Enabling Stable Zn Anode with PVDF/CNTs Nanocomposites Protective Layer Toward High‐Performance Aqueous Zinc‐Ion Batteries

Due to its abundance of natural resources, high theoretical capacity, and suitable redox potential (−0.76 V vs SHE), Zn anode has received extensive attention both from academy and industry. However, the coexistence of Zn and H2O, which is unfortunately thermodynamically unstable, always involves severe metal corrosion, H2 evolution, dendrite growth, resulting in low reversibility of Zn anode. Herein, a phase transfer method is adapted to design a porous conductive protective layer on the Zn anode, denoted as (PVDF (Polyvinylidene fluoride)/CNTs(Carbon Nanotubes)‐PT(phase transfer) @ Zn). Based on in situ characterization, COMSOL simulation, and migration energy barrier calculation, it can be demonstrated that PVDF/CNTs‐PT @ Zn effectively inhibited the production of Zn dendrites and the side reactions triggered by H2O, achieving uniform deposition. Especially, the full picture of Zn deposition is observed using in situ computed tomography (CT). The symmetrical cell using the PVDF/CNTs‐PT @ Zn demonstrates dendrite‐free plating/stripping and possesses much better cycle stability than the bare Zn. A stable rechargeable full battery is demoed through coupling the PVDF/CNTs‐PT @ Zn anode with commercial V2O5. The strategy showcases a feasible pathway to inhibit Zn dendrite and side reactions in aqueous Zn ion battery, opening a promising avenue for the construction of metal anode protection layer.


Introduction
Aqueous Zn ion batteries have become a strong competitor in the field of large-scale energy storage due to their safe and green operation environment, abundant supply of Zn sources (≈300 times higher than lithium), and high theoretical capacities (820 mAh g −1 ). [1]In spite of such attractive advantages, the investigations on aqueous Zn ion batteries are still hindered by the problems such as the coupling degradation of dendrite growth, hydrogen evolution, and corrosion, which jointly affect the sustainable operation. [2]The uneven Zn deposition caused by "tip effect" and "concentration polarization" can generate Zn dendrites. [3]The following growth of Zn dendrites will pierce the separator, resulting in short circuit of the battery. [4]ince the Zn dendrite structure is loose and porous, the contact area between the electrode and the electrolyte will increase, providing additional reaction sites and thereby accelerating hydrogen evolution and corrosion. [5]The resulting bubbles generated by hydrogen evolution will attach to the anode surface and affect Zn nucleation, which not only increases the nucleation overpotential of Zn but also leads to uneven Zn deposition. [6]In addition, the increase of OH − concentration in the electrolyte can further accelerate the corrosion reaction.This accelerated corrosion and uneven Zn deposition will form a rougher Zn anode and in turn further aggravate the generation of Zn dendrites. [7]Recently, researchers have found many strategies to alleviate the above problems, such as interface modification, [8] electrolyte additive strategy, [9] intercalated anode, [10] alloy anode, [11] separator design, [12] hydrogel electrolyte, [13] and salt-tolerance training strategy, [14] etc.For example, Zhou et al. prepared a disordered Zn silicate interface with an ionic conductivity of up to 9.29 mS cm −1 .The abundant channels accelerate the transport of Zn 2+ , redistribute the flux of Zn 2+ , and promote the uniform deposition of Zn. [15] In addition, Linda F. Nazar et al. introduced DOTf into the electrolyte, which was decomposed into triflic superacid to form a SEI with a solid nanostructure. [16]This not only excludes H 2 O but also achieves dense Zn deposition.More recently, Hu et al. prepared a chitosan hydrogel with high mechanical strength, Zn 2+ conductivity, and water binding capacity, which achieved an ideal Zn deposition morphology. [17]However, simple and low-cost manufacturing process is a necessary condition for large-scale production.
Among those strategies, interface modification on the anode is a simple and effective way.Coating a protective layer on the Zn anode can separate the Zn anode from the electrolyte, alleviate the side reactions, and improve the stability of the Zn anode.Polyvinylidene fluoride (PVDF) is a polymer that is alternately connected by-CF 2 -and-CH 2 -.It is widely used in the field of batteries due to its low cost, strong chemical stability, [18] and high density of polarizable F-containing groups. [19]However, PVDF usually suffers from insufficient ionic conductivity and effectiveness during long-term cycling. [20]Therefore, proper modification of PVDF is required to achieve high performance of the battery such as performing a convert from -PVDF to a polar -PVDF. [21]n addition, the combination of molecular sieve [22] and PVDF could induce the uniform deposition of Zn 2+ .And also, the gradient design of PVDF, Sn, and Zn with imprinted 3D structure [23] played a good role in the regulation of polarity, uniform distribution of Zn 2+, and induction of Zn 2+ deposition, which improved the shortcomings of PVDF to a certain extent.However, the combination of PVDF and 3D conductive network can make up for each other's shortcomings and give full play to their respective advantages.This will be of great significance for inhibiting Zn dendrites and promoting uniform deposition of Zn 2+ .
