They should be the default for solar installations and grid-level storage, but are too new. They can also replace lead-acid batteries for many applications. Lithium will still rule microelectronics and wearables, but all lower density stuff should switch to sodium. That being said, for cold environments like Scandinavia and the US Midwest & canada, sodium ion works better in both cold and heat swings than Lithium variants that it might be worth the tradeoff in capacity because in the long cold months, the reduced capacity and performance of lithium chemistries would completely close the gap anyways.