Theoretical Investigations On Low-dimensional Nitrogen/phosphorus-based Materials For Inhibiting The Shuttle Effect In Li-S Batteries And Photocatalyzing Water Splitting

With the worsening environmental pollution isusses and energy crisis brought by traditional fossil fuels,the use of new energy sources has become more widespread.How to make new energy sources more efficient turned to be a hot topic for researchers.One of the feasible approaches is to design and use new energy materials with more excellent properties or modify existing new energy materials to improve efficiency.The discovery of graphene in 2004 triggered a boom research in two-dimensional(2D)materials.More and more 2D materials have been designed theoretically or synthesized experimentally.The family of 2D materials has many members with different properties.They have shown unprecedented application prospects in the fields of optoelectronics,catalysis,spintronics,information storage,and energy conversion,providing more possibilities for human development.With the increasingly environmental protection in our country,the use lithium batteries and hydrogen energy has become more and more extensive.Lithium-sulfur(Li-S)batteries are considered to be the most promising next-generation lithium-ion batteries due to the high energy density and specific capacity.The obviousdisadvantage of shuttle effect which determines the performances of Li-S batteries also exists.Although there are many methods to obtain hydrogen energy,how to prepare hydrogen energy in an energy-saving,environmentally friendly and efficient manner is challenging,and the photocatalysis makes it desirable.In this paper,research is carried out on the theme of inhibiting the shuttle effect of Li-S batteries and designing new and efficient photocatalyst materials.The specific research contents are as follows:(1)The Effect of Nitrogen-Based Anchorage Materials on Lithium Bonds to Inhibit the Shuttle Effects in Li-S BatteriesThe notorious polysulfide shuttle effect is a crucial factor responsible for the degradation of Li-S batteries.A good way to suppress the shuttle effect is to effectively anchor the dissoluble lithium polysulfides(LPSs,Li₂S_n)on appropriate substrates.Previous studies revealed that Li of Li₂S_n is prone to interact with N of the N-containing materials to form Li bonds.In this work,by means of density functional theory(DFT)computations,we explored the possibility to form Li bonds on 10different N-containing monolayers,including BN,C₂N,C₂N₆S₃,C₉N₄,covalent triazine framework(CTF),g-C₃N₄,p-C₃N₄,C₃N₅,S-N₂S,and T-N₂S,by examining the adsorption behavior of Li₂S_n(n=1,2,3,4,6,8)on these two-dimensional(2D)anchoring materials(AMs)to investigate the performance of the formed Li bonds(if any)in inhibiting the shuttle effect.By comparing and analyzing the nitrogen content,the N-containing pore size,charge transfer,and adsorption energies,we found that N content and N-containing pore size correlate with adsorption energies,and the formed Li-N bonds between LPSs and AMs well correspond with the adsorption energies of LPSs.At the same time,we also found that C₉N₄ and C₂N₆S₃ monolayers are promising AMs in Li-S batteries.From the views of Li bonds,this work provides the guidlines for designing 2D N-containing materials as anchoring materials to reduce the shuttle effect in Li-S batteries,and thus improving the performance of Li-S batteries.(2)Electric and Magnetic Properties of Nanowires and Nanosheets Assembled Based on P₈ and N₂"Bottom-up"method is a powerful approach to design nanomaterials with desired properties.The bottle neck of being oxidized of phosphorous structures may be conquered by cluster assembling method.Here,we used P₈ and N₂ as assembling units to construct 1D nanowire(NW)and two-dimensional 2D nanosheet(NS),the stability,electronic and magnetic properties of these assembled nanomaterials were investigated using density functional theory(DFT)calculations.The assembled1D-P₈N₂ NW and 2D-P₈N₄ NS were identified to possess good stability.Moreover,they also exhibit good anti-oxidization property.The 2D-P₈N₄ NS is a direct bandgap semiconductor with the HSE06 gap of 2.61 e V,and shows appropriate band-edge aliments and moderate carrier mobility for photocatalyzing water splitting.The1D-P₈N₂ NW is an indirect bandgap semiconductor,and Mn doping could convert it into a dilute magnetic semiconductor(DMS)with one Dirac cone in the spin-up channel,while the vd W-type sheet composed of Mn₁@1D-P₈N₂ NWs is a ferromagnetic metal.O ur theoretical study is helpful to design stable phosphorus-based nanomaterials with diverse properties and potential applications.(3)Two-dimensional Phosphorus-Based Binary Nanosheets for Photocatalyzing Water SplittingBased on the current research on phosphorus-based binary compounds,we designed three two-dimensional(2D)materials,namely P₂N₂,P₃N and?-P₃B monolayers.The stability,mechanical and electronic properties of these 2D crystals were investigated using density functional theory(DFT)method.Our proposed monolayers are all semiconductors and exhibit structural and mechanical anisotropy.The P₂N₂,P₃N and?-P₃B were identified to possess good thermodynamic,dynamical and thermal stabilities.The band edges and bandgaps of P₂N₂,P₃N and?-P₃B show different variation under external strains,and P₃N with strain modulation become suitable for photocatalyzing water splitting,due to appropriate band alignment,the different spatial distributions of CBM/VBM,long electron-hole(e-h)recombination time,the optical adsorption in visible and UV light region,and high hole mobility.Our systematic studies added new members of phosphorus nitrogen(PN)and phosphorus boron(PB)binary compounds,which are expected to serve as high efficient photocatalysts for water splitting and flexible nanoelectronic devices.(4)One-Dimesional Assembled Boron-Phosphorus Nanowires for Photocatalyzing Water SplittingBased on the powerful approach-cluster assembling method,five boron-phosphorus nanowires(NWs)were proposed,namely P₈B₂-I,P₈B₂-II,P₆B₄,P₈B₆ and P₁₀B₄ NWs.All the five assembled NWs exhibit good thermodynamic,dynamical and thermal stabilities according to our density functional theory(DFT)calculations.These five assembled NWs are wide-bandgap semiconductors,with the indirect/direct bandgaps in the range of 1.04?2.47 e V.Among them,P₈B₂-I,P₈B₂-II and P₈B₆ NWs perfectly satisfy the requirements(bandgaps and band positions)of a good photocatalyst for water splitting,and particularly the P₈B₂-II NW is a promising photocatalyst candidate for water splitting with high efficiency,because of the different spatial distributions of CBM and VBM,the comparable energy differences(?E₁/?E₂)between CBM/VBM level and water reduction/oxidation potential,good performance in hydrogen evolution and oxygen evolution reactions(HER/OER),and consierable optical adsorption in visible and UV light region.Our comprehensive studies may be helpful to design new P-based binary nanomaterials with specific application in photocatalyzing water splitting.