Effect Of Anion Regulation On Structure And Ion Transport Mechanism Of High-concentration Electrolyte

Lithium-ion battery has high energy density and is the research hotspot of the next generation energy storage battery.In recent years,new efforts have been made to develop advanced electrolytes that not only address the current problems of flammability and limited operating temperature range,but also match or even exceed the performance of existing lithiumion battery electrolytes.Compared with traditional dilute solution,high concentration electrolyte has unique physical and chemical properties,better ability to inhibit dendrite formation and cycle stability.The combination of molecular dynamics simulation and experiment can further study the structure and dynamics from the microscopic point of view,which provides a theoretical basis for the selection of different electrolytes.In this thesis,the microstructures of two high concentration electrolytes and the influence of different anions on the transport mechanism were investigated.This paper consists of the following four parts:Section Ⅰ:The basic situation of high concentration electrolyte,the properties and related theories of high concentration ionic liquid electrolyte,high concentration water-in-salt electrolyte and other high concentration electrolytes,and the application of molecular simulation in Lithium-ion electrolyte was introduced.Section Ⅱ:This work explores the effects of anion size and symmetry on coordination and dynamics of lithium ions(Li+)through molecular dynamics simulations.Four types of ILs composed of the same cation N-N-diethyl-N-methyl-N-(2-methoxyethyl)-ammonium(DEME+)and different anions of symmetrical bis(fluorosulfonyl)imide(FSI-)and bis(trifluorome thanesulfonyl)imide(TFSI-),as well as asymmetric(fluorosulfonyl)(trifluoromethylsulfonyl)imide(FTFSI-)and(difluoromethylsulfonyl)(trifluoromethylsulfonyl)imide(DFTFSI")are studied for comparison purpose.The calculation results show that the formation of Li+aggregates in concentrated IL electrolytes could enhance Li+transport due to the Li+transport mechanism gradually shifts from vehicle transport to Li+hopping through the anion ligand layers.Furthermore,it can be seen that the asymmetric anion based electrolytes possess superior performance than that of symmetric anions.In addition,the dual-anion based electrolytes all exhibit faster Li+diffusion coefficient than mono-anion based electrolytes,which arise from the synergistic effect between mixed anions.As such,this work highlights the role of the anion structures and synergism in the rational design of concentrated IL-based electrolytes.Section Ⅲ:By means of molecular dynamics simulation and experiment,the microstructure changes and Li+diffusion behavior of the water-in-bisalt electrolytes formed by introducing the second salt(LiBF4,LiNO3,LiOAc)into the LiTFSI were studied.In the MD simulation,the same force field and concentration(14 mol/kg)were used for all systems to systematically compare their properties,and the key factors affecting the observed differences in diffusion behavior were discussed.By changing the concentration ratio of the second lithium salt,it is found that the Li+diffusion coefficient increases with the increase of the concentration ratio of LiBF4 and LiNO3,while the LiOAc is on the contrary.This is closely related to the influence of the size of the mixed anions and their binding ability with cations on the microstructures of the ion/water network.The second lithium salt is an effective method to enhance and regulate the diffusion of Li+ and can effectively reduce the cost of lithium salt.Section Ⅳ:The main work and conclusion of this paper are summarized,the innovation points and shortcomings of this work are summarized,and the further research and development of high concentration electrolyte are prospected.