Novel strategy to improve the Li-storage performance of micro silicon anodes

Journal of Power Sources 348 (2017) 302e310

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Novel strategy to improve the Li-storage performance of micro silicon anodes Min-Jae Choi a, 1, Ying Xiao a, 1, Jang-Yeon Hwang a, Ilias Belharouak b, c, **, Yang-Kook Sun a, * a b c

Department of Energy Engineering, Hanyang University, Seoul, 133-791, Republic of Korea Qatar Environment and Energy Research Institute (QEERI), Hamad bin Khalifa University (HBKU), Qatar Foundation, Doha, Qatar College of Science and Engineering, Hamad bin Khalifa University (HBKU), Doha, Qatar

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


We proposed a high performance micro silicon as anode for lithium-ion batteries.
The designed CNT-Si structure alleviated volume change of micro Si during cycling.
The CNT-Si electrode exhibited remarkable tap density and battery performance.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 December 2016 Received in revised form 3 March 2017 Accepted 4 March 2017

Silicon (Si)-based materials have attracted significant research as an outstanding candidate for the anode material of lithium-ion batteries. However, the tremendous volume change and poor electron conductivity of bulk silicon result in inferior capacity retention and low Coulombic efficiency. Designing special Si with high energy density and good stability in a bulk electrode remains a significant challenge. In this work, we introduce an ingenious strategy to modify micro silicon by designing a porous structure, constructing nanoparticle blocks, and introducing carbon nanotubes as wedges. A disproportion reaction, coupled with a chemical etching process and a ball-milling reaction, are applied to generate the desired material. The as-prepared micro silicon material features porosity, small primary particles, and effective CNT-wedging, which combine to endow the resultant anode with a high reversible specific capacity of up to 2028.6 mAh g1 after 100 cycles and excellent rate capability. The superior electrochemical performance is attributed to the unique architecture and optimized composition. © 2017 Elsevier B.V. All rights reserved.

Keywords: Micro Si Nanoscale primary particles High tap density High capacity Lithium-ion batteries

1. Introduction

* Corresponding author. ** Corresponding author. Qatar Environment and Energy Research Institute (QEERI), Hamad bin Khalifa University (HBKU), Qatar Foundation, Doha, Qatar. E-mail addresses: [email protected] (I. Belharouak), [email protected] (Y.-K. Sun). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2017.03.020 0378-7753/© 2017 Elsevier B.V. All rights reserved.

Despite being the leading technology for rechargeable batteries, lithium-ion batteries (LIBs) still fall short of the energy density and long-term stability with regard to applications in commercially available electric vehicles and the future generations of mobile electronics [1e3]. The most commonly used graphite anode has a low theoretical capacity of 372 mAh g1, which limits the Li-storage performance in terms of energy and power density. Therefore, in

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order to enhance the performance of LIBs and subsequently promote their range of applications in energy storage especially in electric vehicles (EVs) and plus-in hybrid electric vehicles (PHEVs), intensive research into advanced anodes with high tap density, high capacity, high rate capability, and durability is required. By virtue of its extremely high theoretical gravimetric capacity (3579 mAh g1 at room temperature, corresponding to Li15Si4), large volumetric capacity (2081 mAh cm3), relatively low operating voltage (ca. 0.5 V vs. Li/Liþ), abundant availability, and environmental benignity, silicon (Si) has been identified as a promising candidate and has attracted remarkable attention as an anode for LIBs [4e6]. However, the practical application of Si anodes is impeded by their unstable behavior originating from several disadvantages. Specifically, the slow Li-ion diffusion and poor electron conductivity of Si generally lead to poor rate capability. Additionally, the large volume change associated with repeated Li insertion and extraction into the Si structure (ca. 300% for Li15Si4) results in electrode failure via pulverization of the active material, loss of electrical contact with the current collectors, continuous instability of the solid electrolyte interfacial (SEI) layer, or some combination of these failings [4e8]. The cycling performance of Si-based anodes is still far from satisfactory from a commercial point of view. Thus, it is necessary to develop an electrode formulation that can overcome all of these issues. Until now, many strategies, including compositing with inactive components [9,10]. Designing novel structures (e.g., porous structures, hollow structures, core-shell or yolk-shell structures, 1D structures) [8,11e15]. reducing the Si size [16], and heteroatomdoping [17], have been developed to overcome the abovementioned critical issues. For instance, Wang et al. prepared a nano-Si/carbon composite fiber paper through electrospraying and electrospinning methods [11]. When this composite was applied as an anode in LIBs, a high capacity of 1600 mAh g1 after 600 cycles was retained. Such excellent performance was mainly attributed to the presence of the conductive carbon fiber network, carboncoated Si nanoparticles, and the strong adhesion between carbon and Si nanoparticles. Cui et al. designed a non-filling, carbon-coated porous silicon microparticle that consisted of many interconnected primary nanoparticles [10]. By virtue of its porous structure, small primary particles, and novel carbon coating, the prepared Si-based anode exhibited remarkable cycling stability with high reversible specific capacity of 1500 mAh g1 after 1000 cycles at 0.25 C. Dou et al. designed a yolk-shell-structured Si@C anode to alleviate the mechanical and chemical stability issues [13]. When applied as an anode in lithium-ion batteries, a high capacity of ca. 1100 mAh g1 was delivered after 200 cycles at 250 mA g1. Bradford et al. presented an aligned CNT-Si sheet/C structure, which served as a freestanding, binder-free, and flexible anode [15]. This novel structure provides enough free space to accommodate silicon expansion; as a result, a high capacity of ca. 1494 mAh g1 can be delivered after 45 cycles at 100 mA g1. However, most of the reported works have utilized either nano Si-based anodes with relatively low tap density or microelectrodes with unsatisfactory capacity and poor cyclic life, which will not be beneficial for practical applications. Therefore, efficient approaches must be developed to improve the overall performance of silicon electrodes. Generally, micro materials can introduce high tap density and generate high volumetric capacity when they are utilized as electrodes for LIBs. However, more serious disintegration and long ion/ electron transport pathways will occur within micro anodes [18,19]. Conversely, nanoscale structure with lower tap density can reduce the transport path lengths and alleviate the volume expansion of the electrode more efficiently [20e22]. Considering of these merits and drawbacks, micro anode materials composed of nanoscale primary blocks should be able to harness these advantages while

