the growing energy demand and The limited supply of traditional fossil energy make people eager to use renewable clean energy.As a mature commercial energy storage device,lithium-ion battery has shown great vitality in mobile communications,electric vehicles and other fields^{[1⇓⇓⇓~5]}。 However,with the large-scale and wide application of lithium-ion batteries,the shortage of lithium resources and rising prices have aroused serious concern from the state and industry.in contrast,sodium-ion batteries have attracted much attention because of their more abundant natural reserves and low price,and are expected to be used in large-scale energy storage systems.However,sodium-ion batteries still face many problems and challenges.For example,compared with lithium ion(0.076 nm),sodium ion(0.102 nm)has larger ion size/mass,higher chemical activity,resulting in lower energy density and power density,and poor air stability.As a vital functional component of sodium-ion batteries,cathode materials play a key role in the energy density and cycle performance of batteries。

At present,the common cathode materials for sodium-ion batteries mainly include layered oxides,polyanionic compounds,Prussian blue analogues and organic compounds^[6][7][8][9]。 Layered transition metal oxides,（TMO₂）,have been widely studied due to their high theoretical specific capacity,good stability,and simple synthesis process.According to the coordination environment of Na⁺and the stacking sequence of oxygen layers,Na_xTMO₂materials can be divided into O3,O2,P2 and P3 types,as shown in Fig.1^[10]。 Where"O"and"P"represent Na⁺at octahedral and trigonal prismatic sites,and"2"and"3"represent the least repeating unit of the TMO₆octahedral layer of oxygen ion stacking.The TMO₂consists of alternating layers of TMO₆octahedra and NaO₆octahedra connected by edge-sharing in O3 and O2 layered structures,and prismatic pentahedra in P2 and P3^[11]。

However,there are still many problems that limit the development of layered oxide cathode materials,especially when the cut-off voltage is increased in pursuit of high energy density,which will have a negative impact on its performance.For example,the reversible capacity ofα-NaFeO₂at 3.3 V charge cut-off voltage is 85 mA·h·g^-1,and the cycle performance ofα-NaFeO₂becomes worse with the increase of cut-off voltage;When the cut-off voltage increases to 3.5 V,a large amount of Na⁺is removed,and the Fe³⁺irreversibly migrates to the sodium layer,resulting in the decrease of Na⁺intercalation during discharge and the decline of battery performance^[13]。 For the O3-type NaNi_1/3Fe_1/3Mn_1/3O₂,when the cut-off voltage is above 4.0 V,the transition metal ions migrate to form an irreversible structural evolution,which eventually leads to the transition of O3′and P3′monoclinic phases^[14,15]。 at the same time,when At high potential,the surface degradation of the material becomes more serious,which makes the capacity of the material decay rapidly during cycling,greatly limiting the application of layered oxide cathode materials for sodium-ion batteries in practical production。

Therefore,improving the high voltage resistance of layered cathode materials is of great significance for the development of cathode materials for sodium-ion batteries.Although there are some studies on the high voltage stability of layered oxide cathode materials for sodium-ion batteries,there are few reports on the systematic review of their modification strategies.in this paper,the main degradation mechanisms of layered oxide cathode materials for sodium-ion batteries during high voltage operation are briefly introduced.the strategies for improving the high voltage resistance of layered oxide cathode materials In recent years were systematically reviewed from the aspects of composition,surface and structure design.Finally,the future development direction of layered oxide cathode materials for sodium-ion batteries was prospected,which was expected to provide a reference for the structure design of high-voltage cathode materials for sodium-ion batteries^[16,17]。

the high voltage failure of cathode materials is usually caused by the irreversible phase transition during charge and discharge,which not only causes the structural transformation of cathode materials,but also causes a series of adverse reactions with the electrolyte,and the precipitation of lattice oxygen,and finally the materials suffer irreversible capacity decay during charge and discharge。

