Flexible Paper-like Free-Standing Electrodes by Anchoring Ultrafine

Apr 21, 2017 - Flexible Paper-like Free-Standing Electrodes by Anchoring Ultrafine SnS2 Nanocrystals on Graphene Nanoribbons for High-Performance ...
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Flexible Paper-like Free-Standing Electrodes by Anchoring Ultrafine SnS2 Nanocrystals on Graphene Nanoribbons for High-Performance Sodium Ion Batteries Yang Liu,† Yongzhen Yang,§ Xuzhen Wang,*,†,‡ Yanfeng Dong,† Yongchao Tang,† Zhengfa Yu,† Zongbin Zhao,† and Jieshan Qiu† †

State Key Lab of Fine Chemicals, School of Chemical Engineering, Liaoning Key Lab for Energy Materials and Chemical Engineering and ‡School of Chemistry, Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, China § Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China S Supporting Information *

ABSTRACT: Ultrafine SnS2 nanocrystals−reduced graphene oxide nanoribbon paper (SnS2−RGONRP) has been created by a well-designed process including in situ reduction, evaporation-induced self-assembly, and sulfuration. The asformed SnS2 nanocrystals possess an average diameter of 2.3 nm and disperse on the surface of RGONRs uniformly. The strong capillary force formed during evaporation leads to a compact assembly of RGONRs to give a flexible paper structure with a high density of 0.94 g cm−3. The as-prepared SnS2−RGONRP composite could be directly used as freestanding electrode for sodium ion batteries. Due to the synergistic effects between the ultrafine SnS2 nanocrystals and the conductive, tightly connected RGONR networks, the composite paper electrode exhibits excellent electrochemical performance. A high volumetric capacity of 508−244 mAh cm−3 was obtained at current densities in the range of 0.1−10 A g−1. Discharge capacities of 334 and 255 mAh cm−3 were still kept, even after 1500 cycles tested at current densities of 1 and 5 A g−1, respectively. This strategy provides insight into a new pathway for the creation of free-standing composite electrodes used in the energy storage and conversion. KEYWORDS: SnS2, graphene nanoribbons, energy storage, sodium ion batteries, volumetric capacity

1. INTRODUCTION Sodium ion batteries (SIBs) have been considered as a promising alternative to lithium ion batteries (LIBs) owing to the abundance and low price of sodium.1,2 However, the larger radius of Na+ compared to that of Li+ induces severe volume expansion and polarization, making the electrode materials for LIBs unsuitable for SIBs.1−4 For example, graphite as the commercial anode material for LIBs has very low sodiumstorage capacity in SIBs due to its insufficient interlayer spacing.3,4 To find suitable anode materials for SIBs, carbonaceous materials (e.g., carbon molecular sieves, high surface carbon with ether-based electrolyte, and low-surface-area hard carbon),5−7 metal alloys (e.g., Sn and Sb),8,9 metal oxides (e.g., Fe3O4, SnO2 and TiO2)10−12 and metal sulfides (e.g., FeS2 and SnS2)13,14 have been investigated. Among them, layered metal disulfides such as MoS2, WS2, and SnS2 have received increasing attentions due to their high sodium-storage capacities.14−16 The Na+ storage in these layered metal disulfides (MoS2 and WS2) is mainly realized through conversion reactions.15,16 However, in the case of SnS2, it demonstrates a combined conversion and alloying−dealloying mechanism in which the conversion © 2017 American Chemical Society

reaction give rises to metallic tin and Na15Sn4 is formed during the alloying reaction.14,17 Thus, SnS2 could deliver a high theoretical capacity of 1136 mAh g−1 (5112 mAh cm−3) superior to MoS2 (670 mAh g−1, 3216 mAh cm−3) and WS2 (432 mAh g−1, 3283 mAh cm−3).14−16 Nevertheless, the bare SnS2 exhibits poor rate and cycling stability performance due to its inherent limitations. As a semiconductor, SnS2 has inherent low electronic conductivity, which affects its electrochemical performances.18 Furthermore, the large volume expansion and mechanical stress of SnS2 during the Na+ insertion and extraction process can induce the failure of the electrode, resulting in poor cycling stability.19 To resolve these problems, SnS2−C composites are constructed based on the high capacity of SnS2 and high electronic conductivity as well as the stable structure of carbon materials. For example, Qu et al. constructed SnS2−reduced graphene oxide (SnS2−RGO) composite through a hydrothermal method, which showed Received: February 17, 2017 Accepted: April 21, 2017 Published: April 21, 2017 15484

