Lithium polymer cell

Lithium polymer batteries (Li-Poly or LiPo) are rechargeable batteries which have technologically evolved from lithium ion batteries. Ultimatively 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. The solid polymer electrolyte is not flammable like the organic solvent that the Li-Ion cell uses. Thus these batteries are less hazardous if mistreated.

Cells sold today as polymer batteries are different from this ultimate design. Different from 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 to lithium ion cells is that in the latter cells, 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 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 and approximately three times better than NiCd and NiMH batteries.

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. 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 charge.

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-Po batteries typically require more than an hour for a full charge. Recent design improvements have increased maximum discharge currents from two times the cell capacity to 15 or even 20 times it. 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 should have a dramatic effect on the power tool and electric vehicle industries, and a major effect on consumer electronics; especially electrically-powered model aeroplanes.

When compared to the lithium ion battery, Li-Poly had a greater life cycle degradation rate. However, in recent years, manufacturers are 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.

The lithium in a Li-Poly cell is hazardous and will react violently with water. Cell disposal instructions usually suggest puncturing the cell and immersing it in salt water for several hours.

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. 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, where the lower weights offered can be sufficient justification for the price. However, lithium polymer-specific chargers and electronic motor speed controls are required to prevent excessive discharge and subsequent battery damage. They are also gaining ground in PDAs and laptop computers, where small form factors and energy density outweigh cost considerations.

Technology

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 ion-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 planning to use metallic Li as the anode, whereas others want 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 etc. The difference between the two technologies is that one (Bellcore/Telcordia technology) uses LiMn₂O₄ as the cathode, and the other, more conventional LiCoO₂.

Other more exotic however not yet commercially available Li-polymer batteries use a polymer cathode. For example, Moltec is developing a battery with a plastic conducting carbon-sulfur cathode. However, as of 2005 this technology seems to have 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.

External links

• Kokam's lithium ion polymer technical specifications (http://www.kokam.com/english/biz/rc.html)

• Worley Energy Cells (http://www.worley.com.au/v5/page.aspx?id=117)

• ITN Energy Systems - Lipo(no gel) lifetimes greater than 50,000 cycles (http://www.itnes.com/pages/battery_graph1.html#capacity_vs_cycles)

• Cymbet Corporation - 900 Wh/L Lip cells (http://www.cymbet.com/highlights.php)

• Sanyo - 500 charge cycles (http://www.sanyo.com/batteries/lithpol.cfm)de:Lithium-Polymer-Akku