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Boosting the sodium storage performance of coal-based carbon materials through structure modification by solvent extraction
Journal Pre-proof Boosting the sodium storage performance of coal-based carbon materials through structure modification by solvent extraction
Nan Xiao, Yibo Wei, Hongqiang Li, Yuwei Wang, Jinpeng Bai, Jieshan Qiu PII:
S0008-6223(20)30159-7
DOI:
https://doi.org/10.1016/j.carbon.2020.02.015
Reference:
CARBON 15065
To appear in:
Carbon
Received Date:
25 November 2019
Accepted Date:
08 February 2020
Please cite this article as: Nan Xiao, Yibo Wei, Hongqiang Li, Yuwei Wang, Jinpeng Bai, Jieshan Qiu, Boosting the sodium storage performance of coal-based carbon materials through structure modification by solvent extraction, Carbon (2020), https://doi.org/10.1016/j.carbon.2020.02.015
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Journal Pre-proof
Journal Pre-proof Boosting the sodium storage performance of coal-based carbon materials through structure modification by solvent extraction Nan Xiaoa,*, Yibo Weia, Hongqiang Lia, Yuwei Wanga, Jinpeng Baia, Jieshan Qiua,b,*
a
State Key Lab of Fine Chemicals, Liaoning Key Lab for Energy Materials and Chemical
Engineering, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, No. 2 Ling Gong Road, High Technology Zone, Dalian 116024, Liaoning, China. b
College of Chemical Engineering, Beijing University of Chemical Technology, Beijing
100029, China.
* Corresponding author. E-mail addresses: [email protected] (Nan Xiao), [email protected] (Jieshan Qiu). Tel/Fax: +86-411-84986024
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Journal Pre-proof Abstract: High-performance carbon anodes for sodium-ion batteries (SIBs) were synthesized with bituminous coal as precursor through solvent extraction and carbonization. The solvent extraction endows the coal-based carbon a hierarchical pore structure and enlarged interlayer distance, which can shorten the charge diffusion distance, alleviate the volume variation, and improve the ion diffusion rate. As a result, the modified coal-based electrodes exhibit impressive cycling stability (92% capacity retention after 7000th cycle at 2 A g-1) and excellent rate capability (79 mA h g-1 at 10 A g-1).
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Introduction Driven by the worsening ecological environment, pursuing renewable energy such as
wind and solar radiation has already become an urgent task. Unfortunately, these renewable energies are intermittent and dispersive. Therefore, developing low-cost large-scale energy storage systems is indispensable for realizing the smooth integration of renewable energy into the grid [1, 2] . Li-ion batteries (LIBs) have been successfully developed as the most popular power sources for portable electronic devices and electric vehicles in the recent three decades [3]. However, the uneven and rare lithium resources will lead to an escalating cost of LIBs in the future [4]. As compared with lithium, sodium possesses similar physical and chemical properties but more abundant and cheaper. In this regard, sodium-ion batteries (SIBs) are considered as an ideal alternative for large-scale energy storage applications instead of LIBs [5]. However, due to the large ion radius of sodium, the anode materials for LIBs are not suitable for SIBs [2]. So, a main obstacle for developing SIBs is the absence of suitable anode materials, which significantly limits the SIBs’ performance. According to the sodium storage mechanism, SIB anode materials can be divided into three categories: alloy type, conversion type and intercalation type [6, 7]. Although the first two categories exhibit superior energy density, the large volume variation during the charge/discharge process leads to poor cycle stability [8-10]. Carbon materials, the representative of intercalation type, are considered as ideal SIBs’ anodes due to their low cost, excellent corrosion resistance, high conductivity, and environmental friendliness [11]. Recently, a mass of carbons, including hard carbons [12], heteroatom doped carbons [13], porous carbons [14], and nano carbons [15] have been studied as anodes for SIBs. However, 4
Journal Pre-proof most of their synthesis processes are sophisticated and their precursors are expensive. To address these issues, coal-based carbons have been synthesized as low cost SIB anodes [16]. Coal is an abundant fossil fuel, which is also an excellent precursor of carbon materials due to its low cost and high carbon yield [17]. Recently, Hu et al. successfully prepared SIB anode from anthracite, verifying the strong potential of coal-based carbon materials in sodium storage [18]. However, using anthracite as precursor also brings about two problems: First, as the highest rank coal, anthracite has the highest price and the lowest reserves. The second and more important, the high coal rank leads to small interlayer distance of coal-based carbons, which results in a low reversible capacity, poor rate performance, and inferior stability. In order to achieve its practical application, improving the rate performance and cycle stability of coalbased carbons is required. It is generally accepted that enlarging the interlayer distance can improve the carbons’ sodium storage performance. And coal rank directly determines the coal-based carbons’ microstructure including interlayer distance. According to our previous work [17], a middlerank bituminous coal-based carbon exhibits superior electrochemical performance due to its ordered carbon clusters with relative large interlayer distance. On the other hand, creating suitable hierarchical pore structure to enhance the infiltration of electrolyte, shorten the Na+ diffusion distance and accommodate volume expansion is also efficient to improve sodium storage performance. From the point of view of molecular level, middle-rank coal is an organic complex consisting of cross-linked macromolecular network filled with relatively small molecules. Some parts of the small molecular compounds can be extracted by strong solvent to leave three-dimensional macromolecular network. In the present work, this residual three5
Journal Pre-proof dimensional macromolecular network was used to prepare coal-based anode materials. The obtained coal-based anodes are outstanding with high reversible capacity of 312 mAh g-1, excellent cycling stability with a capacity retention of 92% over 7000 cycles, and outstanding rate performance of 79 mAh g-1 at an ultrahigh current density of 10 A g-1. 2.
Experimental 2.1 Material preparation A typical Chinese middle-rank bituminous coal was used as precursor, the ultimate and
proximate analyses of which are listed in Table 1. The raw coal was ground into powders and passed through a 100-mesh sieve prior to use. The thermal extraction was carried out in a 300 mL stainless-steel autoclave. In a typical run, 10 g raw coal powder was extracted by 100 mL wash oil, which is a high temperature coal tar distillate with boiling range of 230-300 oC, at 360 oC under 2 MPa for 2 h. The obtained residue was washed by toluene to remove excess washing oil and dried at 115 oC for 12 h. The yield of residue is 87.6% of the raw coal. After extraction, the total content of oxygen and sulfur in the residue is increased to 18.78% (by difference), while the contents of carbon, hydrogen and nitrogen are decreased to 75.11, 4.73, and 1.38%, respectively. On the contrast, the total content of oxygen and sulfur in the extractable fraction is decreased to 0.27% (by difference), while the content of carbon increase to 91.84%. Then, the obtained black powder was oxidized in air at 300 oC for 2 h and followed 2 h carbonization at 1100 oC in argon flow. The synthesized carbon material was denoted as ECC-1100. For comparison, a control sample was synthesized with raw coal as precursor and denoted as CC-1100.
