what about pumped storage hydroelectricity? It currently accounts for 95%-99% of potential energy storage capacity, is far cheaper and more efficient than current commercially available chemical battery storage methods.
That's primary hydropower, where there is an external water source constantly carrying organic material into the reservoir. A closed pumped hydro system could have much less organic input (and possibly could exploit the pumping to keep the water oxygenated, so methane would not be produced.)