First-principles study of closo-dodecaborates M2B12H12 (M = Li, Na, K) as solid-state electrolyte materials.

Closo-dodecaborates M2B12H12 are considered among the potential candidates for solid-state electrolyte materials due to their high ionic conductivities. It has been demonstrated that the reorientation of the icosahedral anion B12H122- plays a key role in high cation motion. However, this category of BnHn materials is still not well established with respect to their structural, thermodynamic and diffusion properties. In the present work, the electronic, vibrational and thermodynamic properties of M2B12H12 (M = Li, Na, K) structures are reported using first-principles calculations. The results of structural and electronic properties show that these structures have an insulator character with a large band gap of 5.75, 5.63 and 5.59 eV, respectively, for Li2B12H12, Na2B12H12 and K2B12H12. The thermodynamic stabilities of these systems are confirmed by their phonon calculation results. The primary quantities, such as heat capacity, vibrational entropy and volume variation at finite temperatures, are determined using the quasi-harmonic approximation in order to provide an input for the Gibbs free energy assessment. The calculated enthalpy of formation of the Li2B12H12 structure at 0 K and the proposed one at 300 K are found to be -127.31 and -740.44 kJ mol-1 per H2, respectively. The migration energy barrier of various cations in each system is calculated to be 0.7 (Li+), 1.16 (Na+) and 1.25 eV (K+), where the lowest energy barrier corresponds to the lithium ion migration in Li2B12H12. Additionally, the molecular dynamics simulation of M2B12H12 (M = Li, Na, K) structures demonstrated that these structures are stable above room temperature, except for the Li2B12H12 structure at 600 K, where the most stable is Na2B12H12. Finally, the temperature effect on icosahedral anion reorientation in each structure is elucidated as a function of temperature and cation type.