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sodium storage properties
Journal Pre-proof Filling Few-Layer ReS2 in Hollow Mesoporous Carbon Spheres for Boosted Lithium/ Sodium Storage Properties Xiue Zhang, Chao Shen, Huayu Wu, Yue Han, Xiaoyu Wu, Weiran Ding, Lubin Ni, Guowang Diao, Ming Chen PII:
S2405-8297(19)31054-2
DOI:
https://doi.org/10.1016/j.ensm.2019.11.019
Reference:
ENSM 994
To appear in:
Energy Storage Materials
Received Date: 21 August 2019 Revised Date:
1 November 2019
Accepted Date: 17 November 2019
Please cite this article as: X. Zhang, C. Shen, H. Wu, Y. Han, X. Wu, W. Ding, L. Ni, G. Diao, M. Chen, Filling Few-Layer ReS2 in Hollow Mesoporous Carbon Spheres for Boosted Lithium/Sodium Storage Properties, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.11.019. 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. © 2019 Published by Elsevier B.V.
Filling Few-Layer ReS2 in Hollow Mesoporous Carbon Spheres for Boosted Lithium/Sodium Storage Properties Xiue Zhang, Chao Shen, Huayu Wu, Yue Han, Xiaoyu Wu, Weiran Ding, Lubin Ni, Guowang Diao, Ming Chen* School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P. R. China
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 21773203), the Natural Science Foundation of Jiangsu Province (BK20161329) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions is acknowledged. Notes The authors declare no competing financial interest.
*
Corresponding author at: School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P. R. China. E-mail address: [email protected] (M. Chen)
Filling Few-Layer ReS2 in Hollow Mesoporous Carbon Spheres for Boosted Lithium/Sodium Storage Properties
ABSTRACT A facile and feasible spatial confinement strategy is proposed for the preparation of ultrathin ReS2 nanosheets confined in hollow mesoporous carbon spheres (HMCSs), forming nanosheets filled hollow carbon spheres (NSFHCSs) ReS2@C. Based on the spatially confined growth, the as-synthesized ReS2@C nanocomposites achieve novel filled-structure and well-defined heterogeneous interfaces. In the excellent architecture, the porous carbon framework precisely constrains ultrathin ReS2 nanosheets within its void space, which can generate essential electrical contacts, buffer volume expansion, and prohibit the loss of active materials during the pulverization process. For lithium-ion batteries, the NSFHCSs ReS2@C display high adsorption energy of Li in the heterogenous interface by the first-principle calculation, and achieve superior reversible capacity of 578 mA h g-1 after 1000 cycles at 1 A g-1. Research shows that the capacitive behavior plays a major role in specific capacity contribution of ReS2@C electrode. For sodium-ion batteries, the NSFHCSs ReS2@C demonstrate reversible discharge capacity of 319 mA h g-1 at 0.2 A g-1 after 400 cycles. The enlarged interlayer spacing and few layer structure of ReS2 nanosheets are conducive to enhance the Li+/Na+ mobility and improve the dynamic characteristic of electrode material. Keywords: few-layer ReS2 nanosheets; spatially confined growth; hollow mesoporous carbon nanospheres; lithium-ion batteries; sodium-ion batteries 1
1. Introduction Lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs), which share similar intercalation chemistry despite the different ionic radii (Li+: 0.076 nm; Na+: 0.102 nm), have attracted dramatic attention for the extensive needs of high-efficient energy storage devices [1-5]. However, the development of LIBs/SIBs has been hampered by major challenges, including low capacities and rapid capacity fading [6-8]. Thus, it is necessary to enhance their electrochemical energy storage performance by developing new electrode materials or designing favorable nanostructures [9-13]. Two-dimensional (2D) transition metal dichalcogenides (TMDs), with the large interlayer distance and weak van der Waals forces, are considered to be one of the best candidate materials for alkaline ion batteries [14-16]. ReS2 has a weaker interlayer coupling energy (18 me V, a unit cell) compared to MoS2 (460 me V) [17-20]. The weaker interlayer coupling energy would permit rapid transfer of alkali metal ions within the interlayer. Moreover, the interlayer distance of ReS2 (0.61 nm) is almost twice larger than graphite (0.34 nm). These characteristics enable ReS2 to be ideal anode material for alkali ion batteries [21, 22]. Nevertheless, the practical application of ReS2 to LIBs/SIBs is greatly suppressed due to poor cyclability. One of the key reasons for the poor cycle is the large volume variation during charge/discharge process, leading to mechanical failure and loss of electrical contact at the electrode [23-25]. The other is the poor conductivity of ReS2 [26, 27]. To address these challenges, the synthesis of carbon hybrid materials of ReS2 is 2
