Design and Construction of Micro/nanostructured Materials for Improving Electrochemical Preformance

Author:Song Yan

Supervisor:dong xiang ting


Degree Year:2019





Design and construction of micro/nanostructural materials used for developing new-typed high energy density storage system is an important research subject.Lithium sulfur batteries are a promising energy storage system benefitting from their high theoretical energy density,environmental benignity and cost-effectiveness.However,their practical application is hampered due to rapid capacity degradation and low areal capacity induced by shuttle of intermediate dissolved polysulfides and slow reaction kinetic.To tackle these intrinsic obstacles,designing and assembling micro/nanomaterials as the sulfur cathode,lithium metal anode or separator has become a meaningful approach.Herein,based on the synergistic effect of physical encapsulating and chemical trapping,we design and construct the micro/nanomaterials including hollow CoS@PPy nanocubic,one-dimensional nitrogen-doped hollow carbon framework,nitrogen and phosphorus co-doped carbon framework,two-dimensional Janus nanofiber membrane and three-dimensional hierarchical porous materials.These micro/nanomaterials with unique microstructure accomplish lithium sulfur batteries with high energy density and cyclability,and provide guidance for the practical applications of energy storage systems.1.Integrating the hollow CoS nanocubic and conductive polypyrrole,the CoS@PPy is successfully constructed to boost reaction kinetics and specific capacity of lithium sulfur battery on the basis of synergy of physical restrict and chemical adsorption.Benefiting from the advantage of Co-based zeolite imidazolate framework and the topology conversion reaction,the product presents the hollow structure and porous characteristic.The hollow architecture can provide ample space for loading active sulfur and effectively buffers volume variation of the electrode during cycling.Polypyrrole layer generates adequate anchoring sites for inhibiting shuttle effect of dissolved polysulfides and accelerates multielectron conversion reactions in lithium sulfur battery.More importantly,the CoS@PPy is able to accelerate the redox kinetics and presents low overpotential.Such composite electrode with a sulfur content of 60%can deliver a superior discharge capacity(1165 mAh g-1 at 0.2 C)with a small capacity fading rate of 0.10%per cycle after 200 cycles.2.Inspired by dandelion and cattail with one-dimensional consecutive framework,the nitrogen-doped hollow carbon framework and nitrogen,phosphorus co-doped carbon framework are successfully prepared using a simple calcination treatment in ammonia atmosphere.The nitrogen-doped hollow carbon framework with unique hollow architecture as the carrier for sulfur can effectively buffer volume variation of the active sulfur and confine polysulfides shuttle.For nitrogen and phosphorus co-doped carbon framework,the unique bamboo-like architecture endows the sufficient contact interface between electrode and electrolyte and anchor sites for polysulfides.Moreover,the one-dimensional consecutive carbon framework provides an effective conductive pathway for electrons,which can improve the redox kinetic of sulfur cathode.In the such one-dimensional carbon framework,the heteroatom doping can further improve the electronic conductivity of electrode and suppress the diffusion of polysulfides based on the chemical binding effect.Benefiting from the unique architecture and heteroatom doping,both of the carbon frameworks are able to significantly improve electrochemical performances of lithium sulfur batteries,achieve high specific capacity of over 1000 mAh g-1 and realize cyclability of over 500 cycles with a capacity fading rate of less than 0.05%per cycle.3.Based on the electrospinning technology,Janus membrane with polyacrylonitrile nanofiber and polyacrylonitrile/polypyrrole composite nanofiber interlayer is designed and constructed for improving the cyclability of sulfur cathode and lithium metal anode.In the Janus membrane,the polyacrylonitrile nanofiber layer can inhibit the shuttle effect of dissolved polysulfides based on physical obstruction and chemical immobilization.More significantly,the polypyrrole nanofiber layer can effectively adsorb and regenerate polysulfides.In addition,the nanofiber membrane obtained by electrospinning reveals the advantageous affinity between the separator and electrolyte,which is conducive to enhance the adsorption ability of the membrane for the electrolyte and improve the transmission rate of lithium cations.Benefiting from these merits,the Janus membrane realizes a lithium sulfur battery of higher performance.Moreover,the lithium sulfur battery with the Janus membrane demonstrates a high specific capacity of over 1200 mAh g-1 and outstanding cycle stability.4.A rich nitrogen doping hierarchical porous carbon framework is constructed on account of the polarity difference of the components in the precursor.The carbon framework presents three-dimensional hierarchical porous network and interconnected microchannels.Specifically,both channels and porous structure can be regulated by adjusting the ratio of polyacrylic acid to polyacrylonitrile in the precursor.The porous carbon network effectively triggers fast charge transfer due to the sufficient contact area between electrode and electrolyte.The introduction of nitrogen atoms in the porous carbon network effectively suppresses the translocation of the polysulfide and further expedites the electronic conduction.Taking advantage of the hierarchical porous architecture design,the carbon framework/sulfur composite electrode achieves remarkable specific capacities of 1228 mAh g-1 with a low capacity decay rate of 0.06%per cycle.The composite electrode also supports high sulfur loading of 6.5 mg cm-2 and excellent cycling stability.In addition,a sulfurized polyacrylonitrile cathode material with porous structure is obtained by the same strategy and a porogen of urea.The sulfurized polyacrylonitrile electrode promotes electron transfer process and avoids the formation of lithium polysulfide.This novel electrode material has important application prospects for accomplishing high performance energy storage systems.