I suspect the best an E2L AA head can do is on
a single 3V Lithium primary. It's possible a single LiFePO4 may work, but why risk it? (it ~ destroying something valuable and not easily replaced) What is it you hope to accomplish? More light? More runtime? It already runs on 2x NiMH AA and Alkaline AA, and it will work on one CR123A.
Here's one thread on early backups with members discussing using liion. There are likely to be others.
If you can scare up search terms you can find more threads by using the builtin CPF site search or using Google Search by adding "site:candlepowerforums.com" to the end of the search terms.
But my recommendation is... whatever it is you want, you can find it in another light that is specified for it. A number of other CPF members are cowboys and will have the opposite opinion, to live dangerously, and possibly destroy or damage a valuable classic. But I think if our tools are important to us, we'll diligently take care of them.
Here's some ideas, E Series threaded heads that will run liion in varying voltages. But if you really want to have fun, you can try incan. There are
Elite head designer imposters available, also
without the hex antiroll. You can try to find Surefire lamps (MN01, MN02, MN03), or Mark has decent lamps, but also has LED towers if you just like the beam pattern but don't want to go incan.
I had Vinh mod my E2L AA and DSS heads with 4000K Nichia 219b, then traded them. I hated the Surefire TIR beam profile. I run Surefire Elite heads currently with Surefire MN01 in them. Incan has the benefits of constant current output and 100CRI, perfect color rendition. Surefire, iirc, uses constant current circuits in their LED heads, but also some of the worst LED color temperatures and color rendition ever conceived, which is a shame. But the LEDs can be swapped for something warmer and high CRI by experts like
Vinh.
I was lucky enough to get some Tad Customs E Sockets, and any E1ish lights I have run Tad's lamps, such as A3712, 3.7V 1.2A 80Lm, or A3718, 3.7V 1.8A 140Lm. Couldn't be happier with the brightness, color rendition and color temperature. If I need more brightness, I'll use another light with 1200Lm WA1185 or 1000Lm FM1794. And I learned the hard way not to EDC Surefire, lost a cherry E1e-BK. I'll EDC a 475Lm 4000K Zebralight SC5c Mk II when I'm off the property. I'd hate to lose it, too, but it would't hurt as bad as that E1e, and it's a light for all other occasions.