the most commonly used strategy to relieve volume expansion and enhance the conductivity [28, 29]. Furthermore, the fabrication of hollow structures of ReS2-based composites help compensate for volume variation of the reactants and improve the electrochemical performance. Recently, carbon-encapsulated hollow metal sulfides have been discovered as effective strategies of anode material for LIBs/SIBs applications [28-31]. However, it is worth noting that the unnecessary inner void space of hollow structure greatly decreases the tap density and the energy density. Therefore, the spatially confined growth in hollow carbon nanostructures might be promising approach to ameliorate the electrochemical properties of ReS2, which could promote Li+/Na+ diffusion by constructing interface, increase the energy density, and meanwhile buffer the mechanical stress via the porous hollow structure to maintain the electrode structural stability [31-35]. Herein, we reported the rational design and fabrication to synthesize ultrathin ReS2 nanosheets encapsulated hollow mesoporous carbon spheres (HMCSs), forming nanosheets filled hollow carbon spheres (NSFHCSs) ReS2@C. Uniformly dispersed few-layer ReS2 nanosheets are confined in HMCSs to enhance the space use ratio. The excellent architecture of NSFHCSs can effectively compensate for the volume fluctuation of ReS2 nanosheets and preserve the electrical highways for lithium ions and sodium ions transport. 2. Experimental Section Materials Ammonium perrhenate (NH4ReO4), thiourea (CH4N2S) and hydroxylamine 3
hydrochloride (HONH3Cl) were achieved from Sinopharm Chemical Reagent Company. The purity of other chemicals and reagents meet laboratory requirements. Synthesis of NSFHCSs ReS2@C The hollow mesoporous carbon spheres (HMCSs) were synthesized as described in our previous work and the corresponding characterizations of HMCSs are shown in Figure S1, S2 [35]. NH4ReO4 (0.3 g), CH4N2S (0.4 g) and HONH3Cl (0.2 g) were dissolved in 30 mL of deionized water, then the pre-prepared HMCSs were added into the solution. The precursor solution was transferred into a Teflon-lined stainless steel autoclave and maintained at 200 °C for 12 h. After that, collecting the prepared product, washing and drying at 60 °C. Finally, the NSFHCSs ReS2@C products were heated at 800 °C for 2h in Ar atmosphere. For comparison, the aggregated ReS2 flakes were obtained under the same hydrothermal conditions without adding HMCSs. In addition, all characterization and electrochemical tests are displayed in Supporting Information. 3. Results and Discussion 3.1 Characterization of NSFHCSs ReS2@C
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Figure 1. Schematic illustration of the synthesis process and advantages of the
NSFHCSs ReS2@C composite. The synthetic approach and advantages of NSFHCSs ReS2@C hybrid structure are illustrated in Figure 1. Ultrathin ReS2 nanosheets are confined to grow inside HMCSs via a facile hydrothermal reaction. The morphologies of the ReS2@C nanocomposite are showed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The composites have the uniform spherical structure with a mean size of 420 nm. The ReS2 nanosheets are mainly encapsulated in the inner of carbon spheres to form the nanosheets filled hollow carbon spheres (NSFHCSs) ReS2@C (Figure 2a-d). Because the carbon shells with abundant mesoporous can serve as frameworks, part of ReS2 nanosheets are intimately embedded on the outer layer of the carbon shell. To investigate definite proof of the ReS2 nanosheets filled in the inner of hollow carbon spheres, ReS2@C composites are sliced by a microtome and the cross- section is observed by TEM. The ReS2 nanosheets are totally exposed after the microsection treatment (Figure 2e-f, Figure S3), which confirms that ReS2 nanosheets fill up the whole cavity of hollow carbon 5
sphere. The microsection process causes some defects in the filled area (marked with red boxes) of ReS2 nanosheets. The special NSFHCSs structure is further confirmed by EDX mapping analysis and line-scan spectrum. EDX mappings of NSFHCSs ReS2@C clearly show the homogeneous distribution of C, Re, and S elements (Figure 2g-j), and the line scan further confirms the spatial distribution of the C, Re and S along the lengthwise direction of the hollow carbon sphere (Figure 2k). The high-resolution TEM (HRTEM) image further proves that ultrathin nanosheets are embedded in the carbon shell (Figure 2l). As shown in Figure 2m, the number of layers in ReS2 nanosheets ranges from two to six layers, indicating typically ultrathin nanosheets structure. The lattice fringe spacing of ReS2 in ReS2@C is 0.63 nm, which is larger than that of natural ReS2 (0.62 nm), verifying the expanded interlayer spacing of ReS2 in ReS2@C (Figure 2n).