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Journal Pre-proof Table 1. The proximate and ultimate analyses of raw coal. Proximate analysis (wt %)
Ultimate analysis (wt %)
Sample
Bituminous coal
Mad
Ad
Vdaf
C
N
H
O+S*
2.12
8.49
37.14
86.20
1.60
5.42
6.78
M, moisture; A, ash; V, volatile; ad, air dry basis; d, dry basis; daf, dry and ash-free basis; *, by difference.
2.2 Material characterization The morphology and structure of materials were observed by field emission scanning electron microscope (FESEM, FEI NOVA Nano SEM 450) and high-resolution transmission electron microscope (HRTEM, JEM-2010, JEOL). The structures of carbon materials were characterized by Raman spectroscope (Thermo Fisher Scientific-DXR using 532 nm laser excitation) and X-ray diffraction (XRD, Rigaku D/max-2400) using Cu Kα radiation (λ=1.5406 Ȧ) over the range of 5–80o (2θ). X-ray photoelectron spectroscopy (XPS) spectra were acquired on a ThermoFisher ESCALAB 250Xi X-ray photoelectron spectrometer. Nitrogen adsorptiondesorption isotherms were determined by nitrogen physisorption on a Micromeritics ASAP 2020 analyzer. The specific surface areas were estimated according to the Brunauer–Emmett– Teller (BET) model, and the pore size distribution was calculated by the density functional theory method. 2.3 Electrochemical measurements All the electrochemical tests were conducted in coin cells (CR2016). The working electrode was prepared by spreading the mixed slurry of the active material, carbon black and 7
Journal Pre-proof polyvinylidene fluoride in N-methyl pyrrolidine with a weight ratio of 7: 2: 1 onto copper foil current collector, and then dried at 80 oC in a vacuum oven for 12 h. The electrolyte was a solution of 1.0 M NaClO4 in ethylene carbonate and dimethyl carbonate (1: 1 in volume). A sodium foil was used as the counter electrode and glass fiber was used as the separator. All the assembled operations were operated in an argon-filled glovebox. The discharge and charge tests were carried out on a Land BT2000 battery test system (Wuhan, China) in a voltage range of 0.01–2.8 V at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out on an electrochemical workstation (CHI 660D, CHI Company). CV was carried out at a scan rate of 0.1 mV s-1 in voltage range of 0.01– 2.80 V. EIS was carried out with an amplitude of 5.0 mV and the frequency was ranged from 100 kHz to 0.01 Hz. 3.
Results and discussion The SEM images shown in Fig. 1a and b reveal the granular morphology of CC-1100 and
ECC-1100. As shown in Fig. 1a, the particle size distribution of CC-1100 is in the range of 1 to 50 μm, while the particle size distribution of ECC-1100 decreases to less than 30 μm (Fig. 1b), indicating that the integrality of the coal particles was destroyed by the extraction. Compared with nano-sized materials, micron-sized materials will be of great benefit to high compaction density and consequently high volume specific capacity. Moreover, CC-1100 presents a dense structure (Fig. 1c) while ECC-1100 exhibits a loose structure (Fig. 1d) with abundant macropores, indicating the pore-forming effect of the thermal extraction process. The abundant macrospores are supposed to act as the reservoir for electrolyte storage, shorten the Na+ diffusion distance and accommodate volume expansion, which will improve the carbons’ 8
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Fig. 1. SEM images of CC-1100 (a, c) and ECC-1100 (b, d).
To further investigate the microstructure of CC-1100 and ECC-1100, TEM and HRTEM were carried out and the results are shown in Fig. 2. By comparing the TEM images of CC1100 and ECC-1100 (Fig. 2a and b), the loose structure of ECC-1100 is further confirmed. As shown in Fig. 2c, with a middle-rank coal as precursor, CC-1100 exhibits a typical turbostratic microstructure with nanosized carbon clusters as basic structural unit, indicating its nongraphitic nature. By extracting, the carbon clusters in ECC-1100 become smaller and more disordered, as shown in Fig. 2d, indicating the effect of the thermal extraction on regulating the coal-based carbon’s microstructure. The contour curve of the carbon cluster is demonstrated by red boxes, which are marked in Fig. 2c and d. According to the corresponding scale, the 9
Journal Pre-proof carbon interlayer distance of ECC-1100 is 0.39 nm, which is larger than that of CC-1100 (0.38 nm). The expanded interlayer distance of ECC-1100 is beneficial for the intercalation and deintercalation of Na+ [19] .
Fig. 2. TEM images of CC-1100 (a) and ECC-1100 (b); HRTEM images and contrast profiles indicate interlayer distance of CC-1100 (c) and ECC-1100 (d).
