Study On The Synthesis And Modification Of LiFePO₄Cathode Material For Lithium-ion Batteries

Rechargeable lithium ion batteries exhibiting high energy and high power density have rapidly conquered the consumer market of advanced portable electronics in recent years and are now considered as the next generation of power sources for future electric vehicles (EVs), hybrid EVs, and plug-in hybrid EVs. To meet the dramatically increased demand for the emerging large-scale applications of these EV, HEV and PHEV, electrode materials for LIBs with high safety, high power density and long cycle life are urgently required. Among the known cathode materials, olivine lithium iron phosphate (LiFePO4, LFP) is considered as an appealing candidate. LiFePO4possesses advantages of potentially low cost, rich resources, and environmental friendliness. It also has a high lithium intercalation voltage of3.4V compared with lithium metal and a high theoretical capacity of170mAhg’. Although it displays plenty of attractive characters,the low electronic/ionic conductivity becomes a major drawback which is unfavorable to high rate capability. In order to further improve the power performance of LiFePO4, much effort has been made to improve the electrochemical properties of LiFePO4by decreasing the particle size and coating LiFePO4particles with conductive carbon.In our present work, we combine ultrafine balling, CNTs coating and spray drying processes to synthesize a spherical micro sized LiFePO4/CNTs composite materials. In a secondary composite material, CNTs not only dispersed on the surface but also between primary nano LiFePO4particles to form an ideal carbon nanotube network. Compared with conventional carbon coated micro or nano sized LiFePO4materials, it is found that this CNTs hybrid material exhibits significantly improved rate capability, cycling performance, liquidity and tap density in production. With3 wt.%CNTs content and2%pyrolyzed carbon content, the LiFePO4shows good chemistry performance that the discharge special capacity rise from54.1mAh/g to127.1mAh/g under IOC, good chemistry performance under50C too,99.0mAh/g. With nearly80times cycle under10C, the material shows few loss rates.In our second work, hydrothermal methods has been used to synthesis sphere-like LiFePO4, the next heating treatment step was carried out in inert atmosphere in order to obtain the crystalline phase and to carbonize the surfactant, so obtaining a carbon film that homogeneously covers the grains. Under650℃treated6hours, the material has a first discharge capacity of137.2mAh/g under0.1C,96.7mAh/g under5C and with less than5%loss rates with80cycles.In order to study the influence from different solvent to LiFePO4, we use three different mixture solution such as ethanol water solution、ethanol glucose water solution、(ethylene) glycol glucose water solution. Under heating treatment, the material using (ethylene) glycol glucose water solution has a good level of disperse and a good chemistry performance, a first discharge capacity of151.6mAh/g under0.2C,53.2mAh/g under IOC and a good cycling performance with more than140mAh/g under100cycles.