High-Performance Sodium-Ion Capacitor Constructed by Well

Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 12188−12199

pubs.acs.org/journal/ascecg

High-Performance Sodium-Ion Capacitor Constructed by WellMatched Dual-Carbon Electrodes from a Single Biomass Haolin Liu,†,‡ Xiao Liu,†,‡ Huanlei Wang,*,† Yulong Zheng,† Hao Zhang,† Jing Shi,† Wei Liu,† Minghua Huang,† Jinglin Kan,† Xiaochen Zhao,§ and Dong Li† †

School of Materials Science and Engineering, Ocean University of China, 238 Songling Road, Qingdao 266100, China College of Marine Science and Biological Engineering, Qingdao University of Science and Technology, 53 Zhengzhou Road Qingdao, Shandong 266042, China

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S Supporting Information *

ABSTRACT: The concept of combining the advantages of batteries and supercapacitors to obtain hybrid capacitors with both high energy and high power is considered to be promising. However, development of hybrid capacitors is still hindered by the matching problem between the cathode and the anode. Here, we report a Na-ion capacitor with well-matched carbon anode and cathode originated from the same precursor: garlic, which is a green and abundantly presented biomass in the world. The hard carbon (GDHC) anode based on ion intercalation is prepared by simple pyrolysis at high temperature, which demonstrates a high reversible capacity of 260 mA h g−1 at 0.05 A g−1 with an intercalating capacity of 148 mA h g−1, a high initial Coulombic efficiency of 50.7%, and an excellent cycling stability of 80% after 10 000 cycles at 2 A g−1. The porous carbon (GDPC) cathode based on ion adsorption is prepared by a simple carbonization−activation method. GDPC with developed porous architecture and high surface area (1682 m2 g−1) provides a superior capacity of 152 mA h g−1 at 0.05 A g−1. By tuning the electrode potential for the balancing of anode and cathode, the assembled sodium-ion capacitor displays highly favorable performance, i.e., 156 and 31 W h kg−1 at 355 and 38910 W kg−1, and retains 73% of its initial capacity after 10 000 cycles at 1.5−4.2 V. We firmly believe that this work provides a practical strategy for designing advanced sodium-ion capacitors with both the anode and the cathode prepared by a facile process. KEYWORDS: Sodium-ion capacitors, Biomass, Hard carbon, Porous carbon, Capacitive storage

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INTRODUCTION Unparalleled attention has been received in electrical energy storage due to the increased demand for renewable energy resources and the rapid development of consumer electronics.1−3 Therefore, it is very critical to find high-performance electrical energy-storage devices. The lithium-ion battery (LIB) with excellent reversible capacity and high energy density has been considered as the ideal choice for meeting the growing requirements.4,5 However, the scarcity and uneven distribution of lithium resources further limit the large-scale application of LIBs.6−9 Since sodium has a similar chemistry with lithium and the sodium source is abundant in the geographic distribution, a sodium-ion battery is considered to be a promising alternative candidate for energy-storage systems.10,11 It is well known that although the battery offers © 2019 American Chemical Society

high energy density, the power density and cycling life are not ideal. Therefore, it is more desirable to meet both high energy and high power in a single device.12,13 In order to reach this balance, a sodium-ion capacitor (SIC) has been designed by integrating a capacitor-type cathode with a battery-type anode.14,15 In such a device abundant sodium ions are inserted in the anode to store charges, while reversible ion adsorption occurs on the surface of the cathode.16 Although various hybrid ion capacitors have been reported, balancing the electrochemical performance of the anode and cathode is still the biggest challenge. Received: March 8, 2019 Revised: May 13, 2019 Published: May 24, 2019 12188

DOI: 10.1021/acssuschemeng.9b01370 ACS Sustainable Chem. Eng. 2019, 7, 12188−12199

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ACS Sustainable Chemistry & Engineering

garlic. In this work, we take different treatment approaches for garlic in order to employ the same precursor to obtain both the anode and the cathode with high performance. A hard carbon with a high degree of graphitization formed by direct pyrolysis of garlic at high temperature is used as the anode for the sodium-ion capacitor. When used as an anode for sodium-ion batteries, the hard carbon displays a high reversible capacity of 260 mA h g−1, a long cycle life (80% capacity retention at 2 A g−1 after 10 000 cycles), and an outstanding initial Coulombic efficiency of 50.7%. A porous carbon prepared by a combined carbonization−activation method is used as the cathode for the sodium-ion capacitor. The garlic-derived porous carbons with high surface area (approximately 1680 m2 g−1), abundant porosity, and functional groups can promote ion diffusion rate, reduce the ion diffusional pathways, and improve the charge storage,40 leading to a high capacity of 152 mA h g−1 and an excellent cycling stability (73% capacity retention at 5 A g−1 after 10 000 cycles). By using these wellmatched anodes and cathodes, we finally design a superior SIC device with a remarkable energy of 156 W h kg−1.

