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Firms are exploring sodium batteries as an alternative to lithium
(www.economist.com)
This is a most excellent place for technology news and articles.
I'm not thinking of reactors, though I am aware that molten sodium is used as a coolant fluid. It seems that I was remembering an off-hand comment in a MinutePhysics video from a few years ago. Molten sodium batteries do exist, but regular sodium batteries also exist.