Development of Si/C6 Negative Electrode for High Capacity Lithium-Sulfur Batteries

Abstract

Recently, lithium-sulfur batteries are attracting attention as large-capacity energy storage devices for renewable energy. Lithium-sulfur battery uses elemental sulfur (S8) as positive electrode and Li metal as the negative electrode, respectively. Although large theoretical capacity (1,672 mAhg-1) of the sulfur active material is expected as advantage, formation of Li dendrites at the Li metal surface during charge process is concerned as serious problem. In this study, we applied a negative electrode with Si to suppress the precipitation of Li dendrites. The theoretical capacity of Si is extremely high at 4,200 mAhg-1 and high energy density can be expected. However, Si has low cycle performance owing to their volume changes with Li+ insertion and desorption. In this study, several cells ([Li | 1M-LiFSI EC/DEC(=3/7) | x wt%-Si/C6] ) using nano-Si mixed electrodes with C6 ( x wt%-Si/C6 (x=5,10,25,50) ) were prepared and evaluated.As example, showing how the 5wt%-Si/C6 electrode is fabricated. C6:Si:Acetylene Black:PVdF was mixed into the NMP solvent at a weight ratio of 81:4:7:8. The mixed slurry was applied onto copper foil and dried at 353 K. The 5wt%-Si/C6 electrode was punched to φ 16 mm and pressed with a uniaxial force. [Li| 1M-LiFSI EC/DEC(=3/7) | x wt%-Si/C6] cells were prepared and investigated by charge-discharge test at 303 K.Fig. 1 shows the charge-discharge profiles at the first cycle of the [Li| 1M-LiFSI EC/DEC(=3/7) |x wt%-Si/C6] (x=5,10,25,50) cells (a) and the relationship between the number of cycles and discharge capacity (b) . The initial discharge capacity was higher than the theoretical capacity of C6 (ca. 360 mAhg-1). However, remarkable decrease in discharge capacity is confirmed in accordance with an increase in the mixing amount of Si. Miniaturization might be occurred due to the volume change of Si during the charge-discharge reaction. As a result, the reaction at the electrode/electrolyte interface should be degraded. Irreversible reactions might be occurred after 2nd cycle. In addition, the lowest discharge capacity was obtained at 10wt%. As a result, isolation of active material was also suggested. Therefore, it is necessary to consider improving the cycle characteristics by the use of additives and improvement of the negative electrode itself. In the presentation, we will report for improvement methods of charge-discharge properties, such as new binder and composites with electric conductive additives. Figure 1