science.sciencemag.org/content/366/6465/645/suppl/DC1

Supplementary Material for

Reversible epitaxial electrodeposition of metals in battery anodes

Jingxu Zheng, Qing Zhao, Tian Tang, Jiefu Yin, Calvin D. Quilty, Genesis D. Renderos, Xiaotun Liu, Yue Deng, Lei Wang, David C. Bock, Cherno Jaye, Duhan Zhang,

Esther S. Takeuchi, Kenneth J. Takeuchi, Amy C. Marschilok, Lynden A. Archer*

*Corresponding author. Email: laa25@cornell.edu

Published 1 November 2019, Science 366, 645 (2019) DOI: 10.1126/science.aax6873

This PDF file includes:

Materials and methods Figs. S1 to S31 Tables S1 to S4 References

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Materials and Methods Materials

ZnSO4 · 7H2O, Zn foil, 1-ethyl-3-methylimidazolium chloride and AlCl3 were purchased from Sigma Aldrich. Deionized water was obtained from Milli-Q water purification system. The resistivity of the deionized water is 18.2 M cm at room temperature. ZnSO4 · 7H2O was dissolved into the deionized water to prepare the 2M ZnSO4 electrolyte for Zn electrodeposition. Graphene dispersion in N-Methyl-2-Pyrrolidone (4 wt%) was purchased from ACS Material. The flake diameter is 1~3 μm; the thickness is 3~5 nm. The graphene dispersion was sheared by doctor blade with a shear strain of 0.5 and a shear rate of 1 s-1 on stainless steel substrate. Immediately after the shearing, the graphene coated stainless steel was transferred into vacuum for drying at room temperature. The areal mass loading of graphene is ~0.2 mg/cm2. Ketjen Black (KB) carbon was purchased from AkzoNobel.

α-MnO2 was synthesized via a conventional hydrothermal route, in which a solution that contains 5mM KMnO4 and 0.3M HCl was kept at 140℃ for 18 hours. The Au nanosheets were synthesized using the method reported in Ref (32). Characterization of materials

Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were performed on FEI F20 to identify the crystallographic features of Zn deposits. Field-emission scanning electron microscopy (FESEM) was carried out on Zeiss Gemini 500 Scanning Electron Microscope equipped with Bruker energy dispersive spectroscopy (EDS) detector to study the electrodeposition morphology of Zn under multiple conditions. Shear rheology measurement was performed using Anton Paar MCR 501 at room temperature. Atomic force microscopy (AFM) was performed on Cypher ES (Asylum Research Inc.). Cyclic voltammetry (CV) was performed on a CH 600E electrochemical workstation.

Carbon K-edge near-edge X-ray absorption fine structure (NEXAFS) measurements were carried out at the National Institute of Standards and Technology (NIST) SST-1 beamline at the National Synchrotron Light Source II at Brookhaven National Laboratory. NEXAFS spectra were obtained using a horizontally polarized X-ray beam incident at 20o,

40o, 55o, 70o, and 90o with a spot size of 10 μm. The entrance grid bias was set to –150 V to enhance surface sensitivity and reduce the low-energy electron background. A toroidal spherical grating monochromator, possessing grating of 1200 lines/mm was used to acquire the C K-edge data. Slit openings of 30 μm × 30 μm along the beamline provided an energy resolution of ~0.1 eV for all spectra. The partial electron yield signals were normalized to the incident beam intensity using the photoemission signal from a freshly evaporated Au mesh located along the incident beam path. The spectra were energy-calibrated with an amorphous carbon mesh located along the path of the incident beam using π* transition of graphite at 285.1 eV. Spectra were calibrated and normalized using standard routines from the Athena software (35).

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Grazing incidence X-ray diffraction (GIXRD) was performed on a Rigaku SmartLab

X-ray diffractometer with a Cu Kα X-ray source and a 0D scintillation detector. The instrument was operated in parallel beam geometry and measurements were collected with a 2θ scan axis. GIXRD patterns were collected from 35-47° 2θ, with a step size of 0.05°, and a scan speed of 0.1°/min. To ensure high electrodeposited zinc intensity while minimizing substrate signal, omega values of 1.0 and 0.8° were used for the bare stainless steel and graphene coated steel samples respectively. 2D-GIXRD was performed on Bruker D8 General Area Detector Diffraction System. XRD simulations were performed in GSAS-II (36) using a step size of 0.05° and instrument parameters consistent with the Rigaku SmartLab used for the GIXRD measurements.

