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 B₁₂H₁₂^2- 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 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.