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The body-centered cubic (bcc) polymorph of NaCB11H12 has been stabilized at room temperature by highenergy mechanical milling. Temperature-dependent electrochemical impedance spectroscopy shows an optimum at 45-min milling time, leading to an rt conductivity of 4 mS cm−1. Mechanical milling suppresses an order−disorder phase transition in the investigated temperature range. Nevertheless, two main regimes can be identified, with two clearly distinct activation energies. Powder X-ray diffraction and 23Na solid-state NMR reveal two different Na+ environments, which are partially occupied, in the bcc polymorph. The increased number of available sodium sites w.r.t. ccp polymorph raises the configurational entropy of the bcc phase, contributing to a higher ionic conductivity. Mechanical treatment does not alter the oxidative stability of NaCB11H12. Electrochemical test on a symmetric cell (Na|NaCB11H12|Na) without control of the stack pressure provides a critical current density of 0.12 mA cm−2, able to fully charge/discharge a 120 mA h g−1 specific capacity positive electrode at the rate of C/2.

A central goal in current battery research is to increase the safety and energy density of Li-ion batteries. Electrolytes nowadays typically consist of lithium salts dissolved in organic solvents. Solid electrolytes could facilitate safer batteries with higher capacities, as they are compatible with Li metal anodes, prevent Li dendrite formation and sulphur shuttling, and eliminate risks associated with flammable organic solvents. Less than 10 years ago, LiBH4 was proposed as a solid state electrolyte. It showed a high ionic conductivity, but only at elevated temperatures. Since then strategies have been developed to extend the high ionic conductivity of LiBH4 down to room temperature, and other light metal hydrides have been explored as solid electrolytes [1]. Using LiBH4 as an example we will discuss how the properties of solid electrolytes can be modified by forming nanocomposites with metal oxides, leading to an enhancement of the room temperature ionic conductivity of more than three orders of magnitude[2]. DSC measurements combined with solid state NMR allow to identify how the nanoconfinement and presence of interfaces modify the phase stability and the Li+ mobility [3]. Systematic studies show how the ionic conductivity can be optimized by tuning the nanostructure and interfaces in these nanocomposites. Finally, promising results have been obtained in using these materials as solid-state electrolytes in next generation all-solid state lithium-sulphur batteries. [4] [1] de Jongh et al. J. Appl. Phys. A (2016), 122:251. [2] Blanchard et al., Adv. Funct. Mater. 25 (2015), 182. [3] Verkuijlen et al., J. Phys. Chem. C 116 (2012) 22169. [4] Blanchard et al, J. Electrochem. Soc. (2016).