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.
Yeah. AA batteries suck though! I'd be happy if they used 14500 cells or something. but the form factor was probably the issue, the controller is a chonk in the middle already with the flat battery pack
18650 is awesome, a good balance of weight to capacity. They are the standard cells used in laptops, vapes, small powerbanks, power tool batteries, and so on. They can also go into a fairly standard charger for AA and AAA batteries and give a lovely nominal 3.7V.
That said, pouches are better for inside a device like a controller. The weight of a battery is significantly influenced by the casing. A pouch is almost entirely capacity, a cell like an 18650 or AA is largely the metal of the casing. If you have the pouch inside the plastic of the device you can save that weight.
Pouch cells suck, there are no standard sizes and they like to puff up and break open the case of whatever they are inside of.