Preparation And Modification For Lithium Iron Phosphate As Cathode Material Of Lithium Ion Battery

As a kind of polyanionic-based cathode material, Lithium iron phosphate possesses a lot of advantages such as high theoretical specific capacity (170mAh/g), low cost and high thermal stability. However, its practical application has been limited due to low electronic conductivity (10-9?10-10S/cm) and poor ionic conductivity (10-12?10-14cm2/s). At present, ion doping, conductive agent coating and particle size reduction are mostly used to promote the conductivities of lithium iron phosphate and electrochemical performance. In this work, lithium iron phosphate was synthesized by microwave-assisted sol-gel method and polymer-network gel method. The sintering technology for the process was researched. Alkyl quaternary ammonium salt surfactants were applied as modifying agent for the samples to improve the properties. The effects of surfactant type, surfactant concentration, sintering temperature and sintering time on the performance of samples were explored. XRD and TEM were used to analysis the phase structure and morphology. The reaction process during elevated temperature, the types of functional groups and chemical state were obtained by TG/DTA, FTIR and XPS respectively. The electrochemical performance was measured by the electrochemical workstation and the battery testing system. The results indicate that:1. The microwave-assisted sol-gel method can be used to prepare lithium iron phosphate and the samples with higher purity and crystallinity were obtained in the condition of 9min under the microwave power with 528W. The initial discharge specific capacity reached the value of 73.9mAh/g at the rate of 0.1 C. The surfactants of DTAB, TTAB and CTAB were devoted to optimize the particle of lithium iron phosphate, in which DTAB exhibited the best effect on the particle size adjusting and aggregation controlling. With the participation of DTAB, the as-synthesized sample with a sintering time of 20min under 528W possesses the smallest grain size and narrow particle size distribution.2. The lithium iron phosphate composite with nitrogen and carbon co-doping was successfully synthesized via polymer-network gel method. The lithium ion diffusion coefficient and electronic conductivity were significantly improved and reached 3.14×10-9cm2/s and 2.79×10-4S/cm respectively, meanwhile, the Rct value reduced to 23.3?. The initial specific capacity is 124mAh/g and 127.5mAh/g at the rates of 0.05C and 0.1C respectively. As the current density reduced again to 0.1C, the capacity returns to the value of 130mAh/g (>124mAh/g), indicating excellent stability during electrochemical cycling.3. The effect of surfactants on the electrochemical performance of LiFePO4 prepared by sol-gel method was investigated. With the sintering temperature of 700? for 8h, the as-prepared sample modified by 0.1250mol/L DTAB possesses the most outstanding performance. The specific surface area of that sample was improved to be 18.35m2/g, which much higher than 5.18 m2/g of blank sample, and the charge transfer rate was enhanced while the polarization of sample was reduced, in addition, the initial discharge specific capacity can be reached to the value of 147.4mAh/g at 0.05C.