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

Solid electrolyte is expected to be an alternative to liquid electrolyte in Li-ion batteries. The former 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 it’s correlation with dopants/defects by taking LISICON (Li₄SiO₄) as a test case. As a first step, using density functional theory (DFT), we compute the formation energies of different defects in LISICON to determine the thermodynamically stable 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 same, leading to anisotropy in ionic conductivity. We observe that interplanar jumps are minimum in bc planes that limits 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.