Electron Spectroscopy Study of Li[Ni,Co,Mn]O₂/Electrolyte Interface: Electronic Structure, Interface Composition, and Device Implications

In recent years, there have been significant efforts to understand the role of the electronic structure of redox active materials according to their performance and thermodynamic stability in electrochemical storage devices and to develop novel materials with higher energy density and higher power. It is generally recognized that transition metal compounds used as a positive electrode determine the specific capacity and the energy density of rechargeable batteries, while the charge transfer resistance at the electrolyte–electrode interface plays a key role in delivering the power of the electrochemical cell. In the present work, we study the stability of Li_xNi_0.2Co_0.7Mn_0.1O₂ thin films through the evolution of the occupied and unoccupied density of states as a function of the charging state of the electrode as well as the physicochemical conditions influencing the ionic transport across the electrode–electrolyte interface. A comprehensive experimental quasi in situ approach has been applied by using synchrotron X-ray photoelectron spectroscopy (SXPS) and O K-edge and Co, Ni, Mn L-edges XANES. Our experimental data demonstrate the change of the Fermi level position with Li⁺ removal and Ni²⁺ → Ni⁴⁺ and Co³⁺ → Co⁴⁺ changes of oxidation state for the charge compensation in the bulk of the material. As is evidenced by the experimentally determined energy band diagram of Li_x≤1.0Ni_0.2Co_0.7Mn_0.1O₂ vs the evolution of the Fermi level, no hole transfer to the O2p bands is observed up to a charging state of 4.8 V, which evidences the thermodynamic stability of Li_x≤1.0Ni_0.2Co_0.7Mn_0.1O₂ under high charging voltage in contrast to LiCoO₂. A very thin solid electrolyte interface layer (less than 30 Å thickness) on the Li_x≤1.0Ni_0.2Co_0.7Mn_0.1O₂ film is formed in a decomposition reaction of the electrolyte also involving the transition metal oxide. The enhanced concentration of lithium in the interface layer correlates evidently with the electron transfer to the transition metal sites changing their electronic configuration. It is concluded that Li_x≤1.0Ni_0.2Co_0.7Mn_0.1O₂ can serve as a high energy density cathode material, but the delivery of high power, which is a critical parameter for an electric vehicle, is strongly influenced by the physicochemical conditions at the solid electrolyte interface, which can suppress Li⁺ diffusion or even block the Li⁺ paths across the interface.