Enhanced Room-Temperature Ionic Conductivity of NaCB11H12 via High-Energy Mechanical Milling

The body-centered cubic (bcc) polymorph of NaCB11H12 has been stabilized at room temperature by high-energy 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 Na-23 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 vertical bar NaCB11H12 vertical bar 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.