Preparation And Modification Of P2-type Layered Oxide Cathode Material For Sodium-ion Battery

P2-type layered oxide cathode material is considered to be one of the most promising cathode materials for sodium-ion batteries due to its advantages of high energy density,rich variety,simple synthesis method and low pollution.However,due to the large ion radius and heavy mass of sodium ions,the P2-type layered oxide cathode material shows poor structural stability and sluggish Na⁺diffusion kinetics during the sodiation/desodiation process,which lead to P2-type layered oxide materials usually show poor cycle performance and rate performance.In addition,some side reactions between the electrode material and the electrolyte during the cycle process will also affect the electrochemical performance of the material.In order to solve these problems faced by P2-type layered oxide materials as cathode materials for sodium-ion batteries,this article successfully improved these conditions through morphology control,ion substitution and surface coating strategies,and improved the electrochemical performance of P2-type layered oxides cathode materials.The innovative strategies adopted in this article are as follows:（1）P2-type 10 mol%F-Na_0.67Li_0.1Fe_0.4Mn_0.5O₂microsphere was successfully synthesized by hydrothermal method and the subsequent two sintering processes.The relatively small specific surface area of the microsphere can effectively reduce the contact area of the material and the electrolyte,thereby reducing the side reaction between the active particles and the electrolyte.Moreover,the introduction of lithium and fluorine can maintain the structural stability of the material.Therefore,the material exhibits excellent electrochemical performance,and shows an initial specific capacity of 182.0 m Ah g^-1 at a current density of 20 m A g^-1,the capacity retention can reach 90.0%after 50 cycles.It also dilivers excellent rate performance,the discharge specific capacity is 128.7 m Ah g^-1at a current density of 400 m A g^-1.（2）P2-type Na_0.67Ni_0.28Mg_0.05Mn_0.67O₂ material with bulk Mg substitution and surface Na Ti₂（PO₄）₃ coating was prepared by sol-gel method and subsequent wet coating.The study found that Mg substitution can stabilize the bulk structure of the material and increase the interlayer spacing.The NASICON type Na Ti₂（PO₄）₃ coating layer can stabilize the phase interface between the material and the electrolyte,prevent HF corrosion in the Na PF₆ electrolyte and promote Na⁺migration at the interface between the cathode and the electrolyte.In addition,Mg substitution and Na Ti₂（PO₄）₃ coating can also suppress particle cracking and flaking.Electrochemical performance test results show that the material exhibits excellent cycle performance,and its capacity retention is 77.4%after200 cycles at a rate of 1 C.The rate performance is also excellent,and it shows a discharge capacity of 106.8 m Ah g^-1 at a rate of 5 C.（3）A series of P2-type Na_0.67Ni_0.33Mn_0.67-xTi_xO₂（x=0,0.1,0.2,0.3）materials were synthesized by sol-gel method and high temperature sintering.Ti substitution can reduce the volume change of the material during the cycle to inhibit the formation of particle cracks,which reduces the contact area of the active material and the electrolyte under high voltage and thus reduces some side reactions between the material and the electrolyte.In addition,Ti substitution enlarges the interlayer spacing.At the same time,it can suppress the Na⁺/vacancy ordering and P2-O2 phase transition,and maintain the structural stability of the material.Therefore,the Ti-substitued material exhibits excellent electrochemical performance,especially for Na_0.67Ni_0.33Mn_0.47Ti_0.2O₂,which delivers a capacity retention of 70.2%after 200 cycles at a rate of 1 C,and shows a discharge capacity of 51.9 m Ah g^-1at 10 C.