Novel Method of Fabricating Free-Standing and Nitrogen-Doped 3D

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Energy, Environmental, and Catalysis Applications

A Novel Method of Fabricating Free-Standing and Nitrogen-Doped 3D Hierarchically Porous Carbon Monoliths as Anodes for HighPerformance Sodium-Ion Batteries by Supercritical CO2 Foaming Jie Gong, Guoqun Zhao, Jinkui Feng, Guilong Wang, Yongling An, Lei Zhang, and Bo Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21660 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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ACS Applied Materials & Interfaces

A Novel Method of Fabricating Free-Standing and Nitrogen-Doped 3D Hierarchically Porous Carbon Monoliths as Anodes for High-Performance Sodium-Ion Batteries by Supercritical CO2 Foaming

Jie Gong, Guoqun Zhao, Jinkui Feng, Guilong Wang, Yongling An, Lei Zhang, Bo Li Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan, Shandong 250061, PR China

Abstract Sodium-ion battery (SIB), a promising candidate for large-scale energy storage systems, has recently attracted significant attention due to the low-cost and high availability of sodium resource. Hard carbon with free-standing structure and plenty of active sites is considered to be the most potential anode material for SIBs. However, keeping a balance between the excellent-performance and low-cost for the large-scale commercial production of carbon anodes is still in great difficulty. Herein, a free-standing nitrogen-doped 3D hierarchically porous carbon monolith (denoted as 3DHPCM) anode for SIBs is successfully fabricated via a novel supercritical CO2 foaming technology and thermal treatment. Thanks to the tunable macro-meso-microporous and disordered structures, the 3DHPCM exhibits a high reversible specific capacity (281 mA h g-1 after 300 cycles at 50 mA g-1), superior rate performance (67 mA h g-1 at 10 A g-1), and excellent long-term cycling stability (175 mA h g-1 after 3000 cycles at 500 mA g-1). Remarkably, the 3DHPCM with such high performance is fabricated via an environmentally friendly strategy from low-cost polyacrylonitrile (PAN) and polymethyl methacrylate (PMMA). Therefore, the strategy has great potential in practical application for fabricating high performance hard carbon anodes and other composite electrodes for SIBs and more energy storage devices.

Keywords: sodium-ion batteries; carbon anodes; CO2 foaming; hierarchically porous carbon monoliths; tunable porous structure 

Corresponding author at: Key Laboratory for Liquid–Solid Structural Evolution & Processing of Materials

(Ministry of Education), Shandong University, Jinan, Shandong 250061, PR China. Tel.: +86(0)53188393238; fax: +86(0)53188392811. E-mail address: [email protected] (Guoqun Zhao). 1

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1. Introduction With the overuse of non-regenerative fossil fuels and the increasingly serious global environmental problem, it is necessary to develop renewable energy resources such as solar, wind and tidal energies. To effectively utilize these intermittent renewable energies into grid-scale, electrochemical energy storage devices have been widely concerned due to their reliability and geographically independency.1,2 Lithium-ion batteries (LIBs) have been widely applied in our daily life, such as portable electronic devices, electric vehicles and so on, owing to their high energy density and long cycle life.3 However, the larger-scale application of LIBs is hampered by the limited storage and uneven geographical distribution of lithium resource. Due to the widespread geological distribution of sodium and the similar electrochemical properties between lithium and sodium, sodium-ion batteries (SIBs) have recently attracted great interest as promising low-cost candidates for next-generation electrochemical energy storage devices.4-7 One of the main challenges for developing SIBs is looking for suitable anode materials.8,9 Hard carbon material is regarded as one of the most promising anode materials of SIBs for commercial production. This is not only attributed to their advantages including low-cost and scalability, but their large interlayer distance and disordered structure with lattice defects and voids that could accommodate sodium ions and provide high electrochemical activity.10-12 Since 2011, many hard carbon materials have been investigated based on various precursors such as sugar,13 cellulose,14 wood,15 cotton,16 polyacrylonitrile (PAN),17 and polyaniline (PANI).18 For example, Hu’s group reported several biomass-based hard carbon anodes with diverse specific surface area, porous structure, defects, and heteroatom doping.16,19,20 Despite obtaining specific capacities as high as 300 mA h g-1, because the large ionic radius of sodium ion (102 pm for sodium ion and 76 pm for lithium ion) induces the unfavorable ion transport and structure instability of the hard carbon materials during cycling, the long-term cycling stability and rate capability still need to be improved urgently.21 Some recent researches are focusing on nanostructure design and heteroatom doping to decrease the transport distance of sodium ion and provide more active sites for sodium ion storage, so as to improve the electrochemical performance of hard carbon electrode materials.22 Zhao and co-workers designed nitrogen-doped and defective hard carbon nanoshells as anode materials, which delivered a reversible sodium storage of 174 mA h g-1 at 100 mA g-1 after 200 cycles.23 In addition, several groups have reported that sulfur-doped and/or phosphorus-doped hard carbons in forms of microtubes, nanosheets, and nanofibers enable high sodium storage performances.24,25 However, the fabrication processes are always complicated and not environmentally friendly, which are not suitable for practical applications. Therefore, it is still a severe challenge to obtain optimal hard carbon electrode materials by keeping a balance between the electrochemical performance and the fabrication cost. In addition to the design of the electrode materials themselves, the fabrication of a 3



