Three‐dimensional electrically conductive–hydrophobic layer for stable Zn metal anodes

The interrelated side reactions and dendrites growth severely destabilize the electrode/electrolyte interfaces, resulting in the difficult application of aqueous Zn ion batteries (AZIBs). Hydrophobic protective layer possesses natural inhibition ability for side reactions. However, the conventional protective layer with plane structure is difficult to attain joint regulation of side reaction and Zn nucleation. Herein, a novel three‐dimensional (3D) electrically conductive and hydrophobic (3DECH) interface is elaborated to enable stable Zn anode. The as‐prepared 3DECH interface presents a uniform 3D morphology with hydrophobic property, large specific surface area, abundant zincophilic sites, and excellent electroconductivity. Therefore, the 3DECH interface achieves uniform nucleation and dendrite‐free deposition from synergetic benefits: (1) increased nucleation sites and reduced local current density through the special 3D structure and (2) uniform electric potential distribution and rapid Zn2+ transport due to the electroconductive alloy chemistry, thus coupling the hydrophobic property to obtain a highly reversible Zn anode. Consequently, the modified anode achieves a superior coulombic efficiency of 99.88% over 3500 cycles, and the pouch cells using modified anode and LiMn2O4 (LMO) cathode retain a capacity of 84 mAh g−1 after 700 cycles at a reasonable depth discharge of 36%, without dendrite piercing and “dead Zn.”

1][32][33] However, the reduced electrode/electrolyte contact of hydrophobic interface results in fewer Zn 2+ nucleation sites and a higher local current density, leading to the risk of dendrite growth, especially in two-dimensional (2D) materials with limited specific surface area. 34,35Therefore, it is challenging to consider a single property of the interfacial film to construct a long-cycling Zn-metal anode.Compared with 2D layers, three-dimensional (3D) interface coupling of electrical conductivity has a larger specific surface area, reduced local current density, and the ability to dictate electrical field distribution and Zn ion flux, which is promising to regulate Zn deposition behavior. 36,379][40] Consequently, the highly reversible Zn anode can be achieved through simultaneously consideration of conductivity, specific surface area, and wettability.
Herein, the optimal anode based on a 3D electrically conductive and hydrophobic layer is designed to enable long-life AZIBs.The 3D CuZn 5 alloy (CZ) film decorated Zn (CZ@Zn) was obtained in situ by heat-induced metal diffusion between the bulk Zn and Cu nanoparticles precursor.Based on in/ex situ evidence and theoretical calculation, this synergistic design of chemistry and structure has several inherent advantages.First, the layer offers hydrophobicity to electrodes, which helps to isolate the active water and corrosion media in the electrolyte, thus inhibiting side reactions.Second, the 3D electrically conductive CZ layer with a large specific surface area reduces local current density and optimizes electric potential distribution.This enhancement endows a uniform deposition morphology and excellent kinetic performance of electrodes, compensating for the issues of hydrophobic interfaces through additional nucleation sites.As a result, the obtained composite anode based on in situ layer can work smoothly over 2000 h with low overpotential in symmetric cells at a current density of 5 mA cm −2 .To validate the practical application, a pouch cell pared with LiMn 2 O 4 (LMO) cathode maintains a capacity of 84 mAh g −1 after 700 cycles.

RESULTS AND DISCUSSION
Figure 1A displays the two-step synthesis of CZ@Zn composite anode by complexing agent-assisted surface ion exchange and heat treatment strategy, the reaction equations are as follows: Step The Cu nanoparticles were modified on the surface of Zn foil to form an initial Cu@Zn precursor sample by ion exchange between the copper citrate ligand ions ([C 6 H 8−x O 7 ⋅yCu] 2y−x ) and Zn foil (10 µm) in aqueous solution.Complex ions are selected to increase the reductive activation energy and reduce the Cu nucleation and growth rate compared with that of Cu 2+ ions, contributing to the dense growth of Cu metal coatings. 41The scanning electron microscope (SEM) image shows that the uniform Cu nanoparticles completely cover the surface of Zn foil with a thickness of approximately 170 nm (Figure S1).The composition of Cu@Zn was investigated by inductively coupled plasma emission spectroscopy, as shown in Table S1.In the Cu@Zn sample, the mass percentages of Zn and Cu elements were 98.54 and 1.32%, respectively.The smaller weight share of Cu elements is consistent with the ultrathin-modified layer, further reflecting that the complexing agent-assisted ion exchange method can precisely control the solution growth process of metal layers.
