Humid hydrophobic ionic liquids—widely used as electrolytes—have narrowed electrochemical windows due to the involvement of water, absorbed on the electrode surface, in electrolysis. In this work, we performed molecular dynamics simulations to explore effects of adding Li salt in humid ionic liquids on the water adsorbed on the electrode surface. Results reveal that most of the water molecules are pushed away from both cathode and anode, by adding salt. The water remaining on the electrode is almost bound with Li+, having significantly lowered activity. The Li+-bonding and re-arrangement of the surface-adsorbed water both facilitate the inhibition of water electrolysis, and thus prevent the reduction of electrochemical windows of humid hydrophobic ionic liquids. This finding is testified by cyclic voltammetry measurements where salt-in-humid ionic liquids exhibit enlarged electrochemical windows. Our work provides the underlying mechanism and a simple but practical approach for protection of humid ionic liquids from electrochemical performance degradation.

Water-in-salt electrolytes are an appealing option for future electrochemical energy storage devices due to their safety and low toxicity. However, the physicochemical interactions occurring at the interface between the electrode and the water-in-salt electrolyte are not yet fully understood. Here, via in situ Raman spectroscopy and molecular dynamics simulations, we investigate the electrical double-layer structure occurring at the interface between a water-in-salt electrolyte and an Au(111) electrode. We demonstrate that most interfacial water molecules are bound with lithium ions and have zero, one, or two hydrogen bonds to feature three hydroxyl stretching bands. Moreover, the accumulation of lithium ions on the electrode surface at large negative polarizations reduces the interfacial field to induce an unusual “hydrogen-up” structure of interfacial water and blue shift of the hydroxyl stretching frequencies. These physicochemical behaviours are quantitatively different from aqueous electrolyte solutions with lower concentrations. This atomistic understanding of the double-layer structure provides key insights for designing future aqueous electrolytes for electrochemical energy storage devices.

Prussian blue analogues (PBAs) are promising cathode materials for monovalent- and multivalent-ion batteries due to the large frame structure. However, the effect of vacancy in lattice on the electrochemical performance has not been clearly elucidated, hindering the further development of PBAs. Here we identify two types of different functional vacancies (defined as structural vacancy and defective vacancy) in Mn-based PBAs (MnHCF) through Synchrotron-based X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculation. In-situ X-ray diffraction (XRD) shows that MnHCF with structural vacancy has a slighter structural evolution than MnHCF with defective vacancy during cycles. The electrochemical results indicate that the defective vacancy in MnHCF deteriorates the cyclic performance, whereas the structural vacancy favors ion diffusion and helps to stabilize the structure. Moreover, the structure with gradient structural vacancy was successfully introduced to K-rich MnHCF, realizing a high capacity and a remarkable improvement in long-term cyclic stability for alkali metal-ion (Li+/Na+/K+) batteries.