Molybdenum-doped tin oxide nanoflake arrays anchored on carbon foam as flexible anodes for sodium-ion batteries

Journal Pre-proofs Molybdenum-Doped Tin Oxide Nanoflake Arrays Anchored on Carbon Foam as Flexible Anodes for Sodium-Ion Batteries M.Y. Wang, X.L. Wang, Z.J. Yao, D. Xie, X.H. Xia, C.D. Gu, J.P. Tu PII: DOI: Reference:

S0021-9797(19)31247-0 https://doi.org/10.1016/j.jcis.2019.10.063 YJCIS 25557

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Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

30 August 2019 15 October 2019 17 October 2019

Please cite this article as: M.Y. Wang, X.L. Wang, Z.J. Yao, D. Xie, X.H. Xia, C.D. Gu, J.P. Tu, MolybdenumDoped Tin Oxide Nanoflake Arrays Anchored on Carbon Foam as Flexible Anodes for Sodium-Ion Batteries, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.10.063

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Molybdenum-Doped Tin Oxide Nanoflake Arrays Anchored on Carbon Foam as Flexible Anodes for Sodium-Ion Batteries

M.Y. Wang, a X. L. Wang,*a Z. J. Yao, a D. Xie, b X. H. Xia,a C. D. Gu, a J. P. Tu*a

a State

Key Laboratory of Silicon Materials,

Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China b

Guangdong Engineering and Technology Research Center for Advanced Nanomaterials,

School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan 523808, China.

* Address correspondence to [email protected] (X.L. Wang); [email protected] (J.P. Tu)

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Abstract Tin oxide (SnO2) has been widely used as an anode material for sodium-ion storage because of its high theoretical capacity. However, it suffers from large volume expansion and poor conductivity. To overcome these limitations, in this study, we have designed and prepared Mo-doped SnO2 nanoflake arrays anchored on carbon foam (Mo-SnO2@C-foam with 38.41 wt.% SnO2 and 3.7 wt.% Mo content) by a facile hydrothermal method. The carbon foam serves as a three-dimensional conductive network and a buffer skeleton, contributing to improved rate performance and cycling stability. In addition, Mo doping enhances the kinetics of sodium-ion transfer, and the interlaced SnO2 nanoflake arrays is beneficial to promote the conversion reactions during the charge/discharge process. The asprepared composite with a unique structure demonstrate a high initial capacity of 1017.1 mAh g−1 at 0.1 A g−1, with a capacity retention over three times higher than that of the control sample (SnO2@C-foam) at 1 A g−1, indicating a remarkable rate performance.

Keywords: Tin oxide; Carbon foam; Doping; Anode; Sodium-ion battery

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1. Introduction Because of the limited availability of lithium resources, lithium-ion batteries (LIBs) cannot meet the growing energy storage requirements. On the other hand, Na-ion batteries (SIBs) are regarded as next-generation commercial energy storage devices because sodium resources are sustainable and their costs are low [1-3]. However, because the radius of Na+ is much larger than that of Li+, the anode materials previously used for LIBs such as graphite cannot accommodate Na+; therefore, several studies have been performed to develop suitable anode materials for SIBs [4]. In recent years, metal oxides, particularly tin oxide (SnO2), have been widely employed as anode materials for SIBs because of their high capacities [5]. The sodium storage mechanism involves Na+ insertion and extraction from SnO2 via alloying/dealloying reactions, resulting in the formation of sodium oxide (Na2O) and an NaxSn (0 ≤ x ≤ 3.75) intermediate phase, as given below [6, 7]: SnO2 + 4Na → Sn + 2Na2O

(1)

Sn + xNa ↔ NaxSn (0 ≤ x ≤ 3.75)

(2)

During the reactions, the SnO2 first converts to tin (Sn); then, Sn reacts with Na+ (Equation 2), leading to large volume expansion [8]. Nonetheless, because of the poor intrinsic conductivity of SnO2, SnO2 anodes exhibit unsatisfactory cycling and rate performances. Various strategies have been adopted to overcome these limitations. One of the strategies involves fabrication of hybrid nanostructures of SnO2 and conductive materials such as carbon [9-14] and polymers [15, 16]. Researchers have focused on SnO2 size reduction to minimize aggregation or volume changes. Accordingly, SnO2 nanostructures with various morphologies such as nanorods [17], nanosheets [18], and nanoflakes [19-21].

