Unlocking the NaCl-AlCl3 phase diagram for low-cost, long-duration Na-Al batteries
J. Mark Weller, Minyuan M. Li, Evgueni Polikarpov, Kee Sung Han, Neil J. Kidner, Anant Patel, Mai Thanh Nguyen, Meghan Stout, Michael Gossett, Keeyoung Jung, David Reed, Vincent Sprenkle, Guosheng Li
Abstract
Sodium and aluminum are a natural combination of inexpensive, abundant elements as a redox pair for battery energy storage. 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. This enables higher specific capacity and average discharge voltages than previous Na-Al batteries, 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 V vs ∼1.6 V for neutral NaAlCl4) contributed by the acidic chloroaluminate cathode reaction, unlocking an additional specific energy of ∼119 Wh kg−1 by utilizing the conversion of NaAlCl4 to NaAl2Cl7, which adds to the neutral melt reaction between NaAlCl4/Al and Na (∼493 Wh kg−1 theoretical). By significantly increasing the cathode thickness and therefore accessible areal capacity 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.