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Free-Standing T-Nb2O5/Graphene Composite Papers with Ultrahigh Gravimetric/Volumetric Capacitance for Li-Ion Intercalation Pseudocapacitor Lingping Kong, Chuangfang Zhang, Jitong Wang, Wenming Qiao, Licheng Ling, and Donghui Long ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b04737 • Publication Date (Web): 29 Sep 2015 Downloaded from http://pubs.acs.org on September 30, 2015
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Free-Standing T-Nb2O5/Graphene Composite Papers with Ultrahigh Gravimetric/Volumetric Capacitance for Li-Ion Intercalation Pseudocapacitor Lingping Kong, Chuanfang Zhang, Jitong Wang, Wenming Qiao, Licheng Ling and Donghui Long*
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China *
Corresponding author: Donghui Long, Tel: +86-21-64252924. Fax: +86-21-64252914.
E-mail:
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ABSTRACT The free-standing electrodes with high gravimetrical/volumetric capacitance will open up the potential applications in miniaturized consumer electronics. Herein, we report a simple synthesis technology of free-standing orthorhombic Nb2O5 (T-Nb2O5)/graphene composite papers for Liintercalating pseudocapacitive electrodes. Through a facile polyol-mediated solvothermal reaction, the Nb2O5 nanodots are homogenously decorated onto the surface of reduced graphite oxide (rGO), which can form a homogeneous Nb2O5/rGO colloidal suspensions that can be easily fabricated into flexible composite papers. The heat-treated T-Nb2O5/graphene composite papers exhibit a nanoporous layer-stacked structure with good ionic-electric conductive pathways, high T-Nb2O5 loading of 74.2% and high bulk density of 1.55 g cm-3. Such TNb2O5/graphene composite papers show a superior pseudocapacitor performance as freestanding electrode, evidenced by an ultrahigh gravimetric/volumetric capacitance (620.5 F g-1 and 961.8 F cm-3 at 1 mV s-1) and excellent rate capability. Furthermore, organic electrolyte based asymmetric supercapacitor is assembled based on T-Nb2O5/graphene composite papers, which can deliver a high energy density of 47 W h kg-1 and power density of 18 KW kg-1.
KEYWORDS: orthorhombic Nb2O5 • graphene • free-standing • Li-ion intercalation • pseudocapacitors
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Pseudocapacitance, a faradaic process involving surface or near surface redox reactions, offers a means of achieving high energy density at high charge-discharge rates.1,2 General, three faradaic mechanisms can result in pseudocapacitive features. They are reversible surface underpotential deposition (e.g. H2 on Pt),3 Faradic reactions of transition metal oxides (e.g., MnO2, RuO2, NiCo2O4)4-6 and doping-dedoping in conductive polymers (e.g., polyaniline, polypyrrole).7-9 The first two processes are primarily surface or near-surface reversible redox reactions, while the third process is more of a bulk process. Recently, Dunn et al discovered that Li+ intercalation into orthorhombic Nb2O5 (T-Nb2O5) was a capacitive-type process.10 The Li+ intercalation reaction could occur not only at the surface but in the bulk crystals, and the total kinetics of intercalation reaction are not solid-state diffusion-limited. They proposed that the high-rate capability of T-Nb2O5 was attributed to its crystal structure which permitted rapid Li+ transport, since the mostly empty octahedral sites between (001) planes provided natural tunnels throughout the a-b plane.11 Ganesh et al. further suggested that the origin of intercalation pseudocapacitance was due to the unique open channels of NbOx sheets (similar to nano-porous structure) that reduced the energy barrier and facilitated the local charge transfer between lithium and oxygen structures.12 Thus, the unique Li+ intercalation behavior in non-aqueous electrolyte represents a new pseudocapacitive feature, which is typical of Li-ion batteries at capacities but at rates closer to those of supercapacitors.13 This may open the door to a new energy storage concept simultaneously possessing battery and supercapacitor properties. Theoretically, 2 mole Li+ could insert into one Nb2O5 to form Li2Nb2O5, with a maximum capacity of 728 C g-1 or ~200 mA h g-1.14 However, the poor electric conductivity (~3.4×10-6 S cm-1 at 300 K) may limit its electrochemical utilization and high-rate capability, especially in relatively thick practical electrode.15-18 Compositing Nb2O5 with conductive carbon materials
