Applications of Carbon Nanotubes in Developing High Performance Lithium Sulfur Batteries

Abstract

Carbon nanotubes (CNTs) play very important roles in the field of lithium batteries. In this talk, research progress on the applications of CNTs as conductive additives and polysulfide barriers will be presented. In one aspect, CNTs are used as conductive additives and growth template for synthesis of nano sulfur−CNT composite material. The conductive CNT matrix not only avoids self-aggregation and ensures dispersive distribution of the sulfur nanocrystals, but also offers three-dimensional continuous electron pathway, provides sufficient porosity in the matrix to benefit electrolyte infiltration, confines the sulfur/polysulfides, and accommodates the volume variations of sulfur during cycling. Furthermore, abundant mesopores are introduced to CNTs through controlled oxidation in the air to obtain porous carbon nanotubes (PCNTs) that demonstrate improved dispersive behavior, enhanced conductivity, and higher mechanical strength. Large surface area and abundant adsorption points on tubes are introduced, which allow high sulfur loading, provide dual protection to sulfur cathode materials, and consequently alleviate the capacity fade especially during slow charge/discharge processes. Electrodes with a high sulfur loading of 70wt% demonstrate excellent reversible capacities and improved high-rate capability, revealing efficient suppression of polysulfide dissolution. In another aspect, ultrathin and lightweight interlayers based on CNT film and metal oxides/ sulfides/phosphides are developed as efficient polysulfide-trapping shields for high-performance Li–S batteries. A simple layer-by-layer procedure is used to construct a sandwiched vein-membrane interlayer by loading MnO2/MoS2/MoP2 nanoparticles on cross-stacked CNT films. The CNT film acts as a skeleton to support the MnO2/MoS2/MoP2 nanoparticles and to ensure their uniform distribution, and also physically hinders the polysulfide migration. The conductive CNT film also serves as a current collector and provides abundant electron pathways. Chemical adsorption of polysulfides is achieved by MnO2, MoS2, and MoP2 nanoparticles on the CNT film. The synergetic effect of the CNT film and the MnO2/MoS2/MoP2 nanoparticles enables the production of Li–S cells with high sulfur loadings, low capacity decay rate, high rate performance, and low self-discharge rate. In-situ Raman spectra, X-ray photoelectron spectroscopy results, electrochemical impedance spectroscopy results, and density functional theory calculations demonstrate the strong interaction between metal oxides/sulfides/phosphides and polysulfides and confirm the polysulfide-trapping ability of the CNT based interlayer. These results demonstrate the great potential of CNTs in developing high-performance Li–S batteries.