Was literally just talking to my wife about this yesterday. Sodium Ion is good shit. Eco friendly, doesn't have the degradation or fire issues of Lithium, and is WAY cheaper. Only issue is energy density. But if you have an application where that isn't an issue, or can expand to use more cells (Electric bikes come to mind), they're a fantastic solution. I'd love to see some commuter cars start using this tech. Would get rid of a few of the issues that current electric cars currently have.
The Wikipedia page(en.wikipedia.org) has more information. Apparently they can be discharged to 0V without damage, good for shipping more safely, and just not ruining them if they are left discharged or a load is left connected.