Coating-dependent electrode-electrolyte interface for Ni-rich positive electrodes in Li-ion batteries

Surface chemistry modification of positive electrodes has been used widely to decrease capacity loss during Li-ion battery cycling. Recent work shows that coupled LiPF6 decomposition and carbonate dehydrogenation is enhanced by increased metal-oxygen covalency associated with increasing Ni and/or lithium de-intercalation in metal oxide electrode, which can be responsible for capacity fading of Ni-rich oxide electrodes. Here we examined the reactivity of lithium nickel, manganese, cobalt oxide (LiNi0.6Mn0.2Co0.2O2, NMC622) modified by coating of Al2O3, Nb2O5 and TiO2 with a 1 M LiPF6 carbonate-based electrolyte. Cycling measurements revealed that Al2O3-coated NMC622 showed the least capacity loss during cycling to 4.6 VLi compared to Nb2O5-, TiO2- coated and uncoated NMC622, which was in agreement with smallest electrode impedance growth during cycling from electrochemical impedance spectroscopy (EIS). Ex-situ infrared spectroscopy of charged Nb2O5- and TiO2-coated NMC622 pellets (without carbon nor binder) revealed blue peak shifts of 10 cm−1, indicative of dehydrogenation of ethylene carbonate (EC), but not for Al2O3-coated NMC622. X-ray Photoelectron Spectroscopy (XPS) of charged TiO2-coated NMC622 electrodes (carbon-free and binder-free) showed greater salt decomposition with the formation of lithium-nickel-titanium oxyfluoride species, which was in agreement with ex-situ infrared spectroscopy showing greater blue shifts of P-F peaks with increased charged voltages, indicative of species with less F-coordination than salt PF6− anion on the electrode surface. Greater salt decomposition was coupled with the increasing dehydrogenation of EC with higher coating content on the surface. This work shows that Al2O3 coating on NMC622 is the most effective in reducing carbonate dehydrogenation and accompanied salt decomposition and rendering minimum capacity loss relative to TiO2 and Nb2O5 coating.