We have no idea what will work in the end. If anything will work at all.
All we know is nothing we tried so far works for solid state electrolyte, and we have a good understanding that if we can manage to get it to work, it should be significantly better than what we have with liquid electrolyte.
This is the materials science where you just have to keep trying different compounds for different elements and see if anything sticks. I.e. see how blue LED was developed. Problem here though is that there are way, way more factors involved in lithium batteries compared to blue LEDs. And that means way more possible permutations, and therefore way more testing.
So as far as we know, we could hit a correct chemistry tomorrow. Or in a day. Or in a month. Or in a decade. Or in a century. Or never, because working chemistry for this doesn't exist.
That's how inventing new things works. This isn't a game of Civilization, where you go through a certain amount of research points and you're guaranteed to get a new working invention as an outcome.