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Moreover, composites containing various carbon structures such as nanotubes [13], nanofibers [22], and hollow spheres [6, 23, 24] have been prepared. Carbon nanotubes can provide ion transfer channels and confine the volume change of active materials inside the tubes. Nanofibers are easy to construct, stable, and highly conducting. In addition, hollow spheres can accommodate volume expansion. Another strategy involves doping of elements, including antimony [25], cobalt [26-28], nitrogen [29], and nickel [30]. Notably, enhanced electrochemical performance has been achieved via cation doping [31]. For instance, Wang’s group [32] demonstrated the influence of Mo content on the electrochemical performance of Mo-doped hollow SnO2 spheres. The hollow spheres with 14 at.% Mo-doping content displayed a specific capacity of 801 mAh g−1 at 0.1 A g−1; with an increase in the current density to 1.6 A g−1, the specific capacity increased to 530 mAh g−1, which was 2.86 times higher than that of the undoped sample. This result indicated that Mo doping increased the conductivity of the SnO2 spheres, thereby enhancing the ion transfer kinetics. In this work, by combining the benefits of both conductive structures and cationic doping, we have designed a novel composite material: 3D carbon foam-supported Mo-doped SnO2 nanoflake array. Through a facile hydrothermal treatment, Mo is uniformly incorporated into the nanoflakes in the form of Mo6+ [33]. The lamellar SnO2 nanosheets increase the contact area between the electrode and electrolyte. In addition, the carbon foam serves as a 3D conductive network, which not only improves Na-ion transfer but also buffers the volume change. Furthermore, Mo doping accelerates Na-ion transfer [33]. Accordingly, the flexible Mo-SnO2@C-foam displays enhanced cycling and rate performances, making it suitable for high-energy storage devices.

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2. Experimental 2.1 Chemicals Urea (CH4N2O, ≥ 99.0%), sodium molybdate dihydrate (Na2MoO4·2H2O, ≥ 99.0%), sodium hydroxide (NaOH, ≥ 96.0%), hydrochloric acid (HCl, 36.0%–38.0%), and ethanol absolute (C2H6O, ≥ 99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Thioglycolic acid (C2H4O2S, ≥ 98.0%) and tin chloride dihydrate (SnCl2.2H2O, 98.0%) were purchased from Aladdin Chemistry Co., Ltd. All reagents were employed without further purifications. A compressible melamine sponge was purchased from Henan 3D New Materials Co., Ltd. 2.2 Material preparation The fabrication process of Mo-SnO2@C-foam is illustrated in Figure 1. Lamellar Modoped SnO2 nanosheets were decorated on a pretreated carbon foam by a one-step hydrothermal process, and to enhance the conductivity, the obtained sample was annealed. Mo-SnO2@C-foam was prepared as follows. First, the purchased sponge was washed in ethanol under ultrasonication and dried. Subsequently, it was annealed for 1 h under an Ar atmosphere at 800 °C with a heating rate of 5 °C min‒1 and rinsed with 1 M HCl solution to obtain a pure carbon foam. Mo-doped SnO2 nanoflake arrays were then uniformly decorated on the pretreated carbon foam skeleton by a facile hydrothermal method, and subsequently annealed. In brief, 1 g of urea was dissolved in deionized water (80 mL) and mixed for 2 min. Then, 20 mL of thioglycolic acid and 1 mL of HCl were added to the resulting solution. Next, 0.238 g of SnCl2.2H2O and 0.022 g of Na2MoO4·2H2O were added. Subsequently, the mixture solution was transferred into a Teflon-lined autoclave, and the pretreated carbon

5

foam was immersed in the solution. Then, the autoclave was heated in an electric oven at 120 °C for 12 h. Next, the Teflon-lined autoclave was rapidly cooled down for about 10 min under flowing water. The as-prepared composite was cleaned with deionized water, ethanol, and NaOH (45 °C for 10 h); then it was dried, and eventually, annealed for 3 h under an Ar atmosphere at the temperature of 500 °C. The control sample, SnO2@C-foam, was synthesized by a similar procedure, as mentioned above, but without adding Na2MoO4·2H2O.

Figure 1. Illustrations of fabrication Mo-SnO2@C-foam composite. 2.3 Characterization The morphologies of the synthesized composites were investigated with a scanning electron microscope (SEM, Hitachi S4800). A transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN) equipped with an energy-dispersive X-ray spectroscope (EDS) was further used to study the structures and elemental compositions of the composites. In addition, the crystalline phases of the composites were characterized by the X-ray diffraction (XRD, Rigaku D/Max-2550PC, 2θ = 10−80°, λ = 0.154184 nm, copper Kα radiation). The content of SnO2 in the composites was determined with a thermal analyzer (Netzsch STA 449C) from room temperature to 900 °C in air (heating rate: 10 °C min−1) by thermogravimetric analysis (TGA). The pore size distribution and specific surface area were determined by density 6

