Molecular dynamics simulation and neutron scattering studies of nonaqueous electrolyte solutions

Li, Zhixia

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https://hdl.handle.net/2142/102931 Copy

Description

Title

Molecular dynamics simulation and neutron scattering studies of nonaqueous electrolyte solutions

Author(s)

Li, Zhixia

Issue Date

2018-12-03

Director of Research (if dissertation) or Advisor (if thesis)

Zhang, Yang

Committee Member(s)

Kozlowski, Tomasz

Department of Study

Nuclear, Plasma, & Rad Engr

Discipline

Nuclear, Plasma, Radiolgc Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Molecular Dynamics Simulation
• Neutron Scattering
• Redox flow battery
• Nonaqueous electrolyte solutions

Abstract

Nonaqueous electrolyte solutions have been widely investigated for a variety of applications due to their outstanding properties such as high conductivity and excellent stability. However, establishment of predictive models for nonaqueous electrolytes remains challenging. Molecular packing and clustering effects in complex liquid systems such as redox-active electrolyte solutions are still poorly understood especially at high concentrations. Here, neutron scattering is used to probe the dynamics at molecular level in nonaqueous organic electrolytes over a wide temperature range. Two model solution systems were chosen: one containing highly symmetric electrolyte molecules prone to crystallization and one containing a de-symmetrized liquid electrolyte preferring disordered states. In the latter case, complete supercooling (preservation of a disordered state below the melting point without crystallization) was observed to very low temperatures at high concentrations. However, upon heating, localized cold crystallization occurs, leading to a burst nucleation of microcrystalline solids within liquid-like components. Our findings indicate the clustering in these materials and point out limits in solvation and molecular crowding in concentrated nonaqueous electrolyte fluids. Although molecular dynamics (MD) simulation is promising method to predict numerous properties of nonaqueous electrolytes, quantitative predictions depend critically on the prescribed force fields. We show that several quantum-mechanically refined force fields for the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) - acetonitrile electrolyte yield structures agreeing well with the experimental neutron pair distribution function (PDF), yet produce dramatically different dynamics disagreeing with NMR measurements. Such glaring discrepancies indicate that inadequate representation of long-range interactions leads to excessive frustration in the free energy landscape. Better agreement is achieved by proportionally scaling down the atomic charges of the ions. This simplification enabled the simulation of concentration dependences of ionic diffusion for 0.2-2 M LiTFSI solutions without sacrificing fit quality of the PDFs. We argue that not only structures but also dynamics constitute important checkpoints towards to computationally design functional electrolytes.

Graduation Semester

2018-12

Type of Resource

text

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http://hdl.handle.net/2142/102931

Copyright and License Information

Copyright 2018 Zhixia Li

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