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Lithium-ion battery profile
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Lithium-ion battery profile

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 self-discharge 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.

 

Battery specifications

Energy/weight

>160 Wh/kg

Energy/size

>350 Wh/L

Power/weight

>1800 W/kg

Charge/discharge efficiency

99.9%

Self-discharge rate

5%-10%/month

Time durability

(24-36) months

Cycle durability

>500 cycles

Nominal Unit Cell Voltage

3.6 / 3.7 V

Lithium-ion battery 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 lithium ion battery electric car models in 2007.

Lithium-ion battery advantages

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%-10% 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.

Lithium-ion battery 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)

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 5%-10% 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.

Lithium-ion battery 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:

LiCoO2 + C6 Li1-xCoO2 + LixC6

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 LixCoO2 being oxidized from Co3+ to Co4+ during charging、and reduced from Co4+ to Co3+ during discharge.

Solid electrolyte interface

A particularly important element for activating Li-ion batteries is the solid electrolyte interface (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.

Lithium-ion polymer battery profile

Lithium-ion polymer batteries、or more commonly lithium polymer batteries (abbreviated Li-poly or LiPo) are rechargeable batteries which have technologically evolved from lithium-ion batteries. Ultimately、the lithium-salt electrolyte is not held in an organic solvent as in the lithium-ion design、but in a solid polymer composite such as polyethylene oxide or polyacrylonitrile.

The advantages of Li-poly over the lithium-ion design include and being more robust to physical damage. Lithium-ion polymer batteries started appearing in consumer electronics around 1996.

Early in its development、lithium polymer technology had problems with internal resistance. 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 March 2005 Toshiba announced a new design offering a much faster (about 1–3 minutes) rate of charge. These cells have yet to reach the market but should have a dramatic effect on the power tool and electric vehicle industries、and a major effect on consumer electronics.

 

Battery specifications

Energy/weight

130-200 Wh/kg

Energy/size

>300 Wh/L

Power/weight

up to 2800 W/kg

Charge/discharge efficiency

99.8%

Self-discharge rate

5%-10%/month

Time durability

(24-36) months

Cycle durability

>500 cycles

Nominal Unit Cell Voltage

3.6 / 3.7 V