Tungsten (W) was coated onto a silicon (Si) anode at the nanoscale via the physical vaporization deposition method (PVD) to enhance its electrochemical properties. The characteristics of the electrode were identified by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis, and electron probe X-ray microanalysis. With the electrochemical property analysis, the first charge capacities of − 1 − 1 the W-coated and uncoated electrode cells were 2558 mAh g and 1912 mAh g , respectively. By the 50th cycle, the capacity ratios were 61.1 and 25.5%, respectively. Morphology changes in the W-coated Si anode during cycling were observed using SEM and TEM, and electrochemical characteristics were examined through impedance analysis. Owing to its conductivity and mechanical properties from the atomic W layer coating through PVD, the electrode improved its cyclability and preserved its structure from volumetric demolition. Keywords: Lithium-ion battery, Silicon anode, Electrochemical reaction, Physical vaporization deposition Background pursued approaches for coating the subject with vari- Silicon (Si) is one of the most attractive energy ous materials [13–16]. Conductive materials such as source elements that can be used as an anode carbon, metal alloys, and even conductive polymers − 1 because of its high-specific capacity (4200 mAh g ), have been employed to restrain the expansion effect, which is 10 times higher than that of graphite . and they have provided not only a buffering effect However, Si experiences problematic volumetric but also charge transportation enhancement. However, expansion during charging and discharging processes, these research methods have limitations regarding and the expansion causes a 300% change in lattice their use in commercial applications because of their volume [2–5]. This results in cracking and disintegra- detailed fabrication procedures. tion of the electrode, leading to active material loss, a Physical vaporization deposition (PVD) produces a decrease in electrical contact, and eventual degrad- uniform coating on a substrate at the nanometer to ation of electrical properties. Additionally, the low visible scale through the process of atomic deposition electrical conductivity of Si is a barrier to its use as [17–20]. This versatile technique can be applied in an electrode material. various fields to enable the deposition of every Therefore, methods for improving the electrochem- inorganic material type and even some organic ical properties of Si electrodes are of high interest, materials. Additionally, because this method induces and extensive research has been conducted to solve less resistance than chemical deposition with a tight the problems associated with the Si electrode, such as layer formed by heterogeneous nucleation and using electrodes with a carbon (C) composite growth , mechanical properties such as wear composition, multidimensional structures, and metal- resistance and hardness are improved greatly. alloyed forms [6–12]. In particular, for active material In this study, a Si electrode was coated with methods used in shockproofing, many studies have tungsten (W) using the PVD method to provide a buffer layer and increase its conductivity. Among all metals in pure form, W has the highest tensile * Correspondence: firstname.lastname@example.org Department of Materials Science and Engineering, Korea University, strength and superior hardness [22, 23]. In addition, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Son et al. Nanoscale Research Letters (2018) 13:58 Page 2 of 7 Fig. 1 Schematic of physical vapor deposition for W coating Fig. 2 Scanning electron microscopy images and energy dispersive X-ray profile of pristine uncoated a and c and coated b and d Si electrode surface Son et al. Nanoscale Research Letters (2018) 13:58 Page 3 of 7 Fig. 3 a Scanning electron microscopy image and electron probe X-ray microanalysis measurement results of b C, c Si, and d W Hornik et al. studied theeffectofW PVDby Experimental magnetron sputtering on ceramic substrates and Fabrication of Electrodes showed that the W coating can function suitably for Si electrodes were fabricated using a casting method substrates with low hardness or wear resistance. By with 40 wt% Si nanopowder (≤100nm),40wt% applying a W nanolayer to the electrode surface, the Denka Black as a conductive material, and carboxy- electrochemical properties and morphologies of the methyl cellulose as a binder. These substances were Si electrode were examined using various analytical dissolved in deionized water to form a slurry. The techniques. This W nanolayer application showed slurry was then coated onto a piece of copper foil improved electrochemical properties and sustained (50 μm) and dried at 70 °C for 1 h. The W coating structural safety. of the Si electrode was conducted using the PVD Fig. 4 a Transmission electron microscopy image and b depth profiling of W-coated Si electrode Son et al. Nanoscale Research Letters (2018) 13:58 Page 4 of 7 Fig. 5 a EIS analysis for