A Lithium thionyl chloride cell (Li/SOCl2) consists of three major components:
- Metallic Lithium (Li) anode or a lithium alloy anode (negative)
- High surface area carbon cathode (positive)
- Thionyl chloride electrolyte, which is composed of thionyl chloride (SOCl2) and an electrolyte salt (LiGaCl4 or LiAlCl4)
In a Li/SOCl2 cell the cathode and anode are physically separated by a layer of non-woven glass separator paper. The following is a diagram of a Li/SOCl2 cell.
A Li/SOCl2 cell operates by oxidation of the Li anode and reduction of thionyl chloride at the cathode. A solid electrolyte interphase (SEI), which is a film of lithium chloride (LiCl), forms automatically on the Li anode when the electrolyte is introduced to the cell. The SEI is necessary for the cell to function.
The oxidation at the Li anode produces lithium ions that travel through the electrolyte to the cathode. An electron forms, which is available to do work in the external circuit.
Li ===> Li+ + e-
At the cathode, thionyl chloride is reduced on the surface of the carbon producing chloride ions, sulfur dioxide, and sulfur,
2 SOCl2 + 4e- ===> 4 Cl- + SO2 + S
The lithium ions, from the anode, combine with the chloride ions, from the cathode, to form insoluble solid lithium chloride (LiCl) which is deposited in the carbon cathode at the reaction site. The sites where the LiCl is deposited in the cathode becomes inactive and are no longer availible for further reaction.
It is important to understand that the carbon cathode does not take part in the cell reaction. The carbon cathode only provides active sites where the reactions take place. When all the sites become blocked with LiCl or become inaccessible, the cell stops functioning.
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