Robust and Conductive Na2Ti2O5–x Nanowire Arrays for High

Sep 28, 2017 - Figure 2b shows the HR-TEM image of Na2Ti2O5–x nanowires. A well-crystallized structure with lattice fringes of about 0.74 nm can be ...
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Article Cite This: Chem. Mater. 2017, 29, 9133-9141

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Robust and Conductive Na2Ti2O5−x Nanowire Arrays for HighPerformance Flexible Sodium-Ion Capacitor Lan-Fang Que, Fu-Da Yu, Ke-Wu He, Zhen-Bo Wang,* and Da-Ming Gu MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92 West-Da Zhi Street, Harbin, 150001 China S Supporting Information *

ABSTRACT: Hybrid capacitors, especially sodium-ion capacitors (SICs), which combine the complementary merits of high-energy batteries and high-power capacitors, have received increasing research interest and have been expected to bridge the gap between the rechargeable batteries and EDLCs. The biggest challenge for SICs is to overcome the kinetics discrepancy between the sluggish faradaic anode and the rapid nonfaradaic capacitive cathode. To boost the Na+ reaction kinetics, robust and conductive Na2Ti2O5−x nanowire arrays have been constructed as an accessible and affordable SIC anode. It is found that the utilization of oxygen vacancies (OVs) can endow Na2Ti2O5−x high electrical conductivity, introduce intercalation pseudocapacitance, and maintain the crystal structure integrity. It exhibits high reversible discharge capacity (225 mAh g−1 at 0.5C), superior rate capability, and ultralong lifespan when utilized as self-supported and additive-free anode for SIB, remaining almost 100% capacity retention after 20 000 cycles at 25 C. When assembled as flexible hybrid SIC (4.5 cm3) with rGO/AC film cathode, a high-level energy density of 70 Wh kg−1 at power density of 240 W kg−1 based on active materials can be achieved, and high volumetric energy density (15.6 Wh L−1) and power density (120 W L−1) based on the whole packge volume can be delivered with superior cycle stability (5000 cycles, 82.5%).

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INTRODUCTION It is imperative to develop efficient flexible energy storage devices to meet the increasing demand for portable and wearable electronic devices.1−4 Hybrid capacitors, which combine the complementary merits of high-energy batteries and high-power capacitors, have received increasing research interest, especially for hybrid SICs due to the convenience and abundance of sodium.5−7 Although considerable efforts have been invested to construct flexible SICs with high-level energy/ power outputs and stable cycle performance, the development of this field is still in its infancy.8,9 Consisting of two asymmetric electrodes, dual mechanisms have been carried out simultaneously in one hybrid capacitor: slow intercalation/ deintercalation of cations in faradaic battery-type anode and fast surface adsorption/desorption of anions in nonfaradaic capacitive cathode. The major challenge in achieving remarkable-performance SICs is the lack of accessible and affordable anodes that can reversibly store a substantial number of Na-ions fast and stably to overcome the kinetics discrepancy between the sluggish faradaic anode and the rapid nonfaradaic capacitive cathode. Due to the high specific capacity and attracitve operating voltages, sodium titanate compounds are suitable for fabricating high-voltage SICs in order to achieve a high energy-power combination.10−13 However, sluggish Na+ diffusion kinetic, intrinsic low conductivity, and large bandgap (3.7 eV) have so far been major obstacles for its application in hybrid systems.14 Considerable efforts have been made to overcome these © 2017 American Chemical Society

intrinsic drawbacks in order to enhance the rate performance. One main method is to shorten the Na+ diffusion length and enable faster Na+ transport by rational electrode design. Nanostructures including nanorods and nanowires, nanosheets and nanobelts, and 3D nanoarchitectural structures, have been proposed by researchers.15,16 Since these initial proposals, a series of energy storage systems based on well-ordered nanoarchitectures with comparable electrochemical properties have been reported, revealing that self-supported nanoarrays growing on flexible substrate directly can effectively enhance electronic conductivity and strengthen electrode stability.17−19 Most sodium titanate arrayed architecture preferentially grows along the longitudinal axis, thereby limiting the transport of Na+ and electrons along this direction. It is proposed that introducing defects such as oxygen vacancies (OVs) not only benefits electron transfer but also offers more diffusion channels by narrowing the band gap dramatically, likewise Na3V2−x (PO4)3, SnO2−x, and TiO2−x.20−22 However, this attractive strategy is mainly applied to LIBs but limited with respect to SICs. Meanwhile, it is difficult to achieve welldesigned nanostructures and OVs doping simultaneously. Therefore, it is desirable and significant to design conductive OVs self-doped sodium titanates with rational nanoarrays for high-rate performance anodes. Received: July 11, 2017 Revised: September 28, 2017 Published: September 28, 2017 9133