free-standing electrode, in which all the materials can take part in the charge storage, is also critical to improve the energy density for practical application.26-29 3D hierarchically porous carbon with inter-connected macroporous open structure is considered promisingly as free-standing anode material. First, the 3D structure can provide efficient and stable transport paths for ions and improve high electrical conductivity.30 Besides, the interior macroporous structures can serve as electrolyte reservoirs and the mesoporous/microporous structures will offer a large number of active sites to accelerate the ion adsorption. These structure properties can facilitate both the cycle stability and rate capability.31 So far, various 3D hierarchically porous carbons such as templated carbons,32 biomass-derived carbons,15,33 and electrospun nanofibers34,35 have been made to improve the electrochemical performances for SIBs. Hu et al. reported a wood carbon anode by directly carbonizing a parallel cutting wood. When tested as a free-standing anode for SIBs, it delivered a reversible capacity of 270 mA h g-1 at a current density of 10 mA g-1 and showed a stable cycle performance after 100 cycles.15 Although the biomass-derived carbon is environmentally friendly and inherits the natural structure of the biomass precursor, it still faces the obstacle of untunable porous structure and low carbon yield, which is not suitable for practical application. Recently, flexible nanofiber films made via electrospinning have been reported for improving the sodium storage performance. Wang et al. fabricated multichannel carbon nanofibers with a reversible specific capacity of 222 mA h g-1 at 100 mA g-1 after 100 cycles.34 However, fast capacity degradation occurred during the long-term cycling test. Although electrospinning strategies have shown great potential in fabricating free-standing carbon nanofiber films anodes for SIBs, the structures of common electrospun films are unstable for simple surface-touching during cycling, which results in higher electrical resistance to limit the cycling stability and rate performance.35 Therefore, it is the most urgent affair for SIBs to develop a tunable and environmentally friendly method to fabricate 3D hierarchically porous carbon anode materials with high electrochemical performance. In this paper, a brand new strategy is developed to fabricate a lightweight and free-standing nitrogen-doped 3D hierarchically porous carbon monolith (denoted as 3DHPCM) anode for SIBs via an environmentally friendly supercritical CO2 foaming technology followed by carbonization. Remarkably, we for the first time achieve the tunable macro-meso-microporous structure of 3DHPCM by adjusting the process parameters of supercritical CO2 foaming and the precursor ratio of PAN and PMMA (polymethyl methacrylate). Benefiting from the synergistic effect of hierarchical porous structure and ion/electron transport dynamics superiority, the 3DHPCM anode shows a reversible capacity of up to 281 mA h g-1 (capacity retention up to 98.3%) at a current density of 50 mA g-1 after 300 cycles and retains 67 mA h g-1 at a high current density of 10 A g-1. It also exhibits an ultra-long cycling stability with the reversible capacity of 175 mA h g-1 4


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(capacity retention up to 90.9%) after 3000 cycles at a current density of 500 mA g-1. These excellent electrochemical performances, combined with the low-cost raw materials and porous structure tunable preparation procedures make the strategy a promising one for the large-scale commercial production of hard carbon anode materials for SIBs. 2. Experimental Section 2.1. Materials: Polyacrylonitrile (PAN, Mw=250000, Shandong Lufa Carbon Fiber Composites Co., Ltd.). Polymethyl methacrylate (PMMA, CM-107, Mw=96800, Taiwan Chimei Industrial Co., Ltd.). Dimethylsulfoxide (DMSO, AR, Sinopharm Chemical Reagent Co., Ltd.). CO2 (99.9%, Jinan Deyang Special Gas Co., Ltd.). All materials were not further purified before use. 2.2. Preparation methods: 2.2.1.

Preparation of precursors:

PAN and PMMA (weight ratio=8:2) were dissolved in 50 ml DMSO to form a 10 wt% homogeneous solution by mechanical stirring at 65 °C for 10 h. Then, the solution was transferred into a culture dish and dried at 60 °C for overnight to obtain a PAN/PMMA/DMSO sheet with thickness of about 0.5 mm. After that, the sheet was cut into small pieces with the same size (10 mm × 30 mm) as the hot-pressing mold cavity, followed by taking 12 pieces and hot pressing them together at 160 °C under 10 MPa of pressure for 10 min to obtain a rectangular sample with a size of 10 mm × 30 mm × 4.5 mm. Finally, the sample was punched to several cylinders with sizes of Φ9 × 4.5 mm as foaming precursors. Herein, the hot pressing process is used for the preparation of blocky precursors with good plasticity. And the as-prepared precursors would be suitable for the next foaming process in the cylindrical foaming device to produce blocky PAN/PMMA foams. So, the prepared foams could be cut into many sheets at one time. 2.2.2.

