Role of inorganic layers on polysulfide decomposition at sodium-metal anode surfaces for room temperature Na/S batteries

Abstract

Sodium metal is a promising anode material for room-temperature sodium sulfur batteries. Due to its high reactivity, typical liquid electrolytes (e.g. carbonate-based solvents and a Na salt) can undergo reduction to form a solid electrolyte interphase (SEI) layer, with inorganic components such as Na₂CO₃, Na₂O, and NaOH, covering the anode surface along with other SEI organic products. One of the challenges is to understand the effect of the SEI film on the decomposition of soluble sodium polysulfide molecules (e.g., Na₂S₈) upon shuttling from the cathode to anode during battery cycling. Here, we use ab initio molecular dynamics (AIMD) simulations to study the role of an inorganic SEI used as a model passivation layer in polysulfide decomposition. Compared to other film chemistries, it is found that the Na₂CO₃ film can suppress decomposition with the slowest reduction rate and the smallest amount of charge transfer towards Na₂S₈. The Na₂CO₃ film can maintain its structural properties during the simulations. In contrast, Na₂O and NaOH allow some decomposed polysulfide fragments to be inserted into the SEI layer. Moreover, the decomposition of Na₂S₈ on both Na₂O and NaOH SEI layers is more reactive with more charge transfer to Na₂S₈ when compared to that of Na₂CO₃. Thus, the ability of the SEI to suppress polysulfide decomposition is in the order: Na₂CO₃ > NaOH ∼ Na₂O. Analyses of the density of states reveal that the Na₂S₈ molecule receives electrons from the Na metal directly in the presence of n-type semiconductor films of Na₂CO₃ and NaOH, while the charge migration behavior is different in a p-type semiconductor Na₂O with the SEI film donating its electrons to the polysulfide solely. Thus, this work adds new insights into charge transfer behavior of inorganic thin film SEIs that could be present at the initial stages of SEI formation.