The lithium-ion SM-BTR2-1Internal Shimano Battery for Di2 is meant for the new Dura-Ace 9070 groupset, but can be retrofitted to work for Ultegra Di2.
It can be mounted in your seatpost of elsewhere in the frame. Some tightening bands and snap rings are included, but mounting this battery inside different manufacturer's seatposts is different in each case. Some creativity is required to mount it very tightly (ie. you may need some foam, etc).
The battery provides a long service life and a high battery performance (up to 2,000km with one charge).
Note: If mounted in your seatpost, you will need to charge this battery using the SM-BCR2 external charger. It makes no sense to remove your battey (and seatpost) every time you need to recharge, so instead, plug the USB connection from the charger into your bike's "A" junction, and the charge will pass through the Di2 wiring to the battery. Cool!