Nb₂O₅/graphene nanocomposites for electrochemical energy storage

Abstract

The development of electrode materials for energy storage, with high energy and power densities along with good cyclic stability, still remains a big challenge. Here we report the synthesis of Nb₂O₅/graphene nanocomposites, through a simple hydrothermal method, with Nb₂O₅ nanoparticles anchored on reduced graphene oxide (RGO) sheets. The fabricated Nb₂O₅/graphene electrodes exhibited an excellent electrochemical performance when studied as anodes for lithium-ion batteries, with a superior reversible capacity and high power capability (192 mA h g⁻¹ under 0.1C rate over 50 cycles). Signature curve studies showed high power capability of the Nb₂O₅/graphene electrode with ∼80% of the total capacity retained at 16C rate compared to ∼30% retention for pristine Nb₂O₅ nanoparticles. To achieve further improvement in energy density and power capability, Li-ion hybrid electrochemical capacitors (Li-HECs) were fabricated with the Nb₂O₅/graphene nanocomposite as the anode and rice husk-derived activated porous carbon as the cathode, in non-aqueous electrolyte. The Li-HECs showed enhanced electrochemical performance with high energy density of 30 W h kg⁻¹, at a specific power density of 500 W kg⁻¹. The Nb₂O₅/graphene nanocomposites show promising results and hence have great potential for application in efficient electrochemical energy storage devices.

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1. Introduction

With the ever-increasing demand for energy storage, lithium ion batteries and supercapacitors have been the attractive technologies that have revolutionized the portable electronic industry and high power device applications.^1,2 It is interesting to note that the material design of these electrochemical energy storage devices would vary with different energy domains, for application in Li-ion batteries and supercapacitors.^2–4 Recently, with the advancements in micro/nanoelectronics, lot of efforts are being made to develop lighter, smaller and powerful rechargeable batteries with operating voltages in the range 1.9–2.5 V, thanks to microfabrication technology enabling reduction of the operating voltage of these devices to ∼2 V or less.⁵ Electrode materials with high electrical conductivity for fast electron transport, and large surface areas with short diffusion paths for fast lithium ion diffusion, are essential to meet the high power requirements of these power sources.^6,7

Under such voltage and power requirements, several materials have been studied as alternative electrodes for rechargeable lithium batteries, among which, some second row transition metal oxides (MoO₃, Nb₂O₅)^8,9 have been explored recently due to their excellent structural stability and the higher valence state of Nb₂O₅ and MoO₃. These metals contain two or more electrons available for lithium insertion/extraction, thus offering the possibility of attaining an improved specific capacity over the existing ones. Among them, Nb₂O₅ show a theoretical capacity of 201 mA h g⁻¹ with the insertion of 2 lithium, forming Li_xNb₂O₅, is very promising as a cathode material for 2 V rechargeable lithium batteries, especially for microelectronic device applications. Recently, lithiated Nb₂O₅ electrode materials were explored for understanding its structural and electrochemical properties for energy storage applications.¹⁰ Nb₂O₅ exists in several crystal structures depending on the synthesis temperature which include pseudo-hexagonal (500 °C), orthorhombic (600–800 °C), metastable tetragonal (1000 °C), and monoclinic (1100 °C). Kodama et al.,⁵ have investigated the variation of electrochemical performance with the change in crystal structure of Nb₂O₅ and found that orthorhombic Nb₂O₅ (T-Nb₂O₅) is the most favorable phase, shown to have excellent electrochemical performance due to an enhanced reversible capacity (no capacity fading) and excellent reversible structural change during the charge/discharge process, although it has a lower capacity than the monoclinic phase.¹¹ The presence of the 3-dimensional structure in orthorhombic Nb₂O₅ facilitates insertion of large amount of Li ions into the interstitial sites, and retains the original crystal lattice upon de-intercalation with very small change in the unit cell volume. Moreover, the chemical diffusion coefficient of Li⁺ ions in T-Nb₂O₅ is about 10⁻¹⁰ cm² s⁻¹ which is the same as for the graphite electrode, and these factors favour the orthorhombic phase as a potential cathode for lithium batteries.^12,13 Furthermore, charge carrier conduction from the electrode to the current collectors and electrolyte diffusion during lithium intercalation are critical factors for improving the working efficiency of electrochemical systems like lithium batteries and supercapacitors.¹⁴ Though there has been recent studies on high pseudocapacitive energy storage in Li intercalated Nb₂O₅ nanocrystals,^15,16 the particle–particle electronic conduction pathways are very critical in achieving high power capability at the nanoscale. Moreover, the fabrication of nanostructures involves a serious issue of heavy aggregation of the nanocrystals due to van der Waal’s forces between the particles that limit the special properties and cause structural instability thereby reducing the applicability. To prevent aggregation, nanocomposites consisting of nanoparticles decorated onto a conducting substrate may be exploited, which preserves the unique properties of the nanoparticles with a high surface area along with imparting high electrical conductivity.¹⁷ One such substrate is graphene which offers large anchoring sites for the inorganic nanostructures due to its large surface area of ∼2630 m² g⁻¹ and a highly 2-dimensional environment for fast electron transport through the conducting zero bandgap graphene layers.^18–23

