Kinetic Monte Carlo Simulations of Anisotropic Lithium Intercalation into LixFePO4 Electrode Nanocrystals

The kinetic anisotropy of lithium ion adsorption and lithium absorption for LixFePO4 olivine nanocrystals is simulated and reported. The kinetics depend on the orientation of the electrolyte/LixFePO4 interface with respect to the far-field ionic flux. As a consequence of these kinetics and a Li miscibility gap in LixFePO4, the particle geometry and orientation also have an effect on the morphology of the two-phase evolution. These processes accompany the charge and discharge behavior in battery microstructures and a direct influence on battery behavior is suggested. A kinetic Monte Carlo (KMC) algorithm based on a cathode particle rigid lattice is used to simulate the kinetics in this system. In these simulations the adsorption kinetics of the electrolyte/electrode interface are treated by coupling the normal flux outside the particle from a continuum numerical simulation of Li-ion diffusion in the electrolyte to the atomistic KMC model within the particle. The interfacial reaction depends on local concentration and the potential drop at the interface via the Butler–Volmer (B–V) relation. The atomic potentials for the KMC simulation are derived from empirical solubility limits (as determined by OCV measurements). The main results show that the galvanostatic lithium-uptake/cell-voltage has three regimes: 1) a decreasing cell potential for Li-insertion into a Li-poor phase; 2) a nearly constant potential after the nucleation of a Li-rich phase Li(1-β)FePO4; 3) a decreasing cell potential after the Li-poor phase has been evolved into a Li-rich phase. The behavior in the second regime is sensitive to crystallographic orientation.