the uncoated and the W-coated Si electrode before cycles and b the equivalent plot − 2 5 method (Fig. 1)atDongwoo Surface Tech Co., Ltd. performed at frequencies of 10 to 10 Hz with an AC Ar gas was used as the plasma generator at 100 °C, amplitude of 5 mV (SOLATRON SI1280B) to compare and W deposition was conducted for 5 min. The the coating effect. deposited W electrode surface was examined by scanning electron microscopy (SEM), transmission Results and Discussion electron microscopy (TEM), electron probe X-ray Figure 2 shows SEM images of pristine uncoated (a) microanalysis (EPMA), and energy dispersive X-ray and W-coated (b) Si electrodes. Because the spectroscopy (EDX). electrode consisted of Si nanopowder with a size less than 100 nm, the powder retained its original size. However, owing to the physical deposition of W onto Test Cell Procedure the coated electrode, each particle seemed to be The test cell was assembled with a CR2032-type coin covered with a W layer, and the overall size of the cell in a dry room. The Si anode electrodes were particles increased to approximately 100 to 120 nm. punched out to a size of 14Φ, and the counter EDX analysis of the elements in the red box of the electrodes were punched from lithium foil to a size SEM image (Fig. 2b) revealed the presence of W of 16Φ. The measured weight of W nanolayer (Fig. 2d). Additionally, EPMA confirmed that the corresponding to a 14Φ-sized electrode is approxi- deposited W was uniformly distributed (Fig. 3). mately 0.0001 g. The electrolyte used was 1 M LiPF TEM analysis with depth profiling was conducted to with a mixture comprising equal volumes of ethylene examine the thickness of the W layer. Figure 4 confirms carbonate, dimethyl carbonate, and ethylene methyl that the W layer (white) deposited onto the Si nanoparti- carbonate (Soulbrain, Republic of Korea). All cells cles (black) had a depth of approximately 40 nm. The W were fabricated in a dry room. The assembled cell layer also covered the gaps between Si powder and other was aged for 24 h at 40 °C. electrode materials. From the above tests, it is apparent Galvanostatic electrochemical tests were performed that the W layer coated via the PVD method was well using a WBCS 3000 instrument (WonATech Inc., formed at the nanometer scale . Republic of Korea). Charging and discharging processes An electrochemical impedance spectroscopy (EIS) were performed between 0 and 1.5 V with specific test was performed for further analysis. Figure 5 current rates for each process. After the cycles, surface shows the impedance results for (a) the uncoated Si observations of W-coated and uncoated Si electrodes and W-coated Si electrodes and (b) the equivalent were conducted. Additionally, impedance tests were circuit. The figure shows the equivalent circuit based on the Randles circuit structure, and Table 1 lists the Table 1 Results of impedance analysis fitting data results of impedance fitting. In the equivalent circuit, Uncoated electrode W-coated electrode R indicates the sum of the ohmic resistances of the electrode and electrolyte, and R and C represent R 20.53 19.37 s ct dl the charge transfer resistance and double-layer R 12.36 30.81 sei capacitance, respectively. The constant phase element R 123.3 24.56 ct (CPE) is connected to R in series [25, 26]. R and ct sei −4 −6 C 7.609 × 10 7.4236 × 10 sei C , which are related with the resistance and sei Son et al. Nanoscale Research Letters (2018) 13:58 Page 5 of 7 Fig. 6 Charge/discharge capacity profiles for uncoated and W-coated Si electrodes at a rate of 0.1 C and cutoff voltage range from 0 to 1.5 V over 50 cycles capacitance of the electrode surface , are in and stimulates faster charge transfer. The discharge parallel. capacities of the W-coated Si electrode at the 10th, By comparing the initial states, as shown in Fig. 5 20th, and 50th cycles were 1843, 1676, and − 1 and Table 1,the values of R and R decreased 1137 mAh g , respectively, and the retention ratios s ct owing to the W coating, whereas R increased of the same cycles were 99.1, 90.1, and 61.1%, sei because of the increase in surface resistance. This respectively. Those values for the uncoated Si − 1 result indicates that, because of the uniform coating electrode were 1132, 790, and 452 mAh g and of the W layer, the electrical conductivity was en- 63.9, 44.6, and 25.5%, respectively. The coated cell hanced, which may contribute to increased capacity clearly showed improved capabilities. This result is and stable cyclability. However, the increases in R attributable to the W coating, which forms a buffer sei and ion diffusion impedance are also observed, imply- layer and enhances electrical conductivity. The ing that the W layer can act as an ion permeability uncoated Si electrode was exposed to structural inhibiter. destruction, while the W-coated Si electrode was The specific capacities of the bare and W-coated protected by the W nanolayer, preventing the forma- cells at a rate of 0.1 C over 50 cycles are plotted in tion of cracks overall and