Silicon–Carbon Nanocomposite Semi-Solid Negolyte and Its Application in Redox Flow Batteries

Nonaqueous redox flow battery (RFB) is a promising technology to improve the energy density of RFBs for both stationary grid energy storage and electric vehicle applications. Despite tremendous advancement made in high-capacity flow posolyte, lithium metal has been dominantly exploited as a nonflowable counter electrode in reported lithium-based RFBs to date due to the lack of efficient and high-capacity flow negolyte. Here, we report a silicon–carbon nanocomposite semi-solid negolyte, achieving a high reversible capacity (>1200 mAh g–1) and stable cycle life (>100 cycles). A facile process of incipient wetness impregnation followed by polymerization/carbonization is developed to synthesize monodispersed Si–C nanocomposite with ultrathin graphitic carbon coating. Exploiting Si–C nanocomposite as the negolyte effectively suppresses the volume change of Si particles and enhances the electrical conductivity of the negolyte. Coupling with highly concentrated LiI (5.0 M), we demonstrate the first lithium metal-free Si-based negolyte in a full all-flow cell configuration with a stable cycle life (>60 cycles), high Coulombic efficiency (>90%), and the highest full cell voltage (3.0 V) reported for Li metal-free RFBs. Our successful demonstration of lithium metal-free all-flow battery highlights the promising potential of Si-based negolyte to replace lithium metal for high-energy-density RFBs.