Multivalent metal ion hybrid capacitors: a review with a focus on zinc-ion hybrid capacitors

Abstract

Multivalent metal ion hybrid capacitors have been developed as novel electrochemical energy storage systems in recent years. They combine the advantages of multivalent metal ion batteries (e.g., zinc-ion batteries, magnesium-ion batteries, and aluminum-ion batteries) with those of supercapacitors, and are characterized by good rate capability, high energy density, high power output and ultralong cycle life. Herein, after a brief introduction to supercapacitors and multivalent metal ion batteries, we reviewed the recent progress in research on multivalent metal ion hybrid capacitors, with a focus on zinc-ion hybrid capacitors, from the perspectives of design concept, configuration, electrochemical behavior and energy storage mechanism. An outlook of the future research regarding multivalent metal ion hybrid capacitors was also presented. This review will be beneficial for researchers around the world to have a better understanding of multivalent metal ion hybrid capacitors and develop novel electrochemical energy storage systems to meet the demands of rapidly developing electric vehicles and wearable/portable electronic products.

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Liubing Dong

Liubing Dong was born in 1989. He received his PhD degree in materials science and engineering from Tsinghua University in 2018. He is now a postdoctoral research associate at the Center for Clean Energy Technology, University of Technology Sydney. His current research mainly focuses on supercapacitors (especially flexible supercapacitors), multivalent-ion hybrid capacitors and rechargeable aqueous zinc-ion batteries.

Guoxiu Wang

Guoxiu Wang is the Director of the Centre for Clean Energy Technology and a Distinguished Professor at the University of Technology Sydney (UTS), Australia. Professor Wang is an expert in materials chemistry, electrochemistry, energy storage and conversion, and battery technologies. His research interests include lithium ion batteries, lithium-air batteries, sodium-ion batteries, lithium–sulfur batteries, supercapacitors, hydrogen storage materials, fuel cells, 2D materials such as graphene and MXenes, and electrocatalysis for hydrogen production.

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1. Introduction

Supercapacitors are characterized by fast charge–discharge rate, high power density, a broad application temperature range and ultralong service life. Thus they have become an indispensable kind of electrochemical energy storage device in consumer electronics, the transportation field and electricity systems.^1–3 Nevertheless, their low energy density severely limits their large-scale applications. By contrast, batteries (e.g., widely used lithium-ion batteries at present) exhibit relatively high energy density but low power output, poor rate performance and inferior cycling stability.^2,4 To integrate these advantages of supercapacitors and batteries into one device, the concept of hybrid capacitors was developed. In a hybrid capacitor, one electrode is a battery-type faradaic electrode as an energy source and the other is a capacitive electrode as a power source.² Similar to rechargeable batteries (e.g., lithium-ion batteries, sodium-ion batteries and potassium-ion batteries), hybrid capacitors are generally named based on their internal shuttling cations, such as lithium-ion hybrid capacitors, sodium-ion hybrid capacitors and potassium-ion hybrid capacitors. These univalent metal ion hybrid capacitors, especially the former two types, have been widely studied.^5–13

In recent years, portable electronics and electric vehicles have developed rapidly and set higher demands for energy density, power density, security and production cost of electrochemical energy storage devices.^14–16 In the above-mentioned batteries/hybrid capacitors, the alkali metals Li, Na and K are extremely reactive and the introduced organic electrolytes are flammable, which brings enormous security risks. Besides these issues, a shortage of lithium resources and their uneven distribution in the earth have made costs higher for lithium-ion batteries/hybrid capacitors. Against that background, multivalent ion storage mechanisms have been proposed and applied to design novel electrochemical energy storage systems. Compared with the energy storage systems based on univalent metal ions (Li⁺, Na⁺, K⁺, etc.), the systems based on multivalent metal ions (e.g., Zn²⁺, Mg²⁺, Ca²⁺ and Al³⁺) may possess some distinct advantages, such as fast charge transfer dynamics and high capacity and energy density.¹⁷ On the basis of multivalent ion storage mechanisms, rechargeable zinc-ion batteries,^18,19 magnesium-ion batteries,^20–22 calcium-ion batteries,^23–25 and aluminum-ion batteries^26–29 have been developed and are being widely studied. Some reviews also have separately presented these multivalent metal ion batteries.^30–34 However, multivalent metal ion hybrid capacitors, which are also designed based on multivalent ion storage mechanisms and exhibit many unique merits as novel energy storage devices, have not yet been systematically overviewed.

Herein, we reviewed the recent progress in research on multivalent metal ion hybrid capacitors. Considering that multivalent metal ion hybrid capacitors are constructed by integrating one capacitor-like electrode with one battery-like electrode (which refers to electrodes for multivalent metal ion batteries herein), the energy storage mechanisms and typical characteristics of separate supercapacitors and multivalent metal ion batteries were expounded first. The design concept, configuration, electrochemical behavior, energy storage mechanism and applicability of multivalent metal ion hybrid capacitors, especially aqueous zinc-ion hybrid capacitors, for micro-scale devices were then comprehensively elaborated. An outlook of the development directions for multivalent metal ion hybrid capacitors was also provided based on our overview of the published studies. We believe this overview can effectively collate the research systems for multivalent metal ion hybrid capacitors and thus provide guidance for designing, fabricating and exploiting novel high-performance electrochemical energy storage devices.

2. Introduction to supercapacitors and multivalent metal ion batteries

2.1 Supercapacitors

2.1.1 Electric double layer capacitance. Supercapacitors, also known as “electrochemical capacitors” or “ultracapacitors”, were developed on the basis of conventional capacitors.^35,36 Conventional capacitors such as parallel-plate capacitors are composed of two plate electrodes which are separated by insulating dielectrics (Fig. 1a). The capacitance of parallel-plate capacitors (C; unit: F) is derived through eqn (1),where ε (F m⁻¹) is the dielectric constant of the insulating dielectrics, S (m²) is the area of the plate electrodes and d (m) is the distance between the two electrodes. The dielectric constant of general insulating dielectrics is as small as 10⁻¹² to 10⁻¹¹ F m⁻¹ and the area of the plate electrodes is always limited; thus, the capacitance of parallel-plate capacitors is generally smaller than 1 μF.

Fig. 1 Schematic illustrations of (a) parallel-plate capacitors, (b) carbon electrode based EDLCs, (c) Stern model of the electric double layer formed on carbon–electrolyte interfaces, and (d) pseudocapacitive behavior of a MnO₂ electrode.

Porous carbon materials with high surface areas were used earlier to construct supercapacitors.^36–39 When carbon bulk is immersed in an electrolyte, the electrolyte will infiltrate open pores inside the carbon bulk, accompanied by the formation of a carbon–electrolyte interface. Each sufficiently small part of the interface can be regarded as a parallel-plate capacitor (Fig. 1b). It is noted from a microcosmic perspective that a capacitance of 3–40 μF cm⁻² can be obtained on the carbon–electrolyte interface.^40,41 Carbon morphology and electrolyte composition are decisive to the capacitance values. Considering the large specific surface area of porous carbon materials, the carbon–electrolyte interface contains a large multiplicity of “parallel-plate capacitors”, and thus is able to store a large amount of charges by electrostatic accumulation. Such a way of storing electricity is logically called electric double layer capacitors (EDLCs). Deeper research has shown that charge distribution on the carbon–electrolyte interface is not exactly the same as that in parallel-plate capacitors. Instead, on the electrolyte side, under the influence of electrostatic forces, ion diffusion and some other factors, charged ions are considered to form a Stern layer and diffuse layer,^42,43 as illustrated in Fig. 1c. Despite this, the concept of an electric double layer is still a good approximation and remains widely used today.

Various carbon materials have been studied and used as EDLC electrodes, such as activated carbon particles (ACs),^44–47 carbon nanotubes (CNTs),^48–52 graphene nanosheets,^53–57 carbon nanofibers (CNFs),^58–61 activated carbon fibers (ACFs)^62–67 and some other forms of carbon materials.^68–71 To achieve a high capacitance, the specific surface area of these EDLC electrode materials is required to be high (e.g., 1000–4000 m² g⁻¹). On the other hand, specific surface area is not the sole factor determining the capacitance of carbon materials. The pore structures and electrical properties of carbon materials also have a crucial effect on their capacitance values.^62,72–75 This is because the above parameters influence the transport behaviors of electrons and electrolyte ions in different ways, while charge storage on carbon–electrolyte interfaces in EDLCs needs the co-participation of electrons and ions. Pores which are too small do not allow the electrolyte to access, and relatively poor electrical conductivity impedes the electron transmission from current collectors to carbon–electrolyte interfaces. It is worth mentioning that besides the three factors of specific surface area, electrical resistance and pore structure, the organizational architecture of the carbon materials in the corresponding EDLC electrodes has to be taken into account. For instance, ACFs themselves are highly conductive and possess a high specific surface area and abundant pores on their surface, but ACF constructed felt electrodes may show inferior capacitive performance, associated with the high porosity and poor electrical conductivity of ACF felt electrodes.^62,76,77 Similarly, densely stacked graphene electrodes hinder the infiltration of electrolyte ions onto the surface of each graphene nanosheet and thus lead to a low capacitance, regardless of the exceptional physical properties of the graphene nanosheets themselves.^56,78,79 Therefore, constructing sufficient channels for fast transport of electrons and electrolyte ions inside electrodes is critical to achieve good electrochemical performance of carbon materials.

2.1.2 Pseudocapacitance. Pseudocapacitance is another kind of charge storage mechanism in supercapacitors. For EDLC electrodes, their double layer capacitance originates from the electrostatic accumulation of charges on carbon–electrolyte interfaces. Even if the specific surface area of one carbon electrode is up to 2000–4000 m² g⁻¹, its capacitance is generally lower than 400 F g⁻¹. The capacitance and energy density of supercapacitors can be enhanced by exploring pseudocapacitance. Reversible and fast surface redox of electrode materials (such as the electrochemical redox of MnO₂ in neutral aqueous electrolytes) is a typical form of pseudocapacitive behavior (Fig. 1d). In some other nano-sized electrode materials, pseudocapacitance can be derived in other ways. For instance, lithium-ion insertion into Nb₂O₅ in organic electrolytes corresponds to intercalation-type pseudocapacitance.^80–82 A detailed discussion can be found in the ref. 83.

