Chromium-containing polyanionic cathode materials for sodium-ion batteries: progress, challenges and opportunities

Abstract

Sodium-ion batteries (SIBs) have emerged as promising candidates for next-generation energy storage systems due to their abundant resources, low cost, and environmental friendliness. Among various cathode materials, chromium-containing polyanionic compounds have attracted significant attention for their high working voltage, excellent diffusion kinetics, and safety. This review provides a comprehensive overview of the recent progress in chromium-containing polyanionic cathode materials for SIBs, emphasizing the multifunctional roles of chromium in enhancing electronic conductivity, stabilizing crystal structures, and enabling high-voltage redox activity. The interplay between material composition, crystal architecture, and sodium storage behavior is discussed. Challenges such as poor high-voltage durability and interface degradation are identified, with emphasis on strategies including structural modulation, defect regulation, and interface engineering. Moreover, strategies to overcome the bottlenecks in material development, such as improving high-voltage stability, optimizing energy density, and enhancing interfacial performance, are proposed. These findings not only deepen the understanding of chromium-containing polyanionic materials but also provide a theoretical foundation for the development of efficient and safe energy storage solutions.

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Jin-Ling Liu

Jin-Ling Liu is a PhD candidate at the College of Environmental and Resource Sciences and College of Carbon Neutral Modern Industry at Fujian Normal University. Her research focuses on electrode materials for sodium-ion batteries.

Yan Zhuang

Yan Zhuang is a Master's candidate at the College of Environmental and Resource Sciences and College of Carbon Neutral Modern Industry at Fujian Normal University. His main research direction is cathode materials for sodium-ion batteries.

Yi-Fei Liu

Yi-Fei Liu is a Master's candidate at the College of Environmental and Resource Sciences and College of Carbon Neutral Modern Industry at Fujian Normal University. His main research direction is cathode materials for sodium-ion batteries.

Xiao-Tong Wang

Xiao-Tong Wang received her bachelor's degree from Qufu Normal University in 2019. She is currently pursuing her doctorate under the guidance of Prof. Xing-Long Wu of Northeast Normal University. Her main research direction is the recycling and revalorization of spent Li-ion batteries.

Zhen-Yi Gu

Dr Zhen-Yi Gu received his PhD from Northeast Normal University (NENU) in 2024 and is currently a postdoctoral fellow at NENU, under the joint supervision of Prof. Xing-Long Wu and Prof. Yichun Liu. His current research interests focus on polyanionic cathode materials and electrolytes for sodium-ion batteries and exploration of new battery design.

Denglong Chen

Prof. Denglong Chen is a professor at the College of Environmental and Resource Sciences and College of Carbon Neutral Modern Industry at Fujian Normal University, China. He received his PhD in Polymer Chemistry and Physics from Fujian Normal University. His focus is primarily on the research of new energy materials.

Xing-Long Wu

Prof. Xing-Long Wu is currently a Professor of Materials Science and Chemistry at Northeast Normal University (NENU), China. He received his PhD from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2011. After completing two years of postdoctoral work in ICCAS, he moved to NENU as an Associate Professor in 2013, and became a Full Professor in 2018. His current research interests focus on the advanced materials for energy storage devices including metal ion batteries and dual-ion batteries, and the reuse and recycling of spent lithium-ion batteries.

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1. Introduction

Sodium-ion batteries (SIBs) have gained prominence as a secondary battery technology due to their abundant sodium resources, low cost, superior safety and environmental friendliness, making them highly suitable for large-scale energy storage and electric transportation applications.^1–4 The overall performance of SIBs is influenced by the choice of cathode and anode materials.⁵ Since most anode materials exhibit high capacity and low voltage,^6–10 the development of advanced cathode materials is essential for enhancing the performance of SIBs.^11,12

In recent years, the most extensively researched cathode materials for SIBs include transition metal oxides^13–15 and polyanionic compounds,^16–19 and Prussian blue analogs, as shown in Table 1.^20,21 Among them, PACs have demonstrated great application potential because of their stable structural frame, adjustable operating voltage and excellent thermal stability.^22,23 The chemical formula of PACs can be written as Na_xM_y[(XO_m)ⁿ⁻]_z (M is a transition metal and X is a non-metallic element such as P, S, B, Si, etc.), usually composed of (XO_m)ⁿ⁻ and an MO₆ polyhedral frame structure connected by strong covalent bonds, with remarkable stability and fast Na⁺ transport.^24,25 Compared to other materials, polyanionic compounds exhibit higher redox potentials primarily due to their unique inductive effects. The introduction of atoms with high electronegativity (X) to form M–O–X bonds weakens the strength of the M–O covalent bonds. This leads to a decrease in the energy of the antibonding orbitals and an increase in the energy gap between the antibonding orbitals and the vacuum level, thereby enhancing the redox potential. Typical synthesis methods for phosphate cathode materials are solid phase and hydrothermal methods.^26–28 The former requires a high sintering temperature and has the advantage of a simple process that is easy to industrialise. The latter can control the morphology, but the reaction requires high-pressure conditions with a lower yield and safety. Additionally, they offer advantages such as high stability and safety (Fig. 1).^29–31 However, transition metal ions are often separated by non-conductive polyanionic groups in the structural framework, causing electron transfer to follow the M–O–P–O–M mode instead of the faster M–O–M mode. This results in an extremely low intrinsic electronic conductivity of the material, thereby limiting the practical application of polyanionic cathodes.³² To enhance the electrochemical performance, numerous modification strategies have been reported. Common approaches include carbon coating or modification, design of nanostructures and morphologies, and ion doping or substitution.³³

