Lithium Ion Battery Cathode Material Li < Sub > 2 < / Sub > Fesio < Sub > 4 < / Sub > / C Synthesis And Modification Of Research

Lithium iron silicate, Li2FeSiO4, is one of the most promising positive electrode materials for next-generation lithium-ion batteries due to its low cost, environmental benignity and high theoretical capacity (166mAh g-1for one Li+exchange;332mAh g’1for two Li+exchange). However, pristine Li2FeSi04is insulating in nature with a low conductivity of around10-14S cm-1, inducing poor electrochemical performance. To overcome the intrinsic low electrical conductivity of pristine Li2FeSiO4, considerable efforts have been made by particle size reduction, carbon coating and super-valence ion doping. Therefore, two parts of work, vanadium substitution at Fe/Si sites and carbon-decorating with porous texture, have been done to modify its electrical conductivity to achieve a better electrochemical performance of Li2FeSiO4. The main points of this article include:1) Li2FeSiO4/C cathode materials doped by vanadium at Fe/Si sitesLi2FeSiO4/C composites doped by vanadium at Fe/Si sites have been prepared via a sol-gel method assisted by citric acid. Effects of Vanadium substitution at different sites on the structure of LiFeSiO4/C are examined by X-ray diffraction (XRD), X-photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). These results show that vanadium doping at Fe/Si sites have different influence on the cell parameter:the cell volume is expanded when V-doping at Si sites, while shrunk when V-doping at Fe sites; The oxidation state of vanadium doped at Fe sites is+3, whereas is+5when doped at Si sites. Electrochemical measurements show that the Li2FeSi0.9V0.1O4/C sample exhibits the best electrochemical performance with initial discharge capacity of159mAh g-1and excellent cyclability with capacity of145mAh g-1at30th cycle, which can be ascribed to higher lithium ion diffusion coefficient, however, the initial discharge of the Li1Fe0.7V0.1SiO4/C sample is only90%of the undoped Li2FeSiO4, which can be attributed to the loss of active Fe content. Anyway, we proposed a new method of VO43-substitution for SiO44-to enhance the conductivity without reduction of Fe content, improving the electrochemical performance of Li2FeSi04.2) Optimization of mesoporous Li2FeSiO4/C nanocomposite by a sol-gel method based on p123for lithium ion battery.A sol-gel method based on p123was employed for preparing mesoporous, carbon-decorated Li2FeSiO4nanocomposite cathode material by altering sinter temperature (600℃,650℃,700℃. They are labeled as MP-LFS600, MP-LFS650, MP-LFS700, respectively.) and the amount of carbon (7.03%、9.22%and11.15%. They are labeled as MP-LFS7.03, MP-LFS9.22, MP-LFS11.15, respectively.). The structure and morphology of the mesoporous materials were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N2adsorption/desorption techniques (BET). The results indicate that the obtained Li2FeSiO4materials can be all indexed to monoclinic structure, P21/n and the uniform nanometer-sized Li2FeSiO4particles of about20nm are embedded in irregular mesoporous amorphous carbon with pore size between3.5and14nm for MP-LFS600. MP-LFS9.22has the largest total specific surface area,80.01m2g-1. Electrochemical measurements show that the mesoporous material MP-LFS9.22exhibits the best electrochemical performance especial rate performance, with a high initial discharge capacity of178.4mAh g-1at1/16C and reversible discharge capacity of100mAh g"1or so at5C after100cycles. It can be ascribed to the small size of the nanocomposite to reduce the diffusion distance of Li+in Li2FeSiO4; the large specific surface area to increase the effective contact area between the nanocomposite Li2FeSiO4/C and electrolyte; carbon-decoration for improving the electronic conductivity of Li2FeSiO4; the texture with mesoporous can be infiltrated by the electrolyte in favor of Li+migration in the electrode.