Si-based composite interconnected by multiple matrices for high-performance Li-ion battery anodes

Chemical Engineering Journal 381 (2020) 122619

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Si-based composite interconnected by multiple matrices for highperformance Li-ion battery anodes Seung-Su Leea,b, Ki-Hun Nama, Heechul Jungc, Cheol-Min Parka,

T

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a

School of Materials Science and Engineering, Kumoh National Institute of Technology, Gumi, Gyeongbuk 39177, Republic of Korea IT & New Application Battery Center, LG Chem Research and Development Campus, Daejeon 34122, Republic of Korea c Advanced Materials Group, Samsung SDI, Suwon, Gyeonggi 16678, Republic of Korea b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

composite interconnected • Aby Si-based multiple matrices is developed. Si-based composite is fabricated • The by a combination of two simple solidstate methods.

Si-based composite contains Si, a • The Li-inactive Cu Si, and multiple 3

•

carbon-based matrices. The Si-based composite exhibits excellent electrochemical performances.

A R T I C LE I N FO

A B S T R A C T

Keywords: Li-ion batteries Anode materials Si-based anodes Multiple carbon network matrices Nanostructured composite

To obtain anode materials with high performance Li-ion batteries, a nanostructured Si-based composite, including Si, a Li-inactive conducting Cu3Si matrix, and multiple carbon-based matrices (carbon nanotube, graphite, and a pyrolytic carbon coating) is developed by facile step-by-step combination of solid-state synthetic technologies. First, various SixCuy alloys with different weight compositions are synthesized by a simple ballmilling process. Among the SixCuy alloys, Si80Cu20, which is comprised of Si and Cu3Si, displays the highest electrochemical performance. To further improve the electrochemical performance of Si-Cu3Si, an interconnected composite with 1D-structured CNT and 3D-structured graphite, Si-Cu3Si-CNT/G, is prepared via an additional ball-milling process. The Si-Cu3Si-CNT/G composite is finally coated via the pyrolysis of polyvinyl chloride, thus forming the carbon-coated Si-Cu3Si-CNT/G-C composite. This final product, Si-Cu3Si-CNT/G-C, is comprised of well-dispersed nanocrystalline Si and Cu3Si (Li-inactive conducting matrix) within the multiple interconnected carbon matrices. The Si-Cu3Si-CNT/G-C displays excellent electrochemical performance, with a high first reversible capacity of 1237 mA h g−1, a high initial coulombic efficiency of 82.8%, a long cycle durability of 1084 mAh g−1 over 100 cycles, and a high rate capability of ~1000 mAh g−1 at 1C-rate, which confirms its commercial application as a high-performance Si-based anode for Li-ion batteries.

1. Introduction Recently, the demand for Li-ion batteries (LIBs) with high energy densities has been rapidly increasing to meet the needs of energy

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storage systems and electric vehicles [1–7]. Although graphite has been commercialized as an anode in LIBs, its theoretical capacity (LiC6: 372 mAh g−1) falls short of the requirements for higher-energy-density LIB anodes. Therefore, Si-based anodes have been proposed as an

Corresponding author. E-mail address: [email protected] (C.-M. Park).

https://doi.org/10.1016/j.cej.2019.122619 Received 13 May 2019; Received in revised form 8 August 2019; Accepted 23 August 2019 Available online 24 August 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

