What is in focus: the ‘hidden resistance variation’ of cylindrical cells
Panasonic’s US patent US12080916B2 claims that when negative-electrode graphite particles simultaneously meet two property windows, ‘internal porosity of 5% or less’ and ‘breaking strength of 25-55 MPa’, the cell-to-cell variation in internal resistance shrinks in cylindrical cells whose negative electrode directly contacts the outer can. It is not an eye-catching topic like the form-factor race, but it is a quiet patent tied directly to quality uniformity when large cylindrical cells are mass-produced.
Start with the industry context. While Tesla’s 4680 cell has drawn attention for its ‘tabless’ design, Panasonic has also been preparing 4680 mass production. On September 9, 2024, Panasonic Energy announced that it had finalized preparations for mass production of 4680 cylindrical automotive lithium-ion batteries at its Wakayama factory, with mass production to start after final evaluation. In the same announcement, the company stated that the 4680 cell has five times the capacity of the 2170 cell and described its 30 years of know-how in cylindrical lithium-ion battery development.
The tension is that as cells grow larger and volumes rise, ‘resistance that differs slightly from cell to cell’ becomes a weakness of the whole pack. This article answers three questions: why the mechanical properties of graphite particles connect to variation in electrical resistance, what US12080916B2 claims and how far, and how competitors can read and avoid this scope of rights.
Basic principle: graphite particles that spring back
A cylindrical lithium-ion cell is made by winding the positive electrode, negative electrode and separator into a spiral ‘jelly roll’ and inserting it into a metal can. In the structure this patent addresses, the uncoated exposed part of the negative current collector (copper foil) forms the outer circumference of the jelly roll, and this exposed part touches the inner surface of the outer can directly, doubling as the negative terminal. If this contact is not uniform from cell to cell, internal resistance variation widens, which can lead to imbalance in which current concentrates in particular cells within a pack.
Source: reconstructed by the editorial team from the specification of US12080916B2
Panasonic’s solution is to control the mechanical properties of the graphite particles. As an analogy, it is the difference between a good mattress spring and a sponge. Particles with few pores and moderate hardness do not break easily when the electrode is rolled (pressed) and retain their spring-back force. According to the original explanation, this rebound force pushes the negative electrode outward after the jelly roll is inserted into the can, making the contact between the exposed part and the can’s inner surface tighter on its own. Conversely, porous particles absorb the force into their pores like a sponge, weakening this effect.
The patent also specifies how the two properties are measured.
- Internal porosity: particle cross-sections are imaged by scanning electron microscope (SEM) at 3,000-5,000x, the ratio of internal pore area to particle area is obtained by binarization analysis, and ten particles are averaged.
- Breaking strength: a load is applied with a 50 μm diamond indenter on a micro-compression tester (Shimadzu MCT-W201), the inflection point of the load-deformation curve is taken as the breaking point, the value is calculated as St=2.8P/πd², and five particles are averaged.
- Particle size: the specification gives a D50 (median particle size) of 5-30 μm, preferably 10-25 μm, for the graphite particles.
That the measurement methods are fixed in the specification matters for claim interpretation. The same graphite can yield different values under different measurement conditions, and infringement is likely to be argued, in principle, on values measured by the method the patent specifies.
Comparison: resistance variation in examples and comparative examples
The original table contained only two examples and three comparative examples. The table below reorganizes all five examples and six comparative examples from the patent specification.
| Item | Graphite type | Internal porosity (%) | Breaking strength (MPa) | Average internal resistance (mΩ) | Standard deviation (mΩ) |
|---|---|---|---|---|---|
| Example 1 | C | 5 | 30 | 16.7 | 0.24 |
| Example 2 | D | 5 | 50 | 16.4 | 0.61 |
| Example 3 | E | 5 | 55 | 16.5 | 0.64 |
| Example 4 | C/G=30/70 | mixed | mixed | 17.8 | 0.80 |
| Example 5 | C/G=15/85 | mixed | mixed | 18.2 | 0.88 |
| Comp. Ex. 1 | A | 5 | 10 | 18.8 | 1.51 |
| Comp. Ex. 2 | B | 5 | 15 | 17.3 | 1.21 |
| Comp. Ex. 3 | F | 5 | 60 | 18.8 | 1.45 |
| Comp. Ex. 4 | G | 15 | 10 | 20.3 | 1.43 |
| Comp. Ex. 5 | H | 15 | 30 | 20.4 | 1.71 |
| Comp. Ex. 6 | I | 15 | 60 | 19.2 | 1.93 |
Source: compiled by the editorial team from the example and comparative example data in the specification of US12080916B2
What this table says: Examples 1-3, which meet both conditions, show standard deviations of 0.24-0.64 mΩ, whereas Comparative Examples 1-6 show 1.21-1.93 mΩ, more than twice as large. Even Example 5, which blends only 15% of in-range graphite (0.88 mΩ), has smaller variation than every comparative example.
