Pseudocapacitive Na-Ion Storage Boosts High ... - ACS Publications

Oct 21, 2016 - Key Laboratory for Photonic and Electric Bandgap Materials, Heilongjiang University of Science and Technology, Harbin 150022,. China. Â...
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Pseudocapacitive Na-Ion Storage Boosts High Rate and Areal Capacity of Self-Branched 2D Layered Metal Chalcogenide Nanoarrays Dongliang Chao,†,¶ Pei Liang,§,¶ Zhen Chen,∥ Linyi Bai,† He Shen,† Xiaoxu Liu,*,‡,ϕ Xinhui Xia,⊥ Yanli Zhao,† Serguei V. Savilov,# Jianyi Lin,*,∥ and Ze Xiang Shen*,†,∥ †

School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore Key Laboratory for Photonic and Electric Bandgap Materials, Heilongjiang University of Science and Technology, Harbin 150022, China § College of Optical and Electronic Technology, China Jiliang University, Hangzhou 310038, China ∥ Energy Research Institute, Nanyang Technological University, 637553, Singapore ⊥ State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China # Department of Chemistry, Moscow State University, Moscow 119992, Russia ϕ School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China ‡

S Supporting Information *

ABSTRACT: The abundant reserve and low cost of sodium have provoked tremendous evolution of Na-ion batteries (SIBs) in the past few years, but their performances are still limited by either the specific capacity or rate capability. Attempts to pursue high rate ability with maintained high capacity in a single electrode remains even more challenging. Here, an elaborate self-branched 2D SnS2 (B-SnS2) nanoarray electrode is designed by a facile hot bath method for Na storage. This interesting electrode exhibits areal reversible capacity of ca. 3.7 mAh cm−2 (900 mAh g−1) and rate capability of 1.6 mAh cm−2 (400 mAh g−1) at 40 mA cm−2 (10 A g−1). Improved extrinsic pseudocapacitive contribution is demonstrated as the origin of fast kinetics of an alloying-based SnS2 electrode. Sodiation dynamics analysis based on first-principles calculations, ex-situ HRTEM, in situ impedance, and in situ Raman technologies verify the S-edge effect on the fast Na+ migration and reversible and sensitive structure evolution during high-rate charge/discharge. The excellent alloying-based pseudocapacitance and unsaturated edge effect enabled by self-branched surface nanoengineering could be a promising strategy for promoting development of SIBs with both high capacity and high rate response. KEYWORDS: 2D layered SnS2, self-branched structure, pseudocapacitance, high rate and areal capacity, sodium-ion battery, unsaturated-edge effect

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spacing, short diffusion length, and open nature of a massive exposed surface.11,12 As a typical 2D material, SnS2 possesses the advantages of earth abundance and alloying-based high theoretical capacity (1136 mAh g−1 for a combined conversion and alloying reaction). SnS2 nanosheets prepared by Sun et al. using a hydrothermal method at 160 °C for 2 h showed a high reversible capacity of 733 mAh g−1 at 0.1 A g−1 with a capacity retention of 647 mAh g−1 after 50 cycles.13 Several nanostructured SnS2/graphene composites, also fabricated through a

n the pursuit of cost-effective, large-capacity, and high-rate energy storage technologies, one of the most appealing alternatives is the room-temperature sodium-ion battery (SIB), which shows especially notable advantages over the lithium-ion battery (LIB) in terms of cost and supply restriction of Li.1−5 Among an enormous variety of carbonaceous material, metal/alloys, and metal oxide/sulfide anodes, alloying-based materials stand out on account of their high capacity for sodium-ion batteries.6−10 Unfortunately, the achievement of a high specific capacity may often be at the expense of rate capability. Two-dimensional (2D) materials have drawn considerable attention in mechanical, catalytic, and electrochemical energy storage and conversion applications due to their generally good electrical conductivity, large interlamellar © 2016 American Chemical Society

Received: August 18, 2016 Accepted: October 21, 2016 Published: October 21, 2016 10211

