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Enhanced cycle stability of silicon coated with waste poly(vinyl butyral)-directed carbon for lithium-ion battery anodes
Journal of Alloys and Compounds 698 (2017) 525e531
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced cycle stability of silicon coated with waste poly(vinyl butyral)-directed carbon for lithium-ion battery anodes Sung-Woo Park a, Jae-Chan Kim a, Mushtaq Ahmad Dar b, Hyun-Woo Shim a, Dong-Wan Kim a, * a
School of Civil, Environmental and Architectural Engineering, Korea University, Seoul 136-713, Republic of Korea Center of Excellence for Research in Engineering Materials, Advanced Manufacturing Institute, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2016 Received in revised form 27 November 2016 Accepted 18 December 2016 Available online 19 December 2016
Waste poly(vinyl butyral) (W-PVB) derived from the windshield glass of end-of-life vehicles is one of the most difficult waste resources to recycle. Therefore, almost all of the W-PVB is buried in landfill sites. Herein, carbon-coated Si (CCSi) as an anode for lithium ion batteries was synthesized using W-PVB as a carbon source via simple carbonization at a relatively low temperature. Commercial Si was well dispersed in ethyl alcohol solutions of W-PVB, leading to the formation of uniform carbon layers on the surface of the Si particles during carbonization. The amorphous carbon layers derived from W-PVB effectively mitigated the pulverization of the Si particles and side reactions between Si and the electrolytes, leading to a stable cycling performance with a retention as high as 77.5% without significant capacity fading. Furthermore, the anode exhibited high coulombic efficiency and an excellent rate capability of 910 mA h g1 at a current density of 840 mA g1. This facile and cost-effective synthesis of CCSi is expected to be applicable to other polymer-based industrial wastes as a new recycling strategy. © 2016 Elsevier B.V. All rights reserved.
Keywords: Waste poly(vinyl butyral) Carbon coating Silicon anode Composite materials Li-ion batteries
1. Introduction Lithium-ion batteries (LIBs) are currently the most commonly used type of secondary battery for portable devices owing to their stable cycle performance and safety in use. Recently, the increasing demand for large- and medium-scale rechargeable batteries for plug-in hybrid electric vehicles (PHEVs), pure electric vehicles (PEVs), and energy storage systems (EESs) has necessitated the development of next-generation LIBs that exhibit higher energy and power density than existing ones. In this context, it is important to develop the superior anode and/or cathode materials because the battery performance can be largely depended upon a choice of electrode materials. Great effort has been devoted to developing high capacity anode materials along with various compositions and nano/microstructures to achieve a higher energy density, and to replacing the commercial graphite due to the inevitably limited capacity (372 mA h g1) that cannot satisfy the highly desired energy density. In particular, based on conversion
* Corresponding author. E-mail address: [email protected] (D.-W. Kim). http://dx.doi.org/10.1016/j.jallcom.2016.12.242 0925-8388/© 2016 Elsevier B.V. All rights reserved.
and/or alloying-dealloying reaction with lithium ions, binary (or ternary) metals oxides and oxysalts have been extensively considered as high capacity anode materials because of their high theoretical capacities (660e4200 mA h g1) and volumetric energy density [1e7]. These materials show high Li-storage performances along with the lower operation potentials (e.g., alloying reaction at 1 V vs. Li metal) than graphite (insertion-deinsertion reaction at 0.15e0.25 V vs. Li metal) [1]. In recent, Si has been also received tremendous interest and widely investigated as an anode material for next-generation LIBs, because it may be alloyed with lithium to form Li4.4Si anodes that have a highest theoretical capacity (~4200 mA h g1), which is ~10 times higher than that of currently used graphite anodes. However, Si exhibits a large volume expansion of over 300% during lithiation, and does not recover its initial shape upon delithiation [8]. This irreversible volume change induces significant capacity fading in Si anodes as a result of cracking and pulverization. Furthermore, it is difficult to form uniform solideelectrolyte interphase (SEI) layers on the surface of Si. Irregular SEI layers derived from several charge-discharge processes disturb the smooth migration of lithium ions and electrons, leading to a low coulombic efficiency and decreased electrochemical performance [9]. Therefore, the
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various surface engineering of electrode materials, including the Sibased anode materials, are required to overcome the problems above and to improve the electrochemical performances. For examples, Chowdari group reported metal oxides (i.e., NiO and AlPO4) coating on active materials to prevent the capacity fade and to provide the improved cycling performance with a high capacity and coulombic efficiency [10,11]. Coating the surface of Si is a well-known strategy for mitigating the adverse effects of volume change. For example, alumina-coated Si anodes exhibit high coulombic efficiency because the alumina increases the fracture resistance of the anode and prevents the Si from undergoing side reactions with the electrolyte [12]. Moreover, metals such as cobalt-, copper- and silver-coated Si anodes demonstrate high rate capabilities due to their improved surface electrical conductivity, reducing electrical resistance of electrodes [13e15]. However, carbon coating is the most widely employed protection strategy, and this is due to a number of reasons. First, carbon exhibits electrical conductivity superior to metal oxides and other semiconductors [16]; second, carbon coating can be achieved using simple methods such as pyrolysis and mechanical milling [17,18]; third, special carbon materials such as graphene, reduced graphene oxide show unique abilities of high conductivity and flexibility, which is suitable for coating materials [19]; finally, carbon is a low-cost material easily and globally obtained. Consequently, the carbon coating of Si anodes is the most promising and cost-effective approach to the enhancement of their electrochemical performance. The mitigation of pollution from industrial waste has been a significant issue for decades, and recycling of waste resources is necessary to protect the environment. Waste poly(vinyl butyral) (W-PVB) is one of the most difficult waste resources to exploit. Because PVB possesses good adhesive strength and high light transmittance, it is therefore used as an adhesive in the production of laminated glass, which also involves the use of additives called plasticizers. The separation of PVB from glass is expensive, and its purification is difficult owing to the different types and quantities of plasticizers added by different manufacturers [20]. Consequently, almost all the PVB used in laminated glass is buried in landfills and not recycled. Therefore, the development of a suitable high value-added materialization strategy would facilitate the recycling of W-PVB specifically, and polymer-based industrial waste generally. In this study, we synthesized carbon-coated Si (CCSi) using WPVB as a carbon source by a simple and cost-effective method. Furthermore, the electrochemical properties of CCSi as an anode material for LIBs were analyzed, and the effect of increasing the carbon to W-PVB ratio on the properties was explored. Commercial Si powder was well dispersed in ethyl alcohol solutions of W-PVB,
which led to the formation of uniform carbon layers on the surface of the Si particles during carbonization. The optimized CCSi anodes exhibited high reversible capacity, enhanced rate capability, and stable cycling performance. Furthermore, cycling tests confirmed that the carbon-shelled Si particles maintained a spherical shape close to that of the initial Si particles without any significant deterioration. 2. Experimental 2.1. Synthesis of CCSi A schematic illustration of the overall process is shown in Fig. 1. W-PVB was separated from automotive windshield glass though shredding and hammer crushing, and then cleaned using water several times before drying at room temperature. The prepared WPVB was dissolved in ethyl alcohol (99.9%, Samchun Chemical), and Si powder (98%, <50 nm, Alfa Aesar) and dispersed in the PVB solution by stirring overnight. The mass ratios of Si to W-PVB used were 1:20, 1:40, and 1:100. To form Si/W-PVB films, the homogeneous Si/W-PVB suspensions were poured into petri dishes and fully dried at 50 C in a dry oven. Finally, the Si/W-PVB films were placed in an alumina crucible in a tube furnace under argon and carbonized by raising the furnace temperature to 500 C at a heating rate of 50 C min1, and maintaining that temperature for 4 h. The final products prepared using Si to W-PVB mass ratios of 1:20, 1:40, and 1:100 were designated CCSi_#1, CCSi_#2 and CCSi_#3, respectively. 2.2. Material characterization The X-ray diffraction (XRD) patterns of the samples were recorded on an Ultima III diffractometer with CuKa radiation (l ¼ 1.5406 Å). Furthermore, Raman spectroscopy was conducted to confirm the formation of carbon using a combined Raman Fouriertransform infrared (FT-IR) spectrometer (HORIBA Jobin Yvon, LabRAM ARAMIIS IR2). The morphologies of the samples were characterized using a field emission scanning electron microscope (FESEM, Hitachi, S-4300), and the carbon layers on the Si particles were verified by a transmission electron microscopy and energy dispersive spectrometry (TEM and EDS, TECNAI, G2-F30 S-Twin). Thermogravimetric analysis (TGA, NETZSCH, STA 409 PC) was performed to ascertain the mass ratio of carbon in each sample. 2.3. Electrochemical measurements The electrochemical properties of the samples were measured using Swagelok cells composed of the working electrode, a
Fig. 1. Schematic illustration of the fabrication of Si/C composites.
