Constructing And Srengthening Multiple Conversion Pathways To Promote The Kinetics Of Slow Reactions In Lithium-sulfur Batteries

Lithium-sulfur（Li-S）batteries become one of the most promising candidates in virtue of high specific energy density(2600 Wh kg^-1),but sluggish conversion kinetics of polysulfides（LPS）and shuttle effect hamper its application.Promoting the reduction from LPS to Li₂S is a good way to inhibit the shuttle effect.Herein,by applying electrocatalysts on the commercial PP separator,this research aims at improving the conversion kinetics of LPS to Li₂S and exerting the synergistic effect of multifunctional catalytic sites to suppress the shuttle effect and improve the cycling stability of Li-S batteries.Herein,this research explores and clarifies the possibility and strategy of stimulating multiple LPS conversion pathways,providing fast conversion pathways for intrinsically sluggish LPS conversion.Besides,bimetallic carbide C/Ni₃Zn C_0.7 is used as a model catalyst to explore the valence modulation of bimetallic sites by engineering the donor defects（P dopants）and acceptor defects（Ni vacancies caused by Na BH₄etching）.The contribution mechanism of bimetallic catalytic active sites to LPS multipath conversion is clarified.And the key role of Ni²⁺/Ni（0）pair in promoting the reduction of LPS is revealed.Moreover,a bimetallic catalyst is designed to simultaneously strengthen the discharging（LPS reduction）and charging（Li₂S oxidation）processes.The promotion mechansims of different metal sites on the inherent solid（S₈）-liquid（LPS）-solid（Li₂S）redox pathway and the thiosulfate-mediated pathway are revealed,realizing the bidirectional catalysis for Li-S batteries.Tris（2-carboxyethyl）phosphine hydrochiloride and porous carbon（PCN/TCEP）trigger the generation of thiosulfate during the discharging process,thereby constructing multiple LPS conversion pathways.On the one hand,PCN/TCEP accelerates the breaking of disulfide bonds（-S-S-）of LPS and improves the kinetics of the intrinsic solid（S₈）-liquid（LPS）-solid（Li₂S）reduction reaction.On the other hand,PCN/TCEP triggers the generation of thiosulfate and stimulates a new thiosulfate-mediated LPS conversion pathway.Such two LPS conversion pathways synergistically promote the reduction kinetics from LPS to Li₂S,regulate the uniform deposition of Li₂S,and effectively inhibit the shuttle of LPS.When PCN/TCEP is used to modify the commercial PP separator,the Li-S battery exhibits the long-term cycling stability with a decay rate of 0.0356%over 700 cycles at 1.0 C.By engineering donor defects（P dopants）and acceptor defects（Ni vacancies caused by Na BH₄ etching）,valence states of the key Ni²⁺/Ni（0）redox pair in C/Ni₃Zn C_0.7 catalysts are modulated,and the kinetic characteristics of LPS conversion via multiple pathways are promoted.The introduction of donor defects（P dopants）in C/Ni₃Zn C_0.7-P significantly decreases the electron cloud density of Zn sites,meanwhile the introduction of acceptor defects（Ni vacancies）in C/Ni₃Zn C_0.7-B optimizes the valence states of Ni sites with more Ni²⁺species.The impacts of bimetallic sites on Li-S batteries are different:Zn sites serve as"electron reservoirs"to assist the valence regulation of Ni sites,while Ni²⁺/Ni（0）is a key redox pair that promotes LPS multipath conversion.Particularly,the introduction of acceptor defects promotes the electron transfer between the Ni²⁺/Ni（0）pair and LPS and the generation of thiosulfate intermediates,thereby strengthening the thiosulfate-mediated LPS conversion pathway and effectively inhibiting the shuttle of LPS.When C/Ni₃Zn C_0.7-B1 is used to modify commonly used PP separators for Li-S batteries,an improved cycling stability（a decay of 0.0179%over 1400 cycles at 1.0 C）is achieved.Besides,when the areal sulfur loading increases to 4.0 mg_s cm^-2,the specific capacity can reach 519.9 m A h g^-1 after100 cycles at a current density of 1.0 m A cm^-2,but the capacity decay rate needs to be further improved.Such valence modulation strategy reveals the structure-activity relationship between the valence states,the real-time regulation ability of catalysts and the electrochemical performance.The introduction of Co or Zr doping sites synergistically strengthens the bidirectional electron transfer between the catalytically active sites in the bimetallic catalysts(Co_0.30Ni_0.70C/C or Zr_0.30Ni_0.70C/C)and LPS,thereby enhancing the bidirectional multipath oxidation/reduction of LPS.Density functional theory（DFT）analysis shows that Co or Zr doping weakens the chemical bonds related to the terminal sulfur atoms in Li₂S₄ on the atomic level,meanwhile Zr doping also weakens the Li-S bonds in Li₂S.During the discharge/charge process,the Co doping site itself exhibits weak catalytic activity,but it acts as a promoter to strengthen the bidirectional electron transfer between the Ni²⁺/Ni（0）redox pair and LPS and improve the catalytic activity of Ni sites.However,in addition to acting as a promoter,Zr doping site also serves as both the electron acceptor and electron donor to interact with LPS.Furthermore,the introduction of Zr doping sites not only improves the kinetics of intrinsic solid（S₈）-liquid（LPS）-solid（Li₂S）redox,but also strengthens the reversible thiosulfate-mediated LPS redox.Therefore,the Zr_0.30Ni_0.70C/C catalyst accelerates the bidirectional conversion of LPS,inhibits the shuttle effect,and endows Li-S batteries with improved electrochemical performance.When Zr_0.30Ni_2.70C/C is used to modify the commercial PP separator,the Li-S battery exhibits an initial specific capacity of553.2 m A h g^-1 under a sulfur loading of 5.0 mg_s cm^-2 and a small decay rate of 0.0756%over 200 cycles at 1.0 m A cm^-2.This work shows that the new thiosulfate-mediated LPS conversion pathway could be triggered and promoted by the addition of catalysts in Li-S batteries,constructing multiple LPS conversion pathways.And valence modulation is an effective way to improve the catalytic activity of catalysts.The coupling effect and synergistic effect of multifunctional sites are conducive to simultaneously promoting bidirectional Li-S redox,thereby inhibiting the shuttle of LPS and improving the electrochemical performance of Li-S batteries.