Fabrication of Binder-Free Pencil-Trace Electrode for Lithium-Ion

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Fabrication of Binder-Free Pencil-Trace Electrode for Lithium-Ion Battery: Simplicity and High Performance Hyean-Yeol Park, Min-Sik Kim, Tae Sung Bae, Jinliang Yuan, and Jong-Sung Yu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04641 • Publication Date (Web): 15 Apr 2016 Downloaded from http://pubs.acs.org on April 17, 2016

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Fabrication of Binder-Free Pencil-Trace Electrode for Lithium-Ion Battery: Simplicity and High Performance Hyean-Yeol Park,† Min-Sik Kim,† Tae-Sung Bae,‡ Jinliang Yuan*,§ and Jong-Sung Yu*,† †

Department of Energy Systems Engineering, DGIST, Daegu, 42988, Republic of Korea

‡

Korea Basic Science Institute, Jeonju, Jeonbuk 561-756, Republic of Korea

§

Department of Energy Sciences, Faculty of Engineering, Lund University, Box 118, 22100, Lund,

Sweden.

Abstract: A binder-free and solvent-free pencil-trace electrode with intercalated clay particles (mainly SiO2) is prepared via a simple pencil-drawing process on grinded Cu substrate with rough surface and evaluated as an anode material for lithium-ion battery. The pencil-trace electrode exhibits a high reversible capacity of 672 mA h g-1 at 100 mA g-1 after 100 cycles, which can be attributed to the unique multi-layered graphene particles with lateral size of few micrometers and the formation of LixSi alloys generated by interaction between Li+ and an active Si produced in the electrochemical reduction of nano-SiO2 in the clay particles between the multi-layered graphene particles. The multi-layered graphene obtained by this process consists of 1 up to 20 and occasionally up to fifty sheets, and thus can not only help accommodating the volume change and alleviating the structural strain during Li ion insertion and extraction, but also allow rapid access of Li ions during charge-discharge cycling.

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Drawing with a pencil on grinded Cu substrate is not only very simple, but also cost-effective and highly scalable, easily establishing graphitic circuitry through a solvent-free and binder-free approach.

1. Introduction Electric energy storage devices are widely needed with the growing demand of world energy consumption. Lithium-ion batteries (LIBs) are one of the most promising power sources for portable electronic devices and electric vehicles (EVs) due to their high-energy density, long cycle life, and environmental friendliness.1,2 With the four main parts of LIBs including anode, cathode, electrolyte, and separator, the reversible intercalation of Li ions between anode and cathode via electrolyte demonstrates the basic mechanism for power generation. Hence, the efficiency of anode electrode immensely contributes to the Li storage capacity.3 Apart from the improved cathode performance, the development of an efficient anode is also highly required for the balanced overall cell performance. Carbonaceous materials, mainly graphite, have been widely used as anode materials for LIBs due to their low cost, abundant resources and stability. Graphite is a naturally occurring metamorphic rock and is the most stable and main carbon form with layered structure. Moreover, due to such layered characters, it can be exfoliated by mild force and easily deposited onto the rough surface. Influenced by the properties combined with its shiny black color, people have been using graphite for making marks and for writing onto the surfaces.4 However, the traditional graphite anode materials could not satisfy the requirements of high power and high energy of LIBs due to its limited lithium storage capacity with relatively low theoretical capacity (372 mA h g-1), and poor rate capability. It has motivated many researchers to develop new materials with high specific capacity such as Sn-based,5,6 Sb-based,7,8 and Si-based materials.9-11 Among them, Si-based materials have been the subject of great attention due to their significantly high theoretical specific capacity (4200 mA h g-1 for Li22Si5 alloy), high natural abundance, low cost, and high safety.12,13 However, usage of Si-based anodes is generally restricted by the large volume change during Li insertion/extraction in Si, which results in the electrode pulverization 2 Environment ACS Paragon Plus

