Pseudocapacitive Sodium Storage in Mesoporous Single-Crystal-like

Pseudocapacitive Sodium Storage in Mesoporous Single-Crystal-like TiO2− Graphene Nanocomposite Enables HighPerformance Sodium-Ion Capacitors Zaiyuan Le,† Fang Liu,† Ping Nie,† Xinru Li,† Xiaoyan Liu,† Zhenfeng Bian,‡ Gen Chen,† Hao Bin Wu,*,† and Yunfeng Lu*,† †

Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, California 90095, United States Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Department of Chemistry, Shanghai Normal University, Shanghai, 200234, China

‡

S Supporting Information *

ABSTRACT: Sodium-ion capacitors can potentially combine the virtues of high power capability of conventional electrochemical capacitors and high energy density of batteries. However, the lack of high-performance electrode materials has been the major challenge of sodium-based energy storage devices. In this work, we report a microwave-assisted synthesis of single-crystal-like anatase TiO2 mesocages anchored on graphene as a sodium storage material. The architecture of the nanocomposite results in pseudocapacitive charge storage behavior with fast kinetics, high reversibility, and negligible degradation to the micro/nanostructure. The nanocomposite delivers a high capacity of 268 mAh g−1 at 0.2 C, which remains 126 mAh g−1 at 10 C for over 18 000 cycles. Coupling with a carbon-based cathode, a full cell of sodium-ion capacitor successfully demonstrates a high energy density of 64.2 Wh kg−1 at 56.3 W kg−1 and 25.8 Wh kg−1 at 1357 W kg−1, as well as an ultralong lifespan of 10 000 cycles with over 90% of capacity retention. KEYWORDS: sodium-ion capacitor, TiO2, nanocomposite, pseudocapacitive, energy storage

E

electrode can inherit the merits of both ECs and LIBs, thus delivering high energy/power densities and long lifetime.9 While lithium-ion capacitors (LICs) have been successfully demonstrated based on several host materials for lithium ions, developing sodium-ion capacitors (SICs) are much economically favorable, especially in large-scale applications due to the wide availability of sodium source.10−16 However, the lack of proper sodium storage materials has been the major challenge for sodium-based energy storage devices.17−21 For example, as a remarkable anode material for LIBs, graphite is thermodynamically unfavorable for sodium insertion, leading to negligible capacity.11,20,22 Hard carbon demonstrated a reversible capacity of 250 mAh g−1, yet the low working potential leads to safety concerns.23 High capacity anode materials based on alloying and conversion mechanisms have been under develop-

lectrochemical capacitors (ECs) hold great promise for future energy storage applications, such as grid/voltage stabilization, regenerative braking for automobile, and uninterruptible power supply (UPS).1−3 Compared with lithium-ion batteries (LIBs), one of the dominant rechargeable battery systems, ECs deliver ∼10-fold higher power density and ∼100-fold longer cycle life; however, the relatively low energy density of ECs (1−2 times of magnitude lower than that of LIBs) is a major limitation for their practical applications.4 Commercially available ECs are mostly electric double-layer capacitors (EDLCs) using porous carbons as electrode materials, which store charges based on double-layer ion desorption/adsorption.1,5 Even with organic electrolytes to boost the cell voltage, the low capacitance (typically below 300 F g−1) of carbon-based electrodes limits the energy density of EDLCs to ∼10 Wh g−1.6,7 One promising strategy to solve the problem is to build hybrid cells using electrodes with different charge storage mechanisms.8 For example, ion capacitors based on an EDLC-type carbon electrode and an insertion-type © 2017 American Chemical Society

Received: December 12, 2016 Accepted: March 10, 2017 Published: March 10, 2017 2952

DOI: 10.1021/acsnano.6b08332 ACS Nano 2017, 11, 2952−2960

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Figure 1. (a) XRD patterns of MWTO and MWTOG nanocomposite. (b) Representative SEM images of MWTOG nanocomposite. (c, d) TEM and HRTEM images of MWTOG with mesoporous single-crystal-like structure (inset of d shows the SAED pattern). (e, f) Nitrogen sorption isotherms and pore size distribution of MWTO and MWTOG nanocomposite.

