Preparation And Modification Of LiFe_xMn_1-xPO₄/C Material For Lithium-ion Batteries By Solvothermal Method

Olivine structured LiMPO₄（M=Fe,Mn,etc.）cathode materials have attracted much attention due to its high safety,strong stability,long cycle life,and other characteristics.At present,LiFe PO₄ material has been commercialized on a large scale,but gradually cannot meet the needs of people.LiMn PO₄ materials with higher voltage platform and energy density began to be valued.However,the inherent disadvantages of LiMn PO₄,such as low electron conductivity and lithium-ion diffusion rate,seriously affect the application of the material.In this paper,use solvothermal method to study the preparation of LiFe_xMn_1-xPO₄/C material,by studying the effects of different synthetic solvents,synthesis temperatures,and synthetic time on the microstructure and performance of the material,the best process parameters are determined.Using XRD,SEM,FTIR,Raman,TG,and other characterization methods to analyze the microstructure of the materials.Assemble the materials into button battery,and then study the electrochemical properties of the material,such as cyclic voltammetry plots,electrochemical impedance spectrum,rate performance,charge-discharge curve and so on.The main research work of this paper is as follows:The effects of Fe and Mn on the morphology and electrochemical properties of LiFe_xMn_1-xPO₄/C were investigated.The energy densities at 0.1 C of the prepared LiFe PO₄/C,LiFe_0.8Mn_0.2PO₄/C,LiFe_0.5Mn_0.5PO₄/C,LiFe_0.2Mn_0.8PO₄/C,and LiMn PO₄/C are 405.4 m Wh·g^-1,454.9 m Wh·g^-1,471.8 m Wh·g^-1,396.6m Wh·g^-1,and 171.9 m Wh·g^-1,respectively,and the capacity retention rates at a rate of 0.5C after 150 cycles were 92.1%,93.1%,94.7%,71.1%,and 65.6%,respectively.The results show that the presence of Mn in the material can increase the energy density of the material,while the presence of too many Mn will reduce the electronic conductivity and ion diffusion rate of the material,which will reduce the material performance.LiFe_0.5Mn_0.5PO₄/C has the best electrochemical performance.The mixed solution of ethylene glycol and deionized water is used as the reaction solvent to prepare LiFe_0.5Mn_0.5PO₄ through solvothermal method,and ball-mill the LiFe_0.5Mn_0.5PO₄ with glucose,then sinter the mixture to finally get LiFe_0.5Mn_0.5PO₄/C.The influence of different ratio of solvent,synthesis time,synthesis temperature and sintering temperature on the material was investigated to obtain the best process parameters.The results show that when the volume ratio of ethylene glycol and deionized water in the mixed solvent is 1:1,the size of the prepared nanoparticles is uniform and the dispersion degree is the best,which is conducive to the electrochemical performance of the materials.In this solvent,the granular LiFe_0.5Mn_0.5PO₄ was synthesized by reacting at 180℃for 18 h,then sintering the LiFe_0.5Mn_0.5PO₄（ball-milled with glucose）at 600℃for 4 h to obtain LiFe_0.5Mn_0.5PO₄/C.The particle size of the obtained LiFe_0.5Mn_0.5PO₄/C cathode material was 50-100 nm.And the first charge and discharge capacity of the battery can reach 124.7 m Ah·g^-1 at 0.1 C.In the process of preparing LiFe_0.5Mn_0.5PO₄ material,it was found that the material was always oxidated which could affect the electrochemical performance.Ascorbic acid,oxalic acid and citric acid were used as additives in the preparation process to avoid oxidation.The experimental results show that the specific discharge capacity of LiFe_0.5Mn_0.5PO₄/C cathode material prepared without antioxidant is 79.5 m Ah·g^-1 at 0.2 C,and the capacity retention rate is 88.2%after50 cycles,while the material using ascorbic acid(The addition amount is 10%of the molar mass of LiFe_0.5Mn_0.5PO₄)as oxidant has a discharge specific capacity of 119 m Ah·g^-1 at 0.2C,and the capacity retention rate is 96.9%after 50 cycles,which is higher than that of oxalic acid 92.4 m Ah·g^-1（88.1%after 50 cycles）and citric acid 89.1 m Ah·g^-1（91%after 50 cycles）.Nano-bubble water containing Ar-H₂ mixed gas and ethylene glycol are used as solvent for solvothermal reaction for the first time.On the one hand,nano-bubble water can be used as antioxidant to reduce the oxidation of materials during solvothermal reaction;On the other hand,a large number of nano-bubbles can provide enough specific surface energy,which can reduce the agglomeration of materials in the synthesis process.The capacitance of the prepared material at0.1C is increased by 42.9%compared with that of the material without antioxidant and by 15%compared with that of the material with ascorbic acid.