Based on the description on their site, the controller includes a built-in battery: "8.39 Wh Li-ion battery, 35+ hours of gameplay… "
That was disappointing for me. Specially condidering the Steam Frame’s controllers make use of AA batteries: “One replaceable AA battery per controller, 40hr battery life”
AA Batteries might not be as convenient to use, but being able to replace them is a great advantage. All my Xbox360 controllers still work fine, but none of my PS3’ Dualshock 3s.
The official docking station could be used to recharge (rechargables) AA batteries so the functionality could remain the same.
Where have Steam said they are partnering with a company to offer replacement parts? As far as I’ve read, Valve have not provided any details around sourcing replacement batteries for the Steam Controller. All they’ve said is that the controller is able to be serviced by opening via the screws and clips - ie it’s not glued closed like many user-hostile companies, eg Google Stadia.
This is in no way the same as old smart phones (eg Galaxy S1-S5) toolless battery replacement, where you would just slide off the battery cover, pop out the old battery insert new, click battery cover back on. PS3 Dualschock 3 controllers are also exactly as user-serviceable - clips and screws, no glue. So I agree with OP: batteries that are built for toolless user replacement to a standard format are far superior. This is just asking for e-waste.