(M = Fe, Co, Ni) Codopants for Li+ and Na+ Storage - ACS Nano

Sep 25, 2017 - ... achieve the best Li-ion and Na-ion storage properties. For example, the Fe0.3Nb0.7S1.6Se0.4 nanosheets depict excellent rate capabi...
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NbS2 Nanosheets with M/Se (M = Fe, Co, Ni) Codopants for Li+ and Na+ Storage Jianli Zhang,†,‡ Chengfeng Du,‡ Zhengfei Dai,‡ Wei Chen,† Yun Zheng,‡ Bing Li,§ Yun Zong,§ Xin Wang,† Junwu Zhu,*,† and Qingyu Yan*,‡ †

Key Laboratory for Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, China ‡ School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 § Institute of Materials Research and Engineering (IMRE), Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634 S Supporting Information *

ABSTRACT: Transition metal (M = Fe, Co, Ni) and Se codoped two-dimensional uniform NbS2 (MxNb1−xS2−ySey) nanosheets were synthesized via a facile oil-phase synthetic process. The morphology of MxNb1−xS2−ySey can be adjusted by tuning the amount of metal and Se introduced into NbS2. Among them, the optimized Fe0.3Nb0.7S1.6Se0.4 nanosheets, with lateral sizes of 1−2 μm and approximately 5 nm thick, achieve the best Li-ion and Na-ion storage properties. For example, the Fe0.3Nb0.7S1.6Se0.4 nanosheets depict excellent rate capabilities with fifth-cycle specific capacities of 461.3 mAh g−1 at 10 A g−1 for Li storage and 136 mAh g−1 at 5 A g−1 for Na storage. More significantly, ultralong cyclic stabilities were achieved with reversible specific capacities of 444 mAh g−1 at 5 A g−1 during the 3000th cycle for Li storage and 250 mAh g−1 at 1 A g−1 during the 750th cycle for Na storage. Post-treatment high-resolution transmission electron microscopy was studied to prove that the reversible Li-ion storage in NbS2 was based on a conversion reaction mechanism. KEYWORDS: NbS2 nanosheets, oil phase, Li storage, Na storage, conversion reaction mechanism mAh g−1) based on the intercalation mechanism (Nb2O5 + 2Li+ + 2e− ↔ Li2Nb2O5), significantly limit its practical application. In contrast to Nb2O5, niobium disulfide (NbS2) merits in higher theoretical capacity (683 mAh g−1) based on the possible conversion reaction mechanism (NbS2 + 4Li+ + 4e− ↔ Nb + 2Li2S) and conductivity (due to lower band gap), which can be a promising candidate for LIBs and SIBs.23 Moreover, its favorable layered crystal structure allows flexible approaches to prepare two-dimensional (2D) nanostructures and to be integrated into various applications. In addition, heteroatom doping of sulfides with elements such as selenium (Se), iron (Fe), cobalt (Co), and nickel (Ni)

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or the past decades, rechargeable batteries including lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) have been extensively investigated as the most promising portable energy sources for electronic devices and electric vehicles owing to their high energy density, long durability, and no memory effect.1−10 However, further improvement in terms of energy density, rate capability, durability, and safety is still of great importance to advance their practical applications. Therefore, increasing research attention has been devoted to exploring new electrode materials toward high capacity, fast charging, and durable cycling.11−18 As an attractive electrode material, niobium oxide (Nb2O5) has been widely investigated for LIBs and SIBs because of its safe operation, high rate capability, and stable cycle performance.19−22 However, the constraints of Nb2O5, such as poor electrical conductivity and limited theoretical capacity (202 © 2017 American Chemical Society

Received: August 29, 2017 Accepted: September 25, 2017 Published: September 25, 2017 10599

DOI: 10.1021/acsnano.7b06133 ACS Nano 2017, 11, 10599−10607

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Figure 1. (a) XRD of NbS2-based nanosheets with different heteroatom doping. SEM images of (b) flower-like NbS2, (c) NbS1.6Se0.4, and (d) Fe0.3Nb0.7S1.6Se0.4 nanosheets.

the extrinsic pseudocapacitive contribution is verified by kinetic analysis, which should be the origin of excellent rate performance for Fe0.3Nb0.7S1.6Se0.4 nanosheets.

