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Lithium-ion battery

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Battery specifications
Energy/weight 160 Wh/kg
Energy/size 270 Wh/L
Power/weight 1800 W/kg
Charge/discharge efficiency 99.9%[1]
Energy/consumer-price 2.8 Wh/US$
Self-discharge rate 5%-10%/month
Time durability (24-36) months
Cycle durability 1200 cycles
Nominal Cell Voltage 3.6 V
Charge temperature interval

Lithium-ion batteries (sometimes abbreviated Li-ion batteries) are a type of rechargeable battery commonly used in consumer electronics. They are currently one of the most popular types of battery for portable electronics, with one of the best energy-to-weight ratios, no memory effect and a slow loss of charge when not in use. They can be dangerous if mistreated, however, and unless care is taken their lifespan may be reduced. A more advanced lithium-ion battery design is the lithium polymer cell.


[edit] History

Lithium ion batteries, first proposed in the 1960s, came into reality once Bell Labs developed a workable graphite anode[2] to provide an alternative to lithium metal, the lithium battery. Following groundbreaking cathode research by a team led by John B. Goodenough[3] then at Oxford University, now at the University of Texas, Austin, the first commercial lithium ion battery was released by Sony in 1991. Used in numerous commercial applications these batteries revolutionized consumer electronics. One of the latest uses is in hybrid electric cars and eventually electric vehicles. Tesla, Reva and Kewet are all releasing new lithium ion battery electric car models in 2007.

[edit] Advantages and disadvantages

[edit] Advantages

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.

Li-ion batteries are lighter than other equivalent secondary batteries—often much lighter. The energy is stored in these batteries through the movement of lithium ions. Lithium is the third lightest element, giving a substantial saving in weight compared to batteries using much heavier metals. However, the bulk of the electrodes are effectively "housing" for the ions and add weight, and in addition "dead weight" from the electrolyte, current collectors, casing, electronics and conductivity additives reduce the charge per unit mass to little more than that of other rechargeable batteries. The forte of the Li-ion chemistry is the high open circuit voltage in comparison to aqueous batteries (such as lead acid, nickel metal hydride and nickel cadmium).[citation needed]

Li-ion batteries do not suffer from the memory effect. They also have a low self-discharge rate of approximately 5% per month, compared with over 30% per month in nickel metal hydride 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.[4] What looks like a self-discharge in these batteries is a permanent loss of capacity, described in more detail below. On the other hand, "smart" Li-ion batteries do self-discharge, due to the small constant drain of the built-in voltage monitoring circuit. This drain is the most important source of self-discharge in these batteries.

[edit] Disadvantages

A unique drawback of the Li-ion battery is that its life span is dependent upon aging from time of manufacturing (shelf life) regardless of whether it was charged, and not just on the number of charge/discharge cycles. So an older battery will not last as long as a new battery due solely to its age, unlike other batteries. This drawback is not widely publicized.[5]

At a 100% charge level, a typical Li-ion laptop battery that is full most of the time at 25 degrees Celsius or 77 degrees Fahrenheit, will irreversibly lose approximately 20% capacity per year. However, a battery stored inside a poorly ventilated laptop may be subject to a prolonged exposure to much higher temperatures than 25 °C, which will significantly shorten its life. The capacity loss begins from the time the battery was manufactured, and occurs even when the battery is unused. 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% charge level, these figures are reduced to 2%, 4%, 15% at 0, 25 and 40 degrees Celsius respectively.[6]

Li-ion batteries can even go into a state that is known as Deep Discharge. At this point, the battery may take a very long time to recharge. For example, a laptop battery that normally charges fully in 3 hours may take up to 42 hours to recharge. Or, the deep discharge state may be so severe that the battery will never come back to life. Deep discharging only takes place when products with rechargeable batteries are left unused for extended periods of time (often 2 or more years) or when they are recharged so often that they can no longer hold a charge. This makes Li-ion batteries unsuitable for back-up applications compared to lead-acid batteries, and even to Ni-MH batteries.

Because the maximum power that can be continuously drawn from the battery depends on its capacity, in high-powered (relative to C, the battery capacity in A·h) applications, like portable computers and video cameras, rather than showing a gradual shortening of the running time of the equipment, Li-ion batteries may often just abruptly fail.[citation needed]

Low-powered cyclical applications, like mobile phones, can get a much longer lifetime out of a Li-ion battery.[citation needed]

A stand-alone Li-ion cell must never be discharged below a certain voltage to avoid irreversible damage. Therefore all systems involving Li-ion batteries are equipped with a circuit that shuts down the system when the battery is discharged below the predefined threshold.[7] It should thus be impossible to "deep discharge" the battery in a properly designed system during normal use. This is also one of the reasons Li-ion cells are rarely sold as such to consumers, but only as finished batteries designed to fit a particular system.

When the voltage monitoring circuit is built inside the battery (a so-called "smart" battery) rather than the equipment, it continuously draws a small current from the battery even when the battery is not in use; furthermore, the battery must not be stored fully discharged for prolonged periods of time, to avoid damage due to deep discharge.

