Unlocking the multi-electron transfer reaction in NASICON-type cathode materials

The growing concern about scarcity and large-scale applications of lithium resources has attracted efforts to realize cost-effective phosphate-based cathode materials for next-generation Na-ion batteries (NIBs). In previous work, a series of materials (such as Na4Fe3(PO4)2(P2O7), Na3VCr(PO4)3, Na4VMn(PO4)3, Na3MnTi(PO4)3, Na3MnZr(PO4)3, etc) with ∼120 mAh g−1 specific capacity and high operating potential has been proposed. However, the mass ratio of the total transition metal in the above compounds is only ∼22 wt%, which means that one-electron transfer for each transition metal shows a limited capacity (the mass ratio of Fe is 35.4 wt% in LiFePO4). Therefore, a multi-electron transfer reaction is necessary to catch up to or go beyond the electrochemical performance of LiFePO4. This review summarizes the reported NASICON-type and other phosphate-based cathode materials. On the basis of the aforementioned experimental results, we pinpoint the multi-electron behavior of transition metals and shed light on designing rules for developing high-capacity cathodes in NIBs.


Introduction
Large-scale applications based on Na-ion batteries (NIBs) are expected to integrate intermittent renewable energy sources because of the low cost, wide distribution, and abundant reserves of sodium resources [1][2][3][4][5][6][7], where the development of electrode materials is one of the most significant tasks for the improvement of NIBs. In terms of cathode materials, polyanion compounds have high safety and chemical/electrochemical stability [8], which could match the urgent requirement of grid energy storage devices. Among the phosphate-based cathodes, NASICON-type materials have attracted growing attention due to their high Na + ion conductivity [9][10][11][12]. It took 30 years from identifying the crystal structure to realizing reversible charge/discharge behavior in the NIBs (figure 1(a)). As early as 1968, Hagman's group [13] reported the NaMe 2 (PO 4 ) 3 (Me = Ge, Ti, Zr) structure. They mentioned that the crystal's 3D framework is built up of the corner link of MeO 6 octahedra and PO 4 tetrahedra, and the oxygen atoms octahedrally surround the sodium atoms. In 1976, Goodenough and Hong et al [14,15] found fast alkali-ion transport in a series of materials conforming to the chemical formula Na 1 + x Zr 2 P 3−x Si x O 12 (0 ⩽ x ⩽ 3). These compounds were * Authors to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. named NASICON (sodium (Na) super (S) ion (I) conductor (CON)), benefiting from the three-dimensional diffusion tunnel. Subsequently, Nadiri et al [16] used Fe 2 (MoO 4 ) 3 for the positive electrode and AClO 4 (1 M, A = Li/Na) in propylene carbonate as the electrolyte to fabricate half cells, revealing the intercalation behavior of alkali metal ions in the NASICON framework. In 1988, reversible electrochemical (de)intercalation was successfully realized in ATi 2 (PO 4 ) 3 for the first time [17]. However, much research focused on LIB material systems after the first commercial lithium-ion battery was issued in 1991. In 2002, Uebou et al reported electrochemical sodium insertion/extraction of the 3D framework of Na 3 V 2 (PO 4 ) 3 [18], which was synthesized by Delmas in 1978 [19]. However, the insufficient electrochemical data attracted limited attention to such materials until Hu's group first proposed the carbon coating approach to significantly improve the cycling and rate performance [20].
Since then, Na 3 V 2 (PO 4 ) 3 has been regarded as a promising cathode candidate earning wide investigation, and several modification strategies have been explored to optimize its electrochemical performance [20][21][22]. However, vanadium's high cost and low resource sustainability became one of the most serious bottlenecks contrary to the requirements of large-scale applications [23]. In 2013, Hu's group proposed Mn 2+/3+/4+ redox couples in NASICON-type cathodes, and kinds of Mn-rich compounds were designed (such as Na 3 MnTi(PO 4 ) 3 and Na 3 MnZr(PO 4 ) 3 , etc) [24]. Subsequently, the reversible redox couples of Mn 2+/3+/4+ in a NASICON-type cathode have been realized with a high operating potential in Na 3 MnTi(PO 4 ) 3 (∼3.6 V and ∼4.0 V) [25]. Furthermore, Fe-rich NASICON-type cathode materials (such as Na 3 Fe 2 (PO 4 ) 3 ) have attracted great interest due to their wide sources, low costs and abundant reserves on Earth. However, the limited thermodynamic equilibrium potential of Fe 2+/3+ restricted the research of Fe-based NASICON-type cathodes. It is exciting that researchers found that P 2 O 7 4− and F − can increase the redox potential based on Fe 2+/3+ due to the strong electronegativity of such ions. As a result, a series of new structures has been discovered for phosphate-based mixed-polyanion cathodes [26][27][28]. However, the low transition metal mass fraction of the above compounds leads to a limited theoretical specific capacity, as shown in figure 1(b). Therefore, realizing the multi-electron transfer reaction is crucial for advanced next-generation NIBs.
In this review, we summarized redox couples with electrochemical activity in NASICON-type cathodes and other polyanionic compounds. Based on reported voltage profiles and previous accumulations on multi-electron transfer reactions, we pinpoint the reversibility of redox couples in NASICON-type cathodes closely related to the crystal structure. As a result, we demonstrate a cascade of guiding lines for enabling better designs of high-capacity polyanionic NIB cathodes.

