Theoretical Study On Interaction Mechanism Between Ionic Liquids And Polysulfides Via Molecular Dynamics Simulations

The Lithium-Sulfur batteries have a great prospect of application due to their much higher energy density than traditional lithium-ion batteries,abundant sulfur in nature and environmental friendliness.However,the parasitic lithium polysulfides shuttle phenomenon during charge and discharge processes,severely hinders their commercialization.Ionic liquids have wide electrochemical windows,good chemical stability as well as weak lewis acidity and baiscity,which have been found to suppress the lithium polysulfides solubility.However,there is a lack of understanding of the interaction microstructures between ionic liquids and different polysulfides and the effects of anions and cations on how to suppress the shuttle behaviors.In this paper,simulation systems consisting of ionic liquids and polysulfides were constructed.The diffusion properties of long-chain polysulfide in ionic liquid-based electrolytes and the aggregation behaviors of short-chain polysulfides in ionic liquids were studied using molecular dynamics simulations and quantum chemistry calculations.The main results are summarized as follows:(1)The force fields of ionic liquids and polysulfides were constructed based on the quantum chemistry calculations and existing force field parameters.The structures of ionic liquids and polysulfides were optimized by DFT methods,and the atomic charges were obtained by the Restricted Electrostatic Potential method.The parameters such as bond lengths,angles,dihedrals and force constants in cations were derived from existing OPLS-AA force field or substitutions from references;Additionally,parameters for equilibrium bond lengths of polysulfide were obtained from references,and parameters of angles and force constants were calculated by optimizing structures and collecting vibration frequency respectively.Besides,the van der Waals and electrostatic parameters of Li and S were determined by adjusting different LJ parameters and verifying properties with molecular dynamics simulations;The deviations between the simulated densities and experimental values of ionic liquids and polysulfide are less than 5%,which indicate that our force fields are reasonable.Furthermore,the force field takes different structural properties of both long-chain and short-chain polysulfides into account,thus has a certain universality.(2)The microstructures and diffusion properties of long-chain Li2S8 in ionic liquid electrolytes were studied systematically using molecular dynamics simulations and quantum chemistry calculations.The interaction energy results show that the interaction of[OTf]-and Li2S8 is stronger than that of[TFSI]-with Li2S8,while the interaction energy between[P13]+ and Li2S8 is higher than that between[PP13]+and Li2S8.The sequence of the interactions of ion pairs is[PP13]-based ionic liquid>[P13]-based ionic liquid,which can be attributed to the "double Li-O bridging" and"single Li-O bridging" structures in two systems;Furthermore,the radial distribution functions,spatial distribution functions and coordination number of the pure ionic liquid-Li2S8 systems were analyzed using molecular dynamics simulations.It was found that S atoms in S82-mainly interact with the methyl group in side chain of cations and the Li-O interaction in the[OTf]-based ionic liquid is stronger than that in the[TFSI]-based ionic liquid;Then,the diffusion properties of Li+and S82-in ionic liquid-based electrolyte systems were studied.The results show that Li+is mainly coordinated with solvent DME.Li+shows a higher self-diffusion coefficient and leaves the solvated shell earlier in[OTf]-based systems,also the residence time of Li+-O[OTf]-is shorter than that of Li+-O[TFSI]-.From analysis of the microstructures and diffusion properties,we put forward a reasonable hypothesis on the diffusion mechanism,and conclude that the coordination structures of[OTf]-with stronger association with Li+are more likely to be destroyed in the process of Li+diffusion,which speeds up the exchange rate of Li+through the solvated shells and is beneficial to the diffusion of Li+.(3)The microstructures of short-chain Li2S and Li2S2 in ionic liquids and the formation of clusters were analyzed by quantum chemistry calculations and molecular dynamics simulations.From the optimized configurations using DFT methods,it can be seen that ionic liquids and Li2S/Li2S2 always tend to form a "cation-short chain polysulfide-anion" sandwich-like structures;From micro structural analysis by molecular dynamics simulations,it was found that the methyl group in side chain of cations mainly interacts with S in Li2S/Li2S2,and the Li-S interaction in short-chain polysulfides is much stronger than Li-O interaction in anions,which will cause the aggregation of short-chain polysulfides;The results of cluster size distribution under different concentrations of short-chain polysulfides show that the number of monomers in systems decreases and the proportion of large clusters increases with the increase of concentration.Short-chain polysulfides are more likely to form large clusters in[TFSI]-based ionic liquids,and the average size of clusters in[P13]-based ionic liquid is larger than that in[PP13]-based ionic liquid,while the proportion of large clusters in Li2S2 system are higher than Li2S systems.Moreover,the size and structure of Li2S clusters can be generally correlated with the coordination ability of anions.In other words,stronger coordination ability of anions will bring smaller proportion of large Li2S clusters.However,the configuration characteristics and interaction forms of anions-Li2S will also affect the sizes and structures of clusters.