Mind the Solid Side: Electrochemical Thermodynamics, Kinetics, and Hysteresis in Energy Materials

Bulk electrochemical phase transformations (EPTs) govern the operation of many modern electrochemical systems, from batteries to electrocatalysts. These EPTs involve a coupled interplay of ion transport from a liquid into an electrode film, electron transfer, and phase transformations in the film. Yet, despite their importance, a unified thermodynamic and kinetic understanding of EPTs remains lacking. Grounded in the theory of phase transitions and mixtures, this mini‐review introduces, for the first time, a general thermodynamic framework that classifies EPTs into three main categories: regular solutions, Fickian diffusion, and spinodal decomposition. It highlights how galvanostatic charge–discharge and cyclic voltammetry modeling can elucidate reaction mechanisms using prototypical examples from electrochemical ion insertion systems such as Ni(OH) 2 , MnO 2 , and LiFePO 4 . The concepts presented here provide a unifying foundation for interpreting solid‐to‐solid EPTs across mesoscopic and macroscopic scales and inspire new strategies for diagnosing and designing high‐performance energy materials.