Synthesis And Modification Study Of Li₂FeSiO₄Cathode Material For Lithium Ion Batteries By Sol-gel Method

The Li₂FeSiO₄has been considered as one of the most promising cathode materials forlithium ion batteries because of its high safety, environmental non-toxicity, low cost andabundant resources. However, Li₂FeSiO₄has low electronic conductivity and low lithium iondiffusion rate, which greatly limit the application of such material. In order to solve aboveproblems, in this paper, the crystal structure, electrochemical properties and electrode kineticsof this material were investigated by X-ray powder diffraction（XRD）, Scanning electronmicroscope （SEM）, Infrared spectroscopy（FTIR）, Cyclic voltammetry（CV）, Electrochemicalimpedance spectroscopy（EIS） and charge-discharge tests. Some measures such asoptimization of the preparation conditions, coating and doping modification were taken toimprove the discharge and cycling performance.The citric acid assisted sol-gel method was employed to synthesize Li₂FeSiO₄cathodematerial. The precursor was characterized by TG-DSC, and the structure and electrochemicalperformance of calcined Li₂FeSiO₄were investigated by XRD, SEM, grain size analysis andelectrochemical methods. The preparation conditions were optimized by orthogonalexperiments and the results suggested the Li₂FeSiO₄prepared at700℃for10h and a Li/Simolar ratio of2.04:1as the optimum condition.Li₂FeSiO₄/C composites were synthesized by sol-gel method with citric acid, sucrose,oxalic acid and polyethyleneglycol as carbon source. Effect of carbon source and carboncontent on crystal structure, microstructure and electrochemical performance has been studiedby XRD, SEM and electrochemical methods. The experimental results showed the bestcarbon source was citric acid. Using citric acid as carbon source, the grain size and tap densityof Li₂FeSiO₄decreased with increasing carbon content, however, the conductivity graduallyincreased to2.46×10^-5S·cm^-1.The sample with10%carbon exhibited the best performance,with an initial discharge specific capacity of98mAh·g^-1between2.0⁴.5V at C/10rate and acapacity retention ratio of93.88%after15cycles.The Li₂FeSiO₄was modified by body-doping, Li₂FeSiO₄material doped with Ni wasprepared via a sol-gel method. The effect of Ni content on crystal structure, microstructureand electrochemical performance has been investigated by XRD, SEM and electrochemical methods. The results suggested Li₂Fe_0.95Ni_0.05SiO₄has the highest discharge capacity and thebest capacity retention ratio. Lithium-ion diffusion coefficient of doped Li₂FeSiO₄rised by anorder of magnitude, which suggested the doping can effectively improve the Lithium-iondiffusion coefficient of the material. The conductivity of the undoped and doped Li₂FeSiO₄are8.45×10-6and9.58×10-6S·cm^-1respectively, showing that the Ni doping does not playvital role in improving the conductivity of Li₂FeSiO₄.