Skip to Main Content
 

Global Search Box

 
 
 
 

Files

File List


Thesis.pdf (171.04 MB)

ETD Abstract Container

Abstract Header

Modeling ion conduction through salt-doped polymers: Morphology, ion solvation, and ion correlations

Shen, Kuan-Hsuan
http://orcid.org/0000-0001-9740-5204
http://rave.ohiolink.edu/etdc/view?acc_num=osu1595422569403378

Abstract Details

2020, Doctor of Philosophy, Ohio State University, Chemical Engineering.
Solid polymer electrolytes are attractive for use in batteries due to their ease of fabrication, electrochemical stability, and mechanical robustness. However, their low ion conductivity leads to insufficiently low charge/discharge rates for many applications. Our research employs coarse-grained molecular dynamics (MD) simulations to understand ion transport in both salt-doped homopolymers and block copolymers under different conditions, to aid the design of future materials with improved conduction. We first focused on how copolymer nanostructures affect particle diffusion using MD simulations and constrained random walk analysis. Diffusion through randomly oriented grains is 1/3 for cylinder and 2/3 for lamellar morphologies versus an homopolymer system, as previously understood. Diffusion in the double gyroid structure depends on volume fraction and is 0.47-0.55 through the minority phase at 30-50 vol.% and 0.73-0.80 through the majority at 50-70 vol.%. Thus, among randomly oriented standard minority phase structures with no grain boundary effects, lamellae is preferable for transport. Then, we applied a coarse-grained model that includes a 1/r4 potential form to represent ion solvation, allowing us to reproduce experimental behavior of ion diffusion in block copolymers and explore their molecular underpinnings. We show that the trend of increasing diffusion with molecular weight becomes more dramatic as ions are solvated in one polymer block more strongly or as the ion-ion interactions get stronger. In contrast to expectations, the interfacial width or the overlap of ions with the nonconductive polymer block do not adequately explain this phenomenon; instead, local ion agglomeration best explains reduced diffusion. Interfacial sharpening, controlled by the Flory parameter and molecular weight, tends to allow ions to spread more uniformly, and this increases their diffusion. To better understand correlated anion and cation motion, which can significantly reduce the overall ion conductivity, we calculate ion conductivity and the degree of uncorrelated ion motion (inverse Haven ratio) ion mobility under an applied electric field as a function of concentration and interaction strengths. In typical electrolytes, correlation in cation-anion motion is often expected to be reduced at low ion concentrations. However, for these polymer electrolytes with strong ion-polymer and ion-ion interactions, we find cation-anion motion is more correlated at lower concentrations when other variables are held constant. We show this phenomenon is related to the slower ion cluster relaxation rate at low concentrations rather than the static spatial state of ion aggregation or the fraction of free ions. Finally, we screen through salt-doped polymer systems with different ion sizes and polymer dielectric constants. We find that an increase in either polymer dielectric constant or ion size disparity reduces contact ion pairs and aggregates. However, cation conductivity (overall conductivity multiplied by cation transference number) does not always monotonically increase with polymer dielectric constant. At low ion size disparity, stronger polymer dielectric strength effectively reduces ion aggregation as well as the inverse Haven ratio, facilitating cation conduction. On the contrary, at high ion size disparity, increasing dielectric constant decreases cation mobility due to increased preferential solvation of cations versus the larger anion.
Lisa Hall (Advisor)
Li-Chiang Lin (Committee Member)
Isamu Kusaka (Committee Member)
197 p.
Chemical Engineering
Polymer Electrolytes; Molecular Dynamics Simulations; Ion Conductivity; Polymer Physics

Recommended Citations

Citations

  • Shen, K.-H. (2020). Modeling ion conduction through salt-doped polymers: Morphology, ion solvation, and ion correlations [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1595422569403378

    APA Style (7th edition)

  • Shen, Kuan-Hsuan. Modeling ion conduction through salt-doped polymers: Morphology, ion solvation, and ion correlations. 2020. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1595422569403378.

    MLA Style (8th edition)

  • Shen, Kuan-Hsuan. "Modeling ion conduction through salt-doped polymers: Morphology, ion solvation, and ion correlations." Doctoral dissertation, Ohio State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=osu1595422569403378

    Chicago Manual of Style (17th edition)

osu1595422569403378
93
© 2020, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.