Lithium-ion
Polymer Batteries
This article is licensed
under the GNU
Free Documentation License. It uses material from the Wikipedia article
"Lithium-Ion
Polymer Battery".
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 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 lower cost manufacturing
and being more robust to physical damage. 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 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 they store more energy than nickel-cadmium (NiCd) and nickel metal hydride
(NiMH) batteries of the same volume.
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 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 are expected to have a dramatic effect on the power
tool and electric vehicle industries, and a major effect on consumer electronics.[citation
needed]
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.[
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. 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 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. 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 then (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, Lenovo's Thinkpad X300 and Ultrabay Batteries, 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.
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.
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:
Anode: Li or carbon-Li intercalation
compound
Separator: Conducting polymer electrolyte
Cathode: LiCoO2 or LiMnO4
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 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.
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 (mAh) to the capacity of the weakest cell.
It is important to recognize
that the cell mismatch results more from limitations in process control and
inspection than from variations inherent in the Lithium Ion chemistry. The use
of cell balancing can improve the performance of series connected Li-ion Cells
by addressing both SOC and C/E issues.[3] SOC mismatch can be remedied by balancing
the cell during an initial conditioning period and subsequently only during
the charge phase. C/E mismatch remedies are more difficult to implement and
harder to measure and require balancing during both charge and discharge periods.
Cell balancing is defined
as the application of differential currents to individual cells (or combinations
of cells) in a series string. Normally, of course, cells in a series string
receive identical currents. A battery pack requires additional components and
circuitry to achieve cell balancing. However, the use of a fully integrated
analog front end for cell balancing[4] reduces the required external components
to just balancing resistors.
This type of solution eliminates
the need for discrete capacitors, diodes and most other resistors to achieve
balance.
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. This is mostly a one shot fix.
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.
Copyright (c) 2008 Advanced
Battery Systems, Inc.
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entitled "GNU
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