Study On Synthesis And Modification Of LiFePO₄Cathode Material For Lithium-ion Batteries By Hydrothermal Stripping Method

The continuous development of new energy is an important foundation forsustainable human development. Lithium-ion battery has many advantages in linewith the development needs of the new area, olivine-structured lithium iron phosphateis the most promising cathode material for Li-ion dynamic batteries, as it consideredbeing with the advantages of good cycle performance, safety, environmentalprotection, the abundant raw materials and low cost. Nowdays, solid-phase methodand liquid-phase method are the major types for the preparation of LiFePO4. Theprocess of solid-phase method is simple and easy to be industrialized, and moremature, but the disadvantage of the large particle size, irregular of crystal, unevendistribution and unstable production batch can all affect the performance of LiFePO4.In addition, the solid-phase method can consume large energy accompany with thetime. In view of the above shortcomings, the liquid-phase method can solve theproblems. Herein, we describe a new method based on hydrothermal strippingtechnique according to coupling principle, and have successfully improved it tosynthesize LiFePO4particles. The samples were directly synthesized by cationexchange between iron (II)-loaded organic phase which is prepared with cheapFeSO4·7H2O and naphthenic acid, and the aqueous phase which is prepared withH3PO4and LiOH.In this paper, the principle and some factors influencing LiFePO4synthesized bystripping iron (II)-loaded naphthenic acid-isooctanol system with LiH2PO4solutionwere studied in detail. The samples were characterized by X-ray diffraction (XRD)and scanning electron microscope (SEM). The results indicated that the mostsignificant factors are the molar ratio of Li+/PO43-, the amount of reducing agent andstripping temperature. By cyclic voltammetry (CV), electrochemical impedancespectra (EIS) and constant current charge-discharge test to exhibit the electrochemicalproperties of the sample synthesized in different kinds of conditions. The resultsdemonstrate that the sample prepared with hydrothermal stripping temperature of250°C in3h, and80°C insulated for5h can own the best electrochemicalperformance.Although LiFePO4has many advantages, following some disadvantages.Ionicconductivity and electronic conductivity of pure LiFePO4are very low, and Li+ diffusion coefficient between the two phases of the LiFePO4/FePO4is not large duringthe charge-discharge process, which is not very conducive for the diffusion of Li+andelectrons. Coated with carbon is an effective means of modification. The effect ofdifferent calcination temperatures, calcination time, the amount of carbon-coated andcarbon source are discussed by XRD, SEM, HRTEM and electrochemical test. Forhydrthomal stripping method, the results show that the glucose is the best carbonsource, and the LiFePO4/C supplemented with10%glucose, calcined at650°C for4hexhibits the best electrochemical performance.Coated with carbon is mainly to improve the electric conductivity among the activematerial particles, and between the active materials and the conductive agent. In orderto improve the inside lattice conductivity of LiFePO4, metal ion doping should beselected. Series of characterization and testing means on the samples which are dopedwith Co2+、Mn2+and Ni2+, and found that the doping of metal ions did not achieve thedesired results.