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Few-Layer Graphene Island Seeding for Dendrite-Free Li Metal Electrodes Hee-Kook Kang,†,§ Sang-Gil Woo,*,† Jae-Hun Kim,*,‡ Ji-Sang Yu,† Seong-Rae Lee,§ and Young-Jun Kim†,⊥ †
Advanced Batteries Research Center, Korea Electronics Technology Institute, Seongnam, Gyeonggi 13509, Republic of Korea School of Advanced Materials Engineering, Kookmin University, Seoul 02707, Republic of Korea § Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea ⊥ SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Gyeonggi 16419, Republic of Korea ‡
S Supporting Information *
ABSTRACT: Li metal batteries such as Li−air and Li−S systems have increasingly attracted the attention of researchers because of their high energy densities, which are enhanced by the use of Li metal negative electrodes. However, poor cycle efficiency and safety concerns, which are mainly related to dendritic Li growth during cycling, need to be addressed. Here we propose a solution to the Li dendrite problems. We distributed chemically prepared nitrogen-doped few-layer graphene (N-FLG) sheets on Cu substrates to create island structures. The island-type FLG on the Cu electrode was prepared via spin-coating using slurries that included a polymer binder. When the electrode was used for Li deposition, Li ions were first inserted into the graphene layers. Then, Li metal nucleation occurred at the N-FLG sheets owing to their high electrical conductivity; meanwhile, an insulating polymer layer on the Cu prevented the growth of metallic Li there. Lastly, Li metal grew from the edges of N-FLG sheets in both the lateral and vertical direction, and Li metal deposits filled the gaps between the N-FLG islands as well as covering the remainder of the electrode surface. Thus, stable cycling with flat voltage profiles was demonstrated over 100 cycles at a current density of 2 mA cm−2. The materials and electrochemical characterization results highlight the effectiveness of this method, which paves the way for the development of robust, dendrite-free Li metal electrodes. KEYWORDS: Li metal batteries, graphene islands, dendrites, nucleation seeds, lateral growth
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INTRODUCTION Nowadays, Li-ion batteries are widely used in various applications such as portable electronics, electric vehicles, and large-scale energy storage. However, because of ever-increasing energy demands, research interest in next-generation secondary batteries has been progressively intensified, and the pressure is on for rechargeable Li metal batteries to transcend their present energy density level because their abilities to store Li ions are innately limited. Although Li metal anodes exhibit excellent high specific capacities and favorably low operating voltages with regard to their energy density, they have two serious flaws relating to Li dendrite morphology development and to their low Coulombic efficiency.1−3 To suppress the Li dendrite growth, many studies have focused on the stabilization of the interface between the Li metal and the electrolyte by modifying the solid electrolyte interphase (SEI) layer by using various artificial protective layers and additives.4−16 However, these approaches © 2016 American Chemical Society
are unsuitable as long-term solutions for repeated cycling because of the continuous consumption of Li ions and additives and the resultant collapse of the surface layers.17,18 Therefore, new, fundamental solutions to the Li dendrite issue are required that lead to stable Li nucleation at the very first stage of Li deposition. Recently, a few studies have focused on the initial nucleation of Li. Ding et al. proposed a self-healing electrostatic mechanism that changes the Li deposition morphology by eliminating the preferential deposition at protrusions on the substrate,17 whereas Yan et al. reported substrate-dependent Li metal nucleation.19 Those studies propose that Li dendrite formation can be suppressed by controlling the site and energy barriers for Li Received: August 4, 2016 Accepted: September 19, 2016 Published: September 19, 2016 26895
DOI: 10.1021/acsami.6b09757 ACS Appl. Mater. Interfaces 2016, 8, 26895−26901
