Integrated study of first principles calculations and experimental measurements for Li-ionic conductivity in Al-doped solid-state LiGe₂(PO₄)₃ electrolyte

Understanding of the fundamental mechanisms causing significant enhancement of Li-ionic conductivity by Al³⁺ doping to a solid LiGe₂(PO₄)₃ (LGP) electrolyte is pursued using first principles density functional theory (DFT) calculations combined with experimental measurements. Our results indicate that partial substitution Al³⁺ for Ge⁴⁺ in LiGe₂(PO₄)₃ (LGP) with aliovalent (Li_1+xAl_xGe_2-x(PO₄)₃, LAGP) improves the Li-ionic conductivity about four-orders of the magnitude. To unveil the atomic origin we calculate plausible diffusion paths of Li in LGP and LAGP materials using DFT calculations and a nudged elastic band method, and discover that LAGP had additional transport paths for Li with activation barriers as low as only 34% of the LGP. Notably, these new atomic channels manifest subtle electrostatic environments facilitating cooperative motions of at least two Li atoms. Ab-initio molecular dynamics predict Li-ionic conductivity for the LAGP system, which is amazingly agreed experimental measurement on in-house made samples. Consequently, we suggest that the excess amounts of Li caused by the aliovalent Al³⁺ doping to LGP lead to not only enhancing Li concentration but also opening new conducting paths with substantially decreases activation energies and thus high ionic conductivity of LAGP solid-state electrolyte.