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3D Nanocomposite Architectures from CarbonNanotube-Threaded Nanocrystals for High-Performance Electrochemical Energy Storage Zheng Chen, Yin Yuan, Huihui Zhou, Xiaolei Wang, Zhihua Gan, Fosong Wang,* and Yunfeng Lu* High-performance energy-storage devices including supercapacitors and batteries hold great promise for various critical applications, such as portable electronics, electrified vehicles, and grid-scale energy storage. However, broader application of such devices is still hampered by low energy density, low power density, and insufficient cycling lifetime.[1–3] Accordingly, extensive effort has been devoted to developing high-performance electrode materials with improved energy capacity, rate-capability, and structural stability, such as high-surface-area porous carbon species and low-dimension redox-active metal oxides.[3–5] Despite significant progress, current-state-of-art carbon species possess capacitances of less than 150 F g−1 (or 50 mA h g−1) in an organic electrolyte[6,7] or 300 F g−1 in aqueous electrolytes,[8] which can only provide a moderate device energy density of about 5–10 Wh kg−1. Transition-metal oxides, on the other hand, possess significantly higher capacitance (up to 1000 F g−1 or capacity > 200 mA h g−1);[9–11] harvesting such high capacitance, however, is difficult mainly due to their slow ion diffusion and poor electronic conductivity. Making nanocomposites of low-dimension oxides and carbons (e.g., carbon black,[12,13] carbon nanotubes (CNTs),[14,15] and graphene)[16] has therefore emerged as the most effective strategy towards better energystorage materials, since the low-dimension structure shortens ion-diffusion length while the carbon constituents enable effective electron transport. In the context of composite structure, to date, four main categories of nanocomposites have been explored; these are i) composites made by physical mixing of carbon species and oxide materials,[12,13,17,18] ii) composites with cable-like structure

Z. Chen,[+] Y. Yuan,[+] H. Zhou, X. Wang, Prof. Y. Lu Department of Chemical and Biomolecular Engineering University of California Los Angeles, CA, USA E-mail: [email protected] Y. Yuan, Prof. Z. Gan, Prof. F. Wang Institute of Chemistry Chinese Academy of Sciences Beijing, P. R. China E-mail: [email protected] Y. Yuan University of Chinese Academy of Sciences Beijing, P. R. China [+]These

authors contributed equally to this work.

DOI: 10.1002/adma.201303317

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formed by coating CNTs or other conductive wires with active oxide layers,[19–22] iii) composites with interpenetrative networks constructed by intertwining growth of nanowires within CNT networks,[23–25] and iv) composites with a sandwich structure made by alternately stacking oxide particles and graphene layers.[16,26–28] These composite architectures possess diverse charge- and ion-transport characteristics and structural stability and exhibit discrete performance in energy storage. For the first category of composites, the charge and ion transport pathways are formed by random mixing of binder, oxide, and carbon particles, which is often poorly controlled and ineffective. For the second category of composites, the cable-like structure facilitates effective charge transport locally. The oxide layers, however, increase the CNT contact resistance, which deteriorates the rate performance, for example, electrodes prepared by coating CNTs with TiO2 exhibit improved rate performance in comparison with those made by physical mixing of TiO2 and CNTs; their charge/discharging time, however, is still more than 10 min.[19] For the third category of composites, the interpenetrative network structure provides better conductivity throughout the whole electrodes. The loose CNTs– nanowire interface, however, reduces electrode stability. Accordingly, electrodes based on interpenetrative networks of V2O5 nanowires and CNTs exhibit excellent rate performance but lose more than 10% of their capacity in the first 20 cycles.[29] For the fourth category of composites, the sandwich structure enables intimate contact between the graphene and the active materials. The two-dimensional structure, however, intrinsically limits three-dimensional mass and charge transport. Consistently, sandwich-structure electrodes made from graphene and Fe3O4 show good cycling performance but still with mediocre rate performance.[16] Despite the extensive efforts made, designing more effective materials architecture remains crucial and challenging. Herein, we report a simple but efficient solution process to fabricate a class of novel composite architecture for high-performance electrochemical energy storage. Unlike previous composite approaches mentioned above which mainly involve physical mixing processes, multistep templating, or in situ growth using salt precursors and conductive additives, our strategy is based on a fast solvation-induced assembly that directly uses hydrophobic oxide nanocrystals (NCs) and CNTs as building blocks. As depicted in Scheme 1, NCs and CNTs are first dispersed in nonpolar solvent (e.g., toluene). Upon addition of polar solvent (e.g., methanol), solvation forces induce the hydrophobic NCs to assemble around the hydrophobic CNTs,