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alternative high-capacity anode for next-generation LIBs [2,8–11]. Sibased anodes have various advantages, including a high theoretical capacity of 3578 mAh g−1 (for Li3.75Si at room temperature), nontoxicity, appropriate operating potential (Li+/Li), and accessibility due to the abundance of Si in nature. Although Si-based anodes have many advantageous features, they suffer huge volume variations (> 300%) during cycling which results in poor cycle performance. Therefore, a significant amount of research effort is focused on mitigation of this volume expansion during cycling in consideration of next-generation Sibased anodes [12–14]. Various attempts have been made to mitigate the volume expansion of Si-based anodes used in LIBs during cycling. A typical improvement involves enhancing the structural control of the Si-based material. Such structural changes include reducing the crystallite size, applying a carbon coating, as well as the use of porous structures, nanofibers, nanotubes, nanowires, etc [12–30]. These approaches elicit better electrochemical performance than that of counterpart anodes based on pure Si. However, commercial application is restricted because these synthetic materials require a long and expensive manufacturing process. To address these issues, the fabrication of nanostructured composites via simple solid-state methods, particularly those based on heattreatment (HT) and ball milling (BM), has been proposed as a solution for the development of Si-based anodes. The products produced through such synthetic methods also have excellent electrochemical properties [31–37]. In addition, composites fabricated by simple solidstate methods have a higher initial coulombic efficiency (ICE) than those synthesized by chemical methods, because the chemically fabricated composites contain several residual impurities. The ICE is a very important factor in the electrochemical performance of LIB anodes. Because the anode receives Li from the relatively expensive cathode, an anode with low ICE results in losses of the cathode. In general, solidstate synthesis methods can be applied in current production processes, imparting advantages such as enhanced simplicity of the process and the possibility for mass production. Among the transition metals, Cu has various advantageous features, including environmental compatibility, inexpensiveness, and high electronic conductivity. Cu can form several compounds with Si which can consequently be used as conducting Liinactive matrices in composites, because Cu-Si compounds are known to be electrochemically Li-inactive materials [38–43]. Therefore, the use of Cu-Si compounds can complement the poor electrical conductivity and large volume variations of Si, and are thus suitable conductive Li-inactive matrices for the commercialization of Si-based anodes. The purpose of this research is to manufacture a Si-based nanostructured composite with excellent electrochemical performance characterized by high initial reversible capacity (IRC), high ICE, long cycle durability, and high rate capability. Based on the opinions of researchers from representative battery manufacturers (i.e., Samsung SDI and LG Chem.), we established a specific target for the electrochemical performance (IRC: > 1000 mAh g−1; ICE: > 80%; capacity retention after 100 cycles: > 85%), with the focus being suitability for commercialization. Si-Cu alloy (Cu3Si) and various structured carbon-based materials have been adopted to improve the electrochemical performance of Si-based anodes, playing key roles as conducting and buffering matrices, enhancing structural stability during cycling, and supporting better conductivity than that of bulk Si. A combination of simple solid-state synthetic methods was adopted to manufacture a high-capacity Si-based composite anode. Therefore, we herein propose an optimized and practical nanostructured Si-based composite anode for LIBs with superior performance.

Fig. 1. Synthesis of various SixCuy alloys with different weight compositions (Si90Cu10, Si80Cu20, Si70Cu30, and Si60Cu40). XRD spectra for Si90Cu10, Si80Cu20, Si70Cu30, and Si60Cu40 alloys.

Si80Cu20, Si70Cu30, and Si60Cu40) were synthesized by the following solid-state synthetic route. The composition of each of these alloys is marked with a red line in the Si-Cu binary phase diagram presented in Fig. S1. The Si (Aldrich; 99.9%, average size: ~150 μm) and Cu (Kojundo; average size: ~75 μm) powders (Si/Cu = 90:10, 80:20, 70:30, and 60:40 wt%) and stainless steel balls (diameters: 3/8 and 3/16 in.) were placed into a hardened-steel vial (capacity 80 cm3) at a ball-topowder weight ratio of 20:1. The vial was assembled in an Ar-filled glove box, and the BM process (Spex-8000M) was carried out for 6 h under an Ar atmosphere. To obtain the carbon-modified composites, SiCu3Si-carbon black (Si-Cu3Si-CB), Si-Cu3Si-graphite (Si-Cu3Si-G), SiCu3Si-carbon nanotube (Si-Cu3Si-CNT), and Si-Cu3Si-carbon nanotube/ graphite (Si-Cu3Si-CNT/G), the same BM process was carried out for

