1.   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 [1-7], 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 [9-12]. 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 NaMe₂(PO₄)₃ (Me = Ge, Ti, Zr) structure. They mentioned that the crystal’s 3D framework is built up of the corner link of MeO₆ octahedra and PO₄ 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}Zr₂P_3-xSi_xO₁₂(0 x 3). These compounds were named NASICON (sodium (Na) super (S) ion (I) conductor (CON)), benefiting from the three-dimensional diffusion tunnel. Subsequently, Nadiri et al [16] used Fe₂(MoO₄)₃ for the positive electrode and AClO₄ (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 ATi₂(PO₄)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 Na₃V₂(PO₄)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].

Figure  1.  (a) Timeline of the development of phosphate-based cathode materials. (b) Potential and specific capacity of different cathode materials. Squares are one-electron reactions of each transition metal (named 1e^-/TM); the circle symbols are 1.5e^-/TM. NOTE: the reactions with 1.5e^-/TM in Na₄VMn(PO₄)₃ and Na₄VFe(PO₄)₃ are irreversible.

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Since then, Na₃V₂(PO₄)₃ has been regarded as a promising cathode candidate earning wide investigation, and several modification strategies have been explored to optimize its electrochemical performance [20-22]. 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 Na₃MnTi(PO₄)₃ and Na₃MnZr(PO₄)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 Na₃MnTi(PO₄)₃ (3.6 V and 4.0 V) [25]. Furthermore, Fe-rich NASICON-type cathode materials (such as Na₃Fe₂(PO₄)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 P₂O₇^4- 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 [26-28]. 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.

2.   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}Zr₂P_3-xSi_xO₁₂(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’(PO₄)₃ [11]. Where the A’ site can be occupied by alkali ions (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺), alkaline earth ions (Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺), transition metals (Cu²⁺, Ag⁺, Pb²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺, Al³⁺, Ge⁴⁺, Zr⁴⁺, and Hf⁴⁺), and ion-molecules (H₃O⁺ and NH₄⁺), 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, MO₆ and M’O₆ octahedrons share all angles with XO₄ tetrahedrons, and MO₆ and M’O6 octahedrons are arranged linearly along the c-axis. The octahedron MO₆ and M’O₆ connect three tetrahedral XO₄ 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.

3.   Electrochemical performance of reported materials

The NASICON-type compounds are much favored by the open 3D diffusion channels and stable skeleton structure. Currently, research focuses on the following directions: optimizing strategies for Na₃V₂(PO₄)₃-based materials (e.g. carbon coating [20, 33-37], morphology control [22, 38], element doping [21, 39-45], etc), and exploring unknown material systems by replacing the M/M’ transition metal sites [25, 46-48]. To date, the NASICON-type cathode materials with various electrochemically active metals (V, Mn, Fe, Cr, Ti, etc) have been extensively explored. Meanwhile, phosphate-based mixed-polyanion compounds (such as Na₃V₂(PO₄)₂F₃, Na₄Fe₃(PO₄)₂(P₂O₇), Na₂FePO₄F, etc) have also been proposed as cathodes in NIBs.

3.1   V-based NASICON cathodes

Vanadium compounds have attracted great attention for their excellent redox, electrochemical, catalytic, and magnetic properties [49-51]. 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. Na₃V₂(PO₄)₃ can be indexed as the rhombohedral phase $R\bar{3}C$ space group [29, 32, 52], and the oxidation state of V in Na₃V₂(PO₄)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 Na₃V₂(PO₄)₃ [32, 52].

The transition metal substitution of the V element in Na₃V₂(PO₄)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 Na₄VM(PO₄)₃ (M = Fe, Mn, Ni) [48]. Similar to LMFP, the operating potential of Na₄VMn(PO₄)₃ can be significantly improved without capacity fading. Immediately, researchers focused on enhancing the electrochemical performance of Na₄VMn(PO₄)₃ [54-60]. However, as shown in figure 2(c), the V^4+/5+ redox couple is irreversible in Na₄VMn(PO₄)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 V⁵⁺ can migrate to Na_vacancy sites and block the sodium ion pathway in Na₃VCr(PO₄)₃. Interestingly, a similar phenomenon was captured in Na₃VSc(PO₄)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 V⁵⁺ is a common issue of the irreversible V^4+/5+ redox couple. It should be noted that the replacing elements of Al³⁺, Cr³⁺, and Ga³⁺ 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)) [63-66]. This finding can be attributed to the small Al³⁺ and the eliminated Jahn-Teller effect of Mn³⁺, 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.