Presently, researchers have mainly focused on improving the electrochemical performance of anode materials for SIC. Unfortunately, sodium ions cannot be effectively inserted into the graphite, owing to the larger radius of sodium ions (0.102 nm).17,18 Subsequently, a wide variety of negative electrode materials have been explored, such as nongraphite carbon nanostructures,19 transition metal oxides,20 metal alloys,21 and ionic compounds,22 but they still exhibit the drawbacks of poor cycling performance, low capacity, and/or significant volume change during the intercalation/deintercalation process. Recently, benefiting from the abundant resource and facile preparation process, hard carbons synthesized from biomass have been given much interest.23 The graphite-like microcrystallites and amorphous region can be observed in the hard carbon structure.24 These microcrystallites in hard carbons are composed of several parallel graphene sheets stacked on top of each other with expanded interlayer spacing (0.36−0.4 nm),25 which is beneficial for the insertion of sodium ions. Moreover, biomass precursors are also rich in a variety of natural heteroatoms (such as N, P, S), which can incorporate into the carbon structure for improving the electrochemical properties.26 Although abundant biomass, such as peanut skin,27 orange peels,28 lotus stem,29 corn cobs,30 macadamia nut shell31 and pomelo peels,32 have been used to prepare hard carbons with excellent electrochemical performance, the initial Coulombic efficiency and the cycling stability are still not ideal. For example, Zheng et al. reported the macadamia nut shell-derived carbon can deliver an initial Coulombic efficiency of 42.8%, but only 70% capacity can be retained after 1300 cycles.31 Hong et al. prepared a porous hard carbon through H3PO4-treated pomelo peels, showing an excellent capacity retention (84% after 220 cycles), but the initial Coulombic efficiency is as low as 27%.32 In addition, development of efficient cathode materials for the sodium-ion capacitor is critical. The activated carbon (AC) is generally used as the cathode for the hybrid capacitor devices, but AC usually exhibits low capacity due to the poor pore connectivity.33 Currently, porous carbons synthesized from biomass by simple activation have attracted much attention due to their high surface area and hierarchical porous strucutre.34 Even so, the capacity of the cathode is still substantially lower than that of the anode. There are usually two ways to solve this problem: the first is to achieve capacity balance by optimizing the cathode-to-anode mass ratio, and the other is adjusting the electrode potentials.35,36 On the basis of a previous report,37 the specific capacity and the voltage window of a hybrid device depends on the electrode potential window of the individual anode/cathode. Optimizing the mass ratio can increase the working voltage of the hybrid device, but the capacity cannot be increased. Furthermore, tuning the electrode potential to obtain equal capacity with equal mass for both the anode and the cathode can not only enlarge the working voltage but also improve the specific capacity of the hybrid device. In this work, we take the strategy of potential regulation to obtain well-matched anode and cathode. Designing high-performance electrodes through green and sustainable precursors is very essential, which can have a price advantage to make large-scale application.38,39 Garlic is a common crop grown and abundantly present in the world. In 2014 garlic was producing approximately 25 million tons worldwide. The carbon materials prepared from garlic could be rich in heteroatoms and defects due to the amino acids, sulfurcontaining compounds, and some mineral elements in the

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EXPERIMENTAL SECTION

Material Synthesis. We employed garlic as the precursor, which is produced from Shandong of China. The purchased garlic was first peeled and cut into small pieces. For synthesis of the cathode, the peeled garlic was soaked with 2 M KOH for 12 h. Then the soaked garlic was dried at 80 °C to obtain the garlic/KOH composite. Next, 4 g of the garlic was heated at 900 °C (3 °C min−1) under N2 for 1 h. Finally, the resultant garlic-derived porous carbon (labeled as GDPC) was liberated by washing with 2 M HCl, filtering with deionized water, and drying at 100 °C overnight. For synthesis of the anode, garlic was dried and ground into powders. Then 4g of the dried garlic powder was carbonized at 1100, 1300, or 1500 °C under N2 for 6 h (3 °C min−1). The resultant hard carbon was collected in the same way as GDPC. The garlic-derived hard carbon product was named as GDHC-X, where X indicates the carbonization temperature. Material Characterization. To investigate the morphology and structure of the GDPC and GDHC, scanning electron microscopy (SEM, Hitachi S4800, 15 kV) and transmission electron microscopy (TEM, JEOL 2010F, 200 kV) were carried out. Xray diffraction (XRD, Bruker D8 Advance) analysis was performed to identify the crystalline structures of the carbons. The Raman spectra were recorded with a confocal laser Raman spectroscopy (Lab Ram HR800) with a laser wavelength of 532 nm. Elemental compositions of the carbons were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XI, Thermo Scientific). The porosity and specific surface area of the carbons were measured at 77 K by using the Micromeritics ASAP 2460 4MP. Electrochemical Measurements. A slurry of 75 wt % active carbon, 15 wt % super P, and 10 wt % polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidinone was coated on stainless steel spacers and dried at 100 °C under vacuum overnight for preparing the working electrode. The typical mass loading of the electrodes was about 1 mg cm−2. The electrolyte was 1 M NaClO4 dissolved in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume) containing 5% volume of fluorinated ethylene carbonate (FEC). The half-cells were assembled by using polyethene as the separator and sodium metal as the counter and reference electrodes. For the Na-ion capacitor (NIC), GDPC and GDHC-1300 were employed as cathode and anode. All of the cell fabrication and disassembly were performed inside an Ar-filled glovebox with a water/oxygen content lower than 1 ppm. Cyclic voltammetry (CV) curves and electrochemical impedance spectroscopy (EIS) measurements were evaluated using an electrochemical workstation (Gamry Interface 1000). The galvanostatic charge−discharge profiles were carried out by using the LAND battery test system (CT2001A). 12189


