Theory, Simulation and Models for Electrolyte Systems with Focus on Ionic Liquids

Abstract

Without rechargeable batteries, the modern world would probably look different. Lithium ion batteries are currently the most popular and widely used ones, almost omnipresent within modern society, in portable devices, electrical vehicles, energy storage stations, etc. The demand for more efficient, more durable, and more sustainable batteries is rapidly growing. The electrolyte is a key element to improve the performance of lithium ion batteries. Traditionally, the electrolyte generally consists of lithium salts and organic solvents, which are volatile and flammable, therefore present safety risks for some applications. Moreover, it is difficult to tune the formula for certain purposes. It has been long practice and a hot research topic to find or innovate novel electrolytes that can ensure the high rate and energy density, stable and safe operations. In this work, we focus on ionic liquid (IL) which consists of an anion and a cation and presents many advantages, such as good thermal and chemical stability, wide electrochemical window, hard to volatile, etc. More importantly, IL is a designable solvent, thus it provides big potential for the variety of IL electrolytes. So far, there are still some unresolved problems in understanding the IL electrolytes, such as the solvation effect, the kinetic behavior, the mechanism of lithium ion transport, etc., Molecular dynamics simulations are powerful tools in investigating these phenomena, processes, and mechanisms at the molecular level, which is the main research method in this work with the support of experimental and theoretical methods. First, two organic solvents (dimethyl carbonate, DMC, and diethyl carbonate, DEC) and four IL solvents (1- alkyl -3- methyl imidazole IL ,viz. [Cnmim][BF4] and [Cnmim][TFSI] (n=2,4)) were computationally investigated for the high concentration lithium electrolytes, 2mol/L lithium bis(trifluoromethylsulfonyl) imide (LiTFSI). The physicochemical properties of the electrolyte solutions, such as density, viscosity, self-diffusion coefficient, and conductivity, were calculated to compare the organic and IL solvent electrolytes. The microstructures of LiTFSI were analyzed further in various solvent electrolytes by evaluating the radial distribution function and ionic coordination number to explore the correlations between structural and physical properties at a micro-scale level. The simulation shows that the IL solvent electrolytes exhibit higher density and viscosity, larger self-diffusion coefficient, and conductivity than the organic solvent electrolytes. The [BF4]-type IL electrolytes have higher conductivity than the [TFSI]-type IL electrolytes, especially the [C4mim][BF4] with the highest conductivity among the IL-based electrolytes. To explore the causes of this phenomenon, the microstructure was analyzed, which revealed that the organic solvents restrict the free movement of the ions, and reduce the conductivity of the electrolytes. Following this study, in order to systematically study the influence of different lithium salt concentrations, the properties and solvation structure of IL electrolytes under four different lithium concentrations (0.3mol/L, 0.5mol/L, 1.5mol/L, 2.0 mol/L) were explored by molecular dynamics simulations and experiment. The result indicated that the density and viscosity of IL electrolytes and the transfer number of lithium ions increased with the increase of the LiTFSI concentration. Furthermore, the effects of the concentration of lithium salt on the ionic associations of Li+ and anion of IL were explored. The structural analysis indicated that strong bidentate and monodentate coordination were found between Li+ and anion of all IL electrolytes. More importantly, the existence of the ion cluster [Li[anion]x](x-1)- in IL electrolytes was found, and the cluster became more closed and compact as the concentration of LiTFSI increases. Due to the discovery of Li-anion clusters in IL electrolytes, a range of IL electrolytes including 1-alkyl-3-methyl imidazole-based IL ([Cnmim][PF6] (n=2,4)) doped with six different concentrations of LiTFSI (0.5M, 1M, 1.5M, 2M, 3M and 4M) are investigated based on the previous work. The transport properties of IL electrolytes by combining molecular dynamics simulations and density functional theory (DFT) were systematically studied, including ion diffusion coefficient, conductivity, lithium-ion transfer number (apparent transfer number, and effective transfer number), effective charge, ion residence time, etc. In the meanwhile, the microstructure of the IL electrolytes is carefully explored to reveal the influence of ion interactions and cluster structure on the transmission properties. It is found that negative lithium transference number and negative effective charge exist, which is caused by strong interactions in IL electrolytes with different lithium concentrations. More importantly, due to the consideration of ion dependence, the negative lithium transference number behavior fundamentally deviates from the apparent transference number obtained using the self-diffusion coefficient analysis. In addition, the calculation of the effective lithium-ion charge shows that the lithium-containing clusters in the IL electrolytes are always negatively charged in a very wide range of concentrations. This indicates that the lithium ions in the IL electrolytes are transferred by the lithium ion-anion charged cluster transfer mechanism. At the same time, the configuration and stability of the charged clusters were quantitatively analyzed by the agglomerated hierarchical clustering algorithm. This discovery is of great significance to the understanding of lithium ion transfer mechanism in lithium ion batteries.