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and poor capacity retention. To overcome this problem, considerable strategies have been devoted to the preparation of less problematic structures such as nanoparticles,14 thin films,15,16 carbon-coated Si powders,17-21 and Si-based composites.22,23 Pencil is essentially composed of graphite with intercalated clay particles acting as a binder, which consist of mainly SiO2 and minor amount of other metal oxides.4,24,25 Although pencil leads exhibit electrical conductivity, they display slightly different properties compared with pure graphite owing to the presence of intercalated clay particles.4,24-26 Pencils are classified into two grades, such as H and B, depending on graphite to clay ratio. H and B grades represent the hardness and blackness of pencil leads, respectively. Hardness mainly arises from higher clay content, while blackness from higher graphitic content. Typically, HB grade pencil consists of 60 - 70 % graphite, and the remainder is mostly all the clay particles. The composition of different types of commercially available pencils has been identified by using inductively coupled plasma mass spectrometry (ICP-MS) and time-of-flight secondary ion mass spectrometry4,24 for applications in supercapacitors,27 piezoresistive sensors,28 and also as UV sensors.29-31 Zhou et al. have fabricated electrodes by using pencil-trace on a ceramic separator for lithium-air battery.32 Drawing with pencil on the paper produces black deposits, which contain exfoliated graphitic material from the pencil lead. This humble deposit includes graphitic particles as well as single or a few layer graphene. Deposited materials also mainly contain multi-layers of graphene, which can provide enough conductive path for the ion movement despite the present of resistive clay metal oxide species in the deposit. Here, we demonstrate a simple solvent-free and binder-free deposition method to fabricate anode material for LIB on grinded copper (Cu) current collector using pencil. The method is carried by tracing with a regular pencil on Cu current collector with rough surface. For efficient cell assembly, commercial batteries have used polymeric binders to certify good adhesion to the current collector in the anodes. However, adoption of a binder in the electrode can lead to thermal runaway and decreased rate capability and cycling lifetime by introducing side reactions between the binder and electrolyte.33 Furthermore, a toxic and flammable organic solvent commonly 3 Environment ACS Paragon Plus

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used for the active material slurry preparation has caused safety hazards by overcharging battery. Hence, binder-free and solvent-free electrode composition can be not only beneficial for the effective charge transfer by providing a direct conductive pathways in the electrode, but also can be directly integrated as an anode due to the formation of dense active material on the electrode. To address all these issues, we propose a simple and efficient binder-free and solvent-free method to fabricate highly conductive electrode material as a pencil trace onto grinded Cu current collector. The present approach can be also efficiently applied to the other fields such as circuit drawing, other electronic devices and electrodes for LIBs.27,29

2. Experimental section 2.1. Electrode preparation. The current collector used in this study was commercially available grinded Cu foil with thickness of 10 µm (Nippon Foil Mfg. Co., Ltd.). To increase surface roughness and surface area, the commercial Cu foil was grinded with abrasive sand papers (Norton, P1000) followed by diamond paste polishing (diameter: 1.0 µm).34,35 The commercial grinded Cu-foil was ultrasonically treated in ethanol and water before proceeding with pencil drawing. To prepare the pencil-trace electrode, the 4B grade pencil made by Tombow Pencil Ltd., Japan was purchased from market. To demonstrate the conception, the pencil-trace electrode was prepared by simple drawing with a 4B grade pencil on grinded Cu current collector of 14 mm diameter with rough surface (Figure 1). The final product was directly applied as a binder-free and solvent-free anode electrode. Such pencil-trace generates the coverage of graphitic layer on the Cu current collector. The average loading density and the thickness of coated active material were calculated as 0.9 mg cm-2 and 10 µm in the pencil-trace electrode, respectively. Commercial graphite and SiO2 powder were also used for electrode preparation and investigation of XRD patterns (purchased from Sigma Aldrich). For comparison, commercial graphite electrode with binder was prepared using a mixture of graphite, acetylene black, and PVDF at a weight ratio of 8:1:1 in N-methyl-2 pyrrolidone solvent to form slurry. The homogeneous slurry was spread with 10 µm thickness onto the identical grinded Cu foil substrate and dried at 80 oC for 12 h. The 4 Environment ACS Paragon Plus