In this work: PVDF/CNTs-phase transfer @ Zn (denoted as: PVDF/CNTs-PT @ Zn) was prepared by phase transfer method.PVDF and CNTs were fully mixed in NMP and directly coated on untreated Zn foil to prepare a porous and conductive protective film for aqueous Zn ion battery anode.The incorporation of uniformly distributed CNTs into PVDF membrane results in the formation of a 3D conductive network.The addition of CNTs significantly boosts the ionic conductivity of the composite and facilitates the establishment of a uniform electric field distribution at the PVDF/CNTs-nPT film.Contrasted with conventional PVDF membranes, the porous architecture formed by PVDF during the phase transfer process offers ample pathways for Zn 2+ transport.This structural configuration, characterized by its extensive specific surface area, contributes significantly to the reduction of interface current distribution.Additionally, it mitigates the volumetric alterations encountered during prolonged cycles of plating and stripping.The presence of this 3D porous structure is therefore instrumental in enhancing the overall electrochemical stability and performance of the membrane.In addition, the abundant F in PVDF repels H 2 O while regulating the concentration distribution of Zn 2+ on the Zn surface through Coulomb interaction.This alleviates the side reaction of Zn and H 2 O aggravated by the increase of specific surface area of 3D porous conductive network.Therefore, PVDF/CNTs-PT @ Zn can simultaneously regulate the concentration field and electric field redistribution, jointly promote the uniform distribution and deposition of Zn 2+ , and inhibit the generation of dendrites, corrosion, and hydrogen evolution side reactions.Therefore, the preparation of high-performance Zn anode by phase transfer method has a good cost effect and provides an idea for industrial production of Zn anode.

Results and Discussion
In this work, a porous protective film (PVDF/CNTs) composed of PVDF (Figure S1, Supporting Information) particles and CNTs was prepared by phase transfer method.The preparation process is shown in Figure 1a.While the PVDF membrane enters the H 2 O, the diffusion of NMP while from the casting solution to the water phase will cause local phase separation due to the well miscibility between water and NMP.PVDF forms initial precip-itation″ in the water in the form of clusters.These PVDF clusters act as growth centers and grow into spherical PVDF particles in the subsequent process.Therefore, a rich porous structure is formed during the diffusion process. [24]This method has the advantages of keeping simple and feasible, suitable for largescale preparation.First, the structure and surface morphology of the material were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM).The SEM image of crosssection through directly breaking after frozen in liquid nitrogen (Figure 1b) showed a flat, porous, and uniform PVDF/CNTs-PT protective film with the thickness of ≈100 μm.The cross section cut by the FIB (Figure 1c) shows that the protective film fits tightly with the electrode.Although after bending, folding, and twisting (Figure S2, Supporting Information), the PVDF/CNTs-PT protective film and Zn can be still firmly anchored together to resist the volume change during plating/stripping process.From Figure 1d, it can be seen that the spherical PVDF particles obtained in the phase transfer process are not isolated and instead, they are connected together by fibrous shape.The obtained PVDF/CNTs-PT protective film is a porous network structure composed of spherical PVDF particles embedded in the 3D CNTs matrix, which is the unique feature of PVDF phase transfer. [24]he uniformly distributed spherical PVDF particles repel H 2 O to attract Zn 2+ under the action of electrostatic repulsion before the hydrated Zn 2+ reach the negative electrode.Therefore, the uniform distribution of Zn 2+ concentration field at the interface is promoted.As the substrate of the protective film, the uniform distribution of CNTs was proved by TEM (Figure 1e).This distribution redistributes the electric field on the zinc surface, thereby making the current density more uniform.More importantly, the 3D porous network structure greatly increases the contact area between the electrode and the electrolyte and promotes the rapid migration of Zn 2+ .XRD analysis confirmed the successful preparation of PVDF/CNTs-PT protective film.It can be demonstrated from Figure S3 (Supporting Information) that PVDF/CNTs-PT @ Zn maintained the original lattice structure of Zn, and the peak at 26.2°corresponded to the carbon in multi-walled CNTs.The chemical structure of PVDF/CNTs-PT @ Zn was analyzed by X-ray Photoelectron Spectroscopy (XPS), no Zn 2P peak was found in the sample (Figure S4a, Supporting Information), [25] indicating that the PVDF/CNTs-PT protective film was well covered.Since PVDF is a polymer rich in-CF 2 -, the C1s spectrum shows a strong peak of -CF 2 -(291.5 eV) (Figure S4b, Supporting Information), which proves that the molecular structure of PVDF is rich in F atoms.In addition, the F 1s spectrum showed a single peak at 689 eV (Figure S4c, Supporting Information).What's important, the C─F bonds show strong polarity due to the large electronegativity of F. As a result, these C─F bonds act as Lewis basic sites and can promote the diffusion of Zn 2+ with lewis acidity, while H 2 O are repelled.This interaction not only reduces the side reaction caused by H 2 O on the surface of Zn anode, but also lowers the deposition barrier of Zn 2+ and promotes the rapid transfer kinetics of Zn 2+ .
