Sulfur-doped carbon nanotemplates for sodium metal anodes

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Sulfur-doped carbon nanotemplates for sodium metal anodes Hyeon Ji Yoon, Seung Ki Hong, Min Eui Lee, Jun Yeon Hwang, Hyoung-Joon Jin, and Young Soo Yun ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00258 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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ACS Applied Energy Materials

Sulfur-doped carbon nanotemplates for sodium metal anodes Hyeon Ji Yoon,1 Seung Ki Hong,2 Min Eui Lee,1 Junyeon Hwang,2 Hyoung-Joon Jin,1,* and Young Soo Yun3,*

1

Department of Polymer Science and Engineering, Inha University, 100, Inha-ro, Nam-gu,

Incheon 22212, Korea 2

Carbon Composite Materials Research Center, Institute of Advanced Composite Materials,

Korea Institute of Science and Technology, 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeollabuk-do 55324 Republic of Korea 3

Department of Chemical Engineering, Kangwon National University, 346, Jungang-ro,

Samcheok-si, Gangwon-do 25913, Korea

KEYWORDS: carbon nanotemplate · macroporous carbon · sulfur doping · metal anode · sodium-ion battery

ACS Paragon Plus Environment

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ABSTRACT Sodium metal is a good candidate as an anode for a large-scale energy storage device because of the abundance of sodium resources and its high theoretical capacity (~1,166 mA h g-1) in a low redox potential (-2.71 V versus the standard hydrogen electrode). In this study, we report effects of sulfur doping on highly efficient macroporous catalytic carbon nanotemplates (MC-CNTs) for a metal anode. MC-CNTs resulted in reversible and stable sodium metal deposition/stripping cycling over ~200 cycles, with average Coulombic efficiency (CEs) of ~99.7%. After heat treatment with elemental sulfur, the sulfur-doped MC-CNTs (S-MC-CNTs) showed significantly improved cycling performances over 2,400 cycles, with average CEs of ~99.8%. In addition, very small nucleation overpotentials from ~6 to ~14 mV were achieved at current densities from 0.5 to 8 mA cm-2, indicating highly efficient catalytic effects for sodium metal nucleation and high rate performances of S-MC-CNTs. These results provide insight regarding a simple but feasible strategy based on bio-abundant precursors and an easy process to design a highperformance metal anode.


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Sodium ion batteries (SIBs) have been in the spotlight as a next generation energy storage device (ESD) because of their similar chemistry to lithium ion batteries (LIBs) and because sodium resources are abundant and ubiquitous.1-3 Nevertheless, a minor difference, the intercalation of sodium ions into a graphite lattice, unlike amicable lithium ion intercalation, is unfavorable in conventional carbonate-based electrolytes and is one of the critical obstacles for sodium ion storage in anodes.4,5 As with many efforts to use alternative anode materials based on (co)intercalation, surface-driven charge storage, conversion reactions, and/or alloying compounds have been developed; the respective materials reveal several intrinsic limitations such as low columbic/energy efficiency, insufficient energy/power density and poor cycling stability, significantly retarding the advancement of SIB technologies for practical application.5-22 Meanwhile, metal anodes have rapidly advanced recently, with growing demands for a highenergy-density ESD for mobile electronic devices and/or electric vehicles.23-27 Metal anodes can deliver a high specific capacity of ~3,860 or ~1,166 mA h g-1 for LIBs or SIBs, respectively, under low redox potentials (-3.04 V and -2.71 V versus the standard hydrogen electrode, respectively), but they suffer from low Coulombic efficiencies (CEs), infinite volume changes, and unexpected metal electrodeposition (dendritic growth). These constraints cause a capacity drop, unstable cycling, and safety concerns. Therefore, most studies have focused on addressing the aforementioned issues through engineering of the electrolyte system28-34, interface stabilization of metal anodes35-38, and/or using host structures for metal deposition/striping process38-43. It is worth noting that nitrogen-doped graphene guided uniform lithium nucleation, resulting in high CEs of ~98% for ~200 cycles through mitigation of dendritic sodium metal growth.41 In addition, Cohn et al. reported that a nanocarbon nucleation layer formed on Al current collectors remarkably reduced the nucleation overpotential of sodium metal by ~12 mV,