Table 1 Electrochemical properties of typical cathode materials

Materials	Voltage range (V)	Discharge specific capacity (mA h g^−1)	Capacity retention	Ref.
P2-Na0.76Ca0.05[Ni0.23□0.08Mn0.69]O2	2–4.3	153.9@0.1C	83% after 100 cycles@1C	14
P2-Na0.67Li0.07Mg0.07Ni0.28Mn0.58O2	2.1–4.3	127.5@100 mA g^−1	90.1% after 100 cycles@100 mA g^−1	15
Na3V2(PO4)3	2.3–4.1	115.2@1C	87.7% after 200 cycles@1C	26
Na3.5MnTi0.5V0.5(PO4)3	1.5–4.3	182.7@0.1C	92.3% after 1000 cycles@10C	16
Na4VMn0.5Fe0.5(PO4)3/C	2–4	120@0.5C	94% after 3000 cycles@20C	17
Na4Fe2.91(PO4)2P2O7	1.7–4.3	110.9@0.2C	∼100% after 10 000 cycles@10C	18
PB/CNT	2–4.2	167@0.1C	78.6% after 500 cycles@5C	20
Na0.647Fe[Fe(CN)6]0.93 0.07·2.6H2O	2–4	130@0.5C	90% after 2000 cycles@20C	21

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Fig. 1 Characteristics of polyanionic cathode materials for SIBs.

Recent studies have demonstrated that the incorporation of chromium in polyanionic cathode materials could reduce the band gap of the material, optimise the crystal structure, broaden the diffusion channels for Na⁺ and improve the electronic conductivity and cycling stability of materials.^34–37 Additionally, the Cr³⁺/Cr⁴⁺ redox couple reached a potential of 4.9 V, effectively boosting the working voltage of batteries.³⁸ The Cr³⁺/Cr⁴⁺ redox pair may be exhibited when the chromium content is high or in combination with other transition metals such as Mn/Fe. Interestingly, researchers have also discovered that a small amount of Cr substitution can induce higher oxidation states of transition metals such as V, increasing the operating voltage while achieving multiple electron transfers, thus further increasing the energy density.³⁹ Consequently, chromium-containing polyanionic cathode materials for SIBs have significant application potential. To date, these materials have achieved a series of research advancements (Fig. 2). However, despite their notable advantages, the development of these materials still faces several challenges. In particular, during high-voltage and long-term cycling operations, material failure issues may arise, including the dissolution of transition metals, phase transitions of the material, and decomposition of the electrolyte. Addressing these challenges by further optimizing the performance of chromium-containing materials is a key issue in current research. In this review, we embark on an in-depth exploration of chromium-containing polyanionic cathode materials in SIBs, examining their mechanisms of action through a fresh and innovative lens. We meticulously review the latest advancements in this field and conduct a thorough analysis of the current challenges. Aiming to address these challenges, we propose innovative research strategies designed to offer unique insights and inspiration for future research directions, propelling the field towards the development of higher-performance cathode materials.

Fig. 2 The development of chromium-containing polyanionic cathode materials.^36,39–46

2. Research progress in the different roles of chromium

2.1. Unveiling intrinsic mechanisms of trace doping for conductivity enhancement

Multiple studies have demonstrated that trace chromium doping can enhance electronic conductivity, thereby exerting multifaceted positive impacts on electrochemical performance. A Na₃V_1.94Cr_0.06(PO₄)₃@C cathode material with a micron flower morphology, synthesized via hydrothermal synthesis, has been recently reported. Examination of the crystal framework structures before and after modification revealed that Cr doping rendered the V–O bond lengths more uniform (Fig. 3a), suppressed geometrical distortion, and augmented the material's structural stability. This trace doping also heightened the electroactivity of the Na2 sites, thereby improving electronic and ionic conductivity. Full batteries assembled with this material exhibited outstanding cycling stability at different temperatures (Fig. 3b) (25 °C, ∼88.2% after 4000 cycles; 0 °C, ∼62.0% after 3000 cycles; −20 °C, ∼82.5% after 1000 cycles; and 50 °C, ∼78.7% after 2700 cycles).⁴⁷ Li et al. reported the band gap of the material decreased from 1.9 eV to 0.3 eV upon chromium doping, which notably boosted the electronic conductivity of Na₃V_1.96Cr_0.03Mn_0.01(PO₄)₂F₃.⁴⁸ Similarly, in the material Na₃V_1.9Fe_0.095Cr_0.005(PO₄)₂F₃, the improvement in electrochemical performance was found to be attributed to the enhancement of electronic conductivity.⁴⁹ Subsequently, Cao's group discovered that Cr doping could optimize the structure of Na₃MnTi(PO₄)₃, extending the Na–O bonds and enlarging the Na-ion diffusion channels.⁴⁵ Density functional theory (DFT) calculations revealed that the Na-ion diffusion energy barrier in Na_3.1MnTi_0.9Cr_0.1(PO₄)₃ (NMTP-Cr_0.1) was significantly lower (Fig. 3c–e), facilitating ion diffusion kinetics. The doped NMTP-Cr_0.1 cathode material exhibited only an 8.1% volume change, achieving a high discharge specific capacity of 167.5 mA h g⁻¹ and an energy density of 517.5 W h kg⁻¹ at 0.1C, compared to a 9.5% volume change and an energy density of 433.7 W h kg⁻¹ for Na₃MnTi(PO₄)₃ (NMTP-Cr₀). Similarly, the high-entropy cathode material Na₃V_1.9(Ca,Mg,Al,Cr,Mn)_0.1(PO₄)₂F₃ with a small amount of chromium reduced the bandgap from 1.68 eV to 0.71 eV, and the diffusion energy barrier of Na⁺ dropped from 1.984 eV to 0.963 eV. Eventually, this led to the increase of the material's average discharge voltage to 3.81 V and its energy density reached 445.42 W h kg⁻¹.⁵⁰ Additionally, Cai et al. found that Cr doping did not alter the crystal structure of Na₃V_2−xCr_x(PO₄)₂F₃/C (NV_2−xCr_xPF/C) and did not generate any impurity phases (Fig. 3f). The doping of Cr³⁺ ions shortened the V–F bonds on the bisoctahedral units, leading to a shorter sodium ion transport pathway on the (001) crystal plane and reduced diffusion energy (Fig. 3g). Consequently, the NV_1.98Cr_0.02PF/C cathode material had a discharge specific capacity of 103.7 mA h g⁻¹ at 10C for the first cycle, and the capacity loss rate was as low as 2.99% after 1000 cycles (Fig. 3h).⁵¹ Interestingly, Cr doping was also found to alter the electron cloud distribution in materials of Na_3−xFe_2–xCr_x(PO₄)P₂O₇, reducing the bandgap and thereby facilitating an augmentation in electronic conductivity, leading to a faster charge transfer process (Fig. 3i). Concurrently, due to charge balance effects, the doped material developed sodium ion vacancies, which effectively promoted the diffusion kinetics of sodium ions.⁵² Therefore, trace chromium doping improved the electronic conductivity of the material by altering the electron cloud distribution, lowering the band gap, and optimising the crystal structure, thus enhancing the electrochemical properties.