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Figure 1. Fabrication and FESEM images of B-SnS2 with different reaction times. (a) Schematics of fabricated B-SnS2 electrode. (b) FESEM images of B-SnS2 obtained 40 min later after adding fresh ethanol. Left inset: Digital photo of GF-supported B-SnS2 electrode that is bent by a small force to show its flexibility and light weight. Middle and right insets: Low- and high-magnification FESEM images of B-SnS2 structure. (c−e) FESEM images of SnS2 nanoarrays obtained by hot bath method before adding fresh ethanol and 15 and 30 min later after adding fresh ethanol, respectively. Scale bar: 200 nm.

hydrothermal reaction at 160−200 °C for 12−24 h, exhibited high initial Coulombic efficiency and good long-term cycling stability as well.6,14,15 It has been demonstrated in the literature that creating vertically aligned 2D arrays by coupling nanoflakes with a backbone could facilitate charge transport and enhance the surface reaction kinetics owing to sufficient electrode/ electrolyte contact of ion pathways, large structural separation for volume change of ion insertion, and strong grip with the current collector, i.e., low contact resistance.16−18 It thus seems that the vertical alignment of a 2D SnS2 nanoarray can be an excellent strategy for Na-ion storage applications with high capacity and high rate capability. However, all of the current SnS2 reports on SIBs are in the form of pure or composite powders, where a binder additive, conductive agent, and current collector are essential for the electrode fabrication. These further decrease the energy/power density of the SIBs,12 and particularly, the widely used binder PVDF has been recently found to accelerate the deterioration of the electrode during sodiation.19,20 On the other hand, most of the vertically aligned 2D materials (i.e., SnS, SnS2, MoS2) in the literature were grown with relatively high temperature and a long reaction time in template-assisted multistep processes or, in particular, with limited mass loading (∼1 mg cm−2) on the heavy inactivating metal Cu foil (∼10 mg cm−2) or 3D Ni foam and carbon cloth (∼15 mg cm−2).

To date, two Na-ion storage mechanisms are involved in various electrode materials: (i) a diffusion-controlled intercalation/conversion/alloying process; (ii) the surface-induced capacitive process, which involves the faradaic contribution from charge transfer with surface/subsurface atoms and the nonfaradaic contribution from the electrical double-layer effect.21,22 With the former it is difficult to achieve a high rate capability due to the large size of Na+ and poor mass diffusion in the bulk, while the capacitive process featuring fast kinetics serves as a typical mechanism behind supercapacitors and is believed favorable for insertion- (TiO2, Nb2O5, LiCoO2, etc.)23−25 or conversion- (MoO2, V2O5, etc.)26−28 based battery materials by increasing the surface area through nanoengineering. Progress on capacitive contribution of an alloying-based SnS2 anode has not been achieved in battery electrodes. Herein, we report a facile hot bath in-processing intervention method for the synthesis of vertically aligned 2D self-branched (big nanoflake core and small nanosheet branches) SnS2 nanoarrays on a 3D lightweight graphene foam (0.8 mg cm−2) backbone. The special architecture possesses significantly increased mass loading together with the highest reported areal reversible capacity and rate capability for SIBs, which are comparable to or even outperform reported Li-ion storage functions. Thanks to the ultrathin and fine 2D SnS2 branches, alloying-based extrinsic pseudocapacitance was greatly en10212

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Figure 2. Structure characterization of B-SnS2 nanoarrays. (a−d) TEM and HRTEM images. (b, c) Layer information from core big flake and branch small sheet, respectively. Inset: SAED pattern. (d) Lateral HRTEM of B-SnS2 nanoarrays. Inset: Size distribution of crystalline grains. (e) EDX mapping of Sn and S elements. (f) Schematic illustrations of (001) and (110) facets of the SnS2 laminar structure. (g) XRD patterns of the branched-SnS2 sample (B-SnS2) and SnS2 flake without branches (SnS2). (h, i) Pore size distribution and the corresponding N2 adsorption/desorption isotherm of the B-SnS2 nanoarray electrode.

(b) self-assembly and oriented crystallization processes (details are discussed in the Supporting Information):29,30

hanced, which was identified by quantitative capacitive analysis as the contribution in alloying-based SnS 2 electrodes responsible for the fast kinetics. Sodiation dynamics analysis and density functional theory calculations were applied to clarify the small Na+ migration barrier for fast kinetics of SnS2 and edge effects of small 2D SnS2 branches for feasible Na+ absorption active sites. Furthermore, ex-HRTEM (highresolution transmission electron microscopy ), in situ impedance, and in situ Raman technologies were performed to investigate the structural evolution, reversibility, and fast rate response of self-branched SnS2 nanoarray electrodes.

