Study On Synthesis And Performance Of Cathode Materials LiFePO₄

Since Padhi et al found lithium iron phosphate(LiFePO₄) could be used as cathode material for the lithium ion batteries, many research groups have been devoted to study the performance of this material. They thought that LiFePO₄ is one of the most promising cathode material for the lithium ion batteries because it is abundant, environmentally benign, stable and safe. In order to overcome the default,one way is to coat the LiFePO₄ with electronically conductive materials like carbon, the other way is to dope the LiFePO₄ with metal ion. Both the two methods have met with success in improving performance. Recently, it has been pointed out carbon coating adversely effect the tap densities. Even small amounts of carbon additive exacerbate this effect, resulting in an electrode with an unrealistically low practical energy density and power density, so it is imperative to optimize carefully the carbon coating in composite LiFePO₄ materials. The electronic conductivity of LiFePO₄ could be improved by eight orders though supervalent ion doping and the metal ion doping dose not influence the physico-chemical characters of LiFePO₄. So we decide to optimize the technological parameters of synthesis of carbon coating LiFePO₄ and use a method combined coating with doping to improving the electronic conductivity of LiFePO₄ in this paper. The main contents are given as following.Firstly,we retrospect the development of cathode materials for lithium ion batteries,focusing on the synthesis and improvement of properties of LiFePO₄. The problems in application and researching direction in the future are analyzed and pointed out. Then the LiFePO₄ has been synthesized and the effect of synthetic temperature is investigated. The results indicated that the LiFePO₄ prepared at the temperature of 700℃has a high capacity because it has well crystallized small and rough surface particles.Carbon coating LiFePO₄ is synthesized by adding the sugar into the precursor before heating in argon and throughly studied in chapter four. The electrochemical test results showed that the electrochemical performance of the LiFePO₄ was improved by coating with carbon, the specific capacity reached 135mAh/g(0.1C) at room temperature. Doping LiFePO₄ is studied in chapter five. The doping ion with high valence benefit the enhancement of the electrochemical performance of Li_0.99M_0.01FePO₄.Combined carbon-coating with metal ion doping to improve the conductivity of LiFePO₄ is studied in chapter six. Carbon coating Li_0.99Zr_0.01FePO₄ is synthesized and its specific capacity reached 151 mAh/g at 0.1C and 110 mAh/g at 4C.