English

• 1. Introduction

  The fast-changing development of portable electronic displays and public traffic facilities has accelerated research advances in high-performance energy storage devices including supercapacitors, metal-ion batteries and their hybrid systems [1-3]. In supercapacitors, the energy storage is realized by means of interfacial cation/anion sorption in the Helmholtz double layers or surface pseudocapacitive reaction [1,4]; the capacitor-type working mechanism merely occurring on the electrode (near-)surface gives supercapacitors with remarkable power density, and the well-maintained electrode structure upon high-rate/successive charging-discharging turns also contributes to a long service lifespan [4,5]. On the contrary, the energy in metal-ion batteries is stored based on the intercalation/deintercalation reaction of cations into the internal electrode structure, which reveals a diffusion-dominated electrochemical process with remarkable energy density [3,6]; meanwhile, the structure distortion induced by the phase evolution of the battery material also degrades the cycling durability of metal-ion batteries [6-8].

  Zinc-ion hybrid capacitors (ZICs) are considered as newly-emerging and competitive candidates for energy storage devices by coupling the characteristic capacitor-level power with complementary high energy of zinc-ion batteries [9-15]. Among various multivalent metal-based devices, more attention to zinc-based systems results from the overwhelming superiorities of Zn element: (1) natural abundance and massive production; (2) high theoretical gravimetric capacity of 820 mAh/g, as well as high compatibility and stability in aqueous solutions; (3) low electrochemical potential of –0.763 V (vs. the standard hydrogen electrode) and the two-electron redox property offering a high-energy density; (4) high safety and nontoxic nature [3,16]. Thus far, ZICs are defined as the hybrid system consisting of a capacitor-type electrode and a battery-type electrode in a zinc-based (e.g., ZnCl₂, Zn(CF₃SO₃)₂, ZnSO₄, Zn(NO₃)₂, ZnAc₂, Zn(ClO₄)₂) electrolyte solution (Fig. 1) [6,10,13,16-20]. In ZICs, the capacitor-type electrodes (e.g., double-layer capacitive carbons, pseudocapacitive materials) inherit the characteristic power/cycling merits of supercapacitors based on a surface-dominated charge-storage mechanism [12,17,19,21]. Compared with conventional ZICs merely using metal Zn as the anode, such broadly-defined ZICs using other battery-type electrode materials (e.g., Mn/V-based oxides, Prussian blue analogues, organic compounds) promise complementary high-energy density relying on Zn²⁺ insertion/extraction into the crystal structure instead of Zn²⁺ deposition/stripping on the Zn anode [10,13,22,23]. However, the practical application of rising ZICs still faces the specific capacity and dynamics (diffusion rate) mismatch between the two electrodes with different surface/diffusion-dominated energy storage mechanisms [6,19,24].

  Figure 1

  

  Figure 1.  Schematic representation of zinc-ion hybrid capacitors.

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  Focusing on these unresolved issues, great research efforts have recently been devoted toward upgrading the performances of present electrodes/electrolytes and exploring novel and high-quality active constituents in ZICs; in other words, the capacitor-type electrodes should increase the electrochemical active surface area by nanostructure engineering (surface accessibility) or doping redox-active constituents (surface pseudocapacitance) to bridge the capacity gap, and the battery-type electrodes should be tailor-designed by surface modification or architecture exploration to accelerate ion/electron diffusion rate in aqueous/non-aqueous electrolytes, thus aiming to matching high power and high energy simultaneously. This mini-review presents recent advances in ZICs referring to the hybrid energy storage mechanism, design strategies of both capacitor-type and battery-type electrode materials, and electrolyte research toward advanced performances (e.g., high operational potential, wide adaptive temperature). Finally, current challenges and future outlook have been proposed for further exploration of next-generation ZICs.

  2. Capacitor-type electrodes in high-power ZICs

  Capacitor-type electrodes store energy by means of physical cation/anion sorption or redox pseudocapacitive reaction on the electrode (near-)surface, and are divided into double-layer capacitive carbons and pseudocapacitive materials. The surface-dominated working mechanism contributes to high-power delivery, wherein the specific capacity is unsatisfactory because of the relatively low surface utilization on electrochemical turns [2,6,25]. Therefore, great research efforts have been devoted toward increasing the electrochemical active surface area by nanostructure engineering (surface accessibility) or doping redox-active constituents (surface pseudocapacitance) to fabricate high-capacity capacitor-type electrodes without sacrificing the high-power density inherent to supercapacitors.

