In these cells, the cathode material that is reduced during discharging is present in the liquid form. Two examples are: SO2 dissolved in a solution of lithium bromide and acetonitrile (pressurized to 3 bar pressure at 20°C); and the liquid SOCl2 (thionyl chloride), which also doubles as the solvent, with lithium aluminum chloride as solute. The high resistance protective film on the lithium anode surface (which provides the long shelf life) can be a problem at low temperatures and even at room temperature.
Liquid cathode cells are generally produced in cylindrical format and with sizes up to about 25 Ah capacity. A bobbin construction is used for low rate cells and a spirally wound format (swiss-roll) for higher discharge rates. The higher currents obtained in the latter case arise from the much higher surface areas of the electrodes. Because of their high cost and incompatibility with standard batteries, they are currently used mostly for military applications, such as munitions, transceivers and surveillance equipment. The cell contents may be toxic, irritant or corrosive and it is important to systems to protect against the dangerous consequences of abuse. A typical design of a lithium/SO2 cell is illustrated here where the different components are shown schematically, while the rise in internal cell pressure with temperature and the importance of a reliable vent arrangement to prevent cell rupture or explosion is shown here.
Liquid cathode cells are capable of the highest current densities of any primary cell, with a high and almost constant voltage during discharge, varying from 2.8V to 3.5V for the different systems. Fewer cells need be connected in series to give an acceptable operating battery voltage, in comparison with conventional systems. Larger cells, up to 20,000 Ah, having flat parallel plates, have also been developed for standby power applications in missile silos, although these are now being decommissioned, which is proving to be an expensive and time consuming exercise.