The structural characteristics of CC-1100 and ECC-1100 were further investigated by XRD and Raman spectroscopy, and the results are shown in Fig. 3a and b. The XRD pattern shows two broad diffraction peaks at around 24o and 43o which are attributed to the (002) and 10
Journal Pre-proof (100) diffraction of carbon materials. The (002) peak position shifts from 23.9° to 23.3°after extraction, corresponding to an increase of the interlayer distance (d002) from 0.37 to 0.38 nm, as shown in Fig. 3a. This XRD result illustrates that extraction can expand the carbon interlayer distance, which coincides with the HETEM images. The Raman spectra presents two characteristic bands of the D band at 1345 cm-1 (the defect-induced band) and the G band at 1593 cm-1 (the crystalline graphite band) [20]. The intensity ratio of D and G bands also can be used to determine the degree of crystallinity in the carbon. The ID/IG ratios of the samples increase from 1.34 to 1.51 after extraction, indicating that the thermal extraction can increase the coal-based carbon’s disorder degree, which is consistent with the HETEM and XRD results. XPS was performed to investigate the effect of extraction on the coal-based carbons’ chemical composition in Fig. 3c. In the XPS survey spectrum of CC-1100, two peaks indicating C 1s (284.6 eV) and O 1s (532.4 eV) are observed. Atomic percentages of the above elements are calculated to be 95.17% and 4.83%. In the case of ECC-1100, the oxygen atomic content increase to 10.34%, which is consistent with the ultimate analysis, i.e. the oxygen species can be enriched in the residue coal [21]. As shown in Fig. 3d, the high-resolution XPS spectrum of O 1s can be resolved into three peaks, C=O carbonyl groups (O-I, 531.3 eV), C-OH phenol groups or C-O-C ether groups (O-II, 532.5 eV) and COOH carboxyl groups (O-III, 533.5 eV) [22]. The area percentages of the three fitting peaks are successively 22.4, 57.8, and 19.8%, representing the content percentages of the corresponding three O pieces. The oxygencontaining groups can create more defects and improve the wettability of the surface, which is beneficial to improve the carbons’ electrochemical performance [24]. A proper pore structure of carbons can provide reservoir for electrolyte storage, shorten 11
Journal Pre-proof the diffusion distance of Na+, accommodate volume expansion, and act as active sites for sodium storage. However, an excess surface area will lower the initial Coulombic efficiency (ICE) and decrease the materials’ compaction density. Nitrogen adsorption-desorption measures were performed to investigate the effect of the extraction on the samples’ pore structure. The nitrogen adsorption–desorption isotherms and the pore size distributions of CC1100 and ECC-1100 are shown in Fig. 3e and 3f. The rapid increase of adsorption volume at low relative pressure (P/P0 =0-0.01) indicates the microporous material nature of CC-1100. Although ECC-1100 exhibits a similar adsorption-desorption curve at low relative pressure, an obvious adsorption uptake close to relative pressure of 1.0 appeared, indicating the macropore forming role of the extraction. The pore volume of ECC-1100 (0.481 cm3 g-1) is almost twice as much as that of CC-1100 (0.270 cm3 g-1), which means the former forms a hierarchical pore structure so as to accommodate more electrolyte and volume variation. The BET specific surface areas of ECC-1100 and CC-1100 are quite similar, 316 m2 g-1 and 296 m2 g-1, respectively, which avoids a slump of material’s ICE. Based on the experimental results, we propose a process to explain the effect of the thermal extraction on the structure of the coal-based carbons. As mentioned above, coal can be considered as an organic complex consisting of cross-linked macromolecular network filled with relatively small molecules. Through thermal extraction, the solvent dissolves the soluble small molecules, diffuses into the raw coal particles, and leaves three-dimensional macromolecular network to form a hierarchical pore structure. Besides, the cross-linked macromolecular network partially retains the molecular structural characteristic of coals’ precursor, i.e., plant debris with high oxygen content, and tends to convert into hard carbon 12
Journal Pre-proof with more disordered microstructure. The extractable fraction, on the other side, has a smaller average molecular weight and higher degree of aromaticity, which tends to convert into soft carbon with small interlayer distance through liquid phase carbonization. Therefore, extracting the small molecule fraction to leave the three-dimensional macromolecular network contributes to form a hierarchical porous coal-based disordered carbon with expanded interlayer distance, which is supposed to improve the sodium storage performance of coal-based carbon.
Fig.3. (a) XRD patterns, (b) Raman spectra, and (c) XPS survey spectra of CC-1100 and ECC1100; (d) high-resolution XPS spectra for O 1s of ECC-1100; (e) nitrogen adsorption– desorption isotherms and (f) the pore size distribution curves of CC-1100 and ECC-1100.
Electrochemical measurements of coal-based carbons were carried in half cells with sodium as the counter electrodes. The galvanostatic charge-discharge profiles of initial three cycles of CC-1100 and ECC-1100 electrodes were measured in voltage range of 0.01–2.8 V vs. Na+/Na at a constant current density of 100 mA g-1 as shown in Fig. 4a and b. The CC-1100 13
Journal Pre-proof electrode delivers a discharge capacity of 386 mAh g-1 and charge capacity of 215 mAh g-1 with an ICE of 55.8%, while the ECC-1100 electrode delivers a discharge capacity of 563 mAh g-1 and charge capacity of 306 mAh g-1 with an ICE of 54.3%. The initial irreversible capacity loss of electrodes is caused by the formation of solid electrolyte interphase (SEI) film and irreversible insertion of Na+. The low ICE of ECC-1100 is comparable to those in recently reported literature and could be attributed to its large specific surface area and abundant oxygen-containing defects [25, 26]. Thanks to the similar specific surface area compared with CC-1100, ECC-1100 exhibits a similar ICE to that of the former. Both the voltage profiles of CC-1100 and ECC-1100 indicate two distinct regions which are similar to nongraphitizable carbon, i.e., a slope between 1.0 – 0.1 V and plateau voltage bellow 0.1 V [27]. It is generally recognized that the plateau is associated with ion insertion into carbon layers. The reversible plateau capacity of ECC-1100 is 172 mA h g-1 (334-162 mA h g-1), which is much higher than that of CC-1100 (115 mA h g-1, 231-116 mA h g-1). The high reversible plateau capacity of ECC-1100 should be attributed to its expanded interlayer distance, which facilitates Na+ insertion and extraction. The electrochemical performance of CC-1100 and ECC-1100 was investigated by CV sweep in the voltage range of 0.01-2.8V. As shown in Fig. 4c and d, during the first cathodic scan, two pronounced reduction peaks appear around 0.5 V and 1.0 V and disappear in the following scan, which can be attributed to the formation of SEI film and the reaction of the electrolyte with functional groups on the carbon surface. Compared to CC-1100, ECC-1100 possesses a pair of larger redox peaks at 0.05 V that can be attributed to the Na+ reversible insertion and extraction, indicating a larger insertion capacity in ECC-1100 due to its larger 14
Journal Pre-proof carbon interlayer distance.
Fig. 4. Charge–discharge profiles of initial three cycles for the (a) CC-1100 and (b) ECC-1100 at a current density of 100 mA g-1; CV curves of (c) CC-1100 and (d) ECC-1100 in a voltage range of 0.01–2.8 V at 0.1 mV s-1.
