Flywheel technology were an area of research over a decade ago when battery research was still in its infancy. IIRC, GM had an electric car concept that had a h

Flywheel technology were an area of research over a decade ago when battery research was still in its infancy. IIRC, GM had an electric car concept that had a huge fly wheel behind the cabin. But your options were slow, heavy (thus inefficient) flywheels or light, fast flywheels. And by fast I mean 100,000 RPM. At those speeds they were using Kevlar to keep it from flying apart, making it impractical because of the price of kevlar.