak centered in the range between 0.5 V − 0.6 V (versus Li/Li+) and at least one reduction peak at lower voltages. For each tested cathode, a major reduction peak is observable at values close to zero as potential and negative values for currents. Additionally, 25Li/RS-HEO and 33Li/RS-HEO exhibit visible secondary peaks at positive potentials, i.e. 0.5 V < x < 0.65 V (versus Li/Li+), thus indicating that the lithiation process occurs along with the formation of a SEI layer and/or of Li₂O, as similarly observed in the 16Li/SP-HEO.

Moreover, by analyzing the peak shapes and positions, a higher symmetry of the lithiation/delithiation mechanism compared to SP-HEO suggests a definitely greater reversibility of the overall process in the Li/RS-HEO series of samples (with the 25 Li/RS-HEO sample exhibiting the most pronounced symmetry, with an oxidation peak at 0.58 V and two reduction peak at 0.93 V and 0.01 V for all the investigated cycles). According to different Li loadings, the various Li/RS-HEO cathodes own different cycle hysteresis: lower Li loading is related to greater cycle hysteresis and viceversa. Moreover, it can be noted that the RedOx reactions occur much more easily in cathodes with lower Li loadings, as for 41Li/RS-HEO a current of almost 5 mA is required to activate the lithiation/delithiation mechanism, contrarily to 16Li-RS-HEO that required just about 2 mA in each investigated cycle. Such behavior could be related to what has been observed via NMR, i.e. the presence of possible short-range ordered for Li+ ions within the rocksalt lattice of 41Li/RS-HEO (possibly involving the presence of Li₂O domains) that is an unfavorable factor for the reaction kinetics thus determining the high currents demanded for the oxidation/reduction of the involved species.

Finally, the oxidation and main reduction peaks potential, being very low compared to the Li/Li+ reference reaction, suggests that all the Li/RS-HEO samples have a fast Li ion transfer rate upon all the reaction process.

To evaluate their specific capacities and cycling stability, full cells with the Li/RS-HEO series of cathodes were cyclically charged/discharged at a constant current (C/20). Primarily, each cell is first rested for 30 min, subsequently charged at a constant current rate for 20 h, and held in stationary condition at the upper voltage limit. At last, the cell is discharged at a constant current rate to the lower voltage limit for 20 h.

Figure 11(a) shows the charging and discharging profiles of 16Li/RS-HEO at its 2nd full cycle (the first is omitted as possibly influenced by non-stationary conditions).

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By firstly analyzing the 16Li/RS-HEO charging profile, the full cell charging process starts from approximately 0,45 V, reaching an upper voltage threshold of around 2.96 V, which corresponds to a specific capacity of almost 500 mAh/g.

After the rest from the charging phase, cell starts to discharge from the upper limit voltage, reaching the lower voltage threshold (i.e. 0.25 V) in 4 h.

Figure 12(b) shows the charging profiles of 16Li/RS-HEO at its 3rd, 4th and 5th full cycles (i.e. right after the full cycle shown in figure 12(a)). From such profiles, a significant instability of the 16Li/RS-HEO specific capacity over full charging cycles is highly noticeable, achieving an outstanding value of almost 1500 mAh g⁻¹ in the 3rd cycle.

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The great variation observed in the specific capacities of 16Li/RS-HEO could be related to the broad reduction peak upon lithiation observed in figure 8, very likely involving a continuous and disordered reduction of the RS-HEO cations, as recently reported in similar systems [33].

Confirming the cyclic voltammetry results, Li/RS-HEO samples with higher Li loadings exhibited relatively higher cycling stability (even though a not negligible capacity fading is still observable, with a minimum of 12,5% capacity loss per cycle for the 41Li/RS-HEO sample), at the expense of lower specific capacities (up to slightly more than 300 mA/g for the 41Li/RS-HEO sample).

We attributed such behavior (i.e. when the Li ratio in the RS/HEO cathodes increases, the specific capacity decreases) to the decreasing ratio between transition metal cations and lithium ions within the system, resulting in fewer redox reactions occurring within the cell upon the lithiation/delithiation process. In other words, as for each synthesized system, we fixed the sum of the molar fractions of the 6 involved cations; an increasing Li content led to a decrease of the sum of the other 5 cations, among which the ones owning the redox behaviour needed to counterbalance Li ions intercalation/deintercalation.

Such results are evident from figure 12, exemplarily showing the charging and discharging profiles of 41Li/RS-HEO at its 3rd, 4th and 5th full cycles.

Very likely, the structural collapse of the variously prepared Li-HEO cathodes after few cycles is primarily due to their chemical composition (abundant in Li-loadings), the formation of lithium oxide at the electrolyte interface, and the inherent lattice disorder. Li-HEOs' high configurational entropy, while contributing to initial capacity and performance benefits due to a more accessible 3D intercalation pattern for lithium ions, also renders these materials structurally unstable already in the short term (i.e. after very few charge/discharge cycles). The abundant extraction of lithium ions (as highlighted by the very high specific capacities upon the first 2–3 cycles) hinder the overall material's stability, leading to structural collapse. Additionally, the formation of lithium oxide during the initial cycles further hinders the reversibility of the charging process and contributes to the observed fast degradation of the electrode's structure.