The Design And Mechanism Study Of Functional Mateirals For High Performance Lithium Sulfur Secondary Batteries

Environment and energy are the two critical concerns in the development of human society.It has been a hot research topic to develop high-performance secondary batteries and realize their application in different fields,such as large-scale energy storage,new energy vehicles and modern defense and military.Lithium-sulfur batteries have much higher theoretical specific energy than traditional lithium-ion batteries.Moreover,sulfur as an active material is natural abundant,inexpensive,easily accessible,and environmentally friendly.However,studies have also shown that polysulfide dissolution,volume expansion and poor stability of lithium metal anode and other issues seriously restricting the practical development of lithium-sulfur batteries.In this dissertation,we focused on the design and preparation of high stability and long cycle life lithium sulfur batteries by constructing key functional materials,optimizing the overall structure of lithium-sulfur batteries,obtained main achievements as follows:(1)In this thesis,a coaxial polydopamine-sulfur-hydroxylated carbon nanotubes sulfur cathode and polydopamine bilaterally-coated separator were prepared by in-situ coating methods.By this way,polysulfide was effectively restricted in the cathodic region,which improves the utilization of the active material.Moreover,the coating of the polar polydopamine enhanced the infiltration performance of the electrolyte,improving the rate performance of lithium sulfur batteries.Furthermore,the high elasticity characteristic of polydopamine effectively buffered the volume change of the sulfur electrode during the electrochemical reaction and promoted the cycle stability of lithium-sulfur batteries.At a current density of 0.2 C,the obtained lithium-sulfur battery based on above concept displayed an initial reversible discharge capacity of 1213.3 m Ah/g.After 100 cycles,the reversible discharge capacity still maintained at 949.2 m Ah/g with a coulombic efficiency of 99.3%.(2)Based on a template-free method,we conceptually designed double-shelled hollow nano-sulfur cathodes with internal polydopamine and external conducting carbon black.The highly viscous polydopamine such as a nano-binder can stick hollow nano-S and conductive carbon blacks together at nano-scale,which improves the timeliness of cathode and further boost their cycle performance.The nano-binder based cathode can not only facilitate electron/ion transfer,trap polysulfides but also prevent carbon black detaching from cathode surface during cycling and ensure their reutilization.Moreover,the hierarchical double-shelled structure can act as physical and conductive barrier to effectively trap the polysulfides and reutilize the migrating polysulfides.In addition,polydopamine coating layer can further prevent the cathode sphere from deformation due to its isotropy and good elastic constant(E=6.748 k Nm kg^-1).Having such constructive characteristics,this“nano-binder”decorated composite exhibited an ultralong cycle performance of 2500 cycles at 0.5 C,with only 0.014%capacity loss per cycle.(3)We successfully prepared a 2D“bubble-like”interconnected carbon fabrics(ICFs)with an ultrathin thickness of?30 nm using a“blowing bubbles”method,and used them as nano sulfur host.The close packing structure of ICFs resulted in the formation nanosulfur dots with sizes of 2-5 nm,which further spontaneously anchored into ICFs.A binder-free interconnected carbon fabrics/nano sulfur/reduced graphene oxides(ICFs/n S/r GO)cathode with a sandwich structure was formed after incorporated with reduced graphene oxide.Such design can not only facilitate fast ion/electron transport but also effectively tether the soluble polysulfide intermediates and prevent agglomeration occurs during the reaction.With 70 wt%high sulfur content and above 2.8 mg/cm² areal sulfur loading,this binder-free cathode delivered a high reversible capacity of 1149.0 m Ah/g at 0.1 C.After 200 cycles,the discharge capacity maintained at 892.3 m Ah/g.Notably,the highest specific energy of pouch Li-S cells assembled with this cathode could be achieved to 315.98 Wh/kg.The highest capacity of the pouch cell was 1.7930 Ah,and maintained at 1.3745 Ah after 51 cycles.(4)The modular assembly of scattered ketjen black nanoparticles into an oval-like secondary microstructure via double“Fischer esterification,”which is a form of surface engineering used to fine-tune the materials surface characteristics,is presented.After carbonization,the obtained oval-like carbon microstructure shows promise as a candidate sulfur host for the fabrication of thick sulfur electrodes.The oval-like structure with large electrolyte/electrode contact area,providing more channels and active sites for lithium ions transportation,thereby increasing the electrochemical performance of the lithium sulfur cells.Through the coating method,a thick sulfur cathode with sulfur loadings abve 7 mg/cm² was prepared.The initial specific discharge capacity and the highest specific discharge capacity of OLCM/S cathode is 6.225 m Ah/cm² and 8.417 m Ah/cm² at 0.1 C with a high sulfur loading of 8.9 mg/cm²,respectively.After 400 cycles,the discharge capacity remained at4.526 m Ah/cm².The large-scale production of advanced lithium–sulfur pouch cells with an energy density of 460.08 Wh/kg@18.6 Ah is also reported.(5)To suppress polysulfide shuttle effect and protect lithium anode effectively,a modularly assembled interlayer was prepared by agglomerating Vulcan XC72 carbon black nanoparticles into ellipsoidal superstructures through a double Fischer esterification reaction.The MAXC interlayer can efficiently trap soluble intermediate polysulfides and provide space to store electrolyte and reaction products.The sandwich has multiple roles:(i)helps to effectively adsorb polysulfides and provide sufficient space for electrolyte and reaction products;(ii)promotes lithium metal anodes to form a dense and stable structure,enabling prolonged cell cycling;(iii)The rich pinhole-like structures in the interlayer can stabilize the lithium ions flow and lead to the uniform deposition of lithium ions.At a high sulfur loading of approximately 5.3 mg/cm²,lithium sulfur coin cells with MAXC interlayer maintained a high reversible capacity of 909.0 m Ah/g and a high areal capacity of 4.75 m Ah/cm² after 100cycles at 0.1 C.A Li/MAXC-MAXC/Li symmetrical cell sustained stable 180-cycles test and delivered only a 20 m V overpotential with a depositing/stripping capacity of 1 mAh/cm²at 1mA/cm².