Lithium Polymer Battery

Lithium Polymer Battery

A lithium-ion battery, also known as the Li-ion battery, is a type of secondary (rechargeable) battery composed of cells in which lithium ions move from the anode through an electrolyte to the cathode during discharge and back when charging. 

A lithium-ion polymer (LiPo) battery (also known as Li-pol, lithium-poly, and other names) is a type of Li-ion battery with a polymer electrolyte instead of a liquid electrolyte. All LiPo batteries use a high-conductivity gel polymer as the electrolyte. Lithium polymer cells have evolved from lithium-ion and lithium-metal batteries. The primary difference between lithium-ion and Li-pol is that instead of using a liquid lithium-salt electrolyte (such as LiPF6) held in an organic solvent, the battery uses a solid polymer electrolyte (SPE) such as polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA) or polyvinylidene fluoride (PVdF). LiPos provide higher specific energies than other lithium batteries, often used in systems where weight is an important factor, such as mobile devices, drones, and some electric vehicles.

The cathode is made of a composite material (an intercalated lithium compound) and defines the name of the Li-ion battery cell. The anode is usually made out of porous lithiated graphite. The electrolyte can be liquid, polymer, or solid. The separator is porous to enable the transport of lithium ions and prevents the cell from short-circuiting and thermal runaway.

Chemistry, performance, cost, and safety characteristics vary across types of lithium-ion batteries. Handheld electronics mostly use lithium polymer batteries (with a polymer gel as electrolyte), a lithium cobalt oxide (LiCoO2) cathode material, and a graphite anode, which offer high energy density.

Li-ion batteries, in general, have a high energy density, no memory effect, and low self-discharge. One of the most common types of cells is 18650 battery, which is used in many laptop computer batteries, cordless power tools, certain electric cars, electric kick scooters, most e-bikes, portable power banks, and LED flashlights. The nominal voltage is 3.7 V.

Note that non-rechargeable primary lithium batteries (like lithium button cells CR2032 3V) must be distinguished from secondary lithium-ion or lithium-polymer, which are rechargeable batteries. Primary lithium batteries contain metallic lithium, which lithium-ion batteries do not.

Composition of Lithium Polymer Battery

A typical lithium-ion cell contains:

• Cathode:  The cathode is the positive or oxidizing electrode that acquires electrons from the external circuit and is reduced during the electrochemical reaction. In the case of lithium batteries, cathode materials are generally constructed from LiCoO2 or LiMn2O4. For the cathode, it is important to hold a large amount of lithium without significant change in structure, have good chemical and electrochemical stability with electrolyte, be a good electrical conductor and diffuser of lithium ions, and be of low cost.
• Anode: The anode is the negative or reducing electrode that releases electrons to the external circuit and oxidizes during an electrochemical reaction. One of the most common anode materials used today is lithiated graphite, Li_xC₆, which is composed of graphite sheets intercalated with lithium. New materials, such as those based on Silicon and other elemental blends, are being researched. Lithiated graphite has a unit cell with an HCP structure.
• Separator. A separator is a permeable membrane placed between a battery’s anode and cathode. The main function of a separator is to keep the two electrodes apart to prevent electrical short circuits while also allowing the transport of ionic charge carriers that are needed to close the circuit during the passage of current in an electrochemical cell. Commercially available liquid electrolyte cells use microporous polyolefin materials, such as polyethylene (PE) or polypropylene (PP). Separators in Li-ions have to be electrochemically and chemically stable relative to the electrolyte and electrode materials. Functional separators that use MOF-coated membranes to perform the dual functions of the electrolyte and separator are being developed to support the design of high-performance Li-metal batteries for high-energy systems in electric vehicles and electric aircraft.
• Electrolyte: The choice of electrolyte in all batteries is critical for performance as well as safety. Most of the electrolytes used in commercial lithium-ion batteries are non-aqueous solutions, in which lithium hexafluorophosphate (LiPF6) salt dissolved in organic carbonates, in particular, mixtures of ethylene carbonate (EC) with dimethyl carbonate (DMC), propylene carbonate (PC), diethyl carbonate (DEC), and/or ethyl methyl carbonate (EMC). A good electrolyte must have low reactivity with other cell components, high ionic conductivity, low toxicity, a large window of electrochemical voltage stability (0-5V), and be thermally stable.

