Porous N-doped carbon nanostructure integrated with mesh current collector for Li-ion based energy storage

Chemical Engineering Journal 374 (2019) 201–210

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Porous N-doped carbon nanostructure integrated with mesh current collector for Li-ion based energy storage

T

Heng-Yi Cheng, Po-Yuan Cheng, Xui-Fang Chuah, Chun-Lung Huang, Cheng-Ting Hsieh, Jiaqi Yu, ⁎ Cheng-Hsien Lin, Shih-Yuan Lu Department of Chemical Engineering, National Tsing-Hua University, Hsin-Chu 30013, Taiwan

HIGHLIGHTS

porous N-doped carbon • Hierarchical nanostructure on stainless steel mesh

• • •

as anode. Bifunctional electrodes for ultrahigh performance lithium ion batteries/capacitors. Li ion capacitors deliver 145 Wh kg−1 at 1.4 kW kg−1 and 58 Wh kg−1 at 27.3 kW kg−1. Excellent cycling stability with 85% capacity retention after 5000 cycle operations.

GRAPHICAL ABSTRACT

A self-assembled mesoporous silica sphere templating process was developed to create hierarchical continuous porous coral reef like N-doped carbon nanostructures on stainless steel meshes as bifunctional electrodes for ultrahigh performance lithium ion based energy storage devices. The lithium ion capacitors (LICs) assembled from using the electrode as both the cathode and anode, exhibited a high energy density of 145 Wh kg−1 at a power density of 1.4 kW kg−1 and maintained an energy density of 58 Wh kg−1 under an ultrahigh power density of 27.3 kW kg−1, outperforming most of the state-of-the-art LICs.

ARTICLE INFO

ABSTRACT

Keywords: Self-assembly Mesoporous silica sphere Hierarchical carbon nanostructure Li ion capacitor Li-ion battery Mesh electrode

A self-assembled mesoporous silica sphere templating process was developed to create hierarchical continuous porous coral reef like N-doped carbon nanostructures on stainless steel meshes as bifunctional electrodes for ultrahigh performance lithium ion based energy storage devices. The coral reef like carbon nanostructure achieved a high specific surface area of 1229 m2 g−1 and a large specific pore volume of 2.21 cm3 g−1, without application of chemical activations. The electrode, when serving as an anode for lithium ion batteries (LIB) or lithium ion capacitors (LIC), delivered an ultrahigh specific capacity of 2058 mAh g−1 at 0.2 A g−1. If used as a cathode for LICs, it generated a high specific capacity of 125 mAh g−1 at 0.1 A g−1. The LICs assembled from using the electrode as both the cathode and anode, exhibited a high energy density of 145 Wh kg−1 at a power density of 1.4 kW kg−1 and maintained an energy density of 58 Wh kg−1 under an ultrahigh power density of 27.3 kW kg−1, among the top tie of the state-of-the-art LICs. The cycling stability of the LIC was outstanding with a 85% capacity retention after 5000 cycle operations at 5 A g−1. The hierarchical continuous porous coral reef like N-doped carbonaceous nanostructure provides micropores as micro-reservoirs of Li ions for local and fast Li ion intercalation/de-intercalation, edge Ndoping for additional redox pseudo-capacitances, and large pore volumes to accommodate the volume expansion/ shrinkage at the charge/discharge cycles and to offer fast mass transfer path for electrolyte ions, altogether leading to the successful applications as bifunctional electrodes for Li ion based energy storage devices.

⁎

Corresponding author. E-mail address: [email protected] (S.-Y. Lu).

https://doi.org/10.1016/j.cej.2019.05.180 Received 23 February 2019; Received in revised form 16 May 2019; Accepted 25 May 2019 Available online 27 May 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

three-dimensionally continuous charge transport path [25,39], but also ample mesh opening space to accommodate active materials. In this work, a self-assembled mesoporous SiO2 sphere templating process was developed to create hierarchical continuous porous coral reef like Ndoped carbon nanostructures on stainless steel meshes as bifunctional electrodes for ultrahigh performance lithium ion based energy storage devices. It is a convenient way to create porous carbon materials by using silica spheres as sacrificial templates [40,41]. Here, mesoporous silica spheres (MSS) of sizes around 50 nm were self-assembled onto stainless steel meshes to serve as a hard template, on which an N-doped carbon coating layer was produced through carbonization of a polydopamine coating layer. The hierarchical continuous porous coral reef like N-doped carbon nanostructure was obtained via etching removal of the MSS from the above product. The coral reef like carbon nanostructure achieved a high specific surface area of 1229 m2 g−1 and a large specific pore volume of 2.21 cm3 g−1, without application of chemical activation, and mass loading as high as 1.436 mg cm−2 was feasible. It was applied as the anode for LIBs and LICs and the cathode for LICs, both showing outstanding performances, achieving an ultrahigh specific capacity of 2058 mAh g−1 at 0.2 A g−1 as the anode and 125 mAh g−1 at 0.1 A g−1 as the cathode. The LIC fabricated by using the coral reef like N-doped carbon nanostructure as both the anode and cathode achieved a high energy density of 145 Wh kg−1 at a power density of 1.4 kW kg−1, among the top tier of the state-of-the-art LICs. The present development proves to be a promising approach for fabrication of ultrahigh performance Li ion based energy storage devices.

