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

Editor's NoteWe are now in the battery era. Batteries are found in almost everything, from mobile phones and laptops to electric vehicles. [Complete Battery Mastery] is a section created to easily convey the basics of batteries, their ecosystem, company information, industry trends, and outlook to general readers, students, and investors interested in the battery industry and related companies. We will meet you every Saturday.

Japan's Panasonic announced at the end of last month that it has signed a contract to supply cylindrical batteries to Japan's Mazda Motor Corporation. The two parties had been negotiating supply since June last year, and the deal was only recently finalized. Prior to this, Panasonic also signed a similar contract with Japan's Subaru Motor Corporation.

Panasonic, Mazda, and Subaru have not disclosed specific contract details, but the industry expects Panasonic to supply 4680 cylindrical batteries to both companies. This would mark the first electric vehicle in Japan equipped with 4680 batteries.

Panasonic has announced that the mass production of 4680 battery cells is scheduled for the first half of fiscal year 2025 (April 1, 2024 ? March 31, 2025). This means they could start mass production of 4680 batteries by September this year at the latest. If on schedule, Panasonic is expected to release 4680 batteries around the same time as LG Energy Solution announced in August.

Domestic battery equipment company Phil Energy revealed at its corporate briefing on the 23rd that it is developing a cylindrical 4600 winder and has begun technical sales demonstrations to global battery companies, including three in Europe, one in the U.S., and four in Korea.

Interest in cylindrical batteries, which once seemed likely to be overshadowed by prismatic and pouch types, is rising again with the advent of so-called 46-pi batteries (a general term for cylindrical batteries with a diameter of 46 mm). Automakers are increasing the use of cylindrical batteries in electric vehicles once more.

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

This figure could increase further as mass production of 46-pi batteries accelerates. Leading battery cell companies such as LG Energy Solution, Samsung SDI, SK On, Panasonic, CATL, and BYD all have plans to produce 46-pi batteries.

The 46-pi cylindrical battery is not significantly different in appearance from traditional cylindrical batteries like 1865 and 2170, and the manufacturing process is similar. However, due to the application of innovative technologies, the manufacturing process is more complex, and the mass production timeline has been delayed accordingly. Let's explore the manufacturing process of cylindrical batteries by comparing it with that of 4680 batteries.

Spreading Electrodes Like Strawberry Jam on Bread...

If you disassemble a cylindrical lithium-ion battery, you can see several layers of thin sheets rolled up like a roll of toilet paper inside a round metal can. The rolled sheets consist of the cathode plate, separator, and anode plate layered together, called a jelly roll. The name comes from its resemblance to a jelly roll cake.

Structure of a cylindrical battery. Photo by Samsung SDI

The cylindrical secondary battery is manufactured by first creating this jelly roll, placing it inside a metal can, injecting electrolyte, and then sealing it.

In the battery industry, the manufacturing process is divided into four stages: electrode process → assembly process → formation process → pack process. While there are differences in detailed processes depending on the shape?cylindrical, prismatic, pouch?the basic process is the same.

The electrode process is the manufacturing of the battery's cathode and anode, also called the plate process. Electrodes have the greatest impact on battery performance and account for a large portion of the cost. The electrode process is further divided into mixing, coating, roll pressing, and slitting/notching.

Mixing is the process of combining cathode and anode active materials with solvents to create a slurry. The slurry is a viscous substance similar to dough. At this stage, binders that increase adhesion between active material particles and conductive agents that facilitate electron movement are added together.

Mixing process. Image source=TSI

The coating process involves applying the prepared cathode and anode slurry thinly onto aluminum and copper foils (current collectors), respectively. It is similar to spreading jam on bread. It is important to coat the slurry uniformly and at a consistent thickness. The equipment that automatically coats slurry onto current collectors is called a slot die coater.

Battery electrode coating equipment. Image source_PNT

After coating, the electrodes are moved to a dryer where solvents are removed using hot air or other methods. Tesla planned to apply a dry process without solvents for the 4680 battery instead of this wet process, but so far, it appears that the dry process is only used for the anode.

The dried electrodes pass between two rolls in a roll pressing process to make them thin and flat. This process reduces the thickness uniformly and improves the bonding strength between the current collector and cathode active material, increasing the battery's energy density.

The rolled cathode and anode then enter the slitting process, where they are cut to battery cell specifications. This process uses equipment called a slitter to cut the electrode width according to the battery cell size.

Structure of the electrode. Image source=LG Energy Solution

The cut electrodes then undergo the notching process. Notching involves cutting away all parts except the lead tab area (the part where the lead tab?a terminal that electrically connects the cathode and anode to the outside?is welded) in the uncoated area of the electrode.

Previously, press equipment with sharp blades was used for notching, but recently, laser equipment has been adopted for more precise cutting.

