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Sodium-Ion Capacitors
Enables readers to quickly understand core issues and field development of sodium-ion capacitors
Sodium-Ion Capacitors summarizes and outlines the dynamics and development of sodium-ion capacitors, covering key aspects of the technology including background, classification and configuration, key technologies, and more, allowing readers to gain an understanding of sodium-ion capacitors from the perspective of both industrial technology and electrochemistry.
Sodium-Ion Capacitors includes information on:
- EDLC-type mechanism of SCs and battery-type mechanism of SIBs, definition and types of pseudocapacitance, and energy storage mechanism of pseudocapacitors
- Cathode materials for sodium-ion capacitors, covering EDLC cathode materials, carbon nanotubes, reduced graphene oxide, and hollow carbon microspheres
- Flexible battery-type anode and capacitive cathode SICs cell configurations, including flexible electrodes based on carbon nanofiber, graphene substrates, carbon cloth, MXenes, and metal foil
- Pre-sodiation technologies, covering operation with Li metal, usage of Li-based alternatives, and the sacrificial additives method
Summarizing the development, directions, potential, and core issues of sodium-ion capacitors, Sodium-Ion Capacitors is an essential resource on the subject for materials scientists, solid-state chemists and electrochemists, and semiconductor physicists in both industry and academia.
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Veröffentlichungsjahr: 2023
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
1 Introduction
1.1 A Brief Development of SICs
1.2 Comparison Between Different Hybrid‐Ion Capacitors
1.3 SICs Energy Storage Mechanism Introduction
1.4 Key Technologies of SICs
References
2 Characteristics of Sodium‐Ion Capacitor Devices
2.1 Basic Features
2.2 Working Principle
2.3 Equations
References
3 Fundamental Understanding of Sodium‐Ion Capacitors Mechanism
3.1 EDLC‐Type Mechanism of SCs and Battery‐Type Mechanism of SIBs
3.2 Pseudocapacitance Mechanism
References
4 Classification of Sodium-Ion Capacitors Cell Configurations
4.1 Battery‐Type Anode and EDLC Cathode SICs Cell Configurations
4.2 Battery‐Type Anode and Pseudocapacitive Cathode SICs Cell Configurations
4.3 EDLC Anode and Battery‐Type Cathode SICs Cell Configurations
4.4 Pseudocapacitive Anode and Battery‐Type Cathode SICs Cell Configurations
4.5 Capacitive Anode and Hybrid Cathode SICs Cell Configurations
4.6 Summary
References
5 Cathode Materials for Sodium‐Ion Capacitors
5.1 Introduction
5.2 EDLC Cathode Materials
5.3 Pseudocapacitive Cathode Materials
5.4 Battery‐Type Cathode Materials
References
6 Anode Materials for Sodium‐Ion Capacitors
6.1 EDLC Anode Materials
6.2 Pseudocapacitive Anode Materials
6.3 Battery‐Type Anode Materials
6.4 Other Novel Materials
References
7 Flexible Sodium‐Ion Capacitor Devices
7.1 Flexible SICs Devices
7.2 Flexible Capacitive Anode and Battery‐Type Cathode SICs Cell Configurations
7.3 Electrolytes in Flexible SICs Devices
References
8 Pre‐sodiation Technologies
8.1 Introduction
8.2 Pre‐lithiation in Lithium‐Ion Batteries
8.3 Pre‐sodiation in Sodium‐Ion Batteries
8.4 Pre‐sodiation in Sodium‐Ion Capacitors
References
9 Conclusions and Future Perspective
9.1 Definitions and Mechanisms
9.2 Configurations
9.3 Electrode Materials
9.4 Key Technologies
9.5 Future Perspective
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Characteristic physical properties of Li
+
, Na
+
, K
+
, M...
Table 1.2 Product characteristics of some of the most advanced commercial l...
Chapter 4
Table 4.1 A summary of dual‐carbon configurations based on carbon‐based ano...
Table 4.2 A summary of SICs devices based on intercalation mechanism anode a...
Table 4.3 A summary of SICs devices based on conversion and alloying reacti...
List of Illustrations
Chapter 1
Figure 1.1 (a) Relative atomic mass and gravimetric specific capacity of met...
Chapter 2
Figure 2.1 Schematic representations of a single electrode system. (a) Capac...
Figure 2.2 An idealized behavior of a sodium‐ion capacitor. (a) Potential pr...
Chapter 3
Figure 3.1 (a) Schematic illustration of the typical configuration (cathode:...
Figure 3.2 Schematic diagram comparing the fundamental charge storage mechan...
Figure 3.3 Classification of electrochemical energy storage mechanisms as a ...
Figure 3.4 Illustration of the three types of mechanisms that give rise to p...
Figure 3.5 Criterion for distinguishing (a) CV, (b) GCD curves, and (c) feat...
Figure 3.6 Criterion of three dimensional Bode plots for three types of elec...
Chapter 4
Figure 4.1 Electrochemical characteristics of different device configuration...
Figure 4.2 Cell structures and charge storage mechanisms of the nine electro...
Figure 4.3 Schematic representations of hybrid combination of a hybrid elect...
Chapter 5
Figure 5.1 Various materials for SIC cathode.
Figure 5.2 Structure diagram of (a) SWCNT and (b) MWCNT.
Figure 5.3 (a) Schematic diagram of the synthesis of 1D CNFs and assembly of...
Figure 5.4 Structure diagram of graphene.
Figure 5.5 (a) The schematic assembly of a soft‐package hybrid SIC by rGO ca...
Figure 5.6 (a) SEM and (b) TEM images of BPC‐5.(c) TEM image and (d) por...
Figure 5.7 (a) KOH activation of carbon spheres, (b) TEM image of AHC, (c) G...
Figure 5.8 (a) HRTEM image and (b) N 1s XPS of NHPAC, (c) schematic of the N...
Figure 5.9 (a) Schematic diagram of the process of realizing high‐volumetric...
Figure 5.10 (a–c) Illustration of the three types of mechanisms that give ri...
Figure 5.11 Schematic illustration of the reaction mechanism during charging...
Figure 5.12 (a) Schematic illustration of the MnSAs/NF–CNs//MnSAs/NF–CNs SIC...
Figure 5.13 (a) Anion storage mechanism in PANI. (b) Charge–discharge profil...
Figure 5.14 (a) Structural characterizations of pyridine‐V...
Figure 5.15 (a) Periodic tables presenting compositions of MAX phases and MX...
Figure 5.16 (a) Cyclic voltammetry curves of MXene Ti
2
CT
x
and Ti
3
C
2
T
x
electr...
Figure 5.17 Schematic representation for the synthesis (selective etching of...
Figure 5.18 (a) The structures of NaMn
1/3
Co
1/3
Ni
1/3
PO
4
. (b) The total DOS (d...
Figure 5.19 (a) Structure of NPF.(b) Charge and discharge curves of the ...
Figure 5.20 (a) Structure of NVOPF. (b) GCD curves of the NVOPF@PEDOT at 0.1...
Figure 5.21 Electrochemical characterizations of Na
0.67
(Mn
0.75
Al
0.25
)O
2
and ...
Figure 5.22 (a) Schematic of the coin‐type ZDC//P2‐NCM SIC; (b) the GCD curv...
Figure 5.23 Schematic of the P2‐layered and tunnel structure.
Chapter 6
Figure 6.1 Summary of the average working potential and practical capacity o...
Figure 6.2 Schematic of the different reaction mechanisms of insertion‐type ...
Figure 6.3 (a) Comparison of structure models of graphite and amorphous hard...
Figure 6.4 Schematic of reaction mechanisms of alloying, conversion, and con...
Figure 6.5 Schematic of classification of the various anodes of SICs.
Figure 6.6 (a) Charge and discharge curves and (b) cycle performance of the ...
