You can't increase RPM very far. The flywheel will tear apart. If you research the topic you'll find they have to use fancy composites to get to 100,000 RPM. A

You can't increase RPM very far. The flywheel will tear apart. If you research the topic you'll find they have to use fancy composites to get to 100,000 RPM. A metal wheel would blow up by 50,000. Train and car regenerative breaking uses flywheels. In trains they don't spin fast, but they're heavy. That's another way of increasing storage density, use a heavier wheel at a moderate speed. Neutron star matter would be perfect.