Amorphous Niobium Sulfides as Novel Positive-Electrode Materials

Abstract

Experimental We mechanochemically synthesized the a-NbSx at room tempera- ture. Appropriate amounts of sulfur (S8, 99.9%, Wako Pure Chemical Industries) and niobium disulfide (NbS2, 99%, High Purity Chemi- cals) were weighed and mixed, and the 1.5-g mixture and 500 4-mm- diameter zirconia balls were placed into a 45-mL zirconia pot in a planetary ball mill (P-7, Fritsch GmbH) in an Ar-filled glove box. The mixture was ball-milled at 510 rpm until the amorphous powder was obtained. The X-Ray diffraction (XRD) patterns for the synthesized sam- ples were recorded using an X-ray diffractometer (Rotaflex RU- 200B/RINT, Rigaku). Prior to the measurements, the samples were covered with Kapton film in a glove box to prevent exposure to air. The conductivities of the prepared samples were measured using a two-electrodecell.Thepelletsusedfortheconductivitymeasurements were prepared by pressing powder samples under 360 MPa at 25 ◦ C. A TITAN3 G2 60-300 electron microscope (FEI, Co., Ltd.) was used for the transmission electron microscopy (TEM) measurements. The a-NbS5 powder was directly dispersed on a Cu-mesh-supported carbon film for the TEM measurements. The working electrodes were prepared from a-NbSx (10 mg), acetylene black (1 mg), and polytetrafluoroethylene (PTFE) powder (0.7 mg). A 1 M solution of LiPF6 dissolved in a 1:1 volumet- ric mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (Tomiyama Pure Chemical Industries, Ltd.) was used as the electrolyte. The counter electrode consisted of a 15-mm-diameter, 0.2-mm-thick Li foil disk. A charge-discharge unit (TOSCAT-3100, Toyo System) was used to perform the electrochemical measurements at 30 ◦ C and 100 μ Ac m −2 between 1.5 and 3.0 V.