On the quantum mechanics requirement - do you not think a dedicated team of experimenters could get quite a long way by systematic trial and error, without a deep theoretical understanding?
To take a simplified example - did Thomas Edison have a deep understanding of what was happening in a lightbulb filament? Or did he just try 10,000 different ways to make a lightbulb until he converged on a good one?
Good to know that steampunk could have been practical!
It's not just about the discovery - though that won the Nobel Prize. You have to be able to reliably lay down sub-100nm thick layers of material. A tall order pre-1960s.
You also need to develop separator films with the correctly sized pores with very few defects or the battery eats itself. Pores big enough for lithium to pass but nothing else. And a stable non-liquid electrolyte.
No one could even conceive of such ideas in 1900 let alone attempt to make such things. They didn't even know where to start looking to grasp the problem, let alone any solutions to try.
To take a simplified example - did Thomas Edison have a deep understanding of what was happening in a lightbulb filament? Or did he just try 10,000 different ways to make a lightbulb until he converged on a good one?
Good to know that steampunk could have been practical!