Following the growth of surface films on lithium and their thermal behavior in standard LiPF₆ solutions using differential scanning calorimetry

In this paper, we report on attempts to use differential scanning calorimetric measurements of aged Li electrodes for the study of the kinetics of the growth of surface films on the active metal. Standard, commonly used alkyl carbonate solutions such as ethylene and di-methyl carbonates with LiPF ₆ were explored. Heating Li samples in solutions after aging by DSC, resulted in well-resolved curves of reaction heats vs temperature. Exothermic reactions occurring at temperatures below 150°C could be attributed to changes related to the surface films and their heat evolved, increased as a function of storage time, and hence these heats represent the thickness of the surface films that grow upon storage. Scanning electron microscopy of the Li surfaces as a function of storage and heating to different temperatures confirmed that the thermal reactions of Li surfaces in these solutions up to 150°C relate to the surface films only. XPS studies revealed that these processes of the surface films change the metastable organic Li salts to more stable inorganic compounds such as LiF and Li₂O. Massive red-ox reactions, between the salt anion and the solvents and between the solution species and the active metal, occur at temperatures higher than 150°C. The kinetics of growth of the surface films on Li show an inverse logarithmic behavior, expected for thin surface films with which the rate-limiting step of their growth depends on ions transport across the film.