Why Do LiPo Batteries & Ultra Thin Batteries Swell?

Why Do LiPo Batteries & Ultra Thin Batteries Swell? Comprehensive Analysis & Prevention Guide by Serui Battery

In the rapidly evolving landscape of portable electronics, electric vehicles, and industrial devices, lithium-ion batteries—especially LiPo batteries (lithium-polymer batteries) and ultra thin batteries—have become indispensable power sources. Their high energy density, compact design, and long cycle life make them the preferred choice for everything from smartphones and wearables to medical devices and industrial sensors. However, one critical issue that plagues users and manufacturers alike is battery swelling (also known as bulging), a common failure mode that not only renders the battery useless but also poses severe safety risks such as fire and explosion. At Serui Battery (www.serui-battery.com), a global leader in high-performance lithium battery technology, we believe that understanding the root causes of LiPo battery and ultra thin battery swelling is the first step toward prevention. In this comprehensive guide, we delve into the science behind battery swelling, explore the key factors contributing to this issue, and share insights from our decades of expertise in manufacturing reliable, safe LiPo battery and ultra thin battery.

The Nature of LiPo Battery & Ultra Thin Battery Swelling: Beyond Normal Expansion

Before diving into the causes, it’s essential to distinguish between abnormal swelling and normal, acceptable expansion. All lithium batteries, including LiPo batteries and ultra thin batteries, produce a tiny amount of gas during their normal lifecycle. This 微量 (trace) gas generation is a byproduct of electrochemical reactions and is typically negligible—designers account for this by incorporating small internal spaces within the battery structure to accommodate it. This “reasonable expansion” falls within industry-standard expansion rates and does not affect the battery’s performance or safety.

Abnormal swelling, however, is a different phenomenon entirely. It occurs when excessive gas accumulates inside the battery, creating internal pressure that deforms the casing or the cell itself. For LiPo batteries and ultra thin batteries, which often feature flexible or thin casings, swelling is particularly noticeable—their lightweight, compact design leaves little room for excess pressure, making them more prone to visible bulging compared to rigid cylindrical or prismatic batteries. For square batteries (a common form factor in many applications), excessive gas production first leads to increased thickness (a quality defect) and, in severe cases, triggers the activation of the pressure relief valve (explosion-proof valve) to prevent catastrophic failure.

Make no mistake: once a LiPo battery or ultra thin battery swells abnormally, it is no longer safe to use. The internal pressure can compromise the battery’s structural integrity, and the gas trapped inside—often flammable or corrosive—poses a significant risk of leakage, fire, or explosion. Such batteries should be immediately disconnected from devices, placed in a fire-resistant container, and disposed of according to local regulations. At Serui Battery, we emphasize that safety is non-negotiable—our LiPo batteries and ultra thin batteries are engineered with strict quality controls to minimize swelling risks, but user awareness remains crucial.

Root Cause: Excessive Gas Generation from Internal Electrochemical Reactions

At the core of all battery swelling issues is excessive gas production resulting from unintended or accelerated electrochemical reactions within the cell. These reactions deviate from the battery’s normal charging and discharging processes, leading to the breakdown of materials and the release of gases such as carbon dioxide (CO₂), carbon monoxide (CO), hydrogen (H₂), and alkanes. Below, we break down the key electrochemical mechanisms that cause LiPo battery and ultra thin battery swelling:

1.1 Electrolyte Decomposition: Triggered by Overcharging and High Temperatures

The electrolyte is the lifeblood of a lithium battery, facilitating the transfer of lithium ions between the positive and negative electrodes. For LiPo batteries and ultra thin batteries, which often use liquid or gel electrolytes based on carbonate solvents (e.g., ethylene carbonate, dimethyl carbonate), electrolyte stability is critical—any decomposition can quickly lead to gas buildup.

• Overcharging: This is one of the most common causes of electrolyte decomposition and LiPo battery swelling. When a battery is charged beyond its maximum safe voltage (typically 4.2V per cell for lithium-ion batteries), the structure of the positive electrode material (e.g., lithium cobalt oxide, lithium iron phosphate) becomes unstable. This instability causes the release of reactive oxygen species, which react violently with the electrolyte’s organic solvents in an exothermic (heat-releasing) oxidation reaction. The result is a rapid production of CO₂, CO, and other gases, along with a sharp rise in temperature—creating a dangerous feedback loop that accelerates further decomposition. For ultra thin batteries, which have thinner electrode layers and less thermal mass, overcharging can lead to swelling even faster, as heat dissipates more slowly.

