Surface modification and carbon coating effect on a high-performance K and S doped LiMn2O4

Abstract

Potassium- and sulphur-doped spinel Li0.99K0.01Mn2O3.99S0.01 (LKMOS) is a high-performance cathode material for lithium-ion batteries. Its synthesis involves facile, efficient and eco-friendly sol–gel procedure, which yields in the uniform nanocrystalline material. In the present work, LKMOS has been successfully modified through a pyrolysis process of a water-soluble polymeric precursor – based on poly(N-vinylformamide) – to obtain spinel with modified surface and carbon coating (mLKMOS). Along with the fabrication of the C-coating nanomaterial undergoes a specific surface change. The combination of these two effects results in the creation of material with greatly increased rate capability under extreme charging currents and improved performance in long-term charge–discharge tests at various temperatures. Pristine and modified materials are characterized by X-ray diffraction, thermogravimetric analysis, elemental analysis and X-ray photoelectron spectroscopy. The electrochemical performance of the materials is examined by galvanostatic charge–discharge tests, electrochemical impedance spectroscopy and cyclic voltammetry. Surface modified nanomaterial-based cathodes exhibit very high rate capability, maintaining up to 75% of the initial 1C discharge capacity at 50C rate and 65% at 100C rate (1C – 148 mAg−1 – current required to provide the theoretical capacity charge of the electrode material within one hour). Electrochemical impedance tests have shown that modifications diminish internal charge-transfer and electronic resistances of the nanomaterial. Moreover, Li+ ion apparent diffusion coefficient in the modified sample is about twofold increased.