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Functional Nanostructured Materials (including low-D carbon)

Highly Lithiophilic Graphdiyne Nanofilm on 3D FreeStanding Cu Nanowires for High-Energy-Density Electrode Hong Shang, Zicheng Zuo, and Yuliang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03633 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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ACS Applied Materials & Interfaces

Highly Lithiophilic Graphdiyne Nanofilm on 3D Free-Standing Cu Nanowires for High-EnergyDensity Electrode

Hong Shang †,‡, Zicheng Zuo *,‡, and Yuliang Li *,‡,§



School of Science, China University of Geosciences (Beijing), Beijing 100083, P. R.

China ‡

Beijing National Laboratory for Molecular Sciences (BNLMS), CAS

Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China §

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

KEYWORDS: Cu Nanowire, Graphdiyne, Lithiophilicity, Dendrite, Lithium metal battery

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ABSTRACT: The sp-hybridization carbon atoms in graphdiyne (GDY) are higher lithiophilic epicenters for lithium (Li) deposition than those of sp2-hybridized ones in traditional carbon materials. The ultrathin graphdiyne nanofilms are constructed in situ on the 3D free-standing Cu nanowires (CuNWs) for improving the lithiophilicity of the surface

efficiently.

The

CuNW

electrode

modified

by

graphdiyne

nanofilms

(GDY@CuNW) shows significant improvements in terms of overpotential for lithium nucleation, battery lifespan, Coulombic efficiency, and suppression of dendritic lithium. The overall improvement of the free-standing electrode exhibits a volumetric capacity high up to 1333 mA h cm−3. Our results have demonstrated that controllable growth of graphdiyne nanofilms should be an effective method to improve the Li plating and stripping process in the Li metal battery.

INTRODUCTION

Metallic lithium as anode in the battery can contribute maximal energy density comparing to the prevailing host materials (graphite, graphene, Si, metal oxides, etc). The Li metal delivers an theoretical specific capacity high up to 3860 mA h g−1 with the

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lowest electrochemical potential.1,2 However, it still has serious defects that the safety and reversibility of metallic Li in the batteries are severely impeded by its dendritic growth.3-5 Solving these problems effectively is an important measure to promote the development of this field, especially some dynamic problems need to be solved due to the thermodynamic characteristic during repeated Li stripping/plating process. In terms of electrolytes, the optimized solvents and Li salts were used to improve the Li+ migration behavior at interface of the Li/electrolyte,6-8 and special additives were engineered for forming a high-strength solid electrolyte interphase (SEI) to prevent the growth of lithium dendrites.9-11 The film of 2D BN/graphene, graphite layer and interconnected hollow carbon nanospheres were used to mechanically supporting and suppressing the dendrite.12-15 3D nickel and copper foams were applied for accommodating the Li metal.16-19 The results indicated the initial nucleation process on the host played significant influence on the reversibility during the Li stripping/plating cycles. Thus, enhancing the lithiophilicity of the electrode is an efficient approach to promote the electrochemical performance. However, the high energy density can hardly be achieved based-on these 3D current collector via traditional methods, and the

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strategies of distributing the atomic-level lithiophilic epicenters uniformly on the electrode haven’t been reported. Graphdiyne, has aroused many potential applications,20 such as in energy storage,21-23 single-atom-metal catalysts for hydrogen evolution reaction,24 metal-free oxygen reduction electrocatalysts,25 and biological fields.26 Recently, the utilization of the large-scale ultrathin GDY films has greatly improved the Li+ migration, thus optimizing the Li nucleation and suppressing the Li dendrites.27 It can be noticed that all these potential applications are benefited from its all carbon framework with novel electronic structure and in-plane atomic-level cavities.28-30 The sp-hybridized carbon network has higher electronic density than the sp2 carbon materials, indicating GDY is a promising lithiophilic material. The ultrathin modification of GDY on the current collector can possibly provide plenty of lithiophilic epicenters, and improve the deposition process of metallic Li. In this paper, the theoretical calculation predicts that the sp-hybridization carbons in the GDY framework are highly lithiophilic, which can improve the nucleation process of lithium plating. For taking full advantage of the high lithiophilicity of GDY, ultrathin GDY

