Just a little word of educated advice:
You can only pack "so much" energy into any given amount of realestate, aka - the size of the battery.
In the past... we'd see some batteries with "spacers" that would fill a larger compartment (Nokia did this years ago, famously... in the early-mid 1990's). The idea was to then sell a full battery as the "Extended" one... when actually they only gave you a crippled one with the phone to start with.
These days - its all about size. The energy density is as high as current technology can handle. Any batter that claims to be "high capacity" when it fits in the exact same space, is in fact... not high capacity.
How do they get away with it? The secret is how they measure the "capacity" in mAh. You can measure the battery at current loads that are approximately the same as the phone uses, and get a true mAh result. Or, you can cheat, and measure the battery at such microscopically low drain conditions that the numbers are skewed.
This is similar to those tiny yet very high wattage computer speakers you'd see years ago. They'd skew the time frame of the reading, to make it "seem like" for that one brief instant (less than a millisecond) that 2w monolithic amp chip was dumping out 150w of power.
Measurement trickery.