The structural transformation of active cathode materials at high voltage is one of the main reasons for the capacity fading of batteries.In the process of Na⁺intercalation/deintercalation,the layered Na_xTMO₂materials undergo complex phase transitions with the ordered rearrangement of Na⁺/vacancies and the migration of transition metal oxide layers^[18]。 For example,for materials such as P2-type Na_0.67Ni_0.33Mn_0.67O₂,Na_xFe_1/2Mn_1/2O₂,an irreversible P2→O2 phase transition occurs when charged to 4.2 V,resulting in a change of the Na⁺coordination environment from trigonal prism to octahedron^[17][19]。 The larger radius of the Na⁺will cause a large volume change and collapse the structure of the layered cathode material,resulting in a more complex structural evolution.At the same time,the large volume change also has a significant impact on the macroscopic properties of particles.When a material expands or contracts,the stress generated inside it may exceed the strength of the material itself,leading to the formation of cracks.These cracks not only destroy the structural integrity of the material,but also affect the electron transport path,restrict the flow of electrons and induce the dissolution of transition metal ions,thus reducing the electrochemical performance of the cathode material^[20]。 For example,when the P2-Na_2/3Ni_1/3-Mn_2/3O₂is cycled in the voltage range of 2.0~4.5 V,the Na⁺is almost completely removed during the first cycle of charging and contributes about 150 mA·h·g^-1of capacity;However,when the voltage rises above 4.2 V,the transition metal layer slips,and the P2 structure is irreversibly transformed into the O2 structure,accompanied by a 23%volume expansion,which eventually leads to a sharp decline in the capacity of the material^[21,22]。 In addition,the Jahn-Teller(J-T)effect of Mn³⁺and Fe⁴⁺can also increase the internal stress of materials,resulting in irreversible structural changes of electrode materials and the decay of battery capacity,and then affecting their electrochemical performance^[23,24]。

high voltage not only causes serious structural transformation of electrode materials,but also has a certain impact on the interface between cathode materials and electrolyte;the interfacial reaction between the electrode and the electrolyte at high voltage and the air stability of the material affect the stability of the cathode/electrolyte interface at high voltage。

It is generally believed that Na_xTMO₂exposed to humid air will produce cracks and electrically insulating substances,which make layered oxides have poor cycle stability and rate performance.Sodiated transition metal oxides are highly hygroscopic and readily react with H₂O and CO₂to form NaOH and Na₂CO₃on the particle surface even when exposed to air for a short time.Excessive sodium extraction from the host structure can lead to irreversible phase transformation of the material and reduce its capacity,and the formation of Na₂CO₃on the surface of the material can hinder the kinetics of Na⁺transport^[25]。 Most P2-Na_xTMO₂materials have large interlayer spacing and good hygroscopicity,so the intercalation of H₂O into P2-Na_xTMO₂materials is widespread.According to Zuo et al.,the mechanism of structural and chemical changes of P2-Na_0.67TMO₂in different atmospheres is shown in equations(1–4 )^[26]。

When the air does not contain CO₂:

When the air contains a small amount of CO₂:

When the air contains a large amount of CO₂:

When the sodium content in the electrode is below the critical sodium content(Nc):

Secondly,the larger interlayer spacing will make more solvent molecules embedded in the material,and the water molecules embedded in the layer will have the following side reactions with the electrolyte(Formula 5,6):^[27,28]

Among them,the product HF tends to react with surface by-products such as Na₂CO₃to further form NaF and H₂O.In addition,HF also reacts with the transition metal,causing the transition metal to dissolve and form a transition metal fluoride on the surface of the positive electrode,resulting in an irreversible loss of active capacity^[29]。 The continuous accumulation of reaction products at the cathode/electrolyte interface will hinder the Na⁺and electron transport,and may lead to the increase of internal impedance and more serious capacity fading of cathode materials^[29]。 According to Komaba et al.,at high potential,the larger interlayer distance(7.00Å)of Na_1−xNi_0.5Mn_0.5O₂will bind electrolyte solvent molecules into the gap between the[Ni_0.5Mn_0.5]O₂layers,which will reduce the storage reversibility of Na^+[30]。

