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Develop the clean technologies of the future with these novel energy storage technologies
Energy storage is a crucial component of the broader battle to develop clean energy sources and transform the power grid in light of advancing climate change. Numerous new energy storage technologies based on electrochemical redox reactions have recently been developed or proposed, promising to reduce costs and enable energy-dense devices and applications of many kinds. This urgent work demands to be incorporated into chemistry, materials science, and industry at every level.
Towards Next Generation Energy Storage Technologies offers a comprehensive overview of these novel technologies and their applications. Beginning with an introduction to the fundamentals of electrochemistry and energy storage, it offers current and future research questions, design strategies, and much more. It is a must-own for scientists and engineers looking to develop the energy grid of the future.
Towards Next Generation Energy Storage Technologies readers will also find:
- Summaries of state-of-the-art research and open challenges
- Detailed discussion of technologies including lithium-ion batteries, all-solid-state batteries, aqueous multi-valence energy storage systems, and more
- Discussion of applications including electric vehicles, aerospace devices, and many others
Towards Next Generation Energy Storage Technologies is ideal for materials scientists, inorganic chemists, electrochemists, electronics engineers, and anyone working on the clean energy grid or electrical devices.
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
Acknowledgments
1 Introduction
2 Fundamentals of Electrochemical Energy Storage Technologies
2.1 Typical Battery Patterns and Corresponding Functions
2.2 Operating Mechanism of Devices
2.3 Critical Parameters and Design Proposal
2.4 Common Investigation Technologies
2.5 Common Design Strategies for High-Performance Electrode Materials
References
3 Lithium-Ion Batteries
3.1 Brief Introduction
3.2 Cathode Materials
3.3 Anode Materials
3.4 Application and Critical Challenges
References
4 Sodium-Ion Batteries
4.1 Introduction
4.2 Energy Storage Mechanism
4.3 Cathode Materials
4.4 Anode Materials
4.5 Electrolyte
4.6 Sodium-Ion Batteries at Extreme Temperatures
4.7 Other Na-Based Technologies
4.8 Summary and Outlook
References
5 All-Solid-State Batteries
5.1 Introduction
5.2 Ion Transport Mechanism
5.3 Key Performance Parameters
5.4 Classification of Solid Electrolytes
5.5 Practical Problems and Critical Challenges
5.6 Practical Advances in Electric Vehicles and Other Areas
References
6 Lithium–Sulfur Battery
6.1 Fundamental Understanding of Li-S Batteries
6.2 Sulfur Cathode
6.3 Electrolyte
6.4 Anode
6.5 Li-S Pouch Cell Analysis
References
7 Aqueous Multivalent Metal Ion Batteries: Fundamental Mechanism and Applications
7.1 Introduction
7.2 Classification Based on Energy Storage Mechanism
7.3 Highly Stable and Energetic Cathodes
7.4 Strategies for Dendrite-Free Metal Anodes
7.5 Strategies for Designing Electrolytes
7.6 Design Strategies for Extreme Temperatures
7.7 Practical Progress in Grid-Scale Energy Storage and Wearable Devices
References
8 Li-O
2
and Li-CO
2
Batteries
8.1 Introduction
8.2 The Mechanism for Li-O
2
and Li-CO
2
Batteries
8.3 Design Strategy of Cathode Materials
8.4 Electrolyte and Electrolyte Stability
8.5 Stable Anode/Electrolyte Interface Construction
8.6 Application Potential Analysis
References
9 Supercapacitors
9.1 Brief Introduction
9.2 Energy Storage Mechanism
9.3 Electrode Materials
9.4 Electrolytes
9.5 Conclusion
References
10 Battery–Supercapacitor Hybrid Devices
10.1 Introduction
10.2 Classification Based on Energy Storage Mechanism
10.3 Key Scientific Problems
10.4 Electrode Materials
10.5 Microgrid Energy Storage
10.6 Summary and Perspectives
References
11 Fuel Cells
11.1 Overview
11.2 Thermodynamics and Kinetics
11.3 Proton Exchange Membrane Fuel Cells
11.4 Alkaline Fuel Cells
11.5 Other Fuel Cells
11.6 Fuel Cell Systems
References
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1 The standard reduction potential of typical materials.
Chapter 4
Table 4.1 Comparison of properties of sodium and lithium.
Chapter 5
Table 5.1 Advantages and disadvantages of solid electrolytes (SEs).
Table 5.2 Ionic conductivity of several typical sulfide solid electrolytes....
Chapter 6
Table 6.1 Advantages and disadvantages of common Li salts for Li-S batteries...
List of Illustrations
Chapter 2
Figure 2.1 Rate of reaction for a chemical process.
Figure 2.2 Typical charge transfer process at an inert electrode surface.
Figure 2.3 Typical process in electrochemical energy storage cells and elect...
Figure 2.4 Design guideline for nonmetal-doped electrode materials [22].
Figure 2.5 A brief introduction of heterostructure materials and correspondi...
Figure 2.6 Band structure changes after constructing the heterostructure.
Chapter 3
Figure 3.1 Structure and general reaction of LIBs.
Figure 3.2 Olivine LFP. (a) LiFePO
4
. (b) FePO
4
.
Figure 3.3 NCM cathode crystal structure.
Figure 3.4 Crystal structures of (a) LiMn
2
O
4
and (b) LiNi
2
O
4
.
Figure 3.5 Intercalation mechanism of graphite anode.
Figure 3.6 Critical development of Li metal anode.
Chapter 4
Figure 4.1 Working principle of the sodium-ion cell.
Figure 4.2 Crystal structures of layered Na-containing oxides (Na
x
TMO
2
) for ...
Figure 4.3 (a) Rietveld refined X-ray diffraction pattern with corresponding...
Figure 4.4 (a) Schematic of the two-step synthesis of P2-Na
0.7
CoO
2
microsphe...
Figure 4.5 TOFSIMS results of pristine bare and NaCaPO
4
-coated Na
2/3
[Ni
1/3
Mn
Figure 4.6 (a) The structural evolution mechanism of high-Na P2 oxides durin...
Figure 4.7 (a) Schematic of the synthesis process for preparing the MgO-Na[N...
Figure 4.8 Crystal structures of (a) olivine NaFePO
4
and (b) maricite NaFePO
Figure 4.9 Polymorphism in Na
2
CoP
2
O
7
: (a) orthorhombic (
P
2
1
/
cn
), (b) triclin...
Figure 4.10 (a) Crystal structure of Na
2
FeP
2
O
7
. (b) Voltage profiles at a C/...
