Low‐Cost Polyanion‐Type Cathode Materials for Sodium‐Ion Battery

Sodium‐ion batteries (SIBs) are considered as important candidate materials for energy storage systems (EES) due to high Earth abundance and low cost of sodium resources. Among all cathode materials for SIBs, polyanion‐type materials can provide high ionic conductivity and high structural stability, simultaneously, due to the 3D network structure. Their rich variety also provides more options for the commercialization of SIBs. However, the majority of polyanion‐type materials are not suitable for commercialization due to either high costs or complex preparation process. Herein, low‐cost polyanion‐type materials that are suitable for the commercialization of SIBs are discussed. The latest developments of polyanion‐type materials are overviewed. Furthermore, the challenges and countermeasures of polyanion‐type materials are summarized. This review can provide valuable insights for the commercialization of polyanion‐type materials.


Introduction
Nowadays, the wide utilization of renewable energy, such as wind energy and tidal energy, has alleviated the urgent challenges of energy depletion and environmental pollution.However, with additional geographical restrictions, these clean energies are hardly continuous and stable.To realize continuous and stable power supply, efficient and cleaner energy storage systems have become urgent problems. [1]Among various energy storage technologies, lithium-ion batteries (LIBs) are being widely used in daily life (e.g., electric vehicles, portable electronic products, etc.) due to their high energy density and superior cycle stability. [2,3]However, Li is rare on the Earth and concentrated in South America, which increases the cost of LIBs. [4]Therefore, it is urgent to develop energy storage devices that can replace LIBs for large-scale application.
Na exhibits similar chemical properties with Li, which is Earth abundant and low cost.Table 1 compares the fundamental chemical properties, Earth abundance, and bulk cost between Na and Li.As shown, Na is a promising supplement or substitute for Li (Table 1) considering similar properties and much lower cost. [5]odium-ion batteries (SIBs) mainly include layered transition metal oxides, Prussian blue crystal compounds, and polyanion-type materials.For layered materials, the irreversible phase transition occurring during the charge and discharge processes can greatly deteriorate the cycle stability. [6]Additionally, poor air stability of layered materials increases the storage and transportation costs. [7]Prussian blue crystal compounds can provide a high capacity. [8]owever, the crystal water in crystal is difficult to remove, which results in poor cycle performance. [8]On the contrary, polyaniontype materials possess tetrahedral and polyhedral 3D network structures, which can provide high cycle stability and high thermal stability. [9]Figure 1 compares the performance of different types of materials.Therefore, polyanion-type materials are regarded as one of the most promising cathode materials in SIBs.
[11] In addition, the high cost (such as NaVPO 4 F [12] ) and complex preparation process of polyanion-type materials also limit their commercialization.In this review, we comprehensively summarized the application potentials of polyanion-type materials for SIBs.We first highlight the specific features of polyanion-type materials, including excellent structural stability, multielectron transfer, and adjustable redox potential.The mechanism of material synthesis and redox process is correspondingly overviewed.In addition, we also put forward the shortcomings of polyanion-type materials and propose corresponding solutions.