The X-ray diffraction (XRD) patterns of Cu@Zn and bare Zn have similar characteristic peaks, indicating that the modification process unable change the structure of metallic Zn (Figure S2).Meanwhile, the absence of characteristic peaks for Cu nanoparticles may be due to the low Cu content.Subsequently, the Cu@Zn precursor was calcined at 350 • C for 1 h with a ramp rate of 5 • C min −1 in an Ar atmosphere and cooled naturally to obtain the CZ@Zn.The morphology of the as-prepared CZ@Zn sample was observed by SEM image.As shown in Figure 1B, the surface of CZ@Zn exhibits a uniform 3D structure with numerous protuberances, which are caused by the different diffusion rates of Zn and Cu atoms during the heat treatment. 42The atomic force microscope images also display the same rough morphology.This 3D rough surface of CZ@Zn contributes to remarkably increased surface roughness (Ra, 5.9-123 nm) and specific surface area (3.83-5.98 m 2 g −1 ) compared with plane Zn (Figure S3).Since part of the metallic Zn participated in the alloying process to form a 3D rough layer, the thickness of CZ@Zn anode was increased to 1.25-1.5 µm after calcination.In addition, the bottom of the 3D alloy remained compact in distinction to the outer surface, which plays as a physical protection barrier to the Zn anode.This compact structure can be ascribed to the small size and dense growth of Cu nanoparticles in the Cu@Zn precursor.The EDS mapping confirms a homogeneous distribution of Cu and Zn elements at the surface and throughout the whole material, respectively, which is consistent with the SEM topography (Figure 1C).Moreover, the chemical natures of CZ@Zn and bare Zn were analyzed.Figure 1D displays the XRD patterns of CZ@Zn and bare Zn.The CZ@Zn anode shows three distinct peaks at 37.7 • , 42.2 • , and 43.4 • well corresponding to the characteristic peaks of CuZn 5 (JCPDS: 00-035-1152), 43 without additional peaks appearing except for Zn.The crystal structure of CZ layer was examined by high-resolution transmission electron microscopy, and the crystal plane spacing of 0.238 nm corresponds to the CuZn 5 (1 1 1) plane (Figure S4). 44he component of CZ@Zn is mainly composed of a 3D CuZn 5 modification layer and using time-of-flight secondary ion mass spectrometry (TOF-SIMS).As illustrated, the TOF-SIMS elemental distribution intensity plot (Figure 1E) and corresponding 3D images (Figure 1F) show that the Cu signal gradually diminishes and the Zn signal significantly increases towards stabilization within 1500 s of etching (1.25 µm), representing the alloy layer region.To verify the wettability between 3D alloy layers and aqueous electrolyte (2 M ZnSO 4 ), the contact angle test was performed as shown in Figure 1G.Because of the enhanced surface roughness, the contact angle of CZ@Zn was increased from 79.8 • to 126.3 • compared with bare Zn, exhibiting a significantly improved hydrophobicity. 45herefore, the CZ@Zn anode not only provides a large specific surface area to reduce local current density, but F I G U R E 2 (A and B) Galvanostatic cycling of Zn||Zn and CZ@Zn||CZ@Zn symmetric cells at 1 mA cm −2 for 1 mAh cm −2 (A), and 5 mA cm −2 for 1 mAh cm −2 (B).(C and D) Top-view SEM images of bare Zn (C) and CZ@Zn (D) electrodes after plating/stripping 20 cycles at 5 mA cm −2 for 1 mAh cm −2 .(E) Coulombic efficiency of Cu||Zn and Cu||CZ@Zn half cells.(F and G) The voltage profiles of bare Zn (F) and CZ@Zn (G) electrodes at 5 mA cm −2 with a Zn plating capacity of 1 mAh cm −2 .also effectively isolates active water molecules to mitigate corrosion and side reactions.
