Cost increases linearly with capacity, which is undesirable at scale. You want sublinear scaling. For example, to store a year’s worth of US electrical generation using batteries would cost approximately 500 trillion dollars.

1) Huge number, but why would you need an entire year of energy in reserve 2) If there was a desire to build that many batteries, cost would surely decrease exponentially as new mines, factories, etc. open up and competition increases.