Electrochemical passivation of LiFePO4 cathodes by polaron crystallization during cell cycling

The nature of intermediate phases during LiFePO 4 cycling and their role in limiting lithium transport remain unresolved. Building on our pioneering application of operando 57 Fe nuclear resonance vibrational spectroscopy to working batteries, we combine this element-specific probe with machine-learning-based spectral analysis to resolve and quantify a vibrational signature of an intermediate component distinct from the LiFePO 4 and FePO 4 end-members. Its maximum molar fraction reaches nearly 20% at cycling rates of C/10, C/5 and C/2, revealing a rateindependent formation of intermediate phase. The intermediate exhibits additional distinct feature in the Fe-projected phonon density of states, suggesting localized Fe 3+ hole-polaron environments, which is supported by atomistic simulations. These observations motivate a working hypothesis linking the persistence of the intermediate to restricted lithium transport. We propose that local accumulation of hole polarons along with elastic stress at phase interface promotes their crystallization into an ordered superstructure at the advancing phase boundary. Ordering restricts polaron motion and through lithium ion–polaron coupling reduces lithium-ion mobility across the intermediate layer. This layer could therefore to certain extent passivate individual particles against further delithiation, allowing transformation to proceed elsewhere in the electrode. The proposed mechanism offers an explanation for the nearly rate-independent intermediate phase fraction and identifies polaron ordering as a potential target for improving high-rate performance, for example through dopants that inhibit polaron crystallization.