Self-supported electrodes composed of silicon nanocrystals in 3D hierarchical carbon network for reversible sodium storage

Silicon as a high capacity alloying anode material in lithium-ion batteries (LIBs) has recently been reported to have a promising specific capacity suitable for sodium-ion batteries (SIBs). However, the low gravimetric capacity and large volume expansion in traditional electrodes arising from the slurry-coating process has restrained the development. Here, we report the fabrication of a self-supported composite composed of silicon nanocrystals in a 3D hierarchical carbon network as an anode for reversible sodium storage by a three-step process involving electrostatically assisted hetero-assembly, vacuum filtration, and thermal treatment. The silicon nanocrystals decorated with a carbon coating are dispersed in interconnected carbon nanotubes with close contact. The structure provides abundant interfacial active sites for capacitive Na storage. Furthermore, the conductive 3D network of carbon nanotubes and carbon coating provide the high-speed pathways for charge transport and buffer to accommodate the volume change during Na⁺ insertion/extraction in silicon nanocrystals. As a binder-free anode in SIBs, the self-supported electrode delivers outstanding electrochemical performance such as stable cyclability with a capacity of 80% at a current density of 0.2 A g⁻¹ after 1000 cycles and high-rate capability with a capacity of 105 mAh g⁻¹ at a current density of 2 A g⁻¹. The self-supported Si-based electrode has great potential in high-performance SIBs.