Such device delivers a high energy density of 64.2 Wh kg−1 at a power density of 56.3 W kg−1, and 25.8 Wh kg−1 at a high power output of 1357 W kg−1. A high capacity retention of 90% is achieved even after 10 000 charge−discharge cycles.

ment;21,24−26 however, they typically show poor kinetics and cycling stability.13 Therefore, sodium-based energy storage systems with high energy/power densities and long cycle life are yet to be developed. Herein, we report a nanocomposite architecture of mesoporous single-crystal-like TiO2 particles (TiO2 mesocages) anchoring on graphene sheets for ultrafast pseudocapacitive sodium storage. Such sodium storage material enables SICs with high energy/power densities and long lifespan. Although ultrafast lithium storage in several polymorphs of TiO2 has been well demonstrated,27−30 pseudocapacitive insertion/ deinsertion of sodium ions is thought to be more challenging due to the larger size of sodium ions (1.02 Å) than that of lithium ions (0.76 Å). Huang and co-workers demonstrated the high capacitive contribution of sodium storage in a TiO2graphene nanocomposite primarily composed of TiO2-B, a lowdensity TiO2 phase suitable for ion insertion.31 However, ultrafast sodium storage in more common and stable phases of TiO2, such as anatase, has not yet been fully understood.32−35 Similar to the cases of lithium storage, insertion/deinsertion behavior of sodium ions in anatase TiO2 shows high dependency on the micro/nanostructures of the materials.36−40 The as-prepared TiO2 mesocage-graphene nanocomposite exhibits several structural and compositional features that benefit the electrochemical charge storage: (1) the mesoporous texture and small primary building blocks of TiO2 mesocages offer fast ion insertion/deinsertion and short diffusion distance; (2) the graphene sheets provide continuous electronic conducting network; (3) the robust submicrometer architecture of TiO2 mesocages ensures stable cycling performance. As a result, the nanocomposite demonstrates ultrafast pseudocapacitive sodium storage capability and negligible capacity fading upon cycling. By coupling with commercial active carbon, highperformance full cells of SICs have been successfully fabricated.

RESULTS AND DISCUSSION Synthesis and Characterizations of TiO2 MesocagesGraphene Nanocomposite. A microwave-assisted solvothermal method has been developed to prepare the TiO2 mesocagegraphene nanocomposite (denoted as MWTOG) with short reaction duration. During the synthesis, oxygen-containing functional groups on graphene sheets serve as the nucleation sites and initiate the growth of TiO2 mesocages, resulting in strong coupling between the TiO2 mesocages and graphene sheets. Moreover, the relatively low oxygen content of the graphene sheets (7−7.5 atom %) maintains a high electronic conductivity and probably assists the formation of mesocage architecture by eliminating excess nucleation sites.41 The crystallographic information on MWTOG is collected by X-ray diffraction (XRD) as shown in Figure 1a. Both of the diffraction patterns of MWTOG nanocomposite and pure TiO2 mesocages (MWTO) match well with anatase TiO2 phase (JCPDS card No. 21-1272). The average grain size (i.e., the size of the primary TiO2 building nanocrystals) is calculated to be ∼8.1 nm from the Scherrer equation, which is consistent with our previous report.42 For the MWTOG sample, the diffraction peaks of graphene cannot be identified, which may be ascribed to the overlapping with the (101) diffraction peak of anatase TiO2 and/or the absence of restacked graphene. The content of graphene in the MWTOG nanocomposite determined by thermogravimetric analysis (TGA) is 12.1 wt % (Figure S1, see the Supporting Information (SI)), which is generally consistent with the nominal content of 10 wt % based on the added amount of graphene and Ti source during synthesis. 2953


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Figure 2. Electrochemical performance of MWTOG electrode in a potential window of 0.01−3 V vs Na/Na+. (a) Galvanostatic charge/ discharge profiles of MWTOG at 5 C with a small voltage hysteresis of 193 mV. (b) Rate performance of MWTOG and MWTO electrodes, ramping from 0.2 to 20 C and back to 5 C. (c) Long-term cycling performance of MWTOG electrodes at 5 C (1675 mA g−1) and 10 C (3350 mA g−1). (d) Charge−discharge profiles of 500th, 5000th, 10 000th, and 15 000th at 10 C.

