Nitrogen functionalized carbon nanocages optimized as high-performance anodes for sodium ion storage

Nitrogen functionalized carbon nanocages optimized as high-performance anodes for sodium ion storage

Electrochimica Acta 304 (2019) 192e201 Contents lists available at ScienceDirect Electrochimica Acta journal homepage:

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Electrochimica Acta 304 (2019) 192e201

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage:

Nitrogen functionalized carbon nanocages optimized as high-performance anodes for sodium ion storage Jinglin Kan, Huanlei Wang*, Hao Zhang, Jing Shi, Wei Liu, Dong Li, Guanghe Dong, Yunpeng Yang, Rongjie Gao** School of Materials Science and Engineering, Ocean University of China, Qingdao 266100, People's Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 December 2018 Received in revised form 1 March 2019 Accepted 1 March 2019 Available online 2 March 2019

Energy storage based on sodium ions is considered to be one of the most promising candidates for largescale applications. However, designing nanostructured anodes with ultralong cycle life, superior rate capability, and high specific capacitance is still a challenge. Herein, we report that nitrogen functionalized carbon nanocages with optimized structures as anodes in sodium-ion half cells demonstrate a superior capacity of 402 mA h g1 at 50 mA g1, excellent rate performance with 101 mA h g1 at 10 A g1, and an ultra-long cycle life by retaining 81% of its initial capacity after 5000 cycles at 10 A g1. It can be observed that carbons with high nitrogen doping level and few-layer graphene domains are proved to be effective for sodium ion storage. Benefiting from the merits of structure and electrochemical performance of nitrogen functionalized carbon nanocages, the sodium-ion capacitors by using identical carbon electrodes achieves a high energy density of 102.5 W h kg1 and an outstanding cycle life of 100,000 cycles with 74.2% of the capacity retention. This work opens a new opportunity for designing highperformance carbon electrodes with scalable production for next-generation energy storage. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Doped carbon Carbon nanocages Templates Few-layer graphene Energy storage

1. Introduction Currently, lithium-ion batteries (LIBs) with high energy density have been applied in portable electronic devices, electric vehicles, and the intermittent renewable energy storing [1e6]. However, due to the resource shortage of lithium reserves, lithium-ion batteries can't meet the growing demand all over the world [7]. Sodium-ion batteries (SIBs), as one of the most promising alternative candidates for lithium-ion batteries, have attracted increasing attention thanks to the abundance and low-cost of sodium [8,9]. Compared to lithium, sodium has similar physicochemical properties. However, the ionic radius of Naþ (102 p.m.) is larger than that of Liþ (76 p.m.), which hinders the direct application of traditional anode materials in LIBs for SIBs, such as the high stability and low-cost graphite anode [10]. The main challenge for SIBs is to design electrode materials with a suitable structure for boosting facile sodium ion storage.

* Corresponding author. . ** Corresponding author. E-mail addresses: [email protected] (H. Wang), [email protected] (R. Gao). 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

With the development of sustainable materials, carbon is considered as one of the most essential materials for catalysis, electronics, water purification, gas storage, adsorption and energy storage, due to its high electrical conductivity and good thermal/ chemical stability [11e19]. Recently, various carbon materials have been investigated as anodes for SIBs, including graphene [20], porous carbon [21,22], carbon nanospheres [23,24], carbon nanofibers [25,26], hollow carbons [27,28], hard carbon [29,30], and so forth. The sodium storage mechanism in carbons can be summarized as physi/chemisorption on the surface and defective sites, intercalation between the graphitic layers, and nanopore filling. As a result, expanding the distance of graphitic layer, increasing the surface area, and introducing heteroatoms have been considered as effective pathways for improving the electrochemical performance of carbons [31e33]. To date, template-assisted carbonization has been certified to be favorable for the preparation of porous carbons, since the microstructure, porous structure, and morphology in the resultant carbons can be effectively controlled by employing various templates. For example, Zhang et al. synthesized hollow carbon microspheres with SiO2 nanospheres as a template, which exhibited a high capacity of 334 mA h g1 at 50 mA g1 [34]. Xiao et al. prepared ordered mesoporous carbon by using SBA-15 as the template,

