Overcharging Li-ion cells can result in metallic lithium buildup on the anode, which can lead to nasty problems like inadvertent short circuiting and cell venting (read: bursting cells and flames).
Nowadays that shouldn't be any concern. Most of these devices have ICs (Integrated Circuts) that prevent overcharging by stopping the charge and when it drops below a certain point it trickles and repeats.
Overcharging Li-ion cells can lead to swollen
The possible reasons why your Li-ion battery became swollen:
- There is contamination or defect within the electrode material.
- You stored the battery/phone in a place that had temperatures above 100F (38C) for a bit of time.
- Your device BMS is not working properly; by charging at a high C-rate and/or charging to a voltage higher than the specs for that particular chemistry. (that's becaue of over charger )
- A leak in the pouch cell outer membrane to allow oxygen and/or moisture to enter.