rGO Nanocomposite for Ultrahigh-Energy-Density

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24114−24121

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Porous Nb4N5/rGO Nanocomposite for Ultrahigh-Energy-Density Lithium-Ion Hybrid Capacitor Shengyuan Li,†,⊥ Ting Wang,‡,⊥ Yunpeng Huang,† Zengxi Wei,§ Guochun Li,† Dickon H. L. Ng,∥ Jiabiao Lian,*,† Jingxia Qiu,† Yan Zhao,† Xiaoyan Zhang,† Jianmin Ma,*,§ and Huaming Li† †

Key Laboratory of Zhenjiang, Institute for Energy Research, Jiangsu University, Zhenjiang 212013, P. R. China Nanyang Environment and Water Research Institute (NEWRI), Interdisciplinary Graduate School (IGS), Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 § School of Physics and Electronics, Hunan University, Changsha 410082, P. R. China ∥ Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, P. R. China

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‡

S Supporting Information *

ABSTRACT: To meet the increasing demands for high-performance energy storage devices, an advanced lithium-ion hybrid capacitor (LIHC) has been designed and fabricated, which delivers an ultrahigh energy density of 295.1 Wh kg−1 and a power density of 41 250 W kg−1 with superior cycling stability. The high-performance LIHC device is based on the uniform porous Nb4N5/rGO nanocomposite, which has an intimate interface between the firmly contacted Nb4N5 and rGO through the Nb(Nb4N5)−O(rGO)−C(rGO) bonds, significantly improving the electron transport kinetics. Moreover, the introduction of rGO nanosheets can prevent the Nb4N5 nanoparticles from agglomeration, not only resulting in a larger specific surface area to provide more active sites but also accommodating the strain during Li ion insertion/deinsertion. Therefore, the Nb4N5/rGO nanocomposite exhibits a higher reversible specific capacity and better rate and cycling performance than the Nb4N5 nanoparticle. In view of the scalable preparation and superior electrochemical characteristics, the Nb4N5/rGO nanocomposite would have great potential practical applications in the future energy storage devices. KEYWORDS: Nb4N5/rGO nanocomposite, porous structure, ultrahigh energy/power densities, lithium-ion capacitor, energy storage

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INTRODUCTION Driven by the surging market of portable electronics, electric vehicles, and industrial equipment, advanced energy storage devices with both high energy and high power densities are in great need. In view of that, novel lithium-ion hybrid capacitors (LIHCs) have emerged in recent years due to the good combination of high power density of supercapacitors (SCs) and high energy density of lithium-ion batteries (LIBs).1−4 Typically, a LIHC consists of a battery-type negative electrode and a capacitor-type positive electrode, which can take advantage of both Faradic and capacitive mechanisms to offer higher energy densities than SCs and higher power densities than LIBs. However, there is still a big challenge in the development of LIHCs, that is, the low energy density at high charge/discharge rate due to the imbalanced kinetics © 2019 American Chemical Society

between the anode and cathode. The kinetics of anode (Faradic Li+ intercalation/deintercalation reaction) is much slower than that of cathode (fast physical anion adsorption/ desorption process). Thus, it is of great importance to explore high-rate anode materials to narrow the kinetics gap for highperformance LIHCs. To mitigate the rate-imbalance issue in LIHCs, various energy-type anode materials have been extensively investigated, including transition-metal oxides (Nb-/V-/Ti-/Mn-/Mo-/Febased oxides, etc.),5−15 MXenes,16,17 and carbonaceous materials.18−21 However, transition-metal oxides still suffer Received: April 11, 2019 Accepted: June 19, 2019 Published: June 19, 2019 24114