2. Experimental 2.1. Materials synthesis Four Si-Cu alloys with different weight compositions (Si90Cu10, 2

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Elements); multi-walled carbon nanotubes (CNT; Aldrich). The optimal amounts of the Si80Cu20 alloy and carbon in the composites were found to be 60% and 40% by weight, respectively (for Si-Cu3Si-CNT/G, the optimal ratio of Si80Cu20 alloy to CNT to G was 60:20:20 wt%), as selected based on parameters of their electrochemical performance. Finally, to obtain the carbon-coated composite (Si-Cu3Si-CNT/G-C), the as-prepared Si-Cu3Si-CNT/G powder was placed into a solution of poly (vinyl chloride) (PVC) in acetone. The resultant solution was dried at 70 °C and stirred at 300 rpm for 10 h. To carbonize the PVC, the dried Si-Cu3Si-CNT/G-PVC mixture was pyrolyzed at 700 °C for 3 h under an Ar atmosphere, completing the production of the final carbon-coated composite product Si-Cu3Si-CNT/G-C. Preliminary electrochemical tests showed that the optimal amounts of Si-Cu3Si-CNT/G and pyrolyzed PVC were 90% and 10% by weight, respectively. The Si:Cu3Si:CNT:G:pyrolyzed C ratio of the Si-Cu3Si-CNT/G-C was 38.5:11.5:20:20:10 (wt%), as calculated based on the remaining C after thermogravimetric analysis (TGA) of PVC (Fig. S2a). 2.2. Material characterization The synthesized samples were characterized by X-ray diffraction (XRD; X-Max/2000-PC with a Cu Kα irradiation source), high-resolution TEM (JEM ARM 200F, JEOL, operated at an accelerating voltage of 200 kV), and EDS (attached to the TEM). To observe the reaction mechanism in the electrodes with Li, ex situ analysis using extended X-ray absorption fine structure (EXAFS)was performed. Cu K-edge EXAFS measurements for the Cu3Si alloy were performed on an 8C-nano XAFS beamline instrument (storage ring: 3.0 GeV) at the Pohang Light Source (PLS, Republic of Korea). The particle sizes and morphologies of the synthesized samples were analyzed using a particle size analyzer (PSA; Mastersizer 2000, Marlern Panalytical) and by scanning electron microscopy (SEM; SNE-4500M). The Brunauer–Emmett–Teller (BET) surface area was determined using a 3FLEX (Micrometritics). Additionally, eletrochemical impedance spectroscopy (EIS) measurements were conducted using a ZIVE-MP2A analyzer (WonATech) over a frequency range from 100 kHz to 10 mHz to compare the electrical conductivities of various samples. 2.3. Electrochemical measurements For electrochemical evaluation of the electrodes, all electrodes were prepared by coating a slurry consisting of the active powder material (80 wt%), CB (Denka Black, 10 wt%) as a conducting agent, and polyvinyl alcohol-polyacrylic acid (PVA-PAA, 10 wt%) in water as a binder. Samples of each mixture were vacuum-dried at 120 °C for 3 h, and the as-prepared electrodes were pressed using a roll-press. The average loadings of Si-based electrodes were approximately 3.0 ± 0.2 mg cm−2 (average active material weight: 2.4 mg, electrode area: 0.79 cm2). Coin-type electrochemical cells were assembled in an Ar-filled glove box using Celgard 2400 as the separator, Li foil as the counter and reference electrodes, and 1 M LiPF6 in ethylene carbonate/ diethyl carbonate (1:1 vol%) with 10% fluoroethylene carbonate (Panax STARLYTE) as the electrolyte. All the cells were tested galvanostatically between 0 and 2 V (vs. Li+/Li) at a current density of 200 mA g−1 using a Maccor automated tester, except for the rate capability tests. Cyclic voltammetry (CV) curves were recorded using a ZIVE-MP2A (voltage range: 0–2 V vs. Li+/Li, scan rate: 0.1 mV s−1). Li was inserted into the electrode during discharging and was extracted from the working electrode during charging. A full cell was assembled using Si-Cu3Si-CNT/G-C anode and LiCoO2 (LCO) cathode with the same condition of half-cell test. LCO cathode was fabricated via LCO powder, conductive carbon agent, and PVDF at a 90:5:5 wt ratio, dissolved in NMP. The full cell was designed with a negative/positive (N/ P) ratio of 1.1. The full cell was cycled with 3.0–4.2 V at 0.1C (1C = 150 mA g−1).

Fig. 2. Electrochemical properties of various SixCuy alloy (Si90Cu10, Si80Cu20, Si70Cu30, and Si60Cu40) electrodes at a current density of 200 mA g−1. Voltage profiles for the 1st and 2nd cycles for the Si90Cu10, Si80Cu20, Si70Cu30, and Si60Cu40 alloys. Table 1 Electrochemical data for the bulk Si and various SixCuy (Si90Cu10, Si80Cu20, Si70Cu30, and Si60Cu40; by wt%) alloys. Electrode

1st discharge/charge capacity [mAh g−1]

2nd discharge/charge capacity [mAh g−1]

Initial coulombic efficiency [%]

Bulk Si Si90Cu10 Si80Cu20 Si70Cu30 Si60Cu40

3880/2615 2945/2307 2749/2359 2454/2062 1857/1594

2507/2115 2070/1271 2247/1923 1821/1573 1517/1299

67.4 78.3 85.8 84.0 85.4

10 min for each of the carbon-modified composites, using the fabricated Si-Cu3Si and various carbon sources: carbon black (CB) as Super P (Alfa Aesar); graphite (G) as mesocarbon microbeads (MCMB; American

