Preparation And Modification Of Transition Metal Layered Oxide Cathode Materials For Sodium-ion Batteries

The layered oxide cathode materials of sodium-ion batteries has attracted the attention of the majority of scientific researchers due to its higher working voltage and theoretical specific capacity.At the same time,because it has a similar structure and synthesis process to the current large-scale commercial lithium-ion batteries layered cathode materials（Li_xTMO₂）,it is generally considered that possesses very broad commercialization prospect.However,the application of layered oxide cathode materials for sodium-ion batteries still faces some challenges,including problems such as irreversible phase transition,lower Na⁺diffusion rate,and poor air stability.This paper aims to improve the electrochemical performance of layered oxide cathode materials for sodium-ion batteries.The P2-Na_0.67Mn_0.67Ni_0.33O₂ materials was synthesized by the solid-phase sintering method,and its electrochemical performance was improved by the methods of lithium-titanium co-doping,sodium content control and surface treatment.The main research contents are as follows:（1）A lithium-titanium co-doped Na_0.67Mn_0.62Ni_0.18Li_0.15Ti_0.05O₂ cathode material was prepared via a simple solid-phase sintering reaction.Co-doping of lithium and titanium can effectively improve the structural stability and Na⁺diffusion rate of the materials.The materials can provide an initial discharge specific capacity of 120 m Ah g^-1 at a current density of 0.2 C,and can exhibit a capacity retention rate of 89%after100 cycles.Meanwhile,it can provide a specific capacity of 95 m Ah g^-1 at a current rate of 5 C,and even at a large rate of 20 C,it can still provide a specific capacity of50 m Ah g^-1,showing excellent rate characteristics.（2）In order to increase the specific capacity of the P2-Na_0.67Mn_0.62Ni_0.18Li_0.15Ti_0.05O₂ cathode materials co-doped with lithium and titanium,the sodium content was adjusted and a series of high sodium content materials were synthesized.The effect of sodium content in the materials on the microscopic morphology,crystal structure and electrochemical performance was studied.Experiments have proved that when the molar ratio of Na/M（M represents a transition metal element）is 0.8,the prepared cathode materials has the best comprehensive electrochemical performance,which can obtain the discharge specific capacity of 132 m Ah g^-1 at a current density of 0.2 C and the capacity retention rate after 150 cycles is still 77%.At the same time,the materials can still provide a specific capacity of 79.6 m Ah g^-1 at a high current of 10 C.（3）To further improve the electrochemical performance of the cathode materials,the Na_0.8Mn_0.62Ni_0.18Li_0.15Ti_0.05O₂ with high sodium-content materials was used as the matrix materials,and the sodium metaphosphate was successfully coated by the low-temperature sintering coating method.Based on the higher sodium-ion conductivity of sodium metaphosphate,it can inhibit the corrosion of the electrolyte to the cathode material,so the rate performance and cycle performance of the high sodium material are significantly improved after coating.Among them,the material with the coating amount of sodium metaphosphate of 5%（Na MNO-0.05）has the best electrochemical performance.The material has a capacity retention rate of 78%after200 cycles at a rate of 0.2 C.And it can still provide a discharge specific capacity of87.2 m Ah g^-1 at a high current density of 10 C.