DOI: 10.1021/acsami.9b06351 ACS Appl. Mater. Interfaces 2019, 11, 24114−24121

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ACS Applied Materials & Interfaces

efficient electron- and ion-transport pathways of the electrode materials. In consideration of these issues, two-dimensional reduced graphene oxide (rGO) nanosheets with large specific surface area and high electrical conductivity were introduced in this work. Moreover, the surface of rGO nanosheets possesses rich functional groups, which are beneficial for combining with metal nitrides. To the best of our knowledge, there are very few reports on the synthesis of the Nb4N5/rGO nanocomposite for LIHCs up till now. Herein, porous Nb4N5 nanoparticles were successfully prepared by a facile ammonia annealing and used to synthesize uniform Nb4N5/rGO nanocomposites via a freeze-drying method with subsequent calcination. The formation of an intimate interface between the firmly contacted Nb4N5 and rGO through the Nb(Nb4N5)−O(rGO)−C(rGO) bonds, which significantly increased the electrical conductivity of the nanocomposite and consequently improved their electrontransport kinetics, was demonstrated. Moreover, the introduction of rGO nanosheets can prevent the Nb4N5 nanoparticles from agglomeration, not only resulting in a larger specific surface area to provide more available active sites for energy storage but also accommodating the strain during the Li ion intercalation/deintercalation process. Therefore, the Nb4N5/ rGO nanocomposite exhibited a higher reversible specific capacity (∼435 mAh g−1) as well as better rate and cycling performance (the capacity retention is 96.2% after 1000 cycles) than the pristine Nb4N5 nanoparticles. More remarkably, the as-fabricated Nb4N5/rGO-based LIHC delivers an ultrahigh energy density of 295.1 Wh kg−1 at 112.3 W kg−1 and an ultrahigh power density of 41 250 W kg−1 with the energy density maintained at 82.2 Wh kg−1, which are the highest values for the reported hybrid supercapacitors. The device also exhibits excellent cycling stability within a high operating voltage of 4.5 V (capacity retention of 92.8% over 4000 cycles at 1.0 A g−1). In view of the scalable preparation and superior electrochemical characteristics, the Nb4N5/rGO nanocomposite would have great potential applications for the future energy storage devices.

from poor rate performance and low power density owing to their inherent low electrical conductivity as well as low Li+ diffusion rate. Regarding carbonaceous materials, even though they exhibit exciting energy density, the large irreversible capacity loss and poor cycle performance severely hinder their practical applications as anode in LIHCs. Therefore, it is highly essential to devise novel anode materials for advanced LIHCs with both high energy and high power densities. In this regard, transition-metal nitrides with superior chemical stability, excellent electrical conductivity, high Li+ diffusion coefficient, and high reversible specific pseudocapacitance are considered to be very promising electrode materials.22−28 Among them, the nitrogen-rich phase Nb4N5 comprising high-valence states (lots of Nb5+ ions) and having a metallic character is extremely desirable for high-performance LIHCs. Moreover, Nb4N5 with the tetragonal I4/m space group has a defective NaCl-type structure with niobium vacancies.29 As shown in Figure 1a, a

Figure 1. Schematic illustration of the crystal structure of tetragonal Nb4N5. (a) A unit cell containing ten N atoms, eight Nb atoms, and two vacancies (one at the body center). (b) A unit cell along the caxis. (c) Three-dimensional perspective view demonstrating the channels along the c-axis.

unit cell of tetragonal Nb4N5 contains ten N atoms, eight Nb atoms, and two vacancies (one at the body center). Furthermore, a unit cell along the c-axis (Figure 1b) indicates the channel with a side length of 2.17 Å and a diagonal length of 3.07 Å, and the three-dimensional perspective view shown in Figure 1c demonstrates the channels along the c-axis, which could accommodate large amounts of lithium ions with a radius of 0.76 Å and result in fast Li+ diffusion. In addition, the energy storage depends highly on the surface area as well as

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EXPERIMENTAL SECTION

In a typical synthesis of porous Nb4N5 nanoparticles, 6.0 mmol of commercial niobium oxalate powder was added into an alumina boat, which was then transferred to a tube furnace for nitridation treatment.

