Structural, physical and electrochemical characteristics of a vanadium oxysulfide, a cathode material for lithium batteries

A vanadium oxysulfide is obtained by a reaction between water solutions of a vanadyl salt and sodium sulfide at room temperature. After drying under mild conditions, the formulation of this phase is V2O3S·3H2O. Thermogravimetric analyses show that it is not possible to remove completely water without losing sulfur. This is in agreement with proton nuclear magnetic resonance experiments which prove that water molecules are tightly bonded to vanadium. Magnetic susceptibility and X-ray absorption spectroscopy measurements allow to define the oxidation states of vanadium and sulfur, (IV) and (−II) respectively. From extended X-ray absorption fine structure spectroscopy at the vanadium K edge and infrared spectroscopy, the local structure around vanadium can be defined as a distorted octahedron, with a vanadyl bond and an opposite sulfur atom. Magnetic susceptibility and X-ray absorption spectroscopy measurements on chemically lithiated compounds show a complex charge transfer from lithium to the host structure upon lithium intercalation. If it appears that vanadium atoms are reduced, a possible role of sulfur atoms in the redox process has to be considered. Cycling tests of lithium batteries whose positive consists of oxysulfide are promising with 70 cycles under a regime of C/8, without noticeable loss in capacity of 120 Ah/kg.