Abstract
Electrochemical cells employing sodium (Na) and zinc (Zn) electrodes and chloride salt electrolytes have been imaged by neutron radiography during cycling. The use of such abundant raw materials in these cells confers a very low energy-normalized cost, but their charge-transfer mechanisms require these components to be entirely molten, necessitating operation at 600°C. Such a high temperature is advantageous in terms of charge-transfer kinetics and ohmic resistance reduction, but it also accelerates self-discharge because ions of both electrode metals are simultaneously present in the molten electrolyte (albeit mostly at opposite ends). Porous ceramic diaphragms are typically used to partition the electrolyte into regions that can only exchange ions by diffusion, thereby retarding this self-discharge process. However, the neutron images presented here reveal large gas bubbles trapped beneath these diaphragms, formed during the cell fabrication process. These arise from the large decrease in volume that accompanies solidification of the electrolyte; a necessary intermediate step between cell fabrication and operation. Cycling data confirm that these bubbles interfere with cell operation by substantially increasing ohmic resistance. These data indicate the need for a new diaphragm design (or some alternative self-discharge suppression mechanism), or a cell fabrication process that prevents bubble formation at the outset.