Ion Transport Behavior in Advanced Electrolytes for Rechargeable Batteries

Abstract

Understanding ion transport behavior at the atomic-level is critical to the design of electrolyte materials for high-performance rechargeable batteries. This thesis presents a comprehensive study of ion transport mechanisms across three representative classes of advanced electrolytes, aqueous zinc-based electrolytes, linear polymer electrolytes, and graft polymer electrolytes, using a combination of molecular dynamics (MD) simulations, theoretical modeling, and experimental validation.

Despite the growing research interest in aqueous zinc-ion batteries (AZIBs), the detailed atomic-level understanding of the structure-property relationship in AZIBs electrolytes remains incomplete. In chapter 3, the effects of salt type and concentration on transport properties of aqueous zinc halide electrolytes were systematically investigated. Classical MD simulations were employed to model ZnCl₂, ZnBr₂, and ZnI₂ solutions across a range of concentrations (0.44 m to 4 m). The results revealed that ionic conductivity exhibits a nonmonotonic dependence on concentration, with an inflection point correlated to the fraction of Zn²⁺ in solvent-separated ion pairs (SSIPs). A universal descriptor based on SSIP fraction was proposed and validated across other divalent salt systems, including Zn(CF₃SO₃)₂, ZnSO₄, and MgCl₂. Further analysis of ion exchange dynamics confirmed the rigidity of the first solvation shell and highlighted the faster relaxation of second-shell anions as a key factor influencing transport.

Solid polymer electrolytes (SPEs) are widely regarded as a promising solution for enabling high-energy-density lithium metal batteries. However, their practical implementation is hindered by inherent challenges such as low ionic conductivity at room temperature and low lithium-ion transference numbers. Decoupling Li⁺ transport from the polymer segmental dynamics is the first step to address these fundamental issues. In chapter 4, we investigated a series of ether-based SPEs to elucidate the impact of backbone oxygen density on Li⁺ transport behavior. A comparative study was conducted on four polymers: polyethylene oxide (PEO), poly(tetrahydrofuran) (PTHF), poly(1,3-dioxolane) (PDOL), and poly(trioxymethylene) (PTOM). The results reveal a clear transition in the dominant Li⁺ transport mechanism from segmental motion to ion hopping with decreasing (PTHF) or increasing oxygen density (PDOL, PTOM) compared to PEO. PDOL and PTOM, exhibit higher amorphous ionic conductivity, Li⁺ transference numbers and hopping activity, which are attributed to their discontinuous coordination (DC) environments and enhanced multichain binding tendencies. These findings are supported by ab initio MD simulations, nuclear magnetic resonance (NMR) measurements, in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and quantum chemical calculations, offering an atomic-level rationale for the decoupling of ion transport from polymer segmental dynamics.

In practical applications, polymer electrolytes are often engineered into more complex architectures such as grafted or crosslinked structures to improve mechanical robustness and suppress crystallinity. Building on this foundation, Chapter 5 focuses on graft polymer electrolytes based on vinyl ether to investigate how variations in sidechain length and chemical composition affect ion transport. A Rouse-based analytical framework was established, using the grafting point as a reference to quantitatively characterize sidechain dynamics through extracted Rouse relaxation times and characteristic hopping rates. It was found that ionic conductivity increases with sidechain length and plateaus beyond 8–12 repeat units, driven by the saturation in sidechain dynamics and a trade-off between enhanced intrachain hopping and suppressed interchain motion. Moreover, tuning the chemical structure of sidechains reveals that oxygen density plays crucial roles in promoting multichain coordination and improving ion transport.

Overall, this thesis deepens the fundamental understanding of electrolyte transport mechanisms and establishes structure–property relationships that can guide the rational design of advanced electrolytes for safe, high-energy-density batteries.

Date of Award	8 Sept 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jun FAN (Supervisor) & Tingzheng Hou (Co-supervisor)