CoSe2-Decorated NbSe2 Nanosheets Fabricated via Cation

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CoSe2-Decorated NbSe2 Nanosheets Fabricated via Cation Exchange for Li Storage Jianli Zhang, Chengfeng Du, Jin Zhao, Hao Ren, Qinghua Liang, Yun Zheng, Srinivasan Madhavi, Xin Wang, Junwu Zhu, and Qingyu Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15457 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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CoSe2-Decorated NbSe2 Nanosheets Fabricated via Cation Exchange for Li Storage Jianli Zhanga,b, Chengfeng Dub, Jin Zhaob, Hao Renb, Qinghua Liangb, Yun Zhengc, Srinivasan Madhavib, Xin Wanga, Junwu Zhua,* and Qingyu Yanb* aKey

Laboratory for Soft Chemistry and Functional Materials, Ministry of Education, Nanjing

University of Science and Technology, Nanjing 210094, China bSchool

of Materials Science and Engineering, Nanyang Technological University, 639798,

Singapore cInstitute

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 *

Corresponding authors:

Junwu Zhu, E-mail: [email protected] Qingyu Yan, E-mail: [email protected] Abstract: Though 2D transition metal dichalcogenides have attracted a lot attention in energy storage applications, the applications of NbSe2 for Li storage are still limited by the unsatisfactory theoretical capacity and uncontrollable synthetic approaches. Herein, a controllable oil-phase synthetic route for preparation of NbSe2 nanoflowers consisted of nanosheets with a thickness of ~ 10 nm is presented. Significantly, a part of NbSe2 can be further replaced by orthorhombic CoSe2 nanoparticles via a post cation exchange process, and the predominantly 2D nanosheet-like 1 ACS Paragon Plus Environment

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morphology can be well maintained, resulting the formation of CoSe2 decorated NbSe2 (denoted as CDN) nanosheets. More interestingly, the CDN nanosheets exhibit excellent lithium-ion battery performance. For example, it achieves a highly reversible capacity of 280 mAh g−1 at 10 A g−1 and long cyclic stability with specific capacity of 364.7 mAh g−1 at 5 A g−1 after 1500 cycles, which are significantly higher than that of reported pure NbSe2. Key words: NbSe2 nanosheets, oil-phase, CoSe2, cation exchange, lithium-ion battery With ever-growing demands for portable electronic devices and electronic vehicles (EVs), advanced energy storage technologies such as rechargeable lithium ion batteries (LIBs) have been extensively investigated.1-2 To meet the demands, exploiting suitable new electrode materials with high-energy density, good rate capability, and long lifespan are important.3-4 2D layered materials, e.g., transition metal dichalcogenides (TMDs),5 layered double hydroxides (LDHs),6 layered metal oxides,7 and metallic metal carbides (Mxenes),8 are regarded as potential anode materials for LIBs due to their high aspect ratio, wide interlayer spacing, and natural electronic properties. Among them, niobium based materials such as NbS2 have been demonstrated as promising alternative anode materials for rechargeable batteries.9 As an analogue, niobium diselenide (NbSe2) with the proposed reaction: 4Li+ + NbSe2 + 4e− ↔ 2Li2Se + Nb, holds a theoretical capacity of 427 mAh g−1, can be a promising host materials for Li ions. However, to the best of our knowledge, CVD and cast followed by exfoliation are the main approaches to obtain NbSe2 nanosheets,10-13 which are constrained by low-production-yield and uncontrollability. Cobalt diselenide (CoSe2) with higher theoretical capacity for Li storage has attracted efforts towards anode materials for LIBs. However, the cubic and orthorhombic CoSe2 are prone to form nanoparticles14 and nanobelts15-16 during the preparation, which suffer from the aggregation and 2 ACS Paragon Plus Environment

