Ion transport processes in mixtures of N-butyl- N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) were characterized by ac impedance spectroscopy and pulsed field gradient NMR. Molar ratios x = n Li-TFSI/( n Li-TFSI + n BMP-TFSI) up to 0.377 could be achieved without crystallization. From the bulk ionic conductivity and the individual diffusion coefficients of cations and anions we calculate the Haven ratio and the apparent lithium transference number. Although the Haven ratio exhibits typical values for ionic liquid electrolytes, the maximal apparent lithium transference number is higher than found in other recent studies on ionic liquid electrolytes containing lithium ions. On the basis of these results we discuss strategies for further improving the lithium transference number of such electrolytes.

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To understand the low-temperature behavior of pyrrolidinium-based ionic liquids (ILs), ILs, including N-butyl-N-methyl pyrrolidinium (Pyr 14 +) and N-methyl-N-propyl pyrrolidinium (Pyr 13 + ) cations and bis(fluorosulfonyl)imide (FSI -), bis(trifluoromethanesulfonyl)imide (TFSI -), bis(pentafluoroethanesulfonyl)imide (BETI -), and (trifluoromethanesulfonyl)(nonafluorobutyl-sulfonyl) imide (IM 14 -) anions, were investigated. Pyr 14 FSI, Pyr 13 FSI, Pyr 14 TFSI, Pyr 13 TFSI, Pyr 14 BETI, Pyr 13 BETI, Pyr 14 IM 14 , and Pyr 13 IM 14 were prepared and intensively studied by means of differential scanning calorimetry (DSC). Four of these ILs (Pyr 14 FSI, Pyr 13 FSI, Pyr 14 TFSI, Pyr13TFSI) were used to prepare binary mixtures, which were examined by DSC measurements. For these mixtures, reduced melting transitions and an enhanced liquidus range were detected, which represents a great advantage for low-temperature applications. In addition, it was observed that the crystallization process of the mixtures is mainly influenced by the anion.

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Some cations of ionic liquids (ILs) of interest for high‐energy electrochemical storage devices, such as lithium batteries and supercapacitors, have a structure similar to that of surfactants. For such, it is very important to understand if these IL cations tend to aggregate like surfactants since this would affect the ion mobility and thus the ionic conductivity. The aggregation behaviour of ILs consisting of the bis(trifluoromethanesulfonyl)imide anion and different N‐alkyl‐N‐methyl‐pyrrolidinium cations, with the alkyl chain varied from C3H7 to C8H17, was extensively studied with NMR and Raman methods, also in the presence of Li+ cations. 2H NMR spin‐lattice and spin‐spin relaxation rates were analyzed by applying the “two step” model of surfactant dynamics. Here we show that, indeed, the cations in these ILs tend to form aggregates surrounded by the anions. The effect is even more pronounced in the presence of dissolved lithium cations.

The use of metallic lithium anodes enables higher energy density and higher specific capacity Li‐based batteries. However, it is essential to suppress lithium dendrite growth during electrodeposition. Li‐ion‐conducting ceramics (LICC) can mechanically suppress dendritic growth but are too fragile and also have low Li‐ion conductivity. Here, a simple, versatile, and scalable procedure for fabricating flexible Li‐ion‐conducting composite membranes composed of a single layer of LICC particles firmly embedded in a polymer matrix with their top and bottom surfaces exposed to allow for ionic transport is described. The membranes are thin (<100 μm) and possess high Li‐ion conductance at thicknesses where LICC disks are mechanically unstable. It is demonstrated that these membranes suppress Li dendrite growth even when the shear modulus of the matrix is lower than that of lithium. It is anticipated that these membranes enable the use of metallic lithium anodes in conventional and solid‐state Li‐ion batteries as well as in future LiS and LiO2 batteries.