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separator film (Celgard 2400), lithium foil as a counter electrode, and a 1 M solution of LiPF6 in ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) (1:1:1 by volume) with 10 wt% of fluoroethylene carbonate (FEC) as the electrolyte. The working electrodes were prepared by coating a Cu foil using a slurry comprising 70 wt% active material, 15 wt% carbon black as a conductive material, and 15 wt% sodium alginate as a binder. The electrodes were cut into 1-cm-diameter disks, and the Swagelok cells were fabricated in an argon-filled glove box. The mass of active materials, which is a total mass of both silicon and pyrolytic carbon, was 2.5e3 mg in average. Galvanostatic charge-discharge testing of the cells was performed on an automatic battery cycler (WBCS 3000, WonaTech) in a voltage range from 0.001 to 2.0 V. 3. Results and discussion The XRD patterns of commercial Si powder and CCSi_#3 are shown in Fig. 2a. The commercial Si powder was identified to be pure Si by reference to JCPDS #27-1402 (2q ¼ 28.4, 47.3, 56.1, 69.1, 76.4, and 88.0 ). In the pattern for CCSi_#3, in addition to the peaks for commercial Si powder, two additional broad peaks are observed at 25e26 and 43e44 . These broad peaks are attributed to pyrolytic carbon with a low graphitization degree generated during the carbonization of W-PVB at low temperature [21,22]. To confirm the graphitization degree of the pyrolytic carbon, the Raman spectrum of CCSi_#3 (Fig. 2b) was obtained. Distinctive peaks appear at ~510, ~1360, and ~1597 cm1, which may be attributed to crystalline silicon, disordered carbon (D), and graphitic carbon (G) respectively. The intensity ratio of the D and
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G bands (ID/IG) is 0.62, which is similar to the value for common pyrolytic carbon with a low graphitization degree [23,24]. Furthermore, TGA curve of W-PVB (Fig. S1) obtained under the equal conditions with carbonization process until 800 C proves that the carbonization temperature is sufficient to carbonize WPVB, showing plateau region after 500 C. Such a small amount of residual carbon (3 wt%) at 500 C may contribute to formation of the uniform carbon layers on the Si surface. The XRD patterns, Raman spectra, and TGA curve demonstrate that the Si/W-PVB film is transformed into Si/pyrolytic carbon with a low graphitization degree and without any impurities upon carbonization at a relatively low temperature (500 C). Fig. 2c and d shows FESEM images of the morphologies of CCSi_#3 and pure Si. The pure Si comprises heaped spherical nanoparticles. Their surface is smooth, and the particles are of various sizes below 100 nm, as shown in inset of Fig. 2c. After carbon coating, the empty spaces between the particles are filled with pyrolytic carbon, forming Si/C composites. The particles present rough surfaces and are larger in comparison with those of pure Si, which might be caused by the pyrolytic carbon coating of the pure Si particles. However, their spherical shape is maintained, as shown in the inset of Fig. 2d. To understand the uniformity of the carbon coating, TEM analysis and EDS line mapping of CCSi_#3 were conducted (Fig. 3). As shown in Fig. 3a, pyrolytic carbon is uniformly coated onto the Si particles. The carbon layers form a spherical shell on the surface of the spherical Si core. The thickness of the carbon layer is ~13 nm, as shown in Fig. 3b. These results indicate that not only does the carbon matrix pack into the empty space between the Si particles forming inner Si/C composites, but
Fig. 2. (a) XRD patterns of pure Si and CCSi_#3. (b) Raman spectra of CCSi_#3. Low- and high-magnification FESEM images of (c) pure Si and (d) CCSi_#3.
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also that the Si particles located on the surface of the Si/C composites are coated with uniform carbon layers. Table 1 shows the carbon contents for CCSi_#1, CCSi_#2, and CCSi_#3 as calculated by weight loss upon the combustion of carbon and weight gain from the oxidation of Si, based on TGA data [25]. As expected, the carbon contents increase to 27.2, 45.3, and 66.7% at a relatively constant rate on increasing the Si:W-PVB ratio to 1:20, 1:40, and 1:100, respectively. The physicochemical changes along with carbon contents are confirmed by TEM and HRTEM images of CCSi_#1, CCSi_#2, and CCSi_#3. As expected, the thickness of carbon layers increased as ~4.8 nm at CCSi_#1, ~7.4 nm at CCSi_#2, and ~13 nm at CCSi_#3, as shown in Fig. S2. Furthermore, the increase of weight ratio of W-PVB induces free carbon excluding coated carbon layers (Fig. S3). The extra free carbon took a possession of space between Si particles with a connection of each carbon coated Si particle, which may contribute to a better and fast charge transfer as electron pathway due to formation of carbon networks in CCSi electrodes. The impedance measurements of the electrodes are carried out to confirm the effects of extra free carbon on charge transfer after the galvanostatic charge-discharge of 5th cycles (Fig. S4). Diameters of semicircle related to charge transfer resistance at interface decrease as the increase of carbon contents constantly, which indicates that both the free carbon networks and thick carbon layers on silicon surface contribute to reduce the charge transfer resistance at electrolyte/electrode interface. The specific surface areas (SSAs) of carbon-coated samples (CCSi_#1, 2, and 3) are calculated by BET method. SSA of each electrode was confirmed with 30.8 m2 g1, 15.1 m2 g1, and 4.2 m2 g1 as increase of weight ratio of W-PVB (Fig. S5). Decrease of SSA as carbon contents can be explained to the formation of non-porous carbon networks between carbon coated Si particles because we do not further treatment for generating pores in pyrolytic carbon. To explore the effect of the carbon content of the samples on their electrochemical properties, the cycling performances of pure Si, CCSi_#1, CCSi_#2, and CCSi_#3 electrodes were measured at a current density of 210 mA g1 over 100 cycles, based on the total mass of Si/C composite, as shown in Fig. 4a. The initial discharge capacity of the samples is 1361, 1998, 2598, and 2821 mA h g1, respectively, which is in the same order as that of the carbon content of the material. For pure Si, a dramatic capacity drop-off occurs after the first cycle, reaching a very low retention of 3.8% after 80 cycles. Similarly, the CCSi_#1 and CCSi_#2 electrodes, which have relatively low carbon contents (27.2 and 45.3%), show continuous capacity fade during cycling; however, after 100 cycles, the electrodes exhibit higher retentions (6.8 and 19.8%) than that of
Table 1 The carbon contents of the samples.
