The Preparetion And Performance Of Lithium Iron Phosphate-Nitrogen Doped Carbon Composite

There are two shortcomings of Lithium iron phosphate (LFP) restrict its application in industry, low electrical conductivity (about 10-9?10-11 S·cm-1) and low ithium ion diffusion efficiency (about 10-11?10-13 cm2·S-1). Low electronic and ionic conductivity of the sample will lead to low specific capacity and rate capacity, how to overcome these shortcomings become the main research content of LFP. Recently, many researchers devoted their efforts to solve the above shortcomings, including doping conductive material, such as carbon and metal ion, and decreasing the size of particles to make it evenly dispersed. In these modification methods, carbon-coated is the most effective method to improve the electrical and ithium ion conductivity. However, just only carbon-coated can not make LFP material has best performance. Under high charge/discharge rate, the uneven distribution of electrode surface charge will reduce the efficiency of the active material. Some researchers reported that the carbon and nitrogen doping can solve this problem, nitrogen-doped carbon can form a good continuous electrically conductive layer that improve the rate performance of material.Here, the LFP materials were synthesized by carbothermal reduction that can synthesize LFP by one-step reduction process, and the raw materials were iron phosphate and lithium carbonate. The reseachs and conclusions of this paper are as follows:The specific capacity is higher when the amount of glucose is 40%, the specific capacity of this sample is 147.1 mAh·g-1 at 0.1 C. The better calcining scheme of carbothermal reduction is heating in 350? 2 h then in 700? 6 h, the specific capacities of the material synthesized by this scheme are 149.1 mAh·g-1,148.5 mAh·g-1,138.0 mAh·g-1,133.1mAh·g-1,117.5 mAh·g-1 and 99.7mAh·g-1 at 0.1C,0.2C,0.5C,1C,2C and 5C rates.Here used urea, melamine, poly aniline and polyvinylpyrrolidone (PVP) that organics of rich-nitrogen as the nitrogen-doped carbon sources to synthesize LFP-nitrogen doped carbon compsite. XPS results showed that the samples do contain nitrogen and carbon element. The characterization and electrochemical tests show that the sample that join PVP has the best performance, the first discharge specific capacity of this sample are 155.5 mAh·g-1,151.4mAh·g-1,133.9mAh·g-1,140.1 mAh·g-1,119.5mAh·g-1 and 107.6mAh·g-1 at 0.1C,0.2C,0.5C,1C,2C and 5C rates.Here discuss the influences of the amount of PVP addition on LFP sample when use PVP as the nitrogen-doped carbon source. Results show that the addition of PVP can improve the performance of the material and the samples when the content of PVP was 30% has the best performance, its first discharge specific capacity are 160.8 mAh·g-1,154.5 mAh·g-1,141.3 mAh·g-1, 144.5mAh·g-1,127.2 mAh·g-1 and 116.1 mAh·g-1 at 0.1C,0.2C,0.5C,1C,2 C and 5 C rates, and its discharge specific capacity can up to 165.1 mAh·g-1 at 0.1C. This sample has a good cycle stability and rate performance.