Features of Polyanion-Type Cathode Materials
Polyanion-type materials refer to compounds with a 3D network structure formed by polyanionic polyhedrons and transition metal ion polyhedrons through strong covalent bonds.
The general formula is Na x M y (X a O b ) z (M = V, Fe, Mn, etc., X = P, S, Si, etc.).Referring to the material composition, polyanion-type materials can be separated into phosphates, pyrophosphates, fluorophosphates, sulfates, and silicates.
For polyanion-type materials, the polyanionic (X a O b ) z nÀ can support and stabilize the crystal structure, which can correspondingly provide high thermal and electrochemical stability.The existence of Na þ and multiple intermediate valence states of transition metals can achieve multielectron transfer.Additionally, diversified transition metal ions in polyanions render the capacity to realize adjustable redox potential and charge-discharge voltage.
However, the polyanion (X a O b ) z nÀ in polyanion-type materials can block the transport of electrons, leading to low electronic conductivity, which can further limit the capacity and magnification performance of materials.Additionally, polyanion-type materials possess a low tap density, which reduces the energy density of materials.
Figure 4 shows in situ synchrotron Fe K-edge X-ray absorption near-edge structure (XANES) and ex situ X-ray diffraction (XRD) studies of t-NaFePO 4 during the initial cycle.Figure 4a-d shows the voltage profile of NaFePO 4 at 2.0-4.0V and the Fe edge energy of operando XANES spectra at the first cycle, respectively.As shown, when Na þ is removed, the Fe K-edge moves to the higher binding energy, which corresponds to increased Fe oxidation state during the charging process.As shown, upon discharge, Fe is reduced to its original state.During the whole chargedischarge process, Fe K-edge XANES shows high reversibility of   Reproduced with permission. [13]Copyright 2013, American Chemical Society.
As shown, t-NaFePO 4 shows excellent reversibility during the charge-discharge process.Overall, t-NaFePO 4 is a promising material due to its excellent electrochemical properties.However, its complex preparation process is not conducive to large-scale production.m-NaFePO 4 is typically considered as electrochemically inactive.However, it is a thermodynamic stable phase.m-NaFePO 4 was reported as an excellent cathode material for the first time in 2014. [17]During the charging process, all Na þ can be deintercalated from the nanosized m-NaFePO 4 and transfer into amorphous FePO 4 (a-FePO 4 ). Figure 5 shows the Na þ diffusion based on quantum mechanics (QM) calculations and bond validity summary (BVS).Figure 5a shows the possible Na sites and diffusion paths in a-FePO 4 . [17]The diffusion of a-FePO 4 is usually disordered, due to the lack of 1D channels.Besides, the activation energy of Na hopping in a-FePO 4 is lower than that in m-FePO 4 (Figure 5b).Furthermore, the activation energies for Na diffusion in a-FePO 4 are lower after Na3 (Figure 5c), which proves that a-FePO 4 is beneficial for the reversible Na þ de/intercalation in m-NaFePO 4 .Figure 5d  In addition, Liu et al. [18] also prepared m-NaFePO 4 with excellent performance by electrospinning, which is facile, convenient, and efficient.Although the performance of m-NaFePO 4 is slightly lower than that of t-NaFePO 4 , the simple preparation method of m-NaFePO 4 makes it more conducive to commercialization.
NaMnPO 4 also shows olivine and maricite phases, which is similar with NaFePO 4 .Boyadzhieva et al. [19] synthesized phospho-olivine NaMnPO 4 by ion-exchange reaction.The voltage platform provided by the Mn 2þ /Mn 3þ redox couple of olivine-type NaMnPO 4 is %3.75 V.The theoretical capacity is 155 mAh g À1 and the practical capacity was up to about 128 mAh g À1 at 0.05 C.However, it demonstrates 55% capacity retention after 20 cycles at 0.05 C. The reason is that the maricite-type NaMnPO 4 shows no ion channels, which is correspondingly electrochemically inactive.At present, the mechanism of NaMnPO 4 is still unclear and requires further investigation.However, the high platform and high theoretical capacity of NaMnPO 4 indicate that it is a promising material for commercialization.
(%80 mAh g À1 at 5 C) and cycle stability (>96% capacity retention after 200 cycles), as shown in Figure 6b,c respectively.Na 3 Fe 2 (PO 4 ) 3 is a promising material that can be prepared by facile methods, such as solid state, [20] sol-gel, [21] and coprecipitation methods, [22] and has demonstrated remarkable electrochemical performance.Elements with a higher redox platform can be doped with Na 3 Fe 2 (PO 4 ) 3 to improve the energy density of the material.Na 3 Cr 2 (PO 4 ) 3 exhibits a flat potential plateau of 4.5 V, [23] Figure 2, which is crucial for high energy density.Figure 6d shows charge/discharge curves of Na 3 Cr 2 (PO 4 ) 3 at different circles.Unfortunately, the unstable Cr 3þ /Cr 4þ redox demonstrates poor cycle stability.Figure 6e shows a cyclic voltammogram of Na 3 Cr 2 (PO 4 ) 3 , which corresponds to a 4.5 V high platform.The substitution of Cr with Ti can improve cycle stability at the cost of reduced capacity (Figure 6f ). [24]This is due to Ti 3þ /Ti 4þ redox reaction at 2.1 V. Thus, inventing ways to make full use of Cr 3þ /Cr 4þ in Na 3 M 2 (PO 4 ) 3 is conducive to improving the energy density.Na 3 V 2 (PO 4 ) 3 (NVP) is also a typical NASICON-type material, which consists of PO 4 tetrahedra and VO 6 octahedra. [25]NVP exhibits a theoretical capacity of 117 mAh g À1 with a platform at 3.3 V.It demonstrates excellent rate performance when combined with carbon materials.However, V element is not only expensive, but also toxic.Therefore, the substitution of V with low-cost and nontoxic elements is necessary.
Mn is a low-cost and nontoxic element with high redox potential, which has made Na x MnM(PO 4 ) 3 (x = 3, 4; M = Ti, Zr, Cr, V) promising material for commercialization.Na 3 MnTi(PO 4 ) 3 was first reported by Goodenough et al. [30] in 2016.Figure 8a shows two curves of the discharge platform corresponding respectively to the redox couples of Mn 2þ /Mn 3þ (3.6 V) and Mn 3þ /Mn 4þ (4.1 V) in the voltage range of 2.5-4.2V. Na 3 MnTi(PO 4 ) 3 exhibits a small volume change during the insertion/extraction of two Na þ , which ensures excellent cycle stability. [30]As shown in Figure 8b, Ti 3þ /Ti 4þ redox couple appears at %2.1 V when the voltage range is extended to 1.5-4.2V.According to in situ XRD, solid-solution reaction and two-phase reaction occur in the voltage range of 1.5-4.2V (Figure 8c). Figure 8d shows the insertion/extraction process of Na þ in the voltage range of 1.5-4.2V. Na 3 MnTi(PO 4 ) 3 deintercalates two Na þ during the first charge process.Three Na þ will be inserted/extracted in the subsequent cycle process. [31]It demonstrates an ultrahigh capability of 160 mAh g À1 at 0.2 C and an outstanding cyclability (%92% capacity retention after 500 cycles at 2 C).Chen et al. [32] prepared a series of Na 3 þ 2x Mn 1 þ x Ti 1-x (PO 4 ) 3 .Na 3.3 Mn 1.15 Ti 0.85 (PO 4 ) 3 delivers a capacity of 181.4 mAh g À1 and an energy density of 560.2 Wh kg À1 at 0.1 C, which is significantly higher than Na 3 MnTi(PO 4 ) 3 (442.4Wh kg À1 ).It is shown that the increase of Mn content increases the working voltage and the theoretical capacity of more than three electron reactions.Hu et al. [33] found that the Na excess in Na 3 þ 2x Mn 1 þ x Ti 1-x (PO 4 ) 3 leads to low Mn 2þ occupation on the Na þ vacancies, which can improve diffusion kinetics and suppress voltage hysteresis.In addition, Zhou et al. [34] prepared V-doped Na 3.1 MnTi 0.9 V 0.1 (PO 4 ) 3 (NMTVP) with highmedium discharge voltage (3.53 V), which is on account of the redox couples of V 4þ /V 5þ (%4.1 V), Mn 3þ /Mn 4þ (%4.0 V), Mn 2þ /Mn 3þ (%3.6 V), and V 3þ /V 4þ (%3.4 V).Hou et al. [35] prepared that F-doped Na 3 MnTi(PO4) 2.83 F 0.5 greatly improves the rate performance.In summary, the multielectron reaction in Na 3 MnTi(PO 4 ) 3 exhibits high specific capacity and high energy density, which are necessary for the commercialization of SIBs.
Goodenough et al. [36] proposed Na 3 MnZr(PO 4 ) 3 as a highvoltage cathode in 2018.Both Mn 2þ /Mn 3þ and Mn 3þ /Mn 4þ redox couples are reversible in Na 3 MnZr(PO 4 ) 3 .Thus, Na 3 MnZr(PO 4 ) 3 demonstrates high-voltage plateaus at 3.5 and 4.0 V with a high capacity of 105 mAh g À1 and excellent cycle stability of 91% capacity retention after 500 cycles at 0.5 C (Figure 8e,f ). Figure 8g shows XRD patterns of different states, which explain that Na 3 MnZr(PO 4 ) 3 undergoes a two-phase reaction with a tiny volume change (5.5%), which is vital to long cycle stability. [36]Besides, it is further demonstrated that Jahn-Teller distortion of Mn 3þ can be inhibited in Na 3 MnZr(PO 4 ) 3 as Mn and Zr are disordered.In accordance, Na 2 MnZr(PO 4 ) 3 is a stable phase. [36]In addition, Shen et al. [37] prepared Na 3 MnZr(PO 4 ) 3 microspheres with embedded dual-carbon networks (NMZP@C-rGO) by spray drying, which provides superior low-temperature performance.NMZP@C-rGO demonstrates a capacity of 90 mAh g À1 at -15 °C, while NMZP@C performs the capacity of only 70 mAh g À1 .It also exhibits excellent cyclic stability of 79.6% capacity retention at -15 °C after 1500 cycles.The design of cathode materials with superior temperature adaptability is significant for industrial application.Na 4 MnCr(PO 4 ) 3 also demonstrates high energy density due to its high capacity and high operating voltage.Ex situ XRD shows the phase transition of Na 4 MnCr(PO 4 ) 3 during charging and discharging processes, which proves high reversibility of the electrochemical reaction (Figure 9a,b).It also shows that there are both solid-solution reaction and two-phase reaction in the processes of Na þ extraction/insertion. [38] As shown in Figure 9c, when the charging range is 1.5-4.3V, there are two platforms corresponding to Mn 2þ /Mn 3þ (3.52 V) and Mn 3þ /Mn 4þ (4.15 V) redox couples, respectively.Na 4 MnCr(PO 4 ) 3 demonstrates a capacity of 108 mAh g À1 at 0.1 C and 72 mAh g À1 at 10 C, with excellent rate performance.Highly reversible redox reaction makes Na 4 MnCr(PO 4 ) 3 capable of maintaining excellent performance after 500 cycles (Figure 9d,e).Figure 9f shows that when the charging range is expanded to 1.4-4.6V, Na 4 MnCr(PO 4 ) 3 possesses a capacity of 160 mAh g À1 because of the three-electron reaction.The increased capacity is induced by the Cr 3þ /Cr 4þ (4.41 V) redox couple.In situ XRD shows that the reaction is highly reversible in the whole phase transition process.Both the single-phase solid-solution reaction and two-phase reaction occur in the charge-discharge process (Figure 9g).Therefore, the main reasons for the decline of circulating performance are the Jahn-Teller effect of Mn 3þ and the decomposition of electrolyte at high voltage.Figure 9h shows the cycle stability of Na 4 MnCr(PO 4 ) 3 at different cut-off voltages and different electrolytes. [39]It shows that the cycle stability of Na 4 MnCr(PO 4 ) 3 can be improved by modifying electrolyte.Na 4 MnCr(PO 4 ) 3 is a promising cathode material for commercialization, where metal-ion doping, morphology design, and surface engineering can may further improve the cycle stability.