In order to evaluate the electrochemical performance of Zn anode protected by different protective layers.The cycling stability of different Zn anodes was measured at 1 mA cm −2 and 1 mAh cm −2 in 2M Zn(CF 3 SO 3 ) 2 .As shown in Figure 2a, compared with the single PVDF protective film, the introduction of CNTs significantly improves the cycle stability of the symmetrical battery. [26]This promotion stems from the improvement on the electron conduction ability and ion conduction ability by the addition of CNTs.In addition, the morphology of the protective film also plays a crucial role in the stable plating/stripping of Zn anode.As shown in Figure S5a (Supporting Information), the PVDF membrane obtained by not phase transfer (PVDF-nPT) cannot provide sufficient channels for the transport of Zn 2+ .However, the PVDF membrane obtained by phase transfer (PVDF-PT) is composed of interconnected spherical PVDF particles (Figure S5b, Supporting Information), which is conducive to the full infiltration of electrolyte and the transfer of Zn 2+ .Compared with the PVDF/CNTs protective film obtained by not phase transfer (PVDF/CNTs-nPT), which is interwoven with intricate CNTs (Figure S5c, Supporting Information), the protective film obtained by phase transfer method (PVDF/CNTs-PT) produces abundant pores between spherical PVDF particles.The widely distributed and dense pores ensure a universal and uniform Zn 2+ transport channel on the surface of the protective layer, while the electroplating/stripping of Zn 2+ will not be affected.Under the combined effect of porous structure generated by phase transfer, uniformly distributed spherical PVDF particles and CNTs network, the PVDF/CNTs-PT @ Zn symmetric battery exhibits a stable polarization voltage and more than 1300 h stable electroplating/stripping.However, the voltage-time curve of the bare Zn electrode was disordered soon, which was obviously a short circuit caused by dendrites and side reactions.The impact of the protective layer's thickness on its cycle performance has undergone systematic evaluation, the results of which are illustrated in Figure S6 (Supporting Information).Through this analysis, it was discerned that a protective layer with a thickness of 100 μm demonstrated the most favorable outcomes.Based on these findings, a thickness of 100 μm has been established as the standard for the protective layers utilized in our research.When the current density/deposition capacity increases to 5 mA cm −2 , 1 mAh cm −2 (Figure 2b), the PVDF/CNTs-PT @ Zn symmetrical battery can still maintain more than 950 h cycle stability.In stark contrast, the bare Zn cannot withstand high current density, and short circuit occurs soon.The rate stability of PVDF/CNTs-PT @ Zn symmetric batteries at different current densities was also confirmed (Figure 2c).PVDF/CNTs-PT @ Zn symmetric batteries showed excellent rate performance, and the voltage hysteresis was lower than that of bare Zn.In the 2 ZnSO 4 , PVDF/CNTs-PT @ Zn still showed non-negligible cycle stability, and the cycle life (more than 1100 h) was much longer than that of bare Zn (Figure 2d).At the same time, it shows higher cycle stability than other reports (Figure 2e).In order to further evaluate the reversibility of electroplating/stripping and the utilization of Zn, the coulombic efficiency of bare Zn//Cu and PVDF/CNTs-PT @ Zn//Cu asymmetric batteries was measured [27] (Figure S7a, Supporting Information).The specific mechanism is that Zn 2+ are first dissolved from the Zn metal anode and deposited on the working electrode Cu during discharge, and then check how much Zn 2+ can be stripped from Cu during charging. [28]t 2 mA cm −2 , the PVDF/CNTs-PT @ Zn//Cu asymmetric battery exhibits more outstanding reversibility and durability than the bare Zn//Cu asymmetric battery.In addition, by comparing the polarization voltage changes between bare Zn//Cu and PVDF/CNTs-PT @ Zn//Cu asymmetric batteries, it can also be confirmed that the cycle performance and reversibility are improved in the presence of a protective layer.In Figure S7b,c (Supporting Information), the voltage hysteresis of PVDF/CNTs-PT @ Zn//Cu asymmetric battery is only 47.9 mV, which is much smaller than that of bare Zn//Cu (154.9 mV), which means that PVDF/CNTs-PT coating can effectively reduce the energy barrier of Zn nucleation/dissolution and help to inhibit dendrite growth. [29]Furthermore, the electrolyte has good wettability on the Zn anode and can evenly distribute the Zn 2+ in the electrolyte to achieve uniform electrodeposition.Therefore, the wettability of the electrolyte on the electrode is of great significance for adjusting the kinetics of Zn deposition.As shown in Figure S8 (Supporting Information), the contact angle of PVDF/CNTs-PT @ Zn is 68°, which is smaller than 78°in bare Zn, indicating that PVDF/CNTs-PT @ Zn has good wettability in 2 Zn(CF 3 SO 3 ) 2 electrolyte. [30]The enhancement of wettability can reduce the interfacial free energy between PVDF/CNTs-PT @ Zn and electrolyte, and promote the uniform nucleation of Zn. [31] For the purpose of testing the corrosion resistance of the PVDF/CNTs-PT protective film in the electrolyte, the bare Zn and PVDF/CNTs-PT @ Zn were immersed in 2 m ZnSO 4 for 7 days for self-corrosion experiments (Figure S9a,b, Supporting Information).Obviously, after immersion, the bare Zn lost its metallic luster and the surface became uneven (Figure S9c, Supporting Information).However, there is no obvious corrosion of PVDF/CNTs-PT @ Zn (Figure S9d, Supporting Information).We further observed the corrosion by SEM.After soaking for a week, the Zn surface became extremely uneven (Figure S10a,c, Supporting Information).On the contrary, PVDF/CNTs-PT @ Zn maintained the uniform and flat surface after immersion (Figure S10b,d, Supporting Information).These loose and unevenly distributed substances were identified as Zn 4 SO 4 (OH) 6 •5H 2 O and ZnSO 4 •xH 2 O (Figure S10e, Supporting Information), which are the typical by-products of harmful side reactions. [32]Compared with bare Zn, the diffraction peak intensity of the side reaction product is quite low, indicating that PVDF/CNTs-PT @ Zn has good corrosion resistance.