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could largerly overcome these issures. For example, Wang et al. physically mixed Nb2O5 nanocrystals with CNTs which significantly improved the rate capability of the composites.19 Zhang et al. synthesized Nb2O5/layered carbide-derived carbon composites using hydrothermal method with consequent heat-treatment in CO2 flow, which exhibited a volumetric charge of 180 C cm-3.20 By evaporation-induced self-assembly and consequent heat-treatment process, Lee et al. prepared m-Nb2O5-C composites exhibiting a high capacity of 115.1 mA h g-1 at current density 5 A g-1.21 Our recently work reported a hydrothermal method with heat-treatment to prepare T-Nb2O5/graphene composites with high capacitance of 626 C g-1 at 1 A g-1 and 220 C g1
at 50 A g-1.22 These results confirm that compositing Nb2O5 with conductive carbon materials,
possible synergetic efforts and better electrochemical properties can be achieved. However, all these Nb2O5 electrodes were typically fabricated with active powder materials, conductive additives and polymer binders on a metal current collector. The addition of conductive agents and binders can, not only result in “dead weight” and “dead volume” of electrodes but can also increase the polarization resistance, which could lead to the decay of rate performance.23-25 Thus, development of synthetic procedures that yield optimized nanostructure of composite material without conductive additives, binders or current collector should be of great interest to boost the electrochemical performance of Nb2O5 electrode. Recent advances in free-standing electrodes have been demonstrated by highly conductive 2D films for bendable, wearable and portable electronics.26,27 In particular, high volumetric capacitance is vital to the development of flexible devices which can provide as much energy as possible in rather limited space.28,29 Numerous free-standing electrodes have been fabricated through mechanical mixing, chemical deposition or hydrothermal reaction of pseudocapacitive species with 2D flexible graphene.30-37 These electrodes are generally consist of
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pseudocapacitive components for high capacitance, and good ionic-electric conductive pathways for rapid ion diffusion and electron transport.38,39 In addition, the electrodes are generally densely stacked without added binders and conductive additives, to achieving high volumetric capacitance. Among all pseudocapacitive components, Nb2O5 is one of new active materials with extraordinarily high theoretical capacitance in organic electrolyte, which should exhibit great potential to improve the properties of flexible electrodes both in terms of pseudocapacitance and working voltage. However, there is no clear research conducted on free-standing Nb2O5 electrode. Herein, we demonstrated a facile strategy to assemble free-standing T-Nb2O5/graphene composite papers with landmark volumetric capacitance for high-energy pseudocapacitor. The key to our synthesis was relied on the polyol-mediated solvothermal process, which could produce ultrafine Nb2O5 nanodots decorated onto reduced graphite oxide (rGO), and still highly dispersed in ethylene glycol (EG). This success has enabled us to fabricate the assembled Nb2O5/rGO composites using the principles of colloidal chemistry. Through a simple vacuum filtration and post heat-treatment, flexible T-Nb2O5/graphene composite papers were obtained with uniform nanoporous channels, high conductivity (2.5 S cm-1), high bulk density (1.55 g cm3
) and high loading of T-Nb2O5 (74.2 wt. %). When evaluated as binder- and conductive agent-
free electrodes, the composite papers exhibited ultrahigh gravimetric and volumetric capacitance of 620.5 F g-1 and 961.8 F cm-3 at 1 mV s-1, which are the highest values among the reported state-of-the-art
organic
electrolyte
based
pseudocapacitors.
Furthermore,
asymmetric
supercapacitor using T-Nb2O5/graphene composite papers as anode and actived carbon (AC) as cathode was assembled, which resulted in very excellent energy density (47 W h kg-1) with high power delivery (18 KW kg-1) and cycle life (~93% retention of initial energy after 2000th cycle).
