3D Printing of Hierarchical Graphene Lattice for Advanced Na Metal

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3D Printing of Hierarchical Graphene Lattice for Advanced Na Metal Anodes Yikang Yu,†,‡,# Zhuoyue Wang,§,# Zhen Hou,†,# Wurui Ta,§ Wenhui Wang,† Xixia Zhao,† Qian Li,† Yusheng Zhao,*,∥ Qiangqiang Zhang,*,§ and Zewei Quan*,†,∥ †

Department of Chemistry, Southern University of Science and Technology (SUSTech), Shenzhen, Guangdong 518055, China Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States § College of Civil Engineering and Mechanics, Key Laboratory of Mechanics on Disaster and Environment in Western China, The Ministry of Education of China, Lanzhou University, Lanzhou 730000, China ∥ Shenzhen Key Laboratory of Solid State Batteries and Guangdong Provincial Key Laboratory of Energy Materials for Electric Power, Shenzhen, Guangdong 518022, China

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S Supporting Information *

ABSTRACT: The expanding energy storage market has created a surge of interest in the exploration of high energy density alternatives to Li-ion batteries, and Na metal batteries have received considerable attention due to their abundant reserves and low cost. Similar to Li metal anodes, the unstable plating/stripping behaviors of Na metal anodes upon cycling hinder their practical applications at room temperature. Herein, a superelastic graphene lattice (GL) with hierarchical structures was fabricated via a 3D printing technique on the basis of the direct inkjet writing strategy. This approach enables the precise tailoring of the multiscale graphene bulk structure, from nanometer GO building elements to macroscopic monoliths. Due to the porestructure design of the GL, the rim regions of the holes demonstrated a highly concentrated current density and could serve as preferred sites for Na deposition. This phenomenon was utilized to regulate the Na deposition; hence, a stable Na metal anode is produced. As a result, a high Coulombic efficiency of 99.84% was realized for a long lifetime of 500 cycles (∼1000 h) at a current density of 1 mA cm−2. These results provide a novel insight into the rational design of graphene-based material structures at multiscale for high-performance Na metal anodes. KEYWORDS: 3D printing, hierarchical structures, multiscale manipulations, Na metal anodes, Na plating/stripping

1. INTRODUCTION The increasing demands of energy storage applications, such as electric vehicles and portable electronic devices, have driven researchers to develop reliable and low-cost systems beyond Li-ion batteries.1 Among various potential technologies, sodium-ion batteries (SIBs) are one of the prominent candidates because of the low cost and earth abundant Na resources.2−6 Although many significant achievements have been made, the insufficient energy density is regarded as one of the weaknesses of SIBs.7−12 Na metal anodes are prized for their high theoretical capacity (1166 mAh g−1) and the lowest electrochemical potential (2.71 V versus the standard hydrogen electrode), which are promising for constructing high energy density of SIBs and are therefore of great interest.13−18 However, the Na plating/stripping process during battery operation suffers from large volume changes, and these changes could easily lead to porous Na deposition and dendrite formation. These issues result in an increasing resistance and rapid lifetime failure, which hinder their practical applications.19 © 2019 American Chemical Society

To address these challenges, some approaches, such as construction of a stable host,20−24 optimization of the electrolyte composition and additives,25−28 introduction of a protective layer,29−33 and employment of a solid-state electrolyte,34−37 have been proposed to stabilize Na metal anodes. In particular, a series of porous hosts have been devised to regulate Li/Na plating/stripping behaviors.38−41 It has been reported that porous structure could significantly affect the current density distribution and thus suppress the formation of dendrites.42−44 On the other hand, improving the Li/Na wetting ability of the substrate could reduce the nucleation overpotential, which is beneficial for stable Na plating/stripping.45−50 Consequently, the excellent affinity for Na metal demonstrated by graphene-based nanostructures44,51−54 may benefit dendrite free deposition. Due to these factors, 3D porous graphene monoliths with specially Received: March 12, 2019 Accepted: April 30, 2019 Published: April 30, 2019 3869

