Facile Strategy to Low-Cost Synthesis of Hierarchically Porous, Active

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Functional Nanostructured Materials (including low-D carbon)

A Facile Strategy to Low-Cost Synthesis of Hierarchically Porous, Active Carbon of High Graphitization for Energy Storage Xiang Deng, Wenxiang Shi, Yijun Zhong, Wei Zhou, Meilin Liu, and Zongping Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04733 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Applied Materials & Interfaces

A Facile Strategy to Low-Cost Synthesis of Hierarchically Porous, Active Carbon of High Graphitization for Energy Storage Xiang Deng†, Wenxiang Shi†, Yijun Zhong#, Wei Zhou†, Meilin Liu*, ‡ and Zongping Shao* ,#, †

†

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical

Engineering, Nanjing Tech University, Nanjing 210009, China ‡

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA

30332-0245, USA #

Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia

Keywords: Carbon, Graphitization, Electrode, Supercapacitor, Sodium-ion Battery

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ABSTRACT: To achieve high energy/power output, long serving life, and low cost of carbon-based electrodes for energy storage, we have developed a unique synthesis method for fabrication of hierarchically porous carbon of high graphitization (HPCHG), derived from pyrolysis of an iron-containing organometallic precursor in a molten ZnCl2 at relatively low temperatures. The as-prepared HPCHG has a large specific surface area (>1200 m2 g-1), abundant micro-/meso- pores, and plenty of surface defects. When tested in a supercapacitor (SC), the HPCHG electrode delivers 248 F g-1 at 0.5 A g-1 and a high capacitance retention of 52.4% (130 F g-1) at 50 A g-1. When tested in a sodium-ion battery (SIB), the HPCHG electrode exhibits a reversible capacity of 322 mA h g-1 at 100 mA g-1 while maintaining ~75% of the initial stable capacity after 2,000 cycles with the applied current density as high as 5000 mA g-1, implying that the HPCHG electrode is very promising for energy storage.


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Introduction The rapid development of personal electronic devices, hybrid/pure electric vehicles, as well as renewable energy utilization has stimulated a large demand for advanced energy storage systems which have the features of low cost, good safety, long life, high energy and/or power density. To date, several types of electrochemical devices have been extensively investigated, including lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and supercapacitors (SCs).1-4 In particular, SIBs have attracted much attention in the past few years5-7 and are considered an ideal alternative to LIBs for large-scale, stationary energy storage.8 On the other hand, SCs are considered to be important part in clean energy technologies because they fill the gap in energy and power density between conventional capacitors and batteries. The combination of SCs and SIBs may provide an ideal solution to the energy storage for smart grids that allows the direct input of intermittent renewable energies, which significantly contribute towards a sustainable future.5, 9, 10

Electrodes are the key components of these electrochemical storage devices and largely determine their performance. Thus, broad commercialization of SIBs and SCs requires the successful development of high-performance electrode materials with low cost. Typically, Carbon plays a critical role in electrochemical energy storage devices due to its abundance, low cost, as well as good stability. Actually, graphite represents the most widely-used anode materials in commercial LIBs,11 and activated carbon is the main electrode material in commercial SCs.12 Additionally, hard carbon is found to be highly promising for SIBs.13 When carbon is used as an anode for an 3 ACS Paragon Plus Environment


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electrochemical double layer capacitor (EDLC), high specific surface area is required since energy storage is realized through surface ion adsorption. Therefore, activated carbon with high specific surface area (SSA) is typically used as the electrode in SCs.14-16 Under optimal conditions, a volumetric capacitance of ~60 F cm-3 and a gravimetric capacitance of ~200 F g-1 have been reported for activated carbon materials.17 A significant challenge for the development of activated carbon materials for SCs is that an increase in the SSA is usually accompanied by a decrease in graphitization, resulting in poorer conductivity and thus lower rate capability. On the other hand, Na+ (0.95 Å) has a larger cation size compared with Li+ (0.68 Å), if the classic intercalation-detercalation mechanism is applied on the electrode, theoretical calculation shows the average interlayer spacing in carbon should be larger than 0.37 nm, allowing easy intercalation of Na+ for energy storage.18,

19

A complicated

synthesis process is needed to create such expanded hard carbon materials, which could be expensive and energy intensive. In addition, due to the large cation size of Na+, the diffusion of Na+ in carbon interlayers is difficult; resulting poor cycling rates.13, 20, 21 Therefore, the researchers are intensively focusing on the development of novel electrode materials with new Na+ storage mechanisms.

