Not to be confused with Lithium battery or Lithium-ion polymer battery
Lithium-ion batteries (sometimes abbreviated Li-ion batteries ) are a type of rechargeable battery in which lithium ions move from the negative electrode ( anode ) to the positive electrode ( cathode ) during discharge, and from the cathode to the anode during charge.
Lithium-ion batteries are common in portable consumer electronics because of their high energy-to-weight ratios , lack of memory effect , and slow self-discharge when not in use. In addition to consumer electronics, lithium-ion batteries are increasingly used in defense, automotive, and aerospace applications due to their high energy density. However, certain kinds of mistreatment may cause conventional Li-ion batteries to explode .
The three primary functional components of a lithium-ion battery are the anode , cathode , and electrolyte , for which a variety of materials may be used. Commercially, the most popular material for the anode is graphite . The cathode is generally one of three materials: a layered oxide (such as lithium cobalt oxide ), one based on a polyanion (such as lithium iron phosphate ), or a spinel (such as lithium manganese oxide ), although materials such as TiS 2 ( titanium disulfide ) originally were also used. Depending on the choice of material for the anode, cathode, and electrolyte, the voltage, capacity, life, and safety of a lithium-ion battery can change dramatically. Recently, novel architectures have been employed to improve the performance of these batteries. Lithium-ion batteries are not to be confused with lithium batteries , the key difference being that lithium batteries are primary batteries , containing metallic lithium, while lithium-ion batteries are secondary batteries , containing an intercalation anode material.
Lithium-ion batteries were first proposed by M.S. Whittingham (Binghamton University), then at Exxon , in the 1970s. Whittingham used titanium(II) sulfide as the cathode and lithium metal as the anode.
The electrochemical properties of the lithium intercalation in graphite were first discovered in 1980 by Rachid Yazami et al. at the Grenoble Institute of Technology (INPG) and French National Centre for Scientific Research (CNRS) in France. They showed the reversible intercalation of lithium into graphite in a lithium/polymer electrolyte/graphite half-cell. Their work was published in 1982 and 1983. It covered both the thermodynamics (staging) and the kinetics (diffusion) aspects of the lithium intercalation into graphite together with reversibility.
Lithium batteries in which the anode is made from metallic lithium pose severe safety issues. As a result, lithium-ion batteries were developed in which the anode, like the cathode, is made of a material containing lithium ions. In 1981, Bell Labs developed a workable graphite anode to provide an alternative to the lithium battery . Following groundbreaking cathode research by a team led by John Goodenough , the first commercial lithium-ion battery was released by Sony in 1991. The cells used layered oxide chemistry, specifically lithium cobalt oxide . These batteries revolutionized consumer electronics.
In 1983, Michael Thackeray, John Goodenough, and coworkers identified manganese spinel as a cathode material. Spinel showed great promise, since it is a low-cost material, has good electronic and lithium ion conductivity, and possesses a three-dimensional structure which gives it good structural stability. Although pure manganese spinel fades with cycling, this can be overcome with additional chemical modification of the material. Manganese spinel is currently used in commercial cells.
In 1989, Arumugam Manthiram and John Goodenough of the University of Texas at Austin showed that cathodes containing polyanions, eg. sulfates, produce higher voltage than oxides due to the inductive effect of the polyanion.
In 1996, Akshaya Padhi, John Goodenough and coworkers identified the lithium iron phosphate (LiFePO 4 ) and other phospho-olivines (lithium metal phosphates with olivine structure) as cathode materials for lithium ion batteries. LiFePO 4 is superior over other cathode materials in terms of cost, safety, stability and performance, and is most suitable for large batteries for electric automobiles and other energy storage applications such as load saving, where safety is of utmost importance. It is currently being used for most lithium-ion batteries powering portable devices such as laptop computers and power tools.
In 2002, Yet-Ming Chiang and his group at MIT published a paper in which they showed a dramatic improvement in the performance of lithium batteries by boosting the material's conductivity by doping it with aluminium , niobium and zirconium , though at the time, the exact mechanism causing the increase became the subject of a heated debate.
In 2004, Chiang again increased performance by utilizing iron-phosphate particles of less than 100 nm in diameter. This miniaturized the particle density by almost a hundredfold, increased the surface area of the electrode and improved the battery's capacity and performance. Commercialization of the iron-phosphate technology led to a competitive market and a patent infringement battle between Chiang and Goodenough.
