[Complete Battery Guide](34) How Are Cylindrical Batteries Made... feat. 4680

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At the end of last month, Panasonic of Japan announced that it had signed a contract with Mazda Motor Corporation for the supply of cylindrical batteries. The two companies have been negotiating supply terms since June last year, only recently reaching an agreement. Prior to this, Panasonic had signed a similar contract with Subaru Corporation in Japan.

Although Panasonic, Mazda, and Subaru have not disclosed specific details of the contract, the industry believes Panasonic will supply the 4680 cylindrical batteries to these two companies. If so, this will result in the first electric vehicles in Japan equipped with 4680 batteries.

Panasonic has announced that it plans to begin mass production of 4680 battery cells in the first half of the 2025 fiscal year (from April 1, 2024 to March 31, 2025). This means that 4680 batteries could be in mass production as early as September this year. If things go as planned, Panasonic will launch its 4680 batteries around the same time as LG Energy Solution, which has announced August as its target date.

On the 23rd, Phil Energy, a domestic battery equipment manufacturer, announced at its company briefing that it is developing a “Cylindrical 4600 Winder (Winding Machine)” and has begun technical sales after demonstrations for global battery manufacturers, including three companies in Europe, one in the United States, and four in South Korea.

Interest in cylindrical batteries, once thought to be eclipsed by prismatic and pouch forms, is rapidly increasing again thanks to the advent of so-called “46-pi batteries” (a term referring to cylindrical batteries with a diameter of 46mm). Automakers are now increasing their adoption of cylindrical batteries in electric vehicles.

According to market research firm SNE Research, the share of cylindrical batteries in the global electric vehicle market declined from 28% in 2019, to 23% in 2020, 16% in 2021, and 12% in 2022. However, it rose again to 13% in 2023.

If mass production of 46-pi batteries gains momentum, this number could rise even higher. Leading battery cell manufacturers such as LG Energy Solution, Samsung SDI, SK On, Panasonic, CATL, and BYD all have ongoing plans to produce 46-pi batteries.

The 46-pi cylindrical battery is not significantly different in appearance or manufacturing process from conventional cylindrical batteries such as the 1865 and 2170. However, because innovative technologies have been applied, the manufacturing process has become more complex and mass production timelines have been delayed. Here, we explain the cylindrical battery manufacturing process and compare it to the 4680 battery process.

Like Spreading Strawberry Jam on Bread...Making the Electrode

Disassembling a cylindrical lithium-ion battery reveals several layers of thin sheets rolled up inside a round metal can, resembling a roll of toilet paper. These rolled-up sheets consist of a cathode, separator, and anode layered together, forming what is called a “jelly roll.” The name comes from the resemblance to a jelly roll cake.

Structure of a cylindrical battery. Photo by Samsung SDI

For rechargeable batteries, the cylindrical cell is first assembled as a jelly roll, placed in a metal can, and then filled with electrolyte and sealed.

In the battery industry, the battery manufacturing process is usually divided into four stages: electrode process → assembly process → formation process → pack process. While there are detailed differences in the individual steps depending on whether it is cylindrical, prismatic, or pouch-type, the main stages are similar.

The electrode process is the stage where the battery cathode and anode are produced, and it is also called the plate process. The electrode is the part of the battery that most strongly influences performance and also accounts for the greatest share of manufacturing cost. The electrode process can be further divided into mixing, coating, roll pressing (calendering), and slitting/notching processes.

Mixing is the process in which the active materials of the cathode and anode are blended with a solvent to form a slurry—a thick, viscous substance similar to dough. During this process, a binder (to increase adhesion between active material particles) and a conductive agent (to aid electron flow) are added.

Mixing process. Image source=TSI

The coating process involves evenly applying the cathode and anode slurry to aluminum and copper foil (current collectors), similar to spreading jam on bread. It is critical to achieve a uniform, consistent thickness of the slurry on the current collector. The equipment that applies the slurry automatically onto the collector is called a slot die coater.

Battery electrode coating equipment. Image source_PNT

Once coated, the electrode is immediately transferred to a dryer, where hot air or other means are used to remove the solvent. Tesla initially planned to use a dry electrode process (which doesn’t require solvents) for its 4680 batteries, but as of now, only the anode appears to use a dry process.

The dried electrode is then passed between two rolls and flattened via roll pressing (calendering). This reduces the thickness and improves the adhesion between the current collector and active material, increasing the battery’s energy density.

Cathodes and anodes that finish roll pressing are cut to fit the appropriate cell size in the slitting process. The slitter trims the electrode into strips of the right width for the battery cell.

Structure of the electrode. Image source=LG Energy Solution

The cut electrode then undergoes the notching process. During notching, parts of the electrode with no active material (the uncoated spaces) are shaped to leave tabs for welding lead tabs (which electrically connect the cathode and anode to the outside), while cutting away unnecessary portions.

Whereas mechanical presses with blades were traditionally used for notching, recently more precise laser systems are being adopted for this stage.

One of the biggest differences in the process between conventional 1865/2170 cylindrical batteries and 4680 batteries is at this notching stage. Tesla's 4680 batteries, for example, use a tabless method—manufactured without conventional tabs. Other 46-pi batteries are also believed to utilize a tabless design.

Tabless Structure of Conventional Cylindrical Battery and 4680 Battery

According to designs released by Tesla, their approach is closer to “multi-tab” rather than purely “tabless,” processing the end of the cell into several tab-like contacts instead of attaching a traditional lead tab. This distributes current and heat more effectively.

In traditional cylindrical batteries, current is concentrated through one cathode tab and one anode tab, which makes them vulnerable to heat. The tabless design used on 4680 batteries addresses this issue. The 4680 battery’s tabless design requires more sophisticated laser notching equipment.