Structure
NASICON-type cathode materials are increasingly attracting attention as phosphate-based compounds due to tunable transition metal sites and fast Na + ion transport pathways. As early as 1976, a series of materials with the chemical formula Na 1 + x Zr 2 P 3−x Si x O 12 (0 ⩽ x ⩽ 3) were named NASICON (an acronym for sodium (Na) Super Ionic CONductor) [14]. Similarly, NASICON-type material structures usually refer to a family of solids with the chemical formula AMM'(PO 4 ) 3 [11]. Where the 'A' site can be occupied by alkali ions (Li + , Na + , K + , Rb + , and Cs + ), alkaline earth ions (Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ ), transition metals (Cu 2+ , Ag + , Pb 2+ , Cd 2+ , Mn 2+ , Co 2+ , Ni 2+ , Zn 2+ , Al 3+ , Ge 4+ , Zr 4+ , and Hf 4+ ), and ion-molecules (H 3 O + and NH 4 + ), may also be vacancies. The M and M' sites are divided by 3d (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn), 4d (Y, Zr, Nb, Mo), 5d (Lu, Hf, Ta), and main-group (Al, Si, In, Ge, As, Sn, Sb) elements to balance the charge appropriately. Phosphorus can be partially or even entirely replaced by S, Si, As, W and Mo, while O can also be replaced by F and Cl. Furthermore, the crystal structure can be rhombohedral, monoclinic, triclinic, orthorhombic, garnet, SW-type, corundum, etc, with different elements. Notably, rhombohedral structures have been extensively reported due to their superior ion diffusion pathway. In this structure, MO 6 and M'O 6 octahedrons share all angles with XO 4 tetrahedrons, and MO 6 and M'O6 octahedrons are arranged linearly along the c-axis. The octahedron MO 6 and M'O 6 connect three tetrahedral XO 4 units to form a basic unit called a lantern [29]. Each lantern is connected to six other lanterns, thereby constructing a stable 3D skeleton structure [30]. In this open 3D framework, interconnected channels provide a high-speed transmission pathway for the ions encapsulated in the 'A' site. Intriguingly, its content is between 1 and 5 [31,32]. In addition, it can be de-intercalated continuously without structural collapse.

