Tin Disulfide Nanoplates on Graphene Nanoribbons for Full Lithium

Nov 12, 2015 - †Department of Chemistry, ⊥Richard E. Smalley Institute for Nanoscale Science and Technology, ¶Department of Materials Science and...
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Tin Disulfide Nanoplates on Graphene Nanoribbons for Full Lithium Ion Batteries Caitian Gao,†,‡,§ Lei Li,†,§ Abdul-Rahman O. Raji,† Anton Kovalchuk,† Zhiwei Peng,† Huilong Fei,† Yongmin He,† Nam Dong Kim,† Qifeng Zhong,† Erqing Xie,*,‡ and James M. Tour*,†,⊥,¶ †

Department of Chemistry, ⊥Richard E. Smalley Institute for Nanoscale Science and Technology, ¶Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States ‡ School of Physical Science and Technology, Lanzhou University, Lanzhou, Gansu 730000, China ∥ State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing, Jiangsu 210096, China S Supporting Information *

ABSTRACT: A nanocomposite material made of layered tin disulfide (SnS2) nanoplates vertically grown on reduced graphene oxide nanoribbons (rGONRs) has been successfully developed as an anode in lithium ion batteries by a facile method. At a rate of 0.4 A/g, the material exhibits a high discharge capacity of 823 mAh/g even after 800 cycles. It shows excellent rate stability when the current density varies from 0.1 to 3.0 A/g with a Coulombic efficiency larger than 99%. In order to demonstrate the anode material for practical applications, SnS2rGONR/LiCoO2 full cells were constructed. To the best of our knowledge, this is the first time that a full cell has been successfully developed using metal chalcogenides as an anode. The full cell delivers a high capacity of 642 mAh/g at 0.2 A/g, superior rate, and cycling stability after long-term cycling. Moreover, the full cell has a high output working voltage of 3.4 V. These excellent lithium storage performances in half and full cells can be mainly attributed to the synergistic effect between the highly conductive network of rGONRs and the high lithium-ion storage capability of layered SnS2 nanoplates. KEYWORDS: layered SnS2, reduced graphene nanoribbons, lithium ion batteries, full cell, synergistic effect

1. INTRODUCTION The development of anode materials with large capacity, highrate capability, and long cycle lifetimes for use in lithium ion batteries (LIBs) is required in order to improve performance in commercial applications, such as in cell phones and electric vehicles.1,2 Graphite is the standard commercialized anode material for LIBs due to its unique stability. However, the theoretical capacity of graphite is only 372 mAh/g which limits its further applications in LIBs.3 Recently, layered sulfides (SnS2, MoS2, WS2) have attracted wide attention due to their 2D structure and their electrical properties which are suitable for catalysis and energy storage applications.4−6 Among these, SnS2 is one of the most promising anode material for LIBs because of its low cost, good chemical stability, and high theoretical capacity (645 mAh/g) attributed to its layered CdI2type structure.7−9 Tin atoms are sandwiched between two layers of hexagonally disposed close-packed sulfur atoms (a = 0.3648 nm, c = 0.5899 nm), and this layered structure is beneficial for the insertion and extraction of Li ions. Various nanostructured SnS2 materials, such as SnS2 nanoplates,10 nanoflowers,11,12 and other nanohierarchical structures,13−15 have been applied in LIBs. However, these SnS2-based anodes © 2015 American Chemical Society

suffer from rapid capacity decay caused by pulverization of the electrode materials due to the large volume changes during extended discharge/charge processes. The same vacillations, which reduce cyclability in SnS2, plague other anode materials such as silicon and numerous metal oxides.16,17 Two effective strategies have been developed to solve this problem. The first strategy is to reduce the size of electrode materials in order to relax the strain during the volume expansion and contraction. The other is to prepare them as composites using a carbon matrix, such as mesoporous carbon,18 graphene,19−22 and carbon nanotubes (CNTs).23,24 These matrices can buffer the volume variation and accelerate ion and electron transport during the discharge/charge processes. The combination of these two approaches could potentially offer a viable way to mitigate the volume changes and further improve the electrochemical performance of LIBs. This study outlines a strategy to prepare nanocomposites of layered SnS2 nanoplates directly grown on reduced graphene Received: August 20, 2015 Accepted: November 12, 2015 Published: November 12, 2015 26549

