You didn't state if you wanted one time flex or many times so here is good place to start if you are considering any temperature. If you want a one time use just apply a rigidizer after forming. For a many time use just apply as is.
There are other approaches like those mentioned above so if you will comeback with more details of whats required you can get a better answer.
I'm trying to install a battery pack into a small backpack. The problem is that the batteries heat up while being used. So I wanted a flexible, lightweight materieral to put between my back and the batteries to reflect or sheild the heat.
I agree those rechargeable batteries explode from too much heat so I would not advise that. You'd have to conduct the heat away from you but still make sure it gets out from the battery. I.e. buy an extruded aluminium heatsink and glue that with thermal epoxy to the battery.
It should be possible to make a spacer, padded on your side that allows for air circulation around the battery so as to keep it at reasonable temperature.
It might be wise to check the safe operating temperature vs the actual operating temperature under the worst possible conditions.
Regards
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Air is the best insulator (well actually a vacuum is), but your batteries may get hot, depending (as people have already pointed out). What kind of batteries are you going to use? What is the energy density and the largest power draw you expect to see. Calculate from this the steady state temp... The battery manufacturer should have a specification max and min temp for charge/use.