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nonadjustable specific surface area, leading inferior rate capabilities and stabilities for LIBs. A novel electrode material designed to combine NbSe2 and CoSe2 is proposed, which possess reasonably higher theoretical capacity for Li storage than that of NbSe2. In addition, the maintained 2D layered structure can provide planar diffusion channels for storing charge-carrying ions in the interlayers. In this work, we demonstrate a facile oil-phase synthetic approach to synthesize the 2D NbSe2 and CoSe2 nanoparticles decorated NbSe2 (CDN) nanosheets (the experimental details can be found in Supporting Information). For NbSe2 nanosheets, the lateral size is 500−800 nm and thickness is around 10 nm. In addition, CDN nanosheets (with the precursor molar ratios of CoCl2:NbSe2, denoted as IC/N, IC/N = 0.2:1, 0.3:1, 0.4:1, 0.5:1, and 1:1) were synthesized through a post cation exchange method. For IC/N = 0.4:1, the CDN nanosheets present enhanced Li storage property. It depicts good rate capability with high specific capacities (607 and 280 mAh g−1 at 0.1 and 10 A g−1, respectively) and excellent capacity retention (364.7 mAh g−1 after 1500 cycles at 5 A g−1). The crystalline phase and structural properties of the synthesized NbSe2 and CDN (IC/N = 0.4:1) were first analyzed by means of X-ray diffraction (XRD) (Figure 1a). For NbSe2, the diffraction peaks can be indexed to the rhombohedral structure (JCPDS No. 01-072-1620, space group: R3m). Interestingly, for CDN (IC/N = 0.4:1), the rhombohedral NbSe2 cannot be detected after cation exchange, of which the crystal structure changes to orthorhombic of CoSe2 (JCPDS No. 00-0530449, space group: Pnnm). The obvious wide peak-pack around 25° probably can be ascribed to the amorphous NbSe2. For different amount of IC/N, the XRD patterns show negligible variation (Supporting Information Figure S1), which infers that the rhombohedral NbSe2 was destroyed as soon as introduced Co precursor. The morphology of NbSe2 can be progressively adjusted by changing the solvent volume ratios of oleylamine (OA) and 1-octadecene (ODE) (denoted as 3 ACS Paragon Plus Environment

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IOA/ODE) from 1:2 to 4:1 (Supporting Information Figure S2). With less OA (IOA/ODE = 1:2) in the oil bath, transition metal ions are prone to connect together, forming irregular nanosheets. Nevertheless, too much OA (IOA/ODE = 4:1) leads to overprotection for Nb ions, which incurs NbSe2 to grow larger nanosheets, forming spherical structure (1−2 μm in diameters, constructed by nanosheets with lateral sizes of 500−800 nm). For IOA/ODE = 1:1, the obtained nanoflowers (~ 1.2 μm in diameters) are composed of nanosheets with average lateral size of 400 nm. The cation exchange procedure is schematically illustrated in Figure 1b. During the cation exchange step, the long-range order structure of NbSe2 might be destroyed, and Co ions combine with Se ions to form CoSe2 crystals, sticking on the nanosheets. The morphologies of the cation-exchanged products are displayed in Figure 1c-d and Figure S3 in Supporting Information. It is clearly showed that the predominantly 2D nanosheet-like morphology can be well maintained after cation exchange. In Figure 1d and its inset, it can be observed that some small particles are attached on the surface of CDN (IC/N = 0.4:1). As showed in Supporting Information Figure S3, when IC/N changes from 0.2:1 to 1:1, the surface of the nanosheets becomes rough. In addition, the corresponding elemental mapping showed in Supporting Information Figure S4 reveals the homogeneous distribution of Nb, Se, and Co, and the ratio of CoSe2:NbSe2 in CDN (IC/N = 0.4:1) is 0.1:1.

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Figure 1 (a) XRD patterns of NbSe2 and CDN (IC/N = 0.4:1) nanosheets. (b) Schematic illustration of the cation exchange process for CDN nanosheets. (c) SEM image of CDN (IC/N = 0.4:1) nanosheets. (d) Detailed SEM image of (c). Inset: NbSe2 nanosheets without cation exchange. The detailed microstructure of NbSe2 and CDN (IC/N = 0.4:1) nanosheets were further analyzed by transmission electron microscopy (TEM). For NbSe2 nanosheets, a typical low-magnification TEM observation (Supporting Information Figure S5a-b) clearly reveals the nanoflower (~ 1.4 μm in diameter) is composed of nanosheets with the average lateral size of ~ 500 nm, agreeable with the above SEM observation. In addition, the observed diffraction rings of the typical selected area electron diffraction (SAED) pattern (inset in Supporting Information Figure S5a) can be indexed to the (012), (015), and (113) planes, indicating the polycrystalline nature of NbSe2 nanosheets. 5 ACS Paragon Plus Environment

