ABSTRACT
In this study, graphene-selenium hybrid microballs (G-SeHMs) are prepared in one step by aerosol microdroplet drying using a commercial spray dryer, which represents a simple, scalable continuous process, and the potential of the G-SeHMs thus prepared is investigated for use as cathode material in applications of lithium-selenium secondary batteries. These morphologically unique graphene microballs filled with Se particles exhibited good electrochemical properties, such as high initial specific capacity (642 mA h g(-1) at 0.1 C, corresponding to Se electrochemical utilisation as high as 95.1%), good cycling stability (544 mA h g(-1) after 100 cycles at 0.1 C; 84.5% retention) and high rate capability (specific capacity of 301 mA h g(-1) at 5 C). These electrochemical properties are attributed to the fact that the G-SeHM structure acts as a confinement matrix for suppressing the dissolution of polyselenides in the organic electrolyte, as well as an electron conduction path for increasing the transport rate of electrons for electrochemical reactions. Notably, based on the weight of hybrid materials, electrochemical performance is considerably better than that of previously reported Se-based cathode materials, attributed to the high Se loading content (80 wt%) in hybrid materials.
Subject(s)
Electric Power Supplies , Electrochemical Techniques/instrumentation , Graphite/chemistry , Lithium/chemistry , Oxides/chemistry , Selenium/chemistry , ElectrodesABSTRACT
A two-step method consisting of solid-state microwave irradiation and heat treatment under NH3 gas was used to prepare nitrogen-doped reduced graphene oxide (N-RGO) with a high specific surface area (1007â m(2) g(-1) ), high electrical conductivity (1532â S m(-1) ), and low oxygen content (1.5â wt %) for electrical double-layer capacitor applications. The specific capacitance of N-RGO was 291â F g(-1) at a current density of 1â A g(-1) , and a capacitance of 261â F g(-1) was retained at 50â A g(-1) , which indicated a very good rate capability. N-RGO also showed excellent cycling stability and preserved 96 % of the initial specific capacitance after 100 000â cycles. Near-edge X-ray absorption fine-structure spectroscopy results provided evidenced for the recovery of π conjugation in the carbon networks with the removal of oxygenated groups and revealed chemical bonding of the nitrogen atoms in N-RGO. The good electrochemical performance of N-RGO is attributed to its high surface area, high electrical conductivity, and low oxygen content.
Subject(s)
Electric Capacitance , Graphite/chemistry , Nitrogen/chemistry , Oxides/chemistry , Electric Conductivity , Electrodes , Models, Molecular , Molecular Conformation , Oxidation-Reduction , Oxygen/chemistry , Surface PropertiesABSTRACT
Considerable effort has been made to develop unique methods of preparing and characterizing nanoparticles and nanocomposites in order to exploit the true potential of nanotechnology. We used a facile, versatile phase-transition method for forming Group 6A nanoparticles on carbonaceous templates to produce homogeneous 5-10 nm diameter Group 6A nanoparticles on carbon nanotubes (CNTs) and reduced graphene oxide (RGO), to obtain nanocomposites. The method involved melting and recrystallizing mixtures of elemental sulfur and either CNTs or RGO on carbonaceous templates. The surface tension and hydrophilicity of the molten Group 6A species surfaces and the oxygen functional groups on the carbonaceous template surfaces were considered in depth to provide important guidelines for forming Group 6A nanoparticles on carbonaceous templates. The surface tension of the molten Group 6A species should be intrinsically low, leading to effective wetting on the carbonaceous template. In addition, the molten Group 6A species hydrophilic surfaces were essential for enabling hydrophilic-hydrophilic interaction for selective wetting at the oxygen functional groups on the carbonaceous template, leading to the heterogeneous nucleation of nanoparticles. Furthermore, the size and morphology (isolated vs layer-like) of the Group 6A nanoparticles were tuned by adjusting the oxidation state of the carbonaceous template. We investigated the potential application of the nanocomposites prepared using this method to cathode materials in lithium-sulfur secondary batteries.
