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

• Cu:Li ratio strongly influenced crystal structure and properties of Cu4-xLixS2.
• Compositions with x = 1 and 2 undergo structural transformation at above 140 °C.
• Thermal conductivity of Cu4-xLixS2 phases increases with decreasing Cu:Li ratio.
• Stability of Cu4-xLixS2/Li half-cells decreases with Cu:Li ratio.
• Initial charge capacity of Cu4-xLixS2/Li half-cells increases with Cu:Li ratio.

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 1.2 W/m K for Cu:Li=3:1 to 0.7 W/m K for Cu:Li=1:3. Cyclic voltammetry of Cu4−xLixS2/Li half-cells showed that high discharge capacity (165 mA h g−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.

Tuning Li content in Cu4−xLixS2/Li half-cells to maintain a Cu/Li ratio equal to unity affords maximum capacity and high stability of the charge–discharge process.Figure optionsDownload as PowerPoint slide