Figure 2. FE-SEM (all scale bars are 100 nm), TEM, and HRTEM images of porous Nb4N5 nanoparticles (a−d) and the Nb4N5/rGO nanocomposite (e−h). 24115


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Figure 3. (a) XRD patterns and (b) Nb 3d, (c) N 1s, (d) O 1s, and C 1s high-resolution XPS spectra of the porous Nb4N5 nanoparticles and Nb4N5/rGO nanocomposite. Black circles are the measured data, and the red curves are the summation of the decomposed curves. The treatment was conducted at 750 °C for 3 h in a flow of NH3 and Ar (50 and 100 sccm) with a heating rate of 2 °C min−1. After being cooled, the resultant product was collected and porous Nb4N5 nanoparticles were obtained. For the preparation of porous Nb4N5/ rGO nanocomposite, 200 mg of aforementioned Nb4N5 nanoparticles and 50 mg of GO nanosheets (Figure S2, GO was prepared using a modified Hummers’ method) were added in 200 mL of deionized water separately. The two mixtures were then ultrasonicated for 0.5 h to obtain the corresponding homogeneous suspensions, i.e., Nb4N5 suspension and GO suspension. The Nb4N5/GO mixed suspension was formed by adding the Nb4N5 suspension into the GO suspension dropwise at room temperature with constant magnetic stirring. Afterward, the Nb4N5/GO mixture was first freeze-dried and then transferred to be annealed at 350 °C in a 5% H2/Ar atmosphere for 0.5 h to form a porous Nb4N5/rGO nanocomposite. The detailed morphological and structural characterizations, as well as the half-cell and LIHC device fabrication and electrochemical measurements, are given in the Supporting Information.

Figure S1a. In addition, the lattice fringe in the high-resolution TEM (HRTEM) image (Figure 2d) is ca. 0.248 nm, which belongs to the (211) d spacing of the tetragonal Nb4N5. The formation of tetragonal Nb4N5 was verified by X-ray diffraction (XRD) measurements (upper curve in Figure 3a). All of the peaks could be indexed to a tetragonal structure of Nb4N5 (JCPDS #51-1327). The narrow sharp peaks suggest that the as-prepared Nb4N5 is highly crystalline, which is well in accordance with the HRTEM result. Further evidence of the formation of Nb4N5 can be obtained from the X-ray photoelectron spectroscopy (XPS) analyses. As shown in the upper curve of Figure 3b, the Nb 3d core-level spectrum of the Nb4N5 nanoparticles comprises three conspicuous peaks. It is noteworthy that Nb4N5 comprises Nb5+ and Nb3+; thus, the multiple peaks can be fitted by two different spin−orbit doublets.25 The higher Nb 3d3/2 binding energy doublet at 208.1 eV belongs to Nb5+−N, and the lower one at 205.4 eV can be assigned to Nb3+−N in Nb4N5. Their corresponding Nb 3d5/2 binding energies are 205.3 and 202.6 eV, respectively. The N 1s core-level spectrum of the Nb4N5 nanoparticles (upper curve of Figure 3c) mainly exhibits an intense peak at 395.2 eV and two weak peaks at 397.8 and 400.1 eV, which are attributed to N−Nb, pyridinic-N, and pyrrolic-N, respectively.26 Moreover, the XPS quantification result indicates that the atomic ratio of Nb to N is about 43.95:56.05, close to the stoichiometry of Nb4N5. Compared with that of the pristine Nb4N5 nanoparticles, the XRD pattern of the nanocomposite (lower curve in Figure 3a) displayed the typical (002) peak of graphite at around 26°, revealing the successful combination of Nb4N5 with rGO,

