Solid state batteries don't over-heat or catch on fire, they operate even when damaged & tolerate a range of temperatures.
Importantly for e-aviation, they can be stacked on top of each other without casings, making them lighter than lithium-ion alternatives.
Unfortunately, they discharge their power slowly. That is not ideal if you need to rapidly increase power consumption, such as when applying thrust.
But NASA researchers have managed to increase their solid-state battery’s discharge rate by a factor of 10 – and then by another factor of 5 - in the last year.