All-solid-state sodium batteries utilize earth-abundant elements and are sustainable systems for large-scale energy storage and electric transportation. Replacing flammable carbonate-based electrolytes with solid-state ionic conductors promotes battery safety. Using solid-state electrolytes (SEs) also eliminates the need for packing when fabricating tandem cells, potentially enabling further enhanced energy density. Na3SbS4, a Na+ conductor, remains stable in dry air and shows high Na+ conductivity (σ ≈ 1.0 × 10–3 S/cm) and is thus a promising SE for applications in sodium batteries. However, upon repeated electrochemical cycling, Na3SbS4-containing Na batteries exhibit decaying capacity and limited cycle life, which is likely associated with the decomposition of Na3SbS4 at the electrode/electrolyte interface. This work presents an in-depth analysis of the decomposition chemistry occurring at the Na3SbS4/anode interface using combined in situ Raman and post-mortem characterization. The results indicate that the SbS4 3– counterion is electrochemically reduced when experiencing Na+ reduction potentials, and this reduction chemistry likely follows multiple pathways. The observed reduction products include SbS3 3–, the Sb2S7 4– dimer, the NaSb binary phase, and Na2S. We also observed the irreversibility of the decomposition and, as a consequence, the accumulation of the degradation products over cycles. Also notable is the heterogeneity of this degradation chemistry across the interface. Through the spectroelectrochemical characterizations, we reveal the possible mechanisms of the Na3SbS4 decomposition at the solid electrolyte/anode interface in an operating device.
Sodium thioantimonate (Na 3 SbS 4 ) and its W-substituted analogue Na 2.88 Sb 0.88 W 0.12 S 4 have been identified as potential electrolyte materials for allsolid-state sodium batteries due to their high Na + conductivity. Ball milling mechanochemistry is a frequently employed synthetic approach to produce such Na + -conductive solid solutions; however, changes in the structure and morphology introduced in these systems via the mechanochemistry process are poorly understood. Herein, we combined X-ray absorption fine structure spectroscopy, Raman spectroscopy, solid-state nuclear magnetic resonance spectroscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy characterization techniques to provide an in-depth analysis of these solid electrolytes. We report unique changes seen in the structure and morphology of Na 3 SbS 4 and Na 2.88 Sb 0.88 W 0.12 S 4 resulting from ball milling, inducing changes in the electrochemical performance of the solid-state batteries. Specifically, we observed a tetragonal-to-cubic crystal phase transition within Na 3 SbS 4 following the ball mill, resulting in an increase in Na + conductivity. In contrast, the Na + conductivity was reduced in mechanochemically treated Na 2.88 Sb 0.88 W 0.12 S 4 due to the formation and accumulation of a WS 2 phase. In addition, mechanochemical treatment alters the surface morphology of densified Na 2.88 Sb 0.88 W 0.12 S 4 pellets, providing intimate contact at the solid electrolyte/Na interface. This phenomenon was not observed in Na 3 SbS 4 . This work reveals the structural and morphological origin of the changes seen in these materials' electrochemical performance and how mechanochemical synthesis can introduce them.
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.
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