The rate performance of the samples is shown in Fig. 5a. The cells were discharged and charged at various current densities from 0.05 to 10 A g-1. The reversible capacities of ECC1100 are retained of 334, 276, 220, 140, 111 and 87 mAh g-1 at current densities of 0.1, 0.2, 0.5, 1, 2 and 5 A g-1, respectively. Even when the current density increases to 10 A g-1, the 15
Journal Pre-proof ECC-1100 can retain a capacity of 79 mA h g-1. A reversible capacity of 318 mA h g-1 can be recovered as the current density returns to 100 mA g-1. In comparison, CC-1100 exhibits an inferior rate performance with reversible capacity of about 53 mAh g-1 at 5 A g-1. Fig. 5b shows the cycling performance of CC-1100 and ECC-1100 at 2 A g-1. The ECC-1100 sample retains a capacity of 124 mAh g-1 corresponding to a capacity retention of 92% after 7000 cycles. The Coulombic efficiency of ECC-1100 approaches nearly 100 % after the first several cycles, indicating the good reversibility of Na+ insertion/extraction process. On the contrast, CC-1100 only retains a capacity of 75 mAh g-1 after 4000 cycles and the capacity becomes unstable. EIS measures were performed to investigate the samples’ kinetic properties. As shown in Fig. 5c, the EIS curves involve a semicircle in high frequency region corresponding to the charge-transfer resistance and a line in low frequency region corresponding to Na+ diffusion resistance in the electrodes. Although the larger semicircle diameter of ECC-1100 indicative of a slightly declining electronic conductivity due to its relatively low degree of crystallinity and loose structure, the Na+ diffusion coefficient of ECC-1100 (1.09×10−13 cm2 s−1) calculated from the line of Z’-ω−1/2
[28] (Fig. 5d), is two orders of magnitude higher than that of CC-
1100 (1.41×10−15 cm2 s−1). Therefore, ECC-1100 exhibits a superior rate performance due to its larger carbon interlayer distance, improved wettability by hydroxyl groups and shortened diffusion distance due to hierarchical porous structure.
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Fig. 5. (a) Rate capability of CC-1100 and ECC-1100 from 0.1 A g-1 to 10.0 A g-1; (b) cyclic performance of CC-1100 and ECC-1100 at a current rate of 2 A g-1; (c) EIS and (d) the line of Z’ ∼ ω−1/2 of CC-1100 and ECC-1100.
Fig. 6a and b shows the CV curves at different potential scan rates of 0.1-5 mV s-1 to study the reaction kinetics of CC-1100 and ECC-1100. The CV profiles display similar shape with visible peaks in both anodic and cathodic processes, demonstrating the coal-based carbons’ high reversibility even at high rates. The integral area of ECC-1100 is much larger than that of CC-1100, indicative of its superior rate performance. In a general way, the relationship between the measured current (i) and the scan rate (v) obeys a power law: i = avb
(1)
where a and b are empirical parameters [29]. The b value can be calculated from the slope of 17
Journal Pre-proof the log(v) versus log(i), which is considered as an indicator of electrodes’ sodium storage mechanism. The b value of 1 represents ideal capacitive behavior, while the b value of 0.5 indicates the diffusion-controlled process [30, 31]. As shown in Fig. 6c, d, e and f, in the charge process, the b values of the CC-1100 electrode are 0.996, 0.986, 0.989, 0.971 and 0.884 at 0.6, 0.8, 1.8, 2.3 and 2.8 V, while which are 0.991, 0.972, 0.986, 0.966 and 0.858 for ECC-1100 electrode, respectively. At 0.3, 0.6 and 1.6 V in the discharge process, the b values of the CC1100 electrode are 0.813, 0.855 and 0.841, while which are 0.801, 0.835 and 0.715 for ECC1100 electrode, respectively. This result suggests that the sodium storage process is a synergistic effect of surface capacitive mechanism and diffusion process, while the former process is more dominant for charge storage. By comparison, the ECC-1100 shows a higher contribution of diffusion-controlled charge storage than that of the CC-1100 over a wide voltage range, which can be attributed to its enlarged interlayer space.
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Fig. 6. Kinetics analysis of the electrochemical behaviour towards Na+ for CC-1100 (a) and ECC-1100 (b); the determination of the b-value of CC-1100 during charge (c) and discharge (e) and the b-value of ECC-1100 during charge (d) and discharge (f) using the relationship between the peak current and scan rate.
To quantitatively separate the capacitive and diffusion controlled charge storage contributions, the current response (i) at a fixed potential (V) is separated into surface capacitive effects (k1v) and diffusion-controlled insertion processes (k2v1/2) using the following 19
Journal Pre-proof equation [32] : i(V) = k1 + k2v1/2
(2)
We can rearrange equation (2) to analyze purposes: i(V)/v1/2 = k1v1/2 + k2
(3)
Fig. 7. Contribution ratio of the capacitive current and cation intercalation-controlled current of ECC-1100 at (a) 0.5 mV s-1 and (b) 5 mV s-1.
k1 and k2 can be easily determined by plotting i(V)/v1/2 versus v1/2 plots. With the value of k1, we are able to separate the capacitive current i(V)=k1v from the total measured current. Fig. 7 a and b compare the CV curves of ECC-1100 at different voltages for the capacitive contribution (the shaded region). When the scan rate is 0.5 mV s-1, the percentage of surface capacitive contribution is 63%, while 81% at a scan rate of 5 mV s-1. The increased percentage of the capacity is contributed by the surface capacitive effects. The enhanced surface capacitive contribution results in the excellent rate performance of the ECC-1100. 4. Conclusions A novel coal-based carbon SIB anode was synthesized by carbonizing the thermally 20
Journal Pre-proof extracted bituminous coal. Through thermal extraction, the solvent dissolves the soluble small molecules, diffuses into the raw coal particles, and leaves three-dimensional macromolecular network to form a hierarchical pore structure. The residual three-dimensional macromolecular network with high oxygen content tends to form a more disordered microstructure with enlarged interlayer distance. The hierarchical pore structure and enlarged interlayer distance can shorten the charge diffusion distance, alleviate the volume variation, and improve the ion diffusion rate. As a result, the modified coal-based electrodes exhibit impressive cycling stability (92% capacity retention after 7000th cycle at 2 A g-1) and excellent rate capability (79 mA h g-1 at 10 A g-1) Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC, No. U1508201).
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J.S. Q. and N.X. supervised the project. Y.B. W. designed the project and carried out all electrochemical measurements and parts of the catalyst’s characterizations. Y.B. W. wrote the manuscript. Y.B. W., H.Q. Li., Y.W. Wang. and J.P. Bai. contributed to the analysis of electrochemical data. Y.B. Wei and H.Q. Li carried out the XPS analysis. Y.W. Wang. performed the TEM measurements. J.P. Bai. performed the SEM measurements. All authors contributed to data analysis and approved the final version of the manuscript.
