The Negative Effect of Cyclic Carbonate-Based Electrolyte on the Electrochemical Performance of Copper Sulfides

Copper sulfides are attractive electrode materials for lithium-ion battery because of their high capacity and energy density. Interestingly, the cycling stability of copper sulfides is poor in the carbonate-based electrolyte but excellent in ether-based electrolyte. The reasons for the different electrochemical performance in these two kinds of electrolyte need to be further investigated. In this study, we have compared the electrochemical performance of Cu2S electrodes in various kinds of carbonate-based electrolyte and the results show that the cyclic carbonate-based electrolyte is a major factor contributing the limited stability. Experiments were done by using commercially available copper sulfides as the electrode material. Some linear carbonate (DMC, EMC) and cyclic carbonate (FEC, PC, VC, EC) were tested as the electrolytes in different compositions. Our results show that the capacity of the Cu2S electrode fades quickly in cyclic carbonate-based electrolytes, but shows a much better electrochemical performance in linear carbonate-based electrolytes. In EMC (1 M LiPF6), it can show a discharge-capacity of 242.8 mAhg-1 with coulombic efficiency of 99.6% after 50 cycles. (As shown in Figure 1 and Table 1) Our study firstly finds that the copper sulfides electrodes can obtain excellent electrochemical performance in carbonate-based electrolyte without any special nanostructure. And the theoretical work was used to reveal the real reason of the negative effect of cyclic carbonate-based electrolyte for the first time. (As shown in Figure 2 and Table 2). From the data as shown in Figure 2 and Table 2, we can draw a conclusion that it is more difficult for nucleophilic attack reaction on linear-based carbonate due to the change of the Gibbs free energies for reaction (b) is a positive value. At the same time, the total energy of the cyclic carbonate molecules is higher than the related energies of linear carbonate molecules which also show that the cyclic carbonate molecules are more unstable. All these indicate that the electrolyte composed of cyclic structure (take FEC, PC, VC and EC for example) may easily react with nucleophilic anions and which is related to the negative effects on the electrochemical performance of copper sulfides. On the basis of the electrochemical study, we can conclude that there is a negative effect of cyclic carbonate-based electrolyte on the electrochemical performance of copper sulfide electrodes. Additional theoretical work for this study reveals the real reason of the negative effect of cyclic carbonate-based electrolyte for the first time. Fig. 1 (a) Discharge–charge profiles of the Cu2S electrode at first cycle in different electrolyte, (b) cycle performance of the Cu2S electrode in different electrolyte, all the experiments are tested between 1.0 and 3.0 V vs. Li/Li+ at a rate of 0.5 C (1 C = 335 mAh g-1).                 Table. 1 The first columbic efficiency and the capacity retention of Cu2S electrode in various composition of electrolytes at room temperature.   Compsition of electrolytes The first columbic efficiency capacity retention 10th 20th 50th 1 M LiPF6 in DMC 92.6% 86.0% 77.3% 55.5% 1 M LiPF6 in EMC 95.2% 95.4% 88.4% 76.5% 1 M LiPF6 in FEC 79.0% 1.3% 0.9% 0.3% 1 M LiPF6 in PC 91.5% 5.8% 1.9% 1.1% 1 M LiPF6 in VC 78.4% 1.1% 0.9% 0.8% 1 M LiPF6 in EC 74.2% 1.3% 0.8% 0.6% Fig. 2 Proposed Reactions between S4 2- and Carbonates.   Species EC DMC S4 2- 1 2 °÷G (a) °÷G (b) G(Hartree/Particle) -342.35 -343.55 -1592.68 -1935.09 -1935.67 -0.05 0.56 Table. 2 Gibbs free energies of the solvents and the change of the Gibbs free energies for reaction. Figure 1