Half of V in NVP is replaced by Mn can lead to the formation of Na 4 MnV(PO 4 ) 3 , which demonstrates a higher operating voltage of 3.5 V than NVP of 3.4 V.However, due to the Jahn-Teller effect of Mn 3þ , Na 4 MnV(PO 4 ) 3 exhibits poor cycle stability.The performance of Na 4 MnV(PO 4 ) 3 can be effectively improved by modification. [40,41]There are two platforms corresponding to Mn 2þ /Mn 3þ (about 3.6 V) and V 3þ /V 4þ (about 3.4 V) respectively in the range of 2.5-3.7 V (Figure 10a).When charged to 4.3 V, V 4þ /V 5þ redox potential appears at about 3.8 V (Figure 10b).Figure 10c shows the reason for deteriorated performance after being charged to high voltage.When it is charged to 3.8 V, the volume change of the material is highly reversible (I, III).Meanwhile, when it is charged to 4.0 V, the volume change of the material becomes larger and highly irreversible (II, IV). [42]s shown in Figure 10d, higher charging voltage can increase the capacity, which simultaneously reduces cycle stability.To obtain better performance, charging voltage of the material needs to be controlled.Although the decrease in voltage results in a loss Reproduced with permission. [27]Copyright 2017, Springer Nature.b) XRD patterns of Na 2 VTi(PO 4 ) 3 during charge and discharge (correspond to the V 3þ /V 4þ couple) and c) discharge and charge (correspond to the Ti 3þ /Ti 4þ and V 2þ /V 3þ couples) at 0.05 C. Reproduced with permission. [28]Copyright 2018, The Royal Society of Chemistry.Electrochemical performances of NTVP/C and NTVP/C-G.d) Rate performances and e) long-term cyclability at 5 C. Reproduced with permission. [29]Copyright 2022, Elsevier.
of capacity, Na 4 MnV(PO 4 ) 3 can still provide higher plateau than NVP and comparable capacity to NVP in the same voltage range.Li et al. [40] prepared graphene aerogel-coated Na 4 MnV(PO 4 ) 3 (NMVP@C@GA) by sol-gel method with excellent rate performance and cycle stability (Figure 10e,f ).Hu et al. [43] found that adjusting the particle size can improve the structural stability and boost kinetics of Na 4 MnV(PO 4 ) 3 .Na 4-3x MnV(PO 4-x F x ) 3 prepared by Ma et al. [44] demonstrates an ultrahigh rate capability of 58 mAh g À1 at 10 C. Cr-doped Na 4-x Mn 1-x Cr x V(PO 4 ) 3 can provide a higher-voltage platform. [45]47] Based on these analyses, Na 4 MnV(PO 4 ) 3 is a promising cathode material for commercialization due to its facile preparation and excellent overall performance.
In addition, Na 3 VCr(PO 4 ) 3 prepared by replacing V with Cr possesses a high-voltage platform and a capacity of %90 mAh g À1 .However, when it is charged to a higher voltage, a metastable phase Na 2-y V 5þ y V 4þ 1-y Cr(PO 4 ) 3 can form, resulting in poor cycle stability.It is found that low temperature can suppress irreversibility and enhance the cycle stability. [48]Therefore, Na 3 VCr(PO 4 ) 3 is a material suitable for low-temperature environment.Although Na 4 FeV(PO 4 ) 3 exhibits great capacity and rate performance, [49] its energy density is affected by the low redox platform of Fe 2þ /Fe 3þ , which is only 2.4 V.However, when it is charged to a higher-voltage range, the cycle stability of Na 4 FeV(PO 4 ) 3 is better than that of Na 4 MnV(PO 4 ) 3 , which can be attributed to the Jahn-Teller effect of Mn 3þ . [42,50]
4.1.Na 2 MP 2 O 7 (M = Fe, Mn) Na 2 FeP 2 O 7 is a material with low cost and excellent performance.Yamada et al. [52] first investigated the application of Na 2 FeP 2 O 7 as SIBs in 2012.They also explored the discharge structure of Na 2 FeP 2 O 7 . [51]As shown in Figure 11a-c, below 550 °C, the desalted NaFeP 2 O 7 remains in β-NaFeP 2 O 7 state (triclinic), which is similar to Na 2 FeP 2 O 7 .However, with the increase of temperature (>560 °C), NaFeP 2 O 7 undergoes an irreversible phase transition from triclinic phase (β-NaFeP 2 O 7 ) into the ground-state monoclinic phase (α-NaFeP 2 O 7 ).Thermal decomposition and oxygen evolution of NaFeP 2 O 7 occur above 600 °C.NaFeP 2 O 7 demonstrates high thermal stability, which provides security for large-scale economic SIB applications.Choi et al. [53] pointed out that Na 2 FeP 2 O 7 exhibits a single-phase reaction at %2.5 V and a continuous two-phase reaction at 3.0-3.25V (Figure 11d,e).The ex situ XRD shows great reversibility during the whole charging and discharging processes (Figure 11e).Carbon coating is a relatively simple strategy to improve the performance of Na 2 FeP 2 O 7 .Zhang et al. [54] first prepared double-carbon synergistically modified Na 2 FeP 2 O 7 .The Na 2 FeP 2 O 7 particles are kept in place by the carbon layer and further anchored in the reduced graphene oxide (RGO) framework, which accelerates the transmission of Na þ and electrons.The NFP@C@RGO provides about 80 mAh g À1 capacity and nearly 100% capacity retention after 300 cycles at 1 C  [38] Copyright 2020, Wiley-VCH.f ) Charge-discharge profiles at 0.05 C of Na 4 MnCr(PO 4 ) 3 within 1.4-4.6V. g) In situ XRD of Na 4 MnCr(PO 4 ) 3 at the first cycle, along with the selected (300) diffraction profiles.h) Comparison of the cycle stability of Na 4 MnCr(PO 4 ) 3 at 0.1 C in different electrolytes containing 5 vol% and 10 vol% FEC under different potential intervals of 1.4-4.3 and 1.4-4.6V. Reproduced with permission. [39]Copyright 2020, Wiley-VCH.
(Figure 11f,g).In addition, the performance of Na 2 FeP 2 O 7 is expected to be improved by changing carbon source, [54,55] Na source, [56] and doping. [57]For example, the double-carbon synergistically modified Na 2 FeP 2 O 7 prepared by Zhang et al. [54] greatly improves the rate performance.Zhang et al. [56] prepared Na 2 FeP 2 O 7 with Na 2 CO 3 , NaH 2 PO 4 , and NaOH as carbon sources, respectively.It is found that different carbon sources have an effect on crystal growth and NaH 2 PO 4 is more conducive to Na 2 FeP 2 O 7 to obtain excellent electrochemical properties.Besides, the Mg-doped Na 2 FeP 2 O 7 prepared by Lu et al. [57] provides nearly 100% capacity retention after 700 cycles.
Na 2 MnP 2 O 7 possesses a theoretical capacity of 97.5 mAh g À1 and a redox potential of up to 3.6 V. Therefore, Na 2 MnP 2 O 7 provides a higher energy density than Na 2 FeP 2 O 7 .Na 2 MnP 2 O 7 can be prepared by a simple solid-state method.However, similar with other Mn-based materials, Na 2 MnP 2 O 7 also shows the problem of Mn dissolution, which deteriorates the performance.Jiao et al. [58] prepared graphene-coated Na 2 MnP 2 O 7 (NMP@GL) by ball milling.NMP@GL delivers a capacity of 93 mAh g À1 at 0.1 C, which is 95.4% of the theoretical capacity (Figure 12a).NMP@GL provides a capacity of 72.1 mAh g À1 at 1 C and the capacity retention rate is 91.7% at 0.2 C after 100 cycles (Figure 12b,c).The full cell couples the NMP@GL cathode with the hard carbon anode (HC).The NMP@GL//HC full cell demonstrates an energy density of 300 Wh kg À1 at 0.2 C, which demonstrates the possibility of its application.Choi et al. [59] systematically studied the structure and mechanism of Na 2 MnP 2 O 7 .It is reported that the corner-sharing crystal structure in triclinic Na 2 MnP 2 O 7 can assist the regulation of Jahn-Teller distortions and enhance the kinetics of Na 2 MnP 2 O 7 .Figure 12d shows the change of Na þ during Na removal process.While charging, Na 2 MnP 2 O 7 takes out half of Na1 first, then takes out all Na3, and finally takes out half of Na4 (Figure 12d).According to Figure 12e,f and discrete Fourier transform (DFT) calculation, Na 2 MnP 2 O 7 undergoes a single-phase reaction at 3.32 V and a continuous twophase reaction at 3.66, 3.98, and 4.15 V. Besides, Kahraman et al. [60] prepared Na 2 Fe 0.5 Mn 0.5 P 2 O 7 by solid-state method, which combines the stability of Na 2 FeP 2 O 7 and the high voltage of Na 2 MnP 2 O 7 to obtain excellent performance.In conclusion, Na 2 MP 2 O 7 (M = Fe, Mn) is a potential Na cathode material with high thermal stability, low cost, high safety, and excellent performance.
demonstrate their feasibility as cathode materials.Na 3.12 Fe 2.44 (P 2 O 7 ) 2 exhibits a capacity of 117.6 mAh g À1 due to its more de/intercalationable Na þ than Na 2 FeP 2 O 7 .Na 3.12 Fe 2.44 (P 2 O 7 ) 2 can be prepared by solid-state method. [62]owever, the high moisture sensitivity and severe surface oxidation of Na 3.12 Fe 2.44 (P 2 O 7 ) 2 lead to inferior electrochemical kinetics and cycle stability.To solve these problems, Deng et al. [63] prepared coral-like Na 3.12 Fe 2.44 (P 2 O 7 ) 2 /C by sol-gel method.The prepared Na 3.12 Fe 2.44 (P 2 O 7 ) 2 /C demonstrates a highly porous structure, which provides remarkable rate performance (about 93 mAh g À1 at 1 C) and cycle stability (95% capacity retention after 200 cycles) (Figure 13a,b).Figure 13c shows that the electrochemical reaction is a single-phase reaction.Chou et al. [64] prepared Na-rich Na 3.32 Fe 2.34 (P 2 O 7 ) 2 /C, which possesses superior rate and cycle stability (Figure 13d,e).Na 3.32 Fe 2.34 (P 2 O 7 ) 2 /C provides a capacity retention of 89.6% after 1100 cycles at 5 C due to the addition of 5% fluoroethylene carbonate(FEC) into the electrolyte (Figure 13e).The in situ XRD shows a single-phase reaction of Na 3.32 Fe 2.34 (P 2 O 7 ) 2 /C (Figure 13f ).In addition, DFT calculation shows that there are two 1D Na þ diffusion channels with low activation energy.Meanwhile, the lattice volume fluctuation is negligible (%2.19%) during the electrochemical cycle, which is the key reason for the excellent cycle stability.Na 3.12 Mn 2.44 (P 2 O 7 ) 2 also demonstrates excellent performance.Zhang et al. [65] synthesized off-stoichiometric Na 3.12 Mn 2.44 (P 2 O 7 ) 2 by sol-gel method.Na 3.12 Mn 2.44 (P 2 O 7 ) 2 provides a reversible capacity of 114 mA h g À1 at 0.1 C and high platform above 3.6 V(Figure 13g).In addition, Na 3.12 Mn 2.44 (P 2 O 7 ) 2 offers excellent rate (Figure 13h) and cycle stability (75% after 500 cycles at 5 C).The ex situ XRD demonstrates a single-phase reaction during the charging and discharging processes (Figure 13i).Although there are few studies on Na 4-α M 2 þ α/2 (P 2 O 7 ) 2 at the current stage, they have demonstrated electrochemical properties and will become competitive cathode materials for large-scale SIBs.

