1Internal short
A tiny metal growth (lithium plating, often from charging in the cold) or a manufacturing burr pierces the separator, so anode and cathode touch inside the cell. Current flows straight through that spot and heats it. The protection board can disconnect the battery from the phone, but it can't stop a short that's inside the cell.
280–120 °C: the anode film breaks down
The thin protective layer on the graphite (the SEI) decomposes, and the electrolyte starts reacting with the anode. Those reactions release heat of their own, so the hot spot grows without any outside help.
3About 130 °C and up: separator fails, gas builds
The polyethylene separator melts near 130 °C. Its pores close first, a built-in shutdown, but as it shrinks the short spreads. The electrolyte breaks down into gases like CO₂ and CO. A pouch cell has no vent, so it swells and pushes the back glass off its glue. This swelling is the warning sign you can see.
4Above about 200 °C: thermal runaway
The cathode starts releasing oxygen, which burns the electrolyte inside the cell. Each reaction heats the next one faster, so the temperature can jump past 600 °C in seconds. The pouch seal splits and vents hot gas, smoke and sparks. Nearby parts (the coil, back glass and board) are scorched.
5How phones catch and prevent it
- A thermistor in the battery pack and the fuel-gauge chip watch temperature and spot unusual self-discharge.
- Charging slows or stops when the battery is too hot (above about 45 °C) or too cold, and the phone shuts itself down if it keeps heating.
- The protection board's switches cut the cell off on over-voltage, over-current or an external short.
- Ceramic-coated separators, charge-voltage limits and slower charging near full reduce the chance of an internal short.
- Factories X-ray cells to check layer alignment, and the battery bay leaves a little room so early swelling shows up before it cracks anything.
- For users: a swollen battery means stop charging, don't press or puncture it, and get it replaced.