Right now and for the foreseeable future, lithium ion batteries are a better alternative to flywheels, though they will see some use for applications that require high power but little total stores energy.

The power:energy ratio is high, but has few applications on the grid. Even lithium ion's typical 4:1 MW:MWh is a bit high.

The really unexplored territory is seasonal storage. Electricity to hydrogen, methanol, etc. have huge round trip efficiency hits, like 30%-50%, but may make a ton of sense.

It's telling that a company with a "complete solution" went bankrupt.