Carbon content (wt%)
Pure Si
CCSi_#1
CCSi_#2
CCSi_#3
0
27.2
45.3
66.7
pure Si. The CCSi_#3 electrodes, which have the highest carbon content (66.7%) exhibit the highest retention of 77.5% without significant capacity fade. The difference of cycling performances by the carbon contents is obviously represented by calculating specific capacities for only Si mass as shown in Fig. S6. The CCSi_#3 electrode demonstrates much better cycling performance than the other electrodes, indicating reversible capacity of 2051 mA h g1 after 100 cycles. The corresponding coulombic efficiencies of each electrode are shown in Fig. 4b. The results demonstrate that as the carbon content increases, the coulombic efficiency over 100 cycles becomes more stable. Remarkably, the coulombic efficiency of the CCSi_#3 electrodes reaches 99% after just seven cycles. The optimized electrochemical performance of the CCSi_#3 electrodes may be attributed to the appropriate amount of pyrolytic carbon that fully covers the surface of the Si particles, which mitigates cracking and pulverization of the Si particles, and the formation of uneven SEI layers as caused by volume change of the Si particles during the charge-discharge process [26]. Fig. 5 shows the galvanostatic charge-discharge curves of 1st, 2nd, 10th, and 50th cycles of all electrodes in a voltage window of 0.001e2.0 V. In the initial discharge curves, the carbon-coated electrodes (Fig. 5bed) exhibit the typical lithiation plateaus of Si anodes at 0.01e0.02 V, as in the pure Si electrode (Fig. 5a), which indicates that the crystalline Si particles coated with pyrolytic carbon react well with lithium [27,28]. After the initial cycle, the lithiation plateaus of all electrodes are shifted by ~ þ0.4 V, indicating the phase transformation of crystalline Si into amorphous Si [29], and the charge-discharge curves show typical behaviors (sloping curves) of Li-intercalating with amorphous LixSi. It is also noted that the average charge-discharge potentials of CCSi_#3 electrode (Fig. 5d) are ca. 0.46 and 0.15 V, respectively, rendering a low average overpotential of 0.155 V, which suggests that the electrode can be a suitable anode for practical applications with the low discharge-charge voltage hysteresis and polarization. It is investigated that the change of irreversible capacity loss (ICL) according to the carbon contents. Initial ICLs of all samples are represented as 18.5% (Pure Si), 6.8%
Fig. 3. (a) TEM image and (b) EDS line mapping of CCSi_#3.
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Fig. 4. (a) Cycling performances of each sample and (b) corresponding coulombic efficiencies at a current density of 210 mA g1 for 100 cycles.
(CCSi_#1), 23.8% (CCSi_#2), and 35% (CCSi_#3). After carbon coating (CCSi_#1), ICL decreases from 18.5% to 6.8%, which means that the coated carbon layers mitigate generation of solid
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electrolyte interface (SEI) caused by the reduction of electrolyte on the silicon surface [30e32]. To further insight of ICL at an initial stage, we measured the cyclic volammorgrams of all samples (Fig. S7). As for the pure Si electrodes (Fig. S7a), the two peaks are observed at around of ~1.0 V and ~0.7 V in the first discharge cycle, indicating the formation of SEI layer caused by reduction of electrolyte on active materials surface, because these peaks are disappeared in the second discharge cycle. In contrast, in the first discharge cycle after carbon coating (CCSi_#1, Fig. S7b), a cathodic peak observed at ~1.0 V is evidently disappeared and two broad peaks are represented at around of ~1.2 V and ~1.3 V. Subsequently, CV curves of more carbon contents showed similar shapes to that of low carbon contents with intenerating the abovementioned cathodic peaks in the first discharge cycles, as shown in Fig. S7c and d. These results mean that the carbon layers coated on silicon surface successfully impede formation of SEI layers at Si/electrolyte interface. Therefore, increase of ICL by up to 35% according to the increase of carbon contents may be contributed to the irreversible insertion of Li-ions into amorphous carbon because the carbon contents of CCSi_#3 electrodes is as high as 66.7% [33]. However, high ICL value of CCSi_#3 electrodes drastically decreases to 5.5% after 2 cycles and 1.7% after 10 cycles; Furthermore, the electrodes show the high discharge capacity of 939 mA h g1 at 50th cycle, indicating that the electrodes successfully prevent the pulverization of electrodes derived from volume expansion. The rate capabilities of the pure Si and CCSi_#3 electrodes were compared at various current densities, as shown in Fig. 6. Although the pure Si electrodes exhibit a high initial discharge capacity of 3014 mA h g1 at a low current density of 84 mA h g1, dramatic capacity drop-off occurs for all current densities (210, 420, 840, and 4200 mA g1). When the current density is returned to 210 mA g1, the pure Si electrodes do not recover their initial reversible capacity. Conversely, the CCSi_#3 electrodes demonstrate good rate capability, exhibiting reversible capacities of 1346, 1194, 1102, and 910 mA h g1 at current densities of 84, 210, 420, and 840 mA g1,
Fig. 5. Comparison of galvanostatic charge-discharge profiles (1st, 2nd, 10th, and 50th cycles) of all samples. (a) pure Si, (b) CCSi_#1, (c) CCSi_#2, and (d) CCSi_#3 electrodes.
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Fig. 6. Rate capabilities of pure Si and CCSi_#3 at different current densities.
respectively. Furthermore, the excellent reversibility of CCSi_#3 electrodes is demonstrated by the fact that they recover a capacity of 1116 mA h g1, which is close to their initial capacity (1194 mA h g1) at a current density of 210 mA g1, after 5 cycles at a current density of 4200 mA g1. To understand the outstanding electrochemical performance of CCSi_#3, TEM analysis of the fully delithiated electrodes after 10 cycles was performed, as shown in Fig. 7. The Si particles located inside the Si/C composites (Fig. 7a) maintain their spherical shape, but are slightly altered and shrink in the initial spherical carbon matrix during the charge-discharge process. Likewise, Si particles located on the surface of the Si/C composites (Fig. 7b) exhibit a slightly smaller spherical shape without degradation of the carbon layers, which is transformed into a rough shell. These results demonstrate that pyrolytic carbon layers coated onto Si particles effectively protect the Si anodes from the pulverization caused by volume change and side reactions, including the irreversible reaction between Si and the electrolyte during the charge-discharge process. This imparts stable cycling performance and high coulombic efficiency to the CCSi_#3 electrodes. Furthermore, the carbon matrix has good electrical conductivity, which contributes to the excellent rate capability [34]. This effective carbon coating derived from W-PVB is not only expected to promote the recycling of W-PVB, but also
presents the possibility of exploiting other polymer-based industrial wastes as carbon sources.