Energy storage devices, as an indispensable part of the sustainable energy infrastructure, have drawn intensive and extensive research attention, targeting for low cost, high energy and power densities, and long cycle life [1,2]. Lithium ion batteries (LIB), operated at high working potentials (4.0 V) and high energy densities (150–200 Wh kg−1), is one of the main energy storage devices [3–5]. LIBs however suffer from short cycle life and low power densities [6,7]. Lithium ion capacitors (LIC), combining the high working potentials and high energy densities of LIB with the high power densities and long cycle life of supercapacitors, have attracted a great deal of research attention in recent years [1,8–14]. In general, LICs are an asymmetric energy storage device. Its anode is commonly composed of materials capable of Li ion intercalation/de-intercalation such as graphite [15], graphene [16], Li4Ti5O12 [17,18], TiO2 [19], etc. or carbonaceous materials integrated with conversion type materials of high theoretical specific capacities such as Fe2O3 [20], Fe3O4 [21], MnO [22,23], SnO2 [16], etc. These conversion type materials often are poor electric conductors and experience large volume expansion/shrinkage during charging/discharging cycles. The former materials are often easy to prepare, with high electric conductivities, and excellent cycling stability, but low theoretical specific capacities (e.g., 372 mAh g−1 for graphite). The latter materials are with high theoretical specific capacities (∼1000 mAh g−1), but require complex synthetic procedures and are with poor electric conductivities and cycling stability [7]. As to the cathode, activated carbons of high specific surface areas are commonly used to generate sorption/desorption capacitances [24]. There have been attempts to fabricate symmetric LICs by using the same carbonaceous materials for both anode and cathode [25]. The advantages of such development are material application convenience and simplified manufacturing procedures to save cost for large scale production [25]. Graphite, although a convenient and popular electrode material for both LIBs and LICs, is restricted by its low theoretical specific capacity [15,26,27]. One way to improve on the situation is through heteroatom doping, particularly N-doping [28–30]. The N doped in graphite can create structural defects, which can serve as micro-voids for Li ion accommodation. It has been shown with DFT simulations that the N of edge-type N-doping adsorbs Li ions to enhance the Li ion storage in the micro-voids [31]. Furthermore, the electronegativity of N (3.5) is higher than that of O (3.0), and can accommodate more Li ions through strong electrostatic attractions [32]. Consequently, there have been a few work, reporting specific capacities higher than 1000 mAh g−1 for N-doped carbonaceous materials [4,29–31,33]. To fully utilize these structural defects, carbon nanostructures with high pore volumes and high specific surface areas are necessary. In this regard, hierarchical continuous porous hollow carbon nanostructures are advantageous. Such nanostructure possesses micro-, meso-, and macro-pores, with micro-pores offering abundant surface areas and active sites, mesopores for fast mass transfer, and macro-pores to accommodate the volume expansion/shrinkage during the charging/discharging cycles for improved cycling stability. Hollow structure, as compared with coreshell ones [32], offers confined conduction path for charges to ensure good high rate capability [34]. N-doped carbonaceous materials have been applied as electrode materials in supercapacitors and Li ion based energy storage devices [35]. The carbon sources adopted however are often toxic [25] or require delicate synthetic procedures, e.g., metal organic framework compounds [31]. The product carbon materials are often in powdery form of low specific surface areas and chemical activation is required to boost their specific surface areas for applications in LICs as the cathode material. Polydopamine, because of its excellent conformable coating characteristic and environmental friendliness, has been commonly used as the carbon source for N-doped carbonaceous materials [32,36–38]. And mesh current collectors, such as stainless steel meshes, offer not only