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nanofilms are in situ prepared on the Cu nanowires to form 3D seamless coating layer with uniformly distributed lithiophilic epicenters. Compared to the traditional Cu foil, CuNWs possess a higher surface area and can offer plenty of reactive positions for catalyzing the cross-coupling reaction of graphdiyne. This light-weight and 3D freestanding current collector (GDY@CuNW) can not only provide many lithiophilic epicenters but also plenty of space for hosting the lithium metal. As a result, a smaller nucleation overpotential than that on the CuNW surface is realized, thus leading to the homogeneous accommodation of metallic Li in the current collector. This modification by GDY brings many significant improvements in terms of overpotential for lithium nucleation, Coulombic efficiency, lifespan, and lithium dendrites suppression. Besides, a volumetric capacity high up to 1333 mA h cm−3 based on this thin electrode can be readily obtained.

RESULT AND DISCCUSION

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Figure 1. (a) The electron density difference map and the adsorption energies for the Li atom on the graphene, 6C- and 18C-hexagon of the GDY, respectively; (b) The electrostatic potential surfaces of the corresponding geometries; (c and d) Schematic process of the Li plating on (c) bare CuNW and (d) GDY@CuNW current collectors.

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Firstly, for theoretically understanding the improvements in lithiophilicity caused by the sp-hybridization carbons in GDY, the adsorption energy of Li atom was calculated by the density functional theory. Figure 1a shows the optimized geometries of Li atom on different carbon materials and their corresponding adsorption energies. On the traditional sp2-hybridization carbon materials, Li atom is accommodated above the center of the 6C-hexagon with a height of 1.754 Å and corresponding adsorption energy of -1.18 eV. On the GDY, Li atom can be absorbed by both 6C- and 18C-hexagon. Similar with the adsorption on the sp2-hybridization carbon materials, the Li atom held by the 6C-hexagon would be on the top of the GDY plane with a height of 1.798 Å and a relatively lower adsorption energy of -1.95 eV. However, the most stable adsorption configuration for Li atom is located at the hole of the 18C-hexagon in the GDY plane with a much lower adsorption energy of -2.60 eV. It is demonstrated that the GDY with rich sp-hybridization carbons are more lithiophilic and beneficial for optimizing the lithium nucleation process than the sp2-hybridization carbon materials. The uniform distribution of lithiophilic 18C-hexagon in the GDY framework provides a new inspiration for solving the critical issues of Li dendrites.

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In order to further understand why the 18C-hexagon cavity is beneficial for anchoring the Li atom, the electron density difference map and the electrostatic potential surfaces (ESP) were also plotted (Figure 1b). It is obvious that the electron density is transferred from the Li atom to the carbon substrates. For the traditional carbons and GDY material, the position of 6C-hexagon has limited periphery for accepting the electron from Li atom. However, the 18C-hexagon in GDY has a larger area to accept electron transferred from the Li atom. Additionally, the most negative polarization (red area) of ESP surfaces is located on the sp-hybridized carbon atoms of 18C-hexagon in GDY, whereas the carbon atoms in 6C-hexagon exhibit weak negative polarization (yellow area). This indicates that the 18C-hexagon with sp-hybridized carbon atoms has higher absorbability of lithium atoms. Thus, we can conclude that the particular 18C-hexagon structure is more advantageous for improving the Li nucleation process than the 6C-hexagon structure in the prevailing carbon materials. As known, the Cu substrate is widely applied as the anode current collector in Li metal battery, and a 3D structure can offer large space for tolerating the volumetric variations during the lithium plating/stripping process. For making full use of the natural