The three participants in the electrochemical reactions in a lithium ion battery are the anode , cathode , and electrolyte .
Both the anode and cathode are materials into which and from which lithium can migrate. The process of lithium moving into the anode or cathode is referred to as insertion (or intercalation ), and the reverse process, in which lithium moves out of the anode or cathode is referred to as extraction (or deintercalation ). When a lithium-based cell is discharging , the lithium is extracted from the anode and inserted into the cathode. When the cell is charging , the reverse process occurs: lithium is extracted from the cathode and inserted into the anode.
During discharge, the anode of a conventional Li-ion cell is made from carbon , the cathode is a metal oxide , and the electrolyte is a lithium salt in an organic solvent .
Useful work can only be extracted if electrons flow through a (closed) external circuit. The following equations are written in units of moles, making it possible to use the coefficient x . The cathode half reaction (with charging being forwards) is:
The anode half reaction is:
The overall reaction has its limits. Overdischarge will supersaturate lithium cobalt oxide, leading to the production of lithium oxide , possibly by the following irreversible reaction:
Overcharge up to 5.2V leads to the synthesis of cobalt(IV) oxide, as evidenced by x-ray diffraction
In a lithium-ion battery the lithium ions are transported to and from the cathode or anode, with the transition metal, Co , in Li x CoO 2 being oxidized from Co 3+ to Co 4+ during charging, and reduced from Co 4+ to Co 3+ during discharge.
See uranium trioxide for some details of how the cathode works. While uranium oxides are not used in commercially-made batteries, intercalation and deintercalation function in the same way as with lithium-based cells.
The cell voltages given in the section above are larger than the potential at which aqueous solutions would electrolyze . Therefore, nonaqueous solutions are used.
Liquid electrolytes in lithium-ion batteries consist of lithium salts , such as LiPF 6 , LiBF 4 or LiClO 4 in an organic solvent , such as ethylene carbonate . A liquid electrolyte conducts lithium ions, acting as a carrier between the cathode and the anode when a battery passes an electric current through an external circuit. Typical conductivities of liquid electrolyte at room temperature (20 o C) are in the range of 10 mS/cm, increasing by approximately 30-40% at 40 o C and decreasing by a slightly smaller amount at 0 o C.
Unfortunately, organic solvents are easily decomposed on anodes during charging. However, when appropriate organic solvents are used as the electrolyte, the solvent is decomposed on initial charging and forms a solid layer called the solid electrolyte interphase (SEI), [ 26 ] which is electrically insulating yet sufficiently conductive to lithium ions. The interphase prevents decomposition of the electrolyte after the second charge. For example, ethylene carbonate is decomposed at a relatively high voltage, 0.7 V vs. Li, and forms a dense and stable interface.
Advantages and disadvantages
- Lithium-ion batteries can be formed into a wide variety of shapes and sizes so as to efficiently fill available space in the devices they power.
- Lithium-ion batteries are lighter than other energy-equivalent secondary batteries —often much lighter. A key advantage of using lithium-ion chemistry is the highopen circuit voltage that can be obtained in comparison to aqueous batteries (such as lead acid , nickel metal hydride and nickel cadmium ).
- Lithium-ion batteries do not suffer from the memory effect . They also have a low self-discharge rate of approximately 0.1% per month, compared with over 30% per month in common nickel metal hydride batteries, approx. 1.25% per month for Low Self-Discharge NiMH batteries and 10% per month in nickel cadmium batteries. According to one manufacturer, Li-ion cells (and, accordingly, "dumb" Li-ion batteries) do not have any self-discharge in the usual meaning of this word. What looks like a self-discharge in these batteries is a permanent loss of capacity (see below). On the other hand, "smart" Li-ion batteries do self-discharge, mainly due to the small constant drain of the built-in voltage monitoring circuit.
Disadvantages of traditional Li-ion technology
- A disadvantage of lithium-ion cells lies in their relatively poor cycle life : upon every (re)charge, deposits form inside the electrolyte that inhibit lithium ion transport, resulting in the capacity of the cell to diminish. The increase in internal resistance affects the cell's ability to deliver current, thus the problem is more pronounced in high-current than low-current applications. The increasing capacity hit means that a full charge in an older battery will not last as long as one in a new battery (although the charging time required obviously decreases proportionally, as well).
