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Lithium Rechargeable Batteries

Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy content. Rechargeable batteries using lithium metal as an electrode are capable of providing both high voltage and excellent capacity, resulting in an extraordinary energy density.

The development of secondary batteries employing lithium metal as the negative electrode has been plagued with safety problems. The positive electrode of these systems is usually a metal or sulfide intercalation compound, but there are also liquid cathode rechargeable batteries using SO2 or SO2Cl2. The major problem with all of these systems is the metallic lithium. Upon cycling, the lithium that is deposited during charging eventually tends to form long "tentacles, called dendrites. These can grow through the separator and short out to the positive electrode. When this happens an explosion often results. The reason for a thin dendrite leading to an explosion lies in the low melting point of lithium, which is 180oC. This means that a shorted dendrite can melt, and molten lithium is violently reactive and is no longer protected by a passivating layer. The severity of an explosion depends somewhat on the nature of the electrolyte.

A Canadian company, Moli Energy of Vancouver, was the first to mass-produce lithium metal rechargeable batteries. These were used in portable telephones in Japan and had been shown to be safe in the laboratory, using a typical duty cycle. After much research during the eighties, it was found that occasional shorts from lithium dendrites could cause thermal run-away. The cell temperature quickly approaches the melting temperature of lithium which results in violent reactions. A large quantity (1.5 million cell) of rechargeable lithium batteries sent to Japan had to be recalled in 1989 after a cellular phone battery exploded and inflicted burns to a man's face. Battery engineers call it "venting with flame." A built-in pressure valve at the bottom of the battery's steel casing pops with a puff of smoke. A second later, the battery takes off like a tiny space shuttle atop a 30 centimeter plume of carmine red flame--the color of burning lithium.

Because of the inherent instability of lithium metal, especially during charging, research shifted to a non-metallic lithium battery using lithium ions from chemicals such as Lithium-Cobalt Dioxide (LiCoO2 ). Although slightly lower in energy density than with lithium metal, the Li-ion is safe, provided certain precautions are met when charging and discharging. In 1991, Sony commercialized the Li-ion and is presently the largest supplier of this type of battery. These intercalation batteries are referred to as lithium-ion because, during discharge, lithium ions are de-intercalated from the carbon negative electrode and intercalated into an oxide positive electrode. To add to the confusion surrounding rechargeable lithium batteries, there are two types of lithium-ion cells:

Lithium metal batteries are also being developed using a solid polymeric electrolyte. The electrodes in these are very thin and can be made into any shape. Hydro Quebec has been developing this technology for several years. These cells are safer because they are all solid, but they also only operate at moderate temperatures, because the conductivity of the solid electrolyte is very low at room temperature.

Although rechargeable lithium batteries offer high energy density, they have not yet matured enough for military use. Research and development are needed in lithium anodes, cathode materials and electrolytes. In addition, lithium rechargeable batteries must overcome their poor power density at low temperature.

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