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Scheme 1. Illustration of the Preparation Process of the GDHC and GDPC and the Mechanisms for the Na-Ion Storage in the Sodium-Ion Capacitors

Figure 1. Typical TEM micrographs of (a) GDHC-1100, (b) GDHC-1300, (c) GDHC-1500, and (d) GDPC.

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RESULTS AND DISCUSSION Structure Analysis of GDHC and GDPC. The schematic illustration of the preparation process for the carbons and the mechanisms for the charge storage in the carbons are shown in Scheme 1. The decision to employ different procedures for synthesis of the anode and cathode was based on their different charge storage mechanisms. As for the anode in sodium-ion capacitors, the capacity is mainly contributed by ion intercalation. We choose the high-temperature pyrolysis of garlic to form the pseudographitic ordering domains in the

carbon structure, and the GDHC with a dilated graphitic layer allows sodium ions to be easily inserted into the carbon. However, for the cathode side, ion adsorption is the main charge storage mechanism. Tuning the porous structure of carbon with high surface area is an effective approach to improve the ion adsorption capacity. Thus, we select the traditional KOH activation technique to form the highly porous structure for GDPC. As observed by TEM images (Figure 1a−c), there are distinct graphitic microstructures in the GDHC samples, which 12190


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Figure 2. (a) XRD patterns and (b) Raman spectra of GDHC and GDPC. (c) XPS survey spectra of GDHC and GDPC. (d) N2 adsorption− desorption isothermal curves of GDHC and GDPC.

Table 1. Physical Properties of GDHC and GDPC pore volume (%)

XPS composition (atom %)

samples

SBET (m2 g−1)a

Vt (cm3 g−1)b

V < 2 nm

V > 2 nm

d002 (nm)

IG/ID

ID/ID + IG

C

N

O

S

GDHC-1100 GDHC-1300 GDHC-1500 GDPC

196 50 37 1682

0.1 0.04 0.03 1.24

77.87 54.65 49.67 39.99

22.13 45.35 50.33 60.01

0.376 0.371 0.365 0.360

0.61 0.81 1.04 0.46

0.62 0.55 0.49 0.68

92.94 91.72 92.37 90.78

1.24 1.32 1.44 1.64

5.46 6.23 5.6 6.74

0.36 0.73 0.59 0.84

a The specific surface area was calculated by using the Brunauer−Emmett−Teller (BET) method. bThe total pore volume was determined by nonlocal density functional theory (DFT) analysis.

increase of carbonization temperature, the peak intensity for GDHC increases accordingly, and the (002) peak shifts to a higher diffraction angle, suggesting improvement of structural ordering. As shown in Table 1, the average intergraphene layer spacing (d002) is calculated to be 0.376, 0.371, and 0.365 nm for GDHC-1100, GDHC-1300, and GDHC-1500 that are much higher than 0.335 nm for graphite. The expanded intergraphitic layer is beneficial for insertion of large-sized Na ions. Raman spectra of the samples are displayed in Figure 2b, and the fitting of Raman spectra is shown in Figure S2. All of the specimens exhibit the D-band (around 1350 cm−1) and Gband (around 1580 cm−1) peaks.45 The integral intensity ratio of IG/ID is being employed to index the structural ordering, which increases from 0.61 to 1.04 with the increase of pyrolysis temperature. Meanwhile, ID/ID + IG, which is being employed to represent the degree of defects in the graphitic layer, shows a completely opposite trend, decreasing from 0.62 to 0.49 with the increase of carbonization temperature. These results further prove that the structural ordering increases with the reduction of defect concentration.25 Undoubtedly, GDPC has the lowest IG/ID and the highest ID/ID + IG values, verifying its amorphous structure with abundant defect concentration. XPS analysis was employed in order to evaluate the fine chemical structure of GDHC and GDPC (Figure 2c and

