Cobalt Decorated Hierarchical Porous Carbon Spheres for High Sulfur Loading Lithium Sulfur Batteries

Abstract

With the growing need of energy storage devices, the market shows a strong requirement of lithium-based batteries. Nowadays, Lithium ion batteries (LIBs) are dominating the battery field because of its high energy density and low safety risks. However, the energy density of LIB is pushed to its principle limitation. To further improve the battery performance, another new battery mechanism has been developed. Lithium sulfur batteries (LSB) have four times higher energy density of LIBs in principle. LSB are considered as the next generation of batteries in electric vehicles and portable devices by many scientists and material engineers. In another hand, there are still a lot of obstacles on lithium sulfur technology. Firstly, sulfur is an insulator, so it must be mixed with other conductive supporters to form the electrode which decrease the energy density respect to total weight. During the oxidation-reduction reaction, soluble polysulfide is generated as an intermediate. It is soluble in electrolyte and diffuse through the separator toward the cathode and react with lithium. In the charging process, the lithium sulfide on the cathode cannot be reduced which is a waste of the active material. The lithium sulfide layer form on anode also deactivated the lithium plate and cause the capacity fading. In this project, cobalt decorated hierarchical carbon sphere (CZ/HPC) is designed to overcome these obstacles. This spherical material has three types porous structure including 3nm mesopores, 150 nm middle-size macropores and large carbon bubbles which is around 2 μm. Spherical structure can provide good ion transfer and high conductivity at the same time. Large carbon bubbles and middle-size macropores provide high sulfur loading in the electrode and the mesoporous structure increase the utility of the sulfur which reflect to high specific capacity. The size of the particle is around 3.5 μm, and larger size of particle made it easier to form thick electrode. The sulfur loading of the battery is over 10 mg/cm2 and the weight percentage of the active material in the cathode is 72.3% which provide excellent energy density of the battery. To limit the shuttle effect, cobalt nanoparticles generated by calcination of ZIF-67 particles, as an effective polysulfide absorption material, is synthesized on the carbon sphere. Spray dry, which is widely used in the industry, is chosen to synthesize CZ/HPC, and the price of the precursor is relatively cheap that gives CZ/HPC a potential of commercialize.