Coherency strain and the kinetics of phase separation in LiFePO4

TL;DR

This paper presents a theoretical study of how elastic coherency influences phase separation, morphology, and electrochemical behavior in LiFePO4 nanoparticles, providing insights relevant for battery performance optimization.

Contribution

It introduces a phase-field model incorporating elastic effects to predict phase boundary orientations, morphology, and solubility in LiFePO4, explaining experimental observations and guiding material design.

Findings

Low-energy phase boundaries along {101} planes predicted

Stripe morphology formation due to elastic relaxation near surfaces

Coherency strain suppresses phase separation, improving rate capability

Abstract

A theoretical investigation of the effects of elastic coherency on the thermodynamics, kinetics, and morphology of intercalation in single LiFePO4 nanoparticles yields new insights into this important battery material. Anisotropic elastic stiffness and misfit strains lead to the unexpected prediction that low-energy phase boundaries occur along {101} planes, while conflicting reports of phase boundary orientations are resolved by a partial loss of coherency in the {100} direction. Elastic relaxation near surfaces leads to the formation of a striped morphology, whose characteristic length scale is predicted by the model and yields an estimate of the interfacial energy. The effects of coherency strain on solubility and galvanostatic discharge are studied with a reaction-limited phase-field model, which quantitatively captures the influence of misfit strain, particle size, and temperature on solubility seen in experiments. Coherency strain strongly suppresses phase separation during discharge, which enhances rate capability and extends cycle life. The effects of elevated temperature and the feasibility of nucleation are considered in the context of multi-particle cathodes.