While with 12.1 wt % of graphene, the total surface area of MWTOG is reduced to 155 m2 g−1, which may be attributed to the wrapping of mesocages by graphene sheets. MWTO possesses rich mesopores centered at around 3.8 nm as shown in Figure 1f, while introducing graphene slightly diminishes these interparticle pores and creates larger nanopore over 10 nm. The total pore volume of MWTOG (0.20 cm3 g−1), however, remains similar to that of pristine MWTO (0.22 cm3 g−1), resulting in sufficient electrolyte uptake for fast ionic transport. Such a porous heterostructure is expected to enhance the electron conductivity effectively by the graphene network, meanwhile shorten the ionic pathway with mesopores throughout the particle. Moreover, it helps maintain a relatively high packing density compared with a conventional nanoparticles (Figure S3, SI).27,28 Electrochemical Sodium Storage Performance of MWTOG. The electrochemical performance of MWTOG is first evaluated in a half cell using sodium foil as both counter and reference electrodes. MWTOG samples with different contents of graphene were synthesized and evaluated, which are denoted as MWTOG-X (X refers to the nominal weight percentage of graphene). Figure 2a presents galvanostatic charge/discharge profiles of the MWTOG-10 electrode within a potential window of 0.01 to 3 V vs Na/Na+. A reversible capacity of 162 mAh g−1 is achieved at a high rate of 5 C (1 C = 335 mA g−1).32 Unlike typical lithium storage in anatase TiO2, where major capacity is contributed by the voltage plateau at 1.75−2.1 V (vs Li/Li+), the main capacity contribution for sodium ion is below 1 V (vs Na/Na+). Such a voltage profile

Figure 1b shows a representative scanning electron microscope (SEM) image of the MWTOG. It can be clearly observed that the spheroid-shaped TiO2 mesocages are uniformly anchored on both sides of graphene sheets. The size of the TiO2 mesocages falls in the range of 100 to 350 nm (inset of Figure 1b). MWTO prepared without graphene exhibits a much broader size distribution (Figure S2, see the SI), implying the important role of graphene in supporting the homogeneous growth of TiO2 mesocages. Transmission electron microscope (TEM) images in Figure 1c further reveal a highly porous feature of the TiO2 particle arising from the voids between TiO2 building nanocrystals. The selected area electron diffraction (SAED) pattern of an individual TiO2 mesocage (inset of Figure 1c) reveals a single-crystal-like anatase pattern along the [001] zone axis with {200} diffraction spots.42 This observation is distinct from most previous reports on porous TiO2 particles, which are typically polycrystalline. Such a highly crystalline anatase framework ensures good mechanical robustness during electrode cycling. Figure 1d shows a highresolution TEM (HRTEM) image depicting the interface between TiO2 and graphene. The strong interaction between these two components offers fast and robust electronic transfer within the hybrid nanostructure of graphene-supported TiO2 mesocages. This feature is especially advantageous for high-rate capability and long-term cycling stability. By using a nitrogen sorption technique as shown in Figure 1e, the pristine crystalline TiO2 mesocages provide a very high Brunauer−Emmett−Teller (BET) surface area of 233 m2 g−1 with a type-IV isotherm, indicating a mesoporous structure. 2954