• High-Temperature Environments: Exposure to high temperatures (above 60℃ for extended periods) is another major trigger for electrolyte decomposition. The carbonate-based solvents in LiPo battery electrolytes are thermally unstable at elevated temperatures, breaking down into gaseous byproducts. This is particularly problematic for ultra thin batteries used in devices that generate heat during operation (e.g., smartphones, laptops, industrial sensors) or for LiPo batteries deployed in harsh environments (e.g., outdoor electronics, automotive applications). Even moderate heat over time can degrade the electrolyte, leading to gradual gas accumulation and eventual swelling.

At Serui Battery, we address this issue through multiple layers of protection. Our LiPo batteries and ultra thin batteries are equipped with advanced battery management systems (BMS) that prevent overcharging by cutting off the charging current once the battery reaches its full voltage. Additionally, we use high-temperature-resistant electrolytes with custom additive packages that enhance thermal stability, reducing the risk of decomposition even in extreme conditions.

1.2 SEI Film Degradation and Regeneration: A Delicate Balance

The Solid Electrolyte Interface (SEI) film is a thin, protective layer that forms naturally on the surface of the negative electrode (typically graphite) during the first few charge-discharge cycles of a lithium battery. This film is critical for battery performance: it allows lithium ions to pass through while preventing the electrolyte from reacting directly with the electrode. However, the SEI film is not permanent—and its degradation can lead to LiPo battery and ultra thin battery swelling.

• Normal Cycle Wear: During normal use, the SEI film undergoes minor wear and tear, with small areas breaking down and regenerating. This regeneration process consumes a tiny amount of electrolyte and produces negligible gas—part of the battery’s normal lifecycle. For ultra thin batteries, which have a higher surface-area-to-volume ratio, the SEI film is more prone to wear, but this is accounted for in the battery’s design.

• Accelerated Degradation from Overcharging or High Temperatures: When a LiPo battery is overcharged or exposed to high temperatures, the SEI film degrades rapidly and unevenly. Large sections of the film break down, exposing the underlying graphite electrode to the electrolyte. This exposure triggers a series of unwanted reactions: the graphite reacts with the electrolyte’s solvents, producing hydrogen gas and alkanes. Additionally, the breakdown of the SEI film allows for increased lithium ion deposition on the electrode surface (a phenomenon known as lithium plating, which we’ll explore next), further exacerbating gas production and swelling.

Serui Battery’s R&D team has spent years optimizing the SEI film formation process for our LiPo batteries and ultra thin batteries. We use electrolyte additives (e.g., vinylene carbonate, fluoroethylene carbonate) that promote the formation of a dense, stable SEI film during the battery’s initial cycling. This robust film resists degradation from overcharging and high temperatures, minimizing gas production and extending the battery’s lifespan.

1.3 Reactions Caused by Moisture and Impurities

Lithium batteries are extremely sensitive to moisture—even trace amounts of water inside the cell can trigger catastrophic reactions. This is especially true for LiPo batteries and ultra thin batteries, which have more complex manufacturing processes involving thin films and flexible packaging, making them more vulnerable to moisture contamination if production controls are not strict.

The primary risk comes from the reaction between water and lithium hexafluorophosphate (LiPF₆), a common lithium salt used in electrolytes. When water (H₂O) comes into contact with LiPF₆, the following reaction occurs:

LiPF₆ + H₂O → LiF + POF₃ + 2HF

The byproducts of this reaction are hydrogen fluoride (HF)—a highly corrosive gas that damages the battery’s electrodes and separator—and other gaseous compounds. The HF not only eats away at the battery’s internal components but also accelerates further electrolyte decomposition, leading to increased gas production and swelling. Over time, the corrosion can weaken the separator, increasing the risk of internal short circuits and thermal runaway.

