Origin of the High Conductivity of the LiI-Doped Li3PS4 Electrolytes for All-Solid-State Lithium-Sulfur Batteries Working in Wide Temperature Ranges

Sulfide electrolytes show great potential as solid electrolytes for all-solid-state batteries due to their high ionic conductivity and low interfacial resistance toward electrode materials. The structure of the P-S framework in the composition of sulfides plays a crucial role in the conductivity. Introducing LiI in the Li3PS4 structure can tailor the P-S frameworks and thus achieve high ionic conductivities. Herein, the dopant of LiI is optimized by high-rotation milling, followed by a sintering route to achieve the highest Li-ion conductivity of 2.3 mS cm-1 for the Li3PS4-50% LiI electrolyte (also denoted as Li7P2S8I) among different compositions. The phase variations and structure evolution of the P-S frameworks in the LiI-doped Li3PS4 electrolytes are carefully investigated in a combination of X-ray diffraction and Raman spectra. All-solid-state Li-S batteries using the prepared Li7P2S8I electrolyte combined with a nano Li2S cathode and a Li-In anode deliver a high initial discharge capacity of 739.7 mA h g-1 and sustain a discharge capacity of 517.7 mA h g-1 after 60 cycles with a capacity retention of 70.0% at 0.13 mA cm-2 at room temperature. When the operating temperature rises to 60 degrees C, it shows a higher discharge capacity of 862.8 mA h g-1 with fast capacity decay. However, it exhibits stable capacities and superior cycling performance under 0 degrees C over 70 cycles. Electrochemical impedance spectroscopy and in situ stack-pressure results find that the electrochemical performance differences of the assembled battery at different operating temperatures are associated with the interfacial resistance caused by volume changes. This work provides the strategy to obtain highly conductive sulfide electrolytes for SSBs and unravels the temperature effects on the electrochemical performance of all-solid-state Li-S batteries.