Figure 6.7 Schematic illustration of the reaction mechanism of Ti
2
CT
x
by ele...
Figure 6.8 (a) Schematic illustration of the Na‐Ti
3
C
2
//AC SIC device. (b) CV...
Figure 6.9 (a) Illustration sodium‐ion storage mechanism for FeVO UNSs anode...
Figure 6.10 (a) Schematic illustration of the solvent influence on realizing...
Figure 6.11 (a) Illustration of dual‐carbon NTHC//APDC SIC configuration. (b...
Figure 6.12 (a) Illustration of the morphology and molecular structure of GD...
Figure 6.13 (a) Schematic illustration of the preparation of N/S‐HCNFs. (b) ...
Figure 6.14 (a) Schematic illustration for preparation of a dual–carbon SIC ...
Figure 6.15 (a) Schematic diagram of preparation process of MnSAs/NF‐CNs. (b...
Figure 6.16 (a) Schematic illustration of the synthesis and related construc...
Figure 6.17 (a) Schematic illustration of NMTiO
2
//NG SIC configuration. (b) ...
Figure 6.18 (a) Stepped layer and lamellar format in [010] zone axis and (b)...
Figure 6.19 (a) Illustration of the various Nb
2
O
5
morphologies prepared by d...
Figure 6.20 (a) Schematic of the advantages of G‐ZVO as high power SIC anode...
Figure 6.21 (a) Illustration of structural merits in the energy‐storage mech...
Figure 6.22 (a) Schematic of MoS
2
molecular structure. (b) Three main types ...
Figure 6.23 (a) Illustration of the preparation procedures of the 3D‐IO FeS‐...
Figure 6.24 (a) Illustration of ordered 2D MoSe
2
/graphene nanocomposite thro...
Figure 6.25 (a) Schematic illustration of the structural evolution mechanism...
Figure 6.26 Illustration of five types of Na
+
reaction mechanisms of org...
Figure 6.27 (a) Illustration of the working principle of the PTCD//PANI OHC....
Figure 6.28 (a) Chemical structure and (b) crystal structure of DSRH molecul...
Figure 6.29 Comparison of the structure and size of (a) zinc‐adeninate octah...
Figure 6.30 Schematic of MOF‐related electrode materials for rechargeable ba...
Figure 6.31 (a) Coordinated structure of the
ab
plane of Cu‐CAT. (b) SEM ima...
Chapter 7
Figure 7.1 Schematic diagram of two types of sodium‐ion capacitors (a) conve...
Figure 7.2 Characterization and Na
+
ion storage property in half‐cells o...
Figure 7.3 (a) Schematic synthetic process of bowl‐like VS
2
arrays on flexib...
Figure 7.4 (a) Schematic illustration of the fabrication of the flexible m‐N...
Figure 7.5 (a) Digital photographs for the appearance and flexibility (rolli...
Figure 7.6 SEM and TEM images of (a) TiO
2
nanosheets/CNF, (b, d) Na
2
Ti
3
O
7
na...
Figure 7.7 FE‐SEM image (inserted for the digital image) (a, b) TEM images,
Figure 7.8 (a) Digital images for the flexibility of MoO
2
SCs@N‐CNFs film wi...
Figure 7.9 Electrochemical performance of the hybrid SICs. (a) Schematic dia...
Figure 7.10 (a) Digital images for the flexibility of the resultant Mo
2
C QDs...
Figure 7.11 (a) Schematic illustration of the configuration of flexible SIC....
Figure 7.12 (a) Digital photographs of the flexible Sb
2
S
3
/Ti
3
C
2
T
x
film. (b) ...
Figure 7.13 AC//NTP quasi‐solid‐state device. (a) Cross‐sectional SEM image,...
Figure 7.14 SIC full device. (a) Schematic illustration of the SIC device us...
Figure 7.15 (a) Photograpy of carbon nanofiber membrane CNF@NVPF. (b) SEM im...
Figure 7.16 (a) Photographs under various bending states of the flexible fre...
Figure 7.17 (a) Photos of the prepared porous P(VDF‐HFP) membrane (b) and it...
Figure 7.18 Quasi‐solid‐state sodium‐ion hybrid capacitor. (a) SEM images an...
Chapter 8
Figure 8.1 Schematic of three different pre‐metallation methods: DC, ESC, an...
Figure 8.2 (a) Schematic of the procedures to fabricate ambient‐air‐stable l...
Figure 8.3 Schematic of the pre‐lithiation mechanism of Li‐Bp‐THF.
Figure 8.4 (a) Schematic illustration of the full cell (Li
3
V
2
(PO
4
)
3
//HC)....
Figure 8.5 (a) Schematic of utilizing a Naph‐Na solution to pre‐sodiate HC e...
Figure 8.6 Ragone plots (a) of assembled SICs [56]. Cycle performance (b) of...
Figure 8.7 (a) Ragone plots of the SICs (NaBH
4
/AC//hard carbon and NaBH
4
/AC/...
Figure 8.8 Ragone plots (A) and cycle performance (B) of YP80F//Na
x
Sn
4
P
3
SIC...
Chapter 9
Figure 9.1 Summary of the key fields of SICs.
Figure 9.2 Summary of the important aspects of SICs in the future perspectiv...
Guide
Cover Page
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
WILEY END USER LICENSE AGREEMENT
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Sodium‐Ion Capacitors
Mechanisms, Materials, and Technologies
Edited by Guoqiang Zou, Xiaobo Ji, and Hongshuai Hou
Editors
Prof. Guoqiang Zou
Central South University
College of Chemistry and Chemical Engineering
No. 932, Lushan South Road
YueLu District
Changsha 410083
China
Prof. Xiaobo Ji
Central South University
College of Chemistry and Chemical Engineering
No. 932, Lushan South Road
YueLu District
Changsha 410083
China
Prof. Hongshuai Hou
Central South University
College of Chemistry and Chemical Engineering
No. 932, Lushan South Road
YueLu District
Changsha 410083
China
Cover Image: © Andrei Kuzmik/Shutterstock
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Preface
Standing at the point where global warming and climate change risks are appearing extremely prominent, the transformation of the global energy configuration to renewable energy system is an inevitable trend in order to achieve the goals in the Paris Agreement and accomplish the reduction of carbon emission. The utilization of renewable energy is expected to be wielded in a larger scale to reduce the dependence on traditional fossil fuels (coal, oil, etc.) and become the major source of power generation. Significantly, the maturity of renewable energy technology and the rapid expansion of scale have greatly reduced the relevant costs in recent years, forming a strong competition with traditional fossil energy in terms of price.
Advanced secondary ion batteries, as the representational technology of energy storage, play a pivotal role for decarbonization. Triumphing toward clean energy conversion in the next few decades, lithium‐ion batteries (LIBs) have been developed in the direction of high‐energy density. However, the limited output performance at high‐power density seriously restricts the corresponding wide application. Importantly, well‐balanced trade‐offs between energy density and power density have been presented by metal‐ion capacitors, in which capacitor‐type electrode with adsorption/desorption behavior and battery‐type electrode with intercalation/conversion/alloying mechanism are selected as cathode and anode, respectively. Owing to the overwhelming advantage of much more abundant content of sodium sources in the crust than lithium, sodium‐ion capacitors (SICs) have attracted extensive interest in constructing next‐generation energy storage systems.
SICs are just like Rubik's cubes with charming diversities; this book comprehensively and systematically describes technology, relevant materials, and development trends of SICs, including characteristic of SICs devices, fundamental understanding of SICs mechanism, classification of SICs cell configurations, cathode materials for SICs, anode materials for SICs, flexible SICs devices, and pre‐sodiation technologies. Meanwhile, this book offers guidelines to construct advanced SICs, which is suitable for scientific and technological workers in the field of energy storage systems, as well as senior undergraduates, graduate students, and teachers of related majors in colleges and universities.