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The high-resolution TEM (HRTEM) image indicates that the observed nanosheet is composed of ~ 16 layers with a thickness of ~ 10 nm (Supporting Information Figure S5c). For the particular as-prepared NbSe2 nanosheets showed in Figure S6 inset (Supporting Information), the thickness were about 40 and 70 nm, as determined by atomic force microscopy (AFM) (Supporting Information Figure S6). From the layered structure showed in above TEM observation, the samples in AFM image might be stacked by 4−7 pieces of nanosheets. The interplanar spacing of 0.623, 0.295, and 0.286 nm showed in Supporting Information Figure S5c-d can be assigned to the (003), (101), and (012) planes of rhombohedral NbSe2 nanosheets, respectively. For CDN (IC/N = 0.4:1) nanosheets, the low-magnification TEM images display a similar morphology as NbSe2 (Figure 2a-b), but is decorated by some nanoparticles. The HRTEM image of the nanoparticle on CDN (IC/N = 0.4:1) nanosheets (Figure 2c) displays two interplanar spacing of 0.306 and 0.257 nm, corresponding to the (011) and (120) crystallographic planes of orthorhombic CoSe2. Moreover, the lattice fringes of 0.259, 0.293, 0.205, and 0.375, ascribing from the (120), (101), (121), and (110) of CoSe2, respectively, can also be found in the HRTEM images (Figure 2d-e). In addition, the observed bright rings in the SAED pattern (Figure 2f) can be indexed to orthorhombic CoSe2.

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Figure 2 (a) and (b) Low-magnification TEM image of CDN (IC/N = 0.4:1) nanosheets. (c−e) HRTEM of decorated CoSe2 nanoparticles. (f) SAED pattern of CDN (IC/N = 0.4:1) nanosheets. The chemical compositions and surface valance states of the obtained NbSe2 and CDN (IC/N = 0.4:1) nanosheets were further examined by X-ray photoelectron spectroscopy (XPS) analysis, see Figure S7 (Supporting Information) and Figure 3. For NbSe2, the high-resolution Nb 3d XPS spectra clearly present two splitting 3d5/2 and 3d3/2 peaks around 207.0 and 209.8 eV (Supporting Information Figure S7a), which suggest that elemental Nb in the composite exists as Nb4+ state.17 Besides, the high-resolution Se 3d XPS spectra were fitted as well (Supporting Information Figure S7b). Two peaks at 54.8 and 60.0 eV corresponding to the Se2- specie and surface oxidation of Se, respectively.9, 18 For CDN (IC/N = 0.4:1), with the cation exchange process, six splitting peaks can be observed in the fitted Nb 3d spectrum (Figure 3a). The peaks associated with Nb5+ 3d5/2 and 7 ACS Paragon Plus Environment

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3d3/2 levels are located at 207.6 and 210.3 eV, respectively.9, 19 The peaks corresponding to Nb4+ 3d5/2 and 3d3/2 can be seen at 204.2 and 207.1 eV, respectively.19 The peaks assigned to Nb(4-δ)+ 3d5/2 and 3d3/2 are at 203.7 and 206.4 eV, respectively.19 It is noted that the high-resolution Se 3d XPS spectra for CDN (IC/N = 0.4:1) show similar profile with that in NbSe2 (Figure 3b), indicating the similar core levels of Se species. The Co 2p spectrum (Figure 3c) displays two peaks located at 779.0 and 794.0 eV associated with Co 2p3/2 and 2p1/2.20-21 These results further confirm the successful cation exchange process to form CoSe2.

Figure 3 High-resolution XPS spectra for CDN (IC/N = 0.4:1) nanosheets. (a) Nb 3d, (b) Se 3d and (c) Co 2p. The electrochemical properties of CDN nanosheets were evaluated in a half-cell configuration versus Li metal. Supporting Information Figure S8 and Figure 4a presents typical cyclic voltammogram (CV) curves of NbSe2 and CDN (IC/N = 0.4:1) nanosheets at a scan rate of 0.1 mV s−1 from 0.005 to 3.0 V. For NbSe2, the first cathodic scan shows three apparent reduction peaks at 1.64, 1.48 and 0.17 V along with a broad peak at 0.67 V. According to the Li storage behaviour for other 2D anode materials, the prominent peaks located at 1.64 and 1.48 V which significantly decay in the following cycles should be ascribed to the insertion of Li ions into NbSe2 interlayer forming LixNbSe2 and further conversion into Nb and Li2Se.22-24 The peaks located at 0.67 and 0.17 V are related to the irreversible reactions and formation of stable solid-electrolyte interphase 8 ACS Paragon Plus Environment

