Characterization of defect thiospinels Cu 1−x[Ti 2]S 4 (0

Abstract

The thiospinel framework [Ti 2]S 4 represents a metastable host selective to Li insertion/extraction reactions in three dimensions that is of technical interest as a battery-cathode material. Synthesis is performed by oxidative extraction of copper from Cu[Ti 2]S 4, but complete extraction of copper from a stoichiometric starting material has not been possible. We report structural, transport, magnetic, 65Cu NMR, and electrochemical data for the defect-thiospinel systems Cu 1−x[Ti 2]S 4, 0 < x ≤ 0.93, obtained by extracting copper with I 2 at 45°C. X-ray data show a linear decrease in lattice parameter with increasing x up to 2.4% (7.2% by volume) at Cu 0.07[Ti 2]S 4. Neutron diffraction shows the residual copper atoms of the x = 0.93 sample remain in the tetrahedral 8a sites of the spinel framework. All samples are metallic or semimetallic and Pauli paramagnetic; 65Cu NMR resonance exhibits a large Knight shift for all x, which indicates an important Cu-4s character at the Fermi energy in the conduction band. Electrochemical lithiation of Cu[Ti 2]S 4 and Cu 0.07[Ti 2]S 4 gave an open-circuit voltage versus lithium that is higher in the former than it is in the latter. These several properties, including the limiting composition x = 0.93, are accounted for by constructing semi-empirical band diagrams for Cu[Ti 2]S 4 and Cu 0.07[Ti 2]S 4.