Lithium-ion polymer batteries,
polymer lithium ion, or more commonly
lithium polymer batteries (abbreviated Li-poly, Li-Pol, LiPo, LIP, PLI or LiP) are
rechargeable batteriesA rechargeable battery is a group of one or more secondary cells. Rechargeable batteries use electrochemical reactions that are electrically reversible. Rechargeable batteries come in many different sizes and use different combinations of chemicals...
(secondary cell batteries). Normally batteries are composed of several identical secondary cells in parallel addition to increase the discharge current capability.
Design origin
This type has technologically evolved from lithium-ion batteries. Ultimately, the
lithiumLithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number three. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
-salt
electrolyteIn chemistry, an electrolyte is any substance containing free ions that behaves as an electrically conductive medium, usually when in a solution...
is not held in an organic solvent as in the lithium-ion design, but in a solid
polymerA polymer is a large molecule composed of repeating structural units typically connected by covalent chemical bonds. While polymer in popular usage suggests plastic, the term actually refers to a large class of natural and synthetic materials with a variety of properties.Due to the extraordinary...
composite such as
polyethylene oxidePoly , also known as poly , polyoxyethylene and under the tradename Carbowax is the most commercially important type of polyether. PEG, PEO or POE refers to an oligomer or polymer of ethylene oxide...
or
polyacrylonitrilePolyacrylonitrile is a resinous, fibrous, or rubbery organic polymer. Almost all polyacrylonitrile resins are copolymers made from mixtures of monomers; with acrylonitrile as the main component. PAN fibers are the chemical precursor of high-quality carbon fiber...
. The only problem with this design is that doesn't work unless heated to above 50 to 60 degrees Celsius. The battery that is currently available is a hybrid design correctly referred to as a Lithium-Ion polymer battery. The flammable electrolyte is still present but held within the polymer. The advantages of Li-ion poly over the lithium-ion design include lower cost manufacturing and being more robust to physical damage. Lithium-ion polymer batteries started appearing in consumer electronics around 1996.
Technology
Cells sold today as polymer batteries have a different design from the older lithium-ion cells. Unlike lithium-ion cylindrical, or prismatic cells, which have a rigid metal case, polymer cells have a flexible, foil-type (polymer laminate) case, but they still contain organic solvent. The main difference between commercial polymer and lithium-ion cells is that in the latter the rigid case presses the electrodes and the separator onto each other, whereas in polymer cells this external pressure is not required because the electrode sheets and the separator sheets are laminated onto each other.
Since no metal battery cell casing is needed, the battery can be lighter and it can be specifically shaped to fit the device it will power. Because of the denser packaging without intercell spacing between cylindrical cells and the lack of metal casing, the energy density of Li-poly batteries is over 20% higher than that of a classic Li-ion battery.
The voltage of a Li-poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged), and Li-poly cells have to be protected from overcharge by limiting the applied voltage to no more than 4.235 V per cell used in a series combination. Overcharging a Li-poly battery will likely result in explosion and/or fire. During discharge on load, the load has to be removed as soon as the voltage drops below approximately 3.0 V per cell (used in a series combination), or else the battery will subsequently no longer accept a full charge and may experience problems holding voltage under load.
Early in its development, lithium polymer technology had problems with
internal resistanceA practical source may be represented as an ideal voltage source in series with an impedance. This impedance is termed the Internal resistance of the source.- Batteries :...
. Other challenges include longer charge times and slower maximum discharge rates compared to more mature technologies. Li-poly batteries typically require more than an hour for a full charge. Recent design improvements have increased maximum discharge currents from two times to 15 or even 30 times the cell capacity (discharge rate in amps, cell capacity in amp-hours). In December 2007
Toshibais a Japanese multinational conglomerate manufacturing company, headquartered in Tokyo, Japan. The company's main business is in infrastructure, consumer products, electronic devices and components.Toshiba-made Semiconductors are among the Worldwide Top 20 Semiconductor Sales Leaders...
announced a new design offering a much faster rate of charge (about 5 minutes to reach 90%). These cells were released onto the market in March 2008 and are expected to have a dramatic effect on the power tool and
electric vehicleAn electric vehicle , also referred to as an electric drive vehicle, is a vehicle which uses one or more electric motors for propulsion. Depending on the type of vehicle, motion may be provided by wheels or propellers driven by rotary motors, or in the case of tracked vehicles, by linear motors...
industries, and a major effect on consumer electronics.
