So you are hoping to run a tesla unit to a differential? There is a reason people use the entire unit as the rpm is a lot higher than an ICE with overdrive. There are better motors to do that with.
The industry is reaching the end of improvement for li-ion. There are a number of prototype solid state chemistries under test now. But because the variables in batteries are so many and can be mutually exclusive, everything needs thorough testing before anything is going to be ready for production. It's estimated that new battery tech takes about 10 years to get from laboratory breakthrough to production. The breakthroughs necessary may or mat not have happened with solid state. If they have we will probably start seeing solid state batteries around 2025. If the chemistries discovered don't pan out, then the breakthrough needed hasn't happened yet.
IF you want to better understand batteries, I'd suggest: https://genesisnanotech.wordpress.com/2019/02/16/todays-gamechanger-professor-yi-cui-stanford-how-far-can-batteries-go-building-a-better-battery-with-nanotechnology-architecture/ I'm "guessing" Li-Ion probably next 20 years (10 years for sure) Solid State the wild card - graphene production too.