V-based NASICON cathodes
Vanadium compounds have attracted great attention for their excellent redox, electrochemical, catalytic, and magnetic properties [49][50][51]. Surprisingly, the vanadium atoms can adopt different oxidation states (from II to V), coordination (from 6 to 4), and environments (octahedral to tetrahedral) in the reported vanadium phosphates. Abundant bonds lead to a wide variety of V-based polyanion compounds. Na 3 V 2 (PO 4 ) 3 can be indexed as the rhombohedral phase R3C space group [29,32,52], and the oxidation state of V in Na 3 V 2 (PO 4 ) 3 is confirmed to be trivalent [31]. As a cathode material, it exhibits a theoretical specific capacity of 117.6 mAh g −1 with 3.4 V operating voltage (vs. Na + /Na). The incompletely occupied Na + sites provide a fast ion diffusion channel, which enables the material to exhibit excellent rate capability [53]. Furthermore, Masquelier et al have made many contributions to elucidate the crystal structure and charge/discharge behavior of Na 3 V 2 (PO 4 ) 3 [32,52].
The transition metal substitution of the V element in Na 3 V 2 (PO 4 ) 3 has also been widely studied due to the high cost of V-based compounds. In 2016, Fe, Mn, and Ni were used to replace V to synthesize a series of materials of Na 4 VM(PO 4 ) 3 (M = Fe, Mn, Ni) [48]. Similar to LMFP, the operating potential of Na 4 VMn(PO 4 ) 3 can be significantly improved without capacity fading. Immediately, researchers focused on enhancing the electrochemical performance of Na 4 VMn(PO 4 ) 3 [54][55][56][57][58][59][60]. However, as shown in figure 2(c), the V 4+/5+ redox couple is irreversible in Na 4 VMn(PO 4 ) 3 . It should be noted that the multi-electron transfer reaction is one of the prerequisites for high capacity NASICON-type cathodes. Therefore, the failure mechanism and how to realize a reversible V 4+/5+ redox couple are crucial. In 2020, Liu et al [61] revealed that the small ion radius of V 5+ can migrate to Na_vacancy sites and block the sodium ion pathway in Na 3 VCr(PO 4 ) 3 . Interestingly, a similar phenomenon was captured in Na 3 VSc(PO 4 ) 3 , and a slightly reversible capacity at the 4.0 V platform was shown at −20 • C [62]. Based on the above finding, we can speculate that the transition metal migration of V 5+ is a common issue of the irreversible V 4+/5+ redox couple. It should be noted that the replacing elements of Al 3+ , Cr 3+ , and Ga 3+ make the V 4+/5+ redox couple reaction reversible, and a platform located at 4.0 V (vs. Na + /Na) occurred in the discharge curve (figure 2(d)) [63][64][65][66]. This finding can be attributed to the small Al 3+ and the eliminated Jahn-Teller effect of Mn 3+ , so the crystal structure can be stable. In addition, the modification of polyanion groups in the NASICON framework also can be considered [67]. However, the above conclusions are inferred based on the reported experimental results, and research on such topics is still limited [68]. Therefore, we must pay attention to such issues and draw a whole picture of failure mechanisms or optimization strategies.
Furthermore, benefitting from the stronger electronegativity of F − , the partial substitution of V-F for V-O can significantly improve the operating voltage of V-based cathode materials [50]. In recent years, fluorine-containing vanadium-based polyanion compounds such as NaVPO 4 F [69,70], Na 3 V 2 (PO 4 ) 2 F 3 [71][72][73][74], and Na 3 V 2 O 2x (PO 4 ) 2 F 3−2x [28,[75][76][77] have been reported as cathodes for NIBs (figures 2(a) and (b)). Although the above cathodes deliver a reversible specific capacity of ∼120 mAh g −1 , the crystal structure will collapse when the V valence exceeds +4. In 2019, Yan et al [78] showed a  [69,70,79,80]. (c) Charge curves of the V 3+ to V 5+ in which the V 4+/5+ redox couple is irreversible, data from [54,62]. (d) Voltage profiles of the reversible V 4+/5+ redox couple reactions, data from [46]. detailed picture of the structural evolution of Na 3 V 2 (PO 4 ) 2 F 3 when more than 2.5 sodium ions were extracted. In addition, they revealed that the Na 0 V 2 (PO 4 ) 2 F 3 phase accommodates sodium in a disordered way and does not convert back to the initial structure. The aforementioned experimental results indicate that more than 1Na/TM can be extracted upon further charging, but the structural collapse occurs simultaneously. Therefore, this is the key problem of V-based high-capacity cathodes.
For the active electrochemical elements, 4d elements have also been reported, except for the above reported 3d transition metal element redox couples. In 2018, NaMo 2 (PO 4 ) 3 was confirmed to achieve stable electrochemical cycling based on the Mo 3+/4+ redox couple [114] with a theoretical specific capacity of 98.2 mAh g −1 at an equilibrium potential of 2.45 V. In addition, the reversible reactions of redox couples such as Nb 4+/5+ [115], Ti 3+/4+ [116], Zr 3+/4+ [117], and Cr 3+/4+ [93] have also been reported in NASICON-type materials. However, the thermodynamic equilibrium potentials of the above compounds are either too low or too high to be used in cathode materials, whereas the relevant research is still in the initial stage.

