First-principles structure prediction of dual cation ammine borohydrides: LiMg(BH₄)₃(NH₃)_x

On-board hydrogen storage for transportation applications continues to be one of the most technically challenging barriers to the widespread commercialization of hydrogen-fueled vehicles. In addition, hydrogen storage is also required for off-board purposes such as stationary power generation and hydrogen delivery and refueling infrastructure. After decades of extensive exploration, research into hydrogen storage materials based on metal borohydrides has become a highly active and exciting area owing to the high theoretical hydrogen capacities of these materials. However, they are thermodynamically too stable and therefore a very high temperature is required for their decomposition. This temperature can be lowered to the tolerable levels by the addition of ammonia and the resulting material is called as Ammine Metal Borohydrides (AMBs). In this study, we aim to search the ground state crystal structures of LiMg(BH₄)₃(NH₃)_x [1] with x = 2, 3, 4 using CrystAl Structure Prediction via Simulated Annealing (CASPESA) method. This approach was successfully located the experimentally determined structure of LiMg(BH₄)₃(NH₃)₂ [1] and other interesting local minima. For x = 3 and 4 cases, our methodology also resulted new crystal phases.