Main Reasons for Bulging (swelling) of Polymer Lithium Batteries

Main reasons for bulging (swelling) of polymer lithium batteries

Bulging (swelling) of polymer lithium batteries is mainly caused by the generation of gas inside the battery or changes in the material structure. Common reasons include the following aspects:

1. Electrolyte decomposition

When the battery is overcharged, over-discharged or in a high temperature environment, the electrolyte (usually lithium salts and organic solvents) will decompose and produce gases (such as CO₂, CH₄, etc.).

Polymer lithium batteries use solid or gel electrolytes, but if the sealing is poor or the process is defective, the electrolyte may still decompose and produce gas.

2. Overcharge or over-discharge

Overcharge: When the voltage exceeds the upper limit (such as 4.2V), the structure of the positive electrode material is destroyed (such as lithium precipitation), the electrolyte is oxidized and decomposed, and a large amount of gas is produced.

Over-discharge: When the voltage is too low (such as <2.5V), the negative copper collector dissolves, and the side reactions inside the battery increase, resulting in gas accumulation.

3. High temperature environment

High temperature will accelerate the decomposition of electrolyte and electrode side reactions, and may shrink or melt the diaphragm, causing internal short circuit and further gas production and bulging.

Long-term high temperature storage (such as > 60℃) will significantly increase the risk of bulging.

4. Manufacturing defects

If there are impurities, electrode burrs, diaphragm defects or poor packaging during the battery production process, it may cause local short circuit or electrolyte leakage, causing bulging.

Improper injection volume or vacuum seal failure can also cause gas residue.

5. Mechanical damage

External force extrusion, puncture or drop may cause internal short circuit of the battery, instantaneous heat generation and gas release, causing bulging.

6. Cycle aging

After long-term charging and discharging, the electrode material (such as graphite negative electrode, ternary positive electrode) will gradually pulverize or form a thickened SEI film, expand in volume and be accompanied by gas side reactions.

Hazards of bulging

Safety risk: bulging may cause excessive internal pressure of the battery, causing leakage, fire or even explosion.

Performance degradation: Battery capacity and internal resistance deteriorate significantly, and charging and discharging capabilities decrease.

How to prevent bulging?

Avoid overcharging/overdischarging: Use a regular charger equipped with a protection circuit (BMS).

Control temperature: Avoid using or storing in high temperature environments (such as exposure in the car).

Prevent physical damage: Avoid hitting or squeezing the battery.

Choose high-quality batteries: Avoid low-quality cells (such as unbranded, uncertified products).

Replace in time: Stop using immediately if a slight bulge is found, and avoid further charging.

Note: The aluminum-plastic package of polymer lithium batteries is softer than the metal shell, and slight bulging may be easier to detect, but it is equally dangerous. If the battery is bulging, do not puncture or disassemble it yourself, and recycle it properly.