Reaction Heterogeneity in LiFePO4 Agglomerates and the Role of Intercalation-Induced Stress

As an important battery cathode material, reaction distribution inlithium iron phosphate (LiFePO4) has been extensively studied in dispersedparticle systems, but remains poorly understood for mesoscopic agglomerates (orsecondary particles) that are used in most commercial batteries. Herein, we applythree-dimensional X-ray spectroscopic imaging to characterize the two-phasestructure in LiFePO4secondary particles during electrochemical cycling.(De)lithiated domains are found to not form the commonly assumed core-shell structure but develop highly anisotropicfilamentary morphology that is rateindependent and symmetric between charging and discharging. Phase-fieldsimulations elucidate that the observed 1D phase growth behavior is not causedby the 1D lithium diffusivity of LiFePO4but the elastic interaction between primary particles, which gives rise to strongerreaction heterogeneity than dispersed nanoparticles. As a result, uniform lithium (de)intercalation does not occur on thesecondary particle surface even at high cycling rates.