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Pore structure regulation of hard carbon: Towards fast and high‐capacity sodium‐ion storage
Journal of Colloid and Interface Science 566 (2020) 257–264
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Pore structure regulation of hard carbon: Towards fast and high-capacity sodium-ion storage Le Yang a,b, Mingxiang Hu b, Hongwei Zhang b, Wen Yang a,⇑, Ruitao Lv b,c,⇑ a Key Laboratory of Cluster Science of Ministry of Education Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China b State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China c Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Nanopore regulation strategy can
improve the sodium storage capacity of hard carbon. The relationship between nanopore and sodium storage capability was investigated. + Micropores hardly accommodate Na + ions and hinder Na ion diffusion. + Mesopores facilitate Na ion intercalation and shorten the ion diffusion pathway.
a r t i c l e
i n f o
Article history: Received 15 November 2019 Revised 9 January 2020 Accepted 22 January 2020 Available online 23 January 2020 Keywords: Micropore Mesopore Pore-structure evolution Hard carbon Anode Sodium-ion battery
a b s t r a c t Hard carbon is regarded as one of the most promising anode material for sodium-ion batteries in virtue of the low cost and stable framework. However, the correlation between pore structures of hard carbon and sodium-ion storage is still ambiguous. In this work, based on precise control of pore-size distribution, the capacity, ion diffusion, and initial Coulombic efficiency were improved. Meanwhile, the relationship between pore structure and capacity was investigated. Our result indicates that the micropores hinder ion diffusion and hardly ever accommodate Na+ ions, while mesopores facilitate Na+ ion intercalation. Hard carbon with negligible micropores and abundant mesopores delivers a maximum capacity of 283.7 mAh g1 at 20 mA g1, which is 83% higher than that of micropore-rich one. Even after 320 cycles at 200 mA g1, the capacity still remains 189.4 mAh g1. Ó 2020 Elsevier Inc. All rights reserved.
⇑ Corresponding authors at: State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China (R. Lv). E-mail addresses: [email protected] (W. Yang), [email protected] (R. Lv). https://doi.org/10.1016/j.jcis.2020.01.085 0021-9797/Ó 2020 Elsevier Inc. All rights reserved.
1. Introduction Sodium, with high terrestrial abundance and low electrode potential, contributes to the low cost and high energy density for
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sodium-ion batteries (SIBs), which were regarded as one of the most promising candidates for large-scale energy storage [1,2]. Unfortunately, commercially utilized graphite anode in LIBs is failed for sodium storage owing to the larger diameter of sodium ion compared with lithium-ion (1.02 Å vs. 0.76 Å) as well as thermodynamic incompatibility between graphite layers and sodium ions [3,4]. It is crucial to develop a high-performance anode materials to realize reversible Na+ ion intercalation/extraction. So far, various materials have been explored as the anode of SIBs [3,5– 15]. Among them, hard carbon constituted by disordered stacking pseudographitic domains, defects, and amorphous carbon is suitable to reversibly accommodate Na+ ions with long lifespan and low electrode potential [13,15–17]. Hard carbon derived from biomass could not only realize a low cost but also relieve environment pollution. Moreover, the biomass materials containing high content of cellulose are beneficial to a high yield after carbonization [18,19]. Different strategies including heteroatom doping (e.g., nitrogen and sulfur) [20–24], morphology design [19,25–32] have been developed to improve the electrochemical properties of hard carbon. Although the pore-size distribution was considered to be related to sodium-ion storage and diffusion in hard carbon [33,34]. However, as far as we have known that the porestructure control of hard carbon was seldom focused, and the detail effect of micropores on Na+ ion storage is still controversial. For example, a traditional viewpoint proposed that the lowpotential (below 0.1 V) capacity can be ascribed to micropore adsorption, which is similar to Li+ storage in hard carbon [19,28,35–37]. In contrast, others demonstrated that the Na+ ion intercalation contribute to the low-potential capacity [14,38–41]. Therefore, it is important to clarify the relationship between pore structure evolution and sodium-ion storage to fully understand the storage mechanism in hard carbon. It should be noted that excluding interference from other structural factors (such as interlayer spacing, domain size, degree of graphitization, and heteroatom) is necessary to realize the goal. In the present work, carbonized walnut shell (CWS) was treated by hexadecyl trimethyl ammonium bromide (CTAB) and/or KOH under hydrothermal condition (as illustrated in Fig. 1a). The pore-size distribution of samples was adjusted independently. After rational control of pore structure, both specific capacity and rate capability are improved without sacrificing initial Coulombic efficiency (ICE), voltage plateau, and cycling stability. We found that hard carbon with negligible micropores and abundant mesopores delivers an excellent cycling capacity of 283.7 mAh g1 at 20 mA g1 with ICE of 64%, which is obviously higher than micropore-rich hard carbon.
tered and washed by deionized water and alcohol several times, then dried at 80 °C overnight. For comparison, just substituting the hydrothermal solution to 1 mol L1 KOH, 10 mg ml1 CTAB or deionized water, CWS-K, CWS-C, and CWS-D were fabricated, respectively. 2.2. Sample characterizations As-synthesized samples were characterized by X-ray diffractometer (XRD, D/Max-2500/PC, Rigaku Corporation, Japan), with radiation sources of Cu Ka. To investigate the morphology and structure of hard carbon, scanning electron microscopy (SEM, JMS-7001-f type, JEOL, Japan) and transmission electron microscopy (TEM, JEM-2010, JEOL, Japan) were used. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250 xi photoelectron spectrometer (Thermo Fisher Scientific, America) using monochromatic Al Ka as the incident X-rays. Raman spectra were recorded with a LabRAM HR Evolution type (HORIBA, Jobin Yvon, Japan) equipped with a 632.5 nm He-Ne laser. To characterize the specific surface areas (SSA) and porous texture, nitrogen adsorption and desorption isotherms were performed on a Belshop-mini type II analyzer (MicrotracBEL, Japan) at 77 K. The SSA was calculated by Brunauer-Emmett-Teller (BET) method, and the pore size distribution was analyzed via a density functional theory (DFT) method. Micropore volume was calculated by SaitoFoley (SF) method, mesopore volume by total pore volume from DFT method subtracting micropore volume. 2.3. Electrochemical tests 75 wt% active materials, 15 wt% polyvinylidene fluoride (PVDF) and 10 wt% carbon black (Super-P) were mixed in N-methyl pyrrolidinone (NMP) solution under vigorous stirring until a uniform slurry is obtained. The slurries were then coated onto a copper foil and then dried at 120 °C in vacuum overnight to remove the solvent. The loading mass of active materials is ~1 mg cm2 on one work electrode. 2032-type coin cells were assembled in the argon-filled glove box with glass microfiber filters (Whatman GF/ D) as the separator, sodium metal sheet as the counter electrode and 1 M NaClO4 in ethylene carbonate/dimethyl carbonate (EC/ DMC, 1:1 v/v) as the electrolyte. LAND CT2001A type battery test system was used for galvanostatic charge-discharge (GCD) tests, with the voltage range of 0.005–3 V. Cyclic voltammetry (CV) measurements were performed on a VSP-300 type electrochemical workstation (Bio-logic Inc.) at a scan rate of 0.1 mV s1. Impedance measurements were also carried out on the electrochemical workstation with a scan frequency from 200 kHz to 100 mHz.