The effect of 3D CZ layer on electrochemical performance and the corresponding Zn plating/stripping behavior was investigated through galvanostatic cycling tests.The symmetric CZ@Zn//CZ@Zn cells were assembled with 2 M ZnSO 4 aqueous electrolyte (symmetric Zn//Zn cells were tested for comparison).At the cycle current density and capacity of 1 mA cm −2 and 1 mAh cm −2 , the Zn//Zn cell exhibited an initial overpotential of 45.8 mV.Subsequently, the voltage hysteresis increased gradually and then suddenly dropped after cycling for about 130 h, suggesting an internal short circuit (Figures 2A and S5a).In contrast, the symmetric battery using CZ@Zn electrodes exhibited a lower initial overpotential of 34.7 mV and cycled stably for more than 2100 h (1050 cycles), demonstrating the reduced nuclear barrier and improved interface stability.At a higher current density of 5 mA cm −2 (Figure 2B), the Zn//Zn cell suffered from a high overpotential of 96.4 mV during early cycles and ultimately failed with a sharp increase in voltage after only 250 h.Encouragingly, the CZ@Zn anode was able to maintain stable operation over 2000 h with a low overpotential of 48.6 mV (Figure S5b).The excellent electrochemical performance of CZ@Zn anode benefits from the dense Zn plating, suppressed side reactions, and fast kinetics.The surface morphologies of bare Zn and CZ@Zn anodes after 20 cycles at 5 mA cm −2 for 1 mAh cm −2 were examined as shown in Figures 2C and D. The bare Zn anode surface produced numerous irregular and agglomerating dendrites, which can be ascribed to the inhomogeneous Zn nucleation on imperfect commercial Zn foil.The massive dendrites not only puncture the separator leading to battery failure, but also exacerbate the competing side reactions.In comparison, the CZ@Zn anodes presented an even and dense surface.
Coulombic efficiency (CE) is a widely used quantifiable indicator for evaluating the reversibility of Zn plating/stripping, which is influenced by the stability and compatibility of interfaces.The asymmetric CZ@Zn||Cu cell was assembled to test the CE at a current density of 1 mA cm −2 and a plating capacity of 1 mAh cm −2 , which was subsequently charged to 0.5 V versus Zn/Zn 2+ (Zn||Cu cell was also tested under the same conditions for comparison).The asymmetric cell based on bare Zn anode underwent a sharp degradation in CE after 27 cycles, while the CZ@Zn||Cu cell cycled stably over 700 cycles with an average CE of 99.47%, which was attributed to the suppressed dendrite growth and side reaction (Figure S6).For higher current density of 5 mA cm −2 , the CZ@Zn anode remained stable cycling over 3500 h with an average CE of 99.88%, demonstrating the excellent electrochemical reversibility of CZ@Zn anode (Figure 2E).In contrast, the Zn||Cu cell fluctuates sharply and eventually failed at less than 200 cycles caused by the continuous formation of Zn dendrites and insulation by-products.Meanwhile, the capacity-voltage curve shows that the voltage hysteresis of Zn anode increases gradually in the stable cycling stage, reaching 235.2 mV at 180 cycles (Figure 2F).Interestingly, the CZ@Zn anode obtained a lower voltage hysteresis after cycling and stabilized at around 81.1 mV (Figure 2G).Therefore, the significantly improved electrochemical performance indicates the superior reversibility and stability of CZ@Zn anode, which may be attributed to the excellent reaction kinetics and interfacial protection provided by the alloy layer.
