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 (LiNi_xMn_yCo_1-x-yO ₂with 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 LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) and LiNi_0.8Co_0.15Al_0.05O₂ (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 reduced at the surface of the uncoated cathode after cycling, whereas the coated cathode showed no changes in surface oxidation states relative to the bulk. The surface coatings also prevented transition metal dissolution and crossover. XPS analysis of the anode harvested from cycled cells with uncoated cathodes showed significant amounts of Mn deposited within the SEI. In contrast, no Mn could be detected on the anodes cycled with coated cathodes. These results affirm that ALD coatings can effectively reduce the reactivity of the NMC surface and prevent detrimental side reactions that shorten battery cycle life.