Prior vacuuming for supercritical fluid synthesis of SnO₂/graphene nanocomposites with superior electrochemical Li⁺ storage performance

Supercritical CO₂ (SCCO₂) fluid, exhibiting gas-like diffusivity, extremely low viscosity, and near-zero surface tension, is used to synthesize uniformly dispersed and tightly anchored SnO₂ nanoparticles (a 3-nm diameter was achieved) on graphene nanosheets (GNSs). Usually, the conventional synthesis processes (in the absence of SCCO₂) results in aggregated SnO₂ clusters; whereas the technique described in this work eliminates this limitation. This study reveals the significance of two crucial factors (the SCCO₂ pressure (i.e., fluid density) and the degassing step (i.e., vacuuming stage) in autoclave before injecting CO₂) on the uniform distribution of the synthesized SnO₂ nanoparticles on GNSs. Increasing the pressure leads to an increase in SCCO₂ density (and viscosity), suppressing the transport of SnO₂ precursors throughout the sample. On the other hand, vacuuming the autoclave before injecting CO₂ improves the uniformity of SnO₂ particle distributions. To assess the electrochemical performance of the synthesized nanoparticles, the specific capacity, rate capability, and cyclic stability were determined for various samples. A capacity of ∼787 mAh g⁻¹ at 100 mA g⁻¹ was achieved for an optimal configuration of the SnO₂/GNS electrodes. The capacity retention was 60% when the charge‒discharge rate increased to 6000 mA g⁻¹.