Enhanced Electronchemical Performance Of Fe-based Poly Anionic Cathode Materials For Lithium-ion Batteries

Polyanionic cathode materials has been recognized as one of the most promising candidate for Li+ion batteries because of its high specific theoretical capacity, high cycling stability as well as high safety. However, the intrinsic low electronic conductivity and poor ionic mobility of polyanion compounds have become major obstacles to improve the electrochemical properties, resulting in the poor rate capability. To enhance the conductivity of polyanion compounds, several effective ways have been proposed, such as conductive surface modification, particle size reduction, supervalent cation doping and so on.In this work, the effects of different carbon addition method, different carbon content and different carbon sourses on the electrochemical performances of LiFePO4cathode material、the effects of Ge film deposition on the LiFePO4/C electrode, and the effects of different carbon content on the structure and electrochemical performances of Li2FeSiO4/C composites were investigated. The main works are presented as follows:Firstly, via solid reaction, LiFePO4/C cathode materials were successfully synthesized. The effects of different carbon addition method, different carbon content and different carbon sourses on the electrochemical performance of LiFePO4cathode material were systematically compared and evaluated by XRD、SEM and Raman methods. As a result, an optimized synthesis condition was summarized to guidance the followed studies.Secondly, LiFePO4/C-Ge electrodes were prepared with vacuum thermal evaporation deposition by depositing Ge films on as-prepared LiFePO4/C electrodes. The effect of Ge film on the electrochemical performances of LiFePO4/C cells was valued systematically by charge/discharge testing, cyclic voltammograms and AC impedance spectroscopy, respectively. It is found that Ge-film-surface modified LiFePO4/C presents excellent electrochemical performance compared to that of the pristine one in terms of cyclability and rate capability. At60℃, LiFePO4/C-Ge film exhibits outstanding cyclability with less than5%capacity fade after50cycles while the pristine one suffers15%. Analysis from the electrochemical measurements shows that the presence of Ge film on the LiFePo4/C electrode would protect active material from HF generated by the decomposition of LiPF6in the electrolyte and stabilizes the surface structure of active material during the charge and discharge cycle. Electrochemical impedance spectroscopy (EIS) results indicate that Ge film mainly reduces the charge transfer resistance Rct of LiFePO4/C electrode, resulting from the suppression of the solid electrolyte interfacial (SEI) film.Lastly, Li2FeSiO4/C composites were synthesized by a simple sol-gel method using tartaric acid as carbon source and the structure of the composite were investigated systematically by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectra and X-ray photoelectron spectroscopy (XPS), respectively. Comparing to the pristine Li2FeSiO4, Li2FeSiO4/C composites exhibit superior electrochemical performances in terms of capacity, cyclability and rate capability as well. Li2FeSiO4/C with12.8wt.%carbon shows a high capacity of220mAhg1at C/16(1C=160mA g-1).Analysis from the electrochemical measurements and XPS results indicate that the presence of Fe4+during the charge/discharge process and more than one electron reaction should be responsible for the obtained results above. In addition, the electronic and electrochemical results also reveal that the improved performances with carbon additive could be mainly attributed to enhanced conductivity, lower charge-transfer resistances, smaller particle size and higher lithium diffusion rate. Moreover, the capacity fading observed at high carbon content would be ascribed to the increasing charge-transfer resistance and ohmic resistance of the inter-particle because of the smaller particle size.