A Phase-Field Model and Simulation of Kinetically Asymmetric Ternary Conversion-Reconversion Transformation in Battery Electrodes

Electrochemical processes in high-energy electrode materials often involve diffusion of multiple species and solid-state phase transformations. Some of these phase transformations involve breaking and rearranging ionic bonds and are referred to as conversion reactions (e.g., the lithium and iron difluoride conversion reaction: 2Li(+) + 2e(-) + FeF2 -> 2LiF + Fe). The phase transformations during conversion processes are governed by fundamental thermodynamics and kinetics in a similar manner to metallurgical systems. In this work, we developed a phase-field model that tracks atomic fractions of three constituent species to simulate the morphological evolution of different phases. The simulations demonstrate that conversion proceeds via a two-stage process consisting of lithiation and decomposition stages, whereas the reconversion process consists of a single-stage delithiation. This asymmetry in evolution paths of conversion and reconversion is likely responsible for the voltage hysteresis commonly observed during lithiation-delithiation cycling of conversion materials.