As batteries were beginning to be mass-produced, the jar design changed to the cylindrical format. The large F cell for lanterns was introduced in and the D cell followed in . With the need for smaller cells, the C cell followed in , and the popular AA was introduced in . See BU-301: Standardizing Batteries into Norms.
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Cylindrical Cell
The cylindrical cell continues to be one of the most widely used packaging styles for primary and secondary batteries. The advantages are ease of manufacture and good mechanical stability. The tubular cylinder can withstand high internal pressures without deforming.
Many lithium and nickel-based cylindrical cells include a positive thermal coefficient (PTC) switch. When exposed to excessive current, the normally conductive polymer heats up and becomes resistive, stopping current flow and acting as short circuit protection. Once the short is removed, the PTC cools down and returns to the conductive state.
Most cylindrical cells also feature a pressure relief mechanism, and the simplest design utilizes a membrane seal that ruptures under high pressure. Leakage and dry-out may occur after the membrane breaks. Re-sealable vents with a spring-loaded valve are the preferred design. Some consumer Li-ion cells include the Charge Interrupt Device (CID) that physically and irreversibly disconnect the cell when activated to an unsafe pressure builds up. Figure 1 shows a cross section of a cylindrical cell.
Typical applications for the cylindrical cell are power tools, medical instruments, laptops and e-bikes. To allow variations within a given size, manufacturers use partial cell lengths, such as half and three-quarter formats, and nickel-cadmium provides the largest variety of cell choices. Some spilled over to nickel-metal-hydride, but not to lithium-ion as this chemistry established its own formats. The illustrated in Figure 2 remains one of the most popular cell packages. Typical applications for the Li-ion are power tools, medical devices, laptops and e-bikes.
In , 2.55 billion cells were produced. Early Energy Cells had 2.2Ah; this was replaced with the 2.8Ah cell. The new cells are now 3.1Ah with an increase to 3.4Ah by . Cell manufacturers are preparing for the 3.9Ah .
The could well be the most optimized cell; it offers one of the lowest costs per Wh and has good reliability records. As consumers move to the flat designs in smart phones and tablets, the demand for the is fading and Figure 3 shows the over-supply that is being corrected thanks to the demand of the Tesla electric vehicles that also uses this cell format for now. As of end of , the battery industry fears battery shortages to meet the growing demand for electric vehicles.
The demand for the would have peaked in had it not been for new demands in military, medical and drones, including the Tesla electric car. The switch to a flat-design in consumer products and larger format for the electric powertrain will eventually saturate the . A new entry is the .
There are other cylindrical Li-ion formats with dimensions of , and . Meanwhile, Tesla, Panasonic and Samsung have decided on the for easy of manufacturing, optimal capacity and other benefits. While the has a volume of approximately 16cm3 (16ml) with a capacity of around mAh, the cell has approximately 24cm3 (24ml) with a said capacity of up to mAh, essentially doubling the capacity with a 50% increase in volume. Tesla Motor refers to their company’s new as the “highest energy density cell that is also the cheapest.” (The nomenclature Tesla advocates is not totally correct; the last zero of the model describes a cylindrical cell harmonizing with the IEC standard.)
The larger cell with a diameter of 26mm does not enjoy the same popularity as the . The is commonly used in load-leveling systems. A thicker cell is said to be harder to build than a thinner one. Making the cell longer is preferred. There is also a made by E-One Moli Energy.
Some lead acid systems also borrow the cylindrical design. Known as the Hawker Cyclone, this cell offers improved cell stability, higher discharge currents and better temperature stability compared to the conventional prismatic design. The Hawker Cyclone has its own format.
Even though the cylindrical cell does not fully utilize the space by creating air cavities on side-by-side placement, the has a higher energy density than a prismatic/pouch Li-ion cell. The 3Ah delivers 248Ah/kg, whereas a modern pouch cell has about 140Ah/kg. The higher energy density of the cylindrical cell compensates for its less ideal stacking abilities and the empty space can always be used for cooling to improve thermal management.
Cell disintegration cannot always be prevented but propagation can. Cylindrical cells are often spaced apart to stop propagation should one cell take off. Spacing also helps in the thermal management. In addition, a cylindrical design does not change size. In comparison, a 5mm prismatic cell can expand to 8mm with use and allowances must be made.