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demonstrating a superior capacity of 407 mA h g1 at 0.1 A g1 [35]. However, there are some shortcomings that cannot be ignored about these templates, such as rigorous washing procedure, and time-consuming and complicated preparation process [36]. From the cost-effective aspect, MgO has been used as a template to prepare porous carbon materials [37,38]. In general, thin carbon layers were coated on the surface of MgO template, then forming porous carbon by further carbonization and the removal of MgO. The existence of MgO can also facilitate the formation of graphitic structures, and those stacked few-layer graphitic structures are helpful for improving electronic conductivity [39]. Recently, Hu's group developed the in situ MgO template method for the synthesis of carbon nanocages [40e42]. However, designing carbon nanocages with high nitrogen doping level for achieving high capacity and superior cycling stability in sodium-ion batteries/capacitors, and exploring sodium storage mechanism of heteroatoms need further investigation. Herein, we reported a modified in situ MgO template method by the combination of a chemical vapor deposition (CVD) process to obtain a new kind of Nitrogen Functionalized Carbon Nanocages (NFCNs) with high surface area, dilated graphitic layer, and high nitrogen doping level. As a result, by optimizing the microstructure, porosity, doping in carbon structures, the as-prepared NFCNs electrode exhibits high specific capacity, excellent rate capability and long cycle life. Hybrid sodium-ion capacitors (SICs) were carefully designed by employing NFCNs as both cathode and anode. The as-built SICs delivered a specific energy as high as 102.5 W h kg1 at a power of 331 W kg1 and maintained a high energy of 40.3 W h kg1 at a power of 12692 W kg1, showing promising potentiality for large-scale energy storage application in the future.


Micromeritics TriStar II 3020 surface characterization analyzer at 77 K. Thermogravimetric (TG) analysis was carried out on Mettler TG/DSC 2HT/1600 in air at a heating rate of 10  C min1 from 25 to 800  C. 2.3. Electrochemical measurements The anode slurry was prepared by mixing 75 wt% active material, 15 wt% super P, and 10 wt% polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidinone. The slurry was coated on the copper foil and then dried at 100  C for 12 h in a vacuum oven. The mass loading of active materials was about 1 mg cm2 1.0 M NaClO4 in 1:1 (volume ratio) ethylene carbonate (EC): diethyl carbonate (DEC) with 5.0% fluorinated ethylene carbonate (FEC) was used as the electrolyte. The half-cells were fabricated by using the as-prepared carbon as the working electrode, polyethene as the separator, and Na metal foil as the counter and reference electrode in the Ar-filled glovebox. The sodium-ion capacitors with cathodes and pre-activated anodes were assembled from identical NFCNs800 sample. During the pre-activation process, the anode in halfcell was charged-discharged for three cycles at 0.1 A g1, and then the half-cell was disassembled and the SIC was assembled using this pre-activated anode and a pristine cathode. Galvanostatic charge-discharge measurements were performed using a LAND CT2001A battery test system. The cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted on an electrochemical workstation (Gamry Interface 1000). All the electrochemical tests were carried out at room temperature. 3. Results and discussion

2. Experimental section 2.1. Material synthesis NFCNs were prepared by a modified in situ MgO template method [43]. In a typical procedure, 2 g basic magnesium carbonate (4MgCO3$Mg(OH)2$5H2O) was first placed into the center of a horizontal tube furnace. When the furnace was heated to the target temperatures (700e900  C) with a heating rate of 5  C min1 in nitrogen flow, the acetonitrile vapor generated by bubbling nitrogen through acetonitrile liquid at room temperature was introduced into the tube for 3 h. Then, the reaction system was cooled down to room temperature in nitrogen. Finally, the as-prepared samples were thoroughly washed with 6 M HCl solution and deionized water to remove the MgO template, and dried at 80  C in an oven overnight. The resulting product is labeled as NFCNs-T, where T indicates the carbonization temperature. 2.2. Material characterization The morphologies and microstructures of NFCNs were measured by scanning electron microscope (SEM, Hitachi S4800, 15 kV) and transmission electron microscopy (TEM, JEOL 2010F, 200 kV). X-ray diffraction (XRD) data was carried out on a Bruker D8 Advance powder diffractometer (Cu Ka radiation). X-ray photoelectron spectroscopy (XPS) was performed by using a multifunctional imaging electron spectrometer (Thermo ESCALAB 250XI). The Raman spectra were measured on a microscopic confocal Raman spectrometer (Lab RAM HR800) with an effective laser power on the sample of 5 mW, an excitation laser wave-length of 532 nm, and a spot size of 1 mm. The pore size distributions and specific surface areas of the samples were examined by

3.1. Structure of NFCNs Fig. 1a illustrates the strategy for synthesis of NFCNs by a modified in situ MgO template method combined with a CVD process. The basic magnesium carbonate was decomposed into cuboidal MgO at the target temperature, which can be used as in situ template for the growth of carbon layers. Once the target temperature was achieved, acetonitrile vapor was introduced into the tube furnace, and nitrogen-doped carbon layer can be covered on the surface of MgO. Higher growth temperature can lead to larger particle size and thicker carbon layer. After removing the template by etching, the as-formed carbon nanocages completely copied the structural outlines of MgO networks, enabling a high porosity and a spatially continuous connectivity inside the final samples. The low-magnification SEM images exhibit that NFCNs have a nanoflake morphology (Fig. S1a, c, e), and the nanoflakes are overlapped. From the high-magnification SEM images (Fig. 1b, c, d), it can be observed that the generated carbon nanocages are connected together to form the nanoflakes. TEM images (Fig. S1b, d, f and Fig. 1eej) further certificate the existence of carbon nanocages in NFCNs. With the increase of growth temperature, the size of nanocages increased from 10 to 20 nm for NFCNs-700 to 18e35 nm for NFCNs-800 and 25e45 nm for NFCNs-900, while the wall thickness of nanocages increased from about 0.5 nm for NFCNs-700 (1e2 graphene layers) to 1e2 nm for NFCNs-800 (3e6 graphene layers) and 3e5 nm for NFCNs-900 (8e15 graphene layers) (Fig. 1h, i, j). In fact, NFCNs-700 sample couldn't maintain a perfect nanocage structure (Fig. 1b, e, h). The explanation can be found that the shell thickness of NFCNs-700 is thin, which is n't strong enough to support the hollow structure for the carbon nanocages after removing the MgO template, thereby causing the internal shrinkage as a result of the strong action of capillary force during