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lithiophilicity of GDY to improve the lithium nucleation and plating process, the ultrathin GDY nanofilms were formed in situ on the CuNWs according to the modified method in our previous report.28 Figure 1c,d schematically illustrate the possible differences in the lithium plating process that might be arisen on the 3D CuNW current collector with and without GDY nanofilms. At the beginning of the plating process, the nonuniform Li nucleation is one of the main issues, leading to the nonuniform growth into lithium dendrites. After decorated by GDY nanofilms, the lithiophilicity of Cu substrate would be significantly tuned by the unique property of GDY as our theoretical prediction. As a result, a more homogeneous growth process of lithium would be produced due to the uniformly distributed lithiophilic epicenters on the GDY, thus benefitting for suppressing the lithium dendrites and improving the long-term cycling performance.

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Figure 2. (a) Raman spectrum; (b) typical XRD pattern (Inset is the enlarged area from 10 to 35°); (c) XPS survey spectrum of GDY@CuNW; (d and e) SEM images of the top view and cross section of CuNW paper; (f and g) SEM images of GDY@CuNW; (h and i) TEM images of GDY@CuNW; (j and k) elemental mapping of C and Cu for GDY@CuNW; (l) SEM image of GDY nanofilms after removing CuNWs; (m and n) TEM images of GDY nanofilms after removing CuNWs.

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For realizing the theoretical prediction, the ultrathin GDY nanofilms were grown in situ on the CuNWs.28 Without GDY, the CuNW paper is fragile. After the cross-coupling reaction of GDY, the GDY@CuNW paper can be arbitrarily bended and cut into pieces as the electrodes (Figure S1). It is demonstrated that the GDY could be readily grown on the CuNWs and bound them together, forming the free-standing paper. Notably, the as-prepared electrode is ultralight (1.6 mg cm−2), which might be propitious to their application. The GDY@CuNW paper was fully characterized. The Raman spectrum in Figure 2a shows four peaks locating at 1390, 1586, 1938, and 2184 cm−1, respectively. Therein, the peak at 1586 cm−1 (G band) is obviously higher than that at 1390 cm−1 (D band), indicating the high quality of GDY nanofilms on CuNWs. The characteristic peak at a Raman shift of 2184 cm−1 is ascribed to the diyne linkages.20 Figure 2b shows a feeble and broad peak around 23° in the X-ray diffraction (XRD) pattern, which is ascribed to the typical interlayer distance of 2D GDY material. Its weak intensity implies that the GDY nanofilms on the CuNW paper is ultrathin. From the X-ray photoelectron spectroscopy (XPS) spectrum in Figure 2c, similar result can be concluded from the few amount (2.19%) of Cu element (binding energy at 935 cm−1 is for Cu 2p3) on the surface.

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The C 1s spectrum at 284.8 eV in Figure S2a can be divided into four peaks at 288.0, 286.5, 285.4, and 284.6 eV, ascribing to the C=O, C-O, C-C (sp), and C-C (sp2), respectively.28 Besides, the O 1s in Figure S2b can be deconvoluted into two peaks (531.6, and 532.5 eV) originated from the oxidation of residual acetylenic groups and the surface-adsorbed oxygen, respectively. Furthermore, the characterization of the GDY nanofilms after the removal of CuNWs presents a typical feature of GDY material (Figure S3). Figures 2d and S4,5 present the morphologies of CuNWs before the growth of GDY nanofilms. The CuNWs have a smooth surface. No connections are observed among adjacent nanowires, that’s the reason why the CuNW paper is fragile before the GDY growth. The CuNW is about 50 nm in width and tens micrometer in length. The thickness of the CuNW paper is about 15 µm (Figure 2e), much thinner than many reported methods. However, it can be found that there are many large voids in the CuNW paper, which is beneficial for the following growth of GDY and the high energy storage of Li metal in the electrochemical application. After the cross-coupling reaction, the ultrathin GDY nanofilms are seamlessly covered on the surface of CuNWs. Over the

