Undecided did a couple of videos on this technology. It seems quite useful for heat storage - as other commenters have noted, it isn't that efficient for pure electric <-> electric storage.

* How a Sand Battery Could Change the Energy Game - https://www.youtube.com/watch?v=G6ZrM-IZlTE

* Sand Batteries for Home Usage - https://www.youtube.com/watch?v=KVqHYNE2QwE