303

avoiding the above-mentioned shortcomings. As a consequence, good cycling stability, high energy density, and high tap density are expected to be achieved, which would be favorable for practical applications. Based on these facts, Wang's group reported a series of works investigating the performance of micro Si anodes [5,18,23]. Related results verified that micro Si, consisting of nanoparticles, can obtain impressive electrochemical performance with high tap density. Besides, carbon nanotubes (CNTs) are considered as an efficient modification and have been widely used in energy storage and conversion area [24e27]. Comparing with other modification materials such as traditional carbon coating, CNTs manifest more appealing characteristics including enhanced three-dimensional electro conductivity networks and strong mechanical strength, decreased diffusion resistance of high aspect ratio structure, excellent connection with host materials, and abundance active activated areas etc. [24,26]. These features will ensure the well electrochemical contact between the electrode and the current collector, fast transportation of ions and electrons, and improved electro conductivity, when applied in lithium-ion batteries. However, the effective hybrid of CNTs with host electrode is always not easy to realize like the commonly used carbon coating [27]. Inspired by the abovementioned works, in this paper, we present a strategy that utilizes CNT-wedging, porous engineering, and the construction of nanoscale units to modify micro Si anodes. A disproportion process, chemical etching, and a Spex-mill treatment were applied to prepare the target material. The porous structure facilitates the immersion of the electrolyte within the micro Si anode and is beneficial for the diffusion of lithium ion [10,23]. The micro-morphology, constructed with small particles, endows the resultant electrode with a large tap density and simultaneously reduces the transport length of electrons [21,28e31]. The wedgedCNTs act as a mechanically strong and flexible buffer during deep galvanostatic cycling, enabling the microparticles to expand and fracture within a small area while retaining their electrical connectivity [32,33]. Most of all, in terms of practical aspect, this novel structure delivers great merits for increasing an energy density, which shows appealing performance including higher initial Coulombic efficiency and specific capacity, better capacity retention and more excellent rate capability among the previous reported micro Si anodes [30,34,35]. Specifically, a high specific capacity of 2028.6 mAh g1, corresponding to a high volumetric capacity of ca. 2607 mAh cm3, can be obtained after 100 cycles, demonstrating the great promise of this material for use in practical applications. 2. Experimental 2.1. Synthesis of micro porous Si, micro porous Si-C, CNT-wedged micro porous Si First, micro porous Si was prepared via deep annealing coupled with an etching method. Typically, commercially-available SiO was used as the raw material and was subjected to a disproportion reaction at 970  C for 30 h under an Ar/H2 (4 vol%) atmosphere. Then, the formed Si/SiO2 powder was collected and immersed in a hydrofluoric acid (HF) solution for 4 h to remove the SiO2 particles. After washing and drying, micro porous Si can be obtained. In order to fabricate the CNT-wedged micro porous Si (denoted as CNT-Si), the prepared porous Si powder was well mixed with CNTs (15 wt %) using a Spex mill for 30 min at a frequency of 20 Hz. As a comparison, micro porous Si coated with carbon was prepared by a similar method. Specifically, pitch was used as the carbon source, and SiO-pitch (10 wt%) was mixed and calcined at 970  C for 30 h under an Ar/H2 (4 vol%) atmosphere. After etching with the HF solution, the pitch-coated porous Si (denoted as C-Si) was generated.

