SYNTHESIS OF NOVEL METAL HALIDES AND THEIR STRUCTURE-PROPERTY RELATIONS

概要

In this thesis, the Ag-Bi-I solid solution system which can be used for absorbers in solar cells and antiperovskite Li-OH-X (X = Cl and Br) compounds for solid electrolytes in all-solid Li-ion batteries were studied. The detailed crystal structures and properties were presented and their relations were discussed.

1. Structure-property relations in Ag-Bi-I compounds: potential Pb-free absorbers in solar cells In Ag2-3xBixI2 (0.33 ≤ x ≤ 0.60) compounds, Ag-rich compositions (x = 0.45 – 0.48) were found to stabilize a rhombohedral CdCl2-type phase, while Bi-rich compositions (x = 0.52 – 0.57) crystalized in a defect-spinel-type crystal structure. Both rhombohedral and cubic crystal structures had similar cubic close-packed I-ion arrangements, and thus the small variation in the Ag/Bi composition caused the structure difference. Both phases showed suitable band-gap energies for potential absorbers in solar cells, but the rhombohedral compounds exhibited shallower valence band energies, larger indirect band gap energies, and higher electrical conductivity with lower activation energy than the cubic compounds.

2. Crystal structures and ionic conductivity in Li2OHX (X = Cl, Br) antiperovskites
The antiperovskite Li2OHCl was found to show a phase transition from an orthorhombic to cubic structure between 27 and 37 °C. Li2OHBr, in contrast, showed a cubic structure and no structural phase transition was observed. The significant size mismatch in Li2OHCl induced Li- ion vacancy ordering, giving rise to the structural phase transition. While the cubic phase showed high Li-ion conductivity, the orthorhombic one exhibited significantly reduced conductivity because of its distorted two-dimensional structure.

3. Ruddlesden-Popper phases of lithium-hydroxide-halide antiperovskites: Two dimensional Li-ion conductors
The n = 2 Ruddlesden Popper (RP) lithium-hydroxide-halide antiperovskite LiBr(Li2OHBr)2 was found to be stabilized in a tetragonal structure. Li-ion vacancies were introduced selectively in the antiperovskite layers but not in the rock-salt type LiBr layers. Ionic conductivity of 1.27×10−7 S/cm at 30 ℃ with the activation energy of 0.57 eV was observed. The Li-ion conduction occurred through the Li-ion vacancies within the antiperovskite layers, yielding the two-dimensional ion conduction.