Synthesis And Properties Of LiFePO₄ Materials For Powered Li-ion Batteries

Olivine structure LiFePO₄ attracted much attention as a promising cathode material for high-power Li-ion batteries of electric vehicles.The overwhelming advantages of LiFePO₄ are,in addition to being inexpensive and naturally abundant,the high theoretical capacity and the excellent safety.However,LiFePO₄ exhibits poor electrochemical characteristics at high current density due to its poor electronic conductivity and low lithium ion diffusivity.In order to overcome these defaults,approaches such as particle size reduction,carbon coating and doping were applied,and showed positive effects to some extents.In this study,low-cost raw materials of ferric iron were used in order to seek feasible commercial applications.LiFePO₄ cathode material was synthesized by thermal decomposition and sol-gel methods and modified by carbon-coating and foreigh ion doping.TG-DTA,XRD,SEM,TEM,EDS,CV,and EIS method were used to characterize structure,morphology and electrochemical properties of the prepared LiFePO₄.Results show that LiFePO₄ synthesized at 700 ℃ by thermal decomposition method is with the best electrochemical performance.Glucose,sucrose and citric acid used as the carbon sources for carbon-coating were also investigated.Experimental results show that the sample with the glucose as the carbon source presented the excellent electrochemical properties.Furthermore,sphere LiFePO₄/C particles were evenly distributed with the size of 300～800nm,the surface of particles was coated by carbon particles,which were inter-connected through carbon fiber.The first discharge capacity of the LiFePO₄/C material reached 162.1mAh·g-1 and remained over 155 mAh·g-1 after 30 cycles at 0.1C rate.However,at 1C rate the first discharge capacity decreases to 118.2 mAh·g-1 rapidly.To improve the high-rate discharge performance of materials,Na,Al,Ce doped LiFePO₄ were prepared and investigated.XRD patterns show that,doped materials show different crystal lattice parameters and cell volume comparing to the un-doped counterpart,with a improved electrochemical properties.Among doped materials,LiCe0.01Fe0.98PO₄ shows the best electrochemical performance.The first discharge capacity reached 167.3 mAh·g-1 at 0.1C,and 138.2 mAh·g-1 at 1C rate.Cyclic voltammetry curves show that Ce doped LiFePO₄ has a good reversibility and EIS results show that Ce doped LiFePO₄ reaction resistance was significantly reduced compared with other doped materials,indicating that Ce doping can enhance LiFePO₄ conductivity.LiFePO₄/C was also prepared by sol-gel method at 650 ℃,where citric acid and glycolic acid,are investigated as carbon-coating source.The result showed that LiFePO₄/C material with citric acid as carbon source demenstrate better electrochemical performance.Further the effect of citric acid content was studied and it was found that the optimized ratio of citric acid to ferric nitrate is 0.75:1,and the resultant material showed the the capacity of 149.5mAh/g at 0.1 C rate.