Why the Lithium Batteries Become Swelling ?

Lithium battery swelling (also known as bulging or expansion) is a common sign of battery failure. It poses safety risks (such as fire or explosion) and typically requires immediate cessation of use. Normally, once a lithium battery swells, it is essentially unusable.

The root cause of swelling is the excessive generation of gas inside the battery, which increases internal pressure and causes the casing or the cell structure to deform. This is most commonly observed in pouch cells. In prismatic cells, significant gas generation can lead to abnormal cell thickness; in severe cases, it may even trigger the venting of the safety valve.

Now let's look at the causes of battery swelling:

I. Internal Chemical Reactions (Root Cause)

1.1 Electrolyte Decomposition

Overcharging: When the voltage is too high, the cathode material structure becomes unstable, releasing reactive oxygen species. This triggers a violent oxidation reaction with the electrolyte, generating significant heat and gases (such as CO₂ and CO).

High temperature environments: The organic solvents in the electrolyte (e.g., carbonate esters) are prone to decomposition and gas generation at high temperatures.

1.2 SEI Film Decomposition & Regeneration

During normal cycling, the Solid Electrolyte Interphase (SEI) film on the anode surface undergoes minor repairs, consuming the electrolyte and producing a small amount of gas. During overcharging or at high temperatures, the SEI film decomposes extensively. The newly exposed anode (e.g., graphite) reacts with the electrolyte, generating hydrogen, alkanes, and other gases.

1.3 Reaction with Moisture Impurities

If moisture is introduced during the manufacturing process, the lithium salt in the electrolyte (e.g., LiPF₆) reacts with water to produce hydrogen fluoride (HF, a corrosive substance) and hydrogen gas, leading to gas expansion and material damage.

1.4 Anode Lithium Plating

During low-temperature charging, high-current charging, or when the anode is aged, lithium ions may be reduced directly to metallic lithium on the anode surface, a process known as lithium plating, instead of intercalating into the graphite layer. This metallic lithium reacts with the electrolyte to produce hydrogen and other gases, and it can also pierce the separator, causing a short circuit.

The explanations above cover why batteries swell—essentially, the main mechanisms of gas generation. Some gas generation is unavoidable. Even with perfect design, optimal material selection, and flawless manufacturing processes, batteries will produce gas. Very small amounts of gas are acceptable, and batteries have internal space to accommodate them. However, unreasonable design and manufacturing defects will accelerate gas generation. Similarly, incorrect usage will also accelerate gas production.

II. Design and Manufacturing Defects

2.1 Production Process Issues

Improper humidity control in the production environment, leading to excessive internal moisture in the cell.

Uneven electrode coating or misalignment during winding/stacking, causing localized stress concentration or short-circuit risks.

Poor sealing of the cell casing, leading to leaks or inadequate airtightness.

2.2 Material Defects

Poor-quality separator (e.g., uneven porosity, low mechanical strength) that shrinks or tears easily.

Improper formulation of electrolyte additives, resulting in poor film-forming characteristics.

2.3 Safety Valve Failure

In some pouch or cylindrical cells, the safety valve is not properly designed and cannot relieve pressure in a timely manner when internal pressure rises.

III. Improper Usage (Common Triggers)

3.1 Overcharging / Over-discharging

Overcharging: The cathode structure collapses, releasing oxygen, and electrolyte decomposition accelerates.

Over-discharging (voltage too low): The copper current collector on the anode can dissolve, leading to short circuits and gas generation.

3.2 High Temperature Environments

Prolonged exposure to high temperatures (e.g., direct sunlight, hot interiors of a car) causes side reaction rates to increase exponentially.

High-current charge/discharge cycles lead to the accumulation of internal heat.

3.3 Mechanical Damage

Crushing, puncturing, or dropping the battery can deform its internal structure, causing micro-short circuits or electrolyte leakage.

3.4 Long-Term Storage

Storing the battery for extended periods at an either too high or too low state of charge causes continuous slow side reactions, gradually generating gas.