Nickel-Dependent Interfacial Chemistry in Layered Oxide Cathodes: Insights from X-ray Photoelectron Spectroscopy and Electrochemical Quartz Crystal Microbalance

Abstract

NMC (LiNi_xCo_yMn_zO₂; x + y + z ≈ 1) cathodes are widely used in Li-ion batteries for long-range vehicle applications. The Ni content in NMC cathodes directly affects practical capacity and, therefore, the energy densities of Li-ion batteries. Ni-rich NMC cathodes, however, suffer a tradeoff between increased energy density and decreased cycle life. In this study, we investigate the tradeoff between capacity and cycle life by thoroughly characterizing the interfacial chemistry of two NMC materials, LiNi_0.6Co_0.2Mn_0.2O₂ (NMC622) and LiNi_0.90Co_0.05Mn_0.05O₂ (NMC9055), in an attempt to understand the impact of Ni content on surface properties and cell performance. In situ electrochemical quartz crystal microbalance (EQCM) measurements were employed on NMC622 and NMC9055 along with LiNi_0.8Co_0.2Mn_0.2O₂(NMC811) as an additional benchmark. Combined with X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), we detected the severe interference of side reactions with lithium re-insertion on NMC9055 discharge compared to NMC622. These experiments indicate that, for lower nickel content NMC622, these reactions are largely constrained to the very surface and reversible in nature, while, for higher nickel content NMC9055, parasitic reactions penetrate deeper into the bulk and contribute to capacity fade.