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intersections among adjacent CuNWs, a 3D GDY network is in situ constructed for binding them together, as a result, the mechanical property of the CuNW network is significantly increased (Figures 2f,g and S6). In the TEM measurement, Figure 2h shows the GDY nanofilms are continuously grown on the surface of Cu nanowires, resulting in a wider diameter ranging from 70 to 120 nm than that of the bare CuNW (50 nm). The interlayer distance is about 0.375 nm, which can be seen from the highresolution TEM image in Figure 2i, consistent with the typical structure of GDY.20,28 The elemental mapping images (Figure 2j,k) demonstrate that the elements C and Cu are uniformly distributed. The mapping area of C element is obviously larger than that of Cu, indicating the CuNW is well covered by the GDY nanofilms. As shown in Figures 2l,m and S7, GDY tubular nanostructures can be obtained after the removal of the CuNWs, and this paper maintains the initial morphologies and continuousness, indicating that the CuNWs are intensively binded together by the GDY network. The HR-TEM image of Figure 2n indicates the GDY network is high quality because of many typical patterns correspongding to the layer-like structure.

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Figure 3. The electrochemical performance of CuNW and GDY@CuNW. (a) The nucleation overpotential curves for metallic Li; (b-d) Voltage-capacity curves at different current densities and cycling capacities; (e) Voltage-time curves (1.0 mA cm−2, 1.0 mA h cm−2).

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The behaviors of Li plating/stripping process on GDY@CuNW and CuNW papers were investigated by the charge-discharge testing. Some remarkable difference can be observed from the first lithium plating curves when using such two current collectors (Figure 3a). In the beginning, a rapid Li nucleation process can be observed on the CuNW electrode, while the GDY@CuNW exhibits a slower voltage drop during the beginning nucleation stage. The reason for such phenomenon is that the graphdiyne material is a promising Li-ion battery anode, which can store Li atom in the natural nanopores.

28,29

Also, it can be concluded that there is a stronger interaction between

the Li atoms and the GDY nanofilms on the CuNWs. The overpotentials can be used to evaluate the process of the Li deposition, and small ones are considered to possess a more uniform plating process. In Figure 3a, nucleation overpotentials for CuNW and GDY@CuNW electrodes are investigated at different current densities and capacities. It is obvious that under the same conditions, the CuNW electrode exhibits larger nucleation overpotentials (60, 78 and 94 mV) than the those of GDY@CuNW electrodes (50, 58 and 81 mV), implying a more uniform and retarded nucleation stage on the electrode modified by the GDY nanofilms. This is contributed by the peculiar 18C-

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hexagon structure in GDY with a plenty of lithiophilic epicenters as aforementioned. After the nucleation process, the Li plating/stripping behaviors were also compared through the following charge-discharge curves. In Figures 3b-d and S8, the overpotential for the Li plating/stripping is increased along with the increased current densities on both GDY@CuNW and CuNW electrodes. In detail, the overpotential for the lithium growth on the bare CuNW electrode has an obviously increasement in Figure 3b after 40 cycles, however, such overpotential variation is almost imperceptible on the GDY modified CuNW electrode even after 200 cycles at 0.5 mA cm−2. Even at higher current densities of 1.0 and 2.0 mA cm−2 (Figure 3c,d), the optimizations in the lithium deposition/dissolution process are also observed with the GDY nanofilms. Furthermore, the long-cycle stability of the two electrodes were investigated in a symmetric cell. The symmetric CuNW and GDY@CuNW electrodes were placed upon Li metal foil respectively and then cycled with a capacity of 1.0 mA h cm−2 at a current density of 1.0 mA cm−2. In Figures 3e and S9, GDY@CuNW exhibits a more stable and excellent performance for almost 350 hours than that of the bare CuNW electrode.

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Figure 4. (a-c) Coulombic efficiencies of CuNW and GDY@CuNW electrodes at different current densities and capacities; (d) The comparison of GDY@CuNW electrode height and volume capacity with prevailing 3D current collectors in literatures; (e) Impedance variations at different cycles (1.0 mA cm−2, 1.0 mA h cm−2); (f and g) SEM images of Li on CuNW; (h and i) SEM images of Li on GDY@CuNW; (j and k)

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SEM images of Li in the framework of GDY@CuNW.