Here, we report a highly conducting Nb₂O₅ decorated graphene nanocomposite with low graphene content (4.5 wt%), synthesized via hydrothermal approach, as an advanced electrode material for electrochemical energy storage. The synthesized Nb₂O₅ nanoparticles of size ∼50 nm, are decorated homogeneously throughout the graphene sheets. Electrochemical characterizations including cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy have been carried out to study the electrochemical performance of these electrodes as anodes for Li-ion batteries. Furthermore, Li-ion hybrid electrochemical capacitors (Li-HECs) were fabricated with the Nb₂O₅/graphene nanocomposite as the anode and rice husk-derived carbon species activated with H₃PO₄ (RHDPC-H₃PO₄) and KOH (RHDPC-KOH) as the cathode, in non-aqueous electrolyte.

2. Experimental methods

2.1 Synthesis of pristine Nb₂O₅ and the Nb₂O₅ anchored graphene nanocomposite

A hydrothermal method was employed to synthesize pristine Nb₂O₅ and the graphene anchored Nb₂O₅ nanocomposite. In a typical reaction, niobium chloride (0.9 g) was dissolved in 15 mL of ethanol under ultrasonication for 0.5 h. Graphene oxide (0.045 g) was dispersed in 15 mL of deionized water to obtain a graphene oxide (GO) dispersion. The GO dispersion was added into the niobium chloride solution and ultrasonicated for 2 h to obtain a pale green colored solution. The solution was transferred to a 50 mL Teflon-lined autoclave and heated at 160 °C for 12 h. The resulting black precipitate was filtered and washed repeatedly with ethanol and deionized water and then dried at 60 °C overnight in an air oven. The synthesized Nb₂O₅/GO nanocomposite was then annealed at 700 °C under argon atmosphere to form the Nb₂O₅ anchored graphene nanocomposite. GO was synthesized by following the procedure reported elsewhere.²⁴ The pristine Nb₂O₅ nanoparticles were synthesized by following the above mentioned procedure without the graphene oxide.

2.2 Material characterization

All the synthesized samples were characterized using the following instruments. The crystal structure was analyzed using an Emperean, PANalytical XRD instrument with Cu Kα radiation. Morphology analysis was performed using a Nova NanoSEM 450 FESEM with a voltage range of 10–30 kV and a JEOL JEM 2100, (200 kV) with a LaB6 electron gun. The graphene content in the nanocomposite was determined using a SDT Q600 Thermogravimetric analyzer and Raman analysis was carried out using a LaBRAM HR Raman spectrometer, Horiba Jobin Yvon, with a 633 nm He–Ne laser source. Surface area measurements were investigated using N₂ adsorption–desorption isotherms measured at 77 K up to a maximum relative pressure of 1 bar, with a Micromeritics 3-Flex surface characterization analyzer. Electrical conductivity measurements were studied using the van der Pauw method.

