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Rechargeable secondary batteries operating through fluoride-ion shuttling between the positive and negative electrodes, referred to as fluoride shuttle batteries (FSBs), offer a potentially promising solution to overcoming the energy-density limitations of current lithium-ion battery systems. However, there are many technical issues that need to be resolved to achieve high-quality fluoride-carrying electrolytes and ensure reversible transformations between a metal and its fluoride counterpart at both electrodes. Here, we introduce novel lactone-based liquid electrolytes consisting either of CsF or KF, which are prepared by a solvent substitution method. Although the maximum fluoride-ion concentration achieved by the method is approximately 0.05 M, these systems behave as strong electrolytes where CsF(KF) is almost fully dissociated into Cs+(K+) and F− ions to give a maximum ionic conductivity of 0.8 mS.cm−1. Hence, the solvent supports electrochemically active fluoride ions that can drive reversible metal/metal-fluoride transformations at room temperature for a wide range of metal electrodes. However, irreversible reductive reactions of the solvent, also promoted by the fluoride ions, limit currently the negative potential window to approximately −1.5 V vs the standard hydrogen electrode.

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An electrolyte capable of operating at room temperature has been developed for fluoride shuttle batteries, which are regarded as a promising type of high energy density batteries. This electrolyte uses a reasonably priced and widely available potassium salt called potassium bifluoride (KHF2) as the source of fluoride ions, propylene carbonate, which has a high dielectric constant, as the solvent, and the 18-crown-6 ether, which is capable of dissolving potassium salts, as an additive. The ionic conductivity of this electrolyte reaches 1.72 mS cm–1, which is sufficient to enable the operation of fluoride shuttle batteries. The charge/discharge test results using the bismuth fluoride (BiF3) electrode showed a discharge capacity of 263 mAh g–1 and a charge capacity of 184 mAh g–1. By the measurement of nuclear magnetic resonance (NMR), the state of the fluoride ions in the electrolyte is determined to be [(FH)­F]−. It is considered that hydrogen fluoride (HF) generated by the equilibration reactions of [(FH)­F]− dissolves metal fluorides that act as the active material of the electrodes, creating a mechanism that enhances the redox reaction of the metal fluorides.

Lithium ion micrometer diffusion in a garnet-type cubic Li 7 La 3 Zr 2 O 12 (LLZO) studied using 7 Li NMR spectroscopy The Journal of Chemical Physics 146, 024701 (2017) The lithium ion diffusion coefficient of a 93% Li β-alumina single crystal was measured for the first time using pulsed field gradient (PFG) NMR spectroscopy with two different crystal orientations. The diffusion coefficient was found to be 1.2 × 10 −11 m 2 /s in the direction perpendicular to the c axis at room temperature. The Li ion diffusion coefficient along the c axis direction was found to be very small (6.4 × 10 −13 m 2 /s at 333 K), which suggests that the macroscopic diffusion of the Li ion in the β-alumina crystal is mainly two-dimensional. The diffusion coefficient for the same sample was also estimated using NMR line narrowing data and impedance measurements. The impedance data show reasonable agreement with PFG-NMR data, while the line narrowing measurements provided a lower value for the diffusion coefficient. Line narrowing measurements also provided a relatively low value for the activation energy and pre-exponential factor. The temperature dependent diffusion coefficient was obtained in the temperature range 297-333 K by PFG-NMR, from which the activation energy for diffusion of the Li ion was estimated. The activation energy obtained by PFG-NMR was smaller than that obtained by impedance measurements, which suggests that thermally activated defect formation energy exists for 93% Li β-alumina single crystals. The diffusion time dependence of the diffusion coefficient was observed for the Li ion in the 93% Li β-alumina single crystal by means of PFG-NMR experiments. Motion of Li ion in fractal dimension might be a possible explanation for the observed diffusion time dependence of the diffusion coefficient in the 93% Li β-alumina system.