Recent advances in carbon materials for flexible zinc ion batteries

WU Li-sha, ZHANG Ming-hui, XU Wen, DONG Yan-feng

（Department of Chemistry, College of Sciences, Northeastern University, Shenyang 110819, China）

Abstract： The ever-growing demands for wearable devices has stimulated the development of advanced flexible energy storage devices. Aqueous rechargeable zinc ion batteries (ZIBs) have gained much attention due to their low cost and intrinsic safety. Carbon materials with excellent conductivity, high mechanical strength, and light weight, can be used to construct flexible ZIBs (FZIBs).Here, we summarize the recent advances in carbon materials (e.g., carbon nanotubes, carbon fibers, graphene) for high-performance FZIBs with one-dimensional cable-shaped, two-dimensional planar, and three-dimensional sandwich configurations. Ways for constructing different types of FZIBs for better electrochemical performance are emphasized. The vital roles of carbons as the conductive materials and current collectors of cathodes, the current collectors and host materials of anodes, and modifiers of functional separators are discussed. The challenges and prospects of advanced carbon materials for next-generation FZIBs are also briefly discussed.

Key words: Carbon materials；Flexible zinc ion batteries；Cathodes；Anodes；Separators

1 Introduction

The increasing demands for flexible and wearable electronics have triggered research enthusiasm for advanced energy storage devices. Zinc metal possesses high theoretical capacity (820 mAh g-1and 5 855 mAh cm-3), excellent chemical stability, and low cost, enabling aqueous zinc ion batteries (ZIBs) to hold great promise in portable and bendable device applications[1,2]. Moreover, aqueous electrolytes display high ionic conductivity (e.g., 13.31 mS cm-1for 2 mol L-1ZnSO4[3,4]) and environmental friendliness,and avoid the safety concerns of commercial lithium ion batteries[5]. However, amphoteric oxide cathodes of ZIBs suffer from low conductivity and poor structural stability, and the random growth of zinc dendrites may pierce through separators and lead to shortcircuits, jointly causing limited energy/power density and undesirable durability[6-10]. Furthermore, flexibility is a vital indicator to define a flexible battery to bear deformations[11,12]. In general, traditional ZIBs are constructed by metal Zn foils and cathodes consisting of active materials, conductive additives, and polymer binders on metal current collectors, and the assembled ZIBs with inferior interface interaction cannot withstand repeated mechanical deformations,such as bending, twisting, folding, and stretching.Therefore, it is highly needed to reasonably select functional materials to construct intimate interfaces to ensure rapid electron transfer and mechanical flexibility.

The pursuit for high-performance and mechanically flexible ZIBs (FZIBs) calls for the exploration of both flexible battery components (e.g., cathodes, anodes, and separators) and advanced battery configurations. Carbon materials possess high conductivity, excellent mechanical property, suitable porous structure,and light weight[13-19], which could be hybridized with active materials to achieve synergistic effects for high-performance FZIBs. Significantly, carbon materials have the ability to construct highly flexible electrodes[20,21], and the absence of polymer binders and additional conductive additives can not only simplify the fabrication procedures but also remarkably enhance the energy density of corresponding FZIBs.Notably, carbon materials are highly desired for the development of various flexible battery configurations, involving 1D cable-shaped[22,23], 2D planar[24,25],and 3D sandwiched batteries[26,27], in which both active materials/current collectors and electrodes/electrolytes interfaces are stable and robust enough to withstand repeated deformations, thus ensuring the integrity of FZIBs to work normally. In this regard, carbon materials play great roles in constructing high-performance FZIBs. However, there is still a lack of a timely and latest review to keep pace with the surging development of FZIBs.

Herein, we summarize the recent progress of carbon materials (e.g., graphene, carbon nanotubes(CNTs), and carbon fibers) for FZIBs with different battery configurations, such as the 1D cable-shaped,2D planar, and 3D sandwiched FZIBs, and the vital roles of carbon materials have been elaborated as conductive materials and current collectors for cathodes,current collectors and host materials for Zn anodes,and modifiers for functional separators (Fig. 1). Especially, the construction strategies of carbon-based FZIBs and the enhanced flexibility and electrochemical performance are emphasized. Finally, prospects on carbon materials for advanced FZIBs are also briefly discussed.

2 Configurations of FZIBs

Before we introduce the progress of various carbon materials for FZIBs, 1D cable-shaped FZIBs, 2D planar FZIBs, and 3D sandwiched FZIBs are firstly clarified based on the configurations of FZIBs. Obviously, different strategies should be applied to construct specific configuration of FZIBs. Therefore, it is necessary to briefly introduce the unique features and differences of the three configurations.

2.1 1D cable-shaped FZIBs

Generally, 1D cable-shaped FZIBs can be categorized into the coaxial type and twisted structure according to the connection pattern of cathodes and anodes[22]. In coaxial FZIBs (Fig. 2a), the core electrode is usually wrapped by a separator, which is further as a substrate for the synthesis of outer electrode, resulting in a core-shell structure. As for the twisted structure (Fig. 2b), a cathode fiber and an anode fiber are intertwined together into a double-helix structure[28].Notably, the key to construct such 1D cable-shaped FZIBs is to prepare 1D Zn anodes. Obviously, commercial Zn fibers can be directly employed as core anodes, however, metal Zn fibers may suffer from metal fatigue after repeated bending cycles. Alternatively,Zn on conductive fiber-shaped substrates can be fabricated via electrochemical deposition route, and in this regard, conductive carbon nanomaterials undoubtedly play great roles to construct 1D cableshaped FZIBs. Importantly, the cable-shaped FZIBs could be integrated with elastic substrates to achieve excellent stretchability (Fig. 2c). Meanwhile, 1D cable-shaped FZIBs usually display miniaturization,light weight, and exceptional flexibility, and they can be deformed into any states and even woven into fabrics, making them suitable to construct wearable and flexible electronics (Fig. 2d). Such configuration of FZIBs can be further integrated with industrial textile technologies, showing promising application in future wearable devices[29-32].

2.2 2D planar FZIBs

The 2D planar FZIBs can be fabricated on a variety of flexible substrates (e.g., cloth, polyethylene terephthalate (PET), and paper) for the construction of highly flexible structures. For a 2D planar battery, the cathode and anode are positioned side by side with insulated gaps lying between adjacent finger microelectrodes, avoiding the usage of additional separators(Fig. 2e)[33]. Furthermore, electrolytes are placed on the top of or embedded in microelectrodes, which significantly provides large active area and reduces ion transfer resistance[34]. Moreover, electrolyte ions transfer in a transverse direction, enabling efficient ion diffusion even with the increase of the battery thickness.Meanwhile, several 2D planar batteries can be easily connected in serial or in parallel on planar substrates,favoring the achievement of high energy and power density for powering ultrathin energy-consuming devices (e.g., electrical display screen) (Fig. 2f)[35]. In addition, the 2D planar battery could be integrated with microelectronics[36](e.g., solar cells[37],sensors[38]) to achieve the generation and use of power without delay for extended promising applications[39].By virtue of the unique interdigital structure and flexible substrates, the 2D planar FZIBs possess great mechanical robustness and simultaneously avoid the short-circuit issues under deformation conditions.