DOI: 10.1021/acsaem.9b00540 ACS Appl. Energy Mater. 2019, 2, 3869−3877

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Figure 1. Three-dimensional-printed GL with multiscale hierarchical structures. (a) Schematic of the additive manufacturing of hierarchical GL and its photo (inset). (b) A flyweight GL (bulk density: 1 mg cm−2) laying on a dandelion flower. (c) Optical microscope image of a porous bulk GL with a long-range periodic order. (d−h) Images of GLs at multiple scales. (d) Long-range order in the bulk lattice and (e) SEM images showing the structural hierarchy of the multiscale GL down to (g) pore sizes of tens of micrometers. (h) GO nanosheet as a building block showing features at the micrometer level in an AFM image.

designed microstructures are believed to enable a significant performance improvement of the Na metal anodes. Nevertheless, most previous reported preparations of those porous structures are mainly based on the template method or random assembly of graphene; it is difficult to achieve precise structural control at the microscale.40,55−61 Graphene suspensions combined with polymer or silica powders have been previously reported to prepare a 3D graphene structure with direct ink writing, and these graphene bulks exhibit a superior mechanical property and enable a wide range of applications.62−67 For example, Zhu and Chen et al. reported the 3D printed cellulose nanofiber structure for a stable lithium metal anode. With this 3D-printed Li anode and a LiFePO4 cathode, a full cell performace of 85% capacity retention has been achieved even after 3000 cycles at 10 C.68 Another example is the 3D printed fabrication of a sulfur/ carbon cathode for Li−S batteries, which delivers a stable capacity of 564 mA h g−1 at a high sulfur-loading of 3 mg cm−2.69 Electrode structure plays an important role in advancing electrochemical device performance, as the rationally constucted structure could facilitate efficient ion diffusion.70 In this work, hierarchically structured graphene lattices (GLs) with specific pore arrangements were fabricated on the basis of a 3D printing technique using graphene oxide (GO) nanosheets as basic building blocks.71,72 This 3D printing technique enables the rational design of 3D graphene-based material nanostructures by providing a controllable and designable strategy for structure manipulation on multiple scales. When GLs were employed as hosts for Na deposition, the Na was successfully deposited into the interior of the oriented pore structure. The current density simulation verified that several times higher of the current density was exhibited at the rim regions of the pore than other regions, and thus, the rim regions could serve as “hot spots” to regulate Na deposition. Compared with commercial Cu/Al current collectors, an improved Na plating/stripping cycling perform-

ance in symmetrical cells as well as a high Coulombic efficiency of 99.84% for 500 cycles at a current density of 1 mA cm−2 were achieved using GLs.

2. EXPERIMENTAL SECTION 2.1. 3D Printing of Graphene Lattice. The printable ink was first prepared by a graphene oxide (GO) precursor with a concentration of 10 mg/mL. The GO precursor was synthesized by a modified Hummers method using natural graphite flakes (50 mesh), as we reported in previous publications.73,74 Then, the viscoelasticity and rheological properties of the fluidic GO precursor were regulated by adding 2 vol % ethylenediamine (EDA) agent (concentration ∼1%) to make the GO ink printable with shear-thin behavior. After that, the as-prepared GO ink, which was a hydrogel, was printed into the designed architectures by a direct inkjet writing process. These printed hydrogel-like lattices were treated at the hydrothermal conditions of 80 °C for 24 h to further promote and strengthen the order assembly among GO sheets at the microscale via π−π stacking interaction. After dialysis in a 20 vol % ethanol solution for 24 h, these lattices were followed by freeze-drying under vacuum conditions to remove extra solvents (e.g., water, EDA). Finally, the dry aerogels, which formed 3D-printed GO lattices, were thermally annealed to obtain a highly graphitized graphene lattice that had a regular arrangement of multiscale hierarchical structures, robust mechanical properties, and excellent electrical conductivity, as expected. 2.2. Preparation of Graphene Paper. Graphene oxide papers were prepared by a facile vacuum-assisted filtration method using an as-prepared graphene oxide suspension with the same ratios of the EDA additives. The prepared freestanding graphene oxide papers were cut into disks with a diameter of 16 mm and further heated to 250 °C (Ar atmosphere) for 1 h to prepare the graphene paper (GP). 2.3. Characterization. Scanning electron microscopy (SEM, HITACHI SU8010, Japan) was conducted at 5 kV and used to characterize the morphology of the Na deposition on the Al, Cu, GP, and GL. A vacuum transfer box was used to avoid the oxidation of Na. However, brief and unavoidable oxidation of Na may still occur, which may contribute to slight surface features on the Na metal in the SEM images. Specific surface areas were measured using the Brunauer−Emmett−Teller (BET) method (ASAP 2020). X-ray photoelectron spectroscopy (XPS) experiments were conducted 3870

