Preparation And Sodium Storage Properties Of Na_xMnO₂ Cathode Materials For Sodium Ion Batteries

Sodium ion battery has become a research hotspot due to the similar chemical properties to lithium,the abundant reserves on earth and its low cost.However,the radius of sodium ion is larger than that of lithium ion,so it is very important to find suitable electrode materials.Among the cathode materials of sodium ion battery,the layered sodium manganate material has wide electrochemical window,high theoretical capacity with low cost.However,the conductivity of the material is poor,and its electrochemical performance is poor at high current density.The continuous dissolution of manganese ions in the recycling process due to Jahn-teller effect leads to poor cycling performance of the material.In this paper,the effect of micromorphology of Na_xMnO₂ on its electrochemical performance is explored,and the capacity and stability of the material are improved by increasing the ratio of Na and Mn,and the electrochemical performance is further modified by conductive polymer.The main research methods and results are as follows:（1）The hollow microspheres of MnO₂ are prepared by the reaction between MnCO₃and KMnO₄ at room temperature.The Na_0.7MnO_2.05 hollow microspheres with uniform size are synthesized after heated with NaOH at high temperature.The hollow microsphere composite Na_0.7MnO_2.05 coated by polypyrrole is obtained after polymerization of pyrrole.The specific surface area of the hollow sphere material is large,the contact between the electrode material and the electrolyte is better,the transport path of sodium ion is shortened.The reaction kinetics of the electrode is improved,and the utilization rate of the positive active material is increased.The coating of polypyrrole improves the conductivity of the electrode material,improving the kinetics of the electrode and leading to high rate performance of the material.On the other hand,it effectively prevents the dissolution of manganese,greatly improving the cyclic stability of the material.The specific capacity of the cathode is 100.5 mAh g^-1 at the current density of 2 A g^-1,and the capacity retention rate is 88.6%after 100cycles at the current density of 0.1 A g^-1.It shows the great potential of layered metal oxides as cathode for sodium ion batteries.（2）The MnCO₃ solid cube obtained by the hydrothermal reaction between KMnO₄and glucose is used as the precursor,and the porous cube of Na_0.91MnO₂ with uniform size was obtained by heated with NaOH at high temperature.Then,the porous Na_0.91MnO₂ cube composite coated by polypyrrole is obtained by polymerization of pyrrole.The high sodium content of Na_0.91MnO₂ results in super high capacity.At the same time,due to the widening of the layer spacing,it is more conducive for the ion intercalation/deintercalation and shows better cycle stability.The one-step synthesizd porous structure greatly simplifies the experimental steps.The specific surface area of porous electrode material is large resulting in better contact with electrolyte.The transport path of sodium ion is shortened,and the kinetics of electrode is improved.On the other hand,it can effectively prevent the dissolution of Mn and improve the cycle stability of the material.The mechanism of sodium storage of Na_0.91MnO₂ was studied by ex-situ XRD,and the mechanism of polypyrrole coating improving electrochemical performance was studied by CV method.The initial capacity of the composite cathode reached 208 mA g^-1 at 0.1 A g^-1.After 200 cycles,the capacity remained 178 mA g^-1.At the high current density of 2A g^-1,the capacity reached 139 mA g^-1.For sodium ion full battery with Na_0.91MnO₂@PPy cathode and porous carbon anode,after 500 cycles at 0.5 A g^-1,the capacity of 106.4 mAh g^-1 was maintained.