CH3C(S)NH 2 + H 2O → CH3C(O)NH 2 + H 2S

(1)

Sn 4 + + 2H 2S → SnS2 + 4H+

(2)

When the raw materials’ concentration decreased via the addition of fresh ethanol, the crystal nucleation and in-plane growth slowed down, thus leading to the formation of small sheets with more exposed edges.30−32 The structural properties of B-SnS2 were further studied by TEM, HRTEM, X-ray diffraction (XRD), EDX, and Brunauer− Emmett−Teller (BET) measurements. Figure 2a−c reveal that both the big-flake core (b) and small-sheet branch (c) constitute B-SnS2 nanoarrays. The lattice fringes of 6.0, 3.2, and 2.7 Å respectively correspond to the (001), (100), and (101) planes of SnS2 (see Figure 2b,c,f). The big flake is ca. 12 layers in thickness, while ca. 4 layers for the branch small sheet. The results in Figure S1 and Table S1 show that the thickness and morphology of both big flakes and small shells of SnS2 nanoarrays remain little changed even when extending the reaction time to 80 min. On the other hand, the mass loading on the B-SnS2 electrode increases with reaction time due to the continuous anchoring of small-sheet branches and is more than 3 times that of the SnS2 big-flake electrode (noted as SnS2 electrode for the below comparison) that was prepared without adding fresh ethanol. Figure 2d suggests the polycrystalline character of the SnS2 with crystalline grains of ca. 5 nm. The EDX and XRD results identify the homogeneous distribution and phase purity of SnS2 nanoarrays (2T-type layered

RESULTS AND DISCUSSION Synthesis and Characterizations of Self-Branched SnS2 Nanoarrays. As illustrated in Figure 1 and Scheme S1, self-branched SnS2 nanoarrays (B-SnS2) are grown on graphene foam (GF) using a facile hot bath method at 80 °C (details in the Supporting Information). SEM images show that the size of SnS2 nanoarrays is tunable from big flakes (Figure S1a,b) to small sheets (Figure S1c,d) by controlling the concentration of the solution. The SnS2 nanoarrays in Figure 1c, which were generated before diluting the reaction solution with fresh ethanol, are vertical big flakes of 200−500 nm in size. After adding fresh ethanol, small sheets (ca. 50 nm) begin depositing onto the lateral surface of big flakes and form a self-branched nanoarchitecture finally 40 min later (Figure 1b and e). On the basis of the experimental conditions, a plausible mechanism of the growth of SnS2 nanoarrays is proposed: (a) hydrolysis of CH3C(S)NH2 (eq 1) and in situ metathesis reactions (eq 2); 10213

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Figure 3. Electrochemical performance of SnS2 nanoarrays with (B-SnS2) and without (SnS2) branches. (a) third-cycle CV comparison between SnS2 and B-SnS2 with the same electrode area. (b) Third galvanostatic discharge−charge profile comparison between SnS2 and BSnS2 electrodes. (c) Galvanostatic discharge−charge profiles of the B-SnS2 electrode at different rates. (d) Rate capability comparison between SnS2 and B-SnS2 electrodes. (e) Survey of specific capacity, rate capability, and areal capacity of selective top performance SIB and high areal capacity LIB anodes (detailed performances shown in Supporting Information Table S2). Red and blue regions represent high rate and specific/areal capacity, respectively. (f) Schematic diagram of energy component with different edge status of the SnS2. Sn−S, Sn, and S edge geometries represent the SnS2 end with Sn edge with S coverage, pure Sn, and pure S edge, respectively. (g) Edge formation energies of different edge status as a function of S chemical potential calculated with DFT.