Figure  2.  (a) Crystal structures and (b) voltage profiles of the typical V-based NASICON-type cathode materials, data from [69, 70, 79, 80]. (c) Charge curves of the V³⁺ to V⁵⁺ 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].

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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 NaVPO₄F [69, 70], Na₃V₂(PO₄)₂F₃ [71-74], and Na₃V₂O_2x(PO₄)₂F_3-2x [28, 75-77] 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 detailed picture of the structural evolution of Na₃V₂(PO₄)₂F₃ when more than 2.5 sodium ions were extracted. In addition, they revealed that the Na₀V₂(PO₄)₂F₃ 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.

3.2   Mn-based NASICON cathodes

Manganese-based electrode materials are attractive due to their excellent stability, resource non-criticality, and high electrode potential [24, 81]. Currently, Mn-rich NASICON-type phosphates provide high electrode potentials and robust anionic redox-free frameworks, such as Na₃MnTi(PO₄)₃ [25, 81-89], Na₃MnZr(PO₄)₃ [47, 90], and Na₄MnCr(PO₄)₃ [91-95] (figure 3(a)). In 2013, Pan et al [24] first proposed Na₃MnTi(PO₄)₃ and Na₃MnZr(PO₄)₃ as cathode materials for NIBs. Subsequently, Gao et al [25] successfully achieved a discharge capacity of 80 mAh g^-1 in Na₃MnTi(PO₄)₃. It should be noted that the Mn^2+/3+ and Mn^3+/4+ redox couples can be entirely activated within the voltage range of 2.5-4.2 V. The corresponding thermodynamic equilibrium potential is 3.6 V and 4.0 V, respectively. As shown in figure 3(b), the voltage profiles of Na₄MnAl(PO₄)₃, Na₃MnTi(PO₄)₃, and Na₃MnZr(PO₄)₃ exhibit initial capacity fading and voltage hysteresis (Al³⁺ < Ti⁴⁺ < Zr⁴⁺). However, the significant capacity fading of Mn-rich NASICON-type cathodes arises in the initial cycle, and an outstanding cycling performance is displayed in the following cycles [86, 89]. This result means that the failure mechanism of Mn-based cathodes is different from that of V-based ones (V⁵⁺ migrates to Na_vacancy sites during charging). For the high-capacity cathode, Wang et al [93] found that Cr³⁺ can not only activate the Mn^2+/4+ redox couple, but Cr^3+/4+ is also electrochemically active in Na₄MnCr(PO₄)₃. Therefore, Na₄MnCr(PO₄)₃ exhibits a high reversible capacity of 150.3 mAh g^-1, which is close to the 1.5e^-/TM transfer reaction, as shown in figure 3(c). Unfortunately, the high operating voltage and poor stability limit its application, and more study is needed to optimize the electrochemical performance.

Figure  3.  (a) Crystal structures and (b) voltage profiles of the typical Mn-based NASICON-type cathode materials, data from [25, 47, 96]. (c) Charge/discharge curve of Na₄MnCr(PO₄)₃ within the range of 1.5-4.5 V.

