The positive role of (NH₄)₃AlF₆ coating on Li[Li_0.2Ni_0.2Mn_0.6]O₂ oxide as the cathode material for lithium-ion batteries

Different amounts of (NH₄)₃AlF₆ (1, 3, and 6 wt%) are successfully coated on the surface of the layered lithium-rich cathode Li[Li_0.2Ni_0.2Mn_0.6]O₂ using a wet coating method. The morphology and structure of the as-prepared materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). It is confirmed that the (NH₄)₃AlF₆ was uniformly coated onto the surface of the Li[Li_0.2Ni_0.2Mn_0.6]O₂. The electrochemical performance of the coated materials at room temperature and 50 °C is investigated systematically. The material coated with 3 wt% (NH₄)₃AlF₆ exhibits the highest reversible capacity of 220.3 mA h g^-1 (0.2C, 50 cycles) as well as the best cycling performance with a capacity retention of 83.4% (0.2C, 50 cycles), attributed to the suppression of unexpected surface side reactions by the protective layer of (NH₄)₃AlF₆. Electrochemical impedance spectroscopy (EIS) analysis reveals that the lower charge transfer resistance of the coated sample may contribute to its excellent rate capability. Furthermore, the coated sample also shows enhanced cycling performance at elevated temperature owing to an improved thermal stability, confirmed by differential scanning calorimetry (DSC).