Li-ion batteries are not as durable as nickel metal hydride or nickel-cadmium designs and can be extremely dangerous if mistreated. They are usually more expensive.

Li-ion chemistry is not safe as such, and a Li-ion cell requires several mandatory safety devices to be built in before it can be considered safe for use outside of a laboratory. These are: shut-down separator (for overtemperature), tear-away tab (for internal pressure), vent (pressure relief), and thermal interrupt (overcurrent/overcharging).[7] The devices take away useful space inside the cells, and add an additional layer of unreliability. Typically, their action is to permanently and irreversibly disable the cell.

Despite these safety features, Li-ion batteries are the subject of frequent recalls (see #Controversy).

The number of safety features can be compared with that of a nickel metal hydride cell, which only has a hydrogen/oxygen recombination device (preventing damage due to mild overcharging) and a back-up pressure valve.[citation needed]

There is ongoing research to develop alternative Li-ion chemistries that would be safe with fewer or no safety devices, such as Valence Technology.[8]

A lithium ion battery from a mobile phone
A lithium ion battery from a mobile phone

[edit] Specifications and design

  • Specific energy density: 150 to 200 W·h/kg (540 to 720 kJ/kg)
  • Volumetric energy density: 250 to 530 W·h/L (900 to 1900 J/cm3)
  • Specific power density: 300 to 1500 W/kg (@ 20 seconds[9] and 285 W·h/L)

A typical chemical reaction of the Li-ion battery is as follows:

\mathrm{Li}_{\frac12} \mathrm{Co} \mathrm{O}_2 + \mathrm{Li}_{\frac12}\mathrm{C}_6 \leftrightarrows \mathrm{C}_6 + \mathrm{Li}\mathrm{Co}\mathrm{O}_2

[citation needed]

[Note that in the above reaction the ½ subscripts relate to the empirical formula; naturally, the atoms themselves are never split.] The actual ion involved in the above reaction is LixCoO2 . It is important to note that lithium ions themselves are not being oxidized; rather, in a lithium ion battery the lithium-ion complexes are used to transport the transition metals to and from the cathode or anode, with the transition metal being involved in the oxidation-reduction reaction on the surface of the anode or cathode (depending on the battery terminal).

Lithium-ion batteries have a nominal open-circuit voltage of 3.6 V and a typical charging voltage of 4.2 V. The charging procedure is done at constant voltage with current limiting circuitry. This means charging with constant current until a voltage of 4.2 V is reached by 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 reach 90% in as little as 10 minutes.[citation needed]

Lithium ion internal design is as follows. The anode is made from carbon, the cathode is a metal oxide, and the electrolyte is a lithium salt in an organic solvent.[citation needed]

[edit] Solid electrolyte interphase

A particularly important element for activating Li-ion batteries is the solid electrolyte interphase (SEI). Liquid electrolytes in Li-ion batteries consist of solid lithium-salt electrolytes, such as LiPF6, LiBF4, or LiClO4, and organic solvents, such as ether. A liquid electrolyte conducts Li ions, which act as a carrier between the cathode and the anode when a battery passes an electric current through an external circuit. However, solid electrolytes and organic solvents are easily decomposed on anodes during charging, thus preventing battery activation. Nevertheless, when appropriate organic solvents are used for electrolytes, the electrolytes are decomposed and form a solid electrolyte interface at first charge that is electrically insulating and high Li-ion conducting. The interface 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.[citation needed]

See uranium trioxide for some details of how the cathode works. While uranium oxides are not used in commercially made batteries, the way in which uranium oxides can reversibly insert cations is the same as the way in which the cathode in many lithium-ion cells work.[citation needed]

[edit] Guidelines for prolonging Li-ion battery life

  • Unlike Ni-Cd batteries, lithium-ion batteries should be charged early and often. However, if they are not used for a longer time, they should be brought to a charge level of around 40%. Lithium-ion batteries should never be "deep-cycled" like Ni-Cd batteries.[6]
  • Li-ion batteries should be kept cool. Ideally they are stored in a refrigerator. Aging will take its toll much faster at high temperatures. The high temperatures found in cars cause lithium-ion batteries to degrade rapidly.
  • Lithium-ion batteries should never be depleted to empty (0%).
  • According to one book,[10] lithium ion batteries should not be frozen (should not be stored under -40 °C), because most lithium-ion battery electrolytes freeze at approximately −40 °C (this is much colder than the lowest temperature reached by household freezers, however).
  • Li-ion batteries should be bought only when needed, because the aging process begins as soon as the battery is manufactured.[6]
  • When using a notebook computer running from fixed line power over extended periods, the battery can be removed and stored in a cool place so that it is not affected by the heat produced by the computer.[6]

[edit] Storage temperature and charge

Storing a Li-ion battery at the correct temperature and charge makes all the difference in maintaining its storage capacity. The following table shows the amount of permanent capacity loss that will occur after storage at a given charge level and temperature.

