Ionic conductivity and the formation of cubic CaH2 in the LiBH4–Ca(BH4)2 composite

• The LiBH4–Ca(BH4)2 composite forms a physical mixture rather than a solid solution.
• The formation of defect-rich, cubic CaH2 in a CaF2-like structure is observed.
• A new layer containing cubic CaH2 is conducting and may lead to a short-circuit.

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.

An Arrhenius plot of the ionic conductivity of the LiBH4–Ca(BH4)2 composites (red, blue, green). The ionic conductivity of ball milled (gray) and non-milled (black) LiBH4 is shown for comparison. The filled symbols are measured during heating runs and the empty symbols are measured during subsequent cooling runs. The conductivity of the composites is in all cases higher during cooling, most probably due to the formation of an electronically conducting layer containing defect-rich cubic CaH2. Such layer formation could eventually lead to a short circuit in the cell and reveals a general issue of chemical stability that should be attended to in the development of solid electrolyte materials.Figure optionsDownload as PowerPoint slide