Research on New Solid Superionic Conductors Containing Cu+, Ga+, In+, and T1+

摘要

An investigation was made on a series of solid electrolytes of the formula A2MX4 (A(+) = Cu(+), Ag(+) In(+), or T1(+); M2(+) = Zn2(+), Cd2(+), or Hg; X = Br(-) or I(-)). Electrical measurements on these materials show consistently lower conductivity for the d(10)s(2) ions, In(+) and Tl(+), than for the d(10) ions, Cu+ and Ag+. A single crystal x-ray investigation of the structure for T12ZnI4 demonstrates that the cation resides in a low symmetry 5-coordinate environment. Raman spectroscopy and x-ray powder diffraction suggest a similar structure for the corresponding In(+) salt. By contrast, structural data in the literature indicate that Cu(+) and Ag(+) reside in tetrahedral sites in their respective compounds. These comparisons suggest that the lower conductivity of the D(10)s(2) ions arise from the structural differences. Ab initio Hartree-Fock LCAO-MO calculations were performed on MI4 arrays. The ground state structure was taken as the cation residing in the center of a tetrahedral I4 array and the transition state was modeled by the ion in the face of the I4 array. Lower energies were found for the movement of the d(10) ions into the face of the iodide tetrahedron. The results demonstrated that the detailed electronic structure of the cation can have a profound influence on ion transport. The polymer electrolyte systems studied have the approximate composition MX-4PEO(M-alkali metal ion, X-large anion, and PEO is repeat unit of polyethylene oxide). They show promise for solid state electrochemical devices.