the reaction between the cathode material and the electrolyte interface will directly affect the performance of the cathode material,and the structural transformation and new substances produced in this process will often accelerate the failure of the cathode material,resulting in capacity fading during cycling^[31]。 Especially at high voltages,this electrode failure is more serious^[32]。 NaPF₆is the main electrolyte salt for sodium-ion batteries because of its high ionic conductivity and good compatibility,which has a decisive impact on the interface chemistry and is closely related to the electrochemical performance^[33]。 The weak interaction between Na⁺and PF₆^-promotes the kinetics of sodium desolvation and storage,and the PF₆^-solvation structure induces preferential decomposition of anions to generate a thin and inorganic compound-rich cathode/electrolyte interfacial layer,which ensures interfacial stability and inhibits solvolysis,thus ensuring electrode stability and promoting charge transfer kinetics^[34]。 Under ideal conditions,the NaPF₆does not degrade or form a specific positive electrode/electrolyte interface layer on the surface of the electrode material,but the stability of the electrolyte becomes poor when the voltage is above 4.In the process of deep sodium removal,the highly active cathode surface will promote the excessive decomposition of electrolyte（NaPF₆）,produce HF,and cause serious side reactions with the bare electrode,resulting in the destruction of electrode materials at the electrolyte interface^[16][35]。 At the same time,the cracks caused by the volume change further expose the new surface to the electrolyte,resulting in the destruction of the stability of the cathode/electrolyte interface layer,causing harmful side reactions,gas release and dissolution of transition metals,and thus significantly accelerating the capacity and voltage decay of the electrode material^[36]。 The concentration of HF,the hydrolysate of NaPF₆,increased with the increase of water content in the electrolyte,and when the water content increased by 6.25 times,the concentration of HF increased by 500 times.At the same time,the degradation products of PO₂F₂^-and PO₃F₂^-increased by 2-and 3-fold,respectively.And the presence of HF and H₂O also easily causes the degradation of the solvent in the electrolyte^[37]。 Therefore,it is crucial to design a stable interface between the cathode/electrolyte to build a sodium-ion battery with excellent cycling stability。

the irreversibility of anion redox at high voltage also limits the practical application of cathode materials,although the participation of anion redox may help to increase the capacity of batteries.However,the oxygen released in this process will accelerate the decomposition of electrolyte and the precipitation of gas,resulting in carbonate and other substances,which will have a negative impact on the structure of cathode materials and the performance of batteries^[38]。 Jia et al.Studied the oxygen plasma redox behavior of O3-Na_0.6Li_0.2Fe_0.4Ru_0.4O₂cathode materials by in situ Raman spectroscopy and in situ differential electrochemical mass spectrometry,and found that when the voltage was lower than 4.0 V,almost no O₂could be detected^[39]; When the voltage is higher than 4.0 V,the redox of oxygen ions is excited,which will inevitably produce O₂,accompanied by the formation of CO₂gas 。

to sum up,under high voltage,a series of interactions will occur in cathode materials,among which complex phase transitions,severe interfacial reactions,and oxidation and oxygen loss of electrolyte make it very difficult To develop layered oxide cathode materials with high energy density and long cycle performance。

Component design is a common modification method to improve the high voltage resistance of layered oxide cathode materials.Finding suitable doping ions and optimizing their concentrations and doping sites can significantly improve the electrochemical performance of cathode materials.doping can be divided into three forms:cation doping,anion doping and cation/anion co-doping。

Layered oxide cathode materials are easily eroded by atmosphere(especially humid air)or electrolyte,resulting in structural changes and capacity fading,or the formation of a cathode/electrolyte interface layer affecting the transmission of Na⁺,resulting in poor electrochemical performance in the charge-discharge process,especially at high voltages^[59]; Coating a protective layer on the surface of these materials is an effective strategy to improve the high voltage stability of cathode materials.Surface coating can effectively prevent the damage of electrolyte to the material,reduce the absorption of CO₂and H₂O while maintaining the structural stability of the material,thus preventing the formation of Na₂CO₃or NaHCO₃on the surface of the material,and thus effectively inhibiting the adverse interface reaction under high voltage^[60]。 At present,the commonly used surface coatings of cathode materials are mainly metal oxides such as（SnO₂,CuO,MgO,Al₂O₃,CeO₂,and ZnO,etc.)and metal salts（AlF₃,AlPO₄,Mg₃(PO₄)₂）,etc^{[61][62][63][64][65][66][67][68][69]}。

Liu et al.Found that the P2→O2 phase transition of Na_2/3Ni_1/3Mn_2/3O₂cathode materials at high voltage will lead to the peeling of transition metal oxide layer,which will lead to the capacity fading^[70]; Therefore,they coated 12 nm thick Al₂O₃on the surface of Na_2/3Ni_1/3Mn_2/3O₂materials to adapt to the large volume change caused by P2→O2 phase transformation,thereby reducing the spallation of transition metal oxide layers;In addition,the Al₂O₃coating is also able to suppress the unfavorable interfacial reaction at high voltage.The results show that the cathode material has a capacity retention rate of 73.2%after 300 cycles in the voltage range of 2.0-4.3 V.Similarly,Kaliyappan et al.Used atomic layer deposition(ALD)to coat Al₂O₃coatings with different thicknesses on the surface of Na_2/3(Mn_0.54Ni_0.13-Co_0.13)O₂materials,and the study showed that the Al₂O₃/Na_2/3(Mn_0.54Ni_0.13-Co_0.13)O₂composite electrode coated twice by ALD had better electrochemical performance,and showed a high specific discharge capacity of 123 mA·h·g^-1at 2.0~4.5 V and 1 C^[64]。 The Al₂O₃coating prevents the direct contact between the material and the electrolyte,which minimizes the consumption of active material in the electrolyte.In addition,Al₂O₃has a high band gap energy(9.00 eV),which can inhibit the phase change during the charge-discharge process and improve the cycle stability of the electrode at high voltage 。

