Can I damage my IPhone by overcharging
No you cannot overcharge it because careful account is made of the charge in the cells by internal circuitry which will not charge the battery when it is fully charged. Many of the technologies involved in modern, power dense batteries react very badly indeed to overcharging, so much so that a little circuit, usually built into the battery itself, acts as a kind of charge "accountant" to ensure it's impossible for things to get out-of-hand.