They already exist in Chinese phones, the new one plus has insane battery life because of its 6000mah battery, while still being as thin as a normal phone.
Other phones targeting the Chinese market have reached 8000.
But companies like Apple and Samsung like to just sit on their laurels and sell the same thing again.
I believe it's been 5 years that some Chinese phones already have Silicon Carbon battery... Samsung/Apple was crazy slow on this, and later this year everyone will get "Shocked" when apple supposedly show up their new phone with the new battery...
Other phones targeting the Chinese market have reached 8000.
But companies like Apple and Samsung like to just sit on their laurels and sell the same thing again.