Why Do Pouch Cells Seem to Swell So Much? — The Structural Reason for Swelling

What’s the Story: It’s Not That Only Pouches Swell, It’s That Only Pouches Show It

Swelling in pouch cells (the cell bulging during charge-discharge and aging) is not a material defect unique to pouches; it is the volume change and gas generation that occur in every lithium-ion cell, made visible on the outside because there is no rigid casing to restrain them. In prismatic and cylindrical cells, the metal can simply holds out against the same forces from within. This article unpacks the difference using published research and patents.

Starting with the industry context: pouch cells are lighter and use space better than prismatic or cylindrical cells, so they are widely used in electric vehicles and IT devices. But the absence of a rigid casing becomes a weakness in its own right. That weakness is swelling — the pouch bulging as charge and discharge cycles repeat. To consumers, a “swollen battery” easily looks like a “dangerous battery,” and this sometimes spreads into distrust of the pouch form factor as a whole.

The tension is that swelling has more than one cause. Reversible and irreversible expansion are mixed together, and the countermeasures are scattered across the cell, pack, and material levels. This article answers three questions: why do pouches visibly swell, how does the same phenomenon appear differently by form factor, and in what configurations are rights to gas-management technology claimed?

Basic Principle: Breathing Electrodes and Escaping Gas

During charge and discharge, electrode material volume changes minutely, and side reactions can also generate gas. In prismatic and cylindrical cells, the rigid can suppresses this change so it is not visible from the outside, but a pouch has no structure to restrain it, so the swelling shows up directly on the outside. A review paper published in the journal Batteries on September 28, 2025 (Maddipatla et al.) divides swelling into two branches.

Fig. 1. A concept diagram dividing swelling into reversible and irreversible expansion and summarizing how each appears in pouch and can-type cells.
Source: reconstructed by the editorial team from Maddipatla et al., Batteries 11(10), 356 (2025)

Reversible expansion: Expansion caused by lithium entering and leaving the electrode lattice (intercalation and deintercalation); on discharge, the cell returns to nearly its original dimensions. According to the review, graphite’s lattice has been measured to expand about 10% in the fully lithiated state (LiC₆), but at the electrode level, pores and binder absorb part of it, limiting the expansion to about 3-4%.
Irreversible expansion: Expansion that accumulates over time, such as gas generation and interphase-layer growth. The cell does not return to its original size and keeps swelling.

As an analogy, the electrode is like breathing lungs, and gas is like an air pocket building up inside the body. The chest rising and falling with each breath is normal, but an air pocket that keeps growing without draining is a warning sign. A pouch is a body wearing only thin clothing, so both movements show on the outside; a can-type cell wears rigid armor, so the same movements turn into pressure inside.

This difference is also reflected in manufacturing. The review paper explains that in pouch cells, excess gas produced during initial charging (formation) is removed by a degassing step before final sealing, whereas cylindrical and prismatic cells do not go through degassing and become sealed systems with elevated internal pressure. It also notes that silicon anodes can expand by up to 300% at full lithiation, so commercial anodes limit silicon content to 5-15% and pair it with mechanical containment.

Comparison: Casing and Swelling Response by Form Factor

We keep the original form-factor comparison table and add one more column of process information from the review paper.

Form Factor	Casing	Swelling response	Formation gas handling
Prismatic	Rigid metal can	Can physically suppresses internal pressure	Sealed without degassing; internal pressure rises
Cylindrical	Rigid metal can	Same	Sealed without degassing; internal pressure rises
Pouch	Flexible aluminum laminate film	Weak restraint, so volume change shows through directly	Excess gas removed by degassing before final sealing

Fig. 2. Comparison of casing structure and swelling and formation-gas handling by form factor
Source: compiled by the editorial team from the original comparison table and Maddipatla et al., Batteries 11(10), 356 (2025)

What this table says: All three form factors undergo the same expansion and gas, but can-type cells trap them as internal pressure while pouches handle them through degassing and external restraint. For graphite electrodes, reversible expansion has been reported at about 3-4% (about 10% at the lattice level).

One distinction is worth drawing here. What the degassing step removes is the initial gas generated all at once during formation. Gas that accumulates gradually during use arises after sealing is complete, so degassing does not solve it. Thus even if a pouch cell is flat when it ships, swelling in the latter part of its life must be managed with separate material and structural measures. The “gas pocket” presented in the original article can be seen as a means addressing the former, while the “pack restraint frame” and “electrolyte-separator optimization” mainly address the latter.

Insight: the pouch’s “visible swelling” is both a weakness and a diagnostic signal. In can-type cells, gas and expansion accumulate only as internal pressure and are hard to notice from the outside, whereas a pouch reveals its aging state through changes in thickness. The review paper’s treatment of swelling as a subject of diagnostic applications follows the same logic. So in packs using pouches, the goal is not only to hide swelling; sensing design that distinguishes reversible from irreversible expansion can become added value.

Patent Perspective: Technology That “Holds” Gas and Technology That “Releases” It

Technologies for dealing with pouch swelling fall broadly into three levels.

