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- Herausgeber: Wiley-VCH GmbH
- Kategorie: Fachliteratur
- Sprache: Englisch
- Veröffentlichungsjahr: 2024
This book comprehensively reviews latest advances in Aqueous Zinc Ion Batteries and clarifies the relationships between issues and solutions for the sustainable design of Aqueous Zinc Ion Batteries.
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
1 Introduction for Aqueous Zinc‐Ion Batteries
1.1 History of Aqueous Zinc‐Ion Batteries
1.2 Main Challenges for Aqueous Zinc‐Ion Batteries
References
2 Theoretical Fundamentals of Aqueous Zinc‐Ion Batteries
2.1 Electrochemical Reaction Mechanism of Cathodes
2.2 The Mechanism of Zinc Metal Anode
References
3 Cathode Materials for Aqueous Zinc‐ion Batteries
3.1 Manganese‐Based Cathode Materials
3.2 Vanadium‐Based Cathode Materials
3.3 Prussian Blue Analogs
3.4 Organic Materials
References
4 Anode Materials for Aqueous Zinc‐Ion Batteries
4.1 Structural Design
4.2 Surface Modifications
References
5 Electrolytes for Aqueous Zinc‐Ion Batteries
5.1 Development of Electrolytes for Aqueous Zinc‐Ion Batteries
5.2 Issues and Solutions of Electrolytes for Aqueous Zinc‐Ion Batteries
References
6 Separators for Aqueous Zinc‐Ion Batteries
6.1 Performance Requirements and Properties of Separator
6.2 Commercial Separators
6.3 Constructing High‐Performance Separators
6.4 Separator‐Free AZIBs
References
7 Development of Full Zinc‐Ion Batteries
7.1 Types of AZIBs
7.2 Performance Parameters of AZIB
7.3 Assembly Process of Full Battery
7.4 Aqueous Zinc‐Ion Battery Manufacturers
7.5 Summary and Outlook
References
8 Advanced Characterization Tools and Theoretical Research Methods
8.1 Characterization Techniques
8.2 In Situ Characterization Techniques
8.3 Theoretical Research Methods
8.4 Conclusion
References
9 Conclusion and Future Perspectives
Index
End User License Agreement
List of Tables
Chapter 5
Table 5.1 Summary of recently reported electrolyte engineering for expanding...
Table 5.2 Summary of recently reported electrolyte engineering for dendrite‐...
Chapter 6
Table 6.1 Several examples of separators with high
Table 6.2 Summary of the basic properties of GEs, matching cathode materials...
Chapter 7
Table 7.1 Composition ratio of zinc paste cathode.
Table 7.2 Composition ratio of coating.
List of Illustrations
Chapter 1
Figure 1.1 (a) Multi‐angle comparison of zinc‐ion and lithium‐ion batteries....
Figure 1.2 The configuration of AZIBs. Source: Zhang et al. [6]/John Wiley...
Figure 1.3 Operating voltage vs. specific capacity of various cathode materi...
Figure 1.4 The dendrites (a), hydrogen evolution (b), and corrosion (c) of z...
Figure 1.5 Relevant physical properties of the separators. SEM images of (a)...
Figure 1.6 The challenges and solutions for AZIB electrolytes. Source: Zhang...
Chapter 2
Figure 2.1 (a) Schematics of the chemistry of the zinc ion battery. Source: ...
Figure 2.2 Rechargeable Zn
0.25
V
2
O
5
·
n
H
2
O system. Source: Kundu et al. [6]/Spr...
Figure 2.3 (a) Charge/discharge curves at different rates in the first cycle...
Figure 2.4 (a) Turning point in charge/discharge profiles. (b) Capacity stat...
Figure 2.5 (a) Schematic of the Zn metal/Zn
0.25
V
2
O
5
battery and the expanded...
Figure 2.6 Schematic illustrations of Zn
2+
and H
2
O co‐insertion mechanis...
Figure 2.7 (a) Schematic diagram of Zn/V
2
O
5
battery with Li
+
/Zn
2+
an...
Figure 2.8 Schematic diagram of the conversion reaction energy‐storage mecha...
Figure 2.9 (a) The galvanostatic discharge profiles of Zn//MnO
2
battery and ...
Figure 2.10 (a) The possible Zn‐ion storage mechanisms of CuI electrodes. St...
Figure 2.11 (a) Discharge/charge curves of Zn//VOPO
4
batteries, which can be...
Figure 2.12 (a) Changes in free energy during Zn nucleation. Source: Zhang e...
Figure 2.13 (a) Fitting of continuous and discontinuous nucleation curves. A...
Figure 2.14 (a) The heterogeneous nucleation model. (b) The effects of conta...
Figure 2.15 Relationship between overpotential and critical nucleus radius. ...
Figure 2.16 Diffusion nucleation behavior of Zn in different dimensions. Sou...
Figure 2.17 Electric double layer structure at the electrode interface durin...
Figure 2.18 Crystal reconstruction and growth behavior during zinc electrode...
Figure 2.19 (a) The continued growth of zinc on a crystal plane with steps a...
Chapter 3
Figure 3.1 Schematic representation of various MnO
2
polymorphs characterized...
Figure 3.2 (a) Crystal structure of α‐MnO
2
. (b) The XRD spectrum of α‐MnO
2
e...
Figure 3.3 (a) The phase change taking place between α‐MnO
2
and layered Zn‐b...
Figure 3.4 β‐MnO
2
/Zn full batteries: (a) CV curves for the first three cycle...
Figure 3.5 The demonstration of multistep phase transformation in the γ‐MnO
2
Figure 3.6 Kinetics and thermodynamics of the redox reactions in Zn//MnO
2
ba...
Figure 3.7 (a) The mechanism of Zn
2+
insertion/extraction in ZMO. (b) Pr...
Figure 3.8 (a) XPS curves of Mn 3s in the MnO tested in pre/post‐initial cyc...
Figure 3.9 (a) XRD curves of layered MnO
2
. (b) The mechanism of Zn
2+
ins...
Figure 3.10 (a) Zn
2+
transport in Zn–Mn
3
O
4
battery. (b) Zn
2+
diffusi...
Figure 3.11 Zn
2+
transport in δ‐MnO
2
bulk (a) and MnO
2
nanosheets (b). S...
Figure 3.12 3D micro‐flower‐like MnO
2
Cathode: SEM image (a), schematic diag...
Figure 3.13 MMO‐3.2 cathode: TEM image (a), rate performance (b), and cycle ...
Figure 3.14 (a) The demonstration of electronic transport in ZMO@PCPs. (b) T...
Figure 3.15 (a) The demonstration of the problems and the ideally correspond...
Figure 3.16 K‐V
2
C@MnO
2
: Schematic illustration (a) and SEM image (b). (c) Ba...
Figure 3.17 cw‐MnO
2
: HRTEM images (a) and its inside migration pathway and b...
Figure 3.18 Illustration of PANI‐intercalated MnO
2
. Source: Huang et al. [61...
Figure 3.19 (a) The mechanism of CS SEI film formation. (b) The activation e...
Figure 3.20 (a) Illustration of oxygen‐deficient σ‐MnO
2
for Zn ion storage. ...