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Fig. 1. Preparation and characterization of NFCNs. (a) Schematic diagram for the synthesis of NFCNs. Typical SEM images for (b) NFCNs-700, (c) NFCNs-800, and (d) NFCNs-900. Typical TEM images for (e) NFCNs-700, (f) NFCNs-800, and (g) NFCNs-900. High-resolution TEM images for (h) NFCNs-700, (i) NFCNs-800, and (j) NFCNs-900.

the drying process. In addition, lattice fringes can be clearly observed in the high-resolution TEM images and the interplanar spacing decreased from 0.35 nm for NFCNs-700 and NFCNs-800 to 0.34 nm for NFCNs-900, demonstrating a higher degree of ordering with higher carbonization temperature. This observation can be further confirmed by the following XRD, Raman, and TG analysis. The structure of NFCNs was investigated by using XRD and Raman analysis. As shown in Fig. 2a, the XRD patterns of NFCNs show two broad diffraction peaks that are indexed as (002) and (100) of the pseudographitic domains [44]. Based on the (002) peaks, the interlayer distance of graphitic layers is about 0.34e0.35 nm for NFCNs, in consistent with the TEM results. Compared to the intergraphene spacing of conventional graphite

(0.3354 nm), the slightly dilated interlayer spacing of NFCNs is also critical to introduce more defective sites for providing extra sodium storage capacity. Furthermore, the XRD patterns also demonstrate a progressively higher degree of ordering with the increase of carbonization temperature, which can be inferred from the sharper (002) peak and the narrower intergraphene spacing for NFCNs-900. This conclusion is in agreement with the following Raman observation (Fig. 2a and Fig. S2). The integrated intensity ratios (IG/ID) were employed to evaluate the relative level of ordering in the carbons. As Table 1 demonstrated, the IG/ID value increased from 0.36 for NFCNs-700 to 0.48 for NFCNs-800 and 0.65 for NFCNs-900. As indicated by TEM, XRD, and Raman analysis, more disordered carbon can be converted to graphitic structure at higher

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Fig. 2. Structural characterizations of NFCNs. (a) XRD patterns and Raman spectra of NFCNs. (b) TG curves of NFCNs. (c) N2 adsorption-desorption isothermal curves of NFCNs. The inset is the volumes for micropores, mesopores, and macropores in NFCNs. (d) XPS survey spectra of NFCNs. (e) N1s core level XPS spectrum of NFCNs-800. (f) Relative surface concentrations (%) of nitrogen species obtained by fitting the N1s XPS spectra of NFCNs.

Table 1 Physical properties of NFCNs. Samples

NFCNs-700 NFCNs-800 NFCNs-900 a b



(m2 g1)a

(cm3 g1)b





1387 1152 390

3.02 5.41 2.93

5.47 1.06 1.54

68.67 77.51 70.79

25.86 21.43 27.67

0.35 0.35 0.34

Pore vol (%)



0.36 0.48 0.65

XPS composition (at%) C



88.1 88.3 87.7

5.9 8.8 9.2

6.0 2.9 3.1

The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. The total pore volume was determined by the non-local Density Functional Theory (DFT) analysis.

carbonization temperature, resulting in the higher degree of graphitization. On the other hand, more lattice defects can be

introduced into the carbon structure with the increase of N-doping amount, which can make the carbon more disorder. However, the