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pencil-trace and commercial graphite electrodes were roll-pressed to improve the adhesion to the current collector. In the commercial graphite electrode with binder, the active material loading was calculated as 1.2 mg cm-2. 2.2. Surface characterization. Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4700 microscope operated at 30 kV. High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) images were obtained using a JEOL FE-2010 microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were determined with an AXIS-NOVA (Kratos) X-ray photoelectron spectrometer using monochromatic Al Kα (150 W) source under base pressure of 2.6 x 10-9 Torr. The XPS analysis was carried out to examine the oxidation state and chemical configuration of elements. The purity of the samples (i.e., standard graphite, SiO2, and pencil) was analyzed with X-ray diffraction (XRD) analysis using a Rigaku Smartlab X-ray diffractometer with CuKα radiation (l=1.5406 Å) operating at 40 kV and 30 mA. Raman spectrum was obtained using Nanofinder 3.0 with a He-Ne laser (1.017 mW, 631.81 nm) to recognize the molecular structure of the carbon material. Thermal gravimetric analysis (TGA) was performed with a Bruker TG-DTA 3000 SA analyzer at a heating rate of 10 °C min-1 in air (60 mL min-1) from room temperature to 1000 °C to study the amount of clay in pencil lead. 2.3. Cell construction and electrochemical performance measurement. To test the electrochemical performance of the binder-free pencil-trace electrodes , the coin cells were assembled in an argon-filled glove box (H2O and O2 less than 0.1 ppm) using CR 2032-coin cell (Hohsen Corp., Japan) with lithium metal (purity, 99.9 % and 150 mm thick) as a counter electrode and reference electrode and Celgard 2400 as a separator. The electrolyte used was 1.0 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1 by volume, Soulbrain Pte. Ltd.). The electrochemical performances of pencil-trace electrode without binder and commercial graphite electrode with binder were examined by galvanostatic charge-discharge cycling using the coin cell and characterized in a BaSyTec multichannel battery test system. The instrument was programmed to read in each 10 s step. The cells were galvanostatically cycled at a rate of 100 mA g-1 (0.1 C rate) in the 5 Environment ACS Paragon Plus

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voltage range of 0.01 - 3.0 V (vs. Li/Li+) at room temperature. Cyclic voltammetry (CV) experiments were performed with a potentiostat (Biologic VMP3) in the potential range of 0.01 - 3.0 V at the scanning rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 100 kHz - 10 mHz with ac amplitude of 0.01 V.

3. Results and discussion Figure 1 shows the schematic representation for the fabrication of pencil-trace electrode as an anode material in a half cell configuration along with metallic lithium. The binder-free electrode employs pencil drawing as an anode electrode on the rough surface of a grinded Cu current collector. The schematic layout of the LIB has the following electrochemical cell structure, ‘‘pencil-trace electrode | organic electrolyte | separator | Li’’. As shown in Figure 1, the prepared pencil-trace is used as an anode electrode, while the metallic Li as a cathode. 1.0 M LiPF6 in EC/DMC is used as organic electrolyte. The Li ions migrate between the two electrodes through the electrolyte. Figure 1 exhibits a schematic of the operating principle of LIB. During the lithiation process, Li ions are taken out from the cathode, travel through the electrolyte and separator and intercalate into the anode with pencil-trace. At the same time, cathode releases electrons, which travel through the external circuit and are accepted by anode compounds. The reverse process take place during the delithiation process. The morphologies and microstructures of the pencil-trace were analyzed using scanning electron microscopy (SEM) and high resolution-transmission electron microscopy (HR-TEM). Figure 2a shows that the pencil leads are composed of irregular grains with graphitic flakes. Figures S1a and S1b of Supporting Information (SI) show that the pencil-trace is not only composed of graphitic layers stacked to form a compact structure, but also appears to be corrugated with rough surface of piles of graphitic layers. In Figure S2 of SI, the cross-section SEM image of pencil-trace electrode shows a stacked graphite flake morphology and a thickness of active material reaches 10 µm, which is produced by the pencil drawing process. While drawing, the rough surface structure of grinded Cu current collector 6 Environment ACS Paragon Plus