4. Conclusions CCSi was synthesized using W-PVB separated from the windshield glass of end-of-life vehicles via simple carbonization at a relative low temperature. As the weight ratio of W-PVB to Si increased (20:1, 40:1, and 100:1), the carbon contents of the CCSi samples increased at a relatively constant rate (27.2, 45.3, and 66.7%) and their electrochemical performances improved. The CCSi_#3 samples, which had a carbon content of 66.7%, exhibited enhanced cycling performance, with a retention of 77.5% after 100 cycles, an excellent rate capability of 910 mA h g1 at a high current density of 840 mA g1, and a high columbic efficiency of ~99% after seven cycles. This enhanced electrochemical performance is attributed to the uniformly coated pyrolytic carbon layers that mitigate both side reactions between Si and the electrolyte, and pulverization induced by volume change during the chargedischarge process. This facile and cost-effective synthesis of CCSi anodes using W-PVB separated from windshield glass demonstrates the possibility of using polymer based-industrial waste as a carbon source.
Fig. 7. TEM images of CCSi_#3 (a) inside the carbon matrix and (b) on the surface of the carbon matrix.
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Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT, and Future Planning (No. 2016R1A2B2012728) and by the R&D Center for Valuable Recycling (Global-Top R&BD Program) of the Ministry of Environment (No. R2-17_2016002250005). Authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this research grant no. (RG#1437-006). FESEM and TEM analyses were performed at Korea Basic Science Institute (KBSI) in Seoul. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.12.242. References [1] M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Metal oxides and oxysalts as anode materials for Li ion batteries, Chem. Rev. 113 (2013) 5364e5457. [2] M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Nano-(V1/2Sb1/2Sn)O4: a high capacity, high rate anode material for Li-ion batteries, J. Mater. Chem. 21 (2011) 10003e10011. [3] C.T. Cherian, M. Zheng, M.V. Reddy, B.V.R. chowdari, C.H. Sow, Zn2SnO4 nanowires versus nanoplates: electrochemical performance and morphological evolution during Li-cycling, ACS Appl. Mater. Inter. 5 (2013) 6054e6060. [4] A.K. Jibin, M.V. Reddy, G.V. Subba Rao, U.V. Varadaraju, M.V.R. Chowdari, Pb3O4 type antimony oxides MSb2O4 (M¼Co, Ni) as anode for Li-ion batteries, Electochim. Acta 71 (2012) 227e232. [5] C.T. Cherian, M.V. Reddy, G.V. Subba Rao, C.H. Sow, B.V.R. Chowdari, Li-cycling properties of nano-crystalline (Ni1-xZnx)Fe2O4 (0 x 1), J. Solid State Electrochem. 16 (2012) 1823e1832. [6] M.V. Reddy, K.Y.H. Kenrick, T.Y. Wei, G.Y. Chong, G.H. Leong, B.V.R. Chowdari, Nano-ZnCo2O4 material proparation by molten salt method and its electrochemical properties for lithium batteries, J. Electrochem. Soc. 158 (2011) A1423eA1430. [7] B. Das, M.V. Reddy, B.V.R. Chowdari, SnO and SnO$CoO nanocomposite as high capacity anode materials for lithium ion batteries, Mater. Res. Bull. 74 (2016) 291e298. [8] W.-J. Zhang, A review of the electrochemical performance of alloy anodes for lithium-ion batteries, J. Power Sources 196 (2011) 13e24. [9] M.T. McDowell, I. Ryu, S.W. Lee, W.C. Wang, W.D. Nix, Y. Cui, Studying the kinetics of crystalline silicon nanoparticle lithiation with in situ transmission electron microscopy, Adv. Mater. 24 (2012) 6034e6041. [10] K.S. Tan, M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Effect of AlPO4-coating on cathodic behaviour of Li(Ni0.8Co0.2)O2, J. Power Sources 141 (2005) 129e142. [11] M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Cathodic behaviour of NiOcoated Li(Ni1/2Mn1/2)O2, Electrochim. Acta 50 (2005) 3375e3382. [12] Y. He, X. Yu, Y. Wang, H. Li, X. Huang, Alumina-coated patterned amorphous silicon as the anode for a lithium-ion battery with high coulombic efficiency, Adv. Mater. 23 (2011) 4938e4941. [13] V.A. Sethuraman, K. Kowolik, V. Srinivasan, Increased cycling efficiency and rate capability of copper-coated silicon anodes in lithium-ion batteries, J. Power Sources 196 (2011) 393e398. [14] B.Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P.A.V. Aken, J. Maier, Reversible storage of lithium in silver-coated three-dimensional macroporous silicon, Adv. Mater. 22 (2010) 2247e2250.
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[15] T. Cetinkaya, M. Uysal, M.O. Guler, H. Akbulut, Developing lithium ion battery silicon/cobalt core-shell electrodes for enhanced electrochemical properties, Int. J. Hydrogen Energy 39 (2014) 21405e21413. [16] S.H. Ng, J. Wang, D. Wexler, S.Y. Chew, H.K. Liu, Amorphous carbon-coated silicon nanocomposites: a low-temperature synthesis via spray pyrolysis and their application as high-capacity anodes for lithium-ion batteries, J. Phys. Chem. C 111 (2007) 11131e11138. [17] M.-S. Wang, L.-Z. Fan, Silicon/Carbon nanocomposite pyrolyzed from phenolic resin as anode materials for lithium-ion batteries, J. Power Sources 224 (2013) 570e574. [18] H. Tian, X. Tan, F. Xin, C. Wang, W. Han, Micro-sized nano-porous Si/C anodes for lithium ion batteries, Nano Energy 11 (2015) 490e499. [19] M. Tokur, H. Algul, S. Ozcan, T. Cetinkaya, M. Uysal, H. Akbulut, Closing to scaling-up high reversible Si/rGO nanocomposite anodes for lithium ion batteries, Electrochim. Acta 216 (2016) 312e319. [20] M. Tupy, P. Mokrejs, D. Merinska, P. Svoboda, J. Zvonicek, Windshield recycling focused on effective separation of PVB sheet, J. Appl. Polym. Sci. 131 (2014) 39879e39887. [21] Y.J. Hwang, S.K. Jeong, K.W. Nahm, J.S. Shin, A.M. Stephan, Pyrolytic carbon derived from coffee shells as anode materials for lithium batteries, J. Phys. Chem. Solids 68 (2007) 182e188. [22] H.-L. Zhang, S.