DOI: 10.1021/acs.chemmater.7b02864 Chem. Mater. 2017, 29, 9133−9141

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Chemistry of Materials

Figure 1. Illustration of (a) two series flexible SICs, (b) the corresponding SEM images of Na2Ti2O5−x nanowire arrays anode, and (c) flexible rGO/ AC film. filtration method and reduced by HI solution. First, the homogeneous GO was synthesized from graphite flakes by a modified Hummers method. Then, GO was ultrasound dispersed for 60 min, and mixed with AC particles for another 60 min ultrasonic dispersion. The mass ratio of GO and AC is 10:1. Finally, 3.5 mg cm−2 of rGO/AC film was obtained by vacuum filtration and reduced by HI solution through immersing into HI solution (45%) at 70 °C for 6 h, washing with distilled water and ethanol, and drying for 24 h. As a comparison, the rGO film was prepared without the addition of AC. Materials Characterization. The morphologies were observed by a field emission transmission electron microscopy (FETEM) (JEM2100) and a field emission scanning electron microscopy (FESEM) (S-4800, HITACHI), respectively. X-ray diffraction (XRD) patterns were recorded with an X-ray diffractometer [Bruker D8 ADVANCE, with Cu Kα radiation (= 1.5418 Å), Germany]. X-ray photoelectron spectroscopy (XPS) spectra were studied by a PerkinElmer PHI 5000C ESCA system with Al Kα radiation operated at 250 W. A Renishaw Via-Reflex spectrometer (532 nm radiation) with a resolution of 2 cm−1 was used to record the Raman spectra of the samples. The X-band electron paramagnetic resonance (EPR) spectra were recorded at room temperature (Varian E-112). Electrochemical Measurement. The as-prepared Na2Ti2O5, Na2Ti2O5−x nanowire arrays and rGO, rGO/AC films were directly fabricated as binder/additive-free anodes/cathodes by cutting them into small thin round slices. Half-cell configurations were assembled using Na-metal foil as counter and reference electrodes in coin type cells. Note that Na2Ti2O5−x nanowire arrays were grown on both sides of Ti foil, so one Na2Ti2O5−x@Ti foil was used as two anodes in the full flexible cell. One Na2Ti2O5−x@Ti foil with two rGO/AC films laminated onto both sides of a separator was sealed with Al-plastic film as a flexible SIC device. Before sealing, vacuumizing had been done to release air or argon of the flexible SIC package. The separator was a polypropylene membrane, and the electrolyte was a 1 mol L−1 NaClO4 solution in a 1:1 (v/v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). All operations were done in an Ar-filled glovebox. The optimized mass ratio of the Na2Ti2O5−x and rGO/AC in a hybrid configuration is 1:3.5. Galvanostatic discharge−charge experiments and the galvanostatic intermittent titration technique (GITT) test were performed at different current densities with a multichannel battery tester (NEWWARE, China). Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were measured by the electrochemical workstation (CHI660E, Chenhua, Shanghai).

Herein, we report a novel flexible hybrid SIC with high volumetric energy density by optimizing both anodes and cathodes from rational architecture design and an initial structure modified to overcome the imbalance kinetics. As illustrated in Figure 1, Na2Ti2O5−x nanowire arrays with porous rooftop network and highly ordered nanowires in the 3D architecture are fabricated on Ti-foil directly as the additive-free anodes, which can (1) contribute to the ion/electron transport and good connectivity with the current collector; (2) effectively improve the volumetric and gravimetric specific capacity; (3) enhance electrical conductivity and introduce intercalation pseudocapacitance to boost the Na+ reaction kinetics; and (4) maintain the crystal structure integrity, with all these virtues triggering superior rate capability and cycle stability. Impressively, both sides of the anode have been utilized to maximize the volumetric energy density of the hybrid device. The modified and composite reduced graphene oxide and activated carbon (rGO/AC) film which with the overall conductive framework is used as the flexible cathode, which can achieve higher capacitance than the pure reduced graphene oxied film. As expected, the Na2Ti2O5−x nanowire arrays exhibits superior rate capability and ultralong lifespan when utilized as additvefree anode for SIB, achieving high reversible capacity of 225 mAh g−1 at 0.5 C, 73 mAh g−1 at 50 C, remaining almost 100% capacity retention after 20 000 cycles at 25 C. When assembled as flexible hybrid SIC (4.5 cm3) with rGO/AC cathode, high volumetric energy density (15.6 Wh L−1) and power density (120 W L−1), and superior cycle stability (5000 cycles, 82.5%) can be achieved.

