Rod-like Ordered Mesoporous Carbons with Various Lengths as Anode Materials for Sodium Ion Battery

Electrochimica Acta 218 (2016) 285–293

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Rod-like Ordered Mesoporous Carbons with Various Lengths as Anode Materials for Sodium Ion Battery Lvqiang Yua , Huaihe Songa,* , Yutong Lia , Yaxin Chena , Xiaohong Chena , Jisheng Zhoua , Zhaokun Maa , Xingyun Wanb , Ping Tianb , Jiao Wub a State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing, 100029, PR China b Infinitus (China) Company Ltd., Guangzhou, PR China

A R T I C L E I N F O

Article history: Received 30 July 2016 Received in revised form 21 September 2016 Accepted 25 September 2016 Available online 26 September 2016 Keywords: ordered mesoporous carbon length pore size anode sodium ion battery

A B S T R A C T

To investigate the pore structural effects on the electrochemical performance of ordered mesoporous carbons (OMCs) as anode materials for sodium ion battery (SIB), we prepared OMCs with various rod lengths from 350 nm to 1300 nm, and different pore sizes from 4.7 nm to 6.5 nm by changing the hydrochloric acid concentration in the P123/silica/glycerol composite system. The reversible capacities of OMCs with the average length of 350 nm, 700 nm, 900 nm and 1300 nm were 214 mA h g 1, 217.4 mA h g 1, 232.6 mA h g 1, and 228.9 mA h g 1 at 50 mA g 1, respectively. Furthermore, the OMC with largest pore size (6.5 nm) presented the highest capacity and even remained 100 mA h g 1 after 1000 cycles at 500 mA g 1 with the coulombic efficiency of nearly 100%. We confirmed that the carbon with ordered mesostructure, bigger pore size and shorter length of pore channel exhibited higher capacity. The results propose an effective direction and strategy for designing mesoporous materials to enhance the electrochemical performance of SIB. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction As one of the promising electrochemical energy storage devices, lithium ion batteries (LIBs) have been widely developed all over the world to meet the demands of portable electric devices and electric vehicles [1–3]. However, large scale application of LIBs has been restricted because of the limitation and high cost of lithium resources. As an alternative, sodium ion batteries (SIBs) have many advantages including natural abundance and low cost [4]. However, it is more difficult for sodium ion intercalation and deintercalation in the rigid lattice since the sodium ion is much larger and heavier than lithium ion [5]. Up to now, it still has a large challenge to design electrode materials to enhance the electrochemical performance of SIBs [6]. Various carbon materials, such as graphene and hard carbon, have been applied in the anode materials for SIBs. Wang et al. [7] synthesized reduced graphene oxide by modified Hummer’s way, and found that this material presented a reversible capacity of 174 mA h g 1 at 40 mA g 1 and 93.3 mA h g 1 at 200 mA g 1. Zhou et al. [8] reported that the highly disordered carbon (HDC) showed a reversible capacity of 225 mA

* Corresponding author. E-mail address: [email protected] (H. Song). http://dx.doi.org/10.1016/j.electacta.2016.09.124 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

h g 1 at 100 mA g 1. The mesoporous carbon with large pore size synthesized by Liu et al. [9] exhibited 125 mA h g 1 at 100 mA g 1. Cao et al. [10] prepared mesoporous soft carbon from mesophase pitch, showing a reversible capacity of 331 mA h g 1 at 30 mA g 1. Ordered mesoporous carbons (OMCs), one of non-graphitic and mesoporous carbons as electrodes for SIB, have attracted much attention owing to its high electrical conductivity, large surface area and uniform mesoporous channels, which can facilitate the migration of electrolyte and diffusion of ion. As to the cathode, Jiang et al. [11] compared the Na3V2(PO4)3@ mesoporous carbon with the Na3V2(PO4)3@ordered mesoporous carbon in SIB, and found that the later showed better performance because of the uniform pore size and interconnected pore structure. To the best of our knowledge, a little report on OMC as anode materials for SIBs has been published. Kim et al. [12] reported that the Nb2O5/OMC electrode showed the reversible capacity of 175 mA h g 1 at 50 mA g 1. Jo et al. [13] prepared ordered porous carbons with different pore sizes through evaporation induced self-assemble, and found that the initial irreversible capacity was more than 500 mA h g 1 with a low first coulombic efficiency. Furthermore they confirmed that carbon electrode with medium size exhibited a higher reversible capacity of 134 mA h g 1 at the current density of 25 mA g 1, and that the electrode performance of mesoposous