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charge and discharge, while the spherical structure with minimized electrode–electrolyte interface offers mechanical and chemical robustness that leads to improved stability. Besides high capacity and excellent cycling stability, the advantageous features provided by our 3D nanocomposite architecture also enable electrodes with extremely high rate capability, which is essential for high-rate batteries and pesudocapacitor applications. By comparison, previous composite structures often provide moderate conductivity, poor composite interface, and structural instability, which lead to mediocre charge-storage performance. Experimentally, NCs of anatase TiO2 and mass-produced CNTs were used as a model system. TiO2 has long been considered a promising energy-storage material owing to its impressive lithium-storage capacity, low cost, and superior safety,[30] however, despite the great efforts made, current TiO2-based electrodes still exhibit moderate rate performance and cycling stability. The CNTs used are low-cost and contain intertwined network structures,[31] which allow efficient encaging of the NCs. The NCs were synthesized from titanium isopropoxide using oleic acid (OA) as the capping ligand.[19] TiO2 NCs and CNTs were first dispersed in toluene; upon addition Scheme 1. Schematic of how the NC/CNT nanocomposite electrode is formed. of methanol, NCs and CNTs were assembled and precipitated out from the mixture, which which leads to the formation of spherical assemblies threaded lead to the formation of robust TiO2/CNTs composite spheres by the CNTs. Subsequent calcination converts the spheres into after calcination. robust CNT-threaded porous particles, from which the porous Figure 1A shows a representative transmission elecstructure is created from the packing voids between the NCs tron microscopic (TEM) image of the NCs, which possess a and the CNTs, as well as calcination of the capped ligands on narrow size distribution with an average diameter of ca. 5 nm the NCs. This 3D structure is quite different from our previous (Figure S1A, Supporting Information). The NCs are highly work,[19] which reported a cablelike structure with a thin layer crystalline, as revealed in the high-resolution TEM image (10–100 nm) of TiO2 nanocrystals deposited on preformed (HRTEM, inset in Figure 1A). In addition, since these NCs are CNT scaffolds, leading to free-standing flexible electrode films. capped with OA ligands, they can be stable in solvent (toluene) Herein, the nanocomposites are in the form of micrometerfor a few months without appreciable precipitation (Figure S1B, size particles which can be easily fabricated into electrodes by Supporting Information). Figure 1B presents a scanning eleca slurry-coating process compatible with existing battery manutron microscope (SEM) image of the CNTs used for building facturing. More importantly, by developing such 3D spherical 3D spherical NC/CNT architectures. The CNTs, with diameter architecture, the capacity, rate capability, and cycling stability of 20–30 nm and length of 5–10 μm, intrinsically show interhave been significantly enhanced compared with those of pretwined network structures. Figure 1C shows a SEM image of vious composite structures. the as-prepared NC/CNT composites, which present a spherical Such a 3D spherical architecture offers unique characterismorphology with sizes ranging from a few hundred nanometers tics needed for high-performance electrodes. For electron conto a few micrometers. A representative composite particle is ductivity, such a structure not only enables intimate contacts presented in Figure 1D, in which an inset high-magnification between the oxide and the CNTs locally; close packing of such SEM image clearly shows the CNTs threading through the NC spherical particles also enables entanglement and intimate networks. Figure 1E and 1F show TEM images of a typical comcontacts of the CNTs, which ensures excellent network conducposite particle penetrated by CNTs, which further confirms the tivity through the whole electrode. For ion transport, the porous formation of CNT-threaded nanocrystal particles. structure enables effective channels for electrolyte transport, Figure 2A shows thermogravimetric analysis (TGA) curves of while the active framework made from NCs provides shortened the NCs, CNTs, and the NC/CNT composite in air. No appreciable ion-diffusion length. For electrode stability, the porous strucweight loss can be observed for the CNTs at temperatures below ture helps to buffer the stress that may be generated during 600 °C; the weight loss of the NCs is about 20%. Accordingly, 340