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2.2. Material characterization The composition and phase of the samples were characterized by X-ray diffraction (XRD, SmartLab, Rigaku) using Cu Ka radiation with a scan step of 4 min1. The microstructures of the samples were investigated by scanning electron microscopy (SEM, NOVA NANO SEM 450) and transmission electron microscopy (TEM, JEOL 2100F). Raman spectra were recorded by an Invia Raman spectrometer using an excitation laser of 514.5 nm. X-ray photoelectron spectra (XPS, PHI 5000 VersaProbe) were applied to analyze the surface composition of the samples. Quantachrom Autosorb-1 instrument was applied to obtain the BrunauereEmmetteTeller surface areas of the as-synthesized sample. 2.3. Electrochemical measurement The working anode was fabricated by mixing 60 wt% active material, 20 wt% super P, and 20 wt% poly(acrylic acid) (PAA) binder. The obtained uniform slurry was coated onto Cu foil, rollpressed, and dried at 80  C for 6 h. For the cell assembly process, 1.0 M LiPF6 in ethylene carbonateediethyl carbonate (EC: DEC, 1: 1 by vol%) with 10 wt% fluoroethylene carbonate (FEC) was selected as the electrolyte. Li metal was used as the counter electrode. Electrochemical testing was carried out using a CR2032 coin-type cell in the voltage window of 0.01e1.5 V. The mass loading of the active material was 1.3 mg cm2. 3. Results and discussion Different samples, including porous Si, the C-Si composite, and the CNT-Si composite, were fabricated using commercial micro SiO in order to evaluate the efficiency of our proposed design strategy. Fig. 1 shows schematic illustrations of the fabrication processes for the three Si-based materials. Micro porous Si was prepared

through a disproportion reaction of commercial SiO under an Ar/H2 atmosphere and then etched with an HF solution (Fig. 1a). During these processes, interconnected Si nanoparticles were generated and embedded in the resultant SiO2 matrix. The subsequent etching process in the HF solution successfully removed the SiO2 component and generated numerous pores. The C-Si composite was obtained by adding a certain amount of pitch in the initial stage while keeping the other conditions the same as those used for the preparation of micro porous Si (Fig. 1b). During the heat treatment process, pitch was carbonized into a carbon coating on the surface of Si/SiO2. The subsequent etching leads to the formation of carboncoated micro porous Si [36,37]. In order to prepare the CNT-wedged porous Si composite, the synthesized micro porous Si was mixed well with a certain amount of CNTs with a Spex-mill (Fig. 1c). Under the appropriate conditions (20 Hz, 30 min), CNTs can be well dispersed within the loose structure and on the surface of the Si nanoparticles to form an interconnected framework. Fig. 2a shows the XRD patterns of the prepared Si-based materials. All of the samples display similar diffraction peaks. The peaks at ca. 28.5 , ca. 47.5 , and ca. 56.3 can be well assigned to the (111), (220), and (311) planes of cubic Si (JCPDS Card No. 27-1402), respectively. The broad peak between 20 and 25 in the XRD pattern of Si-C is related to amorphous carbon resulting from the carbonization of pitch. No obvious peaks belonging to CNTs can be observed in the XRD pattern of CNT-Si, which may be originated from the small content of CNTs and the adequate embedment of CNTs within the Si porous structure. In order to further verify the composition of the prepared samples, the Raman spectra were also analyzed. As seen in Fig. 2b, three peaks centered at ca. 296.1, 502.8, and 928.7 cm1 are observed for all of the samples; these are lower than the peaks of bulk Si, suggesting the nanoscale dimensions of the Si particles and the formation of the Si phase [15,38e41]. In addition, two peaks located at ca. 1341.9 and 1589.3 cm1 can be detected in the spectra of the C-Si composite and CNT-Si composite,

Fig. 1. Schematic of the synthetic procedures of (a) porous Si, (b) C-Si composite, and (c) CNT-Si composite.

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Fig. 2. (a) X-ray diffraction patterns of porous Si, C-Si, and CNT-Si. (b) Raman spectra of porous Si, C-Si, and CNT-Si. (c) XPS spectra of porous Si, C-Si, and CNT-Si.

which correspond to the D band and G band of the carbon materials, respectively. This indicates the successful integration of carbon and CNTs into the porous Si. Compared with porous Si and the C-Si composite, a small shift to a lower frequency can be detected in the Raman spectrum of CNT-Si, which may be caused by the decreased size of the Si nanoparticles prepared through effective ball-milling, the transverse optical mode caused by a phonon confinement effect, and/or a mashing effect of the CNTs [32,42e44]. Fig. 2c shows the Si 2p XPS spectra of porous Si, C-Si, and CNT-Si ranging from 106 to 96 eV. All of the samples display similar spectra, showing peaks centered at ca. 103 eV that are related to the silicon oxide on the surface of Si, which may originate from the natural oxidation of Si in air and the transformation caused by the local heating effect during the test process [2,13,38,41,45]. Additionally, the peaks located at ca. 99 eV can be assigned to Si (0) [2,8,38], which is the dominant species for all of the samples. CNTSi displays similar peaks to the porous Si, indicating there has no obvious surface change after ball-milling treatment. Besides, according to the literature, the obvious shift in C-Si compared with other two samples may be caused by the possible change of the distance between Si ions, the different particle size, the possible charge-transfer, or the electrostatic charge effect during the measurement [46e49]. These results confirm that the proposed method can be used to successfully form Si, C-Si, and CNT-Si. The morphologies and microstructures of the as-obtained samples were first revealed via SEM. From Fig. 3a and b, we can detect that the as-obtained Si powder has a size distribution ranging from 10 to 20 mm and consists of loose aggregates made up of small particles. After being coated with carbon, some sheet-like

