Recent advances and future perspectives for aqueous zinc-ion capacitors

Ion-hybrid capacitors are expected to combine the high specific energy of battery-type materials and the superior specific power of capacitor-type materials and are considered as a promising energy storage technique. In particular, aqueous zinc-ion capacitors (ZIC), possessing the merits of high safety, cost-efficiency and eco-friendliness, have been widely explored with various electrode materials and electrolytes to obtain excellent electrochemical performance. In this review, we first summarize the research progress on enhancing the specific capacitance of capacitor-type materials and review the research on improving the cycling capability of battery-type materials under high current densities. Then, we look back on the effects of electrolyte engineering on the electrochemical performance of ZIC. Finally, we propose research challenges and development directions for ZIC. This review provides guidance for the design and construction of high-performance ZIC.


Introduction
With the development of consumer electronics, transportation electrification and wireless communications, electrochemical energy storage technology has become an essential part of our lives.In particular, the development and utilization of Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.intermittent renewable energy in recent years, provides further development opportunities for electrochemical energy storage techniques.Although lithium-ion batteries (LIB) are the dominant devices for electrochemical energy storage, supercapacitors have complemented LIB and have developed rapidly in the last decade [1].LIB can achieve a specific energy of over 200 Wh kg −1 [2], but are not suitable for application scenarios that require high specific power, such as braking and acceleration for trams, power for forklifts and cranes, and backup power for solid-state drives.Supercapacitors can easily meet the requirement of high specific power, but are limited by their poor specific energy.As a combination of battery and supercapacitor, the ion hybrid capacitor is expected to combine the

Future perspectives
The zinc-ion capacitor (ZIC) has been demonstrated as a promising energy storage technique.Despite the numerous efforts that have been made toward the advancement of capacitor-type materials, battery-type materials and electrolytes, many challenges remain.The most important task of research on capacitor-type materials is to improve their specific capacitance and research of battery-type materials is aimed at improving their rate capabilities.Improving the electrochemical stability of aqueous electrolytes also plays an important role in enhancing the specific energy and Coulombic efficiency of the ZIC.advantages of both high specific energy and power and is considered a promising electrochemical energy storage technique [3,4].
As shown in figure 1(a), supercapacitors are composed of two capacitor-type electrodes and store energy through electrochemical double layer capacitance behaviors or pseudocapacitance behaviors.Cation rechargeable batteries consist of two battery-type electrodes, which enable energy storage and conversion through the ion shuttling behavior between the anode and cathode (figure 2(a)).In contrast, ion hybrid capacitors are composed of a capacitor-type electrode and a battery-type electrode as illustrated in figure 1(c), which produce (de)adsorption or pseudocapacitance behavior and redox reaction during the charging and discharging process respectively.Hence, the key to improve the electrochemical performance of ion hybrid capacitors is to balance their specific energy and specific power, which requires improving the specific capacitance of the capacitor-type electrode and optimizing the cycling stability under a lager current density of the battery-type electrode.
Considering the frequent safety accidents such as explosions and fires of LIB, due to the use of flammable organic electrolytes, as well as the increasing price of lithium salts, aqueous batteries, with intrinsic merits of safety and cost efficiency, are a better choice for stationary electrochemical energy storage systems [1,5].In particular, zinc-ion batteries have become the bellwether of aqueous rechargeable batteries [6,7], since they possess low redox potential, excellent electron conductivity and high safety [8].Similarly, after a series of studies on lithium-ion capacitors [9][10][11], sodiumion capacitors [12,13], potassium-ion capacitors [14,15], and magnesium-ion capacitors [16], zinc-ion capacitors (ZIC) have gradually become the focus of ion hybrid capacitor research.
As an ion hybrid capacitor, the ZIC is expected to combine the large specific energy of battery-type materials and the high specific power of capacitor-type materials, which is promising in many scenarios such as electric vehicle acceleration, backup power supply, high-power industrial equipment.The ZIC has attracted much attention in both the exploration of cathode materials and the modification of the zinc anode in recent years.Hence, it is urgent to systematically summarize research progress on improving the electrochemical performance of ZIC, especially in enhancing the specific capacitance of capacitor-type materials and improving the cycling capability of battery-type materials under high current densities.
Additionally, we also summarize the electrolyte engineering of ZIC.Finally, the research challenges and development direction of ZIC are discussed.

