The thermodynamic aspect of Li ion batteries will be presented to highlight the origin of the formation of parasitic reactions leading to the formation of solid interfaces (SEI) at the positive and negative electrodes ; this aspect will be all the more propitious as the redox potential of the positive and negative electrodes lies outside the thermodynamic stability of the electrolyte.
By way of illustration, we have shown that this scenario is not unique, noting also that lead-acid technology delivers a potential of 2.2 V despite using an electrolyte, water, with a thermodynamic potential of only 1.2 V. Such systems are therefore under kinetic control with, to the great benefit of Li-ion technology, the formation of a protective SEI that evolves little over time due to the slow kinetics of parasitic reactions.
We have demonstrated that the formation of a self-passivating SEI, made up mainly of LiCl, is at the origin of the primary Li-SOCl2 battery : without the formation of this SEI, Li-ion technology would not exist. Although this SEI may be considered a gift of nature, it remains a nightmare for scientists to master.
It is a dynamic interface, whose nature, composition and density change according to the electrolyte (solvent + salt) and electrode material. We have shown that, despite more than 25 years of research, the mystery of the formation, nature and composition of SEI is still not completely solved, despite the development of powerful in situ analytical techniques such as nuclear magnetic resonance, X-ray photoelectric spectroscopy, and even scanning-mode or transmission microscopy.
An example of the complexity of SEI will be illustrated by the Li+ insertion reaction in graphitic carbons, with the interposition of solvent co-intercalation reactions and even exfoliation. Although ignored for many years, we will show that this SEI exists at the positive electrode with a complexity analogous to that of the mechanisms currently being developed.
Finally, we close this lecture with a look at the electrode/electrolyte/current collector interface, focusing on corrosion, anodic oxidation and Al dissolution depending on the nature of the salts. The advantage of LiPF6 over LiTFSI will thus be documented.
