Lithium iron phosphate cells have a working voltage of 3.2. That means if you have an appliance like a camera that uses 2 AA's or an LED flashlight that uses 3 AAA's a single LiFePO4 cell will probably runs the appliance.
I shimmed a LiFePO4 cell with 2 strips of foam tape and stretched the spring on the base of this flashlight I bought as Home Depot and it works fine with a single LiFePO4 cell instead of 3 AAA's.
Step 1: Using Rechargeable Lithium Iron Phosphate AA Cells Instead of Alkalines
AA cell size is 14500. Lithium iron phosphate cells do not have as high a voltage as lithium cobalt, but they also don't catch fire if shorted. LiCo devolve hydrogen gas when shorted, that's what catches fire. LiFePO4 don't gas when shorted but they will get very hot.
LiFePO4 cells have a working voltage of 3.2, compared to to LiCo at 3.7. They require a different charger. The pictured Enedepots charger from eBay China has become unavailable but this one will work fine:
Two AA alkaline cells in series have a fresh voltage of 3.2 and a working voltage of 3.0. That means if you substitute one 3.2 LiFePO4 cell and a dummy cell you have the same voltage, but at 1/2 the capacity/run time. The ebay link below is for 8 cells plus 8 dummy cells.
I've made dummy cells from dead NiCad AA's because they are dried out and won't leak. A nail cut down will also work but steel gets a light coating of rust easily and won't conduct well. You can solder a copper wire to short the cell or solder or tab weld a nickel strip. You can also buy dummy AA cells alone from eBay.
Don't believe any ad for a AA sized LiFePO4 cell with a capacity above 600mah. No such thing. Even better "600mah" cells are really about 550 in real life. Exaggerated claims for capacity are common in the rechargeable battery business, not just these.
The Canon Powershot 120SX camera I took these pictures with was using one LiFePO4 and a homemade dummy cell. I originally did this hack because the camera works so poorly with AA alkalines. It was the last camera in the series that used alkaline cells. The voltage drops off too much under load with alkalines and the camera would shut down even with fresh batteries.