2. Experimental
2.4. Ex-situ XRD characterization
2.1. Material synthesis
After discharging/charging for 10 cycles, the batteries were disassembled and the electrodes were washed by DMC and dried overnight. After being peeled off from copper foil carefully, the electrode materials were used for ex-situ XRD measurements. All processes above were manipulated under Ar protection. For exsitu XRD tests, we used electrodes composed by 95 wt% active materials and 5 wt% PVDF (without Super-P) to minimize the impurity peaks in XRD patterns.
CWS was prepared by the direct carbonization of the walnut shell. The walnut shells were smashed and washed by deionized water, then dried overnight at 80 °C in vacuum. The clean walnut shells were carbonized in the tubular furnace at 1100 °C for 6 h (the heating rate is 10 °C/min) under argon (Ar) flow protection. The as-prepared black powders were washed by deionized water and alcohol several times and then dried at 80 °C overnight. The fabrication process of CWS-CK is described as follows: typically, the as-prepared CWS powders were added into a Teflonlined stainless steel autoclave containing an aqueous solution with 1 mol L1 KOH and 10 mg ml1 CTAB. The autoclaves were sealed and heated to 180 °C for 12 h. The obtained black powders were fil-
3. Results and discussion Illustration of the synthesis procedure is shown in Fig. 1a. Hydrothermal treatment was conducted to change the pore structure of hard carbon. After treatment, the micropores were gradually eliminated, while mesopores were introduced. The addition
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Fig. 1. (a) Synthesis illustration of the walnut-shell-derived hard carbon. Through the hydrothermal treatment, the decrease of micropores and increase of mesopores changes the Na+ ion diffusion behavior. (b) Scanning electron microscope (SEM) image of CWS-CK, high-resolution transmission electron microscopy (HRTEM) images of (c) CWS and (d) CWS-CK. Here, CWS denotes porous hard carbon sample obtained by direct carbonization of walnut shells at 1100 °C, CWS-CK denotes sample obtained by further hydrothermal treatment for CWS under the presence of both hexadecyl trimethyl ammonium bromide (CTAB) and KOH.
of KOH and CTAB during hydrothermal treatment induced such variation on pore size distribution. Here, KOH was used to produce more mesopores due to its etching effect. CTAB, as a common surfactant, could facilitate the dissolution of some impurities and amorphous carbon, which consequently leads to the collapse of microstructure and the decrease of micropores. Based on such variation, the relationship between pore structure and sodium storage ability of hard carbon was investigated. CWS-CK exhibits a block morphology with micro-scale holes on the surface (Fig. 1b and Fig. S1), these micro-scale holes could facilitate electrolyte penetration in the bulk of hard carbon. Highresolution transmission electron microscopy (HRTEM) images of CWS and CWS-CK are shown in Fig. 1c and d, respectively. Constituted by short-range ordered pseudographitic domains and nanopores, both samples possess a low graphitization degree, corresponding to the typical feature of hard carbon. With the effect of KOH etching, mesopores with an open framework can be observed in CWS-CK, which is shown in the orange loop of Fig. 1d. These mesopores could further reduce the diffusion distance of Na+ ions. XRD measurement was carried out to characterize the phase and structure of hard carbon materials. As shown in Fig. 2a, all samples exhibit two broad diffraction peaks centered at ~23° and ~44°, corresponding to the (0 0 2) and (1 0 0) crystal planes of graphite, respectively. The broad and low-intensity peaks demonstrate the disordered orientation in carbon materials, which is in agreement with the HRTEM analysis. From (0 0 2) peak position, the average interlayer space (d002) can be calculated by the Bragg equation (2dsinh = nk) and the results are shown in Table 1. After hydrothermal treatment, no obvious changes were observed on d002 values. The interlayer spacing of pseudographitic domain is around 3.8 Å, which is much larger than that of graphite and suit-
able for sodium-ion intercalation [19,27]. In some cases, the larger interlayer spacing could facilitate Na+ ion intercalation and relieves the structural damage during sodiation/desodiation process. The graphitization degree was demonstrated using Raman spectra (Fig. 2b). The G band located at ~1596 cm1 corresponds to sp2 carbon plane vibration of graphite, and the D band at ~1356 cm1 is associated with disordered carbon or defective graphitic structures. As shown in Table 1, the values of the integral intensity ratio of D band to G band (ID/IG) are all above 1, revealing the high disordered degree and abundant defects in all samples [38]. The similar ID/IG values indicate that the disordered degree of hard carbon hardly changes during hydrothermal treatment. Elemental contents on the surface were analyzed using XPS (Fig. S2). Apart from carbon, nitrogen and oxygen derived from biomass are also detected. CAO, C@O, CAOH, C@N functional groups could be found in the high-resolution XPS spectra of C1s, O1s (Fig. S3), respectively. For all samples, the contents of nitrogen and oxygen are similar (Table. 1). The functional groups such as C@O, C@N could reversibly accommodate Na+ ions via pseudocapacitive binding, which was considered as the source of capacity delivered at 0.1–0.5 V [13]. Moreover, the N, O heteroatoms in carbon materials are beneficial to electrical conductivity [42,43]. Therefore, we consider that the high contents of nitrogen and oxygen (nearly 10% in total) make a positive effect on sodium storage ability. In Fig. 2c, the nitrogen adsorption-desorption isotherms of CWS and CWS-K correspond to I type curves (IUPAC classification) with a sharp uptake at an ultralow relative pressure (P/Po < 0.01), which indicates that the samples mainly comprise of micropores. For CWS-C, CWS-CK, and CWS-CK, a hysteresis loop at P/Po of 0.5–0.9 is the result of mesopore capillary condensation. The SSA of CWS, CWS-K, CWS-C and CWS-CK is 32.7, 16.3, 8.8, and 9.4 m2 g1,
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Fig. 2. Characterization and pore structure analysis of as-synthesized hard carbon samples. (a) X-ray diffraction (XRD) patterns, (b) Raman spectra, (c) N2 adsorption/ desorption isotherms, (d) the pore size distribution, (e) cumulative pore volume and (f) cumulative surface area of CWS, CWS-C, CWS-K and CWS-CK samples. Here, CWS-C or CWS-K denote samples obtained by further hydrothermal treatment of CWS with CTAB or KOH, respectively.