-He Liu, F. Li, S. Bai, C. Liu, J. Tan, H.-M. Cheng, Electrochemical performance of pyrolytic carbon-coated natural graphite spheres, Carbon 44 (2006) 2212e2218. [23] X.-Y. Zhou, J.-J. Tang, J. Yang, J. Xie, L.-l. Ma, Silicon@carbon hollow core-shell heterostructures novel anode materials for lithium ion batteries, Electrochim. Acta 87 (2013) 663e668. [24] M.-S. Wang, L.-Z. Fan, M. Huang, J. Li, X. Qu, Conversion of diatomite to porous Si/C composites as promising anode materials for lithium-ion batteries, J. Power Sources 219 (2012) 29e35. [25] L. Wu, X. Hu, J. Qian, F. Pei, F. Wu, R. Mao, X. Ai, H. Yang, Y. Cao, Sb-C nanofibers with long cycle life as an anode materials for high-performance sodium-ion batteries, Energy Environ. Sci. 7 (2014) 323e328. [26] S.-H. Ng, J. Wang, D. Wexler, K. Konstantinov, Z.-P. Guo, H.-K. Liu, Highly reversible lithium storage in spheroidal carbon-coated silicon nanocomposites as anodes for lithium-ion batteries, Angew. Chem. Int. Ed. 45 (2006) 6896e6899. [27] R.Z. Zhou, H. Guo, Y. Yang, Z. Wang, X. Li, Y. Zhou, N-doped carbon layer derived from polydopamine to improve the electrochemical performance of spray-dried Si/Graphite composite anode material for lithium ion batteries, J. Alloy. Compd. 689 (2016) 130e137. [28] X. Shen, W. Jiang, H. Sun, Y. Wang, A. Dong, J. Hu, D. Yang, Ionic liquid assist to prepare Si@N-doped carbon nanoparticles and its high performance in lithium ion batteries, J. Alloy. Compd. 691 (2017) 178e184. [29] W.-H. Lee, D.-Y. Kang, J.S. Kim, J.K. Lee, J.H. Moon, Uniformly dispersed silicon nanoparticle/carbon nanosphere composites as highly stable lithium-ion battery electrodes, RSC Adv. 5 (2015) 17424e17428. [30] S.-H. Ng, J. Wang, D. Wexler, K. Konstantinov, Z.-P. Guo, H.-K. Liu, Highly reversible lithium storage in spheroidal carbon-coated silicon nanocomposites as anodes for lithium-ion batteries, Angew. Chem. Int. Ed. 45 (2006) 6896e6899. [31] M.V. Reddy, B.L.W. Wen, K.P. Loh, B.V.R. Chowdari, Energy storage studies on InVO4 as high performance anode material for Li-Ion batteries, ACS Appl. Mater. Inter. 5 (2013) 7777e7785. [32] B. Das, M.V. Reddy, B.V.R. Chowdari, Li-storage of Fe3O4/C composite prepared by one-step carbothermal reduction method, J. Alloy. Compd. 565 (2013) 90e96. [33] S. Chen, M.L. Gordin, R. Yi, G. Howlett, H. Sohn, D. Wang, Silicon core-hollow carbon shell nanocomposites with tunable buffer voids for high capacity anodes of lithium-ion batteries, Phys. Chem. Chem. Phys. 14 (2012) 12741e12745. [34] N. Dimov, S. Kugino, M. Yoshio, Carbon-coated silicon as anode material for lithium ion batteries: advantages and limitations, Electrochim. Acta 48 (2003) 1579e1587.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced cycle stability of silicon coated with waste poly(vinyl butyral)-directed carbon for lithium-ion battery anodes Sung-Woo Park a, Jae-Chan Kim a, Mushtaq Ahmad Dar b, Hyun-Woo Shim a, Dong-Wan Kim a, * a
School of Civil, Environmental and Architectural Engineering, Korea University, Seoul 136-713, Republic of Korea Center of Excellence for Research in Engineering Materials, Advanced Manufacturing Institute, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2016 Received in revised form 27 November 2016 Accepted 18 December 2016 Available online 19 December 2016
Waste poly(vinyl butyral) (W-PVB) derived from the windshield glass of end-of-life vehicles is one of the most difficult waste resources to recycle. Therefore, almost all of the W-PVB is buried in landfill sites. Herein, carbon-coated Si (CCSi) as an anode for lithium ion batteries was synthesized using W-PVB as a carbon source via simple carbonization at a relatively low temperature. Commercial Si was well dispersed in ethyl alcohol solutions of W-PVB, leading to the formation of uniform carbon layers on the surface of the Si particles during carbonization. The amorphous carbon layers derived from W-PVB effectively mitigated the pulverization of the Si particles and side reactions between Si and the electrolytes, leading to a stable cycling performance with a retention as high as 77.5% without significant capacity fading. Furthermore, the anode exhibited high coulombic efficiency and an excellent rate capability of 910 mA h g1 at a current density of 840 mA g1. This facile and cost-effective synthesis of CCSi is expected to be applicable to other polymer-based industrial wastes as a new recycling strategy. © 2016 Elsevier B.V. All rights reserved.
Keywords: Waste poly(vinyl butyral) Carbon coating Silicon anode Composite materials Li-ion batteries
1. Introduction Lithium-ion batteries (LIBs) are currently the most commonly used type of secondary battery for portable devices owing to their stable cycle performance and safety in use. Recently, the increasing demand for large- and medium-scale rechargeable batteries for plug-in hybrid electric vehicles (PHEVs), pure electric vehicles (PEVs), and energy storage systems (EESs) has necessitated the development of next-generation LIBs that exhibit higher energy and power density than existing ones. In this context, it is important to develop the superior anode and/or cathode materials because the battery performance can be largely depended upon a choice of electrode materials. Great effort has been devoted to developing high capacity anode materials along with various compositions and nano/microstructures to achieve a higher energy density, and to replacing the commercial graphite due to the inevitably limited capacity (372 mA h g1) that cannot satisfy the highly desired energy density. In particular, based on conversion
* Corresponding author. E-mail address: [email protected] (D.-W. Kim). http://dx.doi.org/10.1016/j.jallcom.2016.12.242 0925-8388/© 2016 Elsevier B.V. All rights reserved.