DOI: 10.1021/acsaem.9b00540 ACS Appl. Energy Mater. 2019, 2, 3869−3877

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Figure 2. Morphologies of deposited Na (4 mAh cm−2) on the different substrates of Al, Cu, GP, and GL. The porous structure of deposited Na is shown on (a−c) Al and (d−f) Cu substrates. (g−i) SEM of the Na morphology on the GP surface, where the preferred deposition is demonstrated. (j−l) SEM of the Na morphology on the GL surface in the region around the holes. with a Physical Electronics PHI5802 instrument using a magnesium X-ray anode (monochromatic Kα X-rays at 1253.6 eV) as the source. Raman spectra were obtained on a Raman microscope (HORIBA LabRAM HR Evolution) with a 325 nm excitation wavelength. Atomic force microscopy images and optical microscopy images were obtained from a Bruker AFM instrument (Dimension Fastscan) and Olympus stereomicroscopes (SZX2), respectively. The compression tests were performed on an electronic universal testing machine with a loading rate of 1 mm/min, and the compression processes were added by a stepwise increase in the strain of 30, 60, and 90%. The obtained strain versus stress curves were used to evaluate the resilience capability of the GL after a certain compression deformation. Moreover, at a compression strain of 80%, cyclic compression was carried out to investigate the fatigue resistance and structural robustness for a GL, which would serve as a Na metal host in the battery systems. 2.4. Electrochemical Measurements. Electrochemical measurements were performed using CR2032-type coin cells. Symmetric NaAl, Na-Cu, Na-GP, and Na-GL coin cells were assembled in an argonfilled glovebox, and 4 mAh cm−2 of Na was first deposited on these current collectors at a current density of 1 mA cm−2 to test the galvanostatic cycling stabilities. A metallic Na foil (diameter of 16 mm) was used as the counter/reference electrode, and these current collectors were used as working electrodes (diameter of 16 mm). An electrolyte quantity of 70 μL (1 M NaPF6 in diglyme) was used in each cell. The cycling voltage profiles were collected by first plating 4 mAh cm−2 of Na on current collectors at 1 mA cm−2 and further stripping/plating 1 mAh cm−2 of Na at a current density of 1 and 2 mA cm−2. To test the Coulombic efficiency, a fixed amount of 1 mAh cm−2 of Na was deposited on these current collectors at a current density of 1 mA cm−2 and then stripped away at a voltage of 0.2 V vs Na/Na+ for each cycle. Electrochemical impedance spectroscopy (EIS) measurements were conducted in the frequency range from 100 kHz to 0.1 Hz (Bio-Logic electrochemical workstation).

3. RESULTS AND DISCUSSION Herein, we report a flyweight graphene lattice (GL) architecture fabricated by direct 3D printing using viscous graphene oxide ink, as shown in Figure 1a. These obtained hydrogel-like lattices were then freeze-dried at −60 °C before thermal annealing at 250 °C in an Ar atmosphere. The final products are illustrated in Figure 1b and c. The length-scale breakdown of the multiscale ultralight GL with hierarchical architecture is shown in Figure 1d−h. At the macroscopic dimension (∼2 cm, Figure 1d), the bulk GL consisted of a long-range periodic order of pore structures. The pore size was precisely controlled with an average radius of approximately 500 μm via a 3D printing strategy (Figure 1e). The crosssection view (Figure 1f) reveals that these GL architectures are constructed by the organization of several printing layers (∼250 μm), which forms a continuous and seamless interface. The self-assembly of the GO sheets and subsequent freezecasting process gave rise to the orderly oriented porous internal structure at the microscale, with pore sizes ranging from hundreds of nanometers to dozens of micrometers (Figure 1g). These architectures were built from GO nanosheets with lateral diameters from hundreds of nanometers (∼500 nm) to several micrometers (∼10 μm). Due to the highly porous structures, the GL had a high Brunauer− Emmett−Teller (BET) specific surface area of 52.8 m2 g−1, measured by N2 isothermal adsorption−desorption analysis (Figure S1). The 3D hierarchical feature ranges from nanometers to centimeters, which is a span of over 3 orders of magnitude, and enables control of the graphene bulk architectures for desired applications. 3871

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Figure 3. Simulation results of the current density distribution on the surface of the porous graphene structure. (a) Morphology of the deposited Na on the GL hole and (b, c) an enlarged view. (d, e) Current density distribution simulation of the single hole graphene structure. (f) Plot of the current density as a function of position x showing the variation in the current density in the horizontal direction.

