Preparation And Electrochemistry Study Of Layered P2-Type Manganese-Based Cathode Materials For Sodium-ion Batteries

Layered manganese-based Na-ion cathode was promising material for Na-ion batteries owing to its low cost, abundant Mn and Na resources and environmental friendliness. Unfortunately, the wide practical application of manganese-based P2-type material has been hindered by these issues:（1） The radius of Na+ is 1.5 times that of Li+,seriously influenced the rate performance of the electrode materials.（2） The other comes from the low electronic conductivity cause a large polarization at high current densities, resulting poor rate performace.（3） The unique Jahn-Teller effect and invetibale Mn3+ dissolution exsist in Mn-based cathode, causing rapid decrease of storage capacity. In this thesis, a substitute-modified stratege have been considered to overcome these drawbacks in Layered sodium manganese oxides.Firstly, we successfully synthesized Na_0.4Mn_0.54Co_0.46O₂ and Na0.4Mn0.4Co0.6O2 nanosheets via two-step annealing route and studied the structure information.Comparing with P2-type Na0.4Mn0.4Co0.6O2, the layered Na_0.4Mn_0.54Co_0.46O₂ nanosheets cathode materials showed a better cycle performance. The Na_0.4Mn_0.54Co_0.46O₂ cathode showed a high second charge capacity of 194 mAh/g and delivered a specific capacity of 125 mAh /g at a current of 20 mA/g after 60 cycles.To further solve the problem of manganese dissolution and improve the average discharge voltage, a nickel substituted materials have been designed and synthesized through two different methods. P2-type Na0.5Mn0.53Ni0.22Co0.25O2（S1） cathode material was obtained by a typical sol-gel method and another kind of P2-type Na0.54Mn0.52Ni0.22Co0.26O2（C1） was synthesized through a conventional co-precipitation method as a contrast. Comparing the SEM image and electrochemical test results, the S1 cathode with smaller sized showed a batter electrochemical performace. S1 sample with smaller size reduce the diffusion distance of Na+ and electrons. The P2-type S1 sample showed a high reversible capacity of 148 mAh/g and after 180 cycles delivered a specific capacity of 106 mAh/g at 20 mA/g. S1 and C1 materials exhibited a superior electrochemical performance than prvious synthesized Na_0.4Mn_0.54Co_0.46O₂,obviously proved that nickel doped is efficiency to improve the average voltage and electrochemical performance.To improve the rate performance of P2-type materials, a new P2-type Na0.72Mn0.80Co0.15Mg0.05O2 layered material was synthesized via a coprecipitation method and used as cathode for Na-ion batteries. The inactive Mg ions were doped in the MeO2（Me=Mn, Co） layers to suppress the structure change and keep structural integrity of the layered framework. The partly-substituted Mn ions with Mg and Co ions improved the average potential and decreased the concentration of Jahn-Teller active Mn3+ ions. Na0.72Mn0.80Co0.15Mg0.05O2 cathode showed a capacity of 90 mAh/g at 30mA/g after 200 cycles. Such a superior rate capability and cycling performance ensure P2-Na_0.72Mn_0.80Co_0.15Mg_0.05O₂ a bright future as cathode for Na-ion batteries.