Incorporation of halogens (Cl, Br, and I) in an Li-P-S-O system for exploring new sulfide solid electrolytes with high conductivity and superior electrochemical performance in solid-state batteries

Solid electrolytes (SEs) with high conductivity and better stability against lithium metal are the most important requirements for successful commercialization of solid-state battery (SSB) technology. Therefore, in the search for new SEs with the above-mentioned qualities, halogen elements (Cl, Br, and I) were explored in an Li-P-S-O system to prepare Li10GeP2S12 (LGPS)-structured SEs. Among all the prepared SEs, Li3.3PS3.7O0.3Br0.1 and Li3.3PS3.7O0.3I0.1 compositions showed the highest conductivities of 0.77 mS cm-1 and 0.93 (approximate to 1.0) mS cm-1, respectively, at 25 degrees C. These compounds also showed superior stability against lithium metal in symmetric and full SSB cells tested with an LiNi0.8Co0.15Al0.05O2 (NCA) cathode and graphite anode. Generally, the instability of sulfide-based SEs in an ambient environment is a major issue, whereas our prepared compounds Li3.3PS3.7O0.3Br0.1 and Li3.3PS3.7O0.3I0.1 showed 5 times higher stability under ambient environment conditions as compared to non-doped Li3.2PS3.7O0.3. In situ EIS study performed on the as-prepared full SSBs and after charge-discharge cycles revealed the contribution of interfacial reactions to impedance. Higher interfacial resistances in the full SSB with the Li-metal anode compared to those with the graphite anode were also revealed. The in situ EIS study confirmed that the Li3.3PS3.7O0.3I0.1 solid electrolyte formed the most stable interface with NCA, graphite and Li metal. Finally, postmortem analysis of the full SSB cell and an SE-Li interfacial study conducted using SEM and elemental mapping revealed morphological changes in the electrodes and electrolytes before and after charge-discharge cycles. In summary, the present work reports new sulfide-based solid electrolytes with high conductivity, better air/moisture stability, and excellent interfacial stability.