Boosting the stable sodium-ion storage performance by tailoring the 1D TiO2@ReS2 core-shell heterostructures

Journal Pre-proof Boosting the stable sodium-ion storage performance by tailoring the 1D TiO2@ReS2 core-shell heterostructures Xinqian Wang, Biao Chen, Jing Mao, Junwei Sha, Liying Ma, Naiqin Zhao, Fang He PII:

S0013-4686(20)30086-4

DOI:

https://doi.org/10.1016/j.electacta.2020.135695

Reference:

EA 135695

To appear in:

Electrochimica Acta

Received Date: 30 October 2019 Revised Date:

6 January 2020

Accepted Date: 11 January 2020

Please cite this article as: X. Wang, Biao Chen, Jing Mao, J. Sha, L. Ma, N. Zhao, F. He, Boosting the stable sodium-ion storage performance by tailoring the 1D TiO2@ReS2 core-shell heterostructures, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135695. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Credit author statement Biao Chen and Xinqian Wang: Investigation, Formal analysis, Writing Original Draft, Visualization, Writing - Review & Editing. Jing Mao: Methodology, Validation, Visualization, Writing - Review & Editing. Junwei Sha and Liying Ma: Investigation, Formal analysis. E. Naiqin Zhao and Fang He: Conceptualization, Formal analysis, Resources, Writing - Review & Editing, Supervision, Project administration, Funding acquisition.

Boosting the Stable Sodium-Ion Storage performance by Tailoring the 1D TiO2@ReS2 Core-Shell Heterostructures Xinqian Wang, † Biao Chen, † Jing Mao, Junwei Sha, Liying Ma, Naiqin Zhao, * and Fang He* X. Wang, B. Chen, J. Mao, J. Sha, Prof. L. Ma, Prof. N. Zhao, Prof. F. He School of Materials Science and Engineering and Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin, 300350, P.R. China. E-mail: [email protected]; [email protected] Prof. N. Zhao, Prof. F. He Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, Tianjin University, Tianjin 300072, P.R. China. Prof. N. Zhao Collaborative Innovation Centre of Chemical Science and Engineering, Tianjin, 300072, P.R. China. † These authors contributed equally to this work.

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Abstract: ReS2 has been considered as an emerging transition metal dichalcogenides (TMDs) material for sodium-ion batteries (SIBs). However, its electrochemical performance is severely limited by the structural aggregation and damage during deep charge-discharge. Here, a new 1D TiO2@ReS2 core-shell structure is reported for boosting the stable performance as the TiO2 has durable structural stability. The 1D TiO2 nanotubes with rough surface and large surface area are helpful to grow few-layer (≤ 4 layers) ReS2 nanosheets onto their surface. In the obtained 1D TiO2 nanotube@ReS2 nanosheet (1D TiO2-NT@ReS2-NS) core-shell heterostructures, the exposed ultrathin ReS2 nanosheets offer high contact area for rapid Na+ diffusion, whilst the TiO2 nanotubes work as robust backbone for accommodating volume change and strain. Moreover, the chemical interfacial interaction between TiO2 and ReS2 gives rise to favorable synergistic effect, leading to enhanced electrical conductivity, Na+ diffusion kinetics, and structural stability at both electrode and materials levels. These findings can be supported by various characterization technologies such as X-ray photoelectron spectrum and high-resolution transmission electron microscopy. As a result, the 1D TiO2-NT@ReS2-NS electrode displays a desirable long-life span cycling performance of 118 mAh g-1 at 1 A g-1 after 1000 cycles in sodium-ion batteries. This work not only reports a stable SIBs anode material, but also provides fundamental understanding for designing and fabricating electrode materials for alkali metal ion batteries. Keywords: 1D TiO2@ReS2; core-shell heterostructures; synergistic effect; sodium-ion battery

