A lithium-ion cell is composed of four main parts:
- a positive electrode (cathode),
- a negative electrode (anode),
- an electrolyte material and
- a porous separator in between that.
The cathode varies between different types of cells but is always a lithium compound mixed with other materials. The anode is almost always graphite, and sometimes includes trace amounts of other elements.
The electrolyte is generally an organic compound containing lithium salts to transfer lithium ions.
The porous separator allows lithium ions to pass through itself but it does not allow to pass the electrons through itself. So the electrons (i.e. current) can travel from cathode to anode and vice-versa through the external electric circuit.
Lithium Ion Cell Working
When the cell is discharged, lithium ions move from the anode to the cathode by passing through the electrolyte. This discharges electrons on the anode side, powering the circuit and ultimately any device connected to the circuit.
When the cell is recharged, this process is reversed and the lithium ions pass back from the cathode to the anode.
The actual process is quite simple. The major differences (and where things get more complicated) are in the shape of the cells and their slight chemical changes.
Form Factors of Lithium Cells
Lithium battery cells are available in several different form factors, yet their basic construction is always the same. The main difference between various shapes of lithium cells is the way they are assembled.
Pouch cells: Pouch cells are the simplest form of lithium battery cells. They look like a tin foil bag (or pouch) and have two terminals at the edge of the pouch.
Inside the pouch, there is a cathode and anode on opposite sides separated by the porous separator and with the electrolyte on either side of the separator. This cathode-electrolyte-anode sandwich is folded back and forth many times within the pouch to increase the capacity of the battery.
There are no standard sizes for pouch cells. They are produced by many different companies and are often designed to exact sizes for specific products, such as cell phones, to ensure that they take advantage of the maximum possible usable space.
The advantage of pouch cells is that they are lightweight and cheap to produce. The main disadvantage is that they have no exterior protection and thus can be damaged if they aren’t enclosed in some form of protective case. A lack of a hard exterior case means they are the lightest and most space-efficient way to produce a lithium battery cell.
Pouch cells perform better when they are contained in some type of rigid or semi-rigid structure that can apply a slight amount of pressure to the cells. This helps keep all of the layers of the cells in close contact.
When a pouch cell ages, it can begin to expand. This is often due to small interior shorts that occur over time as the battery ages, creating gas that expands up the cell. Because pouch cells are entirely sealed, the gas has no chance to escape and thus creates a puffy, pillow-like appearance.
The expansion of the pouch cell results in a reduction in the performance of the cell. Some degree of gas buildup can be retained by the pouch structure, but when the gas buildup becomes too great, the pouch can rupture explosively. The rupture releases a large amount of flammable gas. However, this is a rare phenomenon.
Prismatic Cells: Prismatic cells are quite similar to pouch cells, except that they have the addition of a rigid rectangular case outside of the cell. This gives the cell a rectangular prism (or prismatic) shape.
Prismatic cells are therefore slightly less space-efficient than pouch cells but are also more durable than pouch cells.
Unlike pouch cells that have thin tab terminals, prismatic cells often have threaded terminals that allow a nut or bolt to be used for connections. This makes it easier to join prismatic cells into larger battery modules.
There aren’t standard dimensions for prismatic cells, but they often come in various capacities with 5 – 10 Ah increments
Cylindrical Cells: Cylindrical cells are AA-style batteries. They come in a variety of sizes (most are larger than standard AA batteries) but all have the same cylindrical shape and rigid metal case.
Cylindrical cells are produced by rolling up what amounts to the same contents of a pouch cell, then placing it inside of a metallic cylinder with a positive and negative terminal at either end of the cylinder.
These cells are not as space-efficient. However, they are the most robust type of lithium battery cell and don’t require any external frame or support.
Unlike pouch cells and prismatic cells, cylindrical cells are produced in standard sizes.
