Dave5

A growing number of companies have recently produced battery systems using common rocks that can connect directly to wind and solar, or electricity sourced through the grid. When the energy is captured, the system turns it into heat, which is then stored in the rocks. Later, when users need power, the heat is then converted into superheated steam or electricity that can be used for energy. For example, Brenmiller Energy uses crushed volcanic rock; Antora Energy makes use of graphite; and Rondo uses bricks.

https://www.thedailybeast.com/the-green-battery-of-the-future-is-made-of-rocks?ref=scroll