Preparation,Modification And Electrochemical Properties Of Na_0.44MnO₂ Cathode Material For Sodium-ion Batteries

Sodium ion battery has the advantages of abundant resources,low cost,good safety,and fast charge and discharge speed,which has attracted the research enthusiasm of researchers.In particular,the sodium ion manganese based cathode material Na_0.44MnO₂ is suitable for large-scale energy storage due to its stable three-dimensional tunnel structure and long cycle life.However,there has been still defects in practical applications,such as the complex phase transition during the charge and discharge process and the poor electronic conductivity,which affect the cycle stability and rate performance of the material.In view of the above problems,this thesis optimized the synthesis route of Na_0.44MnO₂ material and adopted the composite method to improve the charge-discharge performance and rate performance of the material.Firstly,Na_0.44MnO₂ material was prepared by rheological phase reaction method,and the effects of calcination temperature and calcination time on the structure and electrochemical properties of the material were investigated.The experimental results showed that the Na_0.44MnO₂ material prepared at 800°C for 3 h has good crystal structure,uniform morphology,and the best electrochemical performance.The initial discharge specific capacity is 125.1 m Ah g^-1 at the current density of 0.5 C,and it still maintains 120.6m Ah g^-1 after 100 cycles,showing good cycle stability and rate performance.Kinetic analysis shows that the Na_0.44MnO₂ material prepared at 800°C for 3 h has a higher sodium ion diffusion coefficient,indicating that the sodium ion diffuses quickly in the material,which is beneficial to the electrochemical performance of the material.In order to improve the electronic conductivity of Na_0.44MnO₂ material,the Na_0.44MnO₂material composited with polypyrrole（PPy）was prepared by ultrasonic-assisted solution dispersion method,and the composite ratio was optimized.The experimental results showed that the Na_0.44MnO₂/PPy（7 wt.%）composite had the best electrochemical performance.The initial discharge specific capacity was 135.2 m Ah g^-1 at the voltage range of 2.0-4.0 V and current density of 0.5 C,and the capacity retention rate was 94.5%after 100 cycles.Under the current density of 1 C,the capacity retention rate was 82%after 500 cycles,showing a good cycle stability.The main reason is that PPy adheres to the surface of the material,inhibits Mn³⁺side reaction,and improves the structural stability and electronic conductivity of the material.Similarly,the Na_0.44MnO₂ material composited with carbon quantum dots（CQDs）was prepared by ultrasonic-assisted solution dispersion method,and the composite ratio was optimized.The experimental results showed that Na_0.44MnO₂/CQDS（3 wt.%）material has the best electrochemical performance.At the voltage range of 2.0-4.0 V and 1 C current density,it still maintains the specific discharge capacity of 112 m Ah g^-1 after 500 cycles,and has good rate performance.The main reason is that CQDs is attached to the surface of the bulk material,which reduces the polarization and charge transfer resistance of the electrode material,indicating that CQDs composite can effectively improve the electrochemical performance of Na_0.44MnO₂ material.