If you find that your mobile phone's battery appears bloated or misshapen, the most likely cause is that you have inadvertently damaged it in one of two ways. Mobile phone batteries are quite delicate and sensitive to environmental conditions. To complicate matters, you can damage the battery without knowing it because your mobile phone may continue to operate normally for a while after the battery itself is compromised.
Most cell phone batteries contain a lithium-ion mixture. Lithium is an alkaline metal, and its ion simply has more electrons than protons. The battery also contains a positive and negative electrode, and when you use the battery, the lithium ions move from one to the other, creating an electric charge. Under normal circumstances, the lithium ions can continue to absorb and dissipate energy. However, if you expose the battery to certain conditions, its chemistry can change, causing undesirable effects.
Your cell phone battery is extremely sensitive to water. The plastic casing provides some protection; however, if you expose the metal electrodes to water, they can corrode and become ineffective. Water leaking into the batteries case can cause it to appear bloated. Discard such a battery immediately because the lithium solvent can be harmful if you come into contact with it. If the phone has been submerged in water, this is usually enough to cause water damage to the battery if there is not a tight seal between the battery and the phone.
The lithium solvent that carries the charge from electrode to electrode is extremely sensitive to heat. If you expose the phone to temperatures that considerably exceed 77 degrees Fahrenheit, the battery may lose its ability to hold a charge over time. This is considered normal. However, if you consistently expose it to higher temperatures, then you can cause the lithium-ion mixture to rupture the cell, which can cause the battery to appear bloated. Most lithium-ion batteries contain fail-safes that prevent them from operating in extreme temperatures, but there is no guarantee that these will work in every case.
Prevent damage of this sort from occurring by not exposing your phone to extreme temperatures and unsafe environments. For example, do not bring your mobile phone near pools or inside of saunas. On hot summer days, keep the phone and other electronics out of direct sunlight.