lithium polymer battery (LiPo)

What is a lithium polymer battery (LiPo)?

A lithium polymer battery is a rechargeable battery with a polymer electrolyte instead of a liquid electrolyte. Often abbreviated as LiPo, LIP, Li-poly or lithium-poly, a lithium polymer battery is rechargeable, lightweight and provides higher specific energy than many other types of batteries.

Construction of lithium polymer batteries

Polymers, a term originating from Greek words meaning "many parts," are natural and synthetic substances made from very large molecules that themselves consist of simpler chemical units. For example, DNA is a polymer of nucleotides.

True LiPo batteries use a highly conductive semisolid (gel) or solid polymer for the electrolyte and lithium for one of the electrodes. Commercially available LiPo batteries are hybrids: gel polymer or liquid electrolyte in a pouch format.

The battery includes a polymer barrier that also contains the electrolyte, a substance that allows lithium ions to move between the battery's electrodes, also known as its anode and cathode. This barrier also serves to separate the electrodes and can be used to shut down the battery if it becomes too hot, for example, during charging or discharging.

When the barrier is designed to perform the shutdown function, it contains at least one polyethylene layer and at least one polypropylene layer. The polyethylene layer stops the current flow when the temperature rises beyond a certain threshold, and the polypropylene layer provides mechanical support for the barrier.

Lithium polymer battery chemistries

There are numerous types of LiPo batteries, each with different strengths and weaknesses. They are defined by their active materials, also known as their chemistries:

• Lithium cobalt oxide.
• Lithium-ion manganese oxide.
• Lithium-ion ternary.
• Lithium iron phosphate.

The structure of the polymer layers in these different chemistries varies, resulting in different energy densities watt-hours per kilogram (Wh/kg), power densities watt per kilogram (W/kg), battery weight and safety levels. As a result, LiPos have more potential applications than many other types of rechargeable batteries.

Applications of lithium polymer batteries

LiPos for commercial use offer reduced thickness, flexibility and weight. These qualities make LiPos suitable for applications where a lightweight battery is desirable, including smartphones, tablets and wearables. LiPos are also used in some drones and electric vehicles. They are also commonly used in radio-controlled hobby devices.

More broadly, however, lithium ion (Li-ion) batteries remain more popular than LiPo due to their better discharge abilities. That said, pouch-type standard Li-ion batteries require external casing to prevent expansion that would otherwise become a performance and safety issue.

Lithium ion batteries vs. lithium polymer batteries

Li-ion and LiPo batteries share some common qualities:

• Both are rechargeable.
• They use the same materials for cathodes and anodes.
• Both have a barrier that separates the electrodes and enables charged ions to move between them.
• They often require special smart chargers and circuit protection for the safest use.
• Li-ion and LiPo are subject to problems such as overcharging, over-discharging and internal shorts.
• Electrolyte vaporization is another problem that's common to both types of batteries. It can affect the contacts between a Li-ion or LiPo battery's internal layers, affecting its cycle time and reliability. In the case of LiPos, vaporization can cause the battery to physically inflate, damaging the battery and affecting its performance and lifetime.

That said, there are also several differences between these two battery types.

Notably, the insulator between electrodes in LiPos is made of a micro-porous polymer. In contrast, a traditional porous film separator is used for Li-ions.

Another difference is in the battery packaging. Li-ion batteries usually come in a stainless steel or aluminum case, often cylindrical in shape and sealed using a laser welding process, to protect the battery from damage. In contrast, LiPo batteries are packaged in an aluminum foil pouch, which is why these batteries are known as pouch (or soft) cells.

The LiPo pouch is easier to fabricate and lighter than Li-ion cases. Also, the battery includes wafer-thin layers. Due to these features, LiPo batteries cost less than Li-ion batteries. These features also allow for the mass production of LiPo batteries in many different configurations and make LiPos more suitable than Li-ions for lightweight systems. The chief drawback of LiPo pouches is that they are more susceptible to physical damage such as ruptures, which may cause the battery to stop working. Electrolyte leaks and fires are also a possibility with LiPos.

Lithium ion batteries vs. lithium polymer batteries: Which is the better choice?

There are benefits and drawbacks to both LiPos and Li-ions. It is also worth noting that, due to advancements in technology over the years, the current generation of LiPo batteries are no longer drastically different from Li-ion batteries. This is because they don't use a true lithium polymer solid, as the original Bell Laboratory designs of the 1970s did, because the solid polymers don't perform well at room temperature.

However, LiPos still have worse low-temperature discharge (zero degrees Celsius to 60 degrees Celsius) than traditional Li-ions, so discharging LiPo batteries to ultra-low voltages can be dangerous. Deep, fast discharges may cause expansion, combustion or even explosion. They are also more expensive than Li-ion batteries and have a shorter lifespan. Li-ion cells have better performance for very high-drain uses. Also, they have a high discharge rate, making them suitable for applications where a lot of power is needed very quickly.

That said, LiPos are more lightweight and offer higher energy density than Li-ions. They can also be produced in different shapes and sizes, allowing for greater applicability for real-world use cases. Additionally, LiPos with solid polymer electrolytes offer further performance advantages for certain applications.

The choice between Li-ion and LiPo depends on the application. Today's LiPo batteries have a shorter shelf life but a longer self-discharge time, which makes them better for devices that might sit unused a few days at a time. LiPos also have an advantage when physical space constraints are a concern, as they fit more easily into tiny and slim devices, such as smartphones.

Lithium ion batteries are increasingly being used in uninterruptible power supply systems to reduce data center real estate and energy consumption. Read how Li-ion batteries are used in data centers.