can i put a battery of 2800mah on the iphone 5?

There are suppliers who have batteries that can be combined with the iPhone, but Apple does not offer anything like that. Be sure to check and see that the voltage and current supplied are compatible with the iPhone input.

You cannot replace the iPhone battery with a larger battery.

If you are talking about an external battery you can use any capacity battery as long as the voltage and polarity is correct. You could use a 100,000 mah battery as the phone will only draw the amperage that it needs. Voltage and polarity is critical, amperage is not as long as you have enough.