Unveiling the Role of Al2O3 in Preventing Surface Reconstruction During High-Voltage Cycling of Lithium-Ion Batteries

Recent achievements in high energy batteries have been made by using Ni-rich NMC cathodes (LiNixMnyCo1-x-yO2 with x > 0.5) in conjunction with higher cell voltages. However, these gains have come at a cost of fast capacity fade and poor rate performace. In our previous study, we showed that Al2O3 ALD coatings on LiNi0.8Mn0.1Co0.1O2 (NMC811) and LiNi0.8Co0.15Al0.05O2 (NCA) cathodes prevented surface phase transitions, reduced impedance, and extended cycle life in high voltage cells. Here, neutron diffraction (ND), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) are used to fully investigate the mechanism by which ALD surface coatings mitigate NMC811 cathode degredation. Refinement of ND patterns indicated no changes in the bulk crystal structure of cycled cathodes with or without the Al2O3 coating. Rather, the improved performance of ALD-coated cathodes is clearly due to surface stabilization. EELS established that all three transition metal oxidation states were reduce...