Applications of Lithium Polymer Batteries

The most common Li-ion type in small consumer electronics (laptops-cell phones) is lithium-cobalt-oxide (LCO) due to its high specific energy. Tesla Motors uses laptop-sized LCO cells in their EVs combined with a liquid cooling system safety issues, but low specific power and life span prevent this type to be a good choice for EVs. 

On the other hand, lithium-iron-phosphate (LiFePO4/LFP) does not experience thermal runaway and has almost no fire hazards since no oxygen is released at high temperatures. LiFePO4 cells have a good life span, low costs per Ah and kW, good power capabilities, and are extremely safe, but the specific energy is low, and the performance is poor at low temperatures.

See also: Lithium-ion Batteries and Grid Energy Storage

Types of Lithium-ion Batteries

The cathode is made of a composite material (an intercalated lithium compound) and defines the name of the Li-ion battery cell. There are two kinds of electrodes: intercalation and conversion electrodes. The intercalation electrodes are materials that function as host materials where lithium ions can intercalate. A typical example is LiCoO₂ and its derivatives. Conversion-type cathode materials are some of the key candidates for the next generation of rechargeable Li and Li-ion batteries.

One of the most common lithium batteries are 

• Lithium Cobalt Oxide (LiCoO₂).  LiCoO₂ is the most commonly used cathode material. LiCoO₂ batteries have very stable capacities, although their capacities are lower than those based on nickel-cobalt-aluminum (NCA) oxides. However, cobalt is relatively expensive compared to other transition metals, such as manganese and iron, despite the attractive electrical properties of LiCoO2 cathodes. Currently, we can find this type of battery in mobile phones, tablets, laptops, and cameras.
• Lithium Manganese Oxide (LiMn₂O₄). LiMn₂O₄ is a promising cathode material with a cubic spinel structure. LiMn₂O₄ is one of the most studied manganese oxide-based cathodes because it contains inexpensive materials. A further advantage of this battery is enhanced safety and high thermal stability, but the cycle and calendar life is limited. This type of battery is found in power tools, medical devices, and powertrains.
• Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO₂) – NMC. Nickel manganese cobalt (NMC) batteries contain a cathode made of a combination of nickel, manganese, and cobalt. NMC is one of the most successful cathode combinations in Li-ion systems. It can be tailored to serve as energy cells or power cells like Li-manganese. NMC batteries are used for power tools, e-bikes, and other electric powertrains.
• Lithium Iron Phosphate (LiFePO₄) – LFP. LiFePO₄ is one of the most recent cathode materials to be introduced. As of 2017, LiFePO₄ is a candidate for large-scale production of lithium-ion batteries, such as electric vehicle applications, due to its low cost, excellent safety, and high cycle durability. The energy density of an LFP battery is lower than that of other common lithium-ion battery types, such as Nickel Manganese Cobalt (NMC). Because of their lower cost, high safety, low toxicity, long cycle life, and other factors, LFP batteries are finding a number of roles in vehicle use, utility-scale stationary applications, and backup power. Working voltage = 3.0 ~ 3.3 V. Cycle life ranges from 2,700 to more than 10,000 cycles depending on conditions.
• Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO₂) – NCA. In 1999, Lithium nickel cobalt aluminum oxide battery, or NCA, appeared in some special applications, and it is similar to the NMC. It offers high specific energy, a long life span, and a reasonably good specific power. NCA’s usable charge storage capacity is about 180 to 200 mAh/g. The capacity of NCA is significantly higher than that of alternative materials such as LiCoO₂ with 148 mAh/g, LiFePO₄ with 165 mAh/g, and NMC 333 (LiNi_0,33Mn_0,33Co_0,33O₂)with 170 mAh/g. Voltage of these batteries is between 3.6 V and 4.0 V, at a nominal voltage of 3.6 V or 3.7 V. Another advantage of NCA is its excellent fast charging capability. Nevertheless, its weak points are the the limited resources of cobalt and nickel and the high cost.

Advantages and Disadvantages of Lithium Polymer Batteries

There are many advantages to using a Li-ion cell. As a result, the technology is being used increasingly for a huge number of widely varying applications. A lithium-ion battery offers advantages over other battery types in several areas. 

Other Types of Batteries

The following list summarizes notable electric battery types composed of one or more electrochemical cells. Four lists are provided in the table. The first list is a battery classification by size and format. Then, the primary (non-rechargeable) and secondary (rechargeable) cell lists are lists of battery chemistry. The third list is a list of battery applications. The final list is a list of different battery voltages.