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic illustration of the hydrothermal exfoliation process of expanded graphite, (b) FE-SEM image, (c) TEM image, and (d) HRTEM image of synthesized N-FLG with FFT patterns, (e) XPS C 1s spectrum, and (f) XPS N 1s spectrum of N-FLG. was added as a reducing agent. The mixture was mechanically stirred for 6 h at 80 °C, and then the dispersed mixture was maintained for 6 h at 150 °C for the hydrothermal reaction. The resulting product was washed with deionized water and ethanol and was dried at 60 °C in a vacuum oven. The island-type N-FLG sheets on the Cu substrates were prepared via a simple spin-coating method. The N-FLG sheets and polyvinylidene fluoride (PVDF, Kureha KF-9300, molecular weight: 1.2 × 106) binder were dispersed in an N-methyl-2-pyrrolidone (NMP) solution for 30 min via a low-power sonication process. This diluted slurry mixture was coated on Cu foil by using a spin-coater; the samples were subsequently dried at 60 °C in a vacuum oven. The detailed sample preparation process is given in the Supporting Information. Material Characterization. The surface morphology and crystal structure of the N-FLG sheets were examined using a field emission scanning electron microscope (FE-SEM, JEOL JSM-7000F) and a highresolution transmission electron microscopy (HR-TEM, JEOL ARM200F) with fast-Fourier transformation (FFT) analysis. For the SEM analysis of the cycled electrodes, the remaining electrolyte was dried in a vacuum oven at room temperature for 2 h. All samples were protected from air during transfer to the SEM chamber by tight sealing using a polyvinyl bag. X-ray photoelectron spectroscopy (XPS, Thermo Scientific Sigma Probe) was used to analyze the chemical state of the N-FLG. Electronic characteristics of the N-FLG were examined by Raman spectroscopy (Bruker Senterra Grating 400) and Raman mapping (with a laser wavelength of 532 nm). The surface morphology and electrical current distribution in the island-type N-FLG sheet electrodes were analyzed by noncontact atomic force microscopy (AFM) and contact scanning spreading resistance microscopy (SSRM) with a Pt-coated Au tip, respectively. Bias voltages of −0.05, 0, and 0.05 V were applied to the electrode to evaluate the current distribution and electrical conductivity. Electrochemical Measurement. To perform the electrochemical tests, beaker-type three-electrode cells were used. The cells were composed of a working electrode (pure Cu or N-FLG coated), as well as Li metal foil counter and reference electrodes. The average N-FLG loading and electrode area were 0.002 mg cm−2 and 1.13 cm2, respectively. The electrolyte was 1 M Li bis(trifluoromethane sulfone) imide (LiTFSi) in 1,2-dimethoxyethane (DME):1,3-dioxolane (DIOX) (1:1 volume) without any additives. A polypropylene membrane (Asahi Kasei Chemicals, 16 μm thickness) was used as a separator. A galvanostatic Li deposition and dissolution test was carried out with a constant current density of 2 mA cm−2 in the voltage range of −1.0 to 2.0 V vs Li+/Li at room temperature. The initial charge capacity was 2 mAh
nucleation. Other studies have investigated the artificial Li storage by functionalizing empty space; various structural substrates were used, such as one-dimensional TiO2 nanotube arrays,20 Li7B6,21 Cu fibers,22 three-dimensional hollow carbon spheres,23 conductive porous carbon media,24 and polymer nanofibers.25,26 The results of these studies demonstrate that highly efficient deposition−dissolution of Li can be achieved without dendritic growth by controlling the complex interplay of an even distribution and resultant lowered effective current densities as well as by utilizing the empty inner space for Li deposition. On the basis of these previous results, we concluded that effective nucleation seeding is clearly required for uniform Li deposition. In this study, we prepared island-type nitrogendoped few-layer graphene (N-FLG) sheets a few hundred micrometers wide with fewer than six layers to act as Li nucleation sites. The N-FLG sheets were obtained via a simple hydrothermal exfoliation process from low-cost expanded graphite, which is an environmentally friendly process suitable for mass production.27 These N-FLG sheets were uniformly distributed within a nonconducting polymer matrix, and thus, the initial Li metal nucleation and growth were limited to the islandtype graphene seed layers on the Cu substrates. Then, the Li growth can proceed laterally at the edges of the seed layers because the insulating polymer layer prevents Li metal nucleation. As a result, Li metal fills the gaps between the island-type N-FLG sheets on the Cu substrate. The few-layered graphene with some defects was obtained from a hydrothermal reaction and can initiate Li nucleation and furthermore improve the surface stability for uniform Li growth. To verify the usefulness of this scheme, we investigated the materials and electrochemical properties of N-FLG deposited Cu. As a result, stable cycling with flat voltage profiles was achieved more than 100 cycles at a current density of 2 mA cm−2.
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EXPERIMENTAL SECTION
Material Preparation. N-FLG was obtained using an ammoniamediated hydrothermal process. Expanded graphite (ExG) was used as the starting material. ExG (40 mg) was dispersed in a mixed solvent (110 g) consisting of ethanol and water (98:2 by weight); 3 mL of NH4OH 26896
DOI: 10.1021/acsami.6b09757 ACS Appl. Mater. Interfaces 2016, 8, 26895−26901
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Schematic illustration of the spin-coating process (b)−(d) FE-SEM images of N-FLG sheets on Cu substrates.
