Simulations of Phase Transformation Dynamics in LiFePO4 Particles in Battery Electrode

Lithium iron phosphate (LFP) has been a successful material for Li-ion battery cathodes due to its enhanced safety and rate capability. For cathodes consisting of LFP nanoparticles, the current-voltage response in macroscopic electrochemical measurements exhibits a feature indicative of solid-solid phase transformation during the charge-discharge processes. However, the Li concentration evolution within individual nanoparticles remains to be elucidated.

In this work, we used phase field modeling to investigate the effects of a variety of material parameters and their anisotropies on the morphological evolution of the lithiated and delithiated phases in cathode particles. An elliptical particle of 45 and 35 nm in diameters along the longest and shortest axes was used in the simulations. This particle was assumed to be immersed in electrolyte and to be placed in contact with a current collector. To simulate the lithium concentration evolution during the electrochemical process, six coupled governing equations were solved self-consistently. These equations describe lithium transport in (1) the particle and (2) the electrolyte, electrostatic potential in (3) the particle and (4) the electrolyte, (5) the electrochemical reaction rate at the particle surface, and (6) mechanical equilibrium in the particle.

The highly anisotropic material properties of LFP reported in the literature were adopted as the simulation parameters, including lithium diffusivities, interfacial energies, surface insertion rate, electronic conductivity, misfit strain, and elastic moduli. To examine the individual and combined effects from these parameters, the simulation parameters were altered from the reported values in our parametric studies. By exploring the material parameter space, we found that the anisotropy of electronic conductivity is the dominant factor in determining phase evolution in a bare LFP particle (without conductive coating). In contrast, for a carbon-coated particle, the anisotropy in electronic conductivity loses its significance and the lithiation process becomes more isotropic. 

This work was supported as part of the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012583.