  2.1 Double-layer capacitive carbons

  Large-surface-area carbon-based materials featuring bonus conductivity/stability merits, have been commonly applied as the capacitor-type electrodes in ZICs, where large carbon surfaces can ensure considerable sorption capacity of cations/anions through the generation of the Helmholtz double layers [4,12,19,26,27]. Unfortunately, the electrolyte-accessible carbon surface upon actual electrochemical turns is far less than the specific surface area parameter measured under the gaseous atmosphere [2,25,28]. Consequently, an effective solution to gain high specific capacity is nanostructure engineering of carbon-based electrodes for improved electrochemical surface accessibility, including activated carbons (ACs), porous carbons, hollow carbon spheres, 2D graphene/carbon nanosheets, 1D carbon nanotubes (CNTs)/carbon nanofibers, etc. [10,12,19,27-34]. In terms of porous structure engineering, customized design of hierarchical porous carbon materials (HPCs) is highly praised as the alternative electrode to commercial AC products in current ZICs [29,35-39]. Interconnected meso-/macropores in HPCs can decrease the interfacial resistance as electrolyte-diffusion shortcuts into deep sorption sites (i.e., micropores) on the interior surface, thus improving surface accessibility and specific capacity [35,36,40]. For instance, Wang's group prepared a metal-organic framework (MOF) as the precursor to obtain sharpened pencil-like HPC as a capacitive electrode in ZICs, and multi-level pore distribution offered sufficient storage and rapid transfer rate, thus achieving high-energy delivery of 130.1 Wh/kg for the as-built HPC//Zn device [29]. Xu's group reported the gas-steamed MOF synthesis to prepare porous carbon cages with large openings on their walls [34]. Active sites exposed on its unique open-wall structure guaranteed high-speed mass transfer of electrolyte-ions to the accessible surface, thus achieving a remarkable capacity of 225 mAh/g at 0.1 A/g for the coin-type ZIC cell. In terms of dimensional structure engineering, sp² carbon-based materials like graphene and CNTs also hold great promise as capacitive electrodes especially for portable ZICs, because of their prominent electrical conductivity, outstanding mechanical flexibility/resilience and accessible pores on the open surface [10,19,31,41-43]. In 2016, CNTs were paired as the cathode with a Zn anode into a ZnSO₄-based ZIC, in which a specific capacitance of 20 mF/cm² @ 10 mV/s and a stable cycling performance (up to 5000 cycles) were demonstrated [10]. Very recently, the surface chemistry of reduce graphene oxide cathode was regulated to upgrade the charge-storage capacity (245 F/g@0.5 A/g), and an energy density of 266 µWh/cm² was realized for the 3D printed ZIC configuration [31]. Both experimental results and theoretical calculations confirmed reversible H⁺ sorption and charge transfer on C atoms of the graphitic domains accompanied by the sp²-sp³ re-hybridization and the domain distortion/restoration (Fig. 2a), giving a supplementary capacity contribution beyond O-containing functionalities.

  Figure 2

  

  Figure 2.  (a) Schematic diagram for H⁺ sorption on C atoms of the graphitic domains. Copied with permission [31]. Copyright 2022, Wiley-VCH GmbH. (b) Schematic illustration showing the Zn²⁺/H⁺ co-action storage mechanism. Copied with permission [17]. Copyright 2020, Wiley-VCH GmbH.

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  2.2 Pseudocapacitive materials