- Also, high charge levels and elevated temperatures (whether resulting from charging or being ambient) hasten permanent capacity loss for lithium-ion batteries. The heat generated during a charge cycle is caused by the traditional carbon anode, which has been replaced with good results by lithium titanate. Lithium titanate has been experimentally shown to drastically reduce the degenerative effects associated with charging, including expansion and other factors. See "Improvements of lithium-ion technology" below.
- At a 100% charge level, a typical Li-ion laptop battery that is full most of the time at 25 °C or 77 °F will irreversibly lose approximately 20% capacity per year. However, a battery in a poorly ventilated laptop may be subject to a prolonged exposure to much higher temperatures, which will significantly shorten its life. Different storage temperatures produce different loss results: 6% loss at 0 °C (32 °F), 20% at 25 °C (77 °F), and 35% at 40 °C (104 °F). When stored at 40%�C60% charge level, the capacity loss is reduced to 2%, 4%, 15% at 0, 25 and 40 degrees Celsius respectively.
The internal resistance of lithium-ion batteries is high compared to other rechargeable chemistries such as nickel-metal hydride and nickel-cadmium . It increases with both cycling and chronological age. Rising internal resistance causes the voltage at the terminals to drop under load, reducing the maximum current that can be drawn from them. Eventually they reach a point at which the battery can no longer operate the equipment it is installed in for an adequate period.
High drain applications such as power tools may require the battery to be able to supply a current that would drain the battery in 1/15 hour if sustained; e.g. 22.5 A for a battery with a capacity of 1.5 A·h ). Lower-power devices such as MP3 players , on the other hand, may draw low enough current to run for 10 hours on a charge (e.g. 150 mA for a battery with a capacity of 1500 mA·h). With similar battery technology, the MP3 player's battery will effectively last much longer, since it can tolerate a much higher internal resistance. To power larger devices, such as electric cars, it is much more efficient to connect many smaller batteries in a parallel circuit rather than using a single large battery.
Li-ion batteries are not as durable as nickel metal hydride or nickel-cadmium designs, and can be extremely dangerous if mistreated. They may explode if overheated or if charged to an excessively high voltage. Furthermore, they may be irreversibly damaged if discharged below a certain voltage. To reduce these risks, lithium-ion batteries generally contain a small circuit that shuts down the battery when it is discharged below about 3 V or charged above about 4.2 V. In normal use, the battery is therefore prevented from being deeply discharged. When stored for long periods, however, the small current drawn by the protection circuitry may drain the battery below the protection circuit's lower limit, in which case normal chargers are unable to recharge the battery. More sophisticated battery analyzers can recharge deeply discharged cells by slow-charging them .
Other safety features are also required for commercial lithium-ion batteries:
- shut-down separator (for overtemperature),
- tear-away tab (for internal pressure),
- vent (pressure relief), and
- thermal interrupt (overcurrent/overcharging).
These devices occupy useful space inside the cells, and reduce their reliability ; typically, they permanently and irreversibly disable the cell when activated. They are required because the anode produces heat during use, while the cathode may produce oxygen. Safety devices and recent and improved electrode designs greatly reduce or eliminate the risk of fire or explosion.
These safety features increase the cost of lithium-ion batteries compared to nickel metal hydride cells, which only require a hydrogen/oxygen recombination device (preventing damage due to mild overcharging) and a back-up pressure valve.
Many types of lithium-ion cell cannot be charged safely below 0 °C.
About 1% of lithium-ion batteries are recalled.
Specifications and design
- Specific energy density: 150 to 200 Wh /kg (540 to 720 kJ /kg)
- Volumetric energy density: 250 to 530 Wh/l (900 to 1900 J/cm3)
- Specific power density: 300 to 1500 W/kg (@ 20 seconds and 285 Wh/l)
Because lithium-ion batteries can have a variety of cathode and anode materials, the energy density and voltage vary accordingly.
