Title Page

ABSTRACT

Contents

Abbreviation 21

CHAPTER 1. Introduction 24

1.1. Research background 24

1.2. Objectives of the research 27

1.3. Thesis structure 28

References 29

CHAPTER 2. Literature Review 30

2.1. Introduction 30

2.2. Principals of Aqueous Zinc Ion Batteries 32

2.2.1. Reactions of Water Molecules 32

2.2.2. Zn deposition process 35

2.2.3. Zinc Manganese-Based Reactions 37

2.3. Issues, Mechanisms and Strategies 45

2.3.1. Electrochemical Stability Window 47

2.3.2. Side reactions, corrosion and formation of by-products 49

2.3.3. Zn dendrite 53

2.3.4. Advancements in Zn Dendrite Protection 58

References 67

CHAPTER 3. Experimental Sections 79

3.1. Overview 79

3.2. Chemicals and materials 82

3.3. Material preparations 82

3.4. Characterizations 85

3.4.1. Fourier-transform Infrared spectroscopy (FT-IR) 85

3.4.2. Raman spectroscopy 86

3.4.3. ²H nuclear magnetic resonance (NMR) 86

3.4.4. X-ray diffraction (XRD) 87

3.4.5. Scanning electron microscope (SEM) 88

3.4.6. In situ optical microscopy 88

3.4.7. Contacting angle measurements 88

3.5. Theoretical calculations 89

3.5.1. Ab-initio (VASP) simulations 89

3.5.2. Quantum Chemistry Calculations 90

3.5.3. Molecular Dynamic (MD) Emulations 90

3.6. Electrochemical measurements 92

3.6.1. Cyclic voltammetry (CV) 92

3.6.2. Galvanostatic charge/discharge curves 93

3.6.3. Linear Sweep voltammetry (LSV) 93

3.6.4. Polarization curve test 93

3.6.5. Electrochemical impedance spectroscopy (EIS) 93

References 94

CHAPTER 4. MSM as Electrolyte Additives for aqueous Zn-ion Batteries 98

4.1. Introduction 98

4.2. Results and Discussion 100

4.2.1. Stability of the Zn Electrode 100

4.2.2. Reversibility of the Zn Electrode 101

4.2.3. Chemical and physical properties of electrolytes 104

4.2.4. Theoretical Calculation 109

4.2.5. Mechanism Analysis 109

4.3. Conclusion 110

References 111

CHAPTER 5. GBL as Electrolyte additives for aqueous Zn-ion batteries 122

5.1. Introduction 122

5.2. Results and Discussion 124

5.2.1. Stability of the Zn Electrode 124

5.2.2. Reversibility of the Zn Electrode 126

5.2.3. Scanning Electron Microscope 126

5.2.4. Chemical and physical properties of electrolytes 130

5.2.5. Molecular Dynamics (MD) Simulation 130

5.2.6. Quantum Chemistry Computation 132

5.2.7. Ab-initio theoretical calculation 135

5.2.8. Mechanism Analysis 136

5.3. Conclusion 142

References 148

CHAPTER 6. Pyrrolidone as Electrolyte additives for aqueous Zn-ion batteries 152

6.1. Introduction 152

6.2. Results and Discussion 154

6.2.1. Stability of the Zn Electrode 154

6.2.2. Reversibility of the Zn Electrode 154

6.2.3. Chemical and physical properties of electrolytes 158

6.2.4. Mechanism Analysis 160

6.3. Conclusion 161

References 162

CHAPTER 7. Conclusion and Prospects 173

7.1. Conclusion 173

7.2. Prospects for future work 176

References 179

Appendices 180

Appendix A. Publications 180

Appendix B. Conferences & Activities 180

Table 1.1. Comparison of monovalent and multivalent metal electrodes. 26

Table 2.1. Properties of several aqueous Zn2+ electrolytes.[이미지참조] 46

Table 3.1. Chemicals and materials used in this work. 82

Table 4.1. Boiling point, flash point and solubility (25 ℃) of selected rganic additives in electrolyte of AZIBs. 113

Table 4.2. Comparison of accumulative plated capacity with current density of selected organic additives in electrolyte of AZIBs. 114

Table 4.3. Comparison of Coulombic efficiency (CE) and cumulaltive plated capacity for Zn half cells with selected organic additives in electrolyte of AZIBs. 117

