Inside Li Polymer Battery Swelling and Chemical Changes

Mari Chen

Hello everyone, I am Mari Chen, a content creator who has been deeply involved in the lithium battery industry and the chief content officer of yungbang . Here, I will take you through the technical fog of lithium batteries - from material innovation in the laboratory to battery selection on the consumer side; from cutting-edge battery research and development to safety guidelines for daily use. I want to be the "most knowledgeable translator" between you and the world of lithium batteries.

Why Li Polymer Battery Swelling Happens

Gas Buildup in Batteries

Swelling in lithium polymer ion batteries happens when gas forms inside the battery. This gas comes from unwanted chemical changes in the polymer layers. When lithium moves between the electrodes during charging and discharging, the polymer electrolyte can break down. This breakdown releases gases like ethylene and carbon dioxide. These gases have nowhere to go, so they build up inside the sealed battery. As the pressure rises, the battery swells and the polymer pouch expands.

Gas buildup is a warning sign. If a li polymer battery looks puffy, it means gas has collected inside the polymer layers.

The main reason for gas buildup is the reaction between lithium and the polymer electrolyte. Overcharging, overheating, or damage can make these reactions worse. When the battery gets old, the polymer materials become less stable. This instability leads to more gas and more swelling. Even small amounts of water or moisture can react with lithium, causing hydrogen gas to form. This makes the swelling problem worse.

Key Chemical Reactions

Many chemical reactions inside lithium polymer ion batteries cause gas generation and swelling. The most important reactions happen at the anode, cathode, and the solid electrolyte interphase (SEI) layer. The table below shows where these reactions occur, what causes them, and which gases they produce:

Reaction Site	Chemical Reaction / Cause	Gases Generated	Notes
Anode (Graphite)	Electrolyte reduction during first charge forming SEI	C2H4 (ethylene), C3H6, CO, CO2	SEI formation blocks further gas generation in later cycles
Anode (Contaminants)	Water contamination and moisture reduction	H2	Water traces from electrolyte or insufficient drying
Anode (Salt)	LiPF6 salt decomposition at high temperature	HF	Toxic gas formed during thermal stress
Cathode	Electrolyte oxidation (electrochemical and chemical)	CO2, CO	Depends on cathode potential and surface area
Cathode	Hydrolysis of LiPF6 at high voltage	HF	Salt degradation contributes to gas generation
Cell-level	Transition metal dissolution, cross-electrode reactions	CO2, carbon deposits	Cross-electrode reactions influence gas evolution
Overcharge	Excessive lithium de-intercalation, oxygen release	CO2, CO, H2, CH4, C2H4, C2H6	Overcharge accelerates electrolyte oxidation and gas production
Overdischarge	Copper dissolution and SEI degradation	CO2	Gas generation linked to electrode degradation
Internal short	Direct contact between anode and cathode	Various gases	Caused by dendrite growth or mechanical damage

Electrolyte decomposition plays a big role in swelling. When the polymer electrolyte breaks down, it releases ethylene gas. This gas reacts with lithium to form inactive materials. These materials do not help the battery work and take up space inside the polymer layers. As more gas forms, the battery loses capacity and swells even more. Studies show that gas from electrolyte breakdown can cause up to 19% of the battery’s capacity loss after many cycles.

The SEI layer forms on the anode during the first charge. This layer protects the battery, but it can also swell by soaking up more electrolyte. A thick, soft SEI layer makes the polymer battery swell more. If the SEI is rich in organic materials, it swells even more than an inorganic-rich SEI. Swelling of the SEI leads to lower battery efficiency and faster aging.

Lithium also reacts with water, moisture, and other battery parts. These reactions create hydrogen and other gases. Overcharging, overheating, and physical damage make these reactions happen faster. As the battery ages, the polymer and electrolyte become less stable, leading to more gas and swelling. Poor manufacturing or using the wrong charger can also cause lithium to react in ways that produce gas.

Swelling in lithium polymer ion batteries is a sign that chemical reactions inside the polymer layers have gone wrong. Watching for swelling helps prevent bigger problems like leaks or fires.