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3.3   Fe-based NASICON cathodes

Fe-based phosphate compounds play a dominant role in NIB cathode research, which is encouraged by the success of LiFePO₄ [97]. As shown in figures 4(b) and (c), the low cost and high resource abundance [23] of Fe/Na match the requirements of large-scale energy storage devices. However, the thermodynamically stable structure of NaFePO₄ is an electrochemically inactive maricite phase [98]. Furthermore, the electrode potential of the Fe^2+/3+ reversible redox couple is 2.4 V (vs. Na⁺/Na) in NASICON-type compounds [99-101]. The operating potential is too low for the cathode material. Subsequently, anion groups with stronger electronegativity are used to increase the thermodynamic equilibrium potential of the Na-Fe-P-O system (e.g. P₂O₇^4- [102-104], F^- [105, 106], etc). In 2012, Barpanda et al [104] found that Na₂FeP₂O₇ has a high theoretical specific capacity of 97 mAh g^-1, with the thermodynamic equilibrium potential of the Fe^2+/3+ redox couple raised to 3.0 V (vs. Na⁺/Na). However, the high molecular mass of P₂O₇^4- results in a limited capacity. Kim et al [26] developed a new mixed-polyanion cathode for NIBs, Na₄Fe₃(PO₄)₂(P₂O₇). Na₄Fe₃(PO₄)₂(P₂O₇) [107-109] shows a robust open framework with 3D Na⁺ ion diffusion paths. It is a new striking Fe-based polyanion material due to its high theoretical capacity (129 mAh g^-1), with an average discharge potential of 3.1 V (vs. Na⁺/Na). In addition, Na₂Fe₂(SO₄)₃ [27, 110], Na₃Fe₂(PO₄)(P₂O₇) [98, 105, 106], and Na₂FePO₄F [98, 105, 106] have also been considered candidates for advanced commercial NIB cathode materials (figure 4(d)).

Figure  4.  (a) Crystal structures of the typical Fe-based NASICON-type cathode materials. (b) Abundance in Earth’s crust [111] and (c) prices [112] of elements. (d) Potential vs. specific capacity plots for Na₄Fe₃(PO₄)₂(P₂O₇) [113], Na₂Fe₂(SO₄)₃ [108], Na₂FePO₄F [105], and Na₃Fe₂(PO₄)₃ [101] normalized to the theoretical specific capacity per one-electron Fe²⁺ Fe³⁺ transition from experimental data.

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For the active electrochemical elements, 4d elements have also been reported, except for the above reported 3d transition metal element redox couples. In 2018, NaMo₂(PO₄)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.

4.   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 LiFePO₄ (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.

4.1   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 Na₀V₂(PO₄)₂F₃ 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 V⁵⁺ 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. Na₄VNi(PO₄)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 entire voltage platform of the V^4+/5+ redox couple. Meanwhile, blocking the migration channel of V⁵⁺ to the Na site is crucial too.

4.2   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 V⁵⁺ easily migrates to the alkali sites, which may be the predominant issue for the irreversibility of V^4+/5+ redox couples (e.g. Na₄VMn(PO₄)₃, Na₄VFe(PO₄)₃, Na₄VNi(PO₄)₃, Na₃VSc(PO₄)₃, etc). Interestingly, the substitution of Al³⁺ and Ga³⁺ 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 Al³⁺, Ti⁴⁺, and Zr⁴⁺, which cannot be used for high energy density materials (the +4 valence state of Ti and Zr is too high, and Al³⁺ is electrochemically inactive) [24]. Excitingly, recent results have shown that Cr³⁺ 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.

Figure  5.  A voltage map of NASICON electrodes, Na_xMM(PO₄)₃, where M and M = Ti, V, Cr, Mn, Fe, Co, and Ni. The text in each box represents the redox couple and the corresponding voltage vs. Na⁺/Na [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 Na_xV₂(PO₄)₃) is V^2+/3+, and the electrode potential is 1.5 V). Reproduced from [120] with permission from the Royal Society of Chemistry.

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4.3   Polyanion frameworks

The polyanion groups are various, such as BO₃^3-, CO₃^2-, C₂O₄^2-, SiO₄^4-, PO₄^3-, SO₄^2-, 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_xMM’(PO₄)₃, more polyanion frameworks also need to be explored.

5.   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 LiFePO₄, and V is 22.35 wt% in Na₃V₂(PO₄)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 Na₄VMn(PO₄)₃, Na₄VFe(PO₄)₃, Na₄MnCr(PO₄)₃, etc.). Overall, the characteristics of polyanionic compounds differ substantially from those of traditional layered oxide materials, and in-depth research is needed.