Herein, we propose an advanced energy-storage system: all-graphene-battery. It operates based on fast surface-reactions in both electrodes, thus delivering a remarkably high power density of 6,450 W kg−1total electrode while also retaining a high energy density of 225 Wh kg−1total electrode, which is comparable to that of conventional lithium ion battery. The performance and operating mechanism of all-graphene-battery resemble those of both supercapacitors and batteries, thereby blurring the conventional distinction between supercapacitors and batteries. This work demonstrates that the energy storage system made with carbonaceous materials in both the anode and cathode are promising alternative energy-storage devices.
The global energy paradigm is rapidly transforming from fossil fuels to sustainable energy resources, including solar, wind and geothermal energies1,2. However, power production from those energy resources is not always coincident with energy demand. Therefore, the development of large-scale energy-storage systems that resolve this discrepancy is vitally important. Considerable efforts have been expended on the development of high-performance energy-storage devices such as lithium-ion batteries (LIBs), supercapacitors and lithium ion capacitors (LICs)3,4,5,6,7,8,9,10,11,12,13,14,15. A major hurdle remains: the development of a novel energy storage system that combines high energy and power density.
LIBs are promising candidates because of their high energy density, which is attributed to the high operating potential and the large charge-storage capability of conventional LIB electrode materials4,5. However, the power density of available LIBs is not suitable for large-scale applications and the cost is too high. These issues – low power density and high cost – are closely related to the fundamental electrode reaction characteristic of LIBs, which relies on intercalation-based reactions of transition metal oxides accompanied by intrinsically slow solid-state lithium diffusion16,17 and generally low electronic conductivity4,17. In this respect, there have been intensive researches on LICs using a lithium intercalation electrode in one electrode and a capacitive electrode in the other electrode to achieve battery-level energy density combined with the power density of supercapacitors18,19,20,21,22. However, despite the recent advancement of LICs, the imbalance in kinetics between two electrodes still remain as a major drawback that should be overcome. It is because the intercalation reaction in one electrode is generally far slower than the surface reaction in the other electrode.
One approach to resolving the aforementioned challenge is to adopt fast surface electrode reactions in both electrodes while maintaining high energy density23,24,25,26. Recently, we demonstrated that the functionalization of graphene to enable fast surface reaction could lead to exceptionally high Li storage capability4. The functional groups on the graphene cathode acted as radical centers to store Li ions at acceptably high potential4,27.
In this study, we expanded the advantages of exploiting fast surface electrode reactions of functionalized graphene cathodes by matching them with reduced graphene oxide anodes, thereby introducing the all-graphene-battery. While similar concept was introduced in the literature, they still used lithium particles or foils in the anode side incorporating with graphene28. Also, the fabrication of graphene was not mass-scalable. To realize, herein, all-graphene-battery, mass-scalable functionalized graphene and prelithiated reduced graphene oxide are used in cathode and anode, respectively, without utilizing lithium metals. All-graphene-battery delivers exceptionally high power density because both the anode and cathode exhibit fast surface reactions combined with porous morphology and high electrical conductivity. Furthermore, their similar chemistry and microstructure maximizes the performance of each electrode in the full cell without introducing a power imbalance, which is commonly observed in LICs11. Moreover, it retains high energy density, attributable to the wide potential difference between the anode and cathode. We demonstrate that this advanced all-graphene-battery is capable of delivering an energy density of 130 Wh kg−1total electrode at a power density of 2,150 W kg−1total electrode. It combines the LIBs and supercapacitors in terms of operating mechanism and performance and thus bridges the performance gap between the two29,30. This work may shed new light on the development of energy storage devices using surface reactions on both the anode and the cathode.