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Figure 2. (a-b) TEM, (c-d) SEM images of NSFHCSs ReS2@C, (e-f) TEM images of NSFHCSs ReS2@C cross-section after microtoming, (g) Annular dark-field STEM image and the EDX elemental mappings of C (h), Re (i), S (j), (k) EDX line-scan profiles, and (l-n) HRTEM images of NSFHCSs ReS2@C, (o) The model of NSFHCSs ReS2@C structure. The mass proportion of ReS2 in ReS2@C composites can be determined to be 88.2 wt% by TGA (Figure S4). The specific surface area of NSFHCSs ReS2@C obtained through the N2 adsorption and desorption profile is as high as 261.7 m2 g-1 (Figure S5a). The adsorption model of ReS2@C is attributed to Type IV and the existence of a hysteresis loop is related to the mesoporous [36, 37]. The predominant micropore and mesoporous size ranged for ReS2@C are centered at approximately 1.9 and 3.7 nm, respectively (Figure S5b). These holey structures in the basal plane of the ReS2 sheets can remarkably reduce the transportation barrier and efficiently provide highway for diffusion. The large surface area and the porous structure are key to accelerating the ion/electron fast transmission through the electrolyte, and improving electrochemical performance of electrode material in LIBs and SIBs [38, 39]. For comparison, pure ReS2 nanosheets were prepared by hydrothermal method without HMCSs, the morphology of which is shown in Figure S6. Obviously, pure ReS2 nanosheets exhibit agglomeration, resulting in the specific surface area only 69.9 m2 g-1 (Figure S7).
7
Figure 3. (a) XRD patterns, (b) Raman spectra, (c) XPS spectra of NSFHCSs ReS2@C, (d, e, f) high resolution spectrum of Re 4f, S 2p, and C 1s of the NSFHCSs ReS2@C, respectively. The compositions of as-prepared samples were investigated by X-ray diffraction (XRD) and Raman spectra. The diffraction peaks of samples can be consistent with JCPDS card No. 89-0341 for pure triclinic ReS2 (Figure 3a) [22, 40]. Compared with pure ReS2 nanosheets, the (002) diffraction peak of ReS2 in ReS2@C is broadened with low intensity, indicating the formation of fewer-layered ReS2 nanosheets [24, 41]. The XRD result is consistent with HRTEM images (Figure 2m, n). As shown in Figure 3b, the peaks at approximately 150 and 158 cm-1 are indicative of the in-plane (Eg) vibrational modes of ReS2, and the signals at approximately 209 cm-1 are attributed to the out-of-plane (Ag) vibrational modes of ReS2 [17, 20]. Meanwhile, the Raman spectra also display several additional peaks in the range of 100-400 cm-1 due to the unique asymmetry in the distorted 1T structure for ReS2. Moreover, two strong peaks at 1350 and 1590 cm-1 are assigned to the D-band and G-band of HMCSs in ReS2@C [42, 43]. The survey of X-ray photoelectron spectroscopy (XPS) spectrum shows the elemental composition and 8
chemical states of ReS2@C composites. Figure 3c verifies the presence of C, Re, and S elements in ReS2@C. Figure 3d shows the high-resolution spectrum of Re 4f, in which the peaks at 43.8 and 41.4 eV are characteristic of Re 4f5/2 and Re 4f7/2 of Re4+, respectively [20, 24]. The two S 2p peaks located at 161.9 and 163.1 eV are associated with S 2p3/2 and S 2p1/2 binding energies, severally (Figure 3e). The C 1s peaks at 284.7, 286.6 and 289.1 eV are ascribed to C-C, C-O and C=O, respectively (Figure 3f). The XPS result confirms the existence of ReS2 and carbon spheres, corresponding to the XRD and Raman results.