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RESULTS AND DISCUSSION The morphology of the as-synthesized samples was observed via field emission scanning electron microscopy (FE-SEM) and the corresponding transmission electron microscopy (TEM) images. Figure 2a,b shows that the as-prepared Nb4N5 consists of uniform nanoparticles with a rather narrow size distribution. Figure 2c further displays that the size of the nanoparticles was about 25 nm and there were many worm-hole-like pores on their surfaces. The formation of pores is probably due to the release of H2O, CO2, and N2 during the nitridation process. The porous structure is further demonstrated by the nitrogen adsorption−desorption isothermal and Barrett−Joyner−Halenda (BJH) pore size distribution curves (inset), as shown in 24116


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Figure 4. CV curves of the Nb4N5 nanoparticle (a) and Nb4N5/rGO nanocomposite (b) electrodes. (c) b-Value determination and (d) Nyquist plots with the fitted impedance data for the Nb4N5 (black) and Nb4N5/rGO electrodes (blue). The inset shows the equivalent electrical circuit model. (e, f) CV response with separation between total currents and capacitive currents (red shaded region) at 1.0 mV s−1. (g) Rate and cycling performance with the corresponding Coulombic efficiency of the Nb4N5 (black and red circle) and Nb4N5/rGO (blue and green square) electrodes.

groups, such as −COOH, −OH, C−O−C, and CO. Afterward, the GO nanosheets could be successfully transformed to rGO to a significant extent by thermal treatment; meanwhile, the intimate interface would be formed between Nb4N5 and rGO, as indicated by the HRTEM image of the nanocomposite (Figure 2h). Moreover, the XPS spectra of the nanocomposite further demonstrated the coexistence of Nb4N5 and rGO as well as a certain electron interaction between them. In comparison with that of the Nb4N5 nanoparticles, the best peak fit to the Nb 3d core-level spectrum of the nanocomposite (lower curve of Figure 3b) involves the common contributions of Nb3+−N (204.6 eV for Nb 3d5/2

which was further demonstrated by FE-SEM, TEM, HRTEM, and XPS characterizations. As displayed in the FE-SEM images (Figure 2e,f), the Nb4N5 nanoparticles are uniformly dispersed on the surface of rGO with continuously cross-linked networks, further observed by TEM images (Figure 2g). As is well-known, GO nanosheets contain many hydrophilic functional groups on their defect sites and edges and hydrophobic basal planes,30,31 keeping them highly dispersible and making it easy to capture the Nb4N5 nanoparticles homogeneously during the mixing and freeze-drying process. As displayed in Figure S2, the Fourier transform infrared (FTIR) and XPS results demonstrated the existence of functional 24117


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ACS Applied Materials & Interfaces and 207.4 eV for Nb 3d3/2), Nb5+−N (207.3 eV for Nb 3d5/2 and 210.1 eV for Nb 3d3/2), and Nb−O (208.0 eV for Nb 3d5/2 and 210.8 eV for Nb 3d3/2); the N 1s core-level spectrum (lower curve of Figure 3c) can be fitted with three subpeaks due to the spin−orbit coupling.32 In addition, the O 1s corelevel spectrum (upper curve in Figure 3d) has two peaks at 530.8 and 533.1 eV, assigned to O−Nb and O−C, respectively. The C 1s core-level spectrum (lower curve in Figure 3d) could also be fitted by two peaks at 284.6 and 285.7 eV, which is the characteristic of graphite-like sp2-hybridized C−C and C−O in rGO, respectively. Furthermore, it is noteworthy that the binding energy peaks of both Nb 3d and N 1s for the nanocomposite obviously shift to a higher energy direction, which is due to the collective contribution of electron transfer from Nb4N5 to rGO.9,33 In general, the binding energy (Eb) could be calculated by the following equation E b = Vc + Vn

percentage of capacitive contribution can be quantified using eq 5 and its derivative eq 614

