BaTiO3 Nanocomposite Anode: Evidence

Jan 27, 2016 - †Energy Material Laboratory and §Analytical Engineering Group, Samsung Advanced Institute of Technology, 130 Samsung-ro, Yeongtong-g...
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Silicon/Carbon Nanotube/BaTiO3 Nanocomposite Anode: Evidence for Enhanced Lithium-Ion Mobility Induced by the Local Piezoelectric Potential Byoung-Sun Lee,† Jihyun Yoon,‡ Changhoon Jung,§ Dong Young Kim,† Seung-Yeol Jeon,‡ Ki-Hong Kim,§ Jun-Ho Park,† Hosang Park,† Kang Hee Lee,† Yoon-Sok Kang,† Jin-Hwan Park,† Heechul Jung,*,† Woong-Ryeol Yu,*,‡ and Seok-Gwang Doo† †

Energy Material Laboratory and §Analytical Engineering Group, Samsung Advanced Institute of Technology, 130 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 443-803, Korea ‡ Nano & Smart Composite Materials Laboratory, Department of Materials Science and Engineering and Research Institute of Advanced Materials, Seoul National University, 1 Gwanangno, Gwanak-gu, Seoul 151-742, Korea S Supporting Information *

ABSTRACT: We report on the synergetic effects of silicon (Si) and BaTiO3 (BTO) for applications as the anode of Li-ion batteries. The large expansion of Si during lithiation was exploited as an energy source via piezoelectric BTO nanoparticles. Si and BTO nanoparticles were dispersed in a matrix consisting of multiwalled carbon nanotubes (CNTs) using a high-energy ball-milling process. The mechanical stress resulting from the expansion of Si was transferred via the CNT matrix to the BTO, which can be poled, so that a piezoelectric potential is generated. We found that this local piezoelectric potential can improve the electrochemical performance of the Si/CNT/BTO nanocomposite anodes. Experimental measurements and simulation results support the increased mobility of Li-ions due to the local piezoelectric potential. KEYWORDS: silicon anode, large volume expansion, piezoelectric particle, local electric potential, Li-ion mobility

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effort has been made to minimize the side effects that result from the volume expansion (and subsequent pulverization) by forming Si-based composite materials with other active conducting materials (e.g., carbonaceous materials such as polymer-based hard carbon, graphene, or carbon nanotubes)7−10 or with inactive materials (e.g., copper or titanium nitride (TiN))11,12 or by designing novel (but complicated) architectures to provide a buffering space.13,14 Although much progress has been made, a trade-off is still involved between electrochemical performance and processing costs because excellent electrochemical properties require complex and expensive processes,15,16 whereas poor electrochemical performance has been attributed to crude processes.17,18 Here, we describe a low-cost but high-performance processing method,

ue to the large energy density and long lifespan, Li-ion batteries (LIBs) have been successfully commercialized and are widely used in mobile electronic devices. There is growing demand, however, for advanced LIBs with a higher specific capacity and longer cycle life to power electric vehicles and high-end electronic devices.1 The low specific capacities of existing electrode materials (e.g., commercialized graphitic carbon anodes (372 mAh g−1) and lithium cobalt oxide cathodes (140 mAh g−1)) are recognized as major obstacles to meeting this demand,2 motivating research into novel electrode materials. Silicon (Si) has been investigated as an anode material because of its high specific capacity (4212 mAh g−1) and low working potential (∼0.5 V).3 The high specific capacity of Si results from the microstructural changes and subsequent large volume change during lithiation.4 However, the large volume expansion is recognized as a critical limitation because the resulting pulverization of the Si leads to a significant deterioration in performance.5 As such, this volume expansion is widely considered as a negative feature of Si,6 and much © XXXX American Chemical Society

