Rare-Earth Metal Trigger Enhanced Generation Kinetics of Lithium Borohydride

Abstract

The lack of efficient and economic synthesis techniques for LiBH4 limits its popularization as a high-capacity hydrolysis material to supply hydrogen for fuel cells. Reversibly converting the low-cost hydrolytic products, typically hydrated lithium borate (LiBO2·xH2O), into LiBH4 via Mg reductants appears attractive, bringing the hope to realize a closed loop for LiBH4 hydrolysis and regeneration; however, the strong B–O bonds in LiBO2·xH2O and the weak reducing capacity pose a severe kinetics obstacle with low yield. Herein, we propose an optimized method to produce LiBH4 by facile ball milling under ambient conditions, with Li2B4O7·3H2O and LiOH·H2O as reactants, Mg-RE that was obtained by introducing cheap rare-earth metals (RE) to normal Mg as reductants, and minor MgH2 as reaction triggers and supplementary hydrogen sources. The usage of Mg-RE can not only improve the contacting efficiency between reactants due to its grind-aiding induced by hard and brittle features but also empower the reducing ability, since the H-philic nature of RE makes Mg-RE prone to hydrogenation, in situ forming highly active intermediate phases that accelerate the conversion of initial H+ to H–. A high LiBH4 yield of 33% can thus be achieved in a relatively short ball milling time of 10 h. This work enriches the synthesis pathways of LiBH4 and boosts its competitiveness for scaled applications.