2. Experimental section 2.1. Materials Commercial stainless steel (type 304, mesh 200) mesh sheets were cut into pieces of 1 cm × 3 cm as substrates. Tetraethyl orthosilicate (TEOS, 98%), ammonium hydroxide (28–30%), and cetyltrimethylammonium bromide (CTAB, 98%) were purchased from Acros. Sodium hydroxide (85%) and dopamine hydrochloride (DA, 98%) were purchased from Sigma. Triethanolamine (TEAH, 99%) and polyvinyl alcohol (PVA, Mw = 61,600) were purchased from SHOWA. Ethanol (EtOH, 99%) was purchased from Honeywell. Poly(acrylic acid) (PAA, Mv = 450,000) was purchased from Aldrich. SuperP carbon black (99+%) was purchased from Alfa. All chemicals and reagents were of analytical grade and used as received without further purification. 2.2. Synthesis of mesoporous silica spheres (MSS) The mesoporous silica spheres were prepared following a previously reported method in literature [42]. Briefly, 150 mL of DI water was mixed with 1.54 g of CTAB and 0.31 mL of TEAH in a water bath maintained at 80 °C under stirring at 1200 rpm for 1 hr. An amount of 15.51 mL of TEOS was added into the above solution for sol–gel reactions under stirring for 2 hr. The solid product was rinsed with EtOH twice to afford the MSS. 2.3. Fabrication of hierarchical porous N-doped carbon nanostructure An amount of 0.5 g of MSS was first dispersed in 5 mL of DI water. A stainless steel mesh piece was dipped into the above suspension and then taken out for evaporation driven self-assembly of the MSS in an oven set at 40 °C for 10 min. This procedure was repeated for a desired number of times to control the loading amount of the MSS on the stainless steel mesh. The MSS-loaded stainless steel mesh was dipped into a solution containing 12.5 mL of DI water, 12.5 mL of EtOH, and 0.375 g of DA to coat DA onto the surface of the MSS. An amount of 0.925 mL of ammonium hydroxide was later added into the above solution under stirring at 130 rpm for a desired length of time for 202

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polymerization of the DA to form PDA coating on the MSS. The polymerization time can be adjusted to control the thickness of the PDA coating. The PDA@MSS loaded stainless steel mesh was rinsed with EtOH and then placed in a furnace, that was pre-purged with 150 sccm of N2 for 30 min. The furnace was heated to a desired temperature at a ramping rate of 1 °C min−1 and held at that temperature for 2 h to carbonize the PDA coating. The carbonization temperature can be adjusted to control the extent of graphitization of the resultant carbon. The N-doped C@MSS loaded stainless steel mesh was immersed in a 4 M NaOH solution for 24 h to etch away the MSS. The product was rinsed with DI water several times to afford the hierarchical continuous porous coral reef like N-doped carbon nanostructure, termed as MSSxn1-n2hr-n3C, with n1 representing the number of the self-assembly cycle of the MSS, n2 for the number of hour spent for the polymerization of DA, and n3 for the carbonization temperature. For example, electrodes fabricated with 5 self-assembly cycles of MSS, 6 h of DA polymerization, and carbonization at 800 °C, are termed MSS-x5-6hr800C. The synthetic process is illustrated in Fig. 1. It is to be noted that, although the fabrication process involves four steps: self-assembly of MSS, PDA coating, carbonization, and etching of MSS, it is well suited for large scale production. First, the preparation of MSS can be carried out in a kilogram scale as demonstrated in Ref. [42]. Second, the four steps are common wet processes that can be conducted at large scales in batch mode.

carbonate:dimethyl carbonate = 3:7) as the electrolyte. For half-cell testing, both counter and reference electrodes were Li chips. As for full cell testing of LICs, pre-lithiated MSS-x3-6hr-800C-A was taken as the anode and MSS-x6-6hr-800C-C as the cathode. Both half-cell and full cell were assembled into a CR2032 coin cell. The assembled CR2032 coin cell was moved out of the dry box and stored for 12 hr to allow electrolyte penetration into the electrode materials for later electrochemical measurements. 2.5. Electrochemical measurements All of the electrochemical tests were conducted at room temperature and the performance parameters were computed based on the mass of the active material. Cyclic voltammetry (CV) and galvanostatic charge/ discharge were conducted by using a CHI 6275D, an NEWARE BTS-5V2 mA, and a BTS-5V-20 mA. The equations for calculation of the specific capacities from the galvanostatic charge/discharge tests are as follow.

C=

I t 3.6

F=

I t V

where C and F are specific capacities with units of mAh g−1 and F g−1, respectively, I is the current density (A g−1), t is the discharging time (s), V is the potential window size (V). For the energy density and power density of the full cell testing, the calculation is based on the total mass of the active materials loaded on the anode and cathode and the equations are as follow.

2.4. Fabrication of Half-cell and LIC For anode half-cells, the above fabricated electrode, for example, MSS-x5-6hr-800C, was immersed in 20 mL of an aqueous binder solution containing 86.4 mg of PAA and 9.6 mg of PVA and then dried in a vacuum oven set at 100 °C for 5 h and at 150 °C for another hour to further enhance the mechanical integrity of the electrode by taking the binder layer as a cushion for the involved compression operation during the cell assembly. This product was termed MSS-x5-6hr-800C-A with A for anodes. As for cathode half-cells, the procedures are the same as above, except the binders were replaced with 96 mg of PAA and 96 mg of SuperP carbon black. The product was termed MSS-x5-6hr-800C-C with C for cathodes. The size of the electrode was controlled at a diameter of 12 mm and the mass loading of the active material was controlled at 0.63 ± 0.06 mg cm−2. The mass loading of the active material was determined from weighing the electrodes before and after the loading of the active material. It is to be noted that the variation of the weight of the stainless steel mesh is negligible after the series of synthetic steps as confirmed with a blank test. The cell assembly was conducted in an Ar filled dry box with 1 M LiPF6 (in solvent of ethylene