structures were generated on the surface of Si particles (Fig. 3c and d), which originated from the carbonization of pitch at high temperatures. Fig. 3e (including the inset one) and f show the SEM images of CNT-Si composite. As can be seen, numerous CNTs surrounded the Si nanoparticles were formed through a ball-milling process. Compared with porous Si and the C-Si composite, the CNT-Si composite displays smaller particles aggregation (200e500 nm) with tighter connections, which are believed to be beneficial for its electrochemical performance. The microstructures of the Si-based materials were further investigated by TEM and HRTEM. As shown in Fig. 4a and b, a loose structure and numerous pores were observed in the Si sample prepared through a disproportion reaction and etching process, which is consistent with the SEM analysis. Additionally, interconnected nanosized particles with an average diameter of ca. 20 nm were shown to comprise the as-prepared porous Si. Fig. 4c and d depict TEM images of the C-Si composite, from which we can clearly detect the carbon coating on the surface of Si. The lowmagnification TEM image (Fig. 4e) of CNT-Si demonstrated that CNTs were distributed among Si particles and generated tightly connection between CNTs and these particles, constructing the micro CNTs wedged Si structure. High-magnification TEM shown in Fig. 4f suggested the uniform coating of CNTs on single Si particle and the well-connections between CNTs and Si building blocks, which have been demonstrated to be beneficial for improving the mechanical stability of the electrode and the electrochemical performance of Si electrode [32,48,49]. Additionally, lattice spacing of 0.31 nm distinguished in HRTEM image (inset in Fig. 4f) is agree with (111) plane of crystalline Si, verifying the formation of Si

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Fig. 3. SEM images of (a, b) porous Si, (c, d) C-Si, and (e, f) CNT-Si.

Fig. 4. TEM images of porous Si (a, b), C-Si (c, d) and CNT-Si (e,f). The inset in Fig. 4f corresponds to the HRTEM image of CNT-Si.

phase. These results indicated the micro features of the as-prepared Si-based materials and the presence of nanoscale building blocks in the CNT-Si composite. Besides, N2 adsorption/desorption isotherms of the CNT-Si composite shown in Fig. S1 indicate that the surface area and total pore volume were 110.0 m2 g1 and 0.205 cc g1,

respectively, confirming the prosperity of the investigated sample. In order to evaluate the efficiency of the proposed strategy, the electrochemical performance of micro porous Si, C-Si, and CNT-Si as anode materials in LIBs was demonstrated using coin-type cells. Fig. 5a shows the initial charge/discharge curves of the samples

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Fig. 5. (a) The initial charge/discharge curves of porous Si, C-Si, and CNT-Si at 0.1 C (0.4 A g1), along with the corresponding efficiencies. (b) Cycling performance of porous Si, C-Si, and CNT-Si at 0.1 C (0.4 A g1) for 100 cycles. The inset one corresponds to the efficiency for first 10 cycles. (c) Rate capability of porous Si, C-Si, and CNT-Si at various current densities.

measured at 0.1 C from 0.01 to 1.5 V. The initial charge and discharge capacities are 2225.7 and 1162.1 mAh g1 for porous Si, 2838.5 and 1682.4 mAh g1 for C-Si, and 2653.1 and 2213.4 mAh g1 for CNT-Si. The corresponding initial Coulombic efficiencies are 52.2%, 59.3%, and 83.4%, respectively. The CNTwedged micro porous Si clearly exhibits an improved Coulombic efficiency compared with those of porous Si and the C-Si composite, indicating that the wedged CNT plays a key role in suppressing the electrolyte decomposition and stabilizing the formed solid electrolyte interface (SEI) film. This value is also higher than most of those that have been reported in the literature for Si-based anodes [9,15,16,30,36,50-52]. Fig. 5b shows the cycling performance and the corresponding Coulombic efficiency of the prepared samples at 0.1 C. After 100 cycles, the CNT-Si composite can deliver a high charge capacity of ca. 2028.6 mAh g1, corresponding to a retained capacity ratio of 91.7%. This capacity is significantly better than those of porous Si (1101.4 mAh g1), the C-Si (1388.7 mAh g1) composite, and most of the previously reported micro Si-based materials. Fig. 5c and Figs. S2aec depicts the rate capabilities of the samples at current densities ranging from 0.4 A g1 to 8 A g1. From the related curves, it can be clearly seen that all of the samples display good rate capabilities. In particular, CNT-Si exhibits better performance than the other two samples. Specifically, charge capacities of 2260.2, 2192.5, 1910.1, and 1747.7 mAh g1 were delivered at 0.4, 0.8, 2, and 4 A g1, respectively. Even at a high rate of 8 A g1, the CNT-Si cell is able to retain a charge capacity of