Received: December 6, 2015 Accepted: January 27, 2016

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Figure 1. Schematic diagrams illustrating microstructural changes in the Si/CNT/BTO nanocomposite anode during lithiation. (a) Si and BTO particles are dispersed in and firmly attached to the dense CNT. (b) Lithiation of the Si nanoparticles results in a large increase in volume, pressurizing BTO nanoparticles and poling them to create a piezoelectric potential. (c) Without piezoelectric poling, the high current density (shown in red) is observed only around the surface of the particles. (d) Permanent and local piezoelectric potential enhances the mobility of the Li-ions in the subsequent discharging and charging processes, resulting in a large current density. The pictures in the box illustrate the microstructural changes in Si (i.e., expansion due to the lithiation) and BTO (i.e., piezoelectric poling due to the expansion of the Si).

can be poled; that is, the piezoelectric potential develops within the piezoelectric material. Our question is this: can this local and permanent piezoelectric potential improve the electrochemical performance of Si-based anode materials? If so, the combination of Si with a piezoelectric material may represent the first exploitation of the volume expansion of Si. To realize this concept, strong adhesion of both Si and the piezoelectric material to the matrix is required, as well as sensitive activation of the piezoelectric material. The matrix should transfer the mechanical stress (or deformation) that results from the large expansion of Si to the piezoelectric material, while maintaining conducting pathways for the Li-ions. Multiwalled carbon nanotubes (CNTs) were used as the matrix material, and crystalline BaTiO3 (BTO) nanoparticles were used as the piezoelectric material. Note that the BTO nanoparticles were used as the additives for the electrochemical advances of the

which we developed by exploiting the large volume expansion of Si. Recently, various energy-generating materials (e.g., piezoelectric, pyroelectric, and ferroelectric materials) and their composites have been actively applied to the new kinds of energy conversion devices.19−21 Among them, piezoelectric materials can be optionally employed in the Li-ion battery to improve the electrochemical performance by creating an electric field in response to mechanical stress (and deformation).22 Xue et al. reported the self-charging Li-ion battery by introducing the piezoelectric poly(vinylidene fluoride) (PVDF) film as a separator.23 Note that the piezoelectricity of the PVDF film was only induced by external mechanical stress.23 In contrast, the piezoelectricity in this study was invoked by internal stress. Namely, the large volume expansion of Si is transferred to a piezoelectric material, which B

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Figure 2. (a) Low- and (b) high-resolution cross-sectional SEM images of the Si/CNT/BTO nanocomposite particle. The BTO is shown by the bright regions, the Si by the dark regions, and the CNT matrix by the gray region, which appears densely packed around the Si and BTO nanoparticles. (c−f) EDS images showing that Si, Ba, Ti, and O appear homogeneously in the CNT matrix.

during lithiation, as shown in Figure 1b. Si expands by approximately 400% when the Li22Si4 phase is formed during lithiation.26 If this deformation is constrained, a large stress (1.7 GPa)27 develops in the Si. This stress pressurizes the BTO particles, which poles them because the BTO is a piezoelectric metal oxide with a piezoelectric constant of d33 = 350 pC· N−1.28,29 This local piezoelectric potential can influence the mobility of Li-ions and hence the electrochemical behavior of the Si/CNT/BTO nanocomposite anode, as shown in Figure 1c,d. We found that the BTO nanoparticles improved the electrochemical performance of the nanocomposite anode through experimental measurements and simulation studies.

cathode materials, but they only focused on the dielectric (or ferroelectric) properties of the BTO.24,25 In the following, we discuss the materials, the processing, and the characterization employed to demonstrate this idea using Si/CNT/BTO nanocomposites, as well as simulation studies that provided insight into the physical processes that occur in this system. The Si and BTO nanoparticles, as well as the multiwalled CNTs, were composited, as shown in Figure 1a. The three components of the Si/CNT/BTO nanocomposites were exploited as follows: the Si nanoparticles provide the large specific capacity; the CNTs serve as a matrix providing conducting pathways; and the BTO nanoparticles provide a local piezoelectric potential. The CNTs form a matrix, such that the other two components (i.e., the Si and BTO nanoparticles) are accommodated in it, and their deformations can be confined. Then, the BTO is poled (i.e., a piezoelectric potential is generated) due to mechanical deformation of Si nanoparticles