398.2 eV and graphitic N at 401.3 eV) in the GL (Figure S3d), originating from the ethylenediamine (EDA) used during preparation of printable GO ink. In the Raman spectra shown in Figure S3f, two characteristic D and G bands peaked at 1354 and 1593 cm−1 were indexed to defects/disorders and crystalline graphitic domains, respectively. The intensity ratio of ID/IG was 1.09, implying a high graphitization of the GL, where most of the sp3 carbons were recovered to sp2 hybridization after thermal annealing. To evaluate the role of the designed pore structure of GL in the sodium plating behavior, we characterized the plated Na metal morphologies on Al foil, Cu foil, graphene paper (GP), and GL. Al foil and Cu foil are typically used as anode current collectors in Na-ion and Li-ion batteries, respectively. Planar GP was prepared by a simple vacuum-assisted filtration method. Four mAh cm−2 of metallic Na was deposited on these substrates at a current density of 1 mA cm−2. The Al and Cu foils with plated Na exhibited similar porous Na morphologies (Figure 2a−f), leading to an increase in resistance and a decrease in cycling stability. Moreover, the GP micrographs contained spherical structures after Na deposition (Figure 2g−i). We conjecture that doped functional groups, such as pyridinic nitrogen and graphitic nitrogen, could serve as uniform sites to regulate Na nucleation and growth, thus giving rise to spherical morphologies.79,80 In contrast, nodule-like Na was formed on the GL branches, which was possessed with seamless interfaces (Figure 2j−l). Moreover, Na was uniformly infused into the holes of the 3D-printed GL (Figure S4 and Figure 3a). Inside these holes, the plated Na had a smooth and compact morphology (Figure 3b and c). The Na first nucleated on the interior of these holes in the patterned GL structure and then grew horizontally to fill in empty voids, which could efficiently suppress the formation of porous Na structures. Such results indicate that the welldesigned pore structure is promising for regulating the deposition behavior of metallic Na anodes.

To ensure the structural integrity during the coin cell assembly process, a superior ductility to resist compression deformation is requested for these printed electrode candidates. The mechanical properties of the GL were tested under a static loading process (loading rate = 1 mm/min), as shown in Figure S2. The GL sample could be compressed into an “ultrathin cake” with a maximum strain of up to 90% and recover its original configuration without noticeable structural fracture or degeneration as long as the applied stress was released gradually (Figure S2a, video S1). The stress versus strain curves with the stepwise compression strain increased from 30 to 90%, as shown in Figure S2b, demonstrating an excellent ability to resist compression deformation. The cyclic test with a compression strain of 80% further verified its excellent performance for overcoming thermal stress fatigue (Figure S2c and d). Unlike bulk graphene aerogels that are mostly formed by the random assembly of graphene oxide sheets,73−76 additive manufacturing of graphene oxide via a 3D printing technique is capable of controlling the microstructure without ruining the structural continuity and robustness. Overall, compared with those previous 3D-printed graphene bulks,62,77,78 our GL demonstrated enhanced mechanical properties with a higher strength (Young’s modulus >100 kPa) and a larger ductility due to its unique hierarchical structures via multiscale design and fabrication process. Such robust structures and the large ductility would allow the GL to resist the structural degeneration during the primary battery assembly and subsequent operation process. X-ray photoelectron spectroscopy (XPS) was further conducted to probe the surface chemical composition and doped functional groups in the GL. The intensity of the peak at 284.6 eV corresponding to sp2-hybridized aromatic carbon increased from the GO (Figure S3a and b) to the GL (Figure S3c and e). In contrast, the oxygen-containing groups of −OH, −COOH, and −O−C−O− peaked at 286.3, 288.2, and 289.1 eV, respectively, exhibited a reduced intensity of GL. The N 1s spectrum confirmed the existence of N-doping (pyridinic N at 3872