Impurities other than moisture—such as metal particles or organic contaminants introduced during manufacturing—can also act as catalysts for unwanted reactions. These impurities create localized “hot spots” within the battery, where reactions proceed faster, generating excess heat and gas. For ultra thin batteries, which have minimal internal space, these hot spots can quickly lead to noticeable swelling.

At Serui Battery, we maintain strict control over our production environment to eliminate moisture and impurities. Our manufacturing facilities feature Class 100 cleanrooms with humidity levels controlled below 1% RH (relative humidity) during critical processes such as electrode coating, cell assembly, and electrolyte injection. All raw materials—including electrolytes, electrodes, and separators—undergo rigorous testing for moisture and impurity content before use. This commitment to quality ensures that our LiPo batteries and ultra thin batteries are free from contaminants that cause swelling.

1.4 Lithium Plating on the Negative Electrode: A Hidden Danger

Lithium plating is a phenomenon where lithium ions are reduced to metallic lithium (Li⁰) on the surface of the negative electrode, rather than intercalating (inserting) into the graphite layers as intended. This metallic lithium is highly reactive and can trigger severe gas production, leading to LiPo battery and ultra thin battery swelling.

Lithium plating typically occurs under the following conditions:

• Low-Temperature Charging: Charging a lithium battery at temperatures below 0℃ significantly slows down the intercalation process. Lithium ions cannot enter the graphite layers quickly enough, so they accumulate on the electrode surface and deposit as metallic lithium.

• High-Current Charging: Charging at a current higher than the battery’s recommended rate (C-rate) overwhelms the intercalation process. Again, lithium ions build up on the surface and plate as metal.

• Negative Electrode Aging: As the battery ages, the graphite structure degrades, reducing its ability to accommodate lithium ions. This leads to increased lithium plating even under normal charging conditions.

Metallic lithium is highly reactive with the battery’s electrolyte. The reaction between lithium metal and the electrolyte’s solvents produces hydrogen gas, which contributes to swelling. Worse, the lithium plating can form needle-like structures (dendrites) that pierce the separator—thin, porous material that keeps the positive and negative electrodes apart. A pierced separator causes an internal short circuit, which generates massive amounts of heat and gas, leading to rapid swelling, fire, or explosion.

For ultra thin batteries, lithium plating is an even greater risk due to their thin electrode design. The reduced thickness of the negative electrode means there is less space for lithium ions to intercalate, making plating more likely under suboptimal charging conditions. LiPo batteries, which often use high-energy-density electrode materials, are also prone to lithium plating if not charged correctly.

Serui Battery addresses lithium plating through a combination of design and user guidance. Our LiPo batteries and ultra thin batteries are engineered with optimized electrode thicknesses and graphite formulations that enhance intercalation efficiency. We also provide clear charging guidelines—recommending appropriate temperature ranges (0℃ to 45℃) and maximum charging rates—to prevent plating. For industrial applications, our batteries can be customized with additional BMS features that adjust charging current based on temperature, further reducing the risk.

Secondary Causes: Design and Manufacturing Defects

While internal electrochemical reactions are the root cause of LiPo battery and ultra thin battery swelling, design flaws and manufacturing defects can accelerate these reactions or create conditions where they are more likely to occur. Even the best electrochemical design can be undermined by poor production practices or subpar materials. At Serui Battery, we recognize that quality control in design and manufacturing is just as critical as technological innovation in preventing swelling.

2.1 Production Process Issues

The manufacturing of LiPo batteries and ultra thin batteries is a highly precise process that requires strict control over every step. Even minor deviations can introduce flaws that lead to swelling.

• Inadequate Humidity Control: As discussed earlier, moisture is the enemy of lithium batteries. If the production environment’s humidity is not properly controlled—especially during cell assembly and electrolyte injection—moisture can seep into the battery. This is particularly problematic for ultra thin batteries, which have thinner packaging and more delicate seals. At Serui Battery, we use advanced dehumidification systems and real-time humidity monitoring to ensure that our cleanrooms meet the strictest standards.

