Ionicity of deep eutectic solvents by Walden plot and pulsed field gradient nuclear magnetic resonance (PFG-NMR).

Deep eutectic solvents (DESs) have attracted considerable attention due to their unique properties. Owing to similar structures and properties, DESs are also called "quasi-ionic liquids" or "ionic liquid analogous". However, for a deeper understanding and application of DESs, a comprehensive investigation on the ionicity of DESs is crucial. In this work, the effects of the structure and components of typical DESs on the ionicity were investigated. Moreover, the ionicity was discussed by using Walden plot, and the validity of applying it to DESs was verified using pulsed field gradient nuclear magnetic resonance (PFG-NMR). We found that the lack of free charged species and high viscosities make it difficult to achieve optimal conductivities for DESs, and thus most of them exhibit "poor ionic" nature. Fortunately, with an in-depth understanding of its microstructure and physicochemical properties, the properties of DESs can be finely tailored by selecting or even designing suitable parent compounds for functional applications. In particular, the ionicity of Li-based DESs was investigated for their potential application as electrolytes in Li-ion batteries.