DOI: 10.1021/acsami.7b02394 ACS Appl. Mater. Interfaces 2017, 9, 15484−15491

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration and digital images for the construction of 2D SnS2−RGONRP. H2SO4 (130 mL) and sonicated for 0.5 h to distribute them evenly. The formed mixture was stirred for 1 h at room temperature with a mechanical agitator. Subsequently, KMnO4 (5.0 g) was slowly added into the mixture for 1 h of reaction at room temperature and then for 2 h of reaction at 70 °C. After the reaction, the obtained brown solution was slowly poured into ice water (500 mL) containing H2O2 (10 mL, 30 wt %). The GONRs was centrifuged and washed with deionized water at least five times and redispersed in water to form a homogeneous dispersion. 2.2. Preparation of SnS2−RGONRP. SnCl2·2H2O (10 mg) was added into GONR dispersion (4 mg mL−1, 5 mL) and sonicated for 30 min to be mixed with each other uniformly. Soon afterward, the mixture was sealed in a glass vial (inner diameter 5 cm) and heated at 95 °C for 6 h. Hydrogel was formed afterward and dried at 80 °C in an oven, and then a paper-like structure was obtained after 2 h. The paper could be easily floated and detached from the bottom of glass vial after adding DI water. After transfer and another drying process at 80 °C, SnO2−partially reduced graphene oxide nanoribbon paper (SnO2− PRGONRP) was obtained. The sulfuration of SnO2−PRGONRP was conducted in a sealed quartz tube under a gas mixture of H2S−Ar (2 vol %) with a flow rate of 100 sccm. After heating at 300 °C for 5 h, the final flexible SnS2−RGONRP was prepared. 2.3. Preparation of SnS2−RGONRA, RGONRs, and SnS2. For comparison, three contrastive samples were prepared. SnS2 loaded on reduced graphene oxide nanoribbon aerogel (SnS2−RGONRA) was synthesized through the similar method for preparing of SnS2− RGONRP. The only difference was that freeze-drying was used to dehydrate SnO 2 −PRGONR hydrogel instead of evaporation. RGONRs were prepared through thermal treatment of GONRs at 300 °C for 5 h under the mixture of H2S−Ar. Pure SnS2 was synthesized through a hydrothermal method with SnCl4·5H2O and thioacetamide as precursor. The whole reaction was conducted in a Teflon lined stainless steel autoclave and heated in an electric oven at 160 °C for 12 h. 2.4. Characterization. The morphology of products was characterized by scanning electron microscopy (SEM, QUANTA 450), field-emission scanning electron microscopy (FESEM, SUPARR 55), and transmission electron microscopy (TEM, Tecnai F30). The crystalline phases of the products were examined using Rigaku D/ MAX-2400 diffractometer (XRD) equipped with a rotating anode and a CuKα radiation source (λ = 0.15418 nm). X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo ESCALAB 250 spectrometer employing an Al−Kα X-ray source. Thermal stability of the samples was assessed by thermal gravimetric analysis (TGA, DTG-60AH) in air with temperature ranging from 30 to 750 °C at a heating rate of 10 °C min−1. 2.5. Electrochemical Measurements. The SnS2−RGONRP was cut into small discs with diameter of 14 mm and directly used as electrodes for SIBs. The average mass loading of whole electrode was found to be 0.78 mg cm−2 (based on the values of 10 samples). SnS2− RGONRA was also directly used as binder-free electrode but has a smaller diameter of ∼10 mm. For the powder samples, slurry was prepared by mixing 70% active electrode material (RGONRs or SnS2), 20% acetylene black, and 10% polyvinylidene fluoride (PVDF) binder. Next, working electrodes were fabricated through slurry coating on Cu