Designing a gas pocket (a dedicated slack space) to collect early gas during formation and then remove it in the degassing step
Physically restraining expansion with a restraint frame that presses on the cells during pack assembly
A materials approach that reduces gas generation itself by optimizing the electrolyte-separator combination

The three levels also have different rights landscapes. Gas pockets and degassing tend to be claimed as manufacturing-process and equipment patents, restraint frames as pack-structure patents, and electrolytes and separators as composition patents, so even adopting a single swelling countermeasure means reviewing different patent classes. Ending an FTO (freedom-to-operate) review with cell-structure patents alone may miss infringement risk at the process and composition levels.

Among these, as a case claiming a structure that vents gas generated during cell use to the outside, we checked LG Energy Solution’s US11177532B2 on Google Patents.

Bibliographic data: US11177532B2 (title “Pouch-type secondary battery having gas discharge means”) has LG Chem Ltd as the original assignee and LG Energy Solution Ltd as the current assignee, with a priority date of April 23, 2018, a filing date of January 24, 2019, and a grant date of November 16, 2021. Its legal status on Google Patents is Active, with an anticipated expiration of January 24, 2039. The specification cites as its problem the swelling and explosion risk caused by gases such as carbon dioxide and carbon monoxide accumulating during charge cycles, overcharge, and short circuits.

Gist of claim 1 (restated): It consists of an electrode assembly of positive electrode, negative electrode, and separator; a pouch case wrapping it; pairs of insulative films surrounding the positive and negative leads at the case edge; and a gas discharge member interposed between the insulative film pairs. The gas discharge member consists of a body with a hollow gas passageway, a spring inside the passageway, and a ball that opens and closes the passageway, and the concave-convex groove channels on the body’s outer surface are arranged so that the inward-facing groove portions do not overlap the insulative films, allowing internal gas to enter.

Fig. 3. A claim map restating the elements of claim 1 of LG Energy Solution patent US11177532B2 and mapping the avoidance potential of each element.
Source: reconstructed by the editorial team from the claims of US11177532B2

The scope of this claim is defined not by the function of “releasing gas from a pouch” but by the specific structure of a spring-and-ball check valve inserted between the lead insulative films and the arrangement of its groove channels. The more numerous and specific the elements, the narrower the claim scope and the wider the room for design-around.

Design-around points: A design that places the gas discharge path somewhere in the sealing portion other than between the lead insulative films, or that uses a different opening means such as a rupture disc or a selectively permeable membrane instead of a spring and ball, may lack an element of claim 1. Restraining expansion with a pack frame outside the cell is on a different level from this claim to begin with.

Design-around scenario (counterargument): If only the opening means is changed while the placement of the discharge member between the lead insulative films is kept, the patentee may argue infringement under the doctrine of equivalents on the grounds that it is substantially the same function and way, as “a means of opening and closing the passageway according to pressure.” Conversely, pouch gas-discharge valves are a topic many companies have worked on for a long time, so from the patentee’s side as well, the claim scope may be interpreted narrowly against prior art. An actual judgment requires a 1:1 comparison of the individual design against the prior literature.

Limitations and Outlook

Swelling in pouch cells is not a material defect; it is a phenomenon that shows up because the form factor’s structure lacks restraint. It is a problem that needs to be managed not only through cell design but together with pack-level restraint structures. The review paper also summarizes that appropriate stack pressure reduces localized lithium plating and swelling non-uniformity and makes aging more even.

The optimistic outlook is that as pack restraint design and gas-suppressing additives mature, the pouch’s light weight and space advantages will outweigh the swelling burden. There is a counter-scenario as well. If high-energy anodes with higher silicon content spread, the magnitude of reversible expansion itself grows, which may put pouches, with their weak casing restraint, at a disadvantage to can-type cells, and the weight and volume of pack restraint parts may offset the pouch’s lightweight advantage.

A challenge on the consumer-perception side also remains. If the same gas and expansion are invisible in can-type cells and visible only in pouches, the impression that pouches are more dangerous may harden regardless of actual risk. Unless product descriptions and service criteria distinguish reversible expansion within the normal range from irreversible expansion that requires replacement, unnecessary replacement requests and warranty costs may increase.

The strategic alternative is to treat swelling not as “a defect to hide” but as “a signal to manage.” Specifying pack restraint pressure as a design variable and adding monitoring to the BMS that tracks cell thickness change split into reversible and irreversible components can secure both explanatory power on safety and a basis for remaining-life assessment. This sensing and discrimination logic may also become a filing area that does not overlap with existing structure-focused gas-discharge patents.

Practical action: Staff planning pouch-cell packs should first check whether their gas-discharge and restraint structure uses a discharge member between the lead insulative films, and if so, prepare an element-by-element comparison chart against claim 1 of US11177532B2.

Items requiring verification

Quantitative comparison data on the thickness increase from pouch-cell swelling by form factor and chemistry
The legal status of US11177532B2’s Korean, European, and other family members and differences in their claims
Comparative data on gas generation during cycling by electrolyte-separator combination
The industry-standard range of pack restraint pressure and the weight and volume burden of restraint parts

Sources

Maddipatla S., Rauf H., Osterman M., Arshad N., Pecht M., Swelling Mechanisms, Diagnostic Applications, and Mitigation Strategies in Lithium-Ion Batteries, Batteries 11(10), 356 (2025-09-28)
Google Patents, US11177532B2 – Pouch-type secondary battery having gas discharge means (LG Energy Solution Ltd)