2.3 Electrochemical measurements

The working electrodes for the lithium ion batteries were fabricated by mixing 80 wt% active material (pristine Nb₂O₅ and Nb₂O₅ anchored graphene nanocomposite), 10 wt% polyvinylidene fluoride binder and 10 wt% acetylene black, using 1-methyl-2-pyrrolidinone as a dispersing solvent. The prepared slurries were coated on stainless steel foil and the electrodes were dried at 120 °C under vacuum overnight to remove the solvent. The electrochemical performance of the Nb₂O₅ anchored graphene nanocomposite was studied vs. Li metal anode using a CR2032 coin-type cell, glass microfiber filter paper as a separator, and 1 M LiPF₆ in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC/DMC, 1 : 1 v/v) as the electrolyte. Electrochemical measurements such as cyclic voltammetry (CV) conducted at a scan rate of 0.05 mV s⁻¹, galvanostatic charge–discharge cycled between a 1.0 and 3.0 V voltage window versus Li⁺/Li and electrochemical impedance spectroscopy (EIS), were carried out using a 16 channel VMP3 biologic electrochemical workstation. The Li-HEC was fabricated with the Nb₂O₅/graphene nanocomposite as the negative electrode and a rice husk derived porous carbon, activated using KOH (RHDPC-KOH) and H₃PO₄ (RHDPC-H₃PO₄), as the positive electrodes. The electrode composition of 80 : 10 : 10 was maintained. An optimized mass loading ratio of 1 : 3 and 1 : 4 (anode : cathode) was used, respectively, with the rice husk derived porous carbon activated with RHDPC-KOH and RHDPC-H₃PO₄.

3. Results and discussion

The Nb₂O₅/graphene nanocomposite was synthesized by anchoring crystalline Nb₂O₅ nanoparticles onto exfoliated graphene oxide (GO) in a homogeneous solution-phase and subsequent conversion of the amorphous Nb₂O₅/GO into a Nb₂O₅/graphene nanocomposite by annealing at 700 °C in the presence of an argon atmosphere (Fig. 1). The mechanism for the synthesis of the Nb₂O₅ decorated graphene is based on the attraction of the positively charged niobium ions (Nb⁵⁺) by the polarized bonds of the functional groups on GO and their subsequent oxidation to Nb₂O₅ followed by thermal reduction of the GO into graphene under an inert atmosphere. Thermally reduced graphene oxide has been demonstrated as an effective matrix for the adhesion of nanoparticles due to the considerable content of oxide functional groups on the basal planes and edges of the 2-D graphene material.¹⁷

Fig. 1 Schematic illustration of the synthesis of Nb₂O₅ anchored graphene nanocomposite through hydrothermal method.

A layered structure of the orthorhombic Nb₂O₅ and Nb₂O₅/graphene nanocomposite was obtained with phase purity, without any impurities of other Nb₂O₅ polymorphs as evidenced from powder X-ray diffraction patterns, and indexed with ICDD no. 30-0873 and space group Pbam (Fig. 2a). The Nb₂O₅ particles thus obtained had a preferential orientation of growth towards the (001) plane, and a small diffraction peak at ∼26° for the Nb₂O₅/graphene nanocomposite was attributed to the disordered graphene sheets which overlap with the (041) plane of Nb₂O₅.¹⁹ This low intensity broad band also reveals the presence of highly disordered graphene sheets with no evidence of restacking of the individual sheets, which is expected to be facilitated by the presence of surface anchored Nb₂O₅ nanoparticles. The GO shows a typical diffraction signature at ∼10° which was not observed for the Nb₂O₅/graphene nanocomposite confirming the complete thermal reduction of graphene oxide into exfoliated graphene sheets. The weight percentage of the graphene in the Nb₂O₅/graphene nanocomposite based on thermogravimetric analysis was estimated to be 4.5 wt% (Fig. S1, ESI†).

Fig. 2 (a) XRD and (b) Raman spectra of the graphene oxide (GO), pristine Nb₂O₅ and Nb₂O₅/graphene nanocomposite.

