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

Abstract

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 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 <i>C</i>/2.