where Vc is the Coulomb force and Vn is the repulsive force (the shielding effect of the outer electrons to the inner electrons, a negative value). The decrease of the outer electron concentration in Nb4N5 will result in the decrease of the shielding effect, thus increasing the binding energy of the Nb 3d and N 1s inner electrons. On the basis of the above analysis, it is reasonable to conclude that an intimate interface is formed during the annealing process and a certain electron interaction exists between the firmly contacted Nb4N5 and rGO through the Nb(Nb4N5)−O(rGO)−C(rGO) bonds, significantly improving the electron- and mass-transport kinetics. Cyclic voltammetry (CV) measurements were conducted to examine the capacitive characteristics of Nb4N5 and Nb4N5/ rGO electrodes, as displayed in Figure 4a,b, respectively. It can be obviously seen that there are broad cathodic peaks between 1.0 and 2.0 V and anodic peaks between 1.5 and 2.5 V in the curves of the Nb4N5 electrode (Figure 4a), indicating a series of intercalation/deintercalation reactions of Nb4N5 with Li ions, as expressed below26,27 (2) +

where x represents the mole fraction of the inserted Li (0 < x < 3). Compared with those of the pristine Nb4N5 electrode, the CV curves of the Nb4N5/rGO electrode (Figure 4b) exhibit steep cathodic slopes and broad anodic peaks in the potential range of 0.01−1.0 V, attributed to the Li+ insertion/ extraction into/from the rGO nanosheets; the reversible Faradic reactions between surface functional groups and electrolyte; and the formation of solid electrolyte interface (SEI) layers.20,34 Moreover, the peak currents increase with the increasing scan rate. According to the power law, the current (i) is proportional to the scan rate (v)35−38 i = avb

(3)

log(i) = b log(v) + log(a)

(4)

(5)

i(V)/v1/2 = k1v1/2 + k 2

(6)

where both k1 and k2 are constants and can be determined by plotting v1/2 versus i(V)/v1/2. Accordingly, the capacitive contribution is about 67.3% for the Nb4N5 nanoparticles (Figure 4e) and 62.5% for the Nb4N5/rGO nanocomposite (Figure 4f) at 1.0 mV s−1. The results indicate that the nanocomposite retains the pseudocapacitive characteristics of Nb4N5. The rate capacities of the as-prepared electrodes are compared in Figure 4g. The charge and discharge capacities of the Nb4N5/rGO nanocomposite electrode are 812.5 and 569.9 mAh g−1 in the first cycle, respectively, indicating that the initial Coulombic efficiency is approximately 70%. The initial capacity loss is due to the formation of the SEI layer and the irreversible side reactions.39 In the following charge/ discharge cycles, the Nb4N5/rGO electrode can deliver reversible specific capacities of ∼435, 360, 322, and 283 mAh g−1 at the current densities of 0.1, 0.25, 0.5, and 1.0 A g−1, respectively, which are much higher than those of the pristine Nb4N5 electrode at all current densities. The improved capacity is ascribed to the introduction of rGO, which can provide a larger specific surface area (SBET) and more active sites for energy storage. As shown in Figure S1, SBET of the Nb4N5/rGO nanocomposite increases dramatically to 45.89 m2 g−1 in comparison with that of the Nb4N5 nanoparticles (24.13 m2 g−1), which also have a porous structure with a pore diameter of around 4 nm. The large specific surface area and the porous structure would be beneficial to provide more contact area with the electrolyte, promote the penetration of the electrolyte, and shorten the diffusion paths of Li+. Moreover, the addition of the rGO nanosheets can increase the electrical conductivity of the nanocomposite and the intimate interface between Nb4N5 and rGO could significantly improve the electron- and mass-transport kinetics. Figure 4d displays the Nyquist plots with the equivalent electrical circuit model (inset), in which the Nb4N5/rGO nanocomposite electrode (blue) displays a much smaller semicircle and a more vertical straight line than the Nb4N5 electrode (black). This trend indicates that the Nb4N5/rGO nanocomposite has a lower charge-transfer resistance (Rct) and a faster Li+ diffusion than the pristine Nb4N5 nanoparticle, attributed to the efficient electrical and ionic transport highways built up in the nanocomposite. As a result, the nanocomposite exhibits an improved rate performance with the rate capacity retention of 65.1%. In contrast, the specific capacity of the Nb4N5 nanoparticles was only 155 mAh g−1 at 1.0 A g−1, equating to 41.8% of that at 0.1 A g−1 (371 mAh g−1). In addition, the introduction of the rGO nanosheets also prevents the Nb4N5 nanoparticles from agglomeration, which can accommodate the strain during Li ion intercalation/deintercalation and consequently ensure an excellent cycling performance. As shown in Figure 4g, the capacity retention of the Nb4N5/rGO nanocomposite after 1000 cycles is 96.2%, which is much higher than that of the pristine Nb4N5 nanoparticle (62.3% capacity retention). Such a superior electrochemical performance would make the Nb4N5/rGO nanocomposite an excellent candidate for high-performance LIHC.

