Investigation About The Solvation Behavior Of Organodisulfides In Electrolyte For Lithium Battery

Energy storage and application play an important role in human society.In the past decade,rechargeable batteries based on metals and their compounds,especially lithium-ion batteries,have gained widespread research and dominated almost all fields of energy storage.In addition to the widely used intercalated electrode materials such as LiFePO4,other cathode active materials such as sulfur,selenium and organic electrode materials have also received more and more attentions and researches.Among them,organic sulfide cathode materials have gradually attracted people’s research interest due to their advantages of high theoretical capacity,flexible structure,and environmental friendliness.Nowadays,the researches about organosulfide cathode materials are mainly focused on the improvement of specific capacity by adding more active sites,or the suppression of active materials’ solubility through molecular design and adsorption.The investigation on the solvate structure of active materials is inadequate.Through the molecular design and the adjustment of coordinate relationship between organodisulfide and Li+in electrolyte.The organosulfide cathode materials with excellent cycling performance and the development of new deep eutectic electrolyte are achieved.The main research contents are as follows:1)Organosulfide small molecules would lose most of the effective capacity during cycling process because of its high solubility in electrolyte.Diphenyl disulfide(DPDS),one of the simplest organosulfide small molecules,also shows low capacity retention due to the high solubility.During the discharge process,the dissolved DPDS diffuse to the cathode through concentration gradient and participate in the electrochemical reaction.While,the amount of diffused DPDS is limited,thus the utilization of active materials is low.Based on this,2,2’-dipyridyldisulfide(DpyDS)and 2,2’difluorodiphenyldisulfide(FDPDS)are selected as contrast for DPDS in lithium half cells.Because of the strong electronegativity and lone pairs of electrons on N and F atoms,DpyDS and FDPDS molecules would coordinate with Li+in electrolyte.As a result,the DpyDS and FDPDS molecules could migrate with Li+ions under electric field.During the discharge process,the diffusion of DpyDS and FDPDS accompanied with extra migration process significantly improves the utilization of active materials.The DpyDS exhibits 1000 cycles with a high Coulombic efficiency of 99%,and the capacity retention is 50%;FDPDS can also provide 1000 cycles,the Coulombic efficiency is above 99%and the capacity retention is 61%.While,the DPDS can only maintain 40%of the specific capacity after 500 cycles.In conclusion,adjusting the solvation structure of organosulfide small molecules through molecular designing can significantly enhance their cycling performance.2)A new class of deep eutectic electrolyte(DEE)based on 2,2’-dipyridyl disulfide(DpyDS)and lithium bis(trifluoromethanesulfonyl)imide(LiTFSI)is reported.The DEE can be simply prepared by mixing LiTFSI with DpyDS by mild heating.According to the result of Raman spectra and theoretical calculation,the pyridine moieties prefer to coordinate with Li+through Li…N interaction.The strong ion-dipole interaction triggers the deep eutectic phenomenon,thus liberating the Li+from LiTFSI and endowing the DEEs with promising ionic conductivity.The DEE shows admirable intrinsic safety,which can’t be ignited by flame.The DEE at the molar ratio of DpyDS:LiTFSI=4:1(abbreviated as DEE-4:1)is electrochemically stable between 2.1 and 4.0 V vs.Li/Li+,and exhibits an ionic conductivity of 1.5 × 10-4 S cm-1 at 50℃.The Li/LiFePO4 half cell with DEE-4:1 can provide a reversible capacity of 130 mAh g-1 and Coulombic efficiency above 98%at 50℃.The DEEs based on Li…N interaction have the potential to become a new class of electrolyte for lithium batteries after further optimization.