Lithium-ion batteries with a lithium iron phosphate cathode and graphite anode have a nominal open-circuit voltage of 3.2 V and a typical charging voltage of 3.6 V. Lithium nickel manganese cobalt (NMC) oxide cathode with graphite anodes have a 3.7 V nominal voltage with a 4.2 V max charge. The charging procedure is performed at constant voltage with current-limiting circuitry (i.e., charging with constant current until a voltage of 4.2 V is reached in the cell and continuing with a constant voltage applied until the current drops close to zero). Typically, the charge is terminated at 7% of the initial charge current. In the past, lithium-ion batteries could not be fast-charged and typically needed at least two hours to fully charge. Current-generation cells can be fully charged in 45 minutes or less; some lithium-ion varieties can reach 90% in as little as 10 minutes.
Stage 1 : Apply charging current limit until the voltage limit per cell is reached.
Stage 2 : Apply maximum voltage per cell limit until the current declines below 3% of rated charge current.
Stage 3 : Periodically apply a top-off charge about once per 500 hours.
The charge time is about three to five hours, depending upon the charger used. Generally, cell phone batteries can be charged at 1C and laptop-types at 0.8C, where C is the current that would discharge the battery in one hour. Charging is usually stopped when the current goes below 0.03C but it can be left indefinitely depending on desired charging time. Some fast chargers skip stage 2 and claim the battery is ready at 70% charge. Laptop battery chargers sometimes gamble, and try to charge up to 4.35v then disconnect battery. This helps to compensate internal resistance and charge up to 100% in short time.
Top-off charging is recommended to be initiated when voltage goes below 4.05 V/cell.
Lithium-ion cells are charged with 4.2 ± 0.05 V/cell,except for military long-life cells where 3.92 V is used to extend battery life. Most protection circuits cut off if either 4.3 V or 90 °C is reached. If the voltage drops below 2.50 V per cell, the battery protection circuit may also render it unchargeable with regular charging equipment. Most battery protection circuits stop at 2.7�C3.0 V per cell.
For safety reasons it is recommended to stay within the manufacturer's stated voltage and current ratings during both charge and discharge cycles.
||This article relies extensively on quotes that were previously collated by an advocacy or lobbying group . Please improve this article or discuss the issue on the talk page .
Improvements focus on several areas, and often involve advances in nanotechnology and microstructures.
- Increasing cycle life and performance (decreases internal resistance and increases output power) by changing the composition of the material used in the anode and cathode, along with increasing the effective surface area of the electrodes (related developments have helped ultracapacitors ) and changing materials used in the electrolyte and/or combinations thereof (e.g., Li-VOx-based cells with polymer electrolyte).
- Improving capacity by improving the structure to incorporate more active materials.
- Improving the safety of lithium-ion batteries.
Manganese spinel cathodes
LG , which is the third-largest producer of lithium-ion batteries, uses the lithium manganese spinel for its cathode. It is working with its subsidiary CPI to commercialize lithium-ion batteries containing manganese spinel for HEV applications. Several other companies are also working on manganese spinel, including NEC and Samsung .
Lithium iron phosphate cathode with traditional anode
Main article: Lithium iron phosphate battery
The University of Texas first licensed its patent for lithium iron phosphate cathodes to the Canadian utility Hydro-Québec . Phostech Lithium inc. was later spun-off from Hydro-Québec for the sole development of lithium iron phosphate.
Valence Technology , located in Austin, Texas, is also working on lithium iron magnesium phosphate cells. Since March 2005, the Segway Personal Transporter has been shipping with extended-range lithium-ion batteries made by Valence Technology using iron magnesium phosphate cathode materials. Segway, Inc. chose to build their large-format battery with this cathode material because of its improved safety over metal-oxide materials. To date Valence has shipped 100,000 batteries to Segway.
In November 2005, A123Systems announced the development of lithium iron phosphate cells based on research licensed from MIT. While the battery has slightly lower energy density than other competing lithium-ion technologies, a 2 Ah cell can provide a peak of 70 Amps without damage and operate at temperatures above 60 degrees C. Their first cell has been in production since 2006 and is being used in consumer products including DeWalt power tools, aviation products, automotive hybrid systems and PHEV conversions.
LiFePO 4 cells are currently available commercially.