matches well with the characteristics of hard carbon materials described by a typical “house of cards” model.41,42 With the increase of carbonization temperature, an increase in parallel carbon hexagonal ordering can occur, and the graphitic microdomains can be grown in all directions, forming a large number of boundaries.43 In fact, not only can the graphitic interlayer allow the intercalation of sodium ions but also these boundary areas can store large amounts of sodium ions. As shown in Figure 1d, the abundant porous structure and irregular graphitic sheets for GDPC can be observed. The unique micro/mesoporous can ensure a reduction of the pore resistance, which is beneficial for accelerating the transmission rate of ions. The presence of rich edge defects in GDPC can also facilitate ion adsorption. From the high-magnification SEM images (Figure S1) it can be observed that all GDHC samples have blocky morphology with a smooth surface and irregular size. However, lots of porous structures can be observed on the surface of GDPC due to the effect of KOH etching. Figure 2a displays the XRD profiles of GDHC and GDPC. All profiles display two broad peaks centered at 24° and 43°, corresponding to the crystallographic planes of (002) and (100) of graphite.44 It is obvious that the peaks of GDPC are relatively weak, indicating an amorphous character. With the 12191


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Figure 3. Half-cell performance of GDHC. (a) CV curves of GDHC-1300, tested at 0.1 mV s−1. (b) Typical discharge−charge curves of the fifth cycle for GDHC-1100, GDHC-1300, and GDHC-1500 at 0.05 A g−1. (c) Sloping region capacity as a function of ID/ID + IG. (d) Evolution of hard carbon characteristics (Vmicro (103 cm3 g−1) and surface area (m2 g−1)) and sloping and plateau region capacities at different carbonization temperatures. (e) Rate performance of GDHC. (f) Cycling performance and Coulombic efficiency of GDHC tested at 2 A g−1 for 10 000 cycles.

the quinone (O−I)-type groups is the most active oxygen functional group, which can generate pseudocapacity by the surface redox reaction with the sodium ions.50,51 Among all of the GDHC samples, GDHC-1300 with the highest oxygen content contains the highest proportion of O−I (41.7%, Table S1) and can be benefited from such surface redox reaction. Furthermore, the doping of nitrogen, oxygen, and sulfur atoms can alter the surface properties of carbon materials, offering rich defects and active sites to improve charge storage. The specific surface areas and pore structures of GDHC and GDPC are evaluated by using N2 adsorption−desorption measurements (Figures 2d and S7). It is clear that all of the GDHC samples are type I/IV isotherms. Compared with GDHC-1300 and GDHC-1500, the N2 adsorption of GDHC1100 displays a rapid rise at a low pressure, which is related to its larger surface area (196 m2 g−1) and pore volume (0.1 cm3 g−1). As shown in Figure S7, GDHC samples exhibit a high portion of micropores, and the more developed micropores under the lower temperature is mainly correlated with the higher degree of defects and weakly stacked hexagonal planes.30 On the other hand, the low surface area of GDHC-

Figure S3−6). As listed in Table 1, the carbons mainly contain C, O, N, and S elements. Other possible heterogeneous atoms may exist in such a small amount that they cannot be detected by XPS analysis or further removed by washing with HCl. The C 1s spectrum exhibits four peaks (Figure S3) located separately at 284.6, 284.9−285.1, 286.3−286.6, and 290.0− 292.0 eV in the carbon frameworks, which can be assigned to the C−C, C−O/CN/C−S, CO/C−N, and COOH.46 The high-resolution N 1s spectra can be fitted by two peaks (Figure S4), including pyrrolic/pyridine (N-5) and quaternary nitrogen (N-Q), which are located at around 399.0−400.0 and 400.0−402.0 eV.47 The S 2p spectrum can be deconvoluted into three peaks (Figure S5) at 163.9−164.2, 165.1−165.4, and 168.5−169.1 eV, corresponding to the S 2p1/2 and S 2p3/2 of C−S−C bonding and the C−S(On)−C bonding.48 XPS analysis shows that GDHC and GDPC have significant oxygen content with 5.5−6.2 atom % for GDHC and 6.7 atom % for GDPC (Table 1). The high-resolution O 1s core-level spectra could be deconvoluted into three peaks (Figure S6), including CO (O−I, 531.0−532.0 eV), C−OH and/or C−O−C (O− II, 533.0 eV), and COOH (O−III, 535.0−536.0 eV).49 Among 12192