and/or alloying-dealloying reaction with lithium ions, binary (or ternary) metals oxides and oxysalts have been extensively considered as high capacity anode materials because of their high theoretical capacities (660e4200 mA h g1) and volumetric energy density [1e7]. These materials show high Li-storage performances along with the lower operation potentials (e.g., alloying reaction at 1 V vs. Li metal) than graphite (insertion-deinsertion reaction at 0.15e0.25 V vs. Li metal) [1]. In recent, Si has been also received tremendous interest and widely investigated as an anode material for next-generation LIBs, because it may be alloyed with lithium to form Li4.4Si anodes that have a highest theoretical capacity (~4200 mA h g1), which is ~10 times higher than that of currently used graphite anodes. However, Si exhibits a large volume expansion of over 300% during lithiation, and does not recover its initial shape upon delithiation [8]. This irreversible volume change induces significant capacity fading in Si anodes as a result of cracking and pulverization. Furthermore, it is difficult to form uniform solideelectrolyte interphase (SEI) layers on the surface of Si. Irregular SEI layers derived from several charge-discharge processes disturb the smooth migration of lithium ions and electrons, leading to a low coulombic efficiency and decreased electrochemical performance [9]. Therefore, the
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various surface engineering of electrode materials, including the Sibased anode materials, are required to overcome the problems above and to improve the electrochemical performances. For examples, Chowdari group reported metal oxides (i.e., NiO and AlPO4) coating on active materials to prevent the capacity fade and to provide the improved cycling performance with a high capacity and coulombic efficiency [10,11]. Coating the surface of Si is a well-known strategy for mitigating the adverse effects of volume change. For example, alumina-coated Si anodes exhibit high coulombic efficiency because the alumina increases the fracture resistance of the anode and prevents the Si from undergoing side reactions with the electrolyte [12]. Moreover, metals such as cobalt-, copper- and silver-coated Si anodes demonstrate high rate capabilities due to their improved surface electrical conductivity, reducing electrical resistance of electrodes [13e15]. However, carbon coating is the most widely employed protection strategy, and this is due to a number of reasons. First, carbon exhibits electrical conductivity superior to metal oxides and other semiconductors [16]; second, carbon coating can be achieved using simple methods such as pyrolysis and mechanical milling [17,18]; third, special carbon materials such as graphene, reduced graphene oxide show unique abilities of high conductivity and flexibility, which is suitable for coating materials [19]; finally, carbon is a low-cost material easily and globally obtained. Consequently, the carbon coating of Si anodes is the most promising and cost-effective approach to the enhancement of their electrochemical performance. The mitigation of pollution from industrial waste has been a significant issue for decades, and recycling of waste resources is necessary to protect the environment. Waste poly(vinyl butyral) (W-PVB) is one of the most difficult waste resources to exploit. Because PVB possesses good adhesive strength and high light transmittance, it is therefore used as an adhesive in the production of laminated glass, which also involves the use of additives called plasticizers. The separation of PVB from glass is expensive, and its purification is difficult owing to the different types and quantities of plasticizers added by different manufacturers [20]. Consequently, almost all the PVB used in laminated glass is buried in landfills and not recycled. Therefore, the development of a suitable high value-added materialization strategy would facilitate the recycling of W-PVB specifically, and polymer-based industrial waste generally. In this study, we synthesized carbon-coated Si (CCSi) using WPVB as a carbon source by a simple and cost-effective method. Furthermore, the electrochemical properties of CCSi as an anode material for LIBs were analyzed, and the effect of increasing the carbon to W-PVB ratio on the properties was explored. Commercial Si powder was well dispersed in ethyl alcohol solutions of W-PVB,
which led to the formation of uniform carbon layers on the surface of the Si particles during carbonization. The optimized CCSi anodes exhibited high reversible capacity, enhanced rate capability, and stable cycling performance. Furthermore, cycling tests confirmed that the carbon-shelled Si particles maintained a spherical shape close to that of the initial Si particles without any significant deterioration. 2. Experimental 2.1. Synthesis of CCSi A schematic illustration of the overall process is shown in Fig. 1. W-PVB was separated from automotive windshield glass though shredding and hammer crushing, and then cleaned using water several times before drying at room temperature. The prepared WPVB was dissolved in ethyl alcohol (99.9%, Samchun Chemical), and Si powder (98%, <50 nm, Alfa Aesar) and dispersed in the PVB solution by stirring overnight. The mass ratios of Si to W-PVB used were 1:20, 1:40, and 1:100. To form Si/W-PVB films, the homogeneous Si/W-PVB suspensions were poured into petri dishes and fully dried at 50 C in a dry oven. Finally, the Si/W-PVB films were placed in an alumina crucible in a tube furnace under argon and carbonized by raising the furnace temperature to 500 C at a heating rate of 50 C min1, and maintaining that temperature for 4 h. The final products prepared using Si to W-PVB mass ratios of 1:20, 1:40, and 1:100 were designated CCSi_#1, CCSi_#2 and CCSi_#3, respectively. 2.2. Material characterization The X-ray diffraction (XRD) patterns of the samples were recorded on an Ultima III diffractometer with CuKa radiation (l ¼ 1.5406 Å). Furthermore, Raman spectroscopy was conducted to confirm the formation of carbon using a combined Raman Fouriertransform infrared (FT-IR) spectrometer (HORIBA Jobin Yvon, LabRAM ARAMIIS IR2). The morphologies of the samples were characterized using a field emission scanning electron microscope (FESEM, Hitachi, S-4300), and the carbon layers on the Si particles were verified by a transmission electron microscopy and energy dispersive spectrometry (TEM and EDS, TECNAI, G2-F30 S-Twin). Thermogravimetric analysis (TGA, NETZSCH, STA 409 PC) was performed to ascertain the mass ratio of carbon in each sample. 2.3. Electrochemical measurements The electrochemical properties of the samples were measured using Swagelok cells composed of the working electrode, a
Fig. 1. Schematic illustration of the fabrication of Si/C composites.
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separator film (Celgard 2400), lithium foil as a counter electrode, and a 1 M solution of LiPF6 in ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) (1:1:1 by volume) with 10 wt% of fluoroethylene carbonate (FEC) as the electrolyte. The working electrodes were prepared by coating a Cu foil using a slurry comprising 70 wt% active material, 15 wt% carbon black as a conductive material, and 15 wt% sodium alginate as a binder. The electrodes were cut into 1-cm-diameter disks, and the Swagelok cells were fabricated in an argon-filled glove box. The mass of active materials, which is a total mass of both silicon and pyrolytic carbon, was 2.5e3 mg in average. Galvanostatic charge-discharge testing of the cells was performed on an automatic battery cycler (WBCS 3000, WonaTech) in a voltage range from 0.001 to 2.0 V. 3. Results and discussion The XRD patterns of commercial Si powder and CCSi_#3 are shown in Fig. 2a. The commercial Si powder was identified to be pure Si by reference to JCPDS #27-1402 (2q ¼ 28.4, 47.3, 56.1, 69.1, 76.4, and 88.0 ). In the pattern for CCSi_#3, in addition to the peaks for commercial Si powder, two additional broad peaks are observed at 25e26 and 43e44 . These broad peaks are attributed to pyrolytic carbon with a low graphitization degree generated during the carbonization of W-PVB at low temperature [21,22]. To confirm the graphitization degree of the pyrolytic carbon, the Raman spectrum of CCSi_#3 (Fig. 2b) was obtained. Distinctive peaks appear at ~510, ~1360, and ~1597 cm1, which may be attributed to crystalline silicon, disordered carbon (D), and graphitic carbon (G) respectively. The intensity ratio of the D and
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G bands (ID/IG) is 0.62, which is similar to the value for common pyrolytic carbon with a low graphitization degree [23,24]. Furthermore, TGA curve of W-PVB (Fig. S1) obtained under the equal conditions with carbonization process until 800 C proves that the carbonization temperature is sufficient to carbonize WPVB, showing plateau region after 500 C. Such a small amount of residual carbon (3 wt%) at 500 C may contribute to formation of the uniform carbon layers on the Si surface. The XRD patterns, Raman spectra, and TGA curve demonstrate that the Si/W-PVB film is transformed into Si/pyrolytic carbon with a low graphitization degree and without any impurities upon carbonization at a relatively low temperature (500 C). Fig. 2c and d shows FESEM images of the morphologies of CCSi_#3 and pure Si. The pure Si comprises heaped spherical nanoparticles. Their surface is smooth, and the particles are of various sizes below 100 nm, as shown in inset of Fig. 2c. After carbon coating, the empty spaces between the particles are filled with pyrolytic carbon, forming Si/C composites. The particles present rough surfaces and are larger in comparison with those of pure Si, which might be caused by the pyrolytic carbon coating of the pure Si particles. However, their spherical shape is maintained, as shown in the inset of Fig. 2d. To understand the uniformity of the carbon coating, TEM analysis and EDS line mapping of CCSi_#3 were conducted (Fig. 3). As shown in Fig. 3a, pyrolytic carbon is uniformly coated onto the Si particles. The carbon layers form a spherical shell on the surface of the spherical Si core. The thickness of the carbon layer is ~13 nm, as shown in Fig. 3b. These results indicate that not only does the carbon matrix pack into the empty space between the Si particles forming inner Si/C composites, but
Fig. 2. (a) XRD patterns of pure Si and CCSi_#3. (b) Raman spectra of CCSi_#3. Low- and high-magnification FESEM images of (c) pure Si and (d) CCSi_#3.
