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The following discussion considers various D size cells operating at around 20°C. Details of the individual electrochemical couples are given in the following section. As a general rule the liquid cathode systems, Li/SO2, Li/SOCl2 and Li/SO2Cl2, plus their derivatives, are capable of higher discharge rates than the solid cathode systems such as Li/MnO2 and Li/CFX. The solid cathode cells do not support currents as high as the liquid cathode ones. This is because the liquid cathode undergoes a discharge at the surface of the electrode (which comprises a high surface area carbon supported on a metal mesh) where the discharge products are deposited. In contrast, discharging at a solid cathode involves diffusion of lithium ions into the bulk of the cathode, which is a slower process.
Manufacturers do not usually recommend the use of cells above a certain current, or rate, even though they are capable, because of possible problems from cell heating. Continuous operation of liquid and solid cathode cells above 2A will lead to a significant rise in cell temperature, so this needs to be borne in mind for a particular battery application: the temperature rise being of more importance for the high pressure Li/SO2 cells.
Liquid and solid cathode cells can be operated at 4A but, if a 24V battery pack containing 8 cells is used at this rate, the temperature rise will be much higher than for a single cell standing alone. This is because the cells are contained in an insulating plastic box and are also encased in a foam material that stops the cells from moving. Consequently, heat dissipation is very slow and heat builds up inside the pack. D size Li/SO2 cells have been developed which can yield 30A pulses (800 W/Kg) on the millisecond time scale, while Li/SOCl2 research cells have been made to yield the equivalent of 75A (1200 W/Kg).
All of the spirally wound systems are likely to be further developed to yield higher rates, through the use of catalysts and thinner electrodes. The latter leads to a higher electrode surface area in a given size cell and hence a higher current. It is possible that the thinner electrodes may prove to be less resistant to physical abuse but little data is available.
The shelf life of primary lithium cells is typically equivalent to a 10% loss of capacity over five years. This compares with a similar loss for alkaline cells over only one year. The long shelf life of lithium batteries arises from the lithium metal surface becoming passivated by reaction with the electrolyte. These films, which are termed the solid electrolyte interface (SEI), are insoluble in the electrolyte and are of the order of a few microns in thickness. They are electronically insulating, but are ionically conducting to lithium ions. The films are mainly composed of the same compound as is produced by the discharge reaction (e.g. lithium chloride in the case of Li/SOCl2 cells). If the product of the reaction between lithium and the electrolyte is soluble, then the lithium rapidly dissolves and an explosion can result. This can, in fact, happen with lithium-sulfur di cells if all of the sulfur dioxide is consumed before all of the lithium metal, because the remaining lithium then dissolves in the lithium bromide/acetonitrile electrolyte, producing hydrogen and heat. All lithium systems are said to be thermodynamically unstable but kinetically stable.
