Lithium-ion battery

Lithium-ion battery

Specific energy	160 Wh/kg
Energy density	270 Wh/l
Specific power	1800 W/kg
Charge/discharge efficiency	99.9%
Energy/consumer-price	2.8-5 Wh/US$
Self-discharge rate	5%-10%/month
Time durability	(24-36) months
Cycle durability	1200 cycles
Nominal cell voltage	3.6 / 3.7 V

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 and unless care is taken their lifespan may be reduced. Although originally intended for consumer electronics, Lithium-ion batteries are growing in popularity with the defense and aerospace industries because of their high energy density.

A more advanced lithium-ion battery design is the lithium polymer cell.

History

Lithium-ion batteries, first proposed in the 1960s, came into reality once Bell Labs developed a workable graphite anode to provide an alternative to lithium metal, the lithium battery. Following groundbreaking cathode research by a team led by John Goodenough (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, as commodity cells. Tesla Motors, Reva and Kewet are all releasing new battery electric car models in 2007.

In 1996, Prof. John Goodenough at the University of Texas discovered the electrochemical utility of the olivine material lithium iron phosphate, LiFePO4. It is an important and emerging cathode material for lithium-ion batteries due in part to its enhanced safety compared to other lithium-ion chemistries. Significant advances in its commercialization were made by a team of researchers from Hydro-Quebec (HQ) and Université de Montréal (UDM) who developed a proprietary fabrication process for Carbon-coating the material.

Advantages and disadvantages

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 has the third smallest atomic mass of all the elements giving the battery 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. A key advantage of using Li-ion chemistry is the high open circuit voltage that can be obtained in comparison to aqueous batteries (such as lead acid, nickel metal hydride and nickel cadmium).

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. 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. 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 publicised.

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% - 60% charge level, these figures are reduced to 2%, 4%, 15% at 0, 25 and 40 degrees Celsius respectively.

As batteries age, their internal resistance rises. This 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 powertools may require the battery to be able to supply a current of 15C - 15 times C - the battery capacity in Ah, whereas MP3 players may only require 0.1C (discharging in 10 hours). With similar technology, the MP3 battery can tolerate a much higher internal resistance, so will have an effective life of many more cycles.

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 where they may become completely discharged.

A stand-alone Li-ion cell must never be discharged below a certain voltage to avoid irreversible damage. Therefore all Li-ion battery systems 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 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 as safe as nickel metal hydride or nickel-cadmium, 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). 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.

Approximately 1% of Li-ion batteries are the subject of 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.

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

A lithium-ion battery from a mobile phone

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)

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 Lithium-Ion variants can reach 90% in as little as 10 minutes.

Electrochemistry

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.

The underlying chemical reaction that allows Li-ion cells to provide electricity is:

$\mathrm {Li} _{1-x}\mathrm {Co} \mathrm {O} _{2}+\mathrm {Li} _{x}\mathrm {C} _{6}\leftrightarrows \mathrm {C} _{6}+\mathrm {Li} \mathrm {Co} \mathrm {O} _{2}$

It is important to note that lithium ions themselves are not being oxidized; rather, in a lithium-ion battery the lithium ions are transported to and from the cathode or anode, with the transition metal, Co, in $\mathrm {Li} _{x}\mathrm {Co} \mathrm {O} _{2}$ being oxidized from Co³⁺ to Co⁴⁺ during charging, and reduced from Co⁴⁺ to Co³⁺ during discharge.

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.

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 Ni-Cd batteries, lithium-ion batteries should be charged early and often. However, if they are not used for a long time, they should be brought to a charge level of around 40% - 60%. Lithium-ion batteries should never be "deep-cycled" like Ni-Cd batteries.
• Lithium-ion batteries should never be depleted to below their minimum voltage, 2.4v to 3.0v per cell.
• 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.
• According to one book, lithium-ion batteries should not be frozen (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.
• When using a notebook computer running from fixed line power over extended periods, the battery should be removed, and stored in a cool place so that it is not affected by the heat produced by the computer.

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
Source: BatteryUniversity.com

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. Some batteries therefore feature an internal safety circuit which will prevent charging in this state, 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. While the battery can be used or charged immediately, some Li-ion batteries will provide more energy when brought to room temperature.

