Effect of microstructure on discharge properties of polycrystalline LiCoO2

The constant-current discharge properties of polycrystalline LiCoO2 are theoretically investigated in relation to grain size, spatial distribution of the crystal orientation for each grain, and grain boundary (GB) diffusivity. We propose implementation of phase-field models for non-Fickian Li diffusion in two-dimensional polycrystalline microstructures, by considering the following: the crystallographic anisotropy of the self-diffusion coefficient of Li, the elastic stress field associated with Li intercalation, and the Li diffusion along or across GB modeled as a thin layer. The simulation results show that the microstructure notably affects the discharge properties at a high discharge rate through modifying the Li diffusivity. Both, the intergranular angle mismatch and GB diffusivity, are crucial parameters for evaluation of the apparent discharge properties. Since the Li diffusivity was promoted so as to alleviate the elastic stress, the discharge capacity was slightly increased. Although the effect of the elastic strain energy is weaker than the effects of intergranular angle mismatch and GB diffusivity, it leads to a nontrivial change in the Li diffusivity during discharge.