• Uneven Electrode Coating: The positive and negative electrodes of LiPo batteries and ultra thin batteries are made by coating a thin layer of active material (e.g., lithium cobalt oxide for the positive electrode, graphite for the negative) onto a current collector (e.g., aluminum foil for the positive, copper foil for the negative). If this coating is uneven—with some areas thicker than others—it creates localized differences in current density during charging and discharging. Thicker areas have higher resistance, generating more heat and increasing the risk of electrolyte decomposition and gas production. For ultra thin batteries, which rely on precise thickness control for their performance, uneven coating is especially detrimental. Serui Battery uses state-of-the-art coating machines with laser thickness monitoring to ensure uniform application of electrode materials.

• Misalignment During Winding or Stacking: For cylindrical and prismatic batteries, electrodes are wound into a jellyroll structure; for many LiPo batteries and ultra thin batteries, they are stacked in layers. If the electrodes or separator are misaligned during this process, it can lead to several issues: localized stress concentrations (which weaken the separator over time), short circuits (if electrodes touch), or uneven ion distribution (which accelerates plating and gas production). Misalignment is a common cause of swelling in ultra thin batteries, where even a tiny shift can have a significant impact. Our production lines feature automated alignment systems and post-assembly inspections to detect and correct misalignment before cells leave the factory.

• Poor Sealing or Encapsulation: LiPo batteries and ultra thin batteries typically use flexible packaging (e.g., aluminum-plastic film) that must be hermetically sealed to prevent moisture ingress and gas leakage. If the seal is incomplete or damaged, moisture can enter the cell, triggering the reactions discussed earlier. Additionally, poor sealing can allow gas to leak out slowly, but in some cases, it can create a one-way valve effect—allowing moisture in but trapping gas inside, exacerbating swelling. Serui Battery uses heat-sealing technology with pressure and temperature monitoring to ensure that every battery is properly sealed. We also perform leak tests on 100% of our products to detect any sealing defects.

2.2 Material Defects

The quality of the materials used in LiPo batteries and ultra thin batteries is critical to their performance and safety. Subpar materials can introduce inherent weaknesses that lead to swelling.

• Low-Quality Separators: The separator is a critical safety component that prevents direct contact between the positive and negative electrodes while allowing lithium ions to pass through. For LiPo batteries and ultra thin batteries, separators are typically made of polyethylene (PE), polypropylene (PP), or a PE-PP composite. A low-quality separator may have uneven pore sizes (which disrupts ion flow), low mechanical strength (which makes it prone to tearing or shrinking), or poor thermal stability (which causes it to melt at moderate temperatures). A defective separator can lead to internal short circuits, accelerated SEI film degradation, and increased gas production. Serui Battery sources separators from top-tier suppliers and tests each batch for pore uniformity, mechanical strength, and thermal stability. We also use separators with ceramic coatings for our high-performance LiPo batteries and ultra thin batteries, enhancing their resistance to tearing and thermal shrinkage.

• Poorly Formulated Electrolytes: The electrolyte’s composition—including solvents, lithium salts, and additives—directly impacts its stability and performance. An electrolyte with an improper additive package may not form a stable SEI film, leading to increased decomposition. Similarly, using low-purity solvents or salts can introduce impurities that act as catalysts for unwanted reactions. For ultra thin batteries, which have a higher electrolyte-to-electrode ratio, the quality of the electrolyte is even more critical. Serui Battery develops custom electrolyte formulations in our in-house R&D lab, tailoring the solvent blend and additive package to the specific needs of our LiPo batteries and ultra thin batteries. We use only high-purity raw materials (≥99.99%) to ensure electrolyte stability.

• Defective Electrode Materials: The active materials in the positive and negative electrodes must meet strict purity and structural standards. For example, a positive electrode material with impurities or a defective crystal structure may release reactive oxygen more easily during overcharging. A negative electrode material with low graphite purity may have reduced intercalation capacity, increasing the risk of lithium plating. Serui Battery works closely with material suppliers to ensure that all electrode materials meet our rigorous specifications. We perform detailed chemical and structural analysis on every batch of active materials to detect defects before they are used in production.

