Operando Full‐Field Strain Reconstruction Reveals Geometry‐Driven Kinetic Desynchronization in LiFePO ₄ Pouch Cells

ABSTRACT Fast‐charging operation in lithium‐ion batteries is influenced by internal mechanical heterogeneity, while its spatiotemporal evolution under high‐rate conditions remains poorly resolved. Existing diagnostic approaches provide only surface‐level information, leaving internal mechanical heterogeneity during fast charging inaccessible. Here, we employ operando distributed optical fiber sensing to reconstruct the strain evolution within LiFePO ₄ pouch cells at millimeter‐scale resolution. The reconstructed strain evolution exhibits a clear transition in deformation behavior with increasing C‐rate. Under low‐rate cycling, the electrode stack undergoes largely synchronous deformation during lithiation and delithiation. As the charging rate increases, this uniform response breaks down, and the deformation becomes kinetically desynchronized. At 1.5 C, pronounced spatial heterogeneity emerges, characterized by strain inversion and amplification of strain gradients near constrained regions. In these regions, the deformation response reflects coupled electrochemical expansion and local thermal loading, with the effective strain–temperature sensitivity increasing from ∼0.71 to ∼2.13 µε °C ⁻¹ . The strain‐concentrated regions exhibit mechanical irreversibility, quantified by a strain asymmetry factor (η) reaching 0.588, and are spatially associated with post‐cycling microstructural evolution, as reflected by an apparent ∼6.8% residual electrode‐thickness difference. Together, these insights provide a mechanistic basis for optimizing cell‐level architectures in next‐generation fast‐charging batteries.