  Pseudocapacitive materials with the co-contribution of both redox pseudocapacitive reaction and physical ion sorption become another strategy to increase the electrochemical active surface area for high specific capacity [17,24,44,45]. Redox pseudocapacitance can be doped by the reversible Faraday reaction between electrolyte-ions and redox-active constituents within a (quasi-)2D space of the electrode surface [6,7,46,47]. Pseudocapacitive electrode materials used in current ZICs mainly include redox-active carbonaceous frameworks, silicene, phosphorene and transition metal compounds like RuO₂, NbPO, BiCuS_2.5, MXenes [17,24,28,34,38,44,48-60]. For instance, Lu's group designed a N/O co-doped HPC as the cathode for a high-performance ZIC with a capacity of 177.8 mAh/g@4.2 A/g and an energy density up to 107.3 Wh/kg [38]. N dopants were evidenced to promote the surface chemical Zn²⁺ sorption via redox pseudocapacitive reaction of O-containing functionalities, finally replenishing the extra pseudocapacitance of the as-assembled N/O co-doped HPC//Zn device through C-O-Zn bonding (cathode: C-OH + Zn²⁺ + e^– ↔ C-O-Zn + H⁺). Alshareef's group attributed the pseudocapacitance of the O-doped porous carbon cathode to the Zn²⁺/H⁺ co-action storage mechanism (Fig. 2b), where hydrogen redox reaction occurred during the discharge process (0.25-0 V) and oxygen pseudocapacitive reaction took action under a high potential of about 1.7-1.9 V [17]. And the H⁺ consumption left behind a dynamic local OH⁻ environment reacting with Zn²⁺ for the precipitation/dissolution of byproducts (alkaline zinc salts). Beyond redox-active carbonaceous frameworks, transition metal compounds are also widely-investigated pseudocapacitive electrode materials including transition metal oxides, carbides, nitrides, sulfides, etc. Very recently, a novel BiCuS_2.5/Ti₃C₂T_x electrode material was designed based on a simple deposition route (Fig. 3a), and the as-assembled ZIC cell exhibited an improved energy density up to 298.4 Wh/kg@7.2 kW/kg [57]. Amorphous BiCuS_2.5 possessed the outstanding electrochemical reaction activity due to reversible redox reactions, and both the XPS analysis and theoretical simulation ascribed the working mechanism to the co-existence of intercalation pseudocapacitance and redox battery-like capacity. Moreover, MXenes as a prominent category of 2D transition metal (Ti, Nb, V, Cr, Mo, etc.) carbides/nitrides, are frequently applied in ZICs as the capacitor-type electrode materials [21,57-65]. 2D MXene structures can provide plentiful electroactive sites for redox pseudocapacitive reactions, in which ion penetration occurs between MXene sheets owing to the unbreakable chemical bonds, ultimately occupying active sites on the MXene surface for energy storage [60,61,66,67]. For example, a 3D-printed additive-free MXene cathode was developed through a facile gelation process for ZIC applications (Fig. 3b), and the resultant cathode delivered a dual-ion storage mechanism to combine the Zn²⁺ sorption process with H⁺ pseudocapacitive behaviors (Ti₃C₂O₂ + e^– + H⁺ ↔ Ti₃C₂O(OH); Ti₃C₂O₂ + 2e^– + 2H⁺ ↔ Ti₃C₂(OH)₂) [58]. The final MXene//Zn ZIC device exhibited a competitive energy-power output (0.10 mWh/cm²@5.90 mW/cm²) as well as a long cycling lifespan.

  Figure 3

  

  Figure 3.  (a) Synthetic illustration of the BiCuS_2.5/Ti₃C₂T_x electrode material. Copied with permission [57]. Copyright 2021, Elsevier. (b) Synthetic illustration of a 3D-printed additive-free MXene cathode. Copied with permission [58]. Copyright 2021, American Chemical Society.

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  3. Battery-type electrodes in high-energy ZICs

  Battery-type electrodes store energy by means of the intercalation/deintercalation reaction of cations into the internal electrode structure, and are divided into conventional metal Zn anodes and other battery-type materials like Mn/V-based oxides, Prussian blue analogues, organic compounds. The broadly-defined ZICs using conventional metal Zn anode or other battery-type electrode materials promise complementary high-energy output, while the diffusion-dominated working mechanism leads to unsatisfactory power density; meanwhile, the structure distortion induced by the phase evolution of the battery material also degrades the ion/electron dynamics (diffusion rate) [6,7,19]. Therefore, great research efforts have been devoted toward accelerating ion/electron dynamics in aqueous/non-aqueous electrolytes by surface modification or architecture exploration to fabricate high-rate battery-type electrodes without sacrificing high-energy density characteristic of zinc-ion batteries.

  3.1 Metal Zn anode

  Metal Zn is the most commonly-used anode material in conventional ZICs, where Zn²⁺ deposition/stripping on the battery-type Zn anode promises complementary high-energy density [9,10,17]. Upon charging, Zn²⁺ electrolyte-ions are deposited on the Zn anode, while Zn stripping occurs with Zn²⁺ diffusion to the electrolyte in the discharge process (Zn ↔ Zn²⁺ + 2e^–) [10,19]. Compared with the high-power capacitor-type electrode, the metal Zn anode still suffers from the low diffusion rate of Zn²⁺ caused by the structure distortion (Zn dendrite/corrosion/passivation) and poor Zn²⁺ deposition/stripping efficiency [22-24]. Consequently, versatile design strategies of metal Zn anode have been proposed in terms of electroplating architecture exploration, polymer/carbon surface coating and substrate/host composite [21-23,42,58,68-71]. For instance, Cha's group constructed a 2D nanostructured Zn anode based ZIC through a voltage-regulated electroplating technique (Fig. 4a) [22]. Such unique Zn architecture gave rise to reduced Zn²⁺-diffusion routes and the enlarged surface area for the power density up to 20 kW/kg and a high energy density of 208 Wh/kg. So far, owing to the heterogeneous/dendritic Zn deposition, Zn dendrite growth on the pristine foil is the main challenge for metal Zn anode in current ZICs to raise safety concerns and impede Zn utilization efficiency [16,68,69,71]. Besides, the dendrite growth triggers disadvantageous Zn corrosion/passivation at the electrolyte-electrode interface, and further decreases the service lifespan of ZICs [71,72]. The Zn anode could be stabilized by a hydrophilic adhesive coating to block the formation of Zn dendrite on the modified Zn surface, and the resultant polymer coating layer presented uniform carbonyl Zn-deposition sites, thereby equipping the assembled ZICs with the capacity retention of 100% after 14000 cycles [68]. Moreover, a series of porous Zn@substrate/host composites using Ti₃C₂ film, Cu foil and MOF-derived materials, are involved as the conductive scaffolds to increase the Zn electrochemical utilization and afford homogeneously-distributed active sites for Zn nucleation [43,58,69,71,73,74]. Xia's group reported the dendrite-free Zn anode with a MOF ZIF-8 derived host [69]. After the calcination, Zn²⁺ within ZIF-8 was transformed into well-dispersed Zn throughout the inherent porous framework to serve as the deposition nuclei. By coupling the as-prepared anode material with an AC cathode, the ZIC cell exhibited excellent energy delivery (140.8 Wh/kg) while maintaining 72% capacity after 20000 turns, which would open up a highly-efficient route for in-situ fabrication of high-rate metal Zn anodes.