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Introduction The rapid development of electronic devices and electrical vehicles stimulate the demand of rechargeable storage batteries.[1-4] Because the sodium element shows low cost and abundant availability, sodium-ion batteries (SIBs) have been widely treated as one kind of promising alternative energy storage technology to lithium-ion batteries (LIBs).[5-10] However, the anode materials for SIBs always suffer from sluggish diffusion kinetics, large volume change, and weak structural stability due to the larger radius of Na+.[11-13] For example, the commercial anode material graphite for LIBs cannot be directly used in SIBs.[14] Therefore, developing new efficient and low-cost anode materials for SIBs is highly urgent. Transition metal dichalcogenides (TMDs) materials show typical layered structure, which is beneficial to fast alkali metal ions diffusion.[15-17] Moreover, the TMDs have high theoretical capacity based on the conversion reaction between one TMD atom and four alkali metal atoms.[15-17] Therefore, TMDs have been widely studied as anode materials for SIBs. In order to improve the performance of TMDs anode materials, preparing interlayer-expanded and metallic phase TMDs have been adopted to improve the alkali metal ionic diffusion kinetics and electrical conductivity, respectively.[18-20] Inspiringly, ReS2, an emerging TMD material, has extremely weak Van der Waals interaction and 1T’ phase.[21-23] Therefore, it has been considered as one kind of promising TMDs anode materials for alkali metal ion batteries.[24-26] However, the obtained electrochemical performance, especially cycling stability of ReS2 is severely limited by the structural aggregation and damage during deep charge-discharge.[27-30] Therefore, it is highly desirable to develop stable ReS2-based anode material for achieving high performance SIBs. Recently, TiO2 anode materials can present low volume variation and robust structural stability via an intercalation storage mechanism in SIBs.[31-34] Moreover, TiO2 also has many advantages such as low-cost, abundance, easy-preparation, non-toxic, and chemical3

stability.[31-36] Therefore, TiO2 has been extensively explored as backbone material to improve the electrode and material stability of TMDs anode materials.[37, 38] For example, TiO2@MoS2,[39-46] TiO2@MoSe2,[47, 48] TiO2@SnS2[49, 50] and TiO2@VS2,[51, 52] core-shell composites have been reported to show enhanced and stable electrochemical performance when evaluated them as anode materials for alkali metal ion batteries. Moreover, it should be noted that 1D TiO2 nanostructures have significant properties of short path lengths for fast electrical/ionic transport and large surface area.[53-55] Therefore, construction of 1D TiO2@ReS2 core-shell composite is highly expected to obtain a positive synergistic effect, in addition, enhance the electrochemical performance of ReS2. However, TiO2/ReS2 composites, including 1D TiO2@ReS2 core-shell structure have not been reported in alkali metal ion batteries. Herein, we prepared the 1D TiO2 nanotubes decorated with few-layer (≤ 4 layers) ReS2 nanosheets composites through a facile hydrothermal method. Both rough surface and large surface area of 1D TiO2 nanotubes play important roles for ReS2 uniform nucleation and growth, and finally formation of the uniform 1D TiO2 nanotube@ReS2 nanosheet (1D TiO2NT@ReS2-NS) core-shell structure. The few-layer ReS2 nanosheets show large contact area with electrolyte for fast Na+ diffusion kinetics, whilst the TiO2 nanotubes exhibit robust structural stability and good electrical transfer channel. In addition, the ReS2 nanosheets anchored onto the TiO2 nanotubes through strong chemical bonds, which gives rise to a synergistic effect: the robust TiO2 backbone in 1D TiO2-NT@ReS2-NS can accommodate volume change and minimize structural pulverization of ReS2 as well as facilitate electrical transport. As a result, the 1D TiO2-NT@ReS2-NS displays significantly enhanced electrical conductivity, Na+ diffusion kinetics, and structural stability. Therefore, the designed 1D TiO2NT@ReS2-NS core-shell nanocomposite exhibit a stable cycling performance of 261 mAh g-1 at 0.2 A g-1 after 100 cycles, along with an ultralow fading of 0.16% per cycle. Even after 1000 cycles at 1 A g-1, it can maintain 118 mAh g-1. 4

Experimental Section: Materials Titanium tetraisopropoxide (Ti(OiPr)4), polyvinyl pyrrolidone (PVP), ethanol, acetic acid, KMnO4, NH4Cl, oxalic acid, hydroxylamine hydrochloride (HONH3Cl), and ammonia (NH3·H2O, 28%) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Ammonium perrhenate (NH4ReO4), hexadecylamine (HDA, 90%), and thiourea were purchased from Beijing Enochai Technology Co., Ltd. All chemicals were used without further purification. Preparation of 1D TiO2 nanowires and 1D TiO2 nanotubes The 1D TiO2 nanowire (TiO2-NW) was prepared by previous literature.[56] Typically, 450 mg of PVP was dispersed into 6 mL ethanol under stirring for 2 h at room temperature. 1.62 mL Ti(OiPr)4 was added into a mixture containing 3 ml ethanol and 3 ml acetic acid. Then, the above two solutions were mixed and stirred to form a transparent solution. Next, the solution was added to a syringe to prepare nanofibers by electrospinning at an ejected rate of 0.001 mm s-1. Finally, the as-prepared nanofibers were calcined at 500 °C for 2 h in air to form TiO2 crystals. The 1D TiO2-NT was prepared according to the method in previous report.[45] 79 mg KMnO4 and 27 mg NH4Cl were added into 30 ml water under stirring for 2 h. The obtained solution was heated at 200 °C for 24 h in a 50 mL Teflon-lined autoclave. After cooling by water, the product was collected by filtration and washed by water and ethanol to obtain MnO2 nanowires. Then, 150 mg of as-prepared MnO2 nanowire was dispersed in 400 mL ethanol by ultrasonication. 0.8 g of HDA and 2 mL ammonia were added into the suspension under stirring. Next, 1 mL Ti(OiPr)4 was added to the above suspension under stirring. After reaction for 24 h, the solid product was collected by centrifugation and calcined at 450 °C for 2 h. Finally, to remove the MnO2, the calcinated products were added into 200 mL oxalic acid