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ACS Sustainable Chemistry & Engineering 1300 (50 m2 g−1) and GDHC-1500 (37 m2 g−1) implies that more graphene planes become hardly accessible to N2 due to the ordering processes at higher temperatures. The decrease of pore volume (0.04 cm3 g−1 for GDHC-1300 and 0.03 cm3 g−1 for GDHC-1500) and micropore volume ratio can be mainly ascribed to closure of pores during thermal annealing at higher temperatures. Obviously, the nitrogen adsorption of GDPC is much higher than that of GDHC, leading to the high surface area (1682 m2 g−1) and pore volume (1.24 cm3 g−1). Remarkably, GDHC also contains abundant mesopores (Figure S7). These features are promising to enhance ion adsorption and facilitate electrolyte diffusion in the hybrid device. Electrochemical Performance of GDHC in Half Cells. The half-cell systems countered with Na metal foil are used to assess the performance of GDHC, which is used as the anode in the sodium-ion capacitors. The CV curves of GDHC samples acquired from 0.001 to 3 V at a scanning rate of 0.1 mV s−1 are shown in Figures 3a and S8a,b. The obvious cathodic and anodic peaks between 0.0 and 0.1 V can be found, indicating the insertion and extraction of sodium ions. Furthermore, the distinct irreversible reduction peaks of all GDHCs during the initial scan, due to the decomposition of electrolyte and the formation of solid electrolyte interface (SEI) film,52 reveals a capacity loss. The irreversible area reduces with the increase of carbonization temperature, suggesting an increase in the initial Coulombic efficiency. This phenomenon is also related with the decrease of specific surface area and the increase of graphitization degree.33 In addition, the CV curve is almost completely overlapped in the following cycle, meaning that insertion and extraction of sodium ion is quite stable. Figure S8c−e displays the galvanostatic charge/discharge (GCD) curves of GDHC, and Figure 3b compares the GCD data of the fifth cycle for different GDHC electrodes tested at 0.05 A g−1. The GCD curves have two obvious voltage regions, which include a sloping region above 0.1 V and a plateau region below 0.1 V. As shown in Figure S8f, when the pyrolysis temperature increases, the slope capacity tends to decrease, being 125, 112, and 104 mA h g−1 for GDHC-1100, GDHC1300, and GDHC-1500. Meanwhile, the plateau region capacity shows the opposite trend, displaying an increasing capacity from 105 mA h g−1 for GDHC-1100 to 148 and 206 mA h g−1 for GDHC-1300 and GDHC-1500. We combined these electrochemical data with the microstructure change data of GDHC at different carbonation temperatures. Figure 3c shows the correlation between the reversible sloping capacity and the defect concentration (ID/ID + IG) in the Raman spectra,25 in which we can find a linear relationship with the coefficient R2 = 0.989. Hence, with the increase of carbonization temperature, the capacity of the sloping region shows an entirely opposite trend to the ordering improvement of hard carbon. Interestingly, compared to the slow reduction of the capacity in the sloping region, the surface area decreases rapidly, which also indicates that the capacity in the sloping region should be mainly related to Na-ion adsorption on surface active sites.53,54 As shown in Figure 3d, it is worth noting that the slope capacity has almost the same trend with surface area and micropore volume, while the plateau capacity has the opposite trend. These results all indicate that the capacity in the plateau region should be derived from the Naion insertion into carbon, not the micropore filling.55 This phenomenon could be well explained by the successfully

proposed “adsorption−insertion” mechanism.25,53 It is worth noting that the low-voltage plateau region is beneficial for maximizing both the voltage and the energy density of the sodium-ion capacitors. In addition, the initial Coulombic efficiency can be improved from 35.4% for GDHC-1100 to 50.7% and 53.8% for GDHC-1300 and GDHC-1500, partially owing to the reduced surface area, consistent with CV analysis. Meanwhile, we argue that the sodium ions can be easily inserted into the carbon structure due to the dilated graphene interlayer spacing of the pseudographitic ordering domains. We analyzed the structural changes of GDHC-1300 by ex situ XRD analysis. The half-cells were first discharged to 1.5, 0.2, 0.1, and 0.001 V vs Na/Na+, and then the cells were disassembled in a glovebox, and finally the active material was cleaned, collected and analyzed. Figure S9 displays the ex situ XRD data and the relationship between voltage and the calculated mean intergraphene layer spacing of GDHC-1300. It is obvious that the (002) peak shifts to lower diffraction angle with the decrease of the discharge voltage, indicating that the intergraphene layer spacing can expand from 0.372 to 0.408 nm. The dilation of the graphene interlayer spacing confirms the intercalation behavior of sodium ions. Figure 3e displays the rate performance of GDHC. The reversible capacities of the GDHC-1300 were about 260, 175, 152, 110, 100, 75, 55, and 42 mA h g−1 at 0.05, 0.1, 0.2, 0.5, 1, 2, 5, and 10 A g−1 (at the fifth cycle for each current). The capacitance tends to decline as the current density increases, which is mainly caused by the weakened diffusion of sodium ions in the plateau area.33 In addition, compared with GDHC1300, the rate performance of GDHC-1100 and GDHC-1500 was barely satisfactory, due to the lower degree of graphitization for GDHC-1100 and the smaller graphitic interlayer spacing for GDHC-1500. We can find that the high reversible capacity can also be restored to the original level when the current returns back to 0.05 A g−1, suggesting that the GDHC structure is not destroyed at high current density. Table S2 compares the electrochemical properties of GDHC-1300 and some reported biomass-derived hard carbons for sodium-ion battery anodes, which illustrates that the electrochemical performance of garlic-derived hard carbon is strongly competitive.31,32,56 Besides rate capability, the GDHC electrodes also show a stable cycling performance at a high current rate of 2 A g−1 (Figure 3f). In particular, the GDHC-1300 can retain 80% of the initial capacity after 10 000 cycles. Moreover, the special capacity retention ratio is also as high as 73% and 75% after 10 000 cycles for GDHC-1100 and GDHC-1500. It is worth noting that GDHC samples exhibit outstanding Coulombic efficiencies, which increase to about 98% after 20 cycles and maintain around 100% for the following cycles, suggesting a fairly stable SEI layer. The GDHC exhibits superior cycle stability compared to other anode materials (Table S2). We can find that the larger intergraphene layer spacing is more conducive to the improvement of cycle performance. For example, Xu et al. reported that the 3D N-doped graphene foams with an intergraphene layer spacing of 0.34 nm only retains 65% capacity after 150 cycles and 0.5 A g−1.57 This phenomenon gives evidence that the lager interlayer distance can promote the Na+ insertion and guarantee the structural stability during cycling.58,59 In addition, the electrochemical impedance spectra of the GDHC were measured and are shown in Figure S10. We obtained the values of Re (equivalent series resistance) and Rct (charge transfer resistance) by fitting 12193


