Effects of Alkyl Phosphonate Surface Coatings on High-Voltage LiMnxNiyCo1-x-YO2 (NMC) Cathode Performance

Abstract

We describe the effects of alkyl phosphonate surface coatings applied to LiMnxNiyCo1-x-yO2 (NMC) cathode materials on battery performance. A homologous series of alkyl phosphonic acids with hydrocarbon chains ranging from ethyl (C2) to octadecyl (C18) were solution grafted onto the surface of NMC cathode particles. For each phosphonic acid, we observed comparable grafting densities onto the NMC surface except for the hexyl phosphonic acid as demonstrated by XPS analyses. Analyses of alkyl phosphonate coatings using diffuse reflectance infrared fourier transform spectroscopy (DRIFTS), showed that crystallinity of the coating layer increases with alkyl chain length. We fabricated cathodes comprising NMC coated with various alkyl phosphonates, incorporated them into half-cells, and assessed their electrochemical performance by subjecting them to 100 charge/discharge cycles and to charge/discharge rate performance tests. Electrochemical impedance spectroscopy (EIS) analyses of alkyl phosphonate-coated cathodes before and after the rate tests revealed that the long chain alkyl phosphonate coatings mitigate electrolyte degradation (1 M LiPF6 in ethylene carbonate/dimethyl carbonate) at the electrode surface. However, cathodes with long chain alkylphosphonate coatings exhibit poor high C-rate performance as compared to the shorter alkylphosphonate coatings. These findings are rationalized in terms of the crystallinities of the different coating layers.