High power cathode using lithium nickel manganese cobalt (NMC)
Imara Corporation , based in Menlo Park, CA is commercializing a new materials-agnostic technology first applied on an NMC material which has the effect of lowering impedance and extending cycle life. These high power-capable cells have high energy density relative to other high power cells in the market. The batteries are being deployed in power tools, outdoor power equipment and hybrid vehicles; Sony and Sanyo use NMC and NCA blended with LMO (spinel) for high-powered applications. NMC has a significant safety advantage over cobalt oxide and 50% greater energy density than FePO4, but suffers from a poor cycle life.
Traditional cathode with lithium titanate anode
Main article: Lithium-titanate battery
Altairnano , a small firm based in Reno, Nevada , has announced a nano-sized titanate electrode material for lithium-ion batteries. It is claimed the prototype battery has three times the power output of existing batteries and can be fully charged in six minutes. However, total energy capacity per cell is about half that of normal lithium-ion cells. The company also says the battery cells have now achieved a life of over 9,000 charge cycles while still retaining up to 85% charge capacity. Durability and battery life are therefore much longer, estimated to be around 20 years, or four times longer than regular lithium-ion batteries. The batteries can operate from -50 °C to over 75 °C and will not explode or experience thermal runaway, even under severe conditions, because they do not contain graphite-coated-metal anode electrode material. The batteries are currently being tested in a new production car made by Phoenix Motorcars which was on display at the 2006 SEMA motorshow. They're also being tested, on a one MW grid scale, in the PJM Interconnection Regional Transmission Organization control area in Norristown , PA as well as by several branches of the U.S. Department of Defense . In addition, the batteries are being demonstrated by Proterra in their all-electric EcoRide BE35 vehicle, a lightweight 35-foot bus. Altairnano is currently working with three different cell chemistries for various energy and power storage applications, with another new cell chemistry expected in the fall of 2009. The nature of their latest cathode materials is currently proprietary.
Combined anode and cathode developments
EnerDel , which is jointly owned by Ener1 and Delphi , is working to commercialize cells containing a titanate anode and manganese spinel cathode. Although the cells show excellent thermal properties and cyclability , their low voltage may hamper commercial success.
In April 2006, a group of scientists at MIT announced a process which uses viruses to form nano-sized wires. These can be used to build ultrathin lithium-ion batteries with three times the normal energy density.
As of June 2006, researchers in France have created nanostructured battery electrodes with several times the energy capacity, by weight and volume, of conventional electrodes.
In the September 2007 issue of Nature, researchers from the University of Waterloo, Canada, reported a new cathode chemistry, in which the hydroxyl group in the iron phosphate cathode was replaced by fluorine. The advantages seem to be two-fold. First, there is less volume change in the cathode over a charge cycle which may improve battery life. Secondly, the chemistry allows the substitution of the lithium in the battery with either sodium or a sodium/lithium mixture (hence their reference to it as an Alkali-Ion battery).
In November 2007, Subaru unveiled their concept G4e electric vehicle with a lithium vanadium oxide-based lithium-ion battery, promising double the energy density of a conventional lithium-ion battery (lithium cobalt oxide and graphite). In the lab, lithium vanadium oxide anodes, paired with lithium cobalt oxide cathodes, have achieved 745Wh/l, nearly three times the volumetric energy density of conventional lithium-ion batteries.
In December 2007, researchers at Stanford University reported creating a lithium-ion nanowire battery with ten times the energy density (amount of energy available by weight) through using silicon nanowires deposited on stainless steel as the anode. The battery takes advantage of the fact that silicon can hold large amounts of lithium, and helps alleviate the longstanding problem of cracking by the small size of the wires. To gain a tenfold improvement in energy density, the cathode would need to be improved as well; however, even just improving the anode could provide "several" times the energy density, according to the team. The team leader, Yi Cui, expects to be able to commercialize the technology in about five years. Having a large capacitive anode will not increase the capacity of the battery as predicted by the author when the cathode material is far less capacitive than the anode. However, current lithium-ion capacity is mainly limited by the low theoretical capacity (372 mAh g ?1 ) of the graphite in use as the anode material, so improvement could be significant and would then be limited by the cathode material instead.
There are trials with metal hydrides as anode material for lithium-ion batteries. A practical electrode capacity as high as 1480 mAh g ?1 has been reported.
In April 2009 a report in New Scientist claimed that Angela Belcher's team at MIT had succeeded in producing the first full virus-based 3-volt lithium-ion battery.