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EXPERIMENTAL SECTION

Synthesis of Na2Ti2O5−x Nanowire Arrays. Na2Ti2O5−x nanowire arrays were prepared via a facile hydrothermal process and a subsequent annealing treatment. Na2Ti2O5·H2O nanowire arrays were fabricated according to our previous study.23 Then, the as-prepared precursor was heated at 450 °C for 2 h in mixed Ar/H2 atmosphere to obtain Na2Ti2O5−x. As a comparison, Na2Ti2O5 was fabricated by treating Na2Ti2O5·H2O at air under the same conditions. Preparation of Reduced Graphene Oxide/Activated Carbon (rGO/AC) Films. The rGO/AC film is prepared by a simple vacuum 9134

DOI: 10.1021/acs.chemmater.7b02864 Chem. Mater. 2017, 29, 9133−9141

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Chemistry of Materials

Figure 2. Microscopy observations and structure characterization of Na2Ti2O5−x nanowire arrays grown on Ti foil: (a) Photographs and SEM images, (b) HR-TEM images, (c) HAADF-STEM EDS mapping of Na2Ti2O5−x, (d) XRD patterns, high resolution XPS spectra of (e) Ti 2p and (f) O 1s.

with Na2Ti2O5·H2O, the right shift of the (200) diffraction peak observed in Na2Ti2O5 and Na2Ti2O5−x is mainly attributed to the dehydration during annealing. Compared with Na2Ti2O5, the defined peaks of Na2Ti2O5−x are much sharper and intense, and generally the peak shape is relative to crystalline, meaning that the Na2Ti2O5−x sample has better crystallinity than Na2Ti2O5. TGA analyses of Na2Ti2O5·H2O in air and N2 conditions are measured and shown in Figure S2 (Supporting Information). According to the curves, no obvious structural change can be observed during heat treatment, expecting the dehydration. Figure 2e compares the high-resolution XPS spectra of Ti 2p of Na2Ti2O5 and Na2Ti2O5−x. Results exhibit two prominent peaks at ≈458.9 and ≈464.7 eV in both samples, suggesting the main species are Ti4+. However, a negative shift can be clearly observed in the XPS spectrum of Ti 2p of Na2Ti2O5−x sample, indicating a surface-bonding change happening after the reduction. The XPS result of Na2Ti2O5−x displays two extra peaks created at ≈457.6 and ≈463.3 eV, correlating with the Ti 2p3/2 and Ti 2p1/2 peaks of Ti3+ reported in the literature,24 demonstrating the successful generation of Ti3+ in the Na2Ti2O5−x sample. Figure 2f displays the O 1s core level XPS spectra of Na2Ti2O5 and Na2Ti2O5−x nanowire arrays. It is clear that an extra peak at 532.5 eV can be found in Na2Ti2O5−x sample, and this can be ascribed to T−OH.25 Raman spectroscopy is also measured and displayed in Figure S3 (Supporting Information); the Na2Ti2O5−x sample exhibits band broadening compared with Na2Ti2O5, indicating a structural variation related to OVs.44 A very intense signal at g = 1.99 (the g value is given by the EPR tester) is observed in the electron paramagnetic resonance (EPR) spectrum of Na2Ti2O5−x, which can be characteristic for Ti3+ ions, while only a very feeble signal can be observed for the Na2Ti2O5 (Figure S4, Supporting Information). These results are in great agreement with the XPS analysis. Electrochemical performance of Na2Ti2O5 and Na2Ti2O5−x half-cells shown in Figure 3 reveals that the Na2Ti2O5−x

The energy density and power density were calculated from P = (ΔE i/m) and E = (P t), where ΔE = (Emax + Emin)/2 and Emax and Emin were, respectively, the initial and final potential of discharge curves of galvanostatic cycling at different current densities. For mass energy density or power density, m was the total mass of active materials on both electrodes, while for volumetric energy density or power density, m was the volume of the overall flexible device.

