Is it the Voltage.... or is it just naturally warm?
Part of this is technical, but it helps to understand that heat is NORMAL, but excess heat and/or voltage is NOT... your phone/battery might be running hot, but it doesn't necessarily mean you have a voltage problem. Knowing the MAX voltage that your battery is reaching while charging is important, and remember, max voltage is not reached until the battery is usually over 80% charged. If your battery or phone is getting warm but NOT EXCEEDING 4.2 Volts at anytime during charge, it shouldn't be a voltage issue.
High charge levels and elevated temperatures (whether from charging or ambient air) hasten capacity loss. Loss rates vary by temperature: 6% loss at 0 °C (32 °F), 20% at 25 °C (77 °F), and 35% at 40 °C (104 °F).
(Many) Lithium-ion battery packs contain fail-safe circuitry that shuts down the battery when its voltage is outside the safe range of 3–4.2 V per cell. ....3.7 V nominal voltage with a 4.2 V max charge.
Typically, lithium-ion cells are charged with 4.2 ± 0.05 V/cell.