Solid State Ion Conductors to Enable Low Temperature Molten Sodium Batteries

Abstract

Molten sodium batteries show clear promise for safe, long-lifetime grid-scale energy storage, and there is growing expectation that they may be able to impact a host of energy storage areas, ranging from vehicle electrification to industrial applications. Key to the successful development and implementation of these technologies, though, are 1) reducing the traditionally high operating temperature of these batteries and 2) identifying robust, zero-crossover solid state separators that will perform effectively at these reduced temperatures. Traditional molten sodium batteries, such as sodium-sulfur and sodium-nickel chloride (ZEBRA), operate near 300°C, where ceramic separators, such as β”-Al2O3 have high ionic conductivity, but these high temperature systems can be cost-prohibitive in large scale applications. Here, we discuss a significant deviation from these chemistries and materials in a sodium-iodide (NaI) battery that utilizes a molten sodium anode and a sodium iodide-based molten salt catholyte, operating at drastically reduced temperatures near 100°C. NaSICONs (Na Super Ion CONductors) show promise as effective ceramic separators in lab-scale demonstrations of this battery chemistry, but challenges to cost, widespread availability of the material, electrochemical compatibility, and mechanical stability of the ceramic have motivated the search for new sodium ion conducting separator materials and designs. We will describe not only critical synthetic considerations, but also important benefits and shortcomings of NaSICON separators for use in this specific battery chemistry. Importantly, we will describe recent efforts build on the promise of NaSICON materials, exploring new composite materials and hybrid structures that meet strenuous demands of ionic conductivity, chemical compatibility, and mechanical robustness for separators in low temperature sodium batteries. Innovative designs that take advantage of controlled particle morphology and organization in polymer-ceramic composites, for example, promise improved mechanical durability and effective ionic conductance in low cost, highly processable systems. Understanding the critical relationships between material composition, microstructure, and electrochemical performance of these emerging materials will be central to the successful development of a new generation of solid-state ion conductors and the realization of effective low temperature molten sodium batteries.Sandia National Laboratories is a multi-mission 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.