Regulating the size and assembled structure of graphene building blocks for high-performance silicon nanocomposite anodes

Silicon-based composites have received significant interest as a high-capacity anode material for high-performance lithium-ion batteries. However, the large volume change during prolonged charge/discharge cycles, poor electric conductivity, and unstable solid electrolyte interface of the Si electrodes lead to performance degradations, such as fast capacity decay and low coulombic efficiency (CE). It's promising but challenging to fabricate Si-based composite anodes with a high Si active material, which enables high energy density, high-rate capability, and good cycling stability. Herein, the size effect of mechanically robust and highly conductive graphene sheets was investigated to effectively regulate the charge transport kinetics, volume changes, first cycle CE, and stable solid-electrolyte-interphase of the Si-anode for improved electrochemical performance. Specifically, our developed nanocomposite electrode (Si@ULG) consisting of Si nanoparticles (NPs) enveloped by ultra-large graphene sheets (ULG) can deliver a specific capacity of 1478 mA h g-1 even after 200 cycles at C/5, with a low capacity loss of 0.23% per cycle. This outstanding cycling performance surpasses that of electrodes wrapped by small (SG) or large graphene sheets (LG). By further assembling ULG sheets as building blocks into a three-dimensional (3D) graphene framework to load a high weight percentage of graphene-wrapped Si materials (e.g., Si@ULG), the as-prepared binder-free 3D Si@ULG-ULG nanocomposite electrode (with a high mass loading of 3 mg cm-2) enabled an areal capacity of 2.1 mA h cm-2 after 200 cycles at C/5, which is much higher than the slurry coating thin-film anodes (e.g., 0.12 mA h cm-2) at low areal mass loading (0.49 mg cm-2). By regulating the size of graphene building blocks and their assembled 3D hierarchical structures, we can improve the charge transport kinetics and mechanical stability of silicon composite electrodes for long cycling battery performance.