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World's largest sodium-ion battery goes into operation - Energy Storage
(www.ess-news.com)
This is a most excellent place for technology news and articles.
The ceiling is going to be lower than with lithium. Sodium ions themselves weigh about 3 times more than lithium, for the same +1 charge. So it's not just that sodium is a certain number of years behind lithium. It's that it'll likely plateau at a point permanently behind where lithium will likely be.