Design and characterisation of alkali metal tetraalkylammonium boranes for solid-state electrolytes
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- The aim of this master thesis was to explore borohydride chemistry for the design of new solid-state electrolyte materials. We combined borohydrides, borofluorides and dodecaborates of alkali metal and of tetraalkylammonium, making ternary systems. A number of new crystalline phases can be obtained in each system, some of them having ionic conductivity and melting points suitable for use in microbatteries. The disorder of the organic cations and of pseudospherical anions contribute to the conductivity via a so-called paddle wheel mechanism. The first part of this study was an extension of the recent work done in the host lab, namely on lithium borohydride – tetraalkylammonium borohydride systems. We repeated the mechanochemical synthesis of two ionically conductive phases and assessed a new method of synthesis by direct melting of the precursors. In situ diffraction studies, in particular done at the European synchrotron, ESRF, allowed us to improve structural models and to compare properties of these electrolytes prepared by different synthetic routes. In the second part, the lithium borofluoride – tetraalkylammonium borofluoride system was studied, finding that new phases do form but their conductivity was too low to justify further investigation. The third chapter was focused on the sodium dodecaborate – tetraalkylammonium dodecaborate system with new distinct crystalline phases found, and the Na-rich rich ones showed high ionic conductivity. The large inorganic anion apparently facilitates the ionic diffusion even further, but the resulting material has a high melting point. This makes melt infiltration of the electrolyte into an electrochemical cell impossible, however other ways of assembling the batteries may also be investigated.