Understanding the Improved Fast Charging Performance of Graphite Anodes with a Fluoroethylene Carbonate Additive by In Situ NMR and EPR

The extreme fast charging (XFC) capability of graphite anodes is becoming increasingly important with the development of electric vehicles due to the usage and safety requirement. In this work, the XFC performance of the graphite anodes is improved by simply adding fluoroethylene carbonate (FEC) into the electrolyte. This robust system is studied by in situ nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) experiments to unravel the kinetic mechanism. The Li local environments in the graphite are detected by in situ NMR, which reveals the phase transitions during XFC without and with the FEC additive and the corresponding Li-ion mobility. The graphite conductivity variation is estimated by in situ EPR, and the plated Li can be clearly observed in the later period of XFC. The kinetics of graphite lithiation is deduced to be surface-controlled during the dilute stages and bulk-controlled during the dense stages. The solid electrolyte interphase (SEI) formed with FEC is more homogeneous and richer in LiF, which delivers a faster Li+ transport ability and results in the improvement of the surface kinetics. The major advantage of FEC additive is in the optimization of the Li plating behavior. Without FEC, the Li deposits grow locally, while the FEC additive consumes more currents to form the SEI and facilitate the uniform deposition of metallic Li on graphite during XFC. These results display the versatility of in situ NMR and EPR technologies in the research of XFC kinetics.