Optimizing the Current Collector for Sodium Iodide-Metal Halide Catholytes in Low-Temperature Molten Sodium Batteries

Low-cost, long-duration energy storage is critically needed for a robust electric grid powered by renewable sources. In the pursuit of meeting this need, low-temperature (<130 °C) molten sodium batteries (MNaBs) with NaI-metal halide molten salt catholytes have been developed. This battery design circumvents many of the safety concerns caused by metal dendrites and flammable organic solvents found in Li-metal or Li-ion batteries. It also drastically reduces the high-temperature material requirements and operating costs compared to traditional MNaBs, such as ZEBRA, which operate near 300 °C. The presented battery operates at 110 °C—just above the melting point of Na (98 °C). It features a molten Na anode, a NaSICON ceramic separator, and a NaI/AlCl3 catholyte. Among the key challenges to widespread utilization of these emerging MNaBs is reducing the overpotential on the cathode while operating at practical current densities over numerous cycles. To improve the performance of the cathode, we examined the electrochemical behavior of a variety of disk electrode materials, including W, Mo, Ta, and glassy carbon (GC) in a 3-electrode configuration. A custom cell was designed to mimic a full battery with separators for both the reference and counter electrodes. Excess molten salt was used to keep bulk concentrations practically constant over the course of the experiments. This enabled experiments that isolated the working electrode from other battery elements, such as a changing catholyte or the NaSICON interfaces. Voltammetry, electrochemical impedance spectroscopy, chronopotentiometry, and chronoamperometry were used to evaluate each material’s charge/discharge kinetics and stability. Instability on charging is hypothesized to be due to iodine (I2) adsorption on rapid iodide (I-) oxidation. Discharge, on the other hand, is limited by the transport of triiodide (I3 -), which depends on the battery’s state of charge. Insights from these studies serve as the foundation for the rational design of high-surface area electrodes for iodide-based molten salt catholytes. After initial testing in the 3-electrode cell, the performance of several high surface area current collectors was evaluated in rate tests and continuous cycling of lab-scale battery cells. 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.