Preparation of 3DHPCMs:

CO2 was used as a physical blowing agent, and the precursors were foamed in a foaming device (composed of an autoclave, ISCO 260D pump, and PID-based temperature control system). The precursor to be foamed was placed into the autoclave, and the autoclave was heated to 140 °C. Then, CO2 was flushed into the autoclave and pressurized to saturation pressure (31.09 MPa). After the saturation for 1.5 h, the pressure was rapidly released to obtain the foam (denoted as PAN/PMMA-8:2 foam). For preparing a sodium-ion battery electrode that can be directly used, the foam was first cut into pieces of approximately 10 mm × 10 mm × 1 mm in size. Then, the pieces were raised to 250 °C in a blast drying oven at a heating rate of 5 °C/min under air-saturated conditions and held for 5 hours to be stabilized. After that, low temperature 5



carbonization (carbonization temperature is 500 °C, heating rate is 5 °C/min, holding time is 1.5 h) and high temperature carbonization (carbonization temperature is 800 °C, heating rate is 5 °C/min, holding time is 1.5 h) were continuously carried out in a tube furnace under argon protection for preparing the 3DHPCMs. For comparison, PAN/PMMA foams with different weight ratio of PAN/PMMA=10:0, 9:1, and 7:3 under the same preparation conditions were also prepared, denoted as PAN foams, PAN/PMMA-9:1 foams, and PAN/PMMA-7:3 foams, respectively. And their carbonized monoliths are denoted as HPCMs, 3DHPCMs-1, and 3DHPCMs-2, respectively. 2.3. Material characterization: The densities of the prepared carbon monoliths were measured via the water displacement method using densitometer (Xiamen Qunlong DX-120E). The microstructures of the foams and carbon monoliths were observed by field-emission scanning electron microscope (SEM, Hitachi SU-70) and transmission electron microscope (TEM, Phillips Tecnai 20U-Twin). The SEM images were analyzed by Nano Measurer software to calculate the average porous diameters and average wall thicknesses of the foams and carbon monoliths. The graphitization degrees of the carbon monoliths were characterized by X-ray diffraction (XRD, Rigaku D/max-c diffractometer) and Raman spectra (LabRAM HR800 Raman spectrometer). The Brunauer-Emmett-Teller (BET) specific surface areas and porous structures were characterized using the isothermal nitrogen adsorption/desorption measurements at 77 k on a surface analyzer (Micromeritics ASAP2020). The elemental compositions of the carbon monoliths were characterized by elemental analyzer (Elementar Vario ELIII) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250). 2.4. Electrochemical measurements: The prepared carbon monoliths were directly used as anodes without adding any binder, conductive agent, or current collector. The mass loadings of the carbon monoliths anodes were from 2-6 mg cm-2. Metallic sodium sheet was used as the counter electrode of the carbon monolith anode, and glass fiber (Whatman C) was used as the separator. The electrolyte was a mixture of 1 M NaPF6 dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 v/v). All the coin cells (CR2016) were assembled in an argon-filled glove box and tested at room temperature. The galvanostatic charge-discharge cycling at different currents between 0.01 and 3 V was carried out using a LAND battery tester (Wuhan, China). Cyclic voltammetry (CV) test was performed on a CHI 660E electrochemical workstation (Shanghai, China) in 0.01-3 V at a scan rate of 0.2 mV s-1. Electrochemical impedance spectroscopy (EIS) test was also performed on the electrochemical workstation in the frequency range of 100 kHz to 0.01 Hz with the amplitude of 5 mV.

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3. Results and Discussion As illustrated in Figure 1, the preparation process of 3DHPCMs can be divided into six main steps, where more details are provided in Experimental Section. (a) PAN and PMMA (weight ratio=8:2) are dissolved in dimethyl sulfoxide (DMSO). (b) The obtained solution is partially dried on the drying plate. (c) The obtained lamellate precursors are cut into small pieces, followed by non-fusion hot pressing and blanking to fabricate blocky precursors. (d) The blocky precursor is loaded into an autoclave, followed by supercritical CO2 foaming under the appropriate temperature and pressure. (e) The acquired foams are cut into sheets. (f) Carbonization treatment to the sheets. It is noteworthy that such foaming procedure can obtain many foamed sheets at one time, and this strategy is propitious to achieve scale production. For comparison, other three carbon monoliths are prepared with different weight ratio of PAN and PMMA of 10:0 (denoted as HPCMs), 9:1 (denoted as 3DHPCMs-1), and 7:3 (denoted as 3DHPCMs-2), respectively. The prepared carbon monoliths all have low densities due to the porous structure, and their densities gradually decrease from 130 mg cm-3 to 78 mg cm-3 as the weight ratio of PAN and PMMA decreases.

Figure 1 Schematic for the preparation of 3DHPCMs. (a) Mechanical stirring. (b) Partially drying. (c) Non-fushion hot pressing and punching. (d) Foaming. (e) Cutting. (f) Carbonization. 7



