PNNL

Abstract

Battery chemistries based on earth-abundant, inexpensive raw materials are crucial for practical and scalable long-duration energy storage (LDES) for a reliable grid. Sodium and aluminum are a natural combination of inexpensive, abundant elements and recent explorations pairing a sodium anode and aluminum cathode have demonstrated reversible, energy dense Na-Al cells with excellent rate capability using the electrochemical reaction between a molten Na anode and a NaAlCl4/Al cathode. In this work, the NaAlCl4/Al cathode is extended beyond the neutral NaAlCl4 composition by unlocking the NaCl-AlCl3 phase diagram to explore the extra accessible capacity hidden in acidic chloroaluminate melts up to and beyond the composition NaAl2Cl7. In doing so, higher specific capacity and average discharge voltages than previous Na-Al batteries are accessible, utilizing two distinct cell reaction mechanisms in one battery. Fundamental aspects of the NaAlCl4-NaAl2Cl7 reaction chemistry are investigated, and Na-metal / chloroaluminate batteries with excellent reversibility and areal capacity are demonstrated. Increasing the voltage window of the chloroaluminate Na-Al battery takes advantage of the higher voltage (~ 2.02 V vs ~1.6 for neutral NaAlCl4) contributed by the acidic chloroaluminate cathode reaction, thereby unlocking an additional specific energy of ~119 Wh kg-1 by utilizing the conversion of NaAlCl4 to NaAl2Cl7, which adds to that of the neutral melt reaction between NaAlCl4/Al and Na (~493 Wh kg-1 theoretical). Cells using this extended chloroaluminate chemistry are demonstrated with high areal capacity (46 mAh cm-2), facilitated by the rapid mass transfer of the chloroaluminate catholyte and fast kinetics of its liquid-solid redox reaction mechanism. By significantly increasing the cathode thickness and therefore areal loading up to 131.7 mAh cm-2, a discharge duration of 28.2 h is achieved with an estimated raw active materials cost of $7.02 kWh-1. These metrics show the great potential of this unlocked chloroaluminate battery for future low-cost, long-duration electrochemical energy storage.