Exploitation and modification of capacitor-type materials
There are two types of capacitance behaviors for capacitortype materials; electrical double-layer capacitance (EDLC) and pseudocapacitance.Increasing the specific surface area of materials is considered as an effective way to enhance the EDLC behaviors, and many efforts have been devoted to increasing the specific surface area of carbonaceous materials.Pseudocapacitance behavior mainly includes surface redox pseudocapacitance and intercalation pseudocapacitance behaviors [2].The improvement of surface redox pseudocapacitance can be achieved by increasing the number of redox activation sites such as by introducing redox group and element-doping.In addition, many efforts have been made to exploit two-dimensional layered materials to enhance the intercalation pseudocapacitance.

Carbonaceous materials
Carbonaceous materials are the most widely used materials for supercapacitors, and the initially reported ZIC are also composed of carbonaceous materials with a zinc anode.The zinc-activated carbon (AC) capacitor, reported almost simultaneously by Kang and Tang's team [17,18], ignited research enthusiasm for ZIC.The typical cyclic voltammetry (CV) curves and galvanostatic charge-discharge (GCD) profiles of the Zn-AC capacitor are displayed in figures 2(a) and (b), which almost contributes to pure EDLC behaviors and represents the typical electrochemical profiles of EDLC capacitors.EDLC behaviors are highly dependent on the specific surface area, so various kinds of carbonaceous materials with large specific surface area have been explored [19].For instance, a hierarchical porous carbon (HPC, figure 2(c)) prepared through the activation of asphalt with KOH, which possesses a much larger specific surface area of 3525 m 2 g −1 than that of commercial AC (1983 m 2 g −1 ).The Zn-HPC capacitor delivered around twice the capacitance than that of the Zn-AC capacitor due to the increase of specific surface area.Carbon quantum dots (figure 2(d)), carbon fibers (figure 2(e)) and graphene (figure 2(f)) have been also explored as cathode materials for ZIC, which represent zero, one and two-dimensional carbonaceous materials.In addition, biomass-derived carbon and organically derived carbon (such as metal-organic framework derived carbon, figure 2(g)) have been developed.This kind of carbon with rich pore structures, has the merits of low-cost and broad resource and is an important source of carbonaceous materials.To further improve the specific surface area and provide more adsorption sites, some special structures have been constructed.For instance, carbon hollow spheres (CHS, figure 2(h)) prepared by the hard-template method were used as cathode materials for ZIC, which delivered superior specific capacitance  and reversibly store SO 4 2− anions [20,21].Bowl-like carbon (figure 2(i)) was prepared through an additional etching process in the preparation procedure of CHS, which delivered over 300 F g −1 at a current density of 100 mA g −1 [22].The exploration of carbonaceous materials with rich porous structures, from zero-dimensional to three-dimensional, show that providing more adsorption sites can effectively enhance EDLC behavior.
The introduction of pseudocapacitance behaviors was considered as an effective strategy to greatly improve the specific energy of supercapacitors [28].As early as the first work on ZIC put forward by Wang' group, it was pointed out that oxidized carbon nanotubes (CNTs) exhibited much better electrochemical performance than un-oxidized CNTs, due to the introduction of a surface redox reaction [29].In addition to increasing the specific surface area, much effort has been devoted to increasing the specific capacitance of carbonaceous materials-based ZIC through another strategy, namely increasing the pseudocapacitance behavior [30].There are two methods to introduce extra pseudocapacitance behaviors in carbonaceous materials; element-doping and surface redox group grafting.Element doping of carbonaceous materials will change the charge distribution of the materials, thus improving the electron conductivity of the materials and introducing many active sites [31,32].Introducing nitrogen dopants into porous carbon (PC) was demonstrated to be able to increase the electron conductivity, improving the chemical adsorption of zinc ions [33,34].N-doped tubular carbon [29] and N-doped metal organic framework (MOF)-derived PC [35] have also been developed as cathode materials for ZIC and delivered enhanced specific capacitance and rate capability.Subsequently, a dual-doping strategy was developed to further improve the electrochemical performance of carbonaceous materials.B/N co-doped layered PC (LPC) was prepared through the pyrolysis of acrylonitrile copolymer guided by the intercalator (H 3 BO 3 ) [36], and the constructed Zn-LPC ion hybrid capacitor delivered a high specific energy of 86.8 Wh kg −1 .B/P co-doped AC was prepared by using red phosphorus and boric acid as phosphorus source and boron source respectively, which exhibited greatly enhanced electrochemical energy storage performance [37,38].It demonstrated that the doping of phosphorus improves the wettability between cathode materials and electrolyte and the doping of boron enhances electron conductivity through the transition of electronic structure and the combination of the two facilitate the enhancement of specific capacitance and performance longevity at high current densities.CHS with P/N dual doping was also demonstrated to promote the reaction kinetics and reduce the diffusion path of zinc ions and enable fast transportation of ions and electrons [39].
Introducing a surface redox group could not only increase adsorption sites but also provide extra redox pseudocapacitance, which could efficiently improve the specific energy of carbonaceous materials based ZIC.For instance, the oxygenenriched three-dimensional PC (OPC) prepared through nitric acid immersion exhibited around twice capacitance than that of original PC.Their CV curves and GCD profiles comparison are shown in figures 3(a) and (b) [40].The charge storage mechanism of zinc-PC with both EDLC capacitance and pseudocapacitance was explored as displayed in figure 3(c) [41].The highly reversible hydrogen and oxygen redox reactions of cathode materials during the charging and discharging processes originates from the chemical state variations of oxygen functional groups, together with the reversible (de)adsorption of hydrogen and zinc ions, contribute to the excellent specific capacitance of ZIC.Pre-oxidized PC derived from polyacrylonitrile delivered much higher specific capacitance than that of PC without oxidization, due to the introduced extra pseudocapacitance [42].Reduced graphene oxide (rGO) with sufficient oxygen functional groups was explored as a cathode material for ZIC [43].The charge storage mechanism is schematically illustrated in figure 3(d), which suggests a discharging process of cathode materials, compromising the electrochemical adsorption of zinc ions and sulfate anions and the chemical bonding of zinc ions with oxygencontaining functional groups on the surface of carbonaceous materials.It demonstrated that these surface oxygen substituents could greatly enhance the charge storage through the rapid redox reaction on the surface of carbon materials providing additional pseudocapacitance and modifying the hydrophilic feature of electrode materials.