RESULTS AND DISCUSSION To effectively transfer the mechanical stress that results from the expansion of the Si to the BTO nanoparticles, the matrix should be sufficiently dense. We used a high-energy ball-milling C

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(210), and (220) planes of crystalline BTO,30 as well as the (111) and (220) planes of crystalline Si.3 The peaks for Si and BTO corresponding to the (311) and (211) planes were almost superposed at around 2θ = 56°. Note that the crystal structure of BTO can be either cubic or tetragonal; however, it is predominantly cubic when the crystalline size is less than 100 nm.31 Because the diameter of the BTO particles was less than 100 nm, we may expect that the crystal structure of the BTO will be cubic. Representative peaks of the carbonaceous microstructure due to CNTs (e.g., C(002)) were not clearly resolved; however, a broad feature appeared at around 2θ = 25°, which is attributed to the CNTs.32,33 Raman spectra were obtained to investigate the microstructural changes of the CNTs following high-energy ball milling, as shown in Figure 3b. Typical crystalline Si and carbon D- and G-peaks appeared at around 519, 1350, and 1596 cm−1,7 and the peaks due to crystalline BTO (i.e., 261, 304, 517, and 715 cm−1) were almost negligible.34 The D-peaks are related to microstructural defects (i.e., disorder), and the G-peaks are related to the graphitic layers;35 the relative intensities of these two peaks increased from ID/IG = 0.12 for the pristine CNTs (see Figure S2) to ID/ IG = 1.36 for the CNTs in the Si/CNT/BTO nanocomposite, which is attributed to damage during the high-energy ball milling. If crystalline or amorphous phases (e.g., silicon carbide (SiC)) were newly formed by side chemical reactions during the process, the crystallographic information on the newly formed phase should be shown on the XRD and Raman data. Fully assigned peaks to BTO, CNT, and Si phases in the XRD and Raman data (Figure 3) indicate that side chemical reactions did not occur during the high-energy ball-milling process. Crystallographic changes of the Si and BTO during the electrochemical reactions were monitored using in situ XRD analysis. Details, including the sample preparation, can be found in the Supporting Information. The Si(111) and BTO(110) peaks exhibited high intensities in the XRD curve during the first discharge process, as shown in Figure 4a. The piezoelectricity of BTO crystals is revealed by the change in the (100) interlayer spacing, accompanied by changes in both the (110) and (111) interlayer spacings. We traced the BTO(110) peak to investigate the microstructural changes of the BTO crystals. We recorded in situ XRD data for 12.0 ≤ 2θ ≤ 15.5°. Note that the 2θ values of the developed XRD peaks are almost half of the ex situ XRD peaks because the Mo Kα source (λ = 0.071 nm) was used as an X-ray source in order to avoid the interferences between X-ray source and Cu current collector. The Si(111) peak shifted toward larger angles 2θ, and the peak width increased during the first discharging process (until the voltage reached 0.01 V) (Figure 4b). The intensity of the Si(111) peak reduced, and eventually, it disappeared as discharging proceeded. This shift and broadening of the Si(111) peak (under the constant current conditions to 0.01 V) are attributed to unreacted crystalline Si, which was compressed by the partially lithiated Si.36 The sudden changes in the peak shift and the broadening of the Si(111) peak at around 0.05 V appear to originate from the rapid volume expansion of the lithiated Si, which in turn results from the amorphous crystalline transition of the Li15Si4 phase.37 Under the constant voltage conditions (i.e., maintaining the voltage at 0.01 V until the current density reached 0.01C, where 1C is defined as 2200 mA g−1, based on the Si and CNT contents), the full width at half-maximum (fwhm) was reduced following Si amorphization. The Si(111) peak was not reconstructed in the subsequent charging process because the amorphization of