Figure 4.11 (a) Crystal structure of Na
4
M
3
(PO
4
)
2
P
2
O
7
; PO
4
and P
2
O
7
units are...
Figure 4.12 Schematic of different sodium storage mechanisms from left to ri...
Figure 4.13 Structure of Na solvent co-intercalated in graphite.
Figure 4.14 Schematic of mechanisms for storage of Na
+
in hard carbon....
Figure 4.15 (a) Contour plot of peak intensities as a function of reaction t...
Figure 4.16 Conversion anode materials for SIBs.
Figure 4.17 Reaction mechanisms for organic electrode materials.
Figure 4.18 IL cations Pyr
13
or BMP (left) and C
2
mim or EMIm (right) used in...
Figure 4.19 Summary of SMBs.
Figure 4.20 Various strategies for dendrite-free SMBs.
Figure 4.21 Theoretical gravimetric and volumetric energy of alkali metal−S ...
Figure 4.22 (a) Schematic of a Na-S (or Se) system, the electrochemical issu...
Chapter 5
Figure 5.1 Schematic diagram of defects.
Figure 5.2 (a) Schematic of predominant Li-ion transport mechanisms in a coo...
Figure 5.3 Transition of lithium-conducting channels in composite solid elec...
Figure 5.4 Projection profiles in the a–b plane: (a) rhombohedral structur...
Figure 5.5 (a) Rhombohedral (R3c) structure of LiTi
2
(PO
4
)
3
. Oxygen is shown ...
Figure 5.6 Schematic representations of (a) an all-solid-state lithium-ion b...
Figure 5.7 (a) Ion conductivities of NASICON-structured Li-ion solid electro...
Figure 5.8 Schematic diagram of evolution of garnet type solid electrolyte f...
Figure 5.9 Two crystal structures of LLZO. (a) Cubic. (b) Tetragonal.
Figure 5.10 Evolution of sulfide solid electrolytes.
Figure 5.11 Classification of sulfide electrolytes.
Figure 5.12 Tiny crystals in glassy and glassy ceramic sulfide solid electro...
Figure 5.13 (a) Crystal structures of Li
6
PS
5
I. (b) The rate-determining step...
Figure 5.14 The typical crystal structure of...
Figure 5.15 (a) structure of Li
3
YCl
6
and (b)
C
2/
m
structure of Li
3
YBr
6
....
Figure 5.16 Polyethylene oxide-based solvent-free polymer electrolytes. (a) ...
Figure 5.17 (a) Arrhenius plots of PVDF–LiFSI, PVDF–LiTFSI, and PVDF–LiClO
4
...
Figure 5.18 Schematic of the dissociation of lithium salts by a FE polymer (...
Figure 5.19 (a) Comparison of possible Li ion conduction pathways in nanowir...
Figure 5.20 (a) A typical image of in situ polymerization of VC into PVCA af...
Figure 5.21 (a) Synthesis of linear block and graft polyester-based copolyme...
Figure 5.22 Schematic of the in situ synthesis route of SSEs based on nitril...
Figure 5.23 A roadmap from traditional lithium-ion batteries to solid-state ...
Figure 5.24 Schematic of the structure of an all-solid-state battery.
Figure 5.25 An overview of composite cathode processing for each design opti...
Chapter 6
Figure 6.1 Overview of the development of Li-S batteries from the first prop...
Figure 6.2 Schematic of operating principles of Li-S battery.
Figure 6.3 Schematic diagram of (a) discharge and (b) charge voltage curves ...
Figure 6.4 Schematic of Li-S batteries with liquid electrolytes and solid-st...
Figure 6.5 A comparison of a working Li metal anode in Li-S batteries and ot...
Figure 6.6 Photographs of a coin cell and a pouch cell and their major diffe...
Figure 6.7 Overview of key issues and strategic solutions for the developmen...
Chapter 7
Figure 7.1 Schematic diagrams of the basic structure and working principle o...
Figure 7.2 Schematic of zinc-ion insertion mechanism in (a) tunneled VO
2
cat...
Figure 7.3 (a) Schematic of the conversion reaction between Al
3+
and Fe
0
...
Figure 7.4 (a) Charge and discharge curves at different rates of...
Figure 7.5 Development of aqueous Zn-S batteries of the redox potential of s...
Figure 7.6 (a) Schematic diagram of various crystal structures of...
Figure 7.7 (a) Schematic illustration of highly coordinated ion complexes of...
Figure 7.8 (a) FTIR spectra for O—H bond in Zn(BF
4
)
2
-based electrolyte.(...
Figure 7.9 Electrolyte additive strategy for AZIBs. (a) DFT computations for...
Figure 7.10 (a) Illustration for PDA polymerization and SEM images of Zn ele...
Figure 7.11 Thermodynamic and kinetic parameters related to the electrochemi...
Chapter 8
Figure 8.1 Schematic illustration of the four different architectures of Li-...
Figure 8.2 Diagram of Mo
2
C/CNT cathode during the discharge–charge process i...
Figure 8.3 Schematic diagram of the influence of different factors on batter...
Figure 8.4 (a) Transmission electron microscope (TEM) image and selected are...
Figure 8.5 Schematic illustration of (a) the preparation procedure of Au@CST...
Figure 8.6 High angle angular dark field-scanning transmission electron micr...
Figure 8.7 (a) The fabrication process of MnO@NC-G. Reproduced with permissi...
Figure 8.8 Mechanism of a soluble catalyst in the decomposition/formation of...
Figure 8.9 O K-Edge XANES total-electron-yield spectra of various charging S...
Figure 8.10 (a) Schematic illustrations of dilute LiTFSI/DMSO-based electrol...
Figure 8.11 (a) Optical photograph, (b) and (c) SEM images, (d) EIS curve, (...
Figure 8.12 Schematic diagram of a lithium–oxygen battery with (a) porous po...
Chapter 9
Figure 9.1 Energy storage mechanism of the supercapacitor.
Figure 9.2 Schematic of EDLCs. (a) the Helmholtz model, (b) the Gouy–Chapman...
Figure 9.3 Schematic of an EDLC made from porous electrode materials.
Figure 9.4 Schematic of temperature distribution, in (a) common heating tech...
Figure 9.5 Schematic of solvated ions residing in pores with the distance be...
Figure 9.6 (a) Schematic of the synthesis route of NiCo-LDH@PANI. (b) Specif...
Figure 9.7 (a) CV curves and (b) GCD curves of the Ni
2
P/NiSe
2
electrode. (c)...
Figure 9.8 Classification of electrolytes for SCs.