E=

t2 t1

P=

V · I dt 3.6

3600E t

where V is the working potential (V), I is the current density (A g−1), t is the discharge time (sec), t1 is the time mark at complete charge, t2 is the time mark at complete discharge. Electrochemical impedance spectroscopy was conducted at 1.5 V (vs. Li/Li+) in the frequency range of 0.01 Hz to 100 kHz with a voltage amplitude of 5 mV. 2.6. Material characterizations All the PDA derived carbon materials were examined with a powder X-ray diffractometer (XRD, Shimadzu XRD-6000, Japan) for crystalline structure. To investigate the extent of carbonization, confocal micro-

Fig. 1. Fabrication process for hierarchical continuous porous coral reef like N-doped carbon nanostructure. 203

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Raman spectroscopy (Raman, Horiba Jobin Yvon LABRAM HR 800UV) was conducted with an excitation wavelength of 532 nm. The specific surface area of the sample was determined from the N2 sorption/desorption isotherms, based on the Brunauer-Emmett-Teller (BET) model and the pore size distribution was calculated by using the non-local density functional theory (NLDFT) (Micrometrics ASAP 2010). The morphology of the sample was characterized with a field emission scanning electron microscope (FE-SEM) (Hitachi S-4800, Japan) and a high-resolution transmission electron microscope (HR-TEM, JEOL, JEM-2010, Japan). Energy dispersive X-ray (EDX, Oxford 6587, Oxford instruments) spectroscopy was conducted to determine the elemental composition of the PDA derived carbon material. The X-ray photoelectron spectroscopy (XPS) was conducted to study the chemical states of the elements in the PDA derived carbon material using a monochromatized Al Kα X-ray as the excitation source (XPS, Thermo ESCALAB 250XI, America).

packed onto stainless steel wire woven meshes through evaporation driven self-assembly to serve as the hard template for the hierarchical continuous porous coral reef like N-doped carbon nanostructure. As shown in Fig. S1(a), not only the stainless steel wire surface but also the large mesh opening of several tens micrometers were uniformly and densely packed with the MSS. The MSS packing was firm with good mechanical integrity. If zoomed in with high magnification SEM imaging, the packing was achieved with continuously and densely assembled MSS as evident from Fig. S1(b). The inset of Fig. S1(b) shows a TEM image of the MSS, revealing clearly the small size and mesoporous nature of the MSS. This MSS packed stainless steel mesh served as an excellent hard template for subsequent coating of DA [43]. DA carries positive charges because of the amino groups and can be readily and conformably coated onto the negatively charged MSS surfaces. DA is thus an excellent and convenient precursor for N-doped carbonaceous materials [32]. Fig. S2 shows the appearance of the stainless steel mesh at four different stages, from plain to after the MSS packing, to after polymerization of the DA coating, and to after carbonization of the PDA coating and NaOH etching of the MSS, showing metallic silver to white, to black, and to dark black colors, respectively. The mechanical

3. Results and discussion Mesoporous silica spheres, MSS, of around 50 nm in diameter were

Fig. 2. Morphology and composition of sample MSS-x5-6hr-800C: (a) low magnification SEM image with inset showing locally enlarged SEM image, (b) TEM image, (c) high magnification SEM image inside crack with inset showing that of electrode surface, (d) HRTEM image with inset showing SAED pattern, (e) elemental mapping of EDX from SEM for N. 204

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integrity of these mesh electrodes was well maintained even after the MSS removal with NaOH etching, as evident from Fig. 2(a). With this, one can load firmly more active materials on mesh electrodes as compared with on flat solid electrodes. If examined with higher magnification as shown in the inset of Fig. 2(a), there are cracks formed on the electrode surface after the etching removal of the MSS. If further zoomed in, one can observe the detailed morphology inside the crack and on the electrode surface as depicted in Fig. 2(c). Both morphologies appear as continuous porous coral reef like nanostructure with pores of 10–20 nm present. The continuous porous coral reef like nanostructure can be more clearly observed with TEM imaging of the carbonaceous material scratched off from the mesh electrode. Evident from Fig. 2(b), the structure is indeed continuous and porous, containing pores with sizes of 10–20 (mesopores) and larger than 50 nm (macropores). The excellent mechanical integrity of the mesh electrode may be attributed to the following three factors. First, the MSS were continuously and densely packed. Second, the PDA, in addition to serve as the precursor of the N-doped carbon, also served as a binding medium to tightly hold the MSS together and also to glue the MSS assembly firmly to the stainless steel wire surfaces. Third, there may be Na4SiO4 formed during the etching of MSS with NaOH (4NaOH + SiO2 Na4SiO4 + 2H2O), and a very minor amount of Na4SiO4, an anticorrosion reagent for steel, may remain to protect the stainless steel wires [44]. The mass loading and morphology of the active materials, the Ndoped carbon, of the mesh electrode can be adjusted through both MSS loading with the self-assembly cycle number and PDA loading with the polymerization time. With increasing self-assembly cycle number, the mass loading increased as presented in Fig S3. Nevertheless, the mass loading dropped when the cycle number was further increased to 7.