Typical pseudocapacitive materials include transition metal oxides/hydroxides (e.g., MnO₂, RuO₂, Fe₂O₃, TiO₂, ZnO and V₂O₅),^63,84–92 conductive polymers (e.g., polyaniline (PANI), polypyrrole (PPy) and polythiophene),^{64,65,93–99} heteroatom-doped carbon materials (e.g., N-doped graphene),^100–106 and some other new nanomaterials (e.g., metal–organic-framework materials, MXene nanosheets, black-phosphorus nanoflakes and MoS₂).^107–122

MnO₂ materials possess a high theoretical capacitance of 1370 F g⁻¹ and a wide window of operating potential such as 0–0.9 V vs. Ag/AgCl in Na₂SO₄ aqueous electrolyte. Two mechanisms have been proposed to explain the pseudocapacitive behaviors of MnO₂, as expressed by eqn (2) and (3),^85,123,124

MnO₂ + C⁺ + e⁻ ⇔ MnOOC (2)

(MnO₂)_surface + C⁺ + e⁻ ⇔ (MnO⁻₂C⁺)_surface (3)

where C⁺ is a proton (H⁺) or a univalent alkali metal cation (Li⁺, Na⁺ and K⁺). The first mechanism expressed by eqn (2) refers to the intercalation/de-intercalation of protons or alkali metal cations in the bulk of MnO₂ materials, while the second mechanism expressed by eqn (3) corresponds to the surface adsorption/desorption of electrolyte cations on/from MnO₂. These two mechanisms both imply a redox reaction between the III and IV oxidation states of manganese. Furthermore, MnO₂ materials are environmentally benign and easily synthesized (at least four strategies can be utilized to synthesize MnO₂ electrode materials, including in situ redox deposition, electrodeposition, electrostatic interaction assembly and chemical co-precipitation).⁹² These merits make MnO₂ the most promising candidate for pseudocapacitive materials of commercialized supercapacitors. Despite this, the poor electrical conductivity of MnO₂ always results in a poor rate performance and unsatisfactory capacitance value in experiments. To enhance the electrochemical performance of MnO₂ electrode materials, the following strategies are typically applied: (i) constructing a nano-structure for the MnO₂ material,^125–127 (ii) compositing with highly conductive materials especially carbon materials (e.g., graphene, CNTs and carbon fiber fabrics)^128–133 and (iii) introducing oxygen vacancies into MnO₂.^134–137 The first strategy is beneficial for expanding the contact area between MnO₂ and the electrolyte and shortening the movement distance for electrons and electrolyte ions, while the latter two strategies are able to optimize the electrical properties of MnO₂ materials.

Conductive polymers are widely used as pseudocapacitive materials in supercapacitors due to their high pseudocapacitance, superior chemical stability, good electrical conductivity (vs. transition metal oxides) and simple synthesis process. Taking PANI as an example, PANI electrode materials can be synthesized through chemical polymerization or electrochemical polymerization of aniline monomers.^{64,65,138–140} Note that these polymerization processes are typically carried out in acid solutions, because in this case, the synthesized PANI materials are doped with protons, which is helpful to optimize the electrical and electrochemical properties of PANI. When used as a supercapacitor electrode, PANI exhibits pseudocapacitive characteristics, attributed to reversible ion doping/de-doping during charge/discharge processes.^141,142 Unfortunately, such an energy storage mode is accompanied by severe swelling and shrinkage of PANI chains. As a consequence, pure PANI electrode materials often exhibit poor cycling stability.^64,143–145 PANI/CNTs, PANI/graphene and some other PANI/carbon composite electrode materials have been studied to prevent aggregation of PANI nanoparticles, facilitate electron transport in electrodes and alleviate the severe volume change of PANI.^{64,77,146–149}

As mentioned above, good electrical properties, high specific surface areas and controllable pore structures endow carbon electrode materials with excellent rate performance and superior cycling performance, whereas the electrostatic charge–discharge mechanism limits carbon materials' capacitance. Heteroatom (e.g., N, O, B and P atoms or combinations of them) doping is an effective way to improve the charge storage ability of carbon materials.^{100–104,150–153} The introduced heteroatom functional groups on the carbon surface contribute a large pseudocapacitance in different ways. Oxygen functional groups are believed to conduct redox reactions between phenol or quinone and acidic or alkaline electrolytes.¹⁰¹ As summarized by Wang et al.,¹⁰¹ nitrogen functional groups can serve as electron donors to attract protons or improve the charge density of space charge layers,^154–156 strengthen oxidation/reduction of quinones¹⁵⁷ and introduce redox reactions of amine groups.¹⁵⁸ P and/or B doping not only directly introduces heteroatom functional groups on the carbon surface but also brings additional surface oxygen groups, and these functional groups endow the carbon materials with pseudocapacitance through complex faradaic reactions with a wide range of potentials.¹⁰⁰

In recent years, MXene nanosheets, black-phosphorus nanoflakes, MoS₂ and some other two-dimensional (2D) nanomaterials have been synthesized and studied as high-performance electrodes for pseudocapacitors. We are giving a brief introduction to MXene materials as an example herein. MXenes possess superior pseudocapacitive performance such as good rate capability and high volumetric capacitance, benefiting from their high metallic conductivity, unique 2D structure and highly reversible surface redox reactions.^{113,116,159,160} MXenes are produced by etching out the A layer from M_n+1AX_n phases (also denoted as MAX), in which M is an early transition metal or a solid solution of early transition metals, A is mainly a group IIIA or IVA element, X is C and/or N, and n = 1, 2 or 3. After etching the A layer, functional groups (OH, O, F, H, etc.) terminate the surface of each MXene layer. Also, the weak bonds between MXene layers allow for the intercalations of inorganic cations (such as H⁺, Li⁺, Na⁺ and K⁺), water molecules, organic compounds and even ionic liquids, which results in different interlayer distances of MXene sheets. All in all, various MAX phase precursors (e.g., Ti₂AlC, V₂AlC, (V_0.5Cr_0.5)₃AlC₂, Ti₃AlCN, Ta₄AlC₃, and Nb₄AlC₃) with different surface functional terminations and adjustable interlayer distances make it possible to synthesize hundreds or more kinds of MXenes with particular physical–chemical features. Considering the ultrahigh volumetric capacitance of MXenes compared with the best all-carbon EDLC materials, the research on MXenes paves a new way for developing high-performance supercapacitors. Nevertheless, MXene materials are relatively easily oxidized by oxygen in air or at higher anodic potentials in electrochemical systems. This means that synthesized MXene materials must be carefully preserved in an inert atmosphere, and symmetric MXene supercapacitors are limited by low operating voltages in aqueous electrolytes (such as ∼0.6 V for titanium carbide MXene electrode-based symmetric supercapacitors), while low operating voltages will result in a modest energy density. Therefore, enhancing the stability of the MXenes will greatly extend the application of MXenes in electrochemical energy storage. In addition, MXene-based composite electrodes, such as MXene/carbon,^{114,161–164} MXene/conductive polymers,^165–169 and MXene/transition metal oxides,^170–172 have been reported for high-performance supercapacitors.

2.2 Multivalent metal ion batteries

2.2.1 Aqueous rechargeable zinc-ion batteries (ZIBs). ZIBs are receiving extensive interest due to their low cost, high safety, and environmental friendliness.^18,19,30 Generally, ZIBs are composed of a zinc metal anode, an aqueous electrolyte such as ZnSO₄ (aq), and a cathode for Zn²⁺ intercalation/de-intercalation. Different from extremely active Li, Na and K metal electrodes, Zn electrodes are stable in air and water, and thus can be directly used as anodes of ZIBs. Meanwhile, as summarized in Table 1, metallic Zn electrodes possess a high gravimetric capacity of 823 mA h g⁻¹ (corresponding to an ultrahigh volumetric capacity of 5845 A h L⁻¹, much higher than the 2046 A h L⁻¹ for Li electrodes and the 3833 A h L⁻¹ for Mg metal electrodes) and a low redox potential of −0.76 V vs. the standard hydrogen electrode. In addition, the high ionic conductivity of the aqueous Zn²⁺-containing electrolyte of ZIBs is conducive to achieving a fast charge/discharge rate. Up to now, diverse kinds of cathode materials have been investigated for rechargeable aqueous ZIBs, such as manganese-based materials,^18,173–178 vanadium-based materials,^19,179–184 Prussian blue materials,^185–189 and other types of cathode materials for zinc ion storage.^190–193

Table 1 Detailed information of Zn, Mg, Ca, Al and Li

	Zn	Mg	Ca	Al	Li
Standard potential (V vs. standard hydrogen electrode)	−0.76	−2.37	−2.87	−1.67	−3.04
Gravimetric capacity (mA h g^−1)	823 (Zn/Zn^2+)	2205 (Mg/Mg^2+)	1337 (Ca/Ca^2+)	2980 (Al/Al^3+)	3860 (Li/Li^+)
Volumetric capacity (A h L^−1)	5845 (Zn/Zn^2+)	3833 (Mg/Mg^2+)	2073 (Ca/Ca^2+)	8046 (Al/Al^3+)	2046 (Li/Li^+)
Ionic diameter (Å)	0.75 (Zn^2+)	0.72 (Mg^2+)	1.00 (Ca^2+)	0.53 (Al^3+)	0.76 (Li^+)
Element abundance in the Earth’s crust (%)	0.0079	2.9	5.0	8.2	0.0017

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2.2.1.1 Cathode materials for ZIBs.
Manganese-based cathode materials for ZIBs. Tunnel structures endow MnO₂ with good Zn²⁺-storage ability. Xu et al. reported α-MnO₂-based ZIBs (Fig. 2a and b).¹⁸ They proved the reversible insertion/extraction of Zn²⁺ ions into/from α-MnO₂: during discharging, the zinc anode rapidly dissolved as Zn²⁺ ions, and Zn²⁺ ions in the electrolyte intercalated into the α-MnO₂ cathode; in the charging process, Zn²⁺ ions reversibly de-intercalated from the cathode, and deposited on the zinc anode. This process can be described by eqn (4).

Zn²⁺ + 2e⁻ + 2MnO₂ ⇔ ZnMn₂O₄ (4)

Fig. 2 (a) Schematics of MnO₂//Zn ZIBs and (b) corresponding CV curves of a Zn anode and MnO₂ cathode. Adapted from ref. 18 with permission from Wiley-VCH. (c) Structure schematic, (d) rate performance and (e) energy storage mechanism of Zn_0.25V₂O₅//Zn ZIBs. Adapted from ref. 19 with permission from Nature Publishing Group.