Table 1 Structure and elemental analysis of different hard carbon samples. Sample
d002 (Å)
ID/IG
SBET (m2 g1)
CWS CWS-C CWS-K CWS-CK
3.85 3.89 3.78 3.87
1.051 1.047 1.084 1.050
32.7 8.8 16.3 9.4
respectively. Compared with reported hard carbon materials, our samples exhibit a relatively low SSA [15,29,44], which is favorable to obtain a high initial Coulombic efficiency by decreasing SEI layer formation [32]. From CWS to CWS-CK, the SSA is obviously decreased. This is because under the assistance of KOH and CTAB, some impurities and amorphous carbon were dissolved, leading to the microstructure collapse and micropore disappearance. The pore-size distributions in Fig. 2d reveal the variation of micropores and mesopores. The micropore volume is in the following order: CWS＞CWS-K＞CWS-C＞CWS-CK. Mesopores ranged from 3 nm
XPS (at.%) C
N
O
91.24 91.41 91.83 91.29
0.67 0.93 0.72 1.32
8.09 7.63 7.45 7.39
to 7 nm obviously increase after hydrothermal treatment, which is in accord with the HRTEM observation. In sharp contrast to CWS with numerous micropores and a few mesopores, CWS-CK exhibits much less micropores and abundant mesopores. In Fig. 2e and f, the cumulative surface area and pore volume of the as-prepared samples are plotted vs. the pore size, respectively. For CWS, a sharp increase in cumulative surface area and pore volume occurs at pore size below 2 nm, but no obvious growth at above 2 nm, indicating nearly all the pores concentrate in 0.6– 2 nm. In contrast, for CWS-CK, negligible accumulation of surface
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area and pore volume is observed below 2 nm, and a gradual rise of pore volume at the range of 2–20 nm indicates that CWS-CK contains abundant mesopores with wide pore size dispersion. The micropore volume of CWS-CK is calculated as 5 104 cm3 g1, which is two orders of magnitude less than 1.4 102 cm3 g1 of CWS. On the other hand, CWS-CK exhibits a larger mesopore volume than that of CWS (1.1 102 cm3 g1 vs. 7 103 cm3 g1). Such the huge difference in micropore and mesopore volume would induce the diverse sodium storage ability, which was minutely discussed below. Notably, the content of heteroatoms, graphitization degree and interlayer spacing among all samples are similar, which is beneficial to minimize the interference from other structural factors. Fig. S4 exhibits the cyclic voltammetry (CV) curves of CWS-CK, a pair of intensive redox peaks with little electrochemical polarization at potential below 0.2 V can be ascribed to Na+ ion reversible insertion/extraction in/from hard carbon [13,19]. A broad reduction peak at around 0.45 V in the first cycle quickly disappears in the second cycle, indicating the main SEI layers formed during the initial sodiation process. The first three discharge/charge cycles at a current density of 100 mA g1 of different samples are displayed in Fig. S5. All profiles exhibit similar shape with a slope at potential 0.7–0.1 V and a plateau below 0.1 V. This plateau at the low potential ensures a high and stable operating potential for practical full cells. The Coulombic efficiencies of the first three cycles are also displayed in Fig. S5. Due to the low SSA, our hard carbon anodes possess high Coulombic efficiency compared with those high-SSA materials. Here, CWSCK delivers a maximum ICE of 64.0%, higher than 56.5% of CWS, the increased ICE can be attributed to the decreased SSA and fewer impurities. The Coulombic efficiency of CWS-CK quickly reaches to 94.51% at the second cycle, indicating few irreversible sodium loss during the followed cycling. In Fig. 3a, cycling capacities of different samples were compared at 100 mA g1. After 100 cycles, the reversible specific capacity of CWS-CK is 244.6 mAh g1, much higher than 141.2 mAh g1 of CWS, 167.9 mAh g1 of CWS-K and 184.5 mAh g1 of CWS-C. From the 5th cycle to 100th cycle, the capacity retention of CWS-CK is 93.1%. Probably caused by the fragile structure and relatively narrower interlayer space (3.78 Å), the capacity retention of CWS-K is just 68%. As shown in Fig. 3b, CWS-CK delivers a capacity of 189.4 mAh g1 after 320 cycles even under a high current density (200 mA g1), exhibiting excellent cycling stability. The obtained samples were discharged/charged at different current densities to evaluate the rate performance (Fig. 3c). The rate performance of CWS-C and CWS-K is much better than that of CWS. Due to the particular pore texture, CWS-CK delivers the best performance of 283.7, 265.8, 245.9, 234.1, and 160.3 mAh g1 at 0.02, 0.05, 0.1, 0.2, and 0.5 A g1, respectively. At an ultrahigh current density of 5 A g1, the reversible capacity still remains
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44.5 mAh g1, and a capacity of 252.7 mAh g1 is recovered when current density returns to 50 mA g1. In comparison to the stateof-the-art hard carbon materials, CWS-CK exhibits advantages on ICE and cycling capacity (Table 2). Sodium storage mechanism was explored by ex-situ XRD (Fig. 4a), from which the interlayer spacing can be detected at different discharge states. As the voltage decreasing from 0.200 V to 0.005 V, the interlayer spacing of both CWS and CWS-CK expands from the initial ~0.38 nm to the final ~0.42 nm, which is close to the equilibrium interlayer spacing of NaC6 of 0.46 nm [40]. This result further demonstrates that Na+ ions intercalating into interlayers is the source of plateau capacity rather than micropore adsorption. Therefore, we attributed the reversible binding of Na+ at carbon layer edges, defects, and functional groups to the highvoltage slope capacity above 0.1 V [14,45] and Na+ ions intercalation into pseudographitic domains to low-voltage plateau capacity below 0.1 V [40]. Fig. 4b exhibits the correlation between nanopores and sodiumion storage capacity. From CWS to CWS-CK, the micropores decrease gradually, but neither plateau capacity nor slope capacity decreases. CWS-CK with the least micropore volume even delivers a highest capacity, implying micropores hardly store Na+ ions. On the other hand, CWS-K and CWS-CK with more mesopores deliver better performance than CWS and CWS-C. It can be observed that sample with more mesopores exhibits higher plateau capacity, which indicates that the additional mesopores as electrolyte reservoir could facilitate Na+ ion to reach the edge of carbon interlayers so that the intercalation is enhanced. Generally, both micropores and mesopores have an obvious effect on capacity, the sample with few micropores and sufficient mesopores exhibits better electrochemical performance. Electrochemical impedance spectroscopy (EIS) was performed to further explore the effect of nanopores on sodium-ion storage (Fig. 4c). In the high-frequency region, the diameter of the semicircle represents the charge transfer resistance (Rct). Generally, micropore-less samples exhibit lower resistance than microporerich samples, such as ~16 ohms of CWS-CK vs. ~100 ohms of CWS, indicating a fast redox reaction in micropore-less samples. In addition, synthesized by hydrothermal treatment with deionized water, CWS-D also exhibits a large Rct (Fig. S6), which can be ascribed to the similar textures of CWS-D and CWS (Fig. S7). In addition, the Warburg coefficients (r) achieving from the slope of fitted lines in Fig. 4d were used to further calculate the sodium ion diffusion coefficients (D) in hard carbon, based on the formula below:
D¼
R 2 T2 2 4
2A F
r2 C 2 n 4
where R is the gas constant, T is the ambient temperature, A is the surface area of the electrode, F is the Faraday constant, r is the slope
Fig. 3. Electrochemical performance of as-synthesized hard carbon samples. (a) cycling performance of different samples at a current density of 100 mA g1, (b) cycling performance of CWS-CK at a current density of 200 mA g1, (c) rate capability of different samples from 20 mA g1 to 5 A g1.
