Lithium-ion (Li-ion) cells come in three basic form factors: cylindrical, prismatic (rectangular brick shape), and flat lithium-polymer (LiPo) cells. The most commonly used Li-ion cell is the cylindrical 18650 cell (Fig. 1). Several million cells per month are manufactured, and they’re used in most notebook computer applications.
The 18650 offers the lowest cost per watt hour. The “18” refers to the cell diameter in millimeters, and the “650” means it’s 65 mm long. Li-ion cylindrical (and prismatic) material layers are rolled like a jelly roll. Li-ion cylindrical (and prismatic) cells are packaged in metal cans. Typical capacities of an 18650 cell range from 2.2 to 3.0 Ahrs.
Prismatic or brick-shaped cells are often cost-effective and available in myriad sizes. They also come in a variety of heights ranging from about 4 mm to about 12 mm. The most common size is the 50-mm length and 34-mm width footprint.
Li-ion prismatic batteries with a thin layered polymer can be housed in a metal can (Fig. 2). Note that the prismatic cell has a pressure vent with the terminals on the metal can. The positive and negative terminals on the polymer cell are tabs protruding from the cell. The typical capacity of a prismatic cell ranges from 1 Ahr to 3 Ahrs.
LiPo cells are sometimes called laminate cells and are available in custom footprint sizes. They can be very thin or quite large depending on their intended use. The primary advantage of LiPo cells is the variety of form factors available. LiPo cells can be encased in flexible aluminum foil laminate pouches, similar to “coffee bag” material, that are just 0.1 mm thick, rather than the 0.25- to 0.4-mm thick aluminum or steel cans traditionally used with cylindrical or prismatic cells.
LiPo cells are constructed by stacking electrode and electrolyte materials in a flat sandwich, rather than winding them in a jellyroll fashion like other cylindrical or prismatic cells (Fig. 3). The length and width can be quite large. Cell capacities can range anywhere from 50 mAhrs for a small cell such as for a Bluetooth headset up to 10 Ahrs or more for an electric vehicle battery.
How Lithium Batteries Work
Lithium ions move from the negative electrode to the positive electrode during discharge and reversely when charged. The three primary functional components of a Li-ion battery are the anode, cathode, and electrolyte, and a variety of materials is used for each. The cathode is generally one of three materials: a layered oxide (such as lithium cobalt oxide), one based on a polyanion (such as lithium iron phosphate), or a spinel (such as manganese).
Li-ion cylindrical and prismatic cells use a discrete porous polymer membrane, usually polyethylene (PE), which is placed between the electrodes. Once assembled, the cell is backfilled with electrolyte solution.
LiPo uses a PE, a polypropylene (PP), or a PP/PE separator. Some LiPos use polymer gel containing the electrolyte solution, which is coated onto the electrode surface. The structure then may be laminated before packaging.
LiPo can either be rolled or stacked like a deck of cards. LiPo batteries can be made very thin, down to around half a millimeter. However, the packaging at the bottom of this range wastes much of the space, so cells typically range from 2 to 6.5 mm thick.
Mechanical Aspects Of Battery Options
One should be aware of a misconception about LiPo and its flexible packaging. This flexibility is often misleading, as LiPo cells should remain flat when installed in a device, not even bending for installation in the battery system.
Bending the cell brings the anode and cathode materials closer together, which can cause preferential plating and shorting. This results in reduced cycle life and presents a potential safety hazard. The soft packaging on polymer cells is easily punctured and has more swelling than metal cans.
Compared to a cylindrical cell, a LiPo cell has less volumetric energy density. This is because cylindrical cells do not bulge due to their extremely strong shape, so very high electrode densities can be used. Also the selection of materials is easier because small amounts of gas produced by a cylindrical cell have no effect on its performance or shape.
The same is not true of LiPo cells. However, this disadvantage in energy density can be overcome by the advantage in packing density. In addition to the lost space between cells, cylindrical cells are a fixed size, mostly 18 mm in diameter, so they may not be able to use all the space available in an application.
Electrical Aspects of Battery Options
The voltage performance does not depend on the packaging but on the active materials inside. From highest to lowest voltage, these include manganese spinel, cobalt oxide (CO), nickel manganese cobalt (NMC), and iron phosphate. The vast majority of Li-ion cells, including LiPo cells, are CO, NMC, or a blend of the two so the voltage ranges should be the same from 3 V on the low end to 4.2 V at the top of charge.
LiPo and prismatic cells both tend to have better cycle life than cylindrical cells because they are not so tightly constrained, allowing the electrodes to expand and contract more freely during cycling. Consider a 2.7-Ah cylindrical cell with a 1C charge/discharge cycle life. It still retains 90% of its original capacity after 500 cycles. There are newer cell designs coming that achieve 95% after 500 cycles and should exceed 1000 cycles.
LiPo Packaging Considerations
Puncturing a cell is a much larger risk for a LiPo cell compared to one in a steel or aluminum can. A punctured cell can cause an internal short circuit, which will cause the cell to get hot. Even if it does not short the cell, a leak may allow moisture in, eventually causing the cell to self-discharge and die. The cell may also swell from the reaction of the anode with moisture. Special care must be made in handling the cells and in the pack design so no sharp objects could contact the cells.
Edge shorting is another often overlooked issue. The aluminum layer in the packaging is conducting so if it’s exposed at the cut edges of the package, it can short out components that are put in contact with it. In addition, internal corrosion reactions in the cell can occur if the tabs to the aluminum layer are shorted. This could happen if the tabs are bent over the edge of the packaging. Again, careful handling and good pack design is required.
Over-discharge damage is an issue for all Li-ion cells but the resultant gassing in LiPo cells is more obvious. When the cell voltage drops too low (~1.5 V), reactions at the anode start to produce gas. As the voltage continues to drop under 1 V, copper from the anode current collector starts to dissolve and will short out the cell. The battery management system (BMS) embedded in the battery pack should prevent over-discharge.
Overcharge is similar. Gassing occurs at the cathode as the electrolyte starts to decompose at high voltage (~4.6 V). Cylindrical cells have integral pressure-activated current-interrupt devices (CIDs) to stop the overcharge when the gas pressure builds. Polymer cells do not have any CID. Although their swelling helps to prevent further overcharge by increasing the cell impedance, this should only be a final failsafe. An external thermal fuse is usually added for overcharge protection, in addition to the control by the charger.
An external short circuit can cause swelling due to heat and over-discharge. Cylindrical cells have an integral positive thermal coefficient (PTC), a device that expands and creates high impedance when it is heated or self-heats due to the high currents experienced during an external short circuit. Polymer cells do not have this integral PTC, so an external PTC or thermal fuse can be added for shorting protection.
LiPo cells are more expensive per watt hour compared to other types of Li-ion cells for several reasons. The high-quality laminate material and the special tabs that allow sealing against the bag are expensive. Second, the lower speed of manufacturing increases both labor and overhead costs. Finally, while lower production runs allow for size flexibility, it results in lower yields and higher prototyping costs.
When building a battery pack, most pack manufacturers will use several key guidelines when selecting the type of Li-ion cells to use within the pack. If the design requirements emphasize cost-effective power delivery, a usage environment that must tolerate abuse (i.e. shock, drop, vibration), or a packaging footprint with all dimensions greater than 22 mm, then a cylindrical cell is usually a good selection.
But if the design requirements emphasize a very thin profile (4 to 10 mm), a custom or very specific footprint, or a lightweight battery, then a LiPo cell is usually a good selection. Any applications that sit between the sweet spot for LiPo and cylindrical cells are usually good candidates for the prismatic cell.