DOI: 10.1021/acsaem.9b00540 ACS Appl. Energy Mater. 2019, 2, 3869−3877

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Figure 4. Electrochemical performance of the GL current collector for the Na metal anode. The detailed plating/stripping behaviors of (a) Na-Cu, (b) Na-GP, and (c) Na-GL. The current density was fixed at 1 mA cm−2 with a cycling capacity of 1 mAh cm−2, and the stripping upper voltage was 0.2 V. (d) The Coulombic efficiency for the first 500 cycles of Cu, GP, and GL at 1 mA cm−2 (∼1000 h). (e) Voltage profiles of the symmetric cells at a current density of 1 mA cm−2.

Na) to strip Na away during each cycle. A sharp increase up to −0.11 V appeared for the Cu foil at its first deposition (Figure 4a), implying a large barrier for Na nucleation. Fluctuations with a larger voltage polarization (∼110 mV) occurred during cycling on the Cu foil, which exhibited unstable plating/ stripping behaviors. Meanwhile, the GP had an oscillating voltage profile at the first deposition (Figure 4b), which was probably induced by the unstable formation of a solid electrolyte interphase (SEI). In contrast, GL (Figure 4c) demonstrated a much more stable plating/stripping voltage curve, and the voltage polarization of plating/stripping on the GL stabilized at approximately 50 mV within 500 cycles. As shown in Figure 4d, the CE of the Cu and Al foils (Figure S6c) became unstable immediately after the test, while GP exhibited stable electrochemical cycling up to ∼170 cycles. In contrast, a reversible and stable CE for GL was displayed beyond 500 cycles (nearly 1000 h) with an average value of 99.84%. According to the Na deposition SEM images (Figure 2), high specific surface structures were formed on Cu, Al, and GP substrates. As for GL, a much more compact and flat Na structure has been deposited in the holes (Figure 3). In this regard, we conjecture that dead Na as well as reaction between Na and electrolyte are prone to occur with high specific surface structures, and thus induce low and unstable CE. These results validate that a high reversibility of Na plating/stripping behavior was achieved on GL, which originated from its multiscale hierarchical ordered structure. The cycling stability was evaluated using symmetrical cells. Specifically, 4 mAh cm−2 of Na was first deposited onto GL, Al, Cu, and GP substrates at a current density of 1 mA cm−2,

To validate the mechanism of the designed porous structure for the Na metal deposition, current density simulations were conducted to understand the relationship between the porestructure orientations and the current density distributions. In view of the accurate formation process of 3D printing on porous graphene monolith structures, the pore structure was constructed with a pore radius of 500 μm and a pore spacing (distance between adjacent pore centers) of 800 μm. The low local current density of porous structures induced by their large specific surface area was reported as one of the main reasons for the successful suppression of dendrite growth.81,82 In contrast, the current density distribution has been less considered.42 Parts d and e of Figure 3 show the simulation results of the current density distributions of the single hole structure. Compared with those of other sections, a much higher current density at the rim regions with a thickness of 15 μm was observed (Figure 3f), indicating that sodium tended to deposit inside the holes during the electroplating process. Similar results could also be obtained (Figure S5) with different pore densities. These intriguing results are in agreement with the Na deposition behaviors, revealing the necessity of structural arrangement to optimize Na metal anode stability. The reversibility of Na plating/stripping on as-prepared GL was evaluated by tests of the Coulombic efficiency (CE); that is, the GL served as a working electrode and Na foil served as the counter/reference electrode. For comparison, Al foil, Cu foil, and GP were also characterized as current collectors. For the CE tests, a fixed Na capacity of 1 mAh cm−2 was deposited on the substrates and then they were charged to 0.2 V (vs Na+/ 3873