To investigate the regulation effect of CZ@Zn on deposition behavior, in situ and ex situ evidence were explored.The surface morphology evolution of bare Zn and CZ@Zn anodes under the Zn deposited process is observed by in situ optical microscopy at a current density of 3 mA cm −2 .As depicted in Figure 3A, the initial Zn deposition on bare Zn exhibited uneven nucleation and growth, forming sharp bulges and irregular morphology.The height difference of bare Zn up to 22 µm was found in the optical surface profiles after 40 min of deposition, demonstrating the disordered dendrites growth and the disrupted electrode surface (Figure 3B).In contrast, the CZ@Zn anode achieved even Zn deposition in the initial stage, and the dense and smooth surface can remain with the deposition capacity increases.The small height difference in the lateral profile also confirms a uniform Zn deposition of CZ@Zn anode (Figure 3C).Accelerated videos of Zn deposition at Zn and CZ@Zn anodes were also taken (Videos S1 and S2).Similarly, CZ@Zn anode exhibited significantly optimized planar deposition, illustrating the positive effect of the alloy layer to homogenize Zn 2+ ions.Moreover, a large number of bubbles were consistently observed in bare Zn anode late in the deposition process (Videos S1 and Figure S7), indicating the severe HER process with the Zn dendrites growth.The apparent HER would compete with the Zn deposition reaction, resulting in low CE and reduced cycling life.There is no significant bubble observed in CZ@Zn anode, demonstrating that the hydrophobic alloy layer has an inhibitory effect on HER, which may be due to the synergistic effect of deposition modulation and free water segregation.
To carefully observe the surface morphology, the ex situ SEM images of bare Zn and CZ@Zn anodes were employed at a current density of 1 mA cm −2 after plating 0.2, 0.5, and 1 h.A disordered accumulation of Zn flakes with different sizes is observed in bare Zn anode at the initial stage of deposition (capacity of 0.2 mAh cm −2 ).When the deposition time was increased to 0.5 h (capacity of 0.5 mAh cm −2 ), the large Zn flakes tend to pile up more loosely (Figures S8A and B).After plating 1 h (capacity of 1 mAh cm −2 ), the inhomogeneous Zn deposition resulted in significant dendrite growth and defective morphology (Figure 3D).In addition, the glass fibers adhering to the electrode surface indicated that the dendrites caused damage to the glass fiber separator, which may lead to a decrease in cell cycling life.For the CZ@Zn anode, the Zn started to plat with a regular vertically aligned network (Figures S8C and D), and then the surface was uniformly covered because of the large specific surface area and rich zincophilic sites provided by the 3D alloy layer.The gaps in the initially formed network were gradually filled with the plating time extended over 0.5 h.After 1 h plating (capacity of 1 mAh cm −2 ), the CZ@Zn anode formed an even and compact Zn layer without obvious dendrites and dead Zn (Figure 3E).Therefore, both in situ optical microscopy and ex situ SEM demonstrate that the plating behavior can be significantly regulated using the CZ@Zn composite anode.