Fabrication of the graphene-based cathode and anode
Simple chemical modification was used to tune the graphene for either anode or cathode. Starting from natural graphite, the modified Hummers method was used to obtain both the functionalized graphene cathode and the reduced graphene oxide anode. The characteristic XRD peak of graphite at 26° shifted to a lower angle after oxidation; this shift was attributed to the extended interlayers in graphite oxide (Figure 1a). XPS analysis revealed that the oxygen content increased from 2.1 to 33% after the oxidation process (Figure S1)31. The resulting graphite oxides were reduced partially or fully to yield cathode graphene (functionalized graphene) or anode graphene (reduced graphene oxide), respectively. In our previous work, we demonstrated that porous functionalized graphene can be obtained in a one-step annealing process at low temperature (120°C) in a controlled atmosphere4. The reduction at low temperature was verified by XRD and XPS analyses, as shown in Figure 1a–c. The reduction at low temperature was verified by XRD and XPS analyses, as shown in Figure 1a–c. XRD peaks were significantly broadened with decrease in intensity after the reduction process, indicating the loss of long-range ordering of the graphene planes32. In addition, the XPS spectra (Figure 1b–c) indicated that functional groups, such as C–O and C = O, were reduced either partially (upper spectrum) or almost fully (lower spectrum) compared to graphite oxide (Figure S1–2). The oxygen contents in each sample were approximately 24.5% and 5.8%, respectively. Raman spectroscopy in Figure S3 also showed that the D band (a disordered band caused by the graphite edges) and G band (associated with in-plane vibration of the graphite lattice) were observed at 1350 cm−1 and 1590 cm−1 in both the functionalized and reduced graphene oxide samples33,34. However, the intensity ratios of the D and G bands (D/G) were 1.11 and 1.04 in the functionalized and reduced graphene oxide samples, respectively, thereby confirming a difference between the two materials35. The partially reduced graphene oxide with some remaining functional groups functioned as a cathode, while the reduced graphene oxide with a minimal amount of functional groups functioned as an anode in the cell.
Both materials exhibited a similar porous morphology, as indicated by FE-SEM and HR-TEM analyses (Figure 1d–g). The pores in the functionalized graphene were formed by rapid gas evolution under the high vapor pressure generated upon treatment with the concentrated HCl (37 wt%) (Figure 1d–e)4. The reduced graphene oxide with minimal functional groups exhibited a similar microstructure, as illustrated in Figure 1f–g. Because the pores were formed by gas evolution, the pores were interconnected and extended from the inside to the surface of the materials, which will be discussed later. The porous electrode structures are expected to be advantageous for electrochemical reactions, because they enable greater penetration of the electrolyte into the interior of the electroactive materials. Moreover, the identical porous structures of both electrodes are expected to be capable of minimizing the power imbalance between the anode and cathode that is a commonly observed drawback of high-power hybrid supercapacitors11. The surface area and pore volume of the samples were further characterized by BET and N2 adsorption/desorption isotherm analyses (Table 1, Figure S4 and S5); the results of both analyses gave similar values for both materials. The slightly lower results observed for the fully reduced graphene originated from partial restacking of graphene layers and locally blocked pores36.
EDS line scanning and mapping analyses were carried out to evaluate the spatial distribution of functional groups on the samples (Figure 2a–e). The graphene surface of the cathode was extensively functionalized (Figure 2a), while far less functionalization was observed on the reduced graphene oxide anode (Figure S6). Mapping images from both HR-TEM and FE-SEM (Figure 2b–e) further indicated that the functional groups were uniformly distributed on the graphene surface. Uniform distribution of functional groups is critical to delivering a large Li capacity because the entire surface of the functionalized graphene cathode can participate in the electrochemical reaction. Cross-sectional images of the functionalized graphene cathode indicated well-developed interconnection of pores within the sample (Figure 2f–g) and verified that the functional groups were uniformly distributed even inside the material (Figure S7).
Li storage capabilities of graphene cathodes and anodes in Li half-cells
Prior to construction of full cells with the functionalized graphene cathode and reduced graphene oxide anode, the electrochemical performances of each electrode were separately characterized in Li half-cell configurations. First, the functionalized graphene cathode was tested in the voltage window between 1.5 and 4.5 V at a current rate of 0.1 A g−1 (Figure 3a). Within this voltage range, the cathode initially delivered approximately 150 mAh g−1 at an average discharge voltage of 2.5 V. The capacity increased slightly to ~200 mAh g−1 over 100 cycles, likely because of the gradual activation of functional groups within the electrode37. Ex situ XPS analysis indicated that the C = O functional group was responsible for Li storage at high potential. As shown in Figure S8, in the discharged state, the XPS peak corresponding to Li–O–C was emerging. In addition, the intensity of the peak associated with C = O decreased simultaneously with an increase in the peak associated with C–O. This behavior indicated that, in this voltage range, the C = O functional group acted as a redox center for storage of Li ions. Note that the graphite oxide did not show satisfactory electrochemical activity as a cathode despite the presence of functional groups, which is attributable to its low electrical conductivity. It is expected that partial reduction of the graphite oxide increases electrical conductivity while maintaining appropriate amount of C = O redox centers to be used as a cathode material. Upon repeated cycles, the high specific capacity was maintained with a coulombic efficiency of ~93%. This outstanding cyclability was attributed to C = O/C–O faradaic surface reactions, which resemble pseudocapacitor that does not accompany a significant lattice expansion or contraction. The rate capability test (Figure 3b) indicated remarkably fast Li storage capability in the functionalized graphene cathode. A capacity of greater than 100 mAh g−1 was delivered at current density of 3,000 mA g−1; furthermore, even at a current density of 8,000 mA g−1, 47% of the initial capacity (~70 mAh g−1) was maintained.