high capacity as well as stable cycling performance when tested as anode material for SIBs.20 Similar results have also been reported by Jiang et al. and Zhang et al.19,21 Although progress has been made in the development of anode materials for SIBs, there are still many challenges in the construction of electrode materials for practical applications. First, in most cases, the gravimetric capacity has been used as the main criterion by which to evaluate the performance of electrode materials.8−21 However, the volumetric energy density of a battery is also one of the most important factors that determines its suitability as a next-generation battery for use in electric vehicles and mobile devices.22−26 Therefore, it is in strong demand to develop new electrode materials delivering high volumetric capacity for SIBs. Second, the slurry-coating method has generally been used to construct electrodes for SIBs with the use of current collector, conductive agent, and polymeric binder. Actually, this process is not only complicated but also reduces the energy density of batteries. The construction of flexible, free-standing electrode containing active material with high volumetric capacity can be an efficient way to solve the two problems. It also satisfies the requirements for bendable, wearable, and implantable electronic devices.27 Recently, free-standing electrodes like MoS2− graphene paper, carbon nanofiber (CNF) film, and MXene− carbon nanotube (CNT) paper have been successfully constructed for SIBs.28−30 However, complicated procedures such as vacuum filtration and electrospinning were needed, and the volumetric capacities of these electrodes were still not satisfactory. As mentioned above, SnS2 has high theoretical capacities in both mass and volume. Our recent research demonstrates graphene oxide nanoribbons (GONRs), a quasione-dimensional (1D) graphene oxide, can facilely assemble to paper-like structure with the evaporation of solvent.31 Herein, Sn2+ in situ reduction and water evaporation induced selfassembly procedures have been developed to construct flexible graphene nanoribbon based paper. The GONRs could be partially reduced by Sn2+ with the in situ formation of ultrasmall SnO2 nanoparticles firmly attached to the surface of nanoribbons. After the sulfuration process, SnO2 was successfully transformed to ultrafine SnS2 nanocrystals with further reduction of GONRs. The obtained ultrafine SnS2 nanocrystal−reduced graphene oxide nanoribbon paper (SnS2/ RGONRP) was used as free-standing electrode for SIBs, showing high volumetric capacity, superior rate capability, and stable cycling performance.

2. EXPERIMENTAL SECTION 2.1. Synthesis of GONRs. The obtained GONRs were synthesized through the unzipping of CNTs with H2SO4 and KMnO4 as oxidants.32 In brief, CNTs (1.0 g) were added into concentrated 15485

DOI: 10.1021/acsami.7b02394 ACS Appl. Mater. Interfaces 2017, 9, 15484−15491

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) XRD patterns of pure SnS2, SnO2−PRGONRP, and SnS2−RGONRP. (b−f) XPS spectra of SnO2−PRGONRP and SnS2−RGONRP. (c) O 1s region; (d) Sn 3d region of SnO2−PRGONRP. (e) S 2p region and (f) Sn 3d region of SnS2−RGONRP. foil. The electrodes were dried under vacuum at 120 °C overnight. Coin-type cells were assembled in a glovebox under argon atmosphere (water and oxygen concentration were less than 0.1 ppm). For SIBs, it consisted of a prepared electrode, glass fiber separator, and sodium foil as the counter electrode. NaClO4 (1 M) dissolved in ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio with 5 wt % fluoroethylene carbonate was used as the electrolyte for SIBs. The galvanostatic charge−discharge tests were carried out on a Land CT2001A battery test system between 0.01−3.0 V using 2016 cointype cells. The specific capacities of electrodes were calculated based on the total mass of active materials. The cyclic voltammograms (CV) and electrochemical impendence spectra (EIS) were conducted using a multichannel electrochemical workstation (VMP-300).