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Figure 4. CV curves at different scan rates of (a) GDHC-1100, (b) GDHC-1300, and (c) GDHC-1500. Cathodic and anodic b values of peak currents for (d) GDHC-1100, (e) GDHC-1300, and (f) GDHC-1500. (g) Normalized contribution ratio of capacitive capacities at different scan rates for GDHC. (h) Capacitive contribution of GDHC-1300 at a scan rate of 1 mV s−1.

the Nyquist plots with the equivalent circuit. The Re increases from 3.7, 3.6, and 3.5 Ω to 7.7, 7.2, and 10.3 Ω after 10 000 cycles for GDHC-1100, GDHC-1300, and GDHC-1500, while the Rct decreases from the original value of 501.7, 411.9, and 471.3 Ω to 272.5, 157.2, and 245.2 Ω after cycling. The increased Re and decreased Rct suggest that the formed SEI films are stable in the following cycles after they are formed, which should be beneficial for improving the cyclability. GDHC-1500 exhibits the highest specific capacity due to the highest degree of graphitization at low current. However, the smaller intergraphene layer spacing of GDHC-1500 could hinder the sodium-ion intercalation at high current, leading to the poor rate performance. Moreover, the rich O−I content and total oxygen content in GDHC-1300 can be also favorable for enhancing the storage capacity of sodium ions by providing additional pseudocapacity. Therefore, considering the reversible capacity, rate capability, and cycling performance, we can draw the conclusion that the GDHC-1300 exhibits the best sodium storage performance among the three samples. As shown in Figure 4a−c, the CV curves were measured at various scan rates in order to analyze the kinetics of GDHC. The obvious oxidation and reduction peaks can be observed for all GDHC samples, suggesting the insertion and extraction processes of sodium ions. We can clearly find that the pair of cathodic and anodic peaks display obvious distortion with increasing scan rate due to polarization of the electrode.60 Meanwhile, Na+ cannot be fully participated in the insertion reaction, and diffusion of Na+ is limited during the increase of scanning rate, leading to a reduction of the total charge

accumulation. The peak current at a specified voltage and scan rate abides by the following equation i = avb

(1)

From eq 1, i is on behalf of the peak current (mA) and v is the scan rate (mV s−1) and a and b are the changeable values. The values of the slope calculated by the linear relationship between log (i) and log (v) are the b values, which can illustrate the primary capacity contribution behaviors. When the b value is close to 0.5, it predicts that the capacity contribution is mainly controlled by diffusion. When the b value approaches 1, it means that the charge storage mechanism is mainly a capacitive-controlled process.61−63 As Figure 4d−f displayed, the cathodic and anodic b values for the GDHC are 0.75−0.81 and 0.85−0.89, demonstrating that the kinetics process is mainly the capacitive-controlled process. The ratios of capacitive contribution can be further determined based on eq 2 i = k1v + k 2v1/2

(2)

where k1v represents the capacitive contribution, and k2v1/2 corresponds to the diffusion-controlled reaction.64,65 By rearranging eq 2 (as indicated by eq 3), we can obtain the values of k1 and k2 by plotting v1/2 versus i/v1/2 (Figure S11a). i/v1/2 = k1v1/2 + k 2

(3)

According to this method we can get the capacitance contribution for current response of these samples. As shown in Figures 4h and S11b, c, the proportion of the capacitive 12194


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Figure 5. Half-cell performance of GDPC. (a) CV curves of GDPC. (b) Dependence of anodic and cathodic current (at 2.75 V) on scan rate. (c) Galvanostatic discharge/charge profiles of GDPC at current densities from 0.5 to 10 A g−1. (d) Rate performance of GDPC. (e) Cycling performance and Coulombic efficiency of GDPC tested at 5 A g−1 for 10 000 cycles.