The facile synthesis of nanostructured carbon materials with well-tailored properties is the key to their practical use in both SCs and SIBs. Furthermore, a new energy storage mechanism is important to overcome the poor rate capability of intercalation-type hard carbon in SIBs. In addition to the intercalation of Na+ in the expanded interlayers of graphitic domains in carbon, it was reported that Na+ can also 4 ACS Paragon Plus Environment

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be stored by absorption at surface defects, edges, and nanovoids and by reaction with surface groups attached to carbon, thus creating capacitive charge storage (CCS).22-24 Such a CCS mechanism may provide a new alternative method to overcome the problems associated with low sodium-ion diffusion and serious structural damage during the Na+ intercalation/de-intercalation cycles.

Herein, we report a facile strategy to low-cost synthesis of hierarchically porous, active carbon of high graphitization (HPCHG), which demonstrated superior performance as an anode in both SCs and SIBs. An iron-containing organometallic coordination compound was used as the carbon precursor, and the iron in the molecular framework also functioned as a catalyst to promote the carbonization at low temperatures. In combination with molten salt activation, a low pyrolysis temperature of 600 °C was required, and the as-obtained carbon material showed high specific area (> 1200 m2 g-1), increased density of surface defects, yet high graphitization. The electrochemical measurements show that a high gravimetric capacitance of 250 F g-1 was reached as the EDLC-type electrode for SCs. Besides, when used as an anode in battery application, a capacitance as high as 322 mAh g-1 was attained at 0.1 A g-1, suggesting that it is a very promising carbon-based electrode for SIBs. In addition, a reversible capacity of 101 mAh g-1 was retained after 2000 cycles at 5 A g-1, which is among the highest reports for carbon electrodes in SIBs.

Results and Discussion



Figure 1 Schematic illustration of the preparation method for the hierarchically porous carbon of high graphitization.

The strategy for facile synthesis of hierarchically porous carbon of high graphitization (HPCHG) is schematically presented in Figure 1. The carbon precursor, Iron (III) acetylacetonate (Fe(acac)3), is an organometallic coordination compound with unique structural features: every Fe atom in the structure is coordinated by three acetylacetone ligands. As reported previously, Fe is a favourable catalyst for carbonization and graphitization of organic substances.25-29 Accordingly, the iron in the Fe(acac)3 structure acted as a catalyst to promote the carbonization of the organic ligands and graphitization of the resulting carbon at relatively low temperatures. On the other hand, molten zinc chloride (ZnCl2) played several roles during the process. First, it acted as a heating media of large thermal capacity (typical heat capacity of ZnCl2 is 80.79 J/mole at 600 oC)30 to provide homogeneous heating of the carbon precursor to the desired temperature for the endothermic carbonization reaction. Second, the molten salt acted as a soft template to introduce pores to the in situ formed carbon, as confirmed in our previous studies.31-33 Third, the molten salt acted as a corrosive medium to create surface defects over the as-formed carbon material. 6 ACS Paragon Plus Environment

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Finally, the molten salt also functioned as an inhibitor to suppress aggregation of carbon during calcination. Thus, the combined catalytic effect of Fe ions and the other effects of the molten salt resulted in hierarchically porous carbon of high graphitization. The typical yield of the porous carbon material is ~23 wt% (after washing away all the metal residues) calculated by the total mass of the raw material iron (III) acetylacetonate. It will be ~39 wt% if we calculate the yield relying on only the total carbon atoms in the synthesis process. Compared to the typical carbon yield of 28-40% as reported in the literatures34-36, our yields are sufficient for the large-scale production of hierarchical porous carbon materials.

Figure 2 a-c) Field-emission Scanning Electron Microscopy (FE-SEM) image of HPCHG sample with different magnifications. d, e) Transmission Electron Microscopy (TEM) image and f) Nitrogen adsorption-desorption isotherm plots of HPCHG sample, the inset is the related BJH pore size distribution (PSD) plot.