This is because of the deteriorated attachment of the loaded material when it gets too thick and material detachment occurs during the carbonization and etching steps. As to the electrode morphology, as evident from the left panels of Fig. S4, the packing of the mesh opening becomes more complete and the coverage gets thicker with increasing self-assembly cycle number, as can be judged from the blurring of the wire pattern of the mesh electrode. The detailed morphology of the mesh electrode surface, when zoomed in with SEM imaging, however remains the same as continuous porous coral reef like nanostructure, as evident from the right panels of Fig. S4. For the control of the polymerization time of DA, the mass loading of the active material increased with increasing polymerization time of DA as expected (Fig. S5). The morphological trend is the same with that of the cycle number case (Fig. S6). The crystalline structure of the PDA-derived carbonaceous material was investigated with HRTEM. It is evident from the HRTEM images of Fig. 2(d) and S7, the carbonaceous materials obtained from carbonization of PDA possess local nanographitic domains from which interlayer distances of larger than the d-spacing of 0.34 nm of graphite in the (0 0 2) planes are determined, 0.42 nm for sample MSS-x5-6hr-700C, 0.40 nm for sample MSS-x5-6hr-800C, and 0.38 nm for sample MSS-x56hr-900C. The larger than normal inter-layer distances, caused by the existence of oxygen containing groups and N-doping of the graphene layers, are advantageous for the intercalation/de-intercalation of Li ions during the charge/discharge cycles. The existence of N can be readily confirmed with the elemental mapping of EDX presented in Fig. 2(e). The mapping also reveals the uniform distribution of N in the material. Note that the inter-layer distances decrease with increasing carbonization temperature since the accompanying more intense thermal

Fig. 3. (a) Raman spectrum with inset showing XRD pattern. (b) N2 sorption/desorption isotherm with inset showing pore size distribution from NLDFT modelling. (c) HRXPS spectrum of N1s. (d) HRXPS spectrum of C1s. 205

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reduction removes increasing amounts of oxygen containing groups and nitrogen atoms. Table S1 summaries the elemental compositions of samples obtained with increasing carbonization temperature, from 700 to 900 °C. It is evident that the O and N concentrations drop with increasing carbonization temperature. Raman spectra and XRD patterns were further recorded to investigate how carbonization temperature affects the extent of graphitization of the PDA-derived carbonaceous materials. Fig. 3(a) shows the Raman spectrum of the PDA-derived carbonaceous material with the XRD pattern included as an inset. The broad diffraction peak of (0 0 2) was located at a 2θ value significantly less than 26° of graphite, in good agreement with the larger than normal inter-layer distance determined from the HRTEM image [34]. Larger inter-layer distances shift the diffraction angle toward the small angle region as predicted by Bragg’s law. Note that the existence of the nanographitic domains gives rise to the broad diffraction peaks of the sample. Nevertheless, the sizes of these nanographitic domains are not large enough to enable showing of the SAED rings. As for the Raman spectrum, the D- and G-bands located at 1346 and 1590 cm−1, respectively are clearly identified. D-band is caused by the structural defects of graphite, whereas G-band is attributed to the vibrations of graphene layers of graphite [45]. The extent of graphitization can be quantified as the ID/IG ratio, with small values indicating well graphitized carbons [32]. The PDAderived carbonaceous material exhibits an ID/IG ratio less than 1, indicating its good extent of graphitization. The graphitic domain size is however too small to show sharp X-ray diffractions or detectable electron diffractions. The extent of graphitization intensifies with increasing carbonization temperature as shown in Fig. S8. The ID/IG of samples fabricated at increasing carbonization temperatures decreases from 0.929 for sample MSS-x5-6hr-700C to 0.909 for sample MSS-x5-