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Fig. 3. Preparation and electrochemical properties of various carbon-modified Si-Cu3Si-based composites. (a) XRD data for Si-Cu3Si-CB, Si-Cu3Si-CNT, and Si-Cu3SiG. (b) Voltage profiles from 1st to 30th cycle for Si-Cu3Si-CB, Si-Cu3Si-CNT, and Si-Cu3Si-G (current density: 200 mA g−1). Table 2 Electrochemical data for the various carbon modified (Si-Cu3Si-CB, Si-Cu3Si-CNT, Si-Cu3Si-G, and Si-Cu3Si-CNT/G, and Si-Cu3Si-CNT/G-C) composites. Electrode

1st discharge capacity [mAh g−1]

1st charge capacity [mAh g−1]

Initial Coulombic efficiency [%]

Capacity retention after Xth cycle [%]

Si-Cu3Si-CB Si-Cu3Si-CNT Si-Cu3Si-G Si-Cu3Si-CNT/G Si-Cu3Si-CNT/G-C

1946 1988 1999 2051 1494

1563 1584 1652 1697 1237

80.3 79.7 82.6 82.7 82.8

33.2 68.1 79.1 84.3 94.9

3. Results and discussion

(X = 30) (X = 30) (X = 30) (X = 30) (X = 50)

which is accompanied by a large volume change, resulting in the collapse of the active material and a subsequent short circuit to the current collector. Fig. S4a shows the XRD pattern of Cu3Si synthesized at the stoichiometric molar ratio (Cu:Si = 3:1, mol%) by BM. The Cu3Si did not react with Li (Fig. S4b), which was also demonstrated by the absence of variations in the ex-situ Cu K-edge EXAFS results during discharge/charge (Fig. S4c). These results demonstrate that the Cu3Si plays a role as a conducting Li-inactive matrix in the SixCuy alloys. As shown in Fig. 2 and Table 1, the Si90Cu10 electrode exhibited a high IRC of 2307 mAh g−1 with a 78.3% ICE, but the reversible capacity decreased significantly to 1271 mAh g−1 in the next cycle. The Si80Cu20 electrode exhibited a high IRC of 2359 mAh g−1 with a high ICE of 85.8%, and a high reversible capacity of 1923 mAh g−1 in the next cycle. The Si70Cu30 and Si60Cu40 electrodes exhibited relatively high ICEs of 84.0% and 85.4%, but their IRCs significantly reduced to 2062 and 1594 mAh g−1, both of which are lower than that of the Si80Cu20. These electrochemical ICEs and IRCs were caused by the increased contents of Li-inactive Cu3Si and enhanced electronic conductivity in these alloys. Although all the SixCuy alloys exhibited better electrochemical performances than that of the bulk Si, Si80Cu20 (Si-Cu3Si) was selected as the starting material for the optimized high-capacity Sibased composite anode because it showed the highest ICE, IRC, and

Various binary compounds such as Cu3Si, Cu4Si, and Cu5Si are shown in the binary Si-Cu phase diagram as shown in Fig. S1 [44–46]. On the basis of the Si-Cu phase diagram, four SixCuy alloys with different weight compositions were synthesized using a simple solid-state BM process. Their compositions are marked with red lines on the Si-Cu binary phase diagram (Fig. S1). Fig. 1 shows the XRD patterns of the synthesized Si90Cu10, Si80Cu20, Si70Cu30, and Si60Cu40 alloys. All of the XRD patterns only exhibited peaks corresponding to crystalline Cu3Si and Si phases. As the Si content decreased, the Si peaks diminished in intensity, and all the Cu was transformed to Cu3Si without generating any other impurities. The four synthesized SixCuy alloys were tested galvanostatically at the constant current rate of 200 mA g−1 over the potential range of 0–2.0 V. Fig. 2 shows the voltage profiles of Si90Cu10, Si80Cu20, Si70Cu30, and Si60Cu40 alloys, for which certain electrochemical parameters for the first and second cycles are also compared with those of bulk Si in Table 1. The bulk Si exhibited a high IRC of 2615 mAh g−1 with a poor ICE of 67.4%, but its capacity decreased dramatically within 10 cycles (Fig. S3). The poor cycle durability of bulk Si was caused by the formation of Li3.75Si at room temperature during cycling, 4