can play a pivotal role in improving their conductivity due to the improved charge-carrier concentrations.24,25 The higher theoretical capacities of sulfides based on Fe,26 Co,27 and Ni28 may further improve the Li/Na storage properties. Besides, to the best of our knowledge, the currently available approaches to produce NbS2 nanosheets are mainly based on high-temperature solid-state reactions (e.g., 700−1050 °C) followed by liquid-phase exfoliation, which is regarded as a low-productionyield process and is difficult to achieve heteroatom doping.29−32 Herein, we report a facile oil-phase synthetic process to prepare NbS2 nanosheets and introduce M (M = Fe, Co, Ni) and Se heteroatoms into the nanosheets with tunable contents. As a result, Fe- and Se-doped NbS2 show a progressive changing on their morphologies. With Se doping, NbS1.6Se0.4 nanosheets exhibit lateral sizes of 300−500 nm and ∼7 nm thick. In addition, Fe0.3Nb0.7S1.6Se0.4 nanosheets are highly uniform with larger lateral sizes of 1−2 μm and thickness of ∼5 nm. Among all the samples prepared, the Fe0.3Nb0.7S1.6Se0.4 nanosheets present the optimized Li/Na storage properties. For Li storage, a high reversible specific capacity of 444 mAh g−1 at 5 A g−1 during 3000th cycle and an excellent rate capability with a fifth-cycle specific capacity of 461.3 mAh g−1 at 10 A g−1 were achieved. For Na storage, the Fe0.3Nb0.7S1.6Se0.4 nanosheets show good rate capability and cyclic stability, delivering a specific capacity of 233.6 mAh g−1 during the 750th cycle at 1 A g−1 and a fifth-cycle specific capacity of 134.8 mAh g−1 at 5 A g−1. Through post-treatment high-resolution transmission electron microscopy (HRTEM) study, we also confirm the conversion Li storage mechanism rather than previously reported intercalation mechanism for NbS2. More importantly,

RESULTS AND DISCUSSION The ingredient and crystal structure of NbS2-based samples are investigated by powder X-ray diffraction (XRD), as shown in Figure 1a. Here, all of the sample composition refers to the original molar ratio among the chemical precursors. The three diffraction peaks of NbS2 and NbS1.6Se0.4 located at 31.1, 55.2, and 64.9° can be indexed to the (100), (110), and (200) planes of the hexagonal crystal structure (JCPDS No. 00-041-0980, space group: P63/mmc), respectively. After the metal (e.g., Fe, Co, Ni) doping, the crystal structure of the samples changes to space group of P6322 (e.g., JCPDS No. 03-065-4108 for FeNb3S6, JCPDS No. 01-070-2816 for CoNb3S6, and JCPDS No. 01-089-5294 for NiNb3S6). The peaks around 31.1, 55.2, and 64.9° can be indexed to (110), (300), and (220), respectively. For different feeding mole ratios of Nb/Fe (labeled as INb/Fe), the XRD patterns are shown in Supporting Information Figure S1. All the samples with different INb/Fe values show similar XRD patterns. It indicates that the hexagonal phase (JCPDS No. 03-065-4108) is the stable crystal structure obtained during the preparation. The morphology of NbS2 can be adjusted by tuning the Se and M (M = Fe, Co, Ni) contents. It can be seen from Figure 1b, without heteroatom doping, pure NbS2 forms nanosheets, which stick together to become a flower-like morphology (∼3 μm in diameter of the flower). After Se is introduced into the samples, the morphology of NbS2−ySey changes progressively 10600

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Figure 2. (a) TEM image of Fe0.3Nb0.7S1.6Se0.4 nanosheets. Inset: SAED of the sample. (b) Delicate image of (a). Inset: Layer of the sample. (c) HRTEM of Fe0.3Nb0.7S1.6Se0.4 nanosheets. (d,e) Mapping of Fe0.3Nb0.7S1.6Se0.4 nanosheets. (f) XPS of Nb 3d and Fe 2p from Fe0.3Nb0.7S1.6Se0.4 nanosheets.

