That might be how Apple was able to give its battery life a boost, even though the size of its battery compartment isn't any different from its predecessors'. Using metal as a pouch also leads to a battery that's stronger and more puncture-resistant. While iFixit wasn't sure what metal Apple used to make the casing, the patent says the layer could be made of aluminum, nickel, steel or other alloys. Whether Apple is planning to use the design for its phones and laptops isn't clear -- but using it on a small device seems to be a good way to test it out.