The Tesla Roadster, which indeed uses Li-ion batteries, stores 56 kWh of electric energy and delivers up to 215 kW of electricpower (a NiMH battery pack with the same electric energy would weigh over twice as much).
The C-Max Energi has an EV mode button on the center stack that lets the driver choose between three modes: electric-only driving, normal hybrid mode (which blends electric and gasoline engine power as appropriate) or EV-later mode, which reserves the battery packpower for later use.