Synergistic Effect of Fluoroethylene Carbonate and Propylene Carbonate on the Calendar Life of Silicon-Based Lithium-Ion Batteries

The energy density and cycling life of silicon-based lithium-ion batteries (LIBs) rapidly approach their designed targets. However, their calendar life still fails to meet the requirements for long-term stability. In this study, Si/C||LiNi0.8Mn0.1Co0.1O2 (NCM811) batteries have been constructed from Si/C composites with no graphite and Ni-rich NCM811. The electrolytes used propylene carbonate (PC) or ethylene carbonate (EC) as the base solvent, supplemented with fluoroethylene carbonate (FEC). After 800 cycles, the PC-based electrolyte battery retained 79.7% capacity compared to 70.2% for the EC-based electrolyte. Following storage at 60 °C for 7 days, the PC-based electrolyte battery exhibited a 95% capacity recovery, 56% resistance growth, and 51% gas generation. The EC-based electrolyte battery showed 91%, 70%, and 101%, respectively. Atomic force microscopy (AFM) analyses and Young’s modulus measurements revealed that the PC-based electrolyte facilitated the formation of a thinner, smoother, and denser solid electrolyte interphase (SEI) on the Si/C surface. Furthermore, for the PC-based electrolyte, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) results showed PC promoted FEC reactions, forming a dense, LiF-rich SEI. In contrast, for the EC-based electrolyte, the EC and FEC jointly reacted to form a thicker SEI. Molecular dynamics (MD) simulations showed that for the PC-based electrolyte, approximately 20.4% of the FEC molecules participated in the Li+ solvation structure, which was more than the 17.1% obtained for the EC-based electrolyte. Thus, the synergistic effect of PC and FEC resulted in an effective formation of a more stable SEI, which enhanced the cycling performance and calendar life of Si/C. This study offers an economical and effective commercial electrolyte solution for high-energy-density Si/C-based LIBs.