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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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.
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.
CAUTION
Keep button cells to out of reach of children. Swallowing a cell can cause serious health problems. See BU-703 Health Concerns with Batteries.Figure 4: Button cells provides small size, most are primary for single-cell use[4]
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.
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.
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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.
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.
[1] Source: Sanyo
[2] Source: Cadex Electronics
[3] Source: Avicenne Energy
[4] Source: Sanyo and Panasonic
[5] Source: Polystor Energy Corporation
[6] Source: A123
[7] Source: Battery Experts Forum
There are many sizes of cylindrical lithium-ion (Li-ion) cells, and the number of sizes continues to grow. Some are optimized for use in simple devices such as toys and flashlights; others are mainly found powering portable electronics and electric vehicles. This FAQ begins by reviewing the broad landscape of cylindrical Li-ions, including protected and non-protected cells for various applications. It then digs more deeply into a comparison of today&#;s two most common formats, the and . It closes looking into the future, including larger formats and improvements in cell and pack construction techniques leading to the development of premium-performance energy storage systems.
Battery protection elements
Various cylindrical Li-ion batteries are offered in protected and unprotected packaging. Most electronic equipment, electric vehicles, and other commercial applications favor unprotected batteries due to their higher capacity ratings and lower prices; in these applications, the battery protection is built into the system, not the battery.
Consumer batteries are offered in both protected and unprotected styles. Protected batteries are safer to use in simple devices such as flashlights and toys (Figure 1). Common battery protection elements protect against excessive currents and overheating high internal pressures, and overvoltage conditions and include:
Common sizes of cylindrical Li-ions include:
Cylindrical cell construction comparison
Cylindrical cells are produced using wound electrodes. That has the advantage of faster production compared with various stacked and pouch formats. Cells such as the and are wound jelly roll formats that have the shape of an Archimedean spiral. The periodic distance between the windings is the jelly roll is dascs, and is a key parameter determining the performance of the cells. In these cells, dascs, is the sum of the thicknesses of the cathode and anode (both double side coated), and twice the thickness of the separator (Figure 2). Cell resistance and heating characteristics are directly related to dascs. Higher dascs values result in higher maximum temperatures during full discharge.
As a result of the greater quantity of active materials, cells have an increased capacity of over 0.9Ah, and cells have an increased capacity of about 1.35Ah compared with cells. Increasing cell size results in a better ratio of energy-storing versus non-energy storage materials. Using even larger formats such as the , , and formats is expected to result in a capacity gain per high-energy cell of 1.8 Ah, 3.1 Ah, and 5.8 Ah, respectively, compared to the (Figure 3).
Some of the similarities and differences between the and include:
Pack level considerations
As designers move from cells to larger or even cells, pack costs are expected to shrink due to needing fewer cells to store a given amount of energy. For example, moving up from cells to cells means that one-third fewer cells will be needed to obtain the same total energy storage (Figure 4).
Fewer cells mean that the battery management system (BMS) for a pack consisting of cells will need to monitor one-third fewer cells, reducing complexity and cost. The percentage of space in the voids between the cells will be about the same for a pack of and a pack of cells. As a result, the amount of cooling liquid in the voids will be similar, and the total pack volume per Wh will also be similar. Of course, a pack using cells will be at least 5mm higher compared with the equivalent energy storage using s, so simple retrofitting will not usually be possible.
Beyond the to the
According to Tesla, the still under development cell promises further increases in performance, including 5X the Whrs and 6X the Watts, to deliver 16% more driving range than cells. The is not just a larger cell; it includes a new &#;tabless&#; electrode design.
As cylindrical cells get larger and larger in diameter, the heat dissipation at the core becomes worse. The new &#;tabless&#; design transfers heat axially through the aluminum and copper current collectors to the bottom of the cell, enabling efficient bottom-side cooling of a battery pack and ensuring more even temperature profiles inside individual cells (Figure 5). Improved thermal management is expected to support faster charging and discharging.
for power tools
Like the proposed cells, the packs of cells designed for power tools use improved packaging to deliver increased performance. For example, a standard 18V battery using cells can produce up to 800 W of power output. The newer packs based on cells can produce up to 1,440 W, an 80% increase. As noted above, the inherently has about 50% greater capacity and energy density than the for discharge rates up to about 3.75C, so where does the added performance come from?
It&#;s the packaging. The packs use welded cell connectors, copper endplates, and power rails to reduce the resistance in the packaging. In addition, improved thermal management transfers about 20% more heat out of the cells. The net result is that runtime gains up to 100% compared with packs and delivers 80% more power.
The combination of cells and the improved packaging technology results in premium performance battery packs that enable new applications beyond portable power tools. Electric power equipment for landscaping and lawn care, battery-powered construction equipment, and high-capacity battery-powered portable inverters are coming onto the market using these premium battery packs. And the higher added value of the premium packs supports the addition to more features such as smart packs that include built-in microprocessors and Bluetooth communications that can be used to enable password control of the packs, enhancing security, users can set notifications, and remotely check battery status.
Summary
There&#;s an expansive and growing selection of cylindrical Li-ion cells that designers can use for specific applications. Each of the many options delivers performance tradeoffs, and some are available in protected and unprotected models giving designers an even wider selection. In the future, larger cell formats are expected to supplant today&#;s and to deliver higher performance. Improvements in cell and battery pack construction are contributing to the development of premium performance energy storage systems.
References
vs. Li-ion cells &#; A direct comparison of electrochemical, thermal, and geometrical properties, Journal of Power Sources
Energy Density of Cylindrical Li-Ion Cells: A Comparison of Commercial to the Cells, Journal of the Electrochemical Society
Safety Limitations Associated with Commercial Lithium-ion Cells, NASA
Tesla Battery Day, Enpower
What is the Difference Between &#;Protected&#; and &#;Unprotected&#; Batteries?, Fenix
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