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resulting increased IG/ID ratio suggests that the temperatureinduced ordering rather than doping-induced disorder plays a dominant role in the carbonization process. To further evaluate the crystalline of the NFCNs, TG analysis was carried out (Fig. 2b). The gradual right shift of weight-loss with the increase of growth temperature indicates the improved crystallinity [42]. The porous structures of NFCNs were demonstrated by nitrogen adsorption-desorption isotherms, and the results are shown in Fig. 2c and Table 1. All samples exhibit type IV isotherms with a hysteresis loop at high relative pressure (P/Po ¼ 0.4e1.0), indicating the coexistence of micro-, meso-, and macropores (Fig. 2c) [45]. The Brunauer-Emmett-Teller (BET) specific surface areas gradually decrease from 1387 m2 g1 for NFCNs-700 to 1152 m2 g1 for NFCNs-800 and 390 m2 g1 for NFCNs-900. Remarkably, NFCNs samples exhibit a high portion of mesopores (Fig. S3 and inset of Fig. 2c), and such a high mesopore volume is beneficial for providing a fast ion transport channel and accommodating the volumetric change during the insertion/extraction of sodium ions. Based on XPS analysis (Fig. 2d), the primary elements in NFCNs were carbon, nitrogen, and oxygen. The carbon and nitrogen are derived from acetonitrile, and the oxygen may come from the template. The C 1s profiles of NFCNs can be fitted into four peaks of C]C/CeC (284.6 eV), CeO/CeN (285e286 eV), C]O (286e289 eV) and COOH (290e291 eV) (Fig. S4) [46]. The nitrogen content is increased from 5.9 at% for NFCNs-700 to 8.8 at% for NFCNs-800 and 9.2 at% for NFCNs-900. The high-resolution N1s spectra of NFCNs700 can be deconvoluted into four types located at around 398, 399, 401e402 and 403 eV, corresponding to pyridinic N (N-6), pyrrolic or pyridonic N (N-5), quaternary N (N-Q) and oxidized N (N-X) (Fig. S5a) [47]. However, N-5 is disappeared in NFCNs-800 and NFCNs-900, and the relative amount of N-6 was also increased (Fig. 2e and f, and Fig. S5b). This can be probably ascribed to the more stable feature of N-6 at high temperatures. The N-6 functionalities refer to N atoms having two C atoms at the extrinsic defects or the edges of the graphene layers, offering one p electron to the p system. Therefore, N-6 itself cannot be used as an active site during the sodiation/desdiation process, but N-6 can create numerous extrinsic defects and active sites owing to the change of neighboring carbons and thus may significantly increase the reversible charge storage capacities [48]. The N-Q species occupy the largest part in NFCNs samples (Fig. 2f), which refer to the N atom replacing the C atom in the hexagonal ring and locate at the center sites of the graphene. The N-Q species can greatly improve electron conductivity and promote fast charge transfer, thus improving rate performance of electrode materials [49]. According to the high-resolution O 1s core level spectra (Fig. S6 a, b, c), there are generally three oxygen peak functionalities: C]O quinone type groups (OeI, 531 eV), CeOH and/or CeOeC groups (O-II, 533 eV) and COOH carboxylic groups (O-III, 536 eV) [50]. The OeI and O-II groups are the dominant functionalities in all the specimens of surfaces, and O-III being a relative minority (Fig. S6d). Although the oxygen atoms doped in the carbon framework are detrimental to the conductivity of the material, the presence of oxygen can improve the wettability and provide more active sites [51]. The wetting properties of the NFCNs electrode were evaluated by the dynamic water contact angle measurements as shown in Fig. S7. The NFCNs samples show better wettability with the small initial contact angle (<30 ) compared with other reported carbon anodes [42,52e54] and the droplet can be completely absorbed by the electrode material within 25s. This result indicates the strong adhesion between NFCNs electrodes and water due to nitrogen/ oxygen doping. Additionally, the initial contact angle of NFCNs-700 (14.5 ) was much smaller than the other two samples, which can be attributed to the higher oxygen content.