Raman spectra of the GO, Nb₂O₅ and Nb₂O₅/graphene nanocomposite are depicted in Fig. 2b. Broad Raman bands ∼420–990 cm⁻¹, centered at ∼680 cm⁻¹, for the pristine Nb₂O₅ and graphene nanocomposite are attributed to the symmetric and asymmetric stretching modes of Nb₂O₅. Other peaks in the lower Raman shift region (<300 cm⁻¹) match with earlier reports.^25,26 The GO and Nb₂O₅/graphene nanocomposite show distinctive D and G Raman bands at ∼1320 and ∼1592 cm⁻¹ respectively. A D/G band ratio above unity illustrates the defective nature of the graphene sheets, caused by the surface decorated Nb₂O₅ nanoparticles. Scanning electron microscopy images of the pristine Nb₂O₅ and Nb₂O₅/graphene nanocomposite are shown in Fig. 3a and b respectively. The Nb₂O₅ nanoparticles are ∼50 nm in size polyhedral particles (Fig. 3a), while the graphene sheets are uniformly decorated with a dense network of Nb₂O₅ nanoparticles in the Nb₂O₅/graphene nanocomposite (Fig. 3b). High-resolution TEM images of the Nb₂O₅ and graphene nanocomposite are shown in Fig. 3c and d respectively. It is to be noted that the Nb₂O₅ nanoparticles seem to agglomerate forming clusters (∼100 nm in diameter), as evidenced in the SEM images (Fig. 3a and S2a, ESI†) and TEM images (Fig. 3c). However, interestingly, the SEM images (Fig. 3b and S2b, ESI†) and TEM images of the nanocomposite (Fig. 3d and S2c, ESI†) show smaller Nb₂O₅ nanoparticles (∼25 nm) anchored onto the graphene sheets, with negligible agglomeration. Formation of such a nanocomposite has resulted in dense packing of the homogeneously distributed Nb₂O₅ nanoparticles on few layer-graphene sheets, which further lead to a much improved electrochemical performance, as explained in the succeeding sections. The Nb₂O₅ nanoparticles exhibit high crystallinity with a lattice spacing of 0.39 nm (inset of Fig. 3d), attributed to the interspacing of the (001) plane, which is consistent with the results obtained from XRD analysis (Fig. 2a). Electrical conductivity measurements carried out using the van der Pauw method on the Nb₂O₅/graphene nanocomposite material showed an excellent conductivity of 0.45 S m⁻¹, compared to that of ∼6 × 10⁻⁷ S m⁻¹, measured for pristine Nb₂O₅. Large enhancement of the BET surface area was also exhibited by the Nb₂O₅/graphene nanocomposite (112 m² g⁻¹) compared to the measured BET surface area of 22 m² g⁻¹ for pristine Nb₂O₅ (Fig. S3a, ESI†). Metal oxide/graphene nanocomposites with different architectures, namely encapsulated, sandwiched, layer-by-layer assembled and anchored, have recently been explored as advanced electrode materials for energy applications. These nanocomposites, as reported in the literature, show noteworthy results for enhancement of the electrochemical properties due to their synergistic effects compared to the respective individual components, but also basically have a very high graphene content (>25 wt%) which allows electrochemical lithium intercalation into the graphene interlayers.^27–29

Fig. 3 SEM images of the (a) Nb₂O₅ nanocrystals and (b) Nb₂O₅/graphene nanocomposite, and TEM images of the (c) pristine Nb₂O₅ nanocrystals and (d) Nb₂O₅/graphene nanocomposite. Inset of Fig. 3d shows high resolution TEM image of Nb₂O₅ nanocrystal.

In the present work, we explore the electrochemical performance of the Nb₂O₅/graphene nanocomposite as an electrode for high-power lithium batteries through the increased surface area of the active Nb₂O₅ nanoparticles which are decorated onto the conducting graphene layers which would enhance the overall battery performance.³⁰ Here, 4.5 wt% of graphene in the composite electrode acts as a dispersing matrix for anchoring the Nb₂O₅ nanoparticles and is not involved in the electrochemical lithiation/delithiation process.

The electrochemical performance of the pristine Nb₂O₅ and Nb₂O₅/graphene nanocomposite was investigated by carrying out half-cell measurements vs. Li metal anode, in a coin cell assembly. The galvanostatic cycling curves and cyclic voltammograms of the pristine Nb₂O₅ are shown in Fig. 4a and b, respectively. Both these studies were carried out in the potential window of 1.0–3.0 V vs. Li/Li⁺, at 25 °C. The CV measurements were performed at a scan rate of 0.05 mV s⁻¹. The electrochemical intercalation–deintercalation reaction of lithium with Nb₂O₅ occurs as follows:¹⁶