  Figure 4

  

  Figure 4.  (a) Schematic illustration of a 2D nanostructured Zn anode based ZIC. Copied with permission [22]. Copyright 2019, Wiley-VCH GmbH. (b) Schematic illustration of a Zn_xMnO₂ nanowires//AC ZIC. Reproduced with permission [75]. Copyright 2020, Wiley-VCH GmbH.

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  3.2 Other battery-type materials

  Compared with conventional ZICs merely using metal Zn as the anode, current ZICs are broadly defined as the hybrid system consisting of a capacitor-type electrode and a battery-type electrode in a zinc-based electrolyte solution [9,13]. Beyond metal Zn, other battery-type electrode materials used in zinc-ion batteries are expected to proceed Zn²⁺ insertion/extraction in ZICs, such as Mn/V-based oxides, Prussian blue analogues (PBA), redox-active organic compounds [13,59,75-80]. Upon discharging, Zn²⁺ electrolyte-ions are inserted into the crystal structure of the battery-type electrode, while Zn²⁺ extraction occurs with subsequent diffusion to the electrolyte in the charge process [13,75]. Such broadly-defined ZICs can readily operate at higher voltages of 2 V for greater energy density than conventional ZICs, and the Zn(-dendrite)-free electrodes offer a high degree of freedom in material discovery and performance improvement. Mn/V-based oxides are the most commonly-used cathode materials in zinc-ion batteries due to various crystal structures and multivalent phases [6,16,81]. As the pioneer of the broadly-defined ZICs, Kang's group assembled the novel γ-MnO₂ nanorods//2 mol/L ZnSO₄//AC ZIC operated under 0-2 V, and the upgraded ZIC in the Mn²⁺-containing ZnSO₄ electrolyte delivered the increased energy density from 34.8 Wh/kg to 83.8 Wh/kg [13]. Also, the SO₄^2– replacement with CF₃SO₃^– in the electrolyte relieved side reactions including continuous Mn²⁺ dissolution and Zn₄(OH)₆SO₄·nH₂O byproduct formation on the MnO₂ electrode. So far, inferior electrical conductivity and structure/phase evolution are the main challenge for emerging battery-type materials in current ZICs to influence the Zn²⁺/electron diffusion rate upon electrochemical turns [75,82]. Consequently, some regulation strategies have been developed, including fabricating stable crystal structures, and compositing with conductive materials (e.g., CNTs, graphene, MXene) [59,63,75,83]. Recently, a Zn²⁺ pre-intercalation strategy was proposed to improve the tunnel structure stability of the battery-type MnO₂-based electrode for a high-performance ZIC with a remarkable battery-level energy density of 969.9 µWh/cm² and a power density up to 20.1 mW/cm² [75]. In this Zn_xMnO₂ nanowires//AC ZIC, Zn²⁺ insertion/extraction within Zn_xMnO₂ nanowire cathode (Zn_xMnO₂ – 2e^– ↔ Zn_yMnO₂ + Zn²⁺) worked with ion sorption on the AC anode to jointly achieve the excellent electrochemical performances (Fig. 4b). Ma's group reported a novel aqueous ZIC based on MXene anode and MnO₂@CNTs cathode, in which the energy and power densities reached 98.6 Wh/kg (77.5 W/kg) and 2480.6 W/kg (29.7 Wh/kg) [59]. Meanwhile, fabricating the MnO₂-based composite cathode with conductive CNTs could alleviate conductivity weaknesses and improve the framework strength for better cycle stability and Coulombic efficiency over 93.3% during 15000 cycles. Moreover, inspired by zinc-ion batteries, more innovative and comprehensive investigations into battery-type electrodes for ZICs are needed to diversify active electrode materials and energy storage mechanisms. For example, the intercalation-type PBA electrodes like CoFe(CN)₆ addressed the low-capacity drawback of PBA-based zinc-ion batteries by multiple high-voltage redox-active centers for improved energy output [77]. The coordination-type redox-active organic compound electrodes (carbonyl/imine compounds, nitronyl nitroxides, triphenylamine derivatives, organosulfur/conductive polymers, etc.) could be equipped with plentiful coordination centers throughout the hierarchical configuration for Zn²⁺ interaction [79,84-89].