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allows exfoliation and adhesion of the graphitic materials from pencil. TEM image in Figure S3a of SI shows the constitution of the pencil lead, where the spherical clay particles are distributed among the graphitic layers. In addition, the particle size distribution histogram of clay particles is shown in Figure S4. The average particle size was ca. 33.4 nm. The thermal gravimetric analysis (TGA) curve of the pencil lead (Figure S3b of SI) exhibits two steps in mass loss, one at 220-250 °C corresponding to the removal of oxygen-containing groups and another at 658 °C probably due to the carbon oxidation.36 The TGA results indicate that the pencil lead composite contains 22 wt % of clay (mainly SiO2 and Al2O3) and 78 wt % of conducting graphite matrix.25 An exothermic peak centered at 660 °C in static air was observed in differential thermal analysis (DTA) plot, which corresponds to combustion reaction of graphite materials in static air.4 Energy dispersive X-ray (EDX) analysis in Figure S3c and XPS spectrum in Figure S3d show the distribution of the constituent elements, C, O, Si, and Al present in the graphitic matrix of the pencil-trace.36 Table S1 of SI summarizes the elemental composition in atomic % determined by XPS and EDX. Interestingly, Figure 2 shows the typical two-dimensional (2D) structure of the carbon nano-sheets. HR-TEM image with the corresponding selected area electron diffraction (SAED) pattern is presented on Figure 2b. In the HR-TEM image of the edge, the carbon nano-sheet is seen to include several atomic layers, and this is a typical character of multi-layered graphene. The SAED pattern contains ring structures, mainly due to multi-layered graphene.37 The results of Figures 2b and c clearly illustrate that the layered structure of graphitic pencil lead can be deposited on the grinded Cu current collector by drawing process to form a 2D nanostructure.32,38 Figure 3a exhibits Raman spectra for the as-made pencil-trace and natural graphite. Raman spectra are characterized by two prominent peaks at 1330 and 1579 cm-1 in Figure 3b, corresponding to D and G bands, respectively. The D and G bands are associated with the edges or disordered layers and the zone center E2g mode corresponding to the ordered sp2 bonded carbon, respectively.39 The intensity ratio (ID/IG) of the D and G band offers a measure of disorder and crystallite size of the graphitic layers.39 Figure 3b shows the higher intensity of D band for the pencil-trace. This can be ascribe to the following 7 Environment ACS Paragon Plus

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reasons: 1) higher edge planes due to decrease in number of stacked graphene layers, and 2) introduction of shearing stress, which can misalign the AB stacking in graphene, which can cause highly disordered graphene.40 On the contrary, natural graphite shows much higher G band intensity compared with that of D band, signifying higher amount of orderly-stacked graphene layer in the graphite. The ID/IG of the pencil-trace is much higher than that of the natural graphite powder, illustrating that the graphitic pencil lead has been peeled off to generate the multi-layered graphene particles during the drawing process. Moreover, the pencil-trace exhibits a down-shifted D band when compared with D band of natural graphite. The difference between the D bands of natural graphite and pencil-trace is around 7 cm-1. This clearly proves that the D band shown in Figure 3b has the property of multi-layered graphene arising from pencil-trace. The edge of graphite illustrates an up-shifted D band as compared to the edge of graphene.41 In addition, Figure 3c exhibits 4-peak fitting of 2D peak of pencil-trace with multi-layered graphene based on natural graphite.