Inside Lithium Polymer Ion Batteries

Battery Structure

Lithium polymer ion batteries have a unique internal structure. Each battery contains three main parts: the positive electrode, the negative electrode, and the polymer electrolyte. The positive electrode often uses lithium cobalt oxide. The negative electrode usually contains graphite. The polymer electrolyte sits between these electrodes. It acts as a pathway for lithium ions to move during charging and discharging.

The polymer electrolyte in lithium polymer ion batteries is different from the liquid electrolyte in other batteries. This polymer can be solid or gel-like. It allows the battery to have a flexible, lightweight design. The battery also includes a separator. This thin layer keeps the positive and negative electrodes apart. The separator prevents short circuits and helps maintain energy stability.

Swelling in lithium polymer ion batteries often starts at the interfaces between the electrodes and the polymer electrolyte. When the battery faces stress, such as overcharging or aging, side reactions can occur. These reactions break down the polymer and create gases. The gases get trapped inside the battery, causing the polymer pouch to swell. Overcharging can also cause lithium plating on the anode. This process releases flammable gases and increases swelling risk.

The structure of lithium polymer ion batteries makes them light and flexible, but it also means that chemical changes can quickly lead to swelling.

Materials and Components

The materials inside lithium polymer ion batteries play a big role in swelling and chemical changes. The graphite anode is very sensitive to lithium intercalation. When lithium ions enter the graphite, they form compounds that can expand and create stress inside the battery. The binder in the anode, such as styrene-butadiene rubber, also affects swelling. Its mechanical properties and how it is pressed during manufacturing can change how much the battery swells.

The polymer electrolyte can break down if it contains impurities like moisture or metal particles. This breakdown creates gas and increases swelling.
The solid electrolyte interface (SEI) forms on the graphite anode. As this layer grows, it uses up the polymer electrolyte and changes the battery’s structure.
Some lithium polymer ion batteries use lithium titanate anodes. These can swell more than graphite anodes because they do not form a strong SEI layer.

The separator material also affects swelling. Polyolefin-based separators can shrink or deform at high temperatures, raising the risk of swelling. Ceramic-coated separators and PMMA triple-layer separators have better thermal stability. They keep their shape and help prevent swelling, even when the battery gets hot.

Separator Type	Key Features	Swelling Risk
Polyolefin-based (PE, PP)	Shrinks at high temperature	Higher
Ceramic-coated	High thermal stability, less shrink	Lower
PMMA triple-layer	Strong, stable, good wettability	Reduced

The choice of polymer, electrode, and separator materials in lithium polymer ion batteries directly affects swelling and energy stability. Careful design and quality materials help reduce chemical changes and keep the battery safe.

Causes of Battery Swelling

Overcharging Effects

Overcharging stands out as a major cause of battery swelling in lithium polymer batteries. When a user charges a polymer battery past its safe voltage limit, the battery faces violent internal reactions. These reactions generate heat inside the polymer layers. The heat causes the polymer electrolyte to break down. As the electrolyte decomposes, it produces gases like carbon dioxide and oxygen. These gases have no escape route, so they collect inside the polymer pouch. The pressure from the trapped gas makes the battery swell and the polymer layers expand.

Researchers have found that overcharging not only increases gas production but also speeds up material breakdown inside the battery. Tests on commercial lithium polymer batteries show that overcharging leads to hazardous gas buildup and visible swelling. The swelling signals that the battery has become unsafe. Overcharging also damages the solid electrolyte interface (SEI) layer on the anode. This damage allows more gas to form and increases the risk of battery failure.

Overcharging a lithium polymer battery can quickly turn a safe battery into a dangerous one. Always use the correct charger and avoid charging past the recommended voltage.

Water and Moisture

Water and moisture create serious problems for lithium polymer batteries. Even a small amount of water can trigger chemical reactions inside the polymer electrolyte. When water enters the battery, it reacts with lithium and the polymer materials. This reaction produces hydrogen gas and heat. The hydrogen gas builds up inside the battery, causing swelling and raising the risk of fire or explosion.