This book is edited by Prof. Zou, Prof. Ji, and Prof. Hou of the Central South University of China. The specific chapter writers divided the work as follows: Chapters 1–4 are mainly written by Professor Guoqiang Zou, Xiaobo Ji, and Hongshuai Hou of Central South University; Chapter 5 is mainly written by Professor Xiong Zhang, and Yanwei Ma of Institute of Electrical Engineering, Chinese Academy of Sciences; Chapter 6 is mainly written by Professor Guoqiang Zou, Xiaobo Ji, and Hongshuai Hou of Central South University; Chapter 7 is mainly written by Professor Huanwen Wang of Faculty of Material and Chemistry, China University of Geosciences; Chapter 8 is mainly written by Professor Chang Liu of School of Chemistry and Chemical Engineering, Hunan Institute of Engineering; Chapter 9 is mainly written by Professor Guoqiang Zou, Xiaobo Ji, and Hongshuai Hou of Central South University.
Due to the wide range of content involved in this book, coupled with the limited time and ability, if there are omissions and inadequacies, please criticize and inform us to correct!
7 November 2022
Guoqiang Zou, Xiaobo Ji, and Hongshuai Hou
Central South UniversityChangsha, China
1Introduction
Peng Cai, Wentao Deng, Hongshuai Hou, Guoqiang Zou, and Xiaobo Ji
Central South University, College of Chemistry and Chemical Engineering, No. 932, Lushan South Road, Yuelu District, Changsha, 410083, China
1.1 A Brief Development of SICs
Nowadays, with the rapid development of daily household appliances, portable instruments, data storage systems, and aerospace facilities, it is even more necessary to develop new energy storage devices with high energy density, high power density, and good cycle stability. In the past, many researchers have been committed to designing excellent energy storage devices that take into account high energy density and high power density, such as rechargeable batteries and electrochemical supercapacitors (SCs) [1–13]. However, for the world, the configuration of electrochemical energy storage devices that provide both high energy density and high power has become an urgent need [14].
The successful commercialization of lithium‐ion batteries (LIBs) in 1991 has received extensive attention from researchers [15–18]. LIBs are characterized by high working voltage, high energy density, and wide working voltage window but poor rate performance [19–22]. In comparison, SCs have higher power density and cycle stability, but their application is limited due to their low energy density defects [23, 24]. Therefore, in response to this defect, the concept of hybrid‐ion capacitors was brought up [25–32]. In 2001, Amatucci used activated carbon (AC) to construct the first lithium hybrid capacitor as the cathode and nanostructured Li4Ti5O12 (LTO) as the anode [17, 33, 34]. The energy density of the container is twice that of traditional carbon‐based SCs, and at the same time it presents a bright magnification prospect. Since then, after the application of lithium hybrid capacitors in assembling a variety of devices, many researchers are still exploring and paying attention to the cost and future reserves of lithium, especially related to the application of large‐scale energy devices and smart grids, and they have gradually proposed the idea of replacing lithium with sodium [32,35–47]. In addition, there is another kind of alkali metal ion capacitor–potassium ion capacitor (PIC), which is rich in resources, but its related technology research is still in the preliminary exploration stage. There are five main factors restricting the development of PICs: (i) low ion diffusion rate in solid electrodes and poor potassium ion reaction kinetics; (ii) large volume changes during potassium insertion/depotassization; (iii) serious side reactions and electrolyte consumption; (iv) dendrite growth and safety hazards; and (v) limited energy density/power density caused by the relatively high atomic mass of K. In addition, some aqueous metal‐ion capacitors (zinc‐ion capacitors) have also received widespread attention. However, due to their lower energy density and longer cycle life, aqueous metal‐ion capacitors are more suitable for applications in biological systems.
From the development of lithium‐ion capacitors (LICs) and other hybrid‐ion capacitors above, it is not difficult to see that mastering the development of LICs is very meaningful for researchers to explore the development of sodium‐ion capacitors (SICs). Therefore, in order to better understand the development features and advantages of SICs, other types of hybrid‐ion capacitors will be introduced in Section 1.2. Other hybrid‐ion capacitors will be introduced in the categories of monovalent hybrid‐ion capacitors and multivalent hybrid‐ion capacitors. Taking the monovalent hybrid‐ion capacitor as an example, the PICs are firstly introduced. Then, a brief introduction to multivalent hybrid‐ion capacitors is given. For example: magnesium‐ion hybrid capacitors (MICs), calcium‐ion batteries (CIBs), zinc‐ion hybrid capacitors (ZICs), and aluminum‐ion hybrid capacitors (AICs). Compared with high power density, the core problems of these emerging systems may lie in other aspects. Therefore, these systems may not be fully developed in the field of hybrid‐ion capacitors. Some basic thinking, attempts, and explorations of hybrid‐ion capacitors in these emerging development fields will also be briefly covered. This chapter hopes to inspire readers to fully understand and master SICs by covering a variety of backgrounds and introductions. Table 1.1 and Figure 1.1 demonstrate the different characteristic aspects of the charge carriers of Li+, Na+, K+, Mg2+, Ca2+, Zn2+, and Al3+ for their respective energy storage systems.
Table 1.1 Characteristic physical properties of Li+, Na+, K+, Mg2+, Ca2+, Zn2+, and Al3+ ion carriers for hybrid‐ion capacitors.
Source: Naskar et al. [48]/with permission of John Wiley & Sons.
Properties
Li
+
Na
+
K
+
Mg
2+
Ca
2+
Zn
2+
Al
3+
Relative atomic mass
6.94
6.94
9.10
24.31
40.08
65.38
26.58
Mass‐to‐charge ratio
6.94
6.94
9.10
12.15
20.04
32.69
8.86
Theoretical gravimetric capacity of ACoO
2
(mAh g
−1
)
274
235
206
260
242
–
268
Theoretical volumetric capacity of ACoO
2
(mAh cm
−3
)
1378
1193
906
—
—
—
—
E
0
(A/A
n
+
aq
) V vs. SHE
−3.04
−2.71
−2.93
−2.37
−2.87
−0.76
−1.66
E
0
(A/A
n
+
PC
) V vs. Li/Li
+
PC
0
0.23
−0.09
—
—
—
—
Shannon's ionic radii (Å)
0.76
1.02
1.38
0.72
1
0.74
0.535
Stokes radii in H
2
O (Å)
2.38
1.84
1.25
3.47
3.10
3.49
4.39
Stokes radii in PC (Å)
4.8
4.6
3.6
—
—
—
—
Molar ionic conductivity in PC (S cm
2
mol
−1
)
8.3
9.1
15.2
—
—
—
—
Molar ionic conductivity in AClO
4
/PC (S cm
2
mol
−1
)
6.54
6.54
—
—
—
—
—
Polarization strength (10
4
pm
−2
)
2.16
1.11
—
4.73
—
—
—
Desolvation energy in PC (kJ mol
−1
)
215.8
158.2
119.2
569.4
—
—
—
Melting point (°C)
180.5
97.8
63.4
650
842
419
660
Coordination preference (O = octahedral, T = tetrahedral, P = prismatic)
O and T
O and P
—
O and T
—
—
—
Figure 1.1 (a) Relative atomic mass and gravimetric specific capacity of metal. (b) Density and volumetric specific capacity of metal. (c) Standard electrode potential (V vs. SHE). (d) Shannon's ionic radii (Å) of different charge carriers for rechargeable batteries, and (e) Relative abundance‐rank and cost of different charge carriers (Li+, Na+, K+, Mg2+, Ca2+, Zn2+, and Al3+) for rechargeable batteries.
Source: Reproduced with permission of Naskar et al. [48] Copyright 2021, Wiley‐VCH GmbH.