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also that the Si particles located on the surface of the Si/C composites are coated with uniform carbon layers. Table 1 shows the carbon contents for CCSi_#1, CCSi_#2, and CCSi_#3 as calculated by weight loss upon the combustion of carbon and weight gain from the oxidation of Si, based on TGA data [25]. As expected, the carbon contents increase to 27.2, 45.3, and 66.7% at a relatively constant rate on increasing the Si:W-PVB ratio to 1:20, 1:40, and 1:100, respectively. The physicochemical changes along with carbon contents are confirmed by TEM and HRTEM images of CCSi_#1, CCSi_#2, and CCSi_#3. As expected, the thickness of carbon layers increased as ~4.8 nm at CCSi_#1, ~7.4 nm at CCSi_#2, and ~13 nm at CCSi_#3, as shown in Fig. S2. Furthermore, the increase of weight ratio of W-PVB induces free carbon excluding coated carbon layers (Fig. S3). The extra free carbon took a possession of space between Si particles with a connection of each carbon coated Si particle, which may contribute to a better and fast charge transfer as electron pathway due to formation of carbon networks in CCSi electrodes. The impedance measurements of the electrodes are carried out to confirm the effects of extra free carbon on charge transfer after the galvanostatic charge-discharge of 5th cycles (Fig. S4). Diameters of semicircle related to charge transfer resistance at interface decrease as the increase of carbon contents constantly, which indicates that both the free carbon networks and thick carbon layers on silicon surface contribute to reduce the charge transfer resistance at electrolyte/electrode interface. The specific surface areas (SSAs) of carbon-coated samples (CCSi_#1, 2, and 3) are calculated by BET method. SSA of each electrode was confirmed with 30.8 m2 g1, 15.1 m2 g1, and 4.2 m2 g1 as increase of weight ratio of W-PVB (Fig. S5). Decrease of SSA as carbon contents can be explained to the formation of non-porous carbon networks between carbon coated Si particles because we do not further treatment for generating pores in pyrolytic carbon. To explore the effect of the carbon content of the samples on their electrochemical properties, the cycling performances of pure Si, CCSi_#1, CCSi_#2, and CCSi_#3 electrodes were measured at a current density of 210 mA g1 over 100 cycles, based on the total mass of Si/C composite, as shown in Fig. 4a. The initial discharge capacity of the samples is 1361, 1998, 2598, and 2821 mA h g1, respectively, which is in the same order as that of the carbon content of the material. For pure Si, a dramatic capacity drop-off occurs after the first cycle, reaching a very low retention of 3.8% after 80 cycles. Similarly, the CCSi_#1 and CCSi_#2 electrodes, which have relatively low carbon contents (27.2 and 45.3%), show continuous capacity fade during cycling; however, after 100 cycles, the electrodes exhibit higher retentions (6.8 and 19.8%) than that of
Table 1 The carbon contents of the samples.
Carbon content (wt%)
Pure Si
CCSi_#1
CCSi_#2
CCSi_#3
0
27.2
45.3
66.7
pure Si. The CCSi_#3 electrodes, which have the highest carbon content (66.7%) exhibit the highest retention of 77.5% without significant capacity fade. The difference of cycling performances by the carbon contents is obviously represented by calculating specific capacities for only Si mass as shown in Fig. S6. The CCSi_#3 electrode demonstrates much better cycling performance than the other electrodes, indicating reversible capacity of 2051 mA h g1 after 100 cycles. The corresponding coulombic efficiencies of each electrode are shown in Fig. 4b. The results demonstrate that as the carbon content increases, the coulombic efficiency over 100 cycles becomes more stable. Remarkably, the coulombic efficiency of the CCSi_#3 electrodes reaches 99% after just seven cycles. The optimized electrochemical performance of the CCSi_#3 electrodes may be attributed to the appropriate amount of pyrolytic carbon that fully covers the surface of the Si particles, which mitigates cracking and pulverization of the Si particles, and the formation of uneven SEI layers as caused by volume change of the Si particles during the charge-discharge process [26]. Fig. 5 shows the galvanostatic charge-discharge curves of 1st, 2nd, 10th, and 50th cycles of all electrodes in a voltage window of 0.001e2.0 V. In the initial discharge curves, the carbon-coated electrodes (Fig. 5bed) exhibit the typical lithiation plateaus of Si anodes at 0.01e0.02 V, as in the pure Si electrode (Fig. 5a), which indicates that the crystalline Si particles coated with pyrolytic carbon react well with lithium [27,28]. After the initial cycle, the lithiation plateaus of all electrodes are shifted by ~ þ0.4 V, indicating the phase transformation of crystalline Si into amorphous Si [29], and the charge-discharge curves show typical behaviors (sloping curves) of Li-intercalating with amorphous LixSi. It is also noted that the average charge-discharge potentials of CCSi_#3 electrode (Fig. 5d) are ca. 0.46 and 0.15 V, respectively, rendering a low average overpotential of 0.155 V, which suggests that the electrode can be a suitable anode for practical applications with the low discharge-charge voltage hysteresis and polarization. It is investigated that the change of irreversible capacity loss (ICL) according to the carbon contents. Initial ICLs of all samples are represented as 18.5% (Pure Si), 6.8%
Fig. 3. (a) TEM image and (b) EDS line mapping of CCSi_#3.
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Fig. 4. (a) Cycling performances of each sample and (b) corresponding coulombic efficiencies at a current density of 210 mA g1 for 100 cycles.
(CCSi_#1), 23.8% (CCSi_#2), and 35% (CCSi_#3). After carbon coating (CCSi_#1), ICL decreases from 18.5% to 6.8%, which means that the coated carbon layers mitigate generation of solid
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electrolyte interface (SEI) caused by the reduction of electrolyte on the silicon surface [30e32]. To further insight of ICL at an initial stage, we measured the cyclic volammorgrams of all samples (Fig. S7). As for the pure Si electrodes (Fig. S7a), the two peaks are observed at around of ~1.0 V and ~0.7 V in the first discharge cycle, indicating the formation of SEI layer caused by reduction of electrolyte on active materials surface, because these peaks are disappeared in the second discharge cycle. In contrast, in the first discharge cycle after carbon coating (CCSi_#1, Fig. S7b), a cathodic peak observed at ~1.0 V is evidently disappeared and two broad peaks are represented at around of ~1.2 V and ~1.3 V. Subsequently, CV curves of more carbon contents showed similar shapes to that of low carbon contents with intenerating the abovementioned cathodic peaks in the first discharge cycles, as shown in Fig. S7c and d. These results mean that the carbon layers coated on silicon surface successfully impede formation of SEI layers at Si/electrolyte interface. Therefore, increase of ICL by up to 35% according to the increase of carbon contents may be contributed to the irreversible insertion of Li-ions into amorphous carbon because the carbon contents of CCSi_#3 electrodes is as high as 66.7% [33]. However, high ICL value of CCSi_#3 electrodes drastically decreases to 5.5% after 2 cycles and 1.7% after 10 cycles; Furthermore, the electrodes show the high discharge capacity of 939 mA h g1 at 50th cycle, indicating that the electrodes successfully prevent the pulverization of electrodes derived from volume expansion. The rate capabilities of the pure Si and CCSi_#3 electrodes were compared at various current densities, as shown in Fig. 6. Although the pure Si electrodes exhibit a high initial discharge capacity of 3014 mA h g1 at a low current density of 84 mA h g1, dramatic capacity drop-off occurs for all current densities (210, 420, 840, and 4200 mA g1). When the current density is returned to 210 mA g1, the pure Si electrodes do not recover their initial reversible capacity. Conversely, the CCSi_#3 electrodes demonstrate good rate capability, exhibiting reversible capacities of 1346, 1194, 1102, and 910 mA h g1 at current densities of 84, 210, 420, and 840 mA g1,
Fig. 5. Comparison of galvanostatic charge-discharge profiles (1st, 2nd, 10th, and 50th cycles) of all samples. (a) pure Si, (b) CCSi_#1, (c) CCSi_#2, and (d) CCSi_#3 electrodes.