Nb₂O₅ + xLi⁺ + xe⁻ → Li_xNb₂O₅ (1)

with a maximum lithium uptake of x = 2. The voltage profile shows a sloppy curve which is characteristic of orthorhombic Nb₂O₅ (Fig. 4a) and agrees with the reported literature.⁹ The discharge voltage decreases with increasing capacity, which is more prominent in the first discharge cycle. The first discharge curve with a line profile indicates the formation of lithiated niobium oxide (Li_xNb₂O₅) during lithium intercalation, where the x value varies between 0 and 2. The CV of pristine Nb₂O₅ (Fig. 4b) showed cathodic peaks at 1.54 V and 1.74 V and anodic peaks at 1.84 V and 2.0 V in the first cycle, which indicates Li ion intercalation/deintercalation process. This revealed that the lithium ion intercalation process in orthorhombic Nb₂O₅ could be a complex two-step process.⁹ With the increase in the number of cycles, broadening and shifting of the cathodic peaks to 1.64 V and 1.83 V occurs, whereas the anodic peaks appear to be stationary.⁹ The shift in the cathodic peaks could be due to the polarization of the electrode in the first cycle.³¹ Moreover, the first few cycles are the formation cycles that form a good electrical contact between the active material, binder, conducting carbon and current collector.

Fig. 4 (a) Voltage profile and (b) cyclic voltammograms of the pristine Nb₂O₅ nanocrystals, (c) voltage profile of the Nb₂O₅/graphene hybrid nanocomposite, (d) capacity vs. cycle no. for the pristine Nb₂O₅ and Nb₂O₅/graphene nanocomposite at C/10 rate and (e) rate capability measurements showing the cycling at different rates for the pristine Nb₂O₅ and Nb₂O₅/graphene nanocomposite electrodes.

The electrochemical characteristics of the Nb₂O₅ decorated graphene nanocomposite electrode vs. Li were studied under the same experimental conditions. It is clear from the voltage profiles that the capacity of the nanocomposite has increased slightly despite the fact that the graphene in the nanocomposite does not intercalate with lithium at these potentials (Fig. 4c). The lithium insertion potential for pure graphene layers was reported to occur close to 0.1 V vs. Li/Li⁺ reference electrode.³² The increase in capacity of the graphene composite may be attributed to the improved surface area of the composite electrode, as evidenced from the BET surface area measurements (Fig. S3a, ESI†), leading to an enhanced electrochemical activity. The decoration of Nb₂O₅ nanoparticles on the graphene sheets has resulted in improved conductivity of the whole electrode material which is also confirmed from the four probe conductivity measurements. With the improvement in conductivity of the material, the kinetic constraint will be greatly reduced resulting in better cycling performances.

The cycling performance of the pristine Nb₂O₅ nanocrystals and Nb₂O₅/graphene nanocomposite were studied at a cycling rate of C/10 (1C = 200 mA g⁻¹) (Fig. 4d). A much higher capacity, closer to the theoretical value, has been obtained for the nanocomposite electrode, when cycled at C/10 rate. The first discharge capacity of the pristine Nb₂O₅ and Nb₂O₅/graphene nanocomposite electrodes was 204 and 195 mA h g⁻¹, respectively, at C/10 rate. A slight increase in the discharge capacity was observed in the initial cycles, which could be due to the nanosize effect of the electrode material, as reported earlier.³³ The reversible discharge capacity of the Nb₂O₅/graphene nanocomposite was higher than the pristine Nb₂O₅, after 50 cycles at C/10 rate, with a value of 192 and 166 mA h g⁻¹, respectively. These capacity values correspond to ∼2.0 and 1.7 moles of lithium per mole of T-Nb₂O₅ (x = 2.0 and 1.7 in Li_xNb₂O₅) inserted for the Nb₂O₅/graphene nanocomposite and pristine Nb₂O₅, respectively, during the discharge cycle. High cyclic stability of the nanocomposite electrode at higher current rates has been demonstrated. The nanocomposite electrode cycled at 1C rate clearly showed an improved performance with ∼160 mA h g⁻¹ for up to 50 cycles, compared to ∼115 mA h g⁻¹ for the pristine Nb₂O₅ nanoparticles (Fig. S4, ESI†). The rate capability of these two electrodes was investigated using galvanostatic cycling measurements under different C rates from C/10 to 10C (Fig. 4e). The specific capacity of the graphene composite at high rates of 10C was 115 mA h g⁻¹ which shows ∼55% capacity retention compared with the C/10 rate. A discharge capacity of 70 mA h g⁻¹ was observed for the pristine sample at 10C rate, with a capacity retention of 35% with respect to the capacity under C/10 rate. The high capacity retention of the graphene composite with highly dispersed T-Nb₂O₅ nanoparticles also results from the presence of a 3-dimensional Wadsley–Roth phase in the flexible Nb₂O₅ crystal structure.³⁴ In pristine Nb₂O₅, large aggregation of the nanoparticles decreases the real surface area and the availability of vacant interstitial sites of the Nb₂O₅ nanoparticles causing low capacity retention at high C rates.