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(0.5 M) under stirring for 24 h at 70 °C, followed by wishing several times with water and ethanol to obtain 1D TiO2 nanotubes. Preparation of 1D TiO2-NT@ReS2-NS core-shell structures 50 mg of 1D TiO2-NT were dispersed in 30 ml water by ultrasonication for 2 h. Then, 53.7 mg of NH4ReO4, 41.7 mg of HONH3Cl and 68.4 mg of thiourea was added into the above suspension under stirring for 1 h. The obtained solution was heated at 220 °C for 24 h in a Teflon-lined autoclave (50 mL). The hydrothermal products were collected by filtration and then calcined at 700 °C for 2 h in Argon atmosphere to improve the crystalline degree and electrical conductivity. For comparison, the ReS2 sphere and 1D TiO2 nanowire loading ReS2 sphere (1D TiO2NW/ReS2-S) composite were also prepared, respectively, without addition of TiO2 nanotubes and using TiO2 nanowires instead of TiO2 nanotubes under the same condition. Characterization The morphologies of the as-prepared materials and the electrodes after electrochemical test were investigated by scanning electron microscope (SEM, HITACHI S4800) and transmission electron microscope (TEM, JEOL JEM-2100f). The crystal structures of ReS2, 1D TiO2NW/ReS2-S, and 1D TiO2-NT@ReS2-NS were detected by the X-ray diffraction (XRD, Bruker D8 Advanced) and the LabRAM Raman spectroscopy. The chemical states of ReS2 and 1D TiO2-NT@ReS2-NS were performed on X-ray photo-electron spectroscopy (XPS, PHI-5000 Versa Probe). The weight content of the 1D TiO2-NT@ReS2-NS was studied by thermogravimetric analysis (TGA, PerkinElmer) with a heating rate of 10 °C min-1 in the air atmosphere. N2 adsorption-desorption curves were recorded at 77 K based on a MicroMeritics ASAP 2020 physisorption analyser. According to the curves, the specific surface area and pore size-distribution were calculated by Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) method, respectively. Electrochemical measurements 6

The sodium-half cells composed of sodium metal, electrolyte, separator, and anode film were assembled into coin-cells (CR 2032). In order to fabricate anode films, homogeneous slurries were prepared in advance by adding 70 wt% active materials (ReS2, 1D TiO2-NT, 1D TiO2NW/ReS2-S, and 1D TiO2-NT@ReS2-NS), 20 wt% conductive agent (carbon black), and 10 wt% binder (sodium carboxymethyl cellulose, SCMC)) in water. Then the homogeneous slurries were coated onto Cu disks (diameter = 12 mm) and dried at 60 °C under vacuum. The films with a mass loading from 0.7 to 1.0 mg cm-2 were chosen to perform the electrochemical performance. The glass microfiber (Whatman) was cut into disks with a diameter of 19 mm. NaClO4 (1 M) in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 vol%) and 5 wt% fluoroethylene carbonate (FEC) was added as electrolyte. The coin-cells were finally assembled into an argon-filled glovebox with 75 µL electrolyte. CHI660D electrochemical workstation was used to measure the cyclic voltammetry (CV) curves. The electrochemical performance including cycling and rate performance were recorded on a battery testing system (LAND CT 2001A). The electrochemical impedance spectroscopy (EIS) measurements were performed on Gamry interface 1000 electrochemical workstation using a frequency between 100kHz to 70 mHz. Results and Discussions The preparation of ReS2, 1D TiO2-NW/ReS2-S, and 1D TiO2-NT@ReS2-NS core-shell structure is illustrated in Fig. 1. Without addition seed crystal of TiO2 materials, the ReS2 was grown though homogeneous nucleation.[37] In order to reduce the surface energy, the ReS2 nanosheets were reunited into spherical structures with multi-layers (~ 9 layers) (Fig. S1). When adding TiO2 seed crystal in the precursor solution, the ReS2 tends to grow onto the surface of TiO2 seed crystal through heterogeneous nucleation.[37] The TiO2 nanowires and TiO2 nanotubes were prepared by calcining at 500 and 450 °C for 2 h, respectively. Both TiO2 nanowires and TiO2 nanotubes show rough surface (Fig. S2 and S3) and similar hydrophilicity. Although the TiO2 nanowires exhibit rough surface that significantly 7