Two more interpretations can be read from the table. First, this window is closed on both sides. Variation was large not only in Comparative Examples 1 and 2 with low breaking strength of 10-15 MPa, but also in Comparative Example 3 at a high 60 MPa (1.45 mΩ). The message is not ‘the harder the better’ but ‘moderately hard’, and the physical cause of the larger variation above the upper limit is hard to establish from public data alone. Second, Comparative Examples 4-6 with 15% porosity also have the highest average resistance, 19.2-20.4 mΩ, suggesting porosity affects both variation and absolute resistance.
Patent perspective: a narrow window for two properties, but a broad content condition
Bibliographic data: The rights holders of US12080916B2 (title “Cylindrical battery”) are Panasonic Energy Co., Ltd. and Panasonic Holdings Corp. Claiming priority from Japanese application JP2019-021656 (February 8, 2019) and proceeding through PCT/JP2020/004405 (filed February 5, 2020), it was granted in the US on September 3, 2024. According to Google Patents its legal status is Active and its anticipated expiration date is October 22, 2041. The inventors are Naoya Morisawa and three others.
Filing history: This patent followed a typical route: Japanese application (2019), PCT international application (2020), US grant (2024). The timing deserves attention. The February 2019 priority date precedes Panasonic’s September 2024 announcement that 4680 mass-production preparations were complete by more than five years. In effect, rights at the level of negative-electrode material properties were secured before competition in large cylindrical cells intensified, and if the rights are maintained until the anticipated 2041 expiration, they may overlap most of the mass-production period of 4680-class cells. Unlike conspicuous design patents on form factors or tab structures, material-property patents do not reveal whether they are practiced from the product’s appearance, so for competitors these are rights whose analysis carries a cost.
Gist of the claims (restated): Claim 1 is a combination of the following four elements. Dependent claim 2 further limits the graphite particles meeting these conditions to 15% by mass or more of the total negative-electrode active material.
Source: reconstructed by the editorial team from the claims of US12080916B2
Insight: claim 1 has no lower limit on content. The original article presented ’15 wt% or more’ as if it were the floor of the rights, but that condition exists only in the dependent claim. Claim 1 is written so that the negative-electrode mixture layer need only ‘contain’ graphite particles meeting the two conditions. Blending only a small amount of such graphite may therefore avoid claim 2 but is a weak means of avoidance in relation to claim 1. Data showing that the effect appears even at a 15% blend, as in Example 5, could also serve as grounds for the rights holder to assert infringement against low-blend products.
Avoidance points: The clearest avoidance route is element (c). If the structure does not have the exposed part of the negative current collector directly contacting the can’s inner surface, but instead welds a negative tab or separate current-collector plate to the can, there is room to lack this element. The second route is to keep graphite properties outside the window, using only graphite with porosity above 5% or breaking strength below 25 MPa or above 55 MPa.
Design-around scenario (counterargument): However, as the comparative data show, property-based avoidance means accepting worse resistance variation, trading off product competitiveness. Graphite properties also scatter from lot to lot, so if part of the supplied graphite falls inside the window, literal infringement risk may arise. Structural avoidance may also invite disputes over the interpretation of (c) if part of the exposed portion incidentally touches the can, so the judgment requires a one-to-one comparison of the actual cell cross-section with the claim language.
Limitations and outlook
The patent itself does not mention the 4680 form factor, and the example data come from the test cells in the specification. It is possible to argue that as cells grow larger (2170 to 4680) the jelly-roll circumference and can-contact area grow, making contact uniformity more important, but public data showing the same degree of improvement in large cells have not been confirmed.
The optimistic outlook is that this property control becomes standard practice for reducing quality variation in mass production of large cylindrical cells. There is also a contrary scenario. If designs such as tabless ones, which connect the entire electrode edge to a current-collector plate, spread, the share of structures that rely on contact with the can’s inner surface could shrink, limiting the business impact of this patent. Tesla’s approach addresses the current path, and Panasonic’s addresses the physical stability of the contact, so the two approaches tackle the same problem at different layers.
The strategic alternative is to view cell design choice and material specification management separately. A manufacturer keeping a can-contact negative-electrode structure should treat the graphite property window as a quality-control item but sort out the rights relationship first; a manufacturer moving to a tabless structure would do better to check FTO for current-collection structure patents before this one.
Practical action: Planning and IP staff sourcing negative-electrode graphite for cylindrical cells should add values measured by the patent’s methods (SEM cross-section binarized porosity, micro-compression breaking strength) to incoming specifications, mark whether the graphite is combined with a can-contact negative-electrode structure, and prepare a comparison chart against claim 1 of US12080916B2.
Items requiring verification
- Grant status and claim-scope differences of the Japanese, Chinese and other family members of US12080916B2
- Whether the graphite property conditions of this patent are actually applied to Panasonic’s 4680 cell
- The specification’s explanation of the physical cause of larger resistance variation when breaking strength exceeds 55 MPa
- The numbers and applicants of Tesla’s tabless-electrode patents and their relationship to this patent
Sources
- Google Patents, US12080916B2 – Cylindrical battery (Panasonic Energy Co., Ltd., Panasonic Holdings Corp.)
- Panasonic Energy press release, Panasonic Energy Ready to Commence Mass Production of 4680 Automotive Lithium-ion Batteries (2024-09-09)