Preparation And Electrochemical Performance Of P2-type Layered Sodium Ion Battery Cathode Materials

Layer oxide cathode materials for sodium ion batteries have attracted much attention because of their advantages such as high specific capacity,high operating voltage and low cost.Among them,P2-type layered oxides are considered as a kind of cathode material with development potential because of their good air stability and low diffusion potential barrier.However,its irreversible phase change occurs during the charge/discharge process,especially the crystal structure of P2-type manganese-based materials is affected by the Jahn-Teller effect caused by Mn³⁺,which leads to the rapid decay of the cyclic capacity of the materials.Meanwhile,the charge-ordered nature of transition metals leads to the ordering of sodium ion vacancies,which severely hinders Na⁺diffusion and leads to insufficient multiplicative performance.Therefore,to address the above problems of P2-type materials,this paper selects P2-Na_0.67MnO₂materials as the research object and designs a series of high-performance layered oxide cathode materials from the aspects of heat treatment process and bulk phase doping modification of the materials,and investigates the crystal structure,ion diffusion kinetics and electrochemical properties.The main research contents and results of this paper are as follows:（1）The cathode material P2-Na_0.67MnO₂ was prepared by sol-gel method to explore the optimal heat treatment in the solid phase sintering process.By comparing the effects of quenching,tempering and annealing processes on the structure and electrochemical properties of the synthesized materials,it was found that all three materials subjected to the heat treatment process had sodium ion vacancy ordering,and the materials after tempering had the optimal crystallinity and electrochemical properties.The capacity retention of the tempered materials was found to be 61.3%after 100 cycles at a current density of 1 C(200 m A g^-1)for all three materials.（2）The P2-Na_0.67Mn_1-xTi_xO₂ materials（0≤x≤0.20）were synthesized by choosing the Ti-doping strategy to further enhance the long-cycle performance of the materials.XPS and CV tests showed that Ti doping effectively reduced the Mn³⁺content in the materials,which suppressed the Jahn-Teller effect induced by Mn³⁺and took advantage of the structural stability maintained by Mn⁴⁺,thus improving the cycling stability of the materials and suppressing the ordering of sodium ion vacancies.It was found that when 10%mol Ti⁴⁺replaced Mn,the retention rate reached 89.8%after 200 charge/discharge cycles(200 m A g^-1).（3）To compensate for the lack of Ti doping for material kinetic performance enhancement,a series of high-purity P2-Na_0.67-yK_yMn_0.9Ti_0.1O₂（0≤y≤0.010）were continued to be synthesized by K⁺doping,and structural refinement showed that inactive K⁺in P2-Na_0.66K_0.01Mn_0.9Ti_0.1O₂ occupied the sodium sites and broadened the sodium ion layer,thus allowing easier diffusion of Na⁺,indicating its good rate performance.The ex-situ XRD analysis showed that the material structure remained stable during charging and discharging,and the capacity retention rate was 87.4%after200 cycles at a current density of 200 m A g^-1,which was 60%higher than that of P2-Na_0.67MnO₂.