Chuan Li, Pei Li, Shuo Yang, and Chunyi Zhi,

1Department of Materials Science and Engineering, City University of HongKong, Hong Kong999077, China

2Center for AdvancedNuclear Safety and SustainableDevelopment,City University of Hong Kong, HongKong999077, China

Abstract: Flexible batteries are key component of wearable electronic devices. Based on the requirements of medical and primary safety of wearable energy storage devices, rechargeable aqueous zinc ion batteries (ZIBs) are promising portable candidates in virtue of its intrinsic safety, abundant storage and low cost. However, many inherent challenges have greatly hindered the development in flexible Zn-based energy storage devices, such as rigid current collector and/or metal anode, easily detached cathode materials and a relatively narrow voltage window of flexible electrolyte. Thus, overcoming these challenges and further developing flexible ZIBs are inevitable and imperative. This review summarizes the most advanced progress in designs and discusses of flexible electrode, electrolyte and the practical application of flexible ZIBs in different environments.We also exhibit the heart of the matter that current flexible ZIBs faces. Finally, some prospective approaches are proposed to address these key issues and point out the direction for the future development of flexible ZIBs.

Key words: flexible electrodes; flexible electrolytes; wearable zinc batteries

1.Introduction

The flexible electronic devices such as artificial electronic skins[1], implantable medical devices[2]and consumer electronics[3]show the fast development inrecentyearsdue tothe boom in wearable/flexibleenergy-storage devices[4−9].Meanwhile, theyalso request high-performance power sources with corresponding flexibility and ultrathinand safeproperties. However, current energy storage devices exhibit characteristics of toothickness andheavyhardness. Toovercome thesedrawbacks, the development of flexibility of energy-storage devices is inevitable andurgent. Inaddition, although lithium-ion batteries (LIBs)possesshighervoltageand energy density, the insecurity of organic electrolyte hinders its wearable application. The aqueous zinc ion batteries (ZIBs) exhibit high safety and abundant reserves[10−13], low standard redox potential of Zn/Zn2+of –0.76 V[14]vs.the standard hydrogen electrode (SHE), and high theoretical volume capacity of metal Zn anode of 5855 mA·h/cm3[5]. Thus, advanced flexible aqueous ZIBs are a promising way to provide an alternative to apply in many aspects.

In this review, we consider different design strategies aiming at preparing flexible ZIBs with the outstanding performances based on proposed literatures. Meanwhile, the developments of flexible current collectors, electrode materials and electrolytes are stated. Wethen discuss variouspractical application scenarios of flexible ZIBs in recent reports. Furthermore, the performancecomparison of various flexible ZIBs is summarizedtoclearly state the promising directions,facing challenges and improved alternatives.Finally, keyscientific problemslimitingthe further development of flexible ZIBs are clarified and the corresponding solutions are also proposed.

2.Schematic designs of flexible ZIBs

FlexibleZIBswere normallyequippedwiththe property of theresistanceto externaldeformation. What itmeanswas that under externalforcesfrom all directions,the electrochemical performance of flexible ZIBs was not affected or very weak. For this purpose, every component of flexible ZIBs including anode, cathode and electrolyte need to exhibit the property of softness. However, parts of traditional batteries generally were constructed employing rigid/fragilematerials.Therefore, fromelectrodes toelectrolytes,theywereall necessaryto design andprepare inreasonable way. In addition,the overall structure of the flexible ZIBs also need to be taken into consideration.

2.1.Flexible electrodes for flexible ZIBs

Asfor anodeof flexibleZIBs,thereweretwo approaches to reach this target. Firstly, thin Zn foil and Zn wire were directly used to be anode of flexible ZIBs[15,16]. However, the deformability of Zn metal was limited due to the performance of shape memory. Under the continuous bending and torsion, metal Znanodewaseasilydamagedand thuscannot beusedin many situations[17]. Secondly,applying flexiblecurrent collectors coated or electroplated Znpowder was aneffectual method, such as electrically depositing onto the surface of the carbon cloth as SEM image exhibited in Fig. 1(a)[18].

Fig. 1. (a) SEM image of the zinc anode by electrically depositing onto a carbon cloth. Adopted with permission from Ref. [18], Copyright 2019, Royal Society of Chemistry. (b) SEM image and the photographs (the inset) of the MnO2/rGO sample on carbon cloth. Adopted with permission from Ref. [19], Copyright 2018, Nature Publishing Group.