Why Serui Battery’s LiPo Batteries & Ultra Thin Batteries Are Resistant to Swelling

At Serui Battery (www.serui-battery.com), we have built our reputation on manufacturing safe, reliable lithium batteries—including LiPo batteries and ultra thin batteries—that minimize the risk of swelling. Our approach combines cutting-edge technology, strict quality control, and a deep understanding of the factors that cause battery failure. Here’s what sets our products apart:

Rigorous R&D Focus on Swelling Prevention

Our team of over 50 engineers and scientists is dedicated to developing battery technologies that address the root causes of swelling. We invest 15% of our annual revenue in R&D, focusing on:

• Electrolyte Innovation: We have developed proprietary electrolyte formulations with high-temperature stability and advanced additive packages that promote stable SEI film formation. Our electrolytes are resistant to decomposition even at temperatures up to 85℃, reducing gas production in harsh conditions.

• Electrode Design Optimization: We use advanced simulation tools to design electrodes with uniform thickness and optimal porosity, ensuring even current distribution and minimizing hot spots. For ultra thin batteries, we have developed thin-film electrode technology that maintains high energy density while reducing the risk of lithium plating.

• Separator Enhancement: We use ceramic-coated separators with high mechanical strength and thermal stability, reducing the risk of tearing and shrinkage. Our separators are also engineered with uniform pore sizes to ensure consistent ion flow.

Strict Quality Control Throughout the Manufacturing Process

We adhere to ISO9001 and ISO14001 standards, with quality control measures at every stage of production:

• Raw Material Inspection: All materials—including electrolytes, electrodes, separators, and packaging—undergo rigorous testing for purity, moisture content, and structural integrity. We reject any batch that fails to meet our standards.

• Cleanroom Production: Our manufacturing facilities feature Class 100 cleanrooms with humidity controlled below 1% RH, eliminating moisture contamination. We use automated production equipment to ensure precision in coating, winding, stacking, and sealing.

• In-Process Testing: Every cell undergoes multiple tests during production, including thickness measurement, voltage testing, and leak testing. We also perform accelerated aging tests on a sample of each batch to simulate long-term use and detect potential swelling issues.

• Final Inspection: Before shipment, every battery is tested for capacity, cycle life, and safety performance. We also perform a visual inspection to ensure there is no initial swelling or defects.

Advanced Safety Features

Our LiPo batteries and ultra thin batteries are equipped with multiple safety features to prevent swelling and mitigate risks:

• Battery Management System (BMS): Our BMS monitors voltage, current, and temperature in real time, cutting off charging if the battery exceeds safe limits. For industrial applications, we offer customizable BMS solutions that include overcharge, over-discharge, and short-circuit protection.

• Pressure Relief Valves: For prismatic and cylindrical batteries, we integrate pressure relief valves that activate if internal pressure exceeds a safe threshold, preventing catastrophic failure. For LiPo batteries and ultra thin batteries, we use flexible packaging with built-in pressure sensors that alert users to abnormal swelling.

• Thermal Protection: We incorporate thermal fuses and positive temperature coefficient (PTC) resistors that reduce current flow if the battery temperature rises too high, preventing thermal runaway.

Comprehensive User Guidance

We believe that user education is a key part of preventing LiPo battery and ultra thin battery swelling. We provide detailed user manuals and technical datasheets for all our products, including:

• Recommended charging parameters (voltage, current, temperature range)

• Storage guidelines (temperature, state of charge)

• Signs of abnormal swelling to watch for

• Proper disposal procedures for swollen batteries

Our customer support team is also available to answer questions and provide guidance on battery use and maintenance. We work closely with our customers to ensure they understand how to use our batteries safely and effectively.

Real-World Applications: Serui’s Swelling-Resistant LiPo & Ultra Thin Batteries in Action

Serui Battery’s LiPo batteries and ultra thin batteries are trusted by customers in a wide range of industries, including consumer electronics, medical devices, industrial sensors, and automotive applications. Here are a few examples of how our products have performed in real-world scenarios:

Consumer Electronics: Smartphones and Wearables

A leading smartphone manufacturer was struggling with swelling issues in their ultra thin batteries, which were causing customer complaints and product returns. After switching to Serui’s ultra thin batteries, which feature our proprietary electrolyte and advanced BMS, the manufacturer saw a 95% reduction in swelling-related issues. Our batteries’ thin-film electrode technology also allowed the manufacturer to increase battery capacity by 10% without increasing thickness, improving device runtime.

