Sodium is very clearly a contender for stationary batteries. It may even be competitive for surface mobile applications like cars... But there is just no way it can compete on air transport.

I keep being intrigued by vanadium redox flow batteries, and I've seen some in the real world in stationary applications. And the prospect of instant charging by spit-swapping in a mobile application is pretty wild.

How scarce is vanadium, though?

Vanadium has an horrible weigh/charge ratio. That's why it's used in stationary applications.

Sodium redox flow batteries are all the hype nowadays, because sodium is much cheaper, sulfur for the oxydizer is also very cheap, weight/charge is very high, and volume/charge (important for stationary applications) is also much higher. But I think it's stuck in a patent minefield.

It’s like 2 percent by mass of spent iron slag.

Now now, no need to be rude.

/