A lithium polymer battery, or more correctly lithium-ion polymer battery, is a rechargeable
battery of lithium-ion technology in a pouch format.
Unlike cylindrical and prismatic cells, LiPos come in a soft package or pouch, which makes
them lighter but also lack rigidity.
The denomination "lithium polymer" has caused confusion among battery users.
It may be interpreted in two ways.
Originally, "lithium polymer" stood for a developing technology using a polymer electrolyte
instead of the more common liquid electrolyte.
The result is a "plastic" cell, which theoretically could be thin, flexible, and manufactured
in different shapes, without risk of electrolyte leakage.
This technology has not been fully developed and commercialized, and research is ongoing.
The second meaning appeared when some manufacturers started applying the "polymer" denomination
to lithium-ion cells in pouch format.
This is the most extended use nowadays, where "polymer" went from indicating a "polymer
electrolyte" to mean a "polymer casing", that is, the soft, external pouch.
While the design is usually flat, and lightweight, it is not a true polymer cell, as the electrolyte
is still in liquid form, albeit it may be "plasticized" or "gelled" through a polymer
additive.
These cells are sometimes known as "LiPo", however, from the technological point of view,
they are the same as the ones marketed simply as "Li-ion", as the underlying electrochemistry
is the same.
This article concerns the second, more extended meaning, while the first meaning is discussed
only in the last section.
The name "lithium polymer" is more widespread among users of radio-controlled models, where
it may indicate a single cell or a battery pack with cells connected in series or parallel.
The more general term "lithium-ion" is used almost everywhere else, including consumer
electronics such as mobile phones and notebook computers, and battery electric vehicles.
History
LiPo cells follow the history of lithium-ion and lithium-metal cells which underwent significant
research during the 1980s, reaching a significant milestone with Sony's first commercial cylindrical
Li-ion cell in 1991.
After that, other packaging techniques evolved, including the pouch format now also called
"LiPo".
Design origin and terminology The original kind of cell named "lithium polymer"
has technologically evolved from lithium-ion and lithium-metal batteries.
The primary difference is that instead of using a lithium-salt electrolyte held in an
organic solvent, the battery uses a solid polymer electrolyte such as poly(ethylene
oxide), poly(acrylonitrile), poly(methyl methacrylate) or poly(vinylidene fluoride).
The solid electrolyte can be typically classified as one of three types: dry SPE, gelled SPE
and porous SPE.
The dry SPE was the first used in prototype batteries, around 1978 by Michel Armand, Domain
University, and 1985 by ANVAR and Elf V Acquitaine of France, and Hydro Quebec of Canada.
From 1990 several organisations like Mead and Valence in the United States and GS Yuasa
in Japan developed batteries using gelled SPEs.
In 1996, Bellcore in the United States announced a rechargeable lithium polymer cell using
porous SPE, but without success in commercialization.
In parallel to the development of these "polymer electrolyte" batteries, the term "lithium
polymer" started being used for liquid electrolyte Li-ion cells in pouch format.
These cells started appearing in consumer electronics around 1995, eventually becoming
known as "LiPo" for some applications.
The confusion in the names may stem from the construction of the basic lithium-ion cell.
A typical cell has four main components: positive electrode, negative electrode, separator and
electrolyte.
The separator itself may be a polymer, such as a microporous film of polyethylene or polypropylene;
thus, even when the cell has a liquid electrolyte, it will still contain a "polymer" component.
In addition to this, the positive electrode can be further decomposed in three parts:
the lithium-transition-metal-oxide, a conductive additive, and a polymer binder of poly(vinylidene
fluoride).
The negative electrode material may have the same three parts, only with carbon replacing
the lithium-metal-oxide.
Therefore, even if a bare, unfinished cell lacks a polymer separator, or any liquid or
solid electrolyte, it may still have a "polymer" component in the active materials of the electrodes.
This polymer, however, is just a small fraction, typically less than 5% by weight, and does
not participate in the electrochemical reactions, being only useful for binding the active particles
together to maintain good conductivity, and help make the slurry mix adhere well to the
copper and aluminium foils that compose the current collectors of the battery cell.
Working principle
Just as with other lithium-ion cells, LiPos work on the principle of intercalation and
de-intercalation of lithium ions from a positive electrode material and a negative electrode
material, with the liquid electrolyte providing a conductive medium.
To prevent the electrodes from touching each other directly, a microporous separator is
in between which allows only the ions and not the electrode particles to migrate from
one side to the other.
Charging
Just as with other kinds of lithium-ion cells, the voltage of a LiPo cell depends on its
chemistry and varies from about 2.7-3.0 V to about 4.20-4.35 V, for cells based on lithium-metal-oxides,
and around 1.8-2.0 V to 3.6-3.8 V for those based on lithium-iron-phosphate.
The exact voltage ratings should be specified in product data sheets, with the understanding
that the cells should be protected by an electronic circuit that won't allow them to overcharge
nor over-discharge under use.
For LiPo battery packs with cells connected in series, a specialised charger may monitor
the charge on a per-cell basis so that all cells are brought to the same state of charge.
Applying pressure on LiPo cells
Unlike lithium-ion cylindrical and prismatic cells, which have a rigid metal case, LiPo
cells have a flexible, foil-type case, so they are relatively unconstrained.