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RESULTS AND DISCUSSION Anode Materials: Na2Ti2O5−x Nanowire Arrays. The Na2Ti2O5−x nanowire arrays are fabricated on Ti foil directly by a facile hydrothermal process and a subsequent reduction reaction. The color of Na2Ti2O5−x sample converts to black after reduced compared with Na2Ti2O5 (which is treated in air), indicating the existence of OVs and the flexibility of this sample is excellent (Figure 2a). According to our previous study,23 we have designed the stable 3D scaffold which composed of porous rooftop and highly aligned nanowires as displayed in Figure 2a, this special architecture can enable facile electrolyte penetration and fast sodium ion diffusion. TEM image of Na2Ti2O5−x nanowire arrays is presented in Figure S1 (Supporting Information). Figure 2b shows the HR-TEM image of Na2Ti2O5−x nanowires. A well-crystallized structure with lattice fringes of about 0.74 nm can be clearly observed, which can be well indexed to the (200) plane of Na2Ti2O5. The element analysis is further characterized by STEM-EDS mapping wherein Na, O, and Ti elements are evenly distributed in the nanometer range (Figure 2c). Structural evolution of Na2Ti2O5·H2O precursor over annealing treatment in different atmosphere is analyzed by Xray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectra. As shown in Figure 2d, XRD patterns collected from Na2Ti2O5 and Na2Ti2O5−x are very similar to Na2Ti2O5·H2O, which can be well indexed to layered H2Ti2O5·H2O (JCPDS No. 47-0124), and the mismatch of (200) lattice plane between Na2Ti2O5·H2O and H2Ti2O5·H2O is due to the different bond length of Na and H.23 Compared 9135

DOI: 10.1021/acs.chemmater.7b02864 Chem. Mater. 2017, 29, 9133−9141

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Figure 3. Comparison of electrochemical properties of Na2Ti2O5 and Na2Ti2O5−x electrodes: (a) the first CV curves of Na2Ti2O5 and Na2Ti2O5−x electrodes, (b) the first three charge−discharge curves at 0.5C, (c) rate capabilities at various rates (inset shows the charge−discharge profiles at various rates), (d) 1000 cycles at 5C, (e) 20000 cycles of Na2Ti2O5−x anode at 25C.

titanates at higher potential vs Na+/Na. This phenomenon will be confirmed and discussed at the following part. When tested in the voltage range of 0.01−2.5 V vs Na+/Na at 0.5C, the Na2Ti2O5−x electrode displays higher reversible discharging capacity (213 mAh g−1 for Na2Ti2O5 and 225 mAh g−1 for Na2Ti2O5−x) and Coulombic efficiency (52% for Na2Ti2O5 and 55% for Na2Ti2O5−x) than the Na2Ti2O5 one (Figure 3b). As displayed, the Na2Ti2O5−x anode delivers excellent rate capability at high C-rates. It delivers capacities of 210, 200, 174, 159, 131, 114, 100, 80, and 72 mAh g−1 at current rates of 0.5, 1, 2.5, 5, 10, 15, 25, 40, and 50 C, respectively. On the contrary, Na2Ti2O5 electrode displays 199, 180, 145, 113, 77, 58, 39, 16, and 9 mAh g−1 at various C-rates. Moreover, it can

electrode exhibits superior properties in various aspects, especially in rate capability and cycle stability. The first CV curves of both electrodes are carried out and presented in Figure 3a. A pair of obvious cathodic/anodic peaks at 0.17/0.65 V can be observed clearly in both samples, associated with the characteristic feature of Na ion insertion/extraction into/from Na2Ti2O5. Unlike the CV curve of Na2Ti2O5, Na2Ti2O5−x exhibits extra slight oxidation/reduction peaks at 1.10/1.30 V, which can be attributed to the reduced Ti3+ species.31 It is reported that the addition of Ti3+ species can introduce a pair oxidation/reduction peaks at 1.10/1.30 V,25 which is mainly attributed to the structure change caused by Ti3+ doping, and this Ti3+ self-doping may trigger Na ions insert into sodium 9136

DOI: 10.1021/acs.chemmater.7b02864 Chem. Mater. 2017, 29, 9133−9141

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Chemistry of Materials

Figure 4. (a) CV curves at various scan rates, b-value determination of the peak anodic and cathodic currents (b) below 0.8 V and (c) in 1.0−1.5 V region, (d) normalized capacity vs v −1/2, (e) ratio of surface charge (Qs) against the total charge stored (Q) at different sweep rates of Na2Ti2O5−x anode, (f) Na+ diffusion coefficients and polarization (inserted) based on GITT test.

