Research On Preparation And Properties Of Porous Graphene And Mxene-based Cathode For Lithium-sulfur Batteries

Lithium–sulfur batteries are regarded as the promising next-generation rechargeable battery systems owing to the high theoretical specific capacity(1675 mAh g^–1)and energy density(2600 Wh kg^–1),however,several issues such as poor electrical conductivity of active material sulfur,sluggish redox kinetics,and severe“shuttle effect”in electrodes still hinder their practical application.Fortunately,the rational designed and constructed cathode plays a key role in adressing the aforementioned issues.Graphene and MXenes,novel two-dimensional materials,owning controlled synthetic method,excellent conductivity and abundant functional surface groups are widely applied in energy storage and conversion field.Nevertheless,two-dimensional architectures deliver limited gravimetric energy density and rate performance because of the irreversible agglomerates via van der waals force andp-pstacking interaction.Therefore,four graphene and MXene-derived three-dimensional porous composites are proposed through self-assembly,physical activation and chemical etching,accompianed with investigation of electrochemical properties as sulfur host in this dissertation.The detailed researches,results,and discussion are concluded as the following aspects:Nitrogen-doped graphene nanoribbon/carbon nanotube hybrid aerogel?NGCA?with large specific surface area(740 m² g^–1)and pore volume(1.39 cm³ g^–1)is synthesized by one-step hydrothermal method.The cell with NGCA-based cathode delivers an initial specific capacity of 1220 m Ah g^–1,excellent rate capability(540 mAh g^–1 at 2.0 C)and superior cycling performance(587 mAh g^–1 after 100 cycles at the rate of 0.5 C).The three-dimensional structure with high porosity promotes the inner diffusion of electrolyte,while the introduced carbon nanotubes short the transport of electrons.In addition,the doped nitrogen affords strong interaction to lithium polysulfides through formation of Li bond?Li···N?,which remits the“shuttle effect”.The above synergistic effects enhance the electrochemical properties of cathode efficiently.A nitrogen-doped porous carbon?NPC?with hierarchical porous structure is prepared by a facile physical activation method,with nitrogen-doped graphene nanoribbon aerogel and carbon dioxide as a precursor and an activating agent,respectively.The NPC with a moderate nitrogen content?5.1 atom%?retains the sponge-like morphology of graphene nanoribbon aerogel,shows high specific surface area(1380 m² g^–1)and pore volume(2.0cm³ g^-1),and possesses hierarchical porous structures.The investigations indicate that large specific surface area and nitrogen-doped of NPC can effectively adsorb the lithium polysulfides,resulting in alleviated the“shuttle effect”.Additionaly,the hierarchical porous structure accelerates the inner diffusion of electrolyte and shorts the transport of electrons.As a result,the cell with S@NPC cathode exhibits a good rate capability and cycling stability.A nanostructured nitrogen-doped porous carbon/MoO₂ composite?NPC/MoO₂?is synthesized through a facile hydrothermal method by using graphene oxide nanoribbon and phosphomolybdic acid hydrate as precursors.The porous nanostructure promotes the diffusion of electrolyte,while MoO₂ nanoparticles immobilize lithium polysulfides via strong chemisorption and enhance the redox kinetics of polysulfides owing to the efficient catalytic activity in liquid–liquid and liquid–solid boundary.The electrochemical mesurements indicate the catalysis of NPC/MoO₂ composite in polysulfide conversion reactions.The first-principles calculations based on the density functional theory?DFT?indicate strong interaction between NPC/MoO₂ and lithium polysulfides.Consequently,the NPC/MoO₂-based cathode exhibits low polarization,high initial discharge capacity(1403 mAh g^-1 at 0.1 C),good rate capabilities(584 mAh g^-1 at 4 C),and superior cycling performance(503 mAh g^-1 at 1 C with capacity fade rate of 0.07%per cycle).A Ti₃C₂/carbon hybrid with expanded interlayer spacing is synthesized by one-step heat treatment in molten potassium hydroxide.A series of experiments indicate that Ti₃C₂/carbon hybrid exhibits effective entrapment to polysulfides through strong chemisorption interaction.Moreover,further electrochemical measurements confirm the accelerated redox kinetics,reduced overpotential for initial nucleation,and enhanced growth kinetics of Li2S in Ti₃C₂/C electrode.The disordered carbon in Ti₃C₂/C optimizes the surface charge transfer kinetics and affords extra deposition sites for Li₂S simultaneously.Consequently,the Ti₃C₂/carbon-based cathode boosts the performance in working lithium–sulfur battery,in terms of ultrahigh initial discharge capacity(1668 mAh g^-1 at 0.1 C),excellent rate performance(520 mAh g^-1 at 5 C),outstanding capacity retention of 530 mAh g^-1 after 500 cycles at 1 C with low capacity fade rate of 0.05%per cycle,and stable Coulombic efficiency?nearly 99%?.The above results indicate that this composite with high catalytic activity is a potential host material for further high-performance lithium–sulfur batteries.