Inventing rope isn't obvious, but it's something a single individual can do. You don't need a team of people that each has studied some field for twenty years and a brilliant idea, you just need a brilliant idea. Inventing a device to cheaply store energy at grid scale is not something a single person can do, ever.
Inventing a better battery is something a single person can do in the lab. I worry that you are conflating inventing things and the engineering of scaling them out. Scaling things almost never was doable by one person, but the initial seed was and usually is.
A lot of money has been poured into battery technology over the last decade or two. We don't have much to show for it. That makes me believe that inventing a better battery is not as simple as it might seem. Coming up with a terrific chemistry is one thing, finding something that is can be manufactured at a good price is something very different.
You can make your own graphene with some charcoal and sticky tape. Graphene is an amazing material, but making it industrially useful is extremely hard.