3.2 Electrochemical Performances of ReS2@C as Anode Materials for Lithium-ion Batteries
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Figure 4. Electrochemical performances of NSFHCSs ReS2@C and pure ReS2 nanosheets as anode materials for LIBs: (a, b) CV profiles over a voltage range of 0.005-3.0 V at a scan rate of 0.1 mV s-1; (c, d) charge/discharge profiles at 1 A g-1; (e) cycling performance at 0.1 A g-1; (f) cycling performance of NSFHCSs ReS2@C at 1 A g-1; (g) rate performances; (h) rate capability of ReS2-based electrodes for LIBs. The ReS2@C material was first evaluated as LIB anode material. Figure 4a shows the cyclic voltammograms (CVs) for the first three cycles of the ReS2@C electrode. Two dominant reduction peaks can be clearly seen at 1.3 and 0.7 V in the first negative scan. When the reduction peak is at 1.3 V, lithium ions are inserted into the structure of ReS2 to form LixReS2 (ReS2 + x Li+ + x e- → LixReS2) [43, 44]. The followed distinct peak located at 0.7 V is ascribed to the reduction of LixReS2 to Li2S and metallic Re (LixReS2 + (4-x) Li+ + (4-x) e- → Re + 2 Li2S), accompanying with the irreversible formation of the solid electrolyte interface (SEI) [45, 46]. In the next anodic sweeps, one tiny peak at 1.8 V is attributed to the conversion of Li2S+Re gradually to LixReS2. The obvious peak at 2.4 V indicates the formation of ReS2 [21, 44]. Additionally, it is shown that the anodic peak of ReS2@C has a larger integral area and a better coincidence of the subsequent curves, whereas the anodic peak of pure ReS2 significantly weakens with the number of scanning increase (Figure 4b). Thus, ReS2@C architecture presents a much better stability and a higher charge-discharge specific capacity than pure ReS2 nanosheets. The charge-discharge voltage profiles of the ReS2@C electrode for the 1st, 2nd, 3rd, 5th and 10th cycles were examined at a current density of 1 A g-1. As illustrated in Figure 4c, the first discharge and charge capacities can reach 1546 and 1157 mA h g-1, respectively. The initial Coulombic efficiency (CE) is approximate 74%, which is 10
much higher than that of the pure ReS2 (775 and 493 mA h g-1, and 63% CE as shown in Figure 4d). The capacity loss of ReS2@C electrode is around 26% mainly due to the electrolyte decomposition and the inevitable formation of SEI films. After the first cycle, it is observed that the discharge curves almost overlap, demonstrating that the ReS2@C electrode offers good cycling stability. Figure 4e exhibits the galvanostatic cycle behaviors of ReS2@C and pure ReS2 nanosheets at 0.1 A g-1, together with the CE of ReS2@C electrode. For ReS2@C, the discharge capacities reach 928 and 719 mA h g-1 at the initial 2nd cycle and after 100th cycles, respectively. The initial capacity fading is supposed to be owing to formation SEI films [46, 47]. Moreover, although the initial CE of ReS2@C is only 74%, it significantly raises to 97% in the 3rd cycles and maintains above 99% after 10th cycles, proving convenient Li+ insertion/extraction and effective electron diffuse in the ReS2@C electrode [31]. Conversely, at the same current density, the capacity of pure ReS2 nanosheets decays along with cycling processes (Figure 4e). Encouraged by the stable circulation, the ultralong cycling performance of the ReS2@C electrode is also tested at a high rate of 1 A g-1 (Figure 4f). Very strikingly, a specific capacity of ReS2@C electrode still remains at 578 mA h g-1 even after 1000 cycles. The NSFHCSs ReS2@C provide enough buffer space to alleviate the volume effect of the ReS2 nanosheets and prevent the exfoliation of ReS2 from the carbon spheres, which are beneficial to the improvement of reversible capacity and cycle stability of ReS2@C electrode. Figure 4g presents the outstanding rate performance of the ReS2@C electrode. 11
Following the current densities raise from 0.1 to 0.2, 0.5, 1, 2, 5, and 8 A g-1, the ReS2@C electrode displays the corresponding reversible capacities of 849, 806, 703, 619, 540, 438 and 365 mA h g-1, respectively. In comparison, these values are superior to pure ReS2 nanosheets and HMCSs (Figure S8a). Notably, as the current rate reverses to 0.1 A g-1, the ReS2@C electrode can still regains a reversible capacity near 838 mA h g-1. This excellent rate capability of our ReS2@C electrode is superior to previous literature reports (as shown in Figure 4h and Table S1), which is related to the well-defined heterogeneous interfaces providing large specific surface area, excellent chemical/mechanical stability, and