Table 5.1. Boiling point, flash point and solubility (25 ℃) of selected rganic additives in electrolyte of AZIBs. 143

Table 5.2. Comparison of accumulative plated capacity with current density of selected organic additives in electrolyte of AZIBs. 144

Table 5.3. Comparison of Coulombic efficiency (CE) and cumulaltive plated capacity for Zn half cells with selected organic additives in electrolyte of AZIBs. 147

Table 6.1. Boiling point, flash point and solubility (25 ℃) of selected rganic additives in electrolyte of AZIBs. 164

Table 6.2. Comparison of accumulative plated capacity with current density of selected organic additives in electrolyte of AZIBs. 165

Table 6.3. Comparison of Coulombic efficiency (CE) and cumulaltive plated capacity for Zn half cells with selected organic additives in electrolyte of AZIBs. 167

Table 7.1. The summary of properties and battery performance of Methyl Sulfonyl Methane, Gamma butyrolactone, and Pyrrolidone. 175

Figure 1.1. Icon collections of electric tools and equipment from the portable flashlight to the rocket, even Metaverse. 24

Figure 1.2. Ragone plot of some conventional commercialized batteries, typical metal ions (including Li+, Na+, K+, Zn2+, Mg2+, Al2+, and Ca2+), and non-metal ion charge carrier-based ABs.[이미지참조] 25

Figure 1.3. (a) Comparison of cost, abundance, and volumetric capacity of metal anodes. (b) Comparison between Zn and other metal anodes in terms of redox potential and ionic... 27

Figure 2.1. Summary and classification of the as-developed electrolyte systems of AZIBs with distinct compositions and features related to the dendrite issue of the Zn... 31

Figure 2.2. Summary and classification on the solubility, mechanism role of the organic compounds used in AZIBs. Reproduced with permission. 31

Figure 2.3. (a) Pourbaix diagram of Zn-H₂O system at 25 ℃. Reproduced with permission. Copyright 2020, Elsevier. (b) Schematic illustration of four major issues that may occur on... 34

Figure 2.4. Illustrations of Zn deposition process, Co is the concentration of Zn2+ cations in the bulk electrolyte, and the Cs the concentration of Zn2+ cations at the interface, with the dashed...[이미지참조] 36

Figure 2.5. Schematic diagram of Zn2+-intercalation mechanism in γ-MnO₂. Reproduced with permission.[이미지참조] 41

Figure 2.6. Schematic diagram of the formation of Zn4SO4(OH)6·4H₂O (ZHS) at the first discharge process.[이미지참조] 43

Figure 2.7. Issues of electrolytes and interplay with Zn anode, cathode. 47

Figure 2.8. Issues associated with aqueous electrolytes and Zn metal anode in aqueous electrolyte. a) Side reactions on Zn anode in aqueous electrolyte; b) stripping resulted holes... 51

Figure 2.9. The contrast of topologies and ion environments between Zn dendrite and its counterparts (Li, Al). (a) SEM image of Li dendrite. (b) SEM observation of Al dendrite. c)... 58

Figure 3.1. (a) SEM image and (b) XRD pattern of β-MnO₂ as synthesized. 83

Figure 3.2. Image capture of binder samples: Ca-SA and Ca-CMC. 83

Figure 3.3. The components structure of Zn symmetric, half and full cells. 84

Figure 4.1. Zn|Zn symmetric cells containing 2M ZnSO₄ with different amount of MSM at the areal capacity and current density of (a) 1 mAh cm-2 at 1 mA cm-2; (b) 5 mAh cm-2 at 5 mA cm-2.[이미지참조] 101

Figure 4.2. Zn|Cu half cells performance and properties. (a) Coulombic efficiency of the cells in 2 M ZnSO₄ electrolytes with different amount of MSM. The half cells after 50 cycles at 1... 102

Figure 4.3. In-situ optical microscopic images for Zn deposition process at the current density of 5 mA cm-2.[이미지참조] 103

Figure 4.4. (a) Anodic and (b) cathodic LSV response curves for aqueous ZnSO₄ and ZnSO₄-MSM electrolyte at 1 mV s-1.[이미지참조] 103

Figure 4.5. Zn|MnO₂ full cells performance. (a) Rate capability at 0.1, 0.2, 0.3, 0.5, 1 A g-1 and back to 0.5 A g-1; (b) Galvanostatic Charge Discharge (GCD) profiles at different current...[이미지참조] 104