2.3 3D sandwiched FZIBs

3D sandwiched FZIB is one of the most widely applied configurations. In this structure, liquid electrolytes in separators or solid electrolytes are sandwiched between cathode and anode (Fig. 2g), and the face-to-face contact mode between electrodes and electrolyte realizes efficient ion diffusion and endows stable interfaces[33]. Moreover, sandwiched FZIBs could be easily stacked face to face to power various electronics (Fig. 2h). Theoretically, the 3D sandwiched FZIBs require the introduction of flexible electrodes or electrolytes. Flexible and lightweight carbon substrates are generally employed for the direct growth of active materials to construct flexible electrodes, while in traditional ZIBs, heavy and smooth metal current collectors (e.g., stainless steel foil, Ti foils) are usually used, in which active materials tend to fall off metal substrates under deformations due to poor interface interaction. Moreover, to ensure the stable connection between active materials and current collectors, free-standing and flexible carbon based films are frequently employed for FZIBs without the employment of polymer binders or conductive additives.

The 1D cable-shaped, 2D planar, and 3D sandwiched FZIBs show glimpses of a future where wearable and portable electronics powered by FZIBs could fulfill the practical and customized requirements in our daily life.

3 Carbons in cathodes

Various conductive carbon materials can be paired with cathode active materials (e.g., MnO2,V2O5) for the construction of FZIBs, in which synergistic effects of both components can be expected to enhance electrochemical performance and exceptional flexibility. To be specific, the vital roles of carbon materials in cathodes lie in the following two aspects:

(1) Conductive materials. The intimate connection between carbons and active materials endows the composite cathodes with rapid electron transfer pathways and robust structure to accommodate volume variation during Zn2+insertion/extraction processes.Moreover, carbon materials could be assembled with active materials to obtain free-standing binder-free films for highly flexible cathodes and FZIBs.

(2) Current collectors. They could not only accelerate electron transfer but also ensure high mechanical flexibility due to the presence of flexible and conductive substrates. Notably, lightweight carbon-based current collectors have the ability to accommodate high-mass-loading active materials (e.g., 15 mg cm-2),which helps to achieve high-energy-density cathodes for FZIBs.

To fully understand the roles of carbons in different battery configurations, various cathodes in FZIBs are summarized in Table 1.

Table 1 A summary of carbon based cathodes for FZIBs.

3.1 Conductive materials

3.1.1 Carbon nanotubes

1D carbon nanotubes with high conductivity(～5 × 105S m-1) are widely applied to integrate with active materials[69,70]. Notably, CNTs can be intertwined into interconnected conductive networks,forming free-standing and flexible films without the need of extra conductive additives and polymer binders, and thus the uniform dispersion and the utilization of active materials are greatly improved.

Generally, CNTs could be directly vacuum filtrated with active materials to obtain free-standing and binder-free composite cathodes. Typically, VO2(B)nanofibers with a length of a few microns were reported to be vacuum filtrated with multiwalled CNTs(MWCNTs) for free-standing VO2(B)-MWCNT films(Fig. 3a), which were further treated with laser engraving process for finger electrodes. The constructed 2D planar FZIBs delivered a favorable energy density of 188.8 μWh cm-2at a power density of 0.09 mW cm-2and realized high mechanical and heat stability under bending states (0°-150°) (Fig. 3b) and a temperature range of 25-100 °C (Fig. 3c)[25]. V2O5/CNT porous composite films were fabricated as flexible cathodes through vacuum filtration, in which V2O5nanofibers were interpenetrated in the CNT network for enhanced electron/ion transfer. As expected,a high diffusion coefficient of Zn2+(～10-8cm2s-1)was achieved and a reversible capacity of 390 mAh g-1was delivered at 1 A g-1, much higher than pure V2O5(263 mAh g-1)[71]. Moreover, metal ions or water molecules pre-insertion is considered as an effective method to enlarge interlayer spacing for enhancing structural integrity[10]. For example, CuV2O6nanobelts were hydrothermally synthesized and subsequently vacuum filtrated with reductive acidified CNTs (RCNTs) featuring enhanced conductivity and hydrophilicity, achieving a free-standing and binderfree composite membrane. High-aspect-ratio CuV2O6and robust RCNT network can be homogeneously and tightly interconnected, resulting in highly flexible films that can be bent without any obvious cracks[43].Moreover, rGO/NaxV2O5·nH2O (rGO/NVO), CNTs,and cellulose fibers were assembled into a free-standing nanocomposite paper with a thickness of～100 μm. Specially, the rGO/NVO possessed a large interlayer spacing of 1.06 nm due to the presence of Na+and water molecules, which could stabilize the V2O5crystal structure and ensure rapid Zn2+diffusion.Meanwhile, the continuous CNT framework enabled a high conductivity of 784 S m-1for the nanocomposite,and the cellulose fibers functioned both as a building block to realize flexible thick film electrodes and electrolyte reservoir for rapid ion transfer. Consequently,the paper cathode exhibited a high tensile strength of 8.51 MPa, and the assembled rGO/NVO based ZIB delivered a high areal capacity of 1.87 mAh cm-2even at a high mass loading of 5 mg cm-2[72]. Similarly,single-walled CNTs (SWCNTs) and hydrophilic cellulose nanofibers could be assembled into a 3D bicontinuous heterofibrous network scaffold (HNS), which enabled the uniform distribution of active α-MnO2rods in the HNS and the construction of free-standing MnO2/HNS cathodes through vacuum filtration and freeze-drying process. Significantly, the porous and conductive HNS network provided continuous electron/ion transfer pathways to achieve excellent mechanical flexibility and simultaneously accelerated redox kinetics, thus endowing the corresponding ZIBs with an unchanged electronic resistance after 5 000 bending cycles, an excellent rate capability of 129 mAh g-1at 20 C, and a high energy (Eelectrode)/power density(Pelectrode) of 91 Wh kg-1/1 848 W kg-1[73]. Besides, proton-type birnessite H0.08MnO2·0.7H2O nanobelts possessing a high aspect ratio of ～30 were reported to integrate with MWCNTs by vacuum filtration, and the as-fabricated binder-free H0.08MnO2·0.7H2O/MWCNT film possessed robust structure and abundant macropores (0.5-5.0 μm) to avoid the self-stacking of active materials, alleviate volume variations, and facilitate the infiltration of electrolyte ions into electrodes, and thus enabled a high Zn2+diffusion coefficient of ～8.18 × 10-14cm2s-1and a long-term life up to 1 000 cycles at 3 A g-1[44].

