Lithium-ion polymer battery

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Battery specifications

A prototype Lithium-Ion Polymer Battery at NASA Glenn Research Center
Energy/weight 130–200 Wh/kg[citation needed]
Energy/size 300 Wh/L[citation needed]
Power/weight up to 7,1 kW/kg[citation needed]
Charge/discharge efficiency 99.8%
Energy/consumer-price 2.8-5 Wh/US$[citation needed]
Self-discharge rate 5%/month[citation needed]
Time durability 24-36 months
Cycle durability >1000 cycles
Nominal Cell Voltage 3.7 V

Lithium-ion polymer batteries, polymer lithium ion, or more commonly lithium polymer batteries (abbreviated Li-poly, Li-Pol, LiPo, LIP, PLI or LiP) are rechargeable batteries (secondary cell batteries). Normally batteries are composed of several identical secondary cells in parallel addition to increase the discharge current capability.

Contents

[edit] Design origin

This type has 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 lower cost manufacturing and being more robust to physical damage. Lithium-ion polymer batteries started appearing in consumer electronics around 1996.

[edit] 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 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 classic Li-ion battery.

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 full charge and may experience problems holding voltage under load.

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 December 2007 Toshiba announced a new design offering a much faster rate of charge (about 5 minutes to reach 90%). These cells were released onto the market in March 2008 and are expected to have a dramatic effect on the power tool and electric vehicle industries, and a major effect on consumer electronics. [1]

When compared to the lithium-ion battery, Li-poly has 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.[citation needed]

[edit] Applications

A compelling advantage of Li-poly cells 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.

3-Cell LiPo for RC-models

Li-poly batteries are also gaining favor in the world of Radio-controlled aircraft as well as Radio-controlled cars, where the advantages of both lower weight and greatly increased run times can be sufficient justification for the price. Some airsoft gun owners have switched to LiPo batteries due to the above reasons and the increased rate of fire they provide. 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 passes through the cell in an instant. Radio-control enthusiasts take special precautions to ensure their battery leads are properly connected and insulated. Furthermore fires can occur if the cell or pack is punctured. Radio-controlled car batteries are often protected by durable plastic cases to prevent puncture. Specially designed electronic motor speed controls are used to prevent excessive discharge and subsequent battery damage. This is achieved using a low voltage cutoff (LVC) setting that is adjusted to maintain cell voltage greater than (typically) 3 V per cell.

Li-poly batteries are also gaining ground in PDAs and laptop computers, such as Apple's MacBook, MacBook Pro, and Macbook Air, Amazon's Kindle, Lenovo's Thinkpad X300 and Ultrabay Batteries, the OQO series of palmtops, the HP Mini and Dell products featuring D-bay batteries. They can be found in small digital music devices such as iPods and other MP3 players as well as gaming equipment like Sony's Playstation 3 wireless controllers[2]. They are desirable in applications where small form factors and energy density outweigh cost considerations.

[edit] Electric vehicles

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.

Hyundai Motor Company plans to use this battery type in its hybrid electric vehicles.[citation needed]

[edit] Technology

There are currently two commercialized technologies, both lithium-ion-polymer (where "polymer" stands for "polymer electrolyte/separator") cells. These are collectively referred to as "polymer electrolyte batteries".

The battery is constructed as:

Typical reaction:

  • Anode: carbon–Lix → C + xLi+ + xe−
  • Separator: Li+ conduction
  • Cathode: Li1−xCoO2 + xLi+ + xe− → LiCoO2

Polymer electrolytes/separators can be solid polymers (e.g., polyethyleneoxide, PEO) plus LiPF6, or other conducting salts plus SiO2, or other fillers for better mechanical properties (such systems are not available commercially yet). Some manufacturers like Avestor (since merged with Batscap) are using metallic Li as the anode (these are the Lithium-metal-polymer batteries), whereas others wish 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. The difference between the two technologies is that one (Bellcore/Telcordia technology) uses LiMn2O4 as the cathode, and the other the 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 had 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 conductive 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.

[edit] Prolonging life in multiple cells through cell balancing

Main article: Battery (electricity)

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.

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

  • 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.
  • 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.

[edit] Capacity Rating

Cell's capacities are rated in Ampere Hours (Ah) or milliamp Hour (mAh). A 1000 mAh battery is the same as a 1 Ah battery, both will supply 1 Amp for 1 hour. The C rating commonly associated with lithium ion batteries refers to the maximum current supply capability as a multiple of the cell's capacity e.g. a 1 Ah, 20C battery should be able to supply 20 A continuously without damage. A useful way to calculate how long a battery will last for under heavy load is to multiply the Ah capacity by 60 to give Ampere-minutes; so a 1 Ah battery becomes a 60 Ampere-minute battery; to calculate how many minutes the battery will last, just divide by the average current drawn; e.g. a 10A average current draw will mean that a 60 Ampere-minute battery will last for 60 / 10 = 6 minutes.

An Ampere Hour is equal to 3600 Coulombs, the standard SI unit for amount of electrical charge. The amount of energy stored depends on the voltage, and is equal to the Voltage * the charge in coulombs, or 3600 * the voltage * the charge in Ampere-hours. So, a 10Ah battery pack with a nominal voltage of 10 volts would deliver 36,000 Coulombs of charge (or 2.2 x 1023 electrons), and 360 kJ of energy.

[edit] Charging

LiPoly batteries must be charged carefully. The basic algorithm is to charge at constant current (1C to 2C depending on manufacturer) until each cell reaches 4.2 V, the charger must then gradually reduce the charge current while holding the cell voltage at 4.2 volts until the charge current has dropped to 10% of the initial charge rate at which point the battery is considered 100% charged.

Balance charging simply means that the charger monitors the voltage of each cell in a pack and varies the charge on a per-cell basis so that all cells are brought to the same voltage.

The charge should not be terminated on reaching a cell voltage of 4.2 V because the capacity reached at that point is only 70% of full capacity; charging at the reducing current necessary to hold the cell voltage at or very near 4.2 V must be continued until the charge current drops to 10% of the initial charge rate.

It is important to note that trickle charging is not acceptable for lithium batteries; Li-ion chemistry cannot accept an overcharge without causing damage to the cell, possibly plating out lithium metal and becoming hazardous. Most manufacturers claim a maximum and minimum voltage of 4.23 and 3.0 volts per cell. Taking any cell outside these limits can reduce the cell's capacity and ability to deliver full rated current.

Most dedicated lithium polymer chargers use a charge timer for safety; this cuts the charge after a predefined time (typically 90 minutes).

[edit] Storage

Unlike NiCad and NiMH, lithium ion batteries can be stored for 1 or 2 months without significantly losing charge. However, if storing for long periods, manufacturers recommend discharging the battery to 3.7 to 3.8 volts per cell and storing in a cool place.[citation needed]

[edit] See also

[edit] References

  1. ^ Toshiba (2007-12-11). Toshiba to Launch Innovative Rechargeable Battery Business. Press release. http://www.toshiba.co.jp/about/press/2007_12/pr1101.htm. Retrieved on 2009-06-25. 

[edit] External links

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Galvanic cells
Non-rechargeable:
primary cells		 Galvanic cell
Rechargeable:
secondary cells
Kinds of cells
Parts of cells
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