Theoretical Design And Research Of Two-dimensional Catalytic Materials For Lithium-sulfur Batteries

With the continuous development of human society,the demand for energy storage equipment is also increasing.As well known,Li-ion batteries have been the most commercialized energy storage system since the 1990s.However,after a series of development and improvement,the performance of Li-ion batteries has almost reached the upper-limit of theoretical prediction.Many studies have pointed out that the low energy density（140～260 Wh/kg）and high production cost（>100$/k Wh）of Li-ion batteries will no longer have the ability to provide long-term power for portable electronic devices and various transportation-based vehicles.Therefore,in order to better meet the needs of the development of human society,it is urgent to develop the next generation of energy storage equipment to replace the Li-ion batteries.Lithium-sulfur（Li-S）batteries have attracted extensive attention and research due to its advantages of ultra-high theoretical specific capacity（～1670 m Ah/g）,energy density（～2600 Wh/kg）and rich sulfur electrode reserves.In addition,the chemical reaction substances involved in Li-S batteries are non-toxic,harmless,and environmentally friendly.However,there are three major challenges hindering the commercialization of Li-S batteries:（1）the insulating properties of sulfur（S₈）in the cathode of the battery and the insoluble short chain lithium polysulfide（Li PSs）products（Li₂S and Li₂S₂）generated during the reaction;（2）in the process of charging and discharging,the soluble long-chain Li PSs（Li₂S₄,Li₂S₆,Li₂S₈）obtained after lithification of the elemental sulfur migrate back and forth between the cathode and anode of the battery,causing the“shuttle effect”;（3）slow reaction kinetics of polysulfides charging-discharging conversion.In recent years,a large number of studies have shown that single atom catalysts（SACs）with TM-N₄ catalytic active centers can greatly alleviate the above problems,thus greatly improve the performance of Li-S batteries.Therefore,based on density functional theory（DFT）calculations and two-dimensional materials which are widely applied in experiments,this paper constructed three types of SACs with TM-N₄catalytic environment to carry out comprehensive theoretical research and exploration.The main research contents are as follows:（1）Phthalocyanine（abbreviated as Pc）was selected as the base material for anchoring all 4d transition metals（TM=Y,Zr,Nb,Mo,Tc,Ru,Rh,Pd,Ag and Cd）to design SACs for theoretical research.First of all,the structure of all substrate materials and polysulfides was constructed and optimized.Subsequently,the binding energy of SACs to S₈/Li PSs and the energy barrier of charge-discharge reaction process were calculated to evaluate and analyze their performance in inhibiting shuttle effect and accelerating charging-discharging reaction.In addition,the electronic structure analysis methods including charge density difference,projected density of states and COHP were also applied to reveal the essence of SACs improving catalytic performance.The results show that the interaction between the d orbital of TM and the p orbital of S is the key factor to enhance the adsorption performance and inhibit the shuttle effect.Zr Pc and Nb Pc are excellent catalytic materials in all aspects of SACs designed.Finally,the results of ab initio molecular dynamics simulation（AIMD）fully predicted the high feasibility of these SACs in experimental preparation and application.（2）Phosphorus carbide（CP）was selected as the substrate material for anchoring 3d transition metals（TMs=Ti,V,Cr,Mn,Fe,Co,Ni,Cu and Zn）to design and screen SACs.Based on the evaluation of structural stability,conductivity,adsorption performance of S₈/Li PSs and charge-discharge reaction performance of SACs,a systematic“four-step screening”process was first proposed and applied for the screening of SACs（TM@N₄-CP）in Li-S batteries.After screening,V@N₄-CP structure shows great potential for development because of its excellent structural stability,conductivity,strong adsorption of S₈/Li PSs and low energy barrier of charge-discharge reaction.In addition,some possible prediction descriptors were further proposed to efficiently predict the adsorption and charge-discharge reaction performance of SACs.Among them,the adsorption energy of SACs to a single S atom（E_a（*S））and the variables relating to the coordination environment（φ）can be used to roughly evaluate the adsorption performance of S₈/Li PS_s.Besides,the value of ICOHP value of TM-S bonds(ICOHP_TM-S)and the adsorption energy of SACs to Li₂S（E_a（*Li₂S））can be used as excellent descriptors for simultaneously predicting the charging-discharging reaction energy barrier.（3）Ten TM-r TCNQ catalysts with TM-N₄ active centers were constructed by bonding N in TCNQ（7,7,8,8-tetracyanoquinodimethane）unit with all 3d transition metal atoms（TM=Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu and Zn）.The stability,conductivity,adsorption of S₈/Li PSs,charge-discharge catalysis and Li-ion diffusion properties of these structures are calculated to comprehensively evaluate their potential application value.The theoretical calculation results fully show that the comprehensive performance of V-r TCNQ structure is the best of all structures,even better than some recently proposed high-efficient SACs.Additionally,the Mn-r TCNQ structure that has been synthesized in the experiment is also very hopeful to be further confirmed in the Li-S batteries experiment.