Lithium polymer cell: Meaning (information, definition, explanation, facts)

Lithium polymer batteries are rechargeable batteries which have technologically evolved from lithium ion batteries. 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 polyacrylonitrile.

There are many advantages of this design over the classic lithium ion design. Because of the solid polymer electrolyte there is no need for the organic flammable solvent. Thus these batteries are less hazardous if mistreated. Furthermore, 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 the holes 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.

Still early in its development, Lithium Polymer technology still has problems with internal resistance and life cycle. Other challenges include longer charge times and the slower maximum discharge rates compared to more mature technologies.

Applications

A compelling advantage of Li-Poly 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.

Li-Poly batteries are also gaining favor is in the world of radio controlled aircraft, where the lower weights offered can be sufficient justification for the price. They are also gaining ground in PDAs and laptop computers, where small form factors and energy density outweigh cost considerations.

The currently commercialized ones are two technologies (both actually Li-ion-polymer) where "polymer" stands for "polymer electrolyte/separator". Let's call them "polymer electrolyte batteries"

The idea is to use an ionically 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 don't require pressure applied to "sandwich" cathode+anode together. Polymer electrolyte seals both electrodes together like a glue.

Design is: anode (Li or carbon-Li intercalation compound)/conducting polymer electrolyte-separator/cathode (LiCoO2 or LiMn2O4)

Typical reaction:

• anode: carbon-Li(x) - xLi+ - xe
• separator: Li+ conduction
• cathode: Li(1-x)CoO2 + xLi+ + xe

Polymer electrolyte/separator can be real solid polymer (polyethyleneoxide, PEO)+LiPF6 or other conducting salt +SiO2 or other filler for better mechanical properties (such systems are not available commercially yet). Some are planing to use metallic Li as anode, whereas others want to go with proven safe carbon intercalation anode.

Both currently commercialized technologies are using PVdF (polymer) gelled with conventional solvent+salt, like EC/DMC/DEC etc. Difference between two technologies is that one (Belcore technology) is using LiMn2O4 as cathode, and other more conventional LiCoO2.

Other more exotic not yet commercially available Li-polymer batteries actually are using polymer cathode, so we could call them "real polymer batteries". For example Moltec is promising to deliver a battery with a plastic conducting carbon-sulfur cathode. They seem to have problems with self-discharge and manufacturing cost.

Yet another proposal is to use organic sulfur containing compounds as cathode in combination with electrically conducting polymer (polyaniline for example). This promises high power capability (e.g. low internal resistance) and high discharge capacity, but have problems with cycleability and first of all - production cost.