Air-Stable Li₃SbS₄–LiI Electrolytes Synthesized via an Aqueous Ion-Exchange Process and the Unique Temperature Dependence of Conductivity

The stability of sulfide-based solid electrolytes (SEs) in ambient air is a critical criterion for the application of all-solid-state lithium-ion batteries. Air-stable Li3SbS4–LiI SEs were synthesized using a unique process in an aqueous solution under ambient air, i.e., an ion-exchange (IE) process. The crystalline structure of Li3SbS4 obtained by this process was confirmed by X-ray diffraction (XRD) patterns. The ionic conductivity of the obtained SE was 8.5 × 10–8 S cm–1 at 50 °C. The SEs of Li3SbS4–LiI were also synthesized via the IE process. The temperature dependence of the Li3SbS4–LiI SEs’ ionic conductivities showed a unique behavior; for example, the conductivities of 60Li3SbS4·40LiI (LSbSI) rapidly increased upon heating from 1.8 × 10–7 S cm–1 at 26.5 °C to 8.4 × 10–3 S cm–1 at 65 °C. The LiI layers on LSbSI are responsible for the unique temperature dependence of conductivity determined by differential scanning calorimetry–XRD measurement. Further, the dehydrated LSbSI obtained by milling and annealing showed a high conductivity of 1.3 × 10–4 S cm–1 at a low temperature of 25 °C. A cathode composite containing the active material of Ti2S and the LSbSI SE obtained via the IE process was prepared by freeze-drying. The all-solid-state cell using the cathode composite, which consists of Li–In/SE/TiS2–LSbSI, showed good performance at 60 °C as a lithium-ion secondary battery.