Under the action of spherical PVDF particles repelling H 2 O to attract Zn 2+ and uniform electric field distribution of CNTs matrix, PVDF/CNTs-PT @ Zn shows good cycle stability and corrosion resistance.This not only proves that the prepared protective film can well inhibit the side reactions on the surface of bare Zn, such as Zn dendrites, corrosion, and hydrogen evolution, but also allows uniform deposition of Zn 2+ (Figure 3a).For further analysis the protection mechanism of PVDF/CNTs-PT, the battery after cycling was disassembled, and the surface composition and structure of the Zn anode were observed.The FIB-SEM shows that the surface of PVDF/CNTs-PT @ Zn anode is relatively more uniform compared with the uneven deposition of bare Zn (Figure 3b,c), and there is a layer of uniform and dense Zn deposition between the protective film and the Zn anode.The XRD pattern (Figure 3d) shows the obvious by-product peaks in addition to the characteristic peaks of Zn metal, which should be attributed to Zn x (OTF) y (OH) 2x-y •H 2 O. [33] These irreversible byproducts affect the transport of electrons and ions at the electrode/electrolyte interface, which further impairs the cycle stability.In stark contrast, almost no peak of irreversible by-product was observed in the XRD pattern of PVDF/CNTs-PT @ Zn electrode, indicating that the PVDF/CNTs-PT protective film can effectively inhibit the generation of by-products and enhance the cycle stability of the electrode.The Tafel curves of bare Zn anode and PVDF/CNTs-PT @ Zn electrode were compared, which further confirmed the excellent corrosion resistance of PVDF/CNTs-PT @ Zn anode.With a greater corrosion potential (Ecorr) value, a better corrosion resistance and lower corrosion current density (Icorr) can be achieved, indicating that the material can inhibit corrosion. [34]Figure 3e shows that the corrosion potential of PVDF/CNTs-PT protective film increased from −0.99 to −0.93 V.In addition, the corrosion current is also significantly reduced, which again proved that PVDF/CNTs-PT @ Zn demonstrated a good corrosion inhibition ability. [35]In order to directly observe the Zn plating and Zn deposition behavior on the anode, the surface of the Zn anode was directly observed by high-resolution computed tomography (CT).Compared with the PVDF/CNTs-PT @ Zn electrode (Movie S1, Supporting Information), the surface of the bare Zn substrate changed to be uneven rapidly (Movie S2, Supporting Information).This indicates that the Zn deposition is uneven and the side reactions of Zn and H 2 O occur, which corresponds well to the above test analysis.The above excellent performance is mainly attributed to the rich F atoms on the surface of the PVDF particles uniformly distributed on the membrane surface, which inhibits the proximity of H 2 O through Coulomb interaction.It not only greatly reduces the side effects such as corrosion caused by H 2 O, but also facilitates the rapid desolvation of hydrated Zn 2+ , which is conducive to the migration of Zn 2+ .and c) PVDF/CNTs-PT @ Zn after 20 cycles at 1 mA cm −2 ,1 mAh cm −2 .d) XRD patterns of the bare Zn and the PVDF/CNTs-PT @ Zn anodes of the symmetric battery (at 1 mA cm −2 , 1 mA h cm −2 ).e) Linear polarization curves of bare Zn and PVDF/CNTs-PT @ Zn anode.f) Ionic conductivity test of the PVDF/CNTs-PT and PVDF-PT protective membrane.g) Transference numbers of Zn 2+ for PVDF/CNTs-PT @ Zn and PVDF-PT @ Zn anodes.h) Nucleation overpotential results of the bare Zn//Cu and PVDF/CNTs-PT @ Zn//Cu asymmetric batteries at a current density of 1 mA cm −2 .i) Chronoamperograms (CAs) of bare Zn and PVDF/CNTs-PT @ Zn anodes at a −150 mV overpotential.