The α-MnO₂ cathode delivered a high capacity of 210 mA h g⁻¹ at a charge/discharge rate of 0.5C, and maintained 68 mA h g⁻¹ at a high rate of 126C. Subsequently, other MnO₂ polymorphs (β-MnO₂, γ-MnO₂, and ε-MnO₂) and other manganese oxides with a manganese valence state of less than +4 (e.g., Mn₂O₃ and Mn₃O₄) were studied for ZIBs.^174–178 Their poor electrical conductivity causes a poor rate capability, and structural transformation and manganese dissolution during charge/discharge processes often result in poor cycling stability. Cation-deficient spinel ZnMn₂O₄ was then prepared and used in ZIBs. Its capacity could retain 94% over 500 charge/discharge cycles at 500 mA g⁻¹.¹⁹⁴

Vanadium-based cathode materials for ZIBs. Vanadium-based cathode materials were recently explored as high-performance ZIB cathodes. For instance, Zn_0.25V₂O₅·nH₂O composed of [V–O] layers and pillars of Zn²⁺ and H₂O molecules was first applied in ZIBs by Nazar's group (Fig. 2c–e).¹⁹ The interlayer distance of Zn_xV₂O₅·nH₂O first increased after being immersed in an aqueous electrolyte as a result of water intercalation, and then in the discharge process, Zn²⁺ ions intercalated into while water molecules extracted from the Zn_xV₂O₅·nH₂O host, leading to a change of the interlayer distance. Zn²⁺ de-intercalation and water molecule re-insertion happened during recharging. Consequently, the Zn_0.25V₂O₅·nH₂O cathode showed good electrochemical performance with a specific energy of ∼250 W h kg⁻¹, a capacity of 220 mA h g⁻¹ at a 15C rate, and a capacity retention of about 80% after 1000 charge/discharge cycles. Other vanadium-based cathode materials such as Zn₂(OH)VO₄,¹⁹⁵ Na_1.1V₃O_7.9@rGO,¹⁹⁶ and H₂V₃O₈ (ref. 180) display good and stable Zn²⁺-ion storage behaviors. The NaV₃O₈-type layered structure (e.g., Na₅V₁₂O₃₂ and HNaV₆O₁₆·4H₂O) and β-Na_0.33V₂O₅-type tunneled structure (e.g., Na_0.76V₆O₁₅) have also been explored by Liang's group.¹⁹⁷ Due to the higher charge density of Zn²⁺, some Na⁺ in Na₅V₁₂O₃₂ are more likely to be displaced by Zn²⁺ to form a new phase of Zn₄V₂O₉, which may result in an unstable crystal structure because the Na⁺ in Na_0.76V₆O₁₅ can act as a strong “pillar” to stabilize the whole structure. Potassium vanadate oxides such as K₂V₈O₂₁, K_0.25V₂O₅, and K₂V₆O₁₆·1.57H₂O were also explored by the same group.¹⁹⁸ Further, other vanadium oxides including V₂O₅·nH₂O,^179,199 VO₂ nanofibers,²⁰⁰ and V₅O₁₂·6H₂O nanobelts²⁰¹ also possess good electrochemical properties as Zn²⁺ storage hosts. Although vanadium-based cathode materials have better cycling stability and higher capacity than manganese-based cathodes as a whole, it is a thorny issue that vanadium-based compounds can be toxic to humans and other living creatures.
Prussian blue analog cathode materials for ZIBs. Prussian blue analog cathodes generally exhibit good cycling performance, but their reversible capacities are low. ZnHCF cathodes studied by Liu et al. showed a discharge capacity of only 65.4 mA h g⁻¹ at 1C.¹⁸⁵ CuHCF cathodes delivered a maximum discharge capacity of 50–60 mA h g⁻¹.^186–188 And for FeHCF cathodes, their specific capacity was reported to be less than 50 mA h g⁻¹.¹⁸⁹
2.2.1.2 Zn anodes for ZIBs. Metallic Zn anodes are generally believed to be stable in neutral or slightly acidic aqueous electrolytes (e.g., ZnSO₄ aqueous electrolyte) and will not form dendrites during repeated stripping/electroplating processes.^{18,19,173,202} But in practical experiments, we sometimes find that Zn anodes do not work normally after tens or hundreds of charge/discharge cycles of ZIBs. The following reasons may give rise to this problem. (i) The Zn electrode actually acts not only as an anode material but also as a current collector. Stripping of Zn inevitably causes morphological changes of the current collector (especially when the mass of the Zn electrode is not excessive), and this subsequently affects the electroplating process of Zn²⁺, forming an uneven Zn electrode surface (Fig. 3a).²⁰³ This effect accumulates in repeated stripping/electroplating processes and finally causes the failure of the Zn anode and the ZIB. (ii) Oxygen dissolved in aqueous electrolytes may corrode the Zn anode under the influence of electric field and form insulating zinc oxides on the Zn anode surface. The strategies of surface coating and three-dimensional (3D) architectural design have been proposed to modify Zn anodes to obtain long-term cycling stability (Fig. 3),^203–206 which will also provide guidance to design ultralong-life and high-rate metal electrode-based hybrid capacitors.

Fig. 3 Schematic diagrams of Zn striping/electroplating on (a) a 3D Zn electrode (upper) and commercial Zn foil (lower). The 3D Zn electrode achieves a long-term cycling stability. Reproduced from ref. 203 with permission from American Chemical Society. (b) Schematic illustration of morphology evolution for a CaCO₃ nanoparticle coated Zn electrode during the Zn striping/electroplating process. Reproduced from ref. 205 with permission from Wiley-VCH.

2.2.2 Magnesium-ion batteries (MIBs). Rechargeable MIBs were proposed in the late 1980's,²⁰⁷ then key breakthroughs in electrolytes and cathode materials were achieved in 2000, and the first prototype system including a Mg anode, a Mo₆S₈ Chevrel phase cathode, and an electrolyte based on organohaloaluminate salt/tetrahydrofuran (THF) was presented.²⁰⁸ Mg is attractive because of its natural abundance on Earth, higher chemical stability than Li metal, and high volumetric capacity density of 3833 A h L⁻¹ (Table 1).^209–212 Despite these considerable advantages, there are still several problems impeding the practical application of MIBs. One critical issue that urgently needs to be solved is to develop a suitable electrolyte which can enable reversible dissolution/deposition of Mg²⁺ from the Mg anode.²¹³ The family of organohaloaluminate/ether electrolytes (e.g., Mg(AlCl₂BuEt)₂ in THF) has a stable electrochemical window to 2.5 V for efficient Mg dissolution/deposition, but these electrolytes are corrosive to cell components.²¹⁴ The second major challenge is to synthesize cathode materials with high reversible Mg²⁺-storage capacity and an appropriate operating potential window, considering that the enhanced electrostatic effects of divalent Mg²⁺-ions tend to cause slow solid-state diffusion kinetics, low capacity and reduced power output for cathode materials.^215,216 Herein, we only provide a brief overview of the research studies on MIBs.
2.2.2.1 Cathode materials for MIBs. Chevrel phases, namely ternary molybdenum chalcogenides (M_xMo₆T₈, where M is a metal element and T is a chalcogen element), are the typical cathode materials for MIBs. They have unique structural features and mutable cation species in the crystal structure.^217–230 Chevrel phases can be viewed as a stack of Mo₆X₈ blocks (Fig. 4a), and each Mo₆X₈ block consists of Mo₆ octahedra inside a cubic cage with eight chalcogen atoms at the corners. Mg atoms occupy twelve interstitial sites formed in the channels between these Mo₆X₈ blocks. Insertion/extraction of Mg²⁺ in/from the Mo₆S₈ cathode can deliver a reversible capacity of 124.4 mA h g⁻¹ (maximum theoretical value), corresponding to the reactions as given by eqn (5) and (6).²¹⁸

Fig. 4 (a) Crystal structure of Mo_xMo₆S₈. Reproduced from ref. 219 with permission from Elsevier. (b) Transmission electron microscopy image of graphene-like MoS₂ and (c) and (d) its electrochemical behaviors as a MIB cathode (G-MoS₂, B-MoS₂, N-Mg and B-Mg represent a graphene-like MoS₂ cathode, bulk MoS₂ cathode, ultrasmall Mg nanoparticle anode, and bulk Mg anode, respectively). Reproduced from ref. 221 with permission from Wiley-VCH.

For these Chevrel phase materials, reduced particle size, incorporation of Cu and substitution of S with Se can further enhance their electrochemical properties.

Molybdenum disulfide (MoS₂) has a 2D layered structure (each layer is composed of covalently bonded S–Mo–S trilayers and these layers are linked into a 3D structure through van der Waals forces). As a cathode material for MIBs, its theoretical capacity is 223.2 mA h g⁻¹, according to the electrochemical reactions as expressed by eqn (7).²²¹

6MoS₂ + 4Mg²⁺ + 8e⁻ ⇔ Mg₄Mo₆S₁₂ (7)

Various MoS₂ materials with different micro-morphologies have been investigated during last decades, such as graphene-like nanosheets (Fig. 4b–d), zigzag nanoribbons, nanotubes and so on.^221–224 In addition to Chevrel phases and MoS₂, some other transition metal sulfides have also been studied as cathode materials for MIBs, for instance, TiS₂,^207,225,226 VS₂,²⁰⁷ ZrS₂,²⁰⁷ CuS,²²⁷ NiS,²²⁷ NbS₃,²²⁸ and CoS.²²⁹ These cathode materials presented high ionic mobility ascribed to anion polarizability, but their electrochemical performance was modest.

Vanadium pentoxide (V₂O₅) was studied as a cathode material for MIBs due to its low cost, high theoretical capacity (589 mA h g⁻¹, for the reduction of V⁵⁺ to V³⁺), and high open-circuit voltage (∼2.66 V).²¹² However, intercalation of Mg²⁺ ions in V₂O₅ is a slow process and the electrical conductivity of V₂O₅ itself is low.²³⁰ To solve these problems, numerous research studies have been carried out to decrease the crystallite size of V₂O₅ and hybridize V₂O₅ with conductive materials.^231–235 Manganese oxides have also been investigated as attractive cathode materials for MIBs owing to their low toxicity and natural abundance. It has been demonstrated that for the Mn₂O₃ cathode, an open-circuit voltage of 2.40 V and a capacity of 224 mA h g⁻¹ could be obtained.²⁰⁷ Besides, MnO₂ cathodes displayed a high capacity (308 mA h g⁻¹ for the reduction of Mn⁴⁺ to Mn³⁺) and an open-circuit voltage of ∼2.9 V.²³⁶ Modification of MnO₂ has also been investigated to optimize the transport behavior of Mg²⁺ in MnO₂ cathodes.^237–240 MoO₃ is an orthorhombic layered material, which exhibited an open-circuit voltage of 2.28 V and an intercalation capacity of 143 mA h g⁻¹ when used as a MIB cathode.²⁰⁷ In addition, Mg²⁺ storage behaviors of Co₃O₄, RuO₂, WO₃, U₃O₈, TiO₂ and lead oxides have been investigated.^{207,241–244} Other classes of materials have been preliminarily studied as cathode materials for MIBs, including transition metal phosphates,^245,246 transition metal silicates,^247,248 organic materials,^249,250 Prussian blue analogs,^251,252 and metal chlorides.²⁵³ Further research is still needed.