(1)

Nb4 N5 + x Li+ + x e− V LixNb4 N5

i(V) = k1v + k 2v1/2

where a and b are appropriate values. Determined by the slope of log(i) versus log(v) (Figure 4c), the b-values for the anodic peaks of the pristine Nb4N5 electrode (black) and Nb4N5/rGO nanocomposite electrode (blue) are approximately 0.81 and 0.83, respectively, suggesting a majorly surface-controlled capacitive behavior in the as-prepared electrodes. Furthermore, by dividing the current response i at a fixed potential into the surface-controlled capacitive behavior (k1v) and diffusioncontrolled Li+ insertion process (k2v1/2), the more precise 24118


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Figure 5. (a) Schematic illustration of the energy storage mechanism of the Nb4N5/rGO//SCCB LIHC. (b) CV curves at various scan rates. (c, d) Galvanostatic charge−discharge curves at different current densities. (e) Specific capacitance as a function of current density. (f) Ragone plots of the as-fabricated LIHC, compared with other hybrid capacitors. (g) Cycling performance at 1.0 A g−1.

nanopowders, while Li+ ions from the electrolyte are either intercalated into the Nb4N5 and the graphite layer of rGO or adsorbed on the surface of the nanocomposite; consequently, the voltage of the LIHC increases and the electrical energy can be stored. The discharge process is the reverse of the charge process. Figure 5b shows the CV curves of the as-fabricated Nb4N5/ rGO//SCCB LIHC at various scan rates. Different from the rectangular curve of the symmetric supercapacitor, the asymmetric CV curve clearly indicates the synergistic effect of the Li+ intercalation pseudocapacitance of the Nb4N5/rGO anode and the electric double-layer capacitance of the SCCB cathode. More remarkably, the shape of the CV curves is still retained as the scan rate is increased, indicating its superior rate capability. Figure 5c,d shows the galvanostatic charge/ discharge curves at different current densities (based on the

The commercial superconductive carbon black (SCCB) nanopowder was applied as a cathode to assemble a LIHC with the pre-lithiated Nb4N5/rGO anode. Figure S3 (Supporting Information) shows the electrochemical performance of the SCCB nanopowder electrode in a half-cell, which reveals that the SCCB nanopowder has a high specific capacitance, remarkable rate capability, and long cycle life. As mentioned above, both the as-prepared Nb4N5/rGO nanocomposite and commercial SCCB nanopowders have excellent electrochemical performance, which should be promising Li-ion battery-type and capacitor-type electrode materials for highperformance LIHC. As illustrated in Figure 5a, a LIHC device was fabricated, consisting of the pre-lithiated Nb4N5/rGO nanocomposite anode, SCCB nanopowder cathode, and LiPF6containing organic electrolyte. During the charge process, PF6− ions are adsorbed physically on the surface of the SCCB 24119



