Effects of Silicon Content in Silicon-Graphite Composite Anode on Capacity Degradation

Silicon, with its theoretical capacity of 4200 mAh g-1,[1] has been exploited as a capacity-booster for mature graphite anodes. However, silicon is prone to fast degradation due to significant volume expansion, which may undermine the intrinsic stability of the graphite, resulting in shorter cycle life. The trade-off between higher capacity and shorter cycle life is therefore worth optimizing. This study investigates the effects of silicon content on the cycling performance of silicon-graphite composite anodes. The composite electrodes were synthesized by mixing both materials at various weight ratios, along with carbon black and polyacrylic acid solution, and coated on a copper foil. A mixture of 1 M of LiPF6 in EC: DMC, 1:1, v/v, and Fluoroethylene carbonate (FEC) was used as the electrolyte. The samples consist of electrodes with silicon content from 8% to 40%. Galvanostatic test was done at various C-rates to determine the specific capacity at each cycle, and the correlation between the voltage and the capacity during the test was used to create differential capacitance curves. Electron microscopy and energy-dispersive X-ray spectroscopy were used to study morphology and to detect agglomeration of silicon particles. The results show that the initial capacity and the capacity retention of each sample indeed depend on the silicon content. As expected, higher silicon contents yield higher initial specific capacities but cause faster degradation. However, the trends do not continue beyond 32% of Si content. This suggests that there is a limit for silicon to enhance the initial specific capacity at a fixed C-rate. Differential capacitance curves show the gradual disappearance of silicon peaks, e.g., a peak at 0.45 V, which indicates that the primary cause of the capacity loss is the degradation of silicon material. Elemental analysis shows the high content of silicon content can promote particle agglomeration. Future tasks will include establishing the relationship between the silicon content and quantifying the amount of silicon agglomeration using X-ray spectroscopy. The results from this study can be used to assess an appropriate amount of silicon in anode material.References[1] Gu, M., He, Y., Zheng, J., & Wang, C. (2015). Nano Energy, 17, 366-383.Figure 1