Invest in batteries

Edit: I mean focus solely on. It's a boring technology prone to disruption, used everywhere

Tesla has nothing obvious here to add to the marketplace --- most of their battery tech has typically been out sourced.

Lots of other companies with far more experience and expertice is battery research, development and manufacturing.

They're seem to be, with their factories and utility-scale systems: https://www.tesla.com/megapack

Preferably, solid state