process to achieve densification (see the Methods section for details). The typical morphology of the resulting Si/CNT/ BTO nanocomposites was random-shaped particles with variable diameters (Figure S1a). High-resolution scanning electron microscope (SEM) images revealed that the nanocomposite surface consists of the broken CNTs, Si nanoparticles, and BTO nanoparticles (Figure S1b). The CNTs were broken during the high-energy ball-milling process, so that the average length was reduced to 118.9 nm (±50.3 nm). The diameters of the nanocomposite particles were mostly distributed in the range of 5−15 μm (the average diameter was 8.8 μm (±5.0 μm)) (Figure S1c). Figure 2a shows an SEM image, revealing the typical cross section of the nanocomposites, and Figure 2b shows a high-resolution SEM image, which reveals that the three components were densely packed following high-energy ball milling, and the Si and BTO regions can be clearly seen, with the broken CNTs forming a matrix between them. Energy-dispersive spectroscopy (EDS) analysis revealed that the Si and BTO nanoparticles were homogeneously distributed in the nanocomposite (see Figure 2c−f). The microstructure of the Si/CNT/BTO nanocomposite was characterized using X-ray diffraction (XRD) and Raman spectroscopy. Figure 3a shows an XRD spectrum, which reveals sharp peaks corresponding to the (100), (110), (111), (200),

Figure 3. Microstructural analyses of the Si/CNT/BTO nanocomposite particles: (a) XRD pattern and (b) Raman spectrum. D

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Figure 4. In situ XRD analysis of the Si/CNT/BTO nanocomposite anode during the electrochemical reaction. (a) Changes in the Si(111) and BTO(110) peaks during the first discharge process. (b,c) Quantitative analysis of the peak center and fwhm of the Si(111) and BTO(110) peaks during the first discharge process. (d) Changes in the BTO(110) peak during the first charge process and (e) changes in the BTO(110) peak during the second discharge process.

broadening were maintained during the first charging and the second discharging processes (see Figure 4d,e, as well as Figure S4), which might be due to only partial recovery of Si.38 The change in the electrical conductivity of the Si/CNT/BTO nanocomposite was measured using a high-resistance powder conductivity meter (HPRM-AH2, Han Tech.). The results support the contribution of the mechanically deformed BTO and resulting piezoelectricity to the electrical properties (see Supporting Information). This brings up an interesting question as to whether the piezoelectricity and resulting

Si was irreversible (Figure 4d). The change in the BTO(110) peak during the first discharge process suggests that the BTO nanoparticles were mechanically compressed by the electrochemically reacted Si (see Figure 4a,c). During the first discharging process (under constant current conditions), the BTO(110) peak shifted to larger angles 2θ and the fhwm increased. This is coincident with the evolution of the Si(111) peak (compare Figure 4b,c), suggesting causality between the lithiated Si (expansion) and the microstructural change (compression) of BTO. Interestingly, the shifted peak and its E

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Figure 5. (a) Voltage profiles of the Si/CNT/BTO nanocomposite and (b) voltage profiles of the Si/CNT nanocomposite. (c) Cycle performance and (d) rate capabilities of the Si/CNT/BTO and Si/CNT nanocomposites.

incorporation of BTO.42 The voltage profiles and dq/dv curves of the Si/CNT/BTO and Si/CNT nanocomposites shown in Figure S6 revealed that the improvements in the kinetic behavior resulted from the piezoelectricity of the BTO. A detailed explanation of this can be found in the Supporting Information. The cycle behaviors of the Si/CNT/BTO and Si/CNT nanocomposites were compared to investigate how BTO nanoparticles affected electrochemical performance, as shown in Figure 5c. The Coulombic efficiency of the Si/CNT/BTO nanocomposite converged to 98% by the fifth cycle and then increased to 99.8% at around the hundredth cycle; the Si/CNT nanocomposite showed lower Coulombic efficiency at the same cycles (between 91.7 and 97.5%). The Si/CNT/BTO nanocomposites exhibited markedly less capacity fading: after 40 cycles, the capacity retention of the Si/CNT/BTO structure was 72.2%, whereas that of the Si/CNT nanocomposite was 25.1%. After 43 cycles, the Si/CNT/BTO nanocomposite maintained stable electrochemical behavior. In contrast, the Si/ CNT nanocomposite exhibited unusual charging behavior; that is, the charge capacity of the Si/CNT nanocomposite was much higher than the discharge capacity (data not shown). The capacity retention of the Si/CNT/BTO nanocomposites after 100 cycles was 63.0%. This could be further improved by adding graphite to the nanocomposite (see Figure S7). Figure 5d compares the rate capability of the Si/CNT/BTO nanocomposite with that of the Si/CNT nanocomposite. After 20 cycles, the charge capacity of the Si/CNT/BTO nanocomposite was 84.9% and that of the Si/CNT structure was 70.3% (both with respect to the initial charge capacities); this finding demonstrates that the poled BTO improved the

