Neutron imaging of high-temperature Na-Zn Cells: implications for cell design and fabrication

TL;DR

This study uses neutron radiography to visualize gas bubble formation in high-temperature Na-Zn electrochemical cells, revealing how bubbles hinder performance and suggesting improvements in diaphragm design or fabrication processes.

Contribution

First neutron imaging of Na-Zn cells during operation, identifying bubble formation issues and their impact on cell resistance, guiding future design improvements.

Findings

Gas bubbles are trapped beneath diaphragms during fabrication.

Bubbles significantly increase cell resistance during cycling.

Improved diaphragm design or fabrication can mitigate bubble formation.

Abstract

Electrochemical cells employing Sodium (Na) and Zinc (Zn) electrodes and a chloride salt electrolyte have been imaged by neutron radiography during cycling. The use of such abundant raw materials confers a very low energy-normalised cost to the Na-Zn system, but its operation requires them to be entirely molten, and therefore to be operated at 600 {\deg}C. To suppress the self-discharge that results from this all-molten configuration, porous ceramic diaphragms are used to partition the electrolyte and thereby impede the movement of the Zn2+ ions responsible towards the Na electrode. Neutron images reveal large gas bubbles trapped beneath these diaphragms, formed during the cell fabrication process due to the large volume change that accompanies melting/solidifying of the electrolyte. Cycling data confirm that these bubbles interfere with cell operation by substantially increasing ohmic resistance. They indicate the need for either a new diaphragm design, or a cell fabrication process that prevents their formation in the first instance.