Direct observation of the (de)lithiation process on the multi-particle LiFePO4 by <i>in situ</i> TEM

The development of nanostructured LiFePO4 (LFP) electrodes represents a prominent research direction in the Li-ion battery field, owing to its intrinsic advantages such as high theoretical capacity and excellent structural stability. Studying the electrochemical reaction mechanisms at the atomic scale by in situ TEM is essential; however, the mechanisms of ion migration on LFP have not yet been fully elucidated. We report atomic-scale in situ TEM studies of delithiation and lithiation in multi-particle LFP coupled to a Li-rich garnet (LLZNO) solid electrolyte. During delithiation, LFP converts to a metastable L0.5FP via a periodicity-doubling mechanism (every second layer) accompanied by the emergence of a solid-solution zone, and we directly observe interparticle Li+ transport that drives reversible LFP–L0.5FP–LFP cycles. Conversely, under reductive bias, lithiation proceeds by an interface-dominated crystalline–amorphous transformation, identifying amorphization as a primary particle-level failure pathway. Tracking the structural evolution of LiFePO4 at the atomic scale during (de)lithiation provides key insights into its kinetic limitations and phase stability, which is essential for optimizing its electrochemical performance.