I think I was mixing those up with thermal energy storage and thermal batteries.

https://en.m.wikipedia.org/wiki/Thermal_energy_storage

https://en.m.wikipedia.org/wiki/Molten-salt_battery#Thermal_batteries_(non-rechargeable)

Sodium batteries do have a high temperature type, but it does look like they are non rechargeable and do generate electricity directly. The thermal energy storage only stores thermal energy rather than electricity, but they use sand.