1582.7 mAh g1. To clarify the rate capabilities of porous Si, C-Si and CNT-Si anodes, the rate capabilities with normalized capacity retentions are presented (Fig. S2d). Normalized capacity retention clearly show the difference of rate capabilities when the discharge current density changes (various current density ranging from 0.4 A g1 to 8 A g1). This performance can be ascribed to the ingenious wedging of CNT within the loose structure of micro porous Si and around the small Si particles, effectively facilitating the transport of electrons and ions. Besides, according to the previous report [25], CNTs have low diffusion resistance owing to their high aspect ratio structure and abundance active activated areas, which could give a positive contribution to the related electrochemical reactions. Here, the higher capacity of CNT-Si than that of C-Si may be ascribed to the presence of three-dimensional conductivity network which facilitates the charge transfer and enhances the electrochemical properties between the resultant anode materials with Li during cycling [25,26]. In order to further verify the great contribution of the designed structure and the capacity contribution of CNTs to the excellent electrochemical performance, pure CNTs were tested as an anode at 400 mA g1 (Fig. S3). Compared with the excellent cycling capacity of the CNTs-wedged Si, the low capacity of the CNTs shown in the figure suggest that the relatively small capacity contribution of the CNTs in the composite. In order to investigate the effects of the reaction parameters on the electrochemical performance of micro Si, samples prepared under various conditions (including different CNT dosages and ball

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Fig. 6. (a) The initial charge/discharge curves and (b) the corresponding cycling performance of porous Si with different concentrations of CNTs and different milling times.

milling times) were tested at 0.1 C. As shown in Fig. 6a, all of the CNT-containing Si samples exhibited a larger first Coulombic efficiency and higher specific capacities compared with the sample that did not contain CNT. Additionally, the CNT-Si prepared with 15 wt% CNT and a ball milling time of 30 min showed the highest value, indicating that appropriate ball milling time and CNT concentration play significant roles in reducing side reactions on the Si electrode and improving the related electrochemical performance. Fig. 6b displays the cycling performance of all of the samples at 0.1 C for 100 cycles. The CNT-containing samples clearly exhibit higher capacities compared to the pure porous Si sample. After 100 cycles, the CNT (15 wt%)-porous Si with a ball milling time of 30 min can deliver a high capacity up to 2028.6 mAh g1, retaining the highest value among all of the samples. Table 1 provides distinct comparisons between the different samples. These results indicate the effect of modifying the CNT concentration in the porous Si anode and the important role of ball milling time on the electrochemical performance. Generally, introducing carbon additive into active material could effectively alleviate the stress, accommodate the large volume expansion/ shrinkage, and facilitate the transport of electron and Liþ ion. However, the presence of carbon additive can also lead to serious drawback such as low Coulombic efficiency and much electrolyte decomposition during cycling [53]. Thus, appropriate balancing between carbon additives and active mass play an important role in improving the electrochemical performances [54]. Besides, appropriate milling time is an important condition to fabricate a uniform CNT-Si composite anode and improve the corresponding electrochemical properties [55,56]. Compared with the optimized conditions (with 30 min milling time), a long milling time (45 min) using high energy milling machine will lead to the damage of particles and the agglomerated separately of CNTs (Fig. S4). Thus, we infer that using the appropriate conditions in the ball milling process (i.e., a dosage of 15 wt% CNTs and a ball milling time of 30 min) may be beneficial for homogenously distributing CNTs among the loose structure of micro Si, thereby contributing to the generation of tight

connections between CNTs and Si nanoparticles. Fig. 7 shows the differential capacity plots (dQ dV1) of the porous Si electrode, C-Si electrode, and CNT-Si electrode after 100 cycles. The significant sharp peaks observed at 0.07e0.11 V during the first discharge process can be assigned to the phase transition from crystalline Si to amorphous lithium silicide (LixSi), which corresponds to the long voltage platform in the first discharge process. Compared with the porous Si and C-Si, this peak shifts from 0.11 V toward a lower value (0.07 V) for CNT-Si; the reason for this shift is ascribed to the different solid/electrolyte interface, which leads to different surface kinetics [57,58]. In the subsequent cycles, two peaks at ca. 0.06e0.09 and 0.20e0.24 V appear during discharge, suggesting the transformation from LiSi to Li7Si3 and then to Li3.17Si; the peaks at ca. 0.29e0.32 V and 0.45e0.49 V are related to the de-alloying of Li ions in the Li3.17Si to Li7Si3 phase transition [59,60]. The CNT-Si electrode retained a larger differential capacity value with well curve overlapping, indicating its high activity and reversibility during the electrochemical reaction process. Additionally, the peak located at ca. 0.17 V, belonging to Li deintercalation from carbon materials, is not be observed in Fig. 7. This indicates the neglected capacity contribution of carbon components in the samples, which was also proved by the cycling performance curve of pure CNTs (Fig. S3). These results further confirm the important role of securely attaching CNTs to the surface of the small Si particles and within the framework of the porous microstructure. Additionally, compared with the reported typical Si-based anodes and commercialized graphite anodes, the current CNT-Si structure exhibits a relatively high tap density and considerable weight and volumetric capacities (Table 2); additionally, the investigated CNTs wedged micro porous Si displays appealing merits including higher initial Coulombic efficiency and higher specific capacity, better capacity retention and more excellent rate capability, compared with the reported micro Si anodes [30,34,35], (Table 3), suggesting the efficiency of our proposed strategy and the great promise of the designed anode in practical LIB applications.