Figure 9.9 (a) Cyclic voltammograms of the electrode in different aqueous el...
Figure 9.10 Schematic showing capacitive and faradic charge storage processe...
Figure 9.11 Evolution of the Li
+
primary solvation sheath in diluted and WIS...
Chapter 10
Figure 10.1 Development of lithium-ion hybrid capacitor.
Figure 10.2 Conceptual presentation of fabrication with LICs.
Figure 10.3 Structures and corresponding potential profiles of four typical ...
Figure 10.4 Energy storage mechanisms in LICs. (a) The charge and discharge ...
Figure 10.5 Major characteristics of LIC electrolyte components.
Figure 10.6 Comparison of the scalability, efficiency, cost, safety, control...
Figure 10.7 Qualitative comparison of batteries and supercapacitors.
Figure 10.8 Applications of BS-HESS in a new power system. Qualitative compa...
Chapter 11
Figure 11.1 Grove gas battery.
Figure 11.2 Operating principle of a fuel cell.
Figure 11.3 Schematic overview of different types of fuel cells.
Figure 11.4 Mechanism of a PEMFC using hydrogen as fuel.
Figure 11.5 Preparation method of MEA.
Figure 11.6 Preparation of a water-repellent substrate.
Figure 11.7 Schematic of various flow fields: (a) parallel flow field; (b) s...
Figure 11.8 Specific energy versus specific power. PTFE: polytetrafluoroethy...
Figure 11.9 Alkaline fuel cell working principle.
Figure 11.10 Electrode fabrication procedure.
Figure 11.11 Basic structure of a circulating electrolyte fuel cell.
Figure 11.12 Basic structure of a static electrolyte fuel cell.
Figure 11.13 Schematic of a fuel cell power generation system.
Figure 11.14 Schematic of a fuel cell stack.
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Acknowledgments
Begin Reading
Index
End User License Agreement
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Towards Next Generation Energy Storage Technologies
From Fundamentals to Commercial Applications
Edited by Minghua Chen
Editor
Prof. Minghua Chen
Harbin University of Science and Technology
School of Electrical and Electronic Engineering
Department of New Energy Materials and Devices
Key Laboratory of Engineering Dielectric and Applications (Ministry of Education)
52 Xuefu Road
Nangang District
Harbin 150080
China
Cover Image: © tommy/Getty Images
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Preface
The excessive use of nonrenewable fossil energy has triggered energy crises and environmental degradation. To address these issues, replacing nonrenewable fossil energy with renewable energy such as solar, wind, and tidal energy has been proposed and rapidly developed. However, in general, renewable energy cannot provide stable electric output due to its susceptibility to the environment. To harvest renewable energy and achieve output with the required power density, incorporating electrochemical energy storage technologies is believed to be an effective strategy that prevents the wasting of renewable energy. In addition, replacing gasoline-powered cars with electric-powered cars is a promising way to reduce dependence on nonrenewable energy sources, with their electricity supply primarily coming from electrochemical energy storage technologies. Hence, developing electrochemical energy storage technologies is critical to cope with the energy crises and environmental degradation, and hence, they have attracted ever-increasing attention in recent years.
The currently available electrochemical energy storage techniques, dominated by lead–acid batteries, lithium-ion batteries (LIBs), and supercapacitors (SCs), have greatly developed and changed our daily lives. The energy density of state-of-the-art commercial technologies (LIBs) can reach about 180∼300 Wh kg−1, which makes them beneficial for applications in electric vehicles and grid-scale energy storage systems. Nevertheless, safety issues (caused by lithium dendrite growth and the use of flammable electrolytes), high cost of raw materials, and the limited energy/power density hinder the widespread application of energy storage technologies in large-scale energy storage systems and long-lasting electric vehicles. To address these challenges, many novel promising energy storage technologies (e.g. solid-state batteries, lithium–sulfur batteries, and lithium–carbon dioxide batteries) based on various electrochemical redox reactions have been proposed recently. These newly emerged technologies are of high energy density, low cost, and high safety and can facilitate a wide range of applications in electric vehicles, grid-scale energy storage systems, and other devices (e.g. power supply of equipment applied in aerospace, deep sea, and polar regions). Substantial work has been devoted to exploring the potential of the aforementioned energy storage technologies, and some of them have stepped into the practical application stage.
To push forward the development of these promising energy storage technologies, this book focuses on the fundamental energy storage mechanisms, critical achievements, and challenges of next-generation energy storage technologies from the perspective of practical applications. This book discusses the recent progress of commercialized energy storage technologies (LIBs and SCs) first and then comprehensively introduces the recent progress of newly emerged energy storage technologies. This book aims to help readers clarify what are the critical issues that hinder the widespread application of these energy storage technologies, translate complex scientific concepts into easily understandable language, and demonstrate the potential of these technologies in practical applications through case studies. In this rapidly evolving field, acquiring up-to-date knowledge is challenging, but we are committed to providing the most up-to-date and comprehensive information to help readers stay informed about the latest developments in energy storage technology.
Lastly, we would like to express our gratitude to all individuals and institutions who have contributed to the writing and publication of this book, as well as to all scientists who have contributed to the development of energy storage technologies. We hope that this book will contribute to the development of energy storage technologies and the goal of achieving clean, sustainable energy.
April 2024
Minghua Chen
Harbin, China
Acknowledgments
The swriting of this book was enriched by numerous scientific discussions. I would like to thank my colleagues for their valuable comments. Their knowledge, experience, and skills have greatly enriched this book and have advanced our understanding of next-generation energy storage technologies. We have much gratitude for the many graduate students who have worked on the examples cited and proofread the chapters.
Minghua ChenHarbin, China
1Introduction
Minghua Chen1 and Yu Li2
1Harbin University of Science and Technology, School of Electrical and Electronic Engineering, Department of New Energy Materials and Devices, Key Laboratory of Engineering Dielectric and Applications (Ministry of Education), 52 Xuefu Road, Nangang District, Harbin, 150080, China
2Harbin University of Science and Technology, School of Electrical and Electronic Engineering, Department of Electrical Theory and New Technology, 52 Xuefu Road, Nangang District, Harbin, 150080, China
With ever-increasing concerns about limited fossil fuel reserves (e.g. oil, coal, and natural gas) and the environmental degeneration caused by the emission of harmful gases (e.g. CO2 and SO2), there is an urgent need to replace polluted unrenewable energy resources with clean renewable energy resources (e.g. wind and solar energy). Nevertheless, the generation of renewable energy is intermittent, and the electricity output is unstable. Harvesting renewably generated electricity using an electrochemical energy storage system is a practical way to address the intrinsic issues with renewable energy. In addition, removable electrochemical energy storage technologies play a huge role in our daily lives. For example, phones, laptops, and smartwatches we use daily are powered by high-energy-density lithium-ion batteries (LIBs). Some advanced vehicle manufacturing companies (e.g. BYD and Tesla) have launched electric vehicles, in an attempt to decrease harmful gas emissions. Nevertheless, the application of electrochemical energy storage technologies in a wide range of fields is still challenging. The properties of energy storage technologies depend on the applications and the market they are being used. For example, portable energy storage devices and energy storage systems used by people in their daily lives should be energy dense and highly safe. Outdoor energy storage systems in cold regions should possess good low-temperature energy storage performance. Energy storage devices used in regenerative braking should have both high energy density and power density.