and the formation of SnO2 nanoparticles. After the sulfuration process, hexagonal-phase SnS2 with (001), (100), (101) (102), (110), and (111) planes appears, which suggests the successful transformation of SnO2 into SnS2. The (001) peak located at 2θ = 14.8° corresponds to layered SnS2 with an interlayer space of 6.0 Å, which is similar to the pure SnS2 or SnS2 grown on graphene.20,21 However, the peak intensity of loaded SnS2 in the hybrid is much weaker compared with that of pure SnS2, possibly due to its small size and uniform dispersion. The appearance of a broad peak in the range of 22−26° is ascribed to the (002) plane of graphite, exhibiting the further reduction of GONRs and the improvement of their graphitization degree.35 XPS was used to investigate the surface feature and chemical compositions of GONRs, SnO2−PRGONRP, and SnS2− RGONRP. The survey spectrum of SnO2−PRGONRP (Figure 2b) shows the distinguishing peaks of Sn, O, and C. After sulfuration, O 1s and O KLL peaks disappear with the appearance of S 2p and S 2s peaks in SnS2−RGONRP, which further confirms the successful conversion from oxide to sulfide. The Sn 3d spectra of SnO2−PRGONRP and SnS2−RGONRP show two peaks located at 487.2/495.6 eV (Figure 2d) and 486.9/495.3 eV (Figure 2f), respectively, which should be ascribed to Sn 3d5/2 and Sn 3d3/2 of Sn4+.36,37 The obvious O 1s peak for SnO2−PRGONRP can be fitted to three kinds of oxygen species namely crystal lattice oxygen of SnO2 (531.0 eV), oxygen functional groups (532.1 eV), and physically absorbed oxygen (533.0 eV) (Figure 2c),38 while for the sample after sulfuration, there are doublet peaks at 162.2/163.5 eV, which can be attributed to S2− in SnS2 (Figure 2e).39,40 In addition, XPS was also used to characterize the oxygencontaining functional groups attached on GONRs, PRGONRP, and RGONRP (Figure S3). There are enormous C−O (epoxy or alkoxy) and CO (carbonyl or carboxyl) groups attached on GONRs, which make them with a low C-to-O ratio of 2.4 and well-dispersed in water (Figures S3a and S1e). The content of oxygen functional groups gets lower due to the partial

3. RESULTS AND DISCUSSION The overall synthetic process of SnS2/RGONRP involves three steps (Figure 1): At first, GONRs were partially reduced by SnCl2 and SnO2−PRGONR hydrogel was formed during the hydrothermal process. In detail, Sn2+ would react with the oxygen functional groups attached on GONRs and be oxidized to SnO 2 nanoparticles anchored in the networks of PRGONRs.33 Second, the hydrogel was directly dried at 80 °C for 2 h, and then a paper-like structure was obtained due to the water evaporation induced self-assembly. The paper could be easily detached from the bottom of glass vial due to the buoyancy of added water. The final step led to the sulfuration of SnO2−PRGONRP (SnO2 + H2S → SnS2 + H2O) and further elimination of the residual oxygen-containing functional groups to form flexible SnS2−RGONRP. Characterizations including X-ray diffraction (XRD; Figures 2a, S1a, and S2a), X-ray photoelectron spectra (XPS, Figures 2b−f and S3) have been used to investigate the compositions of samples. As shown in Figure S1a, GONRs have only one sharp peak at 2θ = 8.5° corresponding to the interlayer expanded tube walls (10.4 Å), indicating the exfoliation of CNTs.34 However, this peak disappears and SnO2 peaks with low intensity arise in the XRD patterns (Figure 2a) after the hydrothermal process due to the partial reduction of GONRs 15486

DOI: 10.1021/acsami.7b02394 ACS Appl. Mater. Interfaces 2017, 9, 15484−15491

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a, b) SEM, (c−e) TEM, and HRTEM images of SnS2−RGONRP. (f−h) Elemental mapping images of C, Sn, and S components based on panel b.

reduction of GONRs via Sn2+ (Figure S3b). The sulfuration process further removes the residual oxygen functional groups; thus, SnS2−RGONRP gets a higher C-to-O ratio of 8.7 (Figure S3c). The elimination of oxygen functional groups and restoration of sp2 in carbon substrates can improve electron transfer, which is favorable for the rate capability of SIB electrodes.41 The content of SnS2 in the SnS2/RGONRP was determined by thermogravimetric analysis (TGA) in air atmosphere (Figure S4). After thermal oxidation, a weight percent of 43.3 wt % is achieved at 750 °C. Considering that the SnS2 has been oxidized into SnO2, the weight fraction of SnS2 in composite is estimated to be about 52.5 wt %. GONRs unzipped from CNTs have high aspect ratios and are used as precursor to construct free-standing paper structure (Figure S1b-d). The morphology and microstructure of SnS2− RGONRP are revealed by SEM, TEM, and HRTEM images. The average thickness of the paper is 8.3 μm (Figures 3a and S5) with an average bulk density of 0.94 g cm−3. Figure 3b shows the 2D paper is constructed by a large amount of interconnected 1D RGONRs. A porous structure is formed that is favorable for the diffusion of electrolyte. It should be noted that the 1D structure of GONRs is essential for the successful construction of an intact paper structure. A contrastive experiment using graphene oxide as precursor has been taken. However, only fragmentary samples were obtained due to the shrinkage and curl of 2D graphene sheets (Figure S6). The elemental mapping images based on Figure 3b are shown in Figure 3f−h, exhibiting the uniform distribution of C, Sn, and S all over the SnS2−RGONRP, while there are C, Sn, and O elements in SnO2−PRGONRP (Figure S7b−d). TEM images (Figure 3c,d) were used to reveal their detailed nanostructures. Figure 3c shows the RGONRs still weave together even after experiencing strong ultrasonic in the preparation of TEM samples, demonstrating their tight connection with each other, which is helpful for the stability of electrodes. The HRTEM images (Figures 3d and S8) exhibit that numerous of ultrafine SnS2 nanocrystals ranging from 1 to 4 nm in size (average diameter 2.3 nm) were closely attached to the surface of RGONRs, which is slightly smaller than the diameter of SnO2 nanoparticles (3.0 nm) attached on PRGONRP (Figures S7f−h

and S9). The (100) crystal plane with lattice spacing of 0.32 nm can be resolved from Figure 3e. The functional groups and large surface area of GONRs contribute to the formation and even dispersion of ultrasmall SnO2 nanoparticles; thus, ultrafine SnS2 nanocrystals can be achieved. The size reduction of SnS2 may be ascribed to the fragmentation of SnO2 with low crystallinity during the sulfuration process. As reported, the ultrafine nanocrystals are beneficial for accommodating volume expansion and shorting transport paths for ions and electrons, which could improve the cycling stability and rate capability of electrodes.8,19,42,43 Based on the flexible property, porous and firm structure, uniform and ultrafine active material loading, SnS2−RGONRP was directly used as free-standing anode for SIBs. Figure 4a shows the cyclic voltammograms (CVs) of the electrode between 0.01 and 3.0 V at a scan rate of 0.1 mV s−1. In the first cathodic scan, a reduction peak at 1.5 V can be assigned to the intercalation of the Na ions into SnS2 interlayers, forming NaxSnS2.17−19,44,45 The major reduction peak around 0.6 V is due to the combination of synergetic conversion and alloying reactions as well as the irreversible formation of the solid electrolyte interphase (SEI).17−19,44,45 During the anodic scan, the broad oxidation peak around 1.2 V is attributed to the desodiation of the Na−Sn alloy phase and the reformation of SnS2.17−19,44,45 The charge−discharge profiles of SnS2− PRGONRP are shown in Figure S10a. The evident voltage plateaus existing in the curves are in agreement with the peaks of CV. The SnS2−PRGONRP electrode delivers an initial discharge capacity of 660 mAh g−1 and a reversible capacity of 493 mAh g−1, giving a Coulombic efficiency of 74.7%. The irreversible part can be ascribed to the formation of SEI film.44,45 From the third cycle, the test current density increases to 1.0 A g−1, but the discharge capacity still keeps over 418 mAh g−1 within 100 cycles (Figure 4b). High Coulombic efficiency over 99% can be kept upon cycling, indicating the excellent cycling stability and reversibility of electrode. It is noteworthy that fluoroethylene carbonate added in electrolyte, which is known for stabilizing the SEI film in SIBs, plays an important role in keeping stable the cycling performance of the electrode (Figure S11).2 The specific capacity of SnS2− 15487