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Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Nan Xiao, Yibo Wei, Hongqiang Li, Yuwei Wang, Jinpeng Bai, Jieshan Qiu PII:
S0008-6223(20)30159-7
DOI:
https://doi.org/10.1016/j.carbon.2020.02.015
Reference:
CARBON 15065
To appear in:
Carbon
Received Date:
25 November 2019
Accepted Date:
08 February 2020
Please cite this article as: Nan Xiao, Yibo Wei, Hongqiang Li, Yuwei Wang, Jinpeng Bai, Jieshan Qiu, Boosting the sodium storage performance of coal-based carbon materials through structure modification by solvent extraction, Carbon (2020), https://doi.org/10.1016/j.carbon.2020.02.015
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Journal Pre-proof Boosting the sodium storage performance of coal-based carbon materials through structure modification by solvent extraction Nan Xiaoa,*, Yibo Weia, Hongqiang Lia, Yuwei Wanga, Jinpeng Baia, Jieshan Qiua,b,*
a
State Key Lab of Fine Chemicals, Liaoning Key Lab for Energy Materials and Chemical
Engineering, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, No. 2 Ling Gong Road, High Technology Zone, Dalian 116024, Liaoning, China. b
College of Chemical Engineering, Beijing University of Chemical Technology, Beijing
100029, China.
* Corresponding author. E-mail addresses: [email protected] (Nan Xiao), [email protected] (Jieshan Qiu). Tel/Fax: +86-411-84986024
1
Journal Pre-proof Abstract: High-performance carbon anodes for sodium-ion batteries (SIBs) were synthesized with bituminous coal as precursor through solvent extraction and carbonization. The solvent extraction endows the coal-based carbon a hierarchical pore structure and enlarged interlayer distance, which can shorten the charge diffusion distance, alleviate the volume variation, and improve the ion diffusion rate. As a result, the modified coal-based electrodes exhibit impressive cycling stability (92% capacity retention after 7000th cycle at 2 A g-1) and excellent rate capability (79 mA h g-1 at 10 A g-1).
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Introduction Driven by the worsening ecological environment, pursuing renewable energy such as
wind and solar radiation has already become an urgent task. Unfortunately, these renewable energies are intermittent and dispersive. Therefore, developing low-cost large-scale energy storage systems is indispensable for realizing the smooth integration of renewable energy into the grid [1, 2] . Li-ion batteries (LIBs) have been successfully developed as the most popular power sources for portable electronic devices and electric vehicles in the recent three decades [3]. However, the uneven and rare lithium resources will lead to an escalating cost of LIBs in the future [4]. As compared with lithium, sodium possesses similar physical and chemical properties but more abundant and cheaper. In this regard, sodium-ion batteries (SIBs) are considered as an ideal alternative for large-scale energy storage applications instead of LIBs [5]. However, due to the large ion radius of sodium, the anode materials for LIBs are not suitable for SIBs [2]. So, a main obstacle for developing SIBs is the absence of suitable anode materials, which significantly limits the SIBs’ performance. According to the sodium storage mechanism, SIB anode materials can be divided into three categories: alloy type, conversion type and intercalation type [6, 7]. Although the first two categories exhibit superior energy density, the large volume variation during the charge/discharge process leads to poor cycle stability [8-10]. Carbon materials, the representative of intercalation type, are considered as ideal SIBs’ anodes due to their low cost, excellent corrosion resistance, high conductivity, and environmental friendliness [11]. Recently, a mass of carbons, including hard carbons [12], heteroatom doped carbons [13], porous carbons [14], and nano carbons [15] have been studied as anodes for SIBs. However, 4
Journal Pre-proof most of their synthesis processes are sophisticated and their precursors are expensive. To address these issues, coal-based carbons have been synthesized as low cost SIB anodes [16]. Coal is an abundant fossil fuel, which is also an excellent precursor of carbon materials due to its low cost and high carbon yield [17]. Recently, Hu et al. successfully prepared SIB anode from anthracite, verifying the strong potential of coal-based carbon materials in sodium storage [18]. However, using anthracite as precursor also brings about two problems: First, as the highest rank coal, anthracite has the highest price and the lowest reserves. The second and more important, the high coal rank leads to small interlayer distance of coal-based carbons, which results in a low reversible capacity, poor rate performance, and inferior stability. In order to achieve its practical application, improving the rate performance and cycle stability of coalbased carbons is required. It is generally accepted that enlarging the interlayer distance can improve the carbons’ sodium storage performance. And coal rank directly determines the coal-based carbons’ microstructure including interlayer distance. According to our previous work [17], a middlerank bituminous coal-based carbon exhibits superior electrochemical performance due to its ordered carbon clusters with relative large interlayer distance. On the other hand, creating suitable hierarchical pore structure to enhance the infiltration of electrolyte, shorten the Na+ diffusion distance and accommodate volume expansion is also efficient to improve sodium storage performance. From the point of view of molecular level, middle-rank coal is an organic complex consisting of cross-linked macromolecular network filled with relatively small molecules. Some parts of the small molecular compounds can be extracted by strong solvent to leave three-dimensional macromolecular network. In the present work, this residual three5
Journal Pre-proof dimensional macromolecular network was used to prepare coal-based anode materials. The obtained coal-based anodes are outstanding with high reversible capacity of 312 mAh g-1, excellent cycling stability with a capacity retention of 92% over 7000 cycles, and outstanding rate performance of 79 mAh g-1 at an ultrahigh current density of 10 A g-1. 2.
Experimental 2.1 Material preparation A typical Chinese middle-rank bituminous coal was used as precursor, the ultimate and
proximate analyses of which are listed in Table 1. The raw coal was ground into powders and passed through a 100-mesh sieve prior to use. The thermal extraction was carried out in a 300 mL stainless-steel autoclave. In a typical run, 10 g raw coal powder was extracted by 100 mL wash oil, which is a high temperature coal tar distillate with boiling range of 230-300 oC, at 360 oC under 2 MPa for 2 h. The obtained residue was washed by toluene to remove excess washing oil and dried at 115 oC for 12 h. The yield of residue is 87.6% of the raw coal. After extraction, the total content of oxygen and sulfur in the residue is increased to 18.78% (by difference), while the contents of carbon, hydrogen and nitrogen are decreased to 75.11, 4.73, and 1.38%, respectively. On the contrast, the total content of oxygen and sulfur in the extractable fraction is decreased to 0.27% (by difference), while the content of carbon increase to 91.84%. Then, the obtained black powder was oxidized in air at 300 oC for 2 h and followed 2 h carbonization at 1100 oC in argon flow. The synthesized carbon material was denoted as ECC-1100. For comparison, a control sample was synthesized with raw coal as precursor and denoted as CC-1100.