Moisture also breaks down the organic solvents in the polymer electrolyte. This breakdown creates harmful byproducts that attack the battery’s electrodes. The electrodes lose capacity and the battery’s performance drops. Water can also bridge the gap between the positive and negative electrodes. This bridge causes internal short circuits, which lead to rapid discharge, heat, and swelling. Saltwater makes the problem worse by introducing corrosive ions that damage metal parts inside the battery.

Water exposure degrades both the cathode and anode, causing permanent capacity loss.
Water inside the battery can create a path for electricity, leading to short circuits and swelling.
Saltwater increases corrosion and can release flammable gases, raising the risk of explosion.

Attempting to recharge a wet lithium polymer battery is extremely dangerous. The risk of short circuits, fire, or explosion rises sharply. To prevent water damage, manufacturers use waterproof casings and high IP ratings for polymer batteries.

Separator Damage

The separator in a lithium polymer battery keeps the positive and negative electrodes apart. This thin polymer layer acts as a barrier, stopping direct contact and preventing short circuits. If the separator gets damaged, the battery becomes vulnerable to swelling and failure.

Separator damage can happen during manufacturing or from physical stress. High temperatures can cause the polymer separator to shrink or melt. When this happens, the electrodes may touch each other. Direct contact between electrodes leads to internal short circuits. These short circuits trigger rapid chemical reactions in the polymer electrolyte, producing gas and heat. The gas builds up, causing swelling and sometimes even rupture of the battery pouch.

Mechanical stress, such as bending or dropping the battery, can also harm the separator. Poor-quality separators or improper assembly increase the risk of damage. Once the separator fails, the battery’s vulnerability to swelling rises sharply. The battery may show signs of swelling, heat, or even smoke.

A healthy separator is key to safe polymer battery operation. Damaged separators often lead to swelling and dangerous battery conditions.

Aging and Degradation

Aging affects every lithium polymer battery over time. As the battery cycles through charging and discharging, the materials inside the polymer layers begin to degrade. The graphite anode and lithium iron phosphate cathode both change shape and size during use. These volume changes create swelling forces inside the battery. In fresh batteries, the anode may swell by about 3.4%. In aged batteries, swelling can reach 7% or more after charging. This mechanical deformation causes the battery’s internal structure to buckle or separate, creating gaps between electrodes.

As the battery ages, the SEI layer on the anode thickens. This layer consumes more polymer electrolyte and raises internal resistance. The battery generates more heat, which speeds up degradation and swelling. Electrolyte decomposition at low charge levels also produces gas, increasing internal pressure. Microcracks and mechanical strain from repeated cycling add to the swelling problem.

Battery swelling increases as the materials inside the polymer layers break down.
Aging leads to higher internal resistance, more heat, and greater risk of swelling.
Structural changes, such as buckling and stratification, make the battery less stable and more likely to fail.

Researchers have measured a 25% increase in internal resistance after hundreds of cycles. This increase matches a rise in mechanical stress and swelling inside the battery. Aging, overcharging, and physical damage all work together to make battery swelling more likely as the battery gets older.

Gas Generation in Li Polymer Batteries

Common Gases Produced

Lithium polymer ion batteries create several gases during normal use and when problems occur. The most common gases include hydrogen fluoride (HF), phosphoryl fluoride (POF3), carbon monoxide (CO), and carbon dioxide (CO2). These gases form when the polymer electrolyte breaks down inside the battery. HF and POF3 come from the decomposition of lithium hexafluorophosphate (LiPF6), a salt found in the polymer electrolyte. CO and CO2 appear as byproducts when the polymer materials degrade or burn.

Overcharging, overheating, and exposure to water all increase the amount of gas produced. When lithium polymer ion batteries get too hot, the polymer electrolyte reacts faster and releases more gas. Water or humidity inside the battery also speeds up the breakdown of LiPF6, which leads to more HF and POF3. These gases are not only flammable but also toxic. They can harm people and damage devices if the polymer pouch breaks.