potential (∼0.82 V) in the second and third scans, corresponding to the formation of a solid electrolyte interphase (SEI) and the chemical changes including the conversion of SnS2 to Sn and Na2S as well as the alloying reaction of tin (NaxSn, x ≈ 0.75).13,15,35 Furthermore, a small peak at 0.01− 0.1 V is pointed to be the reaction between the Na and NaxSn alloy (x ≈ 3.75) due to the multistep alloying feature of Na− Sn.15 In the anodic process, two broad oxidation peaks located at ∼1.35 and ∼1.75 V correspond to the multistep dealloying reaction of NaxSn6 and the restitution from Sn to SnS2, respectively.13,15,35 Compared to the SnS2 electrode without branches, the B-SnS2 electrode exhibits a 3 times larger areal capacitance and a ca. 40 mV smaller polarization (see Figure 3a). The galvanostatic discharge−charge profiles in Figure S5b for the B-SnS2 electrode show a first discharge/charge capacity of 1245/910 mAh g−1 with an initial Coulombic efficiency as high as ca. 73%. In the second and third cycles, the high capacitance of 910 mAh g−1 is maintained with almost 100% Coulombic efficiency. These results are very impressive. The extraordinary reversible specific capacity (∼900 mAh g−1) is comparable to the high-capacity phosphorus sodium ion anodes (Figure 3e).2,36 Figure 3b presents a reversible areal capacity of ca. 3.7 mAh cm−2 of the B-SnS2 electrode. To the best of our knowledge, this areal capacity is outstanding among carbonaceous material, metal/alloy, and metal oxide/sulfide anodes for sodium-ion batteries (see Figure 3e and Table S2) and even

hexagonal SnS2, JCPDS 23-0677) except for the peaks from the GF substrate (JCPDS 75-1621).6,33 Clearly, compared to the SnS2 electrode, the peaks broaden and slightly shift left in the case of B-SnS2, indicating the finer grains and distorted fringes of the branches (Figure 2c,d). Moreover, the intensity ratio of the (001) to (100) peak of B-SnS2 is larger than that of SnS2 (2:1 vs 1.5:1), indicating more exposed facets of the small branches.34 The specific surface area of B-SnS2 is measured to be ca. 169 m2 g−1, which is much higher than that of the SnS2 electrode without branches (96 m2 g−1, Figures 2h,i and S4). The relative pore size distribution derived from the Barrett− Joyner−Halenda (BJH) method shows peaks at 8, 14, 19, and 100 nm, suggesting a highly mesoporous feature of the B-SnS2 electrode. The appearance of the 8 nm mesopores for the BSnS2 electrode also corresponds to the pores between crystalline grains in Figure 2d. The structural characteristics of the B-SnS2 electrode, which include a self-branched ultrathin layered structure, high mass loading, high surface area, and mesoporous iso-oriented nanocrystals, are vital for superior Na+ storage performance, especially for high rate capability and high power/energy density. Na-Ion Storage Performance and Kinetics Analysis. In Figures 3a and S5a cyclic voltammetric curves (CVs) are displayed to investigate the sodium-ion storage behavior of the B-SnS2 electrode. A strong broadened peak is at ∼0.6 V in the first cathodic process, which decays and shifts to more positive 10214

