Peek Inside the Water Mixtures of Ionic Liquids at Molecular Level: Microscopic Properties Probed by EPR Spectroscopy

Many ionic liquids (ILs) can be mixed with water, forming either true solutions or emulsions. This favors their applications in many respects, but at the same time might strongly alter their physicochemical properties. A number of methods exist for studying the macroscopic properties of such mixtures, whereas understanding their characteristics at micro/nanoscale is rather challenging. In this work we investigate microscopic properties, such as viscosity and local structuring, in binary water mixtures of IL [Bmim]BF4 in liquid and glassy states. For this sake, we use continuous wave and pulse electron paramagnetic resonance (EPR) spectroscopy with dedicated spin probes, located preferably in IL-rich domains or distributed in IL- and water-rich domains. We demonstrate that the glassy-state nanostructuring of IL-rich domains is very similar to that in neat ILs. At the same time, in liquid state the residual water makes local viscosity in IL-rich domains noticeably different compared to neat ILs, even though the overwhelming amount of water is contained in water-rich domains. These results have to be taken into account in various applications of IL-water mixtures, especially in those cases demanding the combinations of optimum micro- and macroscopic characteristics.

Validation of Structural Grounds for Anomalous Molecular Mobility in Ionic Liquid Glasses

Ionic liquid (IL) glasses have recently drawn much interest as unusual media with unique physicochemical properties. In particular, anomalous suppression of molecular mobility in imidazolium IL glasses vs. increasing temperature was evidenced by pulse Electron Paramagnetic Resonance (EPR) spectroscopy. Although such behavior has been proven to originate from dynamics of alkyl chains of IL cations, the role of electron spin relaxation induced by surrounding protons still remains unclear. In this work we synthesized two deuterated imidazolium-based ILs to reduce electron–nuclear couplings between radical probe and alkyl chains of IL, and investigated molecular mobility in these glasses. The obtained trends were found closely similar for deuterated and protonated analogs, thus excluding the relaxation-induced artifacts and reliably demonstrating structural grounds of the observed anomalies in heterogeneous IL glasses.

Transient morphology of lithium anodes in batteries monitored by in operando pulse electron paramagnetic resonance

Enhancing lithium-ion battery technology in terms of specific capacity and charging time is key for the advancement of the electrification of transportation. Particularly for fast charging, inhomogeneous deposition of metallic lithium, for example on commercial graphite or metallic lithium anodes, leads to cell degradation and safety issues. To understand the underlying mechanisms and develop counter-measures, non-invasive online detection techniques providing satisfactory time resolution are crucial. Here, we demonstrate in operando pulse electron paramagnetic resonance to observe transient processes during pulsed fast charging in cells with metallic lithium anodes. Sampling timescales of 100 ms enable real-time monitoring of the formation and evolution of porous lithium during and after charging pulses. It was observed that the generated morphology continued to evolve after the end of a charging pulse, whereby surface features were fusing with a time constant that was slower than their formation.