Anisotropic surface reaction limited phase transformation dynamics in LiFePO4

TL;DR

This paper develops a continuum model for ion intercalation in LiFePO4, revealing a new surface-reaction-limited regime where phase boundaries propagate via traveling waves, differing from classical diffusion-based models.

Contribution

It introduces a novel theoretical framework capturing surface-reaction-limited dynamics and predicts traveling wave solutions for phase transformation in LiFePO4.

Findings

Identification of a surface-reaction-limited regime in LiFePO4

Prediction of traveling-wave solutions for phase boundaries

Different charge/discharge dynamics from classical models

Abstract

A general continuum theory is developed for ion intercalation dynamics in a single crystal of a rechargeable battery cathode. It is based on an existing phase-field formulation of the bulk free energy and incorporates two crucial effects: (i) anisotropic ionic mobility in the crystal and (ii) surface reactions governing the flux of ions across the electrode/electrolyte interface, depending on the local free energy difference. Although the phase boundary can form a classical diffusive "shrinking core" when the dynamics is bulk-transport-limited, the theory also predicts a new regime of surface-reaction-limited (SRL) dynamics, where the phase boundary extends from surface to surface along planes of fast ionic diffusion, consistent with recent experiments on LiFePO4. In the SRL regime, the theory produces a fundamentally new equation for phase transformation dynamics, which admits traveling-wave solutions. Rather than forming a shrinking core of untransformed material, the phase boundary advances by filling (or emptying) successive channels of fast diffusion in the crystal. By considering the random nucleation of SRL phase-transformation waves, the theory predicts a very different picture of charge/discharge dynamics from the classical diffusion-limited model, which could affect the interpretation of experimental data for LiFePO4.