3.2. Electrochemical performance of NFCNs in half cells NFCNs with hierarchical porous structure, high surface area, rich heteroatoms, and 3D nanocage-like morphology are quite promising for sodium-ion storage applications. To confirm this, NFCNs were firstly evaluated as anodes in sodium-ion half cells. Fig. 3a and Fig. S8 show representative CV curves for NFCNs specimens at 0.1 mV s1. In the first cycle, the pronounced reduction peaks at around 0.3 V and 0.9e1.1 V for NFCNs during the cathodic process can be observed, which are generally ascribed to the irreversible reactions of sodium ions with the surface functional groups, the decomposition of the electrolyte, as well as the formation of the solid electrolyte interphase (SEI) layer [55]. These peaks can be disappeared for the subsequent cycles. In addition, there is an additional reversible reduction peak near 0.01 V, which can be attributed to the insertion and extraction of sodium-ions into the defective graphene interlayers of NFCNs [56]. After the first cycle, no obvious change in the cathodic or anodic peak was observed, demonstrating excellent electrochemical reversibility during the sodiation/desodiation process. It is worth to note that all the CV curves at high potentials show the quasi-rectangular shape, indicating that the capacitive sodium storage behavior has a major contribution to the total capacity [57,58]. We can infer that Na-ion adsorption-desorption is mainly achieved by physical adhesion occurring on the surface or defect sites of NFCNs and the redox reaction involving nitrogen-oxygen containing functional groups [59]. Ex situ XPS were performed at different voltages to explore the Na-ions storage properties of N configurations in NFCNs-800 electrode during the first charge/discharge cycle (Fig. S9a). The results show that the binding energy of Na 1s gradually decreased during the discharging process and reversibly recovered in the subsequent charging process, demonstrating the reversible sodiation/desodiation process in the carbon matrix or reaction of nitrogen functional groups (Fig. S9b). As shown in Fig. S9c, the peak located at about 397.7 and 400.1 eV appeared when discharged to 1.3 V, indicating the sodiation of N-6 and N-Q groups. At 0.4 V, N-6 and N-Q are further sodiated, and 100% of N-6 and 68% of N-Q were sodiated. At this time, the sodiation of N-X begun. When discharged to 0.001 V, all N configurations completed the sodiation process, and the peak positions of N-6, N-Q and N-X changed from the original values of 398.8, 401.1, and 402.2 eV to lower values of 397.7, 400.1, and 401.8 eV, which can be mainly ascribed to the strong interaction between sodium and nitrogen atoms. During the following charging process, not all of N configurations can return to their initial states. When charged to 1.7 V, 38% of N-Q/Na and 100% of N-X/Na recovered to N-Q and N-X, whereas N-6/Na had not changed. When charged to 3 V, 66% of N-Q/Na recovered to initial states, and N-6/Na is still fully irreversible. These results indicate that the sodiation/desodiation process of the N configurations is a gradual process, which is correlated with the binding energy values [48]. The N-X group is completely reversible, N-Q group is partially reversible, and N-6 groups is highly irreversible, in agreement with the previous report [48]. Ex situ O 1s XPS spectra of pristine, and fully discharged or charged NFCNs are shown in Fig. S10. It can be seen that the fully discharged or charged electrode exhibits much higher intensity than that of pristine one, which may be mainly ascribed to the decomposition of the electrolyte [60]. As shown in Fig. S10, the relative percent of OeI decreased and conversely O-II increased after the fully discharged to 0.001 V. Being fully charged again, the relative percents of OeI and O-II basically return to the initial state, demonstrating the reversible sodium storage correlated with breaking and regenerating of C]O. Because the decomposition of the carbonate-based electrolyte produces a large amount of oxygen functional groups, there will be partial deviation

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Fig. 3. Electrochemical performance of NFCNs electrodes in half-cell configuration within the voltage window of 0.001e3 V vs. Na/Naþ. (a) CV curves of NFCNs-800 for the initial five cycles tested at the scan rate of 0.1 mV s1. (b) The typical galvanostatic charge-discharge profiles of NFCNs-800 electrodes at the current density of 50 mA g1. (c) Rate capability of NFCNs. (d)The cycling performance and coulombic efficiency of NFCNs at the current density of 1 A g1. (e) The long-term cycling performance of NFCNs-800 at the current density of 10 A g1. (f) Normalized contribution ratios of capacitive capacities at different scan rates for NFCNs.

in investigating the conversion of O configurations for the NFCNs in the sodium ion storage process. Fortunately, the evolution of O configurations is consistent with previous reported literature [60,61], which is favorable for understanding the effect of oxygen functional groups. The galvanostatic discharge-charge profiles of NFCNs electrodes at a current density of 50 mA g1 are shown in Fig. 3b and Fig. S11. There are no obvious plateau regions in the curves, suggesting the capacitive-like behavior. Moreover, the profiles exhibit two distinct potential regions after the first cycle, in consistent with previous reports [62,63]: 1) the region above 0.20 V due to the storage of sodium ions at pores or defects, and 2) the slope below 0.20 V due to the intercalation of sodium ions in the short-range ordered graphitic layers. The initial specific discharge/charge capacities of NFCNs-700, NFCNs-800 and NFCNs-900 electrodes are 1625/

309 mA h g1, 2369/451 mA h g1, and 2431/237 mA h g1, giving an initial coulombic efficiency of 19.0%, 18.9% and 9.7%. It is generally accepted that the low coulombic efficiency can be ascribed to the formation of solid electrolyte-interphase (SEI) film and the irreversible reaction of sodium ions with the impurities in NFCNs during the discharge process, coinciding with the CV observations [64]. The 3D carbon nanocage structure also shows excellent rate performance. As shown in Fig. 3c, the reversible capacities of 402, 298, 172, and 101 m Ah g1 can be achieved at 0.05, 0.1, 1, and 10 A g1 for NFCNs-800. For comparison, NFCNs-700 and NFCNs900 display a lower capacity of 224/208, 206/169, 109/91, and 39/ 29 m Ah g1 at 0.05, 0.1, 1, and 10 A g1. The cycling stability performance of NFCNs was firstly evaluated of 1 A g1 for 500 cycles (Fig. 3d). The cycling coulombic efficiency stabilized at about 100% after the initial 30 cycles. The specific