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.

It is important to recognize that the cell mismatch results more from limitations in process control and inspection than from variations inherent in the Lithium Ion chemistry. The use of cell balancing can improve the performance of series connected Li-ion Cells by addressing both SOC and C/E issues. SOC mismatch can be remedied by balancing the cell during an initial conditioning period and subsequently only during the charge phase. C/E mismatch remedies are more difficult to implement and harder to measure and require balancing during both charge and discharge periods.

Cell Balancing

Cell balancing is defined as the application of differential currents to individual cells (or combinations of cells) in a series string. Normally, of course, cells in a series string receive identical currents. A battery pack requires additional components and circuitry to achieve cell balancing. However, the use of a fully integrated analog front end for cell balancing reduces the required external components to just balancing resistors.

This type of solution eliminates the need for discrete capacistors, diodes and most other resistors to achieve balance.

Battery pack cells are balanced when all the cells in the battery pack meet two conditions.

1. 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. This is mostly a one shot fix.

2. 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.

Controversy

File:Dell laptop burnt by bad Sony lithium-ion battery.jpg

Dell laptop burnt by a defective Sony lithium-ion battery

Lithium-ion batteries can rupture, ignite, or explode when exposed to high temperature environments, for example in an area that is prone to prolonged direct sunlight. . Short-circuiting a Li-ion battery can cause it to ignite or explode, and as such, any attempt to open or modify a Li-ion battery's casing or circuitry is dangerous. Li-ion batteries contain safety devices that protect the cells inside from abuse, and, if damaged, can cause the battery to ignite or explode.

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 heatresulting in an exothermic oxidizing reaction increasing the temperature to a few hundred degrees Celsius in a fraction of a second. This causes the neighbouring cells to heat up causing a chain thermal reaction. However, there are problems that go beyond this and so this explanation is not complete.

The mid-2006 Sony laptop battery recall was not the first of its kind, however it is 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. There was an incident in the Philippines involving a Nokia N91, which uses the BL-5C battery.

It is possible to replace the lithium cobalt oxide cathode material in li-ion batteries with lithiated metal phosphate cathodes that are not as sensitive to temperature, and so are less prone to explode. This also extends their shelf life. However, currently these 'safer' li-ion batteries are mainly destined for electric cars and other large-capacity battery applications, where the safety issues are more critical. Unfortunately, a problem with these 'safer' li-ion batteries is that lithiated metal phosphate batteries hold only about 75 percent as much [energy].

Another option is to use manganese oxide or iron phosphate cathode.

New technology, new electrodes

In February 2005, Altairnano, a small firm based in Reno, Nevada, 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 the energy capacity is about half that of normal li-ion cells. 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. 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 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 December 2007, Toshiba announced that it will be launching the battery by March 2008. Analysis of the press release shows that instead of having a higher capacity, the batteries seem to have an energy density well below that of a conventional lithium-ion battery.

Since March 2005, the Segway Personal Transporter has been shipping with extended-range lithium-ion batteries made by Valence Technology using iron 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.

In November 2005, A123Systems announced a new Li-Ion battery system based on research licensed from MIT. While the battery has a lower energy density that other competing Lithium Ion technologies, a 2 Ahr cell can provide a peak of 70 Amps without damage, and operate at temperatures above 60 degrees C. Their first cell is in production (1Q/2006) and being used in consumer products including DeWalt power tools, aviation products, automotive hybrid systems and PHEV conversions.

All these formulations involve new electrodes (anodes or cathodes). 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 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 December 2007, researchers at Stanford university reported creating a lithium ion battery with ten times the current capacity 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 eliminates the longstanding problem of cracking by the small size of the wires.[1]

See also

• Lithium battery
• Lithium iron phosphate battery - LiFePO₄

External links

• How Lithium-ion Batteries Work.
• The Lithium Ion Battery.
• Designing Multi-Cell Li-ion Battery Packs Using the ISL9208 Analog Front End.
• Advantages and Limitations of the Lithium Polymer Battery.
• Charging the Li-ion battery.
• Stanford's nanowire battery holds 10 times the charge of existing ones, Stanford Report, December 18, 2007