good structural flexibility [44]. The electrochemical impedance spectra (EIS) is adapted to explain the electrode kinetics. The ReS2@C electrode exhibits small charge-transfer resistance (Rct), which proves that the interface resistance is reduced and the rapid transfer of electron/ion is promoted (Figure 5a). We also investigated the diffusion behavior by the slope of Zʹ vs ω-1/2 (Figure 5b) [48]. The diffusion coefficient (DLi) values of the electrodes are obtained by Warburg coefficient σ and the corresponding values are exhibited in Table S2, suggesting that this ReS2@C electrode could significantly enhance conductivity and kinetic behavior to a certain extent [49, 50]. Meanwhile, to better understand the electrochemical kinetics, CV curves were also tested at different scan rates to calculate the contribution from diffusion-controlled and capacitive effects (Figure 5c). As shown in Figure 5d, the b values are determined by the slope of the curve log (і) and log (ν) described by following equations [50, 51]: i = a νb
(1) 12
log (i)=log (a) + b log (ν)
(2)
i: current density; v: sweep rate; a, b: constants. When the b value is approximate to 1, indicating that capacitive process plays an important role in electrochemical reaction, and if the b is 0.5, it is diffusion-controlled behavior. The b values of two redox peaks (R and O) are 0.81 and 0.83, respectively, indicating that the capacitive behavior plays a major role in capacity contribution of ReS2@C electrode. By analyzing the relationship between sweep speed and current, the contribution of capacitive behavior and diffusion-controlled at a given voltage can be quantitatively distinguished. As following the equations: i = k1ν + k2ν0.5
(3)
i/ν0.5 = k1ν0.5 + k2
(4)
i: current density at a given voltage; v: sweep rate; capacitive behavior (k1ν); diffusion-controlled (k2ν0.5); k1 and k2 are variable parameters. As shown in Figure 5e, the contribution percentages of the capacitive behavior are 56.8%, 62.3%, 66.8%, 71.2%, 74.2%, 77.1% at the scan rates of 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0 mV s-1, respectively. From the above result, the capacitive behavior dominates the majority of the whole capacity of ReS2@C electrode, which is the reason for its excellent high-rate capability. The ratio of capacitive behavior increases with the sweep rate, and the capacitive contribution reaches 74.2% at 0.8 mV s-1 (Figure 5f). The results prove that the porous structure and void space of the ReS2@C electrode with high conductivity facilitate charge and ion transportation.
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Figure 5. (a) Nyquist plots and applied equivalent circuit model (inset), and (b) graph of the Z´plotted against ω-1/2 at low frequency region for the NSFHCSs ReS2@C and pure ReS2; (c) CV curves of NSFHCSs ReS2@C at different scan rates; (d) log (i) versus log (v) plots at each redox peak; (e) capacitive contribution ratio at various sweep rates; (f) capacitive contribution at 0.8 mV s-1. In order to systematically study the enhanced of lithium storage performance, the first principle calculations were implemented to investigate the adsorption behavior of Li on (002) crystal plane of ReS2 with and without carbon. Through calculation, we found that Li preferentially adsorbs on the Re top site (TRe) with adsorption energy at -0.406 eV, relative to the S top site (TS) with adsorption energy at -0.388 eV (Figure 6a-b). Li adsorbs on hollow site of pure carbon with the adsorption energy at -0.538 eV (Figure 6c) [52, 53]. The strong adsorption of Li located on top of the Re atom is also related to the coordination of three nearest neighbor S atoms with negative charge. Compared to ReS2, the adsorption strength of Li at the heterogeneous interfaces between carbon and ReS2 is greatly enhanced, and the adsorption energy at the two sites are -0.906 eV (TRe) and -0.840 eV (TS), respectively (Figure 6d-e). And Li also prefers to adsorb on the Re top site (TRe) of ReS2@C composite. The above 14
theoretical results indicate that the synergistic effect of heterogeneous interface between ReS2 and carbon significantly enhances the adsorption energy of Li, promotes the stability of Li in the intercalation process and improves electrochemical performance of ReS2@C hybrid [52, 54].
Figure 6. Ball-and-stick models for the most stable configurations and binding energies of Li adsorption via DFT calculation. ReS2 layer (a) TRe site and (b) TS site, (c) graphene layer (hollow site), ReS2@C (d) TRe site and (e) TS site.