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capacity of NFCNs samples exhibit a slight decay for the initial 50 cycles, and then no obvious capacity fading is observed in the following cycles. High reversible capacities of 123, 180, and 113 mA h g1 for NFCNs-700, NFCNs-800 and NFCNs-900 can be maintained after 500 cycles. From EIS analysis (Fig. S12), the value of charge transfer resistance (Rct) decreased from 400.1 U before cycling to 210.1 U after 500 cycles for NFCNs-800 at 1 A g1, indicating that the cycling-induced activation can be beneficial for improving the cycling stability. The SEM micrographs of NFCNs-800 after 500 cycles are shown in Fig. S13. It can be observed that the carbon nanocages can be well-maintained after cycling, demonstrating the robust structure of NFCNs-800. Moreover, long-term cycling stability was also tested at the high current density of 10 A g1 (Fig. 3e). The NFCNs-800 electrode still achieves a high capacity retention of 81.8% after 5000 cycles, further demonstrating the superior cycling stability. To further understand the sodium storage kinetics of NFCNs, CV curves at different scan rates from 0.2 to 1 mV s1 was conducted (Fig. S14 a, b, c). The current (i) in the CV measurement with the scan rate (y) obeys the power law according to the following equations:

i ¼ avb


where a and b are variables. From equation (1), the b values can be calculated from the slope of the linear plot of log (jij) versus log(v) [65]. If b ¼ 1, surface-induced capacitive process is dominated; if b ¼ 0.5, diffusion-controlled process is classified [66]. The calculated b-values are 0.74e0.85 and 0.70e0.80 for anodic and cathodic scans, suggesting that the current response mainly originates from the capacitive-controlled process (Fig. S14 d, e, f). Moreover, the current (i) can be classified as capacitive-controlled behavior (k1v) and diffusion-controlled process (k2v1/2), as shown in the following equation:

i ¼ k1 v þ k2 v1=2



. i v1=2 ¼ k1 v1=2 þ k2


where k1 and k2 are constants [67]. From equation 2 or 3, the contribution from surface capacitive behavior can be determined by linear fitting the curves of i/v1/2 v1/2 at a fixed potential with various scan rates from 0.2 to 1 mV s1. As demonstrated in Fig. 3f, the ratio of capacitive contribution increases from 50.1/53.8/52.9 to 71.1/76.8/73.6% for NFCNs-700/800/900 with the scan rates rising from 0.2 to 1.0 mV s1. The high capacitive contribution of NFCNs is mainly attributed to the easily accessible high surface area, the abundant functional groups, the increased defective sites, and the smooth ion diffusion pathways. By changing the carbonization condition, surface areas, graphitic structures, and nitrogen-doping levels can be easily tuned in NFCNs. We can observe that NFCNs-800 sample displays the most promising electrochemical performance, which can be ascribed to the synergetic effects of microstructure, porosity, and doping. Fig. 4a showed the schematic illustration of sodium ion storage in NFCNs. Firstly, the unique nanocage-like structure of the electrodes can act as a reservoir for the electrolyte to store a great amount of sodium ions apart from surface adsorption, and the hierarchical porous (especially mesoporous) structure can provide an interconnected pathway for the fast ion transport, ensuring the high rate capability. Secondly, the highly graphitized carbon shell can enable NFCNs to maintain a good conductivity for easy electron