It is well known that the deposition and side reactions of metallic Zn are inextricably linked since the rough interface and byproducts caused by corrosion as well as the H 2 bubbles generated by HER can intensify dendrite growth.Therefore, the interfacial compositions of the cycled anodes were analyzed by XRD during the longterm plating/stripping process.As shown in Figure 3F, the bare Zn after 10 cycles showed a sharp peak at around 8.84 • corresponding to the irreversible by-product of ZnSO 4 ⋅3Zn(OH) 2 ⋅4H 2 O (PDF#00-009-0204), 46,47 proving that a severe side reaction between electrode and electrolyte occurred during the cycling process.In contrast, no impurity peaks were seen in the XRD patterns of CZ@Zn, indicating that the cycling process is highly reversible.Furthermore, the characteristic peaks of CuZn 5 were still maintained during the plating/stripping process, indicating that the alloy skeleton remained stable during cycling and can continuously modulate the deposition behavior.The hydrophobic effect of the alloy layer on Zn corrosion is also investigated using the linear polarization experiment in 2 M ZnSO 4 electrolyte.As displayed in Figure 3G, the corrosion potential of CZ@Zn increased from −1.015 to −1.013 V compared with that of bare Zn, revealing a lower corrosion tendency.Meanwhile, the corrosion current of CZ@Zn (94.36 µA cm −2 ) is almost 8.5 times lower than that of bare Zn (797.1 µA cm −2 ), representing a slower corrosion rate. 48These results proved that the 3D alloy layer can effectively alleviate the HER and anodic dissolution.The strong solvation effect will lead to difficult Zn + ions transport at interfaces, thus desolvation process may affect deposition kinetics and morphology.Electrochemical impedance spectroscopy (EIS) is measured in  3H, I, and  S9).The charge transfer resistance (R ct ) of the CZ@Zn electrode was about 2.5 times lower than that of the bare Zn at 20 • C, indicating good room temperature kinetics.The desolvation process can be evaluated by the activation energy (E a ) in the Arrhenius equation (inset of Figure 3J).The calculated activation energy of CZ@Zn is only 14.32 kJ mol −1 , whereas the bare Zn shows a much higher activation energy (25.27 kJ mol −1 ).These phenomena demonstrate that the large surface area of the 3D alloy layer induces the rapid transport of Zn 2+ ions and enhanced deposition kinetics.
Theoretical calculations were carried out to further explore the optimization mechanisms of the CZ layer on Zn anode performance.To elucidate the band structure of CZ layer, the projected density of states was investigated based on the DFT method (Figures 4A and B).At the Fermi energy level (E−E f = 0), the CZ alloy has a slightly higher density of states than the Zn metal, indicating that the CZ@Zn can maintain good conductivity with the hydrophobic alloy layer.The conductive CZ layer is beneficial to promote uniform electrical potential distribution to achieve homogeneous Zn 2+ flux and improved interfacial kinetics.Figure 4C delivers the stable configurations of Zn on the surface of Zn and CZ, respectively.The adsorption energies of Zn 2+ on the Zn and CZ substrates are determined to be −0.68 and −1.408 eV, respectively (Figure 4D), demonstrating the superior zincophilicity of the CZ layer, which can act as nucleation sites for Zn deposition.Together with the 3D CZ layer, the uniform, and sufficient nucleation sites homogenize the Zn 2+ flux and reduce the practical current density, ultimately promoting even Zn deposition and suppressing dendrite growth.Furthermore, the migration barriers of Zn 2+ on the Zn and CZ were calculated (Figures 4E and F).The inner images show the top view of Zn 2+ ion migration pathways.The CZ exhibits a higher migration barrier of 0.62 eV than that of Zn (0.27 eV), indicating that Zn 2+ ions are preferentially removed from the metallic Zn.Hence, the CZ@Zn electrode can maintain structural stability during the Zn plating/stripping process, which is conducive to stable interface and long-life cycle.Combining theoretical calculations and test results, the synergistic effects of CZ layer on regulating Zn deposition and inhibiting side reactions are shown in Figure 4G.The hydrophobicity of CZ layer can prevent H 2 O/O 2 from reaching the bulk Zn surface, resulting in diminished HER and corrosion reactions.In addition, the high conductivity of CZ layer facilitates uniform Zn 2+ flux and reduces interface impedance.What is more, the zincophilic nature and 3D structure of CZ layer can provide numerous and homogeneous Zn nucleation sites, ensuring uniform deposition simultaneously.Further assessments of CZ@Zn anode were carried out in full cells.The LMO//Zn and LMO//CZ@Zn cells were assembled using bare Zn and CZ@Zn anodes (10 µm) paired with LMO cathodes (mass loading: 2 mAh cm −2 ), respectively.The mixed ion system employed here aims to evaluate the influence of CZ modification on the anode side.As seen from the cyclic voltammetry (CV) curves, LMO//CZ@Zn cell exhibits higher current densities than LMO//Zn, revealing the enhanced reactivity and capacity (Figure 5A).In addition, the overpotential gap (ΔEp) of LMO//CZ@Zn cell was calculated to be lower than that for LMO//Zn cell (220 and 260 mV, respectively), indicating superior reversibility.The rate performance of full cells is shown in Figure 5B.The considerable capacities of 124, 116, 103, and 71 mAh g −1 at 0.5C, 1C, 2C, and 5C (1C = 148 mAh g −1 , based on LMO) can be attained by LMO//CZ@Zn cell.However, the full cell with bare Zn anode exhibits low capacities at various rates, with an apparent capacity of only 52 mAh g −1 at 5C.At the same time, the LMO//CZ@Zn cell shows no significant capacity loss or dendrite growth after the test is reverted to 0.5C, while the LMO//Zn cell displays a capacity fading from 116 to 94 mAh g −1 and shows obvious "dead Zn" in the same condition (Figure S10).