The morphologies and structures of the foams and their carbonized monoliths are characterized by the scanning electron microscopy (SEM) and the transmission electron microscopy (TEM). As shown in Figure 2a, b and Figure S1a, b in Supporting Information, both PAN foam and PAN/PMMA-8:2 foam exhibit the honeycomb-like macroporous structures. This is because during the supercritical CO2 foaming procedure, supercritical CO2 can be homogenously mixed with the polymer matrix near the molecular level. And then, when we offer a rapid depressurization to the supercritical CO2, it would nucleate and grow in the polymer matrix, thereby forming a dense cell structure. Moreover, the cell nucleation and growth can be effectively controlled by adjusting the process parameters of supercritical CO2 foaming, thus, the porous structures including porous density, porous size and void fraction of the foams are tunable. The mechanism of supercritical CO2 foaming is illustrated in Supporting Information. In addition, Figure S2 in Supporting Information shows the SEM images of PAN foams made by supercritical CO2 foaming under various saturation pressures, indicating that the porous structures of the foams can be changed by adjusting the process parameters of supercritical CO2 foaming. Notably, different from the closed porous structure of PAN foam, the PAN/PMMA-8:2 foam shows an open porous structure and larger porous size owing to the assistance of PMMA. PMMA acts as an opening agent which reduces the viscoelasticity and weakens the strength of the polymer matrix. During foaming, the walls of the polymer matrix are difficult to support the instantaneous tension from the rapid growing pores, further forming an open porous structure. The porous structures can be tuned by adjusting the weight ratio of PAN and PMMA (Figure S3, Supporting Information). High ratio of PAN/PMMA (PAN/PMMA-9:1 foam) is not beneficial to form open pores, and low ratio of PAN/PMMA (PAN/PMMA-7:3 foam) leads to the pores collapsing. Herein, as shown in Figure 2b and Figure S1b in Supporting Information, the prepared PAN/PMMA-8:2 foam exhibits an inter-connected and stable macroporous open structure.

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Figure 2 SEM images of (a) PAN foam, (b) PAN/PMMA-8:2 foam, (c) HPCM, and (d) 3DHPCM; TEM images of (e) HPCM and (f) 3DHPCM; High-resolution TEM images of (g) HPCM and (h) 3DHPCM; The insets in (g, h) show the SAED patterns of HPCM and 3DHPCM.

It should be stressed here that for intensive investigation, we selected the 3DHPCM carbonized from PAN/PMMA-8:2 foam as the optimal carbon monolith and compared it with the HPCM carbonized from PAN foam. As shown in Figure 2c and d, after carbonization, both HPCM and 3DHPCM shrink but maintain the macroporous structures without any collapse or pulverization compared with their foams. The average porous diameter and average wall thickness of the PAN/PMMA-8:2 foam measured are 16.8 μm and 320 nm, respectively. After carbonization, the average porous diameter and the average wall thickness decrease to 12.7 μm and 230 nm, respectively. During carbonization, the releasing of many small molecule gases (e.g., CO2, CO, NH3, H2O) and the thermal decomposition of PMMA are contributed to the shrinkage of 3DHPCM.36 From the TEM image of Figure 2f, it can be clearly seen that some mesopores with the porous diameter of 10-20 nm are uniformly distributed on the carbon walls of 3DHPCM. These mesopores are derived from the thermal decomposition of agglomerated PMMA during carbonization. The high-resolution TEM images of HPCM and 3DHPCM shown in Figure 2g and h reveal the highly disordered structure of amorphous carbon with large amounts of micropores. However, there are more turbostratic graphite-like structures in HPCM rather than 3DHPCM, indicating a higher degree of graphitization of HPCM. Moreover, the selected area electron diffraction (SAED) patterns in the selected areas (inset in Figure 2g and h) show more fuzzy diffraction ring for 3DHPCM, displaying a corresponding demonstration of the increased 9



disordered carbon in the high-resolution TEM images. Therefore, the thermal decomposition of PMMA can create more porous structures and can hinder the convertion of PAN to an ordered carbon structure during carbonization.37 The TEM images of carbonized samples with different weight ratio of PAN and PMMA (HPCM, 3DHPCM-1, 3DHPCM-2, and 3DHPCM) indicate that the mesoporous size and disordered structure of carbon monoliths could be completely tuned by adjusting the amount of PMMA (Figure S4, Supporting Information). The crystal structures and graphitization extents of HPCM and 3DHPCM are revealed by X-ray diffraction (XRD) pattern and Raman spectroscopy. The XRD patterns of HPCM and 3DHPCM in Figure 3a show two diffraction peaks at 24.3°and 43°, corresponding to the (002) and (100) plane in disordered carbon structure. After the addition of PMMA, the patterns show no obvious peak shift, but with a decreased peak intensity and broaden peak width. The interlayer distance (d002) of HPCM and 3DHPCM calculated by Bragg equation is 0.366 nm, lager than that of graphite (0.336 nm). Since PMMA is immiscible with PAN, causing no structural rearrangement during carbonization, HPCM and 3DHPCM show the same value of d002. However, the addition of PMMA brings more defects and prevents the structure ordered process of PAN during carbonization. As shown in Figure 3b, the Raman spectra of HPCM and 3DHPCM exhibit a typical D band at 1350cm-1 and G band at 1580cm-1. The D band represents the defective structures or the disordered structures, while the G band is ascribed to the ordered graphitic structures.38 The intensity ratios of the D and G bands (RI=ID/IG) are calculated to be 0.91 and 1.13 for HPCM and 3DHPCM, respectively. This further represents that the addition of PMMA decreases the graphitization extent of the carbon monolith, which is consistent well with the results of XRD patterns and the observations of TEM.39

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Figure 3 (a) XRD patterns and (b) Raman spectra of HPCM and 3DHPCM. (c) Nitrogen adsorption-desorption isotherms and (d) the pore size distribution of HPCM and 3DHPCM. High-resolution N1s spectra of (e) HPCM and (f) 3DHPCM.