To push forward the energy structure upgrade, numerous studies have been dedicated to the investigation of advanced energy storage technologies. Various promising energy storage technologies (e.g. high-safety solid-state batteries, aqueous lithium-ion batteries [ALIBs], aqueous zinc-ion batteries [AZIBs], high-energy-density lithium–sulfur batteries [Li-S batteries], metal–air batteries, and metal–CO2 batteries) have been developed. These advanced energy storage technologies are designed for different requirements and markets and show promise for next-generation energy storage technologies. In this book, recent advances in energy storage technologies that can be commercialized are thoroughly summarized and discussed in detail, providing the landscape of the state-of-the-art electrochemical energy storage technologies and their widespread practical applications. First, the fundamental knowledge about energy storage technologies will be introduced, and then the practical progress of the existing commercial energy storage technologies and the corresponding energy storage properties, as well as critical challenges, will be presented. Subsequently, energy storage mechanisms, critical issues, design strategies, and practical progress of next-generation energy storage technologies will be discussed. Finally, future trends in these advanced energy storage technologies will be presented. We hope this book can contribute to the development of electrochemical energy storage technologies.
2Fundamentals of Electrochemical Energy Storage Technologies
Minghua Chen1 and Yu Li2
1Harbin University of Science and Technology, School of Electrical and Electronic Engineering, Department of New Energy Materials and Devices, Key Laboratory of Engineering Dielectric and Applications (Ministry of Education), 52 Xuefu Road, Nangang District, Harbin, 150080, China
2Harbin University of Science and Technology, School of Electrical and Electronic Engineering, Department of Electrical Theory and New Technology, 52 Xuefu Road, Nangang District, Harbin, 150080, China
2.1 Typical Battery Patterns and Corresponding Functions
Of late, numerous electrochemical energy storage systems have been developed to meet the demands of our daily lives, of which five are commonly used and commercialized: lead–acid batteries, lithium-ion batteries (LIBs), nickel metal hydride (Ni-MH) batteries, nickel–zinc (Ni-Zn) batteries, and supercapacitors. Lead–acid batteries are most commonly used in gasoline cars for engine ignition and running the auxiliary electronics when the engine is off. However, they are harmful to the environment, and replacing them with less expensive, safe, and energy-dense electrochemical energy storage technologies is one of the major challenges faced by researchers. Ni-MH batteries and Ni-Zn batteries are typical alkaline batteries using KOH-based electrolytes. Their energy density is slightly higher than that of lead–acid batteries; however, their output voltage is highly restricted by the limited theoretical thermodynamic stable potential (1.23 V). In addition, alkaline electrolytes will continuously etch the current collector and zinc anode, and thus, cycling lifespans of these batteries can rarely be more than 500 cycles without obvious decay [1]. LIBs have been widely used in portable devices, electric vehicles, and grid-scale energy storage systems due to their high output voltage, high energy density, and long cycling lifespans. The lifespan of advanced LIBs can be more than 10,000 cycles. Although the safety of using LIBs is ensured in various portable devices (e.g. phones and laptops), thermal runaway and explosion can occur in grid-scale energy storage systems and electric vehicles due to the growth of lithium dendrite, which induces internal short circuits and releases substantial heat to ignite flammable electrolytes. Dendrite growth is related to overcharging and over-discharging of batteries. These can be prevented by rational battery management, which can be efficiently controlled in a single cell and hardly controlled in the battery pack [2]. The more the number of LIBs, the higher the thermal runaway risk. Supercapacitor is a power-density-superior electrochemical energy storage device that harvests energy via a rapid physical adsorption/desorption process. However, the energy density of supercapacitors is more than tenfold lower than that of batteries, and thus, they cannot be used as a main power source for the functioning of electric equipment [3].
Nevertheless, all the abovementioned electrochemical energy storage technologies have changed our daily lives and have similar component patterns. In general, all of them have two electrodes. If the electrode materials used are different, they can be classified into cathode and anode, which will be introduced in the following sections. Materials used as electrodes should at least be good electronic conductors. Meanwhile, the cathode and anode need to be separated by electrolytes and separators to avoid direct contact between them. The energy storage mechanism of electrochemical energy storage technologies is mainly based on the electrochemical reactions (reversible redox reaction or intercalation) at cathode and anode. It is noteworthy that the redox reactions happened in electrochemical energy storage devices is quite different with chemical reactions. The primary distinction between an electrochemical reaction and a chemical redox reaction is that, in the former, reduction occurs at one electrode and oxidation occurs at the other, while in the latter, both reduction and oxidation occur at the same electrode. This distinction has several implications. In an electrochemical reaction, oxidation is spatially separated from reduction. Thus, the entire redox reaction is divided into two half-cells. The rate of these reactions can be controlled by externally applying a potential difference between the electrodes, e.g. using an external power supply, a feature that is not present in chemical reactors. Furthermore, electrochemical reactions are always heterogeneous; i.e. they always occur at the interface between the electrolyte and an electrode (and possibly a third phase, such as a gaseous or insulating reactant). Even though half-cell reactions occur at different electrodes, reaction rates are coupled by the principles of conservation of charge and electroneutrality.
2.1.1 Cathode
To date, many electrochemical battery systems have been proposed. The term “cathode” is a relative concept. A cathode in one electrochemical system can be an anode in another system. For example, MnO2 is a cathode material in Zn-Mn batteries but an anode material in LIBs. Hence, whether a material is a cathode or anode depends on the relative redox potential of the material, which, in an electrochemical device, can be deduced based on standard reduction potential. Standard reduction potential describes the ability of a material to release electrons. The lower the standard reduction potential, the stronger the ability to release electrons. Therefore, when two materials are assembled into an electrochemical system, one with the higher reduction potential is the cathode, and the other one is the anode. Theoretically, any material can be assembled into electrochemical batteries with appropriate electrolytes. The output voltage of an electrochemical system depends on the potential difference between the cathode and anode. Some electrodes commonly used in diverse electrochemical energy storage technologies are LiCoO2, LiFePO4, LiMn2O4, LiNiCoMnO2, and Ni(OH)2. All these possess unique crystal structures (e.g. layer structure and sodium super ionic conductor (NASICON) structure) and have sufficient space to store ions and ensure rapid ion transportation (Table 2.1).