facilitates the ReS2 nucleation on the surface of TiO2.[37] The small specific surface area of TiO2 nanowire (40.6 m2 g-1 in Fig. S4) cannot provide adequate nucleation sites for ReS2 nanosheets. After hydrothermal process, as shown in Fig. S5, in the obtained 1D TiO2NW/ReS2-S, the ReS2 forms sphere-like structure with fewer layers (~ 7 layers). For TiO2 nanotube seed crystal, it displays a significantly larger specific surface area (129.0 m2 g-1 in Fig. S4) than TiO2 nanowire. Therefore, the TiO2 nanotube can provide significantly more nucleation sites for ReS2 during the hydrothermal process.[37] Therefore, 1D TiO2 nanotubes supporting ReS2 nanosheets core-shell structure was obtained after hydrothermal process. The morphology of 1D TiO2-NT@ReS2-NS core-shell structure has been carefully studied by SEM and TEM images. The SEM images (Fig. 2a and inset) demonstrate that the formed ReS2 nanosheets tightly anchor onto the surface of TiO2 nanotubes. Moreover, the TEM image (Fig. 2b) reveals a 1D hierarchical tubular structure. The high resolution TEM (HRTEM) images (Fig. 2c and 2d) exhibit that the grown ReS2 shows fewer layer (≤ 4 layers) when compared with those in ReS2 spheres and 1D TiO2-NW/ReS2-S. The lattice spacings of 0.61 and 0.27 nm are attributed to the (002) and (-220) planes of ReS2,[27-29] whilst the lattice spacing of 0.32 nm is ascribed to the (110) plane of rutile TiO2.[31-33] The few-layer ReS2 tightly anchors onto the rough surface of TiO2. Moreover, the distributions of ReS2 and TiO2 in 1D TiO2-NT@ReS2-NS can be further studied by the scanning TEM (STEM) image and corresponding elemental mapping images (Fig. 2e). The 1D TiO2-NT@ReS2-NS is composed of Ti, O, Re, and S elements (Fig. S6). The elemental mapping zone for Ti is in good accordance with that for O, whilst the Re mapping zone dovetails well with S mapping zone. This suggests the co-existence of TiO2 and ReS2 in 1D TiO2-NT@ReS2-NS. Moreover, the mapping area for O/Ti is slightly smaller than that for S/Re, indicating the TiO2@ReS2 core-shell structure. The crystal structures of ReS2, 1D TiO2-NW/ReS2-S, and 1D TiO2-NT@ReS2-NS were examined by XRD patterns and Raman spectra. As shown in Fig. 3a, both the XRD patterns 8

of 1D TiO2-NW/ReS2-S and 1D TiO2-NT@ReS2-NS are composed of 1T’ phase ReS2 (JCPDS card No. 89‐0341), anatase TiO2 (JCPDS card No. 21‐1272), and rutile TiO2 (JCPDS card No. 21‐1276). Notably, both the ReS2 and 1D TiO2-NW/ReS2-S show a strong (002) diffraction peak that assigned to the stacking of S-Re-S layers, while the 1D TiO2NT@ReS2-NS exhibits a significantly weak (002) peak. This result reveals that the layer of ReS2 nanosheet in 1D TiO2-NT@ReS2-NS is smaller than those in ReS2 and 1D TiO2NW/ReS2-S,[43-45] which is in good consistent with the above HRTEM results. According to previous literatures,[44, 57] the Raman spectra (Fig. 3b) further confirm that co-existence of ReS2 and TiO2 in 1D TiO2-NW/ReS2-S and 1D TiO2-NT@ReS2-NS. The porous properties of ReS2, 1D TiO2-NW/ReS2-S, and 1D TiO2-NT@ReS2-NS were studied by N2 adsorptiondesorption curves (Fig. S7). The BET surface areas of ReS2, 1D TiO2-NW/ReS2-S, and 1D TiO2-NT@ReS2-NS increase from 6.7 to 48.5 m2 g-1. The high BET surface area facilitates the electrolyte contact with 1D TiO2-NT@ReS2-NS.[58] All the three samples display a large amount of mesopores. The size of pore distribution peak for ReS2 (15.1 nm) is approximately equal to that for 1D TiO2-NW/ReS2-S (14.9 nm). This result is attributed the similar spherical structure of ReS2. The pore distribution peak shifts to lower size for 1D TiO2-NT@ReS2-NS (8.2 nm), suggesting the uniformly distributed structure without aggregation. The weight content of ReS2 in 1D TiO2-NT@ReS2-NS can be calculated by TGA test. As shown in Fig. S8a, during the calcination process in air, the ReS2 is fully oxidized and volatilized. [26-29] The TiO2 has strong chemical stability during the calcination process. [39-44] Therefore, the weight content of ReS2 in 1D TiO2-NW/ReS2-S and 1D TiO2-NT@ReS2-NS is equal to the weight loss after the calcination. The weight contents of ReS2 in 1D TiO2-NT@ReS2-NS and 1D TiO2-NW/ReS2-S are similar and around 34% (Fig. S8). The chemical states of ReS2 and 1D TiO2-NT@ReS2-NS were investigated by XPS spectra. The 1D TiO2-NT@ReS2-NS has four elements of Ti, O, Re, S, while the ReS2 has Re and S

