“Does that mean it has to be heated TO 200-400C to operate, or that it heats up to 200-400C when operating? If the latter is the case the excess heat can be used to generate steam to drive a turbine generator, to partly recharge itself or run other applications.”
It needs to be at that temp to work, according to the paper referenced in the article.
https://onlinelibrary.wiley.com/doi/10.1002/aenm.202203789
It works by diffusion of oxygen ions into and out of lattice vacancies in ceramic materials (plates) of different chemistry. No doubt this diffusion is negligible at room temp.
So it needs to be heated to that high temp to function. No doubt it will also generate heat in use, charging, discharging, and just sitting there. So yes, any heat flow from the battery necessary to keep it at a suitable operating temperature could, in part, be converted to useful work.
So the battery bank may use more power to maintain the 395-755 deg F. temperature than the power it stores?? Scale it up baby!! Energy density per volume/weight is key in my thinking, and the temp thing plus “... the oxygen-ion battery only achieves about a third of the energy density that one is used to from lithium-ion batteries” sounds like a “tech-killer” to me. We’ll see I guess.