To investigate the origin of the improved electrochemical stability, the fundamental mechanism of Na ion storage of Na2Ti2O5−x anode in SIBs is further probed by CV. CV curves in various sweep rates of Na2Ti2O5−x electrodes are evaluated and presented in Figure 4a. As observed, the CV curves display distortion from the basic shape at higher scan rates, and this distortion may be originated from several sources such as increased Ohmic contribution and/or diffusion constraints. Notably, the extra oxidation/reduction peaks at 1.10/1.30 V in Na2Ti2O5−x sample are stronger with the increasing of scan rates. According to

be found that the discharge voltages of Na2Ti2O5 electrode drop rapidly with the increasing of C-rates according to the charge−discharge profiles at various C-rates (inserted in Figure 3c), while the discharge voltages of Na2Ti2O5−x are more constant. Such phenomenon can be attributed to the polarization caused by poor Na+/e− transport. Long-term cycle performance of Na2Ti2O5 and Na2Ti2O5−x anodes at 5C is investigated and displayed in Figure 3d. It is observed that the capacity of Na2Ti2O5 electrode decreases quickly during cycling, whereas Na2Ti2O5−x electrode can maintain about 92% capacity retention after 1000 cycles. The cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) after 1000 cycles of these two samples have been recorded. As observed, the peak shape of Na2Ti2O5−x is sharper and more intense than that of Na2Ti2O5, and the gap between redox peaks of Na2Ti2O5−x is smaller than that of Na2Ti2O5, indicative of more effective redox reaction and lower overall resistance for Na2Ti2O5−x electrode (Figure S5a). It is correlated well with the result of EIS (Figure S5b), in which the Na2Ti2O5−x electrode displays a much lower charge-transfer resistance than that of the Na2Ti2O5 one (295.3 Ω vs 429.7 Ω) based on the modified Randles equivalent circuit. Fast charging/discharging process of Na2Ti2O5−x at 25C has also been tested and shown in Figure 3e. It exhibits an outstanding cycle capability maintaining about 60 mAh g−1 over 20 000 cycles without appreciable decay. When compared with many excellent reports about sodium-titanate-based anodes,19,25−30 our Na2Ti2O5−x anode still shows apparent superiority at rate capability and cycle stability as shown in Figure S6a,b. This signals that our material design is highly effective and the asprepared Na2Ti2O5−x nanowire arrays could be regarded as an excellent candidate for high-capacity and excellent stability anode material for SICs.

i = avb

(1)

log i = b log v + log a

(2)

the relationship between current (i) and scan rate (v) can be analyzed, wherein a and b are appropriate values, adjustable parameters, with b values determined by the slope of the logv− logi plots.32 According to the calculated b value, the Na-ion storage mechanisms can be classified between the surfacecontrolled capacitive (b = 1) and diffusion-controlled insertion processes (b = 0.5). Interestingly, the slopes of Na2Ti2O5−x sample in different peak potential range are distinct. As displayed in Figure 4b, the anodic and cathodic b values below 0.8 V are 0.5 and 0.6, respectively, indicating that diffusioncontrolled insertion process is more pronounced. For the peak potential range between 1.0 and 1.5 V, the anodic and cathodic b values reach up to 0.9 and 0.85, demonstrating a surface charging or pseudocapacitive mechanism. Additionally, the extra pair anodic/cathodic peaks around 1.0−1.5 V are only observed in Na2Ti2O5−x electrodes, meaning that the OVs and Ti3+ self-doping strategy can introduce a pesudocapacitive mechanism to enhance the Na reaction kinetics. A chargestorage mechanism can also be established by the relationship 9137

DOI: 10.1021/acs.chemmater.7b02864 Chem. Mater. 2017, 29, 9133−9141

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Figure 5. HR-TEM images of Na2Ti2O5−x electrodes with different cycled time at 1.0 A g−1: (a) pristine, b) 2nd cycled, (c) 50th cycled, and (d) 500th cycled; the insets are the corresponding SAED patterns.