Two-dimensional layered materials
Intercalation pseudocapacitance is an important kind of pseudocapacitive behavior proposed very recently, which is also expected to enhance the specific energy of supercapacitors [44].The research on intercalation pseudocapacitance mainly focuses on the exploitation of two-dimensional layered materials [45].Hao's group developed a HF-free exfoliation method to obtain various few layer MXene, and first they were applied as cathode materials for ZIC [46].Subsequently, pillared Ti 3 C 2 was applied as a cathode material for ZIC, and kinetic studies were explored through ex-situ Raman spectroscopy as displayed in figure 4(a).It revealed that Ti 3 C 2 store zinc ions through an intercalation pseudocapacitive process with the surficial oxygen terminations surviving as the binding sites for zinc ions [47,48].Considering that the MXene nanosheets are prone to aggregating and restacking due to the interlayer van der Waals interaction, rGO aerogels were introduced as a framework to inhibit the stacking of MXene and construct MXene-rGO aerogels [49].The Zn-MXene-rGO ion hybrid capacitor (as displayed in figure 4(b)) shows a high specific capacitance of 128.6 F g −1 at a current density of 0.4 A g −1 .MXene can form a self-standing film by pumping, similar to graphene.To improve the mechanical strength and Young's modulus and prevent the restacking of MXene nanosheets, nano-fibrillated cellulose was introduced to a composite with MXene [50].The optimized composite film exhibits excellent electron conductivity of 24 930 S m −1 and superior specific capacitance of 326.7 F g −1 at a current density of 1 mA cm −2 .In addition, MXene has also been adopted as a capacitor-type anode for ZIC [51,52].For example, MXene loaded cotton cloth [53] and graphite paper with electrodeposited Ti 3 C 2 T x [54] were utilized as anode materials for ZIC, which store charges through the interactions between the zinc ions and the surficial terminations of MXene.
Few-layer phosphorene (FL-P) possesses an armchair layer structure with a large interlayer spacing of 0.53 nm, sturdy mechanical strength (around 94 G Pa) and a large specific surface area [55,56], making it a promising material for both energy storage and energy conversion devices [57,58].Our previous work prepared FL-P through an electrochemical exfoliation method and applied it as the cathode material for ZIC [59].The constructed Zn-FL-P ion hybrid capacitor (as shown in figure 4(c)) delivered 130 F g −1 after over 9500 cycles at a current density of 500 mA g −1 and superior anti-self-discharge performance that retained over 76% of capacitance after a resting time of 300 h (figure 4(d)).Another two-dimensional material, graphitic carbon nitride, was also adopted as a cathode material for ZIC.The constructed photo-rechargeable Zn-g-C 3 N 4 ion hybrid capacitor could be directly charged by light and deliver a capacitance of 11.4 F g −1 [60].Very recently, few-layer siloxene was prepared through a liquid exfoliation method and adopted as a cathode material for ZIC [61].The CV curves at various scan rates and GCD profiles at different current densities of the constructed Zn-siloxene ion hybrid capacitor are displayed in figures 4(e) and (f) respectively, which delivered a maximum specific capacitance of 6.86 mF cm −2 .