The resulting 4-peaks include phonons with

momenta, such as q1B, q1A, q2A, and q2B, due to the splitting of the phonon band structure of multilayered graphene.42 In general, the intensity ratio between the G and 2D bands (IG/I2D) tend to increase with increasing number of layers, while one prominent peak at 2700 cm-1 shows the characteristic 2D band of the defect-free graphene.43 The pencil-trace has a smaller IG/I2D value compared with that for natural graphite, indicating that the pencil-trace process has been employed for the formation of multilayer graphene. All the results in Figure 2 and Figure 3 demonstrate the presence of some multi-layered graphene nano-sheets in the prepared pencil-trace.31,32 Figure 4 shows the crystal structure of the pencil. The XRD pattern of pencil exhibits the hexagonal crystal structure with space group p63mmc of graphite structure. The XRD intensity of Figure 4 is illustrated in log-scale to visualize the XRD pattern of clay phase in graphite pencil.25 The major phase of clay in pencil matched well to standard pattern of SiO2 (SiO2 with reference code: 00-002-0471 has shown in Figure 3), and the graphite phase of pencil exhibited a sharp peak centered at 26.6 corresponding to the 002 diffraction peak of commercial graphite.44


o

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The electrochemical performance of pencil-trace electrode was carried out by cyclic voltammetry (CV). Figure 5a shows the CV plots of pencil-trace electrodes measured between 3.0 and 0.01 V at the scanning rate of 0.1 mV s-1 for 1st, 2nd, 5th, and 10th cycles. The electrochemical reduction of SiO2 in clay component of pencil is definite at ca. 0.18 V during the first cathodic potential scan, but it weakens in the following cycles. Electrolyte decomposition and formation of the solid electrolyte interphase (SEI) passivation layer at ca. 0.65 V are important reasons for the irreversible capacity due to the reaction of lithium with the electrolyte. The SEI layer becomes stable during the subsequent Li insertion and extraction, which is demonstrated by considerably reduced current response and disappearance of the peak from the 2nd and the subsequent cycles.45 When the composite is subject to initial reduction cycle, the amorphous nano-SiO2 in the pencil-trace is reduced to Si at ca. 0.18 V, and amorphous Li2O or crystalline Li4SiO4 can be also generated.46,47 Moreover, the reduced active Si reacts with Li+ to form LixSi alloys between the multi-layered graphene sheets produced by the pencil drawing process.47,48 During anodic potential scan, two anodic peaks at 0.14 and 0.18 V progress from the 1st cycle, and become gradually more distinct after following cycles, which correspond to the extraction of lithium ions from the pencil-trace electrode with clay (mainly SiO2).49 The cathodic peak at 0.18 V also represents the alloying of Li ions with Si. The observed redox peaks may be related to the alloying/dealloying process of LixSi alloys with different compositions.50 The irreversible formation of Li4SiO4 and Li2O consumes much Li and thus, is an important reason for the irreversible capacity in the initial lithiation process.51-53 These reactions can be summarized as follows:

SiO2 + 4Li+ +4e- → 2Li2O + Si

(1)

2SiO2 + 4Li+ +4e- → Li4SiO4 + Si

(2)

Si + xLi+ + xe- ↔ LixSi

(3)