2.2.2.2 Anode materials for MIBs. The passivation of Mg metal in most conventional electrolytes is an important issue needing to be considered in choosing anode materials for MIBs.²⁵⁴ Bi and Sn as anodes for MIBs have been investigated, because Bi can form intermetallic compounds with Mg and Sn has good compatibility with conventional battery electrolytes.^255–258 In addition, 2D materials including defective graphene and black phosphorus have been studied as anodes for MIBs due to their open morphology and unique physical properties.^259,260 Other materials such as nanoporous-carbon and Li₄Ti₅O₁₂ are also worthy of attention.^261,262
2.2.2.3 Electrolytes for MIBs. In recent decades, researchers have been developing suitable electrolytes which allow reversible insertion/extraction of Mg²⁺ into/from Mg anodes. Previously, electrolytes such as Mg(ClO₄)₂ or Mg(CF₃SO₃)₂ dissolved in THF, acetonitrile (AN) or propylene carbonate were employed in MIBs. However, the high charge density of Mg²⁺ resulted in large de-solvation energies at cathodes.²⁶³ Afterwards, Mg(AlCl₂BuEt)₂ in THF has proven to be a suitable electrolyte for MIBs with cathodes of Chevrel phase materials.²⁰⁸

2.2.3 Calcium-ion batteries (CIBs). Rechargeable CIBs are another attractive alternative divalent system. Calcium is the fifth most abundant element in the Earth's crust and its standard reduction potential is only 170 mV higher than that of Li, enabling a higher working voltage for CIBs (Table 1). Moreover, Ca²⁺ ions show faster reaction kinetics than Mg²⁺ or Al³⁺ due to their lower polarizing character.²⁴ However, pioneering research work by Aurbach et al. pointed out that the electrodeposition of Ca is virtually impossible because calcium ions are difficult to transport through the surface passivation layer formed in conventional organic electrolytes.²⁶⁴ Therefore, compared to other electrochemical energy storage systems, the development of CIBs is still in its initial stage. Since the reversible deposition/stripping of Ca was realized by Ponrouch et al., there are several reports about CIBs in recent years.^{23–25,265–268} These research studies are briefly introduced as follows.
2.2.3.1 Cathode materials for CIBs. According to computational and experimental results, a 3D tunnel structure and/or a layered structure is beneficial for Ca²⁺ insertion/extraction. For example, CaMn₂O₄ was predicted to be a potential cathode material, attributed to its outstanding stability.^267,268 V₂O₅ as a typical layered structure material can accommodate the intercalation of large Ca²⁺ and has an advantage from the perspective of energy density.^269,270 Recently, Chevrel phase materials have also been investigated theoretically for CIBs. Indeed, Mo₆X₈ nanostructured materials have potential for Ca²⁺ insertion even at an average voltage of 1.25 V (vs. Ca/Ca²⁺).^271,272 Unfortunately, no experimental results of Chevrel phases for CIBs have been reported to date. Prussian blue analogs have been investigated as insertion electrodes for CIBs.^23,265,273 These materials have a cubic framework with the general formula of A_xMFe(CN)₆·yH₂O (where A = Li, Na, Mg, Ca, etc. and M = Ba, Ti, Mn, Fe, Co or Ni). However, their capacities are unsatisfactory.
2.2.3.2 Anode materials for CIBs. Ca metal anodes are attractive due to their ultrahigh volumetric and gravimetric capacity. A solid electrolyte interphase (SEI) layer easily forms on the surface of Ca, but it is difficult for Ca²⁺ ions to diffuse through this SEI layer.²⁴ Alloy materials such as Ca–Si alloy and Ca–Sn alloy can provide attractive electrochemical capacities while avoiding the issue of surface passivation; thus, they are possible alternative anodes for MIBs (Fig. 5).^24,25,248

Fig. 5 (a) Working schematics of a CIB with a graphite cathode and a metal foil anode that can form an alloy with Ca; (b) rate performance and (c) cycling behavior of the CIB. Adapted from ref. 25 with permission from Nature Publishing Group.

2.2.3.3 Electrolytes for CIBs. Ponrouch et al.²⁴ reported some organic electrolyte combinations from dissolving Ca(ClO₄)₂, Ca(BF₄)₂, and Ca(TFSI)₂ salts in mixed carbonate solvents (propylene carbonate and ethylene carbonate), which offer a wide electrochemical window. However, these electrolytes require elevated temperatures and still exhibit poor reversibility of Ca²⁺ insertion/extraction. In another recent study, THF-based electrolytes with Ca(BF₄)₂ salts for CIBs were shown to operate at room temperature for Ca²⁺ insertion/extraction, although they showed a lower coulombic efficiency.²⁷⁴

2.2.4 Aluminium-ion batteries (AIBs). Owing to their distinct three-electron redox properties (Al/Al³⁺), rechargeable AIBs are considered as a promising alternative to the present Li-ion batteries.^26,275 Al has several inherent advantages (Table 1). For instance, Al is the most abundant metal element in the Earth's crust. It possesses a higher volumetric capacity of 8046 A h L⁻¹ due to its high density (2.7 g cm⁻³) and ability to exchange three electrons. Moreover, Al is chemically stable and can be handled in an ambient environment. The first battery system (“the Buff cell”) using Al metal as an anode dates back to the year 1857, and the concept of AIBs dates back to the early 1970s.²⁷⁶ However, researchers in the past 30 years have been faced with numerous challenges in realizing the practical application of AIBs, such as cathode material collapse, low discharge voltage and poor cycle life.^27,277,278 The following summarizes the development of AIBs.
2.2.4.1 Electrolytes for AIBs. Aqueous electrolytes based on AlCl₃ or Al₂(SO₄)₃ salts in water have been widely used for AIBs because of their low cost, simple operation and environmental friendliness.^279–281 However, in aqueous electrolytes, a passive oxide layer (Al₂O₃) will form and the reversible deposition of Al³⁺ is hindered by a competing intrinsic hydrogen evolution reaction, leading to a low efficiency of the Al anode.²⁸² To overcome these limitations, non-aqueous electrolytes have been applied in AIBs. Initially, molten salts such as the binary NaCl–AlCl₃ system or the ternary KCl–NaCl–AlCl₃ system were considered as possible electrolytes for AIBs.^283,284 In these molten salt systems, bare Al³⁺ does not exist, only in the forms of AlCl₄⁻ and Al₂Cl₇⁻. The stripping/deposition process of Al follows eqn (8).²⁸³

2AlCl⁻₄ ⇔ Al₂Cl⁻₇ + Cl⁻ (8)

However, these molten salt electrolytes have a high melting point, which limits the practical application of AIBs. Therefore, ionic liquids with high ionic conductivity, low volatility and high electrochemical/chemical stability were then investigated as room-temperature electrolytes for AIB systems.^285,286 The most widespread ionic liquid electrolytes for AIBs are AlCl₃ in imidazolium based ionic liquids, in which imidazolium cations are typically 1-butyl-3-methylimidazolium, 1-ethyl-3-methylimidazolium (EMIM), etc.^{26,27,278,285–287}

2.2.4.2 Cathode materials for AIBs. Graphite cathodes together with Al anodes can be used to construct a low-cost AIB system (Fig. 6). The redox reactions during charge–discharge processes inside Al/graphite AIBs (the operating voltage window is around 0.5–2.5 V) can be expressed by eqn (9) and (10).²⁶

Al + 7AlCl⁻₄ ⇔ 4Al₂Cl⁻₇ + 3e⁻ (9)

C_n[AlCl₄] + e⁻ ⇔ C_n + AlCl⁻₄ (10)

Fig. 6 (a) Schematic of the graphite//Al AIB with an AlCl₃/[EMIm]Cl ionic liquid electrolyte; (b) charge–discharge curves at a current density of 66 mA g⁻¹ and (c) cycling behavior of the pyrolytic graphite//Al cell. Reproduced from ref. 26 with permission from Nature Publishing Group.

Similarly, 3D graphitic foam and fluorinated natural graphite have been studied as cathode materials for AIBs; while the former showed modest capacity and the latter displayed low coulombic efficiency (about 75%).^26,278,288 Furthermore, anatase TiO₂, V₂O₅, CuHCF, Mo₆S₈, chloroaluminate-doped conducting polymers and FeS₂ also allow reversible Al³⁺-storage in AIBs.^{280,287,289–292}

Multivalent metal ion batteries including ZIBs, MIBs, CIBs and AIBs are overviewed above. Considering that multivalent metal ion batteries are often compared to monovalent metal ion batteries such as lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs), we thereby give a very brief introduction to monovalent metal ion batteries herein. The three types of monovalent metal ion batteries work based on a “rocking-chair”-type mechanism: during charge, Li⁺/Na⁺/K⁺ ions move from the cathode to the anode through the electrolyte, and then move back when the batteries are discharged.^293–295 LIBs have been commercialized since 1991 and are widely applied in modern society. Up to now, LiCoO₂, LiMn₂O₄, LiNi_0.8Co_0.15O₂, LiNi_xMn_yCo_1−x−yO₂ and LiFePO₄ have been developed as cathode materials of LIBs, and the anode materials include carbon-based materials (e.g., graphite, which is the current dominant anode material in commercial LIBs), Li₄Ti₅O₁₂ and Si/C composites.^296–298 The electrolytes for LIBs are prepared by dissolving lithium salt (e.g., LiPF₆, LiClO₄ and LiAsF₆) in organic solvents (e.g., dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, ethyl methyl carbonate and a mixture of them).²⁹⁷ The growing demand of LIBs around the world and increasing shortage of lithium resources push up the price of LIBs. This promotes the research studies on SIBs and KIBs. Relatively, SIBs and KIBs possess the following advantages:^299,300 (i) Na element and K element have abundant distribution on the Earth; (ii) standard electrode potentials of Na/Na⁺ and K/K⁺ are very low (−2.71 V for Na/Na⁺ and −2.93 V for K/K⁺, respectively, only 0.11–0.33 V higher than that of Li/Li⁺), which is beneficial for achieving a high energy density; (iii) Na⁺ and K⁺ have higher mobility than Li⁺ (in propylene carbonate solution), making it possible for SIBs and KIBs to realize a good high-rate performance. For SIBs,²⁹³ the cathode materials can be Na layered oxides (e.g., Na_xMO₂, where M = Co, V and Mo), 3D polyanion oxides and so on, and the anode materials contain carbonaceous materials, titanium oxides, some alloys and phosphorus. For KIBs, as summarized by Xu et al.²⁹⁴ the available cathode materials include layered metal oxides, metal hexacyanometalates, polyanionic compounds and organic compounds, and the anode materials are divided into carbon-based materials (e.g., graphite, soft carbon and hard carbon) and alloy-based materials (e.g., Sn-based anode, Bi-based anode and so on).

