Synthesis Of LiFePO₄ By Solid-state Method And The Modification Research

Lithium iron phosphate, LiFePO₄, has recently attracted significant interest because of its high safety, low cost, high stability at a high temperature, environmentally friendly components and preferred cathode material for lithium-ion power batteries. The conductivity and the ion diffusibility of LiFePO₄ are poor due to the restriction of its crystal structure, so the practical specific capacity is low and the discharge performance at high rates is poor. The modification research is focused on the enhancement of the conductivity and the ion diffusibility.In this paper, the starting materials were mixed by ball milling, LiFePO₄ cathode material was prepared by solid-state method and modified by carbon coating and Mg²⁺ doping, and meanwhile, the electrode technology of LiFePO₄ was studied. The preparation technology, modification, structure characterization, electrochemical performance and electrode preparation technology were investigated by using TG-DTA, XRD, SEM, CV, EIS and charge-discharge tests.The results shows that:the grains of the carbon coated LiFePO₄ is smaller and distributed uniformly for the carbon which can prevent the oxidation of Fe²⁺ and the agglomeration of the grains as a nucleating agent; the tap density is reduced greatly as the increasing carbon contents; the carbon content and the sintering temperature have obvious influence on the electrochemical performance of LiFePO₄, when the carbon content is 6% and the sintering temperature 650℃, the obtained has best performance with the specific capacity of 143mAh g^-1.Mg²⁺ doped LiFe1-xMgxPO₄/C was obtained based on carbon coated LiFePO₄. The LiFe1-xMgxPO₄/C has higher specific capacity, flatter potential platform and better cycle performance. The specific capacity of LiFe1-xMgxPO₄/C is enhanced firstly and then is reduced as the increasing contents of Mg²⁺. When the doping content equals 0.04, the obtained has the best electrochemical performance with a specific capacity of 153mAh g^-1.The electrode preparation technology has important effect on the performance of the LiFePO₄ cathode material, such as the adhesive contents, the conductive agent and the electrolyte. The proper adhesive content is 6% and the proper conductive agent content is 15%. The electrolyte with three components of organic solvents (EC:DEC:DMC= 1:1:1) has better performance at a relative lower rate (0.1C), while the electrolyte with two components of organic solvents (EC:DMC=1:1)has better performance at a relative higher rate (1C). The performance of LiPF6 is much better than that of LiBF4.