What is in focus: same chemistry, different containers
Even cells with the same chemical composition differ greatly in volumetric efficiency, heat dissipation and mechanical stability depending on how the electrodes are wound or folded and what casing they are packed into. The form factor (the external format of a cell) is as decisive a variable for performance as the chemistry. This article sets out the structural differences among the three form factors, cylindrical, prismatic and pouch, and examines how form-factor competition shows up in patents.
Starting with the industry context, the three form factors coexist rather than converging on one. Tesla has secured a patent on a structure that eliminates tabs from the electrodes of cylindrical cells (US11749842B2, granted September 5, 2023), and on March 29, 2020, BYD unveiled the “Blade Battery,” in which LFP cells are made in a long blade shape and placed directly in the pack, announcing that pack space utilization increased by over 50% compared with conventional LFP block batteries. One is an attempt to make up for a weakness of the cylindrical format through structural design, the other for a weakness of the prismatic format.
The tension is that once a form factor is chosen, equipment, processes and pack design are locked in with it. Because it is hard for cell makers and automakers to change form factors, structural patents that compensate for each form factor’s weaknesses also gain value. This article answers three questions: how do the three form factors differ structurally, what tradeoffs exist across performance items, and how should form-factor patents be read?
Basic principle: a can, a lunch box and a zipper bag
Secondary battery cells fall broadly into three shapes. The cylindrical cell winds the electrodes into a roll and places them in a metal can, giving high mechanical rigidity. The prismatic cell uses an angular metal can and makes good use of space. The pouch cell is wrapped in aluminum laminate film (a multilayer film with aluminum foil sandwiched between resin films), making it light and giving it high design freedom.
Source: reconstructed by the editorial team from the original article (concept diagram)
By analogy, the cylindrical cell is a drink can, the prismatic cell a lunch box and the pouch cell a zipper bag. A drink can is sturdy because its round wall takes pressure evenly, but when several are packed into a box, gaps remain between the cans. A lunch box has square corners and fills a box without gaps, but its flat walls are weaker than a can against force pushing from inside. A zipper bag is light and can be made in any size, but it cannot hold its own shape and needs a frame to support it from outside.
The way electrodes are layered inside the cell also diverges. Pouch and prismatic cells use both methods.
- Winding: long electrodes are wound into a roll, giving high productivity. Cylindrical cells use this method by design
- Stacking: electrodes are cut into individual sheets and stacked, with the advantage that internal stress is distributed evenly. Adoption is increasing in recent cell designs that require high energy density and high safety at the same time
Winding has one structural weakness. The longer the wound electrode, the farther the current must travel to the tab (the metal strip that draws current from the electrode to the external terminal) at the electrode’s end. The larger the cell, the more easily this path becomes a source of resistance and heat. Tesla’s tabless patent targets exactly this point.
Heat dissipation is also tied directly to shape. A cylinder’s surface-to-volume ratio is the sum of a term inversely proportional to its radius (2/r) and a term inversely proportional to its height (2/h), so enlarging the diameter at the same height reduces the surface available per unit volume to release heat. Making cells larger and fewer simplifies assembly and interconnection, but the path for heat to escape from the center of the cell becomes relatively narrower. This is why current-collection improvements that lower heat generation itself by shortening the current path become important in large cylindrical cells. Pouch cells, by contrast, are thin, so their surface area per volume is large and their broad faces can easily be placed directly against a cooling plate.
Comparison: performance tradeoffs by form factor
The table below keeps the comparison table from the original article. It shows relative tendencies among form factors, not numerical specifications, and actual values vary with cell design and manufacturer.
| Item | Cylindrical | Prismatic | Pouch |
|---|---|---|---|
| Mechanical rigidity | High | Medium | Low (separate support needed) |
| Volumetric energy density | Medium | High | Highest |
| Heat dissipation | Good (relative to surface area) | Average | Good (thin shape) |
| Swelling (expansion) tolerance | Strong | Average | Weak; design margin required |
Source: compiled by the editorial team from the public general sources cited in the original article
What this table says: cylindrical cells are strongest on mechanical rigidity and swelling tolerance, while pouch cells lead on volumetric energy density, so the two axes run in exactly opposite order. Prismatic cells are a compromise that secures medium or better on all four items.
The key relationship to read from this table is that “the harder the casing, the more it holds itself up; the thinner it is, the less space it uses.” The weaknesses of the cylindrical format (empty space between cans and a long current path), of the prismatic format (volume lost to module structures inside the pack) and of the pouch format (lack of mechanical support) can all be compensated by design outside the cell. That is why recent form-factor competition often takes place less in the cell itself than in “the structure where cell and pack meet.” A cell marked weak in one box of the comparison table does not mean that form factor is automatically a poor choice; what decides real competitiveness is who filled that weakness, and with what structure.
| Priority | Favored form factor | Example of compensating direction |
|---|---|---|
| Mechanical stability | Cylindrical | Shorter current path with tabless electrodes (US11749842B2) |
| Space efficiency | Prismatic | Blade-shaped cells placed directly in the pack (BYD Blade Battery) |
| Light weight and design freedom | Pouch | Support structures and swelling margin at pack and module level |
Source: compiled by the editorial team from the original article, Google Patents US11749842B2 and the BYD announcement (2020-03-29)
What this table says: the strengths of each form factor are as summarized in the original article, but the solutions that fill each form factor’s weaknesses come from different layers, such as the internal electrode structure (cylindrical) or the pack layout (prismatic).
