The battery displayed in the I3 concept is constructed of cylindrical lithium-ion cells that carry the Saft brand, along with integrated charge management and state-of-health/state-of-charge measurement. The complete pack weighs approximately 90 kg, occupies 70 L of space, and has a usable capacity of approximately 3kWh to yield an electric-only range of about 10 miles. Air cooling is used, as JCI currently does not believe that liquid cooling makes sense for PHEV packs due to its greater complexity.
JCI is also working on a battery that will provide a range of 40 miles; however, this particular pack presents an issue with packaging volume as it's approximately four times the size as the pack on display. This is in direct contradiction with the overall smaller size of a longer-range plug-in hybrid vehicle; we assume that such a pack would also be rather costly, although that topic didn't get any attention.
Expect to see the first use of this lithium battery in Mercedes' upcoming S-class mild hybrid, albeit in a much smaller form.