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Hybridizing battery and supercapacitor technologies have the potential to overcome the limitations of the currently prevailing energy-storage systems. Combining high-power capacitive electrodes from supercapacitors with the high-energy intercalation electrodes in lithium-ion batteries provides the opportunity to create a single device that can deliver both high energy and high power. Although energy densities in such hybrid systems easily exceed those found in supercapacitors, the kinetic imbalance between capacitive and intercalation electrodes remains a bottleneck to achieving the desired performance. This imbalance is eliminated through the use of graphene-wrapped Li4Ti5O12 from a simple, one-step process as a high-power anode in a new hybrid supercapacitor. The new hybrid supercapacitors are capable of delivering a high specific energy of up to 50 Wh kg−1 and can even maintain an energy of approximately 15 Wh kg−1 at a 20 s charge/discharge rate.
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The development of new high-performance energy-storage devices is vital for satisfying the rapidly increasing demands for new applications that require high-power, high-energy, and cost-effective energy-storage systems (ESSs).1–11 Of the prevailing modern energy-storage devices, lithium-ion batteries (LIBs) can deliver the highest energy density (ca. 150 Wh kg−1) through Faradaic lithium insertion reactions.12–14 However, Faradaic lithium de/intercalation reactions involve the solid-state diffusion of ions in a crystal, which often results in low power density.12, 13 Alternatively, supercapacitors, one of the most important types of energy-storage device, can provide high power densities (ca. 3 kW kg−1), owing to non-Faradaic surface reactions, at the expense of the energy density (ca. 5 Wh kg−1).14, 15 For emerging large-scale ESSs, bridging this performance gap remains a key issue.
Hybrid supercapacitors that combine the advantages of LIBs and supercapacitors have been proposed and intensively studied in recent years as alternative energy-storage devices.14, 16–22 Although conventional electric double-layer capacitors (EDLCs) use only non-Faradaic surface reactions at both electrodes, hybrid supercapacitors adopt Faradaic lithium intercalation reactions at one electrode and non-Faradaic surface reactions at the other electrode (Figure 1 a and b).14, 23 As a result, the overall energy density of the system can be increased while maintaining a high power density (Figure 1 c).20, 23 However, an imbalance in the reaction kinetics between the lithium-intercalation electrode and the capacitive electrode remains a challenge. Indeed, the electrochemical performance of hybrid supercapacitors is limited primarily by the comparatively sluggish reactions in the Faradaic lithium-intercalation electrode.23 Therefore, the intercalation electrode must be properly chosen and/or designed with the fastest possible kinetics in order to fulfill the performance potential of hybrid supercapacitors.
This study describes a novel, high-energy, and high power-density hybrid supercapacitor based on a graphene-wrapped Li4Ti5O12 (LTO) anode and an activated carbon (AC) cathode. By taking advantage of the LTO anode, with a high specific capacity (ca. 175 mAh g−1), small volumetric change (ca. 0.2 %), and a potential of approximately 1.6 V (free of the reductive electrolyte decomposition),23–25 the hybrid system demonstrates remarkable power and energy densities. Furthermore, the simple, one-step process of wrapping LTO particles with graphene helps remedy the intrinsically low electronic conductivity of LTO and dramatically improves the rate capability. The new hybrid supercapacitors are capable of delivering a specific energy of up to 50 W kg−1 and can even maintain approximately 15 Wh kg−1 at a 20 s charge/discharge rate.
2. Results and Discussion
The phase purity of the synthesized LTO was confirmed by using X-ray diffraction (XRD) analysis. Figure 2 a shows that no noticeable impurities were detected. The as-prepared LTO particles ranged from a few hundreds of nanometers to a few micrometers in size, as determined by using field-emission scanning electron microscopy (FESEM) analysis (Figure 2 b and Figure S1). The particle size of LTO was further verified by a particle-size distribution measurement, which showed an average value of 1.3 μm (Figure S2). During the wrapping of LTO with graphene, the surface charges of the LTO particles and the graphene nanosheets were controlled by adjusting the pH value of the solution. LTO particles were positively charged at pH values lower than 4, whereas the graphene nanosheets were negatively charged throughout the pH range of the experiments (pH 2–12; Figure 2 c).26 At a low pH value (pH 4), the negatively charged graphene nanosheets electrostatically attracted positively charged LTO particles, resulting in the uniform wrapping of individual LTO particles, without aggregation. Figure 2 d and Figure S3 show micron-sized LTO particles embedded in graphene nanosheets. The FESEM and high-resolution transmission electron microscopy (HRTEM) images in Figure 3 also confirm that the LTO particles were wrapped with graphene nanosheets. The wrinkles observed on the LTO particles indicate that the surface of the LTO particles was covered by the graphene nanosheets. The graphene layer was around 3 nm thick, corresponding to approximately ten layers of graphene.27, 28 The graphene content in the composite was 7.55 wt. % based on elemental analysis.
