The effect of additives on lithium cycling in propylene carbonate

Abstract

Deposition and discharge of lithium on an inert (nickel) substrate in 1 M LiClO 4/propylene carbonate (PC) has been studied as a function of plating and stripping currrents, cycle number, and electrolyte composition. Water (<0.06 M), nitromethane, and SO 2 were found to enhance the cycling efficiency, which was relatively low in the dry electrolyte without additives. In all cases, marked deterioration in efficiency was noted after 10-20 repeated cycles. Electro-deposited lithium became unavailable for electrochemical stripping initially (over the first 10,000 s of its life) at an average rate of 30-40 μA/cm 2, essentially independent of the additive. This rate of capacity loss decreases with time, and reaches a limiting value of 5-10 μA/cm 2. This rate is also not improved by presence of additives. A mechanism of loss of efficiency during lithium cycling and also on open circuit stand has been formulated: granules of lithium are encapsulated by an insulating film of PC-lithium reaction products, thus severing electrical contact with the substrate. The additives are postulated to form Li +-conductive films on the deposit, allowing deposition onto the granules, and their growth into larger granules. On open circuit stand, the long-term equilibrium favors the formation of an insoluble insulating (Li-PC) film over the Li +-conductive film. It is suggested that such behavior limits the usefulness of propylene carbonate in secondary lithium batteries.