SEI governs battery performance in terms of cycling, power, durability and calendar life. So it's up to us to master its formation and control its evolution, at different temperatures.
In the quest for an ideal SEI, current research aims either to act at electrode level via the use of particle coating and solution or ceramic surface treatment methods, or to act at electrolyte level via the use of additives that will electrochemically decompose at potentials within the electrolyte's thermodynamic potential window, enabling a protective SEI to be tailor-made.
We thus reviewed surface treatment methods before turning to the science of additives. We attempted to classify this vast array of additives according to their operating principle, which can be based either on electrochemically-induced polymerization of monomers, or on the reductive decomposition of heteroelements (e.g. S) in a high oxidation state, or even of F-, Cl- or other-based molecules, leading to the formation of a protective layer (LiF, LiCl or other).
Finally, we demonstrated the importance of triple-bond additives (-C≡N) and their synergy with double-bond additives (-C=C-), as well as the importance of coating, via the in situ decomposition of tris(trimethylsilyl)phosphate (TMSP)-type additives, for the protection of high-potential oxides (LiCoO2, LiMn2O4, ...), whose active sites we mentioned deduced by XPS measurements.
From this overview, it will be clear that the addition of additives is based on a science of trial and error, with the possibility of extracting certain trends that have proved very useful in the rapid development of an optimized electrolyte for Na-ion technology.
It goes without saying that we could not have concluded this extensive research without mentioning the current work dedicated to the search for new additives through high-throughput screening, taking as an example the "Throughput Screening System" developed at the University of Münster (Germany).