Fig. 3 (a) Crystallographic framework of NVP and NVCP-6. (b) Cycling performance of NVCP-6 at different temperatures. Reprinted with permission from ref. 47. Copyright 2023, Tsinghua University Press. (c) Diagram of the Na⁺ migration path of the NMTP-Cr_0.1 crystal. (d) and (e) Na⁺ migration energy barriers in NMTP-Cr0 and NMTP-Cr_0.1. Reprinted with permission from ref. 45. Copyright 2024, Elsevier-B.V. (f) XRD patterns of NV_2−xCr_xPF/C. (g) Schematic diagram of vanadium/chromium double octahedra in NVPF/C and NV_1.98Cr_0.02PF/C. (h) Cycling performance at 10C. Reprinted with permission from ref. 51. Copyright 2023, Elsevier-B.V. (i) EIS and equivalent circuit diagram. Reprinted with permission from ref. 52. Copyright 2023, American Chemical Society.

2.2. Exploring novel mechanisms of minor substitution for the electron transfer increase

Na₃V₂(PO₄)₃ (NVP) is a typical highly concerned polyanionic cathode material for SIBs.⁵³ In recent years, more emphasis has been placed on the activation research of the V⁴⁺/V⁵⁺ redox couple.^54–56 Goodenough et al. prepared Na₃V_1.5Cr_0.5(PO₄)₃ using the sol–gel method.⁵⁷ The CV curves between 1 and 4.4 V clearly showed three pairs of reversible redox peaks corresponding to V²⁺/V³⁺, V³⁺/V⁴⁺, and V⁴⁺/V⁵⁺ (Fig. 4a). Corresponding plateaus were also observed in the charge–discharge curves. The plateaus at 3.6 V and 4.1 V correspond to the oxidation reactions of V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺, respectively (Fig. 4b). During the discharge process, the reduction proceeded from V⁵⁺ to V²⁺, achieving approximately 2.7 electron transfers and a discharge specific capacity of about 150 mA h g⁻¹ at 30 mA g⁻¹. A further study has been done by Chen et al.⁴² They have used advanced characterization techniques and systematic studies to elucidate the function of Cr in the rhombohedral structure as well as the valence electron transition mechanisms involving V and Cr. The doped Cr was randomly distributed in the VO₆ octahedral positions. The moderate amount of Cr doping lowered the diffusion energy barrier of sodium ions, as shown in Fig. 4c and d. The forbidden band gap of Na₃V_1.5Cr_0.5(PO₄)₃ was lower than that of NVP. More importantly, they pointed out that the unpaired electrons in the 3d orbitals of Cr were the key factor for the activation of the V⁴⁺/V⁵⁺ redox couple. The Cr-doped material exhibits excellent electrochemical performance 163.2 mA h g⁻¹ at 0.1C, and it still maintained 72.1% of its initial capacity even after 2650 cycles. The Cr-doped material exhibited minimal polarization at different rates and maintained a high-voltage plateau at 4.1 V (Fig. 4e), exhibiting a higher energy density than NVP (Fig. 4f). Lee et al. proposed a rapid synthesis of in situ carbon-coated Na₃V_1.6Cr_0.4(PO₄)₃/C cathode material using tetra ethylene glycol as the solvent in a polyol medium (Fig. 4g).⁵⁸ The substitution of Cr for V not only enhanced the material's structural stability but also triggered the consecutive redox reactions of V³⁺V⁴⁺V⁵⁺. After chromium substitution, the sodium storage mechanism shifted from a typical two-phase reaction to the coexistence of single-phase solid solution and two-phase reactions, reducing the volume change from 7.8% to 3.9%. Additionally, Na₃V_4/3Cr_2/3(PO₄)₃@C prepared by the spray drying method has a micro/nano hollow spherical structure, and a continuous redox reaction of V²⁺–V⁵⁺ occurred during the sodium ion insertion/de-insertion process, thus possessing a high specific capacity of 175 mA h g⁻¹ at 100 mA g⁻¹ and excellent cycling stability.⁵⁹ Zhang et al. further designed a NASICON Na₃Cr_0.5V_1.5(PO₄)₃ cathode (VC/C–G) loaded with reduced graphene oxide, demonstrating a triple electron transfer reaction at 4.02 V (V⁵⁺/V⁴⁺), 3.43 V (V⁴⁺/V³⁺), and 1.65 V (V³⁺/V²⁺).⁶⁰ GITT tests revealed excellent D_Na⁺ for VC/CG (Fig. 4h). In situ XRD confirmed the primary reaction mechanism during charging and discharging to be the solid solution reaction, accounting for the material's good electrochemical reversibility and rapid Na⁺ diffusion. This is reflected in a specific capacity of 176 mA h g⁻¹ at 0.2C (equivalent to the theoretical value), an energy density reaching up to 470 W h kg⁻¹, and stable cycling for 1000 cycles at 20C (Fig. 4i). Additionally, the material exhibited excellent fast charging performance, achieving 80% SOC in approximately minutes. Interestingly, NVP nanocrystals doped with Cr can be integrated into a dual-carbon matrix through a simple in situ reaction process involving M₄C₃T_x MXene and a V/Cr mixture.⁶¹ This doping modified the arrangement of Na⁺ ions at the Na2 position and induced an additional phase during the electrochemical process. Consequently, the energy barrier for Na⁺ migration was reduced. In synergy with the dual-carbon cladding, a stable V⁴⁺/V⁵⁺ redox at ∼4.0 V was achieved, demonstrating excellent electrochemical performance (1C initial discharge specific capacity of 119.3 mA h g⁻¹, energy density of 410 W h kg⁻¹; 78 mA h g⁻¹ at 200C and ∼230 W h kg⁻¹). Additionally, several high-entropy materials designed containing Cr were able to activate the V⁴⁺/V⁵⁺ redox couple, thereby enhancing the electrochemical performance of the materials.^62–66 Chromium, with one more valence electron than vanadium, activated the high-voltage potentials of V⁴⁺/V⁵⁺ through its unpaired 3d electrons. This doping also enhanced the ordering of Na⁺ and reduced its diffusion energy barriers, thereby effectively improving the energy density of NVP.