Figure 3.21 (a) HRTEM image of Mn
0.61
x
0.39
O. Calculated density of states of...
Figure 3.22 (a) Rietveld refinement results of NM20. (b) Charge distribution...
Figure 3.23 (a) XRD curves of Mn
3
O
4
at the initial cycle. (b) Reaction progr...
Figure 3.24 Different vanadium coordination polyhedra. Source: Reproduced wi...
Figure 3.25 (a) The working principle of metallic zinc//LVO‐250 aqueous ZIBs...
Figure 3.26 Crystal structures of (a) M
x
V
3
O
8
and (b) H
2
V
3
O
8
(V
3
O
7
· H
2
O). So...
Figure 3.27 (a) Schematic of oxygen and vanadium redox mechanisms during cha...
Figure 3.28 (a) Calculated Zn diffusion activation barriers in VO
2
(B). (b) I...
Figure 3.29 The schematic illustration of the electrochemical reaction of Ag
Figure 3.30 (a) Two‐dimensional crystal structure of ZnV
2
O
4
. (b) In situ XRD...
Figure 3.31 (a–c) Schematic diagrams of the structure for Na
3
V
2
(PO
4
)
3
(a) an...
Figure 3.32 (a) Optical images of V
2
O
5
dissolution in different electrolytes...
Figure 3.33 (a) Diffusion energy barriers of VO
2
and VO
1.75
. (b) Schematic i...
Figure 3.34 (a) Crystal structure viewed along the
b
‐axis of CVO, (b,c) Typi...
Figure 3.35 (a, b) The GITT tests and corresponding ion diffusion coefficien...
Figure 3.36 (a) Illustration of the synthesis process of PANI‐intercalated V
Figure 3.37 (a–d) Schematic illustrations of the precursor from solvothermal...
Figure 3.38 (a) Schematic diagram of the VO
x
‐G heterostructure, (b) schemati...
Figure 3.39 (a) Schematic illustration of the fabrication process of pristin...
Figure 3.40 (a) Schematic illustration of the oxidation of V
2
O
3
during the f...
Figure 3.41 Crystal structure of Prussian blue analogs. Source: Zhang et al....
Figure 3.42 Comparison of the characteristics of common synthesis methods fo...
Figure 3.43 TEM images of (a) Mn‐PBA NCs, (b) Mn‐PBA DSNBs, and (c) CuMn‐PBA...
Figure 3.44 (a) The XRD patterns of KMnHCF electrode for KMnHCF‐Zn batteries...
Figure 3.45 The charge/discharge mechanisms of three types of organic cathod...
Figure 3.46 Schematic diagram of charge/discharge mechanism of the aqueous P...
Figure 3.47 (a) Schematic diagram for preparing C4Q. (b) Discharge/charge vo...
Figure 3.48 (a) Oxidation/reduction process of BDB accompanied by anion upta...
Figure 3.49 (a) Reduction and oxidation reactions of the PPy. (b) The charge...
Figure 3.50 (a) Redox couple and ultrafast charging of PTVE. (b) Schematic r...
Figure 3.51 (a) The redox mechanism of PANI/CFs. (b) Schematic diagram of re...
Figure 3.52 (a) Schematic configuration of PA‐COF||Zn. (b) Long‐term cycling...
Figure 3.53 (a) Structural formulae—the PQ monomer (PQ‐Ref) and the PQ trian...
Chapter 4
Figure 4.1 Pourbaix diagram of zinc speciation in the aqueous solution. When...
Figure 4.2 (a) Schematic illustration of rechargeable Zn—Ni batteries with c...
Figure 4.3 Simulations of electric field distribution on (a) carbon cloth an...
Figure 4.4 (a) CE of zinc plating on ZIF‐8‐500 at different current densitie...
Figure 4.5 (a) Nucleation barrier and voltage curve of zinc deposition. Simu...
Figure 4.6 (a) Sand's time and nucleation rate are impacted by applied curre...
Figure 4.7 (a) Schematic illustrations of morphology evolution for bare and ...
Figure 4.8 Illustration for zinc deposition on a (a) bare zinc anode and (b)...
Figure 4.9 Schematic illustrations of ion transport in (a) Nafion and (b) Na...
Figure 4.10 (a) SEM images and (b) voltage curves of zinc deposition on HCS ...
Figure 4.11 (a) Linear polarization curves of PH and SH in 3M KCl electroly...
Figure 4.12 (a) Optical photograph of (002) preferred zinc after deposition....
Chapter 5
Figure 5.1 (a) Schematic of changes in the Zn
2+
solvent sheath, together...
Figure 5.2 (a) DFT calculation models of Arg, Ser, and Glu. (b) Absorption e...
Figure 5.3 The effect of LiTFSI concentration on ion–solvent and ion–ion int...
Figure 5.4 (a) Schematic showing the structure of gum Zn–MnO
2
battery, which...
Figure 5.5 (a) Far‐IR spectra of [EMIm]TFSI, [EMIm]TfO, [EMIm]OMs, and [EMIm...
Figure 5.6 The current issues and the corresponding solutions using electrol...
Figure 5.7 The cathode dissolution tests. Comparison of CV (a) curves and (b...
Figure 5.8 The expanded electrochemical window. (a) The expanded operating v...
Figure 5.9 The water decomposition performances. (a) CV curves measured in t...
Figure 5.10 The characterization of corrosion and passivation. (a) In situ o...
Figure 5.11 The interface reaction in the aqueous electrolyte. (a) The compa...
Figure 5.12 (a) Change in free energy during Zn nucleation. (b) Typical volt...
Figure 5.13 The electrolyte engineering for dendrite‐free Zn anode. SEM imag...
Figure 5.14 The schematic illustration of Zn deposition. (a) Electric double...
Figure 5.15 (a) Schematic illustration of zinc stripping/deposition in 1M Z...
Figure 5.16 Schematic diagram of the interaction of dendrite formation, hydr...
Chapter 6
Figure 6.1 SEM images of (a) PP separator, (b) GF separator, (c) filter pape...
Figure 6.2 The effect of pore structure on the distribution of zinc‐ion conc...
Figure 6.3 (a) Dual‐interface engineering diagram of BTO‐modified GF separat...
Figure 6.4 (a) The zinc deposition schematic diagram, (b) simulation of the ...
Figure 6.5 (a) The cycling performance of Zn||NaV
3
O
8
·1.5 H
2
O full cells with...
Figure 6.6 (a) Electric field distribution and (b) schematic illustration of...
Figure 6.7 (a) Electric field distribution simulation of 3D conductive scaff...
Figure 6.8 (a) SEM images of composite separator decorated with supramolecul...
Figure 6.9 (a) XPS spectra of Zn 2p spectrum after adsorption for pure ZnSO
4
Figure 6.10 Mechanistic sketches of the effect of surface conduction on meta...
Figure 6.11 (a) Schematic illustration depicting the function of MXene‐GF se...
Figure 6.12 (a) Atomic arrangements of (002) plane of Zn and (002) plane of ...
Figure 6.13 (a) Schematic illustration of zinc deposition on post‐stripping ...
Figure 6.14 (a) Illustration of the structure of self‐healing integrated all...
