There is a direct relationship between cell voltage and specific energy, i.e., the higher the voltage, the higher the specific energy. In addition, a higher cell voltage also has the added advantage that a smaller number of cells are required to provide a given bus voltage. For example, 22 Ni-Cd cells are required for a 28 V spacecraft bus, while only 14 Li-TiS2 cells would be needed. In all cases, the theoretical and operating cell voltages of the lithium cell are significantly higher than the Ni-Cd cell. Li-CoO2 cell data are omitted in the Table because the technology is relatively new and therefore actual cell data are limited. An important characteristic for the commercial market is the energy density of a cell (Wh/1). Again, in Table I, the advantages of the rechargeable lithium cells are clear. This is quite apparent for the high temperature systems. The reason for the high energy density is the ability to optimize the packaging of cells in a battery. All of the lithium cells including the molten salt system can be produced in a prismatic shape which minimizes the battery packaging factor. On the other hand, the Na-S molten salt system utilizes a beta alumina solid electrolyte (BASE) which is produced in a cylindrical structure. Another advantage of lithium cells is the ability to stand by on open circuit for lengthy periods compared to the existing aqueous cells. The improved shelf life of the organic, inorganic and polymer electrolyte systems is due to the formation of a film on the lithium surface which prevents Li from open circuit discharge to Li+. Although not completely verified, a shelf-life of five years has been projected. With the molten salt system, as long as the electrolyte is below its melting point the cell will not discharge. One of the disadvantages of the rechargeable lithium cell technology is the limited number (500) of cycles demonstrated. This is mainly due to the irreversibility of the Li^Li+ reaction. Two factors which affect the irreversibility are: (a) the stability of lithium in the presence of the complex electrolytes; and (b) the inability to replate the lithium on the negative electrode in the same form it was removed. However, there is much that is still unknown regarding these properties and efforts are under way to find solutions. Most cycling tests on the lithium cells have been performed at 100% depth of discharge (DOD). The limited cycle life appears to be far from satisfactory when compared with the Ni-Cd cell experience in which many thousands of cycles have been demonstrated in space and ground testing. However, Ni-Cd testing is usually performed at 25% depth of discharge (DOD). The results of Ni-Cd tests at 100% DOD are reported to be 1000+ cycles, which is a reasonable goal for the lithium systems. Cell Features Organic Electrolyte Cells Various levels of development of organic electrolyte cells have been reported [2] in recent years. In this country only two have reached the pilot plant stage. These include the ‘Molicell’ (Li-MoS2), and the ‘Faraday Cell’ (Li-NbSe3). JPL has evaluated samples of these cells as part of its ongoing NASA sponsored effort to develop lithium rechargeable cells. The JPL effort has been based on the Li-TiS2 cell, selected after a lengthy evaluation of several candidate materials. The two newest organic electrolyte cell technologies appearing in the literature are the Li-MnO2 and Li-CoO2 cells. The former has received much attention recently and is approaching the production stage. With regard to the latter, although it would seem that an operating voltage of 3.8 V is
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