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.

With the depletion of energy resources and escalating environmental issues, the development of new energy sources and advanced energy storage devices has become increasingly critical. Zinc-ion batteries have attracted significant attention due to their lower cost and safer chemistry. A major focus of zinc-ion battery research is the development of zinc anodes and electrolyte design. Zinc-free anodes have emerged as a promising strategy, offering higher zinc utilization compared to traditional zinc metal anodes (approximately 10%). Among many zinc-free anodes, Ti₃C₂Tx, a MXene (transition metal nitride/carbide layered material), stands out due to its lower lattice mismatch (~10%), higher conductivity, superior mechanical properties, and good hydrophilicity. However, the higher zinc utilization of zinc-free anodes presents challenges for the stable plating/stripping of zinc. Co-solvent engineering is a convenient and direct approach to improving the stability of zinc-free anodes.

In this study, we propose the use of PEG and IPA as co-solvents added to a 1M Zn(OTF)₂-H₂O electrolyte to enhance the stability of Ti₃C₂Tx zinc-free anodes. We found that the addition of PEG reduced the hydrogen evolution current by 0.42 mA (at -1.3 V vs. Ag wire), indicating suppression of the hydrogen evolution reaction. Furthermore, the addition of PEG in the electrolyte inhibits the 2D diffusion of zinc on the Ti₃C₂ surface and promotes zinc deposition along the (002) crystal direction, increasing the (002)/(001) ratio from 0.59 to 0.86, leading to more uniform zinc deposition. Consequently, the Ti₃C₂ zinc-free anode achieved a coulombic efficiency of 97.67% and a cycle life of 268 hours. Similarly, the addition of IPA reduced the hydrogen evolution current by 0.39 mA (at -1.3 V vs. Ag wire), indicating a weaker hydrogen evolution reaction. Moreover, the addition of IPA in the electrolyte inhibits 2D diffusion on the Ti₃C₂ surface and facilitates zinc deposition along the (002) crystal direction, increasing the (002)/(001) ratio from 0.59 to 0.87. The formation of an SEI containing ZnCO₃, ZnFx, and F-rich organics helps to homogenize the Zn ion gradient. Ultimately, the Ti₃C₂ zinc- free anode achieved a high coulombic efficiency of 98.95% and a cycle life of over 1200 hours in the IPA-containing electrolyte.

The high theoretical volumetric capacity, abundance of magnesium in the earth’s crust, and the relatively good safety features of Mg metal, have drawn great attention to developing Magnesium batteries as a follow-up to the success of Li-ion battery technology. However, the magnesium metal is incompatible with most electrolyte solvents and salts, which passivates the surface of magnesium and causes the battery failure. One of the crucial approaches to address this problem is to seek novel electrolytes which have good metallic Mg compatibility and subsequently improve the cycling performance.

In this research, the feasibility of combing PEO, Mg-alginate and MgCl2 to design a solid polymer electrolyte (SPE) for Mg-ion batteries has been assessed. The SPE membrane shows a conductivity of ~10-5 S·cm-1 at 60 °C and an increased value up to ~10-4 S·cm-1 at 80 °C and above. The XRD results have suggested there is no real interaction in between the two polymers which can cause reconstruction of the polymer structure. Moreover, this electrolyte material is highlighted with an excellent cycling stability of up to 150 hours. At last, a possible model which could explain the Arrhenius behavior of temperature-dependent ion conduction is proposed for this SPE material. Overall, this research demonstrates the potential application of Mg-alginate for Mg energy storage in terms of developing a polymer electrolyte, despite further modification in the future is needed.