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Both the reactions (1) and (2) correspond to spontaneous irreversible lithiation process. The formation of SEI layer and SiO2 reduction accounts for irreversible capacity, while the reaction (3) is responsible for the reversible capacity.46,51 These two mechanisms demonstrate that two types of reactions take place between SiO2 and Li ions. From these reactions, the theoretical capacity of SiO2 can be calculated according to the number of electrons transferred. It can be observed that with increasing electrochemical reduction of SiO2 to Si, the theoretical capacity increases.47,54 Specifically, the reaction (1) generates Li2O and Si, showing the largest reversible capacity of 1961 mA h g-1, whereas the reaction (2) generating Li4SiO4 and Si shows reversible capacity of 980 mA h g-1.47,54 Thus, pencil with intercalated clay (mainly SiO2) indicates a promising Li storage material with high capacity and cycling stability.55-57 The charge and discharge curves of the pencil-trace electrode are shown in Figure 5b. A plateau at 0.65 V can be observed during 1st lithiation, corresponding to the peak in the CV curve in Figure 5a. The 1st lithiation reaction is a spontaneous process in Li metal half-cell, so the first process, in term of energy flow direction, is named as discharge process and is irreversible and substantially different from the subsequent reversible reaction. The discharge and charge capacities of the 1st cycle are 1263 and 834 mA h g-1, respectively. Although the initial coulombic efficiency of pencil-trace electrode is low with 66 % owing to the formation of SEI film and irreversible electrochemical reactions between Li+ and SiO2 in the intercalated clay particles, the coulombic efficiency increases sharply to 87 % during the 2nd cycle, and reaches over 97 % after 30th cycle, indicating a good cycling performance for the penciltrace electrode. In the following charge-discharge processes, the voltage profiles show a similar shape, indicating that the electrochemical performance of pencil-trace electrode become stable. Figure 6a shows comparison of the galvanostatic cycling behavior at 100 mA g-1 for pencil-trace electrode with commercially used graphite electrode. Pencil-trace electrode was found to be very stable up to the 100th cycle with a slight decrease in the reversible Li storage capacity. Compared with commercial graphite, pencil-trace electrode illustrates not only a much higher initial reversible capacity, but also much higher Li storage capacity at the 100th cycle (i.e., the pencil-trace electrode and graphite 10 Environment ACS Paragon Plus

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electrode retain a specific capacity of ca. 672 and 245 mA h g-1 after the 100th cycle). The initial irreversible capacity can be attributed to the formation of SEI film on the surface and irreversible electrochemical reactions between Li+ and SiO2. In addition, long-term cycle performance of penciltrace electrode was investigated at 100 mA g-1 as shown in Figure S5. A highly reversible capacity of ca. 571 mA h g-1 is gained at 200th cycle. It is an obvious deterioration in the cycle performance after 200th cycle in the pencil-trace electrode, which can be also ascribed to various aspects such as lithium deposition, electrolyte decomposition, and further passive film formation on the electrode and current collectors. The presence of stacked graphitic layers with intercalated clay can help accommodating the volume change and maintaining a stable structure during Li ion insertion and extraction.51,58,59 Feldspar represent a group of rock-forming tectosilicate minerals in nature, which are composed of the aluminum silicates of sodium, calcium, and potassium. The possibility of existing as feldspar phase was further examined by XRD. However, XRD pattern of the pencil lead used in the current study does not show any evidence of feldspar phase as shown in Figure 4.60,61 A good rate capability is one of the required electrochemical characteristics of Li ion batteries to power the high energy applications such as hybrid and heavy-duty electric vehicles. To evaluate the rate capability of the pencil-trace electrode, stepwise charged-discharge cycles were performed on a cell from 100 mA g-1 to 200, 300, 500 and 1000 mA g-1 (0.1 C to 1 C) with 10 cycles at each current rate (Figure 6b). A capacity drop is usually observed after switching from a lower current rate to a higher current rate, which can be due to the concentration polarization of Li ions in electrode resulting from a diffusion limited process. Pencil-trace electrode demonstrates an excellent rate capability. After 10th cycles, the capacity reaches 701 mA g-1 at a rate of 100 mA g-1. The capacity reduces with increase in the current rate, but it remains as high as 498 mA h g-1 at a rate of 500 mA g-1. Even at a high current of 1000 mA g-1, it still delivers a high capacity of 435 mA h g-1. The battery based on the pencil-trace anode demonstrates an excellent rate capability, still delivering a high fraction of its initial capacity even at a rate as high as 1000 mA g-1. Furthermore, the capacity recovers almost completely (697 mA h g-1)


