Desigin Of Cathode And Separator Materials And Electrochemical Characterization For High Performance Lithium-Sulfur Battery

With the increasing demand for portable electronics and electric vehicles,new energy storage systems with high energy density and long-cycle life has significantly increased to replace the traditional lithium-ion batteries(LIBs),which are limited by their theoretical energy density.Lithium-sulfur(Li-S) batteries have attracted great interest for next-generation energy storage technology due to its inherently high theoretical specific capacity(1675mAhg^–1) and high theoretical energy density(2600Whkg^–1).Apart from that,sulfur has many advantages,such as low-cost,nontoxic and environmentally friendly.However,the practical application of Li-S batteries still faces several complex challenges:for instance,the poor electronic conductivity of sulfur and Li₂S₂/Li₂S,the large volume changes during cycling,the high dissolution and shuttling behaviors of the intermediate polysulfids(PS)and corrosion and dendrite of Lithium anode.As a result,Li-S batteries show low sulfur utilization and poor cycling stability.In order to solve the above problems,we successfully designed sulfur host for cathodes and functionalization of separator which has been achived the high sulfur loading and long cycle performance of Li-S battery.The main content is as follows:First,the coupling of the sulfur host of the hollow and graphitic carbon flakes(HGCF/S) and the Barium titanate(BT) nanoparticles(HGCF/S-BT) as a cathode material.As highly efficient electrocatalyst and demonstrate much higher redox dynamics towards the conversion reaction of PS and Li₂S of BT nanoparticles,as shown by both electrochemical measurements and DFT calculation.The capacity of HGCF/S-BT retains 453.3mAhg^-1after 1000 cycles at 1C,corresponding to a capacity retention of 59% and an average capacity decay of 0.041% per-cycle;HGCF/S-BT electrode with S loading of 3.4 and 4.5mgcm^–2 delivers an areal capacity of 4.2 and 4.9mAhcm^-2.Moreover,HGCF/S-BT with S of 3.4mgcm^–2 still delivers a reversible capacity of 613mAhg^-1 after 300 cycles at a current density of 0.5C.HGCF/S-BT also shows great promise for practical application in flexible devices as demonstrated on the soft-packaged Li-S batteries.Next,we prepared cobalt(Co) nanoparticles-decorated and nitrogen-doped carbon nanocages confined in a carbon shell(denoted Co-NC@C) through carbonization of polyacrylonitrile(PAN)-encapsulated ZIF-67(ZIF-67@PAN) prepared by a simple“phase-inversion”method.The Co nanoparticles supported on nitrogen-doped carbon frame and confined in the carbon shell can efficiently promote the redox kinetics of PS,as demonstrated by DFT and electrochemical measurements.Therefore,the sulfur cathode based on such confined hollow core/shell nanocages delivers a capacity of 635.8mAhg^–1 at 5C and an outstanding cycling stability with an ultralow capacity decay of 0.018% per cycle for 2700cycles at 1C.Furthermore,a reversible capacity of 552.9mAhg^-1 was delivered at 0.2C over 500 cycles at a high sulfur loading of 5.7mgcm^-2under lean electrolyte condition.Meanwhile,a high areal capacity of 9.5mAhcm^-2 was achieved at a sulfur loading of 10.2mgcm^-2.More intriguingly,a soft-packaged Li-S batteries based on this composite cathode material also exhibited superior cycling stability in folding conditions.At last,by selectively coating a set of 2-demensional(2D)organic molecules,chenodeoxycholic acid(CA),4,4?,4??,4???-(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid)(TTBA),berberine chloride hydrate(BCH)and poly(diallyl dimethyl ammonium chloride)(PDDA)on honey comb-like porous carbon(HC)interlayers for Li-S batteries,respectively.The redox kinetics of PS are remarkably enhanced in the interspace between the 2D molecules and carbon matrix,in the order of CA<TTBA<BCH<PDDA by electrochemical techniques and DFT calculation,with the most efficient molecules featured with positively charged amino groups.Among these 2D molecules,PDDA coating,due to the positively charge amino groups,is the best organic modifier in promoting the redox kinetics of PS,delivering a capacity of 637.6mAhg^-1 at 10C;601.7 and 483.5mAhg^–1 after 1200 cycles at 2C and 4C,corresponding to a capacity decay as low as 0.024% and 0.031%per cycle,and an areal capacity of 8.4mAhcm^-2 at high sulfur loading of 8.9mgcm^-2.Furthermore,the cycling performance of these Li-S batteries with interlayers of super P and CNTs was much enhanced by PDDA coating.In all,we designed sulfur host for cathodes and functionalization of separator to catalyze and convert PS,thereby improving the electrochemical performance of Li-S battery.