DescriptionAll-solid-state batteries equipped with ceramic, highly dense electrolytes have attracted great attention as they would allow the realization of safe, next-generation Li and Na energy storage systems.[1,2] An extremely high ion mobility, which is known for liquid electrolytes, is, how-ever, needed to realize such batteries. Over the last couple of years we witnessed the renewed interest in Na-bearing compounds that exhibit extremely high ion conductivities. This interest is driven by the high abundance of Na (vs. Li) and thus reduced costs to develop stationary systems able to store electricity from renewable sources like solar, wind or tidal. In order to realize such all-solid-state systems it is, of course, necessary to understand the general rules that determine ultrafast ion dynamics in solids. Apart from macroscopic methods, such as AC and DC conductivity spectroscopy, solid-state nuclear magnetic resonance (NMR) spectros-copy offers powerful techniques to throw light on the underlying elementary steps of ion hop-ping in crystalline and amorphous ion conductors.
Here we used a multinuclear approach to obtain first insights into the ion transport in the metal closo borate Na2(B12H12)0.5(B10H10)0.5. We took advantage of 23Na, 11B and 1H NMR spin-lattice
relaxation (SLR) measurements to measure activation energies and jump rates. For 11B NMR, which indirectly senses Na ion motions, we could see the complete diffusion-induced rate peak. Interestingly, the total magnetization transients revealed two spin sub-ensembles characterized by distinct SLR rates R1fast and R1slow. The rates R1fast measured at 160 MHz are in line with those for 23Na NMR recorded at the same frequency. 11B NMR points to an activation energy of only 0.16 eV for local Na+ jump processes. In addition, SLR NMR entails information on the extent of correlated motion on overall dynamics as well as on the influence of rotational motions on translational transport. In the closo-borate structure Na diffusivity is indeed antici-pated to be linked to the reorientations and disorder of the anion sublattice. In the present study, we compared the results with those from a 23Na NMR study on Na-ß"-alumina. Na-ß"-alumina was chosen as a benchmark to visualize the differences in Na+ diffusivity as seen by NMR. Na2(B12H12)0.5(B10H10)0.5 and Na-ß"-alumina as well reveal ion conductivities in the mS/cm range at ambient conditions. Such high conductivities, besides sufficiently high electro-chemical stability, represent one of the most important prerequisites for the development of all-solid state batteries.
 J.C. Bachman, S. Muy et al., Chemical Reviews 2016, 116, 140-162.
 J. Janek, W. Zeier, Nature Energy 1 2016, 16141.
 B. L. Ellis, L. F. Nazar, Curr. Opin. Solid State Mater. Sci. 2012, 16, 168-177.
 M. Bettman, C. R. Peters, Journal of Physical Chemistry 1969, 73, 1774-1780.
 L. Duchêne, R.-S. Kühnel, D. Rentsch, A. Remhof, H. Hagemann, C. Battaglia, Chem. Commun. 2017, 53, 4195-4198.
 W. Jakubowski, D. H. Whitmore, Journal of the American Ceramic Society 1979, 62, 381-385.
 T. J. Udivic, M. Matsuo, Adv. Mater 2014, 26, 7622-7626.
|Period||28 May 2018|
|Held at||Empa, Switzerland|