Figure 4.6. Image captures of 1M / 2M ZnSO₄ with different amount of MSM. 105

Figure 4.7. pH value of 2M, 2MS0.5, 2MS0.7 and 2MS0.9 electrolytes. 105

Figure 4.8. FT-IR spectra of 2 M ZnSO₄ electrolytes without and with different amount of MSM at ATR mode. (a) OH stretching (3600-3000 cm-1) and (b) S＝O, ν (SO42-) (1500-950 cm-1)[이미지참조] 106

Figure 4.9. FTIR spectroscopy for H₂O, H₂O-MSM and MSM. 106

Figure 4.10. Raman spectra of 2 M ZnSO₄ electrolytes without and with different amount of MSM. (a) 500-4000 cm-1 (b) 920-1180 cm-1 and (c) 3050-3650 cm-1.[이미지참조] 107

Figure 4.11. Contact angle of 2M and 2MS0.9 electrolytes with Zn foil. 108

Figure 4.12. Comparison of adsorption energy of H₂O and MSM molecules on Zn (002) crystal plane, vertically and parallelly. Inset shows corresponding absorbed model. 109

Figure 5.1. Zn||Zn plating/stripping and stability. 125

Figure 5.2. Additive effect: Cycle performance for Zn||Zn symmetric cells using 2M ZnSO₄ electrolyte with differing volume percent of GBL at 1 mA cm-2 and 1 mAh cm-2.[이미지참조] 126

Figure 5.3. Zn||Cu half-cell performance and properties. 128

Figure 5.4. Coulombic efficiency for Zn||Cu half cells using 2M ZnSO₄ electrolyte with differing volume percent of GBL at 1 mA cm-2 and 1 mAh cm-2.[이미지참조] 129

Figure 5.5. Nyquist plots between 2M and 2MG1 electrolytes of the cells before and following 50 cycles. 129

Figure 5.6. Chemical and physical properties of electrolytes. 131

Figure 5.7. (a) Anodic and (b) cathodic LSV response curves for aqueous ZnSO₄ and ZnSO₄-GBL electrolyte at 1 mV s-1.[이미지참조] 132

Figure 5.8. FTIR spectroscopy for H₂O, H₂O-GBL and GBL. 133

Figure 5.9. 3D snapshot for (a) ZnSO₄ and (b) ZnSO₄-GBL determined from molecular dynamics (MD) simulation. 133

Figure 5.10. Comparison of coordination number of H₂O molecules for Zn2+ vs. distance in ZnSO₄ and ZnSO₄-GBL electrolyte from MD simulation. Inset shows comparison at low...[이미지참조] 134

Figure 5.11. Theoretical computations for ZnSO₄-GBL electrolyte. 134

Figure 5.12. MSD as a function of time under ZnSO₄ and ZnSO₄-GBL electrolyte. 136

Figure 5.13. Comparison of absorption energy for H₂O and GBL on Zn (002) and Zn (101) crystal plane (more possible absorbed sites for GBL in a parallel direction). Insets show... 137

Figure 5.14. Nucleation overpotential curve for Zn|Zn symmetric cells 2M and 2MG1 ZnSO₄ electrolyte. 137

Figure 5.15. Iso-surface model of charge density difference for ZSC-002 with (a) one H₂O, (b) one GBL in a parallel direction and (c) one GBL vertically (iso-value＝2.5 x 10-4 e Bohr-3).[이미지참조] 138

Figure 5.16. Iso-surface model of charge density difference for ZSC-101 with (a) one H₂O, (b) one GBL in a parallel direction and (c) one GBL vertically (iso-value＝2.5 x 10-4 e Bohr-3).[이미지참조] 138

Figure 5.17. 2D contours for charge density difference for ZSC-002 with (a) one H₂O, (b) one GBL in a parallel direction and (c) one GBL vertically (iso-value＝2.5 x 10-4 e Bohr-3).[이미지참조] 139

Figure 5.18. 2D contours for charge density difference for ZSC-101 with (a) one H₂O, (b) one GBL in a parallel direction and (c) one GBL vertically (iso-value＝2.5 x 10-4 e Bohr-3).[이미지참조] 139

Figure 5.19. Schematic mechanism of some reactions between electrolytes and electrode. 140