Other materials
Organic electrode materials, with merits of cost-efficiency, safety, eco-friendliness and meeting specific energy requirements [62], are promising materials for various energy storage devices, including cation rechargeable batteries and ion hybrid capacitors [63,64].The typical conductive polymer, polyaniline (PANI), owns large specific capacitance but inferior rate capabilities.Graphene has been introduced to create a composite with PANI forming a hydrogel as a self-standing cathode for ZIC [65], which exhibited a specific capacitance of 414 F g −1 .A pseudocapacitive polymer, poly (4,4 ′thiodiphenol, TDP), was electrodeposited on carbon cloth (CC) with porous AC coated as shown in figure 5(a) [66].When serving as cathode materials for ZIC, the TDP cathode exhibited greatly enhanced specific capacitance compared to a bare AC coating cathode and a carbon fabric cathode, as shown in the CV curves comparison of different cathodes (figure 5(b)).A novel phenanthroline covalent organic framework (PA-COF) was explored as a cathode material for ZIC [67].The schematic illustration of the configuration and electrochemical behaviors of the Zn-PA-COF ion hybrid capacitor is presented in figure 5(c), which shows the variation of the and extraction/insertion of Zn 2+ are both involved in the charging and discharging process, as illustrated in figure 5(e) [68].Furthermore, RuO 2 •xH 2 O nanoparticles were explored as cathode materials for ZIC, which achieve charge storage through the surface redox pseudocapacitive behaviors and rapidly finish the charging and discharging process within 36 s with a high specific energy of 82 Wh kg −1 at a specific power of 16.74 kW kg −1 [69].
Due to their unique electronic and physical properties, perovskite oxides are promising materials for a fuel-cell, metal-air battery, as well as a supercapacitor.The oxygenvacancy-mediated redox pseudocapacitance of LaMnO 3 has demonstrated that oxygen anions can be reversibly inserted into and extracted from perovskite oxides and delivered a large specific capacitance of around 380 F g −1 [70].The specific capacitance of perovskite oxides are critically related to the oxygen vacancy concentration.SrCoO 3−δ and Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ were found to exhibit anion intercalation behaviors in aqueous alkaline electrolytes [71].In addition, another advantage of perovskite materials is the possibility of the combination of a solar cell and supercapacitor to construct a self-sufficient energy system [72].
In addition, Fontaine et al proposed a novel strategy to construct a high-energy supercapacitor [73].They grafted organic functional groups with redox activity to ionic liquid (IL) to enable the occurrence of redox reactions when it was adsorbed and desorbed by capacitor-type materials.As shown in figure 5(f), anthraquinone and 2,2,6,6-tetramethylpiperidinyl-1-oxyl are grafted to the anion and cation of IL respectively.The illustrated charge storage process of pure EDLC and IL functioned with a biredox group are compared in figure 5(g).It turned out that the biredox IL delivered twice the specific capacitance than that of pure EDLC from the CV curves shown in figure 5(h).An electrolytic redox reaction was introduced into the electrochemical energy storage process, which greatly increased the specific capacity of supercapacitors.
Subsequently, the redox pair of Br − /Br 3 − in the hydrogel electrolyte was introduced into ZIC to contribute extra faradic capacitance [74].Furthermore, the interactions between Zn 2+ and charge groups in hydrogel could facilitate the uniform deposition of zinc.