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the sintered NC/CNT composite consists of 80 wt% TiO2 and 20 wt% CNTs. Figure S2 and S3 in the Supporting Information show the electrochemical performance and morphology of the composites made with different amounts of CNTs. The composites made with 2 wt% CNTs do not develop such well-defined 3D architecture. The composites made with 10–30 wt% CNTs exhibit similar morphology and structure. Consistent with the morphology evolution, the composites exhibit better rate performance with increased CNT content but with reduced electrode packing density. Herein, we focus on the composite with 20 wt% CNT to demonstrate the architecture design. Figure 2B shows X-ray diffraction (XRD) patterns of as-synthesized NCs and the NC/CNT composite (20 wt% CNT) after removal of the capping agents. The diffraction pattern of as-synthesized NCs matches well with that of anatase TiO2 (JCPDS card No. 21–1272). The peak broadening originates from their small crystallite size, which is calculated from the Scherrer equation to be 5.3 nm, and is consistent with the TEM observations. Sintering the NC/CNT composites in air successfully removed the capping agent (Figure 2A) yet led to no noticeable growth of grain size; the NCs retain the anatase phase with an average size of ca. 5.5 nm. Since the CNTs penetrate through the spherical NC/CNT architectures, the presence of CNTs may retard the relocation and sintering process of the nanocrystals during the heat treatment, preventing undesired NC growth. The pore structure of the composite was further quantified by using the nitrogen-sorption technique. The CNTs show a macroporous structure (Figure S4, Supporting Information) with an open network structure as observed by using SEM (Figure 1B), while the particles made by assembling the NCs (without the CNTs) display a mesoporous structure (Figure 2C). By comparison, the NC/CNT composite exhibits a hierarchically porous structure which benefits electrolyte transport. The first nitrogen-uptake step at a relative pressure below 0.2 is due to the micropores mainly formed by the linkage of NCs during the calcination process; the significant nitrogen-uptakes at relative pressures between 0.4 and 0.8, and above 0.8, suggest the coexistence of mesopores and macropores. Figure 2D compares the pore-size distributions of different materials, where the presence of macropores (> 50 nm) may not show up due to intrinsic limitations of N2 sorption measurement. The pore-size distribution of TiO2/CNT composite with a peak pore size of 4 nm also suggests a hierarchical pore structure, which is consistent with the nitrogen-sorption isotherms. In addition, the CNTs and the NC-only particles have surface areas of 212 and 140 m2 g−1, respectively. The NC/CNT composite with 20 wt% of CNT has a surface area of 204 m2 g−1, which is significantly higher than that estimated based on its composition (146 m2 g−1). This result indicates that the presence of CNTs effectively prevents the growth of NC size during the sintering process, which is essential to retain the shortened ion-transport distance. Moreover, the as-formed hierarchical porous structure with high surface area provides the electrodes with efficient ion transport and abundant active sites, which are essential for fast electrode kinetics. Charge-storage behavior of the composites was first characterized by cyclic voltammetry (CV) using coin-type half cells, which are commonly used to characterize both supercapacitor and battery electrodes.[32] The mass loading of active materials

