Design And Propoerties Of Lithium Sulfur Cathode Materials Based On Carbon Nanostructures

Lithium-sulfur?Li-S?batteries are one of the most promising next-generation energy storage systems.The“shuttle effect”and slow redox reaction kinetics of the active ingredient sulfur limit its commercial application.In order to overcome the inherent problems of the sulfur cathode,great efforts have been made to improve the cathode performances in Li-S batteries.However,it is still a challenge to effectively construct the cathode with high catalytic properties.This article focuses on the exploration of low-cost and efficient catalysts,the construction of reasonable and stable structures and the design of efficient and synergistic components to improve the electrochemical performance of Li-S batteries based on carbon nanostructures.Meanwhile,the adsorption-catalysis synergistic mechanisms were explained.In addition,the action mechanism between catalyst and polysulfides is further understood by means of theoretical calculations and ex-situ X-ray photoelectron spectroscopy.The specific research content and results are as follows:?1?The structural design and study on the electrochemical properties of nitrogen-doped carbon nanosheets loaded with pyrite FeS₂ nanoparticles for Li-S batteries.Aiming at the problems of low sulfur utilization rate and polysulfide dissolution in Li-S batteries,a strategy was proposed that combining highly electron-conducting nitrogen-doped carbon nanosheet with high-adsorption inorganic FeS₂ nanoparticle?NCNs/FeS₂?.The experimental results show that the FeS₂ not only has strong adsorption capacity for lithium polysulfide,but also can catalyze the conversion of lithium polysulfide.In addition,the theoretical calculations further revealed that FeS₂ has a stronger adsorption capacity for polysulfides than other disulfides.The existence of FeS₂ is beneficial to the uniform deposition of the discharge product Li₂S.A solid-liquid-solid conversion mechanism was proposed at the three-phase interface among FeS₂,carbon nanosheets and electrolyte.The NCNs/FeS₂ composite material is applied to lithium-sulfur battery,which shows excellent rate and cycle performance.At a current density of 0.2 C,the S-NCNs/FeS₂ electrode maintained a specific discharge capacity of 782 mAh g^-1 after 160 cycles.When the sulfur loading increased to 2.03 mg cm-²,it remained after 100 cycles It has a specific capacity of745.6 mAh g-¹.?2?Carbon confined synthesis of hollow carbon spheres loaded with defect-rich MoS_2+x composites and their cathode performance for Li-S batteries.Aiming at the problems of large volume changes and polysulfide dissolution during charge and discharge process in high energy density Li-S battery,we proposed a design idea of combining hollow carbon nanostructure with bifunctional layer coating:A hollow carbon nanosphere sulfur-rich defects MoS_2+x double-shell micro-nano structure material(HCNs@c-MoS_2+x)was prepared by in-situ vulcanization using polystyrene sphere as templates.The hollow structure not only can effectively buffer the volume change of sulfur during the charge/discharge process,but also can physically block polysulfide from entering the electrolyte to suppress the shuttle effect.Experiments have confirmed that carbon-synthesized MoS_2+x nanosheets have wide interlayer spacing and abundant sulfur atom bridging defects,which can significantly enhance the adsorption and anchoring of soluble polysulfides and accelerate their catalytic conversion kinetics,thereby greatly improving the rate and cycling performance of Li-S batteries.The S/HCNs@c-MoS_2+x electrode with sulfur load of 3.8 mg cm^-2 maintains the capacity of 802 mAh g^-1 after 140 cycles at a current density of 1.0 C.?3?In situ construction of a nitrogen-doped carbon-coated ZnS-FeS heterostructure and its Li-S battery cathode performance.Aiming at the problems of slow conversion kinetics and shuttle effect of polysulfides in Li-S batteries,we designed on the basis of the hollow nanostructure combined with large heterojunction band gap as an efficient coordination component?ZnS-FeS/NC?to improve the electrochemical properties of Li-S batteries.Theoretical calculation and kinetics analysis reveal the ZnS-FeS heterostructure has excellent electronic/ionic conduction,which can effectively enhance polysulfides adsorption and catalytic conversion.The ex-situ impedance test reveals the heterojunction interface significantly increased the liquid-solid-liquid transformation kinetics.The S@ZnS-FeS/NC electrode with the sulfur load of 3.34 mg cm-² shows a high discharge capacity of 3.57 mAh cm^-2 after 200 cycles at 0.2 C.