Optimization of battery-type materials
Battery-type electrode materials usually have a much higher specific capacitance than that of capacitor-type materials, but are plagued by poor rate performance, i.e. inferior electrochemical cycling performance at large current densities.Hence, the modification of battery-type materials mainly focusses on the improvement of their rate capabilities.

Zinc metal
Zinc metal, with a high theoretical specific capacity (820 mAh g −1 ) and a relatively low redox potential (−0.76 V vs standard hydrogen electrode), has been considered the most promising of anode materials in aqueous energy storage systems [75,76].Since the cathode materials of zinc metal-based ZIC are capacitor-type materials with limited specific capacitance and low mass loading at the laboratory level, and the use of zinc foil is usually excessive, the problems of zinc anodes do not appear at relatively low current densities [77].However, when zinc ion capacitors are designed for practical applications, large current density is required to ensure sufficient specific power density.Many reports indicate that a zinc anode suffers from a variety of problems, including dendrite, corrosion and hydrogen evolution [78][79][80], if the mass loading of the cathode materials and zinc foil are strictly matched and there are high area current densities.Hence, numerous efforts have been devoted to mitigating the problems of zinc anodes in aqueous electrolytes.There are four main strategies to achieve these, including surface coating [81], electrolyte engineering [82], electrode structure [83] and interface design [84,85] and alloys forming [86].
First, the surface coating is designed to regulate the distribution of the electric field and guide the uniform diffusion of zinc ions, as well as play a role of spatial shielding by constructing a similar artificial solid electrolyte interface (SEI) layer.Many inorganic materials, such as TiO 2 [87], CaCO 3 [88], ZnF 2 [89], and organic film, including poly(vinyl butyral) [90], polyamide 6 [91], and poly(ethylene glycol) (PEG) [92], have been adopted as an artificial surficial layer on the surface of the zinc anode.Specifically, self-standing CNTs paper was fabricated to inhibit the formation of zinc dendrites in ZIC [93].The schematic illustrations of the striping and plating process of bare zinc foil and a zinc anode with CNTs paper coated are compared in figure 6(a).The electrochemical performance of symmetric cells as shown in figure 6(b) demonstrated that the Zn coated with CNTs paper exhibited significantly lower voltage hysteresis and improved cycling longevity than that of a bare zinc anode.Second, electrolyte engineering mainly involves the addition of additives [94], which facilitate the formation of SEI from the decomposition of anions or additives [8], and the modification of solvation sheath structure [95], which is beneficial in reducing the overpotential of deposition process [82].
Third, the design of the zinc electrode structure can improve the electric field distribution, weaken the tip effect of deposition, and induce uniform deposition [96] and the design of the electrode electrolyte interface [97] is aimed at reducing interface impedance, improving interfacial wettability and reducing the overpotential and polarization of the deposition process [98].For instance, the CC with zeolitic imidazolate framework-8 (ZIF-8) pre-grown was utilized to modify the surface of a zinc anode, which demonstrated a regulation effect on the nucleation, diffusion and deposition behaviors of zinc ions [99].The deposition process illustration of a zinc anode with and without ZIF-8/CC interfacial structure are compared in figures 6(c) and (d).The CV curves and cycling stability comparison of ZIC, ZIC-CC and ZIC-CC/ZIF-8 are displayed in figures 6(e) and (f), in which ZIC-CC/ZIF-8 exhibited the largest specific capacitance due to the reduction of polarization and the most stable cycling stability, which was ascribed to the efficient inhibition of zinc dendrites.In addition, the electrodeposited nanostructured two-dimensional zinc metal electrode was designed to improve the diffusion capability of charge carriers and enhance the overall electrochemical performance of ZIC [100].Lastly, the alloys forming strategy could modify the surface reaction thermodynamics to guide the nucleation and growth though alloy reaction and construct a porous nanostructure to facilitate the diffusion kinetics of zinc ions [101].The liquid alloy coating is an integrated strategy to ameliorate the interfacial issues of a zinc anode.Introducing a Ga-In alloy interlayer on the surface of the zinc anode could effectively migrate the dendrite formation and reduce the overpotential of zinc deposition [72].