Table 1 Electrochemical performance comparison of porous Si and CNT-Si prepared under different conditions. Materials

0.1 C 1st charge (mAh g1)

0.1 C 1st discharge (mAh g1)

1st efficiency

0.1 C Cycle retention

Porous Si 10 wt% CNT, 10 wt% CNT, 15 wt% CNT, 15 wt% CNT,

2225.7 2897.8 2701.3 2653.1 2759.2

1162.1 2428.8 2114.9 2213.4 2051.3

52.2% 83.8% 79.4% 83.4% 74.3%

94.8% 52.9% 71.1% 91.7% 72.7%

20 20 20 20

Hz Hz Hz Hz

30 45 30 45

min min min min

(1101.3 (1284.7 (1505.2 (2028.6 (1490.6

mAh mAh mAh mAh mAh

g1 g1 g1 g1 g1

100th 100th 100th 100th 100th

Cycle) Cycle) Cycle) Cycle) Cycle)

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Fig. 7. dQ dV1 plots of (a) porous Si, (b) C-Si, and (c) CNT-Si electrodes during 100 cycles.

Table 2 Comparison of this work with reported Si-based anodes and graphite for LIBs with regard to tap density, weight capacity, and volumetric capacity. Materials

Tap density (g cm3)

Weight capacity (mAh g1)

Volumetric capacity (mAh cm3)

References

Nano-Si Ball-milled bulk Si Graphite C-Si CVD C2H2-absorbed porous Si Porous Si Milled Si with CNTs

0.16 0.7 1.16 0.49 0.78 0.285 1.103

1800 1800 360 1950 1544 1170 2364

288 1260 418 956 1204 333 2607

Ref. [61] Ref. [61] Ref. [62] Ref. [63] Ref. [3] This work This work

Table 3 Comparison of electrochemical properties and tap density of micro sized silicon anodes materials. Materials

Initial Coulombic efficiency

Discharge capacity (0.4 A g1) (mAh g1)

Cycle Retention

Rate Capability (mAh g1)

Tap density (g cm3)

References

Milled Si with CNTs

83.4%

2213.4

91.7% (100th [email protected] A g1

1.103

This Work

Semimicro Si-C composite

82.2%

2084

81% (100th Cycle@ 0.4 A g1)

0.488

Ref. [34]

800 (0.5 A/g)

96.4% (200th [email protected] A g1) 96.6% (50th [email protected] A g1)

1964.5@2 A g1 1835.6@4 A g1 1614.6@8 A g1 2000@2 A g1 1400@4 A g1 1100@8 A g1 600@2 A g1 300@8 A g1 [email protected] A g1 [email protected] A g1

Micro-sizednano-porous 81.5% Si/C Micro-sized Si-C 77.0% composite

1544

Ref. [35] Ref. [30]

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4. Conclusions In this work, an ingenious strategy including attaching CNTs on the surface of primary nanoparticles and wedging in the framework of micro porous Si was provided. In order to demonstrate the efficiency of utilizing the proposed porous engineering, CNT-wedging, and nanoscale primary units presented in this work, several samples were prepared. The influences of CNTs dosage and ball milling time on the final electrochemical performance were intensively studied. Related characterizations indicate that the designed CNT-Si structure can alleviate the large volume change of micro Si, improve the electronic conductivity of the electrode, and stabilize the as-formed SEI film. As a consequence, the resulting micro Si has a high tap density and shows an extremely high specific capacity of 2213.4 mAh g1; this value is maintained at 2028.6 mAh g1 after 100 cycles. This tap density is superior to those reported in the literature for micro Si in LIBs. The current work provides a facile and effective approach to modifying micro Si anodes and could be expanded to other types of microelectrodes in the future. Notes The authors declare no competing financial interest. Acknowledgement This work was supported by the Human Resources Development program (No. 20154010200840) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy and also supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20152000000650). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.03.020. References [1] T. Kennedy, M. Brandon, K.M. Ryan, Adv. Mater. 28 (2016) 5696e5704. [2] J.Y. Liao, S.M. Oh, A. Manthiram, ACS Appl. Mater. Interfaces 8 (2016) 24543e24549. [3] G.Q. Tan, F. Wu, Y.F. Yuan, R.J. Chen, T. Zhao, Y. Yao, J. Qian, J.R. Liu, Y.S. Ye, R. Shahbazian-Yassar, J. Lu, K. Amine, Nat. Comm. 6 (2016) 11774. [4] H.S. Kim, E.J. Lee, Y.K. Sun, Mater. Mater. Today 17 (2014) 285e297. [5] R. Yi, M.L. Gordin, D.H. Wang, Nanoscale 8 (2016) 1834e1848. [6] A. Casimir, H.G. Zhang, O. Ogoke, J.C. Amine, J. Lu, G. Wu, Nano Energy 27 (2016) 359e376. [7] Y.Z. Han, P.F. Qi, J.W. Zhou, X. Feng, S.W. Li, X.T. Fu, J.S. Zhao, D.N. Yu, B. Wang, ACS Appl. Mater. Interfaces 7 (2015) 26608e26613. [8] J.W. Liang, X.N. Li, Z.G. Hou, W.Q. Zhang, Y.C. Zhu, Y.T. Qian, ACS Nano 10 (2016) 2295e2304. [9] X.S. Zhou, Y.X. Yin, L.J. Wan, Y.G. Guo, Chem. Commun. 48 (2012) 2198e2200. [10] Z.D. Lu, N. Liu, H.W. Lee, J. Zhao, W.Y. Li, Y.Z. Li, Y. Cui, ACS Nano 9 (2015) 2540e2547. [11] Y.H. Xu, Y.J. Zhu, F.D. Han, C. Luo, C.S. Wang, Adv. Energy Mater. 5 (2015) 1400753. [12] Y. Yao, M.T. McDowell, I. Ryu, H. Wu, N. Liu, L.B. Hu, W.D. Nix, Y. Cui, Nano Lett. 11 (2011) 2949e2954. [13] L. Zhang, R. Rajagopalan, H.P. Guo, X.L. Hu, S.X. Dou, H.K. Liu, Adv. Funct. Mater 26 (2016) 440e444. [14] Y.Y. Kim, H.J. Kim, J.H. Jeong, J. Lee, J.H. Choi, J.Y. Jung, J.H. Lee, H.H. Cheng, K.W. Lee, D.G. Choi, Adv. Energy Mater. 18 (2016) 1349e1353. [15] K. Fu, O. Yildiz, H. Bhanushali, Y.X. Wang, K. Stano, L.G. Xue, X.W. Zhang, P.D. Bradford, Adv. Mater. 25 (2013) 5109e5114.