The reduced graphene oxide anode was evaluated in the voltage range from 0.01 to 3.0 V at a current density of 0.1 A g−1 (Figure 3c). A capacity of 540 mAh g−1 was reversibly obtained over 100 cycles, while the first irreversible capacity was relatively high. Wang et al. reported that Li ions can be stored on both sides of graphene, forming C3Li38. Accordingly, graphene can deliver about twice the capacity of a conventional graphite anode. In our case, >72% of the theoretical capacity was achieved with a coulombic efficiency near 100% after 100 cycles. Because the reduced graphene oxide fabricated in this study was multilayer rather than single-layer graphene, it delivered slightly lower capacity than the theoretical value. The reduced graphene oxide anode also exhibited outstanding rate capability. At 8,000 mA g−1, 220 mAh g−1 of the capacity was delivered without any noticeable capacity degradation (Figure 3d). The high-power capability of both electrodes was attributed to the fast surface reactions on the graphene that enabled rapid Li storage capability. The linear relationship between current peaks and scan rates measured by cyclic voltammetry (CV), as shown in Figure 3e–f, confirmed that the electrochemical reactions in the functionalized graphene cathode and reduced graphene oxide anode were not diffusion-limited but confined to the surface9. Notably, the CV profile of the reduced graphene oxide anode shows asymmetric behavior in discharge and charge process. While further study is needed, this behavior is attributable to the hysteresis in lithiation and delithiation process.
Electrochemical performance of all-graphene-battery
The similar chemical composition and microstructure of both electrodes in all-graphene-battery maximized the performance of each electrode while avoiding the commonly observed power imbalance in the full cell. All-graphene-battery was prepared by combining a functionalized graphene cathode with a reduced graphene oxide anode in a lithiated state, as shown in Figure 4. The electrochemical properties of all-graphene-battery were evaluated at a current density of 0.05 A g−1 in the voltage window between 0.01 and 4.3 V (Figure 5a–b). The specific capacity was approximately 170 mAh g−1 based on the weight of the cathode, which corresponded to nearly 100% utilization of the functionalized graphene cathode. The graphene anode delivered a capacity of 430 mAh g−1, which corresponded to ~80% utilization. The total capacity was ~120 mAh g−1 when calculated from the total active material weights of both the cathode and the anode. The charge/discharge profiles were not significantly altered upon repeated cycling, thereby indicating that the electrochemical reaction was highly reversible. High coulombic efficiency (~98%) was achieved after 50 cycles. Extended cycle stability tests were conducted at a current density of 0.5 A g−1 (Figure 5c). Approximately 75% of the initial capacity was maintained after 400 cycles with a coulombic efficiency of ~99% (Figure 5c); approximately 56% of the capacity could be still delivered stably after 2,000 cycles, indicating the robustness of the electrode reaction.
A Ragone plot was used to represent the trade-off between energy and power density from discharge profiles at various current rates, which is important for practical applications. (Figure 5d–e). The power (P) and energy (E) were calculated from the equation used in literatures as follows11,39:
where, Emax and Emin are the voltages at the beginning and end of the discharge (V), i is the discharge current (A), t is the discharge time (sec.) and m is the total mass of active materials in the anode and cathode (g).
All-graphene-battery exhibited an energy density of ~225 Wh kg−1. The energy density was comparable to that of conventional LIBs29 and it was retained even at second-level charge/discharge rates providing ~6,450 W kg−1, which also makes all-graphene-battery comparable to supercapacitor systems30. The power and energy of this all-graphene-battery rivaled other high performance energy storage systems previously reported39,40,41,42, which have aroused considerable recent interests. The unique performance of all-graphene-battery is attributed to the following factors: (i) the electrochemical reactions in both the anode and cathode are based on fast surface reactions and are similar to those typically associated with supercapacitors, but (ii) the Li storage capability is not compromised due to the large amount of Li storage sites in graphene-derived materials. Additionally, the interconnected porous structure and high electrical conductivity of graphene-based materials boosted the power capability.
