Correlations between the ionic conductivity and cation size in complex borohydrides
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SHORT COMMUNICATION
Correlations between the ionic conductivity and cation size in complex borohydrides Xiaoxuan Luo 1 & Kondo-Francois Aguey-Zinsou 1 Received: 26 June 2020 / Revised: 8 August 2020 / Accepted: 23 August 2020 / Published online: 28 August 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The ionic conduction in the metal borohydrides is often linked to the energy barriers of BH4− reorientation and cation diffusion. However, the ionic conduction is a complex phenomenon, and limited reports are available to establish straightforward correlations with experimental trends in alkali metal borohydrides. This communication reported the correlations between ionic conductivity and cationic size in complex borohydrides. The ionic conductivities of LiBH4, NaBH4, and KBH4 were found to decrease monotonically with increasing cationic radius. This evidences that borohydrides follow trends observed for other ionic conductors and provide further ground toward designing better borohydride-based electrolytes. Keywords Complex borohydrides . Cation size . Ionic conductivity . Solid-state electrolyte
Introduction Enabling advanced solid-state electrolytes has potential to lead to new and safer batteries of increased power (due to higher transference numbers) and longer life owing to the wide electrochemical window, thermal stability, and reduced flammability of solid-state electrolytes [1]. In this context, complex hydrides including LiBH 4 , LiBH 4 ∙1/2NH 3 , NaCB11H12, and Na2B12H12 have shown to be promising solid-state ionic conductors (conductivity of ~ 10−2–10−3 at 110 °C) for all solid-state Li- and Na-based batteries, respectively [2–7]. In addition, borohydride-based electrolytes demonstrate good compatibility and stability against metal anodes, with a high transference number of ~ 0.9, which effectively prevents the formation of dendrites from metal anodes. However, to date, demonstration of low ionic conductivity of borohydrides at room temperature hinders application. In solid electrolytes, ionic diffusion is often determined by both the valence state and the size of the mobile ionic species Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11581-020-03756-6) contains supplementary material, which is available to authorized users. * Kondo-Francois Aguey-Zinsou [email protected] 1
MERLin, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
[1]. The lower the valence of the conducting ion, the weaker the electrostatic interaction with the surrounding structural counter ions and the faster the ionic conduction is [8]. For instance, in Li2SO4, monovalent cations such as K+ and Na+ have been found to display the highest diffusion coefficient with the lowest activation energies in comparison with their divalent counterparts Mg2+ and Zn2+ [1, 9]. Reduced diffusivity with higher valency has also been observed in hydridebased conductors. For example, Na2(BH4)(NH2) has proven to display a higher ioni
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