transport, the dilated graphitic interlayer spacing (0.35 nm) in the carbon layers can further promote insertion and extraction of sodium ions, and the ultrathin carbon layers (1e2 nm) can greatly shorten the diffusion distance of sodium ions. Thirdly, the nitrogen and oxygen containing functional groups can give an extra pseudoabsorption capacity. Last but not the least, the interlayer intercalation also contains the contribution from the defective sites on the edges and heteroatoms. The lower capacity observed for NFCNs-700 can be attributed to the lower degree of graphitic ordering and less nitrogen doping, while the lower capacity for NFCNs-900 can be ascribed to the thicker carbon layers and narrower graphitic interlayer spacing. As far as we know, the electrochemical performance of NFCNs-800 is competitive with other reported carbon anodes, such as activated carbons, hierarchical porous N-doped carbon, N, S co-doped carbons, carbon nanofibers and hollow nanostructured carbons [21e25]. 3.3. Electrochemical performance of NFCNs-800//NFCNs-800 sodium-ion capacitors NFCNs-800 as the cathode was also evaluated in the voltage range of 2.7e4.2 V vs. Na/Naþ, and the charge storage mechanism of NFCNs-800 cathode is based on ion adsorption. As illustrated in Fig. S15, NFCNs-800 cathode delivered a high capacity of 50 m Ah g1 at 0.1 A g1 and a high capacity retention ratio of 68% from 0.1 to 10 A g1, indicating its excellent electrochemical performance. At present, hybrid SICs are fabricated by using NFCNs-800 as both cathode and anode, which can combine the advantages of batteries and supercapacitors. As shown in Fig. 4a, during the charging process for SICs, cations (Naþ) move to the anode, which can not only adsorb on the surface, defects, and functional groups, but also intercalate into the graphitic layers, and simultaneously anions in the electrolyte (ClO 4 ) can adsorb on the cathode surface. During the discharging process, Naþ and ClO 4 can regress into electrolytes. Considering that the charge balance between cathode and anode is vital for obtaining high-performance SICs, the anode to cathode mass ratio should be optimized. The optimized mass ratio between anode and cathode is certificated to be 1:2 (the detailed information is shown in Fig. S16). The CV curves of the constructed sodium-ion capacitors exhibit a deviation from the ideal rectangular shape (Fig. S17a), indicating the existence of multiple energy-storage mechanisms of Faradaic and non-Faradaic reactions in the device. Moreover, the chargedischarge curves of SICs are not strictly linear (Fig. S17b), further certificating the combined energy storage mechanisms. The energy and power of SICs based on the mass of active materials were calculated according to the previously reported method [66]. It can be found that the SICs exhibits a high energy density of 102.5 W h kg1 at the power of 331 W kg1, and still retains a high energy density of 40.3 W h kg1 at a high power of 12692 W kg1 (Fig. 4b). As shown in Fig. 4b and Table S1, the energy and power combinations of our SICs is remarkedly superior or comparable to that of most reported SICs systems, such as TiO2@CNT@C//BAC [67], SCN-A//SCN-A [68], CS-800//CS-800-6 [69], FeS2-xSex//AC [70], GNaTi2(PO4)3//2D-GNS [71], Gr-Nb2O5//AC [72], PI-2.5//AC(PI-5) [73], MCMB//AC [74], CDC//C-NVP [75], CoHCF//AC [76], and Nb2O5@C/rGO-50//AC [77]. Since commercial packaged energy devices generally include about 30e40 wt% of active material, a factor of 3 is usually employed to extrapolate the energy and power of the hybrid capacitor [78,79]. The maximum energy density is about 34.2 W h kg1 based on the total mass of the hybrid capacitor. Obviously, the performance of our fabricated sodium-ion capacitors can bridge the performance gap between supercapacitors and rechargeable batteries (Fig. 4c). Cycling stability is also a critical factor for the practical application of energy storage systems. The

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Fig. 4. Electrochemical performance of NFCNs-800//NFCNs-800 sodium-ion capacitors. (a) Schematic illustration of charge storage mechanisms in SICs. Energy-power density comparison of the SICs versus state-of-the-art reported (b) hybrid sodium-ion capacitors (active mass normalized) and (c) commercial energy storage devices (total mass normalized). (d) The long-term cycling performance of the SICs at the current density of 50 A g1.

SICs could maintain a reversible retention of 74.2% at the very high current density of 50 A g1 after 100,000 cycles (Fig. 4d). Such ultralong cycle life has been seldomly reported. This confirms that our SICs based on the scalable NFCNs-800 as both the cathode and anode is potentially promising for future energy storage applications. 4. Conclusions In summary, 3D hierarchical N-doped carbon nanocages have been successfully constructed by a modified in situ MgO template method using acetonitrile as the precursor. The as-prepared NFCNs have combined micro/meso/macroporous structures, which

ensures plenty of pathways for sodium in diffusion, leading to excellent rate capability. Importantly, the ultrahigh specific capacity of NFCNs-800 is related with the high nitrogen doping level and the dilated graphitic layer. NFCNs-800 displayed a significantly improved capacity of 402 mA h g1 at 50 mA g1, and achieved a superior rate capability with 101 mA h g1 at 10 A g1. Benefiting from the excellent kinetics feature of NFCNs, a novel highperformance sodium-ion capacitor with NFCNs-800 as both cathode and anode electrodes was assembled, which exhibited an energy density of 102.5 and 40.3 W h kg1 at the power density of 331 and 12692 W kg1, as well as a high capacity retention of 74.2% after 100,000 cycles. We believe that this high-performance and costeffective nitrogen functionalized carbon material can be used as a


