Quantitative <i>Operando</i> EPR Method on Redox-Active Ions in Electrode Materials: Considering the Conversion-Type Reactions as Examples

Electron paramagnetic resonance (EPR), a sensitive probe for unpaired electron spins, has been widely used in the investigation of various electrochemical systems. Despite its success in studying electrochemical processes involving radicals or other species with narrow line widths, interpreting EPR results for systems with broad featureless signals, particularly ions in solid materials, remains challenging. In this work, we develop a quantitative <i>operando</i> EPR method to study redox-active ions in electrode materials. The relationship between the variation in absolute spin amounts and electron transfer amounts is established to assign the signals. Four conversion-type electrode materials, MnO, MnF₂, Mn₂O₃, and MnF₃, are used as typical representatives of Kramers and non-Kramers ions to exemplify this method. The results accurately reveal the electrode processes, including redox mechanisms, local structural variations, and transition metal dissolution. It offers a new strategy for properly interpreting the broad featureless signals from ions in electrode materials, paving the way for understanding electrochemical processes in critical systems such as cathode materials.