The Coulombic efficiency (CE) of GDY@CuNW at 0.5 mA cm−2 with an areal capacity of 0.5 mA h cm−2 is stably maintained over a high value of 96.5% for near 200 cycles. On the contrary, the bare CuNW performs an relatively unstable CE during the whole cycling testing (Figure 4a). When doubling the current density and the capacity, the CE of GDY@CuNW electrode is still maintained at a high value of 96% over 160 cycles (Figure 4b). Even under a tough condition (2.0 mA cm−2, 2.0 mA h cm−2), it can be retained high up to 92% for almost 80 cycles (Figure 4c). These performances under different conditions are obviously improved compared to the CuNWs without the GDY nanofilms. Notably, a volumetric capacity high up to 1333.3 mA h cm−3 can be easily achieved on this ultrathin GDY@CuNW electrode, and its performance is much better than many of the 3D current collectors and host materials (Figure 4d and S10).16-18,31-35 Figure 4e shows the electrochemical impedance curves of the CuNW and the GDY@CuNW electrodes during the test. There is an obvious increasement after only 50 cycles for the CuNW electrode. In contrast, even after 200 cycles, the impedance of the

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electrode modified by GDY nanofilms only has a slight increasement, demonstrating a more stable SEI and electrochemical process. After repeated lithium plating and stripping process, the Li morphologies plated on the electrode were studied by SEM characterization in Figure 4f-i. Owing to the high polarization of the nucleation of metallic Li on the Cu, the nonuniform nucleation was formed, and finally induced many Li dendrites on the CuNWs as the previous report.36,37 The dendric growth of metallic Li is the main reason for the low CE and short lifespan of the battery based on the CuNWs (Figures 4f,g and S11). Contrarily, due to the lithiophilic property of GDY with atomic-level uniform epicenters, the nucleation process of Li is optimized (Figure S12), resulting in a more uniform growth process of metallic Li. As seen in Figure 4h,i, the metallic lithium on the surface of GDY@CuNW is presented as larger dumpies and the Li dendrites are efficiently avoided. Such structrue can remarkably reduce the surface area of Li metal, better for improving the CE of the battery and avoiding the short circuit. Additionally, the metallic Li is successfully anchored in the voids of the GDY@CuNW network (Figure 4j,k), which is the reason for the high volumetric capacity on such thin electrode. Without the Li dendrites, the high

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CE and long lifespan can be easily obtained. After the long-term test, the GDY@CuNW electrode was washed by deionized water. It clearly demonstrates that the electrode remains good integrate, and the GDY nanofilm on the CuNW remains the initial feature (Figure S13), indicating the strong mechanical stability of GDY.

CONCLUSION

In summary, our theoretical and experimental results have demonstrated that the unique 2D GDY has more lithiophilic epicenters than traditional carbon materials, due to its plenty of uniformly distributed sp-hybridization carbon atoms. This peculiar property of 2D GDY is adopted for tuning the surface property of Cu nanowire, and a 3D freestanding CuNW electrode decorated with ultrathin GDY nanofilms is prepared. The GDY nanofilms efficiently optimize the nucleation process of metallic Li and suppress its dendric growth, thus significantly improving the CE and long-term cycling performance. Such electrode can easily perform an ultrahigh volumetric capacity of 1333 mA h cm−3, beneficial for the practical applications in high energy density Li metal battery.

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Additionally, our results are universal and important for solving the key scientific issues in other metal batteries.

EXPERIMENTAL METHODS

GDY@CuNW. The monomer hexakis(ethynyl)benzene (HEB), and the Cu nanowires were prepared following the reported synthetic methods.20,28 The in situ growth of GDY nanofilms by HEB (2 mg) on the free-standing CuNW paper (20 mg, the diameter of the paper is 4 cm) was synthesized following the published method in our group.28 Before the characterization and the electrochemical testing, thermally treated (180 ℃) was applied to remove the moisture on the GDY@CuNW paper in N2 atmosphere.