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RESULTS AND DISCUSSION
Figure 1. (a) Schematic of the fabrication process for T-Nb2O5/graphene composite papers: (i) solvothermal process, (ii) vacuum filtration, (iii) heat treatment. Digital photographs of GO, NbCl5+GO and Nb2O5/rGO colloidal suspensions in EG (b), Nb2O5/rGO composite papers (c) and T-Nb2O5/graphene composite papers (d). Material Synthesis and Characterization. The fabrication procedure of free-standing TNb2O5/graphene composite papers involves three steps, which are illustrated in Figure 1a. (i) Through a polyol-mediated solvothermal reaction of GO and NbCl5 in EG, the ultrafine Nb2O5 nanodots are uniformly anchored on reduced GO (rGO) surface. The obtained Nb2O5/rGO composites are highly dispersed into EG, forming a homogeneous colloidal suspension without any flocks or precipitated particles even after two months (Figure 1b). Such stable suspension is
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rarely achieved in other metal oxide/rGO composites reported previously. This should be due to the EG as the solvent, which could act as a stabilizing agent that effectively restricts particle growth of Nb2O5 and hinder the aggregation of rGO during reduction process. (ii) By simple vacuum filtration of the colloidal suspension, paper-like structure is fabricated. After washing with ethanol and drying in air, Nb2O5/rGO composite papers with shiny black surface and ca. 45 mm in diameter are obtained, as shown in Figure 1c. These papers can randomly bended around glass rod with no noticeable damage, indicating their remarkable flexible and durable properties (inset in Figure 1c). (iii) Further heat-treated the as-assembled papers at 900 oC in N2 flow, the highly crystalline orthorhombic phase of Nb2O5 is achieved. The obtained T-Nb2O5/graphene composite papers still exhibit the good flexible property as shown in Figure 1d. The typical cross-section SEM images of the Nb2O5/rGO composite papers are shown in Figure 2a and 2b. It is clearly that a compact layer-stacked structure with an average thickness of ~20 µm is assembled by Nb2O5/rGO composite sheets. The top-view SEM image (Figure 2c) and TEM image (Figure 2g) further show the ultrafine Nb2O5 nanodots with 1-3 nm size homogeneously anchored on rGO surface. The flexible property of Nb2O5/rGO composite papers can be therefore explained by the highly ordered self-assembly layer structure. After heat treatment, the resultant T-Nb2O5/graphene composite papers maintain a similar layer stacked structure with the thickness of ~11 µm, but relatively loose structure with some open sheet-like channels formed (Figure 2d and 2e). The top-view SEM image (Figure 2f) and TEM image (Figure 2h) reveal that the particle size of Nb2O5 greatly increases to 5-15 nm and Nb2O5 particles are still highly dispersed. HR-TEM image of T-Nb2O5/graphene (Figure 2i) shows the visible lattice fringes with a lattice spacing of 0.39 nm and 0.31 nm, which are consistent with (001) and (180) plane of orthorhombic phase Nb2O5. Apparently, the growth of Nb2O5 particles
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should serve as the spacers to separate the neighbor layers, leading to the formation of open sheet-like channels in T-Nb2O5/graphene composite papers. The Nb2O5 loading in the TNb2O5/graphene composite papers is ca. 74.2 wt. % determined by TG analysis (Figure S1). The bulk density of T-Nb2O5/rGO composite papers is ca.1.55 g cm-3, generally higher than these of other composite papers (0.8-1.3 g cm-3) in the reported references.35,40-42
Figure 2. Cross-section and top-view SEM images of Nb2O5/rGO (a-c) and T-Nb2O5/graphene composite papers (d-f). TEM image of Nb2O5/rGO (g). TEM image (h), HR-TEM and Electron diffraction pattern (i) of T-Nb2O5/graphene.
