Preparation And Characterization Of LiFePO₄/Carbon Nanocomposite With A Core-Shell Structure By A Polyaniline Coating Method

Lithium-ion batteries are considered to be the perfect power source of electric automobile and military equipments. LiFePO₄ positive electrode material have several advantages, such as excellent circulation performance, raw material low in price and high security, so it is a suitable positive electrode material. But there are also some disadvantages on low ionic conductivity and electronic conductivity. In order to solve these problems, LiFePO₄/Carbon nano-composite with core-shell structure was synthesized by polyaniline coating method. The influence factors of synthesizing of nano-FePO₄/PANI and the heat treatment process on the composition, structure and high rate performance of LiFePO₄/C was analyzed.The polyaniline was used to restrict the crystal size of FePO₄ into nano-scale during the synthesizing of nano-FePO₄/PANI. Then the lithium source and carbon source was added. Nano-LiFePO₄ will be coated with carbon when nano FePO₄ would be reduced by the heat treatment at high temperature.During the process of the synthesis FePO₄/PANI, FePO₄ particle size can be controlled by the addition of aniline. When the pH value of the reacting system was 5.0 and the reaction time was more than 4 hours, the aniline could grow on the FePO₄ surface and it could limit FePO₄ crystal growth. When the molar ratio of aniline and FePO₄ is 0.44, the size of FePO₄/PANI was about 50 nm. The size of FePO₄/PANI will not become smaller if the aniline was added to the reacting system continuously.When the sucrose was used as the carbon source, the LiFePO₄ can be coated by carbon completely in order to prevent the LiFePO₄ crystal growth continuously through heat treatment. When the content of carbon was about 8%, the crystal size of LiFePO₄/C is about 55nm and its conductivity was about 10^-2 S·cm^-1. It has perfect charge and discharge performance at high rate. Lithium source may lose about 5% during the heat treatment process. When the lithium source is not enough, there will appear impurity products lacking of lithium. When the Li: Fe ratio was 1.05:1 in the precursor, the Li: Fe ratio in the products was about 1:1. There are fewer impurities in the products and it has good charge and discharge performance at high rate charge.During heat treatment process, the temperature and the heat time is regarded to influence LiFePO₄/C property remarkable. When the temperature is low (T≤500℃) or heat time is short (t≤6 h), the LiFePO₄ crystal size is small, but LiFePO₄ crystallizes are not completely. It is not suitable for the charge and discharge at high rate. When the temperature is high (T≥800℃), the product LiFe(P₂O₇) appeared and influenced the electrochemistry performance of the products. The products prepared at 700℃, heat 12 h has best performance at high rate charge.The nano-LiFePO₄/C was synthesized using optimizing technics. The crystal size of LiFePO₄/C is about 55nm. The content of carbon was about 8%. The conductivity was about 2.1×10^-2 S·cm^-1.The specific capacity of nano-LiFePO₄/C is about 136 mAhg^-1 at 1C and 0.2C discharge rate. The specific capacity is respectively 118, 103, 94 and 87 mAh·g-1 at 10C, 20C, 30C and 40C. It shows better circulation performance than commercial LiFePO₄.