As representative of monovalent hybrid‐ion capacitors, LICs, their development history and successful commercialization path are worth discussing. In 1984, Dr. Yamabe of Kyoto University cooperated with Dr. Yata of Kanebo Co., Ltd. to synthesize a new type of carbonaceous material, namely polyacene semiconductor (PAS), by pyrolysis of phenolic resin [49]. In 1987, Yata et al. reported the intercalation/deintercalation properties of lithium ions in PAS, and research activities on LICs have received much attention since then [50]. In 2005, Fuji Heavy Industries was the first to commercialize LIC based on a porous carbon cathode and a pre‐lithiated PAS anode with a stacked structure [51]. Since then, different companies have attempted to commercialize LICs based on various technologies, and a few promising commercial products are described in Table 1.2. Recently commercialized LICs can achieve gravimetric energy and power densities of 20 Wh kg−1 and 7.5 kW kg−1, respectively, with lifetimes ranging from 100 000 to 800 000 [52].
Table 1.2 Product characteristics of some of the most advanced commercial lithium‐ion capacitors (LICs).
Source: Adapted from Naoi et al. [52].
Company
Device type
Potential (V)
Capacitance (F)
Energy density (Wh kg
−1
)
Cycles
JM Energy Corporation
Prismatic
2.2–3.8
3300
13
300 000
General Capacitor Intl, Inc.
Laminate
2.2–3.8
3000
18
100 000
Taiyo Yuden
Cylinder
2.2–3.8
200
15
100 000
Vina Technology
Cylinder
2.2–3.8
270
—
—
Aowei Technology
Module
2.2–3.8
9000
>20
>30 000
Greenway
21 700
2.0–4.0
333
10.7
50 000
LRNET
Laminate
4
15 000
—
50 000
Asahi Kasei FDK energy
Module
15
600
12 Wh
—
ACT
Laminate
2.0–4.0
5000
15
—
NEC Tokin
Laminate
2.2–3.8
1000
8.0
—
MSR Micro
Laminate/Prismatic
2.2–3.8
92–825 range
—
—
PuriXel, South Korea
Laminate
2.25–3.00
—
—
100 000
Among the many LICs, the most classic is the pioneering work of Amatucci et al. As mentioned before, Amatucci et al. were the first to report LICs via hybridization of EDLCs and LIBs [33]. In 2001, AC(+)//Li4Ti5O12(−) LIC using 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (2 : 1, v/v) was first reported by Amatucci et al. The LICs displayed sloping charge/discharge curves in the 1.5–3.0 V window with 90% capacity utilization at 10 C rate and 10–15% capacity loss after 5000 cycles. In their reports, the Li4Ti5O12 anodes exhibit almost zero volume change upon Li+ ion intercalation/deintercalation and show a terminal lithiation voltage of 1.55 V. In a follow‐up publication in 2004, typical devices with 40 Wh kg−1 large energy density and 4000 W kg−1 high power density were developed by using nanostructured Li4Ti5O12 anode and AC/LiCoO2 composite cathode in 2 M LiBF4/acetonitrile (AN) electrolyte [53]. High durability is achieved in this device due to the utilization of AC EDL cathode material and Li4Ti5O12 nanostructured anode material. The capacity loss after 9000 cycles at full depth of discharge is 20%, which is quite superior compared to conventional LIBs.
1.2 Comparison Between Different Hybrid‐Ion Capacitors
Similarly, among monovalent hybrid‐ion capacitors, PICs have recently drawn attention as promising next‐generation energy storage systems due to their relatively abundant potassium reserves and low cost. The development of PICs is accompanied by the development of potassium‐ion batteries (PIBs). Since 2015, an increasing number of scientific publications on PIBs and PICs, can be observed. PICs exhibit some advantages [54]. For example, potassium does not thermodynamically form Al–K intermetallics, cheaper aluminum foils can be utilized as anode current collectors for PICs. Moreover, the standard electrode potential of K (−2.936 V vs. K+/K) is lower than that of Na (−2.714 V vs. Na+/Na), which may result in a wider voltage window, indicating higher energy density than SICs. Furthermore, due to the weak Lewis acidity of K+, K+ can form smaller solvated ions (3.6 Å) than Li+ (4.8 Å) and Na+ (4.6 Å) in monovalent hybrid‐ion capacitors. Thus, PICs achieve fast ionic diffusion rates and high electrical conductivity in propylene carbonate (PC) solvent. Furthermore, unlike SICs, commercially available graphite can be used as anode materials for PICs by forming intercalation compounds (KC8) with theoretical capacities of 279 mAh g−1. Another significant advantage compared to SICs is that, for most anode materials, the intercalation potential of K ions is about 0.2 V relative to K+/K, which reduces the possibility of metal potassium plating and effectively avoids the risk of dendrite formation during charging. For example, for hard carbon (HC) in SICs, the sodium potential is about 0.05 V relative to Na+/Na, while for K+/K in PICs, the potassium potential is 0.2 V. However, PICs also suffer from more limited cycling stability. Due to the large ionic radius of potassium ions (1.38 Å), among many electrode materials, especially the anodes would show large volume expansion during charge and discharge, resulting in a short lifespan in most PICs (usually <500 cycles). Consequently, more limited cycling stabilities are also great challenges for improving the power densities of PICs. Therefore, PICs are still in the developmental stages.
As discussed before, multivalent hybrid‐ion capacitors may have a different focus than the core issues of monovalent hybrid‐ion capacitors. To understand the features of MICs, a brief overview of the electrodes and electrolytes of MICs is required. As a divalent cation, energy systems based on Mg ions encounter sluggish kinetics due to the strong electrostatic interactions of Mg ions with anions in the host framework in the cathode. Therefore, the design of the cathode active material is a key factor. Chevrel phases (MgxMo6T8, T = S, Se, Te) and disulfides (MoS2, WSe2, etc.) are preferred as promising Mg‐ion cathode active materials [55–59]; since in these host materials, the electrostatic interaction between Mg ions and anions is low. In addition, sufficient channel size in the crystal structure also facilitates the facile intercalation/deintercalation of Mg ions. On the other hand, moderately polar anions (S2−, Se2−, Te2−, etc.) in electrode materials can lead to weaker bond strengths between transition metal cations, resulting in relatively low redox potentials for transition metals [60]. Therefore, Chevrel phases and disulfide show redox potentials below 2 V (vs. Mg2+/Mg). This discussion suggests that higher voltages (i.e. higher energies) and fast kinetics (i.e. higher power) are not easily achieved simultaneously in MICs. Besides these materials V2O5[61], MnO2[62], MoO3[63], MgCo2O4[64], TiS2[65], TiO2[66], sulfur [67], iodine [68], and polyanion‐based materials (MgMnSiO4, MgFeSiO4, etc.) [69, 70] have also been reported as cathode active materials for MICs. To better understand the properties of MICs, some characteristics of metallic magnesium anodes in magnesium‐ion batteries (MIBs) are also introduced. From the anode point of view, metallic Mg electrode is a good choice for