In-Situ Characterization of Molecular Processes at the Anode/Na₃SbS₄ Electrolyte Interface in All-Solid-State Sodium Batteries

Abstract

Sulfur based solid-state Na+ conductors exhibit high ionic conductivity and are promising candidates for electrolytes used in the next generation all-solid-state sodium-ion batteries. Sodium thioantimonate (Na3SbS4), for example, shows an ionic conductivity of 1 mS/cm, comparable to its liquid counterparts. In contrast to the well-known thiophosphate solid-state electrolytes, Na3SbS4 is chemically stable in dry air. However, solid-state Na-ion batteries assembled using Na3SbS4 as the electrolyte show a decaying performance over the charge and discharge cycles. This work characterized the molecular processes occurring at the interface between Na3SbS4 solid electrolyte and the anode. This interfacial chemistry was probed in real-time (in-situ) using Raman spectroscopy while the battery was in operation. Combined with the characterization results obtained from X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), we observed a largely irreversible decomposition of SbS4 3- while Na3SbS4 was directly exposed to negative potentials (vs. Na/Na+). Sb2S3 and elemental Sb are the two major decomposition byproducts formed and accumulated at the Na3SbS4/anode interface. This result unravels the decomposition mechanism at the Na3SbS4/anode interface in all-solid sodium batteries. It provides deep molecular insights into designing ideal protective layers at this critical interface.