Metal oxides
Replacing the zinc anode with capacitor-type materials is another effective way to avoid the dendrite and hydrogen evolution problems of zinc metal [102,103].When using capacitor-type materials including carbonaceous materials and MXene as anode materials, battery-type materials such as metal oxides could be adopted as cathode materials to construct a ZIC.To match with the high specific power of capacitor-type materials, research on battery-type cathode materials focuses on the exploitation and modification of metal oxides.For instance, MnO 2 nanorods (figure 7(a)) are adopted as cathode materials for ZIC matched with an AC anode [104].Their CV curves and GCD profiles in 2 M ZnSO 4 + 0.5 M MnSO 4 electrolyte are shown in figures 7(b) and (c) respectively, which delivered a high specific capacitance at a low current density but poor rate capability, as well as a low Coulombic efficiency, due to the inferior electron conductivity of MnO 2 and self-discharge behaviors.Hence, CNTs [52] and carbon fiber [53] were reported in a composite with MnO 2 to improve its electron conductivity.Specifically, the MnO 2 nanowires were mixed with CNTs and filtered to form a free-standing electrode and constructed with Ti 3 C 2 T x film anode as shown in figure 7(d) [51].As can be seen from the CV curves and GCD profiles of Ti 3 C 2 T x -MnO 2 /CNTs in figures 7(e) and (f) respectively, the as-assembled ZIC delivered greatly improved specific capacitance and rate properties compared with a pure MnO 2 cathode.In addition, V 2 O 5 was also explored as a cathode material for ZIC, which delivered a large specific capacitance at small current densities but inferior rate capabilities, similar to MnO 2 [105].Then, V 2 O 5 film (figure 7(g)) was electrodeposited onto graphite paper to improve the electron conductivity and matched with MXene film to construct a flexible ZIC, which exhibited excellent specific capacitance as well as rate performance as shown in figures 7(h) and (i).In addition to lithium-ions and sodium-ions, the electrochemical intercalation behaviors of other cations such as NH 4 + have also been well studied.Strong hydrogen bonding can be formed between NH 4 + and V 2 O 5 electrode materials exhibiting prominent pseudocapacitive behaviors [106,107].

Electrolyte engineering for aqueous ZIC
The newly emerged aqueous electrolyte engineering plays an important role in improving the specific energy and specific power of ZIC.The introduction of additives and modification of solvation sheath structure can effectively suppress the hydrogen evolution reaction and greatly broaden the operating voltage range of ZIC thus enhancing its specific power.Comprehensive exploration of the interaction between anions in the electrolyte and electrode materials is beneficial to improve the specific energy and anti-self-discharge capability of ZIC.