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in each electrode may strongly affect the storage performance. Here the loading of each TiO2/CNT electrode was controlled to be 1.5–2 mg cm−2, which is comparable to previous highrate TiO2-based lithium-storage electrodes.[33,34] Figure 3A presents representative cyclic voltammograms for electrodes at a sweep rate of 0.5 mV s−1. The electrochemical Li+ insertion/ extraction processes occurring at anatase TiO2 electrode can be expressed by: TiO2 + xLi+ + xe−↔ LixTiO2, where x is the mole fraction of the inserted lithium ions (x ≤ 1).[35] A slight shift of the CV curves was observed during the first few cycles (especially cathodic scans) due to irreversible reactions. The observed capacity loss from the initial scans may be ascribed to irreversible replacement of absorbed protons by lithium ions and other electrolyte-related surface reactions.[33,36] Nevertheless, the charge/discharge efficiency increased from 90% to 100% after the first five cycles. Two well-defined current peaks were observed at 1.7 (cathodic sweep) and 2.1 V (anodic sweep), which correspond to the biphasic transition between tetragonal anatase and orthorhombic lithium titanate; this is consistent with the typical insertion/extraction behavior of lithium in TiO2.[37,38] CV measurement at different sweep rates was also performed to evaluate the rate behavior (Figure S5, Supporting Information). Although a slight peak shift was observed due to increased rates, the current peaks corresponding to characteristic lithium reactions were well retained as sweep rates increased from 1 to 20 mV s−1. The CV curves with large symmetric under-curve areas at high sweep rates indicate that a significant portion of the charge storage is from surface reactions.[34,39] Typical galvanostatic charge/discharge curves of the composite electrodes at a rate of 0.25C (1C = 170 mA g−1) are shown in Figure 3B. A distinct voltage plateau is observed for both the charge and discharge plots, which indicates coexistence of anatase and lithium titanate phases.[34,37] A capacity of ca. 130 mA h g−1 is estimated from the plateau region of the discharge plot, which is ascribed to the diffusion-controlled intercalation of lithium occurring within the crystalline frameworks. The slope region mainly accounts for a surface process with capacity of about 140 mA h g−1, which is consistent with the small NC size and high surface area that provide abundant surface active sites for fast surface/interfacial lithium reactions.[35,37,39] During the charging process, a slope region followed by a plateau at higher voltage was also observed, which indicates a similar lithium-extraction process involving both the diffusion and surface processes. The CNTs within the nanocomposites are only responsible for a small portion of capacity contribution (ca. 20 mA h g−1, Figure S6, Supporting Information). The electrodes deliver a reversible capacity of about 270 mA h g−1 of TiO2, which is amongst the highest TiO2 capacities reported.[17,19,33,34,40] Figure 3C shows charge/discharge curves of the composite electrodes at different cycling rates. The TiO2 in the electrodes delivers a reversible capacity of about 265 and 220 mA h g−1 at rates of 1 and 20C (1C=170 mA/g), respectively. Remarkably, at the extremely high rates of 100 and 200C (charge/discharge in 22 and 17 s), where the discharge plots only show the slope region, the electrodes still retain high capacities of ca. 150 and 12 mA h g−1. Such high capacities at high rates have never been reported with anatase TiO2 or TiO2-B.

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To further dissect this ultrafast charge-storage process, the total electrode charge can be separated into surface-related capacity (qs) and diffusion-controlled charge (qd) by using the Trasatti analysis,[41] where qs is correlated with fast double-layer capacitance and pseudocapacitance and qd mainly depends on slow diffusion processes. For a semi-infinite linear-diffusion process, within a reasonable range of charging rates, qs can be derived by plotting the total charge qT against the square root of the charging time (t) and extrapolating t to zero (Figure S7, Supporting Information). Figure 3D shows the dependence of qs and qd on the discharge rate. The capacitive charge remains constant at different discharge rates (ca. 11 mA h g−1), and the value is close to the capacity obtained from the sloping regions of discharge curves (120–140 mA h g−1). By comparison, the diffusion-controlled capacity decreases rapidly with increasing discharge rate (e.g., 133, 87, and 15 mA h g−1 at 1, 10, and 100C, respectively). Therefore, with increasing charging rate, capacitive contribution to the overall capacitance increases rapidly (e.g., ca. 45% at 0.25C and ca. 90% at 100C). By taking account of the surface area (204 m2 g−1) and the composition of the composites, the surface-normalized specific capacitance of TiO2 is estimated to be ca. 150 μF cm−2 at a rate of 100C, which is about one order of magnitude higher than that of double-layer capacitance (about 5–15 μF cm−2),[6] indicating a pseudocapacitive nature. This result explains the exceptional high rate performance of the TiO2 composite electrodes.[35,39]