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Journal Pre-proof Table 1. The proximate and ultimate analyses of raw coal. Proximate analysis (wt %)
Ultimate analysis (wt %)
Sample
Bituminous coal
Mad
Ad
Vdaf
C
N
H
O+S*
2.12
8.49
37.14
86.20
1.60
5.42
6.78
M, moisture; A, ash; V, volatile; ad, air dry basis; d, dry basis; daf, dry and ash-free basis; *, by difference.
2.2 Material characterization The morphology and structure of materials were observed by field emission scanning electron microscope (FESEM, FEI NOVA Nano SEM 450) and high-resolution transmission electron microscope (HRTEM, JEM-2010, JEOL). The structures of carbon materials were characterized by Raman spectroscope (Thermo Fisher Scientific-DXR using 532 nm laser excitation) and X-ray diffraction (XRD, Rigaku D/max-2400) using Cu Kα radiation (λ=1.5406 Ȧ) over the range of 5–80o (2θ). X-ray photoelectron spectroscopy (XPS) spectra were acquired on a ThermoFisher ESCALAB 250Xi X-ray photoelectron spectrometer. Nitrogen adsorptiondesorption isotherms were determined by nitrogen physisorption on a Micromeritics ASAP 2020 analyzer. The specific surface areas were estimated according to the Brunauer–Emmett– Teller (BET) model, and the pore size distribution was calculated by the density functional theory method. 2.3 Electrochemical measurements All the electrochemical tests were conducted in coin cells (CR2016). The working electrode was prepared by spreading the mixed slurry of the active material, carbon black and 7
Journal Pre-proof polyvinylidene fluoride in N-methyl pyrrolidine with a weight ratio of 7: 2: 1 onto copper foil current collector, and then dried at 80 oC in a vacuum oven for 12 h. The electrolyte was a solution of 1.0 M NaClO4 in ethylene carbonate and dimethyl carbonate (1: 1 in volume). A sodium foil was used as the counter electrode and glass fiber was used as the separator. All the assembled operations were operated in an argon-filled glovebox. The discharge and charge tests were carried out on a Land BT2000 battery test system (Wuhan, China) in a voltage range of 0.01–2.8 V at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out on an electrochemical workstation (CHI 660D, CHI Company). CV was carried out at a scan rate of 0.1 mV s-1 in voltage range of 0.01– 2.80 V. EIS was carried out with an amplitude of 5.0 mV and the frequency was ranged from 100 kHz to 0.01 Hz. 3.
Results and discussion The SEM images shown in Fig. 1a and b reveal the granular morphology of CC-1100 and
ECC-1100. As shown in Fig. 1a, the particle size distribution of CC-1100 is in the range of 1 to 50 μm, while the particle size distribution of ECC-1100 decreases to less than 30 μm (Fig. 1b), indicating that the integrality of the coal particles was destroyed by the extraction. Compared with nano-sized materials, micron-sized materials will be of great benefit to high compaction density and consequently high volume specific capacity. Moreover, CC-1100 presents a dense structure (Fig. 1c) while ECC-1100 exhibits a loose structure (Fig. 1d) with abundant macropores, indicating the pore-forming effect of the thermal extraction process. The abundant macrospores are supposed to act as the reservoir for electrolyte storage, shorten the Na+ diffusion distance and accommodate volume expansion, which will improve the carbons’ 8
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Fig. 1. SEM images of CC-1100 (a, c) and ECC-1100 (b, d).
To further investigate the microstructure of CC-1100 and ECC-1100, TEM and HRTEM were carried out and the results are shown in Fig. 2. By comparing the TEM images of CC1100 and ECC-1100 (Fig. 2a and b), the loose structure of ECC-1100 is further confirmed. As shown in Fig. 2c, with a middle-rank coal as precursor, CC-1100 exhibits a typical turbostratic microstructure with nanosized carbon clusters as basic structural unit, indicating its nongraphitic nature. By extracting, the carbon clusters in ECC-1100 become smaller and more disordered, as shown in Fig. 2d, indicating the effect of the thermal extraction on regulating the coal-based carbon’s microstructure. The contour curve of the carbon cluster is demonstrated by red boxes, which are marked in Fig. 2c and d. According to the corresponding scale, the 9
Journal Pre-proof carbon interlayer distance of ECC-1100 is 0.39 nm, which is larger than that of CC-1100 (0.38 nm). The expanded interlayer distance of ECC-1100 is beneficial for the intercalation and deintercalation of Na+ [19] .
Fig. 2. TEM images of CC-1100 (a) and ECC-1100 (b); HRTEM images and contrast profiles indicate interlayer distance of CC-1100 (c) and ECC-1100 (d).