X-ray photoelectron spectroscopy was used to investigate the valence state of the elements in the Nb₂O₅/graphene nanocomposite, which revealed the presence of Nb⁵⁺ species in the electrode (Fig. S5, ESI†). The stability and the morphological changes of the pristine Nb₂O₅ and graphene composite electrodes upon electrochemical cycling were investigated using SEM of the cycled electrodes (Fig. S6, ESI†), which clearly showed that the Nb₂O₅ nanoparticles had no morphological change after the lithiation/delithiation cycles. This results from the superior structural stability and near zero volume expansion of the Nb₂O₅ nanoparticles.

The superior electrochemical performance of the Nb₂O₅/graphene nanocomposite may be attributed to the following reasons. (a) The presence of flexible, thin graphene sheets that act as a support for the anchoring of Nb₂O₅ nanoparticles. The graphene sheets provide an effective buffering layer for the Nb₂O₅ particles, in addition to their own structural voids, during lithium intercalation and prevent the aggregation of the Nb₂O₅ nanoparticles upon cycling thus realizing a high reversible capacity, capacity retention and power capability.^19,35 (b) The graphene sheets provide high electrical conductivity and deliver conductive pathways between individual Nb₂O₅ nanoparticles which decrease internal resistance leading to a higher reversible capacity.^36,37 (c) The kinetics of the lithium transport depend on the path length and vacant active sites on the surface of the electrode.³⁸ Hence the graphene nanocomposite provides a large electrode/electrolyte interfacial area, and a shorter path length for Li⁺ transport maintaining the structural integrity of the self-supported nanoarchitectural electrode. (d) The anchored Nb₂O₅ nanoparticles efficiently prevent the restacking of the graphene sheets, maintaining a high surface area of graphene sheets.¹⁹

The formation of graphene composite with low amount of graphene content (4.5 wt%) is expected to increase the overall conductivity of the electrode, which was confirmed using electrochemical impedance spectroscopic analysis. The Nyquist plots of the pristine and the graphene composite electrodes before and after cycling at C/10 rate are shown in Fig. 5a. The charge transfer resistance (R₂) of the pristine Nb₂O₅ nanoparticles and the Nb₂O₅/graphene nanocomposite before cycling was found to be 135.3 and 61.2 Ω respectively. The charge transfer resistance after 50 galvanostatic charge/discharge cycles had increased to 150.9 Ω and 67.5 Ω for the pristine Nb₂O₅ nanoparticles and Nb₂O₅/graphene nanocomposite respectively. This shows that the graphene scaffolding provides good electronic conductivity for the niobate nanoparticles resulting in reduction of the overall resistance of the nanocomposite material. A detailed EIS analysis using an equivalent circuit has been carried out (Fig. S7, Table S1, ESI†). The power capability of the electrode depends greatly on its electronic conductivity and the availability of the real surface area of the active material for the lithiation/delithiation process which would also enhance the kinetics of lithium insertion. To further investigate the rate capability of the composite electrode, typical ‘signature curve’ studies have been carried out on the two electrodes (Fig. 5b). For this, both the pristine Nb₂O₅ nanoparticles and the Nb₂O₅/graphene nanocomposite were discharged at a slow rate of C/20 and continuous charging cycles were carried out from 16C to C/32 while giving a rest time of 5 min in between to obtain the signature curve data. However to investigate the role of graphene and its conductivity (similar to a conductive carbon additive) in the rate capability of the Nb₂O₅/graphene composite at high C rates, signature curves were obtained for the pristine Nb₂O₅ electrode (depicted as Nb₂O₅-1) with excess addition of carbon black (Nb₂O₅-2) and an excessive graphene content (Nb₂O₅-3), apart from the defined cell fabrication composition of 80 wt% active material, 10 wt% carbon black and 10 wt% pvdf (Nb₂O₅-1). While the pristine Nb₂O₅ (Nb₂O₅-1) showed only 30% capacity retention at a high current rate of 16C, Nb₂O₅-2 and Nb₂O₅-3 with 5 wt% excess carbon black and 5 wt% excess graphene (thermally reduced graphene oxide) respectively, delivered a capacity retention of 42%.