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Fig. 4. Preparation, electrochemical properties, and morphological characteristics of Si-Cu3Si-CNT/G. (a) XRD profile for Si-Cu3Si-CNT/G. (b) Voltage profiles from 1st to 30th cycle for Si-Cu3Si-CNT/G (current density: 200 mA g−1). (c) Bright-field TEM image. (d) High-resolution TEM image with corresponding SAED patterns. (e) Scanning TEM image with corresponding EDS elemental mapping images (Si: red; Cu: yellow; C: blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

indicating the amorphous nature of 0D-CB. Meanwhile, the XRD patterns of Si-Cu3Si-CNT and Si-Cu3Si-G exhibited carbon (graphite) peaks, which confirmed that the composites were fabricated successfully. The electrochemical performances of Si-Cu3Si-CB, Si-Cu3Si-CNT, and SiCu3Si-G are compared in Fig. 3b and Table 2. Although all composite electrodes exhibited superior electrochemical performance in comparison to that of the Si-Cu3Si alloy, Si-Cu3Si-CNT and Si-Cu3Si-G performed better than Si-Cu3Si-CB. The initial discharge/charge capacities of Si-Cu3Si-CB, Si-Cu3Si-CNT, and Si-Cu3Si-G were nearly similar values. However, the capacity retentions after the 30th cycle for Si-Cu3SiCB, Si-Cu3Si-CNT, and Si-Cu3Si-G were 33.2, 68.1, and 79.1% of their IRCs, respectively, which demonstrates that the use of 1D-CNT and 3D-

second reversible capacity among the SixCuy alloys. Recently, nanostructured Si-based composites modified by the addition of various carbon-based materials were reported to exhibit excellent electrochemical performances [31,47–55]. Interestingly, the morphologies of CB, CNT, and G featured 0-dimensional (0D), 1-dimensional (1D), and 3-dimensional (3D) structures, respectively, as shown in Fig. S5. Therefore, 0D-CB-, 1D-CNT-, and 3D-G-modified SiCu3Si composites, herein denoted as Si-Cu3Si-CB, Si-Cu3Si-CNT, and SiCu3Si-G, respectively, were produced by a simple BM process. Fig. 3a shows the XRD patterns of the Si-Cu3Si-CB, Si-Cu3Si-CNT, and Si-Cu3SiG composites produced. The XRD pattern of 0D-CB-modified Si-Cu3SiCB did not change from that of the non-carbon-modified counterpart, 5

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evenly dispersed and interconnected by the 1D-CNT and 3D-graphite carbon matrices (Fig. 4c, d). The dark-field TEM and its corresponding EDS elemental mapping images also confirmed that the Si and Cu3Si nanocrystallites were evenly dispersed within the multiple carbon matrices (Fig. 4e). Based on the results of these analyses, it was concluded that the nanocrystalline (~10 nm) Si and Cu3Si were successfully physically integrated with the multiple carbon matrices. To further investigate the enhanced electrochemical performances of Si-Cu3Si-CB, Si-Cu3Si-CNT, Si-Cu3Si-G, and Si-Cu3Si-CNT/G, EIS analysis results and changes to electrode thickness are shown in Fig. 5a and b, respectively. EIS results for the Si-Cu3Si-CB, Si-Cu3Si-CNT, SiCu3Si-G, and Si-Cu3Si-CNT/G electrodes exhibited smaller semicircles in comparison to that of the Si-Cu3Si (Fig. 5a), which confirms that the carbon-modified composites prepared by BM have enhanced electrical conductivity. Although the EIS results for Si-Cu3Si-CB, Si-Cu3Si-CNT, SiCu3Si-G, and Si-Cu3Si-CNT/G were similar, a slightly improved electrical conductivity in the Si-Cu3Si-CNT and Si-Cu3Si-CNT/G was observed, which means that the CNT matrix affects a little more to the improvement of electrical conductivity than other carbon matrices. Additionally, changes to the electrode thickness during discharge/ charge are compared in Fig. 5b. At a fully discharged state (0 V, 100% state of charge (SOC)), the Si-Cu3Si, Si-Cu3Si-CB, Si-Cu3Si-CNT, SiCu3Si-G, and Si-Cu3Si-CNT/G electrodes swelled by 110.3%, 60.2%, 57.4%, 55.1%, and 48.0% relative to their original states. At a fully charged state (2.0 V, 0% SOC), the thicknesses of the Si-Cu3Si, Si-Cu3SiCB, Si-Cu3Si-CNT, Si-Cu3Si-G, and Si-Cu3Si-CNT/G electrodes were swollen by 50.0, 16.1, 10.6, 10.5, and 9.5% relative to their initial states, respectively. Generally, the carbon-modified composites showed smaller variations than that of Si-Cu3Si, which confirms that the carbon matrices are highly effective in mitigating the large volume variations of Si during cycling. Additionally, the Si-Cu3Si-CNT/G with interconnected 1D-CNT and 3D-G carbon matrices exhibited the smallest degree of volume variation during cycling, which demonstrates that the use of multiple carbon matrices is ideal for suppressing the volume expansion of Si. Although the Si-Cu3Si-CNT/G electrode showed good electrochemical performance, it did not meet our specific target for commercialization (IRC > 1000 mAh g−1; ICE > 80%; capacity retention after 100 cycles > 85%). The pyrolytic carbon coating method is well known for enhancing the electrochemical performance of Si-based anodes [58–62]. Therefore, a pyrolytic carbon coating (using PVC) was applied to the Si-Cu3Si-CNT/G, thus fabricating the pyrolyzed C-coated Si-Cu3Si-CNT/G (Si-Cu3Si-CNT/G-C). Among the various pyrolytic carbon sources available, PVC was selected because it has excellent electrical conductivity when pyrolyzed at high temperatures. As shown in Fig. S2a, b, the carbonization of pyrolyzed PVC was observed above ~500 °C, and the XRD pattern confirmed the amorphous nature of the pyrolyzed PVC after HT at 700 °C for 3 h. Additionally, the pyrolyzed PVC exhibited relatively stable electrochemical performance (Fig. S2c). The three-step procedure for fabrication of the Si-Cu3Si-CNT/G-C is schematically illustrated in Fig. 6a. In the first step, Si-Cu3Si alloy particles were produced by a simple BM process. Si-Cu3Si was further subjected to a BM process with multiple carbon matrices (1D-CNT and 3D-graphite), thus producing the Si-Cu3Si-CNT/G. Lastly, the Si-Cu3SiCNT/G was pyrolyzed with PVC, thereby yielding the final product, SiCu3Si-CNT/G-C. The XRD pattern of Si-Cu3Si-CNT/G-C is shown in Fig. 6b. All the peaks corresponded to Si, Cu3Si, and C, and no other crystalline phases were detected, indicating that the composite was effectively prepared through the simple pyrolysis carbon-coating process. The EIS results for bulk Si and the various Si-based composites are compared in Fig. 6c. The EIS spectrum of Si-Cu3Si-CNT/G-C exhibited the smallest semicircle (Fig. 6c), which confirms that the carboncoating via pyrolysis contributed significantly to the enhanced electrical conductivity. Additionally, the linear relationship between the Warburg impedance and the inverse square root of angular frequency in low frequencies is shown in Fig. S7, and the slopes of the fitted lines are