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when the discharge rate returns to 100 mA g-1 after 50 cycles, further demonstrating the outstanding capacity of this electrode to keep its integrity not only for a long number of cycles, but also at high rates. Although the graphitic materials from pencil can make a good adhesion onto the grinded Cu current collector with rough surface during the drawing, it is difficult to specifically control the loading amount of active material. However, the average loading amount of pencil lead in the binder-free pencil-trace electrode was calculated as 0.9 mg cm-1. On the other hand, the average loading amount of commercial graphite electrode with binder was around 1.2 mg cm-1 since a slurry included a mixture of graphite, acetylene black, and PVDF. The volumetric capacity of pencil-trace electrode was determined to be 1136 mA h cm-3, which is 2.7 times higher than that (416 mA h cm-3) of the commercial graphite electrode. The areal capacity is found to be 1.13 mA h cm-2 for pencil-trace electrode and 0.42 mA h cm-2 for commercial graphite, with a total electrode thickness of 195 µm (i.e., electrode + separator + current collector + Li metal). Moreover, the volumetric energy density of the cells was calculated based on the average cell voltage of 3 V. The volumetric and gravimetric energy densities are calculated to be 174 Wh l-1 and 193 Wh kg-1, respectively for the pencil-trace cell, which are 2.7 and 3.6 times as large as those of commercial graphite cell (64 Wh l-1 and 53 Wh kg-1). We have also recorded the SEM image to investigate changes in the morphology of the pencil electrode before and after cycling in Figure S1c of SI. It can be seen in Figure S1a and S1b that the fresh pencil-trace electrode shows well stacked graphitic layers to form a compact structure, which provide a direct conducting path on the electrode. Compared with fresh electrode, the binder-free electrode is apparently covered with a SEI film around the graphitic layer particles formed during the chargedischarge cycling of cell. We think that the film can maybe prevent the active graphitic layers from making direct contact with the electrolyte, which will provide improved cycling lifetime and rate capability.16,62 In addition, in order to confirm the advantage of prepared electrode, electrochemical impedance spectroscopy (EIS) measurements were conducted on the fresh cells made from commercial graphite with binder and binder-free pencil-trace, and the representative Nyquist impedance plots are shown in 12 Environment ACS Paragon Plus

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Figure 7 and Table S2 of SI. Both of the fresh cells display a well-defined semicircle in the high to medium frequency range and a straight line in the low-frequency range. Equivalent circuits as shown in inset of Figure 7 are proposed for analysis of the impedance spectra. As shown in Figure 7 and Table S2 of SI, some significant kinetic parameters, such as electrolyte resistance (RS), charge transfer resistance (RCT), and Warburg resistance (ZW), are determined by fitting the impedance spectra to the proposed equivalent circuit using the ZView software. Compared with both of the fresh cells, the commercial graphite cell with binder and the binder-free pencil-trace cell show almost similar electrolyte resistance (RS = 6.82 and 6.58 Ω, respectively). On the other hand, RCT and ZW results in the binder-free penciltrace cell show 68.3 and 37.2 Ω, which are lower than 85.6 and 79.1 Ω in the corresponding commercial graphite cell with binder. It obviously represents that the binder-free pencil-trace configuration with high conductivity has less resistance due to the direct and efficient electrical pathways between active material and current collector.63 The excellent conductivity of binder-free pencil-trace electrode can provide long cycle life and excellent rate capability at high rate current density. In summary, an approach to fabricate a binder-free pencil-trace anode electrode has been successfully developed. The development of binder-free cell configuration by drawing of pencil is suggested to improve cycling life time and conductivity because direct pencil-trace on grinded Cu current collector provide an efficient electrical pathways for lithium ion diffusion and transport, while shortening the diffusion lengths. In addition, interestingly, all the above results suggest that pencil with intercalated clay can enhance the Li storage capacity to great extent compared with commercial graphite.

4. Conclusions In summary, we have designed a novel and interesting anode electrode by using pencil-trace on grinded Cu foil, which is a simple, rapid, and cost-effective. This methodology paves the way for use of drawing on current collector with a rough surface to exfoliate other active materials as well as pencil lead. 13 Environment ACS Paragon Plus

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Remarkably, as an electrode material of LIBs, the pencil lead with intercalated clay helps to exhibit enhanced rate capability and better cycling performance. The pencil lead carbon with intercalated clay as anode material provides a strong structural buffer, high electrical conductivity and the formation of low SEI film, which are highly beneficial for their practical application.