coincidence with the XRD result. The HRTEM image (Supporting Information Figure S7b) of the observed NbS1.6Se0.4 nanosheet is composed of ∼12 layers (∼7 nm). For Fe0.3Nb0.7S1.6Se0.4, the low-magnification TEM image shows typical nanosheets with lateral sizes of 1−2 μm (Figure 2a). The diffraction rings of the corresponding SAED pattern can be indexed to the (110) and (300) planes (inset of Figure 2a), which agrees well with the observed peaks in the XRD result pattern. The lattice fringes of 6.11, 2.96, and 2.57 Å shown in Figure 2b,c can be assigned to the (002), (004), and (112) planes of the hexagonal FeNb3S6 phase, respectively. The inset in Figure 2b shows that the observed Fe0.3Nb0.7S1.6Se0.4 nanosheet has 8 layers and is ∼5 nm thick. The elemental mapping result indicates that Fe and Se atoms are homogeneously distributed in the NbS2 nanosheets (Figure 2d,e). The X-ray photoelectron spectroscopy (XPS) analysis of Fe0.3Nb0.7S1.6Se0.4 nanosheets is shown in Figure 2f. For the Nb 3d spectra, the peaks at 207.2 and 209.8 eV correspond to the 3d5/2 and 3d3/2 of Nb5+, respectively.31,33 The peaks at 203.8 and 206.7 eV coincide with the 3d5/2 and 3d3/2 of Nb4+, respectively.31 The peaks at 203.2 and 205.9 eV can be assigned to the 3d5/2 and 3d3/2 of Nb(4−δ)+.31,34 The binding energies at 710.4 and 723.0 eV, accompanied by associated satellite peaks, can be assignable to the 2p3/2 and 2p1/2 of the Fe2+ state,35,36 which agrees with the FeNb3S6 phase reported previously.37 The S 2p can be resolved into two peaks at 160.9 and 162.1 eV, which can be assigned to the 2p3/2 and 2p1/2 of S2− state (Supporting Information Figure S8a), respectively.31 For the Se 3d region, the peak at 54.8 eV indicates the Se2− state, whereas the peak at 58.8 eV corresponds to surface oxidation of Se (Supporting Information Figure S8b).25,38 Based on the Brunauer−Emmett−Teller (BET) analysis of the N2 adsorption−desorption isotherm curves, the specific areas are determined to be 86.5 and 33.3 m2 g−1 for Fe0.3Nb0.7S1.6Se0.4

(Supporting Information Figure S2). The NbS1.6Se0.4 sample presents smaller uniform nanosheets with lateral sizes from 300 to 500 nm without formation of flowers (Figure 1c). In addition, transition metal atoms (e.g., Fe, Co, Ni) can also be introduced into NbS2 together with Se without disrupting the nanosheet-like morphology. As shown in Figure 1d and Figure S3 (Supporting Information), the FexNb1−xS1.6Se0.4 nanosheets display a progressive variety as a function of INb/Fe. With the addition of Fe from 9:1 to 7:3 for INb/Fe, the average lateral size of nanosheets becomes larger. For samples prepared with INb/Fe < 6:4, the nanosheets stick together to form a spherical structure. Among the Fe/Se-doped samples, Fe0.3Nb0.7S1.6Se0.4 exhibits uniform sheet-like morphology with lateral sizes of 1−2 μm. For Co0.3Nb0.7S1.6Se0.4, the nanosheets display an average lateral size of 200 nm, which stick together to form spheres (0.4−1 μm in diameter of the sphere, Supporting Information Figure S4a,b). For Ni0.3Nb0.7S1.6Se0.4, the nanosheets show lateral sizes of ∼300 nm, which also form spheres (0.3−1 μm in diameter of the sphere, Supporting Information Figure S4c,d). For comparison, Fe0.3Nb0.7S2 and Fe0.95S were also prepared via a similar oil-phase method. The Fe0.3Nb0.7S2 exhibits flower-like morphology, which is composed by stacked nanosheets with lateral sizes of 1−3 μm (Supporting Information Figure S5), whereas the Fe0.95S shows inhomogeneous rod-like morphology (Supporting Information Figure S6). The detailed microstructure of NbS2-based nanosheets were further characterized by TEM and HRTEM analyses. The lowmagnification TEM image presents typical NbS1.6Se0.4 nanosheets with lateral sizes of 300−500 nm, which is consistent with the above SEM observation (Supporting Information Figure S7a). In addition, the corresponding selected area electron diffraction (SAED) pattern (inset of Supporting Information Figure S7a) shows the polycrystalline nature of NbS1.6Se0.4 nanosheets. The observed two diffraction rings can be well indexed to (100) and (110) planes, showing a good 10601