Patent perspective: reading the tabless cylindrical cell claim
Bibliographic data: US11749842B2 (title “Cell with a tabless electrode”) is owned by Tesla, Inc., and the inventors are Kunio Tsuruta, Mikel Ehrlich Dermer and Rajeev Dhiman. The priority date is November 5, 2018, the filing date November 4, 2019, publication May 7, 2020 (US20200144676A1), and grant September 5, 2023; according to Google Patents its legal status is Active, with an anticipated expiration of November 4, 2039.
Claim summary (restated): claim 1 covers an energy storage device having all of the following elements.
- A conductive-material region at one end (the proximal end) of the first substrate, divided into a plurality of conductive portions spaced apart from one another by non-conductive portions
- The first substrate, an inner separator and the second substrate are rolled about a central axis to form the cell
- A first cap with a contact surface at one end of the can
- The spaced-apart conductive portions are bent and substantially in contact with the first cap
- These conductive portions form a concentric circular pattern in contact with the first cap
Source: reconstructed by the editorial team from the claims of US11749842B2
Insight: form-factor patents claim not “the whole form factor” but “the one point where current exits.” Rather than broadly covering the whole concept of tabless design, this claim takes as its right a specific combination, “spaced conductive portions + bending + concentric contact pattern,” on top of the cylindrical structure of a wound cell, a can and a cap. Reading this patent as blocking cylindrical cells in general is therefore an overstatement, and conversely it is likely not to reach prismatic or pouch cells literally. FTO (freedom-to-operate) reviews by form factor should be done at the level of the detailed elements of the current-collection structure, not the external shape.
Design-around points: literally, a method that joins the electrode edge to the cap in continuous form without dividing it into separate pieces, a method that does not bend the conductive portions, or a method using a non-concentric contact pattern may each lack one element of claim 1. Stacked prismatic and pouch cells may not satisfy even the element of “a cell rolled about a central axis.”
Design-around scenario (counterargument): however, if a design that changes one element achieves substantially the same function (shortening the current path by connecting the whole electrode edge directly to the cap) in the same way, it may become subject to a doctrine-of-equivalents analysis. In addition, because this patent is part of a family that started from publication US20200144676A1, it cannot be ruled out that other continuation applications or foreign patents based on the same specification have broader or different claims. It is hard to confirm a design-around by looking at the granted claims of a single patent.
The claim category also deserves attention. Claim 1 claims not a manufacturing method but a product, an “energy storage device.” A product claim reaches not only the act of making the product but also using, selling and importing it, so not only cell makers but also companies that sell or import packs or vehicles carrying such cells can become subjects of an infringement-risk review. This is why patent warranty and indemnity clauses in cell supply contracts should be checked.
Limits and outlook
The optimistic outlook is that structural technologies compensating for each form factor’s weaknesses will accumulate and the three form factors will each raise performance in their own domains. There is also a contrary scenario. If structures that place cells directly in the pack, like the Blade Battery, spread, the differences shown by cell-level form-factor comparison tables may be absorbed into pack design and lose meaning. In that case the focus of competition shifts from “which form factor” to “how cell and pack are integrated.”
The strategic alternative is to track form-factor IP in three layers: “internal current-collection structure,” “casing and can structure” and “cell-to-pack integration structure.” Even within cylindrical cells, current-collection patents and pack-layout patents differ in owners and claiming style, so mixing the layers easily leads to gaps in review.
Practical action: planning and IP staff evaluating large cylindrical cells should summarize, based on drawings, how their cell’s electrode edge is treated (whether divided, whether bent, the cap contact pattern) and prepare an element comparison table matching it one-to-one against the five elements of claim 1 of US11749842B2.
Items requiring verification
- Quantitative primary sources for the relative characteristics of each form factor in the comparison table
- Scope of the granted claims of continuation applications and foreign family members derived from the same specification as US11749842B2
- Numbers and claim scope of BYD patents related to the Blade Battery
Sources
- Google Patents, US11749842B2 – Cell with a tabless electrode (Tesla, Inc.)
- Google Patents, US20200144676A1 – Cell with a tabless electrode (publication)
- BYD press release, BYD’s New Blade Battery Set to Redefine EV Safety Standards (2020-03-29)
Related Reading — Cell-to-Pack (CTP): What You Gain and Lose by Eliminating the Module · Why Do Pouch Cells Seem to Swell So Much? — The Structural Reason for Swelling · Electrode Tab Design — The Hidden Variable That Determines Internal Resistance