6hr-800C, and to 0.897 for sample MSS-x5-6hr-900C. The specific surface area and pore volume of the porous coral reef like carbon nanostructure were determined with the N2 sorption/desorption isotherms as shown in Fig. 3(b). The specific surface area is 1229 m2 g−1, ultrahigh considering that no chemical activations were applied to the carbon material, with a large specific pore volume of 2.21 cm3 g−1. In the sorption/desorption isotherm, the rapid pick up in adsorption at low pressures implies existence of micropores and the pronounced hysteresis loop signifies the presence of mesopores. It is evident from the pore size distribution, inset of Fig. 3(b), that the material possesses micro-, meso-, and macro-pores, revealing its hierarchical porous characteristic. The pore volumes contributed by the micro-, meso-, and macro-pores are 13, 58, and 29%, respectively, with meso-pores being the dominant contributor. The hierarchical nanostructure provides large space to accommodate the Li ions and the volume expansion/shrinkage at the charge/ discharge cycles, and offer fast mass transfer path for electrolyte ions. Its micropores mainly come from the structural defects formed during the carbonization of PDA, and its mesopores are generated from the porous structure of the MSS. As to the macropores, they are mainly created from etching of the MSS and MSS clusters. Carbonization temperature did not affect the specific surface area and pore volume much as can be seen from the N2 sorption/desorption isotherms presented in Fig. S10 and data summarized in Table S2. As for the polymerization time of DA, it does affect the specific surface area of the materials when the polymerization time exceeds 8 h (Fig. S11 and Table S3), with sample MSS-x5-6hr-800C achieving the maximum specific surface area of 1229 m2 g−1. The pore size distribution and specific pore volume however remain relatively unchanged (Fig. S12 and Table S3). The long polymerization time produces thick PDA films, leading to thick carbonaceous layers at carbonization and thus decreasing specific

Fig. 4. Electrochemical performances of electrode MSS-x5-6hr-800C-A: (a) cycling voltammograms within potential window of 0.01–3.0 V (vs. Li/Li+) with scan rate of 0.5 mV s−1, (b) galvanostatic charge/discharge curves at 0.2 A g−1, and (c) specific capacities at increasing current density. 206

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surface areas. The high specific surface area and specific pore volume achieved by the material can be mainly attributed to the small sizes of the templating MSS, around 50 nm (47.6 ± 2.4 nm), leading to the high specific surface area of 615 m2 g−1 and large specific pore volume of 1.53 cm3 g−1 of the MSS (Fig. S9), from which even higher specific surface areas and even larger specific pore volumes can be created. The oxygen containing groups and N-doping of the material were further investigated with XPS. The full survey spectra of the materials are presented in Fig. S13, showing the presence of the major dominant element C and two minor constituent elements N and O. The HRXPS spectra of N1s and C1s were shown in Fig. 3(c) and (d), respectively to examine the detailed chemical states of N and C. For N1s, the binding energy peak can be de-convoluted to reveal four different N-doping states, quaternary (401.0 eV), pyrrolic (400.1 eV), pyridinic (398.4 eV), and oxidized pyridinic (403.1 eV) [32], among which pyrrolic, pyridinic, and oxidized pyridinic states are located along the edges of the carbon structure. Both N-doping and carbonization created nano-sized pores as evident from the pore size distributions of Fig. S10 and Fig. S12. The edges of the nanopores are energetically active and prone to association with N for formation of edge-type N-doping. These edge doping states can proceed with pseudo-capacitive redox reactions with Li+ for energy storage [46]. As for C1s, the binding energy peak can be de-convoluted into four constituent peaks, one CeC peak (284.8 eV) for the graphitic structure, one CeN/C]N peak (286.0 eV) for the N-doping, and OeC]O (287.8 eV) and C]O peaks (289.6 eV) for oxygen containing groups [27,47]. With increasing carbonization temperature, the amounts of the less stable edge N-doping states decrease as can be clearly seen from the right panels of Fig. S13. Table S4 summarizes the decreasing trend of the N-doping with increasing carbonization temperature. The electrochemical performances of the product electrodes were characterized with cycling voltammetry, galvanostatic charge/discharge, specific capacity, and cycling stability in terms of specific capacity and Coulombic efficiency. We first characterize them as anodes.

Fig. 4(a) shows the CV loop of electrode MSS-x5-6hr-800C-A within the potential window of 0.01–3.0 V (vs. Li/Li+). There can be observed a pronounced reduction peak at around 0.5 V at the discharge of the first cycle, which disappears from the second cycle on. Note here that discharge/charge is referred to the electrochemical workstation, and it is charge/discharge if referred to the anode. The reduction peak is a strong indication of formation of solid electrolyte interphase (SEI) layer on the electrode surface, retarding the charge transport and transfer at the electrode/electrolyte interface. The discharge plateau around 0.5 V observed at the first galvanostatic cycle shown in Fig. 4(b) is another indication of the SEI formation [3,48]. The discharge capacity drops sharply from 5026 mAh g−1 of the first discharge to 2247 mAh g−1 of the second discharge at 0.2 A g−1, which is the consequence of the SEI formation [49,50]. The situation however stabilizes from the second cycle on, as evident from the almost overlapping CV loops and galvanostatic charge/discharge curves exhibited in Fig. 4(b) and (c), respectively. The electrochemical reversibility of the electrode is excellent, exhibiting almost overlapping specific capacity versus current density curves (Fig. 4(c)) and excellent re-generation of the high specific capacity achieved at the starting low current density of 0.2 A g−1. The effects of the DA polymerization time and carbonization temperature on the electrode performances were investigated in terms of galvanostatic charge/discharge and specific capacity at increasing current densities as shown in Fig. S14 and Fig. S15, respectively. From the comparison, it can be concluded that there exist an optimal DA polymerization time, i.e., PDA coating amount, and an optimal carbonization temperature for the storage performances of the anodic electrodes, with electrode MSS-x5-6hr-800C showing the highest specific capacities. The continuous porous coral reef like carbon nanostructure offers excellent conduction path for the charge carriers and excellent diffusional environment for mass transfer of involved electrolytes [25]. Fig. S16 shows the Nyquist plot of electrodes MSS-x5-6hr-700C-A, MSS-x56hr-800C-A, and MSS-x5-6hr-900C-A. The curves exhibit typical capacitive characteristics, showing nearly straight lines at low frequencies

