Anyone else remember being able to just pop open the case and swap out the batteries of old Nokia phones? Sounds like an obvious solution if you need to “charge” real quick

This is a very bad idea. We need your phone to be online to be able to locate your phone (to track you). /s

The actual paper is about improving a simulation applied to super capacitors, and is clearer IMO [0].

[0]: https://www.pnas.org/doi/10.1073/pnas.2401656121

Anyone have good resources that explain why we don’t just use super/ultra capacitors as batteries?

Is it going to happen eventually or is it not practical?

The energy densities are generally at least an order of magnitude lower and the cost is at least an order of magnitude higher. They serve different purposes.

My (naive) understanding is that capacitors charge very quickly, but also discharge very quickly. If you want to store a lot of power and deliver it all at once, then capacitors are good fit.

Phones, however, need a steady flow of low power, rather than a short-lived spike of high power.