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total mass of the anode and cathode materials). It is worth noting that the slopes of the curves are not linear especially at low current density, further indicating a good connection between fast Li+ intercalation reaction at the Nb4N5/rGO anode and a rapid PF6− ion physisorption at the SCCB cathode. Based on the galvanostatic discharge curves after IR drop (the IR drops at different discharge current densities are shown in Figure S4), the specific capacitances of the asfabricated LIHC (Figure 5e) were calculated to be 105.3, 97.9, 84.5, 76.5, 68.3, 57.3, 42.4, 39.7, and 34.8 F g−1 at 0.05, 0.1, 0.25, 0.5, 1.0, 2.5, 5.0, 10, and 20 A g−1, respectively. The energy and power density values of the Nb4N5/rGO// SCCB LIHC at different current densities were also calculated based on the total mass of the active materials, and the corresponding Ragone plot is shown in Figure 5f. Remarkably, the as-fabricated Nb4N5/rGO//SCCB LIHC can deliver an ultrahigh energy density of 295.1 Wh kg−1 at 112.3 W kg−1 and remain at 82.2 Wh kg−1 even at an ultrahigh power density of 41 250 W kg−1, which are the highest values for the reported hybrid supercapacitors, as summarized in Table S1 (Supporting Information) and Figure 5f. In addition, the device also exhibits a superior cycling stability with 92.8% capacity retention over 4000 cycles at 1.0 A g−1 (Figure 5g). The exceptional electrochemical performance of the Nb4N5/rGObased LIHC, in terms of high energy/power densities in a wide potential window and long cycle life, would provide great potential practical applications in the future.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.L.). *E-mail: [email protected] (J.M.). ORCID

Jiabiao Lian: 0000-0002-4439-2988 Huaming Li: 0000-0002-9538-5358 Author Contributions

The manuscript was written by S.L. and T.W. and revised by J.L., with contributions from all authors. Author Contributions ⊥

S.L. and T.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the Natural Science Foundation of Jiangsu Province (BK20170549), the National Natural Science Foundation of China (21706103), and the China Postdoctoral Science Foundation (2017M621647). J.L. also appreciates the support from Jiangsu Provincial Program for High-Level Innovative and Entrepreneurial Talents Introduction, the Senior Talent Foundation (16JDG020), and Young Talent Cultivation Plan of Jiangsu University, High-Tech Research Key Laboratory of Zhenjiang (SS2018002) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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CONCLUSIONS In this work, we demonstrate a facile and scalable method to synthesize a uniform Nb4N5/rGO nanocomposite with controllable mass ratio, large specific surface area, porous structure, and intimate interface. The introduction of rGO nanosheets not only helps to increase the surface area and electrical conductivity of the final nanocomposite but also prevents the Nb4N5 nanoparticles from agglomeration, which can provide more active sites for energy storage and accommodate the strain during Li ion insertion/deinsertion. Therefore, the Nb4N5/rGO nanocomposite exhibits a much higher reversible specific capacity and better rate and cycling performance than the pristine Nb4N5 nanoparticle. More remarkably, the Nb4N5/rGO-based LIHC delivers ultrahigh energy and power densities, as well as superior cycling stability within a high operating voltage of 4.5 V, which is believed to have great potential practical applications for fast and efficient energy storage.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06351. Experimental details; nitrogen adsorption−desorption isothermal and BJH pore size distribution curves of the porous Nb4N5 nanoparticle, Nb4N5/rGO nanocomposite, and SCCB nanopowder; TEM image and FTIR and XPS spectra of the GO nanosheet; electrochemical performance of the SCCB electrode and the corresponding analysis; IR drops of the as-fabricated Nb4N5/rGO// SCCB LIHC at various discharge current densities; and electrochemical performance of our Nb4N5/rGO// SCCB LIHC in comparison with other previously reported hybrid capacitors (PDF) 24120


Research Article

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rGO Nanocomposite for Ultrahigh-Energy-Density

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