The effect of passivating films involving the lithium anode in thionyl chloride, bromine trifluoride, molten nitrates and molten perchlorates

The use of lithium metal anodes in oxidizing solvents requires the formation of a stable passivating film at the anode surface. For battery applications, the ideal film would be an excellent ion conductor with all of the charge being transported by the metal ion. The passivating film for lithium anodes consists mainly of LiCl in SOCl₂, LiF or LiBrF₄ in BrF₃ and Li₂O in molten nitrates and molten perchlorates. The discharge of the lithium anode in BrF₃ appears to be limited by the passivating film that forms at the anode. Nevertheless, nearly flat discharge curves are obtained for lithium in BrF₃ at current densities of 20 mA/cm² or less. The discharge characteristics of the lithium anode in molten nitrates improve markedly at temperatures above the melting point of lithium (mp=181°C). Discharge rates as large as 1000 mA/cm² have been attained for liquid lithium anodes in molten nitrates. The liquid lithium anodes can be immobilized in molten nitrates by a lithium-boron matrix or by the use of iron powder. The Li(B) anode is quite stable in the LiNO₃-KNO₃ eutectic melt at temperatures up to 300°C and yields discharge current efficiencies exceeding 90%. This stability is lost in initially Li⁺-free nitrate melts such as NaNO₃-KNO₃ where rapid deterioration of the anode is observed. These experimental results can be explained by a thin passivating Li₂O film on the anode that is destabilized in nitrate melts having low lithium-ion concentrations. There is generally a striking difference in behavior between the lithium anode and the calcium or magnesium anode in oxidizing solvents