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causing highly stable and efficient cycling behaviors exceeding 1,000 cycles.43 These results suggest that catalytic carbon materials play a key role in sodium metal deposition/stripping cycles, and their electrochemical performances could be highly dependent on their microstructure and surface properties. Therefore, it is important to design a better catalytic carbon material using a simple chemistry for the feasibility of sodium metal batteries. In this study, macroporous catalytic carbon nanotemplates (MC-CNTs) were prepared through pyrolysis of cellulose pellicles at 800 °C, which were then heated with elemental sulfur at 800 °C. The resulting sulfur-doped MC-CNTs (S-MC-CNTs) were applied as an anode for a sodium metal anode, which exhibited significantly improved cycling performances, enduring 2,400 cycles with a high average CE of ~99.8%, despite their poor crystalline structure. Herein, the effects of sulfur doping on the surface of MC-CNTs were the focus, especially for the metal deposition/dissolution cycles, which were compared with those of the reference MC-CNTs. Morphologies of MC-CNTs and S-MC-CNTs were observed by field-emission scanning electron microscopy (FE-SEM), as shown in Figure 1a,b. Both samples have a similar macroporous nanoweb structure comprising entangled carbon nanofibers with diameters and lengths of 10~30 nm and > 10 µm, respectively. When sodium metal was deposited in MC-CNTs, the large empty space between the entangled fibers accommodated the deposited sodium metals, which could be electrically connected by the networked carbon nanofibers at a nanoscale. Therefore, moss-like sodium compounds and inactive sodium metals could be mitigated by the unique nanostructure of MC-CNTs and S-MC-CNTs, resulting in enhancement of CEs. Microstructures of both samples were characterized by high resolution transmission electron microscopy (HR-TEM), as shown in Figure 1c,d. Distorted and waved graphitic structures are observed in MC-CNTs, indicating that they have a poor carbon crystallinity without long-range


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graphitic ordering (Figure 1c). These structures have numerous carbon edge sites and topological defects, a relatively unstable and active chemical structure compared with aromatic hexagonal carbon plane.5 After heat-treatment with elemental sulfur, the amorphous carbon structure was not changed, while numerous sulfur atoms (~2.57 wt.%) were homogeneously introduced on the surface of MC-CNTs, which were confirmed by scanning transmission electron microscopy (STEM) and mapping images of electron energy loss spectroscopy (EELS) (Figure 1e-h). Sulfur atoms could be doped on the reactive carbon structures as a chemical formation of C-S-C and/or C=S.44,45 Accordingly, the sulfur-capped defective sites can be chemically more stable.44,45 Further specific microstructure and surface properties of MC-CNTs and S-MC-CNTs were investigated by X-ray diffraction (XRD) patterns, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). In the Raman spectra of both samples, obvious D and G bands were observed at ~1,328 and ~1,570 cm-1 (Figure 2a). The D band is relevant to the disordered A1g breathing mode of the hexagonal carbon structure; the G band corresponds to the six-membered aromatic ring and is related to the E2g vibration mode of the sp2-hybridized C atoms.46 Hence, the presence of D and G bands means that hexagonal carbon structures are welldeveloped, and the D to G intensity ratio (ID/IG) suggests an approximate size of the perfect hexagonal carbon planes. The ID/IG ratio of MC-CNTs is 1.69, which is similar to the 1.72 of SMC-CNTs, indicating both samples have similar hexagonal carbon domains, corresponding to a few nanometers in an in-plane size. In addition, a weak 2D band at ~2,662 cm-1 is observed in the Raman spectra of both samples, which is a result of their poor stacking ordering of hexagonal carbon layers (Figure 2a). XRD patterns support the results of the Raman spectra. Broad graphite (002) peaks were found in the XRD patterns of both samples, indicating a similarly poor graphitic ordering (Figure 2b). Through the results from Raman spectra and XRD patterns, it was


