PNNL

Abstract

The sodium-nickel chloride (ZEBRA) battery is typically fabricated with a thick tubular ß"-alumina solid electrolyte (BASE) and operated at relatively high temperatures (? 300ºC) to achieve adequate electrochemical performance. In the present work, a planar-type sodium-nickel chloride battery possessing a thin BASE (~600 ?m thick) was tested in order to evaluate the feasibility of the battery operation at low temperatures (?200°C). Electrochemical test results revealed that the battery was able to be cycled at C/3 rate at as low as 175°C despite the higher cell polarization at the reduced temperature. Overall, low operating temperature resulted in a considerable improvement in the stability of cell performance. Cell degradation was negligible at 175°C, while 55% increase in end-of-charge polarization was observed at 280°C after 60 cycles. SEM analysis indicated that the performance degradation at higher temperatures was related to the particle growth of both nickel and sodium chloride in the cathode. The cells tested at lower temperatures (e.g., 175 and 200°C), however, exhibited a sharp drop in cell voltage at the end of discharge due to the diffusion limitation, possibly caused by the limited ionic conductivity of NaAlCl4 melt or the poor wettability of sodium on the BASE. Therefore, improvements in the ionic conductivity of a secondary electrolyte and sodium wetting are desirable to further enhance the battery performance at low temperatures.