Computational Investigation of Ionic Diffusion in Polymer Electrolytes for Lithium-Ion Batteries

Citation

Brooks, Daniel James (2018) Computational Investigation of Ionic Diffusion in Polymer Electrolytes for Lithium-Ion Batteries. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ZE9T-V407. https://resolver.caltech.edu/CaltechTHESIS:06012018-042437640

Abstract

Energy storage is a critical problem in the 21^st century and improvements in battery technology are required for the next generation of electric cars and electronic devices. Solid polymer electrolytes show promise as a material for use in long-lifetime, high energy density lithium-ion batteries. Improvements in ionic conductivity, however, for the development of commercially viable materials, and, to this end, a series of computational studies of ionic diffusion were performed. First, pulsed charging is examined as a technique for inhibiting the growth of potentially dangerous lithium dendrites. The effective timescale for pulse lengths is determined as a function of cell geometry. Next, the atomistic diffusion mechanism in the leading polymer electrolyte, PEO-LiTFSI, is characterized as a function of temperature, molecular weight, and ionic concentration using molecular dynamics simulations. A novel model for describing coordination of lithium to the polymer structure is developed which describes two types of interchain motion "hops" and "shifts," the former of which is shown to contribute significantly to ionic diffusion. The methodology developed in this study is then applied to a new problem – the adsorption of CO₂ at the surface of semi-permeable polymer membranes. Finally, a new method, PQEq, is developed, which provides an improved description of electrostatic interactions with the inclusion of explicit polarization, Gaussian shielding, and charge equilibration. The dipole interaction energies obtained from PQEq are shown to be in excellent agreement with QM and a preliminary application of PQEq to a polymer electrolyte suggest that it can provide an improved description of ionic diffusion. Taken as a whole, these techniques show promise as tools to explore and characterize novel materials for lithium-ion batteries.

Thesis Files