Fig. 4 (a) Na₃V_1.5Cr_0.5(PO₄)₃ CV curves at a sweep speed of 0.5 mV s⁻¹. (b) Charge and discharge curves of different cycles at 30 mA g⁻¹. Reprinted with permission from ref. 57. Copyright 2020, Wiley-VCH. The computed total density of states (DOS) and associated partial density of states (PDOS) plot for (c) Na₃V_1.5Cr_0.5(PO₄)₃ and (d) Na₃V₂(PO₄)₃. (e) The charge–discharge curves of Na₃V_1.5Cr_0.5(PO₄)₃ at different current densities. (f) Current density–varying energy density comparison chart. Reprinted with permission from ref. 42. Copyright 2021, American Chemical Society. (g) The synthesis mechanism diagram of Na₃V_1.6Cr_0.4(PO₄)₃/C. Reproduced with permission from ref. 58. Copyright 2021 Elsevier. (h) GITT profiles, D_Na⁺, and overpotentials of VC/C–G. (i) Cycling performances at 20C. Reprinted with permission from ref. 60. Copyright 2022, Wiley-VCH.

2.3. Deciphering high-voltage activity and failure mechanisms from a unique perspective

The Cr³⁺/Cr⁴⁺ redox couple exhibits inherent electrochemical activity, enhancing the cathode material's working voltage and facilitating multi-electron transfer, thus increasing energy density. Herklotz et al. demonstrated the active redox couple Cr⁴⁺/Cr³⁺ within a phosphate polyanion cathode material for the first time, operating at approximately 4.8 V.⁶⁷ The electrochemical activity of the Cr³⁺/Cr⁴⁺ redox couple enhances the operating voltage of cathode materials. The electrochemical activity of chromium in SIB cathode materials, specifically in Na₃Cr₂(PO₄)₃/AB, was not discovered until 2018.⁴¹ Cyclic voltammetry curves showed a distinct redox peak at 4.5 V, corresponding to the Cr³⁺/Cr⁴⁺ redox couple (Fig. 5a). The material exhibited an initial charge specific capacity of 98 mA h g⁻¹ at 0.5C, equivalent to 84% of the theoretical capacity, but demonstrated poor cycling stability (Fig. 5b). Cr L_2,3-edge IPFY XAS confirmed the reversible oxidation of chromium from +3 to +4 throughout the charge–discharge cycle (Fig. 5c and d). The team employed Ti doping to enhance the stability of Na₃Cr₂(PO₄)₃.⁶⁸ A Cr³⁺/Cr⁴⁺ redox pair at 4.5 V was distinctly observed in the charge/discharge and dQ/dV curves. This doping strategy significantly improved the charge/discharge reversibility, particularly from the second cycle. After 50 cycles, the material retained 90% of its discharge specific capacity compared to the second cycle (Fig. 5e). Subsequently, Zhang et al. reported similar findings.⁶⁹ Multiplicity performance plots from 10 to 320 mA g⁻¹ demonstrated that after cycling within the 2.5–5 V window, the capacity recovered to 95.6% of its initial capacity when the current density returned to 10 mA g⁻¹ (Fig. 5f). Moreover, cycling for 1000 cycles at 50 mA g⁻¹ achieved a capacity retention of 76.6% (Fig. 5g). Additionally, the introduction of the highly electronegative F was shown to elevate the operating voltage. In the crystal structure of the Na₃Cr₂(PO₄)₂F₃ cathode material, Na atoms occupied interstitial sites within the three-dimensional Cr₂(PO₄)₂F₃ framework in a disordered manner (Fig. 5h).⁷⁰ The Cr⁴⁺/Cr³⁺ redox potential in the modified material reached as high as 4.7 V.