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confirmed that the carbon basic-structural-units (BSUs) of MC-CNTs and S-MC-CNTs comprise a few-nanometer-sized hexagonal carbon planes with a poor stacking ordering. The surface bonding nature of both samples was similar, except for the presence of sulfur atoms on S-MCCNTs. In the XPS C 1s spectra, sp2 C=C and sp3 C-C bonding were observed at ~284.4 and ~285.0 eV, respectively, and C-O and C=O bonding were confirmed at the higher bonding energy levels (Figure 2c). The sulfur atoms on S-MC-CNTs were bonded with carbon atoms as a chemical state based on C-S bonding, which is confirmed in the XPS S 2p spectrum exhibiting S2p 1/2 and S2p 3/2 peaks at 165.2 and 163.9 eV, respectively (Figure 2d).47,48 The C/S and C/O atomic ratios calculated by XPS were 53.0 and 32.9, respectively, for S-MC-CNTs, and the C/O ratio of MC-CNTs was 36.4 (Figure S1). Porous properties of MC-CNTs and S-MC-CNTs were characterized by nitrogen adsorption and desorption isotherm curves. Both samples exhibited monolayer adsorption quantities of 50~60 cm3 g-1 at a relative pressure below 0.05, indicating that they have considerable micropore volumes. The micropores originate from the open space formed between crumpled hexagonal carbon layers. In addition, at a higher relative pressure level of > 0.8, a dramatic increase in the quantity of nitrogen adsorption was observed. The results reveal that macroporous structures are also well-developed on the samples, which coincides with the FESEM observation. Thus, MC-CNTs and S-MC-CNTs have IUPAC type-I and Type-II hybrid shapes, corresponding to microporous/macroporous dual pore structures. Specific surface areas of MC-CNTs and S-MC-CNTs were 102.4 and 106.8 m2 g-1, respectively (Figure 2e-f), which are approximately 1,000 times higher that of a typical Cu or Al foil. It is expected that the high specific surface area, including numerous catalytic sites and macroporous nanostructures, of both MC-CNTs and S-MC-CNTs could further improve sodium metal deposition/stripping cycling.


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Electrochemical performances of MC-CNTs and S-MC-CNTs as a sodium metal anode were tested under galvanostatic plating/stripping cycles using a cut-off capacity of 1 A h g-1 in an electrolyte of 1 M NaPF6 dissolved in diethylene glycol dimethyl ether (DEGDME) without additives. Areal loading density of the samples was ~1 mg cm-2 and no substrate, binder, or conducting agent was used for the tests. The first sodium metal deposition profiles of both samples at a current density of 50 μA cm-2 show similar voltage overshooting phenomena and very low nucleation overpotentials of sodium metals corresponding to ~6 mV, which is half the value of previously reported results (Figure 3a).41 This suggests that both samples provide sodium

ion

highly

efficient

catalytic

nucleation

sites,

and

S-MC-CNTs

have

a

thermodynamically comparable catalytic activity to MC-CNTs for sodium metal deposition. The cyclic voltammograms of both samples are similar to each other and agree well with their galvanostatic discharge profiles (Figure S2). On the other hand, the areal capacity of S-MCCNTs exhibiting voltage overshooting is much higher than that of MC-CNTs, because sulfur can be an additional redox host for sodium ion storage in an anodic voltage region.49-51 Thus, sulfur can gather more sodium ions on the surface of S-MC-CNTs, which could be kinetically advantageous for the sodium metal deposition/dissolution process. Na metal nucleation can be described by the following first-order linear equation:

+ = , 0 = 0

(1)

where, N0, N, and A denote the number density of the active sites, density of nucleated particles, and nucleation rate, respectively. Three different growth regimes can be distinguished by variations in the ratio of the characteristic times of diffusion 1/DN0 and charge transfer 1/kGN01/2


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(D and kG denote the diffusion coefficient and charge transfer kinetic constant, respectively): kinetic control (kG/DN01/2 > 1).52 The diffusion-controlled growth is generally accompanied by the formation of dendritic structures, while the kinetic-controlled growth leads to identical (homogeneous) metal growth on the overall surface. Therefore, a large number of active sites can significantly reduce the characteristic times of diffusion and charge transfer, leading to low nucleation overpotential and high rate capabilities. Rate performances of MC-CNTs and S-MCCNTs were tested at current densities from 0.5 to 8 mA cm-2 (Figure 3b,c). The overpotentials on both samples continuously increased with increasing current density, and S-MC-CNTs showed a smaller margin of increase at the respect current densities. Even at 8 mA cm-2, S-MC-CNTs showed an overpotential of ~14 mV, which corresponds to ~70% of that of MC-CNTs. Note that S-MC-CNTs show significantly high CEs at various current densities, as shown in Figure 3d. At all the tested current rates from 0.5 to 8 mA cm-2, CEs of > 99.0% were achieved for S-MCCNTs, which was slightly higher than those of MC-CNTs. Furthermore, a CE of 99.7% at 2 mA cm-2 for S-MC-CNTs is a practically useful result. The higher CEs of S-MC-CNTs could be due to reduction of a side reaction and more uniform growth of sodium metal nano-arrays. The large electro-active surface area and numerous catalytic nucleation sites on S-MC-CNTs may guide the homogeneous deposition of sodium metals on the overall surface area, which may mitigate large dendritic growth of sodium metals and the formation of mossy-like byproducts. The sodium metal deposition process on the surface of S-MC-CNTs is depicted as a schematic image (Figure S3). One other important point that draws our attention is the superior cycling stability of S-MC-CNTs which were tested in a current rate of 2 mA cm-2 as a cut-off capacity of 500 mA h g-1 (Figure 3e and Figure S4). MC-CNTs endured over ~200 cycles, with average CEs of ~99.7%.