The most common lithium battery cylindrical cell is the 18650 cell, named for its 18 mm diameter and 65 mm length. The 18650 is the cylindrical cell most commonly used in laptops, power tools, flashlights, and other devices that require cylindrical lithium cells.
Two other common sizes of cylindrical cells are the 14500, which is 14 mm in diameter and 50 mm in length and is the same size as a standard AA battery, as well as the 26650, which is 26 mm in diameter and 65 mm in length.
The 18650, which falls right in the middle of the three most common cylindrical standard sizes, has seen the most widespread use and is available from the highest number of manufacturers.
Lithium Ion Cell Types
All lithium battery cells aren’t created equally. There are a few different chemistries of lithium batteries that have very different properties and specifications. They all have their unique advantages and disadvantages, so let’s compare them here.
Lithium ion (Li-ion): Li-ion is the most common type of lithium battery used in consumer electronics like cellphones, laptops, power tools, etc.
They have the highest energy to weight ratio and are also some of the most energy-dense cells, meaning you can pack a lot of energy into a small volume.
Depending on the exact type, Li-ion cells are relatively safe cells, at least as far as lithium batteries go. Most Li-ion cells won’t just burst into the fire if they are punctured or the cell is otherwise heavily damaged, though this can happen with some types of Li-ion and has been observed many times.
The chance of fire is always present in lithium batteries but is usually caused by negligence or abuse of a lithium cell or battery. Short-circuiting a battery is one common example of such negligence.
Li-ion cells also have relatively long cycle lives. The shortest is rated for around 300 cycles until they reach 70 – 80% of their initial charge capacity, while the longest can last for over 1,000 cycles.
Just based on the manufacturer’s ratings though, Li-ion cells are the middle of the road for cycle life, as compared to the other two major chemistries that we’ll talk about next.
Cost is always an important factor when choosing components for any specific application. Li-ion cells fall in the middle range of lithium cell prices. There are cheaper chemistries (RC lipo) and more expensive chemistries (lithium iron phosphate), which leaves standard Li-ion somewhere in the middle in terms of price.
This is the most widely used lithium battery chemistry. It is also the most widely available in different sizes, shapes, capacities, and slight chemical variations that have different effects on the performance.
One of the most common and easiest to work with formats of Li-ion cells is the 18650 cylindrical cell. There are number of great quality top brand 18650 Li-ion cells plus hundreds of other off-brand and generic 18650 Li-ion cells as well.
Because 18650s are so commonly used in OEM products including
everything from electric vehicles to power tools, they have been developed with a wide range of specifications.
Li-ion is a good option for applications where we have space and weight limitations as well as moderate to high power needs.
Most li-ion cells have a nominal voltage of between 3.6 V to 3.7 V and are usually rated for a discharge-charge voltage range of 2.5 V – 4.2 V. Li-ion cells are usually rated for maximum capacity at this voltage range (i.e. charging to 4.2 V, then discharging down to 2.5 V) but it is recommended to avoid draining Li-ion cells to 2.5 V very often. They can handle it, but it reduces their expected lifetime.
Most battery management systems (BMS) for Li-ion batteries cut off discharge at around 2.7 V – 2.9 V per cell. Discharging below 2.5 V will cause irreparable damage to the cell, resulting in the cell not holding its rated capacity or sustaining its rated discharge current.
Always check the manufacturer’s recommendations for the highest-rated charging voltage. Overcharging a Li-ion cell not only reduces its lifetime but can be dangerous as well.
Lithium Manganese Oxide (LiMn2O4 or Li-manganese): LiMn2O4 gets its name from the use of a manganese matrix structure in the cathode. LiMn2O4 can handle relatively high power in very short bursts and offers high thermal stability. This makes it one of the safer Li-ion chemistries because higher temperatures are required to cause thermal runaway. LiMn2O4 cells can also be tweaked for either higher power or higher capacity at the expense of each other.
The downside to LiMn2O4 is its relatively lower cycle life compared to other Li-ion chemistries.
