Lithium polymer battery

Lithium polymer battery

A Lithium-Ion Polymer Battery used to power a mobile phone
Specific energy	130–200 W·h/kg[citation needed]
Energy density	300 W·h/L[citation needed]
Specific power	up to 10 kW/kg (continuous)[1]
Charge/discharge efficiency	99.8%[citation needed]
Energy/consumer-price	1.5-1.7US$/A·h[2] (2.2-2.5 W·h/US$)
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 (secondary cell) batteries. LiPo batteries are usually composed of several identical secondary cells in parallel to increase the discharge current capability, and are often available in series "packs" to increase the total available voltage.

History

Design origin

This type has technologically evolved from lithium-ion batteries. The primary difference is that the lithium-salt electrolyte is not held in an organic solvent but in a solid polymer composite such as polyethylene oxide or polyacrylonitrile. The advantages of Li-ion polymer over the lithium-ion design include potentially lower cost of manufacture,^{[citation needed]} adaptability to a wide variety of packaging shapes, reliability, and ruggedness, with the disadvantage of holding less charge. Lithium-ion polymer batteries started appearing in consumer electronics around 1995.

Technology

An experimental lithium-ion polymer battery made by Lockheed-Martin for NASA

Cells sold today as polymer batteries are pouch cells. Unlike lithium-ion cylindrical cells, which have a rigid metal case, pouch cells have a flexible, foil-type (polymer laminate) case. In cylindrical cells, the rigid case presses the electrodes and the separator onto each other; whereas in polymer cells this external pressure is not required (nor often used) because the electrode sheets and the separator sheets are laminated onto each other. Since individual pouch cells have no strong metal casing, by themselves they are over 20% lighter than equivalent cylindrical cells.

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.

Early in its development, lithium polymer technology had problems with high internal resistance. Other challenges included longer charge times and lower maximum discharge rates compared to more mature technologies. 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 were expected to have a dramatic effect on the power tool and electric vehicle industries, and a major effect on consumer electronics.^[3] Recent design improvements have increased maximum discharge currents from 2 times to 65 or even 90 times the cell capacity charge per hour.

In recent years, manufacturers have declared 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]}

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.

Some airsoft gun owners have switched to LiPo batteries due to the above reasons and the increased rate of fire they provide.

Radio controlled equipment

3-Cell LiPo battery for RC-models

Li-po batteries are gaining favor in the world of radio-controlled aircraft, radio-controlled cars and large scale model trains, where the advantages of lower weight and increased capacity and power delivery justify the price. A low voltage cutoff (LVC) setting maintains cell voltage greater than (typically) 3.2 V per cell under load.^[4]

As of the beginning of 2013, Li-po products are widespread. They feature charge rates up to 5~15C, and 65C continuous discharge rates, with 130C burst discharge,^[5] although smaller batteries with lower C-ratings, (45C continuous discharge, 90C burst) are more common,^[6] while still delivering 250 cycles.^[7] Vendors include Hyperion,^[4] HobbyKing,^[8] ThunderPower^[9] and BananaHobby.^[10]

Personal electronics

Li-poly batteries are also gaining ground in PDAs, laptop computers and cell phones. Very small GPS tracking units rely on li-poly batteries for days or even weeks of autonomous operation between recharges. Li-poly batteries are also used in small portable media player and tablet computers, as well as wireless controllers for video game consoles. Li-poly batteries are a popular choice for electronic cigarettes. They are desirable in applications where small form factors and energy density outweigh cost considerations.

Electric vehicles

These batteries may also power the next generation of battery electric vehicles. The cost of an electric car of this type is currently significantly higher than of a petrol car, but it is likely that with increased production and technological advances, the cost of Li-poly batteries will go down.

Hyundai Motor Company uses this battery type in some of its hybrid electric vehicles.^[11] On 26 October 2010, a Li-poly powered Audi A2 covered the record distance of 600 km without recharging.^[12] From April 2011 batteries of this type for output exceeding one Megawatt have been responsible for a number of world speed records in drag racing.^[13]

Risks and limitations

A photograph of an Apple iPhone 3GS's Lithium-ion polymer battery, which has expanded due to a short circuit failure.

