Implementing Low Temperature Strategies to Advance “Really Cool” Molten Sodium Batteries

Abstract

Though batteries for portable electronics and electric vehicles continue to advance at an impressive rate, the needs for safe, low-cost, reliable batteries suitable for large-scale implementation remain unsatisfied. Here, we describe advances toward a promising new class of low temperature molten sodium-halide batteries, capable of safe, reliable operation near 100oC, a dramatic reduction from the ~300oC operating temperatures common to traditional molten sodium batteries. While realizing this low temperature goal promises reduced system costs and improved material lifetimes, it introduces a host of new material performance challenges including molten catholyte phase control and electrochemistry, solid state separator performance, and charge transfer at critical solid-liquid interfaces. The new battery design described here relies on highly conductive NaSICON-based ion-conducting membranes to separate a molten sodium anode and, critically, a molten inorganic sodium iodide (NaI)-based halide salt catholyte. Through engineering of novel molten salt catholyte compositions, we have identified several chemistries capable of producing a high voltage (>3V) while avoiding problems of thermal runaway and electrolyte flammability. Our studies further show that catholyte composition not only impacts low temperature electrochemical performance, but also the accessible capacity and even techno-economic viability of the system. Implementing these catholytes in lab-scale prototypes with charge-transfer interfaces engineered for low-temperature operation, we have demonstrated 5 months of stable electrochemical cycling at 110oC. Continued refinement of these material-enabled strategies promises a new class of grid-scale energy storage technologies based on molten sodium.Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and 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.