  In-depth research efforts are still underway to explore dual-ion (co-)intercalation, redox-active coordination, and conversion mechanisms for advanced ZICs [16,90]. Multivalent Zn²⁺ charge carriers can deliver two-electron transfer during redox reaction for a high energy density of current ZICs, where Zn²⁺ insertion/extraction and Zn²⁺/anion sorption occur at the battery-type and capacitor-type electrode interfaces, respectively. Also, the limited Zn²⁺ diffusion rate and strong electrostatic repulsion could degrade the charge storage and dynamics of current ZICs. Thus, the co-action storage mechanisms involving other charge carriers (H⁺/anions/electrolyte-additive ions) with small ion sizes and rapid dynamics are highly-praised. The hydrogen redox reaction results in H⁺ pseudocapacitive behaviors, while the consumption of H⁺ charge carriers could be accompanied by the OH^–-induced precipitation/dissolution of byproducts (alkaline zinc salts). The predominant involvement of anions in the Zn²⁺-based solvated structure is expected to give the anionic co-insertion mechanism for high-performance reaction dynamics and structure reversibility.

  4. Electrolytes in advanced ZICs

  Capacitor-type and battery-type electrodes in ZICs have been discussed in the above two sections for high specific capacity and high diffusion rate, respectively. Further, in this section, new types of electrolytes including aqueous electrolytes, ionic liquid and organic electrolytes, and (quasi-)solid-state electrolytes, are presented to resolve the specific capacity and dynamics (diffusion rate) mismatch for advanced ZICs. The involved zinc salts in the electrolyte solutions are ZnCl₂, Zn(CF₃SO₃)₂, ZnSO₄, Zn(NO₃)₂, ZnAc₂, Zn(ClO₄)₂ and so on [19,20,24,91,92]. Among them, ZnSO₄ and Zn(CF₃SO₃)₂ are the most commonly used in current ZICs. Zhi's group investigated the influence of various anions carriers on the electrochemical behaviors of Zn//TiN ZICs, which demonstrated that SO₄^2– anions with the lowest negative adsorption energy were involved in the two-step adsorption-intercalation process to deliver the highest capacitance [24]. In contrast, the Zn(CF₃SO₃)₂ solution was proven to give the better anodic stability and high-efficiency Zn²⁺ deposition/stripping, owing to the fact that large CF₃SO₃^– anions could block more surrounding H₂O molecules for the decreased solvation effect [19].

  4.1 Aqueous electrolytes

  Aqueous electrolyte ZICs prevail over ionic liquid and organic electrolyte counterparts relying on excellent ionic conductivity, high cost-effective and safe operational conditions [19,54,93]. Water-in-salt electrolytes have been frequently applied to upgrade the operational potential of aqueous zinc-based systems [18,54,94-97]. Super-concentrated salt solutions relieve the electrode-electrolyte interphase from H₂O availability, thereby broadening the electrochemical stability window over the thermodynamical range [54,94,98]. In a hybrid water-in-salt electrolyte (1 mol/kg Zn(TFSI)₂ + 20 mol/kg LiTFSI), where Zn²⁺ was blocked by TFSI^– from H₂ evolution, dendrite-free Zn²⁺ deposition/stripping at nearly 100% Coulombic efficiency was demonstrated [18]. Ji's group developed a 30 mol/kg ZnCl₂ water-in-salt electrolyte to improve the Zn²⁺-insertion potential and drag the anion-insertion potential lower, finally enlarging the operational potential window by 0.35 V in contrast to a 5 mol/kg dilute solution [95]. Recently, a phosphorene-based ZIC with 1 mol/kg Zn(TFSI)₂ + 21 mol/kg LiTFSI hybrid water-in-salt electrolyte delivered an operational voltage of up to 2.2 V and a capacitance of 214.3 F/g over 5000 charging-discharging cycles; the addition of a 0.2 mol/kg ZnCl₂-containing organic solvent further increased the voltage range to 2.5 V [54]. Another solution to upgrade the operational potential of aqueous ZICs is to introduce soluble electrolyte additives into dilute solutions [13,93,99]. The addition of metal salts can shield a small amount of Zn²⁺ in zinc-based aqueous electrolytes, or control the dendrite growth by co-deposition with Zn. Fan's group supplemented Mg²⁺ cations into a common 2 mol/L ZnSO₄ aqueous electrolyte for the invertible Zn₄SO₄(OH)₆·xH₂O formation in ZICs. For the Zn anode, the Mg²⁺ cation-mediated electrolyte was proven to suppress the hydrogen evolution for homogeneous Zn nucleation-deposition under high voltage, while ion sorption and invertible proton adsorption related to the Zn₄SO₄(OH)₆·xH₂O formation led to high capacity for the AC cathode [93]. Besides, the addition of redox couples to electrolytes can further supplement extra charge carriers to take advantage of the "dead" mass of electrolytes for faradaic reactions, and organic anions like OTF^–, TFSI^– are compelled in the Zn²⁺ solvation structure to kinetically prevent H₂O-related side reactions [18,99-102].