The enhanced Zn deposition electrodynamics is also validated via EIS. Figure 5C shows that LMO//CZ@Zn cell exhibits a smaller R ct of 115 Ω than that of the LMO//Zn cell (846 Ω).It can be inferred that the low R ct and fast Zn 2+ diffusion kinetics of CZ@Zn have a significant effect on improving the rate performance of full cells.To further understand the relationship between side reactions and electrochemical properties in LMO//CZ@Zn full cell, differential electrochemical mass spectrometry (DEMS) was performed, which can be used to quantitatively analyze HER.The side reaction produces microbubbles and insulating by-products that exacerbate the dendrites growth and deplete the active Zn.As shown in Figure 5D, during the first charging/discharging process, the H 2 evolution rate of the LMO//CZ@Zn full cell stably remained below 1.5 nmol h −1 .In contrast, the H 2 evolution rate of the LMO//Zn cell gradually increased to approximately 1.8 nmol h −1 as the charging process continued.At the end of one cycle, the H 2 evolution rate of LMO//Zn cell increased from 1.59 to 1.73 nmol h −1 , which can be ascribed to the severe dendrite growth and corrosion damage on the bare Zn anode (Figure 5E).Therefore, the CZ@Zn anode in full cell enables effective suppression of HER.
Fiber cells and pouch cells were also assembled using CZ@Zn anodes to explore the feasibility of practical applications.The 3D alloy layer was successfully modified in situ on the Zn fiber surface using the same method described above (Figure S11), and then a coaxial fiber cell was assembled with the CZ@Zn fiber as displayed in Figure 5F (left).The fabricated fiber cell is flexible enough to successfully light the light-emitting diode when bent at approximately 180 • (Figure 5F, right).This proves that the 3D alloy layer can modify various forms of Zn metal surface, illustrating a high degree of processing adaptability.The pouch cells are assembled using Zn foil and CZ@Zn sample as anode with dimensions of 3 cm × 4 cm × 10 µm and LMO as cathode, which are used to power a small bulb (inset of Figure 5G), and the cycling performances at 1C are shown in Figure 5G.The areal capacity of LMO//CZ@Zn cell reached 133.6 mAh g −1 in the first cycle while the LMO// Zn cell delivered a lower capacity of about 122.8 mAh g −1 .More importantly, the capacity of the LMO//Zn cell begins to fluctuate at 260 cycles and finally declines rapidly to below 40 mAh g −1 after 290 cycles.On the contrary, the LMO//CZ@Zn cell maintained a capacity of 84 mAh g −1 after 700 cycles because of the uniform deposition morphology and the suppressed HER and corrosion.

CONCLUSION
We report a composite Zn anode with hydrophobic and electrically conductive 3D layers by an in situ method.
