Where did you see 14500 LiFePo4?
I've only seen that in RCR123. If you have seen it, please spill because there is much demand here for that.
I was going to suggest eneloop, but I see you are looking at Li-on. Note that 3.6 V batteries don't play nice with the Fenix, if you want medium and low modes. If you just what turbo bright, then it works well.
AW (here) is a good source for batteries.