Fig. 5 (a) First-cycle CV curve of Na₃Cr₂(PO₄)₃/AB. (b) Different cycle charge and discharge curves at 2.5–4.7 V. First-cycle Cr K-edge XANES spectra of Na₃Cr₂(PO₄)₃/AB: (c) charging and (d) discharging. Spectra of Cr₂O₃ were used as a reference for Cr³⁺. Reprinted with permission from ref. 41. Copyright 2018, American Chemical Society. (e) Charge–discharge curves of Na₂CrTi(PO₄)₃ for different cycles within the voltage window 2.5–4.7 V (inset left: dQ/dV curve; right: cycling performance). Reprinted with permission from ref. 68. Copyright 2019, the Royal Society of Chemistry. (f) Rate performance of Na₂TiCr(PO₄)₃ at 2.5–5 V. (g) Cycling performance. Reprinted with permission from ref. 69. Copyright 2020, American Chemical Society. (h) Crystal structure of Na₃Cr₂(PO₄)₂F₃. Reprinted with permission from ref. 70. Copyright 2021, American Chemical Society.

The integration of chromium with other transition metals has been shown to improve capacity.⁷¹ Through DFT theoretical calculations, it was predicted that when possessing the Mn²⁺/Mn³⁺, Mn³⁺/Mn⁴⁺, and Cr³⁺/Cr⁴⁺ redox pairs simultaneously, the average voltage of Na₄MnCr(PO₄)₃ could be as high as about 4 V (Fig. 6a), and the theoretical specific capacity could reach 165 mA h g⁻¹.⁷² Subsequently, Zhang et al. distinctly observed the aforementioned three pairs of redox couples in the charge–discharge curves of the Na₄MnCr(PO₄)₃ cathode material (Fig. 6b).⁷³ XANES further confirmed the reversible changes of Cr³⁺/Cr⁴⁺ (Fig. 6c). The synergistic effect of Mn and Cr enabled the material to achieve an impressive reversible capacity of 160.5 mA h g⁻¹ and a notable average discharge voltage of 3.53 V at 0.05C, with an energy density of 566.5 W h kg⁻¹, surpassing the commercialized LiFePO₄ in lithium-ion batteries (Fig. 6d), setting a new record. Liu's group exploited the anchoring effect of the inert ions Mg²⁺ or Zr⁴⁺. Subsequently, they carried out successive studies on Na₄Mn_0.9CrMg_0.1(PO₄)₃⁷⁴ and Na_3.9MnCr_0.9Zr_0.1(PO₄)₃,⁴³ with the objective of improving the material's electrochemical properties. Fig. 6e illustrated that Zr⁴⁺ in Na_3.9MnCr_0.9Zr_0.1(PO₄)₃ stabilizes the NASICON framework after Na⁺ insertion/de-insertion. Zr⁴⁺ has a larger ionic radius than Cr³⁺, which enables it to expand the unit cell, thereby facilitating the transport of sodium ions and enhancing the cycling stability. After 50 cycles at 0.5C, the doped samples showed superior capacity retention compared to the undoped material (Fig. 6f). In a fascinating development, the doping with Ti⁴⁺ was found to suppress the Jahn–Teller effect of Mn atoms and the charge compensation of Cr³⁺ at high voltages. Subsequently, Liang et al. combined the high-voltage attributes of Na₄MnCr(PO₄)₃ with the stability features of Na₃MnTi(PO₄)₃, thereby designing a novel solid-solution material designated as NMCTP.⁷⁵ As depicted in Fig. 6g, under the entropy-increasing effect of multiple transition metals, the longer TM–O₁ bonds shortened while the TM–O₂ bonds elongated, inducing minimal distortion in NMCTP. This led to a significant enhancement in the bond length symmetry of the TM octahedra, which was conducive to suppressing lattice distortion and improving structural stability. The NMCTP cathode material demonstrated an exceptional specific capacity of 150.3 mA h g⁻¹ and an energy density exceeding 500 W h kg⁻¹ at 0.1C.

Fig. 6 (a) Voltage profiles in Na_xMnM(PO₄)₃ derived from DFT calculations (M = Cr, Ti, and Zr; x = 0, 1, 2, 3, and 4). Reprinted with permission from ref. 72. Copyright 2020, Wiley-VCH. (b) Charge/discharge curves of Na₄MnCr(PO₄)₃ at 1.4–4.6 V. (c) XANES spectra of Cr K-edge in various charge–discharge states. (d) Energy density comparison chart of polyanionic cathode materials for SIBs. Reprinted with permission from ref. 73. Copyright 2020, Wiley-VCH. (e) Schematic of the NASICON structure and effects of Zr⁴⁺ doping on Na⁺ diffusion. (f) Comparison of cycling performance for Na_4−xMnCr_1−xZr_x(PO₄)₃/C (x = 0, 0.05, 0.1, and 0.15) at 0.5C. Reprinted with permission from ref. 43. Copyright 2020, Wiley-VCH. (g) Structural evolution of the TMO₆ octahedron. Reprinted with permission from ref. 75. Copyright 2024, Wiley-VCH.