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Due to their poor electronic conductivity (10−8–10−11 S cm−1)[42] and low lithium diffusion coefficient (10−11–10−17 cm2 s−1),[35,42] TiO2-based capacitors are made only in the form of ultrathin films with extremely low energy densities.[35,43] The formation of CNT-threaded 3D nanocomposite architecture significantly improved electrode conductivity. Direct measurement of the conductivity using pellet samples shows that the conductivity increases to 0.015 (2 wt% CNT), 1.4 (10 wt% CNT), and 2.2 S cm−1 (20 wt% CNT) by effectively threading TiO2 with CNTs. This result is also confirmed by electrochemical impedance spectroscopy (EIS) (Figure S8, Supporting Information) of a single electrode. The resistance of TiO2/CNT electrodes (0.014 Ohm g) was less than 50% of that of the TiO2 electrode (0.03 Ohm g) prepared with carbon black and was close to that of the CNT electrode (0.01 Ohm g). The hierarchically porous, conductive NC-CNT architecture enables superfast charge storage in bulk TiO2 electrodes with dramatically improved energy density.[44] Figure 4E further compares the performance of the composite with a series of TiO2 composites with cablelike,[19] nanosheet,[33] 3-nm TiO2-B,[34] and sandwich[40] structures, as well as to that of the composite made by a simple mixing process.[17] Clearly, the CNT-threaded architecture outperforms the others. Although the electrode made from 6-nm anatase and an extremely high content of carbon black (45 wt%) shows a slightly higher capacity at 100C rate, such electrodes show a rapid capacity loss (50% after 100 cycles).[17] In contrast, the CNT-threaded composites show significantly

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Figure 4. A) Representative galvanostatic charge/discharge curves of TiO2/CNT-AC asymmetric supercapacitors from 1.0–3.0 V at different current densities in 1 m LiPF6 in EC/DMC (a–e: 1, 2, 6, 10, 15 A g−1, the current density is based on the mass of TiO2/CNT electrode and the CNT content is 20 wt% in this electrode). B) Comparison of Ragone plots of various devices, including asymmetric supercapacitors based on a TiO2/ CNT nanocomposite anode and AC cathode, symmetric supercapacitor made from the same AC, and asymmetric supercapacitor devices based on AC cathode and a V2O5/CNT[29] or Li4Ti5O12/CNF[46] anode. All the data are based on the total mass of electrode materials.

enhanced cycling stability (87% retention of its initial capacity after 1000 cycles at a rate of 20C; Figure 4F). The capability to make such robust, high-rate and high-capacity nanocomposites enables the fabrication of high-performance supercapacitors, demonstrated herein by fabricating asymmetric prototypes using the CNT-threaded composites as the anode and a commercial activated carbon (AC) as the cathode. Figure 4A shows the representative galvanostatic charge/discharge curves at different rates. At a current of 0.25 A g−1, the devices provided a maximum cell capacitance of about 54 F g−1, which gives an energy density of 59.6 Wh kg−1 at a power density of 120 W kg−1.