Fluorophosphates
Na 2 FePO 4 F is a material with 2D ion transportation channel (Figure 14a). [66]It is found that the Na removal process of Na 2 FePO 4 F is a two-phase reaction, with Na 1.5 FePO 4 F as the intermediate phase. [67,68]Figure 14c reveals the charging and discharging mechanisms of Na 2 FePO 4 F through in situ XRD patterns, where two platforms correspond to two-phase reactions.The in situ XRD patterns of the charging and discharging process are symmetrical, which indicates that the whole process is highly reversible.Stable structure and highly reversible phase transition provide good cycle stability.Besides, the preparation process of Na 2 FePO 4 F with high performance can be prepared by a facile solid-state method.Xia et al. [5] prepared Na 2 FePO 4 F/carbon/multiwalled carbon nanotubes microspheres (NFPF@C@MCNTs) by ball milling.NFPF@C@ MCNTs exhibit excellent rate (93 mAh g À1 at 1 C) and cycle stability (97% retention rate after 700 cycles), as shown in Reproduced with permission. [51]Copyright 2013, American Chemical Society.d) Charge-discharge curves of Na 2 FeP 2 O 7 at 0.05 C. e) Ex situ XRD of Na 2 FeP 2 O 7 in the first cycle.Reproduced with permission. [53]Copyright 2013, Wiley-VCH.f ) Charge-discharge curves of NFP@C@RGO at different rates.g) Cycle stability at 1 C. Reproduced with permission. [54]Copyright 2017, Elsevier.
Figure 14d,e.The full cell consisted of NFPF@C@MCNTs and HC can provide an excellent energy density of 210 Wh kg À1 .Nowadays, researchers can prepare Na 2 FePO 4 F with excellent performance by a green and economical route, like ball milling. [5,69,70]Chen et al. [67] replaced Fe with a small amount of V to significantly improve the rate performance of Na 2 FePO 4 F. Vlad et al. [71] prepared the metastable crystalline Na 1.2 Fe 1.2 PO 4 F 0.6 by ball milling, which delivers a reversible capacity of over 140 mAh g À1 and discharge plateau of 2.9 V, which reveal the importance of exploring metastable or transient energy storage materials.In conclusion, Na 2 FePO 4 F is a promising material with simple preparation, low cost, and excellent performance.Compared with LiFePO 4 , Na 2 FePO F prepared by ball milling has an enormous cost advantage.
Na 2 MnPO 4 F was reported without electrochemical activity at the initial discovery (Figure 14b). [66,72]However, Yang et al. [73] discovered that binding Na 2 MnPO 4 F with carbon stimulates its activity.Sun et al. [74] prepared double carbon-constrained dual carbon-confined Na 2 MnPO 4 F nanoparticles (Na 2 MnPO 4 F CrGO) by hydrothermal method.Na 2 MnPO 4 F CrGO delivers a reversible capacity of 124 mAh g À1 at 0.05 C and an average discharge potential of 3.6 V (Figure 14f ).However, the cycle stability of Na 2 MnPO 4 F is 77% after 200 cycles at 0.1 C, which is lower than that of Na 2 FePO 4 F (97% capacity retention after 700 cycles), as shown in Figure 14e,g.The large polarization, poor electrochemical activity, and complex preparation methods of Na 2 MnPO 4 F have limited its practical application.
Yang et al. [73] prepared Na 2 Fe 1-x Mn x PO 4 F (x = 0, 0.1, 0.3, 0.7, 1) by the sol-gel method and analyzed their structures and properties.The XRD patterns show that Na 2 Fe 0.9 Mn 0.1 PO 4 F (x = 0.1) has the same 2D layered structure compared with Na 2 FePO 4 F. When x ≥ 0.3, Na 2 Fe 1-x Mn x PO 4 F transits from the 2D layer-structured Na 2 FePO 4 F into the 3D tunnel-structured Na 2 MnPO 4 F (Figure 14h).As shown in Figure 14i, Na 2 Fe 1- x Mn x PO 4 F (x = 0.3, 0.7) demonstrates higher platform and higher capacity compared with Na 2 Fe 1-x Mn x PO 4 F (x = 0, 0.1).Copyright 2019, Elsevier.d) The minimum energy structures at each composition of Na x MnP 2 O 7 .e) Calculated formation energies of Na x MnP 2 O 7 .f ) Ex situ XRD at different potentials of Na x MnP 2 O 7 .Reproduced with permission. [59]Copyright 2013, American Chemical Society.

