Now that is an interesting idea. They are AA size, cost about $1 each, and have 1500 - 3500 mAh storage.
As to sleep operation, I think the Lantern needs to run continuously to download files. Not like our lazy cats
So then, given we use the CR18650 for power hiccups and device moves, we would need a way to put the CHIP into a sleep mode. The CHIP reboot switch is hardwired to do reboots as @Abhishek says , so that won't work, but perhaps a software routine which recognizes a disruption of external USB power to start a sleep sequence that gets cancelled when USB power is restored. Ken