Synthesis and thermal decomposition properties of a novel dual-cation/anion complex hydride Li2Mg(BH4)2(NH2)2

Abstract

A novel dual-cation/anion complex hydride (Li2Mg(BH4)2(NH2)2), which contains a theoretical hydrogen capacity of 12.1 wt%, is successfully synthesized for the first time by ball milling a mixture consisting of MgBH4NH2 and Li2BH4NH2. The prepared Li2Mg(BH4)2(NH2)2 crystallizes in a triclinic structure, and the [NH2] and [BH4] groups remain intact within the structure. Upon heating, the prepared Li2Mg(BH4)2(NH2)2 decomposes to release approximately 8.7 wt% hydrogen in a three-step reaction at 100–450 °C. In addition, a small amount of ammonia is evolved during the first and second thermal decomposition steps as a side product. This ammonia is responsible for the lower experimental dehydrogenation amount compared to the theoretical hydrogen capacity. The XRD and FTIR results reveal that Li2Mg(BH4)2(NH2)2 first decomposes to LiMgBN2, LiBH4, BN, LiH and MgBNH8 at 100–250 °C, and then, the newly formed MgBNH8 reacts with LiH to form Mg, LiBH4 and BN at 250–340 °C. Finally, the decomposition of LiBH4 releases hydrogen and generates LiH and B at 340–450 °C.