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(SEI) layer.25 In the subsequent anodic scan, two anodic peaks at 1.87 and 2.23V possibly correspond to the oxidation of metallic Nb to Nb4+ and the decomposition of Li2Se to Se.9, 22 For CDN (IC/N = 0.4:1), with the decoration of CoSe2 nanoparticles, some reduction peaks below 2.0 V are noticed during the first cathodic scan. The attenuation of these peaks in the subsequent scans reveals more complicated irreversible reactions as well as the formation of SEI layer. The anodic peak at 1.85 V in the following anodic scan should be the overlap of the oxidations of metallic Nb to Nb4+ and Co to Co4+, while the peak around 2.23 V can be attributed to the oxidation from Li2Se to Se. The initial four discharge-charge curves of CDN (IC/N = 0.4:1) nanosheets at 0.1 A g−1 can be seen in Figure 4b. During the first cycle, the discharge and charge capacities are 1187 and 665 mAh g−1, respectively, showing a Coulombic efficiency (CE) of 56%. Figure 4c presents rate capabilities of the NbSe2 and CDN nanosheets for Li storage. Among the CDN nanosheets, CDN (IC/N = 0.4:1) presents the optimized rate capability. Specifically, it shows 5th-cycle specific capacities of 607, 466, 345, and 280 mAh g−1 at 0.1, 1.0, 5.0, and 10 A g−1, respectively. It is considerably better than the rate capability of pure cobalt selenide that is synthesized via this method (Supporting Information Figure S9). Electrochemical impedance spectroscopy (EIS) was carried out to further evaluate the electrochemical reaction kinetics of the NbSe2 and CDN (IC/N = 0.4:1) nanosheets. The Nyquist plots contain a depressed semicircle corresponds to the charge-transfer kinetics-controlled section in high frequency region and a liner Warburg tail in low frequency region (Figure 4d).26-27 The smaller diameter of the semicircle indicates the lower resistance for CDN (IC/N = 0.4:1) nanosheets. Besides, under a current density of 5.0 A g−1, the CDN (IC/N = 0.4:1) nanosheets electrodes for LIBs also achieve good stability that remain a specific capacity of 364.7 mAh g−1 after 1500 cycles (Figure 4e), which is reasonably higher than that of NbSe2. The increased capacity during the initial hundreds cycles for CDN (IC/N 9 ACS Paragon Plus Environment

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= 0.4:1) nanosheets should be derived from the gradually expanded nanosheets, which allows flexible insertion of Li+. The Li storage properties in this work are significantly higher than that of reported pure NbSe228 and CoSe2 composites29-30 (Supporting Information Table S1). The good rate capability for CDN (IC/N = 0.4:1) nanosheets is further confirmed by analyzing the CV curves from different scan rates. The resultant contribution of capacitive process of CDN (IC/N = 0.4:1) at 1.2 mV s−1 reaches 49.9% (Figure 4f-g). Such a high capacitive contribution also explains the fast Li+ diffusion kinetics for CDN (IC/N = 0.4:1) nanosheets.

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Figure 4 Electrochemical properties for LIBs. (a) The initial four CV curves of CDN (IC/N = 0.4:1) nanosheets at 0.1 mV s−1. (b) The initial four discharge-charge curves of CDN (IC/N = 0.4:1) nanosheets at 0.1 A g−1 (c) Rate capabilities of NbSe2 and CDN nanosheets. (d) EIS spectrum of NbSe2 and CDN (IC/N = 0.4:1) nanosheets. (e) Cyclic stabilities of CDN (IC/N = 0.4:1) and NbSe2 nanosheets at 5 A g−1. (f) Capacitive contribution of CDN (IC/N = 0.4:1) nanosheets at 1 mV s−1. (g) For CDN (IC/N = 0.4:1) nanosheets, capacitive contributions at different scan rates. In summary, we have presented a facile oil-phase route to synthesize 2D layered NbSe2 and CDN nanosheets. The obtained samples possess nanosheets of about 400 nm in lateral size, which are prone to form nanoflowers (~ 1.4 μm in diameter). After a post cation exchange process, the CoSe2decorated NbSe2 maintains the nanosheet-like morphology with more active sites can be obtained. By changing the ratio of added Co, the density of CoSe2 nanoparticles decorated on the surface of NbSe2 nanosheets could be tuned. The optimized CDN (IC/N = 0.4:1) nanosheets can provide specific capacities of 607 and 280 mAh g−1 at 0.1 and 10 A g−1, respectively. Meanwhile, under a current density of 5.0 A g−1, the specific capacity remains 364.7 mAh g−1 after 1500 cycles. Supporting Information The supporting information is available free of charge on the ACS Publication website. Full experiment details; XRD, SEM, EDX, TEM, AFM, and XPS characterizations of the obtained samples; Li storage properties (CV curves and rate capabilities) for obtained samples. Acknowledgements This work was financially supported by Natural Science Foundation of China (Nos. 51772152 and 51472122), PAPD of Jiangsu, and Singapore MOE AcRF Tier 1 under grant Nos. RG113/15 and 2016-T1-002-065, and Singapore EMA project EIRP 12/NRF2015EWT-EIRP002-008, National 12 ACS Paragon Plus Environment