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elements (Fig. S9). The fine spectra of Re can be divided into two peaks of Re 4f5/2 and Re 4f7/2. The Re 4f5/2 and Re 4f7/2 peaks of ReS2 are, respectively, located at 44.8 and 42.4 eV (Fig. 3c). Both the binding energies of Re 4f5/2 and Re 4f7/2 peaks for 1D TiO2-NT@ReS2-NS are 0.3 eV smaller than those for ReS2. Moreover, the S 2p peaks for 1D TiO2-NT@ReS2-NS also shift 0.3 eV to right compared with those for ReS2 (Fig. 3c). This significantly negative energy shift of 0.3 eV indicates a strong interfacial intercalation between TiO2 and ReS2 in 1D TiO2-NT@ReS2-NS.[47, 51, 56] The fine spectrum of O 1s (Fig. 3d) reveals the existence of Ti-O-Re bond, suggesting that the TiO2 and ReS2 contact with a Ti-O-Re chemical bond. This phenomenon has been found in other TiO2@MoS2 structure prepared by hydrothermal method.[59] The obtained ReS2, 1D TiO2-NW/ReS2-S, and 1D TiO2-NT@ReS2-NS composites were worked as anode materials of SIBs to evaluate their electrochemical performance. As show in Fig. S10 (Supporting Information), active materials and carbon conductive agent show uniform distribution in the three electrodes. As shown in Fig. 4a and Fig. S11a-c, the cyclic voltammetry (CV) curves of the three electrodes and 1D TiO2-NT electrode were performed at a scan rate of 0.2 mV s-1. The pure ReS2 curves in Fig. S10a exhibit typical peaks that similar to previous report on ReS2.[26-29] In the first discharge curve, the cathodic peaks at 1.14 and 0.29 V of ReS2 (Fig. S11a) are, respectively, ascribed to the Na+ intercalation and conversion process. After discharged to 0.01 V, the original ReS2 nanosheets transformed into Na2S and Re particles.[26-29] These particles were converted into Re and S particles at the following anodic peak at 1.95 V in the charge curve.[26-29] After the first cycle, two new cathodic peaks at 1.71 and 1.29 V appear in the discharge curves, which are attributed to the Na-S conversion mechanism from S to Na2S.[60] For 1D TiO2-NT (Fig. S11b), it exhibits one discharged peak between 0.31 to 1.18 V and one charged peak between 0.41 to 1.25 V, respectively, suggesting the Na+ insertion and extraction process in TiO2 surface.[31-34] Both typical peaks of ReS2 and 1D TiO2-NT exist in the CV curves of 1D TiO2-NT@ReS2-NS (Fig. 10

4a). These are the intercalation/deintercalation reactions in TiO2 and the conversion reactions in ReS2, indicating that TiO2 and ReS2 contribute the Na+ storage capacity through surface storage and faradic charge storage mechanisms, respectively.[27-29, 31] The 1D TiO2NW/ReS2-S (Fig. S11c) shows similar peaks to those of 1D TiO2-NT@ReS2-NS. The galvanostatic discharge–charge (GDC) profiles were also studied to investigate the electrochemical behaviour and performance. Fig. 4b and Fig. S11d-f show the typical GDC profiles at 0.2 A g-1 in 100 cycles. The corresponding cycling performance is displayed in Fig. 4c. The voltage platforms in the GDC profiles are in consistent with the peaks in CV curves. The initial discharge and charge capacities of 1D TiO2-NT@ReS2-NS are 417 and 307 mAh g-1, repetitively. They correspond to a coulombic efficiency (CE) value of 73.6%, which is between that of 1D TiO2-NT (46.5 %) to that of ReS2 (79.9 %) (Fig. S11d, e). The irreversible loss in the first cycle is ascribed to formation of solid electrolyte interphase (SEI) film.[61, 62] The CE values of 1D TiO2-NT@ReS2-NS rapidly increase to above 95% after the second cycle. The GDC profiles of 1D TiO2-NT@ReS2-NS almost overlap after the first cycle (Fig. 4b). Therefore, the 1D TiO2-NT@ReS2-NS can achieve a reversible capacity of 261 mAh g-1 after 100 cycles, along with an ultralow capacity fading of 0.16% per cycle. This excellent cycling stability is comparable to those of reported ReS2-based anodes.[26-29] Although the ReS2 shows a significantly large initial charge capacity (447 mAh g-1) and the 1D TiO2NW/ReS2-S has same composition, they both can only keep 175 mAh g-1 after 100 cycles (Fig. S11d, f). Therefore, only in the designed core-shell structure, the 1D TiO2-NT backbone with durable structural stability (Fig. S11e) can effectively maintain the electrochemical activity and capacity of ReS2. As a result, the designed 1D TiO2-NT@ReS2-NS core-shell structure achieves a positive synergistic effect, resulting in the highest capacity and the best cycling stability among the four samples.

