The sodium–metal halide (ZEBRA) batteries have been considered as one of the most attractive energy storage systems for stationary and transportation applications. Even though the battery technologies have been widely investigated for a few decades, there is still a need to further improve the battery performance, cost and safety for practical applications. In the present work, a novel low-cost Na–ZnCl2 battery with a planar β′′-Al2O3 solid electrolyte (BASE) was proposed, and its electrochemical reactions and battery performance were investigated. Compared to Na–NiCl2 chemistry, the ZnCl2-based chemistry was more complicated, in which multiple electrochemical reactions including liquid-phase formation occurred at temperatures above 253 °C. During the first stage of charge, NaCl reacted with Zn to form Na in the anode and Na2ZnCl4 in the cathode. Once all the NaCl was consumed, further charge with the reaction between Na2ZnCl4 and Zn led to the formation of a NaCl–ZnCl2 liquid phase. During the end of charge, the liquid phase reacted with Zn to produce solid ZnCl2. To identify the effect of liquid-phase formation on electrochemical performance, button cells were assembled and tested at 280 and 240 °C. At 280 °C, with the liquid phase formed during cycling, cells revealed quite stable cyclability. On the other hand, more rapid increase in polarization was observed at 240 °C where only solid-state electrochemical reactions occurred. SEM analysis indicated that the stable performance at 280 °C was due to the suppressed growth of Zn and NaCl particles, which were generated from the liquid phase during the discharge of each cycle.