3. Multivalent metal ion hybrid capacitors

3.1 Zinc-ion hybrid capacitors (ZHCs)

3.1.1 Metallic Zn electrode based ZHCs. As mentioned above, metallic Zn electrodes are characterized by a high gravimetric capacity of 823 mA h g⁻¹, an ultrahigh volumetric capacity of 5845 A h L⁻¹, and a low redox potential of −0.76 V vs. the standard hydrogen electrode (Table 1). They are also environmentally benign and low-cost. More importantly, metallic zinc is much safer than extremely reactive lithium, sodium and potassium metals, and can be directly used as a metal electrode in aqueous electrolytes. In basic electrolytes (e.g., KOH aqueous solution), stripping/plating of Zn/Zn²⁺ often results in the formation of zinc dendrites and insulating zinc oxide/hydroxide and thus poor cycling stability.^301–303 However, in neutral or slightly acidic electrolytes (e.g., ZnSO₄ aqueous solution), zinc electrodes show long-term cycling stability.^{18,19,173,202} This is an important basis for aqueous rechargeable zinc-ion batteries (as stated above in Section 2.2.1).
3.1.1.1 ZHCs with an aqueous electrolyte. Based on the above advantages of Zn metal electrodes, we constructed extremely safe, high-rate and ultralong-life ZHCs in our previous work.²⁰² In the ZHCs (Fig. 7a and b), high surface area AC was used as the cathode, metallic Zn foil was used as the anode and ZnSO₄ aqueous solution was used as the electrolyte. The AC particles had a Brunauer–Emmett–Teller (BET) surface area of 1923 m² g⁻¹ and most of the pores on the AC surface had a size of 1.2–2.7 nm. The constructed AC//Zn ZHCs worked in a voltage range of 0.2–1.8 V. Different from the rectangular-shaped CV curves of EDLCs, redox peaks were observed in the CV curves of the ZHCs (Fig. 7c). Preservation of these redox peaks even at a large scan rate of 100–1000 mV s⁻¹ demonstrated rapid kinetics of electrochemical reactions inside ZHCs. The ZHCs exhibited a high capacity of 121 mA h g⁻¹ (calculated based on the mass of AC) at a charge/discharge current of 0.1 A g⁻¹, as well as a good rate performance (could be charged/discharged within 15 s at a large current of 20 A g⁻¹, corresponding to a discharge capacity of 41 mA h g⁻¹) (Fig. 7d). High capacity and good rate performance endowed the ZHCs with a high energy density of 84 W h kg⁻¹ and a high power output of 14.9 kW kg⁻¹. In addition, the ZHCs presented an excellent cycling performance, with 91% capacity retention after 10 000 charge/discharge cycles at 1 A g⁻¹ (Fig. 7e).

Fig. 7 (a) Schematic of a ZHC with the AC cathode, Zn foil anode and ZnSO₄ aqueous electrolyte; (b) scanning electron microscopy (SEM) image of the AC cathode; (c) CV curves, (d) charge–discharge profiles and (e) long-term cycling stability test of the AC//Zn ZHC. Reproduced from ref. 202 with permission from Elsevier.

The energy storage mechanism for the ZHCs is summarized as follows. For Zn anodes, the primary electrode reaction is stripping/plating of Zn/Zn²⁺, as expressed by eqn (11).

Zn ⇔ Zn²⁺ + 2e⁻ (11)

For the AC cathode, due to its high specific surface area and appropriate pore structure, ion adsorption/desorption inevitably becomes one way for it to store energy, as confirmed by double layer capacitive behaviors of symmetric AC supercapacitors with the ZnSO₄ electrolyte. Specifically, when the operating voltage is higher than the potential of zero charge (pzc), differential capacitance is minimum at the pzc. In Fig. 7c, we can see that the pzc of the AC material is around 1.3 V vs. Zn/Zn²⁺, ZSO₄²⁻ anion adsorption/desorption on/from the AC surface is dominant, and when the operating voltage is lower than the pzc, it is mainly Zn²⁺ cation adsorption/desorption. Besides the aforementioned electrode reactions, a precipitation/dissolution reaction of Zn₄SO₄(OH)₆·5H₂O (also expressed as ZnSO₄[Zn(OH)₂]₃·5H₂O) on both electrodes also contributes capacity to ZHCs. The precipitation/dissolution of Zn₄SO₄(OH)₆·5H₂O, as described by eqn (12),

4Zn²⁺ + 6OH⁻ + SO²⁻₄ + 5H₂O ⇔ Zn₄SO₄(OH)₆·5H₂O↓ (12)

is caused by pH changes of the electrolyte (Zn₄SO₄(OH)₆·5H₂O precipitate forms when the pH value is larger than ∼5.3 and will then re-dissolve when pH decreases.³⁰⁴ Note that the initial pH value of 2 M ZnSO₄ electrolyte is ∼4.5.) The reasons for pH change were not clearly revealed in our previous research. Herein, based on our new thinking, the pH change is more likely a result of reversible hydrogen storage in nanoporous AC at a relatively low potential.³⁰⁵ Furthermore, considering the existence of O heteroatoms on the AC surface (its oxygen content is about 10 wt% according to our latest research), the corresponding oxygen functional groups would contribute pseudocapacitance to the AC cathode based on the following reactions.

{C}⋯O + H⁺ + e⁻ ⇔ {C}⋯OH (13)

{C}⋯O + Zn²⁺ + 2e⁻ ⇔ {C}⋯O⋯Zn (14)

We also utilized other AC materials to produce AC//Zn ZHCs and found that higher specific surface areas of the AC cathode were helpful to enhance the capacity of ZHC devices.²⁰² Overall, metallic Zn electrode-based aqueous ZHCs have shown great potential in achieving a much higher energy density than aqueous symmetric supercapacitors and general asymmetric supercapacitors (e.g., MnO₂//Na₂SO₄ (aq)//AC supercapacitors), benefiting from the following factors: (i) improved capacity due to the utilization of a high capacity Zn electrode and (ii) higher operating voltage (vs. aqueous symmetric supercapacitors and some asymmetric supercapacitors). Besides these factors, excellent cycling stability and fast kinetics of Zn²⁺ stripping/plating on Zn anode surfaces and ion adsorption/desorption on/from cathode surfaces endow metallic Zn electrode-based ZHCs with ultralong lifetime and superior rate performance.

Tian et al. applied CNTs as cathode materials to assemble CNT//ZnSO₄ (aq)//Zn ZHCs and investigated the effect of CNT oxidation on ZHCs' electrochemical behaviors.³⁰⁶ Unoxidized CNTs showed simple electric double layer capacitive behavior. However, after oxidation treatment, an additional reversible faradaic process with participation of divalent Zn²⁺ and oxygen functional groups could be introduced to CNT cathodes according to eqn (6) and (7), thus leading to a notably enhanced capacity of CNT//Zn ZHCs. However, the low specific surface area (120–211 m² g⁻¹) of these CNT cathodes led to a relatively low capacity (about 27 mA h g⁻¹ deduced according to related descriptions in the literature).

Besides carbon materials, conductive polymers of PANI and PPy have been employed as cathodes to prepare ZHCs by Han and Li et al.^307,308 Note that the assembled conductive polymer//Zn devices were called zinc-ion batteries in some literature, but they are categorized as ZHCs herein (a detailed discussion is given in the following). Han et al. designed a graphene@PANI composite hydrogel with a 3D nano-architecture and they directly used this as a free-standing cathode for graphene@PANI//ZnSO₄ (aq)//Zn ZHCs (Fig. 8a–e).³⁰⁷ Zn²⁺ ions were stored in the cathode through two pathways: electric double layer capacitance of graphene and pseudocapacitance of PANI (in fact, due to the nanoscale size and relatively high specific surface area of the deposited PANI particles, PANI also could contribute an electric double layer capacitance besides pseudocapacitance). The ZHCs operated in a voltage range of 0.4–1.6 V (Fig. 8c and d). Compared with the aforementioned AC//ZnSO₄ (aq)//Zn ZHCs, the graphene@PANI//ZnSO₄ (aq)//Zn ZHCs exhibited more obvious redox peaks in CV curves, associated with the redox transition of the leucoemeraldine/emeraldine and emeraldine/pernigraniline states of PANI (i.e., reversible ion doping/de-doping of PANI, which is considered as a pseudocapacitive behavior in traditional supercapacitors as discussed in Section 2.1.2. This is why we classify PANI//Zn as a ZHC.) The capacity of this cathode was 154 mA h g⁻¹ (ca. 437 F g⁻¹, calculated based on the dry weight of the graphene@PANI electrode) at a charge/discharge current density of 0.1 A g⁻¹. The 3D porous structure and highly conductive graphene framework endowed the cathode with a superior rate performance: a capacity of 106 mA h g⁻¹ was achieved at 5 A g⁻¹. The ZHCs showed a maximum energy density and power density of 205 W h kg⁻¹ and 2455 W kg⁻¹, respectively. As mentioned above, conductive polymers often suffer from poor cycling stability as supercapacitor electrodes, because of their severe swelling-shrinkage during charge–discharge processes. A similar problem occurred in the graphene@PANI//Zn ZHC systems: 80.5% capacity retention was achieved after 6000 charge/discharge cycles (Fig. 8e), worse than that of AC//Zn ZHCs (Fig. 7e). Similarly, Li et al. recently reported an aqueous PPy//Zn ZHC (Fig. 8f–h).³⁰⁸ For this ZHC, the cathode was prepared through chemical polymerization of pyrrole on graphite nanosheets/polyvinyl alcohol aerogel, the anode was fabricated by electrodepositing Zn on a graphite paper and the electrolyte was a mixed aqueous solution of ZnCl₂ and NH₄Cl. CV curves at 5–100 mV s⁻¹ for the ZHC presented quasi-rectangular shapes, quite different from the typical CV curves of zinc-ion batteries (clearly separated and intense oxidation peaks and reduction peaks always exist in the CV curves of zinc-ion batteries). In a voltage range of 0.6–1.6 V, the ZHC delivered a discharge capacity of ∼151.1 mA h g⁻¹ at 0.5 A g⁻¹, but the corresponding coulombic efficiency was only about 88%. In addition, this ZHC showed a good rate performance with a capacity value of 87.6 mA h g⁻¹ at 16 A g⁻¹, whereas its capacitance retention was only 76.7% after 1000 charge/discharge cycles, indicating unsatisfactory cycling stability.