functional theory and Brunauer-Emmett-Teller methods with an analyzer (JW-BK112T). Moreover, the surface chemical composition of the composite was determined by X-ray photoelectron spectroscopy (XPS) using a Kratos AXIS Supra photoelectron spectrometer with an aluminum Kα X-ray excitation source; XPS calibration was performed using the carbon peak. 2.4 Electrochemical measurements The Mo-SnO2@C-foam composite additive anode was immersed in the electrolyte: a mixture solution of diethyl carbonate and ethylene carbonate with a volume ratio of 1:1 containing 1 M sodium perchlorate and 5 vol % of fluoroethylene carbonate. Then, typically, a CR 2025 cell was assembled with a commercial glass microfiber filter (Whatman GF/F) as the separator and sodium metal as the counter electrode in a glove box. Cyclic voltammetry (CV) was performed on a CHI 660D electrochemical workstation, from 0.005–2.5 V (vs. Na/Na+). A Neware battery testing system (BTS82-64, Neware Technology Co., Ltd., China) was used to record the cycling and rate performances at room temperature, each for about 20 times. Electrochemical impedance spectroscopy (EIS) was performed from 100 kHz to 10 mHz. Moreover, the undoped SnO2@C-foam composite was utilized as the anode instead of the Mo-SnO2@C-foam composite for comparison.

3.

Results and discussion The SEM images of pure carbon foam are presented in Figure 1a,b, illustrating a network

skeleton with a bare surface. After the hydrothermal treatment, interlaced SnO2 nanoflakes with an average section thickness of around 10 nm and an average diameter ranging from

7

about 85 nm to 158 nm, homogeneously grown on the skeleton, are obtained (see Figure 1c,d). As observed, the Mo-doped sample nearly retains the morphology and size (Figure 1e,f) of the SnO2@C-foam.

Figure 2. SEM images of a,b) C-foam composite; b,c) SnO2@C-foam composite; d,e) MoSnO2@C-foam composite.

To further confirm the successful synthesis and crystallinity of SnO2 in the composites, TEM analysis was carried out. The SnO2 nanoflakes in Mo-SnO2@C-foam have an average section thickness of around 10 nm and an average diameter of about 78–150 nm (see Figure 8

3a,b), measured along the length and width of the nanoflakes compared with the standard size; these results agree with the SEM results. As observed in Figure 3c, the Mo-SnO2@C-foam composite exhibits interplanar spacings of 0.35, 0.26, and 0.23 nm, corresponding to the (110), (101), and (200) crystal planes of SnO2 (JCPDS 41-1445), respectively [34]. As illustrated in Figure 3d, the selected area electron diffraction (SAED) pattern of MoSnO2@C-foam exhibits distinct diffraction rings corresponding to the (110), (101) and (211) planes of SnO2 [25, 30]. In addition, Figure 3e verifies the uniform dispersion of C, Sn, O, and particularly Mo in the composite, the content of Mo in the composite is determined to be 3.7 wt. %, as shown in Figure S1.

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Figure 3. a–c) TEM and high-resolution TEM images, d) SAED pattern, and e) EDS elemental (C, Sn, O, and Mo) mapping images of Mo-SnO2@C-foam composite.

Figure 4 displays the XRD patterns of the synthesized carbon foam, SnO2@C-foam and Mo-SnO2@C-foam. The broad peak at about 25 ° in the XRD pattern of SnO2@C-foam is the characteristic peak of graphite (JCPDS 75-1621), indicating the successful synthesis of carbon foam. The other peaks are consistent with those of tetragonal SnO2 (JCPDS 41-1445). Notably, the Mo-doped sample exhibits the same peaks and no byproduct peaks, implying that Mo doping does not influence the crystal phase of SnO2 [7]. Figure S2 illustrates the nitrogen adsorption-desorption isotherms and the pore size distribution of Mo-SnO2@C-foam. The specific surface area of sample is 48.4 m2 g‒1, and the sample mainly contains mesopores (< 4.6 nm).

