Preparation Of Cathode Materials And Characterization Of Electrochemical Performance For High Performance Lithium-sulfur Batteries

Lithium–sulfur（Li–S）batteries,have been considered as one of the promising technology among next generation battery energy storage systems,owing to their high theoretical specific capacity（1675 m Ah g-1）and specific energy density（2600 Wh kg-1）.Meanwhile,the low-cost,nontoxic,and environmentally friendly cathode active material（elemental sulfur）makes Li–S batteries a hot research topic for next generation energy storage systems.Although sulfur as cathode material offers various advantages,it also has obvious shortcomings and critical challenges.For instance,the insulating property of sulfur and lithium sulfides lead to poor activity of sulfur electrodes;the dissolution of intermediate polysulfides（Li₂Sx,4 ≤ x ≤ 8）in organic electrolytes results in unwanted “shuttle effect” between the anode and cathode（The “shuttle effect” consists of polysulfides molecules migrating from anode to cathode and vice versa,but without carrying any useful net charge,which results in an irreversible loss of sulfur,a low Coulombic efficiency,a low cycling capacity,and an increase in interfacial resistance）.Thus,to overcome the intristic issues of sulfur cathodes mentioned above,this dissertation focus on designing novel sulfur materials and cathodes to improve the electrochemical performance,mitigate dissolution of polysulfides and eliminate “shuttle effect”.The summary of research is listed as follows:1.Mn₃O₄@CNF/S cathode with high sulfur loading:Freestanding paper cathodes（Mn₃O₄@CNF/S）with layer-by-layer structure are synthesized through electrospinning for high-loading lithium-sulfur（Li-S）battery.Sulfur is loaded in a three dimensional（3D）interconnected nitrogen-doped carbonfiber（CNF）framework impregnated with Mn₃O₄ nanoparticles.The 3D-interconnected CNF framework creates an architecture with outstanding mechanical properties.Synergetic effects generated from physical and chemical entrapment could effectively suppress the dissolution and diffusion of the polysulfides.Electrochemical measurements suggest that the rationally designed structure endows the electrode with high utilization of sulfur and good cycle performance.Specifically,the cathode with high areal sulfur loading of 11 mg cm-2 exhibits a reversible areal capacity over 8 m Ah cm-2.Furthermore,the fabrication procedure is low cost and readily scalable.2.Selenium-doped sulfurized polyacrylonitrile cathode with high-rate and long-life:Building sulfur cathodes with only low order Li₂Sn（n ≤ 4）intermediates can effectively prevent the dissolution of polysulfide and the “shuttling effect”,leading to high capacity and long cycle life for rechargeable lithium-sulfur（Li-S）batteries.Here a novel selenium-doped sulfurized polyacrylonitrile（Sex SPAN,x < 0.15,～50wt % Sex S）composites was designed and synthesized,which exhibited excellent electrochemical performance in both carbonate and ether-based electrolytes,delivering a high reversible capacity up to 1300 m Ah g-1 at 0.2 A g-1（0.13 C）,excellent active material utilization ratio（84%）and extremely high rate with capacity up to 900 m Ah g-1 at 10 A g-1（6.5 C）.The Li-Sex SPAN cells can cycle up to 800 cycles with nearly 100 % Coulombic efficiency and 0.029 % capacity decay per cycle.The dissolution and diffusion of polysulfide are successfully suppressed owing to fast reaction kinetics and the unique reaction pathway of Sex SPAN.3.In-situ synthesis of nano-Li₂ S for Li-S battery:Lithium sulfide（Li₂S）is considered as a promising lithium storage material because of its high theoretical specific capacity of 1166 m Ah g-1.The electrochemical performance of Li₂ S can be remarkably improved by introducing carbon to form Li₂S-carbon composites.However,the complex preparation process of Li₂S-carbon composites limits their large-scale applications.Herein,we present a green and facile strategy to synthesize porous carbon coated Li₂S-carbon nanotube composites（Li₂S@C-CNT）via spray and heating the mixture,which is the low-cost and large-scale method for in situ synthesis of Li₂S-carbon composites.For the Li₂S@C-CNT composites,Li₂ S particles are connected with neighboring particles throuth the 3D CNTs network skeleton.The charge barrier is as low as 3.1 V to effectively active the nano-Li₂ S in the first charge process.Furthermore,in the first cycle,the Columbic Efficiency is as high as 97 %,and the specific capacity is as high as 1100 m Ah g-1,which is close to the theoretical specific capacity.The Li₂S@C-CNT cathode with 5 mg cm-2 still deliver 450 m Ah g-1 at a current density of 0.1 C after 200 cycles.This unique nanoarchitecture not only prevents the loss of active materials and polysulfides diffusion,but also significantly facilitates fast electron and ion transport,thus resulting in a high specific capacity and remarkable cycle life.In this work,novel sulfur cathodes have been designed and synthesized,which successfully suppress the shuttle effect,improve the electrochemical reactivity,deal with volume expansion and enhance the utilization of active materials.As a result,excellent electrochemical performance of sulfur cathodes have been achieved.The reaction mechanism of the new sulfur cathodes has also been systematically studied.