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Table 2 Sodium-storage performance comparison of CWS-CK versus state of the art hard carbons reported in literature. Sample
Capacity (mAh/g)
Initial Coulombic efficiency (%)
Cycle Number
Current Density (mA/g)
ref
CWS-CK Pomelo peels derived carbon Cellulose derived nanofibers N-doped carbon nanofibers Porous Carbon Nanosheets Expanded graphite Reduced graphene oxide Carbon nanobubbles Hollow carbon microspheres
189 181 176 132 194 91 93 60 211
64.0 27.0 58.7 41.8 34.8 49.5 20.0 30.0 24.3
320 220 600 200 10 13 250 30 100
200 200 200 200 200 200 200 200 200
This Work [32] [15] [22] [28] [3] [30] [31] [29]
Fig. 4. (a) XRD patterns of CWS and CWS-CK at discharge state of 0.200 V, 0.100 V, 0.050 V and 0.005 V, respectively. (b) Pore volume and capacity of different samples. The symbol-polylines represent the volume of mesopores (red) and micropores (black). Here, the pore volume is calculated by density functional theory (DFT) method. The specific capacity values of CWS, CWS-K, CWS-C and CWS-CK are obtained based on the discharge process of the 5th cycle at 100 mA g1. (c) Nyquist plots (with high frequency region in the insert) of different samples. (d) Plot of real resistance (Z0 ) as function of inverse square root of angular speed (x1/2) for different samples, the straight lines are the results of linear fitting. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
of the fitted line Z0 ~ x1/2, C is concentration of sodium ion in hard carbon electrode, n is the number of electrons per molecule attending the charge transfer reaction. The calculations were shown in Table S1. Here, CWS-CK exhibits a D of 3.32 1014 cm2 S1, one order magnitude higher than micropore-rich samples such as CWS and CWS-K. Combining with the results of pore size distribution, it can be found that the D values exhibit a negative correlation to the volume of micropores. As illustrated in Fig. 1a, this trend possibly caused by the steric hindrance of electrolyte-impermeable micropores, which hinders Na+ ions reach the deeper active sites. Moreover, micropores occupy many ‘‘dead volume” with no capacity contribution, further decreasing the utilization of materials. On the contrary, mesopores provide a positive effect on ion diffusion because electrolyte can easily penetrate. Generally, decreasing micropores and increasing mesopores would promote the utilization of active sites and Na+ ion diffusion in hard carbon, which would realize the improvement of the specific capacity and rate capability without sacrificing other electrochemical performances.
4. Conclusion In summary, we proposed a strategy of pore structure regulation to improve the sodium storage capacity and initial Coulombic efficiency of hard carbon. The capacity of carbon with negligible micropores and abundant mesopores is 83% higher than the carbon with a few mesopores and abundant micropores. In comparison to previous works [39,41,46], our method not only eliminated nearly all of micropores but also kept other structural factor unchanged, which is beneficial to investigate the effects of micropores and mesopores in hard carbon: the micropores (pore size below 2 nm) hinder ion diffusion and hardly ever accommodate Na+ ions, while mesopores (pore size between 2 and 20 nm) facilitate Na+ ion intercalation. The above conclusion supports a mechanism in which the slope capacity is related to the reversible binding of Na+ ion on carbon layer edges, defects, and N, O functional groups, and the plateau capacity is contributed to the intercalation of Na+ ions in carbon interlayers. The nanopore regulation strategy can be
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furtherly applied for other fields including lithium-ion batteries, supercapacitors, electrochemical catalysis, and gas uptake. CRediT authorship contribution statement Le Yang: Conceptualization, Methodology, Data curation, Writing - original draft, Writing - review & editing, Software. Mingxiang Hu: Software, Methodology. Hongwei Zhang: Investigation, Formal analysis. Wen Yang: Supervision, Validation. Ruitao Lv: Supervision, Validation, Project administration. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (51722207, 51972191). This work was also supported by the National Natural Science Foundation of China (No. 21975025, 21575015, 21203008), the Beijing Natural Science Foundation (No. 2172051), the National Key Research and Development Program of China ‘‘New Energy Project for Electric Vehicle” (2016YFB0100204), State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2020.01.085. References [1] S.W. Kim, D.H. Seo, X. Ma, G. Ceder, K. Kang, Electrode materials for rechargeable sodium-ion batteries: potential alternatives to current lithiumion batteries, Adv. Energy Mater. 2 (2012) 710–721. [2] J.Y. Hwang, S.T. Myung, Y.K. Sun, Sodium-ion batteries: present and future, Chem. Soc. Rev. 46 (2017) 3529–3614. [3] Y. Wen, K. He, Y. Zhu, F. Han, Y. Xu, I. Matsuda, Y. Ishii, J. Cumings, C. Wang, Expanded graphite as superior anode for sodium-ion batteries, Nat. Commun. 5 (2014) 4033. [4] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Research development on sodium-ion batteries, Chem. Rev. 114 (2014) 11636–11682. [5] D. Tang, Q. Huang, R. Yi, F. Dai, M.L. Gordin, S. Hu, S. Chen, Z. Yu, H. Sohn, J. Song, Room-temperature synthesis of mesoporous Sn/SnO2 composite as anode for sodium-ion batteries, Eur. J. Inorg. Chem. 2016 (2016) 1950–1954. [6] Z. Liu, X.Y. Yu, X.W. Lou, U. Paik, Sb@C coaxial nanotubes as a superior long-life and high-rate anode for sodium ion batteries, Energy Environ. Sci. 9 (2016) 2314–2318. [7] Y. Zhang, C. Wang, H. Hou, G. Zou, X. Ji, Sodium-ion batteries: nitrogen doped/carbon tuning yolk-like TiO2 and its remarkable impact on sodium storage performances, Adv. Energy Mater. 7 (2017) 1600173. [8] J. Wu, Z. Lu, K. Li, J. Cui, S. Yao, I.U. Haq, B. Li, Q.H. Yang, F. Kang, F. Ciucci, Hierarchical MoS2/carbon microspheres as long-life and high-rate anodes for sodium-ion batteries, J. Mater. Chem. A 6 (2018) 5668–5677. [9] L. Shi, D. Li, P. Yao, J. Yu, C. Li, B. Yang, C. Zhu, J. Xu, SnS2 nanosheets coating on nanohollow cubic CoS2/C for Ultralong life and high rate capability half/full sodium-ion batteries, Small 14 (2018) 1802716. [10] Y. Fang, L. Xiao, J. Qian, Y. Cao, X. Ai, Y. Huang, H. Yang, 3D graphene decorated NaTi2(PO4)3 microspheres as a superior high-rate and ultracycle-stable anode material for sodium ion batteries, Adv. Energy Mater. 6 (2016) 1502197. [11] Y. Matsuo, K. Ueda, Pyrolytic carbon from graphite oxide as a negative electrode of sodium-ion battery, J. Power Sources 263 (2014) 158–162. [12] Y. Lu, Y. Lu, Z. Niu, J. Chen, Graphene-based nanomaterials for sodium-ion batteries, Adv. Energy Mater. 8 (2018) 1702469. [13] L. Wu, D. Buchholz, C. Vaalma, G.A. Giffin, S. Passerini, Apple-biowaste-derived hard carbon as a powerful anode material for na-ion batteries, ChemElectroChem 3 (2016) 292–298. [14] C. Bommier, T.W. Surta, M. Dolgos, X. Ji, New mechanistic insights on Na-Ion storage in non-graphitizable carbon, Nano Lett. 15 (2015) 5888–5892.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Pore structure regulation of hard carbon: Towards fast and high-capacity sodium-ion storage Le Yang a,b, Mingxiang Hu b, Hongwei Zhang b, Wen Yang a,⇑, Ruitao Lv b,c,⇑ a Key Laboratory of Cluster Science of Ministry of Education Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China b State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China c Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Nanopore regulation strategy can
improve the sodium storage capacity of hard carbon. The relationship between nanopore and sodium storage capability was investigated. + Micropores hardly accommodate Na + ions and hinder Na ion diffusion. + Mesopores facilitate Na ion intercalation and shorten the ion diffusion pathway.