Sulfates
Compared with other polyanionic materials, sulfate exhibits higher redox potential, which is conducive to improving the energy density of corresponding batteries.However, compounds containing SO 4 2À usually decompose at above 400 °C.SO 4 2À can be dissolved in water, which makes it unstable in aqueous solution.Therefore, sulfate compounds are usually prepared by lowtemperature solid-state method.Na 2 Fe 2 (SO 4 ) 3 is a promising candidate cathode material of batteries because of its high-voltage platform (3.8 V) and theoretical capacity (120 mAh g À1 ).So far, Na 2 Fe 2 (SO 4 ) 3 possesses the highest potential among all Fe-based SIB cathode materials (Figure 15b).Barpanda et al. [75] first synthesized Na 2 Fe 2 (SO 4 ) 3 via a solid-state method at 350 °C for 24 h.The crystal structure of Na 2 Fe 2 (SO 4 ) 3 is shown in Figure 15a, where two FeO 6 octahedra form a Fe 2 O 10 dimer by sharing an edge.The SO 4 2À anions interconnect these dimers to form a 3D network.Na 2 Fe 2 (SO 4 ) 3 prepared by solid-state method delivers a capacity of over 100 mAh g À1 at 0.1 C and excellent rate performance of 74 mAh g À1 at 1 C. To further enhance the capacity, Wang et al. [76] prepared carbon and graphene oxide (GO)-coated Na 2 Fe 2 (SO 4 ) 3 (NFS@C@GO) via a simple freeze-drying method.NFS@C@GO delivers a capacity of 107.9 mAh g À1 at 0.1 C, which is 90% of the theoretical capacity.NFS@C@GO demonstrates a specific capacity of more than 80 mAh g À1 at 5 C and a capacity retention of 80.1% after 800 cycles (Figure 15c,d).This is because the carbon network formed by the double-layer carbon coating provides a fast electron transfer rate.Figure 15e shows in situ synchrotron XRD patterns of the charge and discharge processes.Symmetric patterns indicate that the entire charge-discharge process is highly reversible.
(Figure 15a,h).Among the prepared materials, NFS@5%CNTs shows the best performance.NFS@5%CNTs exhibit a voltage plateau of 3.6 V and a capacity of 110.2 mAh g À1 at 0.1 C (1 C = 120 mA g À1 ).It also delivers more than 90 mAh g À1 capacity at 1 C and nearly 100% capacity retention after 1000 cycles at 2 C (Figure 15i,j).This full NFS@5%CNTs//HC cell delivers an impressive energy density of 350 Wh kg À1 , which can be applied in practical wide applications.

