Risk assessment for Na-Zn liquid metal batteries

Background Na-Zn liquid metal batteries, which operate at 600 °C, have recently been proposed as inexpensive stationary energy storage devices. As with any other electrochemical cell, their fabrication and operation involves certain risks, which need to be well understood in order to be minimised. Methods A risk assessment according to ISO 12100 is performed at the cell level for operating Na-Zn cells in the laboratory environment. Hazard identification and risk evaluation are systematically addressed, including a thorough literature review, theoretical calculations and selected experiments. Results Cell overpressure is found to be one of the main risks – and might be caused either by mistakes in battery production (humidity) or operation (over-charge/discharge). In terms of cell housing, the weakest component is clearly the feedthrough. Its failure might lead to the release of hazardous aerosols to the environment. In this context, the candidate electrolyte components LiCl and BaCl2 are especially dangerous, and should therefore be reduced or avoided if possible. Conclusions Overall, Na-Zn cells are expected to reach a very high safety level, similar to state-of-the-art ZEBRA technology, as they are not prone to thermal runaway. However, considering the still low TRL level and open questions concerning the durability of certain parts of their housing, the batteries should preferably be operated under a fume hood.