Moreover, other physical assembly strategies(e.g., spray printing, wet spinning, blade coating and rolling assembly) are also developed to enable the successful construction of FZIBs. Typically,KV3O8·0.75H2O (KVO) nanobelts with the V3+/V5+redox reaction and large interlayer spacing were hydrothermally prepared, which were subsequently subjected to assembly with SWCNTs in a spray printing process to achieve free-standing KVO/SWCNT composite films featuring a thickness of 8 μm. KVO nanobelts were well distributed in the CNT network, resulting in a conductive structure and exceptional flexibility for the composite film. Notably, the film can be bent to 180° without any cracks and structural damage. Simultaneously, the 3D sandwiched FZIBs could deliver almost an unchanged discharge capacity of～210 mAh g-1at 1 A g-1for 200 cycles under either flat or bent conditions[45]. In addition, wet spinning is a low-cost and scalable strategy to fabricate fibershaped electrodes. For instance, CNTs, cellulose nanofibers (CNFs), and MnO2nanowires could be mixed as viscous spinning inks, and after wet spinning in the ethanol coagulation bath, the composite cathode was constructed, in which CNTs and CNFs with high aspect ratio and large specific surface area (SSA) were interconnected into a binary fibrous network for the uniform interpenetration of MnO2nanowires (Fig. 3d,e). As demonstrated, the 1D fiber-shaped FZIB delivered high gravimetric and volumetric energy densities (Eelectrode) of 47.3 Wh kg-1and 131.3 mWh cm-3at 42.2 W kg-1at 117.3 mW cm-3, respectively(Fig. 3f), and achieved excellent flexibility to maintain a high capacity retention of 92% after 4 000 bending cycles at the largest bending angle of 90°.Moreover, the flexible fiber electrodes could be bent,looped, typed, and twisted (Fig. 3g), thus endowing the 1D FZIBs with favorable flexibility and mechanical strength to be woven into flexible textiles for practical applications (Fig. 3h)[40]. Besides, blade coating and rolling assembly hold great promise for the fabrication of all-in-one integrated ZIBs with high flexibility. Niu and co-workers[46]developed polyaniline(PANI) nanorods that could be cross-linked with SWCNTs, and the resulting composite slurry was spread over a polyimide substrate through blade coating to obtain porous PANI/SWCNT composite films with a high conductivity of 1 730 S m-1. Significantly, the unique integrated structure of combining all components into one monolith reduced ion transfer distance and ensured structural compatibility and intimate interfacial connection. Correspondingly, the 3D all-inone FZIB showed an elastic strain of ～12.9% with a Young’s modulus of ～34.5 MPa, and could deliver stable capacity under various stretching conditions.Moreover, the constructed FZIBs could be tailored into various desired shapes for the exploration of stretchable, editable and shape-customized configurations.

Chemical assembly can construct a composite structure of CNTs and active materials with strong adhesion, and thus obtaining highly flexible and stable cathodes. Typically, after acid treatment, hydrophilic CNTs acted as a template to reduce KMnO4and as a conductive substrate to support the uniform dispersion of amorphous MnO2nanosheets. The synergy of the porous CNT network and amorphous MnO2with the highly disordered structure provided more active sites and accelerated ion diffusion and reaction kinetics, endowing the free-standing a-MnO2@CNT foams with attractive rate performance, such as a reversible specific capacity of 69.5 mAh g-1at 97.4 C. Significantly, the FZIB delivered a stable specific capacity of 180.7 mAh g-1at 6.5 C under flat state, and maintained 179.0 mAh g-1under various bent conditions,indicative of high mechanical flexibility[47]. Similarly,MnO2nanosheets were hydrothermally assembled on the surface of CNTs to construct a cross-linked hierarchical framework, and the resulted CNT@MnO2ink was 3D-printed on substrates in a precise manner for 2D planar FZIBs, which delivered a stable reversible capacity of 63 μAh cm-2at 0.4 mA cm-2and maintained 97.28% of initial capacity under bending states[41]. In addition, bilayered NaxV2O5·nH2O (NVO)nanobelts with Na+ions and water molecules as pillars embedded in V2O5framework possessed a larger interlayer spacing of ～1.1 nm than pure V2O5(～0.44 nm), thus accommodating more Zn2+and favoring reaction kinetics. CNT networks as structuredirection agent could guide the hydrothermal growth of NVO nanobelts by providing active sites (e.g., oxygen-containing groups and defects) on the surface of CNTs. In the resulted free-standing and binder-free NVO@CNTs composite films, active NVO nanobelts and conductive CNTs were interwoven and well distributed to form a porous and conductive structure, enlarging the contact area between active materials and electrolytes, and enhancing reaction kinetics. As expected, the NVO@CNT composite delivered a high capacity of 330.5 mAh g-1at 2 A g-1at a high mass loading of 12 mg cm-2[74].

Besides, some polymers (e.g., polydopamine,polyaniline) can be self-polymerized on the surface of CNTs with strong interaction, thus achieving stable interface between CNTs and active materials to withstand repeated charge/discharge and various deformations. For instance, polydopamine (PDA) possesses both hydrophilic nature for fast ion diffusion at electrode/electrolyte interface and low solubility in aqueous solutions to avoid the dissolution issues. To further stabilize the structure and enhance conductivity, CNTs were selected as substrates for the selfpolymerization of dopamine, resulting in a robust interface with strong adhesion. After vacuum filtration,the PDA coated CNTs with a thickness of 8 nm was well-interwoven into a flexible and high-rate PDA/CNT membrane. Therefore, based on catechol/ortho-quinone redox reactions and Zn2+adsorption/desorption, the PDA/CNT cathode delivered a high capacity of 126.2 mAh g-1and maintained 96%of initial capacity after 500 cycles[75]. More interestingly, highly conductive CNTs with length of hundreds of nanometers can be modified with various oxygen-containing functional groups, -OH in particular, to enhance wettability and Zn2+storage capability by introducing chemical adsorption into original electric double-layer capacitance. Furthermore, PANI with active redox sites and high chemical stability was in situ polymerized on the surface of hydroxylated CNTs (h-CNT), resulting in a h-CNT/PANI composite cathode. The integration of capacitive chemical adsorption of h-CNTs with redox reactions on PANI endowed the h-CNT/PANI composite cathode with a superior capacity retention of 49% as the current density increased from 0.1 to 5 A g-1to pure PANI cathode (6.2%). Moreover, the constructed FZIB delivered a capacity retention of 90% after bending for 600 cycles, which sheds light on the great flexibility benefiting from the presence of CNT network[48].

3.1.2 Graphene

Graphene nanosheets possess a unique 2D ultrathin structure and physicochemical properties, such as superior conductivity (～107 S m-1), large SSA(～2 630 m2g-1), and exceptional mechanical strength[6,76], which are promising conductive materials for advanced FZIBs with enhanced battery performance[50,77]. Moreover, nano graphene could assemble with active materials into conductive and flexible composite films, which avoids the self-stacking of graphene nanosheets and agglomeration of active materials[78]. Most importantly, graphene nanosheets can be easily assembled into 1D fibers, 2D films and 3D aerogels. With the rapid development of graphene industry, advanced and low-cost graphene based hybrids are expected to be massively produced for practical applications.

Vacuum filtration is also frequently employed to achieve graphene based composite cathodes with enhanced reaction kinetics and desirable flexibility. As a typical example, 1D ultralong MnO2nanowires with high aspect ratios of above 1 000 were reported to be assembled with rGO nanosheets without restacking to obtain a lightweight MnO2/rGO hybrid membrane(Fig. 4a). Notably, the 1D/2D hybrid structure with a high SSA of 159.9 m2g-1not only reduced line-toface ion pathways for fast reaction kinetics, but also enabled the films with extraordinary mechanical flexibility, such as abilities to bear bending, rolling, and multiple folding (Fig. 4b). Consequently, the constructed FZIBs could operate stably and exhibit unchanged reversible redox reactions under flat, bent or even folded conditions (Fig. 4c)[49].