Recent studies performed at SUNY Binghamton by M. S. Whittingham et al. determined that vanadium ions can be incorporated into the iron-containing olivine structure of LiFePO4; a small amount of vanadium (around 5%) enhancing the rate capability of the LiFePO4 olivine cathode material. The resulting compound material had higher electronic and ionic conductivities, and they were of comparable magnitude. The doping reaction kinetics were optimal under reducing atmosphere during the synthesis of the LiFe0.95V0.05PO4 material.
Guidelines for prolonging lithium-ion battery life
- Lithium-ion batteries should never be depleted below their minimum voltage (2.4 to 2.8 V/cell, depending on chemistry). If a lithium-ion battery is stored with too low a charge, there is a risk that the charge will drop below the low-voltage threshold, resulting in an unrecoverable dead battery. Usually this does not instantly damage the battery itself but a charger or device which uses that battery will refuse to charge a dead battery. The battery appears to be dead or not existent because the protection circuit disables further discharging and there is zero voltage on the battery terminals.
- Lithium-ion batteries should be kept cool. Ideally they are stored in a refrigerator.
- Aging will take its toll much faster at high temperatures.
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.
Lithium-ion batteries can rupture, ignite, or explode when exposed to high-temperature environments, e.g. in an area that is prone to prolonged direct sunlight. Short-circuiting a lithium-ion battery can cause it to ignite or explode and any attempt to open or modify the casing or circuitry is dangerous. For this reason they normally contain safety devices that protect the cells from abuse.
Contaminants inside the cells can defeat these safety devices. For example, the mid-2006 recall of approximately 10 million Sony batteries used in Dell , Sony , Apple , Lenovo / IBM , Panasonic , Toshiba , Hitachi , Fujitsu and Sharp laptops was stated to be as a consequence of internal contamination with metal particles. Under some circumstances, these can pierce the separator, causing the cell to short, rapidly converting all of the energy in the cell to heat resulting in an exothermic oxidizing reaction , increasing the temperature to a few hundred degrees Celsius in a fraction of a second. This causes the neighboring cells to heat up, causing a chain reaction.
The mid-2006 Sony laptop battery recall was not the first of its kind; it was, however, the largest to date. During the past decade, there have been numerous recalls of lithium-ion batteries in cellular phones and laptops owing to overheating problems. In October 2004, Kyocera Wireless recalled approximately 1 million batteries used in cellular phones due to counterfeit batteries produced in Kyocera's name.In December 2006, Dell recalled approximately 22,000 batteries from the U.S. market. In March 2007, Lenovo recalled approximately 205,000 9-cell lithium-ion batteries due to an explosion risk. In August 2007, Nokia recalled over 46 million lithium-ion batteries, warning that some of them might overheat and possibly explode. One such incident occurred in the Philippines involving an Nokia N91 , which uses the BL-5C battery.
Replacing the lithium cobalt oxide cathode material in lithium-ion batteries with lithiated metal phosphate leads to longer cycle and shelf life, improves safety, but lowers capacity. Currently these 'safer' lithium-ion batteries are mainly used in electric cars and other large-capacity battery applications, where safety issues are critical.
Another option is to use a manganese oxide or iron phosphate cathode.
A new class of high power cathode materials, lithium nickel manganese cobalt (NMC) oxide has recently been introduced that have a significantly higher temperature tolerance compared to lithium cobalt oxide (see above). [ unreliable source? ]
In the event of a lithium-ion battery explosion, dense white smoke which can cause severe irritation to the respiratory tract, eyes and skin will be generated. All precautions must be taken to limit exposure to these fumes.
Restrictions on transportation
As of January 2008, the US Department of Transportation issued a new rule that permits passengers on board commercial aircraft to carry lithium batteries in their checked baggage IF the batteries are installed in a device. Types of batteries affected by this rule are those containing lithium, including Li-ion, lithium polymer , and lithium cobalt oxide chemistries. Lithium-ion batteries containing more than 25 grams Equivalent Lithium Content (ELC) are exempt from the rule and are forbidden in air travel.
The purpose of this restriction is that it greatly reduces the chances of the batteries becoming short-circuited and causing a fire. A limited number of replacement batteries can be carried in hand luggage providing they are kept in their original protective packaging or in individual containers or plastic bags.