Fabrication And Electrochemical Properties Of The Carbon/Sulfur Composite Cathodes For Lithium-sulfur Batteries

In recent years,lithium–sulfur?Li–S?batteries have been considered as extremely promising candidates for the next-generation of rechargeable batteries.By coupling a sulfur cathode(with a theoretical capacity of 1675 mAh g^-1)and a lithium anode(with a theoretical capacity of 3840 mAh g^-1),the Li–S battery affords an average voltage of about 2.15 V and a high theoretical energy density of 2600 Wh kg^-1,which is able to gain up to 2–3 times higher practical energy density than the state-of-the-art commercial LIBs.In addition,the natural abundance and non-toxicity of sulfur enable Li–S batteries with a more attractive cost advantage and better environmental friendliness compared to LIBs.Despite the overwhelming advantages,the Li–S battery also has several significant technological obstacles.The research in this dissertation mainly focused on the high-performance carbon/sulfur composite cathodes and the development of novel carbon materials as the substrates for sulfur to improve the electrochemical performances of the sulfur cathode.The main results and new findings in this work are summarized as follows:1.The anodic aluminum oxide?AAO?membrane template was used to obtain hollow carbon nanofibers,followed by infusing sulfur into the hollow fibers.This cathode not only facilitates the transportation of electrons and ions,but also shortens the distance of lithium ions diffusion in electrodes,contributing to a high reaction rate on kinetic process.As a result,the binder free and flexible cathode displays a capacity of more than 603.28 mAh g^-1 after 95 cycles at 0.5 C.2.We design a novel method to synthesize yolk-shell carbon/sulfur composites with sulfur fully confined inside the conductive carbon shells,and the filling content of sulfur can be well-controlled and fine-tuned.In the yolk–shell structure the sulfur spheres partially occupy the internal void space of highly conductive carbon,allowing for comfortable accommodation of the volume expansion of sulfur upon lithiation during the battery discharge process.The corresponding yolk-shell carbon/sulfur composite exhibits a high reversible capacity and excellent high-rate capability,which can maintain 695 mAh g^-1 after150 cycles at 1 C,and 633 mAh g^-1 after 150 cycles at 2 C.