Very interesting idea by a company called EV Clinic (originally based in Zagreb, Croatia). It’s a battery that sits in parallel with the existing vehicle battery and adds about 100km range. The one in this video is designed for the Tesla model S, and sits in the sub-boot space (that spot where you might put your charger cable and sensitive things like a laptop).
It includes cooling channels, so can be hooked into the water cooling loop of the car. Of course, this would probably rule it out (for now) in the i3, which uses the AC refrigerant to cool the battery.
Another interesting thing to note: It uses 50Ah Samsung SDI cells (NMC), 96 of them. So this is almost exactly the layout of the 22Ah battery of the original i3… but look how much smaller it is!