Quicky #1 : Using Lithium AA Cells in Fenix LD12

Can we use Lithium AA cells in Fenix LD12 Flashlight?

Technical data:
Fenix LD12 Features a  CREE XP-G2 (R5) LED.

This LED is capable of handling up to 1.5A at 3Volts (max 3.1V).

LD12 datasheet:

• LD12 Flashlight is a 125Lm LED based Flashlight powered by one single AA 1.5V alkaline cell.
• The same LED is found in the 215lm LD22 (and hopefully the same constant current protection) powered by two AA 1.5V cells (3V).

Fenix Flashlights are protected by a Constant Current LED driver circuit [source: Fenix catalog].

Energizer L91 Ultimate Lithium AA cell datasheet:

• 1.5V (1.8V out of the box)
• max discharge: 3A continuous (5A pulse)

Do not confuse the L91 with the L92 which is limited to 1.5A or the Advanced Lithium (EA91 1.5A -EA92 1A) cells; those cells will work, but for a quite shorter time (i don't have enough reliable data to estimate the runtime yet).

--> To know more about these cells have a look at budgetlightforum.com user review here

Is it safe to use one Lithium AA battery to power LD12?

Yes.

• Energizer Ultimate Lithium Cells (L91) are 1.5V Cells (1.8V with no load, out of the box, wich is safe as the LED can run with up to 3.1V power source)
• These Cells can provide "up to" 3A but will NEVER burn the LED as the Constant Current driver will do its job.

But will it work with LD22?

Not sure if it will or not harm it : 

• 2*1.8V=3.6V which is higher than the 3.1V of the LED, the constant current driver should limit voltage to keep current at a fixed threshold, but i don't have an LD22. 
• There SHOULD have no issue, but who will try?