Why Do Pouch Cells Swell?

There are many reasons why pouch-type lithium-ion batteries swell. Based on experimental R&D experience, the causes of swelling can be categorized into three types: first, an increase in thickness caused by the expansion of battery electrode sheets during cycling; second, swelling caused by gas generation resulting from the oxidative decomposition of the electrolyte; and third, swelling caused by manufacturing defects, such as moisture ingress due to poor sealing or damage at the cell corners.

The dominant factors driving changes in battery thickness vary across different battery chemistries. For instance, in batteries using lithium titanate (LTO) anodes, gas generation is the primary cause of swelling; in contrast, for batteries with graphite anodes, both electrode sheet expansion and gas generation contribute to swelling.

Discussion on Factors and Mechanisms Affecting Graphite Anode Expansion

The increase in cell thickness during the charging of lithium-ion batteries is primarily attributed to the expansion of the anode; the expansion rate of the cathode is only 2–4%. Anodes are typically composed of graphite, a binder, and conductive carbon, with the graphite material itself exhibiting an expansion rate of approximately 10%. Key factors influencing the expansion rate of graphite anodes include SEI (Solid Electrolyte Interphase) film formation, the state of charge (SOC), process parameters, and other variables.

(1) SEI Film Formation
During the initial charge-discharge cycle of a lithium-ion battery, a reduction reaction occurs between the electrolyte and the graphite particles at the solid-liquid interface, forming a passivation layer (the SEI film) over the electrode material. The formation of the SEI film leads to a significant increase in anode thickness, contributing to an overall increase in cell thickness of approximately 4%. Over the course of long-term cycling, the physical structure and specific surface area of ​​the graphite influence a dynamic process involving the dissolution of the existing SEI and the formation of new SEI; for example, flake graphite exhibits a higher expansion rate than spherical graphite.

(2) State of Charge (SOC)
During the cycling process, the volume expansion of the graphite anode exhibits a strong periodic functional relationship with the cell's SOC. Specifically, the volume expands gradually as lithium ions intercalate into the graphite (increasing the cell's SOC); conversely, when lithium ions de-intercalate from the graphite anode, the cell's SOC decreases, and the graphite anode volume shrinks accordingly.

(3) Process Parameters
Regarding process parameters, compaction density significantly impacts the graphite anode. During the cold pressing of the electrode sheet, substantial compressive stress develops within the graphite anode coating layer; this stress is difficult to fully relieve during subsequent steps such as high-temperature baking. During charge-discharge cycling, the combined effects of lithium-ion intercalation/de-intercalation and electrolyte-induced swelling of the binder cause the stress within the coating to be released, leading to increased expansion. Furthermore, the compaction density determines the void volume within the anode coating. A large void volume can effectively accommodate the expansion of the electrode sheet.

Conversely, if the void volume is small, there is insufficient space to absorb the expansion; consequently, the expansion is forced outward, manifesting as an increase in the overall volume of the anode sheet.

(4) Other Factors
Other factors influencing anode expansion include binder adhesion strength (specifically the interfacial adhesion between the binder, graphite particles, conductive carbon, and the current collector), charge-discharge rates, the swelling behavior of the binder in the electrolyte, the shape and packing density of the graphite particles, and electrode volume increases resulting from binder degradation during cycling.

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Gas generation within the battery is another major cause of swelling.

Batteries experience varying degrees of gas generation and swelling during cycling at room or high temperatures, as well as during high-temperature storage.

During the initial charge-discharge cycle, a Solid Electrolyte Interphase (SEI) film forms on the electrode surface. The formation of the SEI (Solid Electrolyte Interphase) layer on the anode primarily results from the reductive decomposition of EC (Ethylene Carbonate); the generation of alkyl lithium and Li2CO3 is accompanied by the production of significant amounts of CO and C2H4. Solvents such as DMC (Dimethyl Carbonate) and EMC (Ethyl Methyl Carbonate) also contribute to film formation, yielding RLiCO3 and ROLi alongside gases like CH4, C2H6, C3H8, and CO. In PC (Propylene Carbonate)-based electrolytes, gas generation is relatively higher, primarily due to the production of C3H8 gas from PC reduction. For LiFePO4 pouch cells, gas swelling is most severe following the completion of the initial 0.1C charge cycle.

It is evident that SEI formation is inevitably accompanied by the generation of substantial amounts of gas. The presence of H2O impurities destabilizes the P-F bonds in LiPF6, leading to the formation of HF; this destabilizes the battery system and generates gas. Excessive H2O consumes Li+ ions, producing LiOH, LiO2, and H2, which also results in gas evolution. Gas generation also occurs during storage and prolonged charge-discharge cycling; in sealed lithium-ion batteries, significant gas accumulation causes swelling, thereby impairing battery performance and shortening its service life. The primary reasons for gas generation during storage are as follows:

(1) H2O present in the battery system leads to HF formation, which damages the SEI layer. Oxygen (O2) in the system may oxidize the electrolyte, resulting in the generation of large amounts of CO2;
(2) If the SEI layer formed during initial activation (formation) is unstable, it may degrade during storage; the subsequent repair of the SEI layer releases gases consisting primarily of hydrocarbons. During long-term charge-discharge cycling, changes in the crystalline structure of the cathode material and non-uniform potential distribution across the electrode surface can cause localized high potentials. This reduces the stability of the electrolyte at the electrode interface; the continuous thickening of the surface film increases interfacial resistance, further raising the reaction potential. This triggers electrolyte decomposition and gas generation, while the cathode material itself may also release gas.

