Evolution of the Surface and Subsurface Chemical Environment of LixNi0.8Co0.15Al0.5O2 electrodes

Abstract

The nickel-based layered positive electrode, LiNi0.8Co0.15Al0.05O2 (NCA) has a high discharge capacity of ~ 200 mAhg-1 making it promising for potential electric vehicle applications. However, NCA displays drastic capacity fade with cycling. Accelerated cycling tests at elevated temperatures have shown that NCA electrodes undergo significant power fading. The significant power fading has been largely attributed to the formation of insulating and electrochemically inactive phases at the NCA surface. These include Li2CO3 surface species and a NiO-like rocksalt phase at particle surface due to oxygen loss. The literature of NCA surface and subsurface studies have highlighted the complexity of studying electrodes, with many reactions occurring that may not be inherent to NCA. Here we report on our studies of the surface and subsurface of NCA electrodes with and without a binder additive (i.e. PVDF) as a function of initial charging and cycling. The chemical composition and transition metal oxidation states were determined from a combination of x-ray photoelectron and absorption spectroscopy techniques. Our results reveal dramatic differences in the surface environments between our NCA electrodes with and without binder. Our results suggest that the PVDF may play a more significant role in degrading the surface of the NCA i.e. oxygen loss, than previously thought. We also report our surface analysis of our extended cycled NCA binder free electrodes. This work was supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001294.