The lithium-thionyl chloride cell has the highest energy density of commercially available primary cells. It has a very good low- and high-temperature performance, excellent shelf life and a very high power density in its high-rate configuration. However, the major obstacle to the commercialization of a high-rate type cell, and in particular a high-rate multi-cell battery, is the well known explosion hazard during reversal or charging. This hazard is common to all high-rate nonaqueous lithium batteries. Reversal of one or more cells or the charging of one row of cells by another row connected in parallel is likely to happen during deep discharge. The reason for this is that lithium is deposited at low over-potentials, on itself, or on a passivated cathode. The metallic lithium which deposits under these conditions is likely to have a large active surface area. At a high deposition rate, a powder-like lithium deposit can be formed. The presence of lithium powder in a thionyl-chloride cell can create a very hazardous situation. On prolonged reversals or charging of a lithium cell there is the danger of an internal short circuit due to lithium dendrite growth. This may lead to internal spark which can rapidly melt, or even evaporate, the lithium dendrite, thereby initiating a cell explosion. This explosion process is more likely to happen in a partially discharged cell where less electrolyte is available to cool the spark area.
U.S. Pat. Nos. 4,184,014, 4,093,794 and 4,042,756 and German Pat. No. 2,262,256, claim electrochemical cells having an alkali metal or alkaline earth metal as anode, an oxyhalide as solvent/cathode material and some unspecified electrolyte salt or electrolyte salt consisting of lithium as cation and an anion. Israel Pat. No. 63336 and U.S. Pat. No. 4,461,815 claims a safe cell which even under conditions of abuse is not likely to create a hazard or to explode. It is based on calcium (or magnesium) or dischargeable alloy of same, devoid of an alkali metal in the alloy, on a porous inert current collector, and on the use of an inorganic oxyhalide solvent wherein there is dissolved a suitable calcium salt. The cell according to U.S. Pat. No. 4,461,815 having 99.9% Ca anode and 0.95M Ca(AlCl.sub.4).sub.2 in SOCl.sub.2 solution has a shelf life of about 1.5 to 2 years at room temperature. This cell lost about 10% of its capacity after two weeks of storage at 70.degree. C. [Peled, Elster, Tulman and Kimel, J. Power Sources 14, 93 (1985)].
In the following M designates an alkaline earth metal.