As I said below (or possibly above) this material is only energy-dense when it's in its most compressed state. To get the energy out you have to expand it again, at which point you find that you need a vast apparatus to store your energy.

Remember, the state of the art in energy storage is a metal tank filled with liquid hydrocarbons. It's pretty damn hard to beat for price, reliability, safety [ * ] and energy density. What we really want is a more efficient way of producing those hydrocarbons out of water, CO2 and energy.

[ * ] Relative to most ways of storing large quantities of energy, that is.

Indeed.

What comes close, if you do not care about moving around with energy stored in it and safety at the same time, is the humble fly-wheel.

Also as long as you don't care about energy leakage -- no matter how good your bearings are, they're imperfect. Flywheels store energy for minutes or hours, batteries for days or weeks, non-volatile hydrocarbons for millions of years.