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Figure 3. For LIBs, the initial five CV cycles of (a) NbS1.6Se0.4 nanosheets and (b) Fe0.3Nb0.7S1.6Se0.4 nanosheets. (c) Initial five discharge− charge curves of Fe0.3Nb0.7S1.6Se0.4 nanosheets for LIBs. (d) Rate capabilities of flower-like NbS2, NbS1.6Se0.4, Fe0.3Nb0.7S2, and Fe0.3Nb0.7S1.6Se0.4 nanosheets for LIBs. (e) Cyclic stabilities of Fe0.3Nb0.7S1.6Se0.4 nanosheets for LIBs from 1 to 5 A g−1. (f) EIS of coin cells made from NbS2, NbS1.6Se0.4, Fe0.3Nb0.7S2, and Fe0.3Nb0.7S1.6Se0.4 electrodes.

the following curves demonstrates the good reversibility of the reaction. Among all the metals (e.g., Fe, Co, Ni) and Se-doped MxNb1−xS2−ySey nanosheets, the Fe0.3Nb0.7S1.6Se0.4 presents the best rate capability (Figure 3d and Supporting Information Figures S11 and S12). It achieves fifth-cycle specific capacities of 805 mAh g−1 at 0.1 A g−1 and 461.3 mAh g−1 at 10 A g−1. This value is much higher than the fifth-cycle specific capacities at 10 A g−1 for NbS1.6Se0.4 (220.4 mAh g−1), Fe0.3Nb0.7S2 (175.1 mAh g−1), and flower-like NbS2 nanosheets (61.1 mAh g−1). The cycling performance of Fe0.3Nb0.7S1.6Se0.4 at different current densities is shown in Figure 3e. Interestingly, the discharge capacity increases slowly for the first few hundred cycles, especially at lower current densities. The increased capacity could be ascribed to the gradually expanded and exfoliated interlayers, resulting in more active Li+ storage sites.41 As a result, the reversible specific capacities are maintained to be 745 mAh g−1 at 1 A g−1 during the 100th cycle, 671 mAh g−1 at 2 A g−1 during the 1000th cycle, and 612 mAh g−1 at 3 A g−1 during the 1500th cycle. Remarkably, the specific capacity remains as 444 mAh g−1 at 5 A g−1 during the 3000th cycle with a high capacity retention of 83%. The Nyquist plots of NbS 2 , NbS 1.6 Se 0.4 , Fe 0.3 Nb 0.7 S 2 , and Fe0.3Nb0.7S1.6Se0.4 nanosheets are similar in profile, which are composed of a depressed semicircle at high frequency accompanied by a straight line at low frequency (Figure 3f). The Fe0.3Nb0.7S1.6Se0.4 nanosheets show a smaller diameter of the semicircle, which indicates less charge transfer resistance (Rct). In addition, Fe0.95S presents a fifth-cycle specific capacity of 66.7 mAh g−1 at 10 A g−1, which is much worse than that of Fe0.3Nb0.7S1.6Se0.4 nanosheets (Supporting Information Figure S13). Compared to reported iron sulfides or their composites for Li storage, our Fe0.3Nb0.7S1.6Se0.4 nanosheets achieve higher specific capacity and capacity retention (Supporting Information Table S1).42−44 As shown in Figure S14 (Supporting Information), pure single-walled carbon nanotubes (SWCNTs) were also investigated as anodes for LIBs. During the 10th cycle at 0.1 A g−1, it presents a specific capacity of 216 mAh g−1. In

