I just re-powered an Ergorapido with a 2200mAh LIPO battery, like ones used in drones and RC cars, and it works great. My unit had NiMH (metal hydride) batteries in it and the original charger circuit for those batteries was definitely lacking. The newer models of the Ergorapido use Li-Ion batteries and should be better. Don't get the one with NiMH batteries.
My original problem with the vac was anemic performance. For what it's worth I believe the original problem on my unit was caused by a defective power pack. If you see the blue "charge" lights blinking on and off a lot then you may have this problem. Get a correct replacement power pack from Electrolux to avoid battery damage or worse.
Li-Ion batteries are very sensitive to charge/discharge and can be dangerous if not done properly. To use the LIPO battery pack I had to add a "Protection board" to the vac that manages charge and discharge rates and levels. It was a lot of work to shoehorn everything in the vac body and there were fairly extensive wiring changes in both the vac and sweeper body.
If you look on Youtube you'll find videos of people who have taken a simpler approach for using a LIPO pack in the Electrolux stick vacs. They cut a small hole in the vac body to expose the balance charge connector of the battery pack and just buy a balance charger to plug into that connector.