Figure 6.15 (a) Schematic illustration of ex situ and in situ‐formed SPEs fo...
Chapter 7
Figure 7.1 (a) Development history of Zn‐ion batteries. The first reported Z...
Figure 7.2 (a) Coin cell, (b) cylinder cell, (c) prismatic cell, and (d) sof...
Figure 7.3 Composition of battery voltage and its relationship with working ...
Figure 7.4 (a) Coating, (b) rolling and in situ synthesis (c–f). Source: (a)...
Figure 7.5 Assembly process of soft‐packed cell. Source: Huang et al. [31]/J...
Chapter 8
Figure 8.1 (A) SEM images of (a–c) bare and (d–f) nano–CaCO
3
–coated Zn foils...
Figure 8.2 (A) (a, b) TEM and HRTEM images and energy‐dispersive X‐ray (EDX)...
Figure 8.3 Investigation of the form of cathodic Zn deposition. (a, b) HAADF...
Figure 8.4 (A) Morphology evolution and theoretical simulation of Cu‐Zn@Zn a...
Figure 8.5 (A) LCSM images of Zn plate at different cycling times in 3M ZnS...
Figure 8.6 (A) Volta potential maps via SKP of (a) polished Zn foil, (b) MMT...
Figure 8.7 (a) The galvanostatic charge/discharge profile at 0.3 A g
−
1
Figure 8.8 (A) Raman spectroscopy of concentration gradient ZnSO
4
aqueous so...
Figure 8.9 (a) FTIR spectra for 1 M ZnSO
4
, 1 M ZnSO
4
with 10mM glucose, 1M...
Figure 8.10 (a) Zn 2p and V 2p at selected states; Copyright 2018, Nature. (...
Figure 8.11 (a) Characterizations of 1 m ZnSO
4
+ PEO aqueous electrolytes;
1
Figure 8.12 (A) SXAS of Mn L‐edge spectra. (B) Local structural study of the...
Figure 8.13 (a) EPR to measure oxygen vacancies. (b) EPR spectra of VN@NC@CN...
Figure 8.14 FTIR spectra at various plating stages of (a) GF@SM‐based and (b...
Figure 8.15 (a) 2D contour plots of the in‐situ XRD patterns during two comp...
Figure 8.16 (A) In situ EC‐AFM images of polished Zn foil electrodes before ...
Figure 8.17 In situ optical microscope observations of zinc plating on the s...
Figure 8.18 (a) Models of the electric field distributions for the Zn/CC and...
Figure 8.19 Calculation models of interactions of Zn with (a) Cu, (b) CuZn
5
,...
Figure 8.20 ESP‐mapped molecular van der Waals surface of (a) 1,2‐NQ, 1,4‐NQ...
Figure 8.21 (a) Interfacial model of Zn and ZnS (002) surfaces of the ZnS@Zn...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Aqueous Zinc Ion Batteries
Fundamentals, Materials, and Design
Haiyan Wang, Qi Zhang, Yixin Li, and Yougen Tang
Authors
Prof. Haiyan WangCentral South UniversityYueLu DistrictChangshaCH, 410083
Dr. Qi ZhangCentral South UniversityYueLu DistrictChangshaCH, 410083
Dr. Yixin LiCentral South UniversityYueLu DistrictChangshaCH, 410083
Prof. Yougen TangCentral South UniversityYueLu DistrictChangshaCH, 410083
Cover: © KanawatTH/Shutterstock
All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
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© 2024 WILEY‐VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany
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Print ISBN: 978‐3‐527‐34974‐6ePDF ISBN: 978‐3‐527‐83504‐1ePub ISBN: 978‐3‐527‐83505‐8oBook ISBN: 978‐3‐527‐83506‐5
Preface
Rapid economic development has increased the demand for traditional fossil energy sources (e.g. coal, oil, and natural gas), which has induced a series of serious global problems such as acid rain pollution, greenhouse effect, and land desertification. Fossil energy is formed by the natural deposition of ancient organisms over billions of years and cannot be recovered in the short term. To escape from the dilemma of energy depletion, sustainable and environmentally friendly renewable energy sources (e.g. solar, wind, and tidal) have attracted widespread attention in the past two decades. However, there is a critical scientific problem that needs to be solved in practical applications; that is, renewable energy is susceptible to weather and seasonal influences, and its power generation process is discontinuous and uncontrollable. Therefore, an effective energy storage system is needed as a medium device to coordinate the power input and output in the grid, thus improving the tolerance of the grid to renewable energy generation.
In recent years, electrochemical energy storage technology has developed rapidly and is evolving from a miniaturized application for portable electronic devices to an integrated application for large‐scale energy storage systems and smart grids. Electrochemical energy storage is less affected by terrain and can store or release electrical energy directly through reversible chemical reactions, which is a very promising energy storage technology with the advantages of high energy density and high energy conversion efficiency. Lithium‐ion batteries using organic electrolytes are one of the most widely used electrochemical energy storage systems. However, lithium‐ion batteries have met big challenges in large‐scale energy storage systems that are more focused on low cost, high stability, and high safety due to the limited lithium resources and the use of unsafe organic solvents. Compared with organic electrolytes, aqueous electrolytes are inexpensive and safe, and their ionic conductivity is greater, enabling the operation of batteries with large loads and high power. Aqueous batteries can completely meet the requirements of large‐scale energy storage systems. In recent years, aqueous zinc‐ion batteries have developed rapidly due to their high theoretical capacity (820 mAh g−1), low electrochemical potential (−0.76 V vs. standard hydrogen electrodes), and high natural abundance of zinc resources, which are currently a research hotspot for the application of large‐scale energy storage systems.
This book systematically illustrates the basic theory and frontier development of electrochemical theory, key materials, and battery devices for aqueous zinc‐ion batteries. Chapter 1 provides a general review of the history of zinc‐ion batteries and also provides an overview of the main challenges of cathodes, anodes, separators, electrolytes, and battery devices. Chapter 2 discusses the theoretical fundamentals of aqueous zinc‐ion batteries, including the electrochemical reaction mechanism of cathodes and the deposition/dissolution mechanism of zinc anodes. Chapter 3 lists all the promising cathode materials and summarizes their current problems and corresponding optimization strategies. Chapter 4 is a general overview of the development and modification strategy of the zinc anode in aqueous zinc‐ion batteries. Chapter 5 introduces the research progress, key problems, and solutions in the design of aqueous electrolyte for aqueous zinc‐ion batteries. Chapter 6 illustrates the properties and characteristic parameters of the separator and the strategy to construct high‐performance separators for aqueous zinc‐ion batteries. Chapter 7 presents the progress in the structure and packaging of full aqueous zinc‐ion batteries from the perspective of practical application. Chapter 8 provides advanced characterization tools and theoretical research methods for aqueous zinc‐ion batteries. Chapter 9 summarizes the current challenges of aqueous zinc‐ion batteries and proposes some future directions for their further development. Thanks to Prof. Yougen Tang, Dr. Qi Zhang, and Dr. Yixin Li, we wrote this book together. Also, many thanks to my group members, Dr. Chao Hu, Zefang Yang, Chunlin Xie, Qi Wang, Yihu Li, Wenbin Li, Huimin Ji, Tingqing Wu, Hao Wang, and Zhiwen Cai, for their participation in the compilation of each chapter. All the above colleagues have been engaged in scientific research in aqueous zinc‐ion batteries and have made hard efforts to ensure the quality of this book.
