Visualization of Sodium Metal Anodes via Operando X-Ray and Optical Microscopy: Controlling the Morphological Evolution of Sodium Metal Plating

Because of the abundance and cost effectiveness of sodium, rechargeable sodium metal batteries have been widely studied to replace current lithium-ion batteries. However, there are some critical unresolved issues including the high reactivity of sodium, an unstable solid-electrolyte interphase (SEI), and sodium dendrite formation. While several studies have been conducted to understand sodium plating/stripping processes, only a very limited number of studies have been carried out under operando conditions. We have employed operando X-ray and optical imaging techniques to understand the mechanistic behavior of Na metal plating. The morphology of sodium metal plated on a copper electrode depends strongly on the salts and solvents used in the electrolyte. The addition of a fluorine-containing additive to a carbonate-based electrolyte, NaClO4 in propylene carbonate (PC):fluoroethylene carbonate (FEC), results in uniform sodium plating processes and much more stable cycling performance, compared to NaClO4 in PC, because of the formation of a stable SEI containing NaF. A NaF layer, on top of the sodium metal, leads to a much more uniform deposition of sodium and greatly enhanced cyclability.