Fig. 8 (a) Schematic of a graphene@PANI//Zn ZHC with the ZnSO₄ aqueous electrolyte; (b) SEM image of the graphene@PANI composite hydrogel cathode (inset: structure illustration); (c) CV curves, (d) charge–discharge profiles and (e) cycling performance of the graphene@PANI//Zn ZHC. Adapted from ref. 307 with permission from The Royal Society of Chemistry. (f) SEM image of pyrrole/graphite nanosheets/polyvinyl alcohol aerogel and (g) and (h) its electrochemical behaviors as a cathode of a Zn metal based ZHC. Adapted from ref. 308 with permission from Wiley-VCH.

3.1.1.2 ZHCs with an organic electrolyte. Organic carbon//Zn ZHCs were also studied. Wang et al. utilized coconut shells as raw materials to prepare bio-carbon with the assist of the carbonization-activation method (Fig. 9a).³⁰⁹ The obtained bio-carbon materials had a BET surface area of 2596–3554 m² g⁻¹. They constructed ZHCs based on the bio-carbon cathode, metallic Zn foil anode and organic Zn(CF₃SO₃)₂ electrolyte (the organic solvent was AN or a mixture of dimethoxyethane and 1,3-dioxolane). The organic ZHCs worked in a voltage range of 0–1.8 V, delivering a discharge capacitance of 170 F g⁻¹ (ca. 85 mA h g⁻¹) at a charge/discharge current of 0.1 A g⁻¹ and an energy density of 52.7 W h kg⁻¹ at a power density of 1725 W kg⁻¹ (Fig. 9b and c). The cycling stability of such organic ZHCs (91% capacity retention after 20 000 charge/discharge cycles at 2 A g⁻¹) was comparable to that of our aforementioned aqueous ZHCs.²⁰² Yoo et al. assembled organic ZHCs with an ACF cloth cathode, metallic Zn foil anode and zinc bis(trifluoromethylsulfonyl)imide (i.e., Zn[(CF₃SO₂)₂N]₂)/AN electrolyte.³¹⁰ The ZHCs exhibited quasi box-shaped CV curves and linear charge (and discharge) curves without voltage plateaus in a voltage range of 0–1.8 V. Their capacity reached ∼55 mA h g⁻¹ at a current of 5 mA cm⁻². Above 98% of initial capacity was retained after 10 000 charge/discharge cycles, implying a very good cycling performance.

Fig. 9 (a) Transmission electron microscopy image of a bio-carbon material and (b) and (c) its electrochemical behaviors as the cathode of a Zn metal based ZHC with a Zn(CF₃SO₃)₂/AN electrolyte. Adapted from ref. 309 with permission from Elsevier. (d) CV curves (scan rate: 10–50 mV s⁻¹), (e) charge–discharge profiles and (g) energy storage mechanism illustration of a ZHC with a sponge-like Zn anode, a natural graphite cathode and a biodegradable ionic liquid electrolyte. Adapted from ref. 311 with permission from Springer. Copyright, 2018.

3.1.1.3 ZHCs with an ionic liquid electrolyte. Ionic liquids possess a series of unique characteristics including high ionic conductivity, negligible vapor pressure, excellent thermal and chemical stability, non-flammability, high safety and a wide electrochemical window; therefore, ionic liquid electrolytes have been widely studied in the fields of rechargeable batteries and supercapacitors. Liu et al. reported a ZHC composed of a sponge-like Zn anode, a natural graphite cathode and a biodegradable ionic liquid electrolyte (Fig. 9d–f).³¹¹ The electrolyte was prepared by dissolving Zn(OAc)₂ in the mixture of choline acetate (70 wt%) and water (30 wt%). The constructed ZHC showed deviated rectangular shaped CV curves in a voltage range of 0.5–1.8 V, including a redox process at around 1.2 V (Fig. 9d). According to galvanostatic charge–discharge tests performed in a voltage range of 0–1.8 V, a discharge capacity of ∼73 mA h g⁻¹ at 0.2 A g⁻¹ and an energy density of 52.6 W h kg⁻¹ at a power density of 145 W kg⁻¹ were realized (Fig. 9e). Unfortunately, such a ZHC exhibited rapidly dropped capacity at large currents (e.g., 2 A g⁻¹). The authors considered this to be caused by side reactions occurring at higher charge/discharge rates between the Zn electrode and the ionic liquid electrolyte.^311,312 The energy storage mechanism of the ZHC was proposed as follows: during the charge process, Zn²⁺ ions were reduced on the Zn anode, while anions accumulated on and intercalated into the pseudocapacitive graphite cathode; reversed processes occurred during discharging (Fig. 9f).

Various ZHCs with aqueous electrolytes, organic electrolytes and ionic liquid electrolytes are summarized above. In comparison, aqueous ZHCs exhibit good rate performance benefiting from high ionic conductivity of aqueous electrolytes. Also, aqueous ZHCs are safe, low-cost and environmentally benign. In addition, it is worth noting that in conventional supercapacitors such as symmetric AC//AC supercapacitors and asymmetric MnO₂//AC supercapacitors, application of organic electrolytes and ionic liquid electrolytes is beneficial for achieving a wider operating voltage window than with aqueous electrolytes, thus delivering a higher energy density.^313–316 For example, Lewandowski et al. studied a symmetric carbon//carbon supercapacitor in different electrolytes and found that when aqueous, organic or ionic liquid electrolyte was applied, the supercapacitor could be charged up to 1, 2.5 and 3–3.5 V, respectively.³¹⁴ In contrast, the reported Zn metal electrode-based ZHCs possess similar operating voltages in aqueous, organic and ionic liquid electrolytes. One important reason is that the dissolution/deposition potential of Zn/Zn²⁺ on Zn anodes is not strictly restricted by the hydrogen evolution of water. Despite this, by controlling the surface characteristics of cathode materials (e.g., changing the surface functional groups of carbon cathodes), a wider operating voltage window of ZHCs is still possible to be realized in organic and ionic liquid electrolytes. Furthermore, heteroatom-doped carbon materials and conductive metal–organic-framework materials exhibit good capacitive performance in conventional supercapacitors (e.g., with a H₂SO₄ electrolyte) and are worth studying as ZHC electrodes, while the operating voltage window and specific electrochemical behavior of these material-constructed ZHCs need to be investigated in future research studies.

3.1.2 Metal oxides//carbon ZHCs. In aqueous rechargeable zinc-ion batteries, Zn²⁺ ions can reversibly intercalate/de-intercalate into/from the bulk of manganese oxides and vanadium oxides, such as MnO₂, Mn₂O₃, Mn₃O₄, V₂O₅, V₂O₅·nH₂O, H₂V₃O₈ and transition metal ion-stabilized vanadium oxides.^{18,19,177–182,193} The interaction between Zn²⁺ ions and carbon materials is characterized as a typical double layer capacitive behavior or pseudocapacitive behavior (in the presence of heteroatom functional groups on the carbon surface).^202,306 This affords us an opportunity to design a series of metal oxide//carbon ZHCs.

In our previous research, we first studied MnO₂//AC ZHCs in which MnO₂ nanorods were used as the cathode material, high specific surface area AC particles were used as the anode and several types of aqueous solutions (including ZnSO₄ solution, ZnSO₄/MnSO₄ mixture solution and Zn(CF₃SO₃)₂ solution) were used as the electrolyte (Fig. 10).³¹⁷ These ZHCs operated in a voltage range of 0–2.0 V. The corresponding CV curves were composed of a quasi-rectangular characteristic with redox peaks, and in galvanostatic charge–discharge profiles, the charge and discharge curves were not linear, which were indicative of an integration of battery behavior and capacitive behavior. MnO₂ and AC materials possess different Zn²⁺-storage abilities; thus, the electrochemical properties of MnO₂//ZnSO₄ (aq)//AC ZHCs with different mass ratios of the MnO₂ cathode to the AC anode should be investigated. When the mass ratio was 1 : 2.5, the assembled ZHC exhibited a maximum discharge capacity of 54.1 mA h g⁻¹ and an energy density of 34.8 W h kg⁻¹ (calculated based on the total weight of MnO₂ and AC), much higher than the figures for asymmetric MnO₂//AC supercapacitors with a Na₂SO₄ aqueous electrolyte. We would like to emphasize that even though MnO₂//ZnSO₄ (aq)//AC and MnO₂//Na₂SO₄ (aq)//AC systems have the same cathode material (i.e., MnO₂), anode material (i.e., AC) and operating voltage window (i.e., 0–2.0 V), they are essentially different in energy storage mechanisms and electrochemical performance. MnO₂//Na₂SO₄ (aq)//AC forms an asymmetric supercapacitor. Its energy storage is realized by electrostatic accumulation of charges on the AC surface and pseudocapacitive behavior of MnO₂ (fast and reversible redox reactions occur on the MnO₂ surface, as illustrated in Fig. 1d). In contrast, MnO₂//ZnSO₄ (aq)//AC forms a hybrid capacitor. Its energy storage is mainly through electrostatic accumulation of charges on the AC surface and Zn²⁺-ion intercalation/de-intercalation into/from MnO₂ channels. In addition, formation/dissolution of Zn₄(OH)₆SO₄·nH₂O on both the MnO₂ cathode and the AC anode is another way to store electrochemical energy for MnO₂//ZnSO₄ (aq)//AC ZHCs, as pointed out in our previous research.³¹⁷ Nevertheless, the formation/dissolution process of Zn₄(OH)₆SO₄·nH₂O is only partially reversible, which adversely affects the capacity and cycling performance of MnO₂//ZnSO₄ (aq)//AC ZHCs. Meanwhile, manganese dissolution from the MnO₂ cathode also goes against ZHCs' cycling stability. As a result, up to 35% capacity decay was detected after 3000 charge/discharge cycles of MnO₂//ZnSO₄ (aq)//AC ZHCs.³¹⁷ To minimize the above issues and optimize ZHCs' electrochemical performance, electrolyte composition was regulated, including (i) introducing Mn²⁺ in the ZnSO₄ (aq) electrolyte to suppress manganese dissolution and (ii) replacing SO₄²⁻ anions with CF₃SO₃⁻ anions to inhibit generation of Zn₄(OH)₆SO₄·nH₂O. Overall, the former strategy improved the discharge capacity of ZHCs and the latter strategy significantly extended ZHCs' lifetime.

Fig. 10 (a) Schematic of a MnO₂//AC ZHC; (b) SEM image of a MnO₂ nanorod cathode and its electrochemical properties inside the MnO₂//AC ZHC with the aqueous electrolyte of (c) and (d) ZnSO₄ and (e) and (f) Zn(CF₃SO₃)₂. Adapted from ref. 317 with permission from Elsevier.