The morphologies of the as-synthesized carbon samples were first examined under SEM in Figure 2a-c. The HPCHG sample is made up of loosely packed particles with diameters ranged from 0.3 to 1.0 µm. A closer look at these fine particles reveals that they were mainly constructed from randomly cross-linked carbon frameworks with abundant nanopores. Such a morphological structure resembles a foam-like porous carbon structure. The corresponding transmission electron microscopy (TEM) images in Figure 2d presents that the foam-like carbon was constructed from carbon filaments with diameters of 3-8 nm, which were severely entangled to create plenty of nanopores. Lattice fringes in the curved carbon are clearly seen in the high-resolution TEM (HR-TEM) image shown in Figure 2e, suggesting the existence of graphitic carbon domains in the as-synthesized products. In contrast, the graphitized carbon prepared without ZnCl2 activation (SEM image shown in Figure S1) is composed of thick and curved carbon rods with a diameter of several hundred nanometres. The hierarchically porous carbon (HPC) sample prepared using zinc(II) acetylacetonate as the carbon precursor in combination with molten ZnCl2 activation shows a morphology similar to that of HPCHG (Figure S2); however, the degree of graphitization was much lower; no obvious lattice fringes are found in the HR-TEM image of the HPC sample (Figure S3), which suggests that the nanoporous structure of the carbon is attributed to the templating effect of the molten salt whereas the high degree of graphitization is due to the catalytic effect of the Fe ions.


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Next, the microstructural features of the various samples were investigated by nitrogen adsorption/desorption isotherms shown in Figure 2f and Figure S4. The high intercept of HPCHG at P/Po is close to 0, which suggests the presence of abundant micropores in its structure, and the high rise at high P/Po indicates the presence of rich meso-or macro-pores. The specific surface area for the HPCHG sample, calculated from the isotherms is ~1134 m2 g-1. According to the BJH plot in the inset of Figure 2f, the total pore volume (VTotal) of the sample is ~1.99 cm3 g-1 and the average pore diameter (DAverage) is about 7.0 nm. These structural features allow easy penetration of the liquid electrolyte for fast interface reaction when used as an electrode for SC and SIB. Interestingly, the HPC sample shows an even higher specific surface area of 1430 m2 g-1 compared to HPCHG sample (Figure S4a), providing an indirect support that iron is a good catalyst for graphitization of carbon. Instead, the carbon of high graphitization (CHG) shows a much lower specific surface area (377 m2 g-1), implying the importance of molten salt activation to increasing the specific surface area. Both soft templating and surface corrosion effects from the molten salt likely contributed to the improved specific surface area of the HPCHG and HPC samples compared with the CHG.31, 37-39



Figure 3 a) X-ray diffraction (XRD) result of HPCHG sample, the inset is the TEM picture showing the selected area electron diffraction (SAED) pattern. b) Raman spectra of HPCHG sample. c) X-ray photoelectron spectroscopy (XPS) and d) High-resolution of the C1s spectrum of HPCHG.

The phase structure of the as-prepared HPCHG sample was investigated by X-ray diffraction (XRD) characterization. The XRD pattern of HPCHG in Figure 3 demonstrated two broad peaks at 2-theta of approximately 25.3° and 43.4°, which can be assigned to the (002) and (100) crystallographic planes in a disordered carbon structure, indicating the formation of a pure carbon material. No trace of iron residues was detected, which suggests the successful elimination of the iron catalyst after the acid wash. The corresponding selected area electron diffraction (SAED) pattern also 10 ACS Paragon Plus Environment

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provides the concentric rings of the (002) and (100) lattice planes. To understand the carbonization mechanism during the molten-salt assisted calcination process, XRD patterns of HPCHG, CHG and HPC before and after the acid wash process are comparatively studied with the results presented in Figure S5. Before the acid wash, the HPCHG sample demonstrated a mixture of crystalline phases of ZnFe2O4 (PDF 089-7412) and ZnO (PDF 079-0207). The HPC sample showed mainly the crystalline phase of ZnO, whereas the CHG sample demonstrated the main compositions of metallic Fe (PDF 065-4899) and Fe3C (PDF 034-0001). Before acid washing, the oxygen in ZnO/ZnFe2O4 in the HPCHG and HPC samples originate from the acetylacetonate ligands since only the acetylacetonate ligands contain oxygen atoms. The taking up of oxygen from the acac ligands with the formation of Fe2O3 and ZnO then promoted the formation of graphitic carbon material.39-41 In HPCHG, the as-produced Fe2O3 further reacted with ZnO to form a more stable ZnFe2O4 spinel phase. Thus, mixed phases of ZnFe2O4 and ZnO were observed in Figure S5a.42, 43 Interestingly, the iron was further converted to metallic iron and Fe3C in the CHG sample (Figure S5e), which implies the critical role of molten ZnCl2 in the formation of the oxide phases. It is likely that without the application of molten ZnCl2, the oxygen-contained gaseous intermediates from the thermal decomposition of organic ligands quickly escaped to the surrounding atmosphere, whereas by applying molten ZnCl2, the oxygen was well confined in the molten salt media and immediately reacted with surrounding ZnCl2 to form ZnO.39 As a result, different compositions of iron-based products were observed in the HPCHG, HPC and CHG samples in Figure 11 ACS Paragon Plus Environment


