Their patent[1] says they use sodium chloride as the source of sodium ions. I don't know how they'll deal with the hot salt corroding stuff. Probably ceramics, superalloys, and/or special coatings. Like other types of engines, there are tradeoffs between unit cost, maintenance cost, and unit lifetime. Some applications will prefer a high unit cost with a long lifetime and low maintenance (generators at construction or mining sites). Others will prefer cheaper engines that require more maintenance (drones). And others will prefer very cheap engines that only need to work for a few hours (cruise missiles).

If I had to bet, I'd say this idea is not likely to succeed. But the upside of success is so high that it's worth pursuing.

1. https://patents.google.com/patent/US12136898B2/en?oq=US12136...