DOI: 10.1021/acsami.5b07768 ACS Appl. Mater. Interfaces 2015, 7, 26549−26556

Research Article

ACS Applied Materials & Interfaces oxide nanoribbons (SnS2-rGONRs). Graphene oxide nanoribbons (GONRs) in their reduced (r) forms possess high aspect ratio with predominantly sp2 carbons in the basal planes, revealing the advantages of both high aspect ratio carbon nanotubes and flat graphene surfaces.25,26 Here, GONRs were made by oxidative opening of multiwalled carbon nanotubes (MWCNTs) into elongated ribbons using a previously reported method.25,27 The layered SnS2-rGONRs were then prepared by a chemical vapor deposition (CVD) method. In this composite material, the layered SnS2 nanoplates have a naturally high capacity for Li-ion storage, whereas the rGONRs function as a substrate for loading SnS2 which not only improves the long-range conductivity of the composite but also mitigates the pulverization of SnS2 nanoplates caused by volume changes during the discharge/charge processes. Therefore, when used as an anode in LIBs, SnS2-rGONRs exhibit a high discharge capacity of 1100 mAh/g after 110 cycles at 0.1 A/g and deliver a capacity of 822 mAh/g after 105 cycles at 3.0 A/g with the current density varying from 0.1 to 3.0 A/g, indicating high capacity and good rate performance. More importantly, the nanocomposites still maintain a discharge capacity of 823 mAh/g after 800 cycles at a current density of 0.4 A/g, demonstrating that SnS2-rGONR is a superb candidate for anodes in LIBs. To test the practicality of SnS2-rGONR anodes, we assembled a full lithium ion battery by coupling the SnS2-rGONR anode with commercial LiCoO2 (LCO) as the cathode. As far as we know, this is the first time that the full cell has been successfully constructed with a metal chalcogenide anode. The full cell shows a high capacity of 642 mAh/g at 0.2 A/g, with good rate performance and cyclic stability. More importantly, a high output working voltage of ∼3.4 V is seen in the full cell, further confirming that SnS2-rGONRs is a promising anode material for industrial applications.

Figure 1. (a) Schematic illustration of the synthesis of SnS2-rGONRs. (b) Low magnification SEM image of the SnS2-rGONRs. (c) Higher magnification SEM image of the SnS2-rGONRs.

2. RESULTS AND DISCUSSION Figure 1a schematically illustrates the synthesis process for SnS2-rGONRs. GONRs were first synthesized by unzipping multiwalled carbon nanotubes (MWCNTs) using our previously published method which used a mixture of concentrated sulfuric acid (H2SO4), phosphoric acid (85%). and potassium permanganate (KMnO4).25,27 Next, SnO2 nanoparticles were uniformly grown on the surface of GONRs via a wet chemical process in water to form SnO2GONRs. Finally, SnO2-GONRs were converted to SnS2rGONRs using sulfur and hydrogen in a CVD furnace at 380 °C for 1 h at ambient pressure. Figure S1 shows field emission scanning electron microscope (FESEM) images of rGONRs with widths of ∼300 nm and lengths up to 10 μm indicating a high aspect ratio. Figure 1b,c shows the SEM images of SnS2rGONRs at different magnifications. The rGONRs maintained their high aspect ratio in the SnS2-rGONRs composite as shown in Figure 1b. The high resolution FESEM image in Figure 1c shows ∼200 nm SnS2 nanoplates vertically grown around the rGONRs. The morphology of SnS2-rGONRs was also investigated by transmission electron microscopy (TEM) as shown in Figure 2. Figure 2a shows a ribbon of SnS2-rGONR with well-distributed SnS2 nanoplates that cover the rGONR which is consistent with the FESEM images. TEM elemental mapping images in Figure 2b−e show that carbon, oxygen, sulfur, and tin had a homogeneous distribution. Further inspection of the composite by high resolution TEM (HRTEM) showed that SnS2 nanoplates are hexagonal and implant into rGONR (Figure

Figure 2. TEM characterization of SnS2-rGONRs. (a) Scanning TEM (STEM) image of a single SnS2-rGONR and corresponding elemental mapping of (b) carbon, (c) oxygen, (d) sulfur, and (e) tin. (f) TEM image of a single SnS2 nanoplate on rGONR. (g) HRTEM image of a representative single SnS2 nanoplate implanted in rGONR. (h) HRTEM image of the side of a SnS2 nanoplate in SnS2-rGONRs. The scales in (c), (d), and (e) are the same as (b).

