21700 Battery

21700 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. 

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. On the other hand, they are expensive and must use monitoring electronics to ensure over-charge and deep-discharge protection. One of the most common types of cells is the 18650 battery and the 21700 battery. The 21700 battery is a high-capacity 3.7V lithium-ion battery that is more recent than the 18650 battery. It was developed especially for electric cars and e-bikes (bicycles driven or supported by electric motors). Recently, Tesla Inc. together with Panasonic announced a new model of li-ion battery cell–21700, they also stressed that, at present, this is the highest energy density and low cost of a Li-ion battery that goes to mass production.

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.

Dimensions and Weight of 21700 Battery

21700 batteries (21 mm x 70.0 mm) have a nominal diameter of 21.0 millimeters. The overall height is 70.0 millimeters. 18650 batteries measure 65mm in length and 18 mm in diameter. The dimensions of the two types vary greatly, with one being significantly longer than the other. This is because of their different capacities—the larger capacity of the 21700 lithium-ion allows it to be larger in size and have high power.

21700 batteries have a typical capacity of about 4000 – 5000 mAh. As with all similar batteries, the 21700 battery’s capacity depends on the current drain and actual cutoff voltage of the powered device.

Cells with a cylindrical shape are made in a characteristic “jelly roll” manner, which means it is a single long “sandwich” of the positive electrode, separator, negative electrode, and separator rolled into a single spool. One advantage of cylindrical cells compared to cells with stacked electrodes is the faster production speed. One disadvantage of cylindrical cells can be a large radial temperature gradient inside the cells developing at high discharge currents.

Lithium-ion vs Lithium polymer batteries

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.

Composition of 21700 Lithium-ion Batteries

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. 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. 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.

See also: Types of lithium-ion batteries

Cell → Module → Pack

The composition of an EV battery might vary slightly depending on the types of electric vehicles, but generally, EV batteries are composed of

• Electrochemical Cells. An electric cell is essentially a source of DC electrical energy. It converts stored chemical energy into electrical energy through an electrochemical process. 
• Battery Modules. A battery module is an assembly of battery cells, which is put into the frame by combining a fixed number of cells to protect the cells from vibration, heat, or external hazards. A battery module will always incorporate many discrete cells connected in series and parallel to achieve the module’s total voltage and current requirements.
• Battery Pack. The final shape of an electric vehicle battery installed to an electric vehicle. The collection of data from the pack sensors and activation of the pack relays are accomplished by the pack’s battery monitoring unit (BMU) or the battery management system (BMS).

An electric car battery is composed of many electrochemical cells. The actual battery cells can have different chemistry, physical shapes, and sizes, as preferred by various pack manufacturers. To operate an electric vehicle, an enormous amount of power a thousand times stronger than that of a smartphone is required. That is why EVs need dozens of battery cells up to as many as thousands. The large stack of cells is typically grouped into smaller stacks called modules. Several of these modules are placed into a single pack. The cells are welded within each module to complete the electrical path for current flow. Modules can also incorporate cooling mechanisms, temperature monitors, and other devices.

Characteristics of 21700 Lithium-ion Batteries

To compare and understand the capability of each battery, some important parameters are characteristic of each battery, also within a type of battery. These parameters are a reference when a battery is needed, and specific qualities are required since batteries are used in all types of devices and for infinite purposes.

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.