Modification and exploitation of electrolyte
Aqueous electrolytes are more safe and cost-efficient choices than organic electrolytes for ZIC, while severely limited by their narrow electrochemical stability window (ESW), caused by the intrinsic hydrogen evolution and oxygen evolution reactions.The proposal of 'water-in-salt' electrolytes a few years ago greatly facilitated the development of high-energy aqueous energy storage techniques [108].The ultrahigh concentration of electrolyte salts could break the hydrogen bond networks between water molecules and force the anions and cations to participate in the solvation sheath structure in the form of 'contact ion pairs', thus depleting the solvated water molecules and reducing the water-related parasitic reactions [109].Our previous work explored the electrochemical performance of ZIC in WIS electrolyte, which exhibited greatly widened ESW and significantly enhanced specific capacitance [59].The ZnCl 2 -based WIS electrolyte was applied in ZIC and it revealed that the desolvation energy of the Cl − participated solvation sheath was considerably lower than that of the original solvation sheath [110].In addition, a molecularcrowding electrolyte with ultrahigh concentration of PEG was designed recently [111], in which water activity has been severely suppressed due to sufficient hydrogen bonds between the PEG and water molecules.The super-concentrated sugar electrolyte can expand the ESW to over 2 V and when it was applied in aqueous ion hybrid capacitors, the specific energy and Coulombic efficiency were effectively improved [112].In addition, small dipole molecule-containing electrolytes are promising choices for high-voltage aqueous energy storage devices [113].The introduction of various additives [85,94] could induce the formation of SEI and optimize the solvation sheath structure, thus to reduce the overpotential during the deposition process of zinc in ZIC, which has already been introduced in the zinc anode part.These studies have made crucial contributions to broadening the ESW of aqueous electrolytes, which could effectively improve both of the specific energy and specific power of ZIC when serving as electrolytes for aqueous ZIC.
Electrolyte cation additive strategy is also an effective method to improve the electrochemical performance of ZIC.The introduction of Mg 2+ in to a ZIC system could contribute to the specific capacitance, due to the adsorption of Mg 2+ and reversible proton and exhibit a positive effect in inhibiting hydrogen evolution reaction and dendrite formation [114].

Effects of anions in electrolyte
ZIC achieve energy storage through EDLC and pseudocapacitance behaviors, which involve both cations and anions.Hence, the comprehensive study of the interaction between ions in electrolytes, especially the easily overlooked anions, and electrode materials is essential to improve the electrochemical performance of ZIC.Our previous work explored the effects of the interaction between anions and the cathode materials on the specific capacitance and anti-self-discharge capability of a Zn-TiN ion hybrid capacitor [68].It indicated that among the various aqueous electrolytes, including zinc acetate, zinc chloride and zinc sulfate, SO 4 2− showed the most stable structure of TiN-SO 4 2− after adsorption, revealed by the density functional theory based on the first-principal, which explained the highest specific capacitance and most excellent anti-selfdischarge capability of the Zn-TiN ion hybrid capacitor in ZnSO 4 electrolyte.It means that the capacitance contributed by the adsorption of anions and the self-discharge behaviors are closely related to the stability of the anion-cathode material structure, which was also revealed by following research about the anti-self-discharge behavior [115] and the dual-ion adsorption behaviors of ZIC [116].The effects of electrolyte salts on the Coulombic efficiency of ZIC has been explored, which demonstrated that it achieved the well-balanced state between specific energy and cycling life in 3M Zn(CF 3 SO 3 ) 2 electrolyte.In addition, the type of electrolyte salt has an important on the ion conductivity of the electrolyte at low temperatures.Zn(ClO 4 ) 2 salty ice is least affected by the dramatic drop of ion conductivity caused by the phase separation in frozen aqueous electrolyte [117].

Conclusions and future perspectives
In summary, the ZIC has been demonstrated as a promising energy storage technique to improve the specific energy and specific power of an energy storage device.Despite the numerous efforts that have been made to advance capacitortype materials, battery-type materials and electrolytes, many challenges remain.Further exploration and innovations are required to promote the eventual commercial development of ZIC.

Capacitor-type materials
The most important task of research on capacitor-type materials is to improve their specific capacitance.Current capacitortype materials for ZIC are mainly divided into carbonaceous materials, two-dimensional materials and other materials.Increasing the specific surface area and introducing extra surface redox pseudocapacitive behaviors are two effective strategies to enhance the specific capacitance of carbonaceous materials.The exploitation of two-dimensional materials boosts the understanding and utilization of intercalation pseudocapacitive behaviors.At present, the specific capacitance of capacitive materials for zinc ion capacitors is still at a low level at the device level.Introducing electrolytic redox reaction and surface redox pseudocapacitive behaviors into capacitor-type materials with large specific surface area will be an attractive research direction for ZIC in the future.