Lithium Cobalt Oxide (LiCoO2, Li-co or Li-cobalt): It uses a layered cobalt structure in its cathode. LiCoO2 is known for its relatively low cost and high capacity, but generally has a lower current rating and only moderate cycle life. It also has a lower thermal runaway temperature, making it somewhat less safe than other Li-ion chemistries. LiCoO2 is also the basis for the much more dangerous RC lipo batteries.
In RC lipo batteries, the chemistry is altered to produce a much more powerful cell capable of sustaining an extremely high discharge current. This increased power comes at the expense of safety, weight, and cycle life.
Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2 or NMC): LiNiMnCoO2 is a fairly new chemistry that is still undergoing constant
development. NMC falls in the sweet spot of improving upon the
drawbacks of many previous types of Li-ion chemistries while retaining their benefits. NMC shares many of the advantages of both LiCoO2 and
LiMn2O4 chemistries.
By combining cobalt and manganese, then including nickel, NMC cells have demonstrated relatively high power, capacity, and safety.
By adjusting the ratio of cobalt, manganese, and nickel in the cathode as well as including other trace elements in both the cathode and anode, NMC cells can be tweaked for improved performance in nearly any measurement category.
Other chemistries are capable of achieving better performance in some categories, but NMCs have some of the highest all-around performance figures of any lithium battery chemistry.
This makes NMS an excellent “all-around” chemistry. It doesn’t have the highest performance in any single category, but it has some of the highest average performance of any chemistry.
An example of an NMC cell is the Samsung INR18650-25R, which is optimized for relatively high power and medium capacity.
Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2, NCA, or NCR): LiNiCoAlO2 is very similar to the NMC chemistry above, but with aluminum swapped for manganese in the cathode. The addition of aluminum helps NCA cells achieve the highest capacity of all Li-ion chemistries.
The downsides are a slight decrease in cycle life and power as compared to most other chemistries.
An example of an NCA cell is the Panasonic NCR18650B cell, which was used for most or all of Tesla’s early electric vehicles.
Like NMC, NCA is a very promising chemistry for the future development of Li-ion cells. It is best suited for high capacity, energy-dense purposes. This is why it was selected by Tesla for use in their electric cars. NCA excels in packing the most energy into the smallest space.
However, continued research and incremental improvements are helping to increase the power of this type of cell, making it quite competitive with NMC.
Lithium polymer (li-poly or lipo or RC lipo): There is an entire class of Li-ion cells used for radio-controlled (RC) toys and vehicles that are generally referred to as lipo batteries. These are extremely high-power Li-ion cells that are specifically used in the RC industry for their ability to provide the highest possible current.
The most common usage of the term lipo nowadays is to refer to these RC batteries.
Lipo batteries are the dangerous ones. These are the ones that are itching to burn your house down if you don’t follow proper charging and discharging procedures. They can be safe, but they are also incredibly volatile when used improperly.
Let’s look at what makes lipo batteries special. These cells are specialty chemistry based on lithium cobalt that is suited for high power applications. They can provide super high discharge rates for long periods and insanely high discharge rates for short periods.
RC lipo cells are almost exclusively used in the remote control vehicle industry for applications such as RC drones, helicopters, planes, cars, etc. These devices require very high discharge rates from a small and lightweight battery. RC lipo cells aren’t the lightest cells (those are variants of conventional Li-ion cells), but they can provide much higher power for only a slightly higher weight.
RC lipo cells are also the cheapest lithium cells available. They cost much less than Li-ion and LiFePO4 cells making them attractive for other applications such as electric bicycles.
One major drawback is that RC lipo cells have very short cycle lives. Another issue with RC lipo cells is their more complicated charging process. While Li-ion and LiFePO4 cells are pretty easy to charge, especially when using a battery management system (BMS), RC lipo cells require more expensive balance chargers to ensure that all cells in a battery are maintained at the proper voltage and balanced with one another.
