Sodium? Anybody remember the junior high chemistry demonstration when the teacher threw sodium into a container of water?
Perhaps that’s what they mean by “metastable?”
Sodium ion batteries use sodium ions not sodium metal. Is table salt flammable? It’s 50% sodium and 50% chlorine where Cl is a toxic gas that eats your lungs up it was used during WWI as a gas weapon. However in ion form neither element is harmful.
Solid state cells have no flammable liquid electrolytes which is the source of lithium ion batteries flammability.