[deepdriver]

Sand and basalt-based thermal batteries sound promising. Essentially just heat up rocks in insulated containers, then run Rankin- or Stirling-cycle generators off the stored heat. Power-to-weight's probably awful, but that doesn't matter for grid applications, just cost per kWh:

https://newatlas.com/energy/sand-battery-polar-night/

https://scitechdaily.com/gridscale-storing-renewable-energy-...