S5a, c, and e. The XRD patterns of the HPCHG, HPC and CHG samples after acid washing exhibit a position shift of the (002) diffraction plane. The average interlayer distance of HPCHG (0.354 nm) is larger than the graphite-like CHG sample (daverage=0.337 nm).

As demonstrated by HR-TEM in Figure 2e, the graphitic domain already appeared in the HPCHG sample even at a low pyrolysis temperature of 600 °C. The graphitization degree of the related samples was further characterized by Raman spectroscopy with the results shown in Figure 3b and Figure S6. For comparison, the commercial graphite and activated carbon (XFP01, XF NANO) with surface areas of 1800 m2 g-1 were also measured. All three samples showed two characteristic peaks at ~1340 and ~1580 cm-1, which are typically recognized as the D and G band.44, 45 To be specific, the D band originated from defected and disordered carbon, whereas the G band is related to graphitic part inside the carbon. The graphitization of the sample can be revealed by the intensity ratio value of the D and G peak (ID/IG), which was obtained from the Raman spectra by applying the Lorentz fitting method.33, 45, 46 For the graphite material, an ID/IG value of 0.16 was demonstrated, which is in good agreement with the numbers reported in the literatures,47, 48 whereas the value for the activated carbon was found to be 1.26. ID/IG values of 0.88, 0.73 and 1.02 were calculated from the Raman spectra of the HPCHG, CHG and HPC samples, respectively. The above results clearly suggest a high graphitization of HPCHG and CHG samples despite a low pyrolysis temperature of 600 °C, which is even lower than previous reports for iron catalysts (700-850 °C).26, 27, 29, 49 We believe that a 12 ACS Paragon Plus Environment

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better catalytic effect was provided by the atom level dispersion of Fe in the carbon precursor (iron(III) acetylacetonate) to result in a more efficient carbonization process. The smaller ID/IG value of CHG compared with HPCHG is an indicator of increased surface defects from molten ZnCl2 etching, whereas the larger ID/IG value of the HPC sample compared with the HPCHG sample implies that Fe is a much better atom catalyst than Zn for the graphitization of carbon.

The surface functional group of carbon can contribute to capacitive energy storage; however, its stability is a large concern. X-ray photoelectron spectroscopy (XPS) was also carried out to reveal the surface composition and chemical bond of the HPCHG sample in Figure 3c. The results confirm that HPCHG is a pure carbon material without any metal residues. In addition, the sample demonstrated low surface oxygen content of only 5.4 wt%, which is lower than most of the hard carbon materials that are synthesized at < 1000 °C and suggests that good carbonization is attributed to Fe atom catalysis. The corresponding high-resolution spectrum of C1s can be deconvolved into four peaks, as shown in Figure 3d. The dominant peak is centred at 284.6 eV, which is the characteristic C-C group of carbon-based materials.50 The other three peaks on 285.4, 286.8, and 289.1 eV, were attributed to the oxygen-containing functional groups C-O, C=O and O-C=O, respectively.51 Figure S7 shows the deconvolution result of O 1s, yielding three peaks according to the related literature52-53: the C-O peak located at ~533.4 eV, the C=O peak at ~531.2 eV, and the O-C=O peak at ~535.3 eV. The three peaks fit well with the results showing in the C1s and further support the surface groups in HPCHG sample are 13 ACS Paragon Plus Environment


mainly in the form of C-O, C=O, and O-C=O. However, it is also noted that the oxygen content is relatively low (5.4 wt%), suggesting the contribution of energy storage from the surface reaction with oxygen-containing functional groups in HPCHG should be limited.