This book aims to provide a comprehensive overview of aqueous zinc‐ion batteries in terms of basic theory, frontier science, current status, and development trends of practical applications, which can be used as a reference for science and technology workers engaged in scientific research and technology development in the field of electrochemical energy storage materials and devices. There are inevitably omissions in this book since aqueous zinc‐ion batteries are still developing rapidly and new knowledge and research advances are emerging. We hope that experts will offer valuable comments on the additions and revisions.
Prof. Haiyan Wang
Hunan Provincial Key Laboratory of Chemical Power SourcesCollege of Chemistry and Chemical EngineeringCentral South UniversityChangsha, P.R. China
1Introduction for Aqueous Zinc‐Ion Batteries
1.1 History of Aqueous Zinc‐Ion Batteries
Aqueous zinc‐based batteries can be traced as far back as the voltaic battery, which first used zinc metal as the negative electrode. Subsequently, alkaline zinc‐based batteries such as alkaline Zn–MnO2, Zn–Ni, Zn–C, Zn–Ag, and Zn–Air were developed successively [1]. Among them, alkaline Zn–MnO2 batteries have dominated the primary battery market since their commercialization. Earlier attempts to develop rechargeable Zn‐based batteries were plagued by fast capacity fading and poor coulombic efficiency, mainly due to the uncontrollable growth of Zn dendrites and the formation of insulating, irreversible by‐products (e.g. ZnO) in alkaline electrolytes (e.g. concentrated KOH solution). However, these batteries mentioned above cannot be called aqueous Zn‐ion batteries (AZIBs) because the reaction mechanism of AZIBs is the plating/stripping of Zn2+ at the anode and the intercalation/deintercalation at the cathode in an aqueous solution. The AZIBs differ from the traditional alkaline Zn battery that is based on dissolution/precipitation reactions at the Zn anode (Zn + 4OH− ↔ Zn(OH)4−2 + 2e− ↔ ZnO + 2OH− + H2O + 2e−) and distinguished from other batteries with a Zn anode but no intercalation of Zn ions in cathode reactions [2]. The early investigations of AZIBs date back to 1986, when Yamamoto and Shoji first replaced the alkaline electrolyte with a zinc sulfate electrolyte and tested the electrochemical behavior of rechargeable Zn|ZnSO4|MnO2 batteries [3]. But its research boom was overshadowed by lithium‐ion batteries due to lower energy density and poor cycle stability (Figure 1.1a). Studies on rechargeable AZIBs slowed down until 2012, when Kang and coworkers revisited zinc‐ion battery chemistries with mildly acidic aqueous electrolytes [5]. As shown in Figure 1.1b, a rechargeable AZIB generally consists of a metallic Zn anode, a Zn2+ storage cathode, and a Zn2+‐salt electrolyte, operating via the reversible Zn2+ intercalation/deintercalation (cathode) and Zn plating/stripping (anode) upon discharging/charging [4]. AZIBs have been intensively investigated as potential energy storage devices on account of their low cost, environmental benignity, and intrinsically safe merits. With the exploitation of high‐performance cathode materials, electrolyte systems, and in‐depth mechanism investigation, the electrochemical performances of ZIBs have been greatly enhanced. For example, much work has been done on the modification of zinc anodes, and it mainly focuses on issues such as the inhibition of zinc dendrite growth and the occurrence of side reactions. In terms of electrolytes, in addition to ZnSO4, electrolytes with ZnCl2, Zn(CF3SO3)2, and Zn(CH3COO)2 as the main salts have been developed, while much work has been done in optimizing electrolytes, such as solvents, additives, and concentrations. Cathode materials have also been expanded from the earliest manganese‐based materials to vanadium‐based materials, Prussian blue and analogs, and conductive polymer materials. The main purpose of these modification strategies is to improve the cycle stability and energy density of AZIBs, which has greatly promoted the development of AZIBs.
Figure 1.1 (a) Multi‐angle comparison of zinc‐ion and lithium‐ion batteries. (b) Schematic illustration of the working principle of rechargeable zinc‐ion batteries. Source: Zhang et al. [4]/Royal Society of Chemistry.
1.2 Main Challenges for Aqueous Zinc‐Ion Batteries
Recently, AZIBs have attracted much attention due to their advantages of large theoretical capacity, low cost, and environmental friendliness. The research on each component of AZIBs has increased significantly over the past decade [6]. However, the AZIBs have not been widely industrialized because their overall performance is not comparable to that of commercial lithium‐ion and lead‐acid batteries. The main reason is that the key materials involved in AZIBs, such as electrolytes, separators, anode materials, cathode materials, and current collector materials, cannot meet the cycle life and energy density requirements of practical batteries (Figure 1.2). Herein, the challenges and perspectives for the further development of AZIBs are reviewed, which are instructive for the research toward next‐generation batteries for household appliances, electric vehicles, and large‐scale energy storage systems.
1.2.1 Cathode
It is important to develop promising cathode materials with excellent electrochemical performance since the overall performance of AZIBs is determined by the cathode materials. Manganese‐based, vanadium‐based, and polyanionic materials are the common cathodes applied in AZIB systems (Figure 1.3) [7]. However, these cathode materials exhibit low specific capacity and low‐voltage platforms, and their wide application is limited due to the rapid capacity fading caused by cation dissolution, irreversible phase transition, and by‐product generation. In addition, the current cathode materials usually cannot meet the requirement of some advanced features for practical AZIBs such as low‐temperature performance, low cost, and nonbiotoxicity. Therefore, researchers have attempted to explore the large‐scale preparation of commercial cathode materials with outstanding performance to solve the above problems using inexpensive modification strategies.
Figure 1.2 The configuration of AZIBs. Source: Zhang et al. [6]/John Wiley & Sons.
Figure 1.3 Operating voltage vs. specific capacity of various cathode materials currently used for AZIBs. Source: Xu and Wang [7]/Springer Nature/Licensed under CC BY 4.0.
1.2.2 Anode
Zinc foil is the most common anode material used in AZIBs. However, the electrochemical performances of zinc foils are not sufficient for large‐scale applications due to the severe dendrite growth, corrosion, and hydrogen evolution reaction (HER) (Figure 1.4) [8]. The corrosion and HER on the anode surface in AZIBs generally occur in neutral or mildly acid electrolytes, which give rise to decreased capacity, increased impedance, and electrolyte leakage. The by‐products generated from corrosion and HER will hinder the uniformity of ion transmission to induce more dendrites and further exacerbate the corrosion and HER due to the increased specific area of zinc anodes. The zinc foil anode suffers from electrode perforation and joint detachment during the deep charge and discharge processes, which also limits its wide application.
Figure 1.4 The dendrites (a), hydrogen evolution (b), and corrosion (c) of zinc metal anodes. Source: Xie et al. [8]/John Wiley & Sons/Licensed under CC BY 4.0.