a r t i c l e
i n f o
Article history: Received 15 November 2019 Revised 9 January 2020 Accepted 22 January 2020 Available online 23 January 2020 Keywords: Micropore Mesopore Pore-structure evolution Hard carbon Anode Sodium-ion battery
a b s t r a c t Hard carbon is regarded as one of the most promising anode material for sodium-ion batteries in virtue of the low cost and stable framework. However, the correlation between pore structures of hard carbon and sodium-ion storage is still ambiguous. In this work, based on precise control of pore-size distribution, the capacity, ion diffusion, and initial Coulombic efficiency were improved. Meanwhile, the relationship between pore structure and capacity was investigated. Our result indicates that the micropores hinder ion diffusion and hardly ever accommodate Na+ ions, while mesopores facilitate Na+ ion intercalation. Hard carbon with negligible micropores and abundant mesopores delivers a maximum capacity of 283.7 mAh g1 at 20 mA g1, which is 83% higher than that of micropore-rich one. Even after 320 cycles at 200 mA g1, the capacity still remains 189.4 mAh g1. Ó 2020 Elsevier Inc. All rights reserved.
⇑ Corresponding authors at: State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China (R. Lv). E-mail addresses: [email protected] (W. Yang), [email protected] (R. Lv). https://doi.org/10.1016/j.jcis.2020.01.085 0021-9797/Ó 2020 Elsevier Inc. All rights reserved.
1. Introduction Sodium, with high terrestrial abundance and low electrode potential, contributes to the low cost and high energy density for
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sodium-ion batteries (SIBs), which were regarded as one of the most promising candidates for large-scale energy storage [1,2]. Unfortunately, commercially utilized graphite anode in LIBs is failed for sodium storage owing to the larger diameter of sodium ion compared with lithium-ion (1.02 Å vs. 0.76 Å) as well as thermodynamic incompatibility between graphite layers and sodium ions [3,4]. It is crucial to develop a high-performance anode materials to realize reversible Na+ ion intercalation/extraction. So far, various materials have been explored as the anode of SIBs [3,5– 15]. Among them, hard carbon constituted by disordered stacking pseudographitic domains, defects, and amorphous carbon is suitable to reversibly accommodate Na+ ions with long lifespan and low electrode potential [13,15–17]. Hard carbon derived from biomass could not only realize a low cost but also relieve environment pollution. Moreover, the biomass materials containing high content of cellulose are beneficial to a high yield after carbonization [18,19]. Different strategies including heteroatom doping (e.g., nitrogen and sulfur) [20–24], morphology design [19,25–32] have been developed to improve the electrochemical properties of hard carbon. Although the pore-size distribution was considered to be related to sodium-ion storage and diffusion in hard carbon [33,34]. However, as far as we have known that the porestructure control of hard carbon was seldom focused, and the detail effect of micropores on Na+ ion storage is still controversial. For example, a traditional viewpoint proposed that the lowpotential (below 0.1 V) capacity can be ascribed to micropore adsorption, which is similar to Li+ storage in hard carbon [19,28,35–37]. In contrast, others demonstrated that the Na+ ion intercalation contribute to the low-potential capacity [14,38–41]. Therefore, it is important to clarify the relationship between pore structure evolution and sodium-ion storage to fully understand the storage mechanism in hard carbon. It should be noted that excluding interference from other structural factors (such as interlayer spacing, domain size, degree of graphitization, and heteroatom) is necessary to realize the goal. In the present work, carbonized walnut shell (CWS) was treated by hexadecyl trimethyl ammonium bromide (CTAB) and/or KOH under hydrothermal condition (as illustrated in Fig. 1a). The pore-size distribution of samples was adjusted independently. After rational control of pore structure, both specific capacity and rate capability are improved without sacrificing initial Coulombic efficiency (ICE), voltage plateau, and cycling stability. We found that hard carbon with negligible micropores and abundant mesopores delivers an excellent cycling capacity of 283.7 mAh g1 at 20 mA g1 with ICE of 64%, which is obviously higher than micropore-rich hard carbon.
tered and washed by deionized water and alcohol several times, then dried at 80 °C overnight. For comparison, just substituting the hydrothermal solution to 1 mol L1 KOH, 10 mg ml1 CTAB or deionized water, CWS-K, CWS-C, and CWS-D were fabricated, respectively. 2.2. Sample characterizations As-synthesized samples were characterized by X-ray diffractometer (XRD, D/Max-2500/PC, Rigaku Corporation, Japan), with radiation sources of Cu Ka. To investigate the morphology and structure of hard carbon, scanning electron microscopy (SEM, JMS-7001-f type, JEOL, Japan) and transmission electron microscopy (TEM, JEM-2010, JEOL, Japan) were used. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250 xi photoelectron spectrometer (Thermo Fisher Scientific, America) using monochromatic Al Ka as the incident X-rays. Raman spectra were recorded with a LabRAM HR Evolution type (HORIBA, Jobin Yvon, Japan) equipped with a 632.5 nm He-Ne laser. To characterize the specific surface areas (SSA) and porous texture, nitrogen adsorption and desorption isotherms were performed on a Belshop-mini type II analyzer (MicrotracBEL, Japan) at 77 K. The SSA was calculated by Brunauer-Emmett-Teller (BET) method, and the pore size distribution was analyzed via a density functional theory (DFT) method. Micropore volume was calculated by SaitoFoley (SF) method, mesopore volume by total pore volume from DFT method subtracting micropore volume. 2.3. Electrochemical tests 75 wt% active materials, 15 wt% polyvinylidene fluoride (PVDF) and 10 wt% carbon black (Super-P) were mixed in N-methyl pyrrolidinone (NMP) solution under vigorous stirring until a uniform slurry is obtained. The slurries were then coated onto a copper foil and then dried at 120 °C in vacuum overnight to remove the solvent. The loading mass of active materials is ~1 mg cm2 on one work electrode. 2032-type coin cells were assembled in the argon-filled glove box with glass microfiber filters (Whatman GF/ D) as the separator, sodium metal sheet as the counter electrode and 1 M NaClO4 in ethylene carbonate/dimethyl carbonate (EC/ DMC, 1:1 v/v) as the electrolyte. LAND CT2001A type battery test system was used for galvanostatic charge-discharge (GCD) tests, with the voltage range of 0.005–3 V. Cyclic voltammetry (CV) measurements were performed on a VSP-300 type electrochemical workstation (Bio-logic Inc.) at a scan rate of 0.1 mV s1. Impedance measurements were also carried out on the electrochemical workstation with a scan frequency from 200 kHz to 100 mHz.
