If they can scale I could imagine them making good battery arrays. The largest batteries in the world are not a chemical batteries currently however. Think 20-40 GWh vs 400 MWh (at a cursory search) being the largest current chemical battery arrays. Pumped storage plants can't be built everywhere, and are massively huge infrastructure projects, but as we switch more and more to solar energy we are going to need to make it through the nights somehow. They currently are often used to provide grid stability by discharging during peak hours, and buying cheap power to pump water back up during peak production/lowest demand when electricity is cheapest. This works especially well already with many renewables, including wind farms when you don't have control over production. If they can build sodium ion battery arrays cheaply enough though, pumped storage plants could maybe just be obsoleted.