The chemical assembly of rGO and vanadium or manganese based oxides can be achieved through calcination of graphene oxides (GO) and vanadium or manganese based precursors. Recently, amorphous V2O5grew on both sides of graphene nanosheets through freeze-drying of VOC2O4and GO and subsequent calcination (Fig. 4d), resulting in a flat and ultrathin 2D A-V2O5/G heterostructure (Fig. 4e), which ensured superior structural stability and offered ion diffusion tunnels for fast electron/ion transfer.Moreover, amorphous V2O5with a disordered structure provided abundant active sites for H+/Zn2+insertion/extraction. Accordingly, a 2D planar FZIB was constructed by a vacuum filtration method (Fig. 4f),which delivered an excellent energy density of 21 mWh cm-3at 526 mW cm-3, and three FZIBs connected in series and parallel exhibited stable charge/discharge processes (Fig. 4g-i), demonstrative of great potential in miniature electronics[42]. Similarly, active VO2could be decorated on graphene framework through freeze-drying and subsequent calcination of a mixture of GO and NH4VO3, during which GO was reduced to rGO and meanwhile NH4VO3was converted to VO2via the reaction of NH4VO3+ C → NH3↑ + VO2+ CO2↑ + H2O↑. After mechanical compression, the VO2/rGO composite film was obtained, which possessed both a high SSA of 251 m2g-1for the easy infiltration of electrolyte ions and a porous structure to ease volume variations caused by ion insertion/de-insertion. The robust VO2and conductive and porous rGO network synergistically improved the electrochemical performance of 3D sandwiched FZIBs, such as reversible capacities of 260 and 194 mAh g-1at 0.2 and 8 A g-1, respectively,and stable operation for 200 cycles without obvious capacity decay at varied bending radius from 4.0 cm to 1.0 cm, demonstrative of great potential in wearable electronics[50]. In addition, through calcination of a mixture of MnO2and GO, MnO were densely decorated between graphene layers to form accordion-like MnO/G heterostructures. The graphene-MnO interphase in MnO/G connected with covalent bonds ensured structural stability and provided rich active sites for reduced Zn2+adsorption energy and favorable Zn2+storage due to the formation of oxygen vacancies in disordered MnO, thus resulting in a high capacity of 398.5 mAh g-1at 0.1 A g-1[51]. In addition, layered MoS2was also reported to be integrated with graphene to construct sandwich-structured MoS2/graphene nanosheets through electrostatic assembly and subsequent hydrothermal reduction, in which the presence of graphene effectively enlarged the interlayer of MoS2from 0.62 to 1.16 nm. Importantly, the synergy of graphene and MoS2resulted in enhanced hydrophilicity and high structural stability, and thus enabled high capacities of 285.4 and 141.6 mAh g-1at 0.05 and 5 A g-1, respectively, and a high capacity retention of 88.2% after 1 800 cycles[77].

3.2 Current collectors

3.2.1 Carbon nanotubes

Individual CNTs can be highly aligned into interconnected network featuring a robust structure, high conductivity, and high mechanical strength, which is desirable for current collectors in flexible cathodes.Typically, molten V2O5under the heating temperature of 800 °C could intimately infiltrate the porous framework of CNT films (CNTFs), forming CNTF/V2O5films with high chemical stability and compact structure. Notably, the structurally stable and flexible CNTF@V2O5composite film would not re-disperse in water even under ultrasonication for 1 h and could sustain structural integrity without cracks under frequent folded or crumped states. As a result, the corresponding ZIB delivered a reversible capacity of 356.6 mAh g-1at 0.4 A g-1and high capacity retention of 80.1% after 500 cycles at 2.0 A g-1[79].Moreover, Xu and co-workers[56]electrodeposited MnO2nanoflakes on CNT films to obtain a binderfree CNT@MnO2film, in which MnO2nanoflakes were densely and uniformly distributed on the surface of CNT threads. The synergy of conductive CNT substrate and nanostructured MnO2contributed to highperformance of 3D Zn//MnO2flexible batteries, such as a high energy density up to 16.5 mWh cm-3at 0.2 mA cm-2and long cycling life over 1 000 times without obvious capacity decay. Furthermore, CNT layers were reported to be coated on the surface of flexible textiles by a dipping-drying method to dramatically enhance the conductivity to 10-102S m-1,and then active interconnected MnO2nanosheets were electrodeposited on the modified textiles. The textile electrodes with a 3D hierarchical branched structure enabled the Zn//MnO2battery to deliver a desirable energy density of 12 mWh cm-3at 13 mW cm-3, and capacity retention of over 80% after soaking in water for 200 min[57].

More interestingly, interconnected CNT bundles can assemble into 3D porous CNT foams as conductive frameworks to provide sufficient surface for the uniform growth of MnO2nanoparticles by KMnO4etching (Fig. 5a). Notably, as-fabricated MnO2/CNT composite foam not only exhibited a high porosity of 98.4% to effectively buffer the volume variations and offer ion transfer pathways during MnO2/Mn2+redox deposition/dissolution processes(Fig. 5b), but also withstood 70% strain compression for repeated 200 times, indicative of extraordinary robustness and flexibility. Based on both MnO2/Mn2+and MnO2/Mn3+redox reactions due to the presence of 0.005 mol L-1Mn2+additive in the mild aqueous ZnSO4electrolytes, the Zn//MnO2battery with a high discharge plateau displayed an energy density (Ematerial)of 602 Wh kg-1(Fig. 5c), and ultralong lifespan of 16 000 cycles. Moreover, the assembled 3D sandwiched FZIB could maintain 83.5% of initial capacity after 2 500 cycles (Fig. 5d), and 2 connected batteries in series with an open-circuit of 3.8 V could power a rotor, indicative of the great potential for practical applications[58].