An ideal Zn anode protective layer also requires high ionic conductivity to ensure uniform plating/stripping on the Zn anode. [36]For the benefit of measuring the ionic conductivity, the PVDF/CNTs-PT protective film was installed in a self-made stainless-steel device for testing (Figure S11, Supporting Information).Data derived from electrochemical impedance spectroscopy (EIS) and thickness measurements, as presented in Figure 3f, reveal that the ionic conductivity of the PVDF/CNTs-PT protective film is ≈1.98 mS cm −1 .This value notably exceeds that of the PVDF-PT film, which is measured at 0.737 mS cm −1 .In a parallel examination, the ionic conductivity of the protective film fabricated without the phase transfer process was also assessed.The resultant PVDF/CNTs-nPT protective film demonstrates a noteworthy ionic conductivity of 1.05 mS cm −1 , as shown in Figure S12a (Supporting Informa-tion).This is in marked contrast to the relatively low conductivity of 0.077 mS cm −1 observed in the PVDF-nPT film (Figure S12b, Supporting Information).These findings clearly illustrate the significant enhancement in ionic conductivity that can be attributed to the incorporation of CNTs into the protective film.Building upon the results of the previously conducted experiments, our research indicates a pivotal enhancement in ionic conductivity attributed to the porous structure developed during the phase transfer process.This assertion is substantiated by the comparative analysis of ionic conductivities, where PVDF/CNTs-PT outperforms PVDF/CNTs-nPT, and PVDF-PT exceeds PVDF-nPT, as depicted in Figure S13 (Supporting Information).The phase transfer method generates a porous structure that effectively overcomes the inherent limitations associated with traditional PVDF membranes.In contrast to the dense and ion-restrictive nature of standard PVDF membranes, as evidenced in Figure S5a (Supporting Information), the newly formed porous structure promotes ion transit, thus rendering it more conducive for utilization in zinc anodes.This enhancement is further amplified by the synergistic interplay between the 3D matrix of CNTs and the distinctive porous architecture induced by the phase transfer process, collectively leading to a significant increase in ionic conductivity.Therefore, the relatively high Zn 2+ transfer kinetics greatly lower the concentration polarization caused by the uneven distribution of Zn 2+ concentration and inhibits the generation of Zn dendrites.It is worth noting that the ion transport number derived from the impedance data is the total contribution of anions and cations.Therefore, through potentiostatic polarization and impedance tests, the migration number of Zn 2+ (Figure 3g) calculated by the formula is 0.76.On the contrary, the PVDF-PT protective film has a Zn 2+ migration number of only 0.33 due to poor conductivity (Figure S14, Supporting Information).This result indicates that the PVDF/CNTs-PT protective film promotes the migration of Zn 2+ , and simultaneously, prevents the transfer of harmful ions during the electrochemical reaction. [37]To sum up, it can be demonstrated that the PVDF/CNTs-PT protective film achieves high ionic conductivity and large ion migration number at the same time, indicating excellent Zn 2+ conduction ability of the protective layer.
In order to further understand the migration, nucleation, and growth mechanism of Zn 2+ on the surface of Zn anode, Figure 3h describes the nucleation overpotential of bare Zn and PVDF/CNTs-PT @ Zn symmetric batteries.At a current density of 1 mA cm −2 , the nucleation overpotential of PVDF/CNTs-PT @ Zn transfer electrode is only 21 mV, which is much lower than that of bare Zn anode (60 mV).To elucidate the determinants behind the observed decrease in nucleation overpotential, we performed comparative analyses on the nucleation overpotentials of various protective layers.These included PVDF/CNTs-nPT @ Zn (61 mV), PVDF-PT @ Zn (87 mV), and PVDF-nPT @ Zn (123 mV).The findings, as shown in Figure S15 (Supporting Information), indicate that the integration of a porous structure in conjunction with CNTs plays a significant role in reducing the nucleation overpotential.This decrease suggests a marked reduction in the nucleation barrier for Zn 2+ , [38] thereby promoting a more uniform Zn 2+ deposition on the electrode surface.Moreover, when these results are considered alongside our ionic conductivity test outcomes, it becomes apparent that the improved nucleation efficiency is predominantly due to the accelerated transport of Zn 2+ .This correlation underscores the effectiveness of the combined porous structure and CNTs in enhancing the electrochemical performance of the protective layers.The Zn deposition behavior was also studied by chronoamperometry at a constant overpotential of −150 mV.As shown in Figure 3i, the current density on the surface of the bare Zn anode continues to increase, indicating a serious 2D diffusion and uneven Zn deposition. [39]The process of horizontal Zn 2+ migration and deposition on Zn surface is blocked to some extent under the coupling effect of electric field and concentration gradient, leading to dendritic growth caused by the "tip effect".On the contrary, a 3D conductive network composed of CNTs lowers the local current density of the Zn anode and provides many uniform nucleation sites for Zn 2+ deposition.Therefore, the current density on the PVDF/CNTs-PT @ Zn electrode quickly reaches a stable and low equilibrium value.In order to further explore the role of PVDF/CNTs-PT protective film in the Zn 2+ electroplating process, the charge transfer resistance of the electrode was measured by EIS.As shown in Figure S16 (Supporting Information), the resistance of PVDF/CNTs-PT @ Zn electrode is much smaller than that of bare Zn anode indicating a faster Zn 2+ migration, which proves that PVDF/CNTs-PT is beneficial to Zn plating/stripping.
