Improved performance of high-voltage Li-ion batteries using a novel chemically activated coating process

Aluminium oxide-coated spinel LiNi_0.5Mn_1.5O₄ as cathode material was prepared by a new hydrogen-peroxide-method followed by calcination at different temperatures. The structural and electrochemical properties of these cathodes were investigated using SEM, XRD, TG, and ⁷Li MAS NMR spectroscopy and charge-discharge studies. The morphology of the coating material on the surface and the degree of coverage of the coated particles were investigated by SEM. These investigations have shown that the powder particles of the active material before and after temperature treatment were covered by a fully dense and homogenous coating. XRD reveals that the temperature treatment of the coated active material (between 300 °C and 800 °C) leads to a shift of the main diffraction reflections of the spinel phase toward lower or higher angles, consistent with an expansion or contraction of the unit cell. A clear temperature transition was observed at 700 °C, which indicates a disordered-ordered-disordered structural transition in the studied LiNi_0.5Mn_1.5O₄ spinel. This structural change influences severely the charge–discharge performances. Information about local Al and Li ions arrangements was obtained from the ²⁷Al and ⁷Li MAS NMR spectra. The ²⁷Al MAS NMR studies reveal the existence of the coating phase in the studied samples and exclude the assumption of possible Al ions diffusion into the structure. Our study also shows that coating layers will not necessarily help to achieve optimal electrochemical properties, but rather the coating method and conditions, especially temperature, can improve or restrain the capacity retention.