The reason for this is that when RC lipo batteries stray from their rated voltage range, they become incredibly volatile. It is critically important that RC lipo cells are charged within their specified voltage range.
They should also never be discharged too low. Discharging a RC lipo cell below 2.5 volts and then charging the cell can result in combustion of the cell, especially at higher charging currents. For this reason, RC lipo cells must be monitored carefully during discharge as well to ensure that they are never drained too far.
It is possible to recharge RC lipo cells that have been over-discharged, but it must be done at very low currents and can easily result in fire, depending on how damaged the battery cell was. Ideally, this wouldn’t be attempted, but if it was, it should be done in a monitored environment and away from anything flammable.
RC lipo cells are electro-chemically similar to Li-ion cells and have the nominal voltage of 3.7 V. However, because care must be taken not to over-discharge the cells, it is not recommended to discharge them lower than 3.0 V. Aiming for a higher voltage of 3.2 V is considered safer.
The maximum voltage of RC lipo cells should never exceed 4.2 V. It should also be noted that these voltages are considered the “under load” voltage.
Depending on the current load, a lithium battery cell (of any chemistry) will see a drop in voltage. This drop in voltage is known as voltage sag. An RC lipo cell should never drop below 3.0 V while in use. If discharging stops at 3.0 V under load, the voltage when measured after the load is removed will return to a higher voltage, likely in the 3.3 V – 3.5 V range, though an even safer level for at rest voltage is around 3.7 V.
For this reason, it is critically important to monitor RC lipo cells under load to ensure they never over-discharge beyond a safe limit.
Lithium Iron Phosphate (LiFePO4): Lithium iron phosphate is technically a subset of the more general Li-ion class, but it is unique enough that it is often listed separately. LiFePO4 cells are both heavier and less energy-dense than most Li-ion cells. This means that battery packs built from LiFePO4 cells will be bulkier and more massive than Li-ion or RC lipo batteries of the same voltage and capacity.
The exact amount varies depending on the cell format, but you can expect a LiFePO4 battery to be around twice as large and twice as heavy as a comparable Li-ion battery.
LiFePO4 cells are also some of the most expensive cells. Their cost varies based on many factors including cell size, format, vendor, and location, but you can expect to pay around 20% more for LiFePO4 cells than for Li-ion cells of the same capacity.
Most commonly available LiFePO4 cells also have a lower discharge rate, meaning they can’t provide as much power, though this isn’t always the case. Some cells, such as those made by a high-quality battery company can provide high power levels but cost a premium and are hard to source. Those are mostly sold to OEMs for use in consumer products like power tools.
It is common to hear LiFePO4 being touted for its high discharge rate, but unless you source LiFePO4 cells that are specifically designed for high discharge, most LiFePO4 cells have relatively low discharge rates.
With all of these downsides, there are two big advantages for using LiFePO4 – cycle life and safety. LiFePO4 cells have the longest rated cycle life of all commonly available lithium battery cells. They are often rated for over 2,000 cycles. They are also the safest lithium battery chemistry available.
The electrolyte used in LiFePO4 cells simply can’t oxidize quickly enough to combust efficiently and requires exceedingly high temperatures for thermal runaway, often higher than the combustion temperature of many materials.
The best applications for LiFePO4 cells are projects that require long cycle lives and high safety, don’t have critical space or weight limitations, and don’t require very high levels of power (unless you specifically source high power LiFePO4 cells).
LiFePO4 cells have a nominal voltage of 3.2 V per cell and a discharge-charge voltage range of 2.5 V – 3.65 V. Just like with Li-ion cells, discharging below 2.5 V will cause irreparable damage to the cell, though it isn’t necessarily dangerous, like in the RC lipo cells that we just learned about.
Thanks for reading about “Types of Lithium-ion Cells & Working”. You may like my next article Lithium-ion Battery Specifications.
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