nonaqueous MIBs because of its high negative voltage (−2.37 V vs. SHE) and high theoretical capacity (2205 mAh kg−1). However, complex reactions are prone to occur between the metal magnesium anodes and the electrolytes. Hence, the discussion of anodes and electrolytes needs to be covered. Due to the chemical activity of Mg in conventional electrolytes, it encounters serious problems: Due to the presence of Mg‐ion salts in polar aprotic solvents, the Mg surface tends to form a hard passivation layer that inhibits ion pathways and accompanying electrochemical reactions. To circumvent this problem, researchers are turning to alloyed/dealloyed or Mg insertion/deintercalation types of anodes. However, such materials also suffer from slow kinetics and pulverization due to excessive volume changes during charge/discharge. Among alloying/dealloying materials, Bi, Sb, Bi–Sb, Ge, Si, Sn, and Sn‐based binary alloys (Cu–Sn, Pb–Sn, and In–Sn) have been reported in the literature [71–74]. In addition, the development of non‐Mg metal anodes has promoted the further development of MICs to a certain extent [75, 76]. In 2D materials, the anode of MICs can also use defective graphene and graphene allotrope moieties [77], black phosphorus [78], and Li4Ti5O12[79]. In organic systems, the development of MICs is mainly limited by electrolytes. Furthermore, a discussion of electrolytes in MIBs is essential when it comes to electrolytes for MICs. In the work of Aurbach et al., an unconventional electrolyte system based on organohaloaluminate magnesium salts in tetrahydrofuran (THF) and polyethers of the glyme was developed, in which metallic magnesium electrodes work reversibly with relatively fast kinetics [80]. Grignard reagents (RMgX; R: alkyl or aryl, X: Cl or Br) as electrolytes for MIBs have also been reported in the literature for passivation‐free metal Mg electrodes, but their strong reducibility limits the oxidative stability of cathodes [81]. Another electrolyte system, namely organoborate (magnesium dibutyldiphenyl Mg(BPh2Bu2)2 and tributylphenyl Mg(BPhBu3)2) in THF (>0.4 mol), can realize anode reversible Mg stripping/plating as well as cathode reversible Mg ion insertion/extraction [82]. In recent years, MIBs of aqueous electrolytes have attracted attention due to their safe and cost‐effective properties. For example, in 2019, Zhang et al. constructed devices based on δ‐MnO2@carbon molecular sieve composite as cathode and nanowire VO2 as anode [83]. In 2017, Zhang et al. reported an aqueous anode with carbon‐coated FeVO4 and a todorokite‐type magnesium octahedral molecular sieve (Mg‐OMS‐1) cathode [84]. Nam et al. proposed smart material engineering by introducing crystal H2O into the layered structure of the brucite MnO2 cathode to efficiently screen electrostatic interactions between Mg2+ and host framework anions [85]. The group also demonstrated lower desolvation energies at the cathode and electrolyte interfaces by adding H2O to the nonaqueous electrolyte solution. This is because hydrated Mg ions are allowed to intercalate in their hydrated form, thereby minimizing desolvation energy loss. The intercalated hydrated Mg ions in the host framework further minimize the electrostatic interactions between Mg ions and host anions [86]. Therefore, the birnessite MnO2 cathode in pure aqueous electrolyte exhibits a large reversible capacity of 231.1 mAh g−1 at 2.8 V.
However, the above overview of aqueous/nonaqueous electrode materials is used to complement the basic background of MICs. In 2014, Yoo et al. developed the first MICs utilizing AC cloth and magnesium metal as cathodes and anodes, respectively. To prevent hard passivation films and dendrite growth on the metal anodes, 0.25 M organohalide magnesium aluminate complexes (Mg2Cl3+–Ph2AlCl2−) were utilized in THF electrolytes [87]. In this electrolyte, the Mg electrode works reversibly for thousands of cycles with a CE of approximately 100%. On the other hand, the pores in the cathodes are saturated with large ions (bulky Mg and Al‐based ionic complexes consisting of Cl, alkyl or aryl groups, and THF ligands) before the potential limit is reached. Surprisingly, the introduction of 0.5 M LiCl solves this problem, as small ionic substances will be present in the electrolytes. The full‐cell device exhibited a specific capacitance of 90 F g−1 at 5 mA g−1 within 0.9–2.4 V and maintained 79% of the initial capacitance after 4500 charge/discharge cycles. In the following years, several researchers screened suitable materials and electrolyte systems for advanced MICs. Breakthroughs in MICs are more focused on aqueous electrolytes due to the significant challenges in organic systems. Sun et al. and Maitra et al. reported MnO2 nanowires in MgSO4/Mg(NO3)2 electrolytes and MgNiO2/Mg(ClO4)2 electrolytes for low‐cost aqueous MICs, respectively [88]. In 2017, Zhang et al. demonstrated aqueous MICs with (+)Mg‐OMS‐2/graphene//0.5 M Mg(NO3)2(aq)//AC(−) configuration [89]. The cryptomelane‐type manganese oxide octahedral molecular sieves (OMS‐2) are a unique electrode with 2 × 2 and 1 × 1 tunneling structures of MnO2 in MICs, which is widely used as an active material in magnesium ion batteries. Besides, the low electronic conductivity of OMS‐2 can be mitigated by the preparation of composites containing carbon material. The full cell exhibited a high energy density of 46.9 Wh kg−1 (100 mA g−1) and excellent cycling stability (95.8% capacity retention at 100 mA g−1 after 500 cycles) at 0–2 V. Cao et al. reported the utilization of Mn3O4 and AC as the active materials in cathode and anode, respectively, using aqueous electrolytes of 2 M MgSO4[90]. The MICs showed an energy density of 20.2 Wh kg−1 (125 W kg−1) and excellent cycling stability (95% capacity retention after 6000 cycles at 0.5 A g−1) with the potential of 0–2 V. In addition, the scaled‐up flexible packaging depicts 80% capacity retention after 500 cycles at a current density of 0.5 A g−1. Tian et al. reported Mg2+ ion insertion/detachment in neutral aqueous MgSO4 electrolytes for VN anodes [91]. By increasing the scan rate from 1 to 200 mV s−1, the rectangular‐like CV and the small polarized redox peaks indicate the fast reaction kinetics due to a surface‐controlled process. In order to understand the pseudocapacitance mechanism of VN during charging/discharging, both XPS and CV can confirm that the V(III) to V(II) transition is the key to the VN charge/discharge reaction. Quasi‐solid aqueous MICs with a (+)MnO2@C/MgSO4 gel/VN(−) configuration show a bulk energy density of 13.10 mWh cm−3 (72 mW cm−3), and a bulk power density of 440 mW cm−3 (10.35 mWh cm−3) and excellent cycling stabilities (5000 cycles at 16 mA cm−2) in the range of 0–2.2 V. In addition, the device was further developed as a flexible solar‐charging integrated unit based on screen‐printed micro‐supercapacitors. However, the number of promising MIC systems is indeed limited from the point of view of laboratory prototypes or practical devices.