When compared to the lithium-ion battery, Li-poly has a greater life cycle degradation rate. However, in recent years, manufacturers have been declaring upwards of 500 charge-discharge cycles before the capacity drops to 80% (see
Sanyois a major electronics company and member of the Fortune 500 whose headquarters is located in Moriguchi, Osaka prefecture, Japan. Sanyo targets the middle of the market and has over 324 offices and plants worldwide, together employing more than 11,000 employees.On November 2, 2008, Sanyo and...
). Another variant of Li-poly cells, the "
thin film rechargeable lithium batteryThe Thin film rechargeable lithium battery has been developed by the United States Department of Energy’s Oak Ridge National Laboratory for a variety of technological applications. These batteries have high energy densities, can be recharged thousands of times, and are only 10 micrometres thick...
", has been shown to provide more than 10,000 cycles.
Applications
A compelling advantage of Li-poly cells is that manufacturers can shape the battery almost however they please, which can be important to mobile phone manufacturers constantly working on smaller, thinner, and lighter phones.
Li-poly batteries are also gaining favor in the world of
radio-controlled aircraftA radio-controlled aircraft is a model aircraft that is controlled remotely, typically with a hand-held transmitter and a receiver within the craft...
as well as
radio-controlled carRadio-controlled cars are usually categorized as either "toy" or "hobby" grade. Remote control vehicles are usually of one of two types: control of a vehicle by radio transmission or by a wire connecting between the transmitter and car. This article focuses on the radio-controlled vehicle...
s, where the advantages of both lower weight and greatly increased run times can be sufficient justification for the price. Some airsoft gun owners have switched to LiPo batteries due to the above reasons and the increased rate of fire they provide. However, lithium polymer-specific chargers are required to avoid fire and explosion. Explosions can also occur if the battery is short-circuited, as tremendous current passes through the cell in an instant. Radio-control enthusiasts take special precautions to ensure their battery leads are properly connected and insulated. Furthermore fires can occur if the cell or pack is punctured. Radio-controlled car batteries are often protected by durable plastic cases to prevent puncture. Specially designed electronic motor speed controls are used to prevent excessive discharge and subsequent battery damage. This is achieved using a low voltage cutoff (LVC) setting that is adjusted to maintain cell voltage greater than (typically) 3 V per cell.
Li-poly batteries are also gaining ground in PDAs and laptop computers, such as Apple's
MacBookThe MacBook is a brand of Macintosh notebook computers by Apple Inc. First introduced in May 2006, it replaced the iBook and 12" PowerBook series of notebooks as a part of the Apple Intel transition. Part of the MacBook family, the MacBook is aimed at the education and consumer markets...
,
MacBook ProThe MacBook Pro is a line of Macintosh portable computers by Apple Inc. that was first introduced in January 2006 at the Macworld Expo. The MacBook Pro replaced the PowerBook G4 and was the second model to be announced in the Apple Intel transition...
, and
Macbook AirThe MacBook Air is a Macintosh notebook computer designed by Apple. It is positioned as the ultraportable in Apple's MacBook Family and was introduced at the Macworld Conference & Expo on January 15, 2008. It has been revised twice since the original release....
, Amazon's Kindle, Lenovo's Thinkpad X300 and Ultrabay Batteries, the
OQOOQO was a U.S. computer hardware company that was notable for manufacture of handheld computers. Its systems possess the functionality of a tablet PC in a form-factor slightly larger than a PDA. According to Guinness World Records, the "OQO" is the smallest full-powered, full-featured personal...
series of palmtops, the
HP MiniHP Mini-Note PC is a line of mini notebooks, categorized as Netbook manufactured by Hewlett-Packard under the HP, HP Compaq and Compaq brands. The either contain the HP built Linux or Microsoft Windows XP Operating systems...
and Dell products featuring D-bay batteries. They can be found in small digital music devices such as iPods and other MP3 players as well as gaming equipment like Sony's Playstation 3 wireless controllers[2]. They are desirable in applications where small form factors and energy density outweigh cost considerations.
Electric vehicles
These batteries may also power the next generation of battery electric vehicles. The cost of an electric car of this type is prohibitive, but proponents argue that with increased production, the cost of Li-poly batteries will go down.