Figure 4. XRD patterns of carbon foam, SnO2@C-foam and Mo-SnO2@C-foam. 10

The XPS analysis was conducted to study the surface chemical composition and Mo doping of Mo-SnO2@C-foam. The survey spectrum reveals the existence of Mo, Sn, O, C and N (Figure 5a). The Mo 3d spectrum in Figure 5b contains two peaks located at 235.9 eV and 232.7 eV, corresponding to Mo 3d3/2 and Mo 3d5/2 respectively, implying the incorporation of Mo6+ in SnO2 [35]. As shown in Figure 5c, the Sn 3d spectrum exhibits two peaks at 495.34 eV and 486.93 eV, corresponding to Sn 3d3/2 and Sn 3d5/2, indicating that the oxidation state of Sn is 4+ [36]. The O 1s spectrum in Figure 5d exhibits three peaks, which can be ascribed to C=O (533.1 eV), C-O (532.0 eV), and Sn-O (530.8 eV) [37]. The C 1s XPS profile (Figure 5e) displays C-N (285.6 eV) and C-C (284.6 eV) peaks. The broad C-N peak can be assigned to the intrinsic carbon bonding in the carbon foam, which was carbonized from melamine sponge, as verified by the N 1s spectrum in Figure S3 [38]. The N 1s peak is indexed to pyrrolic N [39, 40]. To determine the content of the active material, TGA analysis of SnO2@C-foam and MoSnO2@C-foam was carried out (see Figure 5f). The content of SnO2 in the SnO2@C-foam is determined to be 38.41 wt.%. The SnO2 content in the Mo-doped sample is the same, because the Mo doping content is negligible.

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Figure 5. XPS analysis results of Mo-SnO2@C-foam composite: (a) survey spectrum; (b) Mo 3d, (c) Sn 3d, (d) O 1s, and (e) C 1s spectra. f) TGA analysis of SnO2@C-foam. The electrochemical performances of Mo-SnO2@C-foam and SnO2@C-foam are compared in Figure 6. The CV curves of SnO2@C-foam (Figure 6a) exhibit the typical redox peaks of SnO2: three peaks at 1.1, 0.8, and 0.005 V in the initial cathodic scan, and two peaks located at 0.2 V and 1.17 V in the anodic scan. The carbon foam exhibits negligible capacity compared to SnO2, because the CV profile of the carbon foam demonstrates no peaks in the

12

potential range of 0.005‒2.5 V (Figure S4). The peak at 1.1 V is attributed to the conversion of SnO2 to Sn, which is an irreversible process, as this peak disappears in the subsequent cycles (see Equation 1). Moreover, the peaks at 0.8 V and 0.005 V suggest the insertion of Na+ into SnO2, resulting in the formation of NaxSn (0 ≤ x ≤ 3.75). Sodium ions are extracted during the anodic process via the dealloying of NaxSn (0 ≤ x ≤ 3.75) at 0.2 V and 1.17 V (Equation 2). Notably, Mo-SnO2@C-foam (Figure 6b) and SnO2@C-foam exhibit similar peaks, indicating that the conversion process occurring in the composites is the same [33]. However, as observed from the CV curves, the peak density and integral area for the Modoped electrode are higher than those for the SnO2@C-foam electrode, suggesting that Mo doping results in enhanced electrochemical kinetics. Moreover, the charge/discharge curves of SnO2@C-foam and Mo-SnO2@C-foam electrodes at current densities of 0.1, 0.2, 0.5, and 1 A g−1 are presented in Figure 6c,d, revealing the corresponding rate performances. The plateaus in the charge/discharge curves are consistent with those in the corresponding CV profiles. Notably, the capacities of Mo-SnO2@C-foam at all current densities are significantly higher than those of SnO2@C-foam (1143, 570, 451, and 336 mAh g−1 vs 954, 284, 198, and 99 mAh g−1 at 0.1, 0.2, 0.5, and 1 A g−1, respectively) owing to Mo doping, as clearly observed in Figure 6e. Moreover, the cycling performances of the synthesized carbon foam, SnO2@C-foam, and Mo-SnO2@C-foam are displayed in Figure 6f. As observed, in terms of capacity, the Mo-doped sample outweighs the SnO2@C-foam sample; the initial capacities of Mo-SnO2@C-foam and SnO2@C-foam are 1017.1 mAh g−1 and 944.9 mAh g−1, respectively. The higher capacity of Mo-SnO2@C-foam suggests that the doped Mo activates the conversion reactions during the charge/discharge process. Mo doping leads to enhanced

13

electron and ion transfer. This results in accelerated conversion reactions (Equation 1 and 2) and polarization reduction, in turn leading to more complete reactions at the same cutoff voltage. It is worth mentioning that the Mo-SnO2@C-foam composite displays improved electrochemical performance as compared to the control sample and anode materials previously reported (Table 1).

