I assume you mean flywheels. Flywheel energy storage seemed appealing until I worked on it in an energy storage startup. Flywheels require expensive protection

I assume you mean flywheels. Flywheel energy storage seemed appealing until I worked on it in an energy storage startup. Flywheels require expensive protection against catastrophic failure. The costly part isn't fragment containment, that's affordable. But angular momentum is conserved. That means when a flywheel crashes, it will exert a very large torque on its containment structure. If the containment breaks loose, it can do a lot of damage in moments. Containment that's safe against torque failure is very expensive. Flywheels also have to run in a vacuum. If the rotor is s composite material, there has to be an active vacuum pump because composites outgas forever. Additional system cost and failure point. If you don't use composite rotors, the energy storage density goes down because tensile strength of metals is so low compared to composite fiber materials. Low energy density increases overall cost/MWHr. Rotor life is limited by static fatigue and by high cycle fatigue, which have different failure mechanisms. For carbon, glass, or aramid fibers, you can count on a rotor MTTF of ~20 years if the rotor is operated so it never exceeds ~35% of the fiber's ultimate tensile strength. But that cuts into energy storage efficiency, driving the cost up more. There are some niches that flywheels can serve with competitive economics (grid intersection phase matching, for example). But overall, it's hard to see how flywheel economics can beat the declining cost curve of electrochemistry. The foreseeable cost floor, set by some of the system issues I listed, is too high.