Influence of Ion Solvation on Charge Storage Behavior of MXene in Aqueous Electrolytes

Abstract

This thesis aims to help understand how ion solvation influences the charge storage behavior of Ti3C2Tx MXene in neutral aqueous electrolytes. This is achieved through two main approaches. First, electrolyte engineering strategy has been employed to tune ion solvation structures through varying ion species and solvents. This enables control over the intercalation behavior of both non-metallic and metallic ions into MXene’s interlayer, as well as the deposition behavior of Zn2+ ions on Ti3C2Tx surface. Second, the electrode architecture is modified by constructing a redox-active Ti3C2Tx/conjugated polyelectrolyte (CPE) heterostructure. This design alters the local interlayer environment and influences the desolvation behavior of ammonium ions.

The thesis is organized into three parts, the first part of this thesis focuses on the intercalation of non-metallic ions into Ti3C2Tx, starting with a systematic study on ammonium (NH4⁺) and tetraalkylammonium ions (TMA⁺, TEA⁺, and TPA⁺) intercalation (chapter 2). These ions, with distinct sizes and solvation structures, provide a platform to understand how solvation influences non-metallic ion storage behavior of flexible 2D materials. Considering the moderate capacitance of Ti3C2Tx for ammonium ion storage, we designed a redox-active heterostructure composed of Ti3C2Tx and a n-type cationic conjugated polyelectrolytes (CPE) (chapter 3). In this chapter, we found that structural tuning at the electrode level can affect ion desolvation, which in turn affects the charge storage behavior.

The second part of the thesis investigates how electrolyte design can be used to control ion solvation structures, with the goal of tuning metallic ion intercalation behavior in Ti3C2Tx. In chapter 4, polyethylene glycol (PEG-400) is introduced as a molecular crowding agent in Li⁺-based aqueous electrolytes. This modification extends the voltage window and tunes the Li+ intercalation behavior at higher potential. In chapter 5, acetonitrile (ACN) was used as co-solvent to tune the solvation environment of Na⁺ ions. By varying the ACN content, the strength of ion-solvent interactions is adjusted, leading to change in charge storage mechanism and electrochemical performance. The third part (chapter 6) examines how ion solvation affects Zn²⁺ deposition behavior on Ti3C2Tx, which is used as a freestanding current collector in anode-free aqueous zinc metal batteries (AZMBs). By introducing Li-salts and propylene carbonate (PC) as electrolyte additives, the solvation structure of Zn²⁺ ions is altered, which directly influences interfacial chemistry at the MXene surface. This modulation leads to the formation of a ZnF2-rich interphase that stabilizes Zn deposition and improves cycling efficiency. These findings demonstrate how tailoring ion solvation can serve as a powerful strategy to control not only intercalation, but also metal deposition behavior in MXene-based charge storage systems.