Flywheels for energy storage -- Pros and cons as compared to batteries

This is a fascinating article looking at the somewhat low-tech (at least in its early incarnations) flywheel for energy storage, relative to batteries.

The charts are great for comparing the Specific Mass and Specific Power of different devices. In particular, while traditional batteries can store more energy in a smaller place, flywheels show promise when energy needs to be quickly absorbed or deployed, for instance in breaking / accelerating.

I was awestruck by this statement:

"[A] flywheel with 1 kg and occupying about half litre could store the energy needed to bring a car moving at 100 Km/h to a standstill."

The article also mentions some interesting tech involved in the newest flywheels: interesting materials and magnets to reduce friction.