I worked for a company that did a huge amount of EV battery research in the 80s and 90s. Early on, liquid sodium sulphur batteries were thought to are in contention. However, the liquid sodium sulphur had to be kept hot (IIRC, about 700F) and leaks were a serious problem. One test car battery leaked onto the asphalt, caught fire, and burned up the prototype car and the a couple of vehicles on either side of it in the parking lot.
Like I said - an ENORMOUS amount of energy stored in an ignitable liquid electrolyte = potential to be on the nightly news! :)