Fig. 5. EIS and electrode thickness variation during Li insertion/extraction for various Si-Cu3Si-based composites. (a) Comparison of EIS data for Si-Cu3Si, SiCu3Si-CB, Si-Cu3Si-CNT, Si-Cu3Si-G, and Si-Cu3Si-CNT/G electrodes. (b) Electrode thickness changes according to the state of charge (SOC) for Si-Cu3Si, Si-Cu3Si-CB, Si-Cu3Si-CNT, Si-Cu3Si-G, and Si-Cu3Si-CNT/G electrodes.

G carbon in the composite was more effective than the use of 0D-CB in improving the cycling durability. Based on these results, a Si-based composite (Si-Cu3Si-CNT/G) with multiple interconnected carbon matrices including 1D-CNT and 3D-G was prepared. The XRD pattern of Si-Cu3Si-CNT/G only exhibited peaks corresponding to Si, Cu3Si, and carbon, with no other impurity peaks, which confirms that the composite comprised of Si, Cu3Si, CNT, and G was fabricated successfully (Fig. 4a). Interestingly, the Si-Cu3Si-CNT/G electrode exhibited better electrochemical performance than Si-Cu3SiCB, Si-Cu3Si-CNT, and Si-Cu3Si-G (Fig. 4b and Table 2). The Si-Cu3SiCNT/G showed an IRC of 1697 mAh g−1 with an excellent ICE of 82.7%. Additionally, the first discharge/charge capacities of BM-treated CNT and graphite were 845/394 mAh g−1 and 854/404 mAh g−1, and exhibited relatively stable cycling durability (Fig. S6a and b). Considering the irreversible capacities corresponding to the proportions of BM-treated CNT and graphite in the Si-Cu3Si-CNT/G, the Si in the SiCu3Si-CNT/G underwent a highly reversible reaction with Li. Furthermore, the capacity retention after 30 cycles was 84.3%, which is significantly higher than those of the other composites. This enhanced capacity retention was attained by the employment of nanosized Si and multiple carbon matrices including 1D-CNT and 3D-G [56,57]. The bright-field TEM and high-resolution TEM (with corresponding selected-area electron diffraction (SAED)) images of Si-Cu3Si-CNT/G are shown in Fig. 4c–e. The bright-field and high-resolution TEM images confirmed that the ~10 nm-sized Si and Cu3Si nanocrystallites were

