Design And Synthesis Of Novel Sulfur Composites For Lithium-Sulfur Batteries

Lithium-sulfur?Li-S?batteries have been considered as most promising candidates for the next generation lithium-ion batteries,as sulfur features high theoretical capacity(1675 mA h g^-1),high specific energy density(2600 Wh kg^-1),cost effectiveness,and nontoxicity.However,the commercialization of lithium-sulfur batteries has been severely hindered by the poor conductivity,shuttle effect,and the volume expansion.Based on this,the research work in this thesis mainly focuses on the design and synthesis of cathode materials for lithium-sulfur batteries.Aiming to solve the above problems,three types of novel sulfur cathode materials have been prepared and their electrochemical properties have been studied.The main research work is as follows:1.In order to eliminate the shuttle effect of the lithium-sulfur batteries,the S and PTCDA was mixed and co-heated to form short-chain S within the organic carbon framework composite.The carbonized organic skeleton provides fast electron transport path,while the linear S was covalently bonded to PTCDA.This unique chemical structure greatly inhibits the formation of long-chain polysulfide ions,thereby inhibiting the shuttle effect.By changing the mass ratio of PTCDA and S to prepare composites with different S contents,when the mass ratio of PTCDA and S is 1.2:1.0,the c-PTCDA/S-1.2 electrode remains 87.3%of the initial capacity over 500 cycles at a current density of 0.5 A g^-1.2.Based on the synergism of physical/chemical adsorption and catalysis effect,the construction of flexible sulfur cathode with NiFe₂O₄ hollow spheres and carbon materials?NiFe₂O₄@S/C?is proposed.In this unique nanoarchiteture,the carbon-based materials have physcial interaction with polysulfides,and act as conductive and flexible skeleton.The hollow sphere NiFe₂O₄ features chemical adsorption and catalytic conversion of polysulfides,thereby effectively suppress the shuttle effect.The as-prepared flexible NiFe₂O₄@S/C electrode delivers excellent electrochemical performance.After 500 cycles,a high capacity of NiFe₂O₄@S/C electrode is aorund 700 mAh g^-1,corresponding to a small capacity decay of0.059%per cycle,even at 3000 mA g^-1,the high specific discharge capacity of 637 mAh g^-1was still achieved.3.To avoid volume expansion problems and potential safety hazards caused by lithium metal anodes,Li₂S@TiN composite with core-shell structure was synthesized by in-situ growth method.In this nanostructure,the TiN coating layer limits the diffusion of polysulfides through physical and chemical confinement.Meanwhile,due to the excellent conductivity and porous structure of the TiN coating layer,electronic/ionic transport and electrochemical reaction process can be accelerated.At the current density of 0.2 C,the Li₂S@TiN electrode delivered specific discharge capacity of 1190 mAh g^-1,and maintained at60.1%of the initial capacity after 200 cycles.