Planet Money covered this recently: https://www.npr.org/templates/transcript/transcript.php?stor...

Here's an entry on the storage facility: https://en.wikipedia.org/wiki/Bath_County_Pumped_Storage_Sta...

"The biggest battery in the world is a giant lake tucked away in the Appalachian Mountains on the border between Virginia and West Virginia"

One of the more promising things is actually water-based storage using back-to-back reservoirs. Water has a lot of mass per volume and is really low tech. And it's a proven strategy that has been operational for many decades.