| Summary: | 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 (LiNi[subscript 0.6]Mn[subscript 0.2]Co[subscript 0.2]O[subscript 2], NMC622) modified by coating of Al[subscript 2]O[subscript 3], Nb[subscript 2]O[subscript 5] and TiO[subscript 2] with a 1 M LiPF[subscript 6] carbonate-based electrolyte. Cycling measurements revealed that Al[subscript 2]O[subscript 3]-coated NMC622 showed the least capacity loss during cycling to 4.6 VLi compared to Nb[subscript 2]O[subscript 5]-, TiO[subscript 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[subscript 2]O[subscript 5]- and TiO[subscript 2]-coated NMC622 pellets (without carbon nor binder) revealed blue peak shifts of 10 cm[superscript −1], indicative of dehydrogenation of ethylene carbonate (EC), but not for Al[subscript 2]O[subscript 3]-coated NMC622. X-ray Photoelectron Spectroscopy (XPS) of charged TiO[subscript 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[subscript 6][superscript −] 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[subscript 2]O[subscript 3] coating on NMC622 is the most effective in reducing carbonate dehydrogenation and accompanied salt decomposition and rendering minimum capacity loss relative to TiO[subscript 2] and Nb[subscript 2]O[subscript 5] coating.
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