between capacity and sweep rate. In a plot of Q versus v−1/2, regions that are linear represent capacity limited by semiinfinite linear diffusion, whereas capacitive contributions are independent of the sweep rate.32 As shown in Figure 4d, solidstate diffusion is not the rate-determining step for charge storage in region 1(v < 1 mV s−1). While at higher scan rates (v > 1 mV s−1), charge storage is mainly diffusion-controlled. In addition, the relative charge store (Q) originated from Na ion insertion (Qb) and capacitive processes (Qs) can be identified and calculated on the basis of Trasatti’s method.32 Q = Q b + Q s = k(v−1/2) + Q s

with the increase of cycle numbers. Although irreversible trapping of Na-ions and expansion of interlayer have been found during the cycling process, the Na2Ti2O5−x electrode structure is stable to withstand expansion/shrinkage during sodium ion intercalation/deintercalation. Taking these analysis results into consideration, the improvement of electrochemical properties through nanoarchitecture design and OVs self-doping can be summarized as these aspects. Adoption of well-ordered and self-supported 3D nanowire arrays can contribute to the ion/electron transport and good connectivity with the current collector. Abandoning polymer binder and black carbon can effectively improve the volumetric and gravimetric specific capacity. Furthermore, OVs self-doping strategy endows Na2Ti2O5−x high electrical conductivity as well as introduces intercalation pseudocapacitance and maintains the crystal structure integrity, which drastically reduces the charge transfer resistance and boosts the Na reaction kinetics, resulting in superior rate capability and cycle stability. Cathode Materials: Reduced-Graphene Oxide/Activated Carbon (rGO/AC) Films. Because of the unique properties of large specific surface area, high electrical conductivity, and particularly the feasibility of designing and developing free-standing electrodes, graphene is chosen as cathode material.34,35 Activated carbon (AC) is used and intercalated along the graphene galleries to avoid the restacked graphene oxide (GO) sheets as well as to enhance the electrical conductivity and improve the specific surface of the flexible film, as illustrated in the Experiment Section. The morphological change and structural evolution of the reduced graphene oxide (rGO) sheets after being modified by AC are observed by SEM and analyzed by X-ray diffraction (XRD) and Raman spectra. Results reveal that the utilization of activated carbon (AC) is capable to hinder the restacking and agglomeration of reduced graphene oxide (rGO) sheets as well as to improve the specific surface and enhance the electrical conductivity of the flexible film (Figure S8). The flexibility of rGO/AC film is excellent (Figure S9). The electrochemical performance of rGO/AC film is evaluated as half-cell assembly and demonstrated in Figure S10. It can be seen that the rGO/AC film displays a higher capacity of 47.5 mAh g−1 than that of rGO electrode (32.8 mAh g−1) when tested at 0.1A g−1 within 2.5−4.2 V vs Na/Na+. The galvanostatic charge−discharge profile of AC electrode is also tested and shown in Figure S11. The capacity of AC electrode is only 35.0 mAh g−1, and the capacities of both pure rGO and AC electrodes are much lower than that of the modified rGO/ AC electrode, suggesting that the increased capacity of rGO/

(3)

As shown in Figure 4e, the relative ratio of Qs gradually increases with increasing sweep rate, and it is 46% at 0.1 mV s−1, and reaches 85% at 1.0 mV s−1. GITT studies on the Na2Ti2O5 and Na2Ti2O5−x electrodes measured after activation by cycling 10 times at 0.5C confirm that Na2Ti2O5−x presents enhanced Na-ion diffusion coefficient and reduced polarization than that of Na2Ti2O5 (Figure 4f).33 Due to the high ratio of pseudocapacitive contribution in the total charge stored and the improved Na-ion diffusion coefficient, Na2Ti2O5−x exhibits remarkable electrochemical properties as an anode for SIBs under extremely high C-rates. The crystal structure changes of Na2Ti2O5−x electrodes during cycle process at 1.0 A g−1 are characterized by XRD and HR-TEM. XRD patterns displayed in Figure S7a show that the peak (200) gradually shifts to a lower angle and tends to be stable with the increase of cycle numbers. TEM images of the Na2Ti2O5−x electrodes with various cycle numbers (Figure 5a− d) exhibit well-maintained crystal structure with increasing lattice fringes, from 0.74 nm for pristine sample to 0.80 nm for the 500th-cycled sample. The values of d200 calculated on the basis of the XRD patterns with background correction and obtained from HR-TEM are compared and summarized in Figure S7b. As observed, after the first cycle, the d200 value increases sharply, while it tends to be stable with the increase of cycling times. Furthermore, the change values of d200 based on XRD patterns are much higher than that based on HR-TEM but with the same variation tendency. The expansion of interlayer distance of 200 plane after cycle process may be partially attributed to the irreversible trapping of Na-ions in the crystal framework, and an energy dispersive X-ray spectrometer (EDS) has been used to confirm the change of Na content during the cycle process.7 Table S1 compares and displays the atomic ratio of Na and Ti in Na2Ti2O5−x electrodes with different cycle times at 1.0 A g−1. A remarkable increase in the Na content in Na2Ti2O5−x electrode can be observed after the first cycle, and this growth still exists with slight fluctuations 9138

