High dispersion of 1-nm SnO₂ particles between graphene nanosheets constructed using supercritical CO₂ fluid for sodium-ion battery anodes

Supercritical CO₂ (SCCO₂) fluid, which has gas-like diffusivity, extremely low viscosity, and near-zero surface tension, is used to synthesize SnO₂ nanoparticles (a 1-nm diameter is achievable), which are uniformly dispersed and tightly anchored on graphene nanosheets (GNSs) and carbon nanotubes (CNTs). The discharge capacity, rate capability, and cyclic stability of the synthesized SnO₂/GNS and SnO₂/CNT nanocomposites are compared. This study also tunes the SCCO₂ temperature (and thus its fluid density) and finds that this factor crucially affects the SnO₂ size and distribution, determining the resulting electrochemical properties. The sodiation/desodiation mechanism of the SnO₂/GNS electrode is examined using synchrotron ex situ X-ray absorption and X-ray diffraction techniques, together with transmission electron microscopy. We confirm that while the oxide conversion reaction is reversible, the sluggish Sn–Na alloying/dealloying reaction is responsible for the lower measured capacity as compared to the theoretical value. The first-cycle efficiency loss is mainly attributed to the trapping of Na in the electrode surface solid electrolyte interphase layer.