The 1:1 ion pair formation constants (K IP 0 ) of 1-alkyl-3-methylimidazolium ([RMeIm] + ; R ) butyl, hexyl, and octyl) and 1-butyl-2,3-dimethylimidazolium ([BuMe 2 Im] + ) ions with tetrafluoroborate ([BF 4 ] -), hexafluorophosphate ([PF 6 ] -), bis(trifluoromethanesulfonyl)amide ([NTf 2 ] -), and 2,4,6-trinitrophenolate (picrate, [Pic] -) ions have been determined conductometrically in dichloromethane at 25 °C. The K IP 0 determinations have also been made for symmetric tetraalkylammonium ions ([R 4 N] + ; R ) methyl, ethyl, propyl, and butyl) for comparison. For a given anion, the K IP 0 value of the [RMeIm] + salt is almost independent of the length of the alkyl chain (R), whereas that of the [R 4 N] + salt decreases with increasing alkyl chain length. Such a difference in the alkyl chain length dependence of the ion pair formation ability can be explained on the basis of the structures of the ion pairs calculated by density functional theory. The K IP 0 values of [BuMeIm] + , [BuMe 2 Im] + , and [Et 4 N] + , which are similar in the van der Waals volume, are in the order of [BuMeIm] + . [BuMe 2 Im] + ≈ [Et 4 N] + , showing that the C2-H atom on the imidazolium ring makes an important contribution to the strong ion pair formation ability of [RMeIm] + . For a given cation, the K IP 0 value is generally smaller for the larger anion, i.e., [BF 4 ]g [PF 6 ]g [NTf 2 ] -> [Pic]for [Et 4 N] + and [BuMe 2 Im] + , and [BF 4 ] -> [PF 6 ]g [Pic]g [NTf 2 ]for [RMeIm] + .
The ion pair formation constants at infinite dilution (K IP 0 ) of bis(trifluoromethanesulfonyl)amide ([NTf 2 ] -) salts in dichloromethane were determined conductometrically at 298.2 K. The cations used were nine monovalent organic ions having similar molecular size, i.e., 1-butyl-3-methylimidazolium (methyltripropylammonium ([MePr 3 N] + ), propyltrimethylammonium ([Me 3 PrN] + ), and tetraethylammonium ([Et 4 N] + ), where the results for [BuMeIm] + , [BuMe 2 Im] + , and [Et 4 N] + were cited from our previous study. It was found from the K IP 0 values that the ion pair stability varies with the cation in the order [BuMeIm
Developing high-performance solid electrolytes that are operable at room temperature is one of the toughest challenges related to all-solid-state fluoride-ion batteries (FIBs). In this study, tetragonal β-Pb 0.78 Sn 1.22 F 4 , a promising solid electrolyte material for mild-temperature applications, was modified through annealing under various atmospheres using thin-film models. The annealed samples exhibited preferential growth and enhanced ionic conductivities. The ratedetermining factor for electrode/electrolyte interface reactions in all-solid-state FIBs was also investigated by comparing β-Pb 0.78 Sn 1.22 F 4 with representative fluoride-ionand lithium-ion-conductive materials, namely, LaF 3 , CeF 3 , and Li 7 La 3 Zr 2 O 12 . The overall rate constant of the interfacial reaction, k 0 , which included both mass and charge transfers, was determined using chronoamperometric measurements and Allen-Hickling simulations. Arrhenius-type correlations between k 0 and temperature indicated that activation energies calculated from k 0 and ionic conductivities (σ ion ) were highly consistent. The results indicated that the mass transfer (electrolyte-side fluoride-ion conduction) should be the rate-determining process at the electrode/electrolyte interface. β-Pb 0.78 Sn 1.22 F 4 , with a large σ ion value, had a larger k 0 value than Li 7 La 3 Zr 2 O 12 . Therefore, it is hoped that the development of high-conductivity solid electrolytes can lead to all-solid-state FIBs with superior rate capabilities similar to those of all-solid-state Li-ion batteries.
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