Fig. 5. Electrochemical performances of electrode MSS-x5-6hr-800C-C: (a) cycling voltammograms within potential window of 2.0–4.2 V (vs. Li/Li+) at increasing scan rates, (b) galvanostatic charge/discharge curves at increasing current densities, and (c) specific capacities at increasing current densities. 207

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material, but also provides pseudo-capacitances from association of Li+ (N + yLi+ + ye− - > [(N)y−(Li+)y]) and dissociation of PF6−([(N)x+(PF6−)x] + xLi+ + xe−- > N + xLi+(PF6−)) [46]. The high specific surface area of 1229 m2 g−1 and large specific pore volume of 2.21 cm3 g−1 enable effective utilization of Li ions for generation of intercalation/de-intercalation and pseudo-capacitances, achieving the ultrahigh specific capacity of 2058 mAh g−1 at 0.2 A g−1, 5.53 folds of the theoretical specific capacity of graphite and outperforming previously reported state-of-the-art carbonaceous material based anodic electrodes [4,5,29–33,45,47,51–54]. At fast charging/ discharging rates, the mass transfer resistances involved for the Li ions to access the inner structural defects and interlayer spacing of the carbon materials however increase substantially, leading to decreasing specific capacitances. Nevertheless, even at a fast charging/discharging

and semi-arcs at high frequencies. The slope of the line is a good measure of the diffusion resistance involved in the electrochemical process, the steeper the line, the smaller the diffusion resistance. As to the size of the semi-arc, it signifies the charge transfer resistance of the electrochemical process, the smaller the arc, the smaller the charge transfer resistance. Evidently, the three electrodes possess the same charge transfer efficiency, whereas electrode MSS-x5-6hr-800C-A exhibit the lowest diffusion resistance, giving it the highest specific capacities among the three. The micropores and structural defects of the carbonaceous material created from the PDA carbonization and N-doping are utilized as the micro-reservoirs for Li ions, boosting the specific capacity to be largely over the theoretical specific capacity of graphite, 372 mAh g−1 [30,31]. N-doping not only creates structural defects of the carbonaceous

Fig. 6. Electrochemical performances of LIC: MSS-x3-6hr-800C-A//MSS-x6-6hr-800C-C: (a) galvanostatic charge/discharge curves at 1.0 A g−1 within potential window 2.0–4.0 V for LICs fabricated at four mass ratios, (b) galvanostatic charge/discharge curves of LIC fabricated at mass ratio 1:6 at increasing current densities, (c) CV loops recorded at increasing scan rates within potential window of 2.0–4.0 V, (d) Ragone plot for present LIC in comparison with state-of-the-art LICs of all carbonaceous electrodes, (e) cycling stability and Coulombic efficiency of present LIC at 5.0 A g−1. 208