• All Li-Ion cells expand at high levels of state of charge (SOC); if uncontained, this may result in delamination, and reduction of reliability and cycle life; the case of cylindrical cells provides that containment, while pouch cells, by themselves, are not contained. Therefore, to achieve the rated performance, a battery composed of pouch cells must include a strong external casing to retain its shape.
• Overcharging a Li-poly battery can cause an explosion 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. Li-poly batteries can be protected by circuitry that prevents over-charge and deep-discharge.
• Compared to the lithium-ion battery, Li-poly is less advantageous in terms of life cycle degradation rate.
• Lithium polymer-specific chargers are required in order 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.^[14]
• While charging the lithium polymer batteries, the individual cells in the pack should be charged evenly. For this purpose, the cells are to be charged with special chargers. This entails special care while charging the batteries in addition to incurring expenses on procuring the chargers specific to lithium polymer batteries.

Technical specifications

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:

• positive electrode: LiCoO₂ or LiMn₂O₄
• Separator: Conducting polymer electrolyte (e.g. polyethyleneoxide, PEO)
• negative electrode: Li or carbon-Li intercalation compound

Typical reaction: (Discharge)

• Negative electrode: (Carbon-Li_x) → C + xLi⁺ + xe⁻
• Separator: Li⁺ conducting
• Positive electrode: Li_1−xCoO₂ + xLi⁺ + xe⁻ → LiCoO₂
• Overall reaction: (Carbon-xLi⁺ + xe⁻) + Li_1-xCoO₂ → LiCoO₂ + Carbon

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 lithium as the negative electrode (these are the lithium-metal polymer batteries), whereas others wish to go with the proven safe carbon intercalation negative electrode.

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 positive electrode, and the other the more conventional LiCoO₂.

Although not yet commercially available, other, more exotic Li-polymer batteries use a polymer positive electrode. For example, Moltech is developing a battery with a plastic conducting carbon-sulfur positive electrode. 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 positive electrode 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.

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 (mA·h) 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.

Charging

LiPoly batteries must be charged carefully. The basic process is to charge at constant current until each cell reaches 4.2 V. The charger must then switch to a constant voltage mode, and the charging current will gradually reduce while the charger holds the cell voltage at 4.2 V. This will continue until the charge current has dropped to a small percentage of the initial charge rate, at which point the battery is considered fully charged. Some manufacturers specify 2%, others 3%, but other values are also possible. The difference in achieved capacity is minute.

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.

Trickle charging is not recommended for lithium batteries.^[15] Most manufacturers claim a maximum and minimum voltage of 4.23 and 3.0 volts per cell, respectively. 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).

As of the beginning of 2013, charging rates of up to 15C (i.e., 15 times the capacity of the battery, or approximately 4-minute charge times) are possible in the relatively new (circa 2009) breed of nanowire technology LiPo batteries. This, however, is the exception to the rule, as the more common 1C charge rate still stands as the recommended standard among RC users. It is also important to note that regardless of the charge rate that a battery can handle, using the lower 1C charge rate will always increase the longevity of any RC LiPo battery ^{[citation needed]}.

Discharging

Also as of mid 2013, discharge rates of up to 70C continuous (i.e.: 70 times the capacity of the battery) and 140C burst are possible (see "Radio controlled equipment" section above). Both of these "C-ratings" are expected to increase as the nanowire battery technology (presumably used in these batteries) continues to mature, and manufacturers and users alike continue to refine their processes and push the limits of this high-performance breed of LiPo battery.

See also

• Thin-film
• Lithium–air battery

References

External links

Wikimedia Commons has media related to Lithium-polymer batteries.

• Electropaedia on Lithium Battery Manufacturing
• Electropaedia on Lithium Battery Failures
• Designing Multi-Cell Li-ion Battery Packs Using the ISL9208 Analog Front End.
• AT&T To Replace 17,000 Batteries
• ProtoTalk.net - Lithium Polymer (Lipo) Battery Guide
• ThunderPower's Safety Warnings
• Proper R/C Li-Po Battery Disposal

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