Batteries exhibit varying degrees of swelling depending on their chemical systems. In batteries using graphite anodes, the primary causes of gas generation and swelling are-as previously mentioned-SEI layer formation, excessive moisture content within the cell, anomalies in the formation process, and poor sealing. In contrast, within the lithium titanate (LTO) anode system, the industry generally attributes gas generation in Li4Ti5O12 batteries to the material's inherent tendency to absorb moisture, although there is no definitive evidence to confirm this hypothesis.

1. Poor sealing: The incidence of swollen cells caused by sealing defects has decreased significantly. Defects can occur on any sealing side, with top sealing and degassing seals being the most common problem areas; top sealing issues usually involve the tab area, while degassing issues often involve delamination. Poor sealing allows atmospheric moisture to enter the cell, triggering electrolyte decomposition and gas generation.

2. Pouch surface damage: During the production line flow, the pouch may sustain accidental or human-induced damage (such as pinholes), allowing moisture to penetrate the cell interior.

3. Corner damage: Due to the specific deformation of the aluminum layer at the folded corners, movement of the gas bag can twist the corner, damaging the aluminum (larger cells with larger gas bags are more prone to this) and compromising the moisture barrier. Applying crepe tape or hot-melt adhesive at the corners can help mitigate this risk. Furthermore, handling cells by the gas bag after top sealing is prohibited; particular care must be taken during operation to prevent the cells from swaying while on aging racks.

4. Excessive internal moisture content: If moisture levels exceed limits, the electrolyte degrades, leading to gas generation after formation or degassing. Primary causes for excessive internal moisture include: high moisture content in the electrolyte, high moisture content in the bare cell after baking, and excessive humidity in the dry room. If excessive moisture is suspected as the cause of swelling, a process traceability check can be conducted.

5. Formation process anomalies: Incorrect formation procedures can lead to cell swelling.

6. Unstable SEI layer: Slight swelling may occur during the charge-discharge cycles of capacity testing. 7. Overcharge/Over-discharge: Due to process, equipment, or protection circuit board (PCB) malfunctions, the battery cell undergoes overcharge or excessive discharge, resulting in severe swelling (gas generation).

8. Short Circuit: Operational errors causing contact between the live cell tabs result in a short circuit; the cell swells, voltage drops rapidly, and the tabs become charred black.

9. Internal Short Circuit: A short circuit between the anode and cathode inside the cell causes rapid discharge and heat generation, accompanied by severe swelling. Causes include design flaws; separator shrinkage, curling, or damage; misalignment of bi-cells; burrs puncturing the separator; excessive fixture pressure; or over-compression by the heat-sealing machine. For instance, insufficient width combined with excessive compression of the cell body by the heat-sealing machine has previously caused anode-cathode shorting and swelling.

10. Corrosion: Corrosion of the cell consumes the aluminum layer, destroying its moisture barrier function and leading to swelling.

11. Abnormal Vacuum Degassing: System or equipment issues result in improper vacuum levels or incomplete degassing. For example, if the heat radiation zone during vacuum sealing is too large, the degassing piercing tool may fail to effectively puncture the pouch, resulting in incomplete gas extraction.

Suppressing abnormal gas generation requires addressing both material design and manufacturing processes.

First, material and electrolyte systems must be optimized to ensure the formation of a dense, stable Solid Electrolyte Interphase (SEI) film, enhance cathode material stability, and inhibit abnormal gas generation.

Regarding electrolyte treatment, small amounts of film-forming additives are often used to make the SEI film more uniform and dense. This reduces gas generation caused by SEI film detachment and regeneration during battery use, thereby mitigating swelling; such methods have been documented in research and applied in practice.

Research indicates that the SEI film components formed by EC (ethylene carbonate) and VC (vinylene carbonate) consist of linear lithium alkyl carbonates. At high temperatures, the lithium alkyl carbonates attached to the lithiated carbon (LiC) are unstable and decompose to generate gases (such as CO2), leading to battery swelling. The SEI film formed by PS consists of lithium alkyl sulfonate; although the film contains defects, it possesses a certain two-dimensional structure and remains relatively stable on the LiC surface even at high temperatures. When PS and VC are used in combination, PS forms a defective two-dimensional structure on the anode surface at lower voltages; as the voltage rises, VC forms a linear-structured lithium alkyl carbonate that fills the defects in the two-dimensional structure, resulting in a stable, network-structured SEI film on the LiC. This type of SEI film structure significantly enhances stability and effectively suppresses gas generation caused by film decomposition.

Furthermore, interactions between the lithium cobalt oxide cathode material and the electrolyte lead to decomposition products that catalyze the breakdown of electrolyte solvents. Consequently, applying a surface coating to the cathode material not only improves structural stability but also minimizes contact between the cathode and the electrolyte, thereby reducing gas generation driven by the catalytic decomposition activity of the cathode. Therefore, the formation of a stable, intact coating layer on the surface of cathode material particles represents a major direction for current development.

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