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.
Neat project and approach! I got fed up with expensive registries and ended up self-hosting Zot [1], but this seems way easier for some use cases. Does anyone else wish there was an easy-to-configure, cheap & usage-based, private registry service?
[1]: https://en.wikipedia.org/wiki/Metal%E2%80%93air_electrochemi...
[2]: https://formenergy.com/technology/battery-technology/