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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.
There are cylindrical cells available. The capacity is pretty low, 18650 cells are around 1.5AH and 26700 cells are around 3.5AH. They discharge down to 1.5V, so you will get less capacity if you use them in something designed for lithium cells.