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

Abstract

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.

Quantitative Operando EPR Method on Graphite Anodes: Electronic Properties, Lithiation Kinetics, and Lithium Deposition

“…The exact moment of Li emergence could be earlier than 4.81 min, where the voltage drops below zero. This finding contradicts with our previous work, in which the Li plating occurs at ca. 350 mAh g –1 on a thinner coating of graphite.…”

“…The microstructure becomes looser after the first cycle and gradually stabilized from the third cycle onward (Figure S12). As evidence, our previous work on unpressed graphite revealed smaller A/B values, closely resembling the A/B curve of the third charge. Such differences in the A/B between the first five charges were employed to correct the DI curves to obtain the D ( E F ) curve during lithiation, as will be discussed in the next section.…”

“…On basis of this, a random search was carried out to find ξ­(A/B) (see more details in Figure S19). The search region indicated by the yellow-shadowed area in Figure c,d is determined according to our previous work, where a recurved DI curve is recorded with lower A/B values, suggesting that the correct DI curve is below the gray dashed line. The search results are shown in Figure e, where the ξ­(A/B) is depicted by a second-order polynomial.…”

“…In situ EPR has been employed to deduce the electronic states of lithiated graphite according to Pauli paramagnetism. , However, recent reinvestigations of graphite anodes using quantitative operando EPR have primarily focused on detecting Li plating. , A more detailed study on the local electronic structures of four lithiation stages was carried out last year, revealing distinct electron environments coupling to Li nuclei by high-frequency EPR. Our recent work demonstrated the power of operando EPR in resolving the graphite signals during XFC compared to in situ NMR . Despite these advancements, the EPR spectral information, particularly the line width, has not been fully interpreted.…”