Besides, CNT based fibers possess superior mechanical strength, high conductivity, and light weight, which hold great promise in the construction of 1D FZIBs. Specially, CNT bundles were firstly prepared on Si wafers with the help of a Fe film catalyst through a typical floating catalytic chemical vapor deposition (CVD) method, which were further wound into CNT fibers with a diameter of 80-100 μm by dryspinning. Subsequently, the as-fabricated CNT fibers were employed as current collectors for the electrochemical deposition of intertwined ε-MnO2nanosheets, and the diameter of MnO2@CNT fibers increased by 20 μm after deposition. A 1D cable-type FZIB was constructed using a MnO2@CNT fiber cathode, Zn wire anode, and ZnCl2based gel polymer electrolyte, which delivered a capacity of 290 mAh g-1at 0.1 A g-1and an energy density (Ematerial) of 360 Wh kg-1at 100 W Kg-1. More importantly, the 1D FZIB could realize stable charge/discharge process during 100 bending cycles[52]. Moreover, Wang and co-workers[53]twisted CNT belts to develop oxidized CNT fibers with hydrophilic surface for the uniform growth of 1D CNTs stitched 2D Zn3(OH)2V2O7·2H2O(ZVO) nanosheets (Fig. 5e), which exhibited exceptional structural stability with a tensile strength of 280 MPa and enhanced conductivity of 98 S cm-1.Specially, the interconnected CNT network and the open frameworks of layered ZVO jointly endowed the 1D fiber-shaped FZIB with a desirable volumetric density (Edevice) of 71.6 mWh cm-3and a desirable capacity retention of ＞96% after repeated 1 000 bending times. Moreover, the presence of flexible CNT fibers enabled the 1D fiber-shaped FZIB to be woven into various flexible fabrics and bent at different states without obvious capacity decay (Fig. 5f, g), showing their great promise in wearable electronics. In addition, ultrathin Co3O4nanosheets (Co3O4NSs) were proposed to be aligned on robust CNT fibers (CNTF)through in situ oxidation and etching of 3D Co-based metal organic framework arrays, and the resulting Co3O4NSs@CNTF showed exceptional conductivity,which endowed the 1D fiber-shaped FZIB with a desirable reversible capacity of 158.70 mAh g-1at 1 A g-1and excellent capacity retention of 97.27%after 10 000 cycles. More interestingly, the as-constructed flexible batteries could be knitted into a sweater and be connected in series to power a watch[54]. Besides, spherical zinc hexacyanoferrate(ZnHCF) with particle sizes of less than 300 nm were arranged on compact CNT fibers, and the as-fabricated ZnHCF@CNT composite was served as outer cathodes to couple with Zn nanosheet arrays on a CNT fiber core, giving birth to a high-voltage coaxial 1D FZIB, which achieved a reversible capacity of 100.2 mAh cm-3at 0.1 A cm-3, an energy density of 195.4 mWh cm-3at 0.2 W cm-3, and a high capacity retention of 93.2% after 3 000 bending times[55].

CNTs with large aspect ratios are easily assembled into 1D CNT fibers, and thus are widely applied in 1D FZIBs[80]. CNT fibers as current collectors possess high conductivity to ensure rapid electron transfer, and meanwhile the functional groups on the surface of CNTs can enhance hydrophilic nature and interface stability between CNT based current collectors and active materials but would slightly sacrifice conductivity. Taking conductivity and stable structures into consideration, CNT fibers with both conductive core and functional groups may be a rational choice for the construction of robust electrodes and flexible FZIBs in the near future.

Despite the great progress of CNTs for FZIBs,the precise synthesis, assembly, and functionalization of CNTs are still challenging. Moreover, some important properties (e.g., high conductivity) should be ensured during their applications, meanwhile, CNTs may possess sufficient wettability with aqueous electrolytes for easy infiltration.

3.2.2 Carbon fibers

Generally, carbon fibers can be interwoven into carbon fiber cloth (CC) or be assembled into carbon nanofiber membrane by electrospinning. Carbon fiber cloth or membrane could be employed as light-weight and high-surface-area current collectors to load active material[81,82].

Typically, high-capacity layered VSe2nanosheets on 1D N doped carbon nanofibers (N-CNFs) were synthesized by electrospinning, carbonization/oxidation, and selenization. The resulting core/shell nanostructures with strong interfacial interaction displayed high mechanical flexibility and excellent electrochemical properties. Moreover, to further stabilize the composite structure and alleviate dissolution issues of active materials during charge/discharge processes,atomic layer deposited Al2O3layers with a thickness of ～5 nm uniformly covered VSe2nanosheets(Fig. 6a), forming Al2O3@VSe2NSs@N-CNFs films with an excellent tensile strength of 230 MPa. Accordingly, the 3D sandwiched FZIB showed a stack energy density of ～125 Wh kg-1and high capacity retention of 86.2% after 2 500 cycles (Fig. 6b). Interestingly, the FZIBs could power an integrated soft robot to walk on land and even in water (Fig. 6c), which exploit multifunction ZIBs and broaden the applied fields of FZIBs[59].

In addition, directly growing active materials on CC substrates could achieve a stable structure and avoid the use of polymer binders (e.g., polyvinylidene difluoride (PVDF)) in traditional cathodes. For example, metastable layered VO2(B) can store Zn2+based on Zn2+insertion/de-insertion. Carbon fiber sheets (CFSs) were reported as current collectors for the uniform and dense dispersion of VO2(B) arrays with a mass loading of 4.5 mg cm-2, achieving a flexible VO2(B)@CFS composite. The stable adhesion between VO2and CFSs enabled two FZIBs connected in series to light up a small LED lamp for 150 h under different bending degrees ranging from 0° to 180°[60]. Moreover, VS2nanosheets with a thickness of 180 nm were densely grown on conductive CC current collectors with a high mass loading of 15.1 mg cm-2for robust and flexible VS2/CC cathodes by a facile hydrothermal strategy. With the use of self-healing poly(vinyl alcohol) (PVA)/Zn(CH3COO)2/Mn(CH3COO)2hydrogel electrolytes, the 3D sandwiched FZIB showed stable capacities of 124, 125 and 114 mAh g-1at 50 mA g-1after 30 cycles at bending angles of 60°, 90° and 180°, respectively[61]. Besides, a hydrated zinc vanadium oxide/carbon cloth(ZnVOH/CC) composite was developed, in which Zn-VOH nanosheets were uniformly and tightly anchored on the surface of CC by hydrothermal treatment and subsequent in situ electrochemical transformation.The strong connection between CC and ZnVOH ensured high structural stability and effectively suppressed the aggregation and dissolution issues of active materials. Meanwhile, the presence of structural water and lattice defects in 2D ZnVOH nanosheets provided abundant active sites and favored rapid reaction kinetics. Consequently, the FZIB delivered a reversible capacity of 184 mAh g-1at 10 A g-1after 170 cycles, and could work at rolled, folded or punched states[62].

