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)2TDI]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.