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Figure 3. (a) CV curves of MWTOG and AC in half cells (top) and full cell of SIC (bottom), indicating the voltage window of full cell from 1 to 3.8 V. (b) Galvanostatic charge/discharge profiles with traingular shape from 0.2 to 10 C. (c) Regone plot of SIC in this work comparing to representative energy storage devices in literature. (d) Long-term cycle life of SIC at 10 C.

achieved. Reversible capacities of 162 mAh g−1 after 7000 cycles at 5 C (1675 mA g−1) and 126 mAh g−1 after 18 000 cycles at 10 C (3350 mA g−1) are obtained without noticeable capacity decay. To the best of our knowledge, such lifespan is among the best reports for sodium-based anodes.31,44−46 Figure 2d shows the voltage profiles for the 500th, 5000th, 10 000th, and 15 000th cycles. The profiles are generally overlapped except for some small upshift of the charge curves, confirming the good reversibility and stability of as-synthesized nanocomposites. Assembly and Electrochemical Performance of Sodium-Ion Capacitor. On the basis of the excellent electrochemical properties of MWTOG in the half-cell test, we assembled full cells of SIC to demonstrate the feasibility of practical applications. The optimized MWTOG-10 sample is used as the anode and coupled with a cathode made of commercial activated carbon (AC, Kuraray YP50 with a BET surface area of 1500 m2 g−1). The working potential window of each electrode is first determined by a cyclic voltammetry (CV) test in half-cell configuration (top of Figure 3a). AC functions as an EDLC material by adsorbing perchlorate anions up to 4 V (vs Na/Na+) during charge. To avoid the risk of electrolyte decomposition and other side reaction, the working voltage window of full cell is set between 1 and 3.8 V (bottom of Figure 3a). It is worth to point out that the maximal voltage of commercial EDLCs using organic electrolyte is usually limited to 2.7 V.47 Such a higher working voltage of the as-assembled MWTOG//AC SIC would offer higher energy density. Figure 3b displays the typical galvanostatic charge/discharge curves

indicates that anatase TiO2 is a more suitable anode material for sodium-based energy storage systems over the lithium-based counterparts. Moreover, the sloping voltage profile even at a low rate of 0.2 C suggests a possible pseudocapacitive behavior.5 The small voltage hysteresis of 193 mV at a high rate of 5 C also indicates the ultrafast sodium ion insertion/ deinsertion kinetics, which will be further discussed shortly. Figure 2b shows the rate capabilities of various MWTOG electrodes and MWTO electrode. All of these electrodes exhibit similar performance at a low rate of 0.2 C, with continuously decreasing capacities due to possible side reactions. The Coulombic efficiency quickly increases and stabilizes at 100% in the first few cycles. The superiority of MWTOG nanocomposites over the graphene-free MWTO becomes much evident as the current rate steadily increases. Specifically, the optimized performance is achieved with MWTOG-10 electrode. This nanocomposite delivers capacities of 268, 189, 167, 152, 125, 111, 104 mAh g−1 at current rates of 0.2, 0.5, 1, 2, 5, 10, and 20 C, respectively. Note that the sodium storage contribution from graphene in these nanocomposites is negligible because of the thermodynamic unfavorable nature and its low content in the nanocomposites.43 In contrast, the very limited capacity of MWTO can be retained at high rates of 10 and 20 C. Such high rate capability of MWTOG can be attributed to the synergistic effect of both the “electron wiring” from graphene and “ion wiring” from the pore channels. The MWTOG-10 electrode also demonstrates very promising durability as shown in Figure 2c. After activated at 0.2 C for one cycle, long-term cycling stability at high rate can be 2955


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Figure 4. (a) Ex situ XRD pattern of pristine MWTOG (a-1), 1st discharge to 0.01 V (vs Na/Na+) (a-2), 1st charge to 3 V (a-3), and at 3 V after 5 cycles at 1 C (a-after cycles). (b, c) TEM and HRTEM images after 15 000 cycles (inset of c shows a FFT pattern). (d) CV curves from 0.1 to 10 mV s−1 (inset shows a small voltage hysteresis of 65 mV at 0.1 mV s−1). (e) Analysis of b-value for anodic and cathodic peak current. (f) Plot of capacity vs v−1/2 to separate diffusion-controlled and capacitive-controlled contributions. Two distinct kinetic regions emerge when sweep rate is varied from 0.1 to 150 mV s−1 (inset represents the capacitive contribution at a sweep rate of 3 mV s−1).

