I have no idea if batteries are Musk's primary focus here, but creating very cheap and dense storage for electricity WOULD be as world-changing as you can get:

I have no idea if batteries are Musk's primary focus here, but creating very cheap and dense storage for electricity WOULD be as world-changing as you can get: if you solve that problem, you can't just replace combustion engines in cars, you can also use solar and wind power for base load on the grid. This would be much more world-changing than cheap rockets.