Possessing the highest Shannon's ionic radii of all multivalent charge carriers (Table 1.1), Ca2+ ions exhibit faster electrochemical reaction kinetics than Mg2+, Zn2+, and Al3+ ions, due to low polarization. However, as mentioned above, the promising classes of electrode materials for CICs are similar to MICs. Different experimental and simulation studies have shown that 3D tunneling and layered structures, such as CaMn2O4, V2O5, graphite, etc., are suitable for Ca2+ intercalation/de‐intercalation. Therefore, these materials can be considered as anode materials for CICs. Inspired by the Mg2+ ion system, a similar Chevral phase (CaMo6T8 [T = S, Se, Te]) is theoretically envisaged for the Ca2+ energy storage systems [92]. The operating voltage of CaMo6S8 is predicted to be 1.4 V (vs. Ca/Ca2+). However, the diffusion of Ca2+ is slower in CaMo6S8 than that of Mg2+. The diffusion energies of Mg2+ and Ca2+ in CaMo6Se8 are both lower than those of CaMo6S8. The diffusion energies of Ca2+ in Mo6S8 and Mo6Se8 are 780 and 520 meV, respectively, which are relatively higher than the diffusion energies of Mg2+ (270 and 180 meV for Mo6S8 and Mo6Se8, respectively). Based on the quantitative diffusion barrier limits for cell operation, the nanostructured Mo6Se8 could be promising as a suitable anode for CICs with an average voltage of 1.25 V. However, no experimental data on the insertion/deinsertion of Ca2+ ions based on the Chevral phase are available in the literature to date. Cubic framework structures of Prussian blue analogues (AxMFe(CN)6·yH2O [where A = Li, Na, K, Mg, Ca, etc., and M = Ba, Ti, Mn, Fe, Co, or Ni]) have also been investigated as insertion electrodes for CICs, but their capacities are not up to the mark [93]. While Ca metal anodes may be attractive for CICs due to their high bulk and weight capacities, surface passivation and subsequent hard SEIs formation in conventional electrolytes hinders reversible Ca2+ stripping/plating. Therefore, in conventional electrolytes (containing Ca ion salts in polar non‐protonic solvents), an efficient Ca ion energy storage system cannot be achieved using metal anodes [94]. On the other hand, alloyed materials such as Ca–Sn and Ca–Si anodes can exhibit reversible alloying/dealloying electrochemical reactions with appreciable capacity by avoiding the formation of hard passivation films [93, 95]. In 1991, Aurbach et al. demonstrated the electrochemical behavior of Ca‐metal electrodes in several organic electrolytes, such as Ca(ClO4)2, Ca(BF4)2, LiAsF6, and tetrabutylammonium salts (BF4− and ClO4−) in THF, PC, AN, and γ‐butyrolactone solvents [96]. During the electrochemical reduction of electrolyte solutions, CaCl2 (in ClO4− salt solutions), Ca(OH)2, CaCO3, and calcium alcohol salts form passivation films which limit the reversible deposition/dissolution of electrodes in Ca‐based nonaqueous electrolytes. In 2016, Ponrouch et al. reported that salts in organic electrolytes mixed with carbonate solvents (PC and EC) containing Ca(ClO4)2, Ca(BF4)2, and Ca(TFSI)2 exhibited a wide electrochemical stability window at high temperatures (e.g. −0.5 to 3.5 V vs. Ca/Ca2+ at 100 °C) [94]. Ultimately, these electrolytes also show irreversibility of Ca2+ stripping/plating. In 2017, Wang et al. developed a THF‐based electrolyte containing Ca(BF4)2 salt that can be operated at room temperature [97]. In this electrolyte, the reversible stripping/plating of Ca2+ was very satisfactory, but with lower anodic stability (3 V vs. Ca/Ca2+) and lower CE. Unlike the reduction products that form hard SEIs in conventional electrolytes, CaH2 was identified as the SEIs component in the THF‐based electrolytes. Unfortunately, the SEI films were unstable and formed continuously during cycling due to CaH2 deposition, resulting in a CE lower than that required for practical applications (99.98%). In 2019, Shyamsunder et al. synthesized a new fluorinated alkoxy borate (Ca(B(Ohfip)4)2·4DME), which is based on the hexafluoroisopropoxy (Ohfip−) ligand [98]. It shows reversible stripping/plating of Ca2+ in 1,2‐dimethoxyethane (DME) solvent at 25 °C with low polarization (170 mV). This salt also exhibits higher anodic stability up to 4.1 and 4.9 V in DME and N,N‐dimethyltriflamide, respectively. In 2019, Li et al. reported a similar electrolyte system, calcium tetrakis(hexafluoroisopropoxy)borate (Ca[B(hfip)4]2) in DME, showing reversible stripping/plating of Ca2+ at room temperature with high oxidative stability up to 4.5 V vs. Ca/Ca2+ and high ionic conductivity (>8 mS cm−1) [99]. Nevertheless, these electrolytes have some limitations and therefore require extensive research on the non‐aqueous and aqueous electrolytes. Ta et al. and Lee et al. report computational simulation studies of Ca electrodeposition and species formation processes in nonaqueous electrolytes and modulate the hydration number of Ca2+ ions by varying the electrolyte concentration, respectively [100, 101]. Different theoretical and experimental studies on electrodes/electrolytes are essential in order to gain a more comprehensive knowledge on the feasibility of practical CICs. In 2019, Wu et al. reported the first CIC devices based on AC cathodes, Sn foil anodes, and a 0.8 M Ca(PF6)2 electrolyte solution in a mixed carbonate solvent (EC, PC, DMC, and EMC) [102]. During the charging of CICs, PF6− anions were adsorbed to the AC surface and Ca2+ were migrated toward the Sn anodes, forming the Ca7Sn6 alloy. During the discharge of CICs, the opposite process occurred. Based on the above mechanism, the full device exhibited an operating voltage of 1.5–4.8 V. Moreover, the CV curves at different scan rates (10–100 mV s−1) indicated that the full‐cell device delivers good CICs performances. Reversible capacities of 92 mAh g−1 (0.1 A g−1) and 82 mAh g−1 (0.4 A g−1) and a capacity retention rate of 84% over 1000 cycles at 0.2 A g−1 were achieved. To our knowledge, this is the only CIC device reported to date that shows promising electrochemical performances at room temperature.
ZICs are developed from ZIBs. ZIBs have received a lot of attention for their low cost, high safety, and environment friendliness. Typically, ZIBs consist of zinc metal anodes, aqueous electrolytes such as ZnSO4 (aq), and cathodes for Zn2+ intercalation/de‐intercalation. Unlike the extremely active lithium, sodium, and potassium metal electrodes, zinc metal electrodes are stable in air and can be used directly as the anodes in ZIBs. At the same time, metallic Zn electrodes have a high weight capacity of 823 mAh g−1 (corresponding to an ultra‐high‐volume capacity of 5845 Ah l−1, much higher than 2046 Ah l−1 for lithium electrodes and 3833 Ah l−1 for Mg metal electrodes) and a low redox potential of −0.76 V compared to standard hydrogen electrodes [103]. In addition, the high ionic conductivity of the aqueous Zn2+‐containing electrolyte of the ZIBs facilitates fast charge/discharge rates. Therefore, ZICs inherit the excellent advantages of ZIBs. As mentioned above, the power density of the ZICs is highly competitive compared to other hybrid‐ion capacitors. The two electrodes of ZICs (AC and Zn) are also very stable (in this neutral/light system) and ensure excellent cycling stabilities [104]. Most importantly, ZICs have the potential to be a hybrid ion capacitor device with good energy density, power density, and excellent long‐term stabilities. ZICs typically utilize porous carbon (or AC) as the cathodes and zinc metal as the anodes. The highly reversible charge storage mechanism of the porous carbon cathode gives ZICs an extremely long service life. Zinc anodes store charge by plating and stripping, thus providing high capacity for ZICs. Given the low redox potential of the zinc anodes, aqueous ZICs with carbon‐based cathodes can provide higher voltages (≈1.6–1.8 V) than symmetrical C//C SCs (≈1 V) [105]. Thanks to the hybrid configuration, ZICs can bridge the energy density gap between SCs and rechargeable batteries. In addition, ZICs offer more stable cycling performances and higher power densities than their ZIB counterparts. Based on the low cost of carbon materials and zinc metal, ZICs hold promise for low‐cost and large‐scale applications, especially for those requiring high power.