2. Experimental
2.4. Ex-situ XRD characterization
2.1. Material synthesis
After discharging/charging for 10 cycles, the batteries were disassembled and the electrodes were washed by DMC and dried overnight. After being peeled off from copper foil carefully, the electrode materials were used for ex-situ XRD measurements. All processes above were manipulated under Ar protection. For exsitu XRD tests, we used electrodes composed by 95 wt% active materials and 5 wt% PVDF (without Super-P) to minimize the impurity peaks in XRD patterns.
CWS was prepared by the direct carbonization of the walnut shell. The walnut shells were smashed and washed by deionized water, then dried overnight at 80 °C in vacuum. The clean walnut shells were carbonized in the tubular furnace at 1100 °C for 6 h (the heating rate is 10 °C/min) under argon (Ar) flow protection. The as-prepared black powders were washed by deionized water and alcohol several times and then dried at 80 °C overnight. The fabrication process of CWS-CK is described as follows: typically, the as-prepared CWS powders were added into a Teflonlined stainless steel autoclave containing an aqueous solution with 1 mol L1 KOH and 10 mg ml1 CTAB. The autoclaves were sealed and heated to 180 °C for 12 h. The obtained black powders were fil-
3. Results and discussion Illustration of the synthesis procedure is shown in Fig. 1a. Hydrothermal treatment was conducted to change the pore structure of hard carbon. After treatment, the micropores were gradually eliminated, while mesopores were introduced. The addition
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Fig. 1. (a) Synthesis illustration of the walnut-shell-derived hard carbon. Through the hydrothermal treatment, the decrease of micropores and increase of mesopores changes the Na+ ion diffusion behavior. (b) Scanning electron microscope (SEM) image of CWS-CK, high-resolution transmission electron microscopy (HRTEM) images of (c) CWS and (d) CWS-CK. Here, CWS denotes porous hard carbon sample obtained by direct carbonization of walnut shells at 1100 °C, CWS-CK denotes sample obtained by further hydrothermal treatment for CWS under the presence of both hexadecyl trimethyl ammonium bromide (CTAB) and KOH.
of KOH and CTAB during hydrothermal treatment induced such variation on pore size distribution. Here, KOH was used to produce more mesopores due to its etching effect. CTAB, as a common surfactant, could facilitate the dissolution of some impurities and amorphous carbon, which consequently leads to the collapse of microstructure and the decrease of micropores. Based on such variation, the relationship between pore structure and sodium storage ability of hard carbon was investigated. CWS-CK exhibits a block morphology with micro-scale holes on the surface (Fig. 1b and Fig. S1), these micro-scale holes could facilitate electrolyte penetration in the bulk of hard carbon. Highresolution transmission electron microscopy (HRTEM) images of CWS and CWS-CK are shown in Fig. 1c and d, respectively. Constituted by short-range ordered pseudographitic domains and nanopores, both samples possess a low graphitization degree, corresponding to the typical feature of hard carbon. With the effect of KOH etching, mesopores with an open framework can be observed in CWS-CK, which is shown in the orange loop of Fig. 1d. These mesopores could further reduce the diffusion distance of Na+ ions. XRD measurement was carried out to characterize the phase and structure of hard carbon materials. As shown in Fig. 2a, all samples exhibit two broad diffraction peaks centered at ~23° and ~44°, corresponding to the (0 0 2) and (1 0 0) crystal planes of graphite, respectively. The broad and low-intensity peaks demonstrate the disordered orientation in carbon materials, which is in agreement with the HRTEM analysis. From (0 0 2) peak position, the average interlayer space (d002) can be calculated by the Bragg equation (2dsinh = nk) and the results are shown in Table 1. After hydrothermal treatment, no obvious changes were observed on d002 values. The interlayer spacing of pseudographitic domain is around 3.8 Å, which is much larger than that of graphite and suit-
able for sodium-ion intercalation [19,27]. In some cases, the larger interlayer spacing could facilitate Na+ ion intercalation and relieves the structural damage during sodiation/desodiation process. The graphitization degree was demonstrated using Raman spectra (Fig. 2b). The G band located at ~1596 cm1 corresponds to sp2 carbon plane vibration of graphite, and the D band at ~1356 cm1 is associated with disordered carbon or defective graphitic structures. As shown in Table 1, the values of the integral intensity ratio of D band to G band (ID/IG) are all above 1, revealing the high disordered degree and abundant defects in all samples [38]. The similar ID/IG values indicate that the disordered degree of hard carbon hardly changes during hydrothermal treatment. Elemental contents on the surface were analyzed using XPS (Fig. S2). Apart from carbon, nitrogen and oxygen derived from biomass are also detected. CAO, C@O, CAOH, C@N functional groups could be found in the high-resolution XPS spectra of C1s, O1s (Fig. S3), respectively. For all samples, the contents of nitrogen and oxygen are similar (Table. 1). The functional groups such as C@O, C@N could reversibly accommodate Na+ ions via pseudocapacitive binding, which was considered as the source of capacity delivered at 0.1–0.5 V [13]. Moreover, the N, O heteroatoms in carbon materials are beneficial to electrical conductivity [42,43]. Therefore, we consider that the high contents of nitrogen and oxygen (nearly 10% in total) make a positive effect on sodium storage ability. In Fig. 2c, the nitrogen adsorption-desorption isotherms of CWS and CWS-K correspond to I type curves (IUPAC classification) with a sharp uptake at an ultralow relative pressure (P/Po < 0.01), which indicates that the samples mainly comprise of micropores. For CWS-C, CWS-CK, and CWS-CK, a hysteresis loop at P/Po of 0.5–0.9 is the result of mesopore capillary condensation. The SSA of CWS, CWS-K, CWS-C and CWS-CK is 32.7, 16.3, 8.8, and 9.4 m2 g1,
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Fig. 2. Characterization and pore structure analysis of as-synthesized hard carbon samples. (a) X-ray diffraction (XRD) patterns, (b) Raman spectra, (c) N2 adsorption/ desorption isotherms, (d) the pore size distribution, (e) cumulative pore volume and (f) cumulative surface area of CWS, CWS-C, CWS-K and CWS-CK samples. Here, CWS-C or CWS-K denote samples obtained by further hydrothermal treatment of CWS with CTAB or KOH, respectively.