1.2.3 Separator
Glass fibers are widely used as the separator in AZIBs due to their low price, large liquid absorption, and low ion transmission impedance. Glass fibers can significantly improve the ion transport of the cathode material for AZIBs. However, zinc dendrites are easily generated on the zinc anode due to their low strength, irregular pore size distribution, and high zinc affinity (Figure 1.5) [9]. Therefore, the glass fiber is not completely suitable for AZIBs. A large number of studies suggested that Nafion films, nonwoven fabrics, cellulose films, and coating‐modified glass fibers can be considered substitutes for glass fibers. However, as there are still some problems with liquid absorption, pore size distribution, strength, and the high cost of these separators, they are not suitable for wide application in AZIBs. In addition, most of the performance evaluations of separators are only carried out in button batteries, and these evaluation results may not be used as test indicators for practical batteries (such as soft pack and box batteries), caused by separators that should possess the strength, wettability, and pore size distribution of the separator, environmental protection, and low cost.
Figure 1.5 Relevant physical properties of the separators. SEM images of (a) glass fiber, (b) filter paper, and (c) filter membrane, (d) stress–strain curves, (e) ionic conductivities (the insert panel is corresponding to EIS curves), (f–h) and water contact angles. Source: Qin et al. [9]/John Wiley & Sons.
1.2.4 Electrolyte
The design of electrolytes is an important method to improve the performance of aqueous batteries (Figure 1.6) [10]. Therefore, deepening the basic understanding of the solvation structure and interfacial chemistry of electrolytes is of great significance to promote the practical application of ZIBs. Compared with the traditional organic electrolyte battery, the aqueous battery possesses the following advantages, such as avoiding fire disasters caused by battery short circuits and lower cost. Meanwhile, aqueous batteries can be assembled and disassembled in the environment because their components are insensitive to air, which is beneficial to battery manufacturing and recycling. In addition, aqueous electrolytes display higher rate capability and power density than organic electrolytes because the ionic conductivity of aqueous electrolytes (about 0.1 S cm−1) is much higher than that of organic electrolytes (1–10 mS cm−1). However, the presence of water makes the electrochemical window of the common electrolyte only 1.23 V (the organic system has more than 3 V), and the freezing point is high (about −10 °C), which results in lower energy density, obvious battery self‐discharge, and poor low‐temperature performance for AZIBs. Therefore, it is of great significance to design electrolytes with low cost, high safety, wide electrochemical window, low freezing point, fast ion transport speed, and good compatibility with electrode materials.
Figure 1.6 The challenges and solutions for AZIB electrolytes. Source: Zhang et al. [10]/John Wiley & Sons/Licensed under CC BY 4.0.
1.2.5 Full Battery Assembly and Practical Application
AZIBs have gradually shown a trend of widespread application due to their low cost, environmental friendliness, intrinsic safety, and relatively high energy and power densities. However, since the energy density, cycle stability, self‐discharge behavior, and operating temperature range of AZIBs have not been uniformly optimized, assembling such batteries into commercial batteries still faces serious challenges [11]. The actual energy density of the battery is significantly lower than the theoretical energy density because the coin cells assembled in the laboratory rarely consider the capacity matching of cathode/anode electrodes and the loading of cathode active materials. The problems of low coulombic efficiency, poor cycle stability, and poor low‐temperature performance in full cells have not been completely solved. Meanwhile, the widespread application of AZIBs requires a structural design that incorporates the essential characteristics of the battery. For example, aqueous batteries are not sensitive to air, so they can be designed as open batteries, which can replenish electrolytes in time to avoid battery failure caused by gas production and irreversible consumption of electrolytes. The choice of current collectors also has a critical impact on the performance and energy density of AZIBs. The carbon‐based and self‐supporting electrodes seem to be only suitable for small‐scale AZIB devices due to their high price and difficulty in large‐scale fabrication and battery assembly. Ni‐based and Ti‐based metal current collectors are not suitable for wide application due to their high price. Therefore, it is of great significance to develop current collectors (such as special copper foil and foamed stainless steel) that are cheap, show excellent performance, and are suitable for assembling large batteries.
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2Theoretical Fundamentals of Aqueous Zinc‐Ion Batteries
2.1 Electrochemical Reaction Mechanism of Cathodes
The energy‐storage mechanism of cathode materials remains complicated and controversial in aqueous zinc‐ion batteries (AZIBs). To date, the recognized energy‐storage mechanisms of zinc batteries are broadly classified into three categories: (i) Zn2+‐insertion/extraction mechanism, (ii) co‐insertion mechanism, and (iii) chemical conversion reaction mechanism [1]. A brief account of these mechanisms is provided in this section, and readers may refer to other literature for an in‐depth mechanistic understanding of charge storage in AZIB.
2.1.1 Zn2+‐Insertion/Extraction Mechanism
Among the three mechanisms, the insertion/extraction of Zn2+ in the host materials is the most essential mechanism in the AZIBs system, similar to that of the traditional rechargeable lithium‐ion batteries (LIBs). In the discharge process, the Zn2+ in the electrolyte is inserted into the cathode, and the Zn in the anode loses electrons to produce Zn2+, which maintains the charge balance of the electrolyte. When charging, Zn2+ is extracted from the cathode to the electrolyte. It migrates to the anode and collects electrons at the interface, finally diffusing inward in the form of atoms [2]. The cathode materials of the tunnel/layer structure store Zn2+ through this mechanism. Reversible Zn2+‐insertion/extraction was first proposed to explain the energy‐storage mechanisms of ZIBs. Kang and coworkers first reported mild aqueous rechargeable ZIBs [3].
Meanwhile, they proposed that Zn2+ can be reversibly inserted into or extracted from α‐MnO2 cathode during discharge/charge processes (Figure 2.1a). And the Zn anode gets dissolving/depositing at the same time. Subsequently, it was proven that the Zn2+‐insertion/extraction mechanism would occur in β‐MnO2, γ‐MnO2, and λ‐MnO2[4]. In addition to reversible Zn2+‐insertion/extraction, a reversible phase transition from the tunneled structure (α‐MnO2) to layered polymorphs was observed. This transition is initiated by the dissolution of manganese from α‐MnO2 during the discharge process to form layered Zn‐birnessite (Figure 2.1b) [5].
Vanadium‐based compounds are mainly layered/tunnel frameworks with a large void space favoring Zn2+‐insertion. In a milestone study, a reversible Zn2+‐insertion mechanism of Zn0.25V2O5·nH2O was revealed by combining operando X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS) analyses [6]. This corresponds to the following equations (Figure 2.2):
Figure 2.1 (a) Schematics of the chemistry of the zinc ion battery. Source: Xu et al. [3]/John Wiley & Sons. (b) Schematic illustrating the mechanism of zinc intercalation into α‐MnO2. Source: Lee et al. [4]/Springer Nature.
Figure 2.2 Rechargeable Zn0.25V2O5·nH2O system. Source: Kundu et al. [6]/Springer Nature.