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ACS Nano comparable to the high areal capacity LIB anodes (multilayer Sn/CNT,37 Si-SHP/CB,38 TiO2/Ni microbattery,39 etc.). Even after 200 cycles, ca. 89% of its capacity is retained (see Figure S6). First-principles plane-wave calculations within density functional theory (DFT) suggest that the high capacity is related to the access of much exposed edges of the B-SnS2 electrode (see calculation details in the Supporting Information). The Sn edge with unsaturated S coverage (Sn−S edge) is calculated to be the most stable structure (see Figure 3f,g), which is coincident with the slightly higher experimental S to Sn ratio by EDX (2.05:1, see Figure S3) and previous observations of high (001)/(100) ratio in XRD (Figure 2g). It was pointed out that the unsaturated S-edge exposure is beneficial for both active site introduction, rapid charge transfer promotion (also proved by smaller impedance of B-SnS2 in Figure S8), and ion absorption.40−42 Notably, compared with the B-SnS2, the SnS2 electrode delivers similar specific capacity but with obvious plateaus, which were typically regarded as the diffusion-controlled feature of battery materials, suggesting the possible existence of capacitive contribution and high rate capability of an ultrathin mesoporous B-SnS2 electrode. Significantly, with increasing the current density to 10 A g−1 (∼40 mA cm−2), a rate capability more than 400 mAh g−1 (∼1.6 mAh cm−2) can be sustained, which is the best areal rate capability among reported SIB anodes based on a comprehensive literature study (see Figure 3e and Table S2). The fast kinetics of 2D SnS2 is also proved by DFT calculations, which show that the Na+ surface diffusion energy barrier of the 2D SnS2 electrode (0.065 eV) is much smaller than that of other 2D electrode materials (e.g., 0.11 eV for MoS2; see Figure S9).43 However, it should be mentioned that the specific capacity gap between B-SnS2 and the SnS2 nanoflake increases from 50 mAh g−1 at 0.2 A g−1 to 300 mAh g−1 at 10 A g−1. A full cell demonstration as a proof of possible practical application in Figure S10 shows a capacity of 112 mAh g−1 at 0.1 A g−1 with an energy density as high as ca. 210 Wh kg−1 (based on total mass of the anode and cathode). The superior rate capability of B-SnS2, which contradicts its heavy mass loading, draws our attention to understand the kinetics origin. Figure 4a displays the CVs of the B-SnS2 electrode from 0.4 to 1.0 mV s−1. The shape is well preserved with increasing scan rate from 0.2 to 1 mV s−1. The degree of capacitive effect can be qualitatively analyzed according to the relationship between measured current (i) and scan rate (v) from the CV curves: i = avb, where a and b both are constants. The value of b is between 0.5 and 1.0, which is determined from the slope of the log i versus log v plot (Figure 4b). It is well known that for a diffusion-controlled process b approaches 0.5, while for a surface capacitance-dominated process b is close to 1.0.11,44 Hence the higher b value of the B-SnS2 electrode (0.89 vs 0.73 of the SnS2 electrode at the cathodic peak) suggests a more favored capacitive kinetics of B-SnS2. In Figure 4c the percentage of capacitive contribution to the current at a fixed voltage is quantitatively determined by separating the current response i from the diffusion-controlled and capacitive contribution at the corresponding voltage.24,25 As a result, 74% of the total capacity is identified as the capacitive contribution for the B-SnS2 electrode, much higher than 58% of the SnS2 sample. The findings are well coincident with the slope feature of galvanostatic profiles, the higher b value, and better kinetics of the B-SnS2 electrode. With the increase of the scan rate, the diffusion contribution is depressed, while the capacitive contribution increases as expected (Figure 4d). At 5 mV s−1

Figure 4. Quantitative capacitive analysis of sodium storage behavior. (a) CV curves at different scan rates of the B-SnS2 electrode. (b) Relationship between logarithm cathodic peak current and logarithm scan rates. (c) Capacitive contribution (green for B-SnS2 and black for SnS2) and diffusion contribution (gray) at 1.0 mV s−1. (d) Normalized contribution ratio of capacitive capacities at different scan rates.

the capacitive contribution tends to be stable at 77% for B-SnS2 and 60% for SnS2 electrodes. Structure Evolution and in Situ Analysis. Ex situ HRTEM images, in situ electrochemical impedance spectroscopy (EIS), and Raman spectra during the sodiation/ desodiation process of the B-SnS2 electrode are displayed in Figure 5 to investigate the surface states and phase evolution of the electrode at different charge/discharge states. Figure 5a is the third discharge/charge profile of B-SnS2 at 200 mA g−1 with labeled points for EIS and HRTEM, where D1.2 for example represents discharge to 1.2 V and C3.0 means charge to 3 V and so on. From the inset in Figure 5c it can be seen that the flake and layer structure of the B-SnS2 electrode is preserved with a more expanded and twisted feature after the cycles. The HRTEM image of the D0.7 sample shows that at a discharge potential of 0.7 V the β-Sn metal is generated and embedded in a Na2S matrix. The process can be described as the conversion reaction of as-prepared SnS2 (i.e., SnS2 + 4Na+ + 4e− ↔ β-Sn + 2Na2S).13,45,46 Partial alloying might also occur at this stage.13,47 Figure 5d shows the formation of Na15Sn4 as the result of full alloying of β-Sn during further sodiation to 0.01 V (D0.01) (4β-Sn + 15Na+ + 15e− ↔ Na15Sn4), as demonstrated by the (011) and (322) fringes for Na15Sn4 and the electron diffraction patterns in the inset.47,48 Corresponding to the structural and phase changes in the discharge−charge process, the in situ EIS spectra in Figure 5b vary. In particular, the fitted charge transfer resistance increases dramatically from 51 ohm at the beginning of sodiation (C3.0) to 193 ohm (D0.01) as the Na2S matrix forms with a gradual volume expansion. During the desodiation process, the dealloying reaction occurs. Metallic Sn separates out and segregates onto the surface in replacement of Na2S (see Figure 5e); hence the charge transfer resistance decreases steeply from 193 ohm to 45 ohm at C1.6 (Figure 5b). Further desodiation refers to the back-conversion to nanoclustered SnS2 (Figure 5f) rather than the previous crystalline SnS2 phase (Figure 2d).13,47 Besides, the charge transfer resistance tends to increase modestly because of the 10215

