In the organic electrolyte cells, electrolyte conductivity is being continuously improved through the use of mixed solvents. For example, in the Li-TiS2 cell, both the conductivity and stability of lithium has been improved by mixing ethylene carbonate (EC) with the existing 2 methyl-tetrahydrofuran (2-MeTHF) solvent in which lithium hexafluoroarsenate (LiAsF6) salt has been dissolved. Separator materials proven to be stable in the environment have been used in the past. However, the role and influence of the separator is a factor that needs to receive some attention. In the polymeric electrolyte cells rate capability is more of an issue because of low conductivity of the polymeric electrolyte. In addition to increased surface area and electrolyte conductivity there are other issues in this cell type. The plate/polymer interface is more critical because of solid contacting solid. Maximizing forces to overcome this problem results in creep of the polymer. The inorganic electrolyte and molten salt Li-FeS2 cells have been successful in demonstrating rates (current density) comparable to the C/2 discharge rate. Cell Size To date the largest rechargeable ambient temperature lithium cells have been the ‘D’ size. Limited numbers of Button cells, ‘AA’, and ‘C’ size cells have been produced for use mainly in the commercial and military markets. Although there is some interest in these sizes for NASA, the goal is to develop cell sizes comparable to the Ni-Cd sizes for NASA spacecraft applications. This includes cell sizes to as large as 50 Ah. The molten salt high temperature systems are, by their very nature, large systems. For the most part they have been developed under the sponsorship of DOE and others in large capacities for electric vehicles and load leveling applications. Safety The reason for limited size, rate and use of lithium cells is the concern for safety. The combination of the highly reactive lithim metal and non-aqueous complex electrolytes under abnormal operating conditions is the cause for this concern. Although the problem is real, much of this concern originated from the ventings and explosions of lithium primary cells in the 1970s. Little was known at that time about the importance of cell design, operating limits, the effects of abuse, and proper handling. Much has been learned about these subjects in the last 15 years. More importantly, scientific work on the fundamental reactions, processes and cell designs has resulted in consumer acceptance of primary lithium low rate cells. However, use of high rate cells is still limited to trained and knowledgeable users. With regard to the rechargeable lithium cells, understanding the reactions, mechanisms and design is needed to minimize concern for safety. One issue discussed above is the need to limit the charge and discharge voltages in order to eliminate unsafe irreversible reactions that produce undesirable products. Either electronic control or internal reaction mechanisms are necessary to prevent these unsafe reactions. A second issue involves the mechanism of replating of lithium back on the metallic electrode during the charge process. Li+ from the electrolyte must find its way back onto the lithium metal through its protective film. Analysis has shown that the process results in a fine particulate lithium powder with very high surface area. Powder scraped from the surface of the lithium metal after a cell has been disassembled can catch fire. This will probably not lead to a venting or fire in a sealed cell where
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