Preparation And Modification Of High Performance Olivine Structure LiFe_xMn_1-xPO₄ As Cathode Material

Olive cathode materials have aroused enormous attention in virtue of their environmental friendlyness,stable structure and low cost.However,the further development and application are seriously restricted by low electronic conductivity and inferior lithium-ion diffusion coefficient.In this paper,the olive Li Fe_xMn_1-xPO₄（0<x≤1）is chosen to improve the electronic conductivity of the cathode material,increase the energy density and power density,and reduce the influence of the Jahn–Teller lattice distortion in the Mn³⁺position.Firstly,in order to improve the electrochemical performance of LiFeO₄ under high current density and prevent the Fe²⁺oxidation during the preparation process,FePO₄ is used as template,and sucrose along with PVDF are acted as carbon sources.The F-doped carbon coating LiFePO₄ composites are successfully synthesized by a typical solid-state method in the carbothermal reduction process.The subsequent results indicate that the F-doped carbon can impove the electronic conductivity of the cathode material,and relieve the corrosion to the cathode material by HF in the high temperature（55℃）.The obtained composite materials show excellent electrochemical performance with a discharge capacity of 120.2 mA h g^-1 at 20 C,and high energy density as well as power density.Secondly,the Jahn–Teller lattice distortion has serious effect at the electrochemical properties and structural stability of the LiMnPO₄ cathode material.The Fe-doping can reduce the influence of the Jahn–Teller lattice distortion at the Mn³⁺section,but the optimal Mn/Fe ratio for LiFe_xMn_1-xPO₄ cathode materials in different synthetic method has not been given yet.A series of Li Fe_xMn_1-xPO₄ cathode material with different Mn/Fe ratios are synthesised by solvothermal method.On the basis of F-doped carbon,the optimum Mn/Fe ratio is investigated.Consequently,with a 6/4 of Mn/Fe ratio,the corresponding LiFe_0.4Mn_0.6PO₄ cathode material with F-doped carbon coating exhibits the best rate capability and cycling stability.Thirdly,the electron conductivity of LiFe_xMn_1-xPO₄ is substantially increased by adopting graphene and F-doped carbon.The reductive graphene oxide and F-doped carbon coating LiFe_0.25Mn_0.75PO₄ positive material is synthesized by a simple ball milling method.The optimal addition of graphene oxide and mixed carbon sources is studied.The effect of graphene on the electrochemical performance of cathode materials is analysed.By a series of tests,the composite materials exhibit excellent rate performance andgood cycling stability especially at high rate current density.In summary,the carbon coating olive cathode materials and the optimal Mn/Fe ratio are investigated to effectively enhance the electron conductivity and relieve the Jahn-Taller effect of Mn³⁺.The prepared olive cathode materials exhibit superior rate performance and enhanced stability.Therefore,the relevant research in this thesis provides a solution to develop high power density and energy density cathode materials for lithium-ion batteries.