Many of these gases have strong odors or cause irritation. If a person smells something strange or sees a swollen polymer battery, they should stop using it right away.

Gas Accumulation and Swelling

Gas does not escape from the sealed polymer pouch. Instead, it builds up inside lithium polymer ion batteries. This buildup increases the pressure and pushes against the flexible polymer layers. The battery starts to swell as the gas fills the space between the electrodes and the polymer separator.

Several factors cause gas to accumulate in lithium polymer ion batteries:

Overcharging breaks down the polymer electrolyte and releases gas.
High temperatures speed up polymer degradation and gas production.
Aging materials inside the polymer layers create more gas over time.
Physical damage disrupts the polymer structure, causing leaks and gas formation.

As gas pressure rises, the polymer pouch expands. The battery may look puffy or feel soft. Swelling signals that the battery is unsafe. If the pressure keeps rising, the polymer pouch can burst, releasing toxic gases and possibly starting a fire.

Electrolyte decomposition inside the polymer layers releases gases and raises internal pressure.
Internal short circuits from damaged polymer separators cause heat and more gas.
Overcharging or overheating destabilizes the polymer and increases swelling.
High temperatures make chemical reactions in the polymer happen faster, leading to more gas.

Swelling in lithium polymer ion batteries always means that gas has built up inside the polymer pouch. People should never ignore these signs.

Risks of Swelling

Safety Hazards

Swelling in lithium polymer batteries creates serious safety concerns. Gas generation inside the cell can lead to increased pressure and bulging. If the battery continues to swell, the risk of thermal runaway rises. This process can cause the battery to heat up quickly and may result in fire or explosion hazards. Some manufacturers recommend disconnecting the charger and powering off the device if swelling appears. Official batteries usually have better safety features, but third-party batteries often lack proper quality control, increasing the risk of failure.

Never ignore a swollen battery. Proper safety considerations include storing batteries in fire-retardant bags and away from flammable materials. Puncturing a swollen battery can release flammable gases and cause an explosion.

Mechanical, electrical, or thermal abuse can damage the separator, leading to direct contact between the anode and cathode.
Rupture of the battery pouch can release toxic and flammable electrolyte, creating additional hazards.
Fires can spread between cells in a battery pack, making the situation worse.

Device Damage

Swelling does not only threaten safety; it also damages devices. The pressure from a swollen battery can push against screens, causing them to bulge or even pop out. Chassis deformation is common, especially in thin devices like smartphones and laptops. In some cases, swelling leads to punctures or burn marks on the device. Device malfunction or complete failure often follows. External impacts, such as drops or collisions, can make swelling worse by letting moisture enter or damaging the battery casing.

Swelling can cause screen displacement and chassis breakage.
Internal short circuits from swelling may trigger rapid voltage drops and device shutdown.
Corrosion from leaked electrolyte can destroy internal components.

Capacity Loss

Swelling affects battery performance over time. Gas buildup inside the cell deforms the battery and damages internal materials. As the battery swells, its capacity drops, and it cannot hold as much charge. Studies show that swelling can increase battery thickness by up to 25% in aged batteries. This physical change leads to faster capacity fade and poor performance. Swelling also stresses the device structure, which can cause malfunctions or total failure. Monitoring for swelling helps maintain both safety and battery life.

Swelling signals a loss of capacity and a higher risk of failure. Replacing swollen batteries protects both the device and user safety.

Preventing Battery Swelling

Charging Tips

Proper charging practices help maintain polymer battery safety and reduce swelling risk. Users should always select high-quality chargers designed for their device. These chargers include safety features that prevent overcharging and overheating. Unplugging the device once fully charged extends battery life and avoids stress on the polymer layers. Many devices now use overcharge protection systems and automated shut-offs to stop charging at the right time. Some advanced tools, such as programmable USB hubs, allow users to monitor and control charging cycles. Safety vents inside the battery casing also help manage pressure and heat buildup, supporting enhanced safety.