As summarized in Section 2.2.1, besides MnO₂, other types of manganese oxides and vanadium oxides are capable of storing Zn²⁺ ions in aqueous rechargeable zinc-ion batteries. In theory, these materials can be used to construct ZHCs with AC anodes. Even though the operating potentials of vanadium oxides (generally 0.2–1.6 V vs. Zn/Zn²⁺) are lower than that of manganese oxides (1.0–1.8 V vs. Zn/Zn²⁺), which will lead to a narrower voltage window of ZHCs, the higher Zn²⁺-storage capacity and good rate performance of vanadium oxide cathodes will be beneficial for ZHCs to achieve high energy density and power density. Similarly, besides AC powder, diverse kinds of capacitor-like electrode materials can be applied as anodes for ZHCs, such as CNTs, graphene, ACFs, CNFs, conductive polymers and so on.

3.1.3 Zinc-ion hybrid micro-supercapacitors (ZHmCs). Micro-supercapacitors are capable of powering miniaturized and wearable electronic products, and have been widely researched in recent years.^318–326 Their micro size, on the one hand, means reduced transport distance of electrons and electrolyte ions, thus promoting high power output. On the other hand, the micro size typically leads to a low loading of electrochemical active materials and a limited energy supply. As reviewed above, ZHCs are characterized by high energy density than general supercapacitors. They also display the characteristics of excellent rate capability, low cost and high safety (especially the ZHCs with an aqueous electrolyte or ionic liquid electrolyte). These characteristics encourage researchers to explore the possibility of applying the ZHC system to fabricate micro-supercapacitors.

Sun et al. designed a ZHmC by integrating an oxidized CNT (oCNT) micro-cathode, a Zn micro-anode and a gelatin/ZnSO₄ electrolyte (Fig. 11a–e).³²⁷ The authors first pasted CNT paper onto polyimide tape, followed by laser etching to form a 6-finger interdigitated pattern and oxygen plasma treatment, and then, the Zn micro-anode was prepared through an in situ electroplating method in ZnSO₄ aqueous solutions (during the electroplating process, one interdigitated oCNT micro-electrode acted as the working electrode and other ones acted as the reference and counter electrodes). The fabricated ZHmC operated in a voltage range of 0.2–1.8 V, and its CV curves could be regarded as a combination of broad redox peaks and fairly rectangular characteristic (Fig. 11b). The authors believed that the charge storage reactions inside the ZHmC were not diffusion-controlled but confined to the electrode surface, judged from the linear relationship between the peak current and scan rates (Fig. 11c). The detailed electrode reactions are the same as those inside macroscopic aqueous carbon//Zn ZHCs as reviewed in Section 3.1.1.1. The ZHmC displayed an areal capacitance of 83.2 mF cm⁻² at a current of 1 mA cm⁻² and a good rate performance with 78% capacitance retention when charge/discharge current was increased from 1 to 10 mA cm⁻², leading to a high areal energy density of 29.6 μW h cm⁻² (corresponding to a volumetric energy density of 0.015 W h cm⁻³) and a high power density of 8 mW cm⁻² (corresponding to a volumetric power density of 4 W cm⁻³) (Fig. 11d). Besides, the capacity of the ZHmC retained 87.4% after 6000 charge/discharge cycles, and more importantly, the electrochemical performance of the ZHmC could recover (or become even higher than) its original level through further in situ electroplating of Zn again. The ZHmC displayed a certain flexibility; it could be bent and twisted (Fig. 11e). After 3000 bending cycles (one cycle: bending from 0° to 45° and then recovery to 0°), ZHmC capacitance retained 89.5% of its initial value. This preliminarily proves that ZHC systems are capable of being applied in the field of wearable/portable electronics.

Fig. 11 (a) Schematic illustration of the preparation process of an oCNT//Zn ZHmC and corresponding SEM images in each step (from left to right: pristine CNT paper, an interdigitated microelectrode and a Zn micro-anode); (b) CV curves at 10–200 mV s⁻¹, (c) relation curve between the peak current and the scan rate, (d) charge–discharge profiles and (e) photographs in bent and twist states of the oCNT//Zn ZHmC. Adapted from ref. 327 with permission from The Royal Society of Chemistry. (f) CV (scan rate: 1–5 mV s⁻¹) and (g) charge–discharge curves of an AC//Zn ZHmC. Adapted from ref. 328 with permission from Wiley-VCH.

Similarly, Zhang et al. reported an AC//Zn ZHmC (Fig. 11f and g), in which an interdigitated Ti/Au pattern served as the current collector, electrochemically deposited Zn nanosheets served as the anode, a coated AC/acetylene black/sodium carboxymethylcellulose electrode served as the cathode, and ZnSO₄ aqueous solution served as the electrolyte.³²⁸ The thickness and loading of Zn nanosheets were controlled by adjusting the electrodeposition time. According to the authors' research, during charge/discharge processes of the AC//Zn ZHmC in a voltage range of 0.5–1.5 V, Zn stripping/plating occurred on the Zn anode and only a physical adsorption/desorption process existed on the AC cathode, because the used AC particles almost did not contain oxygen functional groups, which means that the AC cathode could not contribute pseudocapacitance. Also, the AC//Zn ZHmC worked in a narrower operating voltage window compared with the AC//Zn ZHCs discussed in Fig. 7. Overall, there are some differences between the two devices in the energy storage mechanism. Consequently, CV curves of the ZHmC showed a nearly rectangular shape even at low scan rates of 1–5 mV s⁻¹ (Fig. 11f). Despite this, capacitance of the ZHmC reached 1297 mF cm⁻² at a current density of 0.16 mA cm⁻² (equaling 259.4 F g⁻¹ at a current density of 0.05 A g⁻¹) and 600 mF cm⁻² at 3.12 mA cm⁻², implying a high areal capacitance and good rate performance (Fig. 11g). The maximum areal energy density and power density were 115.4 μW h cm⁻² and 3.0 mW cm⁻², respectively, significantly superior to those of many other previously reported micro-supercapacitors. In addition, for the ZHmC, an excellent cycling stability was also achieved with 100% capacitance retention over 10 000 charge/discharge cycles.

As overviewed above and summarized in Table 2, various ZHCs with different configurations and energy storage mechanisms have been proposed. These ZHCs exhibit attractive electrochemical behaviors (such as the integration of high energy density and high power density). Benefiting from high security and superior electrochemical performance, the ZHCs are considered to have enormous application potential in wearable/portable energy storage, micro-scale devices and some other fields. By contrast, there are only several reports on hybrid capacitors based on multivalent Mg²⁺, Ca²⁺ and Al³⁺, as discussed in the following.

Table 2 Summary of the configuration and electrochemical properties of various ZHCs

Configuration			Electrochemical performance			Ref.
Cathode	Anode	Electrolyte	Voltage (V)	Capacity	Capacity retention
a Calculated based on the total mass of MnO2 and AC.
AC	Zn foil	ZnSO4 (aq)	0.2–1.8	121 mA h g^−1 (0.1 A g^−1)	91% after 10 000 cycles	202
				41 mA h g^−1 (20 A g^−1)
Oxidized CNTs	Zn foil	ZnSO4 (aq); ZnSO4–PVA gel	0–1.8	∼27 mA h g^−1 (10 mV s^−1)	—	306
Graphene@PANI	Zn foil	ZnSO4 (aq)	0.4–1.6	154 mA h g^−1 (0.1 A g^−1)	80.5% after 6000 cycles	307
				106 mA h g^−1 (5 A g^−1)
PPy	Zn/graphite paper	ZnCl2 + NH4Cl (aq)	0.6–1.6	151.1 mA h g^−1 (0.5 A g^−1); 87.6 mA h g^−1 (16 A g^−1);	76.7% after 1000 cycles	308
Bio-carbon	Zn foil	Zn(CF3SO3)2 in AN or the mixture of dimethoxyethane and 1,3-dioxolane	0–1.8	85 mA h g^−1 (0.1 A g^−1)	91% after 20 000 cycles	309
ACF cloth	Zn foil	Zn[(CF3SO2)2N]2/AN	0–1.8	∼55 mA h g^−1 (5 mA cm^−2)	96% after 10 000 cycles	310
Natural graphite	Sponge-like Zn	Zn(OAc)2/choline acetate/water	0–1.8	∼73 mA h g^−1 (0.2 A g^−1)	∼100% after 1000 cycles	311
MnO2 nanorods	AC	ZnSO4 (aq)	0–2.0	54.1 mA h g^−1 (0.1A g^−1)^a	65.3% after 3000 cycles	317
MnO2 nanorods	AC	Zn(CF3SO3)2 (aq)	0–2.0	46.4 mA h g^−1 (0.1A g^−1)^a	93.4% after 5000 cycles	317
Oxidized CNTs	Zn	ZnSO4–gelatin	0.2–1.8	83.2 mF cm^−2 (1 mA cm^−2); 65 mF cm^−2 (10 mA cm^−2)	87.4% after 6000 cycles	327
AC	Zn	ZnSO4 (aq)	0.5–1.5	1297 mF cm^−2 (0.16 mA cm^−2); 600 mF cm^−2 (3.12 mA cm^−2)	100% after 10 000 cycles	328

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3.2 Other multivalent metal ion hybrid capacitors

3.2.1 Magnesium-ion hybrid capacitors (MHCs). Magnesium is the 8th abundant element in the Earth's crust and the 5th most abundant element in seawater, and metallic magnesium electrodes have a high volumetric capacity of 3833 A h L⁻¹. Therefore, magnesium has been explored as an active element for electrochemical energy storage, whereas, for Mg metal anodes, passivation films may form on their surface in many electrolytes, which makes long-term reversible deposition/dissolution of Mg (Mg²⁺) impossible. Yoo et al. elaborated a modified ethereal-Mg organo-haloaluminate complex electrolyte for stable Mg anodes and then designed MHCs.^310,329 In the MHCs, ACF cloth with a BET surface area of 2000 m² g⁻¹ acted as the cathode, Mg foil served as the anode, and 0.25 M Mg₂Cl₃⁺–Ph₂AlCl₂⁻ (in THF) solution containing 0.5 M LiCl was used as the electrolyte. In the electrolyte, the Mg electrode could be deposited/dissolved reversibly for thousands of charge/discharge cycles (100% cycling efficiency). For the ACF cloth//Mg MHCs, during discharging processes, Mg²⁺ and Li⁺ ions from the electrolyte accumulated on the ACF surface (inside pores), and at the same time, magnesium dissolved into the electrolyte from Mg foil; during charging processes, Mg²⁺ and Li⁺ ions were desorbed from the ACF surface, while Mg²⁺ ions in the electrolyte electrodeposited on Mg foil. The authors separately monitored the potential profiles of the cathode and anode at various charge/discharge currents, and pointed out that the kinetics of magnesium deposition/dissolution was fast enough to match with the ACF cathode. In an operating voltage window of 0.9–2.4 V, the MHC showed a capacitance value of 90 F g⁻¹ and 79% capacitance retention after 4500 charge/discharge cycles. Carbon aerogels have also been proven to serve as cathodes for the construction of carbon//Mg MHCs.³²⁹

3.2.2 Calcium-ion hybrid capacitors (CHCs). Ca²⁺-ions have a lower polarization strength than multivalent Mg²⁺ and Al³⁺; thus, Ca²⁺-based electrochemistry exhibits better kinetics and is promising for designing high-power electrochemical energy storage devices.^24,330 However, it is difficult to find appropriate electrode materials and electrolytes for reversible systems based on Ca²⁺-ions and the conventional rocking-chair-type mechanism (as that in Li-ion batteries).

