Multiple-cation compounds always show more desirable controllability than that of monometallic compounds in the field of the hydrogen-storage materials. As in the ammine metallic borohydrides (AMB), Li and Mg appears similar chemical properties, but they show different appearances in preventing the impurities in the hydrogen releasing process. We applied Car-Parrinello molecular dynamic (CPMD) method to investigate the dehydrogenation pathway for LiMg(BH4)(3)(NH3)(2) (AMLB) on the basis of our previous study of Mg(BH4)(2)(NH3)(2) (AMgB) and LiBH4NH3 (ALB). Interestingly, the purity of hydrogen gradually improves from ALB to AMgB to AMLB caused by the effects of the corresponding mixed metals. Furthermore, the two metals in AMLB are in close cooperation to control the motion of [NH3] and [BH4] groups, which suppress almost all the impurities and show enhanced properties in the decomposition. From our results, the improved dehydrogenation performance should depend on the large polarization of the centre metal(s). Furthermore, the purity of the hydrogen is depending on the quantities of the free hydrogen ions. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.