To visualize the deposition behavior of Zn on two different interfaces, the in situ growth process of Zn was observed by optical microscope.As shown in Figure 4a, after 5 min of cycling, uneven protrusions are formed on the surface of bare Zn, and uneven particles are aggregated during the subsequent cycling.As the reaction proceeds, it gradually evolves into a sharp dendritic Zn.However, the PVDF/CNTs-PT @ Zn maintains a clean and smooth interface throughout the deposition process (Figure 4b), indicating a regulation on uniform nucleation and deposition of Zn 2+ .The surface morphology after deposition was further studied.The bare Zn exhibits uneven Zn deposition and obvious dendrites (Figure 4c).In order to overcome the one-sided observation of zinc deposition morphology, we used in situ CT to observe the full deposition on the zinc anode. [40]s shown in Figure 4d, after 50 cycles at 10 mA cm −2 , the bare Zn surface exhibits more and dense bright spots, which is caused by the height difference caused by the presence of Zn dendrites.On the contrary, the PVDF/CNTs-PT @ Zn surface is relatively smooth (Figure 4e) and has fewer bright spots in in situ CT (Figure 4f), indicating that the protective layer significantly reduces the generation of Zn dendrites.This phenomenon can be observed more intuitively from the 3D motion diagram (Movies S3 and S4, Supporting Information).Due to the protective film, a layer of uniformly deposited Zn was formed between the protective film and the Zn sheet, which greatly reduces the risk of Zn dendrites and improves the cycle stability of the battery.In order to verify the effect of the protective layer more clearly, the cross-section of the Zn anode after the cycle was observed by FIB cutting after the protective film was uncovered.Compared with the cross-section of the original Zn sheet (Figure S17a, Supporting Information), the surface and cross-section of Zn after cycle are still very uniform, and there is no corrosion and uneven deposition (Figure S17b,c, Supporting Information).
In order to further simulate the deposition morphology, current density, and ion distribution of Zn 2+ on different electrodes, COMSOL multi-physics finite element simulation method was used to simulate the interface electric field and concentration field.As shown in Figure 4g, the bare Zn anode surface demonstrates uneven protrusions leading to uneven and enhanced field strength.Due to the charge accumulation caused by the uneven distribution of the local electric field, more Zn 2+ is preferentially deposited at the tip. [41]The tip effect causes these protrusions to gradually grow into irregular sharp dendrites, which eventually causes the battery to fail prematurely. [42]However, the electric field intensity distribution of the PVDF/CNTs-PT @ Zn electrode is relatively uniform and low (Figure 4h), which is due to the 3D matrix composed of CNTs not only increases the reaction area but also reduces the local current density.This structure significantly inhibits the generation of tip effect, thereby protecting the Zn anode.The PVDF/CNTs-PT @ Zn protective film enriches SEM image of c) Bare Zn anode.e) PVDF/CNTs-PT @ Zn anode after 20 cycles at 1 mA cm −2 , 1 mAh cm −2 .In situ CT before and after 50 cycles.d).Bare Zn. f).PVDF/CNTs-PT @ Zn. Simulated g,h) electric field and i,j) Zn 2+ distribution of the bare Zn and PVDF/CNTs-PT @ Zn surface.
F to promote the rapid transfer of Zn 2+ , and with the help of 3D CNTs matrix, the Zn 2+ flux on the surface of PVDF/CNTs-PT @ Zn electrode is more uniform than that on the bare Zn surface (Figure 4i,j).Therefore, the introduction of PVDF/CNTs-PT protective film can simultaneously achieve uniform distribution of ion field and electric field, which is conducive to uniform plating/stripping process leading to a dendrite-free morphology.
The improvement of battery performance by PVDF/CNTs-PT protective film can be understood by density functional theory (DFT) calculation.Since PVDF has abundant F atoms, most of the H 2 O is repelled by F through Coulomb interaction outside the PVDF/CNTs-PT protective film.Therefore, the Zn 2+ on the surface of the PVDF/CNTs-PT protective film has only a small amount of H 2 O, so the DFT calculation is Zn 2+ rather than Zn (H 2 O) 6 2+ .The diffusion barriers of Zn 2+ and H 2 O in the PVDF/CNTs-PT protective film was calculated by DFT (Figure 5a; Figure S18, Supporting Information, for detailed models).It can be seen from the calculated data (Figure 5b) that the energy barrier of H 2 O passing through the PVDF/CNTs-PT protective film is much higher than that of Zn 2+ .This indicates that the F of PVDF/CNTs-PT can attract Zn 2+ through Coulomb interaction, thereby effectively reducing the migration barrier of Zn 2+ and promoting the transport of Zn 2+ .This can also significantly repel H 2 O, thereby inhibiting side reactions such as corrosion and hydrogen evolution caused by H 2 O. Ac-cording to the XPS of the Zn anode after cycling (Figure S19, Supporting Information), ZnF 2 is formed in situ due to the presence of PVDF.The XPS spectrum of PVDF/CNTs-PT @ Zn electrode exhibits a pair of peaks, which can be attributed to Zn─F bonds with different binding energies retaining Zn 2P 1/2 and Zn 2P 3/2 , respectively.An additional peak can be detected in the spectrum of F 1s after cycling, confirming the