improved electrical properties could contribute to improved electrochemical performance. The electrochemical performance of Si/CNT/BTO and Si/ CNT nanocomposites was evaluated using galvanostatic tests with a 2032 coin-type half-cell. Figure 5a shows voltage profiles of the Si/CNT/BTO nanocomposite. Li-ions were stably inserted and extracted during repeated cycling, and typical lithiation and delithiation plateaus of Si were observed at approximately 0.25 V during discharging and 0.5 V during charging.2 Because BTO is electrochemically inert to Li-ions,39 the specific capacity is determined by the Si and CNT contents. The contribution of the CNTs to the specific capacity was less than 5%, based on the composition (i.e., 30 wt % CNT) and theoretical capacity (372 mAh g−1),2 and the plateaus of the voltage profiles shown in Figure 5a,b are related primarily to the electrochemical reaction of the Si nanoparticles. The electrochemical performances of the Si/CNT/BTO and Si/ CNT nanocomposites were similar during the first cycle; the respective reversible capacities were 2204.0 and 2262.7 mAh g−1 (based on the Si and CNT content), and the initial Coulombic efficiencies were 86.2 and 87.3%, respectively. In the first discharge curve of the Si/CNT nanocomposites, an overshoot was observed at around 0.1 V, which was probably due to low conductivity and poor reaction kinetics during lithiation.40,41 When the electrochemical cycle was repeated, the electrochemical performance of the Si/CNT nanocomposites deteriorated and the working potential changed significantly due to pulverization of the nanocomposite and subsequent breaking of conducting pathways (see Figure 5b). In contrast, the Si/CNT/BTO nanocomposites exhibited stable lithiation due to the improved kinetics, which we attribute to the F

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Figure 6. Distribution of the local current density of the BTO particles when the lithium intercalation reaction occurred in the anode during the charging step at a constant voltage. (a) Geometry of simulation. (b) Without piezoelectric poling and with piezoelectric poling of (c) −0.05 V, (d) −0.10 V, and (e) −0.15 V. (f) Histograms showing the distribution of the local current density.

K. According to porous electrode theory, we modeled the electrode as an effective medium rather than a single particle.45 The governing equations for these simulations are described in the Supporting Information. A simple battery model was used, as shown in Figure 6a. The battery was modeled using a two-dimensional geometry (details of the boundary conditions, including the local potential of the poled BTO, are given in the Supporting Information and shown in Figure S10). Note that a poling process normally means applying electric fields to ferroelectric materials in order to align dipoles in a specific direction, but the poled BTO particles in this battery model were attributed to piezoelectricity of BTO, resulting from the volume expansion of Si nanoparticles. It was assumed that the poling orientations of the BTO particles were randomly distributed. We built a simplified battery model using a small number of BTO particles with a diameter much larger than those of real particles in experiments. Note that well-dispersed BTO particles used in the actual experiments have diameters less than 100 nm. The electrolyte may create the electrical double layers around BTO particles. If the electrolyte creates the electrical double layers around BTO particles, the thickness of the electrical double layers where piezoelectric potential acts as a local potential is known to be in the range from a few to tens of nanometers. Thus, the potential boundary imposed on BTO can be thought to represent these areas (i.e., a sum of the thickness of individual double layers formed around BTO nanoparticles where piezoelectric potential acts as a local potential). Table S1 lists the input parameters used in these simulations. Charging and discharging simulations of the Li-ion battery were carried out for 30 cycles. Each cycle consisted of five steps: a discharging step with constant current until the cell voltage

