Designing Surface Coating Strategies with Tungsten on Single Crystal NMC Materials by XPS

Abstract Medium‐nickel layered oxide materials are a promising positive electrode material for high energy density lithium ion batteries. These materials can suffer from surface instability at a highly de‐lithiated state and poor rate capability. Surface coatings with multiple elements are a common strategy to overcome some of these challenges, however, the rationale behind adding these elements has never been explored. Here, various mid‐nickel single crystal vendor materials are evaluated by X‐ray Photoelectron Spectroscopy (XPS) and identified various surface coating compounds that can form during synthesis. Among the many common coating elements this study primarily focuses on tungsten (W). Using the in‐house “all dry synthesis” method, single crystal Li_1+x(Ni_0.6Mn_0.3Co_0.1)_1‐xO₂ materials are developed, where W is added at different stages of the synthesis to generate some unique W‐based surface compounds that match with vendor materials. This study systematically evaluates different W‐based surface compounds, compares their overall effect on electrochemical performance and highlights the instability of some of them. This work demonstrates that W is an important element and can significantly improve electrochemical performance when added in a second heating step.