The idea of super-capacitors in cars is positively horrifying, knowing how explosive just the small ones can be. At grid level though, is there anything that speaks against them?

Edit: to answer my own question, they look pretty good [1]. Significantly lower storage density than batteries (roughly 50Wh/L versus 420Wh/L for Li-ion), but still dense enough for this to be workable at grid level. The linked presentation proposes converting decommissioned power plants into grid-level capacitor storage facilities, since the transmission switchyards are often intact. Furthermore, all the technology is available today, and when built, capacitors require almost zero maintenance.

[1]: https://www.energy.gov/sites/prod/files/piprod/documents/Ses...