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Figure 7.（a-f) HRTEM images, (g) SAED pattern, (h-k) The elemental mapping for NSFHCSs ReS2@C after the 1000th discharge cycles. (l) Schematic illustration of the lithiation/delithiation process.
After the 1000th discharge cycles, the structure and morphology of NSFHCSs ReS2@C electrode were explored by TEM (Figure 7a, S9, S10), in which ReS2@C composites have sustained the intact sphere structure. Moreover, the internal ReS2 layers have broken into nanograins around 8 nm, which are uniformly distributed within the carbon framework (Figure 7b). Enough room in HMCSs can accommodate the volume change, avoiding the structure and morphology collapse of the ReS2@C. In HRTEM (Figure 7c-d), the lattice fringes with an interlayer distance of 0.33 and 0.23 nm corresponding to the d-spacing of Li2S (111) and Re (101), respectively [14]. The SAED can be indexed as the mixture of Li2S and Re (Figure 7g). It reveals the lithiation reaction of ReS2 nanosheets involve the conversion from ReS2 phase to Re 16
and Li2S, which is consistent with the CV. Meanwhile, it is clearly seen that an uneven SEI film is formed on the surface of the nanospheres (Figure 7e), which stabilizes the electrode and improves the migration kinetics of Li leading to the prolonged cycle life[46, 55]. Although, ReS2 nanosheets are broken into nanograins, HMCSs intercept the escape of them, which aggregate at the edge of the inner shell due to the confinement effect (Figure 7f, inside the yellow dotted line). Furthermore, the corresponding EDX elemental mappings also demonstrate the presence of C, Re and S elements after the cycle (Figure 7h-k, S11). The internal reaction mechanism is presented as shown Figure 7l.
3.3 Electrochemical Performances of ReS2@C as Anode Materials for Sodium-ion Batteries
Figure 8. Electrochemical performances of NSFHCSs ReS2@C and pure ReS2 nanosheets as anode materials for SIBs: (a, b) CV profiles over a voltage range of 0.005-3.0 V at a scan rate of 0.1 mV s-1; (c, d) charge/discharge profiles at 0.2 A g-1; (e) rate performances (0.1 to 2 A g-1); (f) Cycling performance at 0.2 A g-1; (g) Nyquist plots and applied equivalent circuit model (inset). In addition, the sodium storage behavior of this electrode was investigated as well. Figure 8a displays the CV curves for the initial three cycles. In the first 17
reduction process, the reduction peaks at 1.1, 0.61, and 0.11 V correspond to the multistep conversion reaction between the Na+ and ReS2 and the formation of metallic Re nanocrystals and Na2S (ReS2 + x Na+ + x e- → NaxReS2, NaxReS2 + (4-x) Na+ + (4-x) e- → Re + 2 Na2S). In the following reverse anodic scan, all the positive-going CV curves appear three oxidation peaks associated with a stepwise desodiation process [46, 56, 57]. The CV profiles overlap well from the second scan, suggesting that the ReS2@C nanocomposite has excellent cycling stability for SIBs. The CV of pure ReS2 nanosheets is shown in Figure 8b. The galvanostatic discharge-charge curves of ReS2@C and ReS2 at 0.2 A g-1 are shown in Figure 8c and Figure 8d, respectively. The initial discharge capacity loss in SIBs is related to the irreversible formation of SEI films and the electrolyte decomposition. From the second cycle, highly consistent discharge-charge curves of ReS2@C are obtained, which corroborates remarkable cycling stability. Figure 8e presents the rate performance of ReS2@C electrode at different current densities from 0.1 to 2 A g-1. High reversible capacities of 388, 363, 323, 283, 261, 225 and 190 mA h g-1 could be remained, corresponding current densities are 0.1, 0.2, 0.4, 0.8, 1, 1.5 and 2 A g-1, respectively. As the current density retunes to 0.1 A g-1, the capacity could still be recovered to 375 mA h g-1. Figure 8f shows the cycling performance at 0.2 A g-1. The ReS2@C electrode exhibits high stability within 400 cycles accompanied by the capacity fade in the first 10 cycles. Alike LIBs, highly conductive porous carbon and favorable buffer void of ReS2@C electrode can accommodate the mechanical stresses induced by volume change, thus improving the 18
