Lithium ion polymer battery

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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 polyacrylonitrile. There are many advantages of this design over the classic lithium ion design, including the fact that the solid polymer electrolyte is not flammable (unlike the organic solvent that the Li-Ion cell uses). Lithium ion polymer batteries started appearing in consumer electronics around 1996.

Overview

Cells sold today as polymer batteries have a different design from the older lithium ion cells. Unlike lithium ion cylindrical, or prismatic cells, which have a rigid metal case, polymer cells have a flexible, foil-type (polymer laminate) case, but they still contain organic solvent. The main difference between commercial polymer and lithium ion cells is that in the latter cells, the rigid case presses the electrodes and the separator onto each other, whereas in polymer cells this external pressure is not required because the electrode sheets and the separator sheets are laminated onto each other.

Since no metal battery cell casing is needed, the battery can be lighter and it can be specifically shaped to fit the device it will power. Because of the denser packaging without intercell spacing between cylindrical cells and the lack of metal casing, the energy density of Li-Poly batteries is over 20% higher than that of a classical Li-Ion battery and approximately three times better than NiCd and NiMH batteries.

The voltage of a Li-Poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged), and Li-Poly cells have to be protected from overcharge by limiting the applied voltage to no more than 4.235 V per cell used in a series combination. Overcharging a Li-Poly battery will likely result in explosion and/or fire. During discharge on load, the load has to be removed as soon as the voltage drops below approximately 3.0 V per cell (used in a series combination), or else the battery will subsequently no longer accept a charge.

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-Po 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; especially electrically powered model aircraft.

When compared to the lithium ion battery, Li-Poly had a greater life cycle degradation rate. However, in recent years, manufacturers have been declaring upwards of 500 charge-discharge cycles before the capacity drops to 80% (see Sanyo). Another variant of Li-Poly cells, the "thin film rechargeable lithium battery" has been shown to provide more than 10,000 cycles.

Applications

File:Nokia Battery Hologram.jpg
A rechargeable lithium polymer Nokia mobile phone battery.

A compelling advantage of Li-Poly is that manufacturers can shape the battery almost however they please, which can be important to mobile phone manufacturers constantly working on smaller, thinner, and lighter phones. Another advantage of lithium polymer cells over nickel cadmium and nickel metal hydride cells is that the rate of self discharge is much lower.

Li-Poly batteries are also gaining favor in the world of radio-controlled aircraft, where the advantages of both lower weight and greatly increased run times can be sufficient justification for the price. However, lithium polymer-specific chargers are required to avoid fire and explosion. Explosions can also occur if the battery is short circuited as tremendous current would be available for a short time. Radio control enthusiasts take special precautions to ensure their battery leads are properly connected and insulated. Specially designed electronic motor speed controls are used to prevent excessive discharge and subsequent battery damage. This is achieved using a Low Voltage Cut-off (LVC) setting, that is adjusted to maintain cell voltage at (typically) 3v per cell.

Li-poly batteries are also gaining ground in PDAs and laptop computers, such as Apple's MacBook and small digital music devices such as iPods and other MP3 players, as well as portable gaming devices like the Sony PSP or Nintendo's Game Boy Advance SP, where small form factors and energy density outweigh cost considerations.

These batteries may also power the next generation of battery electric vehicles. The cost of an electric car of this type is prohibitive, but proponents argue that with increased production, the cost of Li-Poly batteries will go down.

Canadian company BionX had been supplying electric conversion kits for some time that used brushless motors and Nickel Metal Hydride batteries, but during 2006, they introduced batteries that used Lithium Ion technology. During 2007 Urban Mover and PowaCycle in the UK, introduced the first commercially available Li-Poly powered electric bikes, as opposed to just supplying conversion kits.

Technology

There are currently two commercialized technologies, both lithium-ion-polymer (where "polymer" stands for "polymer electrolyte/separator"). They are called "polymer electrolyte batteries".

The idea is to use an ion-conducting polymer instead of the traditional combination of a microporous separator and a liquid electrolyte. This promises not only better safety, as polymer electrolyte does not burn as easily, but also the possibility to make battery cells very thin, as they don't require pressure applied to "sandwich" cathode+anode together. Polymer electrolyte seals both electrodes together like a glue.

The design is: anode (Li or carbon-Li intercalation compound)/conducting polymer electrolyte-separator/cathode (LiCoO2 or LiMnO4)

Typical reaction:

Polymer electrolyte/separator can be real solid polymer (polyethyleneoxide, PEO) plus LiPF6 or other conducting salt plus SiO2 or other filler for better mechanical properties (such systems are not available commercially yet). Some are planning to use metallic Li as the anode, whereas others want to go with the proven safe carbon intercalation anode.

Both currently commercialized technologies use PVdF (a polymer) gelled with conventional solvents and salts, like EC/DMC/DEC etc. The difference between the two technologies is that one (Bellcore/Telcordia technology) uses LiMnO4 as the cathode, and the other, more conventional LiCoO2.

Other, more exotic (although not yet commercially available) Li-polymer batteries use a polymer cathode. For example, Moltech is developing a battery with a plastic conducting carbon-sulfur cathode. However, as of 2005 this technology seems to have problems with self-discharge and manufacturing cost.

Yet another proposal is to use organic sulfur containing compounds for the cathode in combination with an electrically conducting polymer such as polyaniline. This approach promises high power capability (i.e. low internal resistance) and high discharge capacity, but has problems with cycleability and cost.

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