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rate of 5 A g−1, the electrode still offers a high specific capacitance of 313 mAh g−1, close to the theoretical specific capacitance of graphite, 372 mAh g−1. The cycling stability and Coulombic efficiency of MSSx5-6hr-800C-C as anode at 2.0 A g−1 are presented in Fig. S17, showing satisfactory stability and 100% Coulombic efficiencies. Electrochemical performances of electrode MSS-x5-6hr-800C used as the cathode of an LIC are discussed next. Unlike most carbonaceous material based cathodes that require chemical activations to boost their specific capacities, electrode MSS-x5-6hr-800C-C was applied as the cathode of the LIC without chemical activations. Fig. 5(a) shows the CV loops within the potential window of 2.0–4.2 V (vs. Li/Li+) at increasing scan rates. The CV loops remain rather rectangular in shape even at the high scan rate of 100 mV/s. This success can be again attributed to the excellent charge conductivities and mass transfer diffusivities offered by the hierarchical continuous porous coral reef like carbon nanostructure. If examined closely, there can be observed broad oxidation and reduction peaks at the positive and negative sweeps of the CV cycles, implying existence of pseudo-capacitances from superficial redox reactions, in addition to the dominant electric double-layer capacitances from the sorption/desorption of electrolyte ions. The galvanostatic charge/discharge curves shown in Fig. 5(b) remain quite symmetric even at the high current density of 10 A g−1, in good agreement with the results of the CV measurements. The specific capacities (gravimetric capacitances) achieved at increasing current densities of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 A g−1 are 125 (2 0 5), 121 (1 9 8), 110 (1 8 0), 101 (1 6 5), 91.4 (1 5 0), 73.9 (1 2 1), and 56.6 mAh g−1 (92.6F g−1), respectively (Fig. 5(c)). The high specific capacity achieved at 0.1 A g−1 is also successfully re-generated at the end of the cycling test. It is to be noted that the micropores created in the present carbonaceous materials are with sizes larger than 1 nm (as shown in the inset of Fig. 3(b)), unlike the dominantly less than 1 nm micropores created from chemical activation of carbonaceous materials, and can thus more effectively accommodate the electrolyte to offer the ultrahigh specific capacities. The present carbonaceous nanostructure was applied as the active materials for both anode and cathode to fabricate the full cell LIC. To construct effective LICs, it is important to balance the charges offered by the anode and cathode through adjusting the active material masses of both electrodes. The following equation offers a rough estimate of the mass ratio of the active materials of the two electrodes based on the charge-balancing concept:

Q=

with the conclusion drawn from the galvanostatic charge/discharge measurements. The LIC achieves outstanding energy densities and power densities, with energy densities of 145, 120, 106, 80.3, and 57.5 Wh kg−1 obtained at power densities of 1.4, 2.8, 5.6, 13.7, and 27.3 kW kg−1. These data were compared to those achieved by recently reported state-of-the-art LICs fabricated based on all carbonaceous materials and based on non-carbonaceous anodes in Ragone plots shown in Fig. 6(d) and Fig. S18, respectively with corresponding results summarized in Table S5 and Table S6, respectively. It is clear from the comparison that the LIC fabricated by using the present hierarchical continuous porous coral reef like N-doped carbon nanostructure as both the anode and cathode, performs the best [15,25,34,55]. The long term cycling stability of the LIC was also investigated at 5 A g−1 for 5000 cycles. The energy density retention rate is excellent at 85% and the Coulombic efficiencies were maintained high above 98%. 4. Conclusions Hierarchical continuous porous coral reef like N-doped carbon nanostructures integrated with stainless steel meshes were created with a self-assembled mesoporous SiO2 sphere templating process. The carbon nanostructure was proved outstanding bifunctional electrode materials for lithium ion based energy storage devices. When applied as the anode of LIB or LIC, it delivers an ultrahigh reversible specific capacity of 2058 mAh g−1 at 0.2 A g−1 (2187 mAh g−1 at 0.1 A g−1), the highest among carbonaceous material based anodes reported to date. If applied as both the anode and cathode of LICs, it exhibits a high energy density of 145 Wh kg−1 at a power density of 1.4 kW kg−1 and the energy density can be maintained at 57.5 Wh kg−1 at an ultrahigh of 17.3 kW kg−1. The cycling stability of the LIC is excellent with an energy density retention of 85% after operation at 5A g−1 for 5000 cycles. Acknowledgements Financial support offered by the Ministry of Science and Technology of Taiwan, ROC under grant MOST 107-2221-E-007-044-MY2 is gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.05.180.

q Q+Q = m+ + m Q+ + Q

References

where Q+ is the specific capacity of the positive electrode (cathode), Q is the specific capacity of the negative electrode (anode), Q is the specific capacity of the full cell, q is the total charge, and m+ and m are the active material masses of the positive and negative electrodes, respectively [24]. A mass ratio of around 1:9 was thus determined from the above equation with relevant specific capacity data. It however has been shown that the optimal mass ratio to achieve the highest specific capacity is often less extreme than that suggested by the above equation [25]. Here, four mass ratios, 1:3, 1:4.5, 1:6, and 1:7, were investigated for the fabrication of LICs: MSS-x3-6hr-800C-A//MSS-x6-6hr-800C-C. Fig. 6(a) shows the galvanostatic charge/discharge curves of the four LICs at 1.0 A g-1 within the potential window of 2.0–4.0 V. Evidently, the LIC fabricated from the mass ratio of 1:6 exhibits the longest charging/discharging times, i.e., the highest specific capacities. This LIC, fabricated with mass ratio of 1:6, was further tested at increasing current densities from 0.5 to 10 A g−1 as shown in Fig. 6(b). The galvanostatic charge/discharge curves remain quite linear even at the high current density of 10 A g−1, indicating its excellent capacitive characteristics. The CV loops of the LIC were also recorded at increasing scan rates from 5 to 100 mV s−1 as presented in Fig. 6(c). The CV loops remain rectangular even at the high scan rate of 100 mV s−1, again implying its excellent capacitive characteristics, in good agreement

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