LAUSR

A batteries (ASSBs) are receiving considerable attention at present because of their better safety and higher energy density compared to conventional Li-ion batteries. By replacing the flammable organic liquid electrolytes with inorganic solid-state electrolytes (SEs), ASSBs have the potential to overcome the inherent safety concerns of liquid cells. Although oxide and sulfide compounds have both been studied as inorganic electrolytes, sulfide-based solid electrolytes are particularly promising candidates. They can exhibit fast ion conductivity at room temperature (i.e., ≥10−3 S·cm−1) owing to the high polarizability of the S2− which weakens the binding energy between the mobile cations and the anion framework. Well-known examples include Li10GeP2S12, and other newer members of the “LGPS” family which have been used as electrolytes in ASSBs that show excellent performance at low temperature. Sulfides have the advantage of being ductile and often form as glass−ceramic phases. Thus, by cold-pressing, grainboundary resistance can be greatly decreased, and even glassy grain-boundary free materials may be obtained. These advantages make them appealing for ASSB applications. Though Li-ion ASSBs are eagerly sought for automotive batteries, sodium-ion ASSBs are more suitable for stationary low-cost energy storage systems due to the vast abundance of sodium vs lithium. However, a critical challenge needs to be overcome; only a handful of processable Na-ion SEs with high conductivities have been reported. The development of new fast Na-ion conductors for Na-ion ASSBs is thus an major focus of recent research. In 2012, Hayashi et al. introduced a breakthrough in sulfidebased Na-ion conductors with the report of the superionic conductor, cubic-Na3PS4 (σi = 0.2 mS·cm −1). Following this seminal discovery, other promising Na-ion thio-phosphate and seleno-phosphate conductors were reported, including cubic cNa3PSe4, 14 Na3PSxSe4−x, 15 and Na10SnP2S12. 16 The existence of defects and a high concentration of mobile carriers, as well as a low energy barrier for mobile ion migration are prerequisites to obtain high diffusivity in solids. For example, theoretical studies revealed that stoichiometric Na3PSe4 and cNa3PS4 present negligible Na -ion diffusivity, but the introduction of either Na interstitials or Na vacancies improve the conductivity. Indeed, defects are believed to play a more crucial role than the framework structure in governing ion diffusion. Recently, our group reported a new sodium superionic conductor, Na11Sn2PS12, which exhibits a high ion conductivity of 1.4 ± 0.04 mS·cm−1 at room temperature and an activation energy of 0.25 eV. Alternating full/partial occupation of Na-ion sites in this unique new framework governs these excellent diffusion properties. A second paper on this material appeared shortly after, reporting even higher conductivity (3.7 mS·cm−1) but also higher activation energy (0.34 eV). It is well established that most alkali thiophosphate superionic conductors have a drawback of being sensitive to oxygen and moisture in air. The theory of hard and soft acids and bases , first explored with the synthesis of Li3.833Sn0.833As0.166S4, 22 suggests that utilization of a “soft” (or more covalent) acid moitie ́ can mitigate the reactivity to a certain extent. Researchers have been prompted to investigate other “soft acid” sulfides that could exhibit suitable stability under ambient conditions, such as those based on antimony. Antimony-based sulfides are even reported to be solution processable using water or methanol. Herein we report the single crystal structure and properties of a Na-ion conductor, Na11Sn2SbS12, that is almost isostructural with Na11Sn2PS12. The excellent statistics afforded by single crystal methods for these two effectively isostructural (Sb, P) materials allow us to identify the reasons responsible for their significant differences in ion conductivity. Such an approach is critical for materials with extensive disorder of mobile ions where partial occupation of lattice sites exist. Refinement of powder data is typically unable to define these subtleties. We show that the major factor governing ion conductivity is a redistribution of sodium ions among the various lattice sites, arising from population of an additional interstitial Na site which strongly affects ion mobility in the framework. Namely, its population “off-loads” Na-ion distribution from the main 3D conduction channels, giving rise to greater vacancy concentrations. The resultant higher atomic displacement parameters (APD) of the Na-ions in Na11Sn2PS12, compared to those in the Sb phase, are correlated to more than doubling of the ion conductivity. Exploration of the role of vacancies in this family of conductors was also studied by examining the nonstoichiometric series Na11−xSn2−xSb1+xS12 (x = 0.2, 0.25, 0.5) that revealed the limit on the Sb-rich side to a strict line phase.