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materials could be enhanced by increasing pore size and decreasing wall thickness. Aimed at inspecting the effects of different structures of OMCs on the performance of OMCs electrodes for SIBs and improving the electrochemical performance of SIBs, herein, we synthesize OMCs (without using the SBA-15 [14] as a hard template) with various lengths from 350 nm to 1300 nm and different pore sizes from 4.7 nm to 6.5 nm by changing the concentration of hydrochloric acid in triblock copolymer/silica/glycerol system. Then we investigate the electrochemical performance of various structural carbons electrodes for SIBs to correlate the relationship between OMC rod lengths/pore sizes with the sodium-storage performance.

Ka radiation (l = 1.5406 Å). Scanning electron microscopy (SEM) was characterized through Zeiss Supra 55 electron microscope at 20 kV. Transmission electron microscopy (TEM) was conducted by a Hitachi H-800 at 200 kV. Nitrogen sorption experiments were performed with an ASAP 2020 Micromeritics Instrument at 77 K. The specific surface area was calculated from the adsorption data by the Brunauer-Emmett-Teller (BET) method. The pore size distribution was derived from desorption branch isotherms through the Barrett-Joyner-Halenda (BJH) method. Fourier transform infrared spectroscopy (FTIR) was measured by Nicolet IS50 infrared spectroscopy instrument. Raman was taken out on an Aramis system (Jobin Yvon) with 532 nm wave length incident laser light.

2. Experimental 2.3. Electrochemical measurements 2.1. Preparation of OMC materials OMCs were synthesized by one-step carbonization from triblock copolymer/silica/glycerol [15,16], as shown in Fig. 1. First of all, the micelle of triblock copolymer and glycerol was formed via a solvent self-assembly. Then, triblock copolymer/ silica/glycerol composites were generated in the existence of inorganic precursor. In detail, OMC was synthesized by crosslinking, solidification, carbonization and etching processes. The steps are shown as following: 3.6 g triblock copolymer (EO20-PO70EO20, P123, M = 5800 g mol 1) was dissolved in 138 ml hydrochloric acid at 40  C under stirring. After the P123 was dissolved completely, 2.9 mL glycerol and 8.3 ml tetraethyl orthosilicate (TEOS, M = 208.33 g mol 1) were added into the solutions. The liquid was kept static for 24 h after stirring for 5 minutes. Then the solution was transferred into a telfon container and heat treated under 100  C for another 24 h. In one way, 1 g products would be mixed with 1 ml sulfuric acid and 10 ml deionized water and then solidified at 160  C for 6 h. The resulting powders were carbonized under N2 atmosphere. At last 15 wt% hydrofluoric acid was used to remove the silica to obtain the last OMCs. In another way, we can obtain the silicas by calcining the composites at 550  C for 2 h after the thermal treatment. Ordered mesoporous silicas and carbons were prepared by changing the concentration of hydrochloric acid, 2.5 mol L 1, 2.0 mol L 1, 1.5 mol L 1 and 1.0 mol L 1. The obtained carbons and the silicas were remarked C1, C2, C3, C4 and Si1, Si2, Si3 and Si4 respectively. 2.2. Materials structure characterization The OMCs and silicas were measured by X-ray diffraction (XRD) on a Rigaku D/max-2500B2+/PCX system at 40 kV and 20 mA by Cu

Electrochemical tests were conducted using CR2025 coin cells with Na foil as the counter electrode. The cells were assembled in an argon-filled glove box (H2O < 1 ppm, O2 < 1 ppm). The working electrodes were manufactured by casting a paste consisted of active materials, acetylene black and carboxyl methyl cellulose at the weight ratio of 8:1:1. Distilled water was used as the solvent to make a slurry paste. The electrolyte was 1.0 M NaSO3CF3 in diglyme. The prepared electrodes were dried at 80  C for 4 h and 120  C for overnight in a vacuum. The galvanostatic charge/ discharge capacitance of each electrode with about 2 mg active material was measured using a Program Testing System (Wuhan LAND Co. Ltd., China). The discharge and charge voltage was ranged from 0.01 to 2.5 V. The cyclic voltammetry (CV) and the electrochemical impedance spectral measurements were carried out on a Zahner-Zennium (Germany) electrochemical working station. 3. Results and discussions The structures of as-prepared silicas and carbons were determined by XRD and Raman tests. As shown in Fig. 2(a), all of the silicas present three peaks implying the highly ordered structures. The peaks appeared in the XRD pattern are indexed as (100), (110) and (200) lattices, which are characteristics of twodimensional space group (p6 mm) [17]. In Fig. 2(b), the ordered structures of C1, C2 and C3 are still reserved after carbonization. However, the ordered structures of carbons become weak with decrease of hydrochloric acid concentration from 2.5 mol L 1 to 1.0 mol L 1, and the very small-angle diffraction peaks of C4 indicate the structure is destroyed in a certain extent after carbonization. The self-assembly behavior of P123 micelle, glycerol

Fig. 1. Synthesis procedure of OMCs by carbonizing the triblock copolymer/silica/glycerol composite directly.

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Fig. 2. Low angle XRD patterns of series of silicas (a) and carbons (b) from 0.5–5 ; Wide angle XRD of carbons from 5–90 (c); Raman spectra (d) and FTIR spectra (e) of carbons.

and inorganic precursor may be changed with the variation of the hydrochloric acid concentration, resulting in the different structures of carbons [18,19]. In Fig. 2(c), two wide peaks at approximately 23 and 44 are indicative of the amorphous phase [20] of as-prepared carbons. Furthermore, Raman spectroscopy (Fig. 2(d)) displays two typical bands at the wavenumbers of 1330 cm 1 (D band) and 1590 cm 1 (G band). The value of ID/IG becomes smaller from 1.01 to 0.94 for C1 to C4, implying carbons become more ordered with the decrease of the concentration of hydrochloric acid [21,22]. All the curves of the four carbons are similar to each other in the FTIR spectra (Fig. 2(e)). The broad bands at around 3430 cm 1 and 2350 cm 1 are mainly caused by the absorbed water molecules and the carbon oxide double bond in the skeleton, respectively. The band at 1620 cm 1 and 1145 cm 1 are caused by hydroxyl group of the surface of carbon material and C O C, respectively [23]. SEM images are given in Fig. 3. It can be seen that the carbons are rod-like type, with the mean lengths of 350 nm, 700 nm and 950 nm for C1, C2 and C3, respectively. C4 are mainly also rod-like type containing some spherical particles with the size of about 1300 nm as shown in Fig. 3(d). The low hydrochloric acid concentration is appropriate for forming the spherical structure with low curvature. From Fig. 3, we can conclude that the rod length of carbons can be tailored by changing the concentration of hydrochloric acid. The colloidal phase separation mechanism (CPSM) could make sense by using the nonionic surfactant to synthesis mesoporous materials [24–26]. The process can be divided into three parts: cooperative formation, interaction between colloids, and phase separation. At first, the P123/ glycerol/silica composites are formed. Gradually, P123/glycerol/ silica liquid phase will be generated with the P123/glycerol/silica amalgamation and condensation. Finally, the phase separation will occur. The electrostatic interaction between EO and hydrolysis of inorganic oligomers and the surface free energy (DGsurf) of liquid crystal phased should be both considered in the process of forming the mesoporous structure. Moreover, although free energy (DG) of forming mesoporous structure plays an important role in forming the final structure, the mesostructure is determined by the competition of DG/DGsurf. The competition of DG/DGsurf are

different from each other in the various acidic mediums. In a high concentration of hydrochloric acid, EO moieties of P123 become more hydrophilic and phase separation will occur early, while the influence of DG is more important than those of the DGsurf, which is suitable for forming the shorter mesoporous structure [27]. However, the EO becomes hydrophobic relatively in a low hydrochloric acid concentration, and the influence of DGsurf increases, resulting in forming longer micelles [27] and spherical structure with lower curvature and surface free energy, and this is why there are some spherical structure in Fig. 3(d1). TEM images are shown in Fig. 4(a–c), C1, C2 and C3 exhibit the ordered pore structures, while the ordered mesostructure of C4 is not well remained concluding from Fig. 4(d). However, we could still recognize the white and black streaks, which are the pore channels and carbon walls, respectively. The results of TEM test are highly corresponding to the XRD measurements. Nitrogen adsorption experiments are carried out to evaluate the porous structures and pore size distributions of the as-obtained products. As shown in Fig. 5, all the samples can be classified as type IV isotherms [28], and the hysteresis loop in the range of P/P0 (0.4–-0.9) indicates the existence of abundant mesoporous structure. The surface area for C1, C2, C3, and C4 calculated from the nitrogen sorption isotherms are 1305.90 m2 g 1, 2 1 2 1 2 1105.53 m g , 1269.87 m g , and 1157.72 m g 1 respectively. The mean pore sizes of C1, C2, C3, and C4 are 4.7 nm, 5.4 nm, 6.5 nm and 5.7 nm, respectively, as shown in Fig. 5(b–e). In a way, the triblock copolymer becomes more hydrophilic in a high concentration of hydrochloric acid, and the ration (VH/VL) of hydrophilic volume to hydrophobic volume becomes larger, resulting in shrinkage of the pore size [29]. In another way, the pore size is bigger than those of carbons carbonized from P123/silica directly [30]. The glycerol can enlarge the volume of PO, and then increase the ratio of VL/VH to magnify the pore size eventually, whose function is similar to butanol [31,32]. The electrochemical behavior of C1 is studied by CV analysis (Fig. 6(a)). The reduction peaks at around 0.75 V in the first cycle is related to the formation of solid electrolyte interface (SEI) layer on the surface of electrode, resulting in an irreversible capacity [33– 35]. The coincidence of the following two cycles indicates the high

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Fig. 3. SEM images and rod length statistics of C1 (a1-a2), C2 (b1-b2), C3 (c1-c2), and C4 (d1-d2).

reversibility of the subsequent reactions. The broad reduction and oxidation range are attributed to the mechanism of sodium insertion and extraction in a wide potential range [36]. The rectangular shape of the CV curves at high potentials actually

implies that a capacitive sodium ion storage behavior is not negligible due to its larger surface area [10,37,38]. From the CV curves, it implies that the sodium ion storage in the carbon can be attributed to the inserted sodium storage in the graphene

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Fig. 4. TEM images of the mesoporous carbons of C1 (a), C2 (b), C3 (c), and C4 (d).

Fig. 5. Nitrogen adsorption isotherms (a) of Si1, C1, C2, C3 and C4; Pore size distributions of C1 (b), C2 (c), C3 (d) and C4 (e).

interlayer and the capacitive sodium storage. In Fig. 6(b), the curves of C1 the selected discharge-charge processes show no obvious plateau. According to the previous studies [39–41], there are two predominant sodium storage mechanisms existing in the disordered carbon: (a) insertion and extraction between graphene

layers, sodium adsorption and desorption in the carbons; (b) sodium insertion into nanopores at low potentials with voltage plateau. So we can confirm the mechanism of sodium storage for the obtained carbons can be classified to reversible binding of carbon layer divacancies and defects between carbon layers

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Fig. 6. CV curve of C1 (a) at the scan rate 0.1 mv s

1

from 0.01–2.5 V; Discharge-charge curves of the selected cycles of C1 (b) at 50 mA g 1.

[37,42], and that micro/mesopores probably also make contribution to the ion-storage. As Table 1 shown, the capacities of C1 and C2 are lower than those of C3 and C4, which would be ascribed to bigger pore sizes of C3 and C4. The first discharge (Na-insertion) specific capacity of C1 is 301 mA h g 1 with coulombic efficiency of 71.09% (the irreversible capacity is 87 mA h g 1 which is much lower than that of OMC (>500 mA h g 1) [13]). Moreover, the highest initial coulombic efficiency in our OMC materials is 77.01% which is much higher than those of reported HDC (55%) [8], mesoporous carbon (40%) [9], mesoporous soft carbon (45%) [10] and hard carbons (70%) [43,44], implying that there are a little side-reactions at the surface of carbons in one way. In another way, the morphologies and pore structure of the carbons with twodimensional hexagonal symmetry maybe enhance the initial coulombic efficiency [45]. Therefore, the materials probably have a huge potential in the full cell of SIB in the future because of the high first coulombic efficiency. Fig. 7(a) and (b) show the cycle performances of samples as anode materials for SIBs at 100 mA g 1 in the voltage of 0.01–2.5 V. The specific capacities of C1, C2, C3 and C4 are 118.9 mA h g 1, 146.1 mA h g 1, 159.3 mA h g 1, and 150.2 mA h g 1, respectively, after 100 cycles at 100 mA g 1, which are much higher than that of OMC [13] and its composites [12]. In Fig. 7(c), C4 exhibits a higher capacity of 260 mA h g 1 at 50 mA g 1 due to the relatively larger pore size. However, C4 decreases to 110 mA h g 1 and it is lower than those of the C2 (130 mA h g 1), and C3 (120 mA h g 1) at 1000 mA g 1, which may be related to the destruction of ordered pore structure. When the current reaches to 50 mA g 1 again, all the carbons return the high capacities of 160 mA h g 1, 207 mA h g 1, 204 mA h g 1, and 200 mA h g 1 for C1, C2, C3 and C4, respectively. The capacities of OMCs are much higher than those of the hard carbon [43] and mesophase pitched-based carbon [10], and the better performances of OMC materials should be ascribed to structure of the ordered pore channel and high surface area. In Fig. 7(f), C3 presents a perfect long term performance with a reversible capacity of 100 mA h g 1 in 1000 cycles with the coulombic efficiency of nearly 100% at 500 mA g 1. Further investigation on the kinetics differences of as-prepared carbons, the impendence spectra of carbons is presented in Fig. 7(d) with an equivalent circuit model (Fig. 7(e)) [46]. Re is the electrolyte resistance, while Cf and Rf are the capacitance and