Electrochemical measurements

Galvanostatic charge/discharge performance of coin cells were tested on Neware battery test systems at room temperature. Electrochemical studies were performed using CR2032 coin cells. The area of electrodes in this study is 1.27 cm2. Electrodes are separated by glass fiber (GF/A, Whatman). Stainless steel and carbon cloth were washed by ultrasonication in deionized water and acetone. In each coil cell, ~100 μL electrolyte was added by pipette. The electrodeposition of Al was performed in a mixture of aluminum chloride and 1-ethyl-3-methylimidazolium chloride (1.5:1, weight ratio).

In the Zn||α-MnO2 full cell experiment, the electrolyte was 2M ZnSO4 + 0.2 M MnSO4. Except the cell that used zinc foil, the zinc anodes are prepared by electrodeposition to precisely control the amount of Zn and therefore the N:P ratio. The electrodeposition was done at 4 mA/cm2.

In a Zn plating/stripping Coulombic efficiency measurement (Zn||stainless steel or graphene coated Zn||graphene coated stainless steel), a certain amount of Zn is plated on the substrate of interest, and a reverse potential is applied to strip the deposited Zn metal. p pp

, which quantifies the reversibility of the metal anode. For example, CE=100% means all the plated Zn can be stripped; while CE=80% means that 80% of plated Zn can be stripped and 20% Zn is electrochemically inactive.

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Fig. S1. Energy dispersive spectroscopy of the Zn deposit under STEM.

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Fig. S2. (A)TEM diffraction pattern and (B) simulated diffraction pattern of [0001]Zn.

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Fig. S3. Atomic arrangements of (0002)Zn and (0002)Graphene. The lattice misfit between graphene and Zn is δ≈7%, indicating that the interface

formed between graphene and Zn is semicoherent, and is hence energy favorable.

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Fig. S4. Shear-thinning behavior of graphene suspension in N-Methyl-2-pyrrolidone.

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Fig. S5. Relaxation behavior of graphene suspension after shearing in N-Methyl-2-pyrrolidone.

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Fig. S6. Supplementary SEM images of graphene coated stainless steel via the shearing method.

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Fig. S7. AFM height line scan of nonaligned (red) and aligned (yellow) graphene substrate.

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Fig. S8. AFM height mapping of graphene substrate (A) with and (B) without shearing, and (C) bare stainless-steel substrate.

The results mean that aligned graphene will not add roughness to the hard stainless-steel substrate on the back; whereas the unaligned graphene substrate without the shearing procedure will significantly increase the surface roughness.

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Fig. S9. Polarized C K-edge spectra of (A) shear aligned graphene and (B) non-aligned graphene prepared without shearing; all spectra have been pre- and post-edge normalized; (C) integrated intensity of the π* resonance versus the angle of incidence; the dash line shows the fitted cos2θ dependence of the π* intensities.

Carbon K-edge NEXAFS spectrum arises from the transition of C 1s core states to 2p-derived partially filled and unoccupied states and is thus a powerful local probe of the electronic structure above the Fermi level. The sharp peak at around 285.5 eV represents the C 1s to the C=C 2p π* transition and the broad peak at 292-294 eV is composed of three C 1s to C-C σ* transitions.(37-39) NEXAFS spectra are sensitive to the polarization of the incident X-rays. In the case of graphene, when the electric-field vector E lies along the basal plane, transitions to orbital states of σ symmetry are enhanced, whereas when E is perpendicular to the basal plane, transitions to the out-of-plane π-network increases in intensity. The strong orientation dependence of transition probabilities thus renders NEXAFS a sensitive probe of the alignment and orientation of graphene samples. Figure S9 depicts polarized C K-edge NEXAFS spectra acquired for the shear aligned graphene and non-aligned control samples prepared without shearing. The π* resonance of the shear aligned sample displayed in Figure S9A clearly showed extensive dichroism. The intensity of the π* peak decreased along with the increasing angle of incidence, while less pronounced dichroism was observed for the nonaligned sample (Figure S9B). Notably, a pronounced peak located at 289.3 eV between the π* and σ* resonances was only observed in the shear aligned graphene sample, which can be attributed to transitions originated from π* C=O states derived from edge carboxylic acid groups formed during sample preparation processes. (40)

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The extent of alignment was further quantified by a dichroic ratio (DR): DR= (equation 1)

where is the integrated intensity of the π* resonance at θ = 90° and is the extrapolate of the integrated intensity at θ = 0° obtained from the fitted cosine curves. A DR value of 0 is expected for a sample with completely random alignment while a DR value of -1 is expected for a perfectly aligned sample.(40, 41) The DR values were calculated to be -0.54 and -0.25 for the shear aligned graphene and nonaligned control sample prepared without shearing, respectively.

