Cathode Materials Design And Intermediate Control For Lithium-sulfur Batteries

Lithium-sulfur(Li-S)battery has become a potential next-generation storage system due to its high theoretical energy density of 2600 Wh kg-1,low cost and environmental friendliness.However,it also suffers from the low conductivity of sulfur species,the large volume change during the electrode process,the dissolution and migration of the long-chain intermediate polysulfides in the electrolyte,and the dendrite growth of lithium anode.To overcome the bottleneck issues of lithium-sulfur system,this dissertation propose some new type adsorbents to control the migration of the intermediate polysulfides.Based on the absorbents,some novel nano-architectures have also been designed.The main research contents and results are listed as follows:1.A perovskite structure La0.6Sr0.4CoO3-?(LSC)was first proposed as a new polysulfide immobilizer to suppress the migration of the intermediate in the electrode process.LSC was fabricated by the electrospinning methode.Due to doping of Sr element,LSC obtains various valence of Co element and creates some oxygen vacancies,which improved the electronic conductivity of LSC and chemical adsorption ability to lithium polysulfides.The author have studied the adsorption mechanism of LSC and polysulfides through the visual absorbent experiment,iodometric titration,X-ray photoelectron spectroscopy(XPS)and the first principle calculation,providing the evidences of Co-S chemical bonds existed in the charge/discharge process.In addition,a coaxial yolk-shell nano fiber structure was designed,which contains LSC core layer,carbon shell layer,and sulfur interlayer.The LSC/S@C composite cathode with a mass loading of 2.1 mgsulfur cm-2 presents a high reversible capacity of 996 mAh g-1 and long-term cycle stability with only 0.039%capacity decay per cycle over 400 cycles at 0.5 C.Even the S loading is increased to 5.4 mgsulfur cm-2,the LSC/S@C composite cathode can still achieve comparable S utilization and good cyclability.2.To address the poor conductivity and shuttle effect of cathode materials,we employ easily-obtained titanium nitride(TiN)as a highly efficient immobilizer to trap polysulfides via a chemical mechanism.The XPS analysis shows Ti-S and N-S bonds form during the charge-discharge process.TiN also possesses high electronic conductivity which helps in achieving a high sulfur utility and an excellent rate capability.The obtained TiN/S composite cathode shows a high reversible capacity of 1012 mAh g-1 and an excellent cyclability with only 0.2%capacity decay per cycle during 200 cycles at 0.5 C rate.The TiN/S cathode also exhibits a high rate capability.Even at a 5 C rate,the discharge capacity is still higher than 550 mAh g-1.3.To overcome the limitation from lithium anode,lithium sulfide(Li2S)can be used as the active materials and lithium source.We designed TiN/PHC@Li2S composite cathode via a very simple slurry soaking way with the porous carbon and TiN composite matrix(TiN/PHC).The novel nanostructure provides an excellent three-dimensional conductive framework with parallel hollow carbon fibers.The interaction mechanism between TiN powder and lithium polysulfides was proved by DFT theoretical calculation.Using this free-standing TiN/PHC@Li2S as a cathode material,the lithium sulfur battery achieves a high reversible specific capacity of 1004 mAh g-1(based on sulfur)at 0.5 C rate and excellent cycle stability with only 0.08%capacity fade per cycle over 400 cycles.4.A Nafion/super P-modified dual functional separator is designed to improve the long-term cycle stability and rate capability of the pure sulfur cathode.The electrostatic repulsion between the SO3-groups and the dissolved negative Sn2-ions,and the trap and reutilizing effect of super P for polysulfides,provide double insurance to confine the polysulfides within the cathode side,leading to great improvement in both reversible capacity and cycling stability of the sulfur cathode as compared to the battery with pristine Celgard separator.With such dual functional separator,a simple elemental sulfur cathode with 70%S content delivers a high initial discharge capacity of 1087 mAh g-1 at 0.1 C and a long-term cyclability with only 0.22%capacity fade per cycle over 250 cycles at 0.5 C.