GDY Nanofilm. A mixed aqueous solution containing FeCl3 (5%) and HCl (5%) was prepared. To obtain the GDY nanofilms, the as-prepared thermally treated GDY@CuNW paper was placed into the aqueous solution overnight to remove the Cu nanowires. Then, the GDY nanofilms were washed with dilute HCl and distilled water for several times. Finally, the sample should be dried in a vacuum drying oven.

Theoretical Calculation. DFT calculations were carried out through the Gaussian 09

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program. The full geometry optimization have been carried out using the B3LYP functional.38,39 Furthermore, Grimme's empirical dispersion correction (B3LYP-GD2) method38,40

was employed to calculate the single-point energy on each optimized

structure. In order to avoid the boundary effect, the GDY cluster including 136 carbon atoms were used to study the Li adsorption on the GDY surface, which is consistent with previous work.21 The adsorption energy Eads is defined as

Eads =ELi/substrate-Esubstrate-ELi where ELi/substrate, Esubstrate and ELi are the Li atom adsorbed on the substrate, the substrate, and the single Li atom, respectively.

Characterization. Raman were obtained at room temperature, with an argon laser excited at 473 nm on an NT-MDT NTEGRA Spectra system. XRD patterns were recorded using Cu Kα radiation on an Empyrean diffractometer. XPS spectra were performed on ESCALab250Xi apparatus, the excitation source was a 200 W monochromated Al Kα radiation. SEM of the samples were characterized using a fieldemission scanning electron microscopy (Hitachi Model S-4800). TEM images were

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characterized by a transmission electron microscopy (JEM-2100F).

Electrochemical testing. The as-obtained thermally treated GDY@CuNW paper (diameter of 1 cm with 0.16 mg cm−2 areal loading amount of GDY nanofilms on CuNW paper) and bare CuNW paper (the diameter is 1 cm) were punched into disks as working electrode. The counter electrode was Li foil (1.6 cm). After assembled in a glove box that contained little O2 and H2O (below 0.5 ppm content), the behavior of the batteries was investigated on a LAND battery system.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at XXX.

Photos before and after the GDY growth on CuNW paper; High-resolution C 1s and O 1s spectra; Characterization of GDY nanofilms; SEM and TEM images for CuNW, GDY@CuNW, and GDY nanofilms; The overpotentials of the 10th cycle at different

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current

densities

for

CuNW

and

GDY@CuNW

paper;

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Voltage-time

curves;

Electrochemical performance for 2D Cu foil and 3D CuNW; SEM images of the SEI and the dendrite/metallic Li on CuNW and the GDY@CuNW; SEM, TEM and EDX elemental mapping of the GDY@CuNW current collector after washing away the SEI.

AUTHOR INFORMATION

Corresponding Authors * Email: [email protected] (Y.L.). * Email: [email protected] (Z.Z.).

ORCID

Hong Shang: 0000-0002-5447-3677

Zicheng Zuo: 0000-0001-7002-9886 Yuliang Li: 0000-0001-5279-0399

Author Contributions

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H.S. and Z.Z. did the synthesis experiment and carried out the characterization and the electrochemical measurements. H.S., Z.Z., and Y.L. directed the study, discussed the results and wrote the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

The authors appreciate the financial support from the National Nature Science Foundation of China (Grant Nos. 21790050 and 21790051), the National Key Research and Development Project of China (2016YFA0200104), the Key Research Program of Frontier Sciences of Chinese Academy of Sciences (Grant No. QYZDY-SSW-SLH015), and the Fundamental Research Funds for the Central Universities (2652018053).

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3D free-standing CuNW electrode decorated with ultrathin GDY nanofilms can efficiently optimize the nucleation process of metallic Li and significantly improve the electrochemical performance.

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