formation of ZnF 2 . [43]The ZnF 2 layer is considered to improve cycle stability, which can be also proved by some existing works. [44]n order to further explore the practical application of PVDF/CNTs-PT protective film, a PVDF/CNTs-PT @ Zn//V 2 O 5 battery was assembled using a commercial V 2 O 5 cathode.The cyclic voltammetry (CV) curves of Bare Zn//V 2 O 5 and PVDF/CNTs-PT @ Zn//V 2 O 5 batteries at 5 mV s −1 scan rate (Figure 5c) show the multi-step redox couples of V 4+ /V 3+ and V 5+ /V 4+ caused by ion insertion/extraction. [45] The higher current of the redox peak and reduction potential during the cathodic process indicate smaller polarization.It is worth noting that at a current density of 1 A g −1 (Figure 5d), the PVDF/CNTs-PT @ Zn//V 2 O 5 battery has a relatively high capacity retention rate and cycle stability compared with the bare Zn//V 2 O 5 battery, indicating that the PVDF/CNTs-PT protective film can well inhibit the capacity decay caused by the rapid dissolution and collapse of commercial V 2 O 5 during the cycle.Therefore, the PVDF/CNTs-PT @ Zn//V 2 O 5 battery has good stability.Comparing the resistance before the cycle (Figure 5e), we can conclude that, the resistance of the battery can be greatly reduced with the PVDF/CNTs-PT protective film, which shows that the PVDF/CNTs-PT protective film is conducive to the rapid transfer of electrons.The rate performance of the battery is also shown in Figure 5f.At 0.1, 0.2, 0.3, 0.5, 1 A g −1 , the discharge capacity of the PVDF/CNTs-PT @ Zn electrode is 338, 323, 308, 303, 283 mAh g −1 , respectively.Even at a high current density of 5 A g −1 , it can still maintain a reversible capacity of 226 mAh g −1 .After cycling, when the current density returned to 0.1 A g −1 , the discharge capacity recovered to 321 mAh g −1 .The rate performance of PVDF/CNTs-PT @ Zn electrode is significantly better than that of bare Zn anode.In order to further demonstrate the advantages and application prospects of PVDF/CNTs-PT @ Zn, we assembled a soft-pack battery (Figure 5g). Figure 5h shows the "IMU" sign containing 21 LED bulbs that can be lit for a long time, indicating the great potential for commercial application of PVDF/CNTs-PT protective film.Through the same steps, PVDF/CNTs-PT protective layer can be introduced on other metal substrates, such as Cu, Al (Figure S20, Supporting Information).PVDF/CNTs-PT @ Al symmetric batteries were assembled to explore the application of PVDF/CNTs-PT protective layer in other battery systems.Compared with bare Al anode, the cycle stability of PVDF/CNTs-PT @ Al anode is improved by six times (Figure 5i).Consistent with previous observations, the cycle performance of the PVDF/CNTs-PT @ Cu symmetric battery demonstrates a marked improvement over that of the bare Cu, as evidenced in Figure S21 (Supporting Information).This is due to the combined effect of 3D CNTs matrix and spherical PVDF particles generated by the phase transfer process.The above experimental conclusions provide a simple and novel method to suppress side reactions and dendrite growth and enhance the cycle stability of the battery.Therefore, PVDF/CNTs-PT protective film has a broad prospect in the practical application of aqueous metal ion batteries.

Conclusion
In this work, a porous network protective film (PVDF/CNTs-PT @ Zn) was successfully fabricated by phase transfer method as a multifunctional ion-electron exchange interface to protect the Zn anode.First, the integration of CNTs into PVDF effectively addresses its inherent poor conductivity, resulting in a more homogeneous distribution of the electric field.Second, the fluorinerich composition of the PVDF/CNTs-PT protective film plays a crucial role in repelling anions and H 2 O through Coulomb interactions.This aspect is instrumental in facilitating the rapid desolvation of hydrated Zn 2+ , thereby accelerating their transfer.Concurrently, this approach effectively addresses the increased likelihood of side reactions associated with the expanded specific surface area introduced by the 3D CNT matrix.Most notably, the porous protective layer, developed through the phase transfer process, surpasses the ionic conductivity limitations typical of conventional PVDF layers.This advancement significantly promotes the high-rate migration of Zn 2+ across the film and ensures a more uniform Zn 2+ distribution on the zinc surface, which is critical for the efficiency and stability of zinc-based electrochemical systems.More importantly, the significant effect of PVDF/CNTs-PT @ Zn on promoting the uniform deposition of Zn 2+ through the above mechanism was confirmed in detail by in situ characterization, simulation, and migration energy barrier calculation.Therefore, the PVDF/CNTs-PT @ Zn symmetrical battery was repeatedly plating/stripping for more than 55 days at 1 mA cm −2 , 1 mAh cm −2 , and stably cycled for 40 days at 5 mA cm −2 , 1 mAh cm −2 , with a low voltage lag.The PVDF/CNTs-PT @ Zn//V 2 O 5 battery with commercial V 2 O 5 showcases excellent rate performance and improved cycle stability.This work provides a simple and economical method to construct a Zn anode protective layer with a 3D porous network structure.It also shed a light on the effective and universal strategy for the design of various metal anode protective layers, which has wide prospect of practical application.

