Diversities of Stoichiometry and Electrical Conductivity in Sodium Sulfides

Aiming at developing and understanding the active materials in the renewable energy-storage sodium-sulfur (Na-S) system, we systemically explored the phase diversity, electronic properties, and chemical bonding of the Na-S system at ambient and high pressure up to 50 GPa, using a combination of first-principles calculations with extensive structural searches. We identified four new stable phases; Na3S, Na5S3, Na2S2 and Na2S3. The previously unidentified Na2S3 with S32- polyanions shows Pnma symmetry with quite a low formation enthalpy relative to Na2S2 and Na2S4. The simulated voltage through Na2S4 to generate Na2S3 in a battery is predicted to be 1.65 V, consistent with the previous measurement. The predicted tetragonal Na5S3 with infinite S chains has mixed valence states of sulfur atoms, which was first observed in alkali metal sulfides. Na3S exhibits a potential one-dimensional (1-D) electride with a chemical formula of [Na3S]+∙e-, where the sulfur atoms possess the highest coordination number (Na12S). Both Na3S and Na5S3 exhibit intrinsic metallic behaviors, clearly differing from the other semiconducting phases. A detailed analysis of electronic structure reveals the distinctly electrical pathways of a 1-D electron gas in the channel voids in Na3S and infinite sulfur chains with metallic S-S bonding in Na5S3. Our results may help to discover new candidates in Na-S system and elucidate the potential electrochemical mechanism in the Na-S battery.