What matters is not joules per unit mass or unit volume, but per dollar, and also watts charging or discharging per dollar. Gravity and compressed air systems e

What matters is not joules per unit mass or unit volume, but per dollar, and also watts charging or discharging per dollar. Gravity and compressed air systems excel on that axis (Energy Vault excepted, of course). Chemical synthesis systems suffer on cost per watt to charge, but hit it out of the park on joules stored per dollar. Further pluses are bulk-shippable storage media that may be bought and sold, and can be discharged using existing gas turbines, even in places where you have not installed any synthesis apparatus.