By themselves the cells are over 20% lighter than equivalent cylindrical cells of the same
capacity.
Being lightweight is an advantage when the application requires minimum weight, such
as in the case of radio controlled models.
However, it has been investigated that moderate pressure on the stack of layers that compose
the cell results in increased capacity retention, because the contact between the components
is maximised and delamination and deformation is prevented, which is associated with increase
of cell impedance and degradation.
Applications
A compelling advantage of LiPo cells is that manufacturers can shape the battery in different
shapes, which can be important in applications requiring a small footprint for the energy
storage system, like thinner and lighter mobile phones and notebook computers.
Radio controlled equipment and Airsoft
LiPo batteries are gaining favour 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.
As of the beginning of 2013, LiPo packs of 1.3 Ah exist, providing 45C continuous discharge,
and short-time 90C bursts.
Bigger packs of 4.5 Ah may feature discharge rates of 70C, with 140C bursts.
LiPo packs also see widespread use in airsoft, where their higher discharge currents and
better energy density compared to more traditional NiMH batteries has very noticeable performance
gain.
The high discharge currents do damage the switch contacts due to arcing, so it is advised
to either use a solid-state MOSFET switch or clean the trigger contacts regularly.
Personal electronics LiPo batteries are pervasive in mobile phones,
tablet computers, very thin laptop computers, small portable media player, wireless controllers
for video game consoles, electronic cigarettes, and other applications where small form factors
are sought and the high energy density outweighs cost considerations.
Electric vehicles Lithium-ion cells in pouch format are being
investigated to power battery electric vehicles.
While it is possible to use a large number of cells of small capacity to obtain required
levels of power and energy to drive a vehicle, some manufacturers and research centres are
looking into large-format lithium-ion cells of capacities exceeding 50 Ah for this purpose.
With higher energy content per cell, the number of cells and electrical connections in a battery
pack may decrease, at the expense of increasing safety risks associated with handling of an
individual cell of such high capacity.
Hyundai Motor Company uses this battery type in some of its hybrid electric vehicles.
Safety
LiPo cells are affected by the same problems as other lithium-ion cells.
This means that overcharge, over-discharge, over-temperature, short circuit, crush and
nail penetration may all result in a catastrophic failure, including the pouch rupturing, the
electrolyte leaking, and fire.
All Li-ion cells expand at high levels of state of charge or over-charge, due to slight
vaporisation of the electrolyte.
This may result in delamination, and thus bad contact of the internal layers of the
cell, which in turn brings diminished reliability and overall cycle life of the cell.
This is very noticeable for LiPos, which can visibly inflate due to lack of a hard case
to contain their expansion.
Compared to cylindrical Li-ion cells, LiPos lack integrated safety devices such as a current
interrupting device or a positive temperature coefficient material that is able to protect
against an over-current or an over-temperature.
Lithium cells with true polymer electrolyte Although the name "lithium polymer" is mostly
applied to lithium-ion cells in pouch format, which still contain a liquid electrolyte,
there are electrochemical cells with actual polymer electrolytes, which however have not
reached full commercialization and are still a topic of research.
Prototype cells of this type could be considered to be between a traditional lithium-ion battery
and a completely plastic, solid-state lithium-ion battery.
The simplest approach is to use a polymer matrix, such as polyvinylidene fluoride or
poly(acrylonitrile), gelled with conventional salts and solvents, such as LiPF6 in ECDEC.
Nishi mentions that Sony started research on lithium-ion cells with gelled polymer electrolytes
in 1988, before the commercialisation of the liquid-electrolyte lithium-ion cell in 1991.
At that time polymer batteries were promising and it seemed polymer electrolytes would become
indispensable.
Eventually, this type of cell went into the market in 1998.
However, Scrosati argues that, in the strictest sense, gelled membranes cannot be classified
as "true" polymer electrolytes, but rather as hybrid systems where the liquid phases
are contained within the polymer matrix.
Although these polymer electrolytes may be dry to the touch, they can still contain 30%
to 50% liquid solvent.
In this regard, an open question remains on how to really define what a "polymer battery"
is.
Other terms used in the literature for this system include hybrid polymer electrolyte,
where "hybrid" denotes the combination of the polymer matrix, the liquid solvent and
the salt.
It was a system like this that Bellcore used to develop an early lithium-polymer cell in
1996, which was called "plastic" lithium-ion cell, and subsequently commercialised in 1999.
A solid polymer electrolyte may be, for example, a compound of lithium bis(fluorosulfonyl)imide
and high molecular weight poly(ethylene oxide), or a high molecular weight poly(trimethylene
carbonate).
The performance of these proposed electrolytes is usually measured in a half-cell configuration
against an electrode of metallic lithium, making the system a "lithium-metal" cell,
but it has also been tested with a common lithium-ion cathode material such as lithium-iron-phosphate.
Other attempts to design a polymer electrolyte cell include the use of inorganic ionic liquids
such as 1-butyl-3-methylimidazolium tetrafluoroborate as a plasticizer in a microporous polymer
matrix like poly(vinylidene fluoride-co-hexafluoropropylene)/poly(methyl methacrylate).
See also Lithium-ion, for more complete information
on this type of battery Lithium iron phosphate battery
Lithium–air battery References
External links 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 Battery Guide
Proper R/C Li-Po Battery Disposal