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The rate performance from 0.1 to 5 A g-1 and long-life span cycling performance at 1 A g-1 were also performed. Fig. 4d and Fig. S12 exhibit the rate performance. Because the ReS2 has large theoretical capacity (446 mAh g-1) and high Na+ diffusion kinetics,[29] the capacities of ReS2 are larger than those of 1D TiO2-NW/ReS2-S and 1D TiO2-NT@ReS2-NS in the beginning process with increasing current densities. However, when the current densities decrease from 5 to 0.1 A g-1, the reversible capacity of ReS2 cannot be recovered to 442 mAh g-1, but it is going down to 173 mAh g-1. This results from its poor structural stability. For 1D TiO2-NT@ReS2-NS, it shows 304, 286, 261, 241, 224, and 195 mAh g-1 at 0.1, 0.2, 0.5, 1, 2, 5 A g-1, respectively (Table S1). They are significantly larger than those of 1D TiO2NW/ReS2-S. More importantly, the 1D TiO2-NT@ReS2-NS still displays a high reversible capacity of 289 mAh g-1 when the current density recovers to 0.1 A g-1, which is significantly larger than those of 1D TiO2-NW/ReS2-S (196 mAh g-1) and even ReS2. The irreversible loss decreases from high current density to low current density, which is ascribed to the durable structural stability of 1D TiO2-NT@ReS2-NS even after repeated cycles at different current densities.[63] Notably, the 1D TiO2-NT@ReS2-NS can even achieve a long-life span cycling performance at 1 A g-1. As shown in Fig. 4e and Fig. S13, after 1000 cycles, the 1D TiO2NT@ReS2-NS can maintain a high capacity of 118 mAh g-1, which is 4.5 and 1.5 times higher than those of ReS2 (26 mAh g-1) and 1D TiO2-NW/ReS2-S (77 mAh g-1), respectively. This is an impressive result when compared with previous literatures based on ReS2 anodes (Table S2). Meanwhile, the 1D TiO2-NT@ReS2-NS also displays the smallest polarization from the GDC profiles (Fig. S13), indicating the largest electron/Na+ transport kinetics and the best electrochemical activity.[64, 65] Therefore, it is believed that the 1D TiO2-NT@ReS2-NS should have excellent structural stability and electron/Na+ transport kinetics. In order to further investigate the electrochemical properties, including electrical conductivity, Na+ diffusion kinetics, and structural stability of 1D TiO2-NT@ReS2-NS, The CV curves at different scanning rates, EIS curves, and morphologies after rate performance 12

were performed. Fig. 5a and Fig. S14a, d display the CV curves from 0.2 to 1 mV s-1 of ReS2, 1D TiO2-NW/ReS2-S, and 1D TiO2-NT@ReS2-NS. The pseudocapacitive property can be studied from the relationship between current density (i) and scanning rate (v) and the following equation (1):[66-68] log(i) = b log(v) + a

(1)

a and b are constants. The b value is between 0.5 and 1, which can represent the pseudocapacitive degree. The larger the b value, the higher the pseudocapacitive degree.[6668] As shown in Fig. 5b and Fig. S14b, e, the b value of peaks 1 and 2 are, respectively, 0.99 and 0.86 for 1D TiO2-NT@ReS2-NS. They are all significantly larger than those for ReS2 and 1D TiO2-NW/ReS2-S. The larger b value of 1D TiO2-NT@ReS2-NS indicates the higher pseudocapacitive contribution and Na+ diffusion kinetics.[64-66] The detailed capacitive contribution can be further quantitatively calculated by equation (2):[66-68] i(V) = k1v + k2v0.5

