How to Choose the Right Lithium-Ion Battery for Mobile Devices

When creating a battery package, most package manufacturers follow a few key principles when choosing the type of lithium-ion battery to use within the package. If the design requirements emphasize cost-effective power delivery, the use environment is relatively harsh, or all dimensions of the package outline exceed 22 mm, then cylindrical batteries are usually the ideal choice. However, if the design requirements emphasize very thin thickness, require custom or very special form factors, or a very light weight battery, then lithium polymer batteries are usually a better choice.

Lithium-ion (Li-ion) batteries come in three basic form factors: cylindrical, prismatic (rectangular brick-shaped), and flat lithium polymer (LiPo) batteries. The most commonly used lithium-ion battery is the cylindrical 18650 cell (Figure 1). Millions of these batteries are produced each month and are used in most laptop computers.

18650 batteries have the lowest cost per watt-hour. "18" refers to the battery diameter in millimeters, and "650" represents the length of 65 millimeters. The cylindrical (or prismatic) layers of lithium-ion batteries are rolled together like an egg roll. Cylindrical (or prismatic) lithium-ion batteries are generally packaged in metal cans. The typical capacity of 18650 batteries ranges from 2200 mAh to 3000 mAh.

Figure 1: The cylindrical 18650 cell is the most commonly used lithium-ion battery, primarily used in laptop computers, where it has the lowest cost per watt-hour.

Prismatic or brick-shaped cells are generally more cost-effective and come in a wide variety of sizes, ranging from about 4 mm to about 12 mm in height. The most common dimensions are 50 mm long and 34 mm wide.

Lithium-ion prismatic cells with thin polymer layers can be packaged in metal cans (Figure 2). It is worth noting that the prismatic cell has a pressure relief hole and the terminals are made on the metal can. The positive and negative electrodes on the polymer cell are protruding from the battery's tabs. The typical capacity of prismatic cells ranges from 1000 mAh to 3000 mAh.

Figure 2: Prismatic or brick-shaped lithium-ion batteries are generally very cost-effective and come in a wide variety of sizes.

Lithium polymer batteries are sometimes called laminated batteries and can be made to order. They can be made very thin or very large, depending on the intended use. The main advantage of lithium polymer batteries is their variable form factor. Lithium polymer batteries can be packaged in flexible aluminum foil laminate bags, similar to "coffee bag" material, only 0.1 mm thick, while cylindrical or prismatic batteries use aluminum or steel cans that are generally 0.25 mm to 0.4 mm thick.

Lithium polymer batteries are made by stacking electrodes and electrolyte materials in a flat sandwich structure, rather than winding them in an egg roll like other cylindrical or prismatic batteries (Figure 3). The length and width can be made very large. Battery capacities range from 50 mAh for small batteries such as those used in Bluetooth headsets to more than 10 ampere hours for electric vehicle batteries.

Figure 3: Lithium polymer batteries are made by stacking electrode and electrolyte materials in a flat sandwich structure, rather than being rolled up like cylindrical or prismatic batteries.

How Lithium Batteries Work

Lithium ions move from the negative electrode to the positive electrode during discharge and from the positive electrode to the negative electrode during charging. The three main functional components of a lithium-ion battery are the anode, cathode, and electrolyte, each of which can use different materials. The cathode material is usually one of the following three: layered oxides (such as lithium cobalt oxide), polyanion-based materials (such as lithium aluminum phosphate), spinels (such as manganese).

Cylindrical and prismatic lithium-ion batteries use a separate porous polymer membrane, usually polyethylene (PE), between the electrodes, which are then backfilled with electrolyte after assembly.

Lithium polymer batteries use polyethylene, polypropylene (PP) or PP/PE separators. Some lithium polymer batteries use a polymer gel containing an electrolyte, which is coated on the electrode surface. This structure may be laminated before packaging.

Lithium polymer batteries can be rolled up or stacked like playing cards. Lithium polymer batteries can be made very thin, down to about half a millimeter. However, this extreme packaging may waste a lot of space, so the battery thickness is generally between 2 mm and 6.5 mm.

Mechanical properties of the battery

People may have misconceptions about lithium polymer batteries and their flexible packaging. This flexible packaging is often misleading because lithium polymer batteries should remain flat when loaded into a device and cannot even bend when installed in a battery system.

Bending a lithium polymer battery brings the anode and cathode materials closer together, potentially leading to unintended plating and short circuits, which shortens the battery's cycle life and creates potential safety risks. The soft packaging on top of the polymer battery is easily punctured and expands more easily than a metal can.

