A Biphasic Interphase Design Enabling High Performance in Room Temperature Sodium-Sulfur Batteries

Room temperature sodium-sulfur batteries possess higher specific energy and improved inherent safety compared to their high-temperature analogs used in stationary grid storage. The viability of room temperature sodium batteries depends critically on the mechanical and ionic transport properties of the solid electrolyte interphase. However, little emphasis has been placed on developing sodium anode interphases that combine high Young’s modulus (stiffness), high critical strain (ductility), and low ionic diffusion barrier for cycling at high rates. Here, we report an artificial biphasic interphase comprising two chemically distinct phases, NaOH and NaNH2, which combines high stiffness and high ductility. In addition, the biphasic interphase exhibits a low diffusion barrier for sodium ions, enabling reversible sodium plating and stripping behavior even at extremely high current densities (up to 50 mA cm−2) in symmetric cell configuration. Stable and reversible cycling of a room temperature sodium-sulfur battery is also demonstrated over 500 cycles.