Besides vanadium-based composites, manganese oxides can also be loaded on CC current collectors.For example, Tan et al.[63]used an in situ carbonization strategy of Mn-metal organic framework to synthesize oxygen defective Mn3O4@C nanorod arrays on CC substrates (Od-Mn3O4@C NA/CC), in which oxygen defects not only adjusted the Mn3+charge density for improved conductivity but also changed the MnO6octahedral structure, and thus enhanced structural stability and suppression of Mn dissolution could be simultaneously achieved. Moreover, the resultant carbon skeleton offered a conductive network for rapid electron transfer. Consequently, the corresponding ZIB achieved a long-term lifespan of 12 000 cycles and a capacity retention of 95.7% at 5 A g-1.Moreover, with acetonitrile as a carbon source and MgO as a template, cross-linked N-doped carbon nanosheets (N-CNSs) could be grown on CC current collectors, which functioned as conductive and porous skeleton for the intimate decoration of MnO2nanoflakes for integrated N-CNS@MnO2arrays with a unique core/shell structure and extraordinary flexibility (Fig. 6d-f). The cross-linked and robust N-CNS arrays could not only provide large surface area to accommodate sufficient MnO2and abundant ion tunnels to favor Zn2+diffusion, but also enrich active sites and enhance electron transfer by introducing N dopant. As expected, the 3D sandwiched FZIB delivered a high energy density of 352.5 Wh kg-1at 542.4 W kg-1(Fig. 6g), and can power LED lamp at normal, bent,and twisted states (Fig. 6h)[64]. Besides, zeolitic imidazole framework-67 (ZIF-67) that hydrothermally grown on CC could be converted to ultrathin MnO2nanosheet based hollow polyhedrons. The resultant robust MnO2/CC composite exposed more active sites and reduced ion diffusion pathways, and thus the corresponding FZIB showed a reversible capacity of 91.7 mAh g-1at 0.1 A g-1and powered LED bulbs under bent states[65]. More interestingly, through the selfpolymerization of dopamine, PDA layers stably adhered to CC substrates due to the presence of abundant catechol groups. Further, ultrathin δ-MnO2nanosheets were in situ grown on CC with strong interaction through the reaction between polydopamine derived N doped carbon and KMnO4(4KMnO4+3C + H2O → 4MnO2+ K2CO3+ 2KHCO3). It should be noted that the remaining unreacted carbon network could effectively improve structural stability and accelerate electron transfer. Therefore, the free-standing C-MnO2@CC cathode could enable the 3D sandwiched FZIB to be bent at different angles without obvious capacity decay and deliver a high energy density of 1.34 mWh cm-2at a power density of 2.95 mW cm-2[66]. Furthermore, Chen et al. reported the hydrothermal growth of MnO2on carbon fibers (CFs)and the resulting MnO2/CF cathodes were further assembled with a Zn/CF anode and a PVA/ZnCl2-MnSO4polymer electrolyte to construct a FZIB,which exhibited a high tensile strength of 293 MPa and an energy density (Ematerial) of 181.5 Wh kg-1at 0.31 kW kg-1[67]. Besides, Cu2+pre-intercalated δ-MnO2(CuMO) nanowires were hydrothermally fabricated on CC, in which Cu2+could not only regulate electronic bandgap and ion state for enhanced electron/ion transfer but also stabilize the MnO2framework through the ionic bonds with oxygen atoms, thus enabling reversible and fast H+/Zn2+insertion/extraction processes and favorable capacity retention of 90.1% after 700 cycles at 5 A g-1[27].

3.2.3 Graphene

Graphene nanosheets could assemble into 2D flexible free-standing films or 3D porous aerogels[83],which could be further employed as light-weight current collectors to load active materials for high-energy-density FZIBs. As a typical example, with the assistance of PVDF, graphene was interconnected into a flexible graphene film, which was further treated in HCl solution to generate abundant oxygen-containing functional groups on the surface of graphene. More interestingly, the graphene film could not only act as a current collector for enhanced electron transfer, but also function as a reducer in the reaction of H2O +3C + 4MnO4-= 4MnO2+ 2HCO3-+ CO32-, resulting in free-standing MnO2needles independently grown on functionalized graphene films (FSM@FGF). The flexible cathode with enhanced charge diffusion endowed the FZIB with a capacity of 145.6 mAh g-1at 0.3 A g-1and stable cycling performance under flat and folded states in water, demonstrative of great flexibility and water-proof properties for further practical applications[68].

4 Carbons in anodes

In traditional ZIBs, commercial Zn foils are frequently employed as both current collectors and Zn anodes simultaneously. Because excessive Zn is usually used, the energy density of traditional ZIBs is not so high. Meanwhile, conventional Zn foils with unsmooth surface in aqueous ZIBs suffer from serious dendrite growth caused by nonuniform electric fields/Zn2+fluxes and possible side reactions between electrolytes and Zn[84]. To address the above challenges, carbon materials with excellent physicochemical properties are introduced to remarkably improve Zn anodes, and several roles of carbon materials can be summarized as follows:

(1) Current collectors for electrochemically deposited Zn. Lightweight and robust carbon materials can greatly improve energy density and enable high mechanical flexibility for FZIBs in comparison with excessive and rigid Zn foils.

(2) Host materials. Carbon materials with high conductivity and large SSA could reduce nucleation barriers, homogenize electric fields, and provide abundant zincophilic sites to induce uniform Zn nucleation and growth for dendrite-free Zn anodes.

The key roles of carbon materials in improved FZIBs are shown in Table 2.

Table 2 A summary of carbon based anodes for FZIBs.

4.1 Current collectors for Zn

4.1.1 Carbon nanotubes

Hollow multiwalled CNT arrays could be interwoven into a 3D conductive and porous CNT network on CC skeleton by a CVD method, which further acted as a robust current collector for Zn plating.CNT skeleton with a high conductivity and high SSA enabled a low nucleation overpotential and more homogeneous electric fields for dendrite-free anodes(Fig. 7a), which endowed the Zn//Zn symmetrical batteries with a reduced voltage hysteresis of 27 mV and long lifespan of 200 h at 2 mA cm-2, together with a high depth of discharge of 28% (Fig. 7b), and the Zn//MnO2battery with a capacity of 167 mAh g-1after 1 000 cycles (Fig. 7c). Moreover, the 3D Zn//MnO2battery with a high mechanical flexibility recorded high capacity retention of ＞97% under flat,bent, or twisted states (Fig. 7d)[86]. In addition, Zn nanosheets (Zn NSs) could be electrodeposited on CNT fibers, and the Zn NSs@CNT fiber anode could offer abundant active sites, large surface area, and reduced ion diffusion path for reversible Zn/Zn2+reaction kinetics and long cycling lifetime[53].

4.1.2 Carbon fibers

3D lightweight CC features high conductivity,desirable mechanical flexibility, and large SSA, and is considered as a promising current collector for electrodeposition of 1D Zn nanowires[61]and 2D Zn nanosheets. For example, Qian et al.[87]adopted activated carbon cloth (ACC) as a substrate for the vertical growth of zincophilic Cu nanosheets under a constant potential of -0.7 V for 600 s, which increased SSA from 36.85 m2g-1of the bare ACC to 100.81 m2g-1and favored the subsequent uniform Zn deposition for dendrite-free Zn@Cu@ACC anodes. The as-fabricated anodes could provide abundant active sites, accelerate charge transfer, and reduce the nucleation overpotential to 76 mV in comparison with ACC(214 mV), thus enabling the Zn//MnO2battery to maintain a high capacity retention of 94.8% and Coulombic efficiency of 97.9% after 1 000 cycles at 1 A g-1.

Moreover, Yuan and co-workers[88]selected commercial CC as a current collector for Zn nanoflake deposition, which further acted as a substrate for the encapsulation of polyacrylamide (PAm) hydrogel matrix through the cross-linking process of acrylamide monomer with N,N′-methylene-tcis-(acrylamide).Further, PANI with a high electronic conductivity of 1.4 S cm-1was grown on the surface and embedded in the porous structure of a hydrogel host by in situ polymerization, and after it was soaked in ZnCl2electrolytes, the integrated and seamless configuration was obtained. This integrated architecture of designing batteries into an entity could not only avoid the use of excessive electrode additives and separator to enable a high energy density of 1.31 mWh cm-2at 0.29mW cm-2but also ensure stable interface between electrode and electrolytes, endowing the integrated ZIB with a high flexibility to withstand frequent 3 000 bending cycles at an angle of 120°, accompanied with a high capacity retention of 97.75%. Notably,as-fabricated PAm hydrogel electrolyte with a porous and interconnected framework possessed a high ionic conductivity of 9.93 mS cm-1at -20 °C, endowing the corresponding batteries with freeze-tolerance properties and operation at a wide temperature range from-20 to 20 °C, such as stable areal capacities of 1.148 mAh cm-2at 20 °C, 0.964 mAh cm-2at 0 °C,and 0.616 mAh cm-2at -20 °C.

