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 graphiteanodes isbecoming increasingly important with the development of electric vehiclesdue to the usage and safety requirement. In this work, the XFC performanceof the graphite anodes is improved by simply adding fluoroethylenecarbonate (FEC) into the electrolyte. This robust system is studiedby in situ nuclear magnetic resonance (NMR) and electron paramagneticresonance (EPR) experiments to unravel the kinetic mechanism. TheLi local environments in the graphite are detected by in situ NMR,which reveals the phase transitions during XFC without and with theFEC additive and the corresponding Li-ion mobility. The graphite conductivityvariation is estimated by in situ EPR, and the plated Li can be clearlyobserved in the later period of XFC. The kinetics of graphite lithiationis deduced to be surface-controlled during the dilute stages and bulk-controlledduring the dense stages. The solid electrolyte interphase (SEI) formedwith FEC is more homogeneous and richer in LiF, which delivers a fasterLi(+) transport ability and results in the improvement ofthe surface kinetics. The major advantage of FEC additive is in theoptimization of the Li plating behavior. Without FEC, the Li depositsgrow locally, while the FEC additive consumes more currents to formthe SEI and facilitate the uniform deposition of metallic Li on graphiteduring XFC. These results display the versatility of in situ NMR andEPR technologies in the research of XFC kinetics.