Medical Devices: Portable Diagnostic Tools

A medical device company required LiPo batteries for their portable diagnostic tools, which are used in hospitals and field settings. The devices are often exposed to varying temperatures, and swelling was a major concern due to the safety risks in medical environments. Serui’s LiPo batteries, with their high-temperature stability and strict quality control, have been used in the devices for over three years with no swelling issues. Our batteries’ long cycle life and low self-discharge rate have also reduced maintenance costs for the company.

Industrial Sensors: Oil and Gas Exploration

An oil and gas company needed ultra thin batteries for their downhole sensors, which operate in high-temperature environments (up to 120℃) and require long-term reliability. Conventional batteries were swelling and failing after a few months of use. Serui’s custom ultra thin batteries, designed with high-temperature electrolyte and ceramic-coated separators, have operated reliably for over two years in these harsh conditions, providing consistent power and no swelling.

Choosing the Right LiPo Battery or Ultra Thin Battery: Key Considerations to Prevent Swelling

When selecting a LiPo battery or ultra thin battery for your application, there are several key factors to consider to minimize the risk of swelling:

1. Quality of the Manufacturer

Choose a manufacturer with a proven track record of producing safe, reliable batteries. Look for certifications such as ISO9001, CE, RoHS, and UN38.3, which indicate adherence to international quality and safety standards. Serui Battery has been manufacturing lithium batteries for over 20 years, with customers in over 50 countries. Our products are certified to meet global standards, and we have a reputation for quality and reliability.

2. Battery Specifications

Ensure the battery’s specifications match your application requirements:

• Voltage and Capacity: Choose a battery with the correct nominal voltage and capacity for your device. Using a battery with insufficient capacity may lead to overcharging if the device’s charging system is not properly matched.

• Operating Temperature Range: Select a battery that can withstand the temperature conditions of your application. For high-temperature environments, choose a battery with a high-temperature rating (e.g., Serui’s LiPo batteries can operate up to 85℃).

• Charging Rate: Use a battery that is compatible with your device’s charging rate. Charging a battery at a rate higher than recommended increases the risk of lithium plating and swelling.

3. Safety Features

Look for batteries with built-in safety features such as BMS, pressure relief valves, and thermal protection. These features help prevent overcharging, overheating, and short circuits—key causes of swelling.

4. User Practices

Even the best battery can swell if used incorrectly. Follow these best practices:

• Use the Correct Charger: Always use a charger that is compatible with the battery’s voltage and current requirements. Avoid using cheap, uncertified chargers.

• Avoid Overcharging: Do not leave the battery charging unattended for extended periods. Disconnect the charger once the battery is fully charged.

• Store Properly: Store the battery in a cool, dry place (ideally between 15℃ and 25℃) with a state of charge between 40% and 60%. Avoid storing fully charged or fully discharged batteries for long periods.

• Inspect Regularly: Check the battery for signs of swelling, leakage, or damage before use. If you notice any abnormalities, stop using the battery immediately.

Conclusion: Trust Serui Battery for Safe, Swelling-Resistant LiPo & Ultra Thin Batteries

Battery swelling is a serious issue that can compromise the safety and performance of LiPo batteries and ultra thin batteries. However, by understanding the root causes—excessive gas production from internal electrochemical reactions—and addressing secondary factors such as design and manufacturing defects, it is possible to minimize the risk. At Serui Battery (www.serui-battery.com), we are committed to producing the highest quality LiPo batteries and ultra thin batteries that are resistant to swelling, with advanced technology, strict quality control, and comprehensive safety features.

Whether you need a LiPo battery for a smartphone, an ultra thin battery for a wearable device, or a custom battery solution for an industrial application, Serui Battery has the expertise and experience to meet your needs. Our team of engineers will work closely with you to design a battery that fits your specifications and delivers reliable performance in even the harshest conditions.

Don’t let battery swelling compromise your products or put your users at risk. Choose Serui Battery’s LiPo batteries and ultra thin batteries for safety, reliability, and peace of mind. Visit www.serui-battery.com today to learn more about our products, request a quote, or speak with a member of our technical team. We look forward to partnering with you to power your success.

At Serui Battery, we don’t just make batteries—we make power you can trust.