Snapshots of the Hydrolysis of Lithium 4,5-Dicyanoimidazolate-Glyme Solvates. Impact of Water Molecules on Aggregation Processes in Lithium-Ion Battery Electrolytes

Despite that 4,5-dicyano-2-(trifluoromethyl)-imidazole lithium salt (LiTDI) exhibits several interesting features in aprotic solvents such as glymes or carbonate esters, little is known about its structural rearrangement after exposure to water. Since the LiTDI salt has been verified as an effective moisture scavenger able to suppress degradation of the LiPF6-based electrolyte, comprehensive knowledge of coordination modes in the LiTDI-H2O system, as well as information about the structure of formed hydrates, is desirable. In the present study, we report the impact of water on the LiTDI glyme-based electrolytes investigated by means of the single-crystal X-ray diffraction technique and Raman spectroscopy. We have found that the exposure of lithium 4,5-dicyanoimidazolate glyme solvates to humid air gives rise to the hydrolysis products arising from stepwise addition of water molecules to the lithium coordination sphere. Several structural motifs have been distinguished as preferred coordination modes in the LiTDI H2O system. A high number of available ether oxygen donor center water molecules cause dissociation of ionic contact pairs and aggregation of cationic species stabilized by crown ethers. Low O:Li molar ratio leads to the formation of LiTDI glyme water solvates and LiTDI hydrates. The air-stable LiTDI trihydrate comprises ionic pairs formed by a lithium cation coordinated to an imidazole nitrogen of TDI. A lithium cation coordinated via nitrile groups and bearing water molecules is a basic motif constituting dimeric species of formula [Li(H2O)(2)TDI](2) which are present in aggregated [Li(H2O)TDI](n), chains making up the structure of a monohydrate. The discovered motifs have been proved to occur in both the solid and melted hydrated systems of LiTDI. They will be helpful for conducting molecular dynamic calculations and for obtaining information how to manipulate the structure of a Li+-solvation sheath in both hydrated and liquid aqueous electrolytes based on heterocyclic anions.