10 000 cycles. The Coulombic efficiency retains at around 100% during the course of measurement, indicating the excellent reversibility of the cell. Such long-term cycling stability has been rarely reported in SICs. Sodium Storage Mechanism in MWTOG Nanocomposite. Unlike the well-studied lithium insertion behavior in TiO2, the sodium insertion mechanism in anatase TiO2 is still under debate.32,51−53 This situation becomes even more complicated with nanocrystals of a few nanometers, in which the ion insertion behavior is significantly altered by the particle size.36,38 Ex situ XRD analysis was performed to study the phase change of the MWTOG electrode upon cycling in a half cell. All the well-define peaks in the fresh electrode (Figure 4a, position 1) mostly vanish after the first discharge to 0.01 V (Figure 4a, position 2), indicating the transformation of anatase into a rather amorphous sodium titanate structure upon sodium insertion. Followed by charging back to 3 V (Figure 4a, position 3), the crystallinity is partially recovered with some alterations in the peak position and relative intensity. This poorly crystalline phase is maintained after 5 galvanostatic cycles at 1 C, suggesting its reversibility during the charge/ discharge after the initial cycle. Note that MWTOG exhibits low Coulombic efficiency in the first cycle as reported in many other anatase TiO2 anodes,51,54,55 which can be hardly explained solely by the formation of a solid−electrolyte interface (SEI). Thus, a certain amount of sodium should be trapped in the TiO2 even after fully extracting sodium. On the

ranging from 0.2 to 10 C. The linear voltage profiles at all these current rates represent a typical capacitive behavior of the asassembled MWTOG//AC full cell. On the basis of the galvanostatic charge/discharge measurements, the energy and power densities of the MWTOG//AC device based on the mass of active materials can be calculated. A high energy density of 64.2 Wh kg−1 can be achieved at a power density of 56.3 W kg−1; at a high power output of 1357 W kg−1 it still delivers an energy density of 25.8 Wh kg−1. The energy/power densities of the as-assembled SIC are displayed in the Ragone plot shown in Figure 3c and compared with several representative electrochemical energy storage devices.45,48−50 Such performance is compared favorably with many electrochemical energy storage devices in literature, including carbon-based EDLC, high-power LIBs, LICs, and several reported SICs, demonstrating the superiority of the asdeveloped MWTOG//AC SIC. Considering the relatively low capacity of the currently used cathode, replacing the commercial AC with more advanced EDLC-type carbons would better match our MWTOG and further boost the performance of the SIC. The cycle life of the MWTOG//AC SIC was also evaluated (Figure 3d) under galvanostatic charge/discharge conditions. At a high rate of 10 C (3.35 A g−1 based on MWTOG anode), the capacity slightly increases in the first ca. 200 cycles, possibly due to improved wetting of electrolyte in the electrodes; afterward the cell retains over 90% of its initial value after 2956


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decreases to 0.54 for the anodic current and 0.6 for the cathodic current. There, the diffusion process is dominant. To further differentiate the contribution from insertion-type capacity and pseudocapacitive charge storage, the total stored charge (QT) in the electrode can be expressed as

