Molecular Simulation Studies On Li~+ Ion Coordination And Transport In Superconcentrated Electrolytes

Nowadays,the demand for rechargeable lithium-ion batteries has increased due to the requirement for high-energy-density storage systems in consumer electronics and electric vehicles.However,Commercial Li PF₆/carbonate organic electrolytes have some disadvantages,such as low coulomb efficiency,irregular growth of metal lithium dendrites,and poor anodizing.The superconcentrated electrolyte formed by further increasing the concentration of lithium salt on the diluted 1 mol/dm^-3standard electrolyte proved to be an effective way to solve these problems.However,these potential advantages are not without drawbacks,as the cost of salt is quite high,the viscosity will increase significantly and the conductivity will decrease.The damaged conductivity in particular presents a fatal flaw.A great deal of experimental work has been done to understand the ionic conductivity in these complex superconcentrated electrolytes at the atomic level in order to suggest possible improvements,but there is no consensus on the molecular level mechanism of charge transport in superconcentrated electrolytes.Herein,we facilely design binary simulation systems of organic electrolytes with different concentrations of LiFSI-DMC and LiFSI-EC and ternary simulation systems of mixed electrolytes consisting of LiFSI-ionic liquid and organic solvent with different molar fractions of ionic liquid[C₃mpyr][FSI].The dielectric constant of EC and DMC is 3.12 and 89.8,respectively.The kinetic properties,migration mechanism and structural properties of lithium ions were systematically studied by means of molecular dynamics.In Chapter 2,we selected the same number and proportion of LiFSI-DMC and LiFSI-EC electrolytes as the research object,and constructed a series of concentration gradient systems from low concentration of 0.5 mol/dm^-3to high concentration of 8.0mol/dm^-3.Our simulation results show that the order of coordination ability is Li⁺（O_E）>Li⁺（O_F）>Li⁺（O_D）.The electrolyte formed by solvents with different polarity has completely different solvation structure in the electrolyte with low concentration,and the solvent with low dielectric constant has a faster ion migration rate.However,when the concentration of lithium salt increases to the range of high concentration electrolyte,the influence of the polarity of the solvent on the electrolyte structure and kinetic properties becomes insignificant.This indicates that the traditional method of improving conductivity by adjusting electrolyte polarity in superconcentrated electrolytes is not suitable.We found migration mechanisms that were inconsistent with traditional expectations.For LiFSI-DMC electrolyte,With the increase of LiFSI concentration,the vehicle diffusion of Li⁺gradually changes to mixed diffusion,and finally the structural diffusion takes the dominant position.For LiFSI-EC electrolytes,with the increase of concentration,the mixed diffusion gradually changed to the structural diffusion.Finally,our study shows that there is a special structure Li⁺-O_（EC）-Li⁺in the superconcentrated electrolyte LiFSI-EC,and the existence of this structure accelerates the migration rate of Li⁺in the superconcentrated electrolyte dominated by structural diffusion,thus negating the influence of the increase of viscosity due to polarity on the conductivity.In Chapter 3,we constructed a superconcentrated electrolyte system of lithium salt-organic solvo-ionic liquid ternary mixture,and studied the influence of the mole fraction of ionic liquid on the structure,kinetics and migration mechanism of organic electrolyte by molecular dynamics simulation.In the mixed system,the molar fraction of ionic liquid is 0.25,0.50 and 0.75.we found that the diffusion rate of all substances in the ternary mixed superconcentrated electrolyte system followed the rule of Li⁺<FSI^-<C₃mpyr⁺,and the migration rate of all particles in the system was increased with the increase of the mole fraction of the ionic liquid,indicating that the ionic liquid allowed Li⁺and FSI^-to move more freely than the organic electrolyte.we found that with the increase of the mole fraction of ionic liquid,the diffusion mechanism of Li⁺in ternary mixed electrolyte gradually changed from structural diffusion to mixed diffusion.Combined with dissociation analysis,It is considered that the ionic diffusion in the ternary mixed electrolyte is a hybrid diffusion mechanism which is combined with the vehicular diffusion driven by anions and the structural diffusion of Li⁺jump transition.Compared with the pure superconcentrated organic electrolyte system,the superconcentrated ternary organic mixed electrolyte system has better conductivity.In conclusion,our simulation results comprehensively uncover the influence of solvation and migration mechanism of lithium ions in super-concentrated electrolytes from the molecular level.We believe that the finding reported in this thesis would provide guidance for the experiment to design the super concentrated electrolyte with better performance.