Very recently, Wu et al. reported a CHC, in which high surface area AC was used as the cathode, Sn foil was the anode and an organic Ca(PF₆)₂ solution (solvent: a mixture of ethylene carbonate, propylene carbonate, dimethyl carbonate and ethyl methyl carbonate in a volume ratio of 1 : 1 : 1 : 1) was used as the electrolyte (Fig. 12).³³¹ The CHC worked in a voltage range of 1.5–4.8 V, much higher than that of other types of multivalent metal ion hybrid capacitors. This is conducive to obtaining a high energy density. When the CHC was charged, PF₆⁻ anions adsorbed on the AC surface, and meanwhile, Ca²⁺ cations from the electrolyte moved onto the Sn foil to form a Ca₇Sn₆ alloy. During subsequent discharging, PF₆⁻ anions desorbed from the AC surface and a dealloying process (i.e., from the Ca₇Sn₆ alloy to Sn) occurred on the anode (Fig. 12a). Correspondingly, the CV curves of the CHC were nonideal rectangular and galvanostatic charge–discharge curves were nonideal straight (Fig. 12b). As a result, the CHC possessed a reversible capacity of 92 mA h g⁻¹ at a charge/discharge current of 0.1 A g⁻¹. Fast ion adsorption/desorption on/from the cathode and fast kinetics of the alloying/dealloying process on the anode endowed the CHC with a good rate capability: only 2 min was needed to fully charge or discharge the CHC. In addition, it was proved that the Sn anode was relatively stable during the alloying/dealloying process, which was beneficial for the CHC to achieve a superior cycling performance: 84% capacity could be retained after 1000 charge/discharge cycles (Fig. 12c). Even though such a cycling stability is much worse than that of many other kinds of multivalent metal ion hybrid capacitors, it is the highest to date for Ca²⁺ based electrochemical energy storage.

Fig. 12 (a) Schematic illustration of a CHC system with an AC cathode, a metal foil anode (e.g., Sn foil) and an organic Ca(PF₆)₂ electrolyte; electrochemical performance of the CHC: (b) CV curves and (c) rate capability and coulombic efficiency. Reproduced from ref. 331 with permission from Wiley-VCH.

3.2.3 Aluminium-ion hybrid capacitors (AHCs). Aluminium is the 3rd abundant metal element in the Earth's crust and metallic aluminum is much more stable than Li, Na and K metals. Besides, one aluminium-ion (Al³⁺) can carry three positive charges, significantly superior to univalent metal ions, while the ionic diameter of Al³⁺ is only 0.535 Å, even smaller than that of Li⁺ (0.76 Å), Na⁺ (1.02 Å) and K⁺ (1.38 Å) (Table 1). These advantages offer opportunities to prepare safe, low-cost and high capacity sustainable energy storage devices based on Al³⁺ ions, such as AIBs and Al³⁺-ion based electrochemical capacitors (i.e., AHCs). A big challenge in fabricating high-performance AHCs is to find a suitable anode with high capacity and good rate performance, considering that a strong electrostatic interaction exists between trivalent Al³⁺ ions and host materials.

Wang et al. first synthesized conductive PPy coated MoO₃ nanotubes (MoO₃@PPy) (Fig. 13a) and used them as an anode material for Al³⁺ storage.³³² In 0.5 M Al₂(SO₄)₃ aqueous electrolyte, the MoO₃@PPy anode delivered a capacity of about 140 mA h g⁻¹ at 1 A g⁻¹ in an operation voltage window of −0.4 to 0.5 V (vs. saturated calomel electrode). Reversible intercalation/de-intercalation of Al³⁺ in MoO₃@PPy was described as follows.

MoO₃ + xAl³⁺ + 3xe⁻ ⇔ Al_xMoO₃ (0 ≤ x ≤ 0.27) (15)

Fig. 13 (a) SEM image of MoO₃@PPy materials; (b) CV curves, (c) charge–discharge profiles and (d) cycling behavior of an AHC system comprising a MoO₃@PPy anode, an AC cathode and an Al₂(SO₄)₃ aqueous electrolyte. Adapted from ref. 332 with permission from The Royal Society of Chemistry.

AC electrodes in an Al₂(SO₄)₃ aqueous electrolyte exhibited typical electric double layer capacitive behavior in a voltage range of 0.5–1.1 V (vs. saturated calomel electrode). The authors then constructed an AHC comprising a MoO₃@PPy anode, an AC cathode and an Al₂(SO₄)₃ aqueous electrolyte (Fig. 13b–d). The AHC functioned from 0 to 1.5 V and its CV curves showed broad redox peaks associated with Al³⁺ intercalation/de-intercalation into/from MoO₃@PPy. Furthermore, the AHC displayed a good rate performance. Its energy density was 28 and 20 W h kg⁻¹ at a power density of 460 and 2840 W kg⁻¹, respectively. After 1800 charge/discharge cycles, the capacity of the AHC retained ∼93%, demonstrating a relatively good cycling stability. The authors attributed the good electrochemical performance of the AHC to the following factors: (i) the small diameter but large charge number of Al³⁺ ions benefited the high capacity, (ii) PPy coating decreased the charge transfer resistance and effectively protected MoO₃ from acid etching, and (iii) the nanotube structure of MoO₃ made it easier for the electrolyte to infiltrate active materials and also buffered the volume change of active materials during Al³⁺ intercalation/de-intercalation. In addition, as pointed out by the authors, the capacity of the used AC cathode in the AHC was relatively low; thus, the energy density of the AHC device could be further enhanced by optimizing the cathode material. Afterwards, Li et al. studied an asymmetric Al³⁺-ion supercapacitor with a single-wall CNT (SWNT)/W₁₈O₄₉ nanowire negative electrode, an SWNT/PANI positive electrode and 1 M AlCl₃ aqueous electrolyte.³³³ Even though such a supercapacitor is beyond the scope of hybrid capacitors, its SWNT/PANI electrode working in a potential range of 0–0.9 V (vs. Ag/AgCl) may be a potential candidate for AHC cathodes.

Based on the above discussions, we can see that various electrode materials and electrolytes are available for constructing ZHCs, whereas only a few configurations have been proved to be feasible for MHCs, CHCs, and AHCs. Besides, ZHCs are superior to the other types of hybrid multivalent metal ion capacitors in terms of rate capability and cycling stability, while CHCs and MHCs especially the former ones possess higher operating voltage, which is beneficial for achieving a higher energy density.

4. Conclusions and future outlook

Multivalent metal ion-based electrochemical energy storage is booming. Research studies on multivalent metal ion hybrid capacitors including ZHCs, MHCs, CHCs and AHCs are reviewed in this work. For each type of multivalent metal ion hybrid capacitor, its design concept, configuration, electrochemical behavior and energy storage mechanism are summarized. Overall, these multivalent metal ion hybrid capacitors integrate various advantages of conventional supercapacitors and multivalent metal ion batteries, exhibiting superior rate performance, high power density, high energy density and long cycle life. These merits give multivalent metal ion hybrid capacitors a promising future in electric vehicles and wearable/portable electronics. Furthermore, in our opinion, future research on multivalent metal ion hybrid capacitors will focus on the following aspects.

(i) Synthesis of high-performance electrode materials for multivalent metal ion hybrid capacitors. Up to now, there have been only a few reports about MHCs, CHCs and AHCs, due to limited choices of battery-like electrodes for these systems. More types of electrode materials for MIBs, CIBs and AIBs should be developed and investigated in corresponding multivalent metal ion hybrid capacitors. Particularly, for practical applications of metal electrode-based hybrid capacitors such as Zn foil-based ZHCs, the energy density of the whole device (instead of the cathode material only) has to be taken into account. This requires the metal electrodes to possess a long-term stability and fast kinetics even though their mass in the hybrid capacitors is relatively low (relative to the mass of the counter electrode). In addition, capacitor-like electrodes beyond AC materials should be further studied for these hybrid capacitors. We hope our overview of electrode materials for supercapacitors (EDLCs and pseudocapacitors) and multivalent metal ion batteries in Section 2 can offer some inspiration.

(ii) Detailing of actual electrochemical processes inside multivalent metal ion hybrid capacitors. Multivalent ion storage mechanisms are a relatively novel concept. They have not been completely understood in battery systems due to the distinct characteristics of multivalent ions (being different from univalent ions in charge number, polarization strength and ionic diameter). Besides this, interactions between multivalent ions and supercapacitive materials such as carbons may be not pure adsorption/desorption. Therefore, comprehensive methods including electrochemical measurements and physical–chemical analysis should be applied to extrapolate the energy storage mechanism of multivalent metal ion hybrid capacitors. This will benefit science by enriching the understanding of multivalent ion storage mechanisms.

(iii) Development of various configurations of multivalent metal ion hybrid capacitors. For ZHCs, Zn electrode based ZHCs and metal oxide//carbon ZHCs have been proposed, while for MHCs, CHCs and AHCs, only metal electrode-based hybrid capacitors have been designed. Other kinds of configurations of multivalent metal ion hybrid capacitors need to be developed to suit diverse applications. The construction of various configurations of multivalent metal ion hybrid capacitors relies on research into high-performance electrode materials, electrolytes and multivalent ion storage mechanisms.

(iv) Study of multivalent metal ion hybrid capacitors for flexible and wearable energy storage. As discussed above, multivalent metal ion hybrid capacitors display a series of merits such as notably enhanced energy density compared to conventional supercapacitors and higher safety than Li-ion batteries. Therefore, multivalent metal ion hybrid capacitors with good deformability based on reasonable electrode/device structure designs have a wide possible range of applications for flexible/wearable energy storage. These flexible hybrid capacitors can also be combined with flexible solar cells, flexible sensors and other electronics for multifunctional smart applications in future society.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial support by the Australian Research Council through the ARC Discovery projects (DP160104340 and DP170100436), the High-level Talents Project of Dongguan University of Technology (KCYKYQD2017016), and the Leading Talents of Innovation and Entrepreneurship of the Dongguan City D2017(16).

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