The nitrogen adsorption/desorption measurements of HPCM and 3DHPCM (Figure 3c) both display the I/IV type of isotherms, indicating that there are a lot of micropores and a small amount of mesopores. Moreover, the isotherms increase significantly when the relative pressure approach to 1, demonstrating that the monoliths possess a certain amount of macropores. The 3DHPCM has a BET specific surface area of 206.38 m2 g-1 and pore volume of 0.163 cm3 g-1 with a mesopore volume contribution of 49.1%, while the BET specific surface area of HPCM is 138.58 m2 g-1 with a pore volume of 0.143 cm3 g-1 and a mesopore volume contribution of 40.6%. In the pore 11



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size distribution curves (Figure 3d and Figure S5a, b in Supporting Information), both HPCM and 3DHPCM exhibit the centrally distributed micropores under 1 nm. But in 3DHPCM, the mesoporous structure is richer than that in HPCM. Figure S6 in Supporting Information exhibits the nitrogen adsorption/desorption isotherms and the pore size distribution of carbonized monoliths tuned by different weight ratio of PAN and PMMA precursors (HPCM, 3DHPCM-1, 3DHPCM-2, and 3DHPCM), and their corresponding porosity parameters are summarized in Table S1 in Supporting Information. It is observed that the BET specific surface area and the total pore volume both increase with the decrease of the weight ratio of PAN and PMMA. For HPCM, the pores only come from the released small molecule gases during carbonization of PAN, while for 3DHPCM, the pores also come from the thermal decomposition of PMMA. PMMA not only produces more micropores in the carbon monoliths, but also aggregates and forms mesopores on the carbon walls.40 PAN is a naturally nitrogen-enriched polymer based carbon precursor. Table 1 exhibits the elemental analysis results for HPCM and 3DHPCM. The 3DHPCM presents a high nitrogen content of 15.02 wt%, while the nitrogen content of HPCM is only 10.36 wt%. This result further demonstrates that the addition of PMMA in PAN reduces the degree of graphitization during heat treatment, leaving more N atoms.

Table 1 Elemental analysis and XPS analysis of HPCM and 3DHPCM. Samples

Elemental analysis (wt%)

XPS analysis (%)

C

H

N

N-6/N

N-5/N

N-Q/N

N-X/N

HPCM

80.63

2.04

10.36

28.26

24.36

40.13

7.25

3DHPCM

75.58

1.46

15.02

34.95

20.83

35.97

8.25

The nitrogen element configurations of HPCM and 3DHPCM are further investigated by X-ray photoelectron spectroscopy (XPS). The results are shown in Figure 3e and f. The high-resolution N1s spectra of HPCM and 3DHPCM can be fitted by four peaks at around 398.3, 399.1, 401.1, and 402.1 eV, which are corresponded to pyridinic (N-6) nitrogen, pyrrolic/pyridine (N-5) nitrogen, graphitic (N-Q) nitrogen and oxidized (N-X) nitrogen, respectively. The ratio of these four peaks is shown in Table 1. Compared to HPCM, 3DHPCM possesses more amount of N-6 nitrogen, which results from the substitution of carbon atom by nitrogen in six membered rings at the edges or defects sites of the graphitic carbon layers. This kind of nitrogen is mainly devoted to providing more active sites to the Na-ion storage capacity for SIBs.41 In addition, the 12


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high-resolution C1s spectra of HPCM and 3DHPCM are shown in Figure S7, which are both fitted to four peaks: C=C bonds (284.8 eV), C-O or C=N bonds (286.1eV), C=O or C-N bonds (287.9eV), and O=C-O bonds (290.1 eV). It further confirms the successful nitrogen-doping of the prepared PAN based carbon monoliths. 38,42 To reveal the superiority of electrochemical performance of the optimal 3DHPCM, all the carbon monoliths prepared from different precursors with various weight ratio of PAN and PMMA (HPCM, 3DHPCM-1, 3DHPCM-2, and 3DHPCM) are directly used as working electrodes in half coin cell configurations. The electrochemical performances of these cells are investigated by galvanostatic charge-discharge. The cycling and rate performances are shown in Figure S8 in the Supporting Information for comparison. The optimal 3DHPCM carbonized from PAN/PMMA-8:2 foam and the HPCM carbonized from PAN foam are selected for the following deeper investigation. The cyclic voltammetry (CV) and galvanostatic charge-discharge cycling of HPCM and 3DHPCM are used to analyze the sodium ion storage behavior between 0.01 and 3 V. Figure 4a and b show the first three CV curves at a scan rate of 0.2 mV s-1. Correspondingly, Figure 4c and d show the initial galvanostatic charge-discharge profiles at a current density of 50 mA g-1. In the CV curves of the HPCM shown in Figure 4a, a peak at 0.17 V only appears in the first reduction scan, which is the main reason for the initial irreversible capacity loss, corresponding to the formation of a solid electrolyte interface (SEI) film.43 In the following cathodic scans, the reduction peaks center at 0.17 V show a small shift to about 0.6 V owing to the activation of the electrode.44 As shown in Figure S9 in Supporting Information, compared to the fresh HPCM electrode, the electrochemical impedance spectra of the HPCM electrode ran after the initial three cycles at a current density of 50 mA g-1 are almost coincident, indicating a stable SEI film forms on the carbon surfaces after the first cycle.45,46 In the lower potential region of both cathodic and anodic scan processes, a pair of sharp redox peaks between 0.01 and 0.15 V are attributed to a reversible sodium ion storage mechanism in hard carbon materials. There is a hot debate regarding to this pair of redox peaks attributed either to the filling of sodium ions into nanopores or to the intercalation of sodium ions into graphitic carbon layers of large d-spacing.47 Herein, our results are in favor of the latter (see details in the following discussions). In addition, a pair of broad humps between 0.2 and 2 V is ascribed to the adsorption of sodium ions on the accessible surfaces and defects.48 Corresponding to the CV curves, the charge-discharge profiles of HPCM shown in Figure 4c exhibit two distinct regions, including a voltage plateau region below 0.1 V related to the sharp redox peaks and a voltage sloping region above 0.1 V connected with the broad humps.