and NbS1.6Se0.4 nanosheets, respectively (Supporting Information Figure S9). The Li storage properties of NbS2-based nanosheets were investigated using a half-cell configuration versus Li metal. The initial five cyclic voltammetry (CV) curves at 0.1 mV s−1 from 0.005 to 3 V for NbS1.6Se0.4 and Fe0.3Nb0.7S1.6Se0.4 anodes are shown in Figure 3a,b. For NbS1.6Se0.4, a series of reduction peaks below 1.77 V can be observed during the first cathodic scan, whereas a sharp anodic peak centered at 2.34 V is noticed. The intensities of the peaks significantly decay in the subsequent cycles, indicating irreversible reactions and the formation of a solid−electrolyte interface (SEI). According to the conversion reaction of Li storage into MoS2 nanosheets,39,40 which has the similar crystal structure to NbS2 nanosheets, the broad cathodic peak around 1.55 V and the two anodic peaks at 1.88 and 2.34 V may be assigned to S−Nb−S/ Li+ reaction. From the second cycle onward, CV curves almost overlap. For Fe0.3Nb0.7S1.6Se0.4, during the first scan, a cathodic peak at 0.68 V can be assigned to the formation of SEI, and an anodic peak at 2.34 V can be assigned to the oxidation of Li2S to S. The cathodic and anodic peaks at 1.42 and 1.83 V in the subsequent scans can be attributed to the reaction between Fe0.3Nb0.7S1.6Se0.4 and Li+ ions. CV of the Co0.3Nb0.7S1.6Se0.4 anode was carried out as evidence for the reaction between metal/Se codoped NbS2-based nanosheets and Li+ ions (Supporting Information Figure S10). For the first cycle, cathodic and anodic peaks can be observed at 0.68 and 2.34 V, which can also be attributed to the formation of SEI, irreversible reactions, and the oxidation of Li2S to S. For the reaction between Co0.3Nb0.7S1.6Se0.4 and Li+ ions, cathodic and anodic peaks are at 1.35 and 1.75 V, respectively. Figure 3c shows the initial five galvanostatic discharge−charge profiles of Fe0.3Nb0.7S1.6Se0.4 anode at 0.1 A g−1. The discharge and charge capacities for the first cycle are 1412 and 924 mAh g−1, respectively, with a Coulombic efficiency (CE) of 65.4%. The initial capacity loss is possibly ascribed to the irreversible reaction and the formation of the SEI layer. The overlapping of 10602

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Figure 4. Ex situ HRTEM images of (a) cubic-phased Li2S obtained after third-cycle discharge to 0.005 V, (b) electrode after third-cycle discharge to 0.005 V, and (c) electrode after third-cycle charge to 3 V. (d) Schematic illustration of the conversion reaction mechanism for pure NbS2 during the discharge process. (e) CV curves of Fe0.3Nb0.7S1.6Se0.4 nanosheets for LIBs at different scan rates. (f) Values of b of Fe0.3Nb0.7S1.6Se0.4 and NbS1.6Se0.4 nanosheets. (g) Capacitive contribution of Fe0.3Nb0.7S1.6Se0.4 and NbS1.6Se0.4 nanosheets at 1 mV s−1. (h) Capacitive contribution comparison of Fe0.3Nb0.7S1.6Se0.4 and NbS1.6Se0.4 nanosheets at different scan rates.

view of the low mass percentage (10%) in the electrode, the added SWCNTs have little influence on the total specific capacity. The Li storage mechanism in the NbS2-based nanosheets is also investigated. Until now, several works have reported the insertion storage mechanism of Li into NbS2 electrodes,31,45 which, however, cannot account for the high specific capacities of NbS2-based nanosheets in this work. Therefore, ex situ HRTEM analysis was carried out on the NbS2 nanosheet anodes during the third-cycle discharge−charge process. After the third full discharge to 0.005 V, cubic-phased Li2S nanodots can be observed (Figure 4a). In addition, the lattice fringes of Nb and NbS can also be recognized in Figure 4b, indicating the coexistence of the conversion reactions from NbS2 to NbS and from NbS to Nb. After charging to 3 V, the lattice fringes of Nb and NbS can still be found in the ex situ HRTEM image (Figure 4c), which indicates that the Li storage via the conversion reaction is not fully reversible, possibly due to the irreversible