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In contrast, the cycling performance is highly improved after sulfur doping. S-MC-CNTs showed stable cycling behaviors during ~2,400 cycles, with average CEs of ~99.8%, which is 8.5 times higher than that of MC-CNTs. The origin of the much-extended cycle life of S-MC-CNTs is due to the chemical/electrochemical stabilities of sulfur-doped carbon edge sites. Because the unsaturated valences of the carbon atoms at the edge sites of MC-CNTs have only two bonds instead of the three that are required for sp2-carbon atoms, the sulfur-terminated MC-CNTs become thermodynamically more stable than MC-CNTs.44 Ex situ XPS depth profiles of MCCNTs and S-MC-CNTs characterized after galvanostatic cycling support this claim (Figure 4). In the C 1s spectra, the width of the main C-C bonding is much broader in MC-CNTs than in SMC-CNTs after the 200th cycle, indicating that the aromatic hexagonal carbon structures are more well-preserved in S-MC-CNTs than MC-CNTs (Figure 4a,b). At the 50th cycle, MC-CNTs and S-MC-CNTs have similar C/Na ratios of 2.7 and 2.5, respectively (Figure 4c,d). However, by the 200th cycle, the C/Na ratio is remarkably reduced to 1.8 for MC-CNTs (Figure 4e), while S-MC-CNTs maintained the initial C/Na ratio even after 1,000 cycles (Figure 4f). In addition, the ex-situ FE-SEM images revealed that the macroporous structure of MC-CNTs is totally clogged by mossy-like products and/or byproducts after 200th cycle, while S-MC-CNTs maintained their initial nanoweb structures consisting of the carbon-based nanofibers (Figure S5). Moreover, the ex-situ FE-TEM images of MC-CNTs after 200th cycle exhibit that their morphologies become worn-out and broaden as well as their carbon microstructures are more amorphized after the repetitive cycles [Figure S6]. In contrast, S-MC-CNTs show the similar morphologies and carbon microstructures to their initial state after 200th cycle [Figure S7], which correspond with the ex-situ characterization data obtained from XPS and FE-SEM. From these results, it is demonstrated that the cycling performances of MC-CNTs declined with the


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formation of byproducts that accelerated with the degradation of unstable carbon structures. However, the reactive carbon edge sites can be preserved for a long cycling process over 2,400 cycles through sulfur doping in the carbon structures. In summary, we prepared MC-CNTs and S-MC-CNTs, and their materials properties and sodium metal storage performances were investigated. Except for the presence of sulfur atoms on S-MC-CNTs, both samples had similar surface properties and microstructures without longrange carbon ordering. On the other hand, S-MC-CNTs showed highly improved electrochemical performances compared to MC-CNTs, particularly in cycling behaviors. S-MC-CNTs exhibited remarkably stable sodium metal deposition/stripping cycling over ~2,400 cycles, 8.5 times longer than that of MC-CNTs without sulfur atoms, with high average CEs of ~99.8%. Ex-situ characterization results reveal that the prolonged cycling mainly originates from more stable carbon structures with sulfur dopants.


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EXPERIMENTAL METHOD Preparation of MC-CNTs and S-MC-CNTs. Bacterial cellulose (BC) hydrogels were incubated and purified by the previously reported method.12 The prepared BC hydrogels were immersed in tert-butanol to exchange the water solvent and were lyophilized at -50 °C and 0.0045 mbar for 72 h. As-prepared BC cryogels were pyrolized in a tubular furnace at 800 °C under Ar flow of 200 ml min-1 for 2 h. A heating rate of 2 °C min-2 was applied for the pyrolysis process. The resulting MC-CNTs were heated with elemental sulfur in a tubular furnace at 800 °C under Ar flow of 200 mL min-1 for 2 h. The weight ratio of sulfur was 500% with respect to MC-CNTs. A heating rate of 10 °C min-2 was applied for the heating process. The final products, S-MC-CNTs, were stored in a vacuum oven without further treatment.

