Hydrothermal Synthesis And Modification Of LiFePO₄for Lithium Ion Battery Cathode Material

Olivine LiFePO₄is considered to be one of the most potential cathode materialsof lithium ion batteries. This material has been used for power source of electricvehicles and hybrid vehicles because it has a wide range of raw materials,environment friendly, good thermal stability, high specific capacity, good cycle andsafety performance, no memory effect and so on. However, there are still someproblems such as low electrical conductivity included in LiFePO₄. In recent years,researchers mainly focus on improving conductive properties, in whichcarbon-coated and metal-ion-doped are the most effective ways to improveelectrochemical performance of LiFePO₄.Based on progress of LiFePO₄cathode material for lithium ion batteries, toimprove electrochemical performance of the material, hydrothermal method isapplied to forming LiFePO₄and its electrochemical properties are improved bycarbon-coating and metal-ion-doping. Scanning electron microscope（SEM） andX-ray diffraction（XRD） are used to analyze surface morphology and crystal structure,also constant current charge-discharge test is employed to test electrochemicalperformance.FeSO₄·7H₂O、H₃PO₄and LiOH·H₂O were used as main raw materials. Precursorwas firstly synthesized by hydrothermal method, then as-formed precursor wascalcined at high temperature, and finally olivine-type LiFePO₄was obtained.Hydrothermal temperature, hydrothermal time, calcination temperature andcalcination time were investigated. Results show that as-prepared LiFePO₄has goodelectrochemical performance with first discharge capacity approximately106.6mA·h/g under condition of150℃and6hours hydrothermal process followed bycalcination temperature of700℃for5hours. In addition, carbon-coated andmetal-ion-doped LiFePO₄specimens were also investigated. It seems thatcarbon-coated and metal-ion-doped specimens can maintain olivine structure, whilecarbon-coated LiFePO₄has improved conductive properties, and incorporation ofions can form lattice defect to reduce the diffusion path of Li⁺. It is shown thatgraphene is a good carbon source, the first discharge is about138.2mA·h/g by adding3wt%graphene. As a specimen with metal-ion doping from nickel acetate,LiFe_0.95Ni_0.05PO₄has a good electrochemical property of152.8mA·h/g of the first discharge capacity.