4.1.3 Graphene

Moreover, 2D graphene nanosheets could be easily assembled into 3D aerogel or foam, which are considered as an ideal current collector for Zn to enable enhanced electrolyte/electrode contact area, low local current density, and reduced side reactions at interface[89,92]. Take a typical example, a flexible and ultralight Ti3C2TxMXene/graphene aerogel (MGA) was synthesized through hydrothermal assembly and freedrying (Fig. 8a, b), during which graphene nanosheets were interconnected into 3D frameworks and MXene nanosheets were uniformly decorated on the surface of graphene host. The MGA skeleton possessed a microporous structure and abundant zincophilic sites for dense electrodeposition of Zn (Fig. 8c, d). More importantly, the fluorine terminations of MXene could react with Zn to form zinc fluoride as solid electrolyte interface to induce uniform Zn nucleation for dendrite-free Zn anodes. As a result, the formation of byproducts (Zn(OH)42-) and hydrogen evolution reactions(3.8 mmol h-1cm-2) have been suppressed, and an ultralong life up to 1 000 h has been delivered at 10 mA cm-2in a symmetrical battery. Furthermore, a 3D FZIB assembled with the LiMn2O4cathode achieved an initial capacity of 110 mAh g-1and maintained a capacity retention of 90.3% at 2 C at repeated folding times (Fig. 8e), together with intact structures without cracks at the folded position(Fig. 8f, g)[21]. Besides, 3D N-doped vertical graphene nanosheets (N-VG) with a size of ～400 nm were grown on CC through plasma-enhanced CVD and subsequent N doping, which acted as current collectors for Zn deposition to obtain robust and dendritefree Zn@N-VG@CC anodes. Notably, the 3D graphene network could enable homogeneous electric fields and the introduction of zincophilic nitrogencontaining functional groups could strengthen the interfacial interaction between Zn2+and CC, thus reducing Zn nucleation overpotential to 83 mV at 5 mA cm-2and inducing uniform Zn nucleation. As a result, the flexible anodes endowed the FZIBs with a high capacity retention of more than 90% under bending and twisting states[89].

In addition, graphene featuring a high conductivity, stability, and desirable rheological properties could be screen-printed on flexible substrates for efficient electron transfer, which functioned as current collectors for the further screen-printing of highly conductive γ-MnO2ink (339 S m-1) and Zn ink(463 S m-1) as interdigital microelectrodes. As expected, the printed integrated 2D planar FZIB achieved a volumetric capacity (Eelectrode) of 19.3 mAh cm-3at 7.5 mA cm-3and a volumetric energy density of 17.3 mWh cm-3at 150 mW cm-3, together with high mechanical flexibility to sustain various deformations,demonstrative of great promise in flexible microscale electronics[85].

4.2 Host materials for Zn

Lightweight and flexible carbon frameworks with large SSA could be used as hosts of nano/microstructured Zn to homogenize electric fields and provide abundant zincophilic sites for Zn species without accumulation, thus enabling flexible and dendrite-free Zn anodes[8,93]. Moreover, carbon materials (e.g., CNTs, graphene) could assemble with nano/micro-structured Zn particles and film-forming agent (e.g., nanocellulose) to construct flexible Zn films as an alternative to commercial Zn foils. As a typical example, uniformly dispersed zinc microspheres, CNTs, and nanocellulose were reported to be assembled into a free-standing ZCN film by a facile vacuum filtration method[90]. Notably, zinc powder with a microsphere structure could offer abundant electroactive sites and 3D diffusion pathways for reversible Zn plating/stripping, and conductive CNTs tightly connected with active materials could enlarge electroactive area and enable a low nucleation barrier of 52.6 mV at 1.0 mA cm-2. Meanwhile, the introduction of nanocellulose enabled a flexible and bendable film with a strength of 3.61 MPa and a hydrophilic cathode favoring Zn2+transfer. With a layer-by-layer vacuum filtration strategy, an integrated FZIB was constructed with the cellulose fiber film as a separator and the CNT/MnO2composite film as a cathode,which demonstrated stable operation for 1 000 cycles.Similarly, Wang et al.[91]prepared free-standing Zn anodes composed of active Zn particles, conductive CNTs, and mechanically flexible poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) by a spin-coating method. Zn particles with a diameter of～13 μm can be embedded in 3D interconnected PVDF-HEP and CNT network for excellent flexibility and improved electrochemical performance, such as a high volumetric energy density (Edevice) of 8.22 mWh cm-3with a battery thickness of 0.4 mm. In addition, through a blade coating method, Zn microspheres with diameters of 3-5 μm were well dispersed on a 3D SWCNT and rGO network for a Zn/SWCNTrGO film with a high conductivity of 597 S m-1, in which rGO nanosheets had the ability to accommodate Zn species and meanwhile CNTs ensured high mechanical strength and rapid electron transfer[46]. Besides, with interfacial assembly of rGO on Zn foam and subsequent mechanical compression, graphene interpenetrated Zn (GiZn) hybrid foils were fabricated as dendrite-free anodes, in which conductive and hydrophilic rGO nanosheets provided abundant zincophilic sites to realize horizontal and uniform Zn deposition. Importantly, rGO nanosheets were well interpenetrated in Zn frameworks with strong interface,which enabled GiZn foils to be bent without breakages, showing great prospect in high-performance FZIBs[94].

5 Carbons in separators

As an essential component for a battery system,separators are used to prevent the direct contact between cathodes and anodes and thus avoid short-circuit issues. Moreover, separator engineering involving modification and functionalization is considered as a novel and efficient strategy for the construction of high-performance energy storage devices[95].

Generally, glass fiber (GF) separators are prevailing in aqueous ZIBs because of their hydrophilic nature, high ionic conductivity, and porous structure.In this respect, in situ modification of GF separators with conductive carbon materials, graphene in particular, offers an innovative avenue to improve the electrochemical performance of ZIBs. Typically, Li et al.,[96]used a plasma enhanced chemical vapor deposition (PECVD) strategy to grow 3D vertical graphene carpets on one side of GF separators to construct Janus separators with an ample porous structure and large SSA for uniform electric field and reduced local current density (Fig. 9a, b). Moreover, the introduction of oxygen and nitrogen functional groups could enhance zincophilicity to regulate homogeneous Zn2+fluxes and stabilize Zn anodes. As a result, the robust Janus separators enabled the Zn//Zn symmetrical battery to stably cycle for 300 h at 0.5 mA cm-2for 0.5 mAh cm-2(Fig. 9c), and the Zn//V2O5battery to deliver a high energy density of 182 Wh kg-1and to withstand deformation without obvious electrochemical decay (Fig. 9d).

