Defect-Induced Li-Ion Trapping and Hopping in a Grain Boundary-Engineered Li1.3Al0.3Ti1.7(PO4)3 Solid-State Electrolyte.

Abstract

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 Li_1.3Al_0.3Ti_1.7(PO₄)₃ (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 L_2,3-, Ti K-, O K-, and P L_2,3-edges from XAS revealed octahedral symmetry in bulk regions of both LATP-CIS and LATP-SPS. However, Ti L_2,3-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 K-edge and P L_2,3-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 L_2,3-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_gb,300 K ∼ 1.36 × 10^-3 S/cm) is two orders higher than that of LATP-CIS (σ_gb,300 K ∼ 3.84 × 10^-5 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.