Figure 4 a) Cyclic Voltammetry (CV) plot of HPCHG at 5-200 mV s-1. b) CV plots of different samples at 100 mV s-1. c) Galvanostatic charge/discharge curves of HPCHG at different current densities. d) Galvanostatic charge/discharge curves of different samples at a high current density of 30 A g-1. The inset picture reveals the current-resistance (I-R) drop during the charge/discharge cycles. e) Rate performances of different carbon samples at various current densities (0.5-50 A g-1).


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f) Long-term test of the HPCHG electrode for 10,000 cycles at a current density of 8 A g-1.

According to above analysis, a unique hierarchical porous foam-like carbon with good graphitization and a low content of surface oxygen-containing functional groups was successfully synthesized based on a facile strategy to low-cost synthesis strategy. The performance of as-synthesized HPCHG for electrochemical energy storage was first investigated as an EDLC-based anode in SC using a three-electrode system with an aqueous electrolyte of 6 M KOH. For comparison, both HPC and CHG were evaluated. Figure 4a presents cyclic voltammetry (CV) curves of the HPCHG electrode at scan rates that range from 5 to 200 mV s-1. The quasi-rectangular shape of all the CV curves and stable high current density response indicate excellent electrochemical behaviour and high stability of its unique porous structure, surface defects and surface functional groups. In Figure 4b, HPCHG had the largest I-V response area among the three electrodes of HPCHG, HPC and CHG in their CVs at a scan rate of 100 mV s-1. From Figure 4c, the quasi-triangular shape of the curves with distinct distortion suggests that, in addition to the main capacitance from EDLC, a certain amount of pseudocapacitance also appeared, which could be ascribed to surface reactions that involve functional O-containing groups.54,

55

According to

Figure 4d, the HPCHG electrode demonstrated the largest charge storage capacity and the smallest IR drops among the three electrodes at 30 A g-1. Here, the specific capacitances in SCs were calculated by the following equation:

(1) 15 ACS Paragon Plus Environment


As a note, C means the specific capacitance, I represents the galvanostatic discharge current, m is the mass of the active material, t is the time for the discharging process, and V represents the operation potential (i.e., 1.0 V). The rate-dependent specific capacitances of the various electrodes are shown in Figure 4e. The specific capacitance of HPCHG (248 F g-1) is much larger compared to CHG (63.3 F g-1) at a current density of 0.5 A g-1. The low capacitance of CHG should be related to the low specific surface area. The HPCHG electrode also demonstrated a high capacitance retention of 52.4% (130 F g-1) at a current density of 50 A g-1, which indicates an efficient charge transfer within the electrode, even under conditions of large current density. In comparison, HPC had a comparable specific capacity of 234 F g-1 at 0.5 A g-1. That is because the HPC sample actually has a higher specific surface area (1430 m2 g-1, Supporting Information Figure S4a) than the HPCHG sample (1134 m2 g-1), implying that the HPC electrode has more apparent surface area for charge storage due to the electrical double layer contributions. In addition, the HPC sample has more surface functional groups (as revealed by the lower d Average and larger ID/IG value), thus showing additional amount of pseudocapacitance bonus. However, when tested at high current densities, the electronic conductivity of the electrode become important and the HPCHG sample with higher graphitization shows better performance as supported by the results in Figure 4e.

Volumetric capacitance is also very important to consider the possible application of supercapacitors. The volumetric capacitance (F/cm3) for the samples at a current density of 0.5 A g-1 for HPCHG, HPC, and CHG is calculated to be 99.2, 93.6, and 16 ACS Paragon Plus Environment

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25.4 F cm-3, respectively. Compared to the volumetric capacitances previously reported56,57 for carbon electrodes in supercapacitors, HPCHG shows promising gravimetric and volumetric capacitances, as summarized in Table S1 of the supporting information.

Additionally, outstanding long-term stability was achieved for HPCHG (Figure 4f) with a specific capacitance of 157.3 F g-1 after 10,000 cycles at 8 A g-1. The electrode delivered almost no capacity decay (approximately 2.4% compared with the initial cycle) and a Coulombic efficiency of approximately 100% through 10,000 cycles, which shows highly stable cycling performance.