[16] Y.H. Huang, Q. Bao, J.G. Duh, C.T. Chang, J. Mater. Chem. A 4 (2016) 9986e9997. [17] Y. Han, N. Lin, Y.Y. Qian, J.B. Zhou, J. Tian, Y.C. Zhu, Y.T. Qian, Chem. Commun. 52 (2016) 3813e3816. [18] J.X. Song, S.R. Chen, M.J. Zhou, T. Xu, D.P. Lv, M.L. Gordin, T.J. Long, M. Melnyk, D.H. Wang, J. Mater. Chem. A 2 (2014) 1257e1262. [19] X.L. Li, M. Gu, S.Y. Hu, R. Kennard, P.F. Yan, X.L. Chen, C.M. Wang, M.J. Sailor, J.G. Zhang, J. Liu, Nat. Comm. 5 (2013) 4105. [20] X.Y. Yu, L. Yu, X.W. Lou, Adv. Energy Mater. 6 (2016) 1501333. [21] Y.M. Chen, X.Y. Yu, Z. Li, U.Y. Paik, X.W. Lou, Sci. Adv. 2 (2016) e1600021. [22] N. Mahmood, T.Y. Tang, Y.L. Hou, Adv. Energy Mater. 6 (2016) 1600374. [23] M. Ashuri, Q.R. Hea, L.L. Shaw, Nanoscale 8 (2016) 74e103. [24] L.H. Zhuo, Y.Q. Wu, J. Ming, L.Y. Wang, Y.C. Yu, X.B. Zhang, F.Y. Zhao, J. Mater. Chem. A 1 (2013) 1141e1147. [25] Z.Y. Wang, D.Y. Luan, S. Madhavi, Y. Hu, X.W. Lou, Energy Environ. Sci. 5 (2012) 5252e5256. [26] X.F. Li, J. Liu, Y. Zhang, Y.L. Li, H. Liu, X.B. Meng, J.L. Yang, D.S. Geng, D.N. Wang, R.Y. Li, X.L. Sun, J. Power Sources 197 (2012) 238e245. [27] H.W. Huang, Y. Liu, J.H. Wang, M.X. Gao, X.S. Peng, Z.Z. Ye, Nanoscale 5 (2013) 1785e1788. [28] Y.Z. Jiang, Y. Li, W.P. Sun, W. Huang, J.B. Liu, B. Xu, C.H. Jin, T.Y. Ma, C.Z. Wu, M. Yan, Energy Environ. Sci. 8 (2015) 1471e1479. [29] R.Z. Hu, D.C. Chen, G. Waller, Y.P. Ouyang, Y. Chen, B. Zhao, B.C. Rainwater, H.H. Yang, M. Zhu, M.L. Liu, Energy Environ. Sci. 9 (2016) 595e603. [30] R. Yi, F. Dai, M.L. Gordin, S.R. Chen, D.H. Wang, Adv. Energy Mater. 3 (2013) 295e300. [31] Y. Li, S. Yu, T. Yuan, M. Yan, Y. Jiang, J. Power Sources 282 (2015) 1e8. [32] J.B. Zhou, Y. Lan, K.L. Zhang, G.L. Xia, J. Du, Y.C. Zhu, Y.T. Qian, Nanoscale 8 (2016) 4903e4907. [33] L.F. Cui, L.B. Hu, J.W. Choi, Y. Cui, ACS Nano 4 (2010) 3671e3678. [34] H. Sohn, D.H. Kim, R. Yi, D.H. Tang, S.-E. Lee, Y.S. Jung, D.H. Wang, J. Power Sources 334 (2016) 128e136. [35] H.J. Tian, X.J. Tan, F.X. Xin, C.S. Wang, W.Q. Han, Nano Energy 11 (2015) 490e499. [36] S.Y. Kim, J.W. Lee, B.H. Kim, Y.J. Kim, K.S. Yang, M.S. Park, ACS Appl. Mater. Interfaces 8 (2016) 12109e12117. [37] S.W. Oh, S.T. Myung, S.M. Oh, K.H. Oh, K. Amine, B. Scrosati, Y.K. Sun, Adv. Mater. 