Research work on ZICs has only recently begun. In 2016, Wang and coworkers reported the first ZICs assembled from oxidized carbon nanotube cathodes and zinc anodes in aqueous ZnSO4 electrolytes [106]. The ZICs showed a low specific capacitance of 53 F g−1, which was attributed to the low specific surface area (SSA) of the oCNT cathode (211 m2 g−1). In 2017, Tang and coworkers used porous carbon with a high SSA (3384 m2 g−1) as ZIC cathodes [107]. Such ZICs provide a capacitance of 170 F g−1 at a current density of 0.1 A g−1, corresponding to an energy density of 52.7 Wh kg−1 at 1725 W kg−1. In order to increase the energy density of ZICs, research work has attempted to expand the voltage window by optimizing the electrolyte. Typically, aqueous ZnSO4 electrolyte‐based ZICs have a voltage window of 0.2–1.8 V. In 2018, Wang et al. developed an ultra‐high concentration water‐in‐salt (WIS) electrolyte [108]. The WIS electrolyte consisted of 20 M (mol kg−1) lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 1 M Zn(TFSI)2. The WIS electrolyte provided a wide voltage window with voltages up to 2.1 V supported by a highly stable WIS electrolyte, and achieved a high average CE of 99.7%. In 2019, Lu ad coworkers assembled ZICs with high energy density by employing nitrogen‐doped graded porous carbon as cathodes [109]. The nitrogen dopant was shown to obtain pseudocapacitance by lowering the energy barrier for the formation of C—O—Zn bonding. In 2019, Zapien and coworkers fabricated the first flexible ZICs using polyacrylamide hydrogel electrolytes [110]. In 2020, Bimbo and coworkers introduced Ti3C2 MXene as an anode material for ZICs [111]. Ti3C2‐based ZICs exhibited a high capacity of 189 mAh g−1 with capacity retention of 96% after 1000 cycles. Zhi and coworkers ZICs were fabricated using phosphene as the cathodes [112]. Phosphene cathodes showed high capacitance of over 300 F g−1. It was worth noting that pseudocapacitances can be used to increase the energy density of ZICs. Qu and coworkers improved the capacitance of ZICs by manipulating the ZICs in an oxygen reduction reaction [113]. Due to the oxygen reduction reaction, the discharge capacities of ZICs were much higher than their charging capacity. Alshareef et al. utilized hydrogen and oxygen pseudocapacitance to increase the capacitances of ZICs. These pseudocapacitance studies provide a new approach to the design of aqueous ZICs with high energy densities [114].
However, the development of practical ZICs requires overcoming multiple challenges [115]. One limitation is the low energy density, which can be improved by increasing the capacitances of the cathode materials and by widening the voltage windows. The voltage windows of ZICs are mainly driven by the electrochemical stabilities of the electrolytes. In alkaline electrolytes, the zinc metal interacts with hydroxide ions, resulting in the accumulation of zinc hydroxide materials on the zinc anodes. Isolated zinc hydroxide materials reduce the overall conductivity and increase the polarization of the zinc anodes. Therefore, alkaline electrolytes should be avoided in ZICs. In neutral and weakly acidic electrolytes (pH above 3), the voltage windows of ZICs are limited by parasitic reactions such as HER, OER, and oxidation of porous carbon cathodes. Another limitation of ZICs is the cycling stabilities of the zinc anodes. Compared to the carbon cathodes, zinc anodes exhibit limited cycling stabilities due to dendrite growth, HER, and corrosion reactions. The formation of zinc dendrites involves a complex process, which is highly dependent on the inhomogeneous electric fields and ion distributions. During charging and discharging, the growth of zinc dendrites increases the SSA of the zinc anodes, thereby accelerating the HER rates. The high HER rate consumes protons in the aqueous electrolytes, leading to the accumulation of hydroxide ions. The hydroxide ions interact with the zinc anode and thus trigger the zinc corrosion reactions. Zinc corrosion results in the accumulation of zinc hydroxide substances on the surfaces of the zinc anodes. These byproducts reduce the overall electrical conductivity and exacerbate the inhomogeneous electric field of the zinc anodes, thereby increasing dendrite growth. Subsequently, the vicious cycles of zinc dendrites, HER, and zinc corrosion continue. Besides metallic zinc anodes, the modification of porous carbon electrodes is also a concern. The capacitances of porous carbon cathodes have been improved by porous structure engineering and pseudocapacitive engineering. Advanced capacitive cathode materials, such as MXenes, phosphorene, and TiN, have proved to be promising candidates for cathode materials for ZICs. In addition to electrodes, the design of electrolytes is also a core area. Aqueous ZnSO4 solutions are the most widely used electrolytes in ZICs. ZIC electrolytes containing ZnSO4 aqueous solutions have a limited voltage window of 0.2–1.8 V. Beyond this voltage range, the electrolyte is subjected to a number of different types of electrodes. Beyond this voltage range, parasitic reactions, such as HER, can occur and damage ZIC devices. To suppress parasitic reactions and expand the voltage window, the electrolyte composition, such as substance, concentration, and solvent, can be optimized. In addition, zinc anodes exhibit poor cycling stabilities compared to porous carbon cathodes, which have an extremely long cycle life. In the case of zinc anodes, the growth of zinc dendrites, ion distributors, and artificial SEIs is suppressed by expanding the electrochemical surface area with a 3D architectural design. The use of low‐potential intercalation anodes also helps to avoid zinc dendrite growth.
Despite the research results achieved with ZICs, the current results are based on laboratory‐scale devices and focus on specific components rather than complete devices. In the reported ZICs, the capacities of the anodes, cathodes, and electrolytes do not match. For studies of ZICs, zinc anodes and electrolytes are often used in excess. The reported energy densities of ZICs are unreliable, as these values are based on the active mass of the porous carbon cathodes. Further research should therefore focus on capacity matching between anodes and cathodes. In other words, the utilization of zinc should be as high as possible. Cycle life, energy density, and power density are only meaningful if there is a limited use of zinc cathodes. From a practical point of view, the resource cost is an important parameter to prove the feasibility of ZIC devices in practical applications. Estimating the actual energy density and the corresponding capital cost of the device is necessary for the further development of ZICs. As the energy density of ZICs can be further increased by various engineering strategies, ZICs are promising energy storage technologies in the future energy storage market. Cost is one of the key parameters determining whether ZICs can be used for practical applications. Although porous carbon and ZICs have a good industrial base, the cost of commercial AC (US$ 57–114 per kg) is much higher than that of ZICs (US$ 2.1 per kg). Based on the above estimated data, the cost of electrode material for ZIC would be US$ 365.6 per kWh (US$ 7.2 per kWh for zinc anodes and US$ 358.4 per kWh for porous carbon cathodes) [116]. The high cost of porous carbon is therefore a bottleneck for the practical application of ZICs. In order to reduce the cost of ZICs, a simple low‐cost porous carbon manufacturing process is required. It is believed that more green synthesis processes will be developed for low‐cost porous carbon materials. The last few years have seen tremendous progress in the electrochemical performances of ZICs. In future, a great deal of effort should be put into the manufacture of ZICs for practical applications. In the next few decades, there will be numerous potential opportunities to scale up ZICs with high performances.