electrochemical performance of the nanocomposites. This improvement was not attributed to the mechanical confinement due to the electrochemically inactive component but rather to the piezoelectricity induced in the BTO. We also fabricated and characterized Si/CNT/Al2O3 nanocomposites with an equivalent composition to the Si/CNT/BTO nanocomposites (i.e., where the Si/CNT/Al2O3 mass ratio was 49:21:30) to confirm the fact that the effect of the BTO to the electrochemical performance enhancement was not mainly attributed to the mechanical confining effect by the electrochemically inactive BTO (see Figure S8). We found that the cycle performance and rate capability of the Si/CNT/Al2O3 nanocomposites were inferior to those of the Si/CNT nanocomposite, which is attributed to the low electrical conductivity of Al2O3. Therefore, it was proven that the piezoelectricity of the BTO contributed to the dramatic electrochemical performance enhancement. Next, we carried out simulations to investigate the mechanism responsible for the improvement in the electrochemical performance of the Si/CNT/BTO nanocomposite anode. Using Comsol Multiphysics software, we investigated the piezoelectric potential of the BTO nanoparticles that was induced by the compressive stress resulting from the volume expansion of the Si nanoparticles during charging. We calculated the negative piezoelectric potentials of BTO nanoparticles (see Figure S9) and used these negative piezoelectric potentials as the electric potential boundaries of the BTO particles during charging. We also used Comsol Multiphysics to simulate a battery system with the piezoelectric BTO nanoparticles as the anode, with a Newman’s porous electrode model43 and a capacity fade model.44 The electrolyte solution was modeled as a binary electrolyte consisting of a single lithium salt and a single solvent at a temperature of 298 G

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Figure 7. Effects of the poled potential of the BTO nanoparticles on the electrochemical performance of the cell. (a) Discharge curve during the 30th cycle. After a constant current density of 24 A/m2 and a further charging step at a constant voltage, the cells were discharged until the voltage reached 3.1 V. (b) Current profiles (inward to positive electrode) of the battery during the 30th cycle. Note that negative current density is related to discharging, while positive current density results from charging. (c) Li-ion intercalation concentration and (d) resistance of SEI layer with various piezoelectric poling potentials of BTO nanoparticles and charge−discharge cycles.

decreased from 4.1 to 3.1 V; further discharging at a constant voltage of 3.1 V until the cell current reached −0.1 A/m2; a charging step at a constant current until the cell voltage increased from 3.1 to 4.1 V; a further charging at constant voltage until the cell current reached 0.1 A/m2; and a final stage during which the open-circuit conditions were maintained until the total cycle time reached 12 000 s. Because the electrolyte solution can diffuse into the electrode domain in the porous electrodes, the intercalation reactions depend on the depth of the electrode, whereas the intercalations always occur near the surface of a solid electrode.46 As shown in Figure 6b, for a battery system without the electrical potential of the BTO nanoparticles (i.e., the piezoelectrically nonpoled case), the local current density near the separator was larger than that near the current collectors. In contrast, the current density of the battery with poled BTO particles in the anode became large over a wider area than for the nonpoled case, as shown in Figure 6c−e. It follows that the charge transfer reactions occur more actively and widely through the negative electrodes (i.e., nearer the current collectors) due to the negative piezoelectric potential of the poled BTO nanoparticles. This behavior was more marked as the piezoelectric potential of the poled BTO particles increased, as shown in Figure 6f. We also investigated the effect of BTO concentration and distribution on the local current density. As the BTO concentration increased, the average current density was gradually increased, which was due to increased area of electrode with piezoelectric potential (Figure S11). On the other hand, the current density profiles became