■ Associated content

Supporting information. Additional SEM and TEM images, XPS and EDS results, and TGA curve. This material is available free of charge via the Internet at http://pubs.acs.org.

■ Author information

Corresponding author *,† J. -S. Yu. E-mail: [email protected]. Tel.: +82-53-785-6443. Fax: +82-53-785-6409. *,§ J. Yuan. E-mail: [email protected]. Tel.: +46-46-222-4813.

Present addresses †

Department of Energy Systems Engineering, DGIST, Daegu, 42988, Republic of Korea

‡

Korea Basic Science Institute, Jeonju, Jeonbuk 561-756, Republic of Korea

§

Department of Energy Sciences, Faculty of Engineering, Lund University, Box 118, 22100, Lund,

Sweden. Notes The authors declare no competing financial interest.


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■ Acknowledgment

This work was generously supported by NRF grant (NRF 2014K2A3A1000240) and Global Frontier R&D program on Center for Multiscale Energy System (NRF 2011-0031571) funded by the Korea government. Authors also would like to thank the Korean Basic Science Institute at Jeonju (SEM and HR-TEM analysis), Daejeon (TEM analysis), and Pusan (XPS analysis). This was also supported by the Swedish Foundation for International Cooperation in Research and Higher Education (STINT).

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Figure 1. A schematic representation of anode electrode prepared from a pencil-trace on grinded Cu current collector, and a schematic illustration of the prepared pencil-trace electrode in lithium half-cell configuration. The circle of inset shows the pencil lead deposited on Cu current collector.


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Figure 2. (a) SEM image of exfoliated pencil-trace. (b) HR-TEM image of exfoliated pencil-trace with monolayer graphene. The inset shows the corresponding SAED pattern. (c) HR-TEM image of the pencil-trace with different graphene sheets numbers.


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Figure 3. (a-c) Raman spectra of the pencil-trace and commercial graphite. (a) Wide spectra of the pencil-trace and commercial graphite. (b) Comparison of D and G peaks in the pencil-trace and commercial graphite and (c) 4-peak fitting of 2D peak in pencil-trace with multi-layered graphene based on commercial graphite.


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Figure 4. XRD patterns of pencil lead, standard SiO2 and graphite.


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Figure 5. (a) CV plots for pencil-trace electrode from 3.0 to 0.01 V versus Li/Li+ at 0.1 mV s-1 for the 1st, 2nd, 5th, and 10th cycles. The anodic curve of the 1st anodic profile between 0.05 and 0.4 V is shown in the inset. (b) Galvanostatic charge/discharge curves at 100 mA g-1 for the fabricated pencil-trace electrode as anode material.


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Figure 6. (a) Comparison of cycling performance in terms of discharge (lithiation) capacity and coulombic efficiency of the battery with commercial graphite and pencil-trace as anode electrode at a specific current of 100 mA g-1. (b) Comparison of rate performances in terms of discharge capacity of battery with commercial graphite and pencil-trace as anode electrode at different current densities from 100 to 1000 mA g-1 and then back to 100 mA g-1.


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Figure 7. Nyquist plots from EIS analysis of fresh cells made from commercial graphite with binder and binder-free pencil-trace. Inset shows the equivalent electronic circuit used for fitting of Nyquist plot.


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[Graphical abstract]

Fabrication of Binder-Free Pencil-Trace Electrode for Lithium-Ion Battery: Simplicity and High Performance

Pencil-trace electrode is successfully made by drawing a pencil over a grinded Cu substrate as a solventfree and binder-free electrode, which helps not only to make a simple and cost-effective electrode, but also to improve its performance in LIB in terms of specific capacity and rate capability.


Fabrication of Binder-Free Pencil-Trace Electrode for Lithium-Ion

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