Preparation And Performance Study Of LiFePO₄ Cathode Material For Lithium Ion Batteries For Electric Vehicles

Olivine-type LiFePO₄ has been widely applied to cathode materials for rechargeable lithium battery because it possesses reversible extraction and insertion of lithium ions.In recent years,it has been extensively used in lithium-ion batteries for large scale energy storage and power source in electric vehicles?EV?and hybrid electric vehicles?HEV?,because it has a significant advantage of high theoretical capacity?170 mAh/g?and moderate operating flat voltage,rich raw material sources,low cost,and environmental friendliness.However,the crystalline structure of LiFePO₄ leads to reduced electronic conductivity and lithium-ion diffusion rate.Consequently,specific capacity and rate capability were limited.Great efforts have been devoted to improve these drawbacks,such as reducing particle size,carbon coating,and doping with supervalent cations.In order to improve the above defects and improve their comprehensive electrochemical performance,the main work is as follows:1.In order to obtain high-performance Nano-Li FePO₄ cathode materials for lithium-ion batteries,CH₃COOLi?2H₂O,FeC₂O₄?2H₂O and NH₄H₂PO₄ were used as raw materials to prepare the precursors of LiFePO₄ by room-temperature solid-state reaction method firstly;then,LiFePO₄ cathode materials were synthesized by high heating solid-state reaction method.The influence of temperature on the crystal structure,microstructure,and electrochemical properties were studied by thermogravimetric analysis,differential scanning calorimetry,X-ray diffraction,scanning electron microscopy,cyclic voltammetry,electrochemical impedance spectroscopy,and charge-discharge performance test.It was indicated that the olivine-phase LiFePO₄ not only presented nano-sized particles in the range of 30-200 nm,but also showed excellent initial discharge specific capacity of about 155 mAh/g at 0.2 C at a sintering temperature of 700 ?.It possessed excellent cycle life of 96%after 100 cycles at 1 C.This preparation method without any solvent and carbon source addition could realize industrialized production of single-phase Nano-Li FePO₄.2.LiFePO₄?xLi₃V₂?PO₄?₃?LFP?xLVP,x = 0,0.05,0.1,0.15,0.2?composites were prepared by the two-step solid-state method.The structure,morphology and electrochemical properties were characterized by X-ray diffraction,scanning electron microscope,cyclic voltammetry,charge-discharge performance test and electrochemical impedance spectroscopy.It was indicated that composites were presented rough in surface of ball-like particles.Two phase of LFP and LVP combination improved electrochemical properties of LFP.The LFP?0.1LVP has the highest discharge capacity of 154 mAh/g at 0.2 C.Compared with LFP,the rate performance of the LFP?0.1LVP increased 13% and 8% at 1 C and 10 C after five cycles.It possessed excellent cycle life of 99.8% after 100 cycles at 1 C especially.The cycle performance and rate performance of LFP has been improved.The all preparation process without any solvent addition was easy operation,energy saving and environmentally friendly.3.LiFePO₄/RGO composite cathode materials were prepared by two-step solid-phase method.The crystal structure,morphology and electrochemical properties were characterized by XRD,SEM,CV,EIS and charge/discharge tests.The results show that the addition of graphene does not change the crystal structure of LiFePO₄,which effectively controls the grain growth and reduces the particle size.LiFePO₄/RGO composite cathode material exhibits excellent rate performance,10 C with 111 mAh/g specific capacity,compared with LiFePO₄ cathode material increased by 14.4%;after 100 cycles after the capacity of no attenuation showed excellent cycle performance.The improvement of electrochemical performance by the combination of graphene and LiFePO₄ cathode material is expected to be a breakthrough in LiFePO₄ cathode material used in power batteries.