Theoretical Studies On Intrinsic Heteroatoms-Doped Porous Organic Polymers For Cathodes Of Lithium-Sulfur Batteries

Materials genome engineering is an emerging frontier technology for materials science.The key point of materials genome engineering is the paradigm transformation from empirical studies to efficient discovery guided by theoretical prediction with the strategic target of reducing the time and cost of development by half to satisfy the urgent demand for new materials in various fields.Lithium-sulfur(Li-S)battery has been considered as one of the most promising energy storage devices owing to its high theoretical energy density,low price,and environmental friendliness.However,the shuttle effect of soluble lithium polysulfides(LiPS)results in poor cycling performances,which considerably impede the industrial applications of Li-S batteries.To inhibit the shuttle effect and improve performances of Li-S batteries,researchers make great efforts on structure and functionality design of sulfur hosts on the basis of physical confinement,chemical adsorption and catalytic conversion strategies.However,the developments of sulfur hosts traditionally depend on lengthy and costly trial and error experiments,which is unfavorable for the rapid discovery and application of advanced sulfur hosts.Therefore,this work proposes new modeling algorithms for intrinsic heteroatomsdoped porous organic polymers to simulate their complex amorphous microscopic structures following the idea of materials genome engineering.It has been proved by experimental efforts that the proposed models can predict structures and performances of amorphous porous organic polymers on inhibition of shuttle effect and provide theoretical guidance for designs andsyntheses of new sulfur hosts.The main research contents are as follows:(1)An atomistic structure modeling algorithm is proposed for amorphous porous organic polymers and applied on two multi-heteroatoms-doped covalent triazine-based frameworks(CTFs),namely,MCTF-1(with imide groups)and MCTF-2(with hydrazide groups).The simulated surface areas,pore volumes,X-ray diffraction(XRD)patterns and nitrogen adsorption isotherms of constructed models are consistent with experimental results.Molecular dynamics simulations are conducted to simulate the diffusion behaviors of LiPS under the electrolyte environment in MCTF-1 and MCTF-2 models to identify active adsorption sites for LiPS,and density functional theory is applied to calculate the adsorption energies.Thus,the physical confinement and chemical adsorption effects of microscopic structures of sulfur hosts on LiPS are explored.Simulation results indicate that MCTF-2 is more capable of inhibiting the shuttle effect of LiPS than MCTF-1 because its active adsorption sites can offer stronger adsorption for LiPS.Thus,MCTF-2 is more appropriate for sulfur hosts.Electrochemical measurements manifest that MCTF-2 can achieve a reversible capacity of 738 mA h g-1 with the capacity retention of 85%after 300 cycles at 0.5 C and shows better cycling performance than MCTF-1.Thus,theoretical predictions are consistent with experimental results.(2)On the basis of previous work,an atomistic structure modeling algorithm based on the synthesis process and characteristics of molecular structures is designed for CTFs.The proposed algorithm has been implemented on MCTF-2.The simulated surface areas,pore,volumes,pore size distributions and XRD patterns of MCTF-2 models are in good agreements with experimental results.Specifically,the simulated and experimental results of surface areas and pore volumes are 1043 m2 g-1,1009 m2 g-1,0.55 cm3 g-1 and 0.52 cm3 g-1,respectively.To explore the influence of microscopic porosities of sulfur hosts on diffusion behaviors of LiPS,the proposed algorithm is applied on new MCTF-3(with hydrazide and biphenyl groups).which has the same active adsorption sites with MCTF-2 but distinct porosity,and the adsorption and diffusion behavior of LiPS in MCTF-2 and MCTF-3 models are simulated.Simulation results demonstrate that increasing the proportions of micropores and active adsorption sites in sulfur hosts can enhance the inhibition of shuttle effect.The simulation work provides theoretical guidance for designs of sulfur hosts.Additionally,electrochemical measurement results indicate that MCTF-2 delivers higher capacity retention and better cycling performance than MCTF-3.Notably,the experimental results are consistent with theoretical analyses.Therefore,the proposed algorithm can offer rational predictive power for amorphous microscopic structures and functionalities of CTFs served as sulfur hosts.And screening new CTFs before synthesis efforts with this algorithm can reduce the time and cost of the development of new CTFs and promote industrial applications of advanced CTFs.(3)To further improve the catalytic effects of intrinsic heteroatoms-doped porous organic polymers,monodispersed single metal atoms are introduced into the polymer matrix.The performances of 20 single-atom catalysts(SACs)with various M-Nx(M=Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn;x=3,4)active centers composed of different transition metal atoms and distinct numbers and types of nitrogen atoms applied in Li-S batteries have been systematically investigated by means of first-principles calculations.From the comprehensive analyses of the stability,the chemical adsorption for LiPS and the catalytic efficiencies for the conversion of LiPS during the discharge process and the oxidation of Li2S in the charge process of M-Nx centers,M-N3 centers generally outperform M-N4 centers,some of which have been successfully employed in Li-S batteries(e.g.,V-N4,Co-N4 and Ni-N4).And V-N3 and Cr-N3 centers can achieve the decomposition barriers of Li2S as low as 1.17 and 1.09 eV,respectively,which manifests promising catalytic effects.The above theoretical calculation efforts provide significant insights for the implementation of new SACs in Li-S batteries.