Table 1 Structure and elemental analysis of different hard carbon samples. Sample
d002 (Å)
ID/IG
SBET (m2 g1)
CWS CWS-C CWS-K CWS-CK
3.85 3.89 3.78 3.87
1.051 1.047 1.084 1.050
32.7 8.8 16.3 9.4
respectively. Compared with reported hard carbon materials, our samples exhibit a relatively low SSA [15,29,44], which is favorable to obtain a high initial Coulombic efficiency by decreasing SEI layer formation [32]. From CWS to CWS-CK, the SSA is obviously decreased. This is because under the assistance of KOH and CTAB, some impurities and amorphous carbon were dissolved, leading to the microstructure collapse and micropore disappearance. The pore-size distributions in Fig. 2d reveal the variation of micropores and mesopores. The micropore volume is in the following order: CWS＞CWS-K＞CWS-C＞CWS-CK. Mesopores ranged from 3 nm
XPS (at.%) C
N
O
91.24 91.41 91.83 91.29
0.67 0.93 0.72 1.32
8.09 7.63 7.45 7.39
to 7 nm obviously increase after hydrothermal treatment, which is in accord with the HRTEM observation. In sharp contrast to CWS with numerous micropores and a few mesopores, CWS-CK exhibits much less micropores and abundant mesopores. In Fig. 2e and f, the cumulative surface area and pore volume of the as-prepared samples are plotted vs. the pore size, respectively. For CWS, a sharp increase in cumulative surface area and pore volume occurs at pore size below 2 nm, but no obvious growth at above 2 nm, indicating nearly all the pores concentrate in 0.6– 2 nm. In contrast, for CWS-CK, negligible accumulation of surface
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area and pore volume is observed below 2 nm, and a gradual rise of pore volume at the range of 2–20 nm indicates that CWS-CK contains abundant mesopores with wide pore size dispersion. The micropore volume of CWS-CK is calculated as 5 104 cm3 g1, which is two orders of magnitude less than 1.4 102 cm3 g1 of CWS. On the other hand, CWS-CK exhibits a larger mesopore volume than that of CWS (1.1 102 cm3 g1 vs. 7 103 cm3 g1). Such the huge difference in micropore and mesopore volume would induce the diverse sodium storage ability, which was minutely discussed below. Notably, the content of heteroatoms, graphitization degree and interlayer spacing among all samples are similar, which is beneficial to minimize the interference from other structural factors. Fig. S4 exhibits the cyclic voltammetry (CV) curves of CWS-CK, a pair of intensive redox peaks with little electrochemical polarization at potential below 0.2 V can be ascribed to Na+ ion reversible insertion/extraction in/from hard carbon [13,19]. A broad reduction peak at around 0.45 V in the first cycle quickly disappears in the second cycle, indicating the main SEI layers formed during the initial sodiation process. The first three discharge/charge cycles at a current density of 100 mA g1 of different samples are displayed in Fig. S5. All profiles exhibit similar shape with a slope at potential 0.7–0.1 V and a plateau below 0.1 V. This plateau at the low potential ensures a high and stable operating potential for practical full cells. The Coulombic efficiencies of the first three cycles are also displayed in Fig. S5. Due to the low SSA, our hard carbon anodes possess high Coulombic efficiency compared with those high-SSA materials. Here, CWSCK delivers a maximum ICE of 64.0%, higher than 56.5% of CWS, the increased ICE can be attributed to the decreased SSA and fewer impurities. The Coulombic efficiency of CWS-CK quickly reaches to 94.51% at the second cycle, indicating few irreversible sodium loss during the followed cycling. In Fig. 3a, cycling capacities of different samples were compared at 100 mA g1. After 100 cycles, the reversible specific capacity of CWS-CK is 244.6 mAh g1, much higher than 141.2 mAh g1 of CWS, 167.9 mAh g1 of CWS-K and 184.5 mAh g1 of CWS-C. From the 5th cycle to 100th cycle, the capacity retention of CWS-CK is 93.1%. Probably caused by the fragile structure and relatively narrower interlayer space (3.78 Å), the capacity retention of CWS-K is just 68%. As shown in Fig. 3b, CWS-CK delivers a capacity of 189.4 mAh g1 after 320 cycles even under a high current density (200 mA g1), exhibiting excellent cycling stability. The obtained samples were discharged/charged at different current densities to evaluate the rate performance (Fig. 3c). The rate performance of CWS-C and CWS-K is much better than that of CWS. Due to the particular pore texture, CWS-CK delivers the best performance of 283.7, 265.8, 245.9, 234.1, and 160.3 mAh g1 at 0.02, 0.05, 0.1, 0.2, and 0.5 A g1, respectively. At an ultrahigh current density of 5 A g1, the reversible capacity still remains
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44.5 mAh g1, and a capacity of 252.7 mAh g1 is recovered when current density returns to 50 mA g1. In comparison to the stateof-the-art hard carbon materials, CWS-CK exhibits advantages on ICE and cycling capacity (Table 2). Sodium storage mechanism was explored by ex-situ XRD (Fig. 4a), from which the interlayer spacing can be detected at different discharge states. As the voltage decreasing from 0.200 V to 0.005 V, the interlayer spacing of both CWS and CWS-CK expands from the initial ~0.38 nm to the final ~0.42 nm, which is close to the equilibrium interlayer spacing of NaC6 of 0.46 nm [40]. This result further demonstrates that Na+ ions intercalating into interlayers is the source of plateau capacity rather than micropore adsorption. Therefore, we attributed the reversible binding of Na+ at carbon layer edges, defects, and functional groups to the highvoltage slope capacity above 0.1 V [14,45] and Na+ ions intercalation into pseudographitic domains to low-voltage plateau capacity below 0.1 V [40]. Fig. 4b exhibits the correlation between nanopores and sodiumion storage capacity. From CWS to CWS-CK, the micropores decrease gradually, but neither plateau capacity nor slope capacity decreases. CWS-CK with the least micropore volume even delivers a highest capacity, implying micropores hardly store Na+ ions. On the other hand, CWS-K and CWS-CK with more mesopores deliver better performance than CWS and CWS-C. It can be observed that sample with more mesopores exhibits higher plateau capacity, which indicates that the additional mesopores as electrolyte reservoir could facilitate Na+ ion to reach the edge of carbon interlayers so that the intercalation is enhanced. Generally, both micropores and mesopores have an obvious effect on capacity, the sample with few micropores and sufficient mesopores exhibits better electrochemical performance. Electrochemical impedance spectroscopy (EIS) was performed to further explore the effect of nanopores on sodium-ion storage (Fig. 4c). In the high-frequency region, the diameter of the semicircle represents the charge transfer resistance (Rct). Generally, micropore-less samples exhibit lower resistance than microporerich samples, such as ~16 ohms of CWS-CK vs. ~100 ohms of CWS, indicating a fast redox reaction in micropore-less samples. In addition, synthesized by hydrothermal treatment with deionized water, CWS-D also exhibits a large Rct (Fig. S6), which can be ascribed to the similar textures of CWS-D and CWS (Fig. S7). In addition, the Warburg coefficients (r) achieving from the slope of fitted lines in Fig. 4d were used to further calculate the sodium ion diffusion coefficients (D) in hard carbon, based on the formula below:
D¼
R 2 T2 2 4
2A F
r2 C 2 n 4
where R is the gas constant, T is the ambient temperature, A is the surface area of the electrode, F is the Faraday constant, r is the slope
Fig. 3. Electrochemical performance of as-synthesized hard carbon samples. (a) cycling performance of different samples at a current density of 100 mA g1, (b) cycling performance of CWS-CK at a current density of 200 mA g1, (c) rate capability of different samples from 20 mA g1 to 5 A g1.
