Li/SOCl2 cells have very high energies and their service life can reach 15-20 years. Moreover, these cells can be stored for long (with capacity losses of 1-2% per year at room temperature) and can be operated in an exceptionally wide temperature range (80C (with a special electrolyte) to 150C).
Low-rate Li/SOCl2 cells are built in the bobbin-type cylindrical configuration, while moderately high-power cells are built in the spirally wound configuration.
The anode is made by a Li metal foil, porous carbon is the cathode support, and SOCl2 is both the active cathode material and the solvent for the electrolyte salt (usually LiAlCl4).
LiCl precipitates on the carbon surface and stops cell operation when pore clogging occurs in cathode-limited cells. SO2 is soluble in the electrolyte, while S is soluble up to 1 mol/L and can precipitate on the cathode towards the end of discharge. LiCl is also the main component of the passivating film formed on the Li anode.
The cell capacity could be improved by adding excess AlCl3 to the electrolyte. In this case, soluble LiAlCl4 is formed instead of LiCl, so that no pore clogging occurs. However, AlCl3 dissolves the LiCl passivating film on Li, thus favouring its corrosion. For this reason, excess AlCl3 is only used in highrate reserve cells.
Cells for low-rate applications are essentially constructed with a bobbintype configuration. The Li anode contacts a steel can. The porous cathode, Teflon-bonded acetylene black, occupies most of the can volume and includes a metallic cylinder as a current collector for larger cells or a pin for smaller cells (i.e. AA size). Bobbin-type cells are cathode-limited, as this is considered safer than the anode-limited type. No hazards have been observed when submitting these cells to short circuits, overdischarge or overcharge. Their capacities range from 0.36 Ah to an impressive 38 Ah for a DD cell.
The spirally wound configuration allows using the Li/SOCl2 couple in applications requiring medium to moderately high rates. In this case, safety devices, such as a vent and a fuse are incorporated to prevent accidents stemming from overpressures or short circuits. Overpressure can be reached on overdischarge: the temperature may rise to 115C and the pressure to 140psig in cathode-limited cells. In the spirally wound configuration, the energy output is reduced (there is more inactive material inside the cell) and the shelf life is also shortened (the reactivity increases with cathode surface area). Furthermore, the upper limit of the operating temperature range cannot overcome 85C.
Several additives can be used for a better performance. In particular, the low-temperature performance may be improved by using LiGaAl4 instead of LiAlCl4. With this salt, a working temperature of 80C is attainable, as demonstrated, for instance, by the cells used in the Mars Microprobe Mission. At the other extreme, Li/SOCl2 cells of the bobbin type can work at temperatures up 150C, or even 180-200C in oil exploration.
Bobbin Li/SOCl2 cells have a high capacity, but their rate capability is not sufficient for some applications, that is GPS, automatic meter readings (AMRs), etc. Some alternative types of Li/SOCl2 cells can be used if both high energy and power are requested. In an approach proposed by Tadiran (Israel), a bobbin-type cell is coupled to a hybrid layer capacitor. The cell manages ordinary loads, while the capacitor can take over when a high current pulse (up to several amperes) is required. The voltage delay effect, also present in Li/SOCl2 cells, may be reduced by applying a conductive polymeric film on the Li anode. The double-layer film on uncoated Li has a porous layer whose thickness increases with storage, while the thickness of the conductive polymer film remains constant, thus limiting the ohmic drop at the start of discharge.
Ordinary bobbin-type cells (-55 to 85C) can be used for CMOS memory backup, medical devices, lighting, emergency locators, tracking, automotive electronics, alarm systems, etc.
High-temperature cells (up to 150C or more) can be used in measurements while drilling (MWD), tyre pressure monitoring systems (TPMS), geothermal applications.
Spirally wound high-power cells can be used in radio communications, space applications, security alarms, GPS and, in general, in military applications.
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