Characterization. The morphologies and microstructures of MC-CNTs and S-MC-CNTs were examined by FE-SEM (S-4300, Hitachi, Japan) and TEM (JEM2100F, JEOL, Japan). XRD (Rigaku DMAX 2500) analysis was performed using Cu-Kα radiation (λ = 0.154 nm) generated at 40 kV and 100 mA. Raman spectra were recorded using a continuous-wave linearly polarized laser (514.5 nm, 2.41 eV, 16 mW). The laser beam was focused using a 100× objective lens, resulting in a spot with a 1-µm diameter. The acquisition time and the number of cycles for the collection of each spectrum were 10 s and 3, respectively. The chemical compositions of the samples were determined by XPS (PHI 5700 ESCA, USA) using monochromatic Al Kα radiation (hν = 1486.6 eV). Pore structures of MC-CNTs and S-MC-CNTs were characterized by nitrogen adsorption/desorption isotherms that were analyzed using a surface area and porosimetry analyzer (Tristar, Micromeritics, USA) at -196 °C.


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Electrochemical characterization. The electrochemical performances of MC-CNTs and S-MCCNTs were characterized by the galvanostatic sodium metal deposition/dissolution process using a Wonatec automatic battery cycler and CR2032-type coin cells. 1 M NaPF6 (Sigma-Aldrich, USA, purity: 98%) was dissolved in a solution of DEGDME (Sigma-Aldrich, USA, purity: 99.5%) and used as the electrolyte. A glass microfiber filter (GF/F, Whatman) was used as a separator. The working electrodes were simply prepared by punching out MC-CNTs and S-MCCNTs into disks with a diameter of 1 cm. The areal mass loading of the active electrode materials was approximately 1 mg cm-2.

Ex situ characterization. (S-)MC-CNTs were galvanostatically cycled with a sodium metal reference/counter electrode at a current rate of 2 mA cm-2 using the Wonatec automatic battery cycler and CR2032-type coin cells. After the metal deposition/dissolution process, the cells were disassembled in an Ar gas filled glove box, and the resulting samples were washed using a saltfree DEGDME solvent. Using the as-obtained (S-)MC-CNTs, ex situ FE-TEM and XPS characterizations were conducted after 50, 200, and 1,000 cycles.


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AUTHOR INFORMATION Corresponding Author *

H.-J. Jin, E-mail address: [email protected]

*

Y. S. Yun, E-mail address: [email protected]

ACKNOWLEDGMENTS This work was supported by the Industrial Strategic Technology Development Program (Project No. 10050477, Development of a separator with low thermal shrinkage and electrolyte with high ionic conductivity for Na-ion batteries) and was funded by the Ministry of Trade, Industry & Energy (MI, Korea). This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1A2B4009601) and (NRF- 2017R1C1B1004167).

ASSOCIATED CONTENT Supporting Information. Additional information, such as ex-situ FE-SEM images and XPS characterization results, as well as schematic images of the sodium metal deposition/dissolution process on S-MC-CNTs are included in the Supporting Information. This material is available free of charge at http://pubs.acs.org.


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Figure 1. FE-SEM images of (a) MC-CNTs and (b) S-MC-CNTs, and HR-TEM images of (c) MC-CNTs and (d) S-MC-CNTs. (e) S-TEM image and EELS mapping data of (f) carbon, (g) sulfur, and (h) carbon/sulfur atoms on S-MC-CNTs.


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Figure 2. (a) Raman spectra, (b) XRD patterns, and (c) XPS C 1s spectra of MC-CNTs and SMC-CNTs. (d) XPS S 2p spectrum of S-MC-CNTs. Nitrogen adsorption and desorption isotherm curves of (e) MC-CNTs and (f) S-MC-CNTs.


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Figure 3. Electrochemical performances of MC-CNTs and S-MC-CNTs as a sodium metal anode. (a) Magnified galvanostatic discharge plots for the first cycle at a low current density of 50 μA cm-2. Galvanostatic plating/stripping profiles of (b) MC-CNTs and (c) S-MC-CNT-based anodes at various current densities from 0.5 to 8 mA cm-2 (A g-1). (d) Coulombic efficiencies of MC-CNTs and S-MC-CNT-based anodes by cycle number, characterized at current densities ranging from 0.5 to 8 mA cm-2 (A g-1). (e) Cycling performances of MC-CNTs and S-MC-CNTbased anodes.


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Figure 4. Ex-situ XPS C 1s spectra of (a) MC-CNTs and (b) S-MC-CNTs after the 200th galvanostatic plating/stripping cycle. Ex-situ XPS carbon and sodium depth profiles of (c) MCCNTs after the (c) 50th and (e) 200th cycle, and S-MC-CNTs after the (d) 50th and (f) 200 cycle.


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Sulfur-doped carbon nanotemplates for sodium metal anodes

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