Liquid Cathode Lithium Cells

These cells tend to offer higher discharge rates because the reactions occur at the cathode surface. In a solid cathode, the reactions take longer because the lithium ions must enter into the cathode for discharge to occur. The direct contact between the liquid cathode and the lithium forms a film over the lithium, called the solid electrolyte interface (SEI). This prevents further chemical reaction when not in use, thus preserving the cell's shelf life. One drawback, though, is that if the film is too thick, it causes an initial voltage delay. Usually, water contamination is the reason for the thicker film, so quality control is important.

LiSO2 Lithium–Sulfur Dioxide

This cell performs very well in high current applications as well as in low temperatures. It has an open voltage of almost 3 V and a typical energy density of 240–280 Wh/kg. It uses a cathode of porous carbon with sulfur dioxide taking part in the reaction at the cathode. The electrolyte consists of an acetonitrile solvent and a lithium bromide solute. Polypropylene acts as a separator. Lithium and sulfur dioxide combine to form lithium dithionite:

2Li +2SO2 —> Li2S2O4

These cells are mainly used in military applications for communication because of high cost and safety concerns in high-discharge situations, i.e., pressure build up and overheating.

LiSOCl2 Lithium Thionyl Chloride

This cell consists of a high-surface area carbon cathode, a non-woven glass separator, and Thionyl chloride, which doubles as the electrolyte solvent and the active cathode material. Lithium aluminum chloride (LiAlCl4) acts as the electrolyte salt.

During discharge the anode gives off lithium ions. On the carbon surface, the thionyl chloride reduces to chloride ions, sulfur dioxide, and sulfur. The lithium and chloride ions then form lithium chloride. Once the lithium chloride has deposited at a site on the carbon surface, that site is rendered inactive. The sulfur and sulfur dioxide dissolve in the electrolyte, but at higher-rate discharges SO2 will increase the cell pressure.

This system has a very high energy density (about 500 Wh/kg) and an operating voltage of 3.3 – 3.5 V. The cell is generally a low-pressure system.

In high-rate discharge, the voltage delay is more pronounced and the pressure increases as mentioned before. Low-rate cells are used commercially for small electronics and memory backup. High-rate cells are used mainly for military applications.