One I'm familiar with (but don't know deeply about) is Iron-air batteries [1]. Form Energy [2], an interesting grid-storage startup uses them. They're not energy-dense whatsoever, but very cheap, which makes them economical for that application.

[1]: https://en.wikipedia.org/wiki/Metal%E2%80%93air_electrochemi...

[2]: https://formenergy.com/technology/battery-technology/