Many studies indicate that co-doping with different elements has a synergistic effect and further enhances the electrochemical performance of materials.^76–78 Interestingly, a recent study achieved 2.61 electron transfers for Fe²⁺/Fe³⁺ (∼2.5 V), V³⁺/V⁴⁺/V⁵⁺, and Cr³⁺/Cr⁴⁺ (∼4.4 V) in Na_3.5Fe_0.5VCr_0.5(PO₄)₃/C cathode materials with an average voltage of 3.43 V.⁴⁴ Notably, the migration energy barriers at different positions (Fig. 7a) indicated that the introduction of Fe reduced lattice strain, while the substitution of Cr activated partial Na1 ions. The crystal structure transformation mechanism, as shown in Fig. 7b, enhanced the reversible capacity. Additionally, the coexistence of solid solution and two-phase reactions during sodium storage resulted in lower volume changes (3.87%) in the material. More importantly, our team synthesized Na_3.5Fe_0.5V_0.5Cr_0.5Ti_0.5(PO₄)₃ (MLNP) using the sol–gel method. The observation of multiple redox couples, including Cr³⁺/Cr⁴⁺ (∼4.5 V), facilitated the transfer of 2.77 electrons. The multilevel redox reactions reduced the migration energy barriers for Na⁺ (Fig. 7c), promoting the uptake of inactive Na1 and breaking the high-voltage barrier of the V⁴⁺/V⁵⁺ redox couple. In situ XRD analysis indicated a reversible solid solution reaction during the Na⁺ extraction/insertion process (Fig. 7d), characterized by a negligible volume alteration of just 1.74%. At 0.1C, the material achieved a high discharge capacity of 176 mA h g⁻¹ and an energy density of 440 W h kg⁻¹.⁷⁹ Furthermore, a high-entropy cathode material Na₄Cr_0.7Fe_0.4Mn_0.3V_0.3Al_0.2(PO₄)₃ (HE-NASICON) with a higher chromium content was recently designed.⁴⁶ The discharge curves and CV curves shown in Fig. 7e revealed the presence of four redox couples at different potentials, with Mn³⁺/Mn⁴⁺ and Cr³⁺/Cr⁴⁺ both appearing at 4.28/4.10 V. The HE-NASICON cathode exhibited a discharge specific capacity of 160.9 mA h g⁻¹ at 0.1C, outperforming Na₄MnCr(PO₄)₃ in terms of capacity at various rates (Fig. 7f). GITT measurement indicated excellent electrochemical kinetics (Fig. 7g). Even after 2000 cycles at an elevated rate of 10C, the material kept 70.7% of its starting capacity. In situ XRD analysis confirmed a single-phase solid solution reaction with an extremely small volume change of only 1.45%.

Fig. 7 (a) Diagram of the Na⁺ migration energy barrier for Na_3.5Fe_0.5VCr_0.5(PO₄)₃/C. (b) Schematic diagram of crystal structure transformation during the first charge–discharge cycle.⁴⁴ Reprinted with permission. Copyright 2023, American Chemical Society. (c) The migration energy barrier of Na⁺ along the Na1 ↔ Na2 pathway. (d) The in situ XRD profile of the MLNP cathode. (e) Discharge curve of HE-NASICON at 0.1C and the CV curve at 0.1 mV s⁻¹. Reprinted with permission from ref. 80. Copyright 2020, Wiley-VCH. (f) Rate performance of HE-NASICON. (g) The second GITT curve and the corresponding Na⁺ diffusion coefficient of HE-NASICON at 0.1C. Reprinted with permission from ref. 46. Copyright 2024, Elsevier.

Despite the numerous advantages of chromium-containing poly-anionic cathode materials, they also face challenges and difficulties. For instance, chromium-based materials exhibit low discharge specific capacities and poor cycling stability. Moreover, for other types containing chromium, the reversibility of the activated Cr³⁺/Cr⁴⁺ redox couple is poor, and the failure mechanisms are not yet clear, which severely limits the application of chromium as an electrode material. Fig. 8a shows that Na₃Cr₂(PO₄)₂F₃ demonstrated a low initial coulombic efficiency along with a discharge capacity of only 55.1 mA h g⁻¹.⁷⁰ In Na₄MnCr(PO₄)₃, more rapid capacity loss at high voltage was observed (Fig. 8b), and the capacity contribution from the high-voltage region increased as the temperature decreased (Fig. 8c).⁷² Liu et al. also demonstrated that the capacity loss in NVCP was primarily attributed to the loss of high-voltage capacity, with improved performance observed at low temperatures.^40,81 They analyzed the failure mechanisms at room temperature and structural recovery using various advanced characterization techniques. In situ synchrotron XRD analysis revealed more complex characteristic peaks than conventional NVP's two-phase reaction, which can be divided into six stages (Fig. 8d). More importantly, at the end of the first discharge, the diffraction peaks did not return to their original positions (Fig. 8e), revealing irreversible phase transitions due to V migration during Na⁺ disinsertion/insertion. Atomic scanning transmission electron microscopy (STEM) before and after cycling confirmed the occupation of the Na1 site by heavier V atoms (Fig. 8f and g). The pre-edge peak intensity in Fig. 8h increased with cycling at 30 °C, reflecting distortion of the VO₆ octahedral structure and further evidencing irreversible structural changes along with the accumulation of degraded phases. However, the largely unchanged peak at −15 °C proved the reversibility of the structure. Then, they proposed a novel method where discharging to a low voltage (<1.7 V) suppressed capacity decay (Fig. 8i) at room temperature. Furthermore, low temperature was found to inhibit V migration, with Fig. 8j illustrating the migration process of V atoms at different temperatures. At 30 °C, V migration to the Na1 site blocked the migration pathway of Na⁺, while an over-discharge strategy enabled the return of migrated V atoms to their original lattice, restoring the structure. Conversely, at −15 °C, V migration was significantly suppressed. However, Zhao et al. demonstrated through impedance analysis and interfacial characterization that no transition metal migration occurred in the Na₄MnCr(PO₄)₃ cathode material.⁸¹ Instead, the capacity decay at high potentials was primarily associated with the evolution of the interface between the cathode and the electrolyte. Therefore, the development of high-voltage-resistant electrolytes was deemed necessary. Previous research has shown that the stability of activated chromium under high pressure is unsatisfactory. Therefore, exploring strategies to enhance its stability represents a meaningful direction for future studies.

