Mobile phone charger does not have a complete charging circuit, the circuit design of control charging is inside the mobile phone, you can not directly use the mobile phone charger to charge the mobile phone battery.
Battery charging has never been charged with a voltage source, should be charged with a constant current source, 4.2V is just the limit voltage to terminate the charge, to make a constant current source with a maximum voltage of 4.2V on the phone, there must be more than 4.2V power supply voltage supply, not necessarily 5V. I have a large collection of mobile phone chargers of 6V, 7V, and even the no-load voltage of more than 8V.
In the same way, the external charging must be matched with a constant current source circuit, the charging circuit output to the battery voltage will not exceed 4.2V, is not directly carried out with 5V voltage adapter.