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forming Na-GL, Na-Al, Na-Cu, and Na-GP anodes, respectively. These symmetrical cells were then cycled at a current density of 1 or 2 mA cm−2 with a cycling capacity of 1 mAh cm−2. As shown in Figure 4e, the voltage profile of the Cu foil oscillated and had a sharp drop in the voltage hysteresis at approximately 120 h (1 mA cm−2), which indicates possible short-circuiting of the cells. At a higher current density of 2 mA cm−2, a sudden increase in the voltage hysteresis occurred, indicating an unstable Na plating/stripping behavior on the Cu foil. Similar to the Cu foils, sudden increase of voltage hysteresis and short-circuiting also occurred for Al foils (Figure S6a and b). Improved performance was observed on GP compared with the Al and Cu foils. GP illustrated a stable cycling behavior for a period of approximately 100 h at current densities of 1 and 2 mA cm−2, while the voltage hysteresis gradually increased upon extended cycling. In contrast, GL was cycled stably for over 600 h at a current density of 1 mA cm−2 with a relatively small voltage hysteresis of ∼40 mV (Figure 4e). Moreover, a stable lifetime of over 400 h was achieved on the GL at a current density of 2 mA cm−2 despite a slight increase of the voltage hysteresis (Figure S6c). The electrochemical performances are summarized in Table S1, and the cycling lifetime of the Na-GL anode is competitive with those previously reported for hosts of Na metal anodes. More importantly, due to the ultralight nature of GL (1 mg cm−2) and high energy property of Na metal, our Na-GL exhibits a specific capacity several times higher than that of hard carbon (Table S2), supporting its potential as an anode material in sodium batteries. To further investigate the polarization of the electrode after cycling, electrochemical impedance spectroscopy (EIS) experiments of GP and GL symmetrical cells were conducted. The semicircle at the high frequency range could be due to SEI interfacial resistance and the charge transfer impedance at the Na surface.83 Compared with GP (∼15 Ω, Figure S7a), GL demonstrates a much smaller interfacial resistance of ∼2.1 Ω (Figure S7b) for the initial Na deposition at 4 mAh cm−2. After 10 cycles at a current density of 1 mA cm−2, the impedance of GP increased while the GL nearly maintained the initial impedance, which is consistent with the stable cycling performance of Na-GL symmetrical cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00540.

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Current density simulation details, mechanical properties test of GL, some materials characterization data (including BET, XPS, and Raman spectra), and supporting electrochemical test data (PDF) Video demonstrating how the GL sample could be compressed into an “ultrathin cake” with a maximum strain of up to 90% and recover its original configuration without noticeable structural fracture or degeneration as long as the applied stress was released gradually (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.Q.). *E-mail: [email protected] (Q.Z.). *E-mail: [email protected] (Y.Z.). ORCID

Yikang Yu: 0000-0002-6101-2393 Qian Li: 0000-0002-4847-4892 Qiangqiang Zhang: 0000-0002-6082-6782 Zewei Quan: 0000-0003-1998-5527 Author Contributions #

Y.Y., Z.W., and Z.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC) (Nos. 51772142, 51702142), Department of Science and Technology of Guangdong Province (No. 2018B030322001), Shenzhen Science and Technology Innovation Committee including fundamental research projects (Nos. JCYJ20170412152528921, KQJSCX20170328155428476, ZDSYS20180208184346531), Science Fund for Distinguished Young Scholars of Gansu Province (No. 18JR3RA263), 2018 Initiative Postdocs Supporting Program (No. BX20180132), peacock technology innovation project (20170328085748757), Development and Reform Commission of Shenzhen Municipality (Novel Nanomaterial Discipline Construction Plan), and start-up fund and Presidential fund from SUSTech.

4. CONCLUSIONS In summary, mechanically robust and flyweight graphene lattices were prepared via 3D printing techniques and were further used as hosts for room temperature Na metal anodes. The 3D printing technique facilitated the controllable manufacture of multiscale microarchitectures, which had a length scale from the nanometer scale of GO building blocks to the centimeter scale bulk, enabling precise control of the multiscale graphene bulk structure. The rim regions of the holes within GL had a high current density and thus could serve as “hot spots” to regulate the preferred Na deposition, which were beneficial to suppress Na dendrites and porous Na deposition. As a result, the Na-GL electrodes were cycled at a current density of 1 and 2 mA cm−2 for long lifetimes of 600 and 400 h, respectively. Moreover, an average Coulombic efficiency of 99.84% was delivered for 500 cycles at a current density of 1 mA cm−2 (∼1000 h). Optimizing the design of the electrode structure to regulate the current density distribution in this paper was verified as a reliable strategy to realize stable Na metal anodes.

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REFERENCES

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