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Figure 5. Mechanism analysis of the electrochemical process. (a) Third charge/discharge profile at 200 mA g−1 with labeled points for EIS and ex situ HRTEM. Insets: Structure evolution illustration during charge/discharge. (b) In situ EIS spectra evolution of the B-SnS2 electrode at different charge/discharge potentials. (c−f) Ex situ HRTEM images of the B-SnS2 electrode at D0.7, D0.01, C1.2, and C3.0, respectively. Scale bar: 5 nm. Insets are the morphology after cycles and relative FFT patterns. (g, h) In situ Raman spectra of electrodes at different charge/ discharge potentials at 5 A g−1.

comparison, no peak vanishing is observed for the SnS2 electrode without branches (Figure 5h), indicating the existence of residual SnS2, which cannot be sodiated even after fully discharging (D0.01) due to the sluggish kinetics of the SnS2 electrode. The findings coincide with the higher pseudocapacitive contribution and specific capacity of the BSnS2 electrode especially at high rate.

comprehensive influences from both removal of metallic Sn and shrinkage of the structure. Different from the previous reports of SnS2 electrodes,13,15,47 no residual Sn and/or Na2S were detected after the full charge process, demonstrating the reversible feature and superior kinetics within the whole sodiation/desodiation processes. Due to the amorphous feature of the back-conversed SnS2, it is hard to detect the products in the cycled samples by XRD measurement. Nevertheless, as can be seen from Figure S11, SnS2 shows characteristic Raman peaks at ∼230 (Eu, weak) and ∼316 cm−1 (A1g, strong).47 Therefore, in situ Raman technology under fast charge/discharge was employed with an in situ Raman reaction cell to confirm the high reversibility of our B-SnS2 electrode. The spectra in the range between 200 and 1000 cm−1 show only peaks characteristic of SnS2 besides those of electrolyte ions ethylene carbonate (EC), diethyl carbonate (DEC), and PF6−. The yellow-shaded peaks are associated with an OC ring bending mode of EC (∼715 cm−1), a symmetric ring breathing mode of EC (∼892 cm−1), and a CH3−O stretching of DEC (∼900 cm−1), respectively, while the blue-shaded peaks refer to the totally symmetric vibration of the PF6− anion at ∼742 cm−1.49,50 They remain little changed during the entire charge−discharge process and hence will not be discussed. On the other hand, the characteristic A1g peak at 316 cm−1 of the B-SnS2 electrode (Figure 5g) is found to decay in intensity and fully eliminated during the sodiation process, suggesting the synchronous consumption of SnS2 and fast response under fast input current. After desodiation, the reappearing broad peak at ∼316 cm−1 indicates the good reversibility of B-SnS2 nanostructures during high-rate charge/discharge (at 5 A g−1). Surprisingly, as a

