Isolation of Grain versus Intergranular Transport in Li1+xTixTa1–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 Lii• were inserted via TiTa′ doping to form Li1+xTixTa1–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 EA 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 Lii• were inserted, providing insight into how Lii• 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 TiTa′ 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 EA. The distribution of the intergranular phase was inhomogeneous (variation in size, stoichiometry), resulting in a wide distribution of relaxation times for the intergranular transport. Li1+xTixTa1–xSiO5 also exhibited wide electrochemical stability, up to 4.9 V, making it suitable for application as a solid electrolyte or cathode coating.