Experimental Evaluation of Influence of Stress on Li Chemical Potential and Phase Equilibrium in Two-phase Battery Electrode Materials

Abstract

We experimentally evaluated the influence of stress on the Li chemical potential (μ_Li) and phase equilibrium in the two-phase battery electrode materials through the emf measurements while applying a mechanical load. In our measurements, we prepared an electrochemical cell by depositing a thin film of a two-phase electrode material (LiFePO₄ or LiCoO₂ in the two-phase region) on each of the solid electrolyte surfaces. Then we applied a mechanical load to the electrochemical cell through four-point bending, and the resulting μ_Li variation in the electrode material was measured as the emf between the two thin films. Our results indicated that μ_Li in the two-phase electrode materials immediately changed just after loading and then gradually changed while maintaining a constant mechanical load. Besides, the loading and unloading led to the μ_Li variation in the opposite direction. Such characteristic μ_Li variations could be explained by considering the change in the phase equilibrium between the two phases, which led to the Li content variation in the two phases and the stress relaxation due to the volume fraction variation of the two phases. Our results can provide valuable insights regarding the influence of stress on the performances of energy storage devices with two-phase electrode materials.