reaction and the formation of SEI layer (Figure 4c). These observations confirm the conversion reaction mechanism of Li storage into NbS2 nanosheets obtained in this work, which is schematically illustrated in Figure 4d. To investigate the good high rate performance of Fe0.3Nb0.7S1.6Se0.4 nanosheets, the redox pseudocapacitancelike contribution was analyzed. The CV curves of the electrode at different scan rates from 0.1 to 5 mV s−1 are plotted in Figure 4e. Obviously, the shape of the CV curves is well preserved at increased scan rates. The current (i) fits a power-law relationship with the scan rate (v): i = avb (a and b both are constants),46,47 which can be used to qualitatively analyze the degree of capacitive effect. The value of b is determined from the slope of log(i) versus log(v) (Figure 4f). For a high surface capacitance dominated process, the value of b is close to 1. The b values are 0.83 for Fe0.3Nb0.7S1.6Se0.4 nanosheets and 0.70 for NbS1.6Se0.4 nanosheets, suggesting large capacitive storage contributions.2 In the CV plots (Figure 4g), the current is 10603

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Figure 5. (a) Initial five CV curves of Fe0.3Nb0.7S1.6Se0.4 nanosheets for SIB. (b) Initial five discharge−charge curves of Fe0.3Nb0.7S1.6Se0.4 nanosheets for SIB. (c) Rate capability of Fe0.3Nb0.7S1.6Se0.4 nanosheets for SIB. (d) Cyclic stability of Fe0.3Nb0.7S1.6Se0.4 nanosheets for SIB at 1 A g−1. Inset: EIS of Fe0.3Nb0.7S1.6Se0.4 nanosheets.

separated based the equation i = k1v + k2v1/2, where the k1 represents capacitive process and the k2 means diffusion process.46,48−50 As a result, the contribution of capacitive process for Fe0.3Nb0.7S1.6Se0.4 nanosheets is ∼81.2% at 1 mV s−1, which is higher than that for NbS1.6Se0.4 nanosheets (∼54.4%, inset of Figure 4g). This is consistent with the better rate capability of Fe0.3Nb0.7S1.6Se0.4 nanosheets. Figure 4h summarizes the percentage of the capacitive contribution for NbS1.6Se0.4 and Fe0.3Nb0.7S1.6Se0.4 nanosheets. Such a high capacitive contribution for Fe0.3Nb0.7S1.6Se0.4 nanosheets, especially at high scan rates, is related to their high surface area, good electrical conductivity, and abundant active sites, which is important for achieving excellent rate performance. We also investigated the Na storage property of Fe0.3Nb0.7S1.6Se0.4 nanosheets. The initial five CV curves at 0.1 mV s−1 of Fe0.3Nb0.7S1.6Se0.4 nanosheets in the potential range of 0.01−3 V are shown in Figure 5a. In the first cathodic scan, a prominent peak is observed at around 0.95 V. The disappearance of the peak in the subsequent cycles can be assigned to the irreversible reactions and the formation of the SEI layer. After the second cycle, a series of cathodic peaks below 1.94 V can be attributed to the sodiation process. According to the above conversion reaction during Li storage, the cathodic peaks around 1.92 and 1.49 V and the anodic peaks around 1.60 and 2.18 V can be assigned to the Fe0.3Nb0.7S1.6Se0.4/Na+ reaction. For the first discharge−charge process, the specific capacities are 844 and 397 mAh g−1, respectively, with a CE of 47% (Figure 5b). The subsequent

discharge−charge curves (Figure 5b) repeated quite well, which suggests a reversible sodiation process. As shown in Figure 5c, the Fe0.3Nb0.7S1.6Se0.4 anode presents a good rate capability with a fifth-cycle specific capacity of 136 mAh g−1 at 5 A g−1. The specific capacity can well recover to 330 mAh g−1 when the current density returns to 0.1 A g−1. The discharge−charge cycling of Fe0.3Nb0.7S1.6Se0.4 at 1 A g−1 is illustrated in Figure 5d. The Fe0.3Nb0.7S1.6Se0.4 nanosheets deliver a specific capacity of 260 mAh g−1 during the 750th cycle. The Nyquist plots of Fe0.3Nb0.7S1.6Se0.4-based Na half-cell is measured (Figure 5d inset). The diameter of semicircle after 750 galvanostatic discharge−charge cycles decreases, which indicates decrease of the charge transfer resistance.