Anode:
Cathode:
Overall:
Alshareef and coworkers reported a simple microwave to synthesize layered metal pyrovanadate nanowires (Zn3V2O7(OH)2·2H2O) with a porous crystal framework [7]. The electrochemical energy storage mechanism can be described by the following equation:
Similar Zn2+‐(de)intercalation storage processes have been demonstrated in most of the reported vanadium‐based cathodes, such as VO2, V5O12·6H2O, Na2V6O16·3H2O, LixV2O5·nH2O, and Ca0.25V2O5·nH2O [8]. Vanadium‐based compounds evolve from V5+ to V4+ or even V3+ through the evolution of the V oxidation state, adapt to the intercalation of Zn2+, and maintain a stable crystal framework. Irreversible phase transitions were also observed in examples of NaV3O8‐type compounds like Na5V12O32, which experienced structural destruction, phase transition, and thus capacity degradation [9].
2.1.2 Co‐Insertion/Extraction Mechanism
The Zn2+ insertion/extraction mechanism is the most ideal and acceptable, so the Zn/MnO2 battery was developed based on the migration of Zn2+ ions between the cathode and anode, which was very early reported in 2012. It was found that not only zinc ions but also other ions were involved in the co‐insertion/extraction mechanism. Because of the sluggish Zn2+ and the strong electrostatic repulsion, other ions or molecules (including H+, Li+, and H2O) in the electrolyte can also be embedded and released when the insertion and extraction of zinc ions are happening in cathode during the process of charge and discharge [10, 11].
2.1.2.1 H+ and Zn2+ Insertion/Extraction Mechanism
For manganese‐based cathode materials, Sun et al. [12] first proposed the co‐insertion mechanism of H+ and Zn2+ in Zn/ε‐MnO2 batteries, and the insertion of H+ and Zn2+ corresponds to the sloped plateau from 1.8 to 1.35 V in region I and the flat plateau at 1.3 V in region II, respectively (Figure 2.1a). Similar mechanisms of the difference crystallographic (α, δ, and β) are observed in these experiments. The joint charge storage of H+ and Zn2+ delivers high‐rate performance and long cycle life in the Zn‐δ‐MnO2 batteries [13]. The first step in fast charge storage is non‐diffusion‐controlled Zn2+‐ion‐storage mechanism in bulk δ‐MnO2 without significant phase transition. In contrast, the following step reaction is proven to be the diffusion‐controlled H+ conversion reaction in Zn(TFSI)2‐based electrolyte (Figure 2.1b). In addition, as evidenced by Gao et al., H+‐insertion can boost the α‐MnO2 electrode [14]. They find that the capacity fading during cycling process is mainly due to the decay of second discharge plateau, which is ascribed to Zn2+ insertion (Figure 2.3a). An increased amount of irreversible ZnMn2O4 is observed on the surface of α‐MnO2, which is less reversible than that of H+. β‐MnO2possessing oxygen defects also allows the insertion of H+, and density functional theory (DFT) computation shows that the β‐MnO2 host structure is much easier for H+ insertion rather than Zn2+[15] (Figure 2.4).
Figure 2.3 (a) Charge/discharge curves at different rates in the first cycle and two discharge regions. (b) Joint non‐diffusion‐controlled Zn2+‐intercalation and H+‐insertion/extraction in δ‐MnO2.
For Zn/vanadium‐based batteries, Wan et al. [16] reported that sodium vanadate hydrate experienced a simultaneous H+ and Zn2+ insertion/extraction process as a cathode, which was mainly responsible for their excellent performance with a capacity of 380 mAh g−1 at 4 A g−1 (a capacity retention of 82% after 1000 cycles). Furthermore, as shown in Figure 2.1b, Wang and coworkers [17] verified the sequential insertion of H+ and Zn2+ at 1.1–0.71 and 0.71–0.32 V plateaus in Zn/V2C@CNT battery. Similar co‐insertion mechanisms of H+ and Zn2+ are also observed in other vanadium‐based battery materials, such as V10O24·12H2O [18], Zn0.3V2O5·1.5H2O [19], and (Ni)VO2[20].
2.1.2.2 Zn2+/H2O Co‐Insertion/Extraction Mechanism
Zn2+/H2O co‐insertion/extraction mechanisms usually exist in vanadium‐based cathodes because the tunnel structure or layered structure of the cathode is beneficial to the insertion/extraction of H2O. Sometimes, the water in such layers of vanadium‐based materials provides a pillar effect for stabilizing structures [21, 22].
Figure 2.4 (a) Turning point in charge/discharge profiles. (b) Capacity statistics of two discharge plateaus after different cycles.
Kundu et al. [6] reported that a single‐crystal layered structure of Zn0.25V2O5·nH2O nanobelt possesses two‐electron redox containing intercalated divalent cations and water. Interlayer metal ions Zn2+ and/or structural water in this layered oxide act as pillars stabilizing the structure (Figure 2.5), providing a capacity retention of more than 80% after 1000 cycles. The water molecules expanding and contracting the layered galleries of Zn0.25V2O5, allow Zn2+‐insertion/extraction in a highly reversible manner, and promise high‐rate performance with a capacity of 220 mAh g−1 at a 15 C rate.
When used as cathode for AZIBs [23], porous V2O3@C material exhibits a capacity of 350 mAh g−1 at 100 mAh g−1 and capacity retention of 90% after 4000 cycles at 5 A g−1. It is worth noting that the Raman spectra and ex situ XPS demonstrate the co‐intercalation of Zn2+ and water, and the electrochemical reaction in the Zn//V2O3 can therefore be illustrated in Figure 2.6.
2.1.2.3 Li+‐ and Zn2+‐Insertion/Extraction Mechanism
An aqueous Zn/V2O5 rechargeable battery with a Li+/Zn2+ co‐insertion mechanism was reported. By XRD, voltammetric, and Raman spectroscopy, it was elucidated that exclusive Li+‐insertion into V2O5 occurs up to mid‐discharge. Still, the co‐insertion of a few zinc ions is likely involved in the second part of the reduction. Besides, Zn2+ acts as pillar species, preventing important structural change and hindering the formation of the distorted δ‐LiV2O5 phase [24].
Except for vanadium‐based materials, Prussian blue analogs, for example, iron hexacyanoferrate (FeHCF), can also store and release Zn2+ and Li+[25]. It is found that Zn2+‐ and Li+‐insertion/extraction cause crystalline distortion and the reduction of interlayer spacing, respectively (Figure 2.7).
2.1.3 Chemical Conversion of Cathodes
Compared with Zn2+ insertion/extraction and co‐insertion/extraction mechanisms, the conversion reactions in the battery would tend to provide higher capacity due to direct charge transfer. Hence, it is very promising and effective to develop and design cathode materials with highly reversible conversion mechanisms [11]. Pan et al. [26]. first proved the role of the chemical conversion mechanism in charge storage of MnO2‐based cathodes. During the discharge process, α‐MnO2 reacted with H+ to form MnOOH. To keep the charge constant in the system, the subsequent OH− reacts with ZnSO4 and H2O to form lamellar ZnSO4[Zn(OH)2]3·xH2O (ZHS). After charging, the aforementioned products are reduced to the original α‐MnO2, indicating that MnO2 has reversible electrochemical behavior with MnOOH/ZHS, which can be expressed as follows:
Figure 2.5 (a) Schematic of the Zn metal/Zn0.25V2O5 battery and the expanded interlayer arrangement of Zn0.25V2O5. (b) Scheme showing reversible water intercalation into Zn0.25V2O5·nH2O and the intercalation or deintercalation of the water and Zn2+ during discharge/charge.