leading to the conserva- Fig. 6. For the first cycle, the charge capacities of tion of the electrode surface. However, the W the W-coated and uncoated Si electrode cells were coating induced irreversible capacity loss during − 1 2588 and 1912 mAh g , respectively. This may be every cycle. Because Li ions must travel through the explained by the high electrical conductivity of W, inactive W layer, which is not an ion-conductive ma- which allows the Si electrode to receive more Li ions terial as discussed in the EIS test, the ion transport Fig. 7 dQ/dV curves for the a uncoated and b W-coated Si electrode under a rate of 0.1 C with a cutoff voltage range of 0 to 1.5 V (vs. Li/Li+) at the 5th, 10th, and 15th cycles Son et al. Nanoscale Research Letters (2018) 13:58 Page 6 of 7 Fig. 8 Voltage profiles for the a uncoated and b W-coated Si electrodes under a rate of 0.1 C with a cutoff voltage range of 0 to 1.5 V (vs. Li/Li+) at the 5th, 10th, and 15th cycles during discharging might be sluggish, resulting in However, a split occurred during the cycles owing to irreversibility. expansion of the entire electrode. Nevertheless, the Figure 7 shows the dQ/dV curves of the 5th, 10th, W-coated Si electrode remained uncracked, indicat- and 15th cycles for both the W-coated and uncoated ing that the atomic deposition by PVD and the in- Si electrodes. The reaction peaks are in the same tense mechanical strength of W effectively sustained voltage regions, which imply that the charging and the expansion [19, 20]. discharging processes occurred with the equivalent reaction [28, 29]. This indicates that the W coating Conclusions did not influence the morphology of the Si electrode W was coated onto a Si electrode using the PVD but covered only the surface layer, and it did not act procedure to improve the electrochemical as an active material. As the cycle number increased, performance of the electrode. The coating layer was the reaction voltage region of the uncoated Si approximately 40-nm thick and was deposited electrode shifted and the polarization increased, uniformly. The capacity retention of the W-coated whereas the reaction voltage region of the W-coated electrode demonstrated enhanced cyclability and was Si electrode remained relatively constant. This sustained at 61.1% through 50 cycles, whereas the implies that the W coating helps retain chemical retention of the uncoated electrode was only 25.5%. stability. This result is also reflected in the voltage The surfaces of the two different electrodes were profile in Fig. 8, which shows the W-coated electrode investigated after cycling, and the observations preserves its capacity with sustained reaction indicated that W acted as a buffer layer. Additionally, voltages. theW-coatedlayer loweredthe resistivityofthe Both the W-coated and uncoated Si electrodes electrode and enhanced the electrical conductivity of were observed by SEM after 10 cycles (Fig. 9). No the cell. We hope that this facile nanolayer cracks were observed on the Si electrode itself, using application through PVD can serve as a reference for nanopowder sizes smaller than 100 nm . future designs of Si-based electrodes. Fig. 9 Scanning electron microscopy images of a uncoated and b W-coated Si electrodes after 10 cycles Son et al. Nanoscale Research Letters (2018) 13:58 Page 7 of 7 Abbreviations 12. Zhongsheng WEN, Shijun JI, Juncai SUN, Feng TIAN, Rujin TIAN, Jingying XIE CPE: Constant phase element; EDX: Energy dispersive X-ray spectroscopy; (2006) Mechanism of lithium insertion into NiSi2 anode material for lithium EIS: Electrochemical impedance spectroscopy; EPMA: Electron probe X-ray ion batteries. Rare Metals 25(6):77–81. microanalysis; PVD: Physical vaporization deposition; SEM: Scanning electron 13. Cui LF, Yang Y, Hsu CM, Cui Y (2009) Carbon––silicon core––shell nanowires as microscopy; TEM: Transmission electron microscopy high capacity electrode for lithium ion batteries. Nano Lett 9(9):3370–3374. 14. Dimov N, Kugino S, Yoshio M (2003) Carbon-coated silicon as anode material for lithium ion batteries: advantages and limitations. Electrochim Acknowledgements Acta 48(11):1579–1587. Not applicable 15. He Y, Yu X, Wang Y, Li H, Huang X (2011) Alumina-coated patterned amorphous silicon as the anode for a lithium-ion battery with high Funding Coulombic efficiency. Adv Mater 23(42):4938–4941. This research was supported by the National Research Foundation of Korea 16. Guo ZP, Wang JZ, Liu HK, Dou SX (2005) Study of silicon/polypyrrole (NRF) funded by the Korean government (MEST) (2016R1A2B3009481 and composite as anode materials for Li-ion batteries. J Power Sources 146(1): 2017M3A9E2093907). 448–451. 17. Dixit S, Popat PP, Rawat SS, Sivarajan S (2016) Multilayer PVD surface Availability of Data and Materials engineered coatings for sheet metal forming tools. Indian J SciTechnol 9(41). 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Nanoscale Research Letters – Springer Journals
Published: Feb 21, 2018
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