Defect-Induced Li-Ion Trapping and Hopping in a Grain Boundary-Engineered LiAlTi(PO) Solid-State Electrolyte.

Understanding lithium-ion dynamics across defect-rich grain boundaries (GBs) is crucial for solid-state electrolytes. This study examines local electronic and structural changes in a LiAlTi(PO) (LATP) solid electrolyte via X-ray absorption spectroscopy (XAS) and their correlation with ion transport properties. GBs were tailored through conventional isothermal sintering (CIS) and spark plasma sintering (SPS). Ti , Ti -, O -, and P -edges from XAS revealed octahedral symmetry in bulk regions of both LATP-CIS and LATP-SPS. However, Ti -edge spectra in total electron yield mode and Ti K-edge white line intensity shifts in LATP-SPS indicate lower oxidation states and structural distortions due to a significant amorphous GB fraction. Modulations in O -edge and P -edge spectra further highlight local structural differences in GB regions of LATP-CIS and LATP-SPS. Electron energy loss spectroscopy (EELS) also reveals variations in Ti -edge splitting and pre-edge peak intensities, consistent with X-ray absorption near-edge spectroscopy analysis. LATP-SPS exhibits a higher Li content in the GB region than LATP-CIS. The GB ionic conductivity of LATP-SPS (σ ∼ 1.36 × 10 S/cm) is two orders higher than that of LATP-CIS (σ ∼ 3.84 × 10 S/cm), while grain conductivity remains similar. Trapping and hopping enthalpy estimations suggest that trapped Li ions contribute ∼27% of activation energy for LATP-SPS compared to ∼17% for LATP-CIS. Enhanced ion diffusion in polycrystalline LATP GBs is predicted from molecular dynamics simulations, where liquid-like ion pair correlations improve mobility. This work highlights the significant influence of GB-induced structural distortions, probed through XAS and EELS, on the ionic conductivity and charge transport in LATP electrolytes.