  4.2 Ionic liquid and organic electrolytes

  Ionic liquid and organic electrolytes are also frequently mentioned in ZICs because of the high-potential and wide-temperature durability [36,103,104]. In such electrolytes, the involved organic solvents are acetonitrile, dimethyl sulfoxide, N, N-dimethylformamide and so on, and well-soluble Zn(CF₃SO₃)₂ is nowadays frequently selected as the electrolyte salt [36,105-107]. Deep eutectic solvents (DESs), as one type of ionic liquids, can be formed by a Lewis base (e.g., urea, ethylene glycol, acetamide, succinonitrile) with a Lewis acid (LiTFSI, Zn(TFSI)₂, etc.) [103,108-111]. A high flexibility in constituent type and molar percentage endows DESs with lower melting/phase-transition points, and the unique H-bonding network in DESs can confine most of H₂O molecules to the matrix for expanded operational potential [108,112]. For instance, a water-in-DES (urea/LiTFSI/Zn(TFSI)₂) electrolyte was developed, wherein the suppressed water reactivity enabled durable Zn²⁺ deposition/stripping with greatly improved cycling stability in contrast with routine electrolytes (Fig. 5a) [108]. Fan's group reported a novel hydrated DES electrolyte consisting of Zn(ClO₄)₂, methylsulfonylmethane and H₂O, where the Lewis base (methylsulfonylmethane) coordinated the Zn²⁺ solvation shell with H₂O molecules [103]. The solvated structure and eutectic interaction in such an electrolyte system resulted in the dendrite-free Zn deposition, which could give inspiring insights into the structure-performance relation between the solvation effect in DES electrolytes and electrochemical parameters like cycle reversibility, potential/temperature durability.

  Figure 5

  

  Figure 5.  (a) Schematic diagrams of metal Zn anode in the aqueous and water-in-DES electrolytes. Copied with permission [108]. Copyright 2019, Elsevier. (b) Raman spectra of ZnCl₂ electrolytes with different concentrations, desolvation energies of [Zn(H₂O)₆]²⁺/[ZnCl(H₂O)₅]⁺, the illustration of the Zn²⁺/ZnCl⁺ desolvation. Copied with permission [20]. Copyright 2020, Wiley-VCH GmbH.

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  4.3 (Quasi-)solid-state electrolytes

  (Quasi-)solid-state electrolytes with fascinating mechanical stability and temperature durability, have aroused great interest in current ZICs to fit the ever-growing demand of versatile flexible and portable electronics [20,21,34,78,92,113-118]. A ZnCl₂ polyacrylamide hydrogel electrolyte was proposed, and the Cl^– desolvation energy induced the smaller [ZnCl]⁺(H₂O)_n−1 (with n = 1-6) clusters for higher cathode capacity (Fig. 5b) [20]. The resultant ZIC using the hydrogel electrolyte delivered the energy density of 217 Wh/kg@450 W/kg, coupled with low-temperature adaptability and mechanical flexibility (92.9% capacitance retention upon 40000 cycles at −20 ℃). A robust quasi-solid-state ZIC was assembled by fabricating a Zn(ClO₄)₂ poly(vinyl alcohol) (PVA) hydrogel electrolyte, where Zn(ClO₄)₂ attracted a large number of free H₂O molecules through H-bonding interactions [92]. Consequently, the flexible device retained the superior storage capacity and cycling reversibility at a low operational temperature of −50℃, revealing exceptional anti-freezing performance. Electrolyte additives like organic molecules and polymers in (quasi-)solid-state electrolytes can suppress Zn dendrite and other side effects by engineering current distribution and ion transfer. Ethylene glycol is applied as a common additive antifreeze and holds great promise to inhibit dendrite formation, due to the low water content and uniform Zn-deposition sites mediated by organic material additions. A series of ZnSO₄-based hybrid electrolytes with ethylene glycol as the antifreeze agent was reported, wherein the solvation interaction between ethylene glycol and Zn²⁺ gave the hybrid electrolytes with a decreased freezing point and high-efficiency Zn²⁺/Zn chemistry (Fig. 6a) [117]. Yang's group designed a self-catalytic nano-reinforced system to synthesize a ZnCl₂ polyzwitterionic hydrogel electrolyte for low-temperature flexible ZICs (Fig. 6b) [118]. The polyzwitterionic network filled with ZnCl₂ reduced the ion-transfer barriers for the high-rate Zn anode to relieve side reactions and extend service durability, which offered innovative fabricating ideas for high-performance ZICs in applications of flexible and portable electronics across a wide adaptive temperature range.