(2)

k1 and k2 are associated with pseudocapacitive and diffusion processes, respectively. Fig. 5c presents typical CV curves composed of the experimental and calculated results at 1 mV s-1 for 1D TiO2-NT@ReS2-NS. The capacitive contribution equals to the ratio of the calculated capacitive area to the experimental area. As shown in Fig. 5d and Fig. S14c, f, the capacitive contributions of 1D TiO2-NT@ReS2-NS are larger than those of ReS2 and 1D TiO2NW/ReS2-S at all scanning rates. The high capacitive contribution of 1D TiO2-NT@ReS2-NS is responsible for the excellent electrical conductivity and Na+ diffusion kinetics.[66-68] This finding reveals that the 1D TiO2-NT@ReS2-NS presents high electrical conductivity and Na+ diffusion kinetics, which are benefits from the core-shell structure. The electrical conductivities of the three electrodes before and after rate performance test were learned by EIS measurements.[69-72] The obtained Nyquist plots and equivalent circuit are shown in Fig. 5d, f and Fig. S16, respectively. The fitted results are summarized in Tables 13

S3 and S4. Before electrochemical test, as shown in Fig. 5e and Table S3, the charge transfer resistance (Rct) for 1D TiO2-NT@ReS2-NS electrode (539.9 Ω) is slightly smaller than those for ReS2 (613.3 Ω) and 1D TiO2-NW/ReS2-S (660.9 Ω), indicating the enhanced electrical conductivity attributed to the 1D core-shell structure. After the rate performance test, as shown in Fig. 5f and Table S4, all diameters are significantly decreased, suggesting the enhanced electrical conductivity derived from the formation of metallic Re nanoparticles (see in Fig. 6). The Rct for 1D TiO2-NT@ReS2-NS (151.2 Ω) is still significantly smaller than those for ReS2 and 1D TiO2-NW/ReS2-S. This result demonstrates the 1D TiO2-NT@ReS2NS shows better electrical conductivity even after deep cycles.[72] The morphologies of ReS2, 1D TiO2-NW/ReS2-S, and 1D TiO2-NT@ReS2-NS after rate performance test were investigated by SEM and TEM to study their structural stability. As shown in Fig. S15, ReS2 electrode presents some cracks (Fig. S15a), demonstrating the large volume change and strain of ReS2 electrode during repeated cycles. Both 1D TiO2-NW/ReS2S and 1D TiO2-NT@ReS2-NS electrodes can keep a smooth surface without forming cracks (Fig. S15c, e), suggesting that the 1D TiO2 nanostructures can effectively accommodate volume change and strain of ReS2 electrode. Moreover, we also carefully study the morphology changes of active materials at nano- and atomic- scale. The ReS2 spheres happen severely pulverization and they are cracked into abundant particles, including Re and Na2S (Fig. S15b and Fig. 6a, b). For the 1D TiO2-NW/ReS2-S, the TiO2 nanowires maintain their original structure very well, while the ReS2 spheres exhibit pulverization and most of the transformed particles are peeled from the TiO2 nanowires due to the weak interfacial interaction (Fig. S15d and Fig. 6c, d). The 1D TiO2-NT@ReS2-NS with strong interfacial interaction keeps well with its original core-shell structure, in which the transformed particles are still anchored onto the surface of TiO2 nanotubes. (Fig. S15f and Fig. 6e, f) Moreover, the STEM image (Fig. 6g) and corresponding elemental mapping images display that the Ti, O, Re, S, and Na elements distribute uniformly. The distribution areas for Ti and O elements are 14

lightly smaller than those for Re and S elements, further demonstrating that the changed materials maintain a core-shell structure. These findings strongly demonstrate that the 1D TiO2-NT@ReS2-NS core-shell structure with strong interfacial interaction shows excellent structural stability on both electrode and material levels since the TiO2 nanotubes can accommodate volume change and strain of ReS2. According to the above results, it can be concluded that the 1D TiO2-NT@ReS2-NS with strong interfacial interaction achieves a favorable synergistic effect, leading to significantly enhanced electrical conductivity, Na+ diffusion kinetics, and structural stability. The positive synergistic effect is attributed to the design of unique core-shell structure. i) The ultrathin ReS2 nanosheets with high specific surface area effectively facilitate the Na+ diffusion kinetics; ii) The TiO2 nanotubes provide fast electrical transport path; iii) The ultrathin ReS2 nanosheets anchored onto the TiO2 nanotubes backbone with strong interfacial interaction effectively accommodate volume change and strain of ReS2 on both materials and electrode levels. As shown in Fig. 7, both the ReS2 electrode and material show severely cracks after repeated cycles, leading to the SIBs failure. The 1D TiO2-NW/ReS2-S can effectively accommodate volume change and strain on electrode level. However, the TiO2 nanowires cannot prevent the loaded ReS2 spheres from pulverization and peeling off on material level due to weak interfacial interaction. Therefore, the 1D TiO2-NW/ReS2-S electrode also presents fast capacity fading. The 1D TiO2-NT@ReS2-NS structure has core-shell structure and strong interfacial interaction. TiO2 nanotubes greatly accommodate volume change and strain of ReS2 nanosheets on both electrode and material levels. The 1D TiO2-NT@ReS2-NS electrode maintains high electrochemical properties and activities even after deep cycles. As a result, the 1D TiO2 nanotubes supporting ReS2 nanosheets core-shell structure boosts a durably stable electrochemical performance in SIBs when compared to pure ReS2 spheres and 1D TiO2 nanowires loading ReS2 spheres composites.