Permanent Capacity Loss versus Storage Conditions
Storage Temperature 40% Charge 100% Charge
0 °C (32 °F) 2% loss after 1 year 6% loss after 1 year
25 °C (77 °F) 4% loss after 1 year 20% loss after 1 year
40 °C (104 °F) 15% loss after 1 year 35% loss after 1 year
60 °C (140 °F) 25% loss after 1 year 40% loss after 3 months

It is significantly beneficial to avoid storing a lithium-ion battery at full charge. A Li-ion battery stored at 40% charge will last many times longer than one stored at 100% charge, particularly at higher temperatures.[6]

If a Li-ion battery is stored with too low a charge, there is a risk of allowing the charge to drop below the battery's low-voltage threshold, resulting in an unrecoverably dead battery. Once the charge has dropped to this level, recharging it can be dangerous. An internal safety circuit will therefore open to prevent charging, and the battery will be for all practical purposes dead.[citation needed]

In circumstances where a second Li-ion battery is available for a given device, it is recommended that the unused battery be discharged to 40% and placed in the refrigerator to prolong its shelf life. Batteries should be allowed to completely warm to room temperature over up to 24 hours before any discharge or charge.

[edit] Controversy

Lithium-ion batteries can easily rupture, ignite, or explode when exposed to high temperatures,[11] or direct sunlight. They should not be stored in a car during hot weather. Short-circuiting a Li-ion battery can cause it to ignite or explode. Never open a Li-ion battery's casing. Li-ion batteries contain safety devices that protect the cells inside from abuse. If damaged, these can also cause the battery to ignite or explode.

Contaminants inside the cells can defeat these safety devices. The mid-2006 recall of 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, rapidly converting all of the energy in the cell to heat.[12] However, there are problems that go beyond this and this explanation is not complete.

The mid-2006 Sony laptop battery recall isn't the first of its kind, but it is the largest. During the past decade there have been numerous recalls of lithium-ion batteries in cellular phones and laptops owing to overheating problems. Last December, Dell pulled about 22,000 batteries from the U.S. market. In 2004, Kyocera Wireless recalled about 1 million batteries used in phones.[13] In March 2007, Lenovo recalled another 205,000 9-cell lithium ion batteries because of an explosion risk.

"It is possible to replace the lithium cobalt oxide cathode material in li-ion batteries with lithiated metal phosphate cathodes that don’t explode and even have a longer shelf life. But for the moment these safer li-ion batteries seem mainly destined for electric cars and other large-capacity applications, where the safety issues are more critical... The fact is that lithiated metal phosphate batteries hold only about 75 percent as much power..."[14]

[edit] New technology

In February 2005, Altairnano,[15] a small firm based in Reno, Nevada, announced a nano-sized titanate electrode material for lithium-ion batteries. Its prototype battery has three times the power of existing batteries and can be fully charged in six minutes. The company also says the battery can handle approximately 20,000 recharging cycles, so durability and battery life are 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 result in thermal runaway even under severe conditions because they do not contain graphite-coated-metal anode electrode material.[16] The batteries are currently being tested in a new production car made by Phoenix Motorcars which was on display at the 2006 SEMA motorshow.

In March of 2005, Toshiba announced another fast charging lithium-ion battery, based on new nano-material technology, that provides even faster charge times, greater capacity, and a longer life cycle. The battery may be used in commercial products in 2006 or early 2007, primarily in the industrial and automotive sectors.[17]

In November 2005, A123Systems announced[18] a new higher power, faster recharging Li-Ion battery system[19][20] based on research licensed from MIT. Their first cell is in production (1Q/2006)[21] and being used in DeWalt power tools and Hybrids Plus Prius PHEV conversions (although the conversion costs more than the original price of the car, mostly due to the price of the batteries).

All these formulations involve new electrodes. By increasing the effective electrode area — thus decreasing the internal resistance of the battery — the current can be increased during both use and charging. This is similar to developments in ultracapacitors. Therefore, the battery is capable of delivering more power (watts); however, the battery's capacity (ampere-hours) is increased only slightly.

In April 2006, a group of scientists at MIT announced that they had figured out a way to use viruses to form nano-sized wires that can be used to build ultrathin lithium-ion batteries with three times the normal energy density.[22]

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.[23]

[edit] References

  1. ^
  2. ^ USPTO link for Bell Labs graphite work
  3. ^ USPTO search for inventions by "Goodenough, John"
  4. ^ . "Gold Peak Industries Ltd., Lithium Ion technical handbook" (pdf).
  5. ^
  6. ^ a b c d e f how to prolong lithium-based batteries
  7. ^ a b Gold Peak Industries Ltd., Lithium Ion technical handbook
  8. ^ . ""Saphion" technology incorporates a phosphate based cathode material".
  9. ^
  10. ^ L.M. Cristo, T. B. Atwater. Characteristics and Behavior of 1M LiPF6 1EC:1DMC Electrolyte at Low Temperatures. Fort Monmouth, NJ: U.S. Army Research. 
  11. ^
  12. ^
  13. ^ Tullo, Alex. "Dell Recalls Lithium Batteries." Chemical and Engineering News 21 Aug 2006: 11.
  14. ^
  15. ^
  16. ^
  17. ^
  18. ^
  19. ^
  20. ^
  21. ^
  22. ^ Science Express (preprint)
  23. ^

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