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Fig. S10. Supplementary low-magnification SEM images of Zn deposits on graphene substrate. J=4 mA/cm2, 2 mins.

The SEM results obtained with low magnifications show that, throughout a millimeter-scale area, Zn metal deposits uniformly on the graphene substrate, implying that the epitaxial electrodeposition of Zn is ubiquitous, and is hence scalable for battery anodes.

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Fig. S11. SEM image of layered, homoepitaxially-templated Zn deposits on graphene (12 mins, J=4 mA/cm2).

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Fig. S12. SEM images of Zn deposits on graphene at high current density (12 s, J=40 mA/cm2)

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Fig. S13. AFM characterization of Zn deposits. (A) Height and (B) phase imaging of Zn on bare stainless steel. (C) Height and (D) phase imaging of Zn on graphene-coated stainless steel. Deposition time: 40 s. Current density J = 4 mA/cm2.

We performed atomic force microscopy (AFM) analysis to further interrogate the epitaxial deposition of Zn on the textured graphene interphase. On a bare substrate without the graphene coating, both the height- (Fig. S12A) and phase-contrast image (Fig. S12B) confirm the plate-like deposition pattern of Zn, and that the plates do not orient parallel to the substrate. In contrast, on a graphene-coated substrate, the plate features are no longer detected (Fig. S12C~D), confirming that the Zn layers are completely parallel to the substrate. AFM Phase imaging provides a map of areal variations in the coating stiffness—— higher stiffness is associated with larger positive phase shift and brighter contrast, which is therefore a more sensitive detection of different phases that exclude the surface roughness. Of note is that the phase imaging of Zn deposition on graphene (Fig. S12D) confirms that there is uniform nucleation and growth of Zn as a result of the precisely controlled epitaxial process.

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Fig. S14. GIXRD characterization of patterns of zinc electrodeposited on (A) bare stainless steel and (B) graphene coated stainless steel.

X-ray diffraction measurements (Fig. S13) were performed to further investigate theZn deposition processes. X-ray diffraction measurements were collected in grazing incidence mode (GIXRD) as this technique is particularly suited for thin films. To optimize thin film intensity while minimizing substrate intensity, omega values of 1.0 and 0.8° were selected for the bare stainless steel electrodeposits and the graphene coated stainless steel electrodeposits respectively. The measurements were performed using Zn electrodeposits formed on bare- (Fig. S13A) and graphene-coated (Fig. S13B) stainless steel at a fixed current density of 4 mA/cm2. Comparing the respective XRD patterns, we observe significantly reduced Zn diffraction intensities on the textured graphene substrate after 20 s, 40 s and 2 mins deposition, which corresponds to 0.02, 0.04, 0.13 mAh/cm2, respectively. The reduced XRD intensities are consistent with expectations for the formation of uniform single/few-atomic-layer Zn deposits on the epitaxial substrate. Similar reduced diffraction peak intensities have been reported in XRD characterization of 2D materials.

More detailed analysis of the (0002)Zn to (1000)Zn XRD peak intensity ratio reveals important information about texturing and orientation anisotropy of the Zn electrodeposits. Specifically, if Zn plates are assumed to orient parallel to the substrate in the epitaxy-templated sample, the (000l)-type reflections, e.g. (0002)Zn, should produce higher XRD intensities, relative to (hki0)-type reflections, e.g. (1000)Zn. This is in fact precisely what is observed by comparing the (0002)Zn to (1000)Zn peak intensity ratio for the Zn deposits on the graphene-coated substrate. For the 12 minute electrodepositions, the (0002)Zn/(1000)Zn ratios were 2.25 and 1.02 for graphene coated steel and bare steel respectively. This indicates that the Zn is much more oriented on the graphene as opposed to the on the steel where it is more randomly oriented. These results therefore support our earlier observations from microscopy experiments. Furthermore, because the intensities measured in the XRD experiment are by definition statistical averages over a bulk material volume, the XRD analysis provides strong support of our epitaxy-templated nucleation hypothesis for Zn electrodeposition on a textured graphene coating.

