I don't think that's necessarily a useful question at this point, because some of the basic thermodynamic assumptions don't work out for deciding between batteries vs. using a fuel-burning generator. Normally, it's pretty obvious that "Hey, if we're going to burn hydrocarbons to produce thrust, we might as well do it directly and not deal with necessary conversion efficiency losses between heat -> electricity -> heat." Since they're dealing with RF-generated gas plasmas, though, there might be efficiency gains with acceptable configurations and lower intermediate losses, etc. over traditional systems that makes those conversions acceptable. I've worked in a lab accelerating gas plasmas using helicon antenna, and as the article mentions, scaling those systems isn't a trivial implementation detail. It's very tough to reason about them from first principles, or even small-scale models because of the non-linearity in the relevant physics.
So, I guess in summary, imho it's probably a bit too early to talking about what kind of battery would be used, since it's unclear from the current system that a usable version would actually use batteries vs. another form of energy storage.
So, I guess in summary, imho it's probably a bit too early to talking about what kind of battery would be used, since it's unclear from the current system that a usable version would actually use batteries vs. another form of energy storage.