Comment by adrian_b

The solid-state batteries with lithium do not have any significant advantage for having a solid electrolyte, but their advantage is in having an anode made of pure lithium, instead of lithium intercalated inside some electrode material.

This greatly increases the energy density, which is very important for a mobile application, but this has little importance for stationary applications, where not the energy per mass or per volume is important, but the energy per dollar.

Moreover, for stationary electric grid applications, besides the cost, the lifetime is extremely important. As mentioned by others, the companies which provide electric energy want lifetimes of the order of 30 years, which can be ensured with sodium batteries.

For now, the main factor that has prevented the use of the lithium batteries with solid electrolytes is the short lifetimes.

There are good chances that we will see cars with lithium batteries with solid electrolytes, but I do not believe that such batteries will ever be used for high-energy stationary applications.