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Australia struggling with oversupply of solar power
(www.abc.net.au)
This is a most excellent place for technology news and articles.
I don't know if the output of the desalination is what we actually need or how much refinement it would need, but the salt output would probably still outpace our ability to use it. Sodium is just 1 factor of building these newer batteries.
e.g Tesla has a factory with a 40gwh storage output when fully scaled, and it's taken years to get there. Cells weren't the only factor in that.