BU-301a: Types of Battery Cells

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 1896 and the D cell followed in 1898. With the need for smaller cells, the C cell followed in 1900, and the popular AA was introduced in 1907. See BU-301: Standardizing Batteries into Norms.

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 18650 illustrated in Figure 2 remains one of the most popular cell packages. Typical applications for the 18650 Li-ion are power tools, medical devices, laptops and e-bikes.

In 2013, 2.55 billion 18650 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 2017. Cell manufacturers are preparing for the 3.9Ah 18650.

The 18650 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 18650 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 2016, the battery industry fears battery shortages to meet the growing demand for electric vehicles.

The demand for the 18650 would have peaked in 2011 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 18650. A new entry is the 21700.

There are other cylindrical Li-ion formats with dimensions of 20700, 21700 and 22700. Meanwhile, Tesla, Panasonic and Samsung have decided on the 21700 for easy of manufacturing, optimal capacity and other benefits. While the 18650 has a volume of approximately 16cm3 (16ml) with a capacity of around 3000mAh, the 21700 cell has approximately 24cm3 (24ml) with a said capacity of up to 6000mAh, essentially doubling the capacity with a 50% increase in volume. Tesla Motor refers to their company’s new 21700 as the “highest energy density cell that is also the cheapest.” (The 2170 nomenclature Tesla advocates is not totally correct; the last zero of the 21700 model describes a cylindrical cell harmonizing with the IEC standard.)

The larger 26650 cell with a diameter of 26mm does not enjoy the same popularity as the 18650. The 26650 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 26700 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 18650 has a higher energy density than a prismatic/pouch Li-ion cell. The 3Ah 18650 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 1980s. 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.

Prismatic Cell

Introduced in the early 1990s, 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 1995, 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 18650 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 18650, 21700, prismatic and pouch cells. The 21700 promises the largest cost decrease over the years and economical production, reaching price equilibrium with the pouch by 2025 (Figure 9).

Automation enables price equilibrium of the 21700 with the pouch cell in 2025. This does not include packaging where the prismatic and pouch cells have a cost advantages.

Fraunhofer predicts the fastest growth with the 21700 and the pouch cell while the popular 18650 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.

• 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

[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