Can the Fenix RC40 use other batteries besides lithium-ion?
No, the Fenix RC40 will only operate with its included proprietary battery that it comes with. The RC40 will not be able to run off any other battery source.
If you are looking for a powerful search and rescue flashlight that can use standard removable lithium-ion batteries, check out the LR40R. It is a 12000 lumen light that comes with a battery pack but can also be powered by 4 standard 18650 batteries.