Button Cell
The button cell, also known as coin cell, enabled compact design in portable devices of the s. Higher voltages were achieved by stacking the cells into a tube. Cordless telephones, medical devices and security wands at airports used these batteries.
Although small and inexpensive to build, the stacked button cell fell out of favor and gave way to more conventional battery formats. A drawback of the button cell is swelling if charged too rapidly. Button cells have no safety vent and can only be charged at a 10- to 16-hour charge; however, newer designs claim rapid charge capability.
Most button cells in use today are non-rechargeable and are found in medical implants, watches, hearing aids, car keys and memory backup. Figure 4 illustrates the button cells with a cross section.
CAUTIONKeep button cells to out of reach of children. Swallowing a cell can cause serious health problems. See BU-703 Health Concerns with Batteries.
Prismatic Cell
Introduced in the early s, the modern prismatic cell satisfies the demand for thinner sizes. Wrapped in elegant packages resembling a box of chewing gum or a small chocolate bar, prismatic cells make optimal use of space by using the layered approach. Other designs are wound and flattened into a pseudo-prismatic jelly roll. These cells are predominantly found in mobile phones, tablets and low-profile laptops ranging from 800mAh to 4,000mAh. No universal format exists and each manufacturer designs its own.
Prismatic cells are also available in large formats. Packaged in welded aluminum housings, the cells deliver capacities of 20–50Ah and are primarily used for electric powertrains in hybrid and electric vehicles. Figure 5 shows the prismatic cell.
The prismatic cell improves space utilization and allows flexible design but it can be more expensive to manufacture, less efficient in thermal management and have a shorter cycle life than the cylindrical design. Allow for some swelling.
The prismatic cell requires a firm enclosure to achieve compression. Some swelling due to gas buildup is normal, and growth allowance must be made; a 5mm (0.2”) cell can grow to 8mm (0.3”) after 500 cycles. Discontinue using the battery if the distortion presses against the battery compartment. Bulging batteries can damage equipment and compromise safety.
Pouch Cell
In , the pouch cell surprised the battery world with a radical new design. Rather than using a metallic cylinder and glass-to-metal electrical feed-through, conductive foil-tabs were welded to the electrodes and brought to the outside in a fully sealed way. Figure 6 illustrates a pouch cell.
The pouch cell offers a simple, flexible and lightweight solution to battery design. Some stack pressure is recommended but allowance for swelling must be made. The pouch cells can deliver high load currents but it performs best under light loading conditions and with moderate charging.
The pouch cell makes most efficient use of space and achieves 90–95 percent packaging efficiency, the highest among battery packs. Eliminating the metal enclosure reduces weight, but the cell needs support and allowance to expand in the battery compartment. The pouch packs are used in consumer, military and automotive applications. No standardized pouch cells exist; each manufacturer designs its own.
Pouch packs are commonly Li-polymer. Small cells are popular for portable applications requiring high load currents, such as drones and hobby gadgets. The larger cells in the 40Ah range serve in energy storage systems (ESS) because fewer cells simplify the battery design.
Although easily stackable, provision must be made for swelling. While smaller pouch packs can grow 8–10 percent over 500 cycles, large cells may expand to that size in 5,000 cycles. It is best not to stack pouch cells on top of each other but to lay them flat, side by side or allow extra space in between them. Avoid sharp edges that can stress the pouch cells as they expand.
Extreme swelling is a concern. Users of pouch packs have reported up to 3 percent swelling incidents on a poor batch run. The pressure created can crack the battery cover, and in some cases, break the display and electronic circuit boards. Discontinue using an inflated battery and do not puncture the bloating cell in close proximity to heat or fire. The escaping gases can ignite. Figure 7 shows a swollen pouch cell.
Swelling can occur due to gassing. Improvements are being made with newer designs. Large pouch cells designs experience less swelling. The gases contain mainly CO2 (carbon dioxide) and CO (carbon monoxide).