Image source=Phil Energy

When notching is complete, so is the electrode process for manufacturing the cathode and anode. The finished cathode and anode are transferred as rolls to the assembly line.

Rolled Up and Inserted in the Can...Assembly Process

The assembly process is where the cathode, anode, and separator are combined to give the battery its exterior form. This is the step where you find the biggest differences depending on the battery’s form—cylindrical, prismatic, or pouch.

In the case of cylindrical batteries, a machine called a winder is used. The cathode roll, anode roll, and two separator rolls are each installed, and, similar to making a roll of toilet paper, are wound together and trimmed to the desired length. The completed product resembles a jelly roll cake, hence the term “jelly roll.”

Schematic diagram of cylindrical battery winding process. Image source=LG Energy Solution

The winder for 4680 batteries needs to be different from that of conventional cylindrical batteries. With a diameter of 46mm, greater accuracy is required for winding without distortion.

Next comes tab welding, where aluminum tabs are attached to the cathode and copper tabs to the anode at the uncoated sections. Initially, ultrasonic welding was used, but recently this step uses lasers.

Cylindrical battery assembly process. Image source=Center for Advanced Life Cycle Engineering (CALCE), University of Maryland

The jelly roll with tabs is placed inside a metal can. At this stage, the anode tab is welded to the bottom of the can, while the cathode tab is welded to the cap (the top of the can). The can is evacuated and the electrolyte is injected. Finally, the can is hermetically sealed through crimping.

Tesla 4680 Cylindrical Battery Disassembly. Image Source=Journal of The Electrochemical Society, 2023

Since Tesla’s 4680 batteries do not use lead tabs, the tab welding step is omitted. Instead, aluminum and copper disks are attached to the cathode and anode, respectively, before welding.

Cylindrical battery internal safety device. Image source=Gangwon-do Fire Headquarters

To enhance safety, various internal safety devices are installed inside the battery. When cylindrical batteries are dismantled, you can find components such as PTC (Positive Temperature Coefficient), CID (Current Interrupt Device), and vents.

The PTC device limits current when resistance rises due to overcurrent. The CID cuts off current if the internal gas pressure rises, and, if pressure continues rising, the vent opens to safely release gas and prevent explosion.

Batteries Also Need “Aging”...Formation Process

Batteries that pass through electrode and assembly processes still lack electrical characteristics and cannot yet function. The formation process is what gives the battery its final “breath” or life.

The first step of the formation process is activation (formation). During this phase, the batteries are aged at constant temperature and humidity, and undergo repeated charge-discharge cycles. This allows sufficient electrolyte to infiltrate the cathode and anode, structurally stabilizing the battery.

Equipment Required for Mars Processing. Image Source = A-Pro

During activation, a thin solid layer called the SEI (Solid Electrolyte Interphase) is formed on the anode surface. SEI has a significant impact on battery performance and lifespan. In pouch-type batteries, an additional degassing step removes gases produced during the aging process.

Lithium-ion battery manufacturing process. Pouch batteries include an additional degassing process in the formation stage to remove gases. Image source=iScience Perspective

The formation process also involves sorting out defective batteries by measuring properties such as resistance, voltage, and capacity. Internal resistance (IR) and Open Circuit Voltage (OCV) tests are conducted to assess battery quality. Lower internal resistance is better. The OCV test checks for microscopic internal short circuits in the battery.

The sequence of the formation process may differ from manufacturer to manufacturer.

Packing the Battery...Pack Process

While manufacturers could sell the batteries as-is after they are fully functional, battery companies add an additional packaging step—the pack process—to suit specific applications. The pack process involves assembling multiple individual cells into modules and inserting them into a battery pack. This adds further market value.

The pack process is divided into the cell-to-cell process—wiring multiple cells using a wiring harness or bus bar—and the module-to-module process, which assembles these cells into modules and connects the modules.

In the cell-to-cell process, multiple cells are interconnected with wiring harnesses or bus bars. These connected cells are modularized, and the modules are then connected—that’s the module-to-module part.

Along with the cells and modules, a Battery Management System (BMS) is also integrated. The BMS continuously monitors parameters such as battery capacity, voltage, and temperature, maintaining performance and safety. Recently, new materials and devices have been added to battery packs and modules to prevent heat transfer and thermal runaway in case of electric vehicle fires. These prevent leakage or explosions in a single cell from spreading to neighboring cells.

The 4680 battery has an advantage over the traditional 2170 cylindrical cell in that fewer batteries are needed for a given volume, simplifying the pack process. For instance, the Model Y originally used 4,400 cells of 2170 batteries but only needs 830 cells of the 4680 battery. This sharply reduces the number of required welds and parts in the pack process.

InsideEvs. Mazda And Panasonic Finally Strike Deal On Cylindrical EV Battery Supply. 2024.3.29
Journal of The Electrochemical Society, Lithium-Ion Cells in Automotive Applications: Tesla 4680 Cylindrical Cell Teardown and Characterization, 2023.12.29
University of Maryland, Safety Analysis of Lithium-Ion Cylindrical Batteries Using Design and Process Failure Mode and Effect Analysis, 2024.2.23
Gangwon-do Fire Headquarters, A Study on the Mechanism of Lithium-Ion Battery Explosion and Ignition, 2020.2.19
LG Energy Solution, Battery Inside
Samsung SDI Blog
Eugene Investment & Securities, Focus on the Cylindrical Battery Market for Electric Vehicle Batteries, 2022.1.30
Kium Securities Research Center, Investment Cycle in Full Swing: Time to Focus on Equipment, 2023.9.13

This content was produced with the assistance of AI translation services.