With respect to cathode of flexible ZIBs, similar to anode bonding with flexible substrates, loading cathode materials on flexiblecurrentcollectors wasa commonway to acquire flexible cathodes. Ingeneral, conductive(such as carbon black, acetyleneblack and carbon nanotube) andbindingadditives (such as carboxymethyl cellulose, polyvinylidene fluoride and polytetrafluoroethylene) were applied to mix with cathode materials. Then, the mixed slurry was coated on the surface of conductive substrates (such as carbon cloth, carbon paper and metal wires) with flexibility. Finally, flexibleelectrode plate after drying was obtained. Nevertheless, this approachexisted some drawbacks.For example,the transport of electrons and ions would be limited due to the existence of conducting additives and binders. Apart from that, under external forces,cathode materials felloff flexiblesubstrates as a result of limited contact area among them. Based on these, many researchers paid attention onin situgrowing method. In otherwords,active materialswerein situformed on the corresponded flexible substrates through hydrothermal method, electrodepositionand so on.Fig. 1(b) displayed the SEMimage of the MnO2/rGO sample on carboncloth byin-situgrowing method, revealing excellent flexibility of cathode part[19].

Recently, researchers tended to use flexible current collectors as flexible basis and then the active materials were integrated ontheir faces to prepareflexible electrodes.With regard to flexible substrates, there were two choices. The one was metal-based current collectors such as titanium or stainless-steel wires. The other wascarbon-based substrates such as carbon cloth, carbon nanotube (CNT) fiber or CNT paper.

2.2.Flexible electrolytes for flexible ZIBs

As a general rule, flexible ZIBs employed hydrogel electrolytes incorporatinginto different salt solutionsuchas ZnSO4,ZnCl2, Zn(ClO4)2, Zn(NO3)2and Zn(CF3SO3)2. The hydrogel electrolyte could be classified into twocategories:i) natural polymers. ii) synthetic polymers.

2.2.1.Natural polymer hydrogel electrolytes

Natural polymers, e.g., gelatin, cellulose and sodium alginate, had excellent physicochemical propertiesthat afforded fabrication of advanced hydrogel electrolytes for flexible electronicdevices: non-toxicity, hydrophilicity,thermodynamic stability,thehigh capacityfor swellingfor highion conductivity and simple craft. Therefore, abundant natural gel electrolyteshad been reported to assemble flexibleZIBs. Zhiet al.adopted a gelatin-based natural polymer hydrogel electrolyte to fabricate flexible ZIBs that had excellent resistance to deformation,namely, aftercutting4 times, bending 800 times,hammering 5 times and other deformations, flexible ZIBs still kept excellent electrochemical performance[20]. Meanwhile,they also explored a sewable Zn-MnO2battery based on a nanofibrillated cellulose/ployacrylamide hydrogel electrolyte, leading to withstanda large shearingforce of43 N[21]. For sodium alginate (SA) natural polymer (Fig. 2(a)), it was usually selected to be as flexible electrolyte because of its superb mechanical performance and simple processing of fabricating.Zhouet al. adopted SA-based hydrogel electrolyte to assembleflexibleZIBsthatexhibitedexcellentmechanical property of bearingahighweightandahighionic conductivityof 1.83× 10–2S/cm[22].

2.2.2.Synthetic polymer hydrogel electrolytes

In fact, most flexible gel electrolytes were chemically synthesized polymer hydrogels. This was ascribed to be able to carry on the structure design reasonably so that flexible hydrogel electrolytes were applied to a variety of scenarios.Inaddition, chemically synthesized polymerhydrogels could be dividedintotwo categories:i) chemically cross-linked hydrogels; ii) physically cross-linked hydrogels. In general, the use of chemically crosslinked hydrogel electrolytes in flexible ZIBs was a mainstream trend such as common polyacrylamide (PAM)-based hydrogel. On the basis of superior interface compatibility andhighlywater content, PAM-based polymer electrolytes had been widelyused in ZIBs. Numerousamide groups (-CONH2) and network structure were beneficial for ion mobility. Zhiet al. reported a quasi-solid-state washable and tailorable elastic yarn ZIBs on the basis of PAM polymer electrolyte (Fig. 2(b)) with ion conductivity of 17.3 ×10–3S/cm at room temperature[23]. Meanwhile,ahighspecific capacityof 302.1mA·h/g and volumetric energy density of 53.8 mW·h/cm as well as excellent cycling stability of 98.5% capacity retention after 500 cycles were obtained by the yarn ZIBs[23].

Although chemically crosslinked hydrogel electrolytes had met mostapplication scenarios, physicallycrosslinkedhydrogelelectrolyteshadunique advantages for flexible ZIBsunder certain circumstances. Just as its name implies, physically crosslinked hydrogels were synthesized through physical interaction including van der Waals' force, hydrogen bonding and electrostatic interactions. The most common example was poly(vinylalcohol) (PVA)-basedhydrogel. Fig.2(c)delivered a self-healingPVAhydrogel electrolyte with high ionconductivity for flexibleZIBs. Aftermultiplecutting/self-healing cycles, the flexible ZIBs still exhibited stable specific capacity of 81.4 mA·h/g[24].

2.3.All-in-one flexible ZIBs

Apart from pursing flexible electrodes and electrolytes,the flexible ZIBs could be fabricated using all-in-one strategy of structural designing. Niuet al. reported a scalable assemblystrategyto prepare flexible ultrathin ZIBsviaall-inone integrated architecture. Theacquired flexibleultrathin ZIBs could be controllablytailored andedited into desired shapes and structures, and the tailored miniature flexible ZIBs still exhibited great electrochemical performance[25]. Fig. 3(a)showed schematic process of flexible ultrathin ZIBs. A scalable assembly strategy was employed to fabricate flexible ultrathinZIBswith all-in-oneintegrated architecture bycombiningblade coating with rolling assemblytechnologies[25].The ultrathin all-in-one flexible ZIBs could maintain a highly capacity retention of 97.3%, and high discharge capacity of 132.1 mA·h/g after 1000 cycles (Fig. 3(b))[25]. In addition, Zhiet al. proposed another approach to prepare all-in-one flexible ZIBs. They fabricated all-solid-state ZIBs enabled byin situconstructed polymer electrolyte (Fig. 3(c)).After 20 000 cycles at 0.5 A/g, the capacity of flexible ZIB remained 105.8 mA/g and the coulombic efficiency maintained more than 99.75 % (Fig. 3(d))[26].

Fig. 2. (Color online) (a) The process diagram of SA-based hydrogel electrolyte. Adopted with permission from Ref. [22], Copyright 2020, Elsevier.(b) The schematic diagram of PAM-based hydrogel. Adopted with permission from Ref. [23], Copyright 2018, American Chemical Society. (c) The structure diagram of fabricating PVA-based self-healing electrolyte. Adopted with permission from Ref. [24], Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

3.Functional application of flexible ZIBs

Flexible ZIBs had been reported into applyingin many situations including mechanical practicability (stretching, compressing, bendingandfolding), self-healing, low temperature,smart transformation and others. In these categories, there were many interesting findings found by researchers. The details wouldbe introducedbelow.

3.1.Mechanical performance

Fig.3.(Color online)(a) Theschematicprocessofdesignand (b)the cycleperformanceofultrathin all-in-oneZIBs.Adopted with permission fromRef. [25], Copyright 2021, JohnWiley & Sons. (c) Schematic illustrationof fabrication procedures and(d) cycle performance of in-plane batteries. Adopted with permissionfrom Ref. [26], Copyright 2020, Wiley-VCH VerlagGmbH & Co. KGaA, Weinheim.

The mechanical practicality wasthe most basic requirement of flexible ZIBs and researches were widely focused on the mechanical properties in the past fewyears. Zhiet al.firstly reported good compressive performance of PAM-based hydrogel electrolyte[27], and then modified the PAM hydrogel by bringinginnaturalpolymer SA[28]. Fig.4(a) exhibited a demonstration of mechanically durable anddevice-level tough Zn-MnO2battery with high flexibility. After walking and car run-over, the discharge capacities of flexible ZIBs were still high similar to initial state. Whereafter, Huanget al.also reported the hydrogel electrolytes on the basis of PAM and SA to assemble flexible ZIB (Fig. 4(d))[29].Fig. 4(e) delivered the tensile strength of 674.28 kPa and compressive strength of 16.29 MPa of Zn-reinforced SA-PAM SE due to the strengthening mechanism of Zn2+cross-linked SA. And the capacity loss per cycle of the flexible ZIB was the lowest after 10 000 cycles (Fig. 4(f))[29]. In addition, integrated all-in-one ZIBs still revealed great electrochemical performance even when they are bent to nearly 360°, reaching a stable capacity of 137 mA·h/g (Figs. 4(g)–4(i))[30]. With regard to flexible ZIB with lightweight MnO2/graphene membrane, it also showed capacity stability under external forces such as folding as shown in Figs. 4(j) and 4(k)[31].

3.2.Self-healing performance

Conventional batteries lost electrochemical performance when damaged. Thus, automatic repair of damaged batteries without affecting the performance of the battery itself was extremely important. Especially for flexible batteries,they were easily damaged when using in many fields. Herein,self-healing performances of various flexible ZIBs were summarized. Zhiet al. employed self-healing carboxylated-polyurethane as the substrate for electrodes to assemble flexible self-healing Zn-MnO2battery[32].Figs. 5(a) and 5(b) showed after self-healing, flexible Zn-MnO2battery still displayed high capacity and powered an electric watch. Meanwhile,theyexploredtheself-healingperformance of flexible ZIBs in an alkaline environment. Figs. 5(c)–5(e)displayed excellent self-healing performance of flexible NiCo-Zn battery by using a self-healable hydrogel electrolyte comprising sodium polyacrylate cross-linked by ferric ions (Fe3+)[33]. Apart from healing hydrogel electrolyte, a self-healing flexible ZIB was also studied by using all-in-one self-healing electrodes. Fig. 5(f) exhibited flexible ZIBs assembled by all-in-one cathode by VS2nanosheets growing on carbon cloth, anode by electrochemically deposited Zn nanowires and a self-healing hydrogel electrolyte. After cutting the ZIB, repaired hydrogel electrolyte and electrodes all restored their original state and thus flexible ZIBs remained excellent electrochemical performance[34].

3.3.Low temperature performance

Due to the freezing, low ion conductivity and slow dynamics of aqueous electrolyte at low temperature, the aqueous metal ion batteries occurred the loss of capacity and power with the drop of temperature. Improving low temperature performance of aqueous batteries including flexible ZIBs had attracted the attention of scientists. Zhiet al. synthesized ethylene glycol-based waterborne anionic polyurethane acrylates(EG-waPUA) and then copolymerized EG-waPUA precursor and AM monomers to fabricate an EG-waPUA/PAM based dual crosslinked hydrogel. Next, the antifreezing Zn-MnO2batteries (AF-battery) on the basis of the hydrogel were assembled and delivered a high specific capacity of 226 mA·h/g at 0.2 A/g at –20 °C[18]. Fig. 6(a) exhibited the demonstration of AF-battery powered a series of electronic devices. Chenet al. also revealed great low-temperature performance of flexible Zn-MnO2battery based on PAM/EG gel electrolyte(Fig. 6(b))[35]. Whereafter, Zhuet al. researchedhighzinc reversibility for flexible ZIBs employing PAAm hydrogel consisting of 2mol/L ZnSO4and4 mol/L LiCl(ZL-PAAm) atlow temperature. Fig. 6(c) showed the voltage-time curves of Zn platingstripping in ZL-PAAm under different temperatures, indicating excellentzinc reversibility of Zn//Zn symmetric cellswith the ZL-PAAm. Meanwhile, the assembled flexible Zn-LiFePO4full cell shows a great cycle performance of 500 cycles at–20°C, further confirming the excellent zinc reversibility benefiting from the ZL-PAAm[36].

Fig. 4. (Color online) (a) Illustrations of the Zn-MnO2 battery i) being placed under foot and ii) going through car run-over. (b) Discharge curve of the battery after 2 days' everyday treading. (c) Discharge curve of the battery after 20 times of random run-over by cars on road. All the discharge curves were recorded at 0.924 A/g (3C rate). Adopted with permission from Ref. [28], Copyright 2019, Elsevier. (d) Schematics of the evolution of the Zn-reinforced SA-PAM SE hydrogel structure. (e) Tensile strength of the Zn-reinforced SA-PAM SE. (f) Capacity loss per cycle of all kinds of flexible ZIBs. Adopted with permission from Ref. [29], Copyright 2020, American Chemical Society. Optical images of a “ZIBs” LED powered by four all-in-one ZIBs in series (g) without bending and (h) under bending. (i) Cycling performance of the all-in-one and stacked ZIBs at 0.5 A/g under flat and different bending states. Adopted with permission from Ref. [30], Copyright 2019, Royal Society of Chemistry. (j) The flexible ZIB is subjected to fold deformation. (k) Galvanostatic charge/discharge curves of the ZIB cell under different mechanical deformations. Adopted with permission from Ref. [31], Copyright 2021, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

3.4.Others

Apart from mechanicalpracticality,self-healingperformance and working at low temperature, a series of flexible ZIBs particularly designed for special situations were studied and recorded. This expanded the application rangeof flexible ZIBs. Zhiet al. firstly reported a smart safe rechargeable flexible ZIBs based on sol-gel transition electrolytes. With the helpof thermal-stimulus responsive polymer of poly(N-isopropylacrylamide) (PNIPAM), the synthesized hydrogel delivered asmart reactionof flexible ZIBs,namely,over acriticaltemperature, the polymer chains precipitated out of solution, resulting in stopping work of flexible ZIBs. Fig. 7(a) exhibited the processof the transformation. Whenthe temperaturewas lower than the critical temperature, the battery restored its original state and worked again without any changing[37]. Therewith,Niuet al. further confirmed the feasibility of smart flexible ZIBs (Fig. 7(b)). Theyemployed asmart thermal-gated PNIPAM/AM-5@GF hydrogel electrolyte to achieve evolution of the pore from an open to closed structure. Meanwhile, it displayedthe phenomenonofasurface wettability transition from hydrophilic to hydrophobic states. The assembled flexible ZIBsshowedthermal-responsiveability andself-protection behavior at high temperature[38]. Under extreme circumstances, flexible ZIBs remained excellent electrochemical performance.Wanget al. fabricated thehydrogel with high ionic conductivity (28.8 mS/cm) that cotton cellulose nanofiber grafted by xanthan gum-polyacrylamide (XG-PAM/CNF).Fig. 7(c) showed the practical submarine-use of flexible ZIBs, indicating use of possibility of flexible ZIBs in extreme environments[39]. Additionally, regulating electrochemical performancewas achievedbyreasonabledesign for flexible gel electrolytes. Fig. 7(d) revealed that particularly designed zwitterionic sulfobetaine/cellulose semi-interpenetrating networks gel(ZSC-gel) provided high ion conductivity (24.6 mS/cm) for flexible ZIBs. And then flexible Zn-MnO2assembled with ZSC-gel showed greatcycling performanceof 10 000cycles[40].Finally, the performance comparison of flexible ZIBs using in different situations was summarized in Table 1. Notably, flexibleZIBs wereused in manyfields andrevealedgreat electrochemical performance. However, production in a large scale remained a challenge and it wasalso necessary todevelop flexible ZIBs for moreapplicationscenarios.

Table 1. The performance comparison of flexible ZIBs using in different situations.

Fig.5. (Color online) (a) Cyclingperformance of theobtainedflexible Zn-MnO2batterybeforehealing andafter fourth healing.(b) Demonstrationof aself-healing flexible Zn-MnO2 battery poweringanelectricwatchbefore and after cuttingandafterhealing. Adoptedwithpermission fromRef. [32],Copyright 2019, American ChemicalSociety. (c)Charging anddischarging profilesof alkaline flexibleNiCo-Zn batteriesbeforeand after multiple cutting/healingcycles. (d)Healingefficiency calculated from (c). (e) Demonstrationof aself-healingflexible NiCo-Zn battery powering an electricwatch beforeandaftercutting and after healing. Adopted withpermissionfromRef.[33],Copyright2018, Wiley-VCHVerlag GmbH&Co.KGaA, Weinheim.(f) Demonstrationof hydrogel electrolytesand thebatteryusingall-in-one electrodesaftereach time of theselfhealing process. Adopted with permissionfromRef. [34], Copyright 2021, Wiley-VCHVerlagGmbH& Co. KGaA, Weinheim.

Fig.6.(Color online)(a) Thedemonstrationof AF-battery powered aseries of electronicdevices. Adopted withpermissionfrom Ref.[18],Copyright 2019,Royal Society of Chemistry. (b) The schematicdiagramof anti-freezinggelelectrolyte based onPAM/EG gelelectrolyte. Adopted with permission from Ref. [35], Copyright 2020, Frontiers Media S.A. (c) The voltage curves of Zn plating-stripping in ZL-PAAm under different temperatures. Adoptedwith permission fromRef. [36], Copyright 2020, Wiley-VCHVerlagGmbH& Co. KGaA, Weinheim.

Fig. 7. (Color online) (a) The process of the smart reaction of flexible ZIBs when temperature changes. Adopted with permission from Ref. [37],Copyright 2018, Science China Press. (b) The schematic diagram of smart rection. Adopted with permission from Ref. [38], Copyright 2020, John Wiley & Sons. (c) The demonstration of practical submarine-use of flexible ZIBs assembled by XG-PAM/CNF hydrogel electrolyte. Adopted with permission from Ref. [39], Copyright 2020, American Chemical Society. (d) Ion conductivity of zwitterionic sulfobetaine/cellulose semi-interpenetrating networks gel (ZSC-gel). Adopted with permission from Ref. [40], Copyright 2020, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

4.Challenges and perspectives

From above review, reported flexible ZIBs exhibit superior flexibility, self-repairability, multiple environmental adaptation, intelligence and ideal electrochemical performance including specific capacity and cyclic stability. This lays a solid foundation for practical application of flexible ZIBs in the future. However, the practical application of flexible ZIBs is not yet commercialized owing to several challenges and issues discussed below. In order to develop practical flexible ZIBs that can be fabricated in a large scale, overcoming or alleviating the following difficulties would be especially vital.

(1) High-performing flexible current collectors and electrodes. At present, widespread-usage flexible current collectors are metal foil and mesh including titanium and stainlesssteel wiresorcarbon-based materialsincludingcarbonpaper and cloth. Nevertheless, because of memory effect of metal materialsand finiteelongationof carbon materials, the metal- and carbon-based current collectors do not fully meet demand in some situations. Therefore, other flexible current collectors are also researched and developed. Although some flexible conductors such as indium tin oxide (ITO) have been developed[41,42], high cost and brittleness restrict its large-scale applications in flexible wearable devices in a large scale. In this regard, therelated research needs to be exploreddeeply.A relatively common flexible electrodes are conductive polymerssuch as polypyrrole (PPy)[43,44], polyaniline (PANI)[45,46]and polyethylene dioxythiophene (PEDOT)[47,48]. However,theseelectrodes areonly employed toassemblesupercapacitor due to lacking to high electroconductibility. Thus, developing conductive polymers with higher electrical conductivity is key issue to obtain fit flexible electrodes for flexibleZIBs.

(2) Flexible hydrogel electrolyte with wide operation potential window. The batterydischarging plateauof aqueous ZIBs is confined because of the narrow electrochemically operation potential window of aqueous electrolyte. Therefore, in order to prepare flexible ZIBs with high discharging voltage,the hydrogel electrolyte with wide operation voltage window is inevitable. Up to now, the hydrogel with high concentrated salts contained provides a potential candidate for achieving high voltage[49−52].However, in viewofcostand craft process, high concentrated salt based the hydrogel is notagoodchoicefor flexibleZIBs. The“water-in-gel”electrolyte achieving wide operation potential window[53]guides a newand valuable direction for high-voltageflexibleZIBs.Meanwhile, the cost and process of fabricating are reduced and simplified. In the future, researchers need to deeply and more detailedly do research in this direction.

5.Conclusion

In summary, flexible ZIBs havebeen fabulously fashionable and generally researched since 2015. The development of flexible ZIBsisstillin itsinfancy even if there are someevolutions to some extent. On the one hand, flexible current collectors or flexible electrodes need to be further developed to make flexible ZIBssuitable for morepractical scenarios. On the other hand, flexible hydrogel electrolytes with wide operation potentialwindow areextremely significant toassemble high-voltage flexible battery. Therefore, it is necessary to overcomeand battle thesekeyissues. Only if thesetwo challenges are solved, can we carry flexible ZIBs forward further.In this review, we have implemented a brief discussion on the challenges and perspectives existed inthedevelopment of flexible ZIBs. We also propose a direction that need to be further researchedinthe futureso thatflexible ZIBscan make a step closer to commercial application.

Acknowledgements

This research was supported by the National Key R&D Programof China under Project 2019YFA0705104.