Enhancing ionic conductivity in lithium lanthanum titanium oxysulfides through tetragonal phase formation

The lithium lanthanum titanium oxides (L_xL_yTO) and oxysulfides (L_xL_yTOS) were synthesized via a solid-state method for use as solid electrolytes in all-solid-state batteries. The powdered products exhibited perovskite-type structures, adopting hexagonal and tetragonal crystal systems using NH₄F flux. Phase identification and unit cell parameters of the synthesized products, as influenced by the synthetic condition, were determined using X-ray diffraction analysis. The ionic conductivities of pressed LLTO-S pellets were measured using electrochemical impedance spectroscopy (EIS) over at temperature range of 50–150 °C, yielding values between 2.3 × 10^–7 and 2.9 × 10^–5 S/cm. The enhanced conductivity observed in L_xL_yTOS pellets was attributed to the presence of the tetragonal phase and the application of high annealing temperatures. The morphologies of L_xL_yTOS samples annealed at 1000 and 1200 °C were characterized, with particular focus on particle grain growth. To decouple bulk and interfacial contributions, bulk ionic conductivity was extracted via EIS using an equivalent circuit model.