Battery Power/Lithium Ion Batteries

Lithium ion batteries (sometimes abbreviated Li-Ion) are a type of rechargeable battery commonly used in consumer electronics. They are currently one of the most popular types of battery, 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 they may have a shorter lifespan compared to other battery types. A more advanced lithium-ion battery design is the lithium polymer cell.

History
Gilbert N. Lewis a prominent American chemist, researched lithium batteries beginning in 1912, but the first non-rechargeable (primary) cells were not commercially produced until the early 1970s. The rechargeable (secondary) lithium-ion battery required nearly 20 years of research and development before being produced on a mass market scale; the first commercial version being produced by Sony in 1991, following initial discoveries by a team at Oxford University led by John B. Goodenough.

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.

The batteries are also lighter than equivalents in other chemistries — often much lighter. This is because lithium ions have an extremely high charge density — the highest of all known naturally occurring ions. Li ions are small and mobile, but more readily stored than hydrogen. Thus a battery based on lithium is smaller than one with hydrogen elements, such as nickel metal hydride, and with fewer volatile gases. The ions need fewer storage intermediaries, so more battery weight is usable as charge, instead of overhead.

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 and 20% per month in nickel metal hydride batteries and nickel cadmium batteries, respectively.

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, 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.

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. This drawback is not widely publicized.

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.

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, 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.

Low-powered cyclical applications, like mobile phones, can get a much longer lifetime out of a Li-Ion battery.

A stand-alone Li-Ion cell must never be discharged below a certain voltage to avoid an 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. 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 not normally 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 (so called "smart battery") rather than equipment, and continuously draws a small current from the battery even if it is not in use, the battery further 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), thermal interrupt (overcurrent/overcharging). 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 subject of frequent recalls (see ).

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

There is an ongoing research to develop alternative Li-ion chemistries, which would be safe with fewer or no safety devices, such as.

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 and 285 W·h/L)

A typical chemical reaction of the Li-ion battery is as follows: $$\mathrm{Li}_{\frac12} \mathrm{Co} \mathrm{O}_2 + \mathrm{Li}_{\frac12}\mathrm{C}_6 \leftrightarrow \mathrm{C}_6 + \mathrm{Li}\mathrm{Co}\mathrm{O}_2 $$

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 one of constant voltage with current limiting. 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.

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. Since the lithium metal which might be produced under irregular charging conditions is very reactive and might cause explosion, Li-ion cells usually have built-in protective electronics and/or fuses to prevent polarity reversal, over-voltage and over-heating.

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 electrolytes after the second charge. For example, ethylene carbonate is decomposed at relatively high voltage, 0.7 V vs. Li, and forms a tight and stable interface. This interface is called an SEI.

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.

Guidelines for prolonging Li-ion battery life

 * Unlike NiCad 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 NiCd batteries.
 * 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, lithium ion batteries should not be frozen. Note that most lithium-ion battery electrolytes freeze at approximately −40 °C, which is much colder than the lowest temperature reached by most household freezers.
 * Li-ion batteries should be bought only when needed, because the aging process begins as soon as the battery is manufactured.
 * 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. (However, a notebook computer's battery prevents sudden loss of the data in memory during power failures and brownouts. Reasonable alternatives are the use of an older lithium ion battery or an external uninterruptible power supply.)

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.

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.

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.

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.

Safety concerns
Lithium-ion batteries can easily rupture, ignite, or explode when exposed to high temperatures, or direct sunlight. They should not be stored in a car during hot weather. Short-circuiting a Li-ion battery can also 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 cause the battery to ignite or explode.

Internal contaminants inside the cells can defeat these safety devices. The mid-2006 recall of 4.1 million Sony batteries used in Dell laptops was 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. See example: Dell laptop "explodes" at Japanese conference

Kuzhikalail M. Abraham, a lithium battery consultant with E-Kem Sciences, says the computer industry's drive to increase battery capacity can test the limits of sensitive components such as the membrane separator, a polyethylene or polypropylene film that is only 20-25 µm thick. He points out that the energy density of lithium-ion batteries has more than doubled since they were introduced in 1991. "When you pack the battery with more and more material, the separator can undergo stress," he says.

The mid-2006 Dell 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.

"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...." (ref: http://www.nytimes.com/2006/09/01/opinion/01cringely.html)

New technology
In February 2005, Altair NanoTechnology, a small firm based in Reno, announced a nano-sized 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.

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.

In November 2005, A123Systems announced a new higher power, faster recharging Li-Ion battery system based on research licensed from MIT. Their first cell is in production (1Q/2006) and being used in DeWalt power tools and Hybrids Plus Prius PHEV conversions.

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 (Amp-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. Science Express (preprint)

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.