Pouch cells are manufactured by adding a temporary “gasbag” on the side. Gases escape into the gasbag while forming the solid electrolyte interface (SEI) during the first charge. The gasbag is cut off and the pack is resealed as part of the finishing process. Forming a solid SEI is key to good formatting practices. Subsequent charges should produce minimal gases, however, gas generation, also known as gassing, cannot be fully avoided. It is caused by electrolyte decomposition as part of usage and aging. Stresses, such as overcharging and overheating promote gassing. Ballooning with normal use often hints to a flawed batch.
The technology has matured and prismatic and pouch cells have the potential for greater capacity than the cylindrical format. Large flat packs serve electric powertrains and Energy Storage System (ESS) with good results. The cost per kWh in the prismatic/pouch cell is still higher than with the cell but this is changing. Figure 8 compares the price of the cylindrical, prismatic and pouch cells, also known as laminated. Flat-cell designs are getting price competitive and battery experts predict a shift towards these cell formats, especially if the same performance criteria of the cylindrical cell can be met.
Historically, manufacturing costs of prismatic and pouch formats (laminate) were higher, but they are converging with cellular design. Pricing involves the manufacturing of the bare cells only.
Asian cell manufacturers anticipate cost reductions of the four most common Li-ion cells, which are the , , prismatic and pouch cells. The promises the largest cost decrease over the years and economical production, reaching price equilibrium with the pouch by (Figure 9).
Automation enables price equilibrium of the with the pouch cell in . This does not include packaging where the prismatic and pouch cells have a cost advantages.
Fraunhofer predicts the fastest growth with the and the pouch cell while the popular will hold its own. Costs per kWh do not include BMS and packaging. The type cell chosen varies packaging costs as prismatic can easily be stacked; pouch cells may require some compression and cylindrical cells need support systems that create voids. Large packs for electric vehicle also include climate control that adds to cost.
Summary
With the pouch cell, the manufacturer is attempting to simplify cell manufacturing by replicating the packaging of food. Each format has pros and cons as summarized below.
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- Cylindrical cell has high specific energy, good mechanical stability and lends itself to automated manufacturing. Cell design allows added safety features that are not possible with other formats (see BU-304b: Making Lithium-ion Safe); it cycles well, offers a long calendar life and is low cost, but it has less than ideal packaging density. The cylindrical cell is commonly used for portable applications.
- Prismatic cell are encased in aluminum or steel for stability. Jelly-rolled or stacked, the cell is space-efficient but can be costlier to manufacture than the cylindrical cell. Modern prismatic cells are used in the electric powertrain and energy storage systems.
- Pouch cell uses laminated architecture in a bag. It is light and cost-effective but exposure to humidity and high temperature can shorten life. Adding a light stack pressure prolongs longevity by preventing delamination. Swelling of 8–10 percent over 500 cycles must be considered with some cell designs. Large cells work best with light loading and moderate charge times. The pouch cell is growing in popularity and serves similar applications to the prismatic cell.
References
The importance of cylindrical batteries is only growing because they are used widely from small electronic devices to EVs. In line with the trend, LG Energy Solution has continued researching and developing cylindrical batteries to improve their capacity and performance.
At the “LGES Cylindrical Li-ion Batteries in The Era of E-mobility” session of LG Tech Conference hosted at LG Sciencepark in Gangseo-gu, Seoul on April 4, there was a presentation on the history and technology trend of cylindrical batteries. The speech delivered information on cylindrical batteries currently being developed by LG Energy Solution. Here’s what Kwan-soo Lee working at the Small Battery Development Center has to say.
LG Energy Solution’s Cylindrical Batteries, from and to 46-Series
LG Energy Solution began its research on lithium-ion batteries in . It launched the development of lithium-ion batteries in and entered into the battery market with the first mass-production of laptop batteries in .
Batteries have been adopted for a variety of applications ever since. We developed the cylindrical battery to provide to manufacturers of electric scooters and power tools in and broadened the application of these batteries in earnest, putting them in LEVs* and then high-performance EVs from .
We produced the battery, an improvement in capacity and efficiency of the battery and adopted it for Energy Storage System(ESS)s in . We then upgraded it to show better performance and have higher safety, starting the supply of the improved version to leading North American EV manufacturers in .
LG Energy Solution has increased the battery sizes and is currently developing the 46-series battery. The 46-series offers 46 mm in diameter and a wide range of height (80 mm – 120 mm), further expanding the application of cylindrical batteries.