basis of above observation, we speculate that the reversible sodium storage in our MWTOG is accomplished between a sodium-poor titanate phase and an amorphous sodium-rich titanate phase. Nevertheless, more efforts are required to reveal the details of the sodium storage mechanism in this material. To further investigate the change of micro/nanostructure of MWTOG, TEM and HRTEM characterizations were carried out for the cell after 15 000 galvanostatic charge/discharge cycles. The spheroid-shaped secondary structure is well maintained as shown in Figure 4b, except for the less discernible pore structure and primary building nanocrystals compared with the pristine MWTOG. HRTEM image shown in Figure 4c reveals continuous lattice fringes with d-spacing of 0.35 nm over several primary nanocrystals, corresponding to the diffraction peak at ca. 26° of the sodium-poor titanate (Figure 4a). Interestingly, another group of lattice fringes with d-spacing of 0.2 nm can be observed in a small area, which is also verified by the fast Fourier transform (FFT) pattern (inset of Figure 4c). Such lattice spacing does not correlate with the XRD pattern of the cycled MWTOG, but is close to that of the “vanished” (200) planes of anatase TiO2. Therefore, even though full recovery of the anatase structure is not observed upon sodium insertion and deinsertion, the MWTOG can well retain the micro/nanostructure during prolonged cycling. Such a feature is critically important for long-term operation of the electrode without much degradation. To investigate the kinetics of the MWTOG electrode, CV curves at sweep rates of 0.1 to 150 mV s−1 were recorded (Figure S4, see the SI). As shown in Figure 4d, a dominating pair of redox peaks exhibits increasing currents as raising the sweep rates. At a very slow sweep rate of 0.1 mV s−1 (inset of Figure 4d, corresponding to a time scale of 8 h), two pairs of redox peaks are observed. The one centered at 0.8 and 0.73 V corresponds to the Ti3+/Ti4+ redox couple accompanied by sodium insertion and deinsertion, which is in good agreement with the galvanostatic charge/discharge results. The other redox peaks close to 0 V might be related to sodium storage in graphene,43 which contributes negligible capacity especially at high scan rates. A very small potential difference of 65 mV is observed between the sodium insertion and deinsertion peaks, revealing the fast kinetics and highly reversible nature of the sodium storage process. Such a small potential difference is close to the theoretical value of 59 mV and much below that of the “zero-strain” lithium titanate (Li4Ti5O12, ∼160 mV).56 The peak current in the CV curves obeys a power-law relationship with the scan rate according to the following equation and can be used to analyze the charge storage mechanism.38,57−59

QT = Q v =∞ + kv−0.5

where Qv=∞ is the capacitive contribution independent of the sweep rates, k is a constant. As shown in Figure 4f, the plot of normalized capacity vs v−0.5 (0.1 to 150 mV s−1) is divided into two regions. For a sweep rate 5 mV s−1, the deviation from the original linear slope indicates the diffusion-controlled kinetics, which might be attributed to inaccessible pores due to inhomogeneous potential distribution,61 uncompensated ohmic drops,61 and resistance of active material.58 Similarly, at a given sweep rate, the capacitive contribution at different potential can be determined.38,45,57,62 For example, at a sweep rate of 3 mV s−1, around 73% of the stored charge is attributed to the capacitive process as indicated by the shadow area shown in the inset of Figure 4f, especially at potentials deviating from the sodium insertion/deinsertion peaks. On the basis of the above discussion, the charge storage in the MWTOG electrode is dominated by the pronounced pseudocapacitive behavior, allowing ultrafast uptake and release of sodium ions with little degradation of the active material. Such exceptional sodium storage capability is mainly related to the following features of the nanocomposite. First of all, the mesoporous structure strongly coupled with conducting graphene support provides both high electronic and ionic transport. Second, by assembling nanocrystals into a mesocage structure, MWTOG is able to maximize the surface/nearsurface charge storage and minimize the sodium diffusion distance in the solid state, while maintaining a high structural robustness. Third, the ultrasmall size of primary nanocrystal induces pseudocapacitive sodium insertion/deinsertion without degrading the active material upon prolonged cycling. Overall, these merits provide MWTOG with the capability to uptake and release a substantial amount of sodium ions in a fast and highly reversible manner, which is not expected in conventional anatase TiO2 materials.

