Three-Dimensional Si Anodes with Fast Diffusion, High Capacity, High Rate Capability, and Long Cycle Life

There are many interfaces in conventional nanostructured silicon anodes for LIBs, including (1) the solid-electrolyte interface (SEI), (2) interfaces between Si nanoparticles (NPs) and binders, and (3) interface between the current collector and active materials (CCAMI). Interfacial layers (e.g., graphene, activated carbon) coated on conventional Cu foil current collectors are often used to improve charge transfer and reduce CCAMI resistance. Indeed, our detailed studies show that the introduction of interfacial graphene layers results in an ∼20-60% increase in capacity after 500 cycles at 0.1 C. While the capacity is enhanced by inclusion of interfacial layers or conductive additives, they do not resolve problems associated with the diffusion of Li⁺ ions in the anode. Such electrodes that cannot accommodate the fast diffusion of Li⁺ ions are prone to plating. Here, we show that the use of freestanding and scalably produced carbon nanotube (CNT) Bucky paper or Bucky sandwich electrodes containing Si NPs (diameter of ∼100 nm) exhibits up to ∼1200 and 1900% increases in the gravimetric capacity after 500 cycles at 0.1 C, respectively, when discharged to 0.1 V. Using detailed electrochemical impedance spectroscopy, we show that the diffusion time constants in the Bucky paper and Bucky sandwich electrodes are increased by 2 orders of magnitude compared to that in the bare Cu foil. Furthermore, we demonstrate that the Bucky paper and Bucky sandwich electrodes can withstand high rates up to 4 C and show long cycle life up to ∼500 cycles at 0.1 C. Finally, we show that the Bucky sandwich electrode architecture with smaller diameter Si NPs (∼30 nm) leads to capacities as high as ∼1490 mAh/g (∼1635 mAh/g) at 0.1 C up to 100 cycles when discharged to 0.1 V (0.01 V).