Figure 5 a) A series of cyclic voltammogram (CV) curves for HPCHG sample with different scan rates (from 0.1 mV s-1 to 10 mV s-1) after the initial activation cycles. b) Linear relationships prediction between log ν and log i of cathodic peaks at 0.01V. c) The plot of normalized capacities (Q) against ν-1/2 for HPCHG.

The HPCHG sample was further tested for its performance as an anode in SIB. A cyclic voltammogram (CV) of the sodium-ion coin cell with a HPCHG anode was first investigated, and the results are presented as an inset in Figure S8. Two irreversible peaks at 0.95 and 0.24 V appeared in the CV curves from the initial 17 ACS Paragon Plus Environment


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cathodic process, which is related to the decomposition of the PC-based electrolyte on the active surfaces with the formation of a stable solid-electrolyte interphase (SEI) and irreversible sodium insertion into the active nanopores.58-60 Figure S8 shows discharge capacities of 1440 and 374 mA h g-1 for the 1st and 2nd cycle, respectively. Since, as previously demonstrated, the average interlayer spacing of the HPCHG is approximately 0.354 nm, and a previous report shows that a spacing of > 0.370 nm is required for the successful intercalation of Na+ into the graphite interlayers,19 so the capacitive energy storage maintains a critical role for the Na+ storage in HPCHG anodes. Further investigation on the series of CV curves shown in Figure 5a provides information

regarding

the

Na+

storage

mechanism

(by

Faradaic

sodium

insertion/extraction or by capacitive sodium storage), which is related to the following equation:24, 61, 62

i = a‫ ݒ‬௕

(2)

In this equation, a and b represent the kinetic-related coefficient values. To be specific, the linear correlation of i and ν (b=1) implies an ideal capacitive process, and a proportional correlation between i and ν0.5 (b=0.5) indicates a diffusion-dominant process and can be assigned to Faradaic Na+ insertion/extraction. In Figure 5b, the b value of the HPCHG sample is calculated to be 0.81 at 0.01 V, which confirms that the capacitive sodium storage makes the biggest contributions on the whole capacity. In addition, the contribution source of the total charge (Q) from CV results can be further considered to be two parts:24, 63


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Q = ܳௗ + ܳ௦

(3)

Qd is the charge resulting from diffusion-dominant process and Qs is the charge from the surface capacitive storage. If the semi-infinite linear diffusion model is applied, the relationship between ν and Qd is described in the following equation:

Q = c‫ି ݒ‬ଵ/ଶ + ܳ௦

(4)

As a note, c represents a constant related to diffusion coefficient. As a result, the capacitive contribution can be estimated by extrapolating the relationship between the normalized capacities (Q) against v-1/2. From Figure 5c, the extrapolated y-intercept can be calculated as ~52.2%, which shows the critical contribution of capacitive storage to the HPCHG sample.24



Figure 6 a) Galvanostatic charge/discharge profiles of HPCHG sample at various current densities. b) Rate performance of HPCHG, HPC, CHG anodes at different current densities. c) Cycling performance with the corresponding Coulombic efficiency of the as-synthesized samples. d) Nyquist plots of HPCHG electrode obtained after initial five activation cycles at 100 mA g-1, which were collected by discharging the cell at 100 mA g-1 to a pre-determined cell potential (e.g., 2.0, 1.5, 1.0, 0.5, 0.01 V) and then keeping at this potential for at least 10 minutes. The inset picture is the relationship between Z' and W-1/2 in the low frequency region. e) Long-term cell performance of HPCHG tested at a high current density of 5000 mA g-1.


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The charge/discharge curves of the HPCHG anode at various current densities and the corresponding rate performances were further studied with the results shown in Figure 6a, b. The electrode delivered favourable specific capacities of 322, 207, 171, 156, 150, 131 and 118 mA h g-1 with the constant current densities of 100, 200, 500, 800, 1000, 2000 and 3000 mA g-1, respectively. Even at a very large current density of 5000 mA g-1, a specific capacity of 101 mA h g-1 was still achieved, which is higher than that of most of the previous carbonaceous anodes for SIBs that were discharged at similar discharge current density (Table S2). The advantageous feature of capacitive Na+ storage was further demonstrated. It is well known that the capacitive process contains a double layer capacitance with fast ion diffusion and superior structural stability. And the surface-induced Na+ storage can promote fast charge/discharge abilities by facilitating the adsorption of Na+ onto nanovoids as well as surface defects/functional groups. On the other hand, the controlled HPC sample had similar specific capacities at low rates but with a serious capacity decay at higher charge-discharge current densities, which proves a critical effect in graphitization degree on the rate performance, and shows the instability of excess oxygen-containing functional groups during the Na+ absorption/desorption. Moreover, the CHG sample exhibited a lower sodium storage capability, which could be due to the lack of sufficient active sites that result from lower surface areas. In Figure 6c, after full activation by the initial ten cycles, the HPCHG electrode demonstrated a stable specific capacity of 170 mA h g-1 at 500 mA g-1 during the following 270 cycles. However, for the HPC sample, the capacity started to fade after approximately 70 21 ACS Paragon Plus Environment


