Synthesis and electrochemical characterization of stoichiometric Cu2S as cathode material with high rate capability for rechargeable lithium batteries

Synthesis of stoichiometric copper sulfide (Cu2S) was studied by spray pyrolysis (SP) with heat treatment for different Cu/S molar ratios in the starting solution and different synthesis and annealing temperatures. The sample prepared via SP at 400 °C and then annealed at 460 °C for 2 h showed the stoichiometric Cu/S ratio, which was verified by inductively coupled plasma-optical emission spectroscopy. The X-ray diffraction (XRD) peaks of the sample were indexed to the monoclinic structure with space group P21/c. The electrochemical performance of the stoichiometric Cu2S electrode was investigated by a cycle test and differential capacity analysis. The Cu2S electrode coated on an Al foil exhibited a first discharge capacity of 335 mAh g−1 at a charge-discharge rate of 0.1 C, which corresponds to 99.4% of its theoretical capacity. However it showed poor reversibility owing to the corrosion of Al foil by Cu. In contrast, the Cu2S electrode coated on Cu foil demonstrated stable cyclability at high charge-discharge rates of up to 30 C. Exsitu XRD analysis showed that a phase transformation from the monoclinic Cu2S structure with the space group P21/c to the tetragonal Cu1.96S structure with the space group P43212 gradually progressed during the initial five cycles.