In addition, to further obtain low-cost separators,Wu and co-workers[97]proposed a functional cellulose nanofiber/graphene oxide (CG) separator, which possessed abundant pores within a range of 10-50 nm,smooth surface, and ample zincophilic oxygen containing functional groups to build strong interaction with Zn and enable uniform Zn2+distribution for dendrite-free Zn anodes. More importantly, the CG separators could preferentially induce the hexagonal Zn growth along (002) plane owing to the low lattice mismatch of 7.4% between Zn(002)and GO(002)(Fig. 9e). As expected, the Zn//Zn symmetrical battery achieved an ultralong lifespan of 1 750 h with a low overpotential of 82 mV at 2 mA cm-2and 1 mAh cm-2, longer than counterpart with commercial glass fiber separators suffering from the limited cycling time of only 35 h (Fig. 9f). Moreover, the CG separators also enabled the 3D sandwiched flexible Zn//MnO2/graphite full battery exhibiting a high energy density of 67 Wh L-1and stable charge/discharge processes after bending for 500 times.

Despite the great progress so far, the research on carbons in separators is still in its infancy. Currently,the widely applied GF separators are fragile and possess large pores, which easily cause the occurrence of Zn dendrite growth and short circuits[98]. Moreover,little attention has been paid to the design of separator/electrolyte and separator/electrode interface, which defines the construction of FZIBs. In this respect, allin-one integrated system that combines all components into one monolith may be an effective strategy to stabilize interfaces and structures to have the ability to bear various deformations[99].

6 Conclusions and perspectives

This review summarizes the recent advancements of carbon materials (e.g., graphene, CNTs, carbon fibers) for FZIBs (e.g., 1D cable-shaped, 2D planar, and 3D sandwiched batteries), and the vital roles of carbon materials in constructing FZIBs are presented as conductive materials and current collectors for cathodes, current collectors and host materials for Zn, and functional separators. Notably, the fabrication strategy and the enhanced electrochemical performance and flexibility by carbon materials are specially emphasized. Despite great progress has been achieved, there are still much room for use of carbon materials in next-generation FZIBs.

(1) Rational selection and controllable synthesis of specific carbon materials for targeted configurations of FZIBs. Various carbon materials with unique physico-chemical properties could construct FZIBs with different configurations. For example, CNTs with large aspect ratio are easily assembled into freestanding membrane or CNT fibers, which favors the design of 1D cable-shaped, 2D planar, 3D sandwiched FZIBs. Active materials could be directly grown on CC that constructed by carbon fibers, and thus CC are widely applied as current collectors for 3D sandwiched FZIBs. Highly stable and conductive graphene nanosheets could be interconnected into a porous network, which plays an important role in the achievement of 2D planar FZIBs. Furthermore, to construct stable interface between carbon materials and key battery materials, functionalization of carbon materials, generally introducing functional groups is usually employed for good adhesion and stable Zn2+storage. Moreover, due to the use of aqueous electrolytes, carbon materials may possess appropriate hydrophilic properties in FZIBs for enhanced Zn2+insertion/extraction while not ruining reaction kinetics[100].Further, considering the construction of high-energydensity FZIBs, minimizing the mass ratio of carbon materials without sacrificing the sufficient roles of carbon materials for the flexibility and enhanced battery performance should be realized in FZIBs. In this regard, carbon material-based free-standing film electrodes may be a feasible way to effectively construct high-energy-density FZIBs. Besides, more carbon materials such as metal-organic frameworks derived carbon materials and graphdiyne also provide opportunities to construct advanced FZIBs[101-104], which have been rarely reported so far. Moreover, material genome engineering and simulation calculations may inject hopes into the development of new carbon materials and energy storage devices.

(2) Appropriate criteria to evaluate the flexibility of FZIBs should be established. Up to now, some standards such as the specific capacity, rate capability,cycling durability, energy density, and power density are widely used to evaluate the electrochemical performance of traditional ZIBs, in which the influence of both the depth of discharge and current density on the cycling life of FZIBs should not be ignored[105].Besides, with regards to FZIBs, bending, twisting, and stretching tests are generally adopted to demonstrate excellent flexibility. However, the currently existing tests and geometry parameters are different and arbitrary, causing difficult comparison with different works and slow development of FZIBs. Therefore, it is essential to provide appropriate criteria to promote the smooth progress of FZIBs. For instance, Li et al.[11]proposed that the following three geometry parameters including the binding angle (θ), the bending radius of curvature (R), and the length of device (L)should be offered to precisely describe the bending states and characterize the FZIBs. Moreover, softness may be a standard to evaluate the wearability of FZIBs in order to fulfill the practical applications[11],such as wearable devices. Besides, precise mechanical computations may be introduced to analyze mechanical changes during various deformation conditions,which could be a base for the practical application of FZIBs.

(3) Rational construction of suitable battery configurations for targeted practical applications. Carbon materials with unique features are expected to construct FZIBs with diverse functionalities, such as electrochromic ZIBs[106]and photo rechargeable ZIBs[107].To pursue the targets of FZIBs in diverse practical applications, the battery system should possess stable electrode/electrolyte interface, integrated battery structure, and superior device compatibility[108-111].Significantly, the smart 1D cable-shaped FZIBs featuring superior wearability, can be directly employed as threads and woven into various flexible fabrics, or as textile electronics through the integration with display textiles for applications[29,30]. More importantly,conventional fibers could be coated by GO layers,which may achieve reversible fusion and fission for wide-spread applications[112]. Moreover, 2D planar and 3D sandwiched FZIBs could be integrated with other electronics (e.g., solar cells, sensors, and nanogenerators) for the achievement of self-powered configuration, in which energy harvested could be directly consumed to drive the other flexible electronics.However, the exploration of FZIBs is still in the initial stage, and developing more configurations are highly desired. In addition, some properties of FZIBs are desired for practical applications. For example, the shelf life, which indicates the storage time without use while not affecting the electrochemical performance of batteries, is usually neglected in the lab level[113],and this gap between lab and industrial level should be narrowed. Moreover, battery operation in extreme conditions (e.g., in a wide temperature range or in water) may be further explored in detail[3,109,114]. Besides,with the rapid evolution of 5G era, more techniques,such as machine learning may be introduced to the accurate design of intelligent FZIBs, which would bring hopes for the advancement of energy storage devices[24].

Overall, the introduction of carbon materials yields improved battery system with enhanced electrochemical performance and excellent mechanical flexibility. To keep pace with the ever-changing requirements, strenuous efforts should be further focused on the rational selection and controllable synthesis of specific carbon materials, appropriate criteria to evaluate the flexibility of FZIBs, and rational construction of suitable battery configurations for targeted practical applications.

Acknowledgements

This work was financially supported by LiaoNing Revitalization Talents Program (XLYC2007129), the Natural Science Foundation of Liaoning Province(2020-MS-095), the Fundamental Research Funds for the Central Universities of China (N2105008), and the fund of the State Key Laboratory of Catalysis in DICP(N-21-03).