* LEV: Light Electric Vehicles. They include electric bikes, scooters, and wheelchairs.
A cylindrical battery has a mechanically stable “thick can” structure, meaning it is basically very safe. This feature allows the application of various and most advanced materials to it ahead of other types of batteries.
These batteries are widely used for devices that require a sudden high output such as power tools as well as LEVs and EVs due to their high energy density and capacity. They can be used for various applications easily and quickly as they come in standardized sizes such as and .
Once, cylindrical batteries were mostly used for electronic devices such as laptops, but some raised questions about their future as electronic devices became slimmer. However, their advantages mentioned earlier put them in LEVs and brought about the expansion of their boundaries into the mobility market. Now, leading EV makers and startups in North America as well as automakers in Europe pay attention to cylindrical batteries, and many global finished car manufacturers are considering adopting them for their EVs, having noticed the potential.
Such moves led to the enlargement of the EV market powered by cylindrical batteries. The prospect for the cylindrical battery market is also promising. The annual growth rate from to is expected to be approximately 41%, with the EVs accounting for the largest share of the cylindrical battery market.
46-Series Batteries, Cylindrical Batteries with Many Advantages
Management of Safety and Thermal Propagation
As mentioned earlier, a cylindrical battery is encased by a hard can and can be vented at a “unit battery” level. This structure allows it to not only maintain a stable shape even after repeated charging and discharging at normal times, but also secure safety when it is exposed to abnormal circumstances such as an internal gas leak or abrupt chemical reaction during charging or discharging.
One of the most common protection tools embedded inside a cylindrical battery is the Current Interrupt Device (CID). When internal gas is created during charging or discharging and puts pressure above a certain level, CID cuts off the electrical circuit and turns it into an open circuit. Also, the vent keeps the battery safe by releasing the gas safely out of the battery.
Safety deserves the highest priority in a battery system. An EV especially needs a device that can prevent chain ignition that can be sparked by thermal propagation, because it has a large amount of battery cells inside. A battery pack of an EV contains from hundreds to thousands of battery cells. If one cell is on flame, the heat might spread to those next to it. Therefore, a large battery system needs a technological approach to effectively manage thermal propagation.
The safety of cylindrical batteries has been proven through many tests. For example, the cell’s side remained without a rupture after the cell had been exposed to high temperature and ignited in a heater test. Also, the cell remained stable without causing a chain ignition through thermal propagation to adjacent cells when the center cell ignited in a honeycomb structure of the cells. To make that possible, the mechanical properties of the can was enhanced and the venting system was optimized.
Significance and Goals of Cylindrical Battery Development
LG Energy Solution is currently developing the 46-series, a line larger than the and batteries. The 46-series cylindrical battery offers more energy, as it can hold more active materials. In particular, nickel content is being increased for higher density and battery capacity.
The 46-series has a simpler pack structure and lower cell counts, but still can provide customers with higher energy efficiency. For example, approximately cells with the size of the batteries can be replaced by only 828 cells with the size of the batteries. As such, the 46-series contributes to a reduction in manufacturing costs and time while improving efficiency of the overall battery system.
As in the case of the 46-series, a change in size and accompanying importance of mechanical structure are emphasized. Structural design and development of a battery from the perspective of mechanical engineering are essential for battery performance and safety. The importance can be compared to that of material composition.
Development of the 46-series is not just about growth in size but can be considered an important step in the path to opening a new chapter of cylindrical battery technology as well as a preparation to meet the rising demand for cylindrical batteries.
Advantages of 46-Series Cylindrical Battery
All eyes of global finished car manufacturers and battery makers are on the 46-series, the new standard of cylindrical batteries. In response, LG Energy Solution is proactively preparing mass-production of the 46-series battery cells.
The 46-series can resolve major disadvantages of its predecessors. The and cylindrical batteries need tabs that work as paths for currents to flow to certain spots in a jelly-roll structure where the electrodes are rolled up. And the electrodes can be deployed efficiently with the employment of various designs. But the design accompanies a rise in resistance. In comparison, the 46-series has the advantages of pouch-type batteries and the cylindrical batteries and is being developed to lower resistance as it is designed to maintain the same energy density with an improvement in current flow. Also, the “directional venting,” a technology that is applied at the unit cell level, which also is an advantage of cylindrical batteries, is employed. This technology rapidly releases the implosion energy of a battery out of it, reducing the cell’s resistance and securing the cell’s safety, and preventing chain ignition at the same time.