Multi-electron transfer reaction
The low mass ratio of the transition metal means that a multi-electron transfer reaction is required to go beyond the electrochemical performance of LiFePO 4 (LFP). We will reveal the issue of reported compounds and show the basic rule for designing high-capacity cathode materials around Na content, transition metal sites, and polyanion frameworks.

Na content and structural stability
The number of alkali metal sites in the NASICON structure is typically 1-4. Recently, researchers found that a new phase with a Na content of 5 will occur when the discharge potential is between 0 and 1 V [118], and the polyanion skeleton structure is unchanged. However, previous results have shown that the discharge voltage platform of the above structures is close to 0 V, which cannot be used as cathode materials. Therefore, we speculate that the highest Na content is ∼4 among NASICON-type cathode materials. Furthermore, Yan et al [78] confirmed that structural collapse occurs in Na 0 V 2 (PO 4 ) 2 F 3 when charged to 4.8 V. Similar phenomena have also been reported in other systems. Theoretical calculations also show that the skeleton structure of NASICON-type compounds is difficult to maintain due to the high formation energy when the Na content is lower than 1. The above finding shows that it is necessary to design a structure with a Na content close to 4 to ensure enough Na + ions for extraction.
Notably, Liu et al [61] suggested that V 5+ with a small ionic radius can diffuse to the Na1 site and induce kinetic hysteresis. According to the above result, the reversibility of the V 4+/5+ redox couple can be realized when maintaining the occupancy of the Na1 site at high voltage. Recently, many works have focused on Na1 and Na2 sites for developing low-cost and high-energy-density V-based NASICON-type cathode materials [63,119]. However, Na + ion diffusion in NASICON frameworks is completed by the cooperation of the Na1 and Na2 sites, which means the Na + in the Na1 and Na2 sites are dynamically evolving throughout the charging and discharging process. Therefore, some V-based NASICON compounds with a Na content of 4 (e.g. Na 4 VNi(PO 4 ) 3 , etc) showed limited reversibility even if there was enough Na + at the Na2 site in the pristine structure. Here, we pinpoint that rather than focusing on precisely controlling the Na + content of Na1 and Na2 sites in the initial structure, it might be more necessary to ensure that Na1 is a thermodynamically/kinetically stable site in the  [120]. The redox couples and the corresponding electrode potentials are shown in boxes (e.g. the redox couple in the range of 3 ⩽ x ⩽ 4 (x of NaxV 2 (PO 4 ) 3 ) is V 2+/3+ , and the electrode potential is 1.5 V). Reproduced from [120] with permission from the Royal Society of Chemistry. entire voltage platform of the V 4+/5+ redox couple. Meanwhile, blocking the migration channel of V 5+ to the Na site is crucial too.