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Fig. 6. Rate capabilities of pure Si and CCSi_#3 at different current densities.
respectively. Furthermore, the excellent reversibility of CCSi_#3 electrodes is demonstrated by the fact that they recover a capacity of 1116 mA h g1, which is close to their initial capacity (1194 mA h g1) at a current density of 210 mA g1, after 5 cycles at a current density of 4200 mA g1. To understand the outstanding electrochemical performance of CCSi_#3, TEM analysis of the fully delithiated electrodes after 10 cycles was performed, as shown in Fig. 7. The Si particles located inside the Si/C composites (Fig. 7a) maintain their spherical shape, but are slightly altered and shrink in the initial spherical carbon matrix during the charge-discharge process. Likewise, Si particles located on the surface of the Si/C composites (Fig. 7b) exhibit a slightly smaller spherical shape without degradation of the carbon layers, which is transformed into a rough shell. These results demonstrate that pyrolytic carbon layers coated onto Si particles effectively protect the Si anodes from the pulverization caused by volume change and side reactions, including the irreversible reaction between Si and the electrolyte during the charge-discharge process. This imparts stable cycling performance and high coulombic efficiency to the CCSi_#3 electrodes. Furthermore, the carbon matrix has good electrical conductivity, which contributes to the excellent rate capability [34]. This effective carbon coating derived from W-PVB is not only expected to promote the recycling of W-PVB, but also
presents the possibility of exploiting other polymer-based industrial wastes as carbon sources.
4. Conclusions CCSi was synthesized using W-PVB separated from the windshield glass of end-of-life vehicles via simple carbonization at a relative low temperature. As the weight ratio of W-PVB to Si increased (20:1, 40:1, and 100:1), the carbon contents of the CCSi samples increased at a relatively constant rate (27.2, 45.3, and 66.7%) and their electrochemical performances improved. The CCSi_#3 samples, which had a carbon content of 66.7%, exhibited enhanced cycling performance, with a retention of 77.5% after 100 cycles, an excellent rate capability of 910 mA h g1 at a high current density of 840 mA g1, and a high columbic efficiency of ~99% after seven cycles. This enhanced electrochemical performance is attributed to the uniformly coated pyrolytic carbon layers that mitigate both side reactions between Si and the electrolyte, and pulverization induced by volume change during the chargedischarge process. This facile and cost-effective synthesis of CCSi anodes using W-PVB separated from windshield glass demonstrates the possibility of using polymer based-industrial waste as a carbon source.
Fig. 7. TEM images of CCSi_#3 (a) inside the carbon matrix and (b) on the surface of the carbon matrix.
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Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT, and Future Planning (No. 2016R1A2B2012728) and by the R&D Center for Valuable Recycling (Global-Top R&BD Program) of the Ministry of Environment (No. R2-17_2016002250005). Authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this research grant no. (RG#1437-006). FESEM and TEM analyses were performed at Korea Basic Science Institute (KBSI) in Seoul. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.12.242. References [1] M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Metal oxides and oxysalts as anode materials for Li ion batteries, Chem. Rev. 113 (2013) 5364e5457. [2] M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Nano-(V1/2Sb1/2Sn)O4: a high capacity, high rate anode material for Li-ion batteries, J. Mater. Chem. 21 (2011) 10003e10011. [3] C.T. Cherian, M. Zheng, M.V. Reddy, B.V.R. chowdari, C.H. Sow, Zn2SnO4 nanowires versus nanoplates: electrochemical performance and morphological evolution during Li-cycling, ACS Appl. Mater. Inter. 5 (2013) 6054e6060. [4] A.K. Jibin, M.V. Reddy, G.V. Subba Rao, U.V. Varadaraju, M.V.R. Chowdari, Pb3O4 type antimony oxides MSb2O4 (M¼Co, Ni) as anode for Li-ion batteries, Electochim. Acta 71 (2012) 227e232. [5] C.T. Cherian, M.V. Reddy, G.V. Subba Rao, C.H. Sow, B.V.R. Chowdari, Li-cycling properties of nano-crystalline (Ni1-xZnx)Fe2O4 (0 x 1), J. Solid State Electrochem. 16 (2012) 1823e1832. [6] M.V. Reddy, K.Y.H. Kenrick, T.Y. Wei, G.Y. Chong, G.H. Leong, B.V.R. Chowdari, Nano-ZnCo2O4 material proparation by molten salt method and its electrochemical properties for lithium batteries, J. Electrochem. Soc. 158 (2011) A1423eA1430. [7] B. Das, M.V. Reddy, B.V.R. Chowdari, SnO and SnO$CoO nanocomposite as high capacity anode materials for lithium ion batteries, Mater. Res. Bull. 74 (2016) 291e298. [8] W.-J. Zhang, A review of the electrochemical performance of alloy anodes for lithium-ion batteries, J. Power Sources 196 (2011) 13e24. [9] M.T. McDowell, I. Ryu, S.W. Lee, W.C. Wang, W.D. Nix, Y. Cui, Studying the kinetics of crystalline silicon nanoparticle lithiation with in situ transmission electron microscopy, Adv. Mater. 24 (2012) 6034e6041. [10] K.S. Tan, M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Effect of AlPO4-coating on cathodic behaviour of Li(Ni0.8Co0.2)O2, J. Power Sources 141 (2005) 129e142. [11] M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Cathodic behaviour of NiOcoated Li(Ni1/2Mn1/2)O2, Electrochim. Acta 50 (2005) 3375e3382. [12] Y. He, X. Yu, Y. Wang, H. Li, X. Huang, Alumina-coated patterned amorphous silicon as the anode for a lithium-ion battery with high coulombic efficiency, Adv. Mater. 23 (2011) 4938e4941. [13] V.A. Sethuraman, K. Kowolik, V. Srinivasan, Increased cycling efficiency and rate capability of copper-coated silicon anodes in lithium-ion batteries, J. Power Sources 196 (2011) 393e398. [14] B.Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P.A.V. Aken, J. Maier, Reversible storage of lithium in silver-coated three-dimensional macroporous silicon, Adv. Mater. 22 (2010) 2247e2250.
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