more uniform as the number of particle increased (Figure S12). The increased BTO concentration resulted in accelerated charge transfer reaction and more uniform distribution of BTO particles brought about a small deviation of the local current density (Figure S13). We conclude that charging is more rapid with the poled BTO anode than with the nonpoled BTO anode because the current density caused by the charge transfer reaction is proportional to the intercalation reaction rate.47 We investigated how poling of the BTO particles affected the discharge curve of the Li-ion battery by changing the magnitude of the piezoelectric potential of the BTO. As the piezoelectric potential increased, the discharge capacity of the battery also increased, as shown in Figure 7a. This trend was maintained as the number of cycles increased (see Figure S14). The overall charge and discharge curve shown in Figure 7b also supports this conclusion. Details of the charge and discharge steps, including the additional stabilization time, are provided in the Supporting Information (see Figure S15). The charging time was shorter with the poled BTO anode than with the nonpoled BTO anode. As the piezoelectric potential increased, the charging time was reduced. It follows that the piezoelectric potential increased the intercalation rate of the Li-ions. The concentration of Li-ions in the negative electrode was investigated, as shown in Figure 7c. Compared with an anode formed using nonpoled BTO nanoparticles, the poled BTO particles appear to have increased the Li-ion concentration. The gradient of the concentration−time curve gives the intercalation rate; the steeper gradient of the poled BTO system is therefore consistent with a shorter charging time, resulting from H

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situ and ex situ XRD and electrochemical studies using a 2032 cointype half-cell were carried out to evaluate microstructural changes and electrochemical reaction kinetics. The source for the ex situ XRD characterization was a Cu Kα source (λ = 0.154 nm), and the source for the in situ XRD measurements was a Mo Kα source (λ = 0.071 nm). A two-electrode 2032 coin-type half-cell was prepared to evaluate the electrochemical performances of the prepared composite anode materials. An anode slurry was formed from the Si/CNT/BTO nanocomposite and Li-PAA binder in deionized water, where the nanocomposite-to-binder mass ratio was 9:1 and was screen-printed onto a copper foil substrate, followed by drying at 120 °C for 2 h. Note that no additional conducting agent was added because the CNTs were incorporated in the nanocomposites. Lithium foil and polyethylene film (Star 20, Asahi Kasei) were used as the reference electrode and the separator, respectively. A mixed solution of ethyl carbonate, diethyl carbonate, and fluoroethylene carbonate (with an EC/DEC/FEC volume ratio of 2:6:2) including 1.3 M LiPF6 (Panax Etec) was used as the electrolyte. The electrochemical performance was then investigated in the voltage range of 0.01−1.5 V. The current density in the first cycle was set at 0.2C and then at 0.5C for the following cycles, where 1C corresponds to 2200 mA g−1. The rate capabilities were evaluated by maintaining the current density for lithiation (i.e., 200 mAh g−1) and varying the current density for delithiation from 200 to 2000 mAh g−1.

the piezoelectric potential of the BTO nanoparticles (see Figure 7c). Because the open-circuit potential depends on the concentration of intercalated Li-ions and thus determines the overpotential, an increase in intercalation results in an increased cell potential.48 It follows that the poled BTO nanoparticles increased the discharging capacity (see Figure 7a). As the cycle proceeded, the nonpoled BTO anode exhibited a low intercalation rate; however, with the poled BTO structure, the intercalation rate was maintained (see Figure S16). Finally, the resistance of the solid−electrolyte interphase (SEI) layer was investigated during cycling by varying the piezoelectric potential of the BTO particles from 0 to −0.3 V in steps of 0.05 V, as shown in Figure 7d. The resistance of the SEI layer decreased as the piezoelectric potential of the BTO nanoparticles increased because of the short charging time. Note that the resistance of the SEI layer was saturated beyond a certain piezoelectric potential. This reduction in the resistance of the SEI layer improved the performance of the battery. The resistance of the SEI layer is related to its thickness. Although the ionic conductivity was sufficiently large for the Li-ions to intercalate/deintercalate into the active materials, a thick SEI layer results in a potential drop due to the low electrical conductivity of the SEI layer (3.79 × 10−7 S/m). As a result, a reduction in the resistance of the SEI layer (see Figure S17) is helpful in maintaining the performance of the battery,49 which suggests that the piezoelectric potential of the BTO nanoparticles improved the performance of the battery.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b07674. Supplementary morphological and electrochemical results are provided, and theoretical information is discussed in a detailed manner (PDF)

