Thionyl chloride is a component of lithium–thionyl chloride batteries, where it acts as the positive electrode (in batteries: cathode) with lithium forming the negative electrode (anode); the electrolyte is typically lithium tetrachloroaluminate. The overall discharge reaction is as follows:
These non-rechargeable batteries had many advantages over other forms of lithium batteries such as a high energy density, a wide operational temperature range, and long storage and operational lifespans. However, their high cost, non-rechargeability, and safety concerns have limited their use. The contents of the batteries are highly toxic and require special disposal procedures; additionally, they may explode if shorted. The technology was used on the Sojourner Mars rover.