In summary, all-graphene-battery based on a functionalized graphene cathode combined with a reduced graphene oxide anode was proposed as an alternative high-performance energy storage technology. Simple chemical modification of graphene was used to tune the material for use as either an anode or a cathode. The similar chemical composition and microstructure of the electrodes in all-graphene-battery enabled maximized performance by each electrode and avoided the power imbalance commonly observed in conventional LICs. As a result, all-graphene-battery delivered a power density of 2,150 W kg−1total electrode and an energy density of 130 Wh kg−1total electrode, thereby positioning its performance in a region inaccessible to LIBs and supercapacitors. By utilizing carbonaceous materials in both the anode and cathode, this work offers a novel approach to the development of energy-storage devices that bridge the performance gap between LIBs and supercapacitors.
Fabrication of functionalized graphene and reduced graphene oxide
Graphite oxide (GO) was fabricated from natural graphite using the modified Hummers method as follows43. NaNO3 (5 g) and H2SO4 (225 mL) were added to graphite (5 g) and stirred for 30 min in an ice bath. KMnO4 (25 g) was added to the resulting solution and then the solution was stirred at 50°C for 2 h. Deionized (DI) water (500 mL) and H2O2 (30 mL, 35%) were then slowly added to the solution and the solution was washed with HCl (750 mL, 10%). Additional washing with concentrated HCl (500 mL, 37%) afforded the GO product as a powder. Next, the GO powder samples were annealed at 120°C for 6 h to synthesize functionalized graphene. The reduced graphene oxide was prepared by high temperature treatment of the functionalized graphene sample at 800°C.
The structure of the samples was analyzed with an X-ray diffactometer (XRD, D2PHASER) using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS, AXIS-HSi) was used to determine the degree of oxidation of the samples. The morphology of the samples was verified using field-emission scanning electron microscopy (FE-SEM, SUPRA 55VP) and high-resolution transmission electron microscopy (HR-TEM, JEM-3000F). Focused ion beam (FIB, AURIGA) analysis was carried out at 30 kV and 600 pA after Pt deposition. The samples were further analyzed using a high-resolution dispersive Raman microscope (LabRAM HR UV/Vis/NIR). The surface area and pore volume were quantified using the Brunauer–Emmett–Teller (BET) method.
Electrodes were prepared by mixing the active material (functionalized graphene or reduced graphene oxide, 70 wt%) with polyvinylidene fluoride binder (PVDF, 20 wt%) and Super-P® conductive carbon black (10 wt%) in an N-methyl-2-pyrrolidone (NMP) solvent. The resulting slurry was uniformly pasted onto Al or Cu foil, dried at 120°C for 2 h and roll-pressed. Test half-cells were assembled in a glove box into a two-electrode configuration with a Li metal counter electrode, a separator (Celgard 2400) and a 1 M lithium hexafluorophosphate electrolyte in a 1:1 mixture of ethylene carbonate and dimethyl carbonate (Techno Semichem). Electrochemical profiles were obtained over a voltage range from 4.5 to 1.5 V for the functionalized graphene cathode and over a voltage range from 3.0 to 0.01 V for the reduced graphene oxide anode using a multichannel potentio-galvanostat (WonATech). Before constructing full cells, each electrode was cycled twice as a formation step in half-cells at a current rate of 50 mA g−1 and then the cells were disassembled and re-collected in a full cell. The reduced graphene oxide was fully discharged (lithiated) up to 0.01 V (vs. Li) before used in the full cells. The mass balance in the test cells was about 2.5:1 (cathode : anode). The mass loading of the electrode including a cathode and an anode was 1.1–1.2 mg. The thickness of the cathode and anode was ~30 μm and ~20 μm, respectively. The full cells were assembled in a glove box into a two-electrode configuration with a separator (Celgard 2400) and a 1 M lithium hexafluorophosphate electrolyte in a 1:1 mixture of ethylene carbonate and dimethyl carbonate (Techno Semichem). The fabricated cells were tested in the voltage range from 0.01 to 4.3 V.
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This work was supported by (i) Human Resources Development program (20124010203320) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy, (ii) the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2009-0094219). (iii) Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (Project no. 20112010100140) grant funded by the Korea government Ministry of Trade, Industry & Energy. and (iV) the development of organic electrode materials for lithium ion batteries project (UE1240220D) funded by the Agency for Defense Development (ADD), Republic of Korea.
The authors declare no competing financial interests.
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Kim, H., Park, K., Hong, J. et al. All-graphene-battery: bridging the gap between supercapacitors and lithium ion batteries. Sci Rep 4, 5278 (2015). https://doi.org/10.1038/srep05278
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