stability of the electrode. In contrast, pure ReS2 nanosheets electrode specific capacity drops dramatically from 420 to 115 mA h g-1 within 60 cycles. The cycling stability of the reported ReS2-based in Table S3. The mechanical stability of NSFHCSs ReS2@C is clearly superior to that of pure ReS2. Additionally, as shown in Figure 8g, the Nyquist plot of NSFHCSs ReS2@C in SIBs also shows similar characteristics to LIBs. However, it is worth noting that the Rct value of NSFHCSs ReS2@C in SIBs is much larger than that in LIBs at the same experimental condition. Regarding the LIBs, the impedance values of SIBs are larger due to the much bigger size of Na ion leading to the sluggish sodium ion diffusion kinetics [43, 48]. Hence, the excellent electrochemical performance of NSFHCSs ReS2@C for both lithium and sodium storage can be attributed to its unique structure and morphology: (1) The few-layer ReS2 nanosheets with an enlarged interlayer spacing could reduce the barriers of Li+/Na+ mobility and increase more active sites. (2) The ultrathin ReS2 nanosheets fully filled in the cavity of hollow carbon sphere might increase the space use ratio and the interspace among ReS2 nanosheets can buffer the volume expansion. (3) The well-defined heterogeneous interfaces could enhance the kinetics of Li+/Na+ transport by shortening the Li+/Na+ diffusion distance. (4) The incorporation of porous carbon framework might improve the intrinsic conductivity and maintain the structural stability and integrity. 4. Conclusion In summary, we designed a unique hybrid nanostructure that ultrathin ReS2 nanosheets are confined in hollow mesoporous carbon nanospheres to form NSFHCSs 19
ReS2@C. The few-layer ReS2 nanosheets fill up the whole cavity of hollow carbon sphere at nanoscale dimensions to enhance the space use ratio compared with the hollow structure. The porous carbon framework not only acts as a nanoreactor to confined growth of the active mass, but also provides enough space to buffer volume changes. Moreover, the confinement effect prevents the loss of active material and improves the cycling stability. Benefiting from the advantageous structure, the NSFHCSs ReS2@C composites exhibit remarkable electrochemical performance as anode materials in LIBs/SIBs, which is an important guarantee for long-term cycle stability. Such a strategy could be readily extended to other metal sulfides for the development of high performance electrodes for better LIBs/SIBs. Conflicts of interest There are no conflicts to declare. 5. References [1] P.K. Nayak, L. Yang, W. Brehm, P. Adelhelm, Angew. Chem. Int. Ed., 57 (2018) 102-120. [2] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater., 23 (2013) 947-958. [3] Y. Sun, N. Liu, Y. Cui, Nat. Energy, 1 (2016) 16071. [4] J. Lu, Z. Chen, F. Pan, Y. Cui, K. Amine, Electrochem. Energy Rev., 1 (2018) 35-53. [5] Y. Cao, M. Li, J. Lu, J. Liu, K. Amine, Nat. Nanotechnol., 14 (2019) 200-207. 20
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Conflict of Interest Statement
The authors declare no competing financial interest.
S2405-8297(19)31054-2
DOI:
https://doi.org/10.1016/j.ensm.2019.11.019
Reference:
ENSM 994
To appear in:
Energy Storage Materials
Received Date: 21 August 2019 Revised Date:
1 November 2019
Accepted Date: 17 November 2019
Please cite this article as: X. Zhang, C. Shen, H. Wu, Y. Han, X. Wu, W. Ding, L. Ni, G. Diao, M. Chen, Filling Few-Layer ReS2 in Hollow Mesoporous Carbon Spheres for Boosted Lithium/Sodium Storage Properties, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.11.019. 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. © 2019 Published by Elsevier B.V.
Filling Few-Layer ReS2 in Hollow Mesoporous Carbon Spheres for Boosted Lithium/Sodium Storage Properties Xiue Zhang, Chao Shen, Huayu Wu, Yue Han, Xiaoyu Wu, Weiran Ding, Lubin Ni, Guowang Diao, Ming Chen* School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P. R. China
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 21773203), the Natural Science Foundation of Jiangsu Province (BK20161329) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions is acknowledged. Notes The authors declare no competing financial interest.
