Research Of Electrode Interface For Regulating The Electrochemical Performance Of Lithium Sulfur Batteries

With the proposal of new energy related industrial policies,the commercialization of electric vehicle,5G,6G technology makes energy storage demand steeply.It is of great significance to develop new energy storage batteries with high energy density,low price,stable cycle and safe reliability to promote the new energy industry.Lithium sulfur（Li-S）battery is composed of sulfur positive electrode,lithium metal negative electrode,diaphragm and liquid electrolyte.Its theoretical specific capacity is up to 1675 m Ah g^-1,and its corresponding energy density can reach 2600 Wh kg^-1,which is 10 times of the current commercial lithium ion batteries.Accordingly,it is regarded as one of the most promising battery systems.However,the commercialization of Li-S battery faces many challenges,especially in the interface of both cathode and anode.The kinetics of lithium polysulfides（Li PSs）conversion at the cathode interface is slow,which leads to polysulfide shuttling,reduced cycling performance and increased the polarization.Thus,lithium ions cannot reach the reaction interface quickly and effectively,resulting in low capacity.The non-uniform nucleation of lithium ions and the non-uniform growth of lithium nuclei at the surface of lithium metal also lead to a series of issues,which restrict the practical application of Li-S batteries.Here,in this dissertation,the interface at both cathode and anode of Li-S battery is regulated.By introducing a catalyst at the interface of the cathode electrode,the direction of improving the kinetics of polysulfide conversion was explored.Through the regulation of atomic nucleation and solid electrolyte interphase（SEI）in the electrochemical process of lithium anode,the direction of improving the quality of SEI and slowing down Li dendrite growth is obtained.The specific research contents of this dissertation are as follows:1.Aiming at the problem of slow polysulfide conversion kinetics at sulfur positive interface,a bottom-up method is proposed.Based on Citric Acid（C₆H₈O₇）and II nickel（II）hexahydrate（Ni（NO₃）₂·6H₂O）,carbon-based materials with Ni-N₄monatomic catalytic sites have been synthesized for catalytic polysulfide conversion at the atomic level.The results show that the Ni-N₄ monatomic catalytic material prepared by this method can significantly reduce the kinetic energy barrier of polysulfide conversion,and can be used as a bifunctional catalyst,which can not only accelerate the transformation kinetics to promote the interfacial redox of Li PSs,but also inhibit the undesirable shuttle effect by fixing Li PSs.The experimental results and DFT theoretical calculation verify that the catalyst interface design can improve the electrochemical performance of the cathode electrode,which provides a stable cycle life of more than 600 times,and the capacity decay rate is only 0.069%per cycle,which confirms the direction of regulating the polysulfide conversion kinetics of the sulfur positive electrode interface by interface catalysis.2.Polyvinylidene fluoride matrix（PVDF）and lithiophilic polyimide（PI）are used as cross-linking polymerization methods to solve the problem of lithium ion inhomogeneous nucleation at the anode interface of lithium metal.It is attached to the surface of lithium metal to change the flux of lithium ions and achieves the purpose of regulating uniform nucleation of lithium ions.The results show that the PVDF framework after PI cross-linking can alleviate the charge mismatch between PVDF material and lithium metal interface,making the prepared solid electrolyte have more uniform Lithium ion flow,and then promoting uniform lithium nucleation.Experimental results and theoretical calculation show that adopting PI molecules fills the PVDF charge density distribution of defect that can make the lithium uniform nucleation.The preparation of Li||Li symmetric cell cycles stability sharply,which confirmed that the electrolyte with lithium metal interface with uniform distribution of charge density is very important to improve lithium ion of homogeneous nucleation.3.Aiming at the issue of Li dendrite growth at the anode interface of lithium metal,the potassium salt of 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide（KHFDF）coupling with bistrifluoromethanesulfonimide lithium salt（Li TFSI）to modified the SEI on the surface of lithium nuclear surface is proposed to inhibit the growth of Li dendrite.Studies show KHFDF salt that has lower Lowest Unoccupied Molecular Orbital（LUMO）than Li TFSI and other solvents in the electrolyte,which makes KHFDF reduce on lithium metal surface and participate in SEI construction preferentially,allowing the regulation of SEI properties.Experimental results show after using KHFDF salt,the quality of SEI is improved,and the preparation of Li||Li symmetric cell at the current density of 1 m A cm^-2 with a capacity of 1 m Ah cm^-2 can be stable cycle more than 1600 hours,The prepared Li-S battery is superior to the unmodified lithium negative electrode in terms of capacity and cycle stability,which further proves that introducing lower LUMO level salt into the electrolyte to change SEI composition and improve the stability of Li metal anode.