Synthsis And Modification Of LiFePO₄ As A Cathode Material Of Power Li-ion Batteries Using FePO₄ As A Raw Material

Lithium-ion battery has become the most favorite power supply for its high energy density, long lifespan and no memory effect. Here the most important requirement of the cathode material for a power battery, in addition to low cost and long cycle life, is good safety performance. As a cathode material of lithium-ion battery, olivine lithium iron（Ⅱ）phosphate （LiFePO₄） has advantages of low cost, long-term cyclability and environmental friendliness. Furthermore, in comparison with these layerd or spinel cathode materials, LiFePO₄ shows a better safety performance. Therefore, LiFePO₄ is the best candidate for power lithium-ion batteries. However, the high-rate performance of is greatly limited by the low electronic conductivity and slow lithium ion diffusion rate. So its practical application is delayed. Traditionally, Solid-state preparative approaches had relied on the use of Fe2+ precursors, typically iron （Ⅱ） oxalate, or iron （Ⅱ） acetate. Apart from being expensive, the multistage preparative strategies and a large amount of CO2 pollution are not considered commercially favorable. Hence, in this study, we aimed at the synthesis of material with high reversible capacity, good high-rate performance and long-term cyclability using the low cost Fe（Ⅲ） by the simple high temperature solid state method. The main contents are as follows:1. The self-made FePO₄ power with high purity and ultrasmall particle size was used as raw material to synthesize the LiFePO₄ material with purer crystallite and smaller particle size. In stead of the traditional Fe（Ⅱ） materials, using the low cost FePO₄ as raw material not only avoided the oxidation of Fe（Ⅱ）, but also solved the side products of large amount of gas and the following security problem;2. A simple high-energy ball milling combined with spray-drying method was developed to synthesize LiFePO₄/C composite using citric acid as carbon source. The sphericalparticulate size distribution is at a broad range of 1-30μm, and the primary particles are distributed with a range of 100-300 nm. The electronic conductivity of LiFePO₄ was enhanced as 10^-2 to 10-3 S/cm by the residual carbon. Furthermore, the spherical particles delivered an improved tap density and these openings on the surface should favor contact between the primary LiFePO₄ particle and the electrolyte. The electrochemical performance, which was especially notable for its high-rate performance, was excellent. The discharge capacities were as high as 160 mAh/g at the rate of 0.1C and 94 mAh/g at the rate of 11C.At the rate of 10C, it exhibited a long-term cyclability, retaining over 92% of its original discharge capacity beyond 2400 cycles. Therefore, the as-prepared LiFePO₄/carbon composite cathode material is capable of such large-scale applications as hybrid and plug-in hybrid electric vehicles.3. A simple and effective method was developed to synthesize a LiFePO₄/PAS （polyacenic semiconductor） composite with particle size of 50-80nm. In comparison with the micro-LiFePO₄/PAS, the nano-LiFePO₄/PAS exhibited much better power and energy densities at high rates. The electronic conductivity of this material was as high as 1.2×10^-1 S/cm due to the conductive network of PAS. It was especially notable that the nano-LiFePO₄/PAS cathode without adding Super P showed similar electrochemical behaviors with the cathode adding Super P at all C-rates. Thus, such cathode without adding Super P would enlarge both the volume energy density and weight energy density of batteries. In addition, this cathode exhibited an excellent long-term cyclability, retaining over 95.4% of its original discharge capacity beyond 500 cycles at 0.2C rate.4. Several layers of PAS coated LiFePO₄ was synthesized from the PF resin. PAS with a quasi-graphene structure can be deposited on the surface of LiFePO₄ particles with an ultra-thin film （< 4nm）. So the electrochemical performance of LiFePO₄ was greatly improved since the thin PAS film coating enhanced its electronic conductivity without influencing the intercalation/ deintercalation of lithium ion and the infiltration of electrolyte. The LiFePO₄/PAS composite with particle size of 100^-200nm delivered a high electronic conductivity of 5.4×10^-1 S/cm. At the rate of 0.1C, the discharge capacity was as high as 166 mAh/g. At a high rate of 5C, its discharge capacity was 116 mAh/g. In addition, this composite showed an excellent long-term cyclability, retaining over 96.5% of its original discharge capacity beyond 500 cycles at 1C rate.5. For the first time, a polythiophene（PTh）in situ polymerization restriction method to synthesize the LiFePO₄/C nanocomposite was reported. The particle size of the FePO₄/PTh was restricted at 30-50nm. Then the LiFePO₄/C composite with particle size in the range 50^-100nm was synthesized from citric acid and the as-prepared FePO₄/PTh. At the rate of 0.1C, the LiFePO₄/C composite showed a reversible capacity of 158 mAh/g. When the depth of discharge （DOD） reached above 90%, the capacity retention was more than 95%. At a high rate of 10C, it delivered a discharge capacity of 103 mAh/g, and it retained over 90% of its original discharge capacity beyond 100 cycles. In addition, the electrochemical performance could not be further improved by a continuative enhancement of electronic conductivity if it was high enough, which was just depended on the diffusion rate of lithium ion. This critical value of electronic conductivity was eatimated as 10^-1 S/cm.