Enhancement of Ionic Conductivity in NASICON-Structured Li3(Zr,Ti)2(Si,Ge)2PO12: An Ab Initio Study

The development of solid electrolytes with high ionic conductivity is crucial for advancing solid lithium-ion battery technology but is still a challenge. In this study, the ionic conductivity of NASICON-structured materials Li₃(Zr,Ti)₂(Si,Ge)₂PO₁₂ are explored through ab initio molecular dynamics (AIMD) simulations. This investigation reveals the significant impact of isovalent substitution on the lithium-ion diffusion pathways and the associated energy barriers. Elemental substitutions, such as replacing Zr with Ti, significantly reduce the Li site energy levels, enhance the polyhedral volume, and change the coordination structure from four-coordinate to five-coordinate, thereby facilitating lithium-ion migration. Conversely, substituting Si with Ge reduces the diffusion channel size and increases fluctuation of Li migration potential surface, leading to less favorable ion transport conditions. Li₃Ti₂Si₂PO₁₂ exhibits a room temperature ionic conductivity of 5.79 × 10⁻² Scm⁻¹, 163% higher than that of the pristine Li₃Zr₂Si₂PO₁₂, with a reduced diffusion barrier of 0.16 eV. Additionally, these analyses reveal that the critical size for effective diffusion channels is vital: below this threshold, ion migration is suppressed; while above it, the channel size no longer limits migration.