Prussian blue analogues (PBAs) are commonly believed to reversibly insert divalent ions, such as calcium and magnesium, rendering them as perspective cathode materials for aqueous magnesium‐ion batteries. In this study, the occurrence of Mg2+ insertion into nanosized PBA materials is shown to be a misconception and conclusive evidence is provided for the unfeasibility of this process for both cation‐rich and cation‐poor nickel, iron, and copper hexacyanoferrates. Based on structural, electrochemical, IR spectroscopy, and quartz crystal microbalance data, the charge compensation of PBA redox can be attributed to protons rather than to divalent ions in aqueous Mg2+ solution. The reversible insertion of protons involves complex lattice water rearrangements, whereas the presence of Mg2+ ion and Mg salt anion stabilizes the proton (de)insertion reaction through local pH effects and anion adsorption at the PBA surface. The obtained results draw attention to the design of proton‐based batteries operating in environmentally benign aqueous solutions with low acidity.

The superior rate capabilities of metal ion battery materials based on Prussian blue analogues (PBAs) are almost exclusively ascribed to the extremely fast solid-state ionic diffusion, which is possible due to structural voids and spacious three-dimensional channels in PBA structures. We performed a detailed electroanalytical study of alkali ion diffusivities in nanosized cation-rich and cation-poor PBAs obtained as particles or electrodeposited films in both aqueous and non-aqueous media, which resulted in a solid conclusion about the exceptionally slow ionic transport. We show that the impressive rate capability of PBA materials is determined solely by the small size of the primary particles of PBAs, while the apparent diffusion coefficients are 3−5 orders of magnitude lower than those reported in earlier studies. Our finding calls for a reconsideration of the apparent facility of ionic transport in PBA materials and deeper analysis of the charge carrier−host interactions in PBAs.

N-ethyl-N-(2-methoxyethyl)-morpholinium bis(trifluoromethylsulfonyl)imide [Et(MEO)Mor][TFSI], N,N-bis(2-methoxyethyl)-morpholinium bis(trifluoromethylsulfonyl)imide [Bis(MEO)Mor][TFSI] and N,N-bis(2-methoxyethyl)-piperidinium bis(trifluoromethylsulfonyl)imide [Bis(MEO)Pip][TFSI] room-temperature ionic liquids (RTILs) were synthesized and their electrochemical properties were studied. Three stages of synthesis of RTILs were used, including the alkylation of secondary and tertiary amines and the anion exchange. The structure of RTILs was confirmed by 1H, [Formula: see text]C, H,H-COSY, 1H–[Formula: see text]C correlation spectroscopy NMR and XPS. Three main approaches based on the Arrhenius, Litovitz, and Vogel–Fulcher–Tammann equations were used to analyze the ion transport properties of RTILs. In addition, the electrical conductivity of the binary acetonitrile solutions of RTILs was studied. It was found that the activation energy and conductivity of RTILs depended on their polarity and ability to associate which affects the mobility of charge carriers and viscosity of the system.