Lithium/Carbon Monofluoride Batteries


Another interesting primary battery is the one using polycarbon fluoride, (CFx)n, as a cathode. This compound, synthesized by direct fluorination of carbon with fluorine gas, has an x value in the range 0.9–1.2, and useful cathode materials have x1. In the following, this cathode material will simply be written as CFx (where x=1 normally).

In large cells, this system has the highest energy, both on a gravimetric and a volumetric basis, of all primary Li batteries. This high energy is maintained at high temperatures. The performance is also good at low temperatures, but at 40C a remarkable voltage drop is observed.

The formation of carbon enhances the electronic conductivity of the cathode. Typical electrolytes useful for this system are LiAsF6 in butyrolactone (BL) or LiBF4 in PC-DME.

Apart from the energy, the Li/CFx system has a number of pleasant features. Its operating voltage is flat and high (2.8V), its capacity is quite high at low-moderate drains, its useful temperature range is wide (–40 to þ85C, and up to 125C for some cells), and its self-discharge rate is the lowest of any primary Li cell. This last feature is particularly notable: a Li/CFx cell loses <0.5% capacity after 1 year of storage at room temperature, and less than 4% per year at 70C. In contrast, this system is not characterized by high power and is better used at low rates. Cost is another weak point – twice that of Li/MnO2 cells.

Pin, coin, cylindrical and prismatic cells are available, with capacities ranging from 25mAh to 5Ah. The pin-type cells use the inside-out design with a cylindrical cathode and a central Li anode. Coin-type cells have a Li foil rolled onto a Cu net, while the cathode, also containing Teflon and acetylene black, is supported on a Ni net. The cylindrical cells mainly use the spirally wound configuration. Prismatic cells are built in high-capacity sizes (typically 40 Ah).

The applications of these cells depend on their design:

• Coin type: low-power consuming cordless appliances, memory backup.

• High-temperature coin type: automotive electronic systems, toll-way transponders, radio frequency identification (RFID).

• Cylindrical type: utility meters, emergency signal lights, electric locks, electronic measurement equipment.

• Prismatic type: biomedical and space systems.

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This article was sent to us by: Giani Rimeollo at 06272010

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