Study On Synthesis And Electrochemical Performances Of Graphene Modified LiFePO₄Cathode Materials

Olivine-structured lithium iron phosphate (LiFePO4)has recently attracted greatinterests as the most potential and promising cathode material for a new generation oflithium ion batteries due to its appealing advantages, such as abundant resources, lowcost, environmental benignity and high safety, etc. However, LiFePO4itself has anumber of weaknesses: relatively low working voltage, electronic conductivity andlithium ion diffusivity, which result in irreversible capacity and poor rate performance,and seriously affect its application process in lithium ion batteries. Recently，carboncoating has been generally adopted to improve the shortcomings of LiFePO4for highpower batteries，and numerous studies show that graphene (G) as a new kind ofcarbon source for the carbon-coating can well make up for the above shortcomingsand greatly improve the electrochemical performance of LiFePO4cathode material.In this paper, anethylene glycol assisted hydrothermal method was adopted tostudy the synthesis of LiFePO4; Ag-G composite, viewed as a modification material,was prepared by chemical reduction and freeze-drying method, which was finallyintroduced to synthesize LiFePO4/Ag-G composite; Graphene oxide (GO) as a novelcarbon source was introduced to synthesize LiFePO4/G composite byco-precipitation-calcination method; X-ray diffraction(XRD), scanning electronmicroscope(SEM), transmission electron microscope(TEM), thermogravimetricanalysis(TGA),cyclic voltammetry (CV), and galvanostatic charge/discharge test wereused to investigate the properties of as-synthesized samples. The concrete contentsand results are as follows.1. The synthesis and characterization of LiFePO4with different conditions(thermal treatment and solvent type) were investigated. LiFePO4prepared by asolvothermal method had integrity crystal structure without thermal treatment, olivestructure LiFePO4was not changed by thermal treatment. The particle size decreased,which indicated that thermal treatment could refine the grain size and produce close packing among the particles. LiFePO4synthesized in ethylene glycol/water(EG/W,1:1) had a more fine particle size and were aggregated to form spindle-like structure,which displayed that the introduction of EG reduced the dielectric coefficient of thesolution and the zeta potential of the particles, and was absorbed on the surface of theparticles resulting in hindering the growing of the particles, which was inclined toaccumulate into spindle secondary particles; The initial discharge specific capacity at0.1C of LiFePO4-HT and LiFePO4-ST were111mAh·g-1and124mAh·g-1, the laterhad more flat voltage plateau, smaller polarization potential and more excellent rateperformances, the main reason was that LiFePO4synthesized in EG/W system had amore fine particle size, higher crystallinity and the spindle-like structure, greatlyreduced the degree of polarization in the discharge-charge process, and facilitated toenhance the rate performance and cycle stability for LiFePO4.2. Combined with the above reflected advantages, Ag-G composite, selected asa modification material, was prepared by chemical reduction and freeze-dryingmethod, which was introduced to synthesize spindle-like LiFePO4/Ag-G composite,and a series of tests were carried out. The results indicated that the introduction ofAg-G did not change the olivine crystal structure, LiFePO4/Ag-G cathode materialprepared by solvothermal method had outstanding electrochemistry capacity and rateperformances， the process of lithium ion intercalation and deintercalation forLiFePO4/Ag-G cathode material was in diffusion control, the initial dischargecapacity was156mAh·g-1at the rate of0.1C and116mAh·g-1at5C, respectively.3. The synthesis, structure and electrochemistry properties of LiFePO4/G andLiFePO4/C with different carbon sources provided by L-AA and GO via aco-precipitation-calcination method were investigated. The results indicated that theintroduction of graphene sheets and carbon particles did not change the olivine crystalstructure, graphene sheets were more effective than carbon particles on deriving fineand uniform particles，which showed the excellent electrochemistry performances ofLiFePO4/G compared with LiFePO4/C, and the initial discharge capacity ofLiFePO4/G at0.1C was134mAh·g-1; After20cycles, the capacity retention was98.8%of the initial discharge capacity.