Herein, we report on the characterization of a Li-S hybrid cell containing a garnet solid electrolyte (Li7La3Zr2O12, LLZO) and conventional liquid electrolyte. While the liquid electrolyte provided ionically conductive pathways throughout the porous cathode, the LLZO acted as a physical barrier to protect the Li metal anode and prevent polysulfide shuttling during battery operation. This hybrid cell exhibited an initial capacity of 1000 mAh/g(S) and high Coulombic efficiency (>99%). The interface between the liquid electrolyte and LLZO was studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy (XPS). These results indicate that a spontaneous interfacial reaction layer formed between the LLZO and liquid electrolyte. XPS depth profiling experiments indicate that this layer consisted of Li-enriched phases near the surface (e.g., Li2CO3) and intermediate Li-La-Zr oxides in subsurface regions. The reaction layer extended well beyond the LLZO surface, and bulk pristine LLZO was not observed even at the deepest sputtering depths used in this study (∼90 nm). Overall, these results highlight that developing stable electrode/electrolyte interfaces is critical for solid-state batteries and their hybrids.
Keywords: LLZO; Li−S battery; electrolyte; hybrid cell.