Figure 3. (a) Schematic of SSRM measurement with overall surface image, (b) AFM image, (c) AFM thickness analysis for N-FLG on Cu. Current mapping images at various applied voltages of (d) −0.05, (e) 0, and (f) 0.05 V. cm−2, and subsequent Li dissolution and deposition capacity was fixed at 1 mAh cm−2.
into few-layer graphene sheets, which is observable in Figure 1b− d. The FLG flakes were observed to have fewer than six graphene layers, and the thickness of FLG sheets was below 3 nm (Figure 1d). The corresponding FFT patterns also exhibited several sets, which consisted of different small spots with a few d-values in (0002), which implies that the interlayer distances between the monolayers varied. The interlayer distances ranged between 3.5 and 7.5 Å, which is much larger than the distances found in the pristine ExG samples. Thus, it is possible that Li metal grows through the space between the graphene layers after the first Li insertion into the interlayers of the FLG sheets. The other FFT pattern is related to the {1100} plane, which shows the typical 6fold symmetry pattern of AB Bernal stacking with a hexagonal
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RESULTS AND DISCUSSION The hydrothermal exfoliation process to produce N-FLG sheets that are a few hundred micrometers wide is briefly illustrated in Figure 1a. The microstructure of pristine ExG was examined by SEM and HR-TEM with FFT analysis (Figure S1). The pristine ExG showed the structure of unexfoliated graphite with over 30 layers stacked in the c axis direction (corresponding to (0002) graphene stacks) and a d-spacing of about 3.4 Å. The pristine graphite showed some surface contaminants and defects. During the hydrothermal exfoliation process, the graphite was separated 26897
DOI: 10.1021/acsami.6b09757 ACS Appl. Mater. Interfaces 2016, 8, 26895−26901
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Voltage profiles for the N-FLG on the Cu electrode during the initial Li deposition with a simplified schematic of the Li metal growth. FESEM images at various charge stages for (b) 0.2 mAh cm−1, (c) 0.6 mAh cm−1, (d) 1 mAh cm−1, and (e) 2 mAh cm−1.
lattice structure.28 A ring pattern with diffuse spots is observed, indicating that the FLG sheets have a polycrystalline character. This can be attributed to crystal defects induced by hydrothermal exfoliation and nitrogen doping.29,30 The material was doped with nitrogen using ammonia during the hydrothermal exfoliation process. The nitrogen doping structure of the FLG sheets was investigated by XPS. The corelevel C 1s spectrum and deconvolution results are presented in Figure 1e. The C 1s spectrum was fitted, resulting in three deconvoluted peaks at 284.7, 285.5, and 287.9 eV. The main peak at 284.7 eV can be assigned to the graphite-like sp2 C bond, indicating that most C atoms are in graphene’s conjugated honeycomb lattice.31,32 The other two small peaks correspond to different structures of the C−N bonds, namely, N-sp2 C and Nsp3 C bonds, respectively.31,33 Figure 1f shows the N 1s spectrum of NFG, which can be deconvoluted into four peaks. The two peaks at 398.4 and 399.7 eV can be attributed to pyridinic and pyrrolic N, respectively.31−36 The N bonds are related to the ptype doping. The peak at 401.2 eV corresponds to graphitic N, which means that the N atoms are in graphene layers where they have replaced C atoms. The peak at 403.7 eV can be assigned to oxidized N.37,38 The dominant p-type N-doping provides nitrogen-induced defects, which create high-energy regions and possibly attract Li+ ions during the electrochemical deposition of Li. To further characterize the N-FLG, Raman spectroscopy was performed, and the analysis results are presented in Figure S2. The exfoliated N-FLGs were deposited on Cu substrates via spin-coating, and the corresponding SEM images are shown in Figure 2. It was observed that the FLG sheets form island structures with wrinkled edges on the Cu foil where they are overlapped. Here, PVDF acts as both a binder to attach the NFLG to the Cu substrate and an insulating barrier in between the N-FLG islands. From the morphology of the corrugated graphene sheets, we know that some free space should exist in the N-FLG sheets, which can be used for the initial deposition of Li. In addition, an open structure is formed by the wrinkled of NFLG edges, which enables the electrolyte to penetrate into the free space created around the N-FLGs.
To further investigate the surface morphology and electrical conductivity of the island-type N-FLG sheets on the Cu substrates, AFM and SSRM techniques were employed. Figure 3a shows the schematic of the AFM apparatus along with an overall surface image. The thickness of the N-FLG on the Cu foil was measured at the edge area of an N-FLG sheet using a C-AFM tip; the results are presented in Figure 3b,c. The thicknesses ranged from 3.5 to 9.4 nm, which is larger than that of assynthesized N-FLG sheets (
