Editors' Choice—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 LiPF 6 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 (LiNi 0.6 Mn 0.2 Co 0.2 O 2 , NMC622) modified by coating of Al 2 O 3 , Nb 2 O 5 and TiO 2 with a 1 M LiPF 6 carbonate-based electrolyte. Cycling measurements revealed that Al 2 O 3 -coated NMC622 showed the least capacity loss during cycling to 4.6 V Li compared to Nb 2 O 5 -, TiO 2 - 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 Nb 2 O 5 - and TiO 2 -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 Al 2 O 3 -coated NMC622. X-ray Photoelectron Spectroscopy (XPS) of charged TiO 2 -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 PF 6 − 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 Al 2 O 3 coating on NMC622 is the most effective in reducing carbonate dehydrogenation and accompanied salt decomposition and rendering minimum capacity loss relative to TiO 2 and Nb 2 O 5 coating.