Figure 5.20. Ignition test for a pristine glass-fibre separator with saturated GBL, H₂O/GBL solution, 2M ZnSO₄ solution (2M) and 2M ZnSO₄-GBL solution (2MG1). 140

Figure 5.21. XRD pattern for β-MnO₂ nano-rod. 141

Figure 5.22. SEM image of β-MnO₂ nano-rod. 141

Figure 5.23. Zn||MnO₂ full cell performance. 142

Figure 5.24. XRD pattern confirms stability of MnO₂ electrodes in ZnSO₄-GBL electrolyte before and following 50 cycles in electrolyte at 0.5 A g-1.[이미지참조] 142

Figure 6.1. Zn|Zn symmetric cells containing 2M ZnSO₄ with different amount of Pyrrolidone at the areal capacity and current density of (a) 1 mAh cm-2 at 1 mA cm-2; (b) 10 mAh cm-2 at...[이미지참조] 156

Figure 6.2. Zn|Cu half cells performance and properties. (a) Coulombic efficiency of the cells in 2M ZnSO₄ electrolytes with different amount of Pyrrolidone. The half cells after 50 cycles... 156

Figure 6.3. In-situ optical microscopic images for Zn deposition process at the current density of 5 mA cm-2.[이미지참조] 157

Figure 6.4. (a) Anodic and (b) cathodic LSV response curves for aqueous ZnSO₄ and ZnSO₄-Pyrrolidone electrolyte at 1 mV s-1.[이미지참조] 157

Figure 6.5. Zn|MnO₂ full cells performance. (a) Rate capability at 0.1, 0.2, 0.3, 0.5, 1 A g-1 and back to 0.5 A g-1; (b) Galvanostatic Charge Discharge (GCD) profiles at different current...[이미지참조] 158

Figure 6.6. FT-IR spectra of 2 M ZnSO₄ electrolytes without and with different amount of Pyrrolidone at ATR mode. (a) 3900-2600 cm-1, O-H stretching and (b) 1500-950 cm-1, ν (SO42-).[이미지참조] 159

Figure 6.7. Raman spectra of 2 M ZnSO₄ electrolytes without and with different amount of Pyrrolidone. (a) 500-4000 cm-1 (b) 920-1180 cm-1 and (c) 3050-3650 cm-1.[이미지참조] 159

Figure 6.8. Contact angle 2M and 2MP3 electrolytes with Zn foil. 160

Figure 7.1. Comparison of different electrolyte design strategies and potential candidates as solvents or additives/cosolvents for AZIBs that can operate at harsh temperatures. (A) Spider... 178

 To greatly improve the ecological environment and achieve the target of carbon neutrality around 2050s, it is significant to develop a kind of low-cost convenient-eco-friendly-green assemble-disassemble-recycle next-generation energy system. It is urgent to develop electrical energy storage which could achieve discharge-charge quickly and effective long life-span of advanced all-size devices and electrics. Rechargeable aqueous zinc batteries have been considered as a promising candidate for large-scale energy storage due to the low cost, intrinsic safety, low toxicity, the abundance of materials, and the unique features of zinc: good conductivity (5.91 microohm), low redox potential (-0.7626 V vs. standard hydrogen electrode, in acidic solution, 298.15 K), high gravimetric capacity (821 mAh g-1) and high volumetric capacity (5855 Ah L-1 compared to 2061 Ah L-1 for Li anode). Nevertheless, state-of-the-art techniques of ZIBs are far from satisfactory of industry standard due to the difficult ability to improve the poor reversibility (caused by evolution of hydrogen, the metal dendrite and by-products, etc.) of Zn anode, matching with the dissolution of cathode materials (such as shuttle effect), resulting in the obvious decline in battery performance.

In the acidic electrolytes, HER inevitably happens and limits the coulombic efficiency (CE) of half-cell or full-cell, significantly influencing on the capacity. Further, the evolution of hydrogen makes the environment instability and annoys the hydroxyl ion (OH-) localized concentration around the surface of zinc anode, which accelerates the corrosion reaction of zinc and form the by-products (e.g., Zn4SO4(OH)6·nH2O). The compound acts as an isolation between zinc anode and electrolyte, not only deteriorate the ability to ion-electron diffusion but also have the side effects on the reversibility of zinc plating/stripping. And the evident Zn dendrite growth has a close relationship with the Coulombic efficiency (CE) during battery cycling and the battery lifespan. In the electrolytes, Zn2+ ion could form the solvation structure like a chelate structure, which is surrounded by six H₂O molecules. These strong bonds make the Zn firmly riveted from desolvation and deposition more difficult, like a high energy barrier.