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Fig. 6. Schematic representation and characteristics of Si-Cu3Si-CNT/G-C. (a) Schematic representation of the three-step fabrication process for Si-Cu3Si-CNT/G-C. (b) XRD data for Si-Cu3Si-CNT/G-C. (c) Comparison of EIS data for bulk Si, Si-Cu3Si, Si-Cu3Si-CNT/G, and Si-Cu3Si-CNT/G-C electrodes. (d) Bright-field TEM image. (e) High-resolution TEM image with corresponding SAED patterns. (f) Scanning TEM image with corresponding EDS elemental mapping images (Si: red; Cu: yellow; C: blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the Warburg constant σ for the samples [63,64]. The results are well agreement with those of calculated Li diffusion coefficient values of Sibased composites (Si: 4.31 × 10-16 cm2 S-1, Si-Cu3Si: 1.16 × 1015 cm2 S-1, Si-Cu3Si-CNT/G: 6.03 × 10-14 cm2 S-1, Si-Cu3Si-CNT/G-C: 3.22 × 10-13 cm2 S-1). Bright-field and high-resolution TEM images of Si-Cu3Si-CNT/G-C are shown in Fig. 6d, e, along with the corresponding SAED patterns. The bright-field TEM image confirmed that the Si-Cu3Si alloy particles interconnected by the multiple carbon matrices of 1DCNT and 3D-G were covered with an amorphous C coating layer. The dark-field TEM and EDS elemental mapping images also confirmed that the nano-sized (~10 nm) Si and Cu3Si nanocrystallites were well

dispersed within the multiple carbon matrices (Fig. 6f). Fig. S8a–c shows the SEM image, PSA, and BET results of the Si-Cu3Si-CNT/G-C, respectively, which confirms that its average particle size was ~10 μm and BET surface area was ~62.8 m2 g−1. Based on the results of this analysis, it was concluded that the Si-Cu3Si-CNT/G-C composite was fabricated successfully and that the Si, Cu3Si, CNT, G, and amorphous carbon phases were well-integrated physically. Fig. 7a shows the voltage profiles of Si-Cu3Si-CNT/G-C at a constant current density of 200 mA g−1. Si-Cu3Si-CNT/G-C exhibited a high IRC of 1237 mAh g−1 with a high ICE of 82.8% (Table 2). Notably, the high ICE accompanied by the IRC above 1000 mAh g−1 for Si-Cu3Si-CNT/G7

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Fig. 7. Electrochemical performance of Si-Cu3Si-CNT/G-C. (a) Voltage profiles from 1st to 100th cycle for Si-Cu3Si-CNT/G-C (current density: 200 mA g−1). (b) ICE and capacity retention for bulk Si, Si-Cu3Si, Si-Cu3Si-CNT/G, and Si-Cu3Si-CNT/G-C electrodes. (c) Changes to the thickness of bulk Si, Si-Cu3Si, Si-Cu3Si-CNT/G, and Si-Cu3Si-CNT/G-C electrodes according to the SOC. (d) Cycling behavior of graphite (cycling rate: 100 mA g−1), bulk Si (cycling rate: 200 mA g−1), and Si-Cu3SiCNT/G-C (cycling rate: 200 mA g−1). (e) Rate capabilities of graphite (MCMB; 1C-rate: 300 mA g−1) and Si-Cu3Si-CNT/G-C (1C-rate: 1200 mA g−1).

(above 99%, except in the initial few cycles). The capacity retentions after 50 and 100 cycles were approximately 94.9% and 88.0% of the IRC. The excellent IRC, ICE, and capacity retention were attributed to the uniform dispersion of the nano-sized (~10 nm) Li-active Si nanocrystallites, the conducting and Li-inactive Cu3Si matrix which prevented agglomeration of the Si nanocrystallites during discharge/ charge, and the buffering effect of the multiple carbon-based matrices on the large volume change. The rate capabilities of Si-Cu3Si-CNT/G-C and commercial graphite were also evaluated, as shown in Figs. 7e and S11 (1C-rate was herein defined as 1200 mA g−1). Even at the high rates of 1C (1200 mA g−1) and 2C (2400 mA g−1), the Si-Cu3Si-CNT/GC exhibited highly reversible capacities of approximately 1000 and 750 mAh g−1, which were superior to those of the commercially available graphite. The high rate capability originated from the welldispersed nanocrystalline Si, which resulted in a short Li-ion diffusion path. Additionally, the employment of the conducting and Li-inactive Cu3Si matrix and CNT, graphite, and pyrolyzed PVC carbon-based matrices also contributed to the high rate capability. Finally, to examine the practical potential of Si-Cu3Si-CNT/G-C anode, a full cell was fabricated using LCO cathode, and its suitability was tested at 0.1 C-rate (Fig. S12). The full cell showed a relatively high areal capacity of ~2.1 mA h cm−2 with an appropriate potential range of 3.0–4.2 V and stable cycling behavior over 50 cycles.

