Mechanical Signals Revealing Spatial Heterogeneity in Lithium Iron Phosphate Electrodes

Abstract Phase-separating electrodes exhibit strong particle-scale heterogeneity; however, electrode-scale reaction-site evolution remains difficult to resolve experimentally. Here, we use in situ stress measurements to probe the spatiotemporal reaction dynamics of lithium iron phosphate electrodes. Systematic nonlinear deviations in the stress response reveal intrinsic spatial heterogeneity in reaction activity. By integrating a domino-cascade phase-transformation mechanism with a coupled electrochemical–mechanical framework, we show that these nonlinear features arise from spatial attenuation of mechanical signal transmission across particle ensembles. For a thin electrode of 37.55 μm, the central strain-transfer efficiency is only 61.2% of that at the edges. Based on this, we reconstruct reaction-site evolution and show that delithiation proceeds as a reaction front propagating from the electrode surface toward the current collector. In thicker electrodes, this effect intensifies, producing stronger nonlinearity and further reducing central strain-transfer efficiency. These results demonstrate that mechanical response signals can capture electrode-scale spatiotemporal reaction dynamics.