is good. The excellent linearity for GDPC at 0.2−10 mV s−1 demonstrates that the porous structure GDPC can promote ion diffusion and transport. Figure 5c shows the GCD curves for GDPC, which display nearly quasi-triangular and symmetrical shape, indicating that the capacitance of GDPC is mainly derived from the surface redox reaction and electric-double-layer capacitive behavior. Figure 5d shows the specific capacity at different current densities. The GDPC delivers a high capacity of 152 mA h g−1 at 0.05 A g−1, and the capacity of GDPC can still retain 44% at 10 A g−1. The GDPC also shows satisfactory cycling stability and Coulombic efficiency, showing a capacity retention ratio of 73% after 10 000 cycles and maintaining a Coulombic efficiency of about 100% after 20 cycles (Figure 5e). As shown in Figure S12, the charge transfer resistance (Rct) is reduced from 397 to 191 Ω after 10 000 cycles, which is beneficial for achieving a stable cyclability. The high surface area, developed porosity, and abundant heteroatom doping of GDPC is favorable for achieving excellent electrochemical performance. Electrochemical Performance of GDPC//GDHC Sodium-Ion Capacitor. GDPC and GDHC-1300 were

contribution for GDHC-1100/1300/1500 is increased to 71.3/ 72.0/73.5% at 1 mV s−1. Also, we also clearly find that the ratio of the surface-induced capacitive contribution is positively correlated with the scan rate (Figure 4g). Electrochemical Performance of GDPC in Half Cells. The electrochemical tests are also carried out in half-cell systems for GDPC, which is used as the cathode of the sodium-ion capacitors. The difference is that the voltage window is set to be 1.5−4.2 V, which can store as many ions as possible into the carbon without decomposition of electrolyte.56 The capacity of the GDPC electrode primarily comes from the reversible adsorption of ClO4− to form double-layer storage and the redox reaction of Na+ through the defects and functional groups to achieve a pseudocapacitive charge storage.66 The CV curves of GDPC show rectangular-like shapes with pseudocapacitive humps at different scan rates from 0.2 to 10.0 mV s−1 (Figure 5a), indicating the charge storage mechanism of GDPC is mainly a capacitive-determined behavior. As shown in Figure 5b, we selected a voltage of 2.75 V to test the relationship between the current and the scan rate of GDPC. It is generally believed that a linear relationship indicates that the ion transfer kinetic reaction in the electrode 12195


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Figure 6. Electrochemical performance of GDPC//GDHC sodium-ion capacitor tested between 1.5 and 4.2 V. (a) Galvanostatic profiles of GDPC//GDHC. (b) CV curves at 5−200 mV s−1. Energy−power performance comparison of GDPC//GDHC versus reported values of (c) hybrid systems (active mass normalized) and (d) commercial energy-storage devices (total mass normalized). (e) Cycling performance and Coulombic efficiency of GDPC//GDHC tested at 30 A g−1 for 10 000 cycles.

the injected charge to the target state. By adjusting the potential, both the anode and the cathode half-cells were disassembled and then assembled into a SIC. It is worth noting that such balancing principle is achieved at low current density. Due to the large capacity loss of anode, the anode and cathode cannot reach the ideal balancing state at high current densities, which suggests that the hybrid device cannot achieve the desired performance at both high and low rates. Therefore, adjusting the mass ratio between the anode and the cathode in a hybrid device should be based on the practical demand. Figure 6 shows the electrochemical performance of the SIC device at 1.5−4.2 V. The GCD curves of SIC are not completely symmetrical (Figure 6a) but overall are similar to the shape of an isosceles triangle, which can be ascribed to the dissimilar energy-storage mechanisms happening in the anode and cathode. Figure 6b displays the CV curves of the SIC from 5 to 200 mV s−1 in the voltage range of 1.5−4.2 V, which are not the regular rectangle curves, further indicating that there are various energy-storage mechanisms occurring during the charge and discharge processes, such as the redox reactions,

combined to fabricate the sodium-ion capacitors. The GDPC can serve as an ideal cathode due to its high reversible capacity and excellent rate performance in a large potential range (1.5− 4.2 V), while GDHC-1300 is used as the anode since it possesses an excellent cycle stability and a high plateau capacity below 0.1 V vs Na/Na+. Before fabrication of the SIC device, we preconditioned both electrodes in half-cells for achieving the ideal electrode potential. When the GDHC-1300 anode operates in the plateau area and the GDPC cathode operates at high potential, we can achieve a large voltage range without decomposition of electrolyte and both electrodes can reach the maximum capacity. Considering the capacity of GDPC at 1.5−4.2 V vs Na/Na+ (152 mA h g−1 at 0.05 A g−1) and the plateau capacity of GDHC-1300 at 0.001−0.1 V vs Na/Na+ (148 mA h g−1 at 0.05 A g−1), the capacity of the anode and cathode is well matched based on a 1:1 mass ratio. Therefore, the GDHC-1300 anode was charged/discharged in the voltage range of 0.001−3 V and then discharged to a cutoff voltage of 0.1 V, while the GDPC cathode was charged/ discharged between 1.5 and 4.2 V and then maintained at 1.5 V. This preactivation process can guarantee the stabilization of 12196


