Study On Ac Impedance Characteristics Of Lifepo₄ As Cathode Materials For Lithium Ion Batteries

Olivine-type LiFePO4 is a new promising cathode material for lithium-ion batteries. This material has many advantages compared with other conventional cathode material. LiFePO4 has high specific capacity, Fe is a resource rich element, the material is environmentally friendly and importantly it is safe when used as an electrode material for Li-ion batteries. The main drawback of LiFePO4 is its low electronic and Li ionic conductivity. Due to the low conductivity, the capacity of the LiFePO4 will decrease dramatically when it is charged and discharged at high rates. The aim of this research work is to improve the electronic conductivity and Li+ diffusivity of LiFePO4, so that the material can be widely adopted in Li-ion battery manufacturing industry. In this paper, the cathode materials was synthesized by two-step solid-state reaction with Li2CO3,Fe2C2O4,(NH4)2HPO4,C12H22O11,etc. The materials' synthesis factors, surface morphology, structure and electrochemical performance were studied through measures by SEM, charge-discharge and EIS tests.The LiFePO4 was modified by carbon surface treatment.LiFePO4/C composite has been synthesized by solid-state reaction using glucose as conductive carbon source. The results showed that the addition of carbon didn't affect the crystal structure of LiFePO4. The electrochemical resistance of LiFePO4 was decreased significantly with the increase of carbon content. The first discharge specific capacity reachs 140mAh/g at 0.1C.The compound materials LiFePO4/C doping metallic ion Mg2+ and Mn2+ and Cu2+ were studied. It was shown that LiFePO4 preserved olivine structure after substitution of Fe by Mg2+ and Mn2+ and Cu2+. The doping composite has a higher charge and discharge specific capacity than pure LiFePO4. The results show that the addition of element didn't affect the crystal structure of LiFePO4. The diffusion coefficient improved markedly to reach 5.8×10-14 cm2/s.The electrochemical performance of the composite is found to be much better than pure LiFePO4.