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Conclusion In summary, this work designed and fabricated 1D TiO2-NT@ReS2-NS core-shell heterostructures via a facile hydrothermal method. This novel core-shell structure shows ultrathin ReS2 nanosheets (≤ 4 layers), a high specific surface area, a lot of mesopores, and chemical interfacial interaction between TiO2 and ReS2. These advantages are beneficial to electrical conductivity, Na+ diffusion kinetics, and structural stability. More importantly, in this core-shell structure, the TiO2 nanotubes greatly accommodate volume change and strain of ReS2 nanosheets on both electrode and material levels. As a result, the 1D TiO2NT@ReS2-NS electrode maintains high electrochemical activities even after deep cycles. Therefore, it achieves a stable cycling performance of 118 mAh g-1 at 1 A g-1 after 1000 cycles and an ultralow fading of 0.16% per cycle at 0.2 A g-1. The above results demonstrate that the rational design of core-shell structure with strong interfacial interaction is an effective strategy to develop advanced anode composites with stable electrochemical performance for SIBs and other alkali metal ion batteries.

Conflict of Interest The authors declare no conflict of interest. Acknowledgements The authors acknowledge the financial support by the National Natural Science Foundation of China (Grant Nos. 51572189, 51972226, 21576202, 51771136, and 51972225).

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Figure 1. Schematic illustration of the preparation of ReS2, 1D TiO2-NW/ReS2-S, and 1D TiO2-NT@ReS2-NS vis hydrothermal method.

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Figure 2. (a) SEM image, (b) TEM image, (c, d) HRTEM images of 1D TiO2-NT@ReS2-NS. (e) STEM image and corresponding elemental mapping images of 1D TiO2-NT@ReS2-NS.

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Figure 3. (a) XRD patterns, (b) Raman spectra of ReS2, 1D TiO2-NW/ReS2-S, and 1D TiO2NT@ReS2-NS. XPS fine spectra of (c) Re 4f and S 2p of ReS2 and 1D TiO2-NT@ReS2-NS and (d) O 1s of 1D TiO2-NT@ReS2-NS.

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Figure 4. Electrochemical performance of SIBs: (a) CV curves of 1D TiO2-NT@ReS2-NS electrode at 0.2 mV s-1. (b) GDC profiles of 1D TiO2-NT@ReS2-NS electrode at 0.2 A g-1. (c) Cycling performance of ReS2, 1D TiO2-NT, 1D TiO2-NW/ReS2-S, and 1D TiO2-NT@ReS2NS electrodes at 0.2 A g-1. (d) Rate performance from 0.1 to 5 A g-1 and (e) long-life span cycling performance at 1 A g-1 of ReS2, 1D TiO2-NW/ReS2-S, and 1D TiO2-NT@ReS2-NS electrodes.

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Figure 5. (a) CV curves at different scan rates, (b) relationship between scan rates and peak current, (c) capacitive contribution pattern (black) at 1.0 mV s-1 and (d) capacitive contribution percentage of 1D TiO2-NT@ReS2-NS electrode. Nyquist plots (e) before and (f) after rate performance test of ReS2, 1D TiO2-NW/ReS2-S, and 1D TiO2-NT@ReS2-NS electrodes.

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Figure 6. TEM and HRTEM images of (a, b) ReS2, (c, d) 1D TiO2-NW/ReS2-S, and (e, f) 1D TiO2-NT@ReS2-NS materials after rate performance test. (g) STEM image and corresponding elemental mapping images of O, Ti, S, Re, and Na.

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Figure 7. Schematic illustration of ReS2, 1D TiO2-NW/ReS2-S, and 1D TiO2-NT@ReS2-NS electrodes before and after repeated cycles.

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Highlights 1. Rational design of 1D TiO2-NT@ReS2-NS core-shell structure through a facile hydrothermal method. 2. 1D TiO2-NT@ReS2-NS exhibited excellent long-life span cycling performance in SIBs. 3. 1D TiO2-NT@ReS2-NS provide fast electrical transport path and facilitate the Na+ diffusion kinetics. 4. The strong interfacial interaction effectively accommodate volume change and strain of ReS2.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.