There is of course a very simple way of storing electricity:

At night, when cheap, use the electricity to pump water from the sea or a lower reservoir or lake into a reservoir or lake at an elevation.

During the day, when you most need the electricity, you use the difference in height to drive a generator and thus give back to the grid. The water would go back into the sea or the reservoir at the lower level.

The best solution would be two reservoirs of fresh water so as not to salinate other areas. The lakes could of course be used for recreation.