You can't. Battery technology has been in a complete standstill for years, and basically any mobile battery from any manufacturer has the same capacity/volume/weight ratios. The thing Nintendo used at 3DS's launch is not "too cheap" or "too low quality". No such thing is even available. It was state of the art, and it still is.Longer battery life with the same (or even smaller) battery is possible when you can increase the energy efficiency and the quality of the battery itself.
It might take 5 or 10 years until batteries get better again.