Thermal and electrochemical behavior of Cu4−xLixS2 (x=1, 2, 3) phases

Abstract

Several compositions of the Cu4−xLixS2 (x=1, 2, 3) series were synthesized via solid-state reaction of the elements. The structural stability at various temperatures and the effect of Li:Cu ratio on the thermal conductivity and the electrochemical performance of Cu4−xLixS2/Li half-cells during charge–discharge process were investigated. Differential scanning calorimetry (DSC) measurements showed a sharp endothermic peak at 140°C for Cu4−xLixS2 samples with x=1 and 2, which is ascribed to a structural phase transition. X-ray diffraction (XRD) measurements on various Cu4−xLixS2 samples at temperatures below and above 140°C indicated a structural phase transition from the room temperature low-symmetry structure to the high temperature cubic structure of Cu2S. The thermal conductivity of Cu4−xLixS2 samples decreases with decreasing Cu:Li ratio and with increasing temperature. The thermal conductivity of Cu4−xLixS2 samples at room temperature decreases from 1.2W/m K for Cu:Li=3:1 to 0.7W/m K for Cu:Li=1:3. Cyclic voltammetry of Cu4−xLixS2/Li half-cells showed that high discharge capacity (165mAhg−1) and stable reversible charge–discharge process is observed for Cu:Li=2:2, whereas other Cu:Li ratios lead to low discharge capacity and poor reversibility. The electrochemical behavior of Cu4−xLixS2/Li half-cells is rationalized by taking into account the competing reactions of Li+ ions with CuS and Cu2S during discharge.