Research On Synthesis And Modification Of Li₂FeSiO₄/C Cathode Material For Lithium Ion Batteries

With the advantage of environmental friendly, abundant resource, low cost, highspecific capacity, good cycle performance and high safety, the novel polyanioncompound Li₂FeSiO₄has been considered to have good research value and applicationprospect. But the poor conductivity has restricted its further development andapplication.For dealing with these two problems, Li₂FeSiO₄cathode material was studied fromthree aspects--synthesis method and process, doping and process, surface coating viamultiple electrochemical test, XRD and SEM. The main content is as follows:1. Li₂FeSiO₄/C was prepared via a solid-state reaction. Many influences on thefinal samples’ performances were investigated, such as the raw material pretreatmentmethod, the temperature and time of pretreatment, and the heating rate. A betterpretreatment method and process were obtained.2. Al-doping Li₂Fe_1-xAl_xSiO₄/C was prepared by solid-state reaction. The effect ofx value on the morphology and electrochemical performance of Li₂Fe_1-xAl_xSiO₄/C wasstudied by SEM and electrochemical test. The results revealed that the right amount ofAl-doping optimized the morphology and improved the electrochemical performance ofLi₂FeSiO₄. Li₂Fe0.97Al0.03SiO₄showed the best comprehensive performance. Its initialdischarge capacity was118.1mAh/g, and the capacity retention ratio after20cycles was93.6%.3. With the combination of liquid pretreatment and solid-state reaction, V-dopingLi₂Fe_1-xV_xSiO₄/C was synthesized. The effects of the selection of raw material, dopingamount, sintering temperature and sintering time on the performances ofLi₂Fe_1-xV_xSiO₄/C were systematically researched. The results suggested that V-dopingdidn’t change the basic structure of Li₂FeSiO₄/C material, but did have an influence onthe crystal growth direction. The appropriate amount of V-doping could improve theperformance of Li₂FeSiO₄/C. Taking Li₂CO₃as lithium source, FeC₂O₄2H₂O as theiron source, the optimized Li₂Fe_0.5V_0.5SiO₄/C was synthesized at650℃for8h, whichhad a better electrochemical performance. 4. Under the optimized contidions, the effect of different carbon sources on theproperties of Li₂Fe_0.5V_0.5SiO₄/C material. The carbon sources included sucrose, glucoseand multiwall carbon nano-tubes （MWCNT）. The result showed that the sample whichas sucrose as carbon source had a better morphology, the sample which carbon sourcewas glucose had a higher initial discharge capacity but a worse cycle performance andthe sample with MWCNT had a lower initial discharge capacity and a bad cycleperformance. On this basis, sucrose and MWCNT were used as composite carbonsource to synthesize Li₂Fe_0.5V_0.5SiO₄/C/CNT. The result revealed that theelectrochemical performances of this sample were better than the samples’ with a singlecarbon source, and it also had a good rate property. The composite carbon source had abetter effect on improving the conductivity.