The Al₂O₃coating can effectively inhibit the unfavorable interfacial reaction at high voltage and the exfoliation of the transition metal oxide layer,which is of great significance for improving the electrochemical performance of cathode materials at high voltage.Other metal oxides,such as In₂O₃,can also achieve similar effects^[71]。 However,metal oxide-coated materials are generally not conducive to Na⁺diffusion and interfacial charge transfer.To this end,Lin et al.Solved these problems through the coating of organic coating.They coated 0.5 wt%permethacrylic acid and acrylonitrile(PMAA-AN)copolymer nanolayer on the Na_0.67Li_0.16Ni_0.33Mn_0.67O₂cathode material,thus improving the interfacial stability and electrochemical performance of the cathode material under high voltage^[72]; The results show that the derivative c-PAN with delocalized sp²-πbond and—CN group formed by PAN linear molecular chain can be used as a transport conductor of Na⁺during the coating process,which is beneficial to improving the migration rate of Na⁺.Therefore,the electrochemical performance of the material at high current density is enhanced,and the material still has a specific capacity of 100.1 mA·h·g⁻¹at a current density of 5 C in a wide voltage range of 1.5 to 4.5 V,showing better rate performance.At the same time,the coating can significantly inhibit the irreversible P2→O2 phase transition of the cathode material at a voltage above 4.2 V and the dissolution of transition metals;And prevent the positive electrode material from being corroded by the electrolyte,thereby reducing the capacity fading of the positive electrode material;The coated cathode material has a capacity retention of 86.0%after 100 cycles in the voltage range of 1.5–4.5 V,and a specific capacity of 100.1 mA·h·g⁻¹at a high current density of 5 C,showing excellent cycling stability and rate capability 。

In addition,fast ionic conductors(such as NaPO₃）)are ideal coating materials because they have three-dimensional Na⁺transport channels,which can effectively inhibit surface side reactions while accelerating Na⁺transport.Jo et al.Prepared the Na_2/3Ni_1/3Mn_2/3O₂cathode material coated by 4.4 wt%NaPO₃nanosheets,and the research results showed that the：NaPO₃coated Na_2/3Ni_1/3Mn_2/3O₂still had good cycle performance and rate performance at a high voltage of 4.2 V in the full cell matched with the hard carbon anode,and the capacity retention rate was still 73%after 300 cycles^[60]。 Even at a high current density of 7 C,the specific discharge capacity of the coated material is significantly higher than that of the uncoated material.This excellent electrochemical performance can be attributed to the fact that the NaPO₃coating can effectively inhibit the side reaction caused by electrolyte decomposition and reduce the by-product HF on the surface of the positive electrode,thus significantly improving the performance of the electrode 。

In addition to NaPO₃coating,Xu et al.Used a simple wet chemical method to coat NaTi₂(PO₄)₃(NTP)on the surface of O3-NaNi_0.3Fe_0.2Mn_0.5O₂materials^[73]。 The results show that the NaTi₂(PO₄)₃coating with special three-dimensional channels is beneficial to the rapid migration of Na⁺,and it can also prevent the direct contact between the electrode and the electrolyte,thus ensuring the stability of the interface.In addition,the NaTi₂(PO₄)₃coating induces partial Ti⁴⁺doping into the NaNi_0.3Fe_0.2Mn_0.5O₂transition metal layer,which increases the stability of the transition metal layer,thereby inhibiting the O3→P3 phase transformation under high pressure,and further reducing the volume change of the material during the whole charge-discharge process,and finally improving the electrochemical performance of NaNi_0.3Fe_0.2Mn_0.5O₂.The material coated with 3 wt%NTP showed the best electrochemical performance,and the specific discharge capacity(125 mA·h·g⁻¹)at 10 C was much higher than that of the uncoated material(70 mA·h·g⁻¹)in the voltage range of 2.0–4.2 V,and the capacity retention was 85%after 300 cycles at 1 C.This surface modification strategy provides a simple and effective method for the design and development of layered oxide cathode materials for Na-ion batteries with high voltage resistance 。