Silicate
Orthosilicates, Na 2 MSiO 4 (M = Fe, Mn), show the possibility of transferring two electrons, [82] which provide a high theoretical specific capacity, as high as 278 mAh g À1 .In addition, the strong Si─O bond secures a high thermodynamic stability of Na 2 MSiO 4 , which sustain up to 1000 °C with less than 5% volume change during desodiation. [83]Na 2 FeSiO 4 has been widely studied due to its low cost and high performance.Feng et al. [84] used spray-drying method to prepare the 3D CNTs-decorated Na 2 FeSiO 4 microspheres(Na 2 FeSiO 4 /CNTs).Na 2 FeSiO 4 /CNTs possess a capacity of 169 mAh g À1 at 0.1 C and excellent cycle  [66] Copyright 2010, American Chemical Society.c) Na storage mechanism study: in situ XRD patterns of Na 2 FePO 4 F. Reproduced with permission. [67]Copyright 2022, Elsevier.d) Rate performance of NFPF@C@MCNTs.e) Cycle stability of NFPF@C@MCNTs at 5 C. Reproduced with permission. [5]Copyright 2022, Wiley-VCH.f ) The initial discharge/charge curves of Na 2 MnPO 4 F CrGO. g) Cycle stability of Na 2 MnPO 4 F C À1 and Na 2 MnPO 4 F CrGO at 0.1 C. Reproduced with permission. [74]opyright 2019, Elsevier.h) XRD patterns of Na 2 Fe 1-x Mn x PO 4 F. i) Charge-discharge curves of Na 2 Fe 1-x Mn x PO 4 F at 10 mA g À1 .Reproduced with permission. [73]Copyright 2011, The Royal Society of Chemistry.stability at different rates (Figure 16a,b).Jiang et al. [85] synthesized carbon-coated Na 2 FeSiO 4 /C by sol-gel method, with a capacity of 181.0 mAh g À1 .Na 2 FeSiO 4 /C demonstrates remarkable capacity retention of 88% after 100 cycles due to the stable Fe-Si framework during the charge/discharge process (Figure 16c).In addition, Abbas et al. [86] revealed that the Na þ storage mechanism of Na 2 FeSiO 4 is a two-electron reaction and Fe 2þ /Fe 3þ /Fe 4þ redox couple is highly reversible during charging and discharging.Na 2 MnSiO 4 can provide higher redox potential than Na 2 FeSiO 4 .Na 2 MnSiO 4 /carbon/graphene (Na 2 MnSiO 4 /C/G) prepared by Zhu et al. [87] exhibits a capacity of 182 mAh g À1 at 0.1 C (Figure 16d).Ex situ XRD shows that the characteristic peaks of Na 2 MnSiO 4 gradually disappear during the charging process, which is related to the stripping of Na þ .When discharging, the characteristic peaks reappear, which indicates that the structural evolution process is reversible (Figure 16h). [88]However, the capacity retention rate of Na 2 MnSiO 4 /C/G after 30 cycles at 1 C is only 64.3%, which is caused by the dissolution of Mn 2þ into the electrolyte.Balaya et al. [89] inhibited the dissolution of Mn 2þ by adding vinylene carbonate (VC) into the electrolyte.Figure 16e shows that the Mn dissolved in the electrolyte with 5 vol% VC is much lower than that in the electrolyte without VC after 30 days.Na 2 MnSiO 4 shows excellent electrochemical properties in electrolyte with 5 vol% VC (Figure 16f,g).In short, the low cost and high capacity of Na 2 MSiO 4 make it worthy further investigation.