Use only manufacturer-certified chargers.
Disconnect the charger when the battery reaches full capacity.
Avoid overnight charging to prevent unnecessary stress on the polymer battery.
Install devices with overcharge protection and automated shut-off features.

Charging the polymer battery correctly is one of the most important safety considerations.

Temperature Control

Temperature plays a key role in polymer battery safety. High temperatures speed up chemical reactions inside the battery, which can cause swelling and reduce battery life. Low temperatures slow down the movement of lithium ions, leading to lithium plating and possible short circuits. Keeping the battery within the recommended temperature range preserves chemical stability and supports battery safety. Many devices use thermal management systems, such as cooling fans or temperature sensors, to keep the polymer battery safe.

Keep the battery temperature below 35°C (95°F).
Avoid leaving devices in direct sunlight or near heat sources.
Use cases or pads that help with heat dissipation.
Monitor battery temperature with built-in sensors or apps.

Storage and Handling

Safe storage and careful handling protect the polymer battery from damage and swelling. Store batteries in a dry, well-ventilated area between 40 and 80 degrees Fahrenheit. Avoid direct sunlight, water, and heat sources. Place batteries where they will not be bumped or knocked over. Cover the terminals to prevent short circuits. Handle batteries gently and follow the manufacturer’s charging instructions. Dispose of any swollen or damaged polymer batteries at a proper recycling facility.

Store batteries in a cool, dry place.
Keep batteries away from water and heat.
Protect battery terminals from contact with metal objects.
Handle batteries with care to avoid drops or pressure.
Replace and dispose of damaged or swollen batteries safely.

Early Warning Signs

Recognizing early warning signs helps prevent serious polymer battery problems. The table below lists common signs that may indicate swelling or other safety issues:

Early Warning Sign	Description
Physical Bulging or Expansion	Battery appears bloated or larger than usual.
Device Deformation	Parts like screens or bases lift or do not sit flat.
Reduced Battery Life	Battery does not last as long as before.
Overheating	Device feels hotter than normal during use or charging.
Difficulty in Charging	Device struggles to charge or shows erratic battery percentage.
Chemical Odor	Strong smell from the battery area.
Leakage or Residue	Visible liquid or residue near the battery compartment.
Performance Issues	Device restarts unexpectedly or lags.
Warning Messages	Device alerts about battery health.

If any of these signs appear, users should turn off the device, disconnect it, and avoid using or charging the polymer battery. Store the battery in a safe, ventilated area away from flammable materials. Seek professional help for removal and disposal. Never puncture or press a swollen battery, as this can cause fire or explosion.

Early action protects both the device and user, ensuring battery safety and enhanced safety for everyone.

Lithium polymer battery swelling happens mainly because of gas buildup from chemical reactions like lithium plating, dendrite formation, and incomplete electrolyte oxidation. Overcharging causes severe but less frequent swelling, while polymer swelling from solvent absorption occurs more often but is usually less dangerous.

Key tips for safe battery use:

Always use quality chargers and avoid overcharging.
Store batteries in cool, dry places away from sunlight.
Regularly check for swelling and replace damaged batteries.
Charge batteries slowly and avoid charging above 80% or discharging below 20%.

Safe habits and regular checks help prevent swelling and protect both devices and users.

FAQ

What should someone do if a Li polymer battery starts to swell?

Remove the device from power. Turn it off. Place the device on a non-flammable surface. Do not puncture or press the battery. Seek professional help for safe removal and disposal.

Can a swollen Li polymer battery be fixed or reused?

A swollen battery cannot be fixed or reused. Swelling means the battery has suffered internal damage. Using it again can cause fire or explosion. Always replace it with a new, safe battery.

How long does a Li polymer battery usually last before swelling?

Most Li polymer batteries last two to three years with proper care. Swelling can happen sooner if the battery faces overcharging, heat, or physical damage. Regular checks help spot early signs.

Is it safe to charge a device with a slightly swollen battery?

Charging a device with a swollen battery is unsafe. Swelling shows internal gas buildup. Charging increases pressure and risk of fire. Replace the battery before using the device again.