A transition to the 46-series could lead to cost savings by expanding production scale. The economic benefits are larger as that allows production of cells with 5 times higher energy capacity within the same time frame as well as more energy on the same production lines. A cut in manufacturing costs can be crucial for customers as well because it raises the possibility for EVs to penetrate into the entire car market. The 46-series can produce even more energy because it speeds up the electrode production line by about 1.5 times while allowing the assembly line to keep its takt time.
For the previously invented cylindrical batteries, slurries were coated onto electrodes at the rate of 40 meters per minute with the intermittent coating method. Periodical discharge and stoppage of slurries in this process created patterns and the electrodes with the patterns on them were wound to make a component. In comparison, for the 46-series, the “zebra coating” method was applied. Since this method proceeds with coating continuously without stoppage, the safety and efficiency in producing electrodes are higher.
* Takt time: The time needed to complete a product.
LG Energy Solution’s Technology Producing High-performance Batteries
There are many things to consider from the customers’ perspective in developing EV batteries. A battery’s energy is directly related to driving distance while different materials and designs have an impact on quick charge. To increase energy capacity, high-capacity materials that have a high nickel content such as silicon-based anode are essential. As for the quick charge, it can impact the long-term life and durability of the battery, although it offers convenience to drivers. Therefore, research on the optimum formulation and stability of cathode and anode active materials is necessary.
Since safety and reliability have to be secured first and foremost, it is important to improve thermal and physical safety of a separator and find the right materials and combination of electrolyte that prevents low voltage and plays an important role in enhancing performance and safety.
Cathode
LG Energy Solution uses NCM-based cathode materials that allow high-capacity energy storage. We became the world’s first to mass-produce batteries consisting of NCM 523 cathode materials for electronic devices in and have been producing batteries consisting of NCMA cathode materials with a nickel content of at least 85% since then. The most important thing to consider in producing high-capacity high-nickel batteries is “How stably the energy is provided.” Structural safety and surface safety have to be secured through effective doping and coating. We have studied relevant materials as well as surface coating and multi-doping techniques for the purpose.
Also, we reinitiated research on single-crystal cathode materials. These materials do not create a crack as they have a higher stress resistance compared to poly-crystal cathode materials and are less likely to produce internal gas.
Anode
Silicon-based anode materials are often used for high-capacity NCMA batteries and offer high energy density, but the volume might expand during charging, which requires adequate control. That’s why research on conductive additives, which will be applied with anode materials, is ongoing in addition to research on silicon anode material itself. These methods include minimizing expansion by adding other materials to silicon or increasing stability by coating the surface of silicon particles.
Separator
A separator keeps the cathode and anode apart, playing a crucial role in preventing a circuit. LG Energy Solution succeeded in mass-production of Safety Reinforced Separator (SRS®), a ceramic-coated separator with enhanced safety, for the first time in the world in . This is a technology of applying ceramic coating and polymer binder onto the surface of the separator and it enhances heat resistance of a separator. Diverse ways are being researched on separators, a necessary effort for higher safety and performance of batteries.
Electrolyte
An electrolyte determines a battery’s voltage and performance. In particular, hydrogen fluoride(HF) scavengers and Elastic Solid Electrolyte Interface(SEI)s are important in resolving the increase in resistance in Hi-nickel cathode materials and instability of the SEI layer in silicon-based anode materials. HF scavengers neutralize hydrogen fluoride, preventing damage to the internal components while Elastic SEIs protect the newly created surface following the expansion of silicon anode materials, stabilizing the SEI layer.
These additives repress resistance even at about 60℃ and have a lower resistance value with time compared to previously developed electrolytes, contributing to improvement of battery safety and performance.
A leader in battery technology development, LG Energy Solution plans to mass-produce 46-series batteries at Ochang Energy Plant. With higher energy density and output, the next-generation cylindrical battery is expected to drive popularity of electricity-powered vehicles!
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