The lithium-storage capability and cycle stability of graphene-wrapped LTO was first measured in a half-cell between 1.0 and 2.5 V at a current rate of 50 mA g−1 (Figure 4). Negligible changes in the charge/discharge profiles were observed during repeated battery cycling, with a specific capacity of approximately 150 mAh g−1, (ca. 2.58 Li+ mol−1 in LTO), demonstrating that the electrochemical reaction is highly reversible. Figure 4 b shows the cycle stability of the graphene-wrapped LTO anode, in which a capacity of approximately 150 mAh−1 was maintained with a Coulombic efficiency close to 100 % for up to 20 cycles.
To evaluate the performance of the as-synthesized graphene-wrapped LTO electrodes in a hybrid supercapacitor system, an electrochemical cell was constructed by using the LTO anode and an AC cathode with an electrolyte of LiPF6 (1 M) in ethylene carbonate/dimethyl carbonate (EC/DMC; 1:1 volume ratio). A typical charge/discharge profile of this system, from 1.0 to 2.5 V at a current rate of 0.1 mA cm−2, is shown in Figure 5 a. Note that the hybrid supercapacitor did not yield the triangle-shaped profile that is typical for symmetric AC/AC supercapacitors.29 This is because the hybrid supercapacitor is asymmetric, employing a battery-type lithium-intercalation electrode on one side, as shown in Figure 1 c.20 The charge/discharge profiles of the hybrid system were not altered, even after extended cycling (inset of Figure 5 a). Furthermore, approximately 75 % of the initial capacity was retained after 1 000 cycles. This shows that the electrochemical reactions occurring at the asymmetric electrodes are highly reversible in the hybrid system. Rate capability tests of the hybrid supercapacitor were performed from 0.1 to 15 mA cm−2, as shown in Figure 5 c, and specific capacitances were around 83.6, 74.0, 56.5, 34.6, and 25.7 F g−1 at 0.1, 1.0, 4.0, 10.0, and 15.0 mA cm−2, respectively. These capacitance values are notably higher than those of hybrid supercapacitors based on conventional LTO anodes.
The Ragone plot in Figure 5 d illustrates the trade-off between energy and power densities in the hybrid supercapacitor described herein (Figures S4 and S5). Power density (P) and energy density (E) were calculated based on the relationships in Equations (1)–(3), (2), (3):30, 31
in which Emax and Emin are the voltages at the beginning and end of the discharge (V), respectively, i is the discharge current (A), t is the discharge time (s), and m is the total mass of electroactive materials in both the anode and the cathode (g). The hybrid supercapacitor could deliver up to 50 Wh kg−1, and it could maintain up to 15 Wh kg−1 at a 20 s charge/discharge rate, providing a power density of approximately 2 500 W kg−1.
This electrochemical performance is superior to those of hybrid supercapacitors based on conventional LTO anodes and is better than other high-energy hybrid supercapacitor systems.14, 16, 20, 24, 32 More realistic estimates of energy and power densities in practical cells could be made by dividing the values based on electrode mass by a factor of 2.33 Figure S6 shows that the energy and power densities of the hybrid supercapacitor satisfy the demands of hybrid electric vehicle (HEV) applications.34 The outstanding electrochemical performance of the new hybrid supercapacitor described above is attributed to the following: 1) The graphene nanosheet coating the surface of the individual LTO particles improves power capability by providing facile electron-transport pathways throughout the electrode (Figure S7). This effectively balances the kinetics between the AC and the LTO electrodes. 2) The use of an LTO anode provides stable cycle performance, owing to an exceptionally small volume change (ca. 0.2 %) during operation, and a potential of approximately 1.6 V, which avoids reductive decomposition of the electrolyte.23–25
A new high-energy and high-power hybrid supercapacitor based on a graphene-wrapped LTO anode and an AC cathode was constructed and evaluated. The graphene nanosheet wrapping significantly improved the rate capability of the LTO anode, which helped overcome the intrinsic kinetic imbalance between non-Faradaic capacitive electrodes (AC) and Faradaic lithium-intercalation electrodes (LTO), which has been one of the most important challenges in hybrid supercapacitor systems. The new hybrid supercapacitor delivered up to 50 Wh kg−1 and could maintain as much as 15 Wh kg−1 at a power of 2 500 W kg−1. This level of electrochemical performance satisfies the demands of HEV applications. Furthermore, the simplicity of the graphene nanosheet coating method, which uses electrostatic attraction between negatively charged graphene and positively charged LTO particles within a specific pH range, will be applicable to various electrode materials.
