Carbon coating of air-sensitive insulating transition metal fluorides: An example study on α-Li3FeF6 high-performance cathode for lithium C ion batteries

Li3FeF6 has been the focus of research of fluorine-based cathode materials for lithium-ion batteries. Because of the low electronic conductivity of Li3FeF6, the decrease of particle size, by an energy-consuming long-time ball milling process with carbon, is necessary to achieve a high electrochemical performance. The most successful method to enhance electrochemical activity, carbon coating, seemed to be impracticable, so far, for sensitive fluorides like Li3FeF6. In this work, carbon coating on Li3FeF6 particles has been successfully achieved for the first time, while avoiding both extended hydrolysis and Fe(III)-Fe(II) reduction. The heat treatment and atmosphere, yielding the maximal transformation of organic carbon to both graphitised and disordered carbon, has been determined. Carbon coating, with a thickness of approximately 2.5 nm, has been achieved by controlled thermal decomposition of glucose, under air, at 300 degrees C. Raman and X-ray photoelectron spectroscopy (XPS) experiments have proved the existence of carbon and Fe2O3 on the surface of Li3FeF6 nanoparticles. XPS spectroscopy indicates the presence of organic residues from glucose decomposition. Attempts to further reduce the organic carbon content results in a decrease of the amorphous carbon coating layer. Optimised carbon-coated Li3FeF6 nanoparticles deliver 122 mAh g(-1) (85% of theoretical capacity) significantly higher than that of a noncoated sample (58 mA h g(-1)). Even more, a significant beneficial effect of carbon coating on both capacity retention and coulombic efficiency is observed. (C) 2019 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.