Hyundai Motor CompanyHyundai Motor Company , a division of the Hyundai Kia Automotive Group, is the world’s fourth largest automaker in terms of units sold and one of the Big Asian Four...
plans to use this battery type in its
hybrid electric vehicleA hybrid electric vehicle combines a conventional internal combustion engine propulsion system with an electric propulsion system. The presence of the electric powertrain is intended to achieve either better fuel economy than a conventional vehicle, or better performance. A variety of types of HEV...
s.
Technical specifications
There are currently two commercialized technologies, both lithium-ion-polymer (where "polymer" stands for "polymer electrolyte/separator") cells. These are collectively referred to as "polymer electrolyte batteries".
The battery is constructed as:
- Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .A widespread misconception is that cathode polarity is always negative...
: LiCoO2 or LiMnO4
- Separator: Conducting polymer electrolyte
- Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID . Electrons flow in the opposite direction to the electric current...
: Li or carbon-Li intercalationIn chemistry, intercalation is the reversible inclusion of a molecule between two other molecules . Examples include DNA intercalation and in graphite intercalation compounds.- DNA intercalation :...
compound
Typical reaction:
- Anode: carbon–Lix → C + xLi+ + xe−
- Separator: Li+ conduction
- Cathode: Li1−xCoO2 + xLi+ + xe− → LiCoO2
Polymer electrolytes/separators can be solid polymers (e.g.,
polyethyleneoxidePoly , also known as poly , polyoxyethylene and under the tradename Carbowax is the most commercially important type of polyether. PEG, PEO or POE refers to an oligomer or polymer of ethylene oxide...
, PEO) plus
LiPF6Lithium hexafluorophosphate is an inorganic chemical compound. It is a white crystalline powder that hydrolyzes readily in contact with water or moisture....
, or other conducting salts plus
SiO2The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with a chemical formula of ' and has been known for its hardness since antiquity. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms...
, or other
fillersFillers are particles added to material to lower the consumption of more expensive binder material or to better some properties of the mixtured material. Worldwide more than 50 million tons of fillers with a total sum of ca...
for better mechanical properties (such systems are not available commercially yet). Some manufacturers like Avestor (since merged with Batscap) are using metallic Li as the anode (these are the
Lithium-metal-polymer batteries), whereas others wish to go with the proven safe carbon intercalation anode.
Both currently commercialized technologies use
PVdFPolyvinylidene Fluoride, or PVDF is a highly non-reactive and pure thermoplastic fluoropolymer.PVDF is a specialty plastic material in the fluoropolymer family; it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low...
(a polymer) gelled with conventional solvents and salts, like EC/DMC/DEC. The difference between the two technologies is that one (
Bellcore/TelcordiaTelcordia Technologies, formerly Bell Communications Research, Inc. or Bellcore, is a telecommunications research and development company based in the United States created as part of the 1982 Modification of Final Judgment that broke up American Telephone & Telegraph.- History :The company was...
technology) uses LiMn
2O
4 as the cathode, and the other the more conventional
LiCoO2Lithium cobalt oxide is a chemical compound commonly used in the positive electrodes of lithium-ion batteries. The structure of LiCoO2 is known theoretically and has been confirmed with techniques like x-ray diffraction, electron microscopy and neutron powder diffraction: it consists of...
.
Other, more exotic (although not yet commercially available) Li-polymer batteries use a polymer cathode. For example, Moltech is developing a battery with a plastic conducting carbon-sulfur cathode. However, as of 2005 this technology seems to have had problems with self-discharge and manufacturing cost.
Yet another proposal is to use
organic sulfur-containing compoundsOrganosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odours, but many of the sweetest compounds known are organosulfur derivatives. Nature abounds with organosulfur compounds—sulfur is essential for life. Two of the 20 common amino acids are...
for the cathode in combination with an electrically
conductive polymerConductive polymers are organic polymers that conduct electricity. Such compounds may be true metallic conductors or semiconductors. It is generally accepted that metals conduct electricity well and that organic compounds are insulating, but this class of materials combines the properties of...
such as
polyanilinePolyaniline is a conducting polymer of the semi-flexible rod polymer family. Although it was discovered over 150 years ago, only recently has polyaniline captured the attention of the scientific community due to the discovery of its high electrical conductivity...
. This approach promises high power capability (i.e., low
internal resistanceA practical source may be represented as an ideal voltage source in series with an impedance. This impedance is termed the Internal resistance of the source.- Batteries :...
) and high discharge capacity, but has problems with cycleability and cost.