Synthesis of Graphene-Wrapped LTO Particles
LTO samples were prepared by using conventional solid-state reactions. A stoichiometric amount of LiOH⋅H2O (Sigma Aldrich,>98 %) and anatase TiO2 precursors (Anatase phase, Sigma Aldrich, 99.7 %) were ball-milled in a container containing acetone for 12 h and dried overnight at 80 °C. The mixture was sintered at 900 °C in ambient air for 12 h.
Graphite oxide was synthesized by using a modified Hummers method.35 Graphite (1 g, Bay Carbon), NaNO3 (1 g, Sigma Aldrich,>99 %), and H2SO4 (46 mL, Sigma Aldrich, >95 %) were stirred together in an ice bath for 30 min, which was followed by the slow addition of KMnO4 (5 g, Sigma Aldrich, >99 %). The mixture was stirred for 2 h at 50 °C and then deionized (DI) water (100 mL) and H2O2 (8 mL, Sigma Aldrich, 30 %) were added. The solution was filtered and washed with HCl (50 mL, Sigma Aldrich, 30 %) and DI water (200 mL). To fabricate graphene oxide, the resultant graphite oxide was dissolved in DI water (1 mg mL−1, 400 mL) and sonicated for 100 min. Graphene-oxide layers were thereby exfoliated from the graphite oxide. To reduce graphene oxide, NH4OH (3.5 mL, Sigma Aldrich, 28 %) and NH2NH2 (0.2 mL, Sigma Aldrich, 35 wt. %) were added to the graphene oxide solution. To fabricate the graphene-wrapped LTO particles, LTO samples were slowly added to the solution, which was then stirred for 6 h at 80 °C. The pH value of the solution was then adjusted to 2; this is the pH value at which LTO and graphene have opposite surface charges.
Characterization of Graphene-Wrapped LTO Particles
Samples were analyzed with an X-ray diffractometer (D2PHASER) using CuKα radiation. The morphology of the materials was examined by using FESEM (SUPRA 55VP) and HRTEM (JEM-3000F). The graphene nanosheet content was quantified with an element analyzer (EA11110-FISONS)
Electrodes were prepared by mixing the graphene-wrapped LTO powders (80 wt. %) with conductive carbon (10 wt. %) and polyvinylidene fluoride (PVDF; 10 wt. %) in N-methyl-2-pyrrolidone (NMP). LTO electrodes without graphene were prepared by mixing LTO particles (72 wt. %) with conductive carbon (18 wt. %) and PVDF (10 wt. %) in NMP as a control. Note that an excess amount of conductive carbon was used to compare the performance against that of the graphene-wrapped LTO with the same amount of carbon. The resultant slurries were uniformly pasted onto an Al foil. The electrodes were dried at 120 °C for 2 h and roll-pressed. For half-cell tests, the cells were assembled into a two-electrode configuration with a separator (Celgard 2400). Li metal was used as a counter electrode. The electrolyte consisted of LiPF6 (1 M) in a 1:1 mixture of EC/DMC (Techno Semichem). Experiments were performed in a glove box. For the electrochemical characterization of the hybrid supercapacitors, a mixture of AC (MSP20, 90 wt. %), conductive carbon (5 wt. %), and polytetrafluoroethylene (PTFE, 5 wt. %) was used for the counter electrode. The electrochemical test cells were assembled with a 1 and 2.5 weight ratio of active materials/counter electrode (i.e. LTO/AC). Electrochemical profiles were obtained from 1.0 to 2.5 V by using a multichannel potentio-galvanostat (WonATech). The cell capacitance (Ccell) was calculated based on Equation (4) and the specific capacitance (Csp) was calculated from Equation (5):
in which i is the applied current (A), t is the discharge time (s), m is the total mass of the active materials in both electrodes (g), and ΔV is the potential difference (V) between the beginning and the end of the discharge.30, 36
This work was supported by 1) the Human Resources Development Program (20124010203320) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean Government Ministry of Trade, Industry and Energy; and 2) the Converging Research Center Program through the Ministry of Education, Science and Technology (2013K000293).