Experimental Section
Reparation of PVDF/CNTs Slurry: PVDF/CNTs protective film was prepared by a simple phase transfer technique.First, PVDF powder (100 mg) was dissolved in NMP (2 mL) solution and stirred for 1 h, then PVP (2 mg) was added and stirred.After stirring fully, CNTs (200 mg) were added to the mixed solution and stirred for 3 h, followed by ultrasonic defoaming.
Preparation of PVDF/CNTs @ Zn-Phase Transfer: The prepared PVDF/CNTs slurry was uniformly coated on the Zn sheet using a scraper.The Zn sheet was quickly transferred to deionized water, phase transfer for 5 min, natural drying to obtain PVDF/CNTs @ Zn-phase transfer.
Assembly of Zn//Zn Symmetric Cells and Zn//V 2 O 5 Cells: The symmetrical cell was assembled using CR2023 battery shell with PVDF/CNTs-PT @ Zn (bare Zn) electrode as anode and cathode.Electrolyte composed of 2 m Zn(CF 3 SO 3 ) 2 or 2 m ZnSO 4 solution and a glass fiber was used as the separator.For the preparation of cathode: commercial V 2 O 5 was mixed with conductive carbon and PVDF at a mass ratio of 7:2:1 in NMP.Then coated on carbon paper and 60 °C dried.The mass loading of active material was ≈1 mg cm −2 .The PVDF/CNTs-PT @ Zn//V 2 O 5 (Zn//V 2 O 5 ) full cell was prepared by dropping 2 m Zn (CF 3 SO 3 ) 2 on a glass fiber separator in air using commercial V 2 O 5 as cathode and PVDF/CNTs-PT @ Zn (bare Zn) as anode.
Assembly of Zn//Cu Asymmetric Cells: PVDF/CNTs-PT @ Zn//Cu and Zn//Cu asymmetric cells were assembled with PVDF/CNTs-PT @ Zn (bare Zn) as the anode, Cu foil as the cathode, and the electrolyte was 2 m Zn (CF 3 SO 3 ) 2 .The coulombic efficiency of zinc plating/ stripping was explored using asymmetric battery with a current of 2 mA cm −2 and a charging cut-off voltage of 1 V.

Assembly of Al//Al Symmetric Cells:
The Al//Al symmetric cell was assembled with the bare Al or PVDF/CNTs-PT @ Al anode.In the glove box (<0.1 ppm water and oxygen) in argon atmosphere, anhydrous aluminum chloride with a molar ratio of 1:1.3 were slowly added to dry 1-ethyl-3methylimidazolium chloride ([EMIm] Cl), stirring under argon protection for 2 h.Then, add 60 μL to the glass fiber separator dropwise.
Assembly of Cu// Cu Symmetric Cells: The assembly of the Cu//Cu symmetric cells was executed using either bare Cu or PVDF/CNTs-PT @ Cu as the anode.The electrolyte for these cells was formulated from a 1 m CuSO 4 solution, and a glass fiber was employed as the separator.This configuration was designed to evaluate the comparative performance of the two anode types in a controlled environment.

Figure 2 .
Figure 2. Performance of symmetrical batteries.a) Cycling performance of plating/stripping reversibility of different symmetric batteries in the 2 Zn(CF 3 SO 3 ) 2 electrolyte at 1 mA cm −2 with a capacity of 1 mAh cm −2 .b) Cycling performance of plating/stripping reversibility of the bare Zn and the PVDF/CNTs-PT @ Zn symmetric batteries in the 2 Zn(CF 3 SO 3 ) 2 electrolyte at 5 mA cm −2 with a capacity of 1 mAh cm −2 .c) Rate cycling performance of bare Zn and the PVDF/CNTs-PT @ Zn symmetric batteries.d) Cycling performance of plating/stripping reversibility of the bare Zn and the PVDF/CNTs-PT @ Zn symmetric batteries in the 2 m ZnSO 4 electrolyte at 1 mA cm −2 with a capacity of 1 mA h cm −2 .e) Lifespan (at 1 mA cm −2 ,1 mAh cm −2 ) comparison between this work and recently published studies.

Figure 5 .
Figure 5. Mechanism simulation and performance of full battery.a) The migration path of Zn 2+ , H 2 O in PVDF.b) The migration energy barrier of Zn 2+ , H 2 O in PVDF.c) Cyclic voltammetry of full batteries, scan rate 5 mV s −1 .d) Long-term cycling stability of the bare Zn//V 2 O 5 and PVDF/CNTs-PT @ Zn//V 2 O 5 batteries at 1 A g −1 .e) EIS spectra curves of the bare Zn//V 2 O 5 battery and the PVDF/CNTs-PT @ Zn//V 2 O 5 battery before cycling.f) Rate performance of the PVDF/CNTs-PT @ Zn//V 2 O 5 battery and the bare Zn//V 2 O 5 battery.g) Soft package battery schematic diagram.h) Application of PVDF/CNTs-PT @ Zn//V 2 O 5 LED Lighting.i) Galvanostatic cycling of symmetric cell of PVDF/CNTs-PT @ Al and Bare Al electrodes at a current density of 0.1 mA cm −2 .