P2-type Transition Metal Layered Oxide Cathode Materials For Sodium-ion Batteries

P2-type layered transition metal oxide cathode material is considered as one of the most promising cathode materials for sodium-ion batteries,which have attracted the attention of researchers due to its advantages of high theoretical specific capacity,simple synthesis method and environmental protection.However,the application of P2-type layered transition metal oxide cathode materials still face some challenges,due to the large ion radius of sodium ions and irreversible side reactions between the electrode material and the electrolyte during the cycle process that lead to the low Sodium-ion diffusion kinetics,poor cycle performance and rate performance.In order to solve these problems,voltage window regulation,co-substitution and surface coating strategies were apllied in this article and the electrochemical performances of P2-type layered transition metal oxide cathode materials are successfully improved.The innovative strategies adopted in this article are as follows:（1）P2-Na_0.67Ni_0.1Fe_0.1Cu_0.1Mn_0.7O₂material was synthesized by the sol-gel method,the P2-O2 phase transition was successfully inhibited by control the voltage window as results cycle performance of the material was improved.The material can provide 115 m Ah g^-1initial discharge capacity at 20 m A g^-1current density at a voltage window of 2-4.0V.The capacity retention rates can reach 93.5%and 93.3%after 100 cycles at 20 m A g^-1and 200 m A g^-1current density.Under the voltage of2.0-4.0V,the material show 65 m Ah g^-1specific capacity at 1000 m A g^-1current density and the specific capacity can reach 99.0%of the initial capacity when the current density is recovered to 20 m A g^-1.（2）In order to inhibit the P2-O2 phase transition of P2-Na_0.67Ni_0.1Fe_0.1Cu_0.1Mn_0.7O₂materials under high voltage window,a series of materials coated with metal oxide were successfully prepared by the low-temperature sintering coating method,and the performance of the material surface Mg O coating was found to be the best.Na NFCM@Mg O-0.05 material can provide 147.1 m Ah g^-1initial discharge capacity at 20 m A g^-1current density,and the capacity retention can reach 83.7%after 100 cycles at 200 m A g^-1current density.At the same time,the material has good rate performance.The specific capacity of Na NFCM@Mg O-0.05material can reach 97.5%of the initial capacity when the current density is recovered from 1000 m A g^-1to 20 m A g^-1.（3）A Co and Ti co-doped modified material Na_0.67Ni_0.1Fe_0.1Cu_0.1Co_0.1Ti_0.1Mn_0.5O₂materials was synthesized by sol-gel method.This titanium-cobalt co-doping strategy can suppress the P2-O2 phase transition under high voltage and maintain the structural stability of the material.The as-prepared samples,shown an initial discharge specific capacity of 118 m Ah g^-1at a current density of 20 m A g^-1.It exhibit a capacity retention rate of 91.7%after 100 cycles at a current density of 200 m A g^-1and a specific capacity of 90 m Ah g^-1at 1000 m A g^-1.