Cathode:
Figure 2.6 Schematic illustrations of Zn2+ and H2O co‐insertion mechanisms of V2O3 cathode.
Figure 2.7 (a) Schematic diagram of Zn/V2O5 battery with Li+/Zn2+ and Li+ electrolyte. (b) Schematic illustration of the crystalline structure evolution under the insertion/extraction of Zn2+ and Li+.
Anode:
Overall:
Similar to the co‐insertion mechanism, the H+ required for the chemical conversion reaction is generated by water decomposition, and the corresponding generated OH− leads to the formation of ZHS [27]. There is another explanation for the formation of ZHS and the energy storage of MnO2 in the charge/discharge process of the battery [28]. During the discharge process, the cathode manganese dioxide is electrochemically reduced to soluble Mn2+, and OH− is generated at the same time, increasing the pH of the electrolyte (Eq. (2.10)). Thus, Zn2+ reacts with the electrolyte to form ZHS and deposits on the electrode surface (Eq. (2.11)). Combining the aforementioned two equations, the total cathodic reaction equations can be obtained (Eq. (2.12)). The corresponding anodic reaction on the negative electrode during the discharge is a typical stripping reaction of zinc metal into the electrolyte (Eq. (2.13)). So, the total electrochemical reaction mechanism equation of α‐MnO2/Zn battery can be described as a reaction equation (Eq. (2.14)). Atomic absorption spectroscopy (AAS) and in situ pH measurement was used to analyze the concentration of manganese and zinc ions in the electrolyte at various stages of the charge–discharge process. Combined with in situ XRD analysis, the key role of pH change in the reaction mechanism of the battery system was further confirmed. Figure 2.8 shows the schematic diagram of the Zn/α‐MnO2 battery discharge process in the ZnSO4 electrolyte.
Figure 2.8 Schematic diagram of the conversion reaction energy‐storage mechanism for the aqueous Zn/α‐MnO2 battery system. Source: Adapted from Lee et al. [28].
Considering the important influence of working voltage, Chao et al. [29] gave a new systematic explanation for the above‐mentioned disputed reaction mechanism of MnO2 through the study of a high‐voltage Zn//MnO2 electrolytic battery. The multi‐redox reactions of the Zn//MnO2 battery exhibited strong voltage dependence, as shown in Figure 2.9a, where the discharge process at an operation voltage of 2.2–0.8 V can be divided into three regions: D1 (2.0–1.7 V), D2 (1.7–1.4 V), and D3 (1.4–0.8 V). In the D1 region of high voltage, an electrolytic conversion reaction was dominant, in which the manganese oxide with +4 valence state was reduced and dissolved as manganese ions in +2 into electrolyte; similar (Eq. (2.6)) reactions occurred in the D2 region; and in the D3 region of relatively low voltage, a classic Zn2+ intercalation chemistry occurred. It is worth noting that the capacity of the D1 region increases with the cycle number, which is the main contributor to the overall capacity and high voltage. The electrolytic conversion reaction can increase the proportion of the D1 reaction by adjusting the pH value of the electrolyte (by adding appropriate H2SO4). As shown in Figure 2.9b, thanks to the unique dual‐electron redox electrolysis reaction of Mn4+ ↔ Mn2+ in the D1 region, the prepared Zn//MnO2 electrolytic battery can achieve a high discharge plateau at about 1.95 V and an imposing capacity.
Figure 2.9 (a) The galvanostatic discharge profiles of Zn//MnO2 battery and the schematic for different reaction mechanisms in the D1, D2, and D3 stages. (b) Schematic representation and charge storage mechanism of the new electrolytic Zn//MnO2 battery in 1 M ZnSO4 and 1 M MnSO4 electrolyte (During charge, the cations of Zn2+ and Mn2+ in the electrolyte are deposited onto current collectors of anode and cathode to form, respectively, Zn metal and MnO2. Zn and MnO2 dissolution occur at the anode and cathode, respectively, in the discharge.) Source: Adapted from Chao et al. [29]/John Wiley & Sons.
Reversible cycles
Cathode:
Anode:
Overall:
In addition to manganese‐based materials, the AZIBs with Co3O4 as the cathode material also have a similar conversion and energy‐storage mechanism [30]. Co3O4 reacts with H+ in water and accepts electrons to form CoO (Eq. (2.15)). At the same time, the subsequent OH− interacts with ZnSO4 and H2O in aqueous electrolytes to form lamellar ZHS (Eq. (2.16)). Cu+‐based materials can also store Zn2+ by direct conversion without Zn2+/H+ intercalation [31]. As shown in Figure 2.10, due to the higher charge density of Zn2+, the barrier of Zn2+ insertion and diffusion based on Zn2+ intercalation is much higher than the formation energy of ZnI2 and Cu metal, so CuI has a direct conversion reaction without Zn2+ insertion.
Different from the cation redox reactions of various cathode materials mentioned above, vanadium‐based compounds can also store energy through some anion redox reactions. For example, Fang and coworkers [32] developed a highly reversible Zn//VNxOy battery. Its energy comes from the insertion/extraction reaction of typical cations (Zn2+ and H+) and reversible cations (V3+ ↔ V2+). On the other hand, the reversible anion (N3− ↔ N2−) redox also provides energy. Based on the VOPO4 cathode and salt‐in‐water electrolyte, Wan et al. [33] designed Zn//VOPO4 battery with highly reversible redox chemistry in high‐pressure region. During the charging/discharging process in the low‐voltage region (0.8–1.8 V), the traditional Zn2+ intercalation reaction and reversible V5+ ↔ V4+ conversion mainly occur. When the voltage window is widened to 2.1 V, the lattice oxygen atoms in VOPO4 undergo an oxygen redox reaction at a high voltage of −1.86 V without the insertion/extraction of zinc ions between 1.8 and 2.1 V (Figure 2.11a). The introduced anionic redox reaction not only increased the average working potential to twice that of the traditional vanadium‐based AZIB, but also enhanced the reversible crystal structure evolution of VOPO4 and ensured its superior electrochemical performance.
Figure 2.10 (a) The possible Zn‐ion storage mechanisms of CuI electrodes. Step 1: Zn2+ intercalation followed by the conversion reaction. Step 2: The direct conversion reaction. (b) The simulation model for Zn‐ion intercalation into CuI lattice. (c) The simulation model for Zn‐ion migration in CuI lattice. (d) The differential charge density of CuI after Zn‐ion intercalation. (e) Slice of the electron density difference map to show the unbalanced charge distribution. (f) The energy barriers of Zn‐ion intercalation, Zn‐ion migration, and the energy changes for the direct conversion reaction. (g) Schematic diagram of Zn‐ion storage mechanism and its pathway in CuI electrode, showing the Zn‐ion storage can be finished by direct conversion reaction. Source: Hao et al. [31]/John Wiley & Sons.
Figure 2.11 (a) Discharge/charge curves of Zn//VOPO4 batteries, which can be divided into two regions of oxygen redox at high voltage and vanadium redox at low voltage. (b) Oxygen and vanadium redox mechanisms during charge/discharge processes. Source: Wan et al. [33]/John Wiley & Sons.
