Here's a working sand battery in Finland. Heated to 500°C, it'll store heat for up to three months. This one heats all the town's municipal buildings, uses no chemicals, it doesn’t degrade and won’t catch on fire.
Seriously cool tech!
https://cleantechnica.com/2025/08/30/finnish-city-inaugurates-1-mw-100-mwh-sand-battery/