Lithium ion polymer batteries or more commonly lithium polymer batteries.docx

1、Lithium ion polymer batteries or more commonly lithium polymer batteriesLithium ion polymer batteryLithium 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. Ultimat

2、ely, the lithium salt electrolyte is not held in an organic solvent like in the proven lithium ion design, but in a solid polymer composite such as PVDF or polyacrylonitrile. There are many advantages of this design over the classic lithium ion design, including the fact that the solid polymer elect

3、rolyte is not flammable (unlike the organic solvent that the Li-Ion cell uses); thus, these batteries are less hazardous if mistreated. Lithium ion polymer batteries started appearing in consumer electronics around 1996. Overview Cells sold today as polymer batteries have a different design from the

4、 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 l

5、atter 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

6、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 approxim

7、ately 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 combinat

8、ion. 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

9、 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 improvemen

10、ts 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

11、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, manu

12、facturers 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 A compelling advantage of Li-Poly is that ma

13、nufacturers 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 discha

14、rge 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 ex

15、plosion. Explosions can also occur if the battery wires are crossed under load. 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 subseq

16、uent 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 Apples MacBook and small digital music devices such as iPods and othe

17、r MP3 players, as well as portable gaming devices like the Sony PSP or Nintendos 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

18、 prohibitive, but proponents argue that with increased production, the cost of Li-Poly batteries will go down. Technology There are currently two commercialized technologies, both lithium-ion-polymer (where polymer stands for polymer electrolyte/separator). They are called polymer electrolyte batter

19、ies. 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 dont r

20、equire 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 LiMn2O4) Typical reaction: Anode: carbon-Li(x) - xLi+

21、- xe Separator: Li+ conduction Cathode: Li(1-x)CoO2 + xLi+ + xePolymer 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 use m

22、etallic 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/Telc

23、ordia technology) uses LiMn2O4 as the cathode, and the other, more conventional LiCoO2. Failed Technologies: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 ca

24、thode. 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 pow

25、er capability (i.e. low internal resistance) and high discharge capacity, but has problems with cycleability and cost. Promising Technologies:The next generation rechargeable battery lithium-ion polymer battery (LiPB) has arisen to meet the demand for increased power density and smaller size with sa

26、fety and cost foremost in mind. Until now, commonly used batteries include the nickel-cadmium (NiCd) and the nickel-metal hydride (NiMH) design typical in cellular phones or even the newer lithium-ion (Li-ion) batteries, powering higher-end devices. Lithium-ion polymer batteries, when high-volume su

27、pplies become available, are expected to challenge and replace NiMH and Li-ion batteries as the preferred power source for portable electronic devices. LiPB technology addresses the most important issues of the rechargeable battery market: power density, size, design flexibility and safety. Figure 2

28、. A type of LiPB battery schematicLiPB technology reveals an exciting new battery structure, discarding the conventional metal-can assembly. The most striking difference between LiPB and Li-ion is that the former uses a solid polymer electrolyte rather than a liquid electrolyte solution. The solid p

29、olymer electrolyte is physically a solid but appears to the ions as a liquid that they can pass through. With no liquid to escape, the solid electrolyte is simply sandwiched between electrodes ( the anode and cathodes formed out of thin sheets). Then the cell is contained in laminated foil and seale

30、d at the edges to form an entire battery. The resulting cell is extremely thin and as flexible as a rubber mat. Using a solid electrolyte is quite appealing because it is naturally spill proof, more resilient under pressure, and capable of being engineered into most any shape because a thin laminate

31、d Figure 2. LiPB battery charge/discharge profilesfoil material, rather than an unyielding metal can, houses each cell. Therefore, lithium-ion polymer batteries are lighter, thinner, flexible and leak resistant thus safer. LiPB Characteristics Thin and Lightweight LiPB technology offers the greatest

32、 energy to size/weight ratio for rechargeable batteries. Batteries as thin as a 0.4 mm credit card with the same power capabilities as standard battery packs on cellular phones have appeared. Lithiums high electrochemical reactivity has made it difficult to work with in the past, yet also why it has held so much potential. Therefore, Lithium batteries are made from Lithium ions rather than Lithium in its natural state. The high energy density of Lithium means less physical material needs to be used, thus reducing battery size and weight, and making it ideal for use in compact electronic de