Design And Mechanism Of Active-Site-Optimization-Oriented Catalytic Materials For Lithium-Sulfur Batteries

Lithium-sulfur batteries(LSB)characterized with unique advantages in terms of energy density,price and cost,which are expected to replace traditional lithium-ion batteries.Therefore,the development and construction of high-performance LSBs system is regarded as one of the most promising solutions to support large-scale energy storage technology.The main bottleneck problems hindering the commercialization of LSB are the shuttle effect of polylithium sulfide(LiPSs)on the cathode and the growth of lithium dendrite on the anode.To solve the problems on cathode,catalytic materials are introduced to promote the conversion rate between LiPSs and Li2S2/Li2S,which is a fundamental breakthrough to overcome the shuttle effect and realize the ideal energy output of LSBs.To realize the efficient utilization of catalytic materials,guided by active site optimization,a series of high-performance catalytic materials were designed and applicated in LSBs by following the strategy of active site addition,coordination environment optimization and active site self-cleaning,respectively.In view of the problem of the anode,following the strategy of satisfying both anodes and cathodes,the dual-function catalytic material was designed to simultaneously realize the acceleration of the LiPSs conversion on the cathode and the uniform deposition of lithium ions on the anode,thus achieving the overall improvement of LSB performance.The specific research contents are as follows:(1)Strategy of increasing active sites.A novel in-situ self-phosphating technique was developed to prepare N.P co-doped carbon supported tungsten phosphide(WP/NPC)ultrafine nanocrystals.Benefitted from the small size and uniform dispersion of WP,the number of active sites was significantly increased.Catalytic performance tests confirmed that the WP/NPC configuration can accelerate liquid-liquid transformation of LiPSs,improve nucleation capacity and decomposition kinetics of Li2S.Guided by theoretical calculation,a novel mechanism of Li-S catalysis chemitry was revealed:a)The excellent interfacial charge state between WP and LiPSs can promote charge transfer,and accelerate nucleation of Li2S;b)The strong interaction between WP and Li2S through W-S bond was similar to a kind of"molecular scissors",which contributed to the activation of Li-S bond and reduced the decomposition energy barrier of Li2S.Benefitted from the abundant active sites and excellent catalytic activity of WP,the LSBs and pouch cells assembled with WP/NPC showed excellent performance.On the one hand,this work had developed a novel strategy of synthesizing phosphide with high active sites.On the other hand,the mechanism of catalytic conversion of LiPSs was further understood.(2)Coordination environment optimization strategy.Basised on the above work,in order to further improve the active site of the material and follow the coordination environment optimization strategy,a unique N,O bicoordination tungsten single atom(W-N2O2 SA)was prepared through a sacrificial template method.The catalytic performance tests demonstrated that the unique coordination of W-N2O2 can promote the interconversion kinetics between S and LiPSs.DFT calculation confirmsed that the W-N2O2 configuration exhibited a lower Li2S decomposition energy barrier than the traditional W SA coordinated with four nitrogens.Further charge analysis confirmed that the introduction of oxygen with more electronegative enhanced the polarity of whole system and increased the amount of charge transfer between the W-N2O2 and Li2S,thus promoted the decomposition of Li2S.Benefited from the advantages of this unique coordination structure,the LSBs assembled with the W-N2O2 showed excellent cycling and rate performance.In this work,we had not only realize the atomic level regulation of the catalytic active sites,but further reveal the structure-activity relationship between the coordination environment and catalytic activity.(3)Active site self-cleaning strategy.All the above work are perfoemed based a series of materials with single active site to accelerate the transformation of LiPSs and inhibit lithium dendrites.However,the active site will gradually become ineffective with the formation of insulating Li2S.On account of this,Zn8 with diverse N,O and Zn active sites and strong LiPSs adsorbability were prepared by sol method and applied in LSBs for the first time.Based on the adsorption difference of inactive sites towards different LiPSs and the steric hindrance effect generated by the ligands,a novel intramolecular tandem transformation of LiPSs within Zn8 was realized,thus achieving the self-cleaning effect of the active sites.It was predicted by theoretical calculation that this novel transformation mechanism can reduce the nucleation/decomposition energy barrier of Li2S.Kinetic analysis also confirmed that this mechanism can accelerate the redox reaction of LiPSs,and induced the unique three-dimensional nucleation mode of Li2S,which contributed to the increase of capacity.All the above advantages were directly confirmed by in-situ XRD.Attributed to this transformation mechanism,the corresponding LSBs delivered superior rate performance,high-current and high-sulfur-loading performance.This work was not only the first report of the application of polynuclear metal clusters in LSBs,but also provide a novel idea for the design of LSBs catalytic materials benefitted by this novel intramolecular relay transformation mechanism.(4)The strategy of satisifing both anode and cathode.Based on the above studies,the slow sulfur conversion kinetics and shuttle effect on the cathode of LSBs were gradually and deeply conducted.However,capacity loss and cycle performance attenuation caused by lithium dendrites still existed on the anode of LSBs.Considering that,a new type of nitrogen-doped graphene-supported tungsten diselenide flakelets(WSe2/NG)were synthesized via further high-temperature selenization method based on the W-N2O2 work,serving as both the sulfur and lithium carriers in LSBs.Electrochemical analysis and theoretical calculations had confirmed that,on the cathode,WSe2/NG can accelerate the conversion kinetics of LiPSs and inhibit shuttle effects.On the anode,WSe2/NG was conducive to the adsorption and migration of Li+,reduced the nucleation overpotential of Li+,promoted the uniform distribution,and thus inhibited the formation of lithium dendrites.The WSe2/NG based lithium-sulfur soft-pack battery can achieve stable cycling under poor lithium and electrolyte conditions,demonstrated the great potential of WSe2/NG in practical applications.This "strategy of satisifing both anode and cathode" provided an effective solution for improving the performance and practicality of LSBs.