Mixed Phosphates
A combination of multiple polyanions can regulate redox voltage through structural diversity.Therefore, several polyanions can be composited to form new materials with excellent properties.
capacity (1 C = 129 mA g À1 ) (Figure 17b).NFPP/HC can provide a capacity of 79 mAh g À1 at 1 C and a capacity retention rate of 63.5% after 4000 cycles at 1 C (Figure 17d).The better rate performance and excellent cycle stability at high rates make NFPP/ HC a promising candidate for fast charge applications.Na 3 Fe 2 (PO 4 )(P 2 O 7 ) has been reported in recent years. [95,96]hang et al. [95] first prepared Na 3 Fe 2 (PO 4 )(P 2 O 7 ) by spray drying.Na 3 Fe 2 (PO 4 )(P 2 O 7 ) demonstrates an initial specific capacity of 110.2 mAh g À1 at 0.1 C (1 C = 119 mA g À1 ) and maintains a specific capacity of 89.7% after 6400 cycles at 2 C (Figure 17e).Na 3 Fe 2 (PO 4 )(P 2 O 7 ) also possesses an average voltage of about 3.1 V. Recently, Cao et al. [96] reported a green and scalable preparation method of Na 3 Fe 2 (PO 4 )(P 2 O 7 ), which extends its commercialization capacity.
(Figure 18a).Na 3 FePO 4 CO 3 only provides a voltage plateau of 2.7 V, which significantly limits the corresponding energy density. [99]Manjunatha et al. [100] employed Na 3 FePO 4 CO 3 for superior performance in aqueous rechargeable SIBs, which demonstrates the application of Na 3 FePO 4 CO 3 in aqueous batteries.Sadrnezhaad et al. [101] prepared Na 3 MnPO 4 CO 3 by modified Hummers method, as shown in Figure 18c.Na 3 MnPO 4 CO 3 coated by reduced graphene oxide (NMCP/rGO) provides excellent specific capacity of 155.1 mAh g À1 as shown in Figure 18d,e.NMCP/rGO exhibits a specific capacity of 180.9 mAh g À1 at 0.01 C (1 C = 191 mA g À1 ), which is 95% of the theoretical capacity.However, NMCP/rGO experiences poor rate (73 mAh g À1 at 1 C) and cycle stability (73 mAh g À1 after 5 cycles), as shown in Figure 18f, which limit its practical application.

