Back in the 1970’s, I remember reading about FORD Motor. Co. doing research into electric cars. Ford scientists developed a battery which used sodium and sulphur as the electrolyte. Unfortunately, the battery had to be kept at the melting point of sulphur. Was it a high capacity battery? Of was the thing simply too dangerous to be used in vehicles? Sodium is highly reactive, and molten sulphur pretty bad as well.
I think you kind of answered your own question, but there is some good info HERE on the difficulties, a few incidents and a newer version of the technology being worked on.