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Table 2 Sodium-storage performance comparison of CWS-CK versus state of the art hard carbons reported in literature. Sample
Capacity (mAh/g)
Initial Coulombic efficiency (%)
Cycle Number
Current Density (mA/g)
ref
CWS-CK Pomelo peels derived carbon Cellulose derived nanofibers N-doped carbon nanofibers Porous Carbon Nanosheets Expanded graphite Reduced graphene oxide Carbon nanobubbles Hollow carbon microspheres
189 181 176 132 194 91 93 60 211
64.0 27.0 58.7 41.8 34.8 49.5 20.0 30.0 24.3
320 220 600 200 10 13 250 30 100
200 200 200 200 200 200 200 200 200
This Work [32] [15] [22] [28] [3] [30] [31] [29]
Fig. 4. (a) XRD patterns of CWS and CWS-CK at discharge state of 0.200 V, 0.100 V, 0.050 V and 0.005 V, respectively. (b) Pore volume and capacity of different samples. The symbol-polylines represent the volume of mesopores (red) and micropores (black). Here, the pore volume is calculated by density functional theory (DFT) method. The specific capacity values of CWS, CWS-K, CWS-C and CWS-CK are obtained based on the discharge process of the 5th cycle at 100 mA g1. (c) Nyquist plots (with high frequency region in the insert) of different samples. (d) Plot of real resistance (Z0 ) as function of inverse square root of angular speed (x1/2) for different samples, the straight lines are the results of linear fitting. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
of the fitted line Z0 ~ x1/2, C is concentration of sodium ion in hard carbon electrode, n is the number of electrons per molecule attending the charge transfer reaction. The calculations were shown in Table S1. Here, CWS-CK exhibits a D of 3.32 1014 cm2 S1, one order magnitude higher than micropore-rich samples such as CWS and CWS-K. Combining with the results of pore size distribution, it can be found that the D values exhibit a negative correlation to the volume of micropores. As illustrated in Fig. 1a, this trend possibly caused by the steric hindrance of electrolyte-impermeable micropores, which hinders Na+ ions reach the deeper active sites. Moreover, micropores occupy many ‘‘dead volume” with no capacity contribution, further decreasing the utilization of materials. On the contrary, mesopores provide a positive effect on ion diffusion because electrolyte can easily penetrate. Generally, decreasing micropores and increasing mesopores would promote the utilization of active sites and Na+ ion diffusion in hard carbon, which would realize the improvement of the specific capacity and rate capability without sacrificing other electrochemical performances.
4. Conclusion In summary, we proposed a strategy of pore structure regulation to improve the sodium storage capacity and initial Coulombic efficiency of hard carbon. The capacity of carbon with negligible micropores and abundant mesopores is 83% higher than the carbon with a few mesopores and abundant micropores. In comparison to previous works [39,41,46], our method not only eliminated nearly all of micropores but also kept other structural factor unchanged, which is beneficial to investigate the effects of micropores and mesopores in hard carbon: the micropores (pore size below 2 nm) hinder ion diffusion and hardly ever accommodate Na+ ions, while mesopores (pore size between 2 and 20 nm) facilitate Na+ ion intercalation. The above conclusion supports a mechanism in which the slope capacity is related to the reversible binding of Na+ ion on carbon layer edges, defects, and N, O functional groups, and the plateau capacity is contributed to the intercalation of Na+ ions in carbon interlayers. The nanopore regulation strategy can be
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furtherly applied for other fields including lithium-ion batteries, supercapacitors, electrochemical catalysis, and gas uptake. CRediT authorship contribution statement Le Yang: Conceptualization, Methodology, Data curation, Writing - original draft, Writing - review & editing, Software. Mingxiang Hu: Software, Methodology. Hongwei Zhang: Investigation, Formal analysis. Wen Yang: Supervision, Validation. Ruitao Lv: Supervision, Validation, Project administration. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (51722207, 51972191). This work was also supported by the National Natural Science Foundation of China (No. 21975025, 21575015, 21203008), the Beijing Natural Science Foundation (No. 2172051), the National Key Research and Development Program of China ‘‘New Energy Project for Electric Vehicle” (2016YFB0100204), State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2020.01.085. References [1] S.W. Kim, D.H. Seo, X. Ma, G. Ceder, K. Kang, Electrode materials for rechargeable sodium-ion batteries: potential alternatives to current lithiumion batteries, Adv. Energy Mater. 2 (2012) 710–721. [2] J.Y. Hwang, S.T. Myung, Y.K. Sun, Sodium-ion batteries: present and future, Chem. Soc. Rev. 46 (2017) 3529–3614. [3] Y. Wen, K. He, Y. Zhu, F. Han, Y. Xu, I. Matsuda, Y. Ishii, J. Cumings, C. Wang, Expanded graphite as superior anode for sodium-ion batteries, Nat. Commun. 5 (2014) 4033. [4] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Research development on sodium-ion batteries, Chem. Rev. 114 (2014) 11636–11682. [5] D. Tang, Q. Huang, R. Yi, F. Dai, M.L. Gordin, S. Hu, S. Chen, Z. Yu, H. Sohn, J. Song, Room-temperature synthesis of mesoporous Sn/SnO2 composite as anode for sodium-ion batteries, Eur. J. Inorg. Chem. 2016 (2016) 1950–1954. [6] Z. Liu, X.Y. Yu, X.W. Lou, U. Paik, Sb@C coaxial nanotubes as a superior long-life and high-rate anode for sodium ion batteries, Energy Environ. Sci. 9 (2016) 2314–2318. [7] Y. Zhang, C. Wang, H. Hou, G. Zou, X. Ji, Sodium-ion batteries: nitrogen doped/carbon tuning yolk-like TiO2 and its remarkable impact on sodium storage performances, Adv. Energy Mater. 7 (2017) 1600173. [8] J. Wu, Z. Lu, K. Li, J. Cui, S. Yao, I.U. Haq, B. Li, Q.H. Yang, F. Kang, F. Ciucci, Hierarchical MoS2/carbon microspheres as long-life and high-rate anodes for sodium-ion batteries, J. Mater. Chem. A 6 (2018) 5668–5677. [9] L. Shi, D. Li, P. Yao, J. Yu, C. Li, B. Yang, C. Zhu, J. Xu, SnS2 nanosheets coating on nanohollow cubic CoS2/C for Ultralong life and high rate capability half/full sodium-ion batteries, Small 14 (2018) 1802716. [10] Y. Fang, L. Xiao, J. Qian, Y. Cao, X. Ai, Y. Huang, H. Yang, 3D graphene decorated NaTi2(PO4)3 microspheres as a superior high-rate and ultracycle-stable anode material for sodium ion batteries, Adv. Energy Mater. 6 (2016) 1502197. [11] Y. Matsuo, K. Ueda, Pyrolytic carbon from graphite oxide as a negative electrode of sodium-ion battery, J. Power Sources 263 (2014) 158–162. [12] Y. Lu, Y. Lu, Z. Niu, J. Chen, Graphene-based nanomaterials for sodium-ion batteries, Adv. Energy Mater. 8 (2018) 1702469. [13] L. Wu, D. Buchholz, C. Vaalma, G.A. Giffin, S. Passerini, Apple-biowaste-derived hard carbon as a powerful anode material for na-ion batteries, ChemElectroChem 3 (2016) 292–298. [14] C. Bommier, T.W. Surta, M. Dolgos, X. Ji, New mechanistic insights on Na-Ion storage in non-graphitizable carbon, Nano Lett. 15 (2015) 5888–5892.
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