DOI: 10.1021/acs.chemmater.7b02864 Chem. Mater. 2017, 29, 9133−9141

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Figure 6. (a) Schematic illustration of the flexible rGO/AC//Na2Ti2O5−x //rGO/AC SIC, (b) voltage range illustration, (c) galvanostatic charge/ discharge profiles of flexible SIC at various current densities, (d) long-term cycle performance at 1.0 A g−1, and (e) cycle stability of rGO/AC// Na2Ti2O5−x //rGO/AC vs other reported SICs. (f) Ragone plots based on the entire volume of a fully packaged SIC vs other reported flxible systmes, (g) a “HIT” pattern formed by 40 LEDs lightened by the flexible full battery under different states, and a blue LED lightened after several bending cycles.

AC film is attributed to the improvement of the inner structure and the enhancement of electrical conductivity, rather than the simple capacity addition that originates from AC. Moreover, the rGO/AC electrode exhibits excellent cycle performance, maintaining almost 35.0 mAh g−1 after 3000 cycles without obvious capacity decay at 1.5 A g−1 (Figure S10b). A deviation of the ideal rectangular shape can be observed in the CV curves of rGO/AC cathode at high scan rates (Figure S12), indicative of a combination of major electric double-layer capacitor (EDLC) and subordinate pseudocapacitor behavior. Hybrid Flexible Na-Ion Capacitor: rGO/AC//Na2Ti2O5−x //rGO/AC. A Na2Ti2O5−x nanowire arrays anode with superior rate capacity and ultralong life span and a rGO/AC film cathode with high surface area can endow this hybrid configuration high energy/power density outputs as well as excellent cycle stability. The schematic of the flexible hybrid device is illustrated in Figure 6a, with both sides of Na2Ti2O5−x nanowire arrays are utilized, meaning that one Na2Ti2O5−x@foil is combined with two rGO/AC cathodes. One flexible hybrid device is labeled as rGO/AC//Na2Ti2O5−x //rGO/AC. As

demonstrated in Figure 6b, a high-voltage output from 1.0 to 3.8 V can be achieved in the rGO/AC//Na2Ti2O5−x //rGO/ AC system with dual mechanisms including sodiation/ desodiation in anode and absorption/desorption in cathode. The CV curves of Na2Ti2O5−x anode, rGO/AC film cathode, and rGO/AC//Na2Ti2O5−x //rGO/AC device at 2 mV s−1 are shown in Figure S13. The CV curves at various scan rates and the relationship between the redox peak current and the scan rate are further explored and displayed in Figure S14. As observed, the redox peak current and the scan rate exhibit a linear relationship, indicating the interfacial electrochemical reactions with fast energy-storage kinetics. The rGO/AC// Na2Ti2O5−x //rGO/AC configuration is galvanostatically cycled between 1.0 and 3.8 V at various current densities ranging from 0.1 to 1.5 A g−1 to investigate the electrochemical properties (Figure 6c), and the representative charge/discharge profiles based on geometric area are also tested and displayed in Figure S15. Unlike symmetrical supercapacitors, the charge/discharge profiles display a nontriangular shape, indicating different energy storage mechanisms, battery-type anode, and capacitor9139