Based on comprehensive evidence from various in/ex situ characterizations and theoretical calculations, the 3D electrically conductive layer reduces local current density and improves the Zn 2+ transfer kinetics due to the increased nucleation sites.Meanwhile, the coupling of hydropho-bic property for the protect film inhibits the corrosion of Zn anode.Therefore, the CZ@Zn electrode achieves an excellent CE of 99.88% over 3500 cycles and a smaller overpotential than bare Zn at a high current density of 5 mA cm −2 .The symmetric CZ@Zn//CZ@Zn cell exhibits 2000 cycles at 5 mA cm −2 with a low 81.1 mV overpotential and no obvious dendrite growth.Furthermore, at the reasonable DOD of 36%, the pouch cell with CZ@Zn and LMO electrodes presented a capacity of 133.6 mAh g −1 in the initial cycle and maintained stable over 700 cycles at 1C, whereas the bare Zn in LMO//Zn almost depleted after 260 cycles.The 3D electrically conductive-hydrophobic layer provides significant insight into regulating the side reactions, Zn deposition, and interface kinetics simultaneously for long-life Zn anodes.

Synthesis of CZ@Zn material
The CZ@Zn material was prepared by in situ surface ion exchange followed by the thermal treatment strategy.In detail, the Cu@Zn precursor was prepared first.CuSO 4 ⋅5H 2 O (5 g; Beijing Tongguang Ltd.) and sodium citrate (11 g; Beijing Tongguang Ltd.) were dissolved in deionized water (500 mL) and stirred for 1 h to form a homogeneous solution.After polishing and cleaning with sandpaper and alcohol, the Zn foil (8 cm × 8 cm) was immersed in the solution for 2 min, keeping the solution stirred at 400 rpm during this time.The sample was then washed with deionized water and alcohol to remove the residual ions on the surface.After drying at room temperature for 24 h Cu@Zn samples were obtained.Finally, Cu@Zn was calcined under an Ar atmosphere at 350 • C for 1 h at a heating rate of 5 • C min −1 , followed by natural cooling to obtain CZ@Zn samples.The CZ@Zn rod was prepared with a similar method except instead of the Zn foil with Zn rod.

Synthesis of LMO cathodes
The LMO slurry was prepared through the coating method.The isopropanol (0.5 mL) was first added in LMO powders (0.8 g) to grind for 10 min.Then the ground LMO was mixed with conductive carbon (0.1 g) and aqueous PTFE (0.1 g) to prepare the LMO slurry after stirring for 30 min at room temperature.The obtained LMO slurry was coated on the Ti mesh and dried in a vacuum oven at 60

Material characterizations
The SEM and EDS were tested with field-emission SEM (Hitachi SU-70).The SEM accelerating voltage is 5.0 kV, while the EDS accelerating voltage is 20.0 kV.The XRD was carried out on the X-ray diffractometer equipment (D8-XRD; Bruker AXS, WI, USA) to analysis the sample composite.The TOF-SIMS of CZ@Zn was tested by ULVAC-PHI, Inc-Japan (PHI nano TOF II) with Ar + pulsed ion beam and extraction voltage of 2 kV.The extracted area is 200 µm × 200 µm.The contact angle of bare Zn and CZ@Zn were examined via KRÜSS-DSA100 to evaluate the wettability between samples and electrolytes.The symmetric cells with bare Zn and CZ@Zn electrodes were observed for their interface morphologies during the Zn plating process using an optical microscope (Leica DVM6).DEMS tests were conducted on a sealed Swagelok cell using a commercial mass spectrometer (Hiden, Beijing).Before testing, the test equipment was purged with Ar for 6 h.