2f), indicating their good contact properties. Figure 2g shows the interplane distances of 0.32 and 0.19 nm, which are attributed to the (100) and (110) planes, respectively. The layered nature of the composite can be seen in Figure 2h where the SnS2 nanoplates consist of ∼15 layers of SnS2 sheets with an interplanar spacing of 0.59 nm that corresponds well with the (001) interplane distance. Compositional analysis of SnS2-rGONRs was performed using X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The XRD patterns of SnS2 26550

DOI: 10.1021/acsami.5b07768 ACS Appl. Mater. Interfaces 2015, 7, 26549−26556

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oxidized into SnO2, the weight fraction of SnS2 in SnS2rGONRs is estimated to be about 90 wt %. The electrochemical lithium storage properties of SnS2rGONRs as an anode material in LIBs were investigated by cyclic voltammetry (CV) and galvanostatic discharge/charge experiments. Figure 4a shows the first five CV curves with a

and SnS2-rGONRs in Figure 3a were indexed well with the 2Ttype layered structure (space group: P3m1; JCPDS CARD no.

Figure 4. (a) Cyclic voltammetry curves of SnS2-rGONRs at a scan rate of 0.4 mV/s in the potential range of 0.01 and 3.0 V (vs Li/Li+). (b) The first three discharge/charge curves of SnS2-rGONRs at a current density of 0.1 A/g in the potential range of 0.01 and 3.0 V (vs Li/Li+). Figure 3. (a) XRD patterns and (b) Raman spectra of rGONRs, SnS2, and SnS2-rGONRs. (c) XPS of SnS2-rGONRs. (d) C 1s XPS fine spectra fitting by four distinguished peaks. (C 1s peak (284.5 eV) used as standard to correct the data.)

potential window of 0.01 to 3.0 V vs Li/Li+ at a scan rate of 0.4 mV/s. In the first discharge cycle, there were five distinct cathodic peaks. The peak at ∼0.10 V represents the reversible Li-ion insertion into rGONRs (eq 1).The weak broad cathodic peak at ∼0.50 V results from a combination of the formation of the solid electrolyte interface (SEI) and reversible Li-ion insertion in Sn (eq 2). The peaks at 1.09, 1.53, and 1.81 V are attributed to the decomposition of SnS2 and the formation of Li2S that may occur in three steps as suggested by Kim et al. (eqs 3a−3c).35 This indicates that lithium can intercalate into the SnS2 layers to some extent without causing phase decomposition.36 In the first anodic scan, the peak at ∼0.61 V is known to represent the redox peak coupling of the reaction (eqs 1 and 2). The peaks at ∼1.90 and ∼2.36 V are attributed to the partial reversibility behaviors of eq 3. In the second cathodic cycle, the broad cathodic peak at ∼0.50 V disappeared and the new peak appeared at ∼0.28 V. This demonstrates that the SEI mainly occurred in the first cycle and the new peak resulted from the reversible Li-ion insertion in Sn. The peak at ∼1.09 V during the first cycle shifted to ∼1.29 V at the second cycle due to the polarization of the electrode materials. The CV curves were mostly overlapped in subsequent cycles, indicating good reversibility of the electrochemical reaction. In the discharge/charge curves, the plateau that emerged at ∼1.10 V during the first discharge process (Figure 4b) corresponds to the first cathodic peak in the CV curves. It is caused by the decomposition of the SnS2 nanoplates (eq 3). During subsequent cycles, this discharge plateau moved to the potential of ∼1.29 V. The changes in the charge/discharge curves were consistent with that in the CV curves. In the first discharge/ charge process, the discharge capacity of the SnS2-rGONRs electrode is 1152 mAh/g, which is much higher than that of the theoretical value. This high value is attributed to the formation of the SEI, electrolyte decomposition, and some side reactions.37

23-0677), and the strong diffraction peaks indicate the high crystallinity of the synthesized materials. The rGONRs show the diffraction peak (002) of graphite at 24.6°. The intensity of this peak became very weak in SnS2-rGONRs due to the overcoating of SnS2. The Raman spectrum of rGONRs in Figure 3b shows characteristic peaks of graphene, namely, the D band at ∼1350 cm−1 and the G band at ∼1580 cm−1.27 The peak at 314 cm−1, which corresponds to the A1g mode of SnS2, confirms SnS2 was successfully prepared.28 Also, the Raman spectrum of SnS2-rGONRs exhibits modes of both SnS2 and rGONRs, indicating that SnS2 was successfully grown on rGONRs. XPS was used to further study the elemental bonding in SnS2-rGONRs. Figure 3c is the survey spectrum showing the distinguishing peaks of Sn, S, O, and C. Furthermore, C 1s (Figure 3d), Sn 3d, and S 2p (Figure S2) were investigated in detail. The main C 1s peak was fitted with four peaks; the peak at 284.5 eV represented C−C bonds and the other peaks centered at 285.7, 287.1, and 288.5 eV were assigned to C−O− C, CO, and O−CO, respectively.29−33 Compared with SnO2-GONR (Figure S3), the intensities of all C 1s peaks of the carbon binding to oxygen in SnS2-rGONR were reduced significantly, which substantiates the deoxygenation process during CVD treatment. In Figure S2a, the peak at 486.8 eV was attributed to Sn 3d5/2 in SnS2-rGONRs compared that of 487.3 eV in SnO2-GONR (Figure S3d). The high-resolution S 2p peak showed the presence of S2− species in the SnS2-rGONRs (Figure S2b). The presence of S2− peaks observed at 161.8 eV also corresponds to the reported SnS2 nanomaterials.34 Although no C−Sn bond was detected using XPS, we postulate that the composite material is formed through strong absorptions because the SnS2 nanoplates remain anchored onto rGONR even after sonication. The content of SnS2 in the SnS2-rGONRs composite was determined by thermogravimetric analysis (TGA) in air atmosphere, as shown in Figure S4. The weight loss is 26 wt %. Considering that the SnS2 had been

C + x Li+ + x e− ⇄ LixC

(1)

Sn + x Li+ + x e− ⇄ LixSn 26551

(0 ≤ x ≤ 4.4)

(2)

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Figure 5. (a) Cycling performance of SnS2-rGONRs at 0.4 A/g. (b) Rate performance of SnS2-rGONRs at various current rates from 0.1 to 3.0 A/g with respect to the cycle number.

SnS2 + 4Li+ + 4e− → Sn + 2Li 2S

The rate capability is also an important parameter in evaluating the electrochemical performance of the electrodes in LIBs. Figure 5b shows the rate characteristics of SnS2rGONRs with varying current density from 0.1 to 3.0 A/g. The stable capacity at different current densities was observed. When the current density was reduced back from high current density to low current density, the capacity not only returned but also increased with the increased cycle numbers. For example, the reversible discharge capacity of SnS2-rGONRs was 1033 mAh/g at the second cycle at a current density of 0.1 A/g. It increased to 1100 mAh/g at the 110th cycle. Furthermore, SnS2-rGONRs can deliver high capacity even at fast discharge and charge rates. For example, the capacity was maintained at 822 mAh/g at a high current density of 3.0 A/g after 105 cycles demonstrating the high capacity and good rate performance of SnS2-rGONRs. To obtain better insight of the kinetic properties, the electrochemical impedance spectroscopy (EIS) of the SnS2rGONRs anode electrodes before and after 800 cycles were measured (Figure S7a). A loop in high-frequency can be clearly observed in both SnS2-rGONRs and SnS2-rGONRs-800. By fitting the Nyquist plots with the equivalent circuit as shown in Figure S7b, RSEI+ct, which represents the SEI surface and charge-transfer resistance, is reduced from 423 Ω of SnS2rGONRs to 132 Ω of SnS2-rGONRs-800, which led to its high rate performance. To study SnS2-rGONRs anodes in full battery applications, we assembled a full cell by using commercial LCO as a cathode material. The morphology, structure, and electrochemical performance of LCO are shown in Figure S8. The particle size of LCO is larger than 10 μm with a large size distribution. The XRD and Raman spectra demonstrate that LCO has a standard layered structure. In addition, the LCO electrode in a half cell delivered a capacity of 123 mAh/g at the second discharge cycle and had a capacity retention of 90% after 100 cycles. On the basis of the electrochemical performances of both the SnS2-rGONRs anode and the LCO cathode, a full cell with high specific capacity and high-output working voltage is

(3)

SnS2 + x Li+ + x e− → LixSnS2

(3a)

LixSnS2 + (y − x)Li+ + (y − x)e− → Li ySnS2

(3b)

Li ySnS2 + (4 − y)Li+ + (4 − y)e− → Sn + 2Li 2S (0 < x < y ≤ 2)

(3c)

The cycling performance of SnS2-rGONRs used as an anode in LIBs was evaluated by the extended discharge/charge experiments at a current density of 0.4 A/g as illustrated in Figure 5a. The reversible discharge capacity was 903 mAh/g at its second cycle and gradually decreased with increasing cycle numbers to 810 mAh/g at the 200th cycle. The capacity increased in subsequent cycles reaching 900 mAh/g by the 300th cycle and maintained an average capacity of 880 mAh/g in the remaining cycles. Finally, after 800 cycles, it reached 823 mAh/g, demonstrating that this anode material has a high capacity retention (>91%). In addition, after the first two cycles, the Coulombic efficiency of SnS2-rGONRs remained at >99%. The specific capacity of the SnS2-rGONRs anode exceeds the theoretical value. This has been observed in other SnS2 studies and mainly attributed to a synergistic effect between the dissimilar materials present.38,39 The morphology of SnS2-rGONRs after 250 cycles (SnS2-rGONRs-250) of discharge/charge processes was analyzed by EFSEM, TEM, and element mapping (Figure S5), indicating no substantial changes compared to the initial morphology of SnS2-rGONRs. The cycling performance of pristine SnS2 nanoplates and rGONRs were also studied as shown in Figure S6. For SnS2 nanoplate anodes, the reversible discharge capacity is 621 mAh/g on its second cycle which is close to its theoretical value. However, this value dropped quickly to 321 mAh/g after only 30 cycles. In the case of rGONRs, the reversible discharge capacity was 372 mAh/g, and the capacity remained at a relatively high value after 100 cycles. Therefore, the dual material SnS2-rGONRs show large improvements in both capacity and cycling stability of the LIBs. 26552

DOI: 10.1021/acsami.5b07768 ACS Appl. Mater. Interfaces 2015, 7, 26549−26556

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(Figure S9b). To overcome the ICL issue, several pretreating procedures such as electrochemical lithiation40 and sacrificial lithium salts41 in the electrolyte were attempted; however, these methods are quite complex. Here, we demonstrated a prelithiation method of directly contacting the anode material with Li foil in electrolyte before assembling the full cell, which is applicable for mitigating ICL in SnS2.42 After prelithiation, the lithium will predope the anode. As expected, the formation of SEI will not consume too much lithium from the cathode and the full cell shows better stability. The details for the prelithiation process and the related data have been described in Figure S10. Figure 7a shows a 3D schematic illustration of SnS2rGONR/LCO full-cell. In a full cell, LCO on Al foil acts as a cathode instead of Li foil in the half cell. CV curves of the full cell are shown in Figure 7b. In the first cycle, no peak was observed and there was only one strong peak at 3.4 V in the cathodic scan. In addition, this peak has no potential shift in subsequent cycles. Figure 7c is a digital image of two tandem LEDs (1.7 V, 40 mA for each) lit by a SnS2-rGONR/LCO full cell indicating the full cell has a high output working voltage. Figure 7d displays the representative discharge/charge voltage profiles of SnS2-rGONR/LCO at various current densities. The main charge and discharge profiles agree well with the simulated profiles between 2.0 and 3.9 V as seen in Figure 6. An output voltage plateau was observed at ∼3.4 V which is consistent with the CV curves. Figure 7e reveals the cycling and rate performances of the SnS2-rGONR/LCO full cell. The ratio of the current densities of the cathode and anode is ∼8. Here, all current densities are calculated according to the anode. First, the cell was charged/discharged at 0.2 A/g for 10 cycles, and then, the rate gradually increased to 1.5 A/g and reduced back to 0.2 A/g for another 10 cycles; finally, the cell was cycled at 0.5 A/g for 200 cycles. On the basis of the mass of the anode material, the discharge capacities at 0.2, 0.5, 1.0, and 1.5 A/g are 642 mAh/g (2nd), 515 mAh/g (12th), 398 mAh/g (22nd), and 336 mAh/g (32nd), respectively. Even when the rate was reduced back to 0.2 and 0.5 A/g, the capacity reached 562 and 468 mAh/g, respectively. In the following subsequent cycles at 0.5 A/g, the discharge capacity slowly dropped to 380 mAh/g after 200 cycles with a high capacity retention of 82%. Furthermore, the ICL was only 2 mAh/g after 200 discharge/ charge cycles, with a Coulombic efficiency of ∼99%. Discussions on the specific energy of lithium ion batteries made of metal chalcogenides and oxides in the literature have been largely speculative and extrapolative. For the second cycle discharge capacity of 642 mAh/g at a current density of 0.2 A/ g, we obtained a corresponding specific energy of 180 Wh/kg for the SnS2-rGONR/LCO system (Figure S11). This energy value is comparable with those of commercial graphite-based lithium ion batteries.43 It is possible to achieve full cell capacity up to the half cell anode capacity of 1020 mAh/g obtained for the second cycle, but irreversible capacity associated with the cathode material during the first cycle still presents a challenge. It should be noted that the above capacity is reported with respect to the anode (SnS2-rGONR) mass and the specific energy is reported with respect to the total electrode (LCO + SnS2-rGONR) mass.

expected. However, several issues should be considered before a full cell is constructed. First, the capacity for anodes and cathodes should be balanced in a full cell configuration. We tested the electrochemical performances of the anode and cathode at 0.2 A/g. As shown in Figure 6a, SnS2-rGONRs anodes delivered a capacity

Figure 6. (a) Second discharge/charge curves of SnS2-rGONRs anode at 0.2 A/g. (b) Second discharge/charge curves of LCO at 0.2 A/g. (c) The simulated charge curve of the full cell. (d) The simulated discharge curve of the full cell.

of 1020 mAh/g at the second discharge cycle which was consistent with the above data obtained in the half cell, whereas the LCO cathodes delivered a capacity of 123 mAh/g (Figure 6b). Therefore, the mass ratio of cathode to anode was calculated to be 8.3 which should be changed with different current densities. Second, the voltage cutoff window in a full cell should be carefully chosen to avoid side reactions at high voltage. To estimate the voltage cutoff window, we simulated the charge and discharge curve in a full cell by using the discharge/charge curves from half cells of both the anode and cathode. Figure 6c shows the second discharge curve of SnS2-rGONRs anode (lower) and the second charge curve of the LCO cathode (upper) in their respective half cells. By subtracting the corresponding data in these two curves, we get the simulated charge curve (middle) in the full cell. Similarly, we plotted the simulated discharge curve in the full cell as shown in Figure 6d. An estimation of the output voltage should be ∼3.4 V. Therefore, we designed the voltage cutoff window of the full cell as 2.0 to 3.9 V. The corresponding specific capacity is ∼600 mAh/g and can be seen from the simulation curve in this voltage range. More capacity can be obtained below 2.0 V; however, the low output voltage in the full cell has fewer applications. Third, the large irreversible capacity loss (ICL) should be addressed, which mainly comes from the SEI formation during the first several cycles, and this influences the battery performance during electrochemical testing. The ICL is as large as 1000 mAh/g in the first cycle in SnS2-rGONR/LCO full cell as shown in Figure S9a. As a result, the cell suffered a rapidly decaying capacity with the increased cycle number

3. CONCLUSIONS In summary, nanocomposites of layered SnS2 nanoplates grown on rGONRs were successfully developed by a facile and scalable method. SEM and TEM analysis revealed ∼200 nm 26553

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Figure 7. (a) Schematic illustration of a SnS2-rGONR/LCO full cell. (b) CV of the first three cycles for the SnS2-rGONR/LCO full cell. (c) Digital photograph of two tandem LEDs lit by a SnS2-rGONR/LCO full cell. (d) Representative discharge/charge voltage profiles at various rates for the SnS2-rGONR/LCO full cell. (e) Capacity retention of the full cell at various rates from 0.2 to 1.5 A/g and then kept at 0.5 A/g for 200 cycles. vol) by bath sonication for 30 min. Then, 4.4 mmol (834 mg) of SnCl2 was added to the above solution and bath sonicated (Branson 2510) for another 10 min, followed by addition of 10 mmol (400 mg) of NaOH. The reaction was kept at 160 °C for 2 h, and after cooling to room temperature, SnO2-GONRs with a mass of ∼750 mg were collected through centrifugation and drying in a vacuum oven (∼6 mmHg) at 60 °C for 20 h.44 In the second procedure, the prepared SnO2-GONRs were placed in the center of a horizontal quartz tube furnace system and 10 mg of sulfur was placed ∼15 cm away from the center, upstream of the horizontal quartz tube. Before heating, the system was evacuated to 16 mTorr, and then, the central temperature was increased to 380 °C and kept there for 1 h under constant Ar flow of 200 sccm and H2 flow of 50 sccm at ambient pressure. Finally, the sample was collected when the furnace was cooled to room temperature. The control samples of pristine rGONRs and SnS2 were obtained using a similar procedure. Materials Characterization. All solid products above were characterized by FESEM (JEOL 6500 field); TEM and scanning TEM (STEM) (200 kV JEOL FE2100); XRD (Rigaku D/Max Ultima II); Raman microscopy (Renishaw Raman RE01 scope); and XPS (PHI Quantera). Assembly and Testing of Lithium Ion Batteries. Slurries for both SnS2-GONRs anode and LiCoO2 cathode were prepared by mixing 80 wt % of active composite, 10 wt % of carbon black (Super P), and 10 wt % of polyvinylidene difluoride (PVDF, Alfa Aesar) with N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich). The slurries were coated on a copper foil and aluminum foil, respectively, and then, the electrodes were dried in a vacuum oven (∼6 mmHg) at 120 °C for 10 h. The mass for each anode is ∼1.0 mg in half cell with the volume density of 1.1 × 103 mg/cm3. In the full cell, to match the large mass of

nanoplates vertically grown on the rGONR surface. When SnS2-rGONRs are applied as an anode material in half cells, they deliver a high specific capacity (1033 mAh/g at 0.1 A/g) with a high-rate capability and excellent cycling stability even after 800 cycles. More importantly, the full cell of SnS2rGONRs/LCO was successfully fabricated and showed a high capacity of 642 mAh/g at 0.2 A/g and a high capacity retention of 82% after 200 cycles. The remarkable electrochemical performance of SnS2-rGONRs in both the half and full cells can be attributed to the robust composite structure, which can maintain the stability of the electrode, and synergy between layered SnS2 and high electrical conductivity and flexibility of rGONR. This research could motivate further exploration of other metal sulfide/oxide-carbon composite material as anodes/cathodes for use in high capacity LIBs.

4. EXPERIMENTAL SECTION Preparation of Graphene Oxide Nanoribbons (GONRs). 150 mg of MWCNTs (NanoTechLabs Inc., USA) were suspended in 36 mL of concentrated H2SO4 by stirring for 1 h. Next, 4 mL of 85% H3PO4 was then added into the solution, and the mixture was kept stirring for another 15 min before the addition of 1.2 g of KMnO4. The reaction mixture was heated to 65 °C for 2 h and then allowed to cool to room temperature. Synthesis of SnS2-Reduced Graphene Oxide Nanoribbons (SnS2-rGONRs). SnS2-rGONRs were synthesized by two main procedures. In the first procedure, 1.0 mL/mg GONRs were dispersed in a 200 mL solution of deionized water and ethylene glycol (1:3 vol/ 26554

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ACS Applied Materials & Interfaces the cathode, the mass of each anode is ∼0.5 mg and the mass of each cathode is ∼4.0 mg. All the mass of the electrodes were measured by a Cahn C31 microbalance (the sensitivity is 0.1 μg) before and after material loading. Electrochemical tests were performed using CR2032 coin-type cells with a lithium metal foil as the counter electrode. The electrolyte was 1 M LiPF6 in a solution of ethylene carbonate and diethyl carbonate (1:1 vol/vol). Celgard 2500 membrane was used as a separator. CV tests were performed on a CHI660D electrochemical station at a current density of 0.40 mV/s. EIS measurements were carried out on the CHI660D at an open circuit potential in the frequency range of 100 kHz to 10 mHz, and the galvanostatic discharge/charge test was carried out on a LAND CT2001A battery system at room temperature. The capacity value was based on the total mass of the active materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07768. Additional SEM, XPS, XRD, and CV figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

C.G. and L.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The AFOSR MURI program (FA9550-12-1-0035), the AFOSR (FA9550-14-1-0111), and the Chinese scholarship council (CSC) provided funding. The authors would also like to thank Celgard LLC for the kind donation of the battery separator material (Celgard 2500).

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