Diffusion of Li-deficient phases in large LiFePO4 single crystals during chemical delithiation

Abstract

Diffusivity of lithium (or Li vacancies) in active materials in a lithium ion battery is a key property to achieve high battery performance. Propagation of Li-deficient phases in large LiFePO₄ single crystals along the (100), (010) and (001) planes (a, b and c planes, respectively) during chemical delithiation was studied by in situ Raman spectroscopy. Li was extracted from one of the b (a, c) planes, and Raman spectroscopy was simultaneously conducted on the other b (a, c) plane on the opposite side. Li-deficient phases were observed 10 minutes after delithiation was started. The results indicate that the diffusivity of Li along the b axis during migration of Li-deficient phases under this condition is in the order of 10⁻⁵ cm² s⁻¹, which are five to seven orders of magnitude higher than Li diffusivities previously reported. The density of Li-deficient phases on the b plane continued to increase even after delithiation had stopped. The diffusivities of Li along the a and c axes were also in the order of 10⁻⁵ cm² s⁻¹, but the diffusivity along the a axis was smaller than those along the b and c axes. The very fast diffusion is thought to be induced by a diffusion mechanism that is strongly coupled to the chemical potential of Li in Li_xFePO₄ depending on the concentration of Li(x).