Enhancing interfacial properties on Silicon-Graphite anode by alkenyl-functionalization of fluorinated cyclotriphosphazene in high-voltage lithium-ion batteries

Abstract

The pursuit for high energy density in lithium-ion batteries has driven the blended adoption of high-voltage cathode and high-capacity Silicon-based anode. Unfortunately, the carbonate-based electrolytes fail to be compatible with these electrode materials and the flammable nature of linear carbonates poses safety hazards to electrolytes. Fluorinated cyclotriphosphazenes can enhance the safety level of electrolytes and possesses the function of high-voltage additives. However, it is difficult to directly employ fluorinated cyclotriphosphazenes in high-voltage lithium-ion batteries containing Silicon-based anode. In this work, an alkenyl-functionalized cyclotriphosphazene- (3-butenoxy)pentafluorocyclotriphosphazene is constructed to enhance the compatibility between electrolytes and Silicon-Graphite anode. The mixed use of (3-butenoxy)pentafluorocyclotriphosphazene and (ethoxy)pentafluorocyclotriphosphazene not only makes electrolytes nonflammable but also renders preeminent performances to 4.45 V LiCoO2/Silicon-Graphite pouch cells at 25 °C. The capacity retention of the cell at 1C after 150 cycles surpasses 80 %, and the discharge capacity ratio of 8C/1C reaches around 53 %. With scanning electron microscopy and X-ray photoelectron spectroscopy technique, it is verified that the mixed flame retardant can build a robust interface film on the anode, which handles the adverse effects caused by the volume expansion of Silicon-Graphite.