As three‐electron redox systems (Al/Al3+), energy storage devices based on aluminum ions have a higher theoretical capacity and higher energy density. However, due to the higher polarization, the trivalent Al3+ suffers from a higher desolvation energy and a higher solid‐state diffusion energy barrier. Considering the ease of practical devices based on divalent ions (Mg2+, Ca2+, and Zn2+), trivalent ions (Al3+) are expected to be more difficult. However, the first reported use of aluminum metal as anodes dates back to 1857, and the concept of AIBs for aluminum ion batteries was introduced in 1972 [117]. Since then, researchers have been working to develop practical aluminum‐based devices by finding solutions for screening suitable cathode active materials with easy Al3+ ion diffusion (solid state), finding suitable cathode active materials for aqueous and nonaqueous media, optimizing the electrolytes for lower desolventization energy, and minimizing the formation of hard SEIs on aluminum metal electrodes. Therefore, the correct choice of electrolyte composition is key to the success of AICs. Aqueous salt electrolytes with AlCl3 or Al2(SO4)3 cannot be used for Al anodes because of the formation of Al2O3 through surface passivation and the intrinsic hydrogen precipitation reaction. Nonaqueous electrolytes can overcome these limitations. Historically, binary (NaCl–AlCl3) and ternary (KCl–NaCl–AlCl3) molten salts have been used as possible electrolytes; however, Al3+ is not present [118]. This chloroaluminate molten salt electrolyte system is divided into three parts, viz. acidic, neutral, and basic species, based on the molar concentration of AlCl3. Molar concentrations of AlCl3 of less than 50% in the electrolyte provide the basic characteristics by containing AlCl4− and Cl− primary anions, while more than 50% provides an acidic electrolyte with AlCl4− and Al2Cl7− anions. There is evidence that reversible Al electrochemical stripping/plating occurs only in acidic compositions [119]. However, the high melting point of such molten chloride aluminate electrolytes limits their practical application. On the other hand, fluorinated salts in organic solvents are not suitable for aluminum‐based systems, but this is negligible for lithium/sodium/potassium‐based systems. For example, aluminum salts containing fluoride anions in the electrolyte (similar to LiPF6 in LICs) form electronically and ionically nonconductive passivated AlF3 layers on aluminum metal electrodes, which hinders the reversibility of aluminum stripping/plating. At the same time, high desolvation energy losses at the electrode/electrolyte interface increase the polarization of the electrochemical reactions in typical organic systems (e.g. ethers). These problems are responsible for the slow kinetics of AICs using conventional fluorinated electrolytes in organic media. Recent research and developments have shown that ILs with high ionic conductivity, low volatility, and high chemical/electrochemical stability can be used as electrolyte solvents in AICs at room temperature [120]. Common IL‐based electrolytes are prepared by mixing AlCl3 with imidazolium chloride in a specific ratio. The most commonly used imidazolium chlorides are 1‐butyl‐3‐methylimidazolium chloride ([BMIM]Cl) and 1‐ethyl‐3‐methylimidazolium chloride ([EMIM]Cl). Herein, the Lewis acidity of the electrolyte is achieved by maintaining the molar ratio of AlCl3/imidazolium salt greater than one. The reaction mechanism for aluminum stripping/plating is similar to that of the molten chloroaluminate electrolyte described above in terms of the main anion species and reversible reactions. Therefore, in the cathodes in AICs devices, graphite‐based materials (3D graphite foam, graphene microflower, etc.), metal sulfides (Mo6S8, FeS2, SnS2, Ni3S2, CuS, Co9S8, etc.), metal oxides (TiO2, V2O5, etc.), Prussian blue analogues (CuHCF), MXene (V2CTx), chloroaluminate‐doped conductive polymers, etc. have been well reported [52]. Due to their high bulk density, high abundance, and environment friendliness, aluminum metal anodes are mostly accepted by conventional AICs. Notably, after removal of the surface oxide films, the thin Al2O3 films in commercial aluminum foils show better electrochemical properties compared to pure aluminum foils [52]. The defective sites in the oxide films facilitate efficient penetration of the electrolytes and minimize dendrite growth. In aqueous aluminum‐based systems, surface modification of aluminum foils by ionic liquid impregnation introduces artificial SEIs to minimize hard surface passivation on the aluminum anodes. Salt‐packed aqueous or solid/quasi‐solid electrolytes are also suitable for aqueous AICs, while the overpotential of the hydrogen precipitation reaction is significantly increased [52]. Interestingly, in addition to aluminum metal electrodes, zinc metal, MoO3@PPy nanotubes, SWCNT/W18O49 nanowires, etc. have also been tried as anode materials in aqueous AICs.
In 2016, Yoo et al. filed a US patent for AICs by preferentially using aluminum (including aluminum foil, aluminum powder, aluminum foam, shell particles with an aluminum coating, and aluminum alloys) anodes, and high surface area porous carbon (including AC, CNT, and graphene) cathodes/[EMIM]Cl with AlCl3 electrolytes [121]. According to the patent description, in addition to aluminum metal, graphite, aluminum‐doped graphite, carbon, silicon, titanium dioxide, molybdenum sulfide, and other intercalation/de‐intercalation materials can also be used as potential anodes. Similarly, conductive polymers, oxides, sulfides, and nitrides are also suitable for potential cathodes. The patent description is not limited to electrolytes in ionic liquids; it also includes organic/aqueous electrolytes and even solid electrolytes. Tian and his colleagues developed low‐cost and high‐performance aqueous AICs by using nanostructured V2O5‐impregnated mesoporous carbon microspheres (MCM/V2O5) in a 1 M Al2(SO4)3 electrolyte. The AICs operated through the intercalation/de‐intercalation of Al3+ and the reduction/oxidation of V5+/V4+ pairs. The CV of the AICs showed typical redox properties due to the pseudocapacitance contribution of the V5+/V4+ pair. The b‐value of the composite electrodes (close to 1) indicated the dominant capacitance/pseudocapacitance contribution in the energy storage. The higher quality loading of V2O5 reduced the b‐value, suggesting that V2O5 led to a diffusion‐controlled mechanism. As‐prepared AICs devices delivered an energy density of 13.2 Wh kg−1 (147 W kg−1), a power density of 5840 W kg−1 (7 Wh kg−1), and good cycling performances (capacitance retention of 90% after 10 000 cycles at 1 A g−1) in the range of 0–1.6 V. The AICs designed by Lei et al. [122] were composed of nitrogen‐doped graphene as the cathode materials, aluminum foil as the anode materials, glass fibers as the separators, and [EMIM]Cl/AlCl3 as the electrolytes. In this device, the capacitive behaviors were derived from the adsorption/desorption of AlCl4− ions in the nitrogen‐doped graphene. The rectangular CV curves of the hybrid devices at various scan rates indicated that the nitrogen‐doped graphene exhibited good properties in AICs. In addition, a weak reduction peak at 1.9 V indicated the presence of AlCl4− intercalation/de‐intercalation in/from the layer structures. The AICs device showed an initial discharge capacitance of 160 F g−1 (95% CE at 0.3 A g−1 within 0.3–2.3 V) increasing to 254 F g−1 (90% CE) after 1000 cycles. The nitrogen‐doped graphene cathode was activated in the first 30 cycles, after which the capacitance showed a slow increase up to 1000 cycles. Wang et al. reported AICs with a (+)AC//0.5 M Al2(SO4)3(aq)//PPy@MoO3(−) structures [123]. CV curves for the full devices of the AICs showed broad peaks associated with the intercalation/de‐intercalation of Al3+ into/out of MoO3@PPy. b values (0.5) also demonstrated the solid‐state diffusion kinetics of MoO3 in the Al2(SO4)3 electrolytes. The device exhibited an energy density of 28 Wh kg−1 (460 W kg−1), a power density of 2840 W kg−1 (20 Wh kg−1), and good cycling performances (93% capacitance retention after 1800 cycles at 2 A g−1) in the 0–1.5 V. Considerable electrochemical performances are achieved due to the PPy coating and the nanotube structure of MoO3. PPy provides a conductive network with reduced charge transfer resistance and also protects MoO3 from acid etching (acid can be produced by hydrolysis of Al2(SO4)3). The nanotube structure facilitates the penetration of electrolytes and buffers MoO3 volume changes due to Al3+ intercalation/de‐intercalation. Furthermore, the fabricated (+)AC//PPy@MoO3(−) devices were able to light up red LEDs, which demonstrated the practicality of the devices. Li et al. reported flexible AICs with (+)SWCNT/PANI//1 M AlCl3 (aq)//SWCNT/W18O49(−) [124]. The CV curves of SWCNT/W18O49 composites revealed broad redox peaks due to the fast pseudocapacitive reactions. This unique network structure contributed to fast ion transport and provided high electrode conductivity (1626 S cm−1). In addition, the W18O49 nanowires exhibited a wide lattice spacing, high aspect ratio, and homogeneous laminar structures, which facilitated the efficient intercalation of Al3+ ions. On the other hand, the SWCNT/PANI cathode provided a combination of capacitive/pseudocapacitive mechanisms. The hybrid device showed a bulk energy density of 19.0 mWh cm−3 (295 mW cm−3), a bulk power density of 1278 mW cm−3 (14.5 mWh cm−3), and an excellent cycling stability (95.9% capacitance retention at 14 mA cm−2 after 6000 cycles) at 0–1.8 V. Nevertheless, the number of successful AICs reported in the literature is indeed limited.
