Isolation of Grain versus Intergranular Transport in Li_1+xTi_xTa_1-xSiO₅ Suggests Concerted Ion Migration in a High-Voltage Stable Electrolyte from High-Throughput Descriptors

LiTaSiO₅, with its suitable conduction channels and wide electrochemical stability window, has been previously suggested as a potential host of concerted Li migration triggered by inserting Li interstitials. However, without proper separation of grain and grain boundary contributions, previous experimental efforts have been unable to isolate and quantitatively characterize the defect chemistry-conductivity relationship within the lattice. In this work, LiTaSiO₅ was identified by descriptor filtering of the Materials Project database, and Li_i^• were inserted via Ti_Ta′ doping to form Li_1+xTi_xTa_1-xSiO₅. The grain and intergranular conductivities were separated using electrochemical impedance spectroscopy with distribution of relaxation times analysis (EIS/DRT). We showed the first clear observation of a monotonic decrease in activation energy E_A from 0.50 to 0.29 eV and a 6× increase in Li⁺ conductivity in the grains to 2.49 × 10^-5 S/cm for x = 0.15 (30 °C) as more Li_i^• were inserted, providing insight into how Li_i^• potentially triggered concerted transport. The necessity of separating grain and grain boundary contributions was further emphasized by observation, via STEM-EDS, of a Si-rich/Ta-poor intergranular amorphous phase that increases in volume with increasing Ti_Ta′ concentration. This phase led to a 19× increased specific grain boundary conductivity to 5.95 × 10^-6 S/cm for x = 0.15 (30 °C) with decreased E_A. The distribution of the intergranular phase was inhomogeneous (variation in size, stoichiometry), resulting in a wide distribution of relaxation times for the intergranular transport. Li_1+xTi_xTa_1-xSiO₅ also exhibited wide electrochemical stability, up to 4.9 V, making it suitable for application as a solid electrolyte or cathode coating.