Table 2.1 The standard reduction potential of typical materials.
2.1.2 Anode
In an electrochemical system, anode is the electrode that reacts at a lower potential among the two electrodes. Metals and graphite are some of the commonly used anode materials in state-of-the-art energy storage technologies. Usually, graphite and other carbon materials are used as host materials to store energy through reversible ion intercalation. The capacity of these materials is usually less than 300 mAh g−1. In metal anodes, energy storage occurs via reversible ion plating/striping. Theoretically, the capacity of metal anodes is much higher than that of carbon materials. However, in metal anodes, irreversible etching and uneven plating/striping lead to the formation of dendrites and “dead metal,” thus presenting them with poor cycling stability and high safety hazard. Designing advanced electrolytes and artificial solid electrolyte interphase (SEI) for ensuring uniform ion plating/striping and protecting irreversible etching is an interesting topic in the field of electrochemical energy storage.
2.1.3 Electrolyte
The electrolyte is an essential component that ensures rapid ionic conductivity inside the battery and prevents direct contact between cathodes and anodes. Electrolytes used in various electrochemical energy storage technologies should generally meet the following basic requirements [4]: (i) high ionic conductivity, at least 1 × 10−3 ∼ 2 × 10−2 S cm−1; (ii) high thermal and chemical stability, with no separation in a wide voltage range; (iii) a wider electrochemical stability window (ESW) to keep the electrochemical performance stable in a wider voltage range; (iv) good compatibility with other parts of the battery, such as electrode materials, electrode current collectors, and separators; and (v) safe, nontoxic, and nonpolluting.
Based on the energy storage mechanism of electrode materials, various electrolytes have been developed, which can primarily be classified into three categories: aqueous electrolytes, organic electrolytes, and solid-state electrolytes. In aqueous electrolytes, water is used as the solvent. The following are the major advantages of aqueous electrolytes: low cost, high ionic conductivity (compared with organic electrolytes and solid-state electrolytes), and being nonflammable and recyclable. Their primary disadvantage is the limited thermodynamic ESW of water. Theoretically, water decomposes above a voltage of 1.23 V. Considering the overpotential of water splitting, the voltage of aqueous electrolytes is less than 2.0 V. Since energy density is highly dependent on output potential, electrochemical energy storage technologies with aqueous electrolytes usually have much lower energy density than those using organic electrolytes and solid-state electrolytes. Recently, numerous strategies have been developed to make the ESW of aqueous electrolytes wider, e.g. water-in-salt (WIS) electrolytes, deep eutectic solvents, and artificial SEI. In particular, using well-designed WIS electrolytes, the ESW of aqueous electrolytes can be extended to 3 V, as proposed in some previously published studies. However, usually, only electrolytes based on the trifluoromethanesulfonimide (TFSI) anion can barely work above 3 V, and the ESW of aqueous electrolytes using inorganic salts can rarely reach above ∼2.5 V in devices. Recent studies suggest that an SEI derived by the decomposition of the TFSI anion is highly porous and dynamically soluble in electrolytes [5]. Since the SEI is not stable, water splitting continuously occurs in LiTFSI-based WIS electrolytes. The continuous generation of gases increases the pressure in the batteries, leading to rapid capacity decay and even explosion. It is proposed that even in high-cost LiTFSI-based electrolytes, electrochemical energy storage devices with aqueous electrolytes can rarely work above 2 V and do not present apparent superiorities over lead–acid batteries [6]. Gel electrolytes are a type of aqueous electrolytes that confine water molecules through strong interactions that can effectively prohibit the decomposition of water at low salt concentrations, which may be a practical way to achieve high output voltage and high energy density in energy storage technologies with aqueous electrolytes. The success of lead–acid batteries can provide a meaningful reference for the development of high-voltage energy storage technologies using aqueous electrolytes.
In organic electrolytes, an organic liquid is used as a solvent to dissolve salts. State-of-the-art LIBs use organic electrolytes, in which LiPF6 is used as the salt and ethylene carbonate (EC)/propylene carbonate (PC)/fluoroethylene carbonate (FEC) is used as the solvent. Each component in the organic electrolytes of LIBs is carefully selected; EC/PC can provide high ionic conductivity with good electrochemical stability and large ESW, while FEC can help form LiF-rich SEI, to ensure stable and rapid ion transportation between the electrode and electrolyte. The critical challenge in using organic electrolytes is safety issues. Dendrites can form even in state-of-the-art technologies. Short circuits induced by dendrites will release a substantial amount of heat to accelerate the decomposition of organic electrolytes and form H2/O2 gas, leading to thermal runaway and even explosion.
Since the thermal runaway of LIBs is attributable to the decomposition and poor thermal stability of organic electrolytes, nonflammable and high-mechanical-strength solid-state electrolytes are developed to ensure the safety of LIBs and other batteries. Solid-state electrolytes can be classified into two groups: inorganic electrolytes and polymer electrolytes. Inorganic electrolytes possess sufficient mechanical strength to prevent the formation of metal dendrites and high thermal stability to achieve intrinsic safety. Their ionic conductivity is 1 × 10−2 ∼ 10−4 S cm−1, which is close to that of liquid electrolytes and meets the criteria for applications in various electrochemical energy storage devices. However, solid–solid electrode–electrolyte interfaces cannot establish an intimate contact, leading to large interface impendence and poor capacity. To ensure good contact, high external pressure is usually required for batteries using solid-state electrolytes, which is hard to achieve in real-world scenarios. Furthermore, the volume expansion of electrodes results in high mechanical strength at electrode–electrolyte interfaces, which leads to the pulverization and falling off of active materials, resulting in poor cycling stability. In contrast, polymer electrolytes are usually soft and can tolerate the volume expansion of active materials and improve the electrode–electrolyte contact, which is conducive to promoting the reversible capacity and cycling lifespans. Nevertheless, polymer electrolytes do not have high enough mechanical strength to prevent short circuits induced by dendrite growth, and their ionic conductivity (∼10-6 S cm−1) is insufficient for a high ion transport pathway for electrochemical reactions. Considering all the abovementioned points, combining the advantages of inorganic electrolytes and those of polymer electrolytes can be a promising way to achieve a practicable solid-state electrolyte, and this has been widely investigated in recent years.