Fig. 8 (a) Charge/discharge curves of Na₃Cr₂(PO₄)₂F₃. The inset showed the first cycle. Reprinted with permission from ref. 70. Copyright 2021, American Chemical Society. (b) Performance of cycling in different voltage ranges. (c) Charge/discharge curves for different temperatures. The inset showed the corresponding discharge specific capacity of the high voltage platform. Reprinted with permission from ref. 72. Copyright 2020, Wiley-VCH. (d) In situ synchrotron XRD patterns of NVCP. (e) In situ XRD patterns of NVCP at 30 °C for the pristine electrode and after one cycle within a selected range. Reprinted with permission from ref. 40. Copyright 2017, American Chemical Society. NVCP Aberration correction of HAADF-STEM images at 30 °C along the [2–21] area axis. (f) Initial and (g) after 5 revolutions of the cycle. (h) Pre-edge region in ex situ XAFS spectra at different temperatures and cycles of NVCP. (i) Charge–discharge curves for different cycles of NVCP over-discharge at 30 °C. (j) Schematic diagram of the migration of V at different temperatures. Reprinted with permission from ref. 79. Copyright 2024, Wiley-VCH.

3. Conclusions and prospects

In summary, chromium-containing polyanionic cathode materials exhibited remarkable advantages in SIBs (Table 2 shows the electrochemical properties of chromium-containing polyanionic cathode materials). This paper systematically summarized the multiple roles of chromium in polyanionic cathode materials and discussed the relevant research progress from three aspects: trace doping, minor substitution, and high-voltage activity. Firstly, trace chromium doping can effectively optimize the crystal structure, and the electronic and ionic conductivity of the materials, and improve their kinetic performance. By regulating lattice parameters and bandgap structures, the doping strategy significantly enhances the rate performance and long-term cycling stability of the materials. Secondly, the minor substitution of chromium not only activates the multi-electron reactions of high-valence transition metals but also improves the electrochemical activity by reducing lattice distortion and optimizing charge distribution.

Table 2 Electrochemical properties of chromium-containing polyanionic cathode materials

Materials	Voltage range (V)	Discharge specific capacity (mA h g^−1)	Capacity retention	Redox couple	Redox voltage (V)	Energy density (W h kg^−1)	Ref.
Na3V1.98Cr0.02(PO4)2F3/C	3–4.5	114.1@0.5C	93.62% after 200 cycles@1C	V^3+/V^4+	3.83/3.65	430	51
				V^4+/V^5+	4.10/4.28
Na3V1.94Cr0.06(PO4)3	2–4	111@1C	82.2% after 7300 cycles at 50C	V^3+/V^4+	3.35/3.43	275.5	47
Na3.1MnTi0.9Cr0.1(PO4)3	1.5–4.3	167.5@0.1C	94.9% after 100 cycles@1C	Ti^3+/Ti^4+	2.0/2.2	517.5	45
				Mn^2+/Mn^3+	3.5/3.7
				Mn^3+/Mn^4+	4.0/4.1
Na3V1.9(Ca,Mg,Al,Cr,Mn)0.1(PO4)2F3	2–4.5	118.5@0.1C	80.4% after 2000 cycles@20C	V^3+/V^4+	3.33/3.41	444.5	52
				Mn^2+/Mn^3+	3.60/3.50
				V^4+/V^5+	4.13/4.19
Na3V1.625Cr0.375(PO4)3	2.5–4.5	122.9@1C	93.4% after 300 cycles@1C	V^3+/V^4+	3.42/3.38	410	61
				V^4+/V^5+	4.02/3.98
Na3VAl0.2Cr0.2Fe0.2In0.2 Ga0.2(PO4)3	2.5–4.4	102@0.1C	86.8% after 5000 cycles@20C	V^3+/V^4+	3.31/3.48	444	63
				V^4+/V^5+	3.96/4.05
Na3V1.5Cr0.5(PO4)3	1–4.2	163.2@0.1C	72.1% after 2650 cycles@5C	V^2+/V^3+	∼1.6	426	42
				V^3+/V^4+	∼3.4
				V^4+/V^5+	∼4.1
Na3V1.5Cr0.5(PO4)3	1.4–4.4	176@0.2C	69% after 2650 cycles@5C	V^2+/V^3+	∼1.65	470	60
				V^3+/V^4+	∼3.43
				V^4+/V^5+	∼4.02
Na3Cr2(PO4)3	2.5–4.7	79@1C	7% after 20 cycles@1C	Cr^3+/Cr^4+	∼4.5	340	41
Na3VCr(PO4)3	2.5–4.3	90@0.1C	77% after 40 cycles@0.1C	V^3+/V^4+	∼3.4	320	40
				Cr^3+/Cr^4+	∼4.1
Na4MnCr(PO4)3	1.4–4.6	160.5@0.5C	86.5% after 600 cycles@5C	Mn^2+/Mn^3+	∼3.6	566.5	73
				Mn^3+/Mn^4+	∼4.2
				Cr^3+/Cr^4+	∼4.4
Na3.9MnCr0.9Zr0.1(PO4)3	1.4–4.5	156.4@0.1C	86.5% after 500 cycles@5C	Mn^2+/Mn^3+	3.73/3.42	555.2	43
				Mn^3+/Mn^4+	4.28/4.07
				Cr^3+/Cr^4+	4.43/4.34
Na4Mn0.9CrMg0.1(PO4)3	1.4–4.5	154.6@0.1C	92.7% after 500 cycles@5C	Mn^2+/Mn^3+	3.72/3.43	558.48	74
				Mn^3+/Mn^4+	4.28/4.08
				Cr^3+/Cr^4+	4.47/4.33
Na3.5Fe0.5VCr0.5(PO4)3	1.8–4.5	148.2@0.1C	95.1% after 1000 cycles@10C	Fe^2+/Fe^3+	2.52/2.44	509.4	44
				V^3+/V^4	3.51/3.37
				V^4+/V^5+	4.08/3.97
				Cr^3+/Cr^4+	∼4.4
Na4Cr0.7Fe0.4Mn0.3V0.3Al0.2(PO4)3	1.5–4.5	165@0.1C	70.6% after 2000 cycles@10C	V^2+/V^3+	1.69/1.53	309.5	46
				Fe^2+/Fe^3+	2.62/2.42
				Mn^2+/Mn^3+	3.60/3.40
				V^3+/V^4	4.28/4.10
				Mn^3+/Mn^4+
				Cr^3+/Cr^4+