CONCLUSIONS In summary, we developed a single-crystal-like TiO 2 mesocages-graphene nanocomposite as a superior host material for electrochemical sodium storage with exceptional high-rate capability and cycling stability. The nanocomposite can deliver a reversible capacity of 126 mAh g−1 at a high rate of 10 C for over 18 000 cycles without noticeable fading. By coupling with a carbon-based cathode, the as-prepared full cell of sodium-ion capacitor exhibits a high energy density of 64.2 Wh kg−1 at a power density of 56.3 W kg−1, and 25.8 Wh kg−1 at a high power output of 1367 W kg−1. Moreover, over 90% of the capacity can be retained after cycling at a high rate of 10 000 cycles. Through in-depth analysis of the sodium storage behavior, the robust architecture of the nanocomposite and the dominating pseudocapacitve charge storage are thought to

i = avb

where i is the peak current, v is the sweep rate, a and b are variables. By plotting logi against logv, the b value can be derived from the slope, which gives two critical conditions: b = 0.5 and b = 1. The former indicates a typical faradaic intercalation process controlled by semi-infinite linear diffusion; the latter represents capacitive charge storage free of diffusion control.60 Figure 4e gives the plots of logi vs logv for both the anodic and cathodic peaks. At low sweep rates ranging from 0.1 to 5 mV s−1, the b-value for anodic and cathodic peaks are 0.89 and 0.88, respectively, suggesting a fast kinetics dominated by pseudocapacitive process. At higher scan rates of 10 to 150 mV s−1, corresponding to charging time below 5 min, the b-value 2957


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E=

∫ IV dt

Em = E /(mc + ma ) where E is the total energy (Wh), I is the constant current (A), V is the voltage (V), t is the discharge time (h), mc and ma represent the mass (g) of active materials for cathode and anode, respectively.

EXPERIMENTAL METHODS

P = E /Δt

Synthesis of MWTOG Nanocomposites. Single-crystal-like TiO2 mesocages-graphene nanocomposite was prepared by a microwaveassisted solvothermal method on the basis of a reported system.42 Graphene was purchased from XFNANO Inc. (XF001W, oxygen content of 7−7.5 atom %) and used as received. In a typical synthesis, a desirable amount of graphene was dispersed in 20 mL of tert-butyl alcohol with probe sonication for 30 min while stirring in a Teflon tube. After that, 1 mL of TiOSO4 was dropwise added in to the above suspension. Then the Teflon tubes were loaded into a microwave reaction system (Ultrawave; Milestone Inc.) and kept at 110 °C for 3 h under 50 bar. The black product was collected by centrifuge, washed with ethanol, and dried in oven at 80 °C overnight. MWTO was synthesized using the same method except for the addition of graphene. Material Characterizations. The X-ray diffraction (XRD) measurements were taken by a Rigaku X-ray powder diffractometer with copper Kα radiation (λ = 1.54 Å). Nitrogen sorption isotherms were measured at 77 K with a Micrometrics ASAP 2020 analyzer. The specific surface areas were calculated by the Brunauer−Emmett−Teller (BET) equation using adsorption branch in a relative pressure (P/P0) range from 0.04 to 0.25. Scanning electron microscopy (SEM) analysis was conducted on a JEOL JSM-6700F. Transmission electron microscopy (TEM) experiments were carried out on a Philips CM120 operated at 120 kV. Electrochemical Characterizations of MWTOG. To fabricate electrodes, the active material powders, carbon black (CB) and poly(vinylidene fluoride) (PVDF) binder were mixed in a mass ratio of 70:20:10 and homogenized in N-methyl-2-pyrrolidone (NMP) to form slurries. The homogeneous slurries were coated on Cu foil substrates and dried at 80 °C under vacuum. The mass loading of active materials was controlled to be 1.5−2 mg cm−2. Coin cells (2032type) were assembled in an argon-filled glovebox, using glass microfiber (Whatman GF/D) as the separator. Sodium discs were freshly made inside a glovebox and 1 M NaClO4 in 1:1 (v/v) mixture of ethylene carbonate and propylene carbonate with 5 wt % fluoroethylene carbonate was used as the electrolyte. The cyclic voltammetry (CV) and galvanostatic charge/discharge measurements were performed on a Bio-Logic VMP3 electrochemical workstation or LAND2000 battery tester. All electrochemical measurements were carried out at room temperature and the C-rate is based on the active composite materials. For postmortem study, the cell was disassembled and the electrode was washed with acetone and methanol. All specific capacities and current densities of MWTOG samples are based on the total mass of nanocomposite. Assembly and Electrochemical Measurements of Full Cells of SIC. SICs were assembled using the MWTOG composite as the anode and a commercial AC (Kuraray YP-50) as the cathode. The electrode fabrication of AC cathode used the same method as the anode. The mass ratio of cathode to anode is controlled as ∼5. Glass microfiber (Whatman GF/D) is used as the separator and the total amount of electrolyte in the full cell was 100 μL. Both anode and cathode were cycled in a half cell vs Na/Na+ to accurately determine the reversible capacity before assembly into a full cell using a 2032type coin cell. Both CV and galvanostatic charge/discharge were measured using the same method as in the half cell. The C-rate was set based on the MWTOG anode. The overall cell energy/power densities were calculated based on the total active mass of cathode and anode materials. The specific energy density (Em, Wh g−1) and power density (Pm, W g−1) were calculated based on the galvanostatic charge/ discharge measurements by the following equations:

