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

-dodecaborates MBH 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 BH plays a key role in high cation motion. However, this category of BH 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 MBH (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 LiBH, NaBH and KBH. 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 LiBH structure at 0 K and the proposed one at 300 K are found to be -127.31 and -740.44 kJ mol per H, 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 LiBH. Additionally, the molecular dynamics simulation of MBH (M = Li, Na, K) structures demonstrated that these structures are stable above room temperature, except for the LiBH structure at 600 K, where the most stable is NaBH. Finally, the temperature effect on icosahedral anion reorientation in each structure is elucidated as a function of temperature and cation type.