Preparation Of Lithium Iron Phosphate Cathode Material And Its Electrochemical Performance Study

Olivine-type LiFePO4 is a promising cathode material for the next generation of lithium-ion batteries. It shows significant advantages such as abundant raw material, nontoxicity, relatively high theoretical capacity and structural stability. However, LiFePO4 has poor electronic conductivity and low diffusion coefficient of lithium, significantly influencing its rate capacity and applications. In this paper, LiFePO4/C composites were synthesized by carbothermal reduction method and LiFeo.98Mgo.o2P04/C composites by solid state reaction. The micro-structures and morphologies of the composites were investigated by XRD and SEM, and the electrochemical performances were evaluated by galvanostatic charge-discharge, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The effects of the synthesis processes on the physico-electrochemical properties of materials were investigated.LiFePO4/C composites were synthesized by heat treatment after wet ball-milling, using Fe2O3 and LiH2PO4 as raw materials. The sample obtained using sucrose carbon source has smaller particle size and better electrochemical properties than those using acetylene black or glucose. De-ionized water differs greatly from absolute alcohol when used as dispersant during wet ball-milling, and the so-obtained samples contain more impurity and present poor electrochemical properties. The results show that calcination time and calcination temperature obviously influence the performances of the products. The material synthesized at 750℃for 6h, with particle size of 200～600nm, shows good electrochemical properties. Its initial discharge capacities are 145 and 112mAh/g at 0.1 C and 1C, respectively.LiFe0.98Mg0.02PO4/C materials were synthesized through pilot scale production, using Li2CO3, FeC2O4·2H2O, NH4H2PO4 as raw materials and sucrose carbon source. The material prepared by stirring raw materials with water has a poor electrochemical performance, with an initial discharge capacity of 122mAh/g at 0.1C rate. LiFe0.98Mg0.02PO4/C composites prepared by wet ball milling-spray drying method, with spherical secondary particles and high tap density, show excellent electrochemical properties. The material prepared by two-step ball milling has a tap density of 1.67g/cm3, and presents large reversible discharge capacity of 151,143 and 132mAh/g at 0.1C, 0.5C and 1C rate respectively. The discharge capacity of LiFe0.98Mg0.02PO4/C material obtained by ball-milling after calcination is slightly lower. The discharge capacities of materials from pressed precursors at 10Mpa and 20Mpa are slightly higher.