ABSTRACT
Solid-state sodium batteries have attracted great attention owing to their improved safety, high energy density, large abundance and low cost of sodium compared to the current Li-ion batteries. Sodium-boranes have been studied as potential solid-state electrolytes and the search for new materials is necessary for future battery applications. Here, a facile and cost-effective solution-based synthesis of Na2B11H13 and Na11(B11H14)3(B11H13)4 is demonstrated. Na2B11H13 presents an ionic conductivity in the order of 10-7 S cm-1 at 30 °C, but undergoes an order-disorder phase transition and reaches 10-3 S cm-1 at 100 °C, close to that of liquids and the solid-state electrolyte Na-ß-Al2O3. The formation of a mixed-anion solid-solution, Na11(B11H14)3(B11H13)4, partially stabilises the high temperature structural polymorph observed for Na2B11H13 at room temperature and it exhibits Na+ conductivity higher than its constituents (4.7 × 10-5 S cm-1 at 30 °C). Na2B11H13 and Na11(B11H14)3(B11H13)4 exhibit an oxidative stability limit of 2.1 V vs. Na+/Na.
ABSTRACT
We report, for the first time, members of the Y xPr2- xO3 system with non-bixbyite or defect fluorite structures. The synthesis, structure, phase transitions, and high temperature reactivity of the trigonal A-type and monoclinic B-type structures are reported along with those of the cubic C-type phase (bixbyite). Combined powder X-ray and neutron diffraction Rietveld refinements are used to report structural details of all three reported phases. Phase transitions are investigated, showing a clear dependence on average cation size. Using neutron diffraction, phase transitions are followed in situ, revealing that all high temperature phases are quenchable. In-situ powder X-ray diffraction experiments in flowing oxygen allow insights into mechanistic details of redox processes in the reported phases. In contrast to the C-type cubic bixbyite, the trigonal A-type and monoclinic B-type structures do not allow for topotactic oxygen uptake, displaying instead a phase transition to either the bixbyite C-type capable of accommodating additional oxide anions or the direct oxidation to the cubic defect fluorite structure. The findings reported here agree with the accepted lanthanide sesquioxide phase diagrams and provide exceptional control of phases. The work is important for the prediction of structures, and the synthetic control needed for rational design of functional materials.
