Understanding ionic diffusivity in (meta)stable (un)doped solid state electrolyte from first principles: A case study of LISICON (Li₄SiO₄)

Considering the flammable nature of the organic liquid electrolyte, the solid-state electrolyte is arising as an alternative for Li-ion batteries. The latter is believed to be safer, capable of delivering higher energy density, faster recharging, higher voltage capability, and longer cycle life. Here, we have studied ionic diffusion and concurrence of dopants/defects on ion transport properties by taking LISICON (Li₄SiO₄) as a test case. As the earliest step, using density functional theory (DFT), the formation energy approach has been employed to determine the thermodynamically stable defected configurations. Following this, we have performed ab initio Molecular Dynamics (AIMD) simulation on (meta)stable (un)doped systems to study the diffusion and ionic conductivity of Li-ions. Our results reveal that jumps between different planes are not the same, leading to anisotropy in ionic conductivity. We observe that interplanar jumps are minimum in bc planes that limit the ionic conductivity. We report that the limited jump rate can be enhanced at room temperature by point defects, viz. Li-vacancy and substitution at Si-sites with different elements viz. P, Ge, Al. We have shown how polarization occurring due to point defects affects the ion transport properties.