Sodium Ion Batteries are, apparently, now in production. As in, this isn't theoretical. Customers are purchasing and using them now.
How Na-Ion batteries differ from Li-Ion batteries:
First, Na is very similar to Li but heavier so in general they operate the same way. But, obviously, there's no Li in a Na-Ion battery. But apparently neither is there any cobalt or copper or graphite.
No Li, Co, and no Cu means Na-Ion batteries are much less costly in terms of materials but also in terms of labor and environmental hazards.
But Na is heavier than Li so what about energy density? Well, there's a claim here that these Na-Ion batteries have comparable energy density to Li-Ion batteries. Which would mean there must be a mass-savings in some of the other constituents of the battery, right?
Well, whatever. It's something I'll be keeping an eye on. If it all works out this could be a very good thing indeed.