This battery idea will go the same way as the 200 MPG gas engine.
Down a rabbit hole.
If there ever was such a thing...it didn't put out much HP. Like a Honda 50.
But as far as a real automobile getting 200 mpg, you have to understand that the efficiency of a gasoline engine, as of 1960, ranged up to 30%. To get up close to 100% is possible, but not with a heat engine but only with a fuel cell generating electricity. Even if you were to surmount all the obstacles which have kept fuel cells out of automobiles ever since their invention during the Apollo space program, you would only turn a 30 mpg car into a 100 mpg car.
Having said that 99% efficiency is possible with a fuel cell, I must add that I first heard the magic carburetor story from an uncle, easily 10 years before the invention of the fuel cell. I assume that the story urban legend long predates my first hearing of it.
Note that cars today do not have carburetors but rather have fuel injection - but even at that we do not see 50 mpg cars much. More like 30mpg for a car I would want to drive.