If L.A. is going to stop burning fossil fuels by 2045 a key goal of Mayor Eric Garcetti’s proposed Green New Deal it must store a lot more of the excess solar and wind energy it produces during the day so it doesn’t have to rely on gas and coal energy to power the city when the sun sets and the wind dies. There’s a growing focus on building big batteries — for example, the kind that use lithium ions. But L.A. needs energy storage that is far bigger than any traditional battery.