It can be seen that the reaction mechanism of AZIBs is very complex. In addition to studying the cathode material itself, the electrolyte environment, working voltage, and other key factors need to be considered. There may be several reaction mechanisms in the process of battery charging and discharging, so it is necessary to conduct a comprehensive and in‐depth study by combining various characterization methods and theoretical calculations.
2.2 The Mechanism of Zinc Metal Anode
Zinc deposition/stripping, hydrogen evolution, and corrosion reactions occur simultaneously at the zinc anode interface during charge/discharge processes in ZIBs. Here, the thermodynamic and kinetic processes involved in these reactions (such as the crystal nucleation and growth of zinc deposition and the thermodynamic basis of hydrogen evolution and corrosion reactions) are mainly introduced to guide the design of zinc anodes with long life and high stability.
2.2.1 Fundamentals of Thermodynamics
Taking the ZnSO4 electrolyte as an example, the main reactions and corresponding electrode potentials on the Zn anode during the deposition or stripping process of AZIBs are as follows [34]:
Zinc deposition reaction:
Electrochemical hydrogen evolution reaction:
Corrosion hydrogen evolution reaction:
Side reactions:
R, T, F, , , , , , and , represent the thermodynamic constant (8.31 J mol−1 K−1), temperature, Faraday's constant (96 500 C mol−1), zinc standard electrode potential (−0.76 V vs. H+/H2), standard hydrogen electrode potential (0 V vs. H+/H2), zinc ion activity coefficient, H+ activity coefficient, zinc‐ion concentration, and H+ concentration, respectively. H+ will be consumed in the electrochemical hydrogen evolution reaction (2.18) and corrosion hydrogen evolution reaction (2.19), while the residual OH− will induce side reactions (2.22) at the zinc anode interface.
To analyze the order of zinc deposition (2.17) and hydrogen evolution reaction (2.18), as well as the spontaneity of corrosion reaction (2.19), it is assumed that 1 M zinc sulfate has a pH of 5 and activity coefficients of 1 at a temperature of 298 K. The calculated ΔG of reaction (2.19) is negative, and the E is 0.46 V, which is also equal to the potential of reaction (2.18) minus the potential of reaction (2.17). Therefore, in the absence of overpotential, the HER tends to preferentially occur in the zinc deposition reaction, and the corrosion reaction will proceed spontaneously. Even though , Zn deposition happens ahead of HER due to the high hydrogen evolution overpotential, sluggish hydrogen evolution kinetics, and low activity of H+ in neutral or mildly acid electrolytes, leading to the reversible deposition/stripping of zinc ions. From the above thermodynamic process, it can be seen that the efficiency of zinc anode deposition/stripping can be improved by increasing the zinc ion activity, reducing the overpotential during the zinc deposition process, adjusting the pH value of the electrolyte, and selecting a substrate with a high hydrogen evolution overpotential [34].
2.2.2 Crystal Nucleation and Growth of Zinc Electrodeposition
From the reaction (2.17), the zinc deposition reaction, determined by the distribution of zinc ions and electrons at the anode interface, is a process of nucleation and growth of zinc ions driven by an electric field in ZIBs. In other words, dendrite‐free zinc deposition can be achieved when the electron‐ion distribution is more uniform on the reaction interface. The electron‐ion exchange process is easier, and the growth direction is more regular on the reaction interface. In this section, current mechanisms and models related to Zn nucleation and growth are introduced to guide the interfacial modification of Zn anodes.
2.2.2.1 Nucleation
The reaction mechanisms of the Zn anode for ZIBs with mild aqueous electrolyte involve the reversible plating/stripping of Zn2+ in the anode, which can be summarized as Zn ↔ Zn2+ + 2e−[35]. Similar to the nucleation and growth processes of lithium deposition, the uniformity of zinc deposition is closely related to the nucleation process.[36–38] During the nucleation process, Zn2+ needs to transcend a barrier corresponding to the nucleation overpotential, as shown in Figure 2.12a,b. Generally, the more difficult Zn nucleation is at the electrode interface, the larger the nucleation overpotential is. The Zn nucleation process may be driven by the following three factors: electrical field, ion concentration, and surface energy. In terms of the electrical field, due to a result of the “tip effect,” the intensity at the tips is much greater than the remaining area, so the Zn near the tips will be subjected to a greater nucleation force (Figure 2.12d). Meanwhile, Zn ion concentration also plays a significant role in the nucleation process because the nucleation barrier is reduced in regions with higher ion concentrations or faster ion transmission. Another influential factor is surface energy. Surface energy, influenced by zincophilicity (Figure 2.12c), defects, lattice matching degree, and so on, can decrease nucleation barriers by creating abundant nucleation sites, eventually inducing uniform deposition on the surface. It can be speculated that Zn dendrites are mainly caused by partial nucleation due to uneven distribution of electrical field and ions, or a small number of nucleation sites by high nucleation barriers on the interface [40].
Figure 2.12 (a) Changes in free energy during Zn nucleation. Source: Zhang et al. [35]/John Wiley & Sons. (b) Typical voltage profile during zinc deposition. Source: Zhang et al. [35]/John Wiley & Sons. (c) Simulation of lithium growth on a substrate with 45° and 135° wetting angles. Source: Cheng et al. [38]/American Chemical Society. (d) Electric field simulation of the plane with different roughness. Source: Yang et al. [39]/John Wiley & Sons.
The nucleation process of zinc can be described by two typical nucleation models, i.e. continuous nucleation and transient nucleation models. Schairfkeer's classical theoretical model is used to describe these two processes. The I–t curve at constant voltage was measured by chronoamperometry and normalized to represent two processes of nucleation [40]:
Equations (2.23) and (2.24) represent the fitting curves of instantaneous nucleation and continuous nucleation, respectively. tm and Im correspond to the maximum current I and the time t at peaks, respectively. The nucleation mode can be determined by how well the experimental curve fits the theoretical curve. As shown in Figure 2.13a, the actual nucleation process is instantaneous at 1.55 V due to the nucleation curve being close to the fitted instantaneous nucleation curve. From the deposition morphology (Figure 2.13b,c), Zn with few nucleation sites and a large nucleus radius undergoes instantaneous nucleation at a current density of 0.2 mA cm−2. However, when continuous nucleation is carried out at a current density of 5 mA cm−2, the number of nuclei is large and the nuclei radius is small [41]. It can be seen that the Zn nucleation model during the charging process is closely related to the magnitude of the current, the composition of the electrolyte, and the structure of the electrode interface. In other words, dense Zn deposits can be obtained through a continuous nucleation process under conditions permitted by the polarization voltage.
Figure 2.13 (a) Fitting of continuous and discontinuous nucleation curves. AFM images of deposited zinc at current densities of (b) 0.2 and (c) 5 mA cm−2. Source: Hou et al. [41]/John Wiley & Sons.
Figure 2.14 (a) The heterogeneous nucleation model. (b) The effects of contact angle and critical Gibbs free energy on nucleation overpotential. Source: Ely and Garcia [42]/IOP Publishing.
Heterogeneous Nucleation Heterogeneous nucleation is the crystal nucleation process of zinc at the hetero‐conducting interface (Figure 2.14
