Are anyone building the battery you described?

No. The holy grail goes undiscovered for now.

Seems to be some progress on learning more about it: https://news.mit.edu/2026/discovery-helps-explain-why-solid-...

Aliens somewhere, presumably.

Isn't their tech powered by portable black holes?

The regulatory approval for those is just too expensive in practice. I mean you blow up one star due to cost cutting and nobody trusts the tech ever again...