C are very high values reported among bulk-type Si-transition metalbased materials for LIB anodes [38–44,65–67]. Considering the irreversible capacities corresponding to the proportions of BM-treated CNT (20 wt%, Fig. S6a), BM-treated graphite (20 wt%, Fig. S6b), and pyrolyzed PVC (10 wt%, Fig. S2c) in the Si-Cu3Si-CNT/G-C, it could be concluded that the Si in the Si-Cu3Si-CNT/G-C underwent a fully reversible reaction with Li. The 1st and 2nd CV curves for the Si-Cu3Si and Si-Cu3Si-CNT/G-C electrodes are compared in Fig. S9. Although the CV peaks of Si-Cu3Si-CNT/G-C were well matched with those of SiCu3Si, the CV peaks were smoothened by employment of the multiple carbon-based matrices. Furthermore, the Si-Cu3Si-CNT/G-C showed an excellent capacity retention (Fig. 7a, b, and d). The ICE and capacity retention for the bulk Si, Si-Cu3Si (fabricated by the 1st step), Si-Cu3SiCNT/G (fabricated by the 2nd step), and Si-Cu3Si-CNT/G-C (fabricated by the 3rd step) are summarized in Fig. 7b. The Si-Cu3Si-CNT/G-C showed a high IRC of 1237 mAh g−1, a high ICE of 82.8%, and stable capacity retention of 88% after 100 cycles. All of these electrochemical parameters (IRC, ICE, and capacity retention) met our specific target for commercialization as a high-capacity Si-based anode for LIBs. Changes in the thickness of bulk Si, Si-Cu3Si, Si-Cu3Si-CNT/G, and Si-Cu3SiCNT/G-C electrodes during discharge/charge are indicated in Figs. 7c and S10. At the fully discharged state (0 V, 100% SOC), the Si-Cu3SiCNT/G and Si-Cu3Si-CNT/G-C electrodes swelled by 48.0% and 39.5%, respectively, to a much lesser degree than the bulk Si (140.4%) and SiCu3Si (110.3%) electrodes. At the fully charged state (2.0 V, 0% SOC), the Si-Cu3Si-CNT/G and Si-Cu3Si-CNT/G-C electrodes were swollen by only 9.5% and 8.3%, respectively, relative to their initial states. The cycling performance of the bulk Si, commercial graphite, and Si-Cu3SiCNT/G-C (voltage range 0–2 V; cycling rate 200 mA g−1) were further compared, as shown in Fig. 7d. The cycling performance and ICE of the bulk Si were very poor, which was caused by mechanical cracking and crumbling resulting from the large volume change due to formation of the Li3.75Si phase. Si-Cu3Si-CNT/G-C exhibited stable cycling behavior over 100 cycles and high coulombic efficiency with repeated cycling

4. Conclusion The novel composite Si-Cu3Si-CNT/G-C has been proposed as a high-capacity Si-based anode material for LIBs. The Si-Cu3Si-CNT/G-C was fabricated by a combination of two simple solid-state synthesis methods (BM and HT), which makes mass production feasible. Within the composite, multiple matrices were interconnected. The nanocrystalline Cu3Si contributed to improving the IRC and ICE via its role as a conducting and Li-inactive matrix. The CNT and graphite contributed to improved cycling durability and electrical conductivity by forming an 8

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interconnected network of different forms of carbon. Additionally, the C-coating using pyrolyzed PVC served to further enhance electrical conductivity and act as a buffering matrix, contributing to improved electrochemical cycle durability. The Si-Cu3Si-CNT/G-C electrode exhibited excellent electrochemical performance, with a high IRC of 1237 mAh g−1, excellent ICE of 82.8%, high capacity retention after 100 cycles of 88.0%, and high rate capability of approximately 1000 mAh g−1 at the 1C-rate. All the electrochemical parameter values satisfied the criteria set within our specific target for commercialization (IRC > 1000 mAh g−1; ICE > 80%; capacity retention after 100 cycles > 85%). These excellent electrochemical properties were achieved based on the conducting Li-inactive matrix composed of welldispersed Cu3Si nanocrystallites among nanocrystalline Si, the multiple carbon network matrices containing variously structured carbons such as 1D-CNT and 3D-graphite, and the carbon coating that contributed to additional buffering and conductivity enhancement. As a result, SiCu3Si-CNT/G-C is a very promising composite material for use as a high-capacity anode for next-generation Si-based LIBs.

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