  Figure 6

  

  Figure 6.  (a) Optimized structure of ZnSO₄-based hybrid electrolytes with ethylene glycol. Copied with permission [117]. Copyright 2020, the Royal Society of Chemistry. (b) Synthetic illustration of a ZnCl₂ polyzwitterionic hydrogel electrolyte for low-temperature flexible ZICs. Copied with permission [118]. Copyright 2021, American Chemical Society.

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  5. Conclusions and outlook

  Zinc-ion energy storage systems hold great promise for the merits of abundant natural reserves, high theoretical gravimetric capacity, low electrochemical potential, operational safety, etc. ZICs are considered as one competitive candidate with the congenerous features of both supercapacitors and zinc-ion batteries, targeting a combination of high-power delivery, high-energy output and high-quality service durability. In this mini-review, recent advances in both electrodes and electrolytes of ZICs are versatilely concluded to remedy the specific capacity and dynamics (diffusion rate) mismatch owing to the hybrid energy storage mechanism. Based on the surface-dominated working mechanism, capacitor-type electrodes are upgraded by porous/dimensional structure engineering (surface accessibility) or doping redox-active constituents (surface pseudocapacitance) to bridge the capacity gap. Meanwhile, battery-type electrodes relying on the diffusion-dominated working mechanism are tailor-designed by surface modification or architecture exploration to relieve the structure/phase evolution especially upon high-rate/successive electrochemical turns. Furthermore, electrolyte research referring to aqueous electrolytes, ionic liquid and organic electrolytes and (quasi-)solid-state electrolytes is presented to broaden ZIC exploitation toward advanced performances like high operational potential and wide adaptive temperature. Despite great achievements in the past few years, the practical application of rising ZICs still cannot meet the ever-growing indicator demand. More forceful and comprehensive efforts are in dire need, and the next-stage research directions are outlined as follows.

  (1) For the role of high-capacity capacitor-type electrodes, more attention should be paid to increasing the electrochemical active surface area (surface accessibility and surface pseudocapacitance) rather than the specific surface area. In terms of surface accessibility, only shallow sorption sites on the electrode skeleton are readily accessed by electrolyte species to give immediate capacitive responses; consequently, in situ optimization strategies of preset precursor skeletons are highly praised to build 3D hierarchical porous carbon architectures, where interconnected meso-/macropores can reduce migration barriers into deep sorption sites on interior surface. In terms of surface pseudocapacitance, the working mechanisms related to heteroatom dopant sites require further clarification, since current ZICs have only reported redox pseudocapacitive reactions between Zn²⁺ and O-containing functionalities; more simulation characterizations and theoretical calculations shall help to understand the interfacial sorption dynamics and the interaction of versatile dopants. Besides, novel and high-quality pseudocapacitive materials should be explored including transition metal compounds and other potential pseudocapacitive layered architectures.

  (2) With regard to high-rate battery-type electrodes, the structure/phase evolution originated from the battery-type working mechanism is the main concern ion/electron diffusion rate degradation. Metal Zn anodes are mostly plagued by Zn dendrite growth and poor Zn²⁺ deposition/stripping efficiency, while other batter-type electrodes are also challenged by continuous cathode dissolution and byproduct (e.g., Zn₄(OH)₆SO₄·nH₂O) formation. Consequently, the next-stage research can be centered on architecture exploration, surface coating and substrate/host composite for better cycling/power properties; further, the above-mentioned robust capacitor-type material structures can also serve as the conductive scaffolds to participate in the tailor-design of emerging battery-type electrodes to remedy the inferior electrical conductivity and unreversible structure/phase evolution. Moreover, inspired by zinc-ion batteries, more innovative and comprehensive investigations into battery-type electrodes for ZICs are needed to diversify active electrode materials (intercalation-type PBAs, coordination-type redox-active organic compounds, etc.) and energy storage mechanisms such as (co-)intercalation/coordination/conversion mechanisms.