The structural characteristics of CC-1100 and ECC-1100 were further investigated by XRD and Raman spectroscopy, and the results are shown in Fig. 3a and b. The XRD pattern shows two broad diffraction peaks at around 24o and 43o which are attributed to the (002) and 10
Journal Pre-proof (100) diffraction of carbon materials. The (002) peak position shifts from 23.9° to 23.3°after extraction, corresponding to an increase of the interlayer distance (d002) from 0.37 to 0.38 nm, as shown in Fig. 3a. This XRD result illustrates that extraction can expand the carbon interlayer distance, which coincides with the HETEM images. The Raman spectra presents two characteristic bands of the D band at 1345 cm-1 (the defect-induced band) and the G band at 1593 cm-1 (the crystalline graphite band) [20]. The intensity ratio of D and G bands also can be used to determine the degree of crystallinity in the carbon. The ID/IG ratios of the samples increase from 1.34 to 1.51 after extraction, indicating that the thermal extraction can increase the coal-based carbon’s disorder degree, which is consistent with the HETEM and XRD results. XPS was performed to investigate the effect of extraction on the coal-based carbons’ chemical composition in Fig. 3c. In the XPS survey spectrum of CC-1100, two peaks indicating C 1s (284.6 eV) and O 1s (532.4 eV) are observed. Atomic percentages of the above elements are calculated to be 95.17% and 4.83%. In the case of ECC-1100, the oxygen atomic content increase to 10.34%, which is consistent with the ultimate analysis, i.e. the oxygen species can be enriched in the residue coal [21]. As shown in Fig. 3d, the high-resolution XPS spectrum of O 1s can be resolved into three peaks, C=O carbonyl groups (O-I, 531.3 eV), C-OH phenol groups or C-O-C ether groups (O-II, 532.5 eV) and COOH carboxyl groups (O-III, 533.5 eV) [22]. The area percentages of the three fitting peaks are successively 22.4, 57.8, and 19.8%, representing the content percentages of the corresponding three O pieces. The oxygencontaining groups can create more defects and improve the wettability of the surface, which is beneficial to improve the carbons’ electrochemical performance [24]. A proper pore structure of carbons can provide reservoir for electrolyte storage, shorten 11
Journal Pre-proof the diffusion distance of Na+, accommodate volume expansion, and act as active sites for sodium storage. However, an excess surface area will lower the initial Coulombic efficiency (ICE) and decrease the materials’ compaction density. Nitrogen adsorption-desorption measures were performed to investigate the effect of the extraction on the samples’ pore structure. The nitrogen adsorption–desorption isotherms and the pore size distributions of CC1100 and ECC-1100 are shown in Fig. 3e and 3f. The rapid increase of adsorption volume at low relative pressure (P/P0 =0-0.01) indicates the microporous material nature of CC-1100. Although ECC-1100 exhibits a similar adsorption-desorption curve at low relative pressure, an obvious adsorption uptake close to relative pressure of 1.0 appeared, indicating the macropore forming role of the extraction. The pore volume of ECC-1100 (0.481 cm3 g-1) is almost twice as much as that of CC-1100 (0.270 cm3 g-1), which means the former forms a hierarchical pore structure so as to accommodate more electrolyte and volume variation. The BET specific surface areas of ECC-1100 and CC-1100 are quite similar, 316 m2 g-1 and 296 m2 g-1, respectively, which avoids a slump of material’s ICE. Based on the experimental results, we propose a process to explain the effect of the thermal extraction on the structure of the coal-based carbons. As mentioned above, coal can be considered as an organic complex consisting of cross-linked macromolecular network filled with relatively small molecules. Through thermal extraction, the solvent dissolves the soluble small molecules, diffuses into the raw coal particles, and leaves three-dimensional macromolecular network to form a hierarchical pore structure. Besides, the cross-linked macromolecular network partially retains the molecular structural characteristic of coals’ precursor, i.e., plant debris with high oxygen content, and tends to convert into hard carbon 12
Journal Pre-proof with more disordered microstructure. The extractable fraction, on the other side, has a smaller average molecular weight and higher degree of aromaticity, which tends to convert into soft carbon with small interlayer distance through liquid phase carbonization. Therefore, extracting the small molecule fraction to leave the three-dimensional macromolecular network contributes to form a hierarchical porous coal-based disordered carbon with expanded interlayer distance, which is supposed to improve the sodium storage performance of coal-based carbon.
Fig.3. (a) XRD patterns, (b) Raman spectra, and (c) XPS survey spectra of CC-1100 and ECC1100; (d) high-resolution XPS spectra for O 1s of ECC-1100; (e) nitrogen adsorption– desorption isotherms and (f) the pore size distribution curves of CC-1100 and ECC-1100.
Electrochemical measurements of coal-based carbons were carried in half cells with sodium as the counter electrodes. The galvanostatic charge-discharge profiles of initial three cycles of CC-1100 and ECC-1100 electrodes were measured in voltage range of 0.01–2.8 V vs. Na+/Na at a constant current density of 100 mA g-1 as shown in Fig. 4a and b. The CC-1100 13
Journal Pre-proof electrode delivers a discharge capacity of 386 mAh g-1 and charge capacity of 215 mAh g-1 with an ICE of 55.8%, while the ECC-1100 electrode delivers a discharge capacity of 563 mAh g-1 and charge capacity of 306 mAh g-1 with an ICE of 54.3%. The initial irreversible capacity loss of electrodes is caused by the formation of solid electrolyte interphase (SEI) film and irreversible insertion of Na+. The low ICE of ECC-1100 is comparable to those in recently reported literature and could be attributed to its large specific surface area and abundant oxygen-containing defects [25, 26]. Thanks to the similar specific surface area compared with CC-1100, ECC-1100 exhibits a similar ICE to that of the former. Both the voltage profiles of CC-1100 and ECC-1100 indicate two distinct regions which are similar to nongraphitizable carbon, i.e., a slope between 1.0 – 0.1 V and plateau voltage bellow 0.1 V [27]. It is generally recognized that the plateau is associated with ion insertion into carbon layers. The reversible plateau capacity of ECC-1100 is 172 mA h g-1 (334-162 mA h g-1), which is much higher than that of CC-1100 (115 mA h g-1, 231-116 mA h g-1). The high reversible plateau capacity of ECC-1100 should be attributed to its expanded interlayer distance, which facilitates Na+ insertion and extraction. The electrochemical performance of CC-1100 and ECC-1100 was investigated by CV sweep in the voltage range of 0.01-2.8V. As shown in Fig. 4c and d, during the first cathodic scan, two pronounced reduction peaks appear around 0.5 V and 1.0 V and disappear in the following scan, which can be attributed to the formation of SEI film and the reaction of the electrolyte with functional groups on the carbon surface. Compared to CC-1100, ECC-1100 possesses a pair of larger redox peaks at 0.05 V that can be attributed to the Na+ reversible insertion and extraction, indicating a larger insertion capacity in ECC-1100 due to its larger 14
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Fig. 4. Charge–discharge profiles of initial three cycles for the (a) CC-1100 and (b) ECC-1100 at a current density of 100 mA g-1; CV curves of (c) CC-1100 and (d) ECC-1100 in a voltage range of 0.01–2.8 V at 0.1 mV s-1.
