Ionic conduction in bimetallic borohydride borate, LiCa3(BH4)(BO3)2

Abstract

Bimetallic borohydride borate LiCa3(BH4)(BO3)2 forms during decomposition of LiBH4–Ca(BH4)2–carbon composite studied for hydrogen storage. We present LiCa3(BH4)(BO3)2 as a material suitable for solid state electrolytes. The compound was studied by in situ Synchrotron Radiation X-ray Powder Diffraction, Electrical Impedance Spectroscopy and Cyclic Voltammetry. The conduction pathways in the crystal were estimated via Topology Analysis. Room temperature Li+ ionic conductivity reaches the value of 2.5×10−6S/cm with an activation energy of Ea=0.25(5) eV. We have investigated the effect of doping on cation sites on the ionic conductivity in the temperature range 293–383K, and showed that the heterovalent doping with Na+ increase RT σLi+ up to 1×10−5S/cm with a saturation at 5at.% of the dopant. Doping with Sr2+ of the Lithium excess containing samples enhances the Li+ conductivity by one order of magnitude in the same temperature range, and with the same saturation limit. In both cases the activation energy increases into the range of 0.47–0.63eV which is also the case of self-doping with excess Lithium. The Li+ ions are mobile in the borohydride borate on a particular pathway composed exclusively from borate anions where Li+ was found in a previous work using ab initio calculations in an octahedral coordination. The pathways create a conduction framework, which is stabilized by the second substructure containing calcium borohydride. The compound is a first example of double anion bimetallic ionic conductor combining borate based substructure favourable for light cation mobility with a stabilizing effect of heavier cation borohydride.