DOI: 10.1021/acs.chemmater.7b02864 Chem. Mater. 2017, 29, 9133−9141

Article

Chemistry of Materials type cathode. In addition, this hybrid device unfolds superior cycle stability, remaining 82.5% capacity retention after 5000 cycles at 1.0 A g−1(Figure 6d). When compared with other reported hybrid SICs, such as Na2Fe2(SO4)3//Ti2CTx (100 cycles, 83%),7 Na2Ti3O7/CT//rGO (2500 cycles, 80.3%),19 AC//V2O5/CNT (900 cycles, 80%),36 NaxH2‑xTi3O7//AC (1000 cycles, 80%),37 AC//Na2Ti3O7 TNT (1000 cycles, 80%),38 and HC//V2CTx (300 cycles, 70%),39 our rGO/AC// Na2Ti2O5−x //rGO/AC device still exhibits noticeable stability preponderance as shown in Figure 6e. Based on the mass of total active materials in both electrodes, a high-level energy density of 70 Wh kg−1 at a power density of 240 W kg−1 can be achieved as shown in Figure S16. Still, an energy density of 24.4 Wh kg−1 can be obtained even at a high power density of 3600 W kg−1. More critically, taking the entire volume of a fully packaged flexible SIC into consideration, the maximum volumetric energy density and power density reach up to 15.6 Wh L−1 and 120 W L−1, respectively (Figure 6f). As compared with other reported flexible devices, such as Li thin film battery,40 MnO2/CNPs symmetric supercapacitor,41 HTiO2 @MnO2 //H-TiO2 @C and GCF-N2//GCF/MnO2-10 micro-SC traditional asymmetric supercapacitor,42,43 and Na2Ti3O7/CT//rGO hybrid SIC,19 our rGO/AC//Na2Ti2O5−x //rGO/AC assembly still delivers the most promising energy delivery as shown in Figure 6f. After charging, a single flexible cell can power a “HIT” pattern formed by 40 LEDs for over 15 min. We also test the brightness change of 40 LEDs under different states to further highlight the practical applicability and flexibility of the device. Still, it can easily drive and keep the brightness even at bended states as observed in Figure 6g. Moreover, a blue LED (3.2 V) can be lightened after repeating the bending tests, indicating the high voltage output and superior flexibility.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-451-86417853. Fax: +86-451-86418616. ORCID

Zhen-Bo Wang: 0000-0001-9388-1481 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (Grant No. 21273058 and 21673064), China postdoctoral science foundation (Grant No. 2014T70350), HIT Environment and Ecology Innovation Special Funds (Grant No. HSCJ201620), and Harbin technological achievements transformation projects (2016DB4AG023) for their financial support.

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REFERENCES

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CONCLUSIONS In summary, we have proposed a novel flexible hybrid SIC with high volumetric energy density by optimizing both anode and cathode from rational architecture design and initial structure modified to overcome the imbalance kinetics. We have improved the rate capability as well as the cycle stability of Na2Ti2O5−x anode by rational nanostructure design and OVs self-doping strategy: (1) enhancing the structure stability of electrode by adopting well-ordered and self-supported 3D nanowire arrays; (2) boosting the Na reaction kinetics by endowing Na2Ti2O5−x high electrical conductivity as well as introducing intercalation pseudocapacitance. The results indicate that the Na2Ti2O5−x electrode achieves prominent overall electrochemical performance with a high reversible capacity (225 mAh g−1, 0.5C) and superior cycle stability (100% after 20 000 cycles at 25C). When assembled as flexible hybrid SIC with rGO/AC cathodes, high volumetric energy density (15.6 Wh L−1) and power density (120 W L−1), and superior cycle stability (5000 cycles, 82.5%) can be achieved.

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Na2Ti2O5−x nanowire arrays; CV curves and Nyquist plots of Na2Ti2O5 and Na2Ti2O5−x electrodes after 1000 cycles, rate capability and cycle stability of Na2Ti2O5−x anode, XRD patterns of Na2Ti2O5−x electrodes and d200 values based on ex situ XRD and HR-TEM, microscopy and cross-sectional SEM images of various conditions, photographs of rGO/AC film in different conditions, charge−discharge profiles, CV curves of various conditions, galvanostatic charge/discharge profiles, Ragone plots, atomic ratio of Na and Ti in Na2Ti2O5−x electrodes (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02864. TEM image of Na2Ti2O5−x nanowire arrays; TGA analyses of Na2Ti2O5·H2O in air and N2 condition; Raman spectra and EPR spectra of Na2Ti2O5 and 9140

DOI: 10.1021/acs.chemmater.7b02864 Chem. Mater. 2017, 29, 9133−9141

Article

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DOI: 10.1021/acs.chemmater.7b02864 Chem. Mater. 2017, 29, 9133−9141