The rate performance of the samples is shown in Fig. 5a. The cells were discharged and charged at various current densities from 0.05 to 10 A g-1. The reversible capacities of ECC1100 are retained of 334, 276, 220, 140, 111 and 87 mAh g-1 at current densities of 0.1, 0.2, 0.5, 1, 2 and 5 A g-1, respectively. Even when the current density increases to 10 A g-1, the 15
Journal Pre-proof ECC-1100 can retain a capacity of 79 mA h g-1. A reversible capacity of 318 mA h g-1 can be recovered as the current density returns to 100 mA g-1. In comparison, CC-1100 exhibits an inferior rate performance with reversible capacity of about 53 mAh g-1 at 5 A g-1. Fig. 5b shows the cycling performance of CC-1100 and ECC-1100 at 2 A g-1. The ECC-1100 sample retains a capacity of 124 mAh g-1 corresponding to a capacity retention of 92% after 7000 cycles. The Coulombic efficiency of ECC-1100 approaches nearly 100 % after the first several cycles, indicating the good reversibility of Na+ insertion/extraction process. On the contrast, CC-1100 only retains a capacity of 75 mAh g-1 after 4000 cycles and the capacity becomes unstable. EIS measures were performed to investigate the samples’ kinetic properties. As shown in Fig. 5c, the EIS curves involve a semicircle in high frequency region corresponding to the charge-transfer resistance and a line in low frequency region corresponding to Na+ diffusion resistance in the electrodes. Although the larger semicircle diameter of ECC-1100 indicative of a slightly declining electronic conductivity due to its relatively low degree of crystallinity and loose structure, the Na+ diffusion coefficient of ECC-1100 (1.09×10−13 cm2 s−1) calculated from the line of Z’-ω−1/2
[28] (Fig. 5d), is two orders of magnitude higher than that of CC-
1100 (1.41×10−15 cm2 s−1). Therefore, ECC-1100 exhibits a superior rate performance due to its larger carbon interlayer distance, improved wettability by hydroxyl groups and shortened diffusion distance due to hierarchical porous structure.
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Fig. 5. (a) Rate capability of CC-1100 and ECC-1100 from 0.1 A g-1 to 10.0 A g-1; (b) cyclic performance of CC-1100 and ECC-1100 at a current rate of 2 A g-1; (c) EIS and (d) the line of Z’ ∼ ω−1/2 of CC-1100 and ECC-1100.
Fig. 6a and b shows the CV curves at different potential scan rates of 0.1-5 mV s-1 to study the reaction kinetics of CC-1100 and ECC-1100. The CV profiles display similar shape with visible peaks in both anodic and cathodic processes, demonstrating the coal-based carbons’ high reversibility even at high rates. The integral area of ECC-1100 is much larger than that of CC-1100, indicative of its superior rate performance. In a general way, the relationship between the measured current (i) and the scan rate (v) obeys a power law: i = avb
(1)
where a and b are empirical parameters [29]. The b value can be calculated from the slope of 17
Journal Pre-proof the log(v) versus log(i), which is considered as an indicator of electrodes’ sodium storage mechanism. The b value of 1 represents ideal capacitive behavior, while the b value of 0.5 indicates the diffusion-controlled process [30, 31]. As shown in Fig. 6c, d, e and f, in the charge process, the b values of the CC-1100 electrode are 0.996, 0.986, 0.989, 0.971 and 0.884 at 0.6, 0.8, 1.8, 2.3 and 2.8 V, while which are 0.991, 0.972, 0.986, 0.966 and 0.858 for ECC-1100 electrode, respectively. At 0.3, 0.6 and 1.6 V in the discharge process, the b values of the CC1100 electrode are 0.813, 0.855 and 0.841, while which are 0.801, 0.835 and 0.715 for ECC1100 electrode, respectively. This result suggests that the sodium storage process is a synergistic effect of surface capacitive mechanism and diffusion process, while the former process is more dominant for charge storage. By comparison, the ECC-1100 shows a higher contribution of diffusion-controlled charge storage than that of the CC-1100 over a wide voltage range, which can be attributed to its enlarged interlayer space.
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Fig. 6. Kinetics analysis of the electrochemical behaviour towards Na+ for CC-1100 (a) and ECC-1100 (b); the determination of the b-value of CC-1100 during charge (c) and discharge (e) and the b-value of ECC-1100 during charge (d) and discharge (f) using the relationship between the peak current and scan rate.
To quantitatively separate the capacitive and diffusion controlled charge storage contributions, the current response (i) at a fixed potential (V) is separated into surface capacitive effects (k1v) and diffusion-controlled insertion processes (k2v1/2) using the following 19
Journal Pre-proof equation [32] : i(V) = k1 + k2v1/2
(2)
We can rearrange equation (2) to analyze purposes: i(V)/v1/2 = k1v1/2 + k2
(3)
Fig. 7. Contribution ratio of the capacitive current and cation intercalation-controlled current of ECC-1100 at (a) 0.5 mV s-1 and (b) 5 mV s-1.
k1 and k2 can be easily determined by plotting i(V)/v1/2 versus v1/2 plots. With the value of k1, we are able to separate the capacitive current i(V)=k1v from the total measured current. Fig. 7 a and b compare the CV curves of ECC-1100 at different voltages for the capacitive contribution (the shaded region). When the scan rate is 0.5 mV s-1, the percentage of surface capacitive contribution is 63%, while 81% at a scan rate of 5 mV s-1. The increased percentage of the capacity is contributed by the surface capacitive effects. The enhanced surface capacitive contribution results in the excellent rate performance of the ECC-1100. 4. Conclusions A novel coal-based carbon SIB anode was synthesized by carbonizing the thermally 20
Journal Pre-proof extracted bituminous coal. Through thermal extraction, the solvent dissolves the soluble small molecules, diffuses into the raw coal particles, and leaves three-dimensional macromolecular network to form a hierarchical pore structure. The residual three-dimensional macromolecular network with high oxygen content tends to form a more disordered microstructure with enlarged interlayer distance. The hierarchical pore structure and enlarged interlayer distance can shorten the charge diffusion distance, alleviate the volume variation, and improve the ion diffusion rate. As a result, the modified coal-based electrodes exhibit impressive cycling stability (92% capacity retention after 7000th cycle at 2 A g-1) and excellent rate capability (79 mA h g-1 at 10 A g-1) Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC, No. U1508201).
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J.S. Q. and N.X. supervised the project. Y.B. W. designed the project and carried out all electrochemical measurements and parts of the catalyst’s characterizations. Y.B. W. wrote the manuscript. Y.B. W., H.Q. Li., Y.W. Wang. and J.P. Bai. contributed to the analysis of electrochemical data. Y.B. Wei and H.Q. Li carried out the XPS analysis. Y.W. Wang. performed the TEM measurements. J.P. Bai. performed the SEM measurements. All authors contributed to data analysis and approved the final version of the manuscript.
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Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: