You can already replace the battery. You can purchase a replacement here, along with all the tools you need to replace it if you don't already have them: https://www.ifixit.com/products/steam-deck-battery
There's also a full video guide on the process.
Valve worked with iFixit to make sure they have replacements available for every part in the deck. Valve was very up front that they wanted to enable people to do their own repairs and upgrades.
True, but I was thinking there might be a smaller, incremental improvement in lithium ion batteries each year. Am I wrong about that? Are we going nowhere with lithium anymore?
There are smaller improvements each year, and from looking at it, it seems like EV automobiles and other markets are helping drive increasingly significant gains actually. I'm not sure on how much of the emergency density gains actually apply to small batteries though, or if it's more about improvements in larger batteries. Either way it's less stagnant than I thought.