Battery-type materials
The research of battery-type materials is aimed at improving their rate capabilities, which mainly focuses on the dendrite and hydrogen evolution issues of zinc anodes, as well as the electron conductivity of metal oxides.The electrochemical performance of battery-type materials is improved by three strategies, including interface improvement, structure construction and material design.The new target for batterytype materials is to ensure the cycling stability of electrode materials under high current densities, while increasing the depth of discharge and the mass loading of active materials.

Electrolyte exploitation
Improving the electrochemical stability of aqueous electrolytes plays an important role in enhancing the specific energy and Coulombic efficiency of ZIC.WIS electrolytes are an effective method to expand the ESW of aqueous electrolyte, while limited by their high cost.Introducing small molecules such as PEG and sugars into aqueous electrolyte seems to be a cost-efficient and eco-friendly strategy to broaden the ESW of aqueous electrolyte.The research on this kind of small molecule electrolyte and aqueous electrolyte additives are still in the early stage, which may be an important research branch of ZIC in following explorations.

Figure 3 .
Figure 3. Pseudocapacitance behaviors of carbonaceous materials.(a) and (b) The CV curves and GCD profiles comparison of Zn-PC and Zn-OPC.Reprinted from [40], Copyright (2020), with permission from Elsevier.(c) The schematic illustration of energy storage mechanism of carbonaceous materials with surface redox group during the charging and discharging process.[41] John Wiley & Sons.[© 2020 Wiley-VCH GmbH].(d) The schematic of surface oxygen substituents enhanced EDLC and surface redox pseudocapacitance charge storage mechanism.[43] John Wiley & Sons.[© 2020 Wiley-VCH GmbH].

Figure 5 .
Figure 5.Other promising capacitor-type materials for ZIC.(a) Schematic illustration of the preparation of a TDP cathode and the configuration of a Zn-TDP ion hybrid capacitor and (b) the CV curves comparison of a TDP cathode, a bare AC coating cathode and a carbon fabric cathode.Reproduced from [66] with permission of The Royal Society of Chemistry.(c) Schematic illustration of the configuration and electrochemical behaviors of Zn-PA-COF ion hybrid capacitor and (d) the initial three CV curves of Zn-PA-COF.Reprinted with permission from [67].Copyright (2020) American Chemical Society.(e) Schematics of the electrochemical process of a Zn-TiN ion hybrid capacitor.[68] John Wiley & Sons.[© 2020 Wiley-VCH GmbH].

Figure 6 .
Figure 6.Modification of zinc anode.(a) Schematic illustrations of the striping/plating process of bare zinc foil and zinc anode with CNTs paper coated and (b) the electrochemical performance of symmetric cells with and without CNTs paper coated.Reprinted from [93], Copyright (2020), with permission from Elsevier.(c) and (d) deposition process illustration of zinc anode with and without ZIF-8/CC interfacial structure respectively; (e) and (f) the CV curves and cycling stability comparison of ZIC, ZIC-CC and ZIC-CC/ZIF-8 respectively.(c-f) reproduced from [99] with permission of The Royal Society of Chemistry.

Figure 7 .
Figure 7.The exploration and modification of metal oxides electrode materials.(a) SEM image of MnO 2 nanorods; (b) and (c) CV curves and GCD profiles of an AC-MnO 2 ZIC respectively.Reprinted from [104], Copyright (2019), with permission from Elsevier.(d) Optical images of a MXene anode and a MnO 2 -CNTs cathode.(e) and (f) The CV curves and GCD profiles of a Ti 3 C 2 Tx-MnO 2 /CNTs ZIC respectively.(g) SEM image of V 2 O 5 film.(h) and (i) The CV curves and GCD profiles of a MXene-V 2 O 5 ZIC respectively.(g-h) reprinted from [54], Copyright (2021), with permission from Elsevier.