Ion Conductivity in a Magnesium Borohydride Ammonia Borane Solid-State Electrolyte

Due to the high cost and limited availability of lithium, Mg-based batteries are currently being investigated as a promising alternative. A critical component in these batteries is the electrolyte, with all-solid-state ones that show superior safety features but must guarantee adequate ionic conductivity to be viable for applications. In this work, a metal borohydride ammonia borane complex, Mg(BH4)2(NH3BH3)2, was theoretically investigated using state-of-the-art ab initio methods based on density functional theory (DFT) approaches and software for the modeling of battery materials. Several features of this compound were first characterized, including its crystal structure and topology, vibrational properties, and infrared and Raman spectra. Theoretical results were compared with experiments showing excellent agreement, thus properly setting the ground for ionic transport analysis. Magnesium ion migration was then investigated by performing climbing image nudged elastic band (CI-NEB) calculations. The most promising migration path occurs along the c-axis and presents two transition states with a calculated migration barrier (including weak van der Waals interactions) in the range of 0.550–0.668 eV. The topological analysis suggests repulsive interactions between Mg and B atoms. It has been confirmed that defect formation energy plays an essential role in correctly evaluating the activation energy for ion migration, as shown by comparing calculated and experimental results for this system. Assuming the formation of Frenkel pairs as the dominant mechanism, the calculated defect formation energy is 1.05 eV (per single defect), which combined with the migration barrier gives a value of the activation energy for migration in the range of 1.60–1.71 eV. The present findings confirm that the activation energy for ion migration in solid-state electrolytes can be reliably estimated by DFT-based methods.