Abstract
This PhD thesis is about the identification of new Mg2+ inorganic solid ionic conductors to be used as solid electrolytes in Mg batteries. The choice was made to study the compounds from the Mg(BH4)2-Mg(NH2)2 binary phases diagram. In this diagram, only Mg(BH4)2, Mg(NH2)2 and Mg(BH4)(NH2) were kwon. Mg(BH4)(NH2) was reported as a good ionic conductor but its synthesis parameters were unknown. First, it has been necessary to determine the synthesis of Mg(BH4)(NH2). After testing different ball milling speed, and different annealing times, atmospheres and temperatures, Mg(BH4)(NH2) could be obtained. During the different synthesis parameters optimization, an unknown additional phase was identified. This phase has an influence on the ionic conductivity of our samples. The hypothesis was made that it was actually a Mg(BH4)2x(NH2)2(1-x)-type compound. From the synthesis parameters determined previously, new synthesis were performed with different stoichiometries. Three new crystalline Mg(BH4)2x(NH2)2(1-x) phases were identified and a trend was observed between the composition of the products and their ionic conductivity. This trend is followed by all the identified compounds and reaches a maximum for x = 0.67 with an ionic conductivity of 4x10-5 S.cm-1 at 100°C, which is one the best Mg2+ ionic conductivity ever reported in an inorganic solid. The solid electrolyte properties of the best ionic conductors identified during this work were characterized. It appeared that these compounds are not stable at high potential. They start to get oxidized around + 1.0 V vs. Mg2+/Mg. Nevertheless, they are stable with Mg and allow its deposition and its stripping