Structure And Electrocheimical Properties Of Cobalt/Nicket Potassium Manganate Cathode For Potassium-Ion Battery

New rechargeable potassium ion batteries（KIBs）,due to the advantages of potassium,such as high resource abundance,low cost and low redox potential,are expected to be widely used in energy storage field.In this thesis,K-Birnessite:potassium cobalt nickel manganese oxides,potassium cobalt manganese oxides and potassium nickel manganese oxides were prepared by traditional solid phase method and used as KIBs cathodes,whose structural characteristics and electrochemical properties were emphatically studied.The molar ratio of potassium to transition metal（K/TM）in the raw material of 0.7,and the temperature of 1000℃,were favorable for the synthesis of K-Birnessite with P2 type layered structure.The K-Birnessite phases were different with different Co content,and the higher the Co content,the smaller the interlayer distance.Excessive Ni content would produce NiO.Their micromorphologies were all sheet and massive,and the sizes were micron and increased with the increase of reaction temperature.Thanks to the ternary synergistic effect of Mn,Co and Ni elements,cycled at current densities of 50 and 1000 mA g-1,potassium cobalt nickel manganese oxide K0.7NCM-1000 delivered capacities of 72.3,and 55.3 mAh g-1,remaining 64.61 and 53.27%over 100 cycles,respectively.In situ XRD confirmed that the structure changed completely reversible during K+ions intercalation and deintercalation,and the dynamic process of elastic shrinkage of TMO2 layer was accompanied,which explained the structural factors for the excellent cyclic stability of the material.The stable structure not only brought low electrochemical activity but also stabilized the diffusion behavior of K+.Potassium cobalt/nickel manganese oxides showed higher capacities and better rate performance.K0.3Mn0.9Co0.1O2（H2O）0.32,K0.26Mn0.8Co0.2O2（H2O）0.21,K0.33Mn0.7Co0.3O2（H2O）0.31,and K0.31Mn0.9Ni0.1O2（H2O）0.26 delivered 98.1,92.36,74.4 and 96.6 mAh g-1 at a current density of 100 mA g-1,retaining 52.7%,61.93%,70.91%,and 54.2%over 100 cycles,respectively.And their initial discharge specific capacities were 81,83.37,56.8,and 88.3 mAh g-1,respectively,when cycled at a larger current density of 500 mA g₁.The electrochemical activity of K0.3Mn0.9Co0.1O2（H2O）0.32 was higher in the high potential region,and the diffusion coefficient of K+was higher in the charging process,promoting the release of K+from the structure.While in the low potential region,K0.26Mn0.8Co0.2O2（H2O）0.21 exhibited higher electrochemical activity and K+had a high diffusion coefficient during discharging process and was easily embedded between interlayers.The highly dense structure of K0.33Mn0.7Co0.3O2（H2O）0.31 reduced the electrochemical reactivity,and the K+had low diffusion coefficient fluctuation.The electrochemical reactivity of the material was improved by the introduction of nickel.During the charging and discharging process,due to the complex interaction between K+/vacancy,crystal water and TMO2 layer in the structure,K0.3,Mn0.9Ni0.1O2（H2O）0.26 underwent complex phase transitions,which promoted the diffusion of K+.Transition metal diversification design had been proved to be an effective strategy to improve the performance of potassium storage of K-Birnessite,which had accumulated valuable experience for designing transition metal oxide cathodes for KIBs.