Fig. 5 (a) Electrochemical impedance spectroscopy plots of the pristine Nb₂O₅ and Nb₂O₅/graphene nanocomposite electrodes before cycling and after 50 cycles at C/10 rate, (b) ‘Signature’ curve plots of the pristine Nb₂O₅ (Nb₂O₅-1) and the Nb₂O₅/graphene hybrid with different compositions of conductive carbon in the Nb₂O₅ electrode. The materials under investigation are Nb₂O₅/graphene nanocomposite: Nb₂O₅/graphene = 80 wt% where the graphene content in the composite is ∼5 wt%, acetylene black = 10 wt% and pvdf = 10 wt%; Nb₂O₅-1 (pristine Nb₂O₅): Nb₂O₅ = 80 wt%, acetylene black = 10 wt% and pvdf = 10 wt%; Nb₂O₅-2: Nb₂O₅ = 75 wt%, acetylene black = 15 wt% and pvdf = 10 wt%; Nb₂O₅-3: Nb₂O₅ = 75 wt%, acetylene black = 10 wt%, graphene = 5 wt% and pvdf = 10 wt%.

However, the Nb₂O₅ decorated graphene composite which possess 4.5 wt% of thermally reduced graphene oxide showed an enormous capacity retention of 80% at 16C rate. This clearly demonstrates the role of graphene in the composite as a conducting substrate, which in addition, acts as an anchoring site for the well dispersed electroactive Nb₂O₅ nanoparticles, thus realizing high surface area for the lithiation/delithiation process and thereby improving the kinetics of lithium intercalation/de-intercalation in the electrode.

The good insertion nature of the Nb₂O₅/graphene nanocomposite makes it a suitable electrode for use in Li-ion hybrid electrochemical capacitor (Li-HEC). A Li-HEC combines both the battery and supercapacitor electrodes, such that it can achieve higher energy density than EDLCs and higher power density than Li-ion batteries, with high cycling stability.^39–41 It contains a faradaic anodic part where Li intercalation takes place and a non-faradaic cathodic part where anionic adsorption occurs.^42,43 Here, a Li-HEC was fabricated with the Nb₂O₅/graphene nanocomposite as the anode and a rice husk-derived activated porous carbon (RHDPC) as the cathode in non-aqueous electrolyte (1 M LiPF₆ in 1 : 1 v/v EC–DMC). The electrochemical performance was studied between 1.0 and 3.0 V at various current densities. At the cathode side, two types of RHDPC⁴⁴ – one activated with KOH (RHDPC-KOH) and the other with H₃PO₄ (RHDPC-H₃PO₄), with varying BET surface areas were used. RHDPC-KOH exhibits high surface area of 1809 m² g⁻¹, while RHDPC-H₃PO₄ shows surface area of 1256 m² g⁻¹ (Fig. S3b, ESI†). The electrode mass loading plays a crucial role in the fabrication of a hybrid device.⁴⁵ The mass ratio was calculated from individual measurements of each electrode with respect to the Li-reference electrode (Fig. S8, ESI†). Electrochemical voltage profiles of the Nb₂O₅–graphene/RHDPC-KOH and the Nb₂O₅–graphene/RHDPC-H₃PO₄ devices were obtained in the voltage range between 1 and 3 V at different current densities (based on the total active mass) and the results are illustrated in Fig. 6a and b respectively. The remarkable features of the discharge curves of the Li-HECs comprise of three important regions, such as, an ohmic drop due to the ohmic resistance offered by the active material, the monotonous curve region due to Li-extraction from the lattice of Nb₂O₅–graphene and simultaneous de-sorption of PF₆⁻ ions from the active sites of the porous carbon and a sudden drop.⁴⁶

Fig. 6 Galvanostatic charge–discharge curves of a Li-HEC with (a) Nb₂O₅–graphene/RHDPC-H₃PO₄ electrodes and (b) Nb₂O₅–graphene/RHDPC-KOH electrodes, at different applied current densities. Cyclic voltammograms of the (c) Nb₂O₅–graphene/RHDPC-H₃PO₄ electrodes and (d) Nb₂O₅–graphene/RHDPC-KOH electrodes; (e) Ragone plot of different Li-HEC configurations in comparison with the reported binder-free Nb₂O₅@graphene with activated carbon (Nb₂O₅–G/AC).⁵⁰

