There's another approach I recently learned about: Converting coal plants to burn metal fuel (iron powder <=> rust):

See e.g.: https://www.sciencedirect.com/science/article/pii/S2666352X2...

Volumetric energy density is pretty decent, and it's easy and safe to store (albeit heavy). The big appeal is that it seems feasible to convert coal plants while touching only their burner (there is a smallscale prototype running in Germany AFAIK). You could also potentially even transport the fuel (not sure how the losses look on that though since energy/weight is not very high; maybe ships or trains could still work though).

Drawback is that regenerating the iron powder needs (green) hydrogen and the cycle efficiency is low.

But this could be very helpful for countries like Germany, Poland or especially China to get dispatchable green power from former coal power stations.