I also have a Fenix E01 and love it.

I use an Energizer Ultimate lithium battery in it (the ones that say 8X on the package, not the 4X ones). Lithium batteries have about a 15 year shelf life, don't leak, and these ones supposedly have 8 times the working life of an alkaline battery.

The combination of the E01 and a lithium battery makes a really great EDC.