NASA says its sodium selenium prototype battery has an energy density of 500 watt-hours per kilogram, which is about double that of conventional lithium-ion batteries.
That battery is up to 40% lighter. Their sodium selenium battery cells can be stacked one on top of the other with no casing around them. Eliminating the casing around individual cells means more energy storage within a given amount of space — a huge advantage when trying to fit batteries into the structure of an aircraft. It also means the cooling systems for the cells can be smaller and lighter.
The solid-state sodium selenium batteries from NASA are able to withstand temperatures twice as hot as conventional lithium-ion batteries. In addition, they are less affected by changes in pressure.
Sounds promising for aircrafts or higher power systems, which have some stricter requierments. Not sure if it is really better in regards of mining/refining.