Management of the liquid alkali-metal in anodes of ZEBRA (sodium–nickel–chloride) or sodium-sulfur batteries is crucial for efficient cell design. Porous carbon materials can take up the liquid metal during battery charging and release it during discharge. Also, porous carbons provide a conductive framework at a low volume penalty even when the anode is stripped completely empty.

We use X-ray computed tomography (XCT) to study wetting interactions between liquid-alkali metals (sodium, potassium and their alloys) and porous carbon foams and felts. We compare as-received carbon samples with samples that underwent a heat treatment at 500 °C in air. The liquid alkali metals show no attraction to the as-received carbon samples. In contrast, the liquid metals are strongly attracted to the heat-treated samples and show spontaneous wetting, likely due to oxygen-rich surface groups introduced during heat treatment. We investigate the carbon materials and their wetting properties with different techniques.
Uniquely, XCT measurements allow us to obtain a non-destructive understanding of these complex wetting interactions revealing pore-sizes, pore-filling and trapped gas.

When in use in a symmetric cell with liquid alkali-metal as electrode, the heat-treated carbon felt shows lower overpotential than the as-received sample. This is due to improved contact between the liquid alkali metal and the solid electrolyte. The heat -treated carbon felt also shows the capability to transport liquid alkali metals vertically over multiple centimeter solely relying on capillary forces.

These results show a possibility of using porous carbon in liquid alkali-metal batteries and showcase the benefits of using X-ray CT to study wetting phenomena in batteries.