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Figure 4 CV curves of the first three cycles of (a) HPCM and (b) 3DHPCM in a voltage range of 0.01-3 V at 0.2 mV s-1. Charge- discharge profiles of the initial three cycles of (c) HPCM and (d) 3DHPCM at 50 mA g-1.

The CV curves of 3DHPCM in Figure 4b show some differences from those of the HPCM. First, the irreversible peak for SEI formation right shifts to 0.3 V compared with that of the HPCM, indicating the changed surface properties of 3DHPCM such as surface area and the different pore structure from those of HPCM.49 Second, the sharp peaks between 0.01 and 0.15 V nearly disappear, indicating that the sodium ion cannot effectively extract from the complex nanopore structures of 3DHPCM.50,51 Coinciding with the CV curves, the charge-discharge profiles of the 3DHPCM in Figure 4d show no obvious voltage plateau region. Third, a broad redox peak can be observed in a wide potential region from 0.3 to 2 V, which can be attributed to the reaction between sodium ions and active sites on the surface of carbon layer, implying that the sodium ion adsorption/desorption takes place in a wide potential range.17,38,48 In addition, the oxidation currents between 0 and 0.5 V in subsequent cycles are obviously higher than that in the first cycle, which should be ascribed to adsorption with charge transfer on both sides of single graphene layers.52 According to the above analysis results of the structure of HPCM and 3DHPCM, 3DHPCM 14


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has lower graphitization extent and more nanopores than HPCM. If the sharp redox peaks in the CV curves and the voltage plateau region in the charge-discharge profiles of the HPCM are attributed to the nanopores filling of sodium ions, there should be sharper redox peaks and longer voltage plateau region in the CV curves and the charge-discharge profiles of the 3DHPCM, respectively, while the results turn out the opposite. Therefore, the sharp redox peaks in the CV curves and the voltage plateau region in the charge-discharge profiles of the HPCM should be attributed to the intercalation of sodium ions into graphitic carbon layers. In conclusion, the sodium ion storage mechanism in 3DHPCM should be mainly ascribed to the adsorption of sodium ions on the accessible surfaces and defects. Noticeably, the absence of the voltage plateau region close to 0 V helps the 3DHPCM to avoid some safety issues. Except for the first cycle, both the CV curves and the charge-discharge profiles of HPCM and 3DHPCM exhibit good repeatability, indicating that both of them have high reversibility during cycling process. At the current density of 50 mA g-1, the HPCM delivers a reversible specific capacity of 144 mA h g-1 (Figure 4c). While the 3DHPCM exhibits a reversible specific capacity of 286 mA h g-1 (Figure 4d), which is about 2 times higher than that of the HPCM. The first reason for such a high reversible specific capacity of 3DHPCM is that the inter-connected and stable macroporous open structure ensures the infiltration of electrolyte into the whole electrode and provides an efficient transport channel for electrons. The second reason is that the abundant surface active sites from the micropores, mesopores and the nitrogen-doping facilitate the sodium ions adsorption. The initial coulombic efficiencies of 3DHPCM (62.2%) is lower than that of HPCM (68.5%). The initial capacity loss is mainly caused by the formation of SEI film. As the results of BET specific surface area measurements, TEM observations, XRD patterns, and Raman spectra shown, the 3DHPCM has larger specific surface area and more defects than those of the HPCM, which would facilitate more electrolytes to be consumed for the formation of greater irreversible SEI film.13,53 In addition, as the results of elemental analysis and XPS spectra shown, the 3DHPCM has more heteroatom-doping than that of the HPCM, which would also lead to more initial coulumbic efficiencies loss originated from irreversible insertion reactions in the vicinity of heteroatom atoms.50,54 However, due to the moderate surface areas, these values are superior to those of many other hard carbons, especially porous and nanostructured hard carbons.55 Figure 5a shows the cycling performance of HPCM and 3DHPCM at the current density of 50 mA g-1. HPCM and 3DHPCM maintain the reversible specific capacities of 132 and 281 mA h g-1 after 300 cycles, corresponding to the capacity retentions of 91.7 and 98.3%, respectively. After the initial ten cycles, the coulombic efficiency of 3DHPCM reaches and retains nearly 100%, indicating the high reversibility. Remarkably, HPCM exhibits poorer cycle stability than 3DHPCM, attributing to the closed macroporous structure and the relatively higher degree of graphitization make the infiltration of electrolyte and the transportation of sodium ions more 15



difficult in this carbon monolith.

