Surface Structure Construction Of LiEePO₄Cathode Material And Their Electrochemical Performances

The lithium ion power battery has become the research hotspot in recent years with the emergency of energy crisis and environmental crisis. LiFePO4has received much attention as a promising storage compound for cathodes in lithium-ion batteries due to it has high specific capacity, good cycle performance, safety, low costs and environmental friendliness. In this paper, the LiFePO4/C, metallic oxide modified LiFePO4/C, metallic oxide and carbon co-coated LiFePO4and symbiotic compounds modified LiFePO4/C cathode materials were synthesized by in-situ solid phase synthetic method and two step method with the cheap chemistry reagents as raw materials. The phase composition and structure of cathode materials were characterized and analyzed by XRD、Raman、 SEM、 TEM, EDS and XPS. The electrochemical performances of cathode materials were tested and studied by charge-discharge technology, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The main results are as follows:1. The effects of carbon source, iron source and calcination method on the electrochemical performances of LiFePO4/C cathode materials. The LiFePO4/C cathode materials were synthesized by in-situ solid phase synthetic method with Fe2C2O4and Fe2O3as iron source, glucose, sucrose, citric acid, polyethylene glycol4000, starch and hexanedioic acid as carbon source, Li2CO3as lithium source and NH4H2PO4as phosphorous source, respectively.(1) There were the obvious effects of carbon sources on carbon structure, particle size and shape of LiFePO4/C cathode materials, the samples synthesized with glucose and sucrose as carbon sources had uniform particle size, well dispersibility and higher graphitization degree of carbon.(2) The cathode materials synthesized with Fe2C2C2O4as iron source and glucose as carbon sources had smaller particle size, well dispersibility and regular shape.(3) With calcination temperature increasing and time prolonging, the crystallinity of LiFePO4increased, particles size become gradually uniform and dispersion become gradually good. When calcination temperature was increased over800℃, the increasing of particles size was significant with the temperature change, and the new phase of Fe2P generated.2. Metallic oxide modified LiFePO4/C cathode materials. ZnO modified LiFePO4/C, W02modified LiFePO4/C, MoO2modified LiFePO4/C and MoO2modified LiFePO4cathode materials were synthesized by in-situ solid phase synthetic method with Fe2C2O4(or Fe2O3), Li2CO3, NH4H2PO4, ZnCO3,(NH4)10W12O41·xH2O,(NH4)6Mo7O24-4H2O and glucose as raw materials. In the synthesis process, the conducting medium composed by metal oxide and carbon on the LiFePO4particles surface and in gap of particles would improve the electrochemical performances of cathode materials, which were formed by the decomposition of metal oxide precursors and carbon sources.(1) The existence of ZnO could improve the charge-discharge specific capacity of cathode materials, but the improvement was very low because of lower electronic conductivity of ZnO.(2) The adding of WO2and MoO2with high electronic conductivity could obvious improve electrochemical performances of cathode materials. The cathode materials with5wt%conductive metal oxides exhibited the best electrochemical performance; they had the maximum value of initial discharge specific capacity of120.1mAh/g and142.6mAh/g at0.1C at room temperature, respectively, the maximum apparent lithium ion diffusion coefficient and the minimum charge transfer resistance. However, because of WO2and MoO2lower crystallinity, partially oxidized of surface and partially dissolved by electrolyte, the cycle performance of cathode materials is poor at different charge-discharge rate.(3) MoO2modified LiFePO4cathode materials without carbon had better charge-discharge performance, when the MoO2content is5wt%, the sample had higher initial discharge specific capacity of76.6mAh/g at0.1C at room temperature, compared with the sample without MoO2, the discharge specific capacity rose59.8mAh/g.3. Metallic oxide and carbon co-coated LiFePO4cathode materials. ZnO and carbon co-coated LiFePO4, WO2and carbon co-coated LiFePO4and MoO2and carbon co-coated LiFePO4cathode materials were synthesized by two step method. In LiFePO4/C, the small amounts of carbon can not form a full coating layer on the surface of LiFePO4, but the adding of metal oxides could repair the bare surface of LiFePO4particles, make the coating layer more full and continuous and improve electrochemical performances of cathode materials.(1) The right amount of ZnO coating could improve charge-discharge specific capacity and apparent lithium ion diffusion coefficient, and decrease charge transfer resistance, and the sample with3wt%ZnO has better cycle performance in different charge-discharge rate.(2) Relative to ZnO coating, the WO2and MoO2coating could improve electrochemical performances even more, including higher charge-discharge specific capacity, higher apparent lithium ion diffusion coefficient and lower charge transfer resistance and good cycle performance. The discharge specific capacity of WO2and carbon co-coated LiFePO4cathode materials are153.7,144.5,137.5and121mAh/g at0.1,0.2,0.5and1.0C after110-cycle, respectively, The discharge specific capacity of MoO2and carbon co-coated LiFePO4cathode materials are157,136.16,131.3and119.1mAh/g at0.1,0.2,0.5and1.0C after110-cycle, respectively. It indicated the positive effects of conductive metallic oxide on electrochemical performances of cathode materials.4. Symbiotic compounds modified LiFePO4/C cathode materials. Symbiotic compounds (Li3PO4, Li4P2O7and Fe2P2O7) modified LiFePO4/C and FeWO4modified LiFePO4/C cathode materials were synthesized by in-situ solid phase synthetic method with Fe2C2O4, Li2CO3, NH4H2PO4,(NH4)10W12O41·xH2O and glucose as raw materials.(1) The FeWO4adding did not contribute to the improved electrochemical performances of cathode materials.(2) The symbiotic compound of Li3PO4adding can improve the charge-discharge performance of LiFePO4cathode materials, but the improvements were very little because of lower lithium ionic conductivity of Li3PO4.(3) The existence of Li4P2O7as fast ionic conductor can improve electrochemical performances of cathode materials, but the cycling performance was very bad.(4) Compared with the adding of Li4P2O7, the existence of Fe2P2O7can not only improve the charge-discharge performance, but also maintain good cycling performance, it could be because of higher lithium ionic conductivity or electronic conductivity of Fe2P2O7.(5) When the Fe2P2O7content is5wt%, the sample had the highest discharge specific capacity of146.7mAh/g at0.1C at room temperature, the discharge specific capacity still reached130mAh/g after110charge-discharge cycle, and the sample had the highest apparent lithium ion diffusion coefficient.