Abstract
Graphene-based advanced electrodes with improved electrochemical properties have received increasing attention for use in lithium ion batteries (LIBs). The conventional synthesis of graphene via liquid phase exfoliation or chemical reduction of graphene oxide (GO) approaches, however, either involves prolonged processing or leads to the retainment of high-concentration oxygen functional groups (OFGs). Herein, bulk synthesis of high-quality reduced graphene oxide using microwave irradiation (MWrGO) within few seconds is reported. The electromagnetic interaction of GO with microwaves is elucidated at molecular level using reactive molecular dynamic simulations. The simulation suggests that higher power microwave irradiation results in significantly less retainment of OFGs and the formation of structural voids. The synthesized MWrGO samples are thoroughly characterized in terms of structural evolution and physicochemical properties. Specifically, a modified ID/IG-in ratio metric for Raman spectrum, wherein the intensity contribution of D’ peak is deducted from the apparent G peak, is proposed to investigate the structural evolution of synthesized MWrGO, which yields a more reliable evaluation of structural disorder over traditional ID/IG ratio. Li-ion half-cell studies demonstrate that the MWrGO is an excellent candidate for usage as high capacity anode (750.0 mAh g−1 with near-zero capacity loss) and high-performance cathode (high capacity retention of ~70% for LiCoO2 at 10 C) for LIBs.
Highlights
Rapid production of high-quality reduced graphene oxide with less oxygen functional groups (OFGs) and large lateral dimensions is pursued worldwide owing to the fascinating intrinsic properties of graphene such as highly flexible but mechanically robust structure, excellent electronic conductivity, and massive theoretical specific surface area (2630 m2 g−1) [1,2]
The preparation of MWrGO anode The slurry of 60 wt% MWrGO and 40 wt% polyvinylidene fluoride (PVDF) was prepared using N-methyl pyrrolidinone (NMP, 99%) and was blade cast on Cu foils, followed by drying at 100 °C overnight
This work demonstrated the feasibility of microwave reduction of solid-state graphene oxide (GO) to synthesize MWrGO within seconds
Summary
Rapid production of high-quality reduced graphene oxide (rGO) with less OFGs and large lateral dimensions is pursued worldwide owing to the fascinating intrinsic properties of graphene such as highly flexible but mechanically robust structure, excellent electronic conductivity, and massive theoretical specific surface area (2630 m2 g−1) [1,2]. The first direct microwave-assisted reduction of solid-state GO was achieved by Ruoff in a domestic microwave oven [13], Following that, the microwavedriven reduction process of solid-state GO was optimized by using an array of microwave secondary susceptors, e.g. graphene, carbon black or graphite They were employed to enhance microwave absorption and trigger super-heating to facilitate the reduction of surrounding GO powder [3,4,15,16]. The low capacity at high current rate was related to the inferior electronic conductivity of rGO caused by inefficient thermal reduction. The effect of rGO prepared using microwave irradiation, i.e. MWrGO, on the electrochemical properties of LiCoO2-based cathode has not been reported and will be further examined in this work. Shown to exhibit an improved capacity retention and rate capability compared to the counterpart containing carbon nanotube (CNT)
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