J. Kan et al. / Electrochimica Acta 304 (2019) 192e201

promising electrode for developing next-generation energy storage devices. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21471139), and the Fundamental Research Funds for the Central Universities (No. 201822008). Appendix A. Supplementary data Supplementary data to this article can be found online at References [1] M.R. Palacin, Recent advances in rechargeable battery materials: a chemist's perspective, Chem. Soc. Rev. 38 (2009) 2565e2575. [2] S. Cho, H.Y. Jang, I. Jung, L. Liu, S. Park, Synthesis of embossing Si nanomesh and its application as an anode for lithium ion batteries, J. Power Sources 362 (2017) 270e277. [3] M. Pramanik, Y. Tsujimoto, V. Malgras, S.X. Dou, J.H. Kim, Y. Yamauchi, Mesoporous iron phosphonate electrodes with crystalline frameworks for lithiumion batteries, Chem. Mater. 27 (2015) 1082e1089. [4] S.M. Hwang, Y.-G. Lim, J.-G. Kim, Y.-U. Heo, J.H. Lim, Y. Yamauchi, M.-S. Park, Y.-J. Kim, S.X. Dou, J.H. Kim, A case study on fibrous porous SnO2 anode for robust, high-capacity lithium-ion batteries, Nanomater. Energy 10 (2014) 53e62. [5] H. Xue, J. Zhao, J. Tang, H. Gong, P. He, H. Zhou, Y. Yamauchi, J. He, Highloading nano-SnO2 encapsulated in situ in three-dimensional rigid porous carbon for superior lithium-ion batteries, Chem. Eur J. 22 (2016) 4915e4923. [6] P. Mei, J. Kim, N.A. Kumar, M. Pramanik, N. Kobayashi, Y. Sugahara, Y. Yamauchi, Phosphorus-based mesoporous materials for energy storage and conversion, Joule 11 (2018) 2289e2306. [7] X. Zhao, W. Cai, Y. Yang, X. Song, Z. Neale, H.-E. Wang, J. Sui, G. Cao, MoSe2 nanosheets perpendicularly grown on graphene with Mo-C bonding for sodium-ion capacitors, Nanomater. Energy 47 (2018) 224e234. [8] D. Kundu, E. Talaie, V. Duffort, L.F. Nazar, The emerging chemistry of sodium ion batteries for electrochemical energy storage, Angew. Chem. Int. Ed. 54 (2015) 3431e3448. [9] D. Gao, R. Du, C. Zhou, B. Han, K. Xia, Q. Gao, J. Wu, Direct implementation of K3Fe(CN)6 as cathode materials of sodium-ion batteries, Mater. Today Energy 10 (2018) 302e306. [10] K. Wang, N. Wang, J. He, Z. Yang, X. Shen, C. Huang, Preparation of 3D architecture graphdiyne nanosheets for high-performance sodium-ion batteries and capacitors, ACS Appl. Mater. Interfaces 9 (2017) 40604e40613. [11] S. Chen, L. Wang, Q. Wu, X. Li, Y. Zhao, H. Lai, L. Yang, T. Sun, Y. Li, X. Wang, Z. Hu, Advanced non-precious electrocatalyst of the mixed valence CoOx nanocrystals supported on N-doped carbon nanocages for oxygen reduction, Sci. China Chem. 58 (2014) 180e186. [12] Y. Li, Z.-Y. Fu, B.-L. Su, Hierarchically structured porous materials for energy conversion and storage, Adv. Funct. Mater. 22 (2012) 4634e4667. [13] G. Li, J. Yu, J. Jia, L. Yang, L. Zhao, W. Zhou, H. Liu, Cobalt-cobalt phosphide nanoparticles@nitrogen-phosphorus doped carbon/graphene derived from cobalt ions adsorbed saccharomycete yeasts as an efficient, stable, and largecurrent-density electrode for hydrogen evolution reactions, Adv. Funct. Mater. 28 (2018) 1801332. [14] M. Sevilla, A.B. Fuertes, R. Mokaya, High density hydrogen storage in superactivated carbons from hydrothermally carbonized renewable organic materials, Energy Environ. Sci. 4 (2011) 1400e1410. [15] H. Ba, Y. Liu, W. Wang, C. Duong-Viet, V. Papaefthimiou, L. Nguyen-Dinh, G. Tuci, G. Giambastiani, C. Pham-Huu, Carbon felt monoliths coated with a highly hydrophobic mesoporous carbon phase for the continuous oil sorption/ filtration from water, Adv. Sustain. Syst. 2 (2018) 1800040. [16] W. Libbrecht, A. Verberckmoes, J.W. Thybaut, P. Van Der Voort, J. De Clercq, Soft templated mesoporous carbons: tuning the porosity for the adsorption of large organic pollutants, Carbon 116 (2017) 528e546. [17] K. Xia, Q. Li, L. Zheng, K. You, X. Tian, B. Han, Q. Gao, Z. Huang, G. Chen, C. Zhou, Controllable fabrication of 2D and 3D porous graphene architectures using identical thermally exfoliated graphene oxides as precursors and their application as supercapacitor electrodes, Microporous Mesoporous Mater. 237 (2017) 228e236. [18] K. Xia, Z. Huang, L. Zheng, B. Han, Q. Gao, C. Zhou, H. Wang, J. Wu, Facile and controllable synthesis of N/P co-doped graphene for high-performance supercapacitors, J. Power Sources 365 (2017) 380e388. [19] L. Zheng, K. Xia, B. Han, C. Zhou, Q. Gao, H. Wang, S. Pu, J. Wu, N/P codoped porous carbon-coated graphene nanohybrid as a high performance electrode for supercapacitors, ACS appl. Nano Mater. 1 (2018) 6742e6751. [20] H. Liu, M. Jia, B. Cao, R. Chen, X. Lv, R. Tang, F. Wu, B. Xu, Nitrogen-doped carbon/graphene hybrid anode material for sodium-ion batteries with

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