Pm = P /(mc + ma ) where P is the total power (W), and Δt is the total discharge time (h).

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b08332. Additional TGA result, SEM image, digital photograph of samples, and CV curves (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zaiyuan Le: 0000-0002-9925-7513 Fang Liu: 0000-0003-0885-5604 Gen Chen: 0000-0003-3504-3572 Hao Bin Wu: 0000-0002-0725-6442 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are grateful for the support of the UCLA Dynavolt Research Center (442531-LY-57914) and the Center of HK Graphene Technology and Energy Storage (442531-LY57759). We thank Professor Bruce Dunn for valuable discussion. We also thank Dr. Wen Wen from the Shanghai Synchrotron Radiation Facility for technical assistance. REFERENCES (1) Nomoto, S.; Nakata, H.; Yoshioka, K.; Yoshida, A.; Yoneda, H. Advanced Capacitors and Their Application. J. Power Sources 2001, 97, 807−811. (2) Lust, E.; Nurk, G.; Jänes, A.; Arulepp, M.; Nigu, P.; Möller, P.; Kallip, S.; Sammelselg, V. Electrochemical Properties of Nanoporous Carbon Electrodes in Various Nonaqueous Electrolytes. J. Solid State Electrochem. 2003, 7, 91−105. (3) Wang, H.; Yoshio, M.; Thapa, A. K.; Nakamura, H. From Symmetric AC/AC to Asymmetric AC/Graphite, A Progress in Electrochemical Capacitors. J. Power Sources 2007, 169, 375−380. (4) Du Pasquier, A.; Plitz, I.; Menocal, S.; Amatucci, G. A Comparative Study of Li-Ion Battery, Supercapacitor and Nonaqueous Asymmetric Hybrid Devices for Automotive Applications. J. Power Sources 2003, 115, 171−178. (5) Simon, P.; Gogotsi, Y.; Dunn, B. Where Do Batteries End and Supercapacitors Begin? Science 2014, 343, 1210−1211. (6) Naoi, K.; Ishimoto, S.; Miyamoto, J.-i.; Naoi, W. Second Generation ‘Nanohybrid Supercapacitor’: Evolution of Capacitive Energy Storage Devices. Energy Environ. Sci. 2012, 5, 9363−9373. (7) Ketabi, S.; Le, Z. Y.; Lian, K. EMIHSO4-Based Polymer Ionic Liquid Electrolyte for Electrochemical Capacitors. Electrochem. SolidState Lett. 2012, 15, A19−A22. 2958


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