Prolonging life in multiple cells through cell balancing
Analog front ends that balance cells and eliminate mismatches of cells in series or parallel significantly improve battery efficiency and increase the overall pack capacity. As the number of cells and load currents increase, the potential for mismatch also increases. There are two kinds of mismatch in the pack: State-of-Charge (SOC) and capacity/energy (C/E) mismatch. Though the SOC mismatch is more common, each problem limits the pack capacity (mAh) to the capacity of the weakest cell.
Battery pack cells are balanced when all the cells in the battery pack meet two conditions:
- If all cells have the same capacity, then they are balanced when they have the same relative State of Charge (SOC.) In this case, the Open Circuit Voltage (OCV) is a good measure of the SOC. If, in an out-of-balance pack, all cells can be differentially charged to full capacity (balanced), then they will subsequently cycle normally without any additional adjustments.
- If the cells have different capacities, they are also considered balanced when the SOC is the same. But, since SOC is a relative measure, the absolute amount of capacity for each cell is different. To keep the cells with different capacities at the same SOC, cell balancing must provide differential amounts of current to cells in the series string during both charge and discharge on every cycle.
Capacity Rating
Cell's capacities are rated in Ampere Hours (Ah) or milliamp Hour (mAh). A 1000 mAh battery is the same as a 1 Ah battery, both will supply 1 Amp for 1 hour. The C rating commonly associated with lithium ion batteries refers to the maximum current supply capability as a multiple of the cell's capacity e.g. a 1 Ah, 20C battery should be able to supply 20 A continuously without damage. A useful way to calculate how long a battery will last for under heavy load is to multiply the Ah capacity by 60 to give Ampere-minutes; so a 1 Ah battery becomes a 60 Ampere-minute battery; to calculate how many minutes the battery will last, just divide by the average current drawn; e.g. a 10A average current draw will mean that a 60 Ampere-minute battery will last for 60 / 10 = 6 minutes.
An Ampere Hour is equal to 3600
CoulombThe coulomb is the SI derived unit of electric charge. It is named after Charles-Augustin de Coulomb.- Definition :If 2 like point charges of equal magnitudes are placed in a vacuum at a distance of 1 metre away from each other and if they repel each other with a force of 9*1000000000 Newton, then...
s, the standard SI unit for amount of electrical charge. The amount of energy stored depends on the voltage, and is equal to the Voltage * the charge in coulombs, or 3600 * the voltage * the charge in Ampere-hours. So, a 10Ah battery pack with a nominal voltage of 10 volts would deliver 36,000 Coulombs of charge (or 2.2 x 10
23 electrons), and 360 kJ of energy (360 kJ = 100 Wh).
Charging
LiPoly batteries must be charged carefully. The basic process is to charge at constant current (1C to 5C depending on manufacturer) until each cell reaches 4.2 V, the charger must then gradually reduce the charge current while holding the cell voltage at 4.2 volts until the charge current has dropped to 10% of the initial charge rate at which point the battery is considered 100% charged.
Balance charging simply means that the charger monitors the voltage of each cell in a pack and varies the charge on a per-cell basis so that all cells are brought to the same voltage.
The charge should not be terminated on reaching a cell voltage of 4.2 V because the capacity reached at that point is only 70% of full capacity; charging at the reducing current necessary to hold the cell voltage at or very near 4.2 V must be continued until the charge current drops to 10% of the initial charge rate.
It is important to note that
trickle chargingTrickle charging, or float charging, means charging a battery at a similar rate as to its self-discharging, thus maintaining a full capacity battery. Most rechargeable batteries, particularly nickel-cadmium batteries or nickel metal hydride batteries, have a moderate rate of self-discharge, meaning...
is not acceptable for lithium batteries; Li-ion chemistry cannot accept an overcharge without causing damage to the cell, possibly plating out lithium metal and becoming hazardous. Most manufacturers claim a maximum and minimum voltage of 4.23 and 3.0 volts per cell. Taking any cell outside these limits can reduce the cell's capacity and ability to deliver full rated current.
Most dedicated lithium polymer chargers use a charge timer for safety; this cuts the charge after a predefined time (typically 90 minutes).
Storage
Sony Corporation recommends long-term storage "discharged" and cool. See Discussion page for the link. They actually show data for long-term storage at 3.3 volts, lower than Buchmann.
Unlike NiCad and NiMH, lithium ion batteries can be stored for 1 or 2 months without significantly losing charge. However, if storing for long periods, manufacturers recommend discharging the battery to 40% of full charge and refrigerating (but not freezing) the cell.
External links