We call such cars "Hybrid Cars" here in Germany. A lot of manufacturers are coming up with them - always proclaiming they have "invented some". But if you take a closer look you'll figure out, that Toyota has the Prius out since years!
The system, the Chevy Volt uses is quite the same the Toyota Prius uses - and that works really well.
Opel (Vauxhall) comes up with a completely electric car, the Opel Ampera. The car is running electrically and if the battery is low a regular engine starts to reload the battery. First test say it's a well functioning system.