| As a secondary batteries, lithium ion batteries show greater advantages at the aspects of high potential (three times of Ni-Cd and Ni-MH), high rate energy, long cycle life and wide work temperature, which is fit for the request of portability, eco-friendness and long life. As soon as the first lithium ion battery was produced, it attracted much attention and was researched deeply. Now, its performance has great improvement, exceed the Ni-Cd and Ni-MH batteries, and become the main product of the market.As the best choice of cathode materials, commercial LiCoO2 has perfect capability, however, it meets the obstacles on the developmental way of lithium ion battery, especially for electric vehicle applications because of its high cost and toxicity. To be the substitution of LiCoO2, LiFePO4 has the advantages of abundant raw material, nontoicity, high safety, high capacity and the excellent abuse tolerance. As the key cathode materials for lithium batteries, intensive attentions have been focused on it. But its rate capability and tap density need to be improved. The industry synthesis methods of LiFePO4 are concentrated in carbothermal (Fe2O3 is the iron source) and solid state reaction (iron oxalate is the iron source). The research approved that the capability of LiFePO4 is much better when we use FePO4/Polymer as the iron source. We get the nanosized core-shell FePO4/Polymer by in-situ polymerization reaction method, then mixed it with Li salt and will get the high capability LiFePO4/C after calcinations.Against the high cost of preparation, bad performance at high current and low tap density, we focused on improving these disadvantages.1. Synthesized FePO4/Polymer composites.Carbon coating is an important method of improving the LiFePO4 capability. Many researchers mix the organic carbon with raw materials directly, and get LiFePO4/C after calcinations. These approaches can increase the particles size during high-temperature treatment and only produce LiFePO4 particles with a partial coating of carbon, thus result in bad rate capability.Herein we report an in situ polymerization restriction method for the synthesis of a LiFePO4/C composite. The restriction process involves the addition of Fe3+ ions to a solution containing PO43- ions and monomer, where it acts as both a precipitator for PO43- and oxidant for aniline. The particles are well coated, less than 200 nm and have good performance. As prepared LiFePO4 material with 5% carbon has capacities of up to 160,150,145,139,125,112,102 mAh/g respectively at 0.1 C,0.5 C,1 C,3 C,6 C, 10 C and 15 C rates, and respectively equivalent with 94.1%,88.2%,85.3%,81.8%, 73.5%,65.9%,60% of theoretical capacity.2. Synthesized spherical LiFePO4/C composites.The theoretical tap density of LiFePO4 is 3.6 g/cm3, much lower than LiCoO2. Even worse, the carbon mixed into the materials improving the conductivity also decrease the pack density, which tap density are commonly about 1.0-1.2 g/cm3. The tap density is so low that the volume of the batteries, made of LiFePO4/C materials, is completely too large and become the bottleneck for practical application. So, enhance the tap density brooking no delay. Thanks to the high density of spherical Ni(OH)2, we synthesize the spherical LiFePO4/C materials to increase its tap density. The result reveals the spherical LiFePO4/C materials not only have outstanding advantages on high pack density and volume capacity, but also have excellent fluidity, dispersibility and machinability performance, which are good for the preparation of electrode pads. Through this method, we get the cathode materials LiFePO4/C with high pack density, high volume capacity and excellent conductivity, which will promote the industry of this material. |
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