CONCLUSION In conclusion, the metal and Se codoped NbS2 nanosheets have been successfully synthesized by a facile oil-phase synthetic process. By tuning the amount of metal salts and Se added into the reaction, the morphology of the obtained nanosheets can be adjusted. The Fe0.3Nb0.7S1.6Se0.4 nanosheets show lateral sizes of 1−2 μm and a thickness of approximately 5 nm, which deliver optimized Li/Na storage properties. For LIBs, it presents fifthcycle specific capacities as high as 808.5 mAh g−1 at 0.1 A g−1 and 461.3 mAh g−1 at 10 A g−1. A specific capacity of 444 mAh g−1 at 5 A g−1 during the 3000th cycle was achieved. In addition, for Na storage, the Fe0.3Nb0.7S1.6Se0.4 nanosheets depict a fifth-cycle specific capacity of 136 mAh g−1 at 5 A g−1 and a specific capacity of 260 mAh g−1 at 1 A g−1 during the 10604

DOI: 10.1021/acsnano.7b06133 ACS Nano 2017, 11, 10599−10607

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ASSOCIATED CONTENT

750th cycle. Post-treatment HRTEM analysis confirms the conversion Li storage mechanism for NbS2. The high rate capability of Fe0.3Nb0.7S1.6Se0.4 nanosheets is further explained via quantitative capacitive analysis.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06133. Characterizations of XRD, SEM, TEM, HRTEM, SAED, XPS, and BET for NbS2-based nanosheets; electrochemical measurements including CV curves and rate capabilities of NbS2-based nanosheets (PDF)