In conclusion, it is necessary to develop strategies for modulating the kinetics of Zn electrodeposition so as to obtain a smoothly homogeneous reversible Zn nucleation. Therefore, one of the strategies is to add the electrolyte additives to minimize the electrolysis of water (e.g., electrolyte decomposition) while suppressing the dendrite growth and parasite reactions as well as controlling regulation of the solvation structure which significantly affect the aqueous Zn batteries.

In the first case, the effects of electrolyte additive, dimethyl sulfone or (methylsulfonyl)methane (MSM) on the electrochemical performance of Zn/MnO₂ was studied. The results show the additive promotes the deposit pattern of Zn ion, intending the dendrite-free Zn plating/stripping highly reversible reaction. Meanwhile, the hydrophilic ability of metal face was optimized by introducing the additive, accelerating the Zn-ion diffusion at the Zn anode/electrolyte interface. Benefiting from these effects, the capacity and cycling life of Zn/MnO₂ batteries were improved to some certain degree.

In the second case, Zn metal plating/stripping mechanism was thoroughly explored in 2M ZnSO₄ electrolyte, demonstrating that the poor performance was ascribed to the formation of a by-product Zn4SO4(OH)6·5H2O flakes, hydrogen evolution reaction, and extremely grievous pulverization dendrite growth. To suppress the dendrite growth and parasitic reactions, a kind of rust remover and reactive organic diluent, Gamma-butyrolactone (C4H6O2, GBL) was introduced to be a new electrolyte additive into ZnSO₄ electrolyte for dendrite-free ZIBs. A small volume of GBL (1% solvent) in 2M ZnSO₄ could markedly improve the performance of Zn plating/stripping behavior under different current density (4200 h for 1 mAh cm-2 at 1 mAh cm-2; 1170 h for 10 mAh cm-2 at 10 mA cm-2; and 140 h for 20 mAh cm-2 at 20 mA cm-2). The high average CE of MnO2-cathodes and a high plating/stripping average CE of 99.7% for Zn anodes demonstrate that the problem of MnO₂ dissolution and dendrite Zn growth have been effectively suppressed. Experiment and theoretical calculations confirmed that GBL could assist to help changing the solvation structure of Zn2+, alleviating the H₂O activation and prohibit the by-product to some extent. Additionally, Zn metal surface was inclined to absorb GBL other than H₂O, which is in favor of hoisting the nucleation overpotential and guiding the uniform deposition. It is noteworthy that 2M ZnSO₄-GBL electrolyte is nonflammable, and conveniently stable, which is promising for next generation green and high-performance Zn-ion batteries. And the outstanding performance was attributed to the i) the nanoarchitecture that provides electrochemically large active sites, leading to the high energy storing performance and ii) the enhanced ionic transport from the hierarchically interconnected 2D-Zn nanostructures.

In the third case, an important chemical raw material, α-pyrrolidone, which can be used as solvent and intermediate of organic synthesis. The effects of α-pyrrolidone on the electrochemical performance of Zn/MnO₂ was studied. The results show the additive promotes the deposit pattern of Zn ion, intending the dendrite-free Zn plating/stripping highly reversible reaction. Meanwhile, the hydrophilic ability of metal face was optimized by introducing the additive, accelerating the Zn-ion diffusion at the Zn anode/electrolyte interface. Benefiting from these effects, the capacity and cycling life of Zn/MnO₂ batteries were improved to some certain degree.

In the future work, I would try more organic molecules as electrolyte additives such as Aspartame, Betaine, and Glucurolactone, etc.

In summary, the chaos of low CE and terrible reversibility of Zn electrode cause of the dendrite growth, hydrogen evolution reaction, by-products and metal corrosion in mild electrolyte severely hinder the further development of aqueous Zn-based batteries at the industry standard. I believe this doctoral research could provide a fundamental understanding of Zn electrode interplay with aqueous-organic media and presents a potential and perspective direction to develop and open up the electrolyte additives in purpose of making advanced aqueous Zn-ion batteries more closely to be commercialized.