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Figure 3. XRD patterns (a), Raman spectra (b), Nitrogen adsorption-desorption isotherms (c) and BJH pore size distributions (d) of Nb2O5/rGO and T-Nb2O5/ graphene composite papers. The XRD patterns of the Nb2O5/rGO and T-Nb2O5/graphene composite papers are shown in Figure 3a. After solvothermal reaction, the obtained Nb2O5/rGO papers show only a broad peak centered at 2θ ≈ 25o, that corresponds to graphitic structure of reduced GO. No any Nb2O5 crystal peaks are observed due to their amorphous feature. Heat-treatment at 900 oC could successfully convert the amorphous Nb2O5 into orthorhombic phase, with the typical diffraction peak located
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at 22.7o, 28.5o, 36.6o, 45.2o, and 50.9o which can be indexed as (001), (180), (181), (2110) and (380) reflections (JCPDS no. 30-0873).11 Raman spectra of the Nb2O5/rGO and TNb2O5/graphene composite papers are shown in Figure 3b. Only two carbon characteristic peaks at 1350 cm-1 (D-bond) and 1585 cm-1 (G-bond) are observed, which are generally related to structural defects and graphitic structure, respectively. The intensity ratio of D-bond to G-bond, ID/IG is 1.12 for Nb2O5/rGO and 1.43 for T-Nb2O5/graphene composites. This result indicates that new graphitic domains are created in graphene during heat treatment, smaller in size to the ones present in rGO.43 In addition, the absent of Raman peaks for Nb2O5 should be due to the tiny Nb2O5 particles which are tightly packed in the layer graphene structure. The textural properties of these composite papers are characterized by N2 adsorption (Figure 3c). The Nb2O5/rGO papers have a type-I isotherm with a high BET surface area of 285.3 m2 g-1. By a sharp contrast, after high temperature treatment, the T-Nb2O5/graphene papers exhibit a type-IV isotherm with a pronounced hysteresis loop at P/P0 of 0.4-0.9, indicating the existence of mesoporous channels. The average pore diameter is determined to be ca. 5 nm, which should originate from the layers space expanded by the T-Nb2O5 nanoparticles. The BET surface area of T-Nb2O5/graphene paper is decreased to 209.6 m2 g-1 and the pore volume increases to 0.263 cm3 g-1, regardless of its dense bulk density. More detailed structural information is given Table S1. The electric conductivities of these papers are measured by using conductivity meter with four point probe head, as listed in Table S2. The electrical conductivity of T-Nb2O5/graphene papers is about 2.5 S cm-1, much higher than 0.12 S cm-1 of Nb2O5/rGO papers. Such enhanced conductivity should provide good electronic conducting pathway through the film, while the nanoporous structure between layers also provide channels for electrolyte spreading to the inner active sites. Therefore, as-prepared T-Nb2O5/graphene composite papers synergistically combine
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many critical features for high performance free-standing electrodes: integrity electrodes with high T-Nb2O5 loading, nanoporous layer-stacked structure with good ionic-electric conductive pathways, high bulk density for high volumetric capacitance. Electrochemical Characterization. Nb2O5 is known to act as an intercalation pseudocapacitive material in nonaqueous Li+ electrolyte, through a Li+ rapid insertion reaction: Nb2O5 + xLi+ + xe- ↔ LixNb2O5. To emphasize the importance of free-standing structure, the obtained composite papers are directly used as electrodes without binder or conductive agent.
Figure 4. CV curves of Nb2O5/rGO (a) and T-Nb2O5/graphene (b) composite papers. Plotting of capacitance versus sweep rate (c). The b-value determination of anode and cathode peak currents (d). Total current (solid line) and capacitive current (shade regions) at 5 mV s-1 (e) and capacitive contribution (f) of T-Nb2O5/graphene composite papers.
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The cyclic voltammetry (CV) of the Nb2O5/rGO and T-Nb2O5/graphene composite papers are firstly investigated. As shown in Figure 4a, amorphous Nb2O5/rGO papers demonstrate very poor electrochemical performance with not obvious intercalation peaks and low current densities. This mainly because the compact layer-stacked structure would hinder the Li+ accessing through these papers and the amorphous Nb2O5 was insensitive to the Li+ intercalation reaction. On the contrary, the heat-treated T-Nb2O5/graphene papers show a pronounced redox response with a couple of symmetric anodic and cathodic broad peaks at ca.1.5 V. There is a little voltage separation (
