Suppression of Phase Separation in LiFePO₄ Nanoparticles During Battery Discharge

Abstract

Using a novel electrochemical phase-field model, we question the common belief that Li x FePO 4 nanoparticles separate into Li-rich and Li-poor phases during battery discharge. For small currents, spinodal decomposition or nucleation leads to moving phase boundaries. Above a critical current density (in the Tafel regime), the spinodal disappears, and particles fill homogeneously, which may explain the superior rate capability and long cycle life of nano-LiFePO 4 cathodes.

Rate-Dependent Morphology of Li2O2 Growth in Li–O2 Batteries

“…The theory predicts a transition starting in the first monolayer from particle growth to film growth when the current exceeds the exchange current for the oxygen reduction reaction, consistent with experimental observations. The mechanism is analogous to the suppression of phase separation in LiFePO 4 nanoparticles, first predicted by the same general theory [27][28][29].…”

“…When the film thickness approaches 5 nm, the active surfaces become passivated, as electronic resistance increases with thickness [24]. In this Letter we model the rate-dependent morphological transition in Li 2 O 2 growth, using the recently developed variational theory of electrochemical kinetics [25][26][27][28][29][30] applied to classical surface-growth models [31][32][33]. The theory predicts a transition starting in the first monolayer from particle growth to film growth when the current exceeds the exchange current for the oxygen reduction reaction, consistent with experimental observations.…”

“…We describe the current density profile I(x, t) using generalized Butler-Volmer kinetics based on nonequilibrium thermodynamics, recently developed by Bazant et al [25] and applied to intercalation dynamics in Li-ion batteries [26][27][28][29][30]48]. Here, we apply the theory for the first time to surface growth, using a different model for the Li 2 O 2 chemical potential,…”

“…This equation generalizes the CH and Allen-Cahn equations for electrochemistry. As in the case of anisotropic LiFePO 4 nanoparticles [26], diffusion can be neglected (M = 0) to yield the Butler-Volmer Allen-Cahn reaction (ACR) equation [25,27], which, using Eqs. normalized by mean height h (see Eq.…”

“…For galvanostatic discharge, the ACR equation is solved subject to the constraint of constant mean current density [27],…”