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is worth noting that this easy-manipulation strategy can be effectively applied for advanced sodium-ion capacitors.

capacitive behavior, and sodium-ion intercalation/deintercalation.36,67 Figure 6c displays the Ragone plot of the GDPC//GDHC. We calculated the values of energy density and power density based on the previously reported method.68 The fabricated SIC can exhibit a remarkable energy in the range from 156 to 31 W h kg−1 with the corresponding power in the range from 355 to 38910 W kg−1 (Figure 6c). Figure 6c and Table S3 also show the specific energy and power of our SIC compared with some other reported hybrid ion devices. We can find that our device is superior to some published sodium-ion capacitors, for example, AC//graphitic microbead,69 Ti2CTx// Na2Fe2(SO4)3,70 GNS//NaTi2(PO4)3,71 SCN-A//SCN-A,38 FeS2−xSex//AC,72 and PSNC-3−800//PSOC-A.56 Meanwhile, it is still competitive with some lithium-ion capacitors, for example, TiO 2 //CNT-AC, 7 3 3DVN-RGO//APDC, 58 LMCMB//SFAC,74 and SCDCS//SCDCS.75 In general, the mass of active material accounts for approximately 30−40 wt % of the total mass in commercial packaging devices, so we divided the original energy and power values by 3 to convert it to the normalized values of the device.76 Obviously, the packaged GDPC//GDHC SIC can exhibit a maximum energy density of 51.8 W h kg−1 at 118 W kg−1, exhibiting that the asbuilt device bridges the gap between lithium-ion batteries and supercapacitors (Figure 6d). We tested the cycling stability of as-built sodium-ion capacitor at 30 A g−1. As Figure 6e demonstrated, the device can retain 78% after 5000 cycles and 73% after 10 000 cycles. We can observe that the capacity drops dramatically after an initial a few hundred cycles, which may be related to the structural degradation by insertion of Na ions, electrolyte decomposition, and other side reactions.77 As we applied a narrow voltage window of 1.5−3.5 V, the capacity retention ratio increases to 84% after cycle 5000 cycles and 78% after 10 000 cycles (Figure S12d). This may be attributed to the fact that a lower voltage window is conducive to reduce the degradation rate of functional groups in GDPC. Moreover, the Coulombic efficiencies of the sodium-ion capacitor at both potential ranges are close to 100% during cycling. EIS data can further illustrate the excellent cycling stability of the sodiumion capacitor (Figure S14). We can observe that the equivalent series resistance (Re) changed from 3.1 to 5.4 Ω after 10 000 cycles, which should be attributed the physical disintegration and/or growth of SEI layer. However, the charge transfer resistance (Rct) decreased from 19.5 to 17.1 Ω, which can be beneficial for obtaining a highly promising cycling performance.

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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/acssuschemeng.9b01370. Additional data including SEM images, fitting Raman spectra, high-resolution XPS spectra, relative surface concentrations, pore size distributions, volumes for micropores and mesopores/macropores, CV curves, galvanostatic charge−discharge curves, specific capacity from the plateau and slope contributions, ex situ XRD data, Nyquist plots, equivalent circuit, capacitive contribution, electrochemical performance of GDPC// GDHC sodium-ion capacitors between 1.5 and 3.5 V, and comparisons of the performance (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. ORCID

Huanlei Wang: 0000-0001-8218-1762 Wei Liu: 0000-0001-8912-5683 Author Contributions ‡

H.L. and X.L. contributed equally to this paper.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (No. 201822008), the National Natural Science Foundation of China (No. 21775142, 21471139 and 51402272), and the Sino-German Center for Research Promotion (Grant GZ 1351).

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REFERENCES

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CONCLUSIONS To summarize, we designed a high-performance sodium-ion capacitor in which both anode and cathode materials are derived from the same precursor of garlic. The hard carbon anode exhibits excellent initial Coulombic efficiency, high specific capacity, and superior cycling stability. We clarify that sodium storage in hard carbon mainly contains two mechanisms: the sodium-ion adsorption on the surface in the high-potential sloping region and the sodium-ion insertion in the low-potential plateau region. On the other side, the porous carbon cathode with large surface area stores charges mainly by an ion adsorption mechanism. Due to the wellmatched capacities between the anode and the cathode by tuning the potential, the assembled SIC device displays an excellent energy of 156 W h kg−1 at a power of 355 W kg−1. It 12197


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