In the german forums (vectric-forum.de or twizy-online.de) there has been some discussion about buying the battery. As Renault/RCI doesn’t offer this possibility, some forum member has contacted Kreisel (a firm that specialises in conversions, and for example offers a VW Caddy with 350km range).
After some mails, they have made a firm offer. If they get 10 purchasers, they will develop a 13kWh battery, including BMS and weighting about 70kg (original Twizy has 6.1 usable or 7kWh total, in 100kg). The price would be 6900 Eur, I think before taxes. It would come with warranty; the battery will keep above 80% of it’s original capacity for at least 180000km.
There is also a second option. Another forum member, which already has developed and used a range extender and added extra chargers to his Twizy, has revealed that at his company they are also developing a new battery. Probably they will let you choose a capacity (12 or 17kWh) and they will also offer a new charger that can charge with 3 or 6kW instead of the 2.2kW that the original charger does. A type 2 plug (Mennekes) would be an option, too. Price is unknow for now, but it is expected to be somewhat cheaper than the Kreisel one.
I would rather prefer a smaller one, more similar to the original Twizy one, so about 50-60% of what they offer if that would mean also a similar reduction in price. The range is not a problem for me right now, and the extra price is a bit hard to justify. Renault’s battery is enough for me, but they are not selling it and the leasing contract is unacceptable.