Structure and Dynamics Property of Li-Ion Electrolyte in High Concentration Solution

Abstract

Electrolyte is an indispensable composition in energy storage since the positive and negative electrodes are interconnected by electrolytes that determines the charge transport during the charging/discharging process. Ideal electrolytes should satisfy the following requirements: i.e., excellent electrochemical stability, wide voltage window, high conductivity, environmental friendliness and easy availability with high purity.[ 1 ,2] In this work, molecular dynamics simulations have been carried out on two organic solvents (dimethyl carbonate, diethyl carbonate) and four imidazolium-based ionic liquids ([Cnmim][X], n=2,4, X=[BF4]- and [TFSI]-) doped with 2 mol/L LiTFSI salt at 298 K and 313 K. Physicochemical properties, including density, viscosity, self-diffusion coefficient and conductivity are investigated. The contribution and influence of independent conductivity type for uncorrelated and correlated ion on the total conductivity are explored. In parallel, the structure of all electrolytes were further analyzed by the radial distribution functions and the ion coordination numbers. Among them, the structures of solute (LiTFSI) were analyzed in details and the effects of different solvents as well as anion and cation of ionic liquids on the interaction of Li+ were explored. The results indicate that the organic solvents restrict the free motion of the ions, reducing the conductivity of the electrolytes, on the contrary, solute is more stable due to the ionic structure of ionic liquids, especially the [C4mim][BF4] with the highest conductivity among the ILs-based electrolytes. More interesting, two conformations of TFSI- (cis and trans) of solute coexist in all electrolytes. Meanwhile, it was found that the effect of Li+ coordination is mainly determined by anion of ionic liquids and the interaction between Li+ and anion of ionic liquids is directly proportional to the length of the side chain of cation for ionic liquids. In this study, the properties and structures of the electrolytes of organic solvents and ionic liquids were compared thoroughly. The advantages of ionic liquid electrolytes are explained in detail and it is helpful to design higher conductive and more stable ionic liquid electrolytes at molecular level.