Industrialization Application
The industrialization of polyanion-type materials conforms to the following points (Table 2). 1) From the safety viewpoint, V-based and Cr-based materials are toxic and unfriendly to the environment.2) From the cost viewpoint, Fe-based and Mn-based materials are the main objects of research because of their lower cost.
3) From the preparation process viewpoint, the simple preparation methods can reduce costs of equipment and process and improve production efficiency.4) From the electrochemical performance viewpoint, the superior energy density, cycle stability, rate performance, and low-temperature performance are necessary for industrialization application.

Summary and Perspective
Polyanion-type materials are considered to be one of the most promising cathode materials for SIBs due to their stable structure, high energy density, and excellent thermal stability.Figure 19 compares the cost of polyanion-type materials with LiFePO 4 , which focuses on Fe-based polyanion-type materials prepared by solid-state methods and includes the price of both precursors and finished products.As shown, the cost of LIBs is 5-10 times of SIBs.The high cost of LIBs is mainly due to the high price of Li 2 CO 3 .Table 3 summarizes the representative polyanionic electrode materials, including structure, redox voltage, and electrochemical performance.
The review shows that 1) Fe-based materials exhibit a low platform but remarkable cycle stability, and Mn-based materials exhibit a high platform but poor cycle stability due to the Jahn-Teller effect of Mn 3þ .2) Na 2 þ 2x Fe 2-x (SO 4 ) 3 demonstrates high plateau but poor air stability and thermal stability.3) Na 2 MSiO 4 (M = Fe, Mn) provides high capacity but low-voltage platform .Reproduced with permission. [100]Copyright 2021, American Chemical Society.b) Charge/discharge curves of Na 3 FePO 4 CO 3 at 10 mA g À1 .Reproduced with permission. [99]Copyright 2014, Springer Nature.c) Composite schematic diagram of NMCP/rGO.Charge/discharge curves of NMCP d) and NMCP/rGO e) at 0.03 C. f ) Discharge curves of NMCP/rGO at different rates.Reproduced with permission. [101]Copyright 2016, Elsevier.and poor rate performance.4) Mixed phosphates provide excellent performance, but their synthesis is difficult to control.5) The applicable range of electrolyte limits high-voltage capacity.
In view of earlier problems, the following solutions are proposed.1) The combination of active material and conductive carbon can improve the conductivity of the material and reduce the grain size.The addition of conductive carbon during the preparation process can improve the electronic conductivity and the diffusion kinetics of Na þ and can inhibit the growth of particles.2) The synthesis of reasonable morphology is controlled to optimize the structure of the material.The porous structures and 3D structures are more conducive to Na transport and electrolyte infiltration.3) Fe and Mn are combined to design new materials.The stability of Fe and the high voltage of Mn are expected to lead to better performed materials.4) Electrolyte is optimized.A high-voltage electrolyte is necessary for using materials under high voltage.5) The failure mechanism is analyzed by advanced characterization techniques for further improvement.
In addition, in order to give full play to the capacity of the cathode materials in the full cell, it is equally important to research suitable negative electrodes, electrolytes, binders, etc.There are  still many challenges in the commercialization of polyanion.A better understanding and development of polyanion materials will provide strong support for the commercialization of SIBs.

Figure 1 .
Figure 1.Performance comparison of different kinds of cathode materials for SIBs.
schematically shows the mechanism of charging and discharging processes of m-NaFePO 4 .At the beginning of charging process, Na þ are extracted from the surface of m-NaFePO 4 and a-FePO 4 can form as a shell of the m-NaFePO 4 .The rest of Na þ can be further extracted through a new fast diffusion path in a-FePO 4 .Correspondingly, all Na þ in the m-NaFePO 4 are deinked and all m-NaFePO 4 transfers into a-FePO 4 .During the discharging process, Na þ intercalation occurred, leading to the formation of a-NaFePO 4 .The subsequent electrochemical cycles proceed by the reversible transfer of Na þ through a-FePO 4 .As shown, a-FePO 4 is necessary for m-NaFePO 4 .

Figure 4 .
Figure 4. a-d) In situ synchrotron Fe K-edge XANES at the first cycle and e,f ) ex situ XRD studies of t-NaFePO 4 at the second cycle.Reproduced with permission.[16]Copyright 2016, The Royal Society of Chemistry.

Figure 5 .
Figure 5. Na diffusion in a-FePO 4 .a) Possible Na sites and diffusion paths in a-FePO 4 .b) The functional relationship between the activation energy of Na migration and the distance between Na sites, and c) the activation energies.d) Mechanism diagram of m-NaFePO 4 during charge/discharge cycling.Reproduced with permission.[17]Copyright 2015, The Royal Society of Chemistry.

Figure 12 .
Figure 12.Electrochemical performances of NMP@GL.a) Charge-discharge curves.b) Charge-discharge curves at different rates.c) Cycle stability at 0.2 C. Reproduced with permission.[58]Copyright 2019, Elsevier.d) The minimum energy structures at each composition of Na x MnP 2 O 7 .e) Calculated formation energies of Na x MnP 2 O 7 .f ) Ex situ XRD at different potentials of Na x MnP 2 O 7 .Reproduced with permission.[59]Copyright 2013, American Chemical Society.

Figure 19 .
Figure 19.The cost of polyanionic materials and LiFePO 4 .a) Price of raw materials.b) Price of electrode material.

Table 1 .
The comparison between Na and Li.

Table 2 .
The specific route of industrial application.Mn-based, low/no V & Cr Preparation method Ball milling, spray drying, etc.

Table 3 .
Summary of representative polyanion-type electrode materials.