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Fig. S15. XRD patterns of simulated zinc with a March-Dollase ratio (0001) of 1.10 (light blue), 12 minute electrodeposited zinc on steel (dark blue), simulated zinc with a March-Dollase ratio (0001) of 0.926 (dark green), and 12 minute electrodeposited zinc on graphene coated steel (light green).

To gain more quantitative information about the observed preferred orientation, theoretical XRD patterns were generated. The XRD patterns were simulated with various March-Dollase ratios along the (0001) axis to determine the level of preferred orientation in both of the 12 minute electrodeposits. March-Dollase ratios along a certain axis of less than 1.0 imply that the crystallites are stacking along that axis, while values greater than 1.0 imply that the crystallites are oriented away from that axis (42). This preferred orientation can be observed in the XRD patterns by changes in the relative intensities in the peaks; specifically stacking along the (0001) axis should lead to a higher (0002)/(1000) intensity ratio while orienting away from that axis should result in a lower ratio. The 12 minute electrodeposit XRD patterns along with the XRD simulations are shown in Fig. S14. The simulated XRD pattern for zinc on steel had a March-Dollase ratio of 1.10 while the simulated pattern for zinc on graphene coated steel had a ratio of 0.926. The simulated intensity ratios of the three peaks are consistent with the observed ratios suggesting the simulated level of preferred orientation is accurate. The lower March-Dollase ratio determined for the zinc on graphene coated steel material is consistent with an epitaxial zinc coating where the zinc is aligned along the mutual (0001) axis of the zinc and graphene.

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Fig. S16. Two-dimensional grazing incident X-ray diffraction patterns of Zn electrodeposits.

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Text following Fig. S16: As revealed by the 2D-GIXRD, the presence of the graphene

substrate leads to fundamental changes in the Zn crystallography.

In the 2D diffraction pattern of Zn deposition on graphene substrates, the strong, discrete

diffraction spots on the (002) ring confirms the existence of the (002)-textured, large

single-crystalline Zn deposits. Of particular note is that, the presence of the strongly single-

crystalline deposits also confirms the homo-epitaxy process, in which the initial small Zn

platelets continuously grow without breaking the original orientation relation set by the

graphene sheet ( =3~4 μm) beneath as an epitaxial substrate (e.g. the one shown in Fig.

S10).

By stark contrast, in the diffraction patterns of Zn on bare stainless steel, the diffraction

rings are continuous, dispersive, which confirms that the Zn platelets are polycrystalline

and are randomly oriented.

Technical note: the diffraction of the stainless steel is close to the (101)Zn diffraction ring,

for both bare stainless steel substrate and graphene-coated stainless steel substrate.

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Fig. S17. SEM images of Zn deposits on KetjenBlack coated stainless steel (40 s, J=4 mA/cm2)

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Fig. S18. Cyclic voltammetry of Zn plating stripping on (A) aligned graphene with shearing, (B) unaligned graphene without shearing and (C) bare stainless steel (scan rate 2 mV/s).

The results show that there is not remarkable difference in peak positions, which is consistent with the galvanostatic potential profiles where the overpotentials of the three types of electrodes do not differ much. It means that, the reaction mechanism (Zn2+ +2e- == Zn) holds regardless of the choice of substrate; instead, the alignment of the reduced Zn (i.e. metallic Zn plates) is dependent on the substrate as evidenced by comparing Fig. 1B~C and Fig. 3 in the main text.

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Fig. S19. (A) Coulombic efficiency of Zn plating stripping on bare stainless steel and (B) corresponding voltage profile. J=1.6 mA/cm2, capacity= 0.8 mAh/cm2.

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Fig. S20. (A) Optical microscopy and (B) scanning transmission electron microscopy images of dead Zn in glass fiber separator when a bare stainless-steel substrate is used. J=1.6 mA/cm2, capacity= 0.8 mAh/cm2, after 5 plating stripping cycles.

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Fig. S21. Optical microscopy images of stainless-steel substrate under (A)(B) pristine and (C)(D) after 5 plating/stripping cycle conditions, and of graphene substrate under (E)(F) pristine and (G)(H) after 5 plating/stripping cycle conditions. J=4 mA/cm2, capacity= 0.8 mAh/cm2.