In addition to coating other materials,the purpose of improving the high voltage resistance of cathode materials can also be achieved by constructing an interface layer.Recently,Moeez et al.Constructed an electrode-electrolyte interface layer on the surface of O3-NaFe_0.5Ni_0.5O₂cathode material by electrochemical pre-intercalation of sodium,as shown in Fig.4,the artificial electrode-electrolyte interface layer,which is thicker and denser than the cathode/electrolyte interface layer,can effectively block the side reactions at the interface under high voltage charging.To prevent the dissolution of transition metal ions and improve the stability of the cathode material interface,this method is expected to become an effective strategy to improve the high voltage resistance of cathode materials^[74]。

to sum up,coating metal oxides,organic compounds or constructing an interface layer on the surface of the cathode material can effectively reduce the side reaction between the cathode material and the electrolyte.To improve the structural stability of the material under high voltage,but usually the coating layer is not uniform in the coating process,so the controllability of the preparation process needs To be studied in the next study。

layered oxide cathode materials for sodium-ion batteries have attracted much attention In recent years due to their low cost,high specific capacity and simple synthesis.However,due to its structural limitations,it is difficult to improve its high voltage resistance.in this paper,the degradation of Layered oxide cathode materials for sodium-ion batteries at high voltage is studied,including the structural change caused by phase transformation during charge-discharge process,the structural damage caused by electrolyte corrosion and the oxygen loss caused by the reaction of lattice oxygen at high voltage.the latest research status of improving the high voltage resistance of cathode materials by composition design,surface design and structure design is reviewed,and the following conclusions are obtained。

(1)element doping can effectively reduce the interlayer slippage of the cathode material,inhibit the irreversible phase change of the cathode material at high voltage,and improve the structural stability of the layered oxide cathode material at high voltage;However,the type and content of doping elements are limited,when the doping content is too small,the effect is not significant,and when the doping content is too much,impurity phases are often generated。

(2)coating a protective layer on the surface of the layered oxide cathode material can effectively inhibit the unfavorable interface reaction and the spalling of the transition metal oxide layer at high voltage,and improve the interface stability and electrochemical performance of the cathode material at high voltage;However,the coating layer is prone to be uneven in the coating process,so the controllability of the preparation process needs to be further studied。

(3)the construction of P2/O3 composite phase materials,the design of micro-nano structure and the controllable preparation of single crystal nano-sheet materials with specific crystal planes can alleviate the structural stress caused by the phase transition of cathode materials,inhibit the slip of transition metal layers,and further improve the rate performance and cycle stability of materials at high voltage;However,the preparation process of the composite phase cathode material is complex and difficult to regulate;In addition,the sheet-like single crystal cathode material prepared by the molten salt method has good high voltage resistance,but the additional washing step will reduce the initial capacity of the cathode material。

Based on the above challenges and problems,the main future research directions of layered oxide cathode materials for sodium-ion batteries are as follows。

(1)the effects of temperature and electrolyte composition on The high voltage resistance of cathode materials were studied。

(2)Explore the influence mechanism of various elements on the sodium storage behavior of layered oxides,and use DFT calculation to assist in screening reasonable multi-element doping and dosage to synergistically improve the electrochemical performance of cathode materials。

(3)Due to the high hygroscopicity of sodiated transition metal oxides,develop an efficient coating process to reduce the contact between air moisture and cathode materials,and further improve the interfacial stability of layered oxide cathode materials for sodium-ion batteries。

(4)Use advanced characterization methods to explore the relationship between the deep-seated reaction mechanism and performance of cathode materials,and make greater breakthroughs in the high voltage resistance of layered oxide cathode materials。

(5)Comprehensive application of bulk doping and surface coating strategies to achieve the stability of the structure and interface of cathode materials,and further improve the high voltage resistance of layered oxide cathode materials for sodium-ion batteries。

(6)multi-strategy comprehensive modification to improve the high voltage resistance of cathode materials,such as coating,multi-ion doping and multi-phase composite modification on the basis of preferred orientation growth modification,morphology modification and particle size modification。