CONCLUSIONS In conclusion, a graphene foam supported self-branched SnS2 (B-SnS2) nanoarray flexible electrode has been synthesized by a simple hot bath in-processing intervention method. When serving as an anode for Na-ion batteries, B-SnS2 delivers a reversible areal capacity as high as 3.7 mAh cm−2 (900 mAh g−1) at 0.8 mA cm−2 (0.2 A g−1) and areal rate capability of 1.6 mAh cm−2 (400 mAh g−1) at 40 mA cm−2 (10 A g−1). Encouragingly, these findings make it possible to obtain a better performance with higher mass loading (3× higher), due to the ultrathin 2D layered structure, mesoporous iso-oriented nanocrystals, and exposed (001) unsaturated S-edges of the B-SnS2 electrode. First-principles calculation reveals that the exposed S-edges provide feasible active sites for Na+ absorption with lower edge formation energy. Quantitative capacitive analysis demonstrates that the pseudocapacitive contribution is responsible for the fast Na+ storage in B-SnS2. It is anticipated that our elaborate approaches of self-branched surface nanoengineering with exposed edge effect and enhanced pseudocapacitive contribution on alloying-based materials should provide clues for inducing the fast kinetics of high-capacity materials. 10216

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EXPERIMENTAL SECTION

AUTHOR INFORMATION

Synthesis and Characterization. Ultrathin B-SnS2 nanoarrays were synthesized by a facile hot bath method. A 0.8 g amount of Tin(IV) chloride and 0.6 g of thioacetamide were first dissolved in 50 mL of ethanol at 80 °C to yield a clear solution. A piece of 3D graphene foam substrate (ca. 0.8 mg cm−2, prepared by the CVD method according to our previous result51) was soaked inside the solution. Then 50 mL of fresh ethanol was added 40 min later. The reaction was carried out for another 40 min before the sample collection and washing with DI water and ethanol. Finally, the obtained sample was dried under vacuum at 60 °C to achieve a 3D GF supported self-branched SnS2 ultrathin freestanding electrode. For the fabrication of GF-supported big-flake SnS2 (without branches), the same procedure was applied as the synthesis of BSnS2 nanoarrays above, except for the second adding of fresh ethanol at 40 min after the reaction. For the GF-supported small-sheet SnS2, the procedure was the same as the synthesis of big-flake SnS2, but the concentrations of the raw materials (tin chloride and thioacetamide) were reduced by half. The crystalline structures of the as-prepared samples were characterized by X-ray diffraction (RigakuD/Max-2550 with Cu Kα radiation) at a scanning rate of 1° min−1. The morphology was observed by field emission scanning electron microscopy (FESEM, JSM-7600F, JEOL). The details of the crystal structure were further investigated by HRTEM (JEOL JEM-2010F at 200 kV). An energy dispersive spectral analysis (EDS) system was also linked with the TEM. The surface area was measured based on BET theory (Micromeritics, ASAP 2020). The pore size distributions were calculated using the BJH method. A splitable test cell with a quartz window was used for in situ Raman analysis, and the spectra were obtained on a WITec-CRM200 Raman system (WITec, Germany) with a laser of 532 nm. Electrochemical Measurements. Electrochemical tests were carried out via CR2032-type coin cells in a glovebox (MBraun, Germany). The working electrode was the as-prepared freestanding GF-supported self-branched SnS2 nanoarrays; the sodium foil served as the counter electrode, 1 M NaPF6 in EC:DEC:fluoroethylene carbonate (1:1:0.03 in volume) as the electrolyte, and a glass fiber as the separator. The cells were galvanostatically charged/discharged in the voltage window of 0.01−3 V versus Na/Na+. EIS and CV measurements were carried out using Solartron 1470E. For the full cell demonstration, Na3(VO)2(PO4)2F nanoparticles were used as the cathode. The active material mass ratio between anode and cathode active material was ∼1:7.

Corresponding Authors

*E-mail (X. Liu): [email protected]. *E-mail (J. Lin): [email protected]. *E-mail (Z. X. Shen): [email protected]. Author Contributions ¶

