Nowadays lithium is a very popular battery for devices because of its energy density, and ability to take many shapes to fit devices. Unfortunately, it is a volatile material if mishandled and can be dangerous to transport.
Lithium reacts when exposed to oxygen and can be very hard to stop once started. As such, special precautions are necessary to safely transport them. Without proper protections, the batteries could accidentally short or get punctured and cause a fire.
The lithium is layered inside the battery pack and if bent can short with the cathode side inside it. This can cause heat to build up and release gas inside the battery until it bursts. If this were to happen on a plane it could cause a lot of damage as it would be difficult to control.
Because of its dangerous nature unfortunately, DigiKey does not ship products containing lithium by air and that can affect shipping times and cost. Checking out with a lithium product will present this notice explaining further about the specific regulations.
Also, our available shipping methods are FedEx and UPS Ground.
Lithium battery diagram, https://www.aranca.com/knowledge-library/articles/ip-research/lithium-polymer-batteries