CONCLUSIONS We prepared Si/CNT/BTO nanocomposites using a costeffective and scalable method (i.e., high-energy ball milling). The mechanical stress resulting from the expansion of Si during lithiation was transferred via the CNT matrix to BTO nanoparticles. The piezoelectric potential of the BTO nanoparticles was maintained due to the dense CNT matrix, as shown using in situ analyses. We demonstrated experimentally that the piezoelectric potential of the BTO resulted in significant improvements to the electrochemical performance of the SI/CNT/BTO nanocomposite anode. Simulation studies revealed that the piezoelectric potential was related to rapid discharging and charging: the increased mobility of the Li-ions resulted in an improvement in the discharge capacity and the cycle performance.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

B.L. and W.Y. designed the concept and experiments. B.L. and J.P. performed the experiments. C.J. and K.K. conducted the in situ XRD coin-cell test. B.L., H.J., K.L., Y.K., and H.P. analyzed the electrochemical behavior. J.Y., D.K., and S.J. analyzed the Bulter−Volmer kinetics. B.L., H.J., and W.Y. co-wrote the paper. J.P., W.Y., and S.D. conceived and guided the project. All authors discussed the results and commented on the manuscript at all stages.

METHODS The Si/CNT/BTO nanocomposites were prepared using a highenergy ball-milling process (SPEX mill). Si nanoparticles were fabricated by pulverizing the Si microparticles (with a diameter of D ∼ 5 μm, Kojundo) using a planetary milling machine. The average diameter of the milled Si nanoparticles was approximately 300 nm. The Si nanoparticles, multiwalled CNTs (with diameters in the range of 15−20 nm, and with lengths in the range of 15−20 μm), and BTO nanoparticles (D < 100 nm, Aldrich) were then premixed using agate for 10 min and ball-milled in Ar for 1 h using a high-energy milling process with stainless steel balls in a milling jar. The BTO nanoparticles and CNTs were used as purchased. The Si/CNT/ BTO mass ratio of the nanocomposite was 49:21:30. Additionally, a Si/CNT nanocomposite (i.e., without BTO) with a Si/CNT mass ratio of 70:30 was prepared for comparison. The morphology and cross section of the nanocomposites were examined using focused ion beam scanning electron microscopy (Helios 450F1, FEI). The cross section was further evaluated using EDS. The microstructure was investigated using wide-angle XRD (X’pert, PANalytical) and Raman spectroscopy (inVia, Renishaw). In

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Midcareer Researcher Program through a NRF grant funded by the Korea government (MSIP) (2013R1A2A2A01067717). The authors thank Prof. Ki-Suk Kang for his valuable discussion. REFERENCES (1) Lee, B.-S.; Son, S.-B.; Park, K.-M.; Yu, W.-R.; Oh, K.-H.; Lee, S.H. Anodic Properties of Hollow Carbon Nanofibers for Li-Ion Battery. J. Power Sources 2012, 199, 53−60. (2) Lee, B.-S.; Son, S.-B.; Seo, J.-H.; Park, K.-M.; Lee, G.; Lee, S.-H.; Oh, K. H.; Ahn, J.-P.; Yu, W.-R. Facile Conductive Bridges Formed between Silicon Nanoparticles inside Hollow Carbon Nanofibers. Nanoscale 2013, 5, 4790−4796. I

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DOI: 10.1021/acsnano.5b07674 ACS Nano XXXX, XXX, XXX−XXX