*
Corresponding author at: School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P. R. China. E-mail address: [email protected] (M. Chen)
Filling Few-Layer ReS2 in Hollow Mesoporous Carbon Spheres for Boosted Lithium/Sodium Storage Properties
ABSTRACT A facile and feasible spatial confinement strategy is proposed for the preparation of ultrathin ReS2 nanosheets confined in hollow mesoporous carbon spheres (HMCSs), forming nanosheets filled hollow carbon spheres (NSFHCSs) ReS2@C. Based on the spatially confined growth, the as-synthesized ReS2@C nanocomposites achieve novel filled-structure and well-defined heterogeneous interfaces. In the excellent architecture, the porous carbon framework precisely constrains ultrathin ReS2 nanosheets within its void space, which can generate essential electrical contacts, buffer volume expansion, and prohibit the loss of active materials during the pulverization process. For lithium-ion batteries, the NSFHCSs ReS2@C display high adsorption energy of Li in the heterogenous interface by the first-principle calculation, and achieve superior reversible capacity of 578 mA h g-1 after 1000 cycles at 1 A g-1. Research shows that the capacitive behavior plays a major role in specific capacity contribution of ReS2@C electrode. For sodium-ion batteries, the NSFHCSs ReS2@C demonstrate reversible discharge capacity of 319 mA h g-1 at 0.2 A g-1 after 400 cycles. The enlarged interlayer spacing and few layer structure of ReS2 nanosheets are conducive to enhance the Li+/Na+ mobility and improve the dynamic characteristic of electrode material. Keywords: few-layer ReS2 nanosheets; spatially confined growth; hollow mesoporous carbon nanospheres; lithium-ion batteries; sodium-ion batteries 1
1. Introduction Lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs), which share similar intercalation chemistry despite the different ionic radii (Li+: 0.076 nm; Na+: 0.102 nm), have attracted dramatic attention for the extensive needs of high-efficient energy storage devices [1-5]. However, the development of LIBs/SIBs has been hampered by major challenges, including low capacities and rapid capacity fading [6-8]. Thus, it is necessary to enhance their electrochemical energy storage performance by developing new electrode materials or designing favorable nanostructures [9-13]. Two-dimensional (2D) transition metal dichalcogenides (TMDs), with the large interlayer distance and weak van der Waals forces, are considered to be one of the best candidate materials for alkaline ion batteries [14-16]. ReS2 has a weaker interlayer coupling energy (18 me V, a unit cell) compared to MoS2 (460 me V) [17-20]. The weaker interlayer coupling energy would permit rapid transfer of alkali metal ions within the interlayer. Moreover, the interlayer distance of ReS2 (0.61 nm) is almost twice larger than graphite (0.34 nm). These characteristics enable ReS2 to be ideal anode material for alkali ion batteries [21, 22]. Nevertheless, the practical application of ReS2 to LIBs/SIBs is greatly suppressed due to poor cyclability. One of the key reasons for the poor cycle is the large volume variation during charge/discharge process, leading to mechanical failure and loss of electrical contact at the electrode [23-25]. The other is the poor conductivity of ReS2 [26, 27]. To address these challenges, the synthesis of carbon hybrid materials of ReS2 is 2
the most commonly used strategy to relieve volume expansion and enhance the conductivity [28, 29]. Furthermore, the fabrication of hollow structures of ReS2-based composites help compensate for volume variation of the reactants and improve the electrochemical performance. Recently, carbon-encapsulated hollow metal sulfides have been discovered as effective strategies of anode material for LIBs/SIBs applications [28-31]. However, it is worth noting that the unnecessary inner void space of hollow structure greatly decreases the tap density and the energy density. Therefore, the spatially confined growth in hollow carbon nanostructures might be promising approach to ameliorate the electrochemical properties of ReS2, which could promote Li+/Na+ diffusion by constructing interface, increase the energy density, and meanwhile buffer the mechanical stress via the porous hollow structure to maintain the electrode structural stability [31-35]. Herein, we reported the rational design and fabrication to synthesize ultrathin ReS2 nanosheets encapsulated hollow mesoporous carbon spheres (HMCSs), forming nanosheets filled hollow carbon spheres (NSFHCSs) ReS2@C. Uniformly dispersed few-layer ReS2 nanosheets are confined in HMCSs to enhance the space use ratio. The excellent architecture of NSFHCSs can effectively compensate for the volume fluctuation of ReS2 nanosheets and preserve the electrical highways for lithium ions and sodium ions transport. 2. Experimental Section Materials Ammonium perrhenate (NH4ReO4), thiourea (CH4N2S) and hydroxylamine 3
hydrochloride (HONH3Cl) were achieved from Sinopharm Chemical Reagent Company. The purity of other chemicals and reagents meet laboratory requirements. Synthesis of NSFHCSs ReS2@C The hollow mesoporous carbon spheres (HMCSs) were synthesized as described in our previous work and the corresponding characterizations of HMCSs are shown in Figