Recent advances in electrospinning nanofiber materials for aqueous zinc ion batteries

Aqueous zinc ion batteries (AZIBs) are regarded as one of the most promising large-scale energy storage systems because of their considerable energy density and intrinsic safety. Nonetheless, the severe dendrite growth of the Zn anode, the serious degradation of the cathode, and the boundedness of separators restrict the application of AZIBs. Fortunately, electrospinning nanofibers demonstrate huge potential and bright prospects in constructing AZIBs with excellent electrochemical performance due to their controllable nanostructure, high conductivity, and large specific surface area (SSA). In this review, we first briefly introduce the principles and processing of the electrospinning technique and the structure design of electrospun fibers in AZIBs. Then, we summarize the recent advances of electrospinning nanofibers in AZIBs, including the cathodes, anodes, and separators, highlighting the nanofibers' working mechanism and the correlations between electrode structure and performance. Finally, based on insightful understanding, the prospects of electrospun fibers for high-performance AZIBs are also presented.


Introduction
6][7] Generally, AZIBs are composed of a Zn anode, mild or weakly acidic electrolyte, separator, and cathode.Zn metal with a high theoretical capacity (820 mA h g −1 ) and low redox potential (−0.76 V vs. the standard hydrogen electrode) is considered an ideal anode for AZIBs. 8,9In addition, the cathode plays a crucial role in the performance of AZIBs, as it serves as a host framework to accommodate Zn 2+ . 10,11So far, cathode materials for AZIBs include manganese, vanadium, Prussian blue analogs, organic compounds, etc. 12 These cathode materials are related to the operation voltage, cycle stability and rate performance of AZIBs. 13,14Herein, the application of suitable cathode materials can improve the performance of AZIBs.
Despite the many advantages of AZIBs, however, many challenges seriously hinder their further application.Firstly, in contrast to lithium/sodium ion batteries, the reaction mechanisms of AZIBs are complicated and immature, 15 and can be categorized into three main types, including Zn 2+ insertion/ extraction, 16,17 H + /Zn 2+ co-insertion/extraction, 18 and chemical conversion reactions. 19Among them, the Zn 2+ insertion/ extraction reaction mechanism is the most commonly acknowledged in AZIBs. 20Secondly, the non-uniform Zn 2+ deposition and the decomposition of active H 2 O molecules belonging to the solvation layer of Zn 2+ will result in uncontrolled growth of Zn dendrites and the formation of by-products on the surface of the Zn anode, ultimately causing battery failure. 21Thirdly, due to the Jahn-Teller effect, the active materials of the manganese-based materials will dissolve in weakly acidic aqueous electrolyte, resulting in material collapse and the rapid degradation of capacity. 22,23In addition, vanadium-based compounds and organic compounds also face the challenge of dissolution. 24Fourthly, as the crucial component of AZIBs, the separator can prevent direct contact between the electrodes and provide the channel for ion migration. 25owever, traditional separators (such as glass ber, lter paper, and non-woven fabrics) cannot meet the requirements for AZIBs of excellent mechanical properties, high wettability, high ionic conductivity, and electrical insulation. 26,27To alleviate these limitations, some novel material preparation technologies and many functional materials have been adopted and fabricated.Among them, the electrospinning nanobers have advantages such as large surface area to volume ratio, high aspect ratio, directional transportation, and short ionic transport lengths, which are desirable in energy storage applications. 28In the previously reported literature, there is no uniform denition of one-dimensional (1D) nanobers. 29Thereby, in this review, single electrospinning nanobers are dened as 1D nanobers.During electrospinning, 1D nanobers deposited and disorderly arranged on the collector can form the twodimensional (2D) nano-lm.Different from the conventional 2D nano-lm, the preparation of three-dimensional (3D) brous structures is complicated.In general, the fabrication strategies of 3D structures include increasing the electrospinning, selfassembly, assembly by post-processing of 2D nano-lm (such as layer-by-layer electrospinning), and direct assembly by an auxiliary factor (like a 3D template). 30These 2D and 3D architecture materials with high exibility and high surface area-tomass ratio are assembled by 1D bers exhibit faster intercalation kinetics in AZIBs.Besides, some unique structures (such as core/shell structures and hierarchical pores), defects, and functional groups can be created and introduced on the electrospinning nanobers, which is benecial for AZIBs. 31or example, Tang et al. fabricated N-doped carbon bers to improve the electronic conductivity of cathode materials. 32iang et al. synthesized zincophilic carbon nanober interlayers by an electrospinning method to uniformize the deposition of Zn 2+ . 33Meanwhile, Fang et al. fabricated a polyacrylonitrile (PAN) nanober separator with high porosity and excellent exibility. 34A brief timeline of the representative works of electrospinning nanobers on AZIBs is summarized in Fig. 1. [35][36][37][38][39][40][41][42] Although electrospinning nanobers are widely applied in AZIBs, there is still no specic review focus on electrospinning nanomaterials' application in AZIBs.Thus, it is necessary to summarize the research progress of AZIBs based on the electrospinning nanomaterials.
Herein, in this review, we rst introduce the principle and processing of the electrospinning technique.Then, the different structures of electrospinning nanobers in AZIBs are summarized.Thirdly, we highlight the development of electrospinning materials in AZIBs, such as cathodes, 39,43,44 anodes, [45][46][47] and separators. 34,48Finally, we propose the challenges, development prospects, and future research directions of the electrospinning materials in AZIBs.

Principle and processing of the electrospinning technique
The electrospinning technique is a novel patented technology invented in 1934 that enables the direct and continuous preparation of polymer nanobers, 49,50 including not only synthetic polymeric compounds such as poly(vinyl pyrrolidone) (PVP), poly(vinylidene uoride) (PVDF), and polyacrylonitrile (PAN), 51 but also natural macromolecules and their derivatives like chitosan and silk protein. 30A common electrospinning apparatus usually comprises a high-voltage power supply, a metallic or plastic syringe, and a collector. 31A "Taylor cone" at the end of the nozzle will form a jet of electrically conductive polymeric precursor solution (or polymer melt) in a classic electrospinning process when the voltage between the collector and needle exceeds a critical value. 50Aer a short distance of stable motion, these jets will go into an unstable movement stage.Experiencing a series of stretching and solvent evaporation, the polymer solution jets will solidify and nally be deposited on the collector, forming polymer bers. 52he structure and morphology of electrospinning nanobers are affected by numerous factors such as the properties of polymer solutions, processing parameters, and ambient parameters. 53The molecular weight of the polymer is a signicant parameter affecting electrospinning nanobers, which directly affects the properties of the precursor solution, such as viscosity, conductivity, and surface tension. 54At the same mass fraction, polymer solutions with higher molecular weight exhibit higher viscosity than those with lower molecular weight.In general, high viscosity usually results in the formation of large diameter nanobers, while low viscosity solutions facilitate the preparation of small diameter nanobers. 50Voltage and feed rate are other important factors affecting the diameter of the nanobers.It is well known that the critical voltage is required to form electrospinning nanobers. 55With the increase of voltage, the diameter of nanobers will decrease at an appropriate concentration of polymer solution. 52In contrast, increasing the feed rate will lead to an increase in ber diameter. 56Besides, the diameter of nanobers is also inuenced by environmental parameters (such as humidity and temperature).A moderately high temperature and a low relative humidity will promote the evaporation of solvent and the solidication of jets, which is favorable for decreasing the diameter of nanobers.These factors are not independent and have a signicant inuence on each other.Therefore, before preparing the nanobers with specic morphology and diameter, the interaction between these parameters needs to be considered.

Electrospinning nanofibers design for zinc ion batteries
Generally speaking, the structure of materials signicantly impacts the electrochemical performance of batteries.For instance, constructing a porous structure cathode material can increase the SSA of the material and facilitate the intimate electrolyte penetration and rapid transfer of Zn 2+ . 57urthermore, the hollow structure has the ability to accommodate the volume changes of the electrodes. 58Different structures of nanobers (e.g., core/shell, porous, hollow, and so on) can be fabricated by the electrospinning method.Thus, the design of different structure nanobers by electrospinning in AZIBs will be discussed and summarized in this section.

Core/shell structure
In the year 2003, nanobers with core/shell structures were prepared by coaxial electrospinning for the rst time. 59Since then, the core/shell-structured electrospinning nanobers have been extensively utilized in energy storage due to their unique features.Compared with normal electrospinning bers, the advantage of core/shell nanobers is to allow many nonspinnable polymers to be used as electrospinnable materials, 60 such as polyaniline and polyvinyl alcohol. 61In the process of electrospinning, two kinds of immiscible solutions were added to two syringes, respectively.Then, under a high voltage electrostatic eld, the shell solution will converge with the core solution at the nozzle, nally forming the core/shell structured bers. 62In AZIBs, the core/shell nanostructure bers are usually used as the electrode material due to the large SSA and excellent charge storage.For example, Long et al. fabricated Mn 3 O 4 nanoparticles (Mn 3 O 4 NPs)/polyacrylonitrile (PAN) composite nanobers by coaxial electrospinning. 63uring annealing, the Mn 3 O 4 /PAN bers were carbonized to Mn 3 O 4 @HCFs nanobers with core/shell structure.

Porous structure
Porous structure electrospinning nanobers have the advantages of large SSA, short ion diffusion length, and fast electrolyte access, and have been widely used in AZIBs. 64Besides, the abundant porosity can accommodate the volume changes caused by ion insertion/extraction, thus mitigating structural distortion during cycling. 65In electrospinning, phase separation and sacricial template methods are usually used to produce porous structures in nanobers.The mechanism of the phase separation method can be categorized into vaporinduced phase separation (VIPS), non-solvent-induced phase separation (NIPS), and thermally induced phase separation (TIPS). 66Usually, the fabrication of porous nanobers involves one or more phase separation methods, while suitable polymers and solvents are also required.Sacricial templates include polymers, metals, metal oxides, and inorganic salts. 67For instance, Liu's group used block copolymer poly(methyl methacrylate)-block-polyacrylonitrile (PMMA-b-PAN) as a raw material to fabricate polymer mats. 68In this polymer mat, the incompatibility between the PMMA block and PAN will result in microphase separation, which will further release and generate abundant micro-/mesopores at high temperatures.This porous structure can shorten the ion diffusion path and facilitate the migration of electrolytes in the electrode.

Hollow structure
The principle of coaxial electrospinning to prepare hollow structure nanobers involves generally soluble or volatile substances (such as oil) as the core layer, and polymer solution as the shell layer, through the coaxial electrospinning process and removal of the core layer to obtain hollow bers. 69,70The construction of hollow structures can signicantly increase the number of active sites, improve the high aspect ratio of nano-bers, and enable accommodating massive deposition at a high current density without a distinct volume change.Additionally, it can be prepared by the Kirkendall effect. 71For example, Xue et al. proposed a hollow TiO 2 and SiO 2 carbon ber.During the carbonization process, hollow porous bers were formed due to the different decomposition and diffusion rates of different molecular weight PVP. 35

Bead-like structure
In recent years, the bead-like structure of electrospinning nanobers has attracted extensive attention on account of its unique geometric shape and chemical performance.Usually, bead-like structure bers are considered the by-products of the electrospinning process.Their formation can be devoted to the axisymmetric instability of the uid jet under an external electric eld. 72,73According to the literature, decreasing the viscosity of the polymer solution (or net charge density of the jets) will facilitate the formation of beads. 74However, the lower surface tension of the precursor polymer solution favors the production of bead-like bers during the process of electrospinning.For instance, the manganese-based metal-organic framework (Mn-MOF) spheres can be wrapped in PAN through the electrospinning technique. 36Aer carbonization in N 2 , the bead-like cathode materials for AZIBs can be achieved by stringing MnO x with carbon ber ropes.

Hierarchical structure
Hierarchically structured bers consist of multiple nanostructures, which can be fabricated by electrospinning and posttreatment technologies. 50Compared to primary structures, the hierarchical structure improves the electrical conductivity of metal oxides and the storage of Zn 2+ . 75For instance, Zhang et al. produced vanadium nitride embedded nitrogen-doped carbon nanober (VN/N-CNFs) composite hierarchical structures by the electrospinning method. 76Additionally, nano-whiskers can be observed in the branches of VN/N-CNFs.

Applications of electrospinning nanofibers in zinc ion batteries
Owing to their versatility and applicability, electrospinning nanobers have been extensively applied in AZIBs.Firstly, electrospinning nanobers possess high mechanical exibility to meet the trend of exible AZIBs.Secondly, the nanober structure can shorten the Zn 2+ diffusion pathway and reduce reaction impedance in cycling.Thirdly, electrospinning nano-bers with electrical conductivity and stability can be used as a collector to uniformize the deposition of Zn 2+ , achieving a "dendrite-free" metal Zn anode.Last but not least, the nano-ber separator with appropriate thickness, high mechanical strength, and controllable pore size can be fabricated by the electrospinning technique, which can facilitate the transfer of Zn 2+ , improve the wettability between the separator and electrolyte, and resist the piercing of the Zn dendrites.Therefore, this section will summarize the application of electrospinning nanobers in the cathodes, anodes, and separators of AZIBs.

Cathodes
In particular, as an important component of AZIBs, the cathode material largely determines the electrochemical behaviors of the battery. 77Therefore, high-performance cathode materials have been the focus of research in the last decade. 78However, cathode materials still face challenges such as poor conductivity, dissolution issues, and volume variation. 23,79Electrospinning carbon nanobers can provide carbonaceous frameworks with high conductivity to improve the conductivity and reaction kinetics of materials. 55,80Besides, the active materials can be embedded in carbon nanobers with a porous structure and large SSA, which greatly prevents the dissolution and volume variation of materials. 81,82For clarity, the application of electrospinning nanobers in cathode materials is described in the following aspects: vanadium-based materials, manganese-based materials, and other cathode materials.
4.1.1Vanadium-based cathodes.Vanadium oxides have become one of the most promising cathode materials because of their various oxidation states, high theoretical specic capacity, and abundant crystal structure. 83,84However, vanadium-based cathodes will dissolve in mild acidic aqueous electrolytes because of the strong polarity of water molecules and anions, resulting in capacity fading.In addition, dissolved substances will deposit on the surface of the Zn anode, reducing the reactivity and utilization of the Zn metal. 85Usually, vanadium-based materials are semiconductors that possess poor electronic conductivity, so highly conductive substances are oen used in the preparation of the cathode electrodes to improve the conductivity of the materials. 86o alleviate these limitations, numerous approaches have been proposed to enhance the electrochemical performance of vanadium-based materials.Among them is preparing V x O y nanobers by the electrospinning technique with excellent ion diffusion pathways, high conductivity, and nanostructures, which promote electron/ion transport and improve the cycling ability of the cathode.For example, to address the problems of dissolution and poor conductivity of VO 2 , Liu et al. prepared self-supported VOC-NF composites by the electrospinning method followed by steam treatment, in which VO 2 nanodots were embedded in carbon nanowires. 87In VOC-NF, the carbon shell with good electrical conductivity not only prevented the dissolution of the vanadium element but also avoided the use of binder and conductive species, resulting in high discharge specic capacity and energy density. 88Therefore, the vanadiumbased cathode exhibited a satisfactory electrochemical performance due to the rapid Zn 2+ diffusion and electron transfer.Generally speaking, the component distribution of the polymer solution determines the content and distribution of active materials in electrospun nanobers. 89The concentration distribution of precursors during the electrospinning process could therefore be adjusted to produce nanobers with a continuous concentration gradient.Niu's group combined a dynamic concentration adjustment technique and electrospinning method to develop continuous gradient composite lms (GCFs) (Fig. 2a). 90The polymer solution was continuously added to the precursor solution to form a continuously diluted resultant precursor solution.In VO-GCFs, VO nanoparticles were gradient distributed throughout the carbon ber matrix aer the electrospinning and annealing process.In particular, the electronic conductivity of VO-GCFs gradually increased with the gradient distribution of VO nanoparticles, which facilitated the rapid transfer of electrons and improved the reaction kinetics and electrochemical performance.Compared with homogeneous or down-graded VO-GCFs, the up-graded cathode exhibited an excellent cycling and rate ability.Hence, at a current density of 5.0 A g −1 , the discharge capacity of the Zn// VO-GCFs battery was nearly unchanged aer 1000 cycles (Fig. 2b).In the rate performance test, the average discharge capacity of the up-graded cathode was 477.1 mA h g −1 at 5 A g −1 .
As the current density became 0.3 A g −1 , the capacity retention of the up-graded cathode reached 81.2% (Fig. 2c).
Constructing a microstructure can efficiently improve the transport kinetics of cathode materials. 12For instance, a hierarchical structure could shorten ion transport pathways, 92,93 a porous structure with a large SSA can provide abundant transfer channels for Zn 2+ , 85 a hollow structure can act as a host to load active materials, 94 etc.Some researchers have prepared many vanadium-based nanobers with special structures to improve the cycling ability of electrodes.For example, Chen et al. successfully produced porous V 2 O 5 nanobers via the electrospinning method followed by calcination. 38This abundant mesoporous structure is conducive to electrolyte permeation and Zn 2+ insertion.In the rst charging process, the V 2 O 5 transformed into Zn pyrovanadate with a highly stable open

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Chemical Science framework, which greatly favors the reversible Zn 2+ insertion/ extraction (Fig. 2d).Therefore, aer 500 cycles, the battery with a V 2 O 5 nanober cathode showed a high capacity of 166 mA h g −1 and an impressive capacity retention of 81% at 2C.Furthermore, Wang et al. fabricated novel hybrid bers with core/shell hybrid bers (Fig. 2e and f), which promoted rapid electron/ion transmission and high mass loading, thus gaining a better energy storage capability and rate performance ability. 91eteroatom doping is an effective method to modify the intrinsic electronic/ionic properties of electrode materials for AZIBs. 95Doped heteroatoms can widen the interlamellar spacing and redistribute the charge of the surface atoms, increasing ion storage and facilitating electron transport. 96,97uring the process of electrospinning, N-containing polymers (such as PVP and PAN) were oen used.These polymers were transformed into N-doped carbon nanobers aer carbonization, which contributed to an increase in the electronic conductivity of materials and provided more active sites for Zn 2+ insertion/extraction.For instance, Zhang et al. fabricated an Ndoped VN-encapsulated carbon nanober (VN/N-CNFs) compound by carbonizing H 2 BDC and VCl 3 /PAN bers. 76The 3D self-supported hierarchical structure of the VN/N-CNFs process was thus directly applicable as an electrode for AZIBs and exhibited ultra-long cycle lifetimes and super-high rates.As shown in Fig. 3a and b ) microbers by the electrospinning method. 98The graphitic N atoms in the composites could promote charge transfer and improve the electrical conductivity and stable cycling ability of N@C/V 2 O 3 .Thus, the battery based on the N@C/V 2 O 3 electrode delivered a specic capacity of 322.3 mA h g −1 and superhigh capacity retention of 91.7% aer 4000 cycles at 10 A g −1 (Fig. 3c).Besides, Yoo et al. produced Fedoped V 2 O 5 nanorods by immersing the PAN ber templates in sol solutions with vanadium salt and iron salt followed by calcination (Fig. 3d). 99As an outstanding cathode for AZIBs, the Fe-V 2 O 5 not only shortened the diffusion distance of Zn 2+ but also provided extra active sites for Zn 2+ storage.
Under thermal treatment, carbon will consume the lattice of materials or surface O atoms to form defects. 24 For example, at high temperatures, vanadium oxide nanobers (VCN) were generated with physical and chemical defects by decomposing VO(acac) 2 /PAN precursor bers. 75The physical defects such as pore pathways and caverns can provide more storage sites for Zn 2+ and abundant chemical defects benet the Zn 2+ insertion/ extraction during cycling (Fig. 3e).As shown in Fig. 3f, compared with V 2 O 5 , the Zn//VCN cell produced higher capacity retention of about 83% and stabler coulombic efficiency (almost 100%) at 5 A g −1 aer cycling over 1000 cycles, which was attributed to the synergistic effect of dual defects.Table 1 summarizes the electrochemical performances of vanadiumbased materials with electrospinning bers.
4.1.2Manganese-based cathodes.Manganese-based materials, including MnO, MnO 2 , Mn 2 O 3 , Mn 3 O 4 , ZnMn 2 O 4 , MnS, and so on, have been widely studied in AZIBs because of their numerous merits such as high operating voltage, cheapness, abundant resources, and nonpoisonous nature. 104Unfortunately, some challenges prevent its practical application. 105anganese-based electrodes are usually constructed of active powder, conductivity agents, binders, and collectors.However, the poor electrical conductivity and random aggregation of manganese-based composites cannot realize fast charging at high current densities. 23arbon nanobers with large SSA and high electrical conductivity can be used as conductive substrates for cathode materials loading, which not only facilitates fast electron transfer but also simply the preparation of electrodes without binders and conductive additives. 44For example, Guo et al. used porous carbon bers (PCF) to support MnO 2 to form a freestanding PFC@MnO 2 electrode. 68Specically, the graphitic PCF fabricated by the electrospinning technique and hightemperature treatment with high electrical conductivity and uniform pores (Fig. 4a) favors the mass loading of MnO 2 (59.1%) and fast charging.As a result, owing to the fast ion/ electron transport ability of PFC@MnO 2 , the Zn//PFC@MnO 2 displayed impressive structural stability at various current densities.Besides, Yang et al. prepared high-exibility nitrogendoped carbon lms through an electrospinning technique and calcination with PAN, PVP, 2-methylimidazole, and zinc acetate as raw materials (Fig. 4b). 106During carbonization, the evaporation of Zn endowed the CNFs with a porous structure, which not only provided abundant reaction sites for the growth of d-MnO 2 but also had a strong electrostatic attraction for Mn 2+ .As displayed in Fig. 4c, the lamellar-like K + -intercalated d-MnO 2

Chemical Science Review
(KMO) was loaded on the surface of CNFs via the hydrothermal method of KMnO 4 , and the resulting KMO/CNFs presented a large surface area to enable expansion of the contact area between KMO and the electrolyte and promote ion transfer.Therefore, even aer 1000 cycles at 3 A g −1 , the KMO/CNFs still exhibited a reversible capacity of 190 mA h g −1 (Fig. 4d).What's more, compared with KMO, KMO/CNF showed lower chargetransfer and ion-diffusion kinetics, which was attributed to the existence of CNFs.Hiralal et al. explored the relationship between the capacity of the battery and the diameter of carbon bers when carbon bers were used as the substrate for the cathode. 37The results showed that decreasing the diameter will enhance the surface area, charge collection area, and conductivity of carbon bers, which will promote electrolyte diffusion in the electrode, resulting in a higher capacity battery.
There is no doubt that using carbon nanobers as a substrate is an effective way to improve the electrical conductivity of manganese-based compounds.However, the construction of a rm and tight interface between active materials and carbon bers is still a great challenge that needs to be addressed in the future.
Embedding active substances in carbon nanober matrixes could inhibit the dissolution of manganese-based materials and construct highways for electrons. 32,107,108For instance, Ding et al. prepared CNF coated bead-like manganese oxide (MnO x -CNFs) via the electrospinning method (Fig. 5a). 36As shown in Fig. 5b, the MnO x particles were tightly encapsulated in the amorphous carbon layer, which effectively relieved its dissolution.Moreover, Wu's group embedded MnS/MnO with the  heterostructures in N-doped carbon bers to form MnS/ MnO@N-CF with high ion and electron conductivity (Fig. 5c). 32As shown in Fig. 5d and e, the MnS/MnS nanoparticles were uniformly dispersed in carbon matrixes, and the edges of active materials were connected by a large amount of amorphous carbon, which was conducive to the storage of electrolyte and the enhancement of the conductivity of the materials.Beneting from the protection of the carbon layer, the structure of active materials remained stable without collapse and pulverization aer cycling, indicating an excellent stable cycling ability of the electrode.However, this strategy will partly reduce the ion transport efficiency and active substance utilization of active materials.As a result, the precise control of the structure of nanobers is essential to achieve a cathode with excellent electrochemical performance.As a typical example, Long et al. fabricated Mn 3 -O 4 @HCFs with core/shell structure by a coaxial electrospinning method and subsequent high temperature treatment. 63This ber consisted of a carbon shell with a thickness of about 70 nm (content of 12.7 wt%) and Mn 3 O 4 nanoparticles (Fig. 5f and g).The amorphous carbon layer not only served as a protective layer between Mn 3 O 4 and the electrolyte, preventing the dissolution of the active substance, but also mitigated the volume expansion of the electrode during cycling.In addition, the void spaces between the carbon shell and the Mn 3 O 4 core can accommodate a large amount of electrolyte, providing space for electrochemical reactions.Therefore, the battery based on the Mn 3 O 4 @HCFs cathode material displayed ultra-stable cycling capability with 96.9% capacity retention and high coulombic efficiency of around 100% aer 1300 cycles at 0.4 A g −1 (Fig. 5h).The precise control of nano-and microstructures can also be achieved by template methods. 109For example, the manganese dioxide precursor was wrapped on the surface of a CNF matrix using a hydrothermal method and then calcining to obtain tunnel-structured a-K 0.19 MnO 2 nanotubes. 110It is worth noting that the CNF as the template will be consumed during the calcining process.Owing to the stability of the structure of a-K 0.19 MnO 2 , the cathode possessed excellent rate and cycling performance.Table 2 summarizes the electrochemical performances of manganese-based materials with electrospinning bers.4.1.3Other cathode materials.In addition to vanadiumbased and manganese-based materials, many other cathode materials were prepared by the electrospinning method.Kim et al. fabricated a freestanding carbon ber (CF) as a current collector to support polyaniline (PANI) via electrospinning and carbonization (Fig. 6a). 39Especially, the CF with high conductivity (resistance about 20 U sq −1 ) was rstly activated by HNO 3 treatment to increase the number of active sites (some groups such as C]O, C-O, and O-C]O), which can promote the in situ polymerization of aniline monomers on the CF surface to achieve a PANI/CF cathode.Due to the high conductivity of the 3D CF, the PANI/CF showed a small electron resistance of about 400 U sq −1 , allowing the fast transfer of electrons.Beneting from the high conductivity and free-standing structure of composites, the PANI/CF can be used as an electrode directly without binder and conductive additives to assemble batteries in arbitrary geometries (Fig. 6b).As displayed in Fig. 6c, the battery with the PANI/CF electrode delivered excellent rate performance at 600C.
Xu et al. synthesized a composite in which hybrid carbon coated Na 3 V 2 (PO 4 ) 3 was interconnected with carbon nanobers (NVP/C/CNF) by electrospinning and sol-gel methods. 113As displayed in Fig. 6d and e, the NVP nanoparticles were randomly wrapped tightly in CNF to form a 3D conductive network to improve the electron conductivity and stable structure ability of the composite.Compared to NVP/C, the NVP/C/ CNF electrode exhibited a more stable cycling ability.The battery based on NVP/C/CNF displayed a high capacity retention of 82.5% aer 100 cycles at 0.1 A g −1 , which is much higher than that of the battery based on NVP/C (52.7%) (Fig. 6f).A comparison of the performance of other cathode materials is presented in Table 3.

Anodes
In aqueous electrolytes, the thermodynamic and electrochemical instability of the Zn metal anode dramatically shortens the service life of AZIBs and limits their practical applications. 114,115Among them, thermodynamic instability is manifested by serious corrosion reactions on the surface of Zn during cycling, which consumes the active Zn and decreases the coulombic efficiency of the Zn anode.The electrochemical instability is presented by uncontrollable dendrite growth, where the formed Zn dendrites will penetrate the separator, ultimately leading to the failure of the cell. 116As a result, various approaches have been proposed to address these above issues, including (1) optimizing the composition and concentration of electrolytes to stabilize the Zn anode; 117,118 (2) protecting the Zn anode surface from direct contact with the electrolyte by forming an interfacial layer and reducing the occurrence of corrosion side reactions; 119,120 and (3) constructing a 3D substrate that can help reduce local current densities and promote the uniform distribution of Zn 2+ , which is advantageous for the homogeneous deposition of Zn and inhibits the growth of dendrites. 78,121Among them, interfacial layer modication and 3D substrate construction are effective and direct strategies to protect the Zn anode.Carbon and polymer bers fabricated by the electrospinning method with high exibility adjustable structures are considered to be an ideal material for use as the protective layer and substrate for the Zn anode.Therefore, we will summarize and discuss the application of electrospinning bers for protective layers and substrates of the Zn anode.
4.2.1 Pure carbon bers.The unique advantages of carbon materials as a substrate or protective layer for the Zn anode can be summarized in the following aspects: (1) the carbon materials with large SSA and porous structure can lower the local current density and accommodate the volume variation of the Zn anode during cycling.(2) A carbon substrate-based anode with high exibility and processibility can be used to assemble exible batteries.(3) A carbon protecting layer can provide abundant ion channels to promote the transfer of Zn 2+ and inhibit the formation of Zn dendrites.As a typical carbon material, carbon bers exhibit high axial strength, low density, good expansion, anisotropy, and excellent corrosion resistance. 122,123In particular, the diameter and porosity of carbon bers can be controlled by the electrospinning method, which has more practical applications in anodes. 124For example, carbon nanober frameworks were prepared by electrospinning  and porosity of the nanobers could be adjusted by electrospinning parameters. 45Interestingly, the plasma treatment improved the surface hydrophilicity of the carbon bers, which was conducive to promoting the uniform deposition of Zn 2+ .Thus, beneting from the coordination of the 3D framework, conductivity, and hydrophilicity of the carbon bers, Zn was homogeneously deposited on the carbon bers without severe aggregation at a current density of 0.5 mA cm −2 with an areal capacity of 5 mA h cm −2 (Fig. 7a).Most importantly, at a 40% depth of discharge (DOD) (an areal capacity of 2 mA h cm −2 ), the Zn@CNF‖Zn@CNF symmetric cell was stably cycled over 193 h at a current density of 2 mA cm −2 (Fig. 7b).As demonstrated in Fig. 7c, compared with Zn@Ti//V 2 O 5 , the battery of Zn@CNF//V 2 O 5 displayed a better cycling ability.
In their study, Baek et al. produced a ZnCNF anode through the electro-deposition of Zn on the surface of electrospun carbon nanobers. 124The 3D porous network of carbon with large SSA (53.04 m 2 g −1 ) and high conductivity (830 S m −1 ) can decrease the local current density during the cycling process and provide more nucleation sites, thus reducing the nucleation overpotential of Zn in the initial stage.Meanwhile, the graphitic carbon with a low lattice mismatch interfacial layer to the Zn (002) plane can promote the preferred orientation of Zn to the (002) plane.Consequently, compared with bare Zn, the ZnCNF showed a smooth and compact anode surface aer cycles (Fig. 7d and e).As shown in Fig. 7f, the symmetric cell demonstrated a stabler plating/stripping behavior with a small voltage hysteresis of 23.9 mV aer 400 cycles at the current density of 0.1 mA cm −2 with an areal capacity of 0.1 mA h cm −2 .
4.2.2Carbon bers with zincophilic materials.Although the pure carbon bers with large SSA can contribute to the homogeneous distribution of the electric eld and conne the Zn in 3D pores to avoid its accumulation during the stripping/ plating processes, the hydrophobic and zincophobic carbon matrixes lead to a high energy barrier of Zn nucleation, which is unfavorable for the uniform growth of Zn. 125 The nucleation behavior of Zn is greatly affected by the surface properties of the substrate.Herein, zincophilic materials (such as functional groups and metal nanoparticles) are introduced on the surface of carbon matrixes to reduce nucleation polarization, achieving a highly reversible Zn cycling process and inhibiting the formation of Zn dendrites.
The functional groups including N, 125,126 C]O, 127 F, 128 and -NH 2 (ref.129) with high electronegativity serve as zincophilic sites to capture the positively charged Zn 2+ , guiding the homogeneous nucleation and plating of Zn.Chen's group fabricated a 3D N-doped carbon nanober lm@Zn (3DN-C@Zn) anode to assemble a 3DN-C@Zn//AlVO-DMF battery.The N doping can improve the hydrophilicity of carbon bers, decreasing the diffusion energy barrier of Zn 2+ . 130Therefore, the 3DN-C@Zn//AlVO-DMF battery was stably cycled over 200 cycles at 1 A g −1 without obvious capacity decay, which is better than that of bare Zn which suffered a short circuit aer three cycles at the same current density.Besides, Zhang's group reported

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a novel N,O co-doped carbon nanober interlayer of a Zn anode via the electrospinning method combined with carbonization treatment. 33At high temperatures, the PAN brous membrane transformed into a freestanding carbon ber interlayer doped with abundant O and N atoms.As the result of theoretical calculation, compared with other sites, the C]O/N Pd (−1.11 eV) and C]O/N Pr dual doping sites (−1.64 eV) showed higher binding energy with the Zn atom, indicating a higher ability to absorb Zn 2+ (Fig. 8a).Therefore, owing to the porous structure of carbon bers and high Zn affinity of N and O heteroatoms, a compact and at Zn deposition layer on the carbon ber interlayer can be observed aer cycling for 400 h at 5 mA cm −2 (Fig. 8b).As exhibited in Fig. 8c and d, at the current density of 5 mA cm −2 and areal capacity of 1 mA h cm −2 , the modied symmetric cell displayed a lower nucleation potential of about 59.5 mV, and a stabler cycling ability (over 1200 h).
In addition, the introduction of zincophilic metal nanoparticles such as Ag, 131 Sn, 121 Co, 132 and In 133 on the substrate can also enhance the zincophilicity of the carbon nanober matrix.These zincophilic metal nanoparticles can be coupled with the carbon bers to stabilize the Zn anode by lowering the nucleation potential of Zn and uniformizing the current density.Yang et al. prepared an Sn modied porous carbon ber (Sn-PCF) framework with a hollow structure to uniformize the deposition of Zn 2+ (Fig. 8e). 134At a high current density of 10 mA cm −2 with an areal capacity of 5 mA h cm −2 , the Sn-PCF@Zn‖Sn-PCF@Zn symmetric cell exhibited a small voltage hysteresis of 47 mV and a long cycle life (over 500 h), which was almost 10 times that of PCF@Zn.In addition, at a current density of 10 A g −1 , Sn-PCF@Zn//Na 2 V 6 O 16 $1.63H 2 O demonstrated a high capacity retention of 73.5% aer 2500 cycles.The reason for the high stable cycle performance of Sn-PCF can be described as the metal Sn possessing a high adsorption ability, which is favorable for regulating the nucleation and deposition of Zn.Besides, the metal Sn can increase the hydrogen evolution energy barrier of the electrode, inhibiting the occurrence of hydrogen evolution reactions.Therefore, owing to the synergetic effect of multifunctional Sn metal and 3D porous carbon, the Zn can be uniformly deposited on the surface of the Sn-PCF (Fig. 8f), and the Sn-PCF@Zn anode had an excellent cycling ability during the test.
Moreover, introducing Cu nanoparticles on the surface of carbon not only improves the conductivity of carbon bers but also promotes the deposition of Zn.Yang et al. reported Cu nanoparticle modied carbon bers (Cu@CNFs) as the protective layer to stabilize the anode. 135Beneting from the large SSA of carbon bers and the zincophilicity of Cu nanoparticles, the Cu@CNFs-Zn exhibited low polarization and high deposition/ dissolution efficiency in cycling.
In addition to doping metal nanoparticles on carbon bers to homogenize Zn 2+ deposition, many researchers have added metal oxides to electrode materials to achieve stable cycling of the Zn anode.For instance, defective ZnO x nanoparticles also demonstrated good affinity for Zn, which can be used to enhance the zincophilicity of electrospun carbon bers. 136Xue et al. fabricated a 3D porous ber with TiO 2 and SiO 2 uniformly distributed in the interior of hollow HSTF. 35Directed by the uniform TiO 2 , the Zn preferred to deposit at the zincophilic TiO 2 seeds inside the bers and was further accommodated in the porous carbon ber matrixes without the growth of Zn dendrites.As shown in Fig. 9a, with the increase in plating capacity, the Zn tended to form a uniform and dense deposition layer in the porous pores rather than the surface of carbon bers.Besides, the inert material of SiO 2 can signicantly reduce the desolvation active energy during cycling and improve the deposition efficiency of Zn.Consequently, at a high current of 20 mA cm −2 , the Zn@HSTF anode demonstrated a highly stable plating/stripping behavior over 2000 cycles (Fig. 9b).Furthermore, the Zn@HSTF//MnO 2 full battery delivered impressive cyclability with 85% capacity retention aer 1000 cycles at 1 A g −1 .
3D carbon bers with functional groups and metal-based nanoparticles could combine the synergistic effects of two zincophilic materials to homogenize the deposition of Zn 2+ .Yu et al. fabricated a 3D conductive ber network (Sn@NHCF) consisting of N-doped hollow carbon and Sn nanoparticles. 58he Sn nanoparticles and doped N element possess high zincophilicity and can reduce the nucleation barrier in cycling.Therefore, even aer 100 cycles, the Sn@NHCF-Zn electrode exhibited a high coulombic efficiency of 99.7% at a current density of 5 mA cm −2 with 5 mA h cm −2 .Typically, Zeng et al. prepared N,P-codoped carbon macroporous bers embedded with atomically dispersed Cu and Zn atoms (Cu/Zn-N/P-CMFs) as the host for the deposition of Zn. 42 It is worth noting that the introduction of N and P atoms not only enhanced the hydrophilicity of carbon bers but also facilitated the dispersion of Cu and Zn atoms.Besides, they produced Cu-p/Zn-N-CMFs by substituting tannic acid for phytic acid, highlighting the crucial function of P. The results showed that in the absence of PA, Cu aggregated from nanoparticles, which will decrease the reversibility of Zn plating/stripping.The results of theoretical calculation further revealed the zincophilicity of Cu, Zn, N, and P atoms, which can decrease the nucleation overpotential of Zn and favor the oriented deposition of the Zn(002) plane to achieve a dendrite-free anode (Fig. 9c and d).As displayed in Fig. 9e, at a plating capacity of 2 mA h cm −2 , the Zn was uniformly deposited on the surface of the substrate with parallel nanoakes.As a result, the Cu/Zn-N/P-CMFs-Zn‖Cu/Zn-N/P-CMFs-Zn cell displayed a small voltage hysteresis (44.9 mV) and a long cycle life (630 h) at a current density of 2 mA cm −2 with 2 mA h cm −2 (Fig. 9f).In contrast, the battery based on the Zn-N-CMFs-Zn electrode suffered a short-circuit aer 110 h due to the serious Zn dendrite growth.Moreover, the Cu/Zn-N/P-CMFs-Zn//MnO 2 exhibited ultralong life up to 2500 cycles with a capacity retention of 88.8% at 1 A g −1 .
4.2.3Polymer bers.Although the excellent conductivity of carbon bers can reduce charge accumulation and facilitate electric eld distribution, the metal Zn tends to deposit inside the layer, easily resulting in a non-uniform plating behavior. 128n addition to carbon bers, the electrospun polymer bers also play an essential role in Zn anode protection.Compared with carbon, the polymer nanober protective layer can be formed in situ by the electrospinning method which avoids the utilization of the binder. 137More importantly, the thickness of the polymer ber layer can be controlled by modulating the electrospinning time.Moreover, the polymer layer has a high exibility and porous structure, and most of the polymer layer is ionically conductive but electronically insulating, which is benecial for transporting Zn 2+ across the interface layer and the uniform deposition of Zn 2+ . 138,139In fact, the polymer possesses numerous polar groups that serve as adsorption sites for Zn 2+ transfer along the polymer chain to the reaction interface. 140dditionally, these groups facilitate the homogeneous distribution of Zn 2+ at the molecular scale by enabling fast ion transport rates.Liu et al. reported an articial interface (TPZA) with high ionic conductivity (19.8 mS cm −1 ) by permeating Znalginate (ZA) into porous thermoplastic polyurethane (TPU) bers (Fig. 10a). 141As shown in Fig. 10b and c, owing to the protection of TPZA, the anode sustained the pristine morphology without the formation of by-products.For comparison, aer 30 days, the Zn anode which was immersed in the electrolyte was randomly covered by the oriented hexagonal Zn 4 SO 4 (OH) 6 $3H 2 O.In addition to the property of anticorrosion, the Zn 2+ can transfer along the polymer chains of Zn-Alg, improving the transfer kinetics of Zn 2+ .Therefore, the Zn@TPZA//Zn@TPZA can be stably cycled over 1200 h at a current density of 5 mA cm −2 with a capacity of 5 mA h cm −2 (Fig. 10d).
A polybenzimidazole (PBI) nanober with abundant Ncontaining functional groups can promote the uniform deposition of Zn.Jian et al. constructed a PBI framework on the surface of Cu foil by an electrospinning method to serve as the substrate for Zn deposition, promoting uniform nucleation of Zn and achieving a dendrite-free Zn anode. 47The PBI nanober host with polar amine groups and porous structure can promote the permeation of electrolytes in the electrode.As illustrated in Fig. 10e, during the plating of Zn, the amine groups can act as nucleation seeds to guide the Zn to evenly deposit on the pores of the PBI nanober substrate to inhibit the formation of Zn dendrites.Consequently, at a current density of 10 mA cm −2 , the Zn@PBI-Cu anode showed a compact surface without the vertical growth of zinc dendrites aer 100 cycles (Fig. 10f).Besides, at a current density of 1 A g −1 , the Zn@PBI-Cu//MnO 2 displayed high capacity retention (close to 100%) and a high coulombic efficiency of about 100% aer 100 cycles (Fig. 10g).Although this polymer ber shows outstanding ability to suppress the growth of Zn dendrites, these nonconductive layers exhibit a huge impedance of interfaces which is not conducive to the rate capability of AZIBs. 142The reported electrospun bers in the anode and their corresponding electrochemical performance are summarized in Table 4.  Review Chemical Science

Separators
High-performance AZIBs depend on the synergy of all components.The separator acts as a carrier for the electrolyte, controlling the transport of ions, which determines the performance of the battery.Glass ber separators are widely applied in AZIBs due to their high wettability, high ionic conductivity (about 17.3 mS cm −1 aer absorbing electrolyte), and abundant porous structure.However, the metal Zn deposit in these pores of the glass ber separator cannot be entirely converted to Zn 2+ in the stripping process, ultimately resulting in the formation of "dead Zn". 27Moreover, the glass ber separator that absorbs excess electrolytes increases the total mass of the battery resulting in a low energy density. 26Although lter paper and non-woven fabric separators possess excellent mechanical properties and high porosity, their further application is prevented by the poor transport regulation ability. 147n ideal separator for AZIBs should not only have excellent ionic conductivity aer taking in the electrolyte but should also regulate the transport of Zn 2+ during the cycling process and prevent the growth of Zn dendrites.Compared to conventional separators, electrospun polymer ber separators have attracted extensive attention because of their thermal stability, mechanical merit, electronic insulation, high mechanical exibility, and controllable structure. 148In addition, the functional groups in the polymer ber can promote the formation of coordination bonds with Zn 2+ , homogenizing the deposition of Zn 2+ and suppressing the formation of Zn dendrites. 414.3.1 Pure polymer separators.Owing to its excellent electrochemical stability, PAN has oen been used to fabricate electrospun ber separators. 149,150To stabilize the Zn anode, Liang's team synthesized a 3D long-range ordered PAN separator. 34Compared to the glass ber separator (640.8%), the lower electrolyte uptake value (430.3%) of PAN lm is advantageous for improving the energy density of the battery.Furthermore, the abundant -CN functional groups in the bers not only promoted the electric eld uniform distribution but also combined with Zn 2+ to guide the uniform deposition of Zn 2+ and effectively inhibit the growth of Zn dendrites.Beneting from the mechanical exibility, the PAN lm was used as the separator and the current collector to prepare novel "paper-like" AZIBs with an all-in-one structure. 151As displayed in Fig. 11a and b, the Zn and MnO 2 nanosheets were closely deposited on both sides of PAN which was modied by carbon nanotubes to form a cell with a thickness of about 97 mm, accelerating the transfer of electrons and achieving rapid kinetics.Therefore, the full cell exhibited a high capacity retention of about 98.7% aer 500 cycles at 1 mA cm −2 .In addition, at a bending angle of 180°, the battery also showed a high discharge capacity aer being cycled at various current densities, indicating an excellent rate performance and outstanding exibility (Fig. 11c).
4.3.2Hybrid polymer separators.Although a pure polymer lm with high porosity and large SSA can be prepared by the electrospinning method, the poor mechanical strength has limited its application in exible devices.Compared with pure polymer separators, hybrid polymer separators are prepared by mixing different types of substances by the electrospinning method (or pure polymer separators are modied by functional materials) which can promote the uniform deposition of Zn 2+ and improve the mechanical strength of separators due to the multi-functional role and synergistic effect of the newly formed hybrids.For example, Saisangtham et al. used highly exible polyurethane (PU) as the raw material to prepare PAN/bio-based PU separators by the electrospinning method. 152Besides, they investigated the effects of electrospinning solution concentration and parameters on the separators.The results revealed that the PAN separator modied by PU had a tensile strength of 44.16 MPa, which is much higher than that of the pure PAN membrane.
Moreover, some functional materials including graphene oxide (GO), 48 sulfonated polysulfone (SPSF), 155 and MXene 154 have been added to regulate the ux of Zn 2+ .Among them is the strong interaction between the functional groups in polydopamine (PDA) and Zn 2+ , which promotes the transport of Zn 2+ on the surface between the separator and electrolyte.Zhou's group developed a PDA functionalized PVDF (PVDF@PDA) to uniformize the homogeneous distribution of Zn 2+ and suppress the formation of Zn dendrites (Fig. 11d). 41These abundant polar functional groups (-OH and -NH-) in the PDA improved the hydrophilicity of PVDF@PDA as well as favoring the formation of Zn-O and Zn-N coordination bonds with Zn 2+ .According to density functional theory calculations, the Zn-O and Zn-N can function as nucleation seeds to decrease the nucleation barrier of

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Zn and guide the ordered deposition of Zn 2+ .Herein, compared with the glass ber separator (Fig. 11e), the surface of Ti foil with the functional separator was even without agglomeration and cracks at 2 mA cm −2 and 5 mA h cm −2 (Fig. 11f).Besides, the PVDF@PDA hybrid separator can effectively prevent the shuttling of V-species by formation of the V-O coordination bond during cycling.Therefore, as demonstrated in Fig. 11g, the Zn// NH 4 V 4 O 10 full cell with the PVDF@PDA separator exhibited a high capacity retention of 92.3% aer 1000 cycles at 5 A g −1 .
Poly(m-phenylenedicarboxamide) (PMIA) with abundant amide groups, electrolyte affinity, and outstanding mechanical strength has been used as the separator for Li metal batteries. 156nspired by this, Hu et al. fabricated a hybrid SPSF@PMIA (SP) nanober separator to stabilize the Zn anode. 155The abundant hydrophilic -SO 3 − in SPSF and the N atom in PMIA with electronegativity will repel anions, which limit the migration of anions and enable the fast transfer of Zn 2+ .Therefore, compared with the batteries with PMIA (glass ber or SPSF), the Zn/SP/Zn showed a higher Zn 2+ transfer number (t Zn 2+ ) of 0.74, which benets the fast ion diffusion and fast charge transfer processes.Besides, owing to the strong ability of -CO-NHin PMIA to absorb Zn 2+ and the zincophilicity of -SO 3 − in SPSF, the battery with the SP separator demonstrated a stable cycling ability and rate performance.Table 5 summarizes the polymer nanober separator performance.

Summary and perspectives
In conclusion, the reasons for the outstanding properties of the electrospun nanobers are as follows.First, electrospun carbon bers with large SSA and high conductivity can improve the electronic conductivity of materials and promote the diffusion of electrolyte in electrodes, which improve the rate performance and cycling ability of the battery.Second, these materials play a momentous role in maintaining the structural stability of electrodes.The porous (or hollow) structure can accommodate the Zn deposition and prevent the volume variation of the anode.In addition, the dissolution of active materials can be suppressed by forming a physical protective layer.Third, electrospun bers with high porosity and exibility can be used as binder-free and bendable electrodes, promising for bendable and wearable devices.
In this review, we summarized the recent progress of electrospinning nanobers in AZIBs, focusing on vanadium-based materials, manganese-based materials, other cathode materials, carbon ber-based and polymer ber substrates, Zn anode protective layer, and polymer separators.In addition, we briey introduced the principle and processing of the electrospinning technique and structural design of the electrospun bers.Despite electrospinning bers having made some research progress in AZIBs, several challenges still remain to be addressed.Therefore, to broaden the application of electrospun nanobers, the following suggestions should be considered.

Precise preparation of functional bers
The microstructure and properties of the electrospun bers are related to the precursor solution, electrospinning parameters,  and subsequent electrospinning process.However, very few studies have investigated the relationship between various parameters and the performance of bers in AZIBs.Besides, various zincophilic units (such as functional groups, metal nanoparticles, metal oxides, and heteroatoms) have been reported to improve the zincophilicity and hydrophilicity of bers to facilitate the homogeneous deposition of Zn.Sometimes, excessive zincophilic materials tend to accumulate together, which not only does not homogenize the Zn deposition but also changes the Zn deposition behavior, resulting in a severe growth of Zn dendrites.Thus, the preparation parameters of electrospun bers should be systematically investigated and optimized.

In-depth investigation of the mechanisms
The working mechanism of the ber material cannot be explained simply as the uniform distribution of the electric eld on the surface of the Zn anode, the regulation of the ux of zinc ions, and the zincophilicity of the modied material.Specic experimental evidence should be provided.Moreover, some advanced characterization techniques including in situ optical microscopy (OM), in situ electron microscopy (EM) and in situ neutron depth proling (NDP) and imaging can be used to elucidate the Zn growth mechanism.Analysis of the Zn metal is crucial to understanding the failure mechanism of AZIBs.
Considering that the dynamics of electrochemical processes are difficult to observe during cycling, theoretical calculation can be used to further understand the mass transfer process of Zn 2+ .

Establishing the test standards
Although electrospun nanober electrodes show an impressive long cycle life at a small current density, it is difficult to meet the requirements of commercial applications.Moreover, different standards were used to test the batteries in previous studies, making it difficult to objectively evaluate various modication strategies.Therefore, it is important to establish unied test standards, which will facilitate the application of AZIBs.Besides, the electrochemical performance of the battery over a wide range of temperatures should be provided to promote the practical application of AZIBs in all climates.

Promoting large-scale commercial application of electrospinning technology
Electrospinning technology provides new insights into improving the performance of batteries.However, it is difficult to apply in industrial production on a large scale due to the use of toxic and corrosive solvents, expensive precursors, and lower production efficiency.Therefore, improving production efficiency, developing low-toxicity and environmentally friendly solvents, and exploring new types of and inexpensive polymer precursors are the main development directions for the future.

Fig. 2
Fig. 2 (a) Schematic illustration of fabricating gradient composite films.(b) Cycling performances and (c) rate capabilities of the upgraded cathode.Adapted from ref. 90, copyright 2022, Elsevier B.V. (d) Schematic illustration of the reaction mechanism of the V 2 O 5 electrode.Adapted from ref. 38, copyright 2019, Elsevier B.V. (e) SEM and (f) TEM images of the hierarchical hybrid fibers with V 2 O 5 .Adapted from ref. 91, copyright 2019, American Chemical Society.

Fig. 3
Fig. 3 (a) A diagrammatic representation of the synthetic procedure and structure of N@C/V 2 O 3 composites.(b) SEM image of samples.(c) The cycling ability and coulombic efficiency of the N@C/V 2 O 3 cathode at 10 A g −1 .Adapted from ref. 98, copyright 2020, Elsevier B.V. (d) An illustration of the synthetic process for Fe-doped V 2 O 5 .Adapted from ref. 99, copyright 2021, Elsevier B.V. (e) TEM image of VCN fibers.(f) Long-term cycling performance of VCN at 5 A g −1 .Adapted from ref. 75, copyright 2020, Elsevier B.V.

Fig. 5
Fig. 5 (a) Schematic illustration of the synthesis process and (b) TEM image of MnO x -CNFs.Adapted from ref. 31, copyright 2022, American Chemical Society.(c) An illustration of the MnS/MnO@N-CF synthesis process.(d) SEM and (e) TEM images of MnS/MnO@N-CF.Adapted from ref. 27, copyright 2022, Elsevier B.V. (f and g) TEM images of Mn 3 O 4 @HCFs.(h) Long cycling performance and coulombic efficiency of the Mn 3 O 4 @HCF electrode at 0.4 A g −1 .Adapted from ref. 59, copyright 2020, Elsevier Ltd.

Fig. 6
Fig. 6 (a) A schematic diagram showing the in situ polymerization of aniline in an aqueous solution to synthesize a PANI/CF cathode.(b) Optical images of ring-, H-, and cylindrical shapes of Zn-PANI batteries.(c) Cycling ability of the cells with different PANI loading.Adapted from ref. 34, copyright 2018, American Chemical Society.(d) SEM and (e) TEM images of NVP/C/CNF.(f) Cycle performance of NVP/ C/CNF and NVP/C at 0.1 A g −1 .Adapted from ref. 109, copyright 2021, American Chemical Society.

Fig. 7
Fig. 7 (a) SEM image of CNFs after being electrodeposited with an amount of Zn at a current density of 0.5 mA cm −2 with a capacity of 5.0 mA h cm −2 .(b) Cycling performance of symmetric cells with different electrodes at 2 mA cm −2 .(c) Cycling ability of full cells at the current density of 0.5 A g −1 .Adapted from ref. 45, copyright 2022, American Chemical Society.(d) The deposition behaviors of Zn 2+ on the different substrates.(e) HRTEM pattern of CNF.(f) Long cycling performance of bare Zn and ZnCNF symmetric cells.Adapted from ref. 124, copyright 2022, John Wiley & Sons Ltd.

Fig. 8
Fig. 8 (a) A comparison of the binding energy between Zn atoms and different adsorption sites.The morphology (b), charge/discharge curves (d), and Zn nucleation overpotential (c) at 5 mA cm −2 with a capacity of 1 mA h cm −2 .Adapted from ref. 33, copyright 2021, Elsevier B.V. (e) The SEM image and (f) Zn plating and nucleation diagrams on Sn-PCF.Adapted from ref. 134, copyright 2022, Elsevier B.V.

Fig. 9
Fig. 9 (a) SEM images showing the top view and cross-sections of the HSTF host after plating with various deposition capacities.(b) Voltage profiles of symmetrical cells at current densities of 20 mA cm −2 and 1 mA h cm −2 .Adapted from ref. 35, copyright 2021, Wiley-VCH.(c) A comparison of the binding energy between Zn atoms and different adsorption sites.(d) Nucleation overpotential of Zn on different substrates at current densities of 2, 3, and 5 mA cm −2 .(e) FESEM image of the Cu/Zn-N/P-CMF framework after Zn plating with capacities of 2 mA h cm −2 .(f) Cycling performance at 2 mA cm −2 and 2 mA h cm −2 for symmetric cells using different composite Zn electrodes.Adapted from ref. 42, copyright 2023, American Chemical Society.

Fig. 10
Fig. 10 (a) A description of the fabrication process of Zn@TPZA.SEM images of (b) bare Zn and (c) Zn@TPZA after immersion in 2 M ZnSO 4 electrolyte for 30 days.(d) Cycling performance of bare Zn and Zn@TPZA anodes at 5 mA cm −2 /5 mA h cm −2 .Adapted from ref. 133, copyright 2022, Wiley-VCH.(e) Schematic illustration of Zn deposition on a PBI nanofiber framework modified Cu electrode.(f) SEM image of Zn@PBI-Cu after 100 cycles at 10 mA cm −2 .(g) Long-term cycling performance of the battery at 1 A g −1 .Adapted from ref. 42, copyright 2020, Royal Society of Chemistry.

Fig. 11 (
Fig. 11 (a) Diagrammatic sketch showing the fabrication procedure of the AZIB.(b) SEM image of the cross-sectional view of the individual AZIB.(c) Rate performance of the AZIB cell at variational bending angles of 30, 60, 90, and 180°.Adapted from ref. 151, copyright 2021, American Chemical Society.(d) Schematic illustration of the fabrication process of the PVDF@PDA separator.SEM images of the Ti foils after Zn deposition at 2 mA cm −2 and 5 mA h cm −2 in Zn/Ti asymmetric cells with (e) a GF separator and (f) PVDF@PDA separator.(g) Long cycling performance of batteries with different separators at 5 A g −1 .Adapted from ref. 41, copyright 2022, The Authors.

Table 1
A summary of electrospinning vanadium-based nanofiber materials for AZIBs

Table 3
A summary of electrospinning nanofibers for other cathode materials of AZIBs aer 100 cycles at 0.1 A g −1 65.0 mA h g −1 at 1.0 A g −1 113 © 2023 The Author(s).Published by the Royal Society of Chemistry Chem.Sci., 2023, 14, 13346-13366 | 13353 Review Chemical Science and calcination treatments, where the diameter (about 200 nm)

Table 5
A summary of electrospinning nanofibers for separators of AZIBsSeparator