Synthesis And Electrochemical Performance Of Novel Sulfur-contained Cathode Materials For Lithium Sulfur Batteries

Nowadays, clean energy has been vigorously developed because of the energy shortage and the environmental problems. Rechargeable lithium sulfur battery as one of the most promising clean energy sources has attracted much attention because sulfur cathode has almost the highest theoretical capacity(1672 m A h g-1) and theoretical energy density(2600 Wh kg-1). In addition, element sulfur possesses the characteristics of high natural abundance, low cost and environmental friendliness. However, it suffers sever capacity fading due to the drawbacks: the dissolution of polysulfides intermediates, the insulating property of pure sulfur(5×10-30 S·cm-1), and the dendrite growth caused with metallic lithium anode. Herein, new types of carbon/sulfur composites and sulfur/conductive polymer composites as cathode materials for lithium sulfur batteries are designed and prepared; their structures and composition are characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, Fourier Transform infrared spectroscopy, and nitrogen adsorption isotherms. The electrochemical performance is evaluated by cyclic voltammetry and galvanostatic charge-discharge properties. The main contents are listed as below:(1) A novel hierarchical structure carbon/sulfur composite is presented based on carbon fiber matrices, which are synthesized by electrospinning. The fibers are constituted with hollow graphitized carbon spheres formed using catalytic Ni nano-particles as hard templates. Sulfur is loaded to the carbon substrates via thermal vaporization. The results exhibit an initial discharge capacity of 845 m A h g-1 at 0.25 C(420 m A g-1), with a retention of 77% after 100 cycles. A discharge capacity of 533 m A h g-1 is still attainable when the rate is up to 1.0 C. The good cycling performance and rate capability are contributed to the uniform dispersion of sulfur, the conductive network of carbon fibers and hollow graphitized carbon spheres.(2) A 3D reduced graphene oxide/carbon nanotubes/sulfur composite is fabricated based on a self-assembled carbon skeleton, which are synthesized via a hydrothermal reduction process. Sulfur is loaded to the carbon substrates via thermal vaporization. The effects of the opening carbon nanotubes on the structures and electrochemical performances of the composites are investigated. When the reduced graphene oxide: carbon nanotubes = 7:1(wt./wt.), the composite exhibits an optimal performance: an initial discharge capacity of 1114.9 m A h g-1 at 0.2 C(300 m A g-1), and a discharge capacity of 797.2 m A h g-1 after 100 cycles with a retention of 71.5%. Adischarge capacity of 608 m A h g-1 is still attainable when the rate is up to 3.0 C(4.0-4.5 A cm-2). The good cycling performance and rate capability are contributed to the uniform dispersion of sulfur, the conductive 3D network of graphene/carbon nanotubes and the channels for electrolyte supplied by opening carbon nanotubes.(3) Sulfur particles with the diameter of 200 nm- 5 μm are synthesized by a novel surfactant assisted dispersing technology. The effects of the surfactant concentration, the feeding rate of formic acid, and the reaction time on the particle size of the product are investigated. Submicron sulfur balls with the diameter of 300-500 nm are prepared under the optimized conditions.The micron and sub-micron core-shell sulfur/polyaniline composites with high sulfur contents are synthesized by the surfactant assisted polymerizaion-coating method.The effects of the PVP concentration and the particle size on the structures and electrochemical performances of the composites are investigated. The submicron core-shell sulfur/polyaniline composite(81 wt.%) exhibits an initial discharge capacity of of 1057 m A h g-1, and a discharge capacity of 743 m A h g-1 after 100 cycles with a retention of 70.3%. A discharge capacity of 487 m A h g-1 is still attainable when the rate is up to 2.0 C. The good electrochemical performances are contributed to the good conductivity of the thin and uniform PANi shell with a thickness of 30 nm, the small particle size, and the stable structure.The reaction mechanism is proposed. The interactions between the surfactants and sulfur are calculated by the quantum chemistry method and confirmed by the experimental results. Polyvinylpyrrolidone(PVP) is the optimal surfactant. The optimal synthesis technology that we proposed has reference value for the future work.