Figure 5 (a) Cycling performances of 3DHPCM and HPCM at current density of 50 mA g-1. (b) Rate performances of 3DHPCM and HPCM. (c) Long term cycling performances of 3DHPCM and HPCM at current density of 500 mA g-1. (d) SEM image (inset is the digital photograph) of 3DHPCM after 3000 cycles. High-magnification SEM images of 3DHPCM (e) before and (f) after 3000 cycles.

As shown in Figure 5b, the 3DHPCM displays superior rate performances than the HPCM. The HPCM shows poor rate performances and exhibits only a specific capacity of 43 mA h g-1 at current density of 2 A g-1. On contrast, the 3DHPCM can reach up to the reversible specific capacities of 283, 242, 215, 183, 148, 121 and 86 mA h g-1 at current densities of 0.05, 0.1, 0.2, 16


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0.5, 1, 2, and 5 A g-1, respectively. Even if the current density increases to 10 A g-1, a reversible specific capacity of 67 mA h g-1 can retain and the specific capacity can be recovered to 280 mA h g-1 when the current returns to 0.05 A g-1. The voltage charge-discharge profiles at various current densities also verify the excellent rate capability of the 3DHPCM electrode (Figure S10, Supporting Information). The excellent rate capability of the 3DHPCM is mainly attributed to the inter-connected and stable macroporous open structure which serves as reservoirs of electrolytes and shortens the transportation distances for both sodium ions and electrons. Our results show that the surface induced capacitive process (SCP) containing the adsorption of sodium ions onto the surfaces and defects can promote the fast storage performance at high current densities.56 It is clear that the 3DHPCM displays a superior rate performance compared with many reported carbon anode materials for SIBs, such as flexible free-standing multichannel carbon nanofibers deliver 62 mA h g-1 at 5 A g-1,34 3D amorphous carbon delivers 66 mA h g-1 at 9.6 A g-1,57 and hard carbon anode delivers 65 mA h g-1 at 5 A g-1.58 For further investigating the electrochemical reaction mechanisms, CV curves of HPCM and 3DHPCM at scan rates between 0.1 to 1.6 mV s-1 are obtained and shown in Figure 6a and b. The relationship between the peak current (𝐼p) and the square root of scan rate (𝑣1/2) in Figure 6c can be used to calculate the apparent diffusion coefficient of sodium ions (𝐷Na + ) in HPCM and 3DHPCM. Based on the CV data and the following equation:59 1/2 𝐼p = 2.69 × 105𝐴𝑛3/2𝐶0𝐷1/2 Na + 𝑣

(1)

where 𝐴 is the surface area of the electrode, 𝑛 is the mole number of electrons during redox reaction, and 𝐶0 is the concentration of sodium ions. The sodium ion diffusion coefficients of HPCM and 3DHPCM at low scan rates from 0.1 to 0.7 mV s-1 are 5.20×10-8 and 7.47×10-8 cm2 s-1, respectively. And the values of HPCM and 3DHPCM at high scan rates from 1 to 1.6 mV s-1 are 6.65×10-8 and 2.02×10-7 cm2 s-1, respectively. Owing to the inter-connected structure and greater active sites, the sodium ion diffusion coefficients of 3DHPCM are higher than that of HPCM. Moreover, the difference between sodium ion diffusion coefficients of HPCM and 3DHPCM increases with the increase of scan rates, indicating faster sodium ion diffusion in 3DHPCM at high rates.

17



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Figure 6 CV curves of (a) HPCM and (b) 3DHPCM at different scan rates (from 0.1 to 1.6 mV s-1). (c) The relationship between the peak current (𝐼p) and the square root of scan rate (𝑣1/2) of HPCM and 3DHPCM. (d) The relationship between logarithm current (ln𝑖) and logarithm scan rate (ln𝑣) of reduction and oxidation peaks at high scan rates from 1 to 1.6 mV s-1. (e) Capacitive (red) and diffusion-controlled (black) contribution to charge storage of HPCM (inset) and 3DHPCM at scan rate of 1 mV s−1. (f) Normalized contribution ratio of capacitive capacities of HPCM and 3DHPCM at different scan rates.