DOI: 10.1021/acsami.7b02394 ACS Appl. Mater. Interfaces 2017, 9, 15484−15491

Research Article

ACS Applied Materials & Interfaces

Figure 4. Electrochemical performance of SnS2−RGONRP for SIBs. (a) Cyclic voltammograms (CVs) of SnS2−RGONRP electrode obtained at a scan rate of 0.1 mV s−1 between 0.01 and 3.0 V. (b) Comparative cycling performance of SnS2−RGONRP, SnO2−PRGONRP, SnS2−RGONRA, RGONRs, and pure SnS2 tested at a current density of 0.2 A g−1 for the first two cycles and then 1.0 A g−1 for the following cycles. (c) Rate capability of SnS2−RGONRP, SnO2−PRGONRP, SnS2−RGONRA, and RGONRs tested at various current densities ranging from 0.1 to 10 A g−1. (d) Comparisons of volumetric capacities of SnS2−RGONRP with pure carbon nanomaterials or their composites with layered metal disulfide, metal alloy, or MXene anodes for SIBs at different current densities from some previous literature (L1: MoS2−RGO paper;28 L2: Ti3C2 MXene−CNT paper;30 L3: Sb−RGO paper;46 L4: nitrogen-doped CNF film;29 L5: folded graphene;47 L6: banana-peel pseudographite;48 L7: porous CNF;49 and this work: SnS2−RGONRP). (e) Long-term cycling performance of SnS2−RGONRP tested at current densities of 1 and 5 A g−1, respectively (the first two cycles were also tested at a current density of 0.2 A g−1).

260 mAh g−1 can be retained when tested at different current densities ranging from 0.1 to 10 A g−1. Obviously, the SnS2− RGONRP shows superior rate capability to SnO2−PRGONRP (329−41 mAh g−1) and RGONRs (286−84 mAh g−1) tested at the same current densities, while the SnS2−RGONRA electrode cannot stand the large current density of 10 A g−1, and only 9 mAh g−1 is obtained at a current density of 5 A g−1. The average bulk density of SnS2−RGONRP is 0.94 g cm−3; thus, the value of its volumetric capacities can be calculated based on the equation Cv = Cm*ρ (Cv: volumetric capacity; Cm: gravimetric capacity; ρ: bulk density).30 The average volumetric capacities of SnS2−RGONRP are 508, 426, 398, 341, 307, and 244 mAh cm−3, tested at different current densities of 0.1, 0.5, 1, 3, 5, and 10 A g−1 (Figure 4d), respectively. These values are superior to many SIB electrodes as reported such as MoS2− RGO paper,28 Ti3C2 MXene−CNT paper,30 Sb−RGO paper,46

RGONRP is much higher than that of SnO2−PRGONRP (171 mAh g−1), SnS2−RGONRA (102 mAh g−1, SnS2 loading on reduced graphene nanoribbon aerogel obtained through lyophilization instead of evaporation, Figure S12), RGONRs (139 mAh g−1, reduced graphene oxide nanoribbons achieved through H2S reduction, Figure S13), and pure SnS2 (8 mAh g−1). The detailed comparisons are shown in their charge− discharge profiles (Figure S10). Meanwhile, the loading of SnS2 on the electrochemical properties of the composites has also been investigated. It is revealed that low capacity or unstable cycling performance is suffered with lower content of 42.5 wt % or higher content of 64.4 wt % (Figures S4 and S14) compared with the optimized one (52.5%). In addition to the specific capacity, rate capability is another important factor for SIBs. The rate performance of SnS2− RGONRP is shown in Figure 4c. A discharge capacity of 541− 15488

DOI: 10.1021/acsami.7b02394 ACS Appl. Mater. Interfaces 2017, 9, 15484−15491

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ACS Applied Materials & Interfaces

Figure 5. (a) Digital image of SnS2−RGONRP electrode after cycling at 1.0 A g−1 100 times. (b,e) SEM, (c) TEM, and (d) HRTEM images featuring the nanostructures of electrode. (f−h) Elemental mapping images of C, Sn, and S components based on panel e.

