Synthesis And Performance Studies Of LiFePO₄ Cathode Materials For Li-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 materials. 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 in 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.LiFePO₄ was prepared by high temperature solid-state method and the effects of calcinating temperature and duration on electrochemical behavior of LiFePO₄ were studied in this work. The results indicated that the LiFePO₄ calcinated at 600℃for 20 hours is better in performance.The LiFePO₄ was modified by Copper surface treatment. LiFePO₄/Cu composite has been synthesized by an electroless deposition method with Cu particles being deposited on LiFePO₄ surface from a Cu solution. The electrochemical performance of the composite is found to be much better than pure LiFePO₄. The initial discharge specific capacity can reach 139mAh/g at 0.1C. At the 40th cycles, the discharge specific capacity can also reach 99mAh/g at 1.0C.The LiFePO₄ was also 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 discharge specific capacity and cycle capability and high-rate charge-discharge capability were all largely improved, the first discharge specific capacity reach 140mAh/g at 0.1C.The effect of doping nickel in place of Fe-site of LiFePO₄ on the electrochemical properties was studied in detail. It was shown that LiFePO₄ preserved olivine structure after partial substitution of Fe by Ni. The doping composite has a higher charge and discharge specific capacity than pure LiFePO₄. The material has a highest charge and discharge specific capacity and best cycle performance when the Ni/Fe substitution amount was at 0.05 mol ratio, the first discharge specific capacific is 131mAh/g at 0.1C.LiFePO₄ cathode material for lithium ion batteries was further prepared under vacuum atmosphere during high temperature calcinations. The XRD analysis indicated that pure olivine-type LiFePO₄ was synthesized. The electric charge transfer resistance of the vacuum synthesized material is found to be lower than that synthesized under nitrogen atmosphere. The electrochemical performance of such prepared LiFePO4 was investigated. The first discharge specific capacity can reach 131mAh/g at 0.1 C rate and 107mAh/g at 1.0C rate.