Did Apple introduce a new battery for the iPhone 6/6s?

[iMore Question]

I bought an iPhone 6s at an Apple Store today and the battery was completely drained when I got home and plugged it in. Another iPhone 6s I bought online came 50% charged (as I've read it should be to preserve the life of the battery). The Apple rep at the store told me that the iPhone 6 has a new battery that is not the same as the old lithium ones. These new batteries go into a "sleep mode" and then hibernation after a certain amount of time. He assured me that it will have no effect on the battery's ability to hold a charge or its lifespan. I am dubious as I can't find anything online about a new battery being introduced for the 6 or 6s and also, if what he said is true, when the phone comes "out of hibernation" the phone should still have some of juice in it, right? Do I need to go back to the Apple Store for an exchange?