resistance of the SEI film in the high frequency, respectively. Cdl and Rct in the middle frequency are the double-layer capacitance and charge-transfer resistance, respectively. Zw is the Warburg impedance related to the diffusion of sodium ion in the low frequency. In Table 2, all the carbon electrodes have the similar Re. However, the lowest Rct of C3 indicates that C3 can promote the penetration of electrolytes into the SEI film more effectively than the others due to its bigger pore size [47], resulting in the better performance than the other carbons as shown in Fig. 7(a). According to the studies previously [48–50], the performance of ion storage in the hard carbon is mainly related to the interlayer distance of the amorphous crystallites. The ability of ion storage of the four as-prepared carbons would decrease from C1 to C4 in turn owning to the increase of ordered structure. However, mesoporous carbon has a considerable ion storage ability not only due to the disorders, but also the pore size [13], defects [50], surface area [50], length of pore channel [51], and the ordered mesoporous structure [51]. The C1 shows a lower capacity although C1 has the largest ID/ IG and surface area, indicating the structure of pore may play an important role in the performance of sodium-storage. Because of the possible factors above, the capacities of the obtained four carbons are in the following order: C3 > C4 > C2 > C1 as shown in Fig. 7(a). As depicted in Fig. 8, the defects [52] at the edge of carbon skeleton, and ion adsorption/desorption [39] both make contribution to the capacity. Moreover, C1 and C2 exhibit lower capacities due to its smaller pore size. As to the C3 and C4, it is the ration of mesopore length to the mesopore size that influences the capability of ion transportation [51,53,54]. It is obvious that the larger the mesopore and the shorter of pore channel is, the less time the ion transportation takes. The pore size of C3 (6.5 nm) is much larger than that of C4 (5.7 nm), and the mean length of the C3 is 0.95 mm, while the C4 is 1.30 mm. The ration of mesopore length to the mesopore size of C3 is smaller than that of C4, resulting in the better performance of C3. 4. Conclusions In summary, we synthesized OMC materials with various mesostructures by changing the concentration of hydrochloric acid in P123/silica/glycerol system. Moreover, we investigated the structural effects on the electrochemical performance of OMC

Table 1 First coulombic efficiency of various carbons at 50 mA g 1. Sample

First discharge (mA h g

C1 C2 C3 C4

301 282.3 326.4 336.6

1

)

First charge (mA h g 214 217.4 232.6 228.9

1

)

Coulombic efficiency (%) 71.09 77.01 71.26 68.04

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Fig. 7. (a) and (b) Cycle performances of C1, C2, C3, and C4 at current density of 100 mA g 1; (c) Rate performances of C1, C2, C3, and C4 at various currents; (d) Electrochemical impedance of C1, C2, C3, and C4; (e) An equivalent circuit model; (f) Long term cycle performance of C3 at current density of 500 mA g 1. Table 2 Parameters of various carbon electrodes at the potential of 1.80 V. Samples

Re (V)

Rf (V)

Rct (V)

C1 C2 C3 C4

7.37 7.73 7.39 7.94

2.80 5.14 3.36 4.40

3.40 5.02 2.57 4.55

materials as anodes for SIBs, and found that the mesoporous carbons with the ordered mesopourous structure, large pore size and short pore length of channel exhibited the higher capacity. The carbon with largest pore size had perfect ultra-long term cycle performance with the reversible capacity of nearly 100 mA h g 1 after 1000 cycles at 500 mA g 1. The results of this work suggest that electrochemical properties of OMC as anode for SIB can be improved by enlarging pore size, shorting channel length, and

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Fig. 8. Schematic illustration of sodium ion storage in the mesoporous carbon, and the possible way of ion transportation in the various carbons.

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