I just went and took mine apart, it's a Logitech Performance MX, and it uses a NiMi battery: 1.2v, 180mA, 1900mAh.

The battery door pops on and off with a fingernail.

They really do look like dead-ringers for AA batteries. I bet you could run the mouse off of a regular AA as long as you didn't try to plug it in!

That is an AA battery. A web search suggests those shipped with Panasonic Eneeloops, which are probably the best-regarded NiMH AA for general use due to their shelf stability and long service life.