The degree of capacitive effect can be analyzed according to the relationship between peak current (𝑖) and scan rate (𝑣) based on the CV curves and the following equation:60 𝑖 = 𝑎𝑣𝑏

(2)

where 𝑎 and 𝑏 are constants related to reaction mechanism. Based on the slopes of the ln𝑖 versus ln𝑣 plots in Figure 6d, at high scan rates from 1 to 1.6 mV s-1, the b values of HPCM are calculated to be 0.58 and 0.62 for the reduction peak and oxidation peak, respectively; And the b values of 3DHPCM are calculated to be 0.75 and 0.82 for the reduction peak and oxidation peak, respectively. The value of 𝑏 approaching 0.5 suggests a diffusion-controlled reaction, while 𝑏 close to 1 indicates a surface capacitance-dominated reaction.,56 The ratio of capacitive capacities contribution can be further quantitatively quantified by separating the current response 𝑖 at a fixed potential 𝑉 into the surface capacitive effects (𝑘1𝑣) and diffusion-controlled reactions (𝑘2𝑣1/2), according to the studies of Dunn:61 18


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𝑖 (𝑉) = 𝑘1𝑣 + 𝑘2𝑣1/2

(3)

As shown in Figure 6e, 79% of the total capacity is identified as the capacitive contribution for 3DHPCM at scan rate of 1 mV s-1, higher than 66% of the HPCM electrode. As the scan rate increases to 1.6 mV s-1, the contribution of surface capacitive further rises to 85% and 73% for 3DHPCM and HPCM, respectively (Figure 6f). The results suggest that the high surface capacitive contribution of 3DHPCM at high scan rates is beneficial for the rate performance.

Figure 7 Comparison of long-term cycling performance of 3DHPCM anode with some previously reported carbon anode materials for SIBs (the current densities in units of mA h g-1 are marked in the parentheses).

As shown in Figure 5c, both HPCM and 3DHPCM show the ultra-long cycling stability after 3000 cycles at a high current density of 500 mA g-1. In particular, the 3DHPCM remains the reversible specific capacity of 175 mA h g-1 which is as high as 90.9% of the initial reversible capacity, and still maintains a high coulombic efficiency nearly 100%. The long-term cycling performance of the 3DHPCM is compared with those of some previously reported carbon anodes 19



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for SIBs, such as graphite,62,63 graphene,64 carbon nanotubes,65 and amorphous carbons,23,34,57,66-69 as schematically shown in Figure 7, where the test current densities in units of mA h g-1 are marked in the parentheses. We can see that over a long cycling life span of 3000 cycles, the 3DHPCM delivers a high reversible capacity among these works. In addition, the capacity retention ratio of the 3DHPCM (90.9% after 3000 cycles at 500 mA g-1) is better than those of carbon materials for SIBs such as N-doping and defective nanographitic domain coupled hard carbon nanoshells (53.3% after 200 cycles at 100 mA g-1),23 elecrospun cross-linked carbon nanofiber films (82% after 500 cycles at 1000 mA g-1),35 and expanded multiwall carbon nanotubes (80% after 100 cycles at 200 mA g-1).65 Moreover, from the digital photograph of the 3DHPCM after 3000 cycles (the inset in Figure 5d), the shape and the structural integrity are well maintained, confirming excellent structural stability. From the scanning electron microscopy (SEM) images of this tested 3DHPCM (Figure 5d,f), it is obvious that the electrode still features the inter-connected macroporous open structure without any fracture and pulverization, except that the surfaces of the 3DHPCM becomes rough, which can be attributed to the formation of SEI layer. The above results demonstrate that the 3DHPCM has an excellent structural stability during long cycling. After 3000 cycles, the electrochemical impedance spectroscopy tests (Figure S11, Supporting Information) show that the charge transfer resistance of 3DHPCM is lower than that of the HPCM. 4. Conclusions In summary, this paper successfully fabricated a lightweight and free-standing nitrogen-doped 3DHPCM anode for SIBs via a brand new supercritical CO2 foaming technology and thermal treatment. The 3DHPCM anode exhibits a high reversible specific capacity (281 mA h g-1 after 300 cycles at 50 mA g-1 with a capacity retention of 98.3%), superior rate performance (67 mA h g-1 at 10 A g-1), and excellent long-term cycling stability (175 mA h g-1 after 3000 cycles at 500 mA g-1 with a capacity retention of 90.9%). The superior sodium ion storage performance of 3DHPCM is attributed to the synergistic effect of hierarchical porous structure and ion transport dynamics superiority. First, the unique inter-connected and stable macroporous open structure provides many reservoirs for electrolyte and ensures an effective short transport path for both electrons and sodium ions, even for an ultra-long cycle. Second, the uniform micropores and mesopores created during carbonization as well as the high nitrogen doping level provide lots of active sites, facilitating the sodium ion adsorption. This type of carbon monolith electrode with prominent

electrochemical

performances

overcomes

the

key

challenge

that

the

macro-meso-microporous structure of free-standing electrodes is difficult to effectively tune. Furthermore, the method provided in this work is environmentally friendly and easy to implement in practical production, which gives a new strategy to fabricate carbon monoliths from many other carbon precursors and is likely to be extended to preparing composite electrodes. 20


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Acknowledgements The authors would like to acknowledge the financial support from the Climbing Program for Taishan Scholars of Shandong Province of China (NO. 20110804) and the Research Award Fund for Shandong Province Excellent Innovation Team (No. 2012-136). Supporting Information Illustration of supercritical CO2 foaming technology and mechanism; SEM and TEM images of the foams and carbon monoliths prepared from different foaming process parameters and precursors; BET analyses and comparative electrochemical performances of the carbon monoliths prepared from different precursors; High-resolution C1s spectra of 3DHPCM and HPCM; EIS analyses of the 3DHPCM and HPCM electrodes; Voltage charge-discharge profiles of the 3DHPCM electrode at various current densities.

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