folded graphene,47 banana-peel pseudographite,48 nitrogendoped CNF film, and porous CNF.29,49 Compared with these samples, SnS2−RGONRP also shows higher gravimetric capacities, as shown in Figure S15. Furthermore, long cycling performance at current densities of 1 and 5 A g−1 is evaluated for SnS2−RGONRP (Figure 4e). Discharge capacities of 334 and 255 mAh cm−3 (355 and 271 mAh g−1) are still kept even after 1500 cycles, which are superior to the free-standing electrodes reported previously.28−30,49,50 It should be noted that the high rate charge−discharge at a current density of 5 A g−1 make the SnS2 −RGONRP electrode suffer partial pulverization and the formation of an unstable SEI layer, which lead to slight decay of capacity during the initial 100 cycles, while the reactivation of SnS2 nanocrystals and the formation of stable SEI layer on the surface of electrode results in the gradual increase of capacity after continuous cycling.51,52 The high capacity, superior rate, and stable cycling performance of SnS2−RGONRP electrode can be ascribed to the following factors. First, the ultrasmall SnS2 nanocrystals resulted from the in situ redox reaction evenly dispersed on the surface of RGONRs can significantly reduce the absolute strain, improve the utilization of active material, and shorten the transport paths for ions and electrons, which could improve the cycling stability and rate capability of electrodes.8,19,42,43 Second, the evaporation induced self-assembly of RGONRs contributes to the creation of 2D paper structure with high density and firm structure. The high density of electrode brings about high volumetric capacity, while the firm structure of SnS2−RGONRP is favorable for the cycling stability. Third, the conductive and interconnected RGONR network provides a pathway for fast charge transfer, which contributes to the high rate capability of electrodes. In contrast, the loose stacking of RGONR sheets makes the SnS2−RGONRA have much-larger impedance than SnS2−RGONRP and, ultimately, poor performance as SIB electrode (Figure S16). Last but not least is the fact that there are numerous nanostructure pores created by the lapping of RGONRs, which can supply facile transport channels for electrolyte and Na+.53 In a word, the combination

of ultrafine SnS2 and compact assembly of RGONRs makes the paper structure suitable anode for high-performance SIBs. The electrode after 100 cycles at 1.0 A g−1 are examined again, as shown in Figure 5. A free-standing structure without any mechanical cracks can be observed, which further proves the toughness of SnS2−RGONRP. No evident particle aggregation is observed on the surface of the electrode through the SEM and TEM observation. As shown in the HRTEM image (Figure 5d), there are still ultrasmall nanoparticles attached to the surface of nanoribbons. However, no clear lattice fringes are observed within the particles, indicating the amorphization process of SnS2 during the charge−discharge, which has been confirmed by other reports.14,44 The elemental mapping results based on panel e reveal that the Sn and S elements are evenly distributed on the surface of RGONRs, confirming the tight connection between SnS2 and RGONRs. In summary, a flexible composite paper-like structure (SnS2− RGONRP) was successfully created and tested as free-standing anode for SIBs. The paper electrode delivered high volumetric capacities of 508−244 mAh cm−3 in the current density range of 0.1−10 A g−1, which were superior to those of many freestanding electrodes reported previously for SIBs. In addition, stable cycling could still be kept even after 1500 cycles when tested at current densities of 1 and 5 A g−1. The remarkable performance can be ascribed to the ultrasmall size of SnS2 nanocrystals and the compact assembly of RGONRs resulting from the strong capillary force during evaporation, which are beneficial to fast charge transfer and the accommodation of electrode volume changes. The evaporation-induced selfassembly plays a crucial role in the formation of high-density, interconnected, and porous graphene nanoribbon based paper structure. This strategy can be expanded to the construction of free-standing electrodes for other energy-storage devices such as LIBs, lithium sulfur batteries, and supercapacitors. 15489

DOI: 10.1021/acsami.7b02394 ACS Appl. Mater. Interfaces 2017, 9, 15484−15491

Research Article

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02394. Figures showing XRD patterns and TEM images of CNTs and GONRs; C 1s spectra; TG curves and cycling performance; digital photograph and SEM images; SEM, TEM, and HRTEM images; charge and discharge curves of different samples; SEM images; and electrochemical impedance spectra. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xuzhen Wang: 0000-0002-0366-8140 Jieshan Qiu: 0000-0002-3124-7183 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (grant nos. U1610105, 51672033, and U1610255), the Natural Science Foundation of Liaoning Province (grant no. 201602170), the Open Fund of Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education (grant no. KLISEAM 201601), and the Open Sharing Fund Projects for Large Equipment Testing, Dalian University of Technology (grant no. 2016-54).

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