Sveinbjörnsson, Dadi Þorsteinn3; Blanchard, Didier1; Mýrdal, Jón Steinar Garðarsson4; Younesi, Reza1; Viskinde, Rasmus2; Riktor, Marit Dalseth5; Norby, Poul1; Vegge, Tejs1
1 Department of Energy Conversion and Storage, Technical University of Denmark2 Atomic scale modelling and materials, Department of Energy Conversion and Storage, Technical University of Denmark3 Risø National Laboratory for Sustainable Energy, Technical University of Denmark4 Center for Atomic-scale Materials Design, Center, Technical University of Denmark5 Institute for Energy Technology
LiBH4–Ca(BH4)2 composites were prepared by ball milling. Their crystal structures and phase composition were investigated using synchrotron X-ray diffraction and Rietveld refinement, and their ionic conductivity was measured using impedance spectroscopy. The materials were found to form a physical mixture. The composites were composed of α-Ca(BH4)2, γ-Ca(BH4)2 and orthorhombic LiBH4, and the relative phase quantities of the Ca(BH4)2 polymorphs varied significantly with LiBH4 content. The formation of small amounts of orthorhombic CaH2 and cubic CaH2 in a CaF2-like structure was observed upon heat treatment. Concurrent formation of elemental boron may also occur. The ionic conductivity of the composites was measured using impedance spectroscopy, and was found to be lower than that of ball milled LiBH4. Electronic band structure calculations indicate that cubic CaH2 with hydrogen defects is electronically conducting. Its formation along with the possible precipitation of boron therefore has an effect on the measured conductivity of the LiBH4–Ca(BH4)2 composites and may increase the risk of an internal short-circuit in the cells.
Journal of Solid State Chemistry, 2014, Vol 211, p. 81-89