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Research Foundation of Singapore (NRF) Investigatorship award number. NRF2016NRFNRFI001-22. The characterizations of TEM, SEM, and XRD was investigated in the Facility for Analysis, Characterization, Testing and Simulation (FACTS), Nanyang Technological University, Singapore. Jianli Zhang gratefully acknowledges the financial support from China Scholarship Council (CSC). Reference (1) Sendek, A. D.; Yang, Q.; Cubuk, E. D.; Duerloo, K.-A. N.; Cui, Y.; Reed, E. J. Holistic Computational Structure Screening of more than 12 000 Candidates for Solid Lithium-Ion Conductor Materials. Energy Environ. Sci. 2017, 10, 306-320, DOI: 10.1039/c6ee02697d. (2) Choi, J. W.; Aurbach, D. Promise and Reality of Post-Lithium-Ion Batteries with High Energy Densities. Nat. Rev. Mater. 2016, 1, 16013, DOI: 10.1038/natrevmats.2016.13. (3) Haro, M.; Singh, V.; Steinhauer, S.; Toulkeridou, E.; Grammatikopoulos, P.; Sowwan, M. Nanoscale Heterogeneity of Multilayered Si Anodes with Embedded Nanoparticle Scaffolds for Li-Ion Batteries. Adv. Sci. 2017, 4, 1700180, DOI: 10.1002/advs.201700180. (4) Zeng, L.; Kawahito, K.; Ikeda, S.; Ichikawa, T.; Miyaoka, H.; Kojima, Y. Metal Hydride-Based Materials towards High Performance Negative Electrodes for All-Solid-State Lithium-Ion Batteries. Chem. Commun. 2015, 51, 9773-9776, DOI: 10.1039/c5cc02614h. (5) Tan, C.; Cao, X.; Wu, X. J.; He, Q.; Yang, J.; Zhang, X.; Chen, J.; Zhao, W.; Han, S.; Nam, G. H.; Sindoro, M.; Zhang, H. Recent Advances in Ultrathin Two-Dimensional Nanomaterials. Chem. Rev. 2017, 117, 6225-6331, DOI: 10.1021/acs.chemrev.6b00558. (6) Zhang, S.; Yao, F.; Yang, L.; Zhang, F.; Xu, S. Sulfur-Doped Mesoporous Carbon from Surfactant-Intercalated Layered Double Hydroxide Precursor as High-Performance Anode

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(28) Hitz, E.; Wan, J.; Patel, A.; Xu, Y.; Meshi, L.; Dai, J.; Chen, Y.; Lu, A.; Davydov, A. V.; Hu, L. Electrochemical Intercalation of Lithium Ions into NbSe2 Nanosheets. ACS Appl. Mater. Interfaces 2016, 8, 11390-11395, DOI: 10.1021/acsami.5b11583. (29) Li, Z.; Xue, H.; Wang, J.; Tang, Y.; Lee, C.-S.; Yang, S. Reduced Graphene Oxide/MarcasiteType Cobalt Selenide Nanocrystals as an Anode for Lithium-Ion Batteries with Excellent Cyclic Performance. ChemElectroChem 2015, 2, 1682-1686, DOI: 10.1002/celc.201500179. (30) Lu, W. Z.; Xue, M. Z.; Chen, X. L.; Chen, C. CoSe2 Nanoparticles as Anode for Lithium Ion Battery. Int. J. Electrochem. Sci. 2017, 12, 1118-1129, DOI: 10.20964/2017.02.32.

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CoSe2-decorated NbSe2 nanosheets is fabricated via a post cation exchange process, and it was used for Li storage. 38x18mm (300 x 300 DPI)

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