Hybrid Solid State “Chaperone” Phases to Improve Solid State Sodium Electrolytes

Abstract

Solid state electrolytes exhibiting high sodium-ion conductivity are promising elements in the advance of high energy density, low temperature molten sodium, solid state sodium, and sodium ion batteries. Traditional solid state Na+-ion conductors, such as β”-Al2O3 or NaSICON are reserved for high temperature operations (~300°C), not only to access higher Na+-ion conductivity, but also to facilitate wetting of molten sodium on the solid electrolyte. Lowering the operating temperature to near or below the ~98°C melting temperature of sodium for new sodium battery applications leads to poor interfacial wetting of the molten sodium to the ceramic electrolyte, increasing interfacial resistance in the cell and ultimately producing poor battery performance. Here, we explore the application of hybrid interfacial “chaperone” phases that dramatically improve physical and charge transfer interfaces between a NaSICON ceramic electrolyte and a metallic sodium anode. Through the application of material such as tin to the NaSICON surface, in-situ transformation during electrochemical cycling leads to the formation of hybrid metallic/intermetallic/ceramic interphases that facilitate efficient sodium wetting and charge transfer. We observe that these chaperone phases dramatically lower cell-limiting interfacial resistance, facilitating higher battery cycling current densities and better overall battery performance. Continued optimization of such functional interphase materials promises new advances in emerging sodium batteries. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.