Bulging (also known as swelling) is a common phenomenon in lithium battery failure, posing safety hazards (such as fire or explosion), and usually requires immediate discontinuation of use.
The essence of bulging is the generation of excessive gas inside the battery, leading to increased pressure and deformation of the casing or internal cells. This is common in pouch batteries. Slightly more gas production in prismatic batteries can result in poor battery thickness, and in severe cases, can cause the explosion-proof valve to open.
The causes of battery bulging include:
I. Internal chemical reactions producing gas (root cause)
1.1 Electrolyte decomposition
Overcharging: When the voltage is too high, the positive electrode material structure becomes unstable, releasing active oxygen, which reacts violently with the electrolyte, generating a large amount of heat and gas (such as CO₂, CO).
High-temperature environment: Organic solvents in the electrolyte (such as carbonates) easily decompose and produce gas at high temperatures.
1.2 SEI film decomposition and regeneration
During normal cycling, the solid electrolyte interface (SEI film) on the negative electrode surface will slightly repair itself, consuming electrolyte and producing a small amount of gas. Under overcharging or high temperatures, the SEI film decomposes significantly, exposing the negative electrode (e.g., graphite) which reacts with the electrolyte to generate hydrogen and alkane gases.
1.3 Moisture Impurity Reaction
If moisture is introduced during production, lithium salts in the electrolyte (e.g., LiPF₆) will react with water to generate HF (corrosive) and hydrogen, leading to swelling and material damage.
1.4 Negative Electrode Lithium Plating
During low-temperature charging, high-current charging, or negative electrode aging, lithium ions may be directly reduced to metallic lithium on the negative electrode surface (lithium plating) instead of embedding in the graphite layer.
Metallic lithium reacts with the electrolyte to generate gases such as hydrogen, and may also puncture the separator, causing a short circuit.
The above describes the reasons why batteries bulge, which are the main causes (mechanisms) of gas production. Gas production is unavoidable; even with perfect design, optimal material selection, and meticulous manufacturing processes, batteries will produce gas. Trace amounts of gas are acceptable, and the battery's internal space can accommodate them.
However, unreasonable design and manufacturing defects can accelerate gas generation, and improper battery use will also accelerate gas production. The following mainly introduces the gas generation problems caused by these factors;
II. Design and Manufacturing Defects
2.1 Production Process Issues
Improper humidity control in the production environment leads to excessive moisture inside the battery cell.
Uneven electrode coating and misalignment of winding/stacking can cause localized stress concentration or short-circuit risks.
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Poor encapsulation results in gas leakage or poor sealing.
2.2 Material Defects (Separator)
Poor quality (uneven porosity, low mechanical strength) makes it prone to shrinkage or tearing.
Improper electrolyte additive formulation results in poor film-forming properties.
2.3 Safety Valve Failure
Some soft-pack or cylindrical batteries have poorly designed safety valves that cannot release pressure in time when it rises.
III. Improper Usage Conditions (Common Causes)
3.1 Overcharging/Discharging
Overcharging: Positive electrode structure collapses, producing oxygen and accelerating electrolyte decomposition.
Over-discharge (low voltage): The copper current collector at the negative electrode dissolves, leading to short circuits and gas production.
3.2 High-Temperature Environment
Prolonged exposure to high temperatures (such as direct sunlight or high temperatures inside a vehicle) causes an exponential increase in the rate of side reactions.
High-current charging and discharging leads to internal heat accumulation.
3.3 Mechanical Damage
Squeezing, puncture, or dropping can cause internal structural deformation, leading to micro-short circuits or electrolyte leakage.
3.4 Long-Term Idle Storage
Storing the battery for extended periods at excessively high or low charge levels causes side reactions to continue slowly, gradually producing gas.