Overlooked issues on oxidation state analysis in electrode materials by X-ray photoelectron spectroscopy

The growing demand for advanced electrochemical storage devices has driven the development of high-performance electrode materials with high energy and power densities. While X-ray photoelectron spectroscopy (XPS) is employed as a powerful tool to characterize electrode materials by analyzing the oxidation and bonding states of transition metals, its high surface sensitivity necessitates ion sputtering (Ar⁺ sputtering is often used) for surface removal. However, Ar⁺ sputtering can cause severe chemical damage, leading to misinterpretation of the electrode surface. This study investigates the impact of Ar⁺ sputtering conditions on the analysis of the oxidation states of various materials used as positive and negative electrodes. It demonstrates how improper Ar⁺ sputtering alters surface composition and oxidation states and potentially misleads analyses of the redox mechanisms involved in electrochemical processes. Furthermore, strategies to mitigate sputtering-induced artifacts are suggested to ensure a more accurate interpretation of XPS data in materials research for secondary batteries.