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This energy density is about twice as high as that of traditional electrical double-layer capacitors (EDLCs) and close to that of graphite anode-based lithium-ion capacitors.[45] Even at a power density of 7 or 13.9 kW kg−1 (charge/discharge in 16 or 5.8 s), the devices still provided an energy density of 31.2 or 22.3 Wh kg−1, respectively. In addition, the asymmetric cell showed a maximum power density of 120 kW kg−1. Such a high power performance is attributed to the low device resistance and efficient ion transport, as indicated by the EIS results (Figure S9, Supporting Information). The asymmetric supercapacitor made from a CNTthreaded composite anode and a commercial AC cathode shows a much smaller equivalent series resistance (ESR) than that of symmetric AC/AC supercapacitors based on two identical AC electrodes with comparable loadings of active materials (8.2 vs. 13.2 Ω for the full cells). Figure 4B further compares the Ragone plots of a series of supercapacitors, including symmetric prototypes made from the same AC electrodes (Figure S10, Supporting Information) and asymmetric prototypes based on an AC cathode and a V2O5/CNT[29] or Li4Ti5O12/CNF anode[46] Clearly, the devices based on the CNT-threaded architecture offer significantly higher energy and power densities than the aforementioned cells, including the state-of-the-art Li4Ti5O12/CNF-AC prototypes, which further confirms the advantages of such well-designed CNT-threaded architecture. Some high-purity single-walled CNT-[47] or graphene-[48] based EDLCs have higher energy density due to their use of 4 V electrolytes, while the high performance of our asymmetric devices is ascribed to the effective electrode design. We expect that the device energy density could be significantly improved if cathodes with high capacitance were available. In summary, we have developed a high-performance pseudocapacitive electrode architecture made from CNTs and in-built NCs. By threading spherical assemblies of active NCs with CNTs, highly robust nanocomposites with effective ion and electron transport pathways and large number of active surface sites were successfully synthesized by using a simple solvation-induced assembly. Such a unique architecture endows the electrodes and devices with high rate, high capacity, and long cycling stability. Moreover, considering that the synthesis of nanocrystals with controlled composition and structure was well developed in the last decade,[49] and that CNTs can now be produced in large scale at low cost,[31] this work offers a powerful strategy towards high-performance energy-storage devices.

Experimental Section Synthesis of TiO2 NCs: The TiO2 NCs were synthesized by using a two-phase hydrothermal reaction. In a typical synthesis, 0.4 mL of tert-butylamine was dissolved in 40 mL of water and the solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave. Subsequently, 4.0 mL of oleic acid (OA) and 2.0 g of titanium (IV) n-propoxide were dissolved in 40 mL of toluene in air and the solution was transferred into the autoclave without any stirring. The autoclave was sealed and maintained at 180 °C for 6 h and cooled down to room temperature with tap water. The crude solution of TiO2 NCs was precipitated with methanol and further isolated by centrifugation and decantation. The purified TiO2 NCs were redispersed in toluene to form a TiO2 NC solution for future use. These NCs contain ca. 20 wt%

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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This research work was supported as part of the Molecularly Engineered Energy Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award DE-SC001342 (Y.L). The authors also acknowledge support from General Motor Inc. Y.Y. acknowledges the sponsorship of the China Scholarship Council. Received: July 18, 2013 Revised: September 15, 2013 Published online: October 31, 2013

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OA capping on the surface according to TGA measurements. Direct calcination of these NCs creates porous TiO2. Synthesis of TiO2 NC/CNT Composites: For the nanocomposites containing 20 wt% of CNTs, 43 mg of CNTs were dispersed in 20 mL of toluene solution containing 0.17 g of TiO2 NC under ultrasonication and stirring. Then 25 mL of methanol was added into the mixture, inducing the assembly of the NCs and the CNTs, which were agitated and separated by centrifuge or filtration. The as-formed wet composites were then dried at 50 °C and annealed at 450 °C for 2 h in air. TiO2/CNT composites with CNT contents of 2, 10, and 30 wt% were synthesized by adding 3.5, 19, and 73 mg of CNTs into the same amount of NC solution, followed by the same annealing treatment. Material and Electrode Characterization: See Supporting Information.

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3D nanocomposite architectures from carbon-nanotube-threaded nanocrystals for high-performance electrochemical energy storage.

Better electrode architecture: spherical assemblies of electrochemically active nanocrystals threaded with carbon nanotubes are made using a simple so...
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