---

In addition, chromium substitution can enhance the energy density while improving the cycling stability. Lastly, the high-voltage electrochemical activity of chromium-containing polyanionic cathode materials can increase the working voltage and energy density, opening up new avenues for the design of high-voltage cathode materials. The multiple roles of chromium in polyanionic cathode materials provide new ideas for the performance optimization of SIBs, and these studies will lay a solid foundation for the development of efficient and safe energy storage systems. However, phase transitions, electrolyte decomposition, and interfacial side reactions may be induced under high-voltage conditions, limiting the feasibility of their practical application. Therefore, further research is needed to address the high-voltage stability of the materials, the maintenance of multi-electron reaction activity, and the resolution of interfacial issues. To break through the bottleneck of chromium-containing polyanionic cathode materials in the application of high-performance for SIBs and promote their practical application in the energy storage field, future research could be further carried out in the following aspects (Fig. 9).

Fig. 9 Future research directions of chromium-containing polyanionic cathode materials.

3.1 Failure mechanism investigation

The failure mechanism of chromium-containing polyanionic cathode materials under high-voltage and long-term cycling conditions limits their widespread application. Future research needs to combine advanced characterization techniques and theoretical calculation methods. Utilizing advanced techniques like in situ XRD, neutron diffraction, electron paramagnetic resonance, and XAS will aid in elucidating the specific structural changes. Furthermore, these methods will assist in defining the reaction mechanisms of the cathode materials. Advanced theoretical calculations should be further employed to reveal the deep-level effects of chromium on lattice stability and charge distribution, providing a theoretical basis for material optimization. In addition, it is necessary to deeply analyze the destruction of interface stability by electrolyte decomposition products and reduce side reactions through interface engineering. The combination of first-principles calculations and machine learning can also be used to establish theoretical models for high-voltage failure, furnishing guidance for the development of more stable cathode materials.

3.2 Low-temperature performance research

Existing research has proven that chromium-containing materials have good low-temperature performance. However, the performance of SIBs often drops significantly under extreme cold conditions. In the future, the sodium ion diffusion channels in the crystal structure can be optimized to reduce the ion migration energy barriers at low temperatures or doping ions can be used to enhance the thermal stability of the crystal framework. Combined with the design of functional interface films, the stable cycling performance of the materials at low temperatures can be ensured.

3.3 Electrolyte optimization

The current commercial electrolytes are prone to decomposition and side reactions at high voltage ranges. Future research should focus on the development of new high-voltage stable electrolytes, such as the introduction of fluorine-containing additives and solid-state electrolytes, to suppress electrolyte decomposition and interfacial side reactions and enhance the long-term cycling stability of the materials. In addition, the development of wide-temperature electrolytes is also needed to ensure good performance of the materials in a wide temperature range.

3.4 Bandgap regulation and electronic structure optimization

By regulating the band structure of the materials, more efficient electronic conductivity and more stable sodium ion storage behavior can be achieved. In the future, strategies such as doping, defect engineering, and surface modification can be used to optimize the bandgap and electronic energy level structure of the materials. This can intrinsically improve the electronic conductivity and achieve more efficient ion and electron transport, thereby improving the overall performance.

3.5 Heterostructure design

In the future, chromium-containing polyanionic materials can be combined with other materials to induce more active sites and construct heterostructured materials with excellent interfacial interactions and ion transport properties. This is capable of not only boosting the electronic and ionic conductivity of the electrodes, but also elevating the rate performance and cycle life of the materials via synergistic effects at the nanoscale.

Chromium-containing polyanionic cathode materials can effectively improve the energy density of SIBs through high-potential redox reactions, multi-electron transfer mechanisms, and stability at low temperatures, thus demonstrating broad application prospects in energy storage systems. Solving the high-voltage stability issue would facilitate their imminent application in high-voltage fields and cold regions.

Author contributions

Jin-Ling Liu: investigation, writing – original draft, and writing – review. Yan Zhuang: investigation, writing. Yi-Fei Liu: investigation. Xiao-Tong Wang: formal analysis. Zhen-Yi Gu: conceptualization and writing – review & editing. Denglong Chen: validation and review & editing. Xing-Long Wu: funding acquisition, validation, visualization, and review & editing.

Data availability

The datasets used in this review are primarily from publicly available literature and research articles cited in the manuscript. No new experimental data were generated. Data can be requested from the corresponding author.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

We gratefully acknowledge the financial support from the National Key R&D Program of China (2023YFE0202000), the National Natural Science Foundation of China (No. 52102213 and 52302222), the Natural Science Foundation of Jilin Province (No. 20220508141RC), and the Fundamental Research Funds for the Central Universities (2412022QD038).

Notes and references

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