cycles, which possibly resulted from the unstable surface reaction with rich oxygen-containing surface groups during repeated Na+ storage and release.

Figure 6d displays the EIS plots of HPCHG which were obtained at various discharging potential. The charge transfer resistance as indicated by the diameter of the impedance loop in the plot, decreases at lower voltages. The decrease in charge transfer resistance suggests that improved sodium-ion transfer kinetics was achieved by adsorption of Na+ onto the active surfaces of the HPCHG electrode. The sodium-ion diffusion coefficient of the electrode is also calculated based on the EIS results according to the equations listed in the supporting information, and the calculated Warburg factor (σ) and sodium-ion diffusion coefficient (DNa) at different stages of discharge are listed in Table S3. The results show that the Na+ diffusion coefficient of HPCHG electrode attains 3.45×10-10 cm2 s-1 at a fully discharged state of 0.01 V, which is higher than other newly-synthesized carbon materials reported in the literatures (Table S4) and further supports the superior rate performance presented in Figure 6b. Moreover, the long-term cycling performance provided in Figure 6e at a high current density of 5000 mA g-1 indicates that HPCHG is highly reversible. The capacity remains to be 98.6 mA h g-1 even after 2,000 cycles with almost 100% Coulombic efficiency of every cycle after the initial several cycles of activation. These data further confirm that HPCHG has efficient Na+ diffusion kinetics, robust frameworks, and high Na+ storage capacity, which is promising as an electrode for fast energy storage and conversion.


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Conclusions In conclusion, hierarchical porous carbon of high graphitization was successfully fabricated at a low pyrolysis temperature of 600 °C by taking the advantages of an Fe-containing carbon precursor in a molten salt-assisted carbonization process. The obtained HPCHG material exhibited promising electrochemical performance when used as the electrode in a SC and a rechargeable SIB, provided by the effective pathways for rapid ion and electron transport from the well-tailored microstructure. When tested in a supercapacitor, the HPCHG displayed a specific capacitance of 248 F g-1 at 0.5 A g-1 and minimal capacity decay after 10,000 cycles at 8 A g-1. When tested in a SIB, the HPCHG electrode showed excellent rate performance with a reversible capacity of 322 mA h g-1 at 100 mA g-1 and a superior capacity of 101 mA h g-1 even at 5000 mA g-1. In addition, it displayed a stable cycling performance for 2,000 cycles, suggesting that HPCHG has potential to be one of the most promising electrode materials for high-performance energy storage devices.



ASSOCIATED CONTENT

Supporting Information

The experimental method, additional material characterization results, additional TEM, XPS, SEM, BET, and XRD results of as-synthesized materials, electrode fabrication and electrochemical test methods, cyclic voltammetry, cycling stability and Galvanostatic charge/discharge plot of HPCHG sample, tables listing the comparison of HPCHG material with the performances of the other recently reported anodes, detailed sodium-ion diffusion coefficient calculation method. These materials are available for free on the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Zongping Shao). *E-mail: [email protected] (Meilin Liu).

ACKNOWLEDGMENT The work was supported by the Six Talent Peaks Project of Jiangsu Province under contract No. XNY-CXTD-001, the National Nature Science Foundation of China under contract No. 21576135, the Priority Academic Program Development of Jiangsu Higher Education Institutions, Jiangsu Natural Science Foundation for


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Distinguished Young Scholars under contract No. BK20170043, Xiang Deng acknowledges the international learning funding from Nanjing Tech University.

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Facile Strategy to Low-Cost Synthesis of Hierarchically Porous, Active

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