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Fig. S22. Coulombic efficiency of Zn plating stripping on bare stainless-steel. J=40 mA/cm2, capacity= 0.8 mAh/cm2.

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Fig. S23. (A) Coulombic efficiency of Zn plating stripping on unaligned graphene substrate and (B) corresponding voltage profile. J=40 mA/cm2, capacity= 0.8 mAh/cm2. (C)(D) SEM images of Zn deposition morphology on unaligned graphene. J=4 mA/cm2, 2 mins.

The deficiencies of unaligned graphene in terms of regulating Zn deposition can be attributed to the following factors. (1) Being unaligned necessarily means that the substrate is rougher than a well aligned substrate (see Fig. S8; the roughness is doubled when the graphene is unaligned). Surface roughness creates local concentrated electric field in the vicinity of regions whose curvature is larger, leading to preferred electrodeposition in these areas. (2) Being unaligned necessarily means that the [0002] directions of the graphene sheets are pointing randomly. As a result, the [0002] directions of the hetero- and homo- epitaxy templated Zn deposits are, as a result, pointing randomly, leading to the failure of forming a compact, smooth deposit.

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Fig. S24. SEM images of Zn deposits on (A)(B)(C) graphene and on (D)(E)(F) bare stainless steel. J=4mA/cm2, 48 mins.

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Fig. S25. Dependence of cell-level energy density on N:P ratio in a Zn||α-MnO2 full cell. Calculation details: The voltage of the Zn||α-MnO2 is 1.3 V. The gravimetrical capacity of α-MnO2 is 150 mAh/g. The cell-level energy density is calculated on the total mass of the Zn anode and the α-MnO2 cathode.

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Fig. S26. Voltage profiles of the Zn||α-MnO2 full cells.

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Fig. S27. SEM images of Zn deposits on (A)(B)(C) graphene substrate and (D)(E)(F) bare stainless-steel substrate from 0.2 M Zn(CF3SO3)2 in dimethylether. Current density J= 4 mA/cm2, 2 mins.

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Fig. S28. Coulombic efficiency of Zn plating stripping in 0.2M Zn(CF3SO3)2 dimethylether and corresponding voltage profiles (A) without and (B) with graphene. J= 4 mA/cm2; Capacity = 0.8 mAh/cm2.

34

Fig. S29. SEM images of Al deposits on bare stainless-steel. Current density = 1 mA/cm2, 10 mins.

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Fig. S30. (A)(B) SEM images of Al deposits on Au coated stainless steel. EDS: (C) SEM image, (D) combined mapping, (E) Al mapping and (F) Au mapping. Current density = 1 mA/cm2, 10 mins.

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Fig. S31. (A)(B) SEM images of Al deposits on Au coated stainless steel. EDS: (C) SEM image, (D) combined mapping, (E) Al mapping and (F) Au mapping. Current density = 1 mA/cm2, 50 mins.

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Ref # Current Density

(mA/cm2)

Cycle number Coulombic Efficiency of Zn Plating/Stripping

(43) 0.5 1000 99.68%

(44) 0.2 50 99%

(45) 1 200 99.5%

(46) 10 5 88%~99%

(47) 1 150 ~96.5%

(48) 0.5 12 ~96%

Present work 40 10000 99.9%

Table S1. Summary of electrochemical performance of metallic Zn anode reported in literature.

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Face-Centered Cubic (FCC) Lattice Parameter a=b=c

Ni 3.54

Cu 3.61

Rh 3.80

Ir 3.83

Pd 3.88

Pt 3.91

Al 4.04

Au 4.07

Ag 4.07

Pb 4.93

Ca 5.56

Sr 6.05

Ce 6.9

Table S2. Lattice parameters of FCC metals.

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Hexagonal Closed Packed (HCP) Lattice Parameter a=b

Be 2.28

Co 2.51

Zn 2.67

Ti 2.95

Mg 3.20

Zr 3.23

Sc 3.30

Gd 3.62

Y 3.67

Table S3. Lattice parameters of HCP metals.

40

Body-Centered Cubic (BCC) Lattice Parameter a=b=c

Fe 2.86

Cr 2.90

V 3.04

Mo 3.14

W 3.16

Nb 3.30

Ta 3.30

Li 3.50

Na 4.20

Eu 4.58

Ba 5.01

K 5.20

Table S4. Lattice parameters of BCC metals.

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