22 (2010) 4842e4845. [38] S.O. Kim, A. Manthiram, J. Mater. Chem. A 3 (2015) 2399e2406. [39] Q. Xu, J.Y. Li, Y.X. Yin, Y.M. Kong, Y.G. Guo, L.J. Wan, Chem. Asian J. 11 (2016) 1205e1209. [40] M. Li, X.L. Hou, Y.J. Sha, J. Wang, S.J. Hu, X. Liu, Z.P. Shao, J. Power Sources 248 (2014) 722e728. [41] R. Epur, M.K. Datta, P.N. Kumta, Electrochim. Acta 85 (2012) 680e684. €gl, M. Antonietti, [42] Y.S. Hu, R.Z. Demir-Cakan, M.M. Titirici, J.O. Müller, R. Schlo J. Maier, Angew. Chem. Int. Ed. 47 (2008) 1645e1649. [43] C. Meier, S. Luttjohann, V.G. Kravets, H. Nienhaus, A. Lorke, H. Wiggers, Phys. E 32 (2006) 155e158. [44] Y. Xiao, M.H. Cao, ACS Appl. Mater. Interfaces 6 (2014) 12922e12930. [45] X. Zhu, H. Chen, Y. Wang, L. Xia, Q. Tan, H. Li, Z. Zhong, F. Su, X. Zhao, J. Mater. Chem. A 1 (2013) 4483e4489. [46] J.H. Yang, R. Wang, L.L. Yang, J.H. Lang, M.B. Wei, M. Gao, X.Y. Liu, J. Cao, X. Li, N.N. Yang, J. Alloy. Comp. 509 (2011) 3606e3612. [47] G.M. Liu, W. Jaegermann, J. Phys. Chem. B 106 (2002) 5814e5819. [48] P. Gao, J.C. Liu, D.D. Sun, W.J. Ng, J. Hazard. Mater. 250 (2013) 412e420. [49] C.Q. Sun, L.K. Pan, Y.Q. Fu, B.K. Tay, S. Li, J. Phys. Chem. B 107 (2003) 5513e5515. langer, D.M. Schleich, T. Brousse, Adv. [50] C. Martin, O. Crosnier, R. Retoux, D. Be Funct. Mater. 21 (2011) 3524e3530. [51] J.H. Lee, C.S. Yoon, J.Y. Hwang, S.J. Kim, F. Maglia, P. Lamp, S.T. Myung, Y.K. Sun, Energy Environ. Sci. 9 (2016) 2152e2158. [52] J.Q. Zhou, T. Qian, M.F. Wang, N. Xu, Q. Zhang, Q. Li, C.L. Yan, ACS Appl. Mater. Interface 8 (2016) 5358e5365. [53] S.M. Oh, J.Y. Hwang, C.S. Yoon, J. Lu, K. Amine, I. Belharouak, Y.K. Sun, ACS Appl. Mater. Interfaces 6 (2014) 11295e11301. [54] P.F. Zhang, M. Chen, X. Shen, Q.H. Wu, X.E. Zhang, L. Huan, G.W. Diao, Electrochim. Acta 204 (2016) 92e99. [55] P. Gu, R. Cai, Y.K. Zhou, Z.P. Shao, Electrochim. Acta 55 (2010) 3876e3883. [56] C.-H. Yim, F.M. Courtel, Y. Abu-Lebdeh, J. Mater. Chem. A 1 (2013) 8234e8243. [57] S.-H. Ng, J.Z. Wang, D. Wexler, K. Konstantinov, Z.P. Guo, H.-K. Liu, Angew. Chem. Int. Ed. 45 (2006) 6896e6899. [58] N. Dimov, S. Kugino, M. Yoshio, Electrochim. Acta 48 (2003) 1579e1587. [59] W. Wang, P.N. Kumta, ACS Nano 4 (2010) 2223e2241. [60] M.K. Datta, P.N. Kumta, J. Power Sources 194 (2009) 1043e1052. [61] M. Gauthier, D. Mazouzi, D. Reyter, B. Lestriez, P. Moreau, D. Guyomard, , Energy Environ. Sci. 6 (2013) 2145e2155. L. Roue [62] T. Zheng, Y.H. Liu, E.W. Fuller, S. Tseng, U. von Sacken, J.R. Dahn, J. Electrochem. Soc. 142 (1995) 2581e2590. [63] A. Magasinski, P. Dixon, B. Hertzberg, A. Kvit, J. Ayala, G. Yushin, Nat. Mater. 9 (2010) 353e358.