Selection of transition metal elements
The elements that can be placed in transition metal sites are shown in figure 5. To facilitate element screening, we propose the following notes. First, the total valence state of the transition metal site is +5, and the Na content can be 4, so multi-element co-union is needed (M and M' are +2 and +3, respectively). Second, previous studies have shown that V 5+ easily migrates to the alkali sites, which may be the predominant issue for the irreversibility of V 4+/5+ redox couples (e.g. Na 4 VMn(PO 4 ) 3 , Na 4 VFe(PO 4 ) 3 , Na 4 VNi(PO 4 ) 3 , Na 3 VSc(PO 4 ) 3 , etc). Interestingly, the substitution of Al 3+ and Ga 3+ in group 13 (IIIA) can realize the reversible reaction of the V 4+/5+ redox couple [41,63]. Therefore, it is essential to understand the relationship between the reversibility of the V 4+/5+ redox couple and the NASICON backbone. Finally, Mn 2+/4+ is a significant step in developing low-cost NASICON-type cathode materials. However, the activation of Mn 2+/4+ redox couples often relies on Al 3+ , Ti 4+ , and Zr 4+ , which cannot be used for high energy density materials (the +4 valence state of Ti and Zr is too high, and Al 3+ is electrochemically inactive) [24]. Excitingly, recent results have shown that Cr 3+ can also activate the Mn 2+/4+ redox couple, and the Cr 3+/4+ redox reaction can be conducted at ∼4.4 V platform [93], suggesting a promising material that deep research is necessary to improve its electrochemical performance. Likewise, exploring other electrochemically active +3-valent elements for activating Mn 2+/4+ is also a feasible strategy.

Polyanion frameworks
The polyanion groups are various, such as BO 3 3− , CO 3 2− , C 2 O 4 2− , SiO 4 4− , PO 4 3− , SO 4 2− , etc. Currently, the reported materials with excellent electrochemical performance are mainly phosphate-based compounds, and the exploration of other anionic groups is still limited. In addition, researchers demonstrated that F, Cl, etc, were able to replace the O sites, which endowed an abundant selection of polyanion frameworks. Therefore, except for focusing on the element replacement of Na x MM'(PO 4 ) 3 , more polyanion frameworks also need to be explored.

Future perspectives
The low cost, wide distribution, and abundant reserves of sodium resources triggered the research of Na-ion batteries (NIBs) in the energy storage devices field. More notably, polyanionic-type NIB cathode materials are expected to meet the expansive demands for large-scale applications, benefitting from their long-term stability and high safety. Since our group first proposed the Mn-rich cathodes, several low cost NASICON cathodes with excellent cycling performance have been reported. However, the low transition metal mass fraction (for example, Fe is 35.4 wt% in LiFePO 4 , and V is 22.35 wt% in Na 3 V 2 (PO 4 ) 3 ) of the above compounds leads to a limited theoretical specific capacity. Additionally, the costs of the total batteries are much higher than the costs of the active materials as additional items, such as electrolytes, binders, casings, and even the electric battery management system, are included. Therefore, a higher capacity is needed for developing advanced polyanionic-type NIB cathode materials, which can further reduce the cost of inactive material. For the next generation of NASICON-type cathode materials, the low transition metal mass ratio means that the multi-electron transfer reaction is essential for high-capacity NASICON-type cathode materials.
Through extensive literature review, we demonstrate the key challenge of realizing the 1.5e -/TM transfer reaction and delivering design rules from Na content, transition metal sites, and polyanion frameworks. Fortunately, the flexible structure gives a promising future in designing multi-electron transfer reaction NASICON-type cathode materials. Although it is important to develop new materials, it is equally essential to focus on the failure mechanism of reported high-capacity systems (such as Na 4 VMn(PO 4 ) 3 , Na 4 VFe(PO 4 ) 3 , Na 4 MnCr(PO 4 ) 3 , etc.). Overall, the characteristics of polyanionic compounds differ substantially from those of traditional layered oxide materials, and in-depth research is needed.