| Summary: | 博士 === 國立中央大學 === 材料科學與工程研究所 === 105 === Supercritical CO2 (SCCO2) fluid, which has gas-like diffusivity, extremely low viscosity, and near-zero surface tension, is used to synthesize SnO2 nanoparticles (a 3-nm diameter is achievable), which are uniformly dispersed and tightly anchored on graphene nanosheets (GNSs). Comparatively, the conventional process (in the absence of SCCO2) produces aggregated SnO2 clusters. This study also tunes the SCCO2 pressure (and thus its fluid density) and vacuum the autoclave before injecting into CO2. The results show that these factors crucially affect the distribution of the produced SnO2 nanoparticles on GNSs, determining the resulting electrochemical properties. Increasing the pressure leads to an enhancement of SCCO2 density (and viscosity), decreasing the transport of SnO2 precursors through out the sample. On the other hand, vacuuming the autoclave before injecting into CO2, the SnO2 particle decoration density on GNSs increased. The discharge capacity, rate capability, and cyclic stability of the synthesized SnO2/GNS nanocomposites are compared. The SnO2/GNS electrode which synthesized during the vacuum process has the best electrochemical performance, which can deliver ~800 mAh g−1 at 100 mA g−1 and retain 60% of this of this capacity when the rate was increased to 6000 mA g−1.
An eco-efficient synthesis route of high-performance carbonate anodes for Li+ and Na+ batteries is developed. With supercritical CO2 (SCCO2) as the precursor, which has gas-like diffusivity, extremely low viscosity, and near-zero surface tension, CoCO3 particles are uniformly formed and tightly connected on graphene nanosheets (GNSs). This synthesis can be conducted at 50 °C, which is considerably lower than the temperature required for conventional preparation methods, minimizing energy consumption. The obtained CoCO3 particles (~20 nm in diameter), which have a unique interpenetrating porous structure, can increase the number of electroactive sites, promote electrolyte accessibility, shorten ion diffusion length, and readily accommodate the strain generated upon charging/discharging. With a reversible capacity of 1105 mAh g−1, the proposed CoCO3/GNS anode shows an excellent rate capability, as it is able to deliver 745 mAh g−1 in 7.5 min. More than 98% of the initial capacity can be retained after 200 cycles. These properties are clearly superior to those of previously reported CoCO3-based electrodes for Li+ storage, indicating the merit of our SCCO2 synthesis, which is facile, green, and easily scaled up for mass production.
Nanocrystalline V2O5 with a bilayer structure is directly grown on a steel substrate electrochemically in VOSO4-based solution as a cathode for sodium-ion batteries. No complicated slurry preparation procedure, involving polymer binder and conducting agent additions, is needed for this electrode synthesis. The incorporation of NaCH3COO in the VOSO4 solution promotes oxide growth and improves oxide film uniformity. The interlayer distance between two-dimensional V2O5 stacks in the structure is as large as ~11.6 Å, which is favorable for accommodating Na+ ions. The growth potential is critical to determine the oxide architectures and thus the corresponding Na+ storage properties (in terms of capacity, high-rate capability, and cycling stability). An optimal charge–discharge capacity of 220 mAh g1 is achieved for V2O5 grown in an activation-controlled potential region (i.e., 0.8 V vs. an Ag/AgCl reference electrode). This V2O5 electrode shows only 8% capacity decay after 500 cycles.
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