  (3) Considering advanced aqueous/non-aqueous electrolytes, high-quality service durability can be realized by electrolyte exploitation to boost the operational potential and widen the adaptive temperature. ZnSO₄ and Zn(CF₃SO₃)₂ are the most commonly used zinc electrolyte salts in current ZICs. Water-in-salt electrolytes not only inherit the environmental friendliness and safe/facile operational condition characteristic of conventional aqueous electrolytes, but also upgrade the operational potential by relieving the electrode-electrolyte interphase from H₂O availability. The supplement of soluble electrolyte additives into dilute solutions can better utilize the "dead" mass of electrolytes and kinetically prevent H₂O-related side reactions. And novel electrolyte salts like ZnCl₂, ZnAc₂, and Zn(ClO₄)₂ are promising to overcome the precipitation/dissolution of zinc salt byproducts (e.g., Zn₄(OH)₆SO₄·nH₂O). Moreover, ionic liquid and organic electrolytes further promote ZIC advances with higher-potential and wider-temperature durability. The synthesis flexibility and unique H-bonding network of DESs result in lower melting/phase-transition points and powerful H₂O constraints in ultimate ZICs. Besides, (quasi-)solid-state electrolytes with fascinating mechanical stability and temperature durability, appear to resolve Zn dendrite growth and unreversible electrode dissolution.

  (4) To keep pace with the fast-changing development of the portable electronics era, fabricating flexible ZICs integrated with new features and smart functions represents a vital research direction for practical applications, such as photo-chargeable, shape-memory, stretchable and biodegradable ZIC devices. The current zinc-ion technology holds a potential position in the market dominance of power-backup applications like Internet hubs or cellular repeater towers, wherein multi-functional flexible ZICs are the emerging candidates to power portable smart equipment. The equipment processability/scalability is highly required for fundamental researches and practical applications, so advanced fabrication routes including 3D printing, inkjet/screen printing and laser etching, are preferred for the development of next-generation ZICs.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (Nos. 21875165, 21905207, 22172111, and 51772216), the Science and Technology Commission of Shanghai Municipality, China (Nos. 22ZR1464100, 20ZR1460300, and 19DZ2271500), Zhejiang Provincial Natural Science Foundation of China (No. LY19B010003), and the Fundamental Research Funds for the Central Universities.

  
  
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• 

  Figure 1  Schematic representation of zinc-ion hybrid capacitors.

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  Figure 2  (a) Schematic diagram for H⁺ sorption on C atoms of the graphitic domains. Copied with permission [31]. Copyright 2022, Wiley-VCH GmbH. (b) Schematic illustration showing the Zn²⁺/H⁺ co-action storage mechanism. Copied with permission [17]. Copyright 2020, Wiley-VCH GmbH.

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  Figure 3  (a) Synthetic illustration of the BiCuS_2.5/Ti₃C₂T_x electrode material. Copied with permission [57]. Copyright 2021, Elsevier. (b) Synthetic illustration of a 3D-printed additive-free MXene cathode. Copied with permission [58]. Copyright 2021, American Chemical Society.

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  Figure 4  (a) Schematic illustration of a 2D nanostructured Zn anode based ZIC. Copied with permission [22]. Copyright 2019, Wiley-VCH GmbH. (b) Schematic illustration of a Zn_xMnO₂ nanowires//AC ZIC. Reproduced with permission [75]. Copyright 2020, Wiley-VCH GmbH.

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  Figure 5  (a) Schematic diagrams of metal Zn anode in the aqueous and water-in-DES electrolytes. Copied with permission [108]. Copyright 2019, Elsevier. (b) Raman spectra of ZnCl₂ electrolytes with different concentrations, desolvation energies of [Zn(H₂O)₆]²⁺/[ZnCl(H₂O)₅]⁺, the illustration of the Zn²⁺/ZnCl⁺ desolvation. Copied with permission [20]. Copyright 2020, Wiley-VCH GmbH.

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  Figure 6  (a) Optimized structure of ZnSO₄-based hybrid electrolytes with ethylene glycol. Copied with permission [117]. Copyright 2020, the Royal Society of Chemistry. (b) Synthetic illustration of a ZnCl₂ polyzwitterionic hydrogel electrolyte for low-temperature flexible ZICs. Copied with permission [118]. Copyright 2021, American Chemical Society.

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