Surface chemistry of metal oxide coated lithium manganese nickel oxide thin film cathodes studied by XPS

The effect of coating the high voltage spinel cathode LiMn _1.5Ni_0.5O₄ with three metal oxide thin layers is discussed. Instead of the typical powder electrodes with poorly defined surface coatings, thin film electrodes were prepared with well-defined oxide coating thicknesses to investigate the influence of coating on surface reactivity via X-ray photoelectron spectroscopy (XPS). ZnO is found to decompose during the first charge whereas Al₂O₃ and ZrO₂ are stable for more than 100 cycles. ZrO₂, however, importantly limits the available Li storage capacity of the electrochemical reaction due to poorer kinetics. Al₂O₃ offers the best results in term of capacity retention. Upon cycling, the evidence of a signal at 75.4 eV in the Al2p binding energy spectrum indicates the partial fluorination of Al ₂O₃ into, perhaps, Al₂O₂F ₂. Moreover, the continuous formation of ethers, esters and Li _xPO_yF_z compounds on the surface of the electrodes is found for all coating materials.