Electrochemical measurements
Symmetric 2025-type coin cells were assembled using two Zn or CZ@Zn electrodes, Whatman glass fiber separators, and 2 mol L −1 ZnSO 4 aqueous electrolyte.The cycling performance of the CE was tested using Zn foil and CZ@Zn with the same electrolyte and separator as the symmetric cells, the Cu foil as the counter electrode.The coin full cells were assembled with LMO cathodes, Zn foil or CZ@Zn anodes, and 150 µL aqueous electrolyte of ZnSO 4 (1 mol L −1 ) and Li 2 SO 4 (2 mol L −1 ).For LMO//CZ@Zn fiber cell, we assembled the CZ@Zn rod, glass fiber separator, and LMO cathode based on titanium foil coaxially with the same electrolyte for the coin full cell.The cell was then sealed in a heat shrink tube with hot melt adhesive at the two ends.The extended anode and cathode materials are used as battery lugs.For the pouch cells, the LMO cathode, separator, and CZ@Zn anode (thickness: 10 µm) were cut into the square of 3 × 4 cm 2 , with the same assemble order as coin cells.Galvanostatic cycling tests were conducted with the Neware battery test system.The CV curves were tested between 1.4 and 2.05 V.The EIS tests from 0.01 Hz to 100 kHz using a CHI 660e electrochemical workstation (ChenHua Instruments Co.).

A C K N O W L E D G M E N T S
This work was supported by the Joint Funds of the National Natural Science Foundation of China (U2130204), Beijing Outstanding Young Scientists Pro-gram (BJJWZYJH01201910007023), and the Young Elite Scientists Sponsorship Program by CAST (YESS20200364).

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

F
I G U R E 1 (A) Schematic illustration of the synthesis process of CZ@Zn electrode.(B) Top-view (left) and side-view (right) SEM images of the CZ@Zn sample.(C) Elemental mapping images of Cu and Zn.(D) XRD patterns of fabricated CZ@Zn and Zn foil.(E and F) The TOF-SIMS elemental distribution intensity plot (E) and corresponding 3D images (F).(G) Contact angle with 2 M ZnSO 4 aqueous electrolyte for bare Zn and CZ@Zn, respectively.

F
I G U R E 3 (A) In situ optical microscopy photos showing the Zn plating process at a current density of 3 mA cm −2 with deposition times of 0, 10, and 40 min, respectively.(B and C) Corresponding optical surface-profilometry image of bare Zn (B) and CZ@Zn (C) electrodes after the Zn plating process.(D and E) SEM images of bare Zn (D) and CZ@Zn (E) electrodes after deposition 1 mAh cm −2 at 1 mA cm −2 .(F) XRD analysis of bare Zn and CZ@Zn after 10 cycles at 1 mA cm −2 for 1 mAh cm −2 .(G) Linear polarization curves showing the corrosion behavior for bare Zn and CZ@Zn electrodes.(H and I) EIS at different temperatures for bare Zn (H) and CZ@Zn (I) electrodes.(J) The corresponding linearly fitted Arrhenius curves and calculated activation energies of bare Zn and CZ@Zn.

F I G U R E 4
Theoretical simulation of the protection effect for CZ@Zn electrode.(A and B) The projected density of states for the Zn (A) and CZ sample (B).(C and D) The calculations models (C) and the corresponding adsorption energies (D) of Zn atom on Zn and CZ, respectively.(E and F) The Zn 2+ -diffusion path (inset) and the corresponding kinetic energy barriers in the Zn (E) and CZ sample (F).(G) Schematic illustration of the positive effect of CZ layer.symmetric cells at different temperatures from 20 to 80 • C and fitted with an equivalent circuit (Figures

F
I G U R E 5 (A) CV measurement of LMO//bare Zn and LMO//CZ@Zn full cells at a scan rate of 1 mV s −1 .(B) Rate performance of LMO//bare Zn and LMO//CZ@Zn full cells.(C) EIS of LMO//bare Zn and LMO//CZ@Zn full cells before the cycle.(D and E) In situ DEMS profiles showing released H 2 gas during the first cycle for LMO//CZ@Zn (D) and LMO//bare Zn (E) cells.(F) Schematic diagram of the fiber cell assembly (left) and optical photograph of LMO//CZ@Zn fiber battery lit a LED (right).(G) The long-term performance of the pouch cells (20 cm 2 ) of LMO//bare Zn and LMO//CZ@Zn at 1C (1C = 148 mAh g −1 ).The inner is an optical image of a bulb lit by the LMO//CZ@Zn pouch cell.