Unraveling Ion Conduction and Interfacial Processes in Sulfide Electrolytes for Emerging Sodium Batteries

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Solid-state sodium (Na) batteries(SSSBs) using sulfide-based solid electrolytes (SEs) has been attracting a lot of attention due to their high theoretical specific capacity, enhanced safety, and abundant resources. Especially, Na3SbS4 shows tremendous potential as a SE owing to its good Na+ ion conductivity of ~1 – 3 mS cm-1. Despite its promise, electrochemical performance of SSSBs based on Na3SbS4 SE remain far from commercialization. The major roadblock thwarting progress in SSSBs is the lack of fundamental understanding of atomic-scale mechanisms underlying (a) interfacial reactions with Na anode, and (b) Na+ ion conduction (especially in the presence of dopants). Here, we employ ab initio molecular dynamics (AIMD) simulations to identify key reactions occurring at the Na|Na3SbS4 interface. Our AIMD simulations indicate that addition of an ionic liquid interlayer results in a stable solid-electrolyte-interphase (SEI) composed mainly of NaF, in excellent agreement with our synthesis and characterization experiments. Similarly, our DFT studies show that Se doping increases the electrochemical stability window of Na3SbS4. In terms of Na+ conduction, we employ AIMD simulations, and nudged elastic band calculations to understand the effect of (a) valence, and (b) size of cation dopants that partially replace Na in Na3SbS4. These cations introduce charge compensating Na-vacancies in the SSE, which in turn, enhance Na+ ion conduction. However, size of the cation dopant has a profound impact on the extent of increase in Na+ ion conductivity. For instance, Ca-doped Na3SbS4 (Na2.75Ca0.125SbS4) showed Na+ conductivity of ~10 times that of pristine Na3SbS4 (rCa2+ / rNa+ = 0.98). On the other hand, larger Ba2+ as dopant (rBa2+ / rNa+ = 1.35) in Na2.75Ba0.125SbS4 hinders Na-ion hops owing to local strain, thereby, yielding a Na+ conductivity ~5 times that of Na3SbS4.We will discuss these findings in the context of developing solid-state electrolytes for emerging SSSBs.