Figure 6. CV curves of a) SnO2@C-foam and b) Mo-SnO2@C-foam at a scan rate of 1 mV s−1; charge/discharge profiles of c) SnO2@C-foam and d) Mo-SnO2@C-foam at different 14

current densities; e) rate performances of SnO2@C-foam and Mo-SnO2@C-foam at different current densities; and f) cycling performances of SnO2@C-foam and Mo-SnO2@C-foam at 0.1 A g−1 and the corresponding Coulombic efficiencies. Table 1. Comparison of electrochemical performances of different SnO2-based anodes reported in the literature. Electrode materials Sb-doped

Capacity (mAh g−1)

Current Density (A g−1)

Cycle Number

Rate capability at 1 A g−1 (mAh g−1)

409

0.1

10

409.6

0.05

100

282

0.05

100

148

[42]

MWNTs@SnO2@C

300

0.05

60

125

[43]

SnO2@PPy nanotube

288

0.1

100

289.7

0.2

100

232.6

[45]

330

0.1

150

125

[46]

270

0.1

100

193

[47]

232

0.05

100

111

[17]

251.5

0.1

100

215

[6]

100

336

SnO2@graphene-CNT SnO2/N-doped graphene N and O dual-doped carbon microspheres

Yolk-shell SnO2 spheres SnO2@rGO N-CNF@SnO2 nanoflowers SnO2 nanorods/3D graphene aerogels SnO2@C hollow nanospheres Mo-SnO2@C-foam

575.8

0.1

273

Ref.

318.7 (0.8 A g−1)

260.2 (0.8 A g−1)

[25]

[41]

[44]

This work

Figure 7a compares the EIS curves of SnO2@C-foam and Mo-SnO2@C-foam, further confirming the enhanced charge transfer kinetics induced by Mo doping. The semicircle in 15

the high-frequency region can be assigned to the charge transfer resistance (Rct). In addition, the straight line in the low-frequency region can be indexed to the Warburg impedance [46]. Rct of Mo-SnO2@C-foam electrode (271.4 Ω) is lower than that of SnO2@C-foam (551.4 Ω), indicating that Mo doping leads to an increase in the Na+ diffusion rate, which can be further verified by determining the Na+ diffusion coefficient (DNa) using the equations given below: 𝐷𝑁𝑎 = 𝑅2𝑇2/2𝐴2𝑛4𝐹4𝐶2𝜎𝑤2 𝑍′ = 𝑅𝑠 + 𝑅𝑐𝑡 + 𝜎𝑤𝜔 ―1/2

(3) (4)

where R is the gas constant, T is 298.15 K, C is the molar concentration of Na+, F is the Faraday constant, n is the number of electrons transferred per mole, A is the surface area of the anode material, and w is the Warburg impendence coefficient. w can be extracted from the slope of the fitting line (Figure 7b), which is obtained by fitting the Z' vs ω curve in the low-frequency area. DNa for Mo-SnO2@C-foam is determined to be 1.89 × 10‒14 cm2 s‒1, which is nearly 10 times larger than that for SnO2@C-foam (1.9 × 10‒15 cm2 s‒1).

Figure 7. a) Nyquist curves of SnO2@C-foam and Mo-SnO2@C-foam electrodes and b) the corresponding fitting lines.

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4.

Conclusion In summary, we prepared molybdenum-doped tin oxide nanoflake arrays anchored on

carbon foam. Compared with the undoped composite, the Mo-doped composite exhibited enhanced rate performance (1143, 570, 451, and 336 mAh g−1 vs 954, 284, 198, 99 mAh g−1 at 0.1, 0.2, 0.5, and 1 A g−1, respectively) and cycling performance as anodes for sodium-ion batteries. Notably, the capacity of the as-obtained molybdenum doped composite, slightly increased after cycling. This was attributed to enhanced electron and ion transfer during the sodium ion insertion and extraction reactions and polarization reduction induced by Mo doping, which resulted in more complete reactions at the same cutoff voltage. Accordingly, the capacity of the as-obtained composite was superior to those of the anode materials previously reported [25, 41, 42]. In addition, the novel structural design contributed to improved electrochemical performance. The carbon foam skeleton well accommodated the volume change and acted as a three-dimensional conductive network, facilitating sodium ion transport. In addition, the tin oxide nanoflakes provided a higher contact area with the electrolyte, thereby preventing tin aggregation. Thus, the scheme for future development of SnO2 as an anode material for SIBs must be based on the following: (i) employment of a cost-effective carbon matrix as a buffer, such as the sponge-derived carbon foam used in this work, (ii) effective design of the structure to avoid Sn aggregation or confine volume expansion, and (iii) doping with cations, particularly transition metal ions.

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Acknowledgments This work was supported by the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037).

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SnO2/MoO3

hybrid

into

foam-like

carbon

nanoflakes

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