EXPERIMENTAL SECTION Chemicals. Niobium(V) chloride (NbCl5, Aldrich, 99%), carbon disulfide (CS2, Aldrich, 99.9%), iron(III) chloride (FeCl3, Acros Organics, 98%), cobalt(II) acetate tetrahydrate (Co(Ac)2, Aldrich, 98%), nickel(II) acetate tetrahydrate (Ni(Ac)2, Aldrich, 98%), selenium powder (Se, Alfa Aesar, 99%), oleylamine (OA, Aldrich, 98%), absolute ethanol, and hexane were of analytical grade. Synthesis of NbS2 Nanosheets and Se/Metal-Doped NbS2 Nanosheets. Typically, 6 g of OA in a three-neck flask was heated to 130 °C for 30 min under argon atmosphere to remove dissolved water and oxygen. Afterward, 1 mmol of NbCl5 was added into OA to form an optically transparent solution. Then the solution was quickly heated to 300 °C, and 3.5 mL of CS2 was slowly injected into the solution at 300 °C. After a 3 h reaction, the resulting black precipitate was centrifuged, washed with hexane and ethanol, and then dried in a vacuum oven at 60 °C. The product was finally obtained after annealing at 400 °C for 3 h in a tube furnace under argon atmosphere. Compared to Se-doped NbS2 nanosheets, Se powder (0.1 to 0.4 mmol) was dissolved in CS2 to add into the reaction. For metal or Se/ metal codoped NbS2 nanosheets, a certain amount of metal salts was added into OA at room temperature, and the total amount of NbCl5 and additional metal salt was 1 mmol. Characterization. XRD patterns of the products were recorded on a Bruker D8 diffractometer using Cu Kα radiation source (λ = 1.5406 Å). TEM, HRTEM, and SAED analyses were performed on a JEOL 2100F microscope operated at 200 kV. Field emission scanning electron microscopy (JSM-7600F) was used to analyze the morphology and microstructure of the nanosheets. XPS (Kratos AXIS Ultra photoelectron spectrometer using Al KR radiation (1486.71 eV)) was performed to analyze the chemical states of elements. BET plots of the N2 adsorption−desorption isotherm were used to compare the specific surface area of the samples. Electrochemical Tests. A total of 70 wt % active material, 10 wt % SWCNTs, 10 wt % super P carbon black, and 10 wt % polyvinylidene fluoride binder was homogeneously mixed in 1-methyl-2-pyrrolidinone (NMP) solvent. The slurry was then coated onto the copper foils and dried in a vacuum oven at 110 °C for 12 h to remove the NMP. The half cells were assembled in an argon-filled glovebox (H2O ≤ 0.1 ppm, O2 ≤ 0.1 ppm) with the coated copper foil as working electrode. For LIBs, electrochemical measurements were carried out via the CR2032 coin-type half cells using Li metal (Na metal, for SIBs) as the counter/ reference electrode, Whatman glass microfiber filter GF/A membrane (GF/D membrane, for SIBs) as the separator, and a solution of 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume) (1 M NaClO4 in EC/DEC = 1:1 with 5 vol % fluoroethylene carbonate, for SIBs) as the electrolyte. The galvanostatic discharge− charge tests were carried out in a NEWARE battery tester at a potential window of 0.005−3 V (0.01−3 V, for SIBs). CV was performed using a Solartron analytical 1470E instrument under various scan rates (0.1−5 mV s−1 for LIBs and 0.1−3 mV s−1 for SIBs) at the same potential window as the discharge−charge tests. Electrochemical impedance spectra (frequency from 0.01 to 105 Hz with ac voltage amplitude 10 mV) were obtained with Gamry instruments. Ex Situ HRTEM for Pure NbS2 Nanosheets. The flowerlike NbS2 was used as the active material for ex situ HRTEM investigation. After the third-cycle discharge to 0.005 V, the batteries were immediately disassembled in the glovebox, followed by being immersed in EC to remove the electrolyte. Afterward, the active materials were dispersed in hexane by ultrasonication for the HRTEM test. Similar process was done for the NbS2 battery charged to 3 V during the third cycle.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yun Zong: 0000-0001-9934-0889 Xin Wang: 0000-0003-4099-4268 Junwu Zhu: 0000-0002-7518-9683 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge the Natural Science Foundation of China (Nos. 51472122 and 51772152), PAPD of Jiangsu, Singapore, MOE AcRF Tier 1 under Grant Nos. RG113/15 and 2016-T1-002-065, and Singapore EMA project EIRP 12/NRF2015EWT-EIRP002-008. The authors also acknowledge the Facility for Analysis, Characterization, Testing and Simulation (FACTS), Nanyang Technological University, Singapore, for use of their TEM, SEM, and XRD facilities. J.Z. gratefully acknowledges the help of Dr. Yubo Luo and Dr. Qinghua Liang and the financial support from China Scholarship Council (CSC). REFERENCES (1) 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. (2) Hu, Z.; Zhu, Z.; Cheng, F.; Zhang, K.; Wang, J.; Chen, C.; Chen, J. Pyrite FeS2 for High-rate and Long-life Rechargeable Sodium Batteries. Energy Environ. Sci. 2015, 8, 1309−1316. (3) Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S. Sodium-ion Batteries. Adv. Funct. Mater. 2013, 23, 947−958. (4) Choi, J. W.; Aurbach, D. Promise and Reality of Post-lithium-ion Batteries with High Energy Densities. Nat. Rev. Mater. 2016, 1, 16013. (5) Qing, R.-P.; Shi, J.-L.; Xiao, D.-D.; Zhang, X.-D.; Yin, Y.-X.; Zhai, Y.-B.; Gu, L.; Guo, Y.-G. Enhancing the Kinetics of Li-rich Cathode Materials through the Pinning Effects of Gradient Surface Na+ Doping. Adv. Energy Mater. 2016, 6, 1501914. (6) Zheng, Y.; Zhou, T.; Zhang, C.; Mao, J.; Liu, H.; Guo, Z. Boosted Charge Transfer in SnS/SnO2 Heterostructures: Toward High Rate Capability for Sodium-ion Batteries. Angew. Chem., Int. Ed. 2016, 55, 3408−3413. (7) Zou, F.; Hu, X.; Li, Z.; Qie, L.; Hu, C.; Zeng, R.; Jiang, Y.; Huang, Y. MOF-derived Porous ZnO/ZnFe2O4/C Octahedra with Hollow Interiors for High-rate Lithium-ion Batteries. Adv. Mater. 2014, 26, 6622−6628. (8) Larcher, D.; Tarascon, J. M. Towards Greener and More Sustainable Batteries for Electrical Energy Storage. Nat. Chem. 2015, 7, 19−29. (9) Lee, J.; Urban, A.; Li, X.; Su, D.; Hautier, G.; Ceder, G. Unlocking the Potential of Cation-disordered Oxides for Rechargeable Lithium Batteries. Science 2014, 343, 519−522. 10605

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DOI: 10.1021/acsnano.7b06133 ACS Nano 2017, 11, 10599−10607