2.2 Operating Mechanism of Devices
2.2.1 Potential and Thermodynamics
The working mechanism of an electrochemical battery depends on the intrinsic properties of the materials used, especially thermodynamic properties. As mentioned earlier, a redox reaction of a battery can occur separately as the oxidation reaction takes place at one electrode and the reduction reaction at another, i.e. the whole battery acting as two half-cells. The energy change of the reaction is given by the change in Gibbs free energy for each half-cell reaction:
where G refers to the Gibbs free energy, μi represents the electrochemical potential of species i, and si is the stoichiometric coefficient of species i. If ΔG is negative, electrons will spontaneously flow from the anode to the cathode. This is the driving force for ensuring spontaneous redox reactions in electrochemical batteries. The anode side usually has a lower electrochemical potential, whereas the cathode side has a higher electrochemical potential. The electrochemical potential of different materials may have different reference potentials, e.g. Li/Li+, Na/Na+, Ag/AgCl, and Hg/HgO. However, these electrochemical potentials based on different reference potentials can be converted to a standard reference potential.
The maximum amount of work (Wmax) that can be performed by an electrochemical cell is equal to the product of the cell potential () and the total charge transferred during the reaction (nF):
Wmax is expressed as a negative number because work is being done by a system (i.e. an electrochemical cell with a positive potential) on its surroundings.
The change in Gibbs free energy (ΔG) is also a measure of the maximum amount of work that can be performed during a chemical process (ΔG = Wmax). Consequently, the relationship between the potential of an electrochemical cell and ΔG can be deduced as follows:
A spontaneous redox reaction is therefore characterized by a negative value of ΔG and a positive value of . When both reactants and products are in their standard states, the relationship between ΔG° and can be expressed as follows:
A spontaneous redox reaction is characterized by a negative value of ΔG°, which corresponds to a positive value of . Thus, equilibrium potential is a function of the intrinsic nature of the species present, as well as their concentrations and, to a lesser extent, temperature. No net current flows at equilibrium, and random thermal collisions between the reactant and product species cause the reaction to occur, sometimes in the forward direction and sometimes in the backward direction. At equilibrium, the rate of the forward reaction is equal to that of the backward reaction. The potential of the electrode at equilibrium is a measure of the electrochemical potential (i.e. energy) of electrons at equilibrium with the reactant and product species. In brief, electrochemical potential can be related to molality mi and activity coefficient γi as follows:
where μθ is independent of concentration, R is the universal gas constant (8.3143 J mol−1 K−1), and T is the temperature in kelvin.
One can electrically control the electrochemical potential of electrons in an electrode by connecting it to an external power supply, thereby perturbing the equilibrium and driving a reaction. Applying a negative potential to an electrode increases the energy of electrons. This increase in the energy of electrons above the lowest unoccupied molecular orbital of a species in the adjacent electrolyte results in a reduction of that species. This reduction current (flow of electrons into the electrode and from there into the reactant) is also called cathodic current, and the electrode at which this occurs is called the cathode. Conversely, applying a positive potential to an electrode decreases the energy of electrons, causing them to be transferred from the reactants to the electrode. The electrode where such an oxidation reaction occurs is called the anode.
2.2.2 Kinetics of Electrode Reactions
As mentioned earlier, the redox reaction is separated into oxidation and reduction that occur in two electrodes. In galvanic cells, the reduction reaction occurs at the cathode side, and the oxidation reaction occurs at the anode side. The rate of reaction can be expressed using the transition state theory (Figure 2.1). For a reaction to occur, it needs to overcome an energy barrier (Gibbs free energy). Considering this, the rate of reaction can be expressed as follows:
Figure 2.1 Rate of reaction for a chemical process.
Figure 2.2 Typical charge transfer process at an inert electrode surface.
The chemical reaction process on inert electrode surfaces involves the transportation of electrochemically active species (usually ions) from the bulk electrolyte to the region that is very close to the electrode surface, preceding chemical reactions (not applicable for all electrochemical reactions however), adsorbing on the electrode surface, ensuring charge transfer, desorbing from the electrode surface, following chemical reactions (not applicable for electrochemical reactions however), and migrating to the bulk electrolyte (Figure 2.2). Adsorption, charge transfer, and desorption processes occur rapidly and cannot be separated based on the existing electrochemical technologies or structural characterization technologies. As for an active electrode, the chemical reaction process is different from that of an inert electrode. The charge transfer process can occur at the internal of the electrode. In addition, ions are solvated and undergo a desolvation process before entering the electrode, which kinetically slows down the reaction rate. To make things more complicated, some side reactions occur before the reversible energy storage reaction, which lead to continuous consumption of electrolytes or the formation of SEI.
In addition to the chemical reaction process, the bias voltage acts as a driving force to initiate electrochemical reactions at electrodes and is another essential parameter that affects reaction kinetics. This driving force is termed “surface overpotential” and is denoted as ηs. The rate of a reaction can often be related to surface overpotential using the Butler–Volmer equation, as follows:
The first term on the right-hand side of the equation is the rate of the oxidation reaction, whereas the second term is the rate of the reduction reaction. The difference between these rates is the net rate of reaction. The parameter i0 is the exchange current density and is analogous to the rate constant used in chemical kinetics. Based on the above equation, the Tafel I – η relationship at very large |η| can be expressed as follows:
This relationship is also called a semi-logarithm relationship. A reaction with a high i0 value is often called fast or reversible, and I − η usually follows a linear relation. In contrast, a low i0 value approaching disappearance indicates the highly polarized electrode and that the chemical reaction is hardly reversible. The I − η relation of a low i0 value usually is a semi-logarithm relation or similar to the charging behavior of a RC circuit. For a high i0 value, a high current density can be obtained with a low surface overpotential. In a reaction involving high exchange current density, both the forward and backward reactions occur rapidly, and the net direction of the reaction depends on the sign of the surface overpotential. The exchange current density of the reaction depends on the concentrations of reactants and products, temperature, the nature of the electrode–electrolyte interface, and impurities that may contaminate the surface. Each of these factors can change the value of i0 by several orders of magnitude; i0 can range from over 1 mA cm−2 to less than 10−7 mA cm−2. The parameters αox and αred, called apparent transfer coefficients, are additional kinetic parameters that inform how an applied potential favors one direction of the reaction over the other. The values of these parameters usually range between 0.2 and 2.