One of the biggest differences between the processes for traditional 1865, 2170 cylindrical batteries and 4680 batteries is the notching process. Tesla manufactures 4680 batteries using a tabless design, eliminating the lead tab. Other 46-pi batteries are also believed to apply tabless designs.

Tabless Structure of Conventional Cylindrical Battery and 4680 Battery

According to Tesla's disclosed design, it appears closer to a multi-tab design rather than completely tabless. Instead of attaching lead tabs as before, the battery ends are processed into multiple tab shapes. This method helps disperse current and heat.

In traditional cylindrical batteries, current concentrates on a single cathode and anode tab, making them vulnerable to heat, which this design improves. The tabless 4680 battery likely requires more precise laser notching equipment.

Image source=Phil Energy

Once the notching process is complete, the electrode process for making the battery's cathode and anode is finished. The completed cathode and anode are moved in roll form to the assembly line.

Roll It Up and Slip It Into the Can... Assembly Process

The assembly process combines the cathode, anode, and separator, giving the battery its shape. The assembly process varies most significantly depending on the battery form?cylindrical, prismatic, or pouch.

In cylindrical batteries, equipment called a winder is used. It mounts rolls of cathode, anode, and two separator rolls, then rolls them up like toilet paper and cuts them to the appropriate size. The resulting shape resembles a jelly roll cake, hence the name jelly roll.

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

The winder for 4680 batteries requires different equipment from conventional cylindrical batteries. Since the diameter is 46 mm, thicker than before, it is important to wind precisely without distortion.

Afterward, tabs made of aluminum and copper are welded to the uncoated areas of the cathode and anode, respectively, in a tab welding process. Initially, ultrasonic welding was used, but recently laser welding has become common.

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

The jelly roll with tabs is inserted into a metal can, with the anode tab welded to the bottom of the can and the cathode tab welded to the cap (lid of the can). The can is then vacuumed, electrolyte is injected, and finally, the can is completely sealed by crimping.

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

In Tesla's 4680 battery, since there are no lead tabs, the tab welding process is omitted. Instead, disks made of aluminum and copper are attached to the cathode and anode, respectively, and then welded.

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

Various safety devices are also installed inside to enhance battery safety. When disassembling manufacturers' cylindrical batteries, devices such as PTC (Positive Temperature Coefficient), CID (Current Interrupt Device), and vent are found.

PTC reduces current flow by increasing resistance when overcurrent occurs. CID cuts off current when gas pressure inside the battery rises. If pressure continues to increase, the vent opens to release gas outside, preventing explosions.

Batteries Also Need Maturation... Formation Process

After the electrode and assembly processes, the battery still lacks electrical characteristics and cannot function properly. The formation process breathes life into the battery.

The first step in the formation process is activation. During activation, the battery is stored under controlled temperature and humidity for a set time and undergoes repeated charging and discharging cycles. This allows the electrolyte to sufficiently penetrate the cathode and anode and structurally stabilizes the battery.

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

During activation, a thin solid electrolyte interphase (SEI) layer forms on the anode surface. SEI greatly affects battery performance and lifespan. In pouch batteries, a degassing process is added to remove gases generated during aging.

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

In the formation process, defective batteries are screened through resistance, voltage, and capacity measurements. Tests such as internal resistance (IR) and open circuit voltage (OCV) are conducted to evaluate battery quality. Lower internal resistance is better. OCV tests check for micro short circuits inside the battery.

The sequence of the formation process may vary by manufacturer.

Packaging the Battery... Pack Process

Although batteries with full vitality could be sold as is, battery companies add value by packaging them according to applications through the pack process. The pack process modularizes manufactured unit cells and places them into packs, enhancing product value and marketability.

The pack process is divided into cell-to-cell and module-to-module processes. Cell-to-cell connects multiple cells using wiring harnesses or bus bars. Module-to-module assembles these connected cells into modules and then connects the modules.

During this process, the battery management system (BMS) is also connected. The BMS monitors battery capacity, voltage, temperature, etc., in real time to maintain performance and safety. Recently, various materials and devices that prevent heat transfer have been applied to packs and modules to prevent electric vehicle fires. These measures prevent gas leaks or explosions in one cell from spreading to adjacent cells.

Compared to existing cylindrical batteries like 2170, the 4680 battery has fewer cells per volume, simplifying the pack process. For example, Model Y uses 4,400 cells of 2170 batteries but only needs 830 cells of 4680 batteries. This significantly reduces the number of welds and parts required 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, Study on Explosion and Ignition Mechanisms of Lithium-Ion Batteries, 2020.2.19
LG Energy Solution, Battery Inside
Samsung SDI Blog
Eugene Investment & Securities, Attention to Cylindrical Battery Market in Electric Vehicles, 2022.1.30
Kiwoom Securities Research Center, Investment Cycle in Full Swing: Time to Focus on Equipment, 2023.9.13