Lithium polymer batteries have lower volumetric energy density than cylindrical batteries because cylindrical batteries have a particularly robust shape that does not deform, so very high electrode densities can be used. Material selection is also easier because the small amount of gas produced by cylindrical batteries has no effect on performance or shape.

This is not the case for lithium polymer batteries. However, this disadvantage in energy density is compensated by the advantage in packaging density. In addition to the lost space between cells, cylindrical batteries are fixed in size, most of which are 18 mm in diameter, so in practical use, not all available space can be used.

Battery electrical characteristics

The voltage of a battery does not depend on the package, but on the active materials inside. Arranged from the highest voltage to the lowest voltage, these materials include manganese spinel, cobalt oxide (CO), nickel manganese cobalt (NMC), and iron phosphate. The active materials used in most lithium-ion batteries, including lithium polymer batteries, are cobalt oxide, nickel manganese cobalt, or a mixture of the two, so the voltage range should be the same, from a minimum of 3V to a maximum of 4.2V.

Both lithium polymer and prismatic cells can be charged more times than cylindrical cells because they are not as tightly constrained, allowing the electrodes to expand and contract more freely during charging cycles. For example, a 2.7 Ah cylindrical cell with a 1C charge/discharge cycle life will still have 90% of its initial capacity after 500 cycles. The latest battery designs that are coming soon can still achieve 95% of the initial capacity after 500 cycles, and the number of cycles can even exceed 1,000.

Lithium polymer battery packaging considerations

Punctures put lithium polymer batteries at much greater risk than batteries enclosed in steel or aluminum cans. A punctured battery can cause an internal short circuit and heat up the battery. Even if the battery does not short, leakage can allow moisture to enter, eventually causing the battery to self-discharge and fail. The battery can also swell due to the reaction of the anode with moisture. Therefore, great care must be taken when handling the battery and packaging design to prevent sharp objects from contacting the battery.

Edge shorts are another problem that can be overlooked. The aluminum layer in the package is conductive, so when it is exposed at the edge of the package, it can short out components that it contacts . Additionally, when the tabs connecting the aluminum layer short out, corrosion reactions will occur inside the battery. This happens when the tabs bend along the edge of the package. Again, battery use and package design require extreme caution.

Over-discharge damage is a problem for all lithium-ion batteries, but gas generation is more pronounced in lithium-polymer batteries. When the battery voltage is too low (around 1.5V), the anode reaction begins to produce gas. As the voltage continues to drop below 1V, the copper at the anode collector begins to dissolve, shorting the entire battery. The battery management system (BMS) embedded in the battery package should prevent over-discharge.

Overcharging creates the same problem. When the battery is at high voltage (around 4.6V), the electrolyte begins to decompose and gas will be generated at the cathode. Cylindrical cells have an integrated pressure-activated current interrupt device (CID) that prevents overcharging when gas pressure is generated. Polymer cells do not have any CID devices. Although their expansion helps prevent further overcharging by increasing the battery impedance, this should only be a last resort. In addition to controlling the charging process through the charger, an external thermal fuse is usually added for overcharge protection.

External short circuit may cause battery expansion due to heating and over-discharge. Cylindrical batteries are integrated with positive temperature coefficient (PTC) devices. When external short circuit causes large current and the device is heated or self-heated, it will expand and form high resistance. Polymer batteries do not have such integrated PTC, so short circuit protection can only be provided by adding external PTC or thermal fuse.

Lithium polymer batteries cost more per watt-hour than other types of lithium-ion batteries for several reasons. First, high-quality stacking materials and specialized tabs that allow the pockets to be sealed are very expensive. Second, manufacturing is slow, which increases labor and overhead costs. Finally, while slow production helps achieve size flexibility, it results in lower yields and higher prototype creation costs.

Conclusion

When making a battery package, most package manufacturers follow a few key principles when choosing the type of lithium-ion battery to use in the package. If the design requirements emphasize cost-effective power supply, the use environment is relatively harsh (i.e., there is the possibility of shock, drop, vibration), or all dimensions of the package outline exceed 22 mm, then cylindrical batteries are usually the ideal choice.

However, if the design requirements emphasize very thin thickness (4 mm to 10 mm), require a custom or very specific form factor, or require a very light weight battery, then LiPo batteries are usually a better choice. Applications that fall between LiPo and cylindrical battery requirements are usually better suited to prismatic cells.