Classical Molecular Dynamics Simulations of Electrolyte Solutions for Next Generation Batteries

In this thesis, classical molecular dynamics simulations are used to provide insight into systems of electrolytes for use in next generation batteries. In the first part of this thesis, a collection of tools are developed to calculate coordination numbers, self-diffusivities, ionic conductivities, molecule pair lifetimes, dielectric constants and viscosities in a reliable, reproducible and automated fashion. These tools include an extension of recently released guidelines for calculating viscosities to include an estimate of the uncertainty.

In the next part of this thesis, ion pair lifetimes and diffusivities are tested as predictors for viscosity in ionic liquids. These predictors were shown to be able to predict low viscosity ionic liquids with an 80% success rate, with significantly reduced computational expense. These methods can be applied to high-throughput screening to reduce the computational expense.

The second half of this thesis looks at the molecular structures in electrolyte solutions for lithium sulfur-batteries. Lithium-sulfur are high capacity batteries, but suffer from poor cycling capacity due to the polysulfide shuttling mechanism. The polysulfide shuttling mechanism can be mitigated by controlling the solubility of polysulfides in the electrolyte solution.

One method of mitigating the polysulfide shuttling mechanism is to include highly-fluorinated ethers in the solution. Molecular dynamics simulations were performed on a system of an electrolyte solution mixed with a fluorinated ether. It was shown that the fluorinated ether aggregated in solution instead of coordinating with the lithium in the electrolyte solution.

The final part of the thesis focuses on using free-energy calculations to determine the role of solvent on the solubility of polysulfides in electrolyte solutions. A combination of Bennett acceptance ratio and adaptive biasing force are used to calculate the relative solubility of polysulfide in various solvents. These relative solubilities showed the same trend as experimental measurements, but showed a significantly larger difference between solvents than experiment.

Adaptive biasing force was also used to calculate the free energy of association between lithium polysulfide in solution. These free energies are compared to other properties such as the dielectric constant and local coordination environment. These findings were used to propose new solvents for use in lithium-sulfur batteries.