A German energy storage company is working to make better lithium ion batteries for EV's/ They have recently shown off their 'Light Battery' concept in an i3, using a modular design to reduce costs and improve safety. This has allowed them to place a 100kWh pack into an i3, which has given it an impressive range of about 435 miles. As the Mini is believed to be using the same powerplant from the i3, this means that this new system could very well work here too.