The Nb₂O₅/graphene nanocomposite electrode delivered a specific capacity of 34 F g⁻¹ at 0.5 A g⁻¹. Cyclic voltammetry performed for the devices (Fig. 6c and d) showed a drastic increase/decrease in current response at above/below 1.5 V which is attributed to the Li-insertion/extraction into or from the Nb₂O₅ lattice. Fig. 6e shows the Ragone plot, representing energy density vs. power density for the two devices with different cathode materials namely, RHDPC-H₃PO₄ and RHDPC-KOH. The Nb₂O₅–graphene/RHDPC-KOH device showed improved electrochemical performance with a maximum energy density of 30 W h kg⁻¹ and power density of 10 kW kg⁻¹ at a current rate of 5 A g⁻¹, compared to the Nb₂O₅–graphene/RHDPC-H₃PO₄ device (Fig. 6e). This could be attributed to the enhanced surface area of the RHDPC-KOH electrode, resulting in more activation sites for accommodating PF₆⁻ ions during the electrochemical processes.^47–49 Li-HEC devices fabricated using pristine Nb₂O₅/RHDPC electrodes showed similar electrochemical behavior, as evidenced from their galvanostatic cycling profiles and cyclic voltammograms (Fig. S9, ESI†). The graphene nanocomposite electrode exhibited better power and energy densities compared to the pristine Nb₂O₅/RHDPC electrodes and the results obtained here are better than the earlier reports on binder-free Nb₂O₅@graphene electrodes.⁵⁰ To further investigate the effect of graphene content on the electrochemical properties of Nb₂O₅–graphene, Li-HEC devices were fabricated with RHDPC-KOH as the cathode and Nb₂O₅–graphene composites with varying graphene content (NG-10%, NG-20% and NG-40%) as the anode. The NG-10%, NG-20% and NG-40% contain ∼4.5 wt%, ∼7.5 wt% and ∼14 wt% of graphene, respectively, as observed from TG analysis (Fig. S10, ESI†). From the Ragone plot (Fig. S11, ESI†), it is clear that the NG-20% composite exhibits the best performance, which could be the optimum loading of graphene in terms of better dispersion. Thus the fabricated Nb₂O₅/graphene nanocomposite showed promising results for use as an electrode material for high-power electrochemical energy storage devices.

4. Conclusions

A self-supported nanoarchitectural electrode with Nb₂O₅ decorated graphene was fabricated through simple strategy, and has been demonstrated as a promising electrode for rechargeable lithium batteries and lithium ion hybrid capacitors. The excellent electrochemical performance of the graphene composite electrode was attributed to the uniform decoration of Nb₂O₅ nanoparticles over the graphene sheets and the realization of high surface area for the lithiation/delithiation process to occur. Graphene nanocomposite with good mechanical flexibility, high surface area and improved structural integrity, enhances the kinetics of lithium intercalation due to short diffusion paths for Li⁺ transport, resulting in high power capabilities. The graphene composite with low graphene content (4.5 wt%) delivered a specific capacity of 192 mA h g⁻¹ compared to 166 mA h g⁻¹ for the pristine sample after 50 cycles at C/10. Signature curve studies clearly revealed high power capability of the graphene composite with a capacity retention of 80% compared to 30% for the pristine electrode at a high current rate of 16C. Li-ion hybrid electrochemical capacitor devices fabricated with Nb₂O₅–graphene as the anode and RHDPC as the cathode, showed excellent electrochemical performances. With such superior electrochemical characteristics, the Nb₂O₅ anchored graphene nanocomposite could be an ideal electrode for high performance electrochemical energy storage devices.

Acknowledgements

The work has been partly supported by the Department of Science & Technology (DST), Govt. of India, through the DST Fast-track scheme for Young Scientists (no. SR/FTP/PS-081/2010), the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy (DAE), Govt. of India, through a DAE Young Scientist Research Award (no. 2012/20/34/5/BRNS) and the Department of Biotechnology (DBT), Govt. of India, under DBT’s Twinning programme for the NE (no. BT/350/NE/TBP/2012).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07895d

‡ Present address: Postdoctoral Research Associate, School of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea.

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