Uncovering coupled ionic-polaronic dynamics and interfacial enhancement in Li$_x$FePO$_4$

TL;DR

This study develops a machine-learned force field to simulate coupled ionic-polaronic dynamics in Li_xFePO4, revealing rapid polaron flips, their correlation with lithium configurations, and enhanced charge fluctuations at phase boundaries, offering insights into interfacial conduction.

Contribution

A novel machine-learned force field accurately captures coupled ionic-polaronic behavior in Li_xFePO4, enabling detailed simulation of complex dynamics and interfacial effects.

Findings

Polaron flips occur on picosecond timescales, much faster than lithium migration.

Polaron charge fluctuations are amplified at phase boundaries.

Simulations reveal strong correlation between polaron behavior and Li-ion configurations.

Abstract

Understanding and controlling coupled ionic-polaronic dynamics is crucial for optimizing electrochemical performance in battery materials. However, studying such coupled dynamics remains challenging due to the intricate interplay between Li-ion configurations, polaron charge ordering, and lattice vibrations. Here, we develop a fine-tuned machine-learned force field (MLFF) for Li$_x$FePO$_4$ that captures coupled ion-polaron behavior. Our simulations reveal picosecond-scale polaron flips occurring orders of magnitude faster than Li-ion migration, featuring strong correlation to Li configurations. Notably, polaron charge fluctuations are further enhanced at Li-rich/Li-poor phase boundaries, suggesting a potential interfacial electronic conduction mechanism. These results demonstrate the capability of fine-tuned MLFFs to resolve complex coupled transport and provide insight into emergent ionic-polaronic dynamics in multivalent battery cathodes.