According to USPS shipping rules, lithium ion batteries can be shipped via air mail if they are inside of the device they are intended to be used with. So maybe this applies to air travel too?
I believe the rule is you can bring batteries containing up to a certain mass of lithium, and they interpret that as up to 100 WH of batteries. That is why you see so many 98 WH (26800 mAH) power banks. People like to bring them when they travel.