Some of the important problems which need to be resolved are capacity decline, low temperature performance and overcharge/overdischarge protection. Specific energies have been reported to be in the range of 50-100 Wh/kg in experimental cells, and have been projected to reach 200 Wh/kg. Inorganic Electrolyte Rechargeable lithium cells containing SO2 have been the most frequently discussed in this category [4]. These cells are sometimes confused with the primary (one discharge) Li-SO2 cells which have been in use for a number of years. However, the chemistry of the rechargeable lithium cell using SO2 is more complex. The specific energy for the rechargeable systems is significantly lower than for the primary cells as expected (See Table I). One important feature of these cells is the ability to withstand overcharge through chemical/electrochemical reactions. These cells also have the capability of operating at higher rates than the other ambient temperature systems. This is due to the high conductivity of the inorganic electrolyte. Higher rates of operation, unfortunately, can lead to irreversible reactions with undesirable products, thus resulting in safety issues. These cells have been constructed in cylindrical shapes, i.e. ‘AA’, ‘C’ and ‘D’ sizes. Two types of Li-SO2 cells have received attention to date. One of these, the Li-CuCl2 cell, is the only cell of this type to be considered for further evaluation. The demonstrated specific energy of this cell is reported to be only 73 Wh/kg despite the 665 Wh/kg theoretical value for this system. In the Li-SO2/LiAlCl4-C cells 82 Wh/kg has been demonstrated in a ‘2/3A’ cell. Molten Salt Lithium Cells There are two types of lithium high temperature cells, both operating at 350-400°C. Both the upper plateau (UP) Li(Al)-FeS2 and Li-FeS cells use a lithium-rich aluminum alloy as the negative electrode and both use the LiCl/KCl eutectic [5]. These systems are compared in Table I with the other well known high temperature rechargeable system, sodium-sulfur (Na-S). The Li-FeS2 cell, once ruled out as a viable high specific energy cell candidate, has been significantly improved in recent years using the upper plateau (UP) voltage only. The recent improvement in the UP Li(Al)-FeS2 system is due to a lower operating temperature (below 400°C), an improved electrolyte using LiCl/LiBr/KBr, the denser packing of active materials made possible by the higher density of the Li2FeS2 product compared to the Li2S and an improved method for preparation of the metal alloy electrode. This system has demonstrated higher practical specific energy (175 Wh/kg) than the well known Na-S system (130 Wh/kg). The cycle life of the UP Li(Al)-FeS2 has reached 1000+ cycles. A typical voltage vs time plot and a cycle life plot are given in Fig. 6. Both require thermal containment for high temperature operation which influences the specific energy of the battery. However, the packaging factor for the prismatic lithium system will have less of an impact than that of the cylindrical cell Na-S battery.
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