D. Chao and P. Liang contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Z.X.S. acknowledge the financial support by Ministry of Education, Tier 1 (No. M4011424.110) and Tier 2 (No. M4020284.110). X.L. would like to acknowledge support from the NSF of China (No. 51307046), NSF of Heilongjiang Province (No. E2016062), the Research Foundation for the Returned Overseas Chinese Scholars of State Education Ministry (No. 20151098) and Heilongjiang Province (No. 2015424), the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (No. 2015082), and the Open Project Program of the Key Laboratory for Photonic and Electric Band Gap Materials of the Ministry of Education of Harbin Normal University (No. PEBM201405). The authors also acknowledge support from the Russian Science Foundation Project (No. 14-43-00072). REFERENCES (1) Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S. P2-Type Nax(Fe1/2Mn1/2)O2 Made from Earth-Abundant Elements for Rechargeable Na Batteries. Nat. Mater. 2012, 11, 512−517. (2) Sun, J.; Lee, H. W.; Pasta, M.; Yuan, H.; Zheng, G.; Sun, Y.; Li, Y.; Cui, Y. A Phosphorene-Graphene Hybrid Material as a HighCapacity Anode for Sodium-Ion Batteries. Nat. Nanotechnol. 2015, 10, 980−985. (3) Wang, Y. X.; Yang, J.; Chou, S. L.; Liu, H. K.; Zhang, W. X.; Zhao, D.; Dou, S. X. Uniform Yolk-Shell Iron Sulfide-Carbon Nanospheres for Superior Sodium-Iron Sulfide Batteries. Nat. Commun. 2015, 6, 8689. (4) Wang, Y.; Yu, X.; Xu, S.; Bai, J.; Xiao, R.; Hu, Y. S.; Li, H.; Yang, X. Q.; Chen, L.; Huang, X. A Zero-Strain Layered Metal Oxide As the Negative Electrode for Long-Life Sodium-Ion Batteries. Nat. Commun. 2013, 4, 2365. (5) Song, J.; Wang, L.; Lu, Y.; Liu, J.; Guo, B.; Xiao, P.; Lee, J. J.; Yang, X. Q.; Henkelman, G.; Goodenough, J. B. Removal of Interstitial H2O in Hexacyanometallates for a Superior Cathode of a Sodium-Ion Battery. J. Am. Chem. Soc. 2015, 137, 2658−2664. (6) Qu, B.; Ma, C.; Ji, G.; Xu, C.; Xu, J.; Meng, Y. S.; Wang, T.; Lee, J. Y. Layered SnS2-Reduced Graphene Oxide Composite–a HighCapacity, High-Rate, and Long-Cycle Life Sodium-Ion Battery Anode Material. Adv. Mater. 2014, 26, 3854−3859. (7) Kundu, D.; Talaie, E.; Duffort, V.; Nazar, L. F. The Emerging Chemistry of Sodium Ion Batteries for Electrochemical Energy Storage. Angew. Chem., Int. Ed. 2015, 54, 3431−3448. (8) Jache, B.; Adelhelm, P. Use of Graphite As a Highly Reversible Electrode with Superior Cycle Life for Sodium-Ion Batteries by Making Use of Co-Intercalation Phenomena. Angew. Chem., Int. Ed. 2014, 53, 10169−10173. (9) Han, X.; Liu, Y.; Jia, Z.; Chen, Y. C.; Wan, J.; Weadock, N.; Gaskell, K. J.; Li, T.; Hu, L. Atomic-Layer-Deposition Oxide Nanoglue for Sodium Ion Batteries. Nano Lett. 2014, 14, 139−147. (10) Chao, D.; Zhu, C.; Yang, P.; Xia, X.; Liu, J.; Wang, J.; Fan, X.; Savilov, S. V.; Lin, J.; Fan, H. J.; et al. Array of Nanosheets Render Ultrafast and High-Capacity Na-Ion Storage by Tunable Pseudocapacitance. Nat. Commun. 2016, 7, 12122.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b05566. Scheme of formation of the big-flake SnS2, small-sheet SnS2, and self-branched SnS2; FESEM images of pure GF and GF backbone small-sheet and big-flake SnS2; cross section of GF backbone big-flake SnS2 and B-SnS2; table of mass loadings of different SnS2 nanoarray electrodes; EDX spectrum of B-SnS2; N2 adsorption/desorption isotherm of pure GF and SnS2 without branches; CVs and galvanostatic discharge−charge profiles for B-SnS2; cycling performance of SnS2 and B-SnS2 and pure GF; CVs of SnS2 without branches; electrochemical ac impedance spectrum; Na diffusion pathways on the SnS2; full cell demonstration of our synthesized B-SnS2; Raman spectrum of B-SnS2 nanoarray; table of electrochemical properties of anodes in top performance sodium-ion batteries; more details on DFT calculations (PDF) 10217

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