Overpotential is the potential difference above the equilibrium potential that is required to overcome the activation energy of the cell reaction to produce a specified current. Hence, the activation energy can also affect the electrode reaction kinetics. As discussed previously, for a reaction to occur, it needs to overcome an energy barrier (Gibbs free energy). The charge transfer process of electrochemical reactions should overcome an energy barrier as well, and this energy barrier is affected by bias voltage. Considering the half-cell of the reduction reaction as an example, the free energy of reduction species (denoted as Red) is invariant with voltage, whereas the reduction product (denoted as Ox + e−) is highly affected by voltage. To determine the impact of voltage on electrode reaction kinetics, it is assumed that the effect of voltage on free energy change follows a linear relationship (this is undoubtedly an oversimplification). Using this linear relationship, the activation free energies for reduction and oxidation will be considered to vary as a function of the applied voltage (V) as follows:
The parameter α is called the transfer coefficient, which typically has a value of 0.5. Physically, this provides an insight into the way the transition state is influenced by the voltage. A value of 0.5 means that the transition state behaves midway between the reactants’ and products’ response to the applied voltage. The free energy on the right-hand side of Eqs. (2.9, 2.10) and 2.10 can be considered the chemical component of the activation free energy change. It is dependent only on the species involved and not on the applied voltage. Substituting these activation free energy terms into the expressions for the oxidation and reduction rate constants gives the following:
These results suggest that the rate constants for a charge transfer process are proportional to the exponential of the applied voltage. So the electrode reaction kinetics can be changed by simply varying the applied voltage. This result provides the fundamental basis for the experimental technique called voltammetry.
Now that the effects of overpotential and activation energy on electrode reaction kinetics are discussed, the following text introduces the effects of mass transfer. Mass transfer is the movement of species in a medium (liquid, gas, or solid). The rate of mass transfer denotes the number of species passing through a unit area within an interval of unit time. Three processes are related to mass transfer, namely diffusion, migration, and convection. Diffusion is the movement of a charged species derived by a gradient of chemical potential (e.g. concentration gradient), which is also called Fick’s first law of diffusion. This can be expressed as follows:
Migration refers to the movement of a species driven by a gradient of electric potential, which can be represented as follows:
Convection refers to the movement of a species in the fluid flow. Natural convection is induced by density gradient, whereas forced convection is caused by stirring or other hydrodynamic means, as shown below:
Hence, mass transfer in an electrochemical cell can be expressed as follows:
Usually, some measures can be taken so that only one kind of mass transfer contributes predominantly and others are negligible. In a solution, natural convection is the predominant mass transfer mode, and it ensures the uniform concentrations of all species. However, the velocity of the fluid decreases sharply upon approaching a solid surface. Thus, within a very thin layer near the electrode surface, convection can be neglected. Diffusion and migration are predominant in the near surface reaction of electrodes (called the Nernst boundary layer). If the excess of an inert electrolyte is added to a solution and its concentration is much higher than that of the electroactive species, the electric field and migration rate in the solution can be greatly reduced. Thus, diffusion will be the only predominant mass transfer mode within the thin layer near the electrode surface (called diffusion layer).
Upon imposing electrochemical polarization (e.g. potential or current perturbation) to an electrode system, interfacial reactions occur, which render the concentration of species in the vicinity of the electrode different from that of the bulk solution. A concentration gradient then arises, and in the meantime, diffusion takes place, trying to eliminate the concentration gradient. The combined processes of the development of concentration gradient due to interfacial reactions and the diffusion to eliminate the concentration dynamically change the concentration, and the concentration profile gradually extends toward the bulk solution:
Due to natural (or forced) convection, the concentration gradient does not extend unlimitedly toward the bulk, and a steady-state profile of concentration is established finally:
It is worth noting that the above discussions about the impact of overpotential and activation energy on reaction kinetics do not consider internal reactions of the electrode. However, in electrochemical energy storage technologies, electrode materials participate in the redox reaction through intercalation, conversion, and alloying reactions. Since the bulk phase of electrode materials can participate in the electrochemical reaction, charge transfer and reaction process in the active electrode can significantly affect electrode reaction kinetics. In this regard, the diffusion kinetics of redox species (e.g. Li+ for LIBs), which can be determined experimentally by diffusion coefficient and theoretically by diffusion barrier, should be taken into account. Hence, to achieve superior energy storage kinetics, both electrodes and electrolytes need to be carefully designed. The typical energy storage processes involved in electrochemical energy storage devices and electrodes are electrochemical double-layer capacitance (EDLC), pseudocapacitance (including underpotential deposition, surface redox pseudocapacitance, intercalation pseudocapacitance), and battery-type faradic reactions (including intercalation, conversion, and alloying), as illustrated in Figure 2.3.
Figure 2.3 Typical process in electrochemical energy storage cells and electrodes. (a) The illustration of electrochemical double layer capacitor, pseudocapacitor, and lithium-ion battery. (b) The diagram of various pseudocapacitors. (c) The energy storage mechanisms of battery-type electrodes.
Source: (a) Reproduced with permission from Jost et al. [7]. © 2014/Royal Society of Chemistry. (b) Reproduced with permission from Liu et al. [8]. © 2020/Elsevier. (c) Reproduced with permission from Bensalah and Dowoud [9]. © 2016/Hilaris, CC BY 4.0.
In energy storage devices, EDLC and pseudocapacitor reactions occur at electrode surfaces or near-surface regions, and they occur rapidly compared with battery-type reactions. Battery-type reactions involve bulk electrode reactions, and their reaction kinetics is slower than that of surface-dominated reactions. The electrode process of these reaction mechanisms involves the transport of electrons/ions and possibly contains phase transition. The difference between the capacitive-type electrode process and the battery-type electrode process is that the former primarily occurs at the surface and electrode/electrolyte interface regions, whereas the latter can occur in the bulk of electrode materials. Since electrodes are electrochemically active, various components are defined to describe the equivalent circuit of an electrode and interfaces, including migration of ions within the electrolyte (Rs), adsorption on the electrode surface (Rad, Cad), charge of the electrochemical double layer (Cdl), charge of the space charge layer (Csc), ions across the electrode–electrolyte interface (Rincorporation), charge transfer (Rct), diffusion within the electrolyte inside pores/cavities (Zdiffusion), bulk diffusion (Rb), diffusion along the grain boundary (Rgb), storage of guest atoms in the lattice (Cchem), phase transition (Cchem), and electron transport (Re).
In sum, compared with classic electrochemical systems with inert electrodes, the diffusion of ions in the bulk electrode, phase/structural transitions (especially in the long term), and multi-interfaces should be considered in electrochemical energy storage systems with active electrodes.
2.3 Critical Parameters and Design Proposal
To achieve practical applications, electrochemical energy storage technologies should have many properties, such as high energy/power density, intrinsic safety, and long lifespan. The energy density of an electrochemical energy storage technology can be calculated based on the following equation:
where Q is the storage charge and U is the output voltage. Hence, high capacity and large voltage window as well as output voltage lead to high energy density. The charge storage ability of a material primarily depends on its theoretical capacity. In general, the theoretical capacity of an electrode material can be calculated using the reversible electron transfer number per mole at the theoretical level and faradic constant. The storage charge of a material can be calculated as follows:
where n is the theoretical electron transfer number per mole, F is the faradic constant, and M is the molecular mass of the material. A high electron transfer number and low molecular mass lead to high capacity. The material with the lowest molecular mass that can be used as an active material for electrochemical energy storage is hydrogen, and the theoretical electron transfer number can reach 8 using the redox couple of NH3/NO3−. However, these theoretically high-capacity electrodes have not been practically applied yet because of complicated reaction mechanisms and optimized electrochemical systems. To obtain a high-energy-density technology, the mass between the cathode and anode should be balanced to achieve a good matching. Considering commercial LiFePO4-based LIBs as an example, the capacity of a LiFePO4 cathode and graphite anode is about 140 and 320 mAh g−1, respectively. To store the same amount of charges, the mass of the cathode material should be twofold higher than that of the anode material. Considering the high mass loading required for practical applications, the mass loading of the cathode material can reach 200 mg cm−2, while that of the anode material is about 100 mg cm−2. Such high mass loading of the cathode material leads to a long transport pathway of electrons from the current collector to the cathode surface, and a partially active material in the cathode cannot participate in the redox reaction, leading to poor rate performance and insufficient energy density. In this regard, the closed specific capacity between cathode materials and anode materials is conducive to achieving good matching.
Power density is an important factor in applications that require high power output, such as electric vehicles and energy storage systems for renewable energy. The power density of electrochemical batteries is usually below 500 W kg−1, much less than that of supercapacitors (>10 kW kg−1). Although some power-type LIBs have been commercialized recently, the charging/discharging process can rarely go beyond 3 C, which is insufficient for some applications. Hence, increasing the power density of electrochemical batteries has become a highly researched topic in the field of electrochemical energy storage. High power density means that rapid redox reactions occur in the electrodes, i.e. rate performance. Usually, the ionic conductivity of electrolytes is sufficient to meet the demand for rapid ion diffusion kinetics. The rate performance is primarily determined by ion diffusion kinetics in bulk electrodes. The diffusion of ions from the initial state to the final state occurs through many transition states and overcomes various energy barriers, which indicates that decreasing the diffusion energy barrier can be an effective way to boost ion diffusion kinetics in electrodes. Research shows that heteroatom doping can tailor the electronic structure of a material and decrease its diffusion energy barrier. In addition, rate performance is influenced by operating temperature. The existing commercial LIBs cannot work below 20 °C. At such low temperatures, the ion diffusion barrier in bulk electrodes and the interfacial impendence are significantly increased, leading to poor rate performance.
Cycling lifespan is a critical parameter for applications that serve for a long period, such as energy storage systems and aerospace equipment. The cycling lifespan of an electrochemical battery depends on the structural stability of electrode materials and the chemical/electrochemical stability of electrolytes. In general, for intercalation electrode materials, repeat ion insertion/extraction results in the collapse of the layer structure. This structural degeneration may be caused by several reasons as follows: oxygen evolution may occur at the cathode side, which causes irreversible phase transition; undue ion extraction can induce the Jahn–Teller effect in LiMn2O4; lithium dendrite growth can occur on the anode side, which can cause “dead Li” formation, leading to irreversible lithium consumption; and so on. In addition, volume expansion may break the SEI on the anode surface, which renders the continuous formation of SEI and consumption of the electrolyte until breakdown. Surface coating and heteroatom doping can effectively prolong the cycling lifespan of electrochemical batteries.
In addition to electrochemical performance under normal conditions, the energy storage performance of electrochemical storage systems in different environments has attracted substantial attention, which are the major issues that should be addressed before being used in wide-ranging applications. State-of-the-art studies primarily focus on the working temperature. Commercial LIBs cannot function at temperatures below −20 °C. Although supercapacitors can work at −40 °C, their energy density is insufficient to be applied as the main power for equipment. Hence, how to achieve high energy/power density below around −40 °C is one of the most important challenges in the field of electrochemical energy storage.
2.4 Common Investigation Technologies
2.4.1 Structural Characterizations
Literature confirms that the nanostructure and crystal structure of a material can significantly affect its electrochemical performance. Briefly, downsizing the nanoparticle size of a material can decrease the distance for ion diffusion between electrodes and electrolytes. Furthermore, decreasing the particle size of a material can increase its specific surface area and contact surface area between electrodes and electrolytes, which can lead to more active materials participating in redox reactions at a relatively high current density. The hierarchical structure of a material further increases its surface area, leading to a high rate performance. Meanwhile, pore structures and hollow structures can provide more space to accommodate the volume expansion of active materials during the charging/discharging process, thus improving structural durability and cycling lifespans of electrodes. With careful design, core–shell composite materials can combine the advantages of different materials, and the shell material can prohibit the structural degeneration of the core material, thus prolonging the cycling lifespans of electrodes. Layered crystal structure materials with large interlayer distances and materials with large frameworks are conducive to rapid ion diffusions, leading to excellent rate performance. It is also worth noting that the structure and components of electrode materials are changed during the continuous charging/discharging process. Hence, understanding the nanostructure and crystal structure of materials is important for the investigation of electrochemical energy storage systems and corresponding materials.
The common structural characterization technologies used in exploring the structure of electrode materials are scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), scanning transmission electron microscopy (STEM), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) test. In general, various electron microscopes are used to analyze the morphology of materials, including nanostructure, size, and phase boundary. These electron microscopes have different resolutions, and the application is also different. For example, SEM is primarily used to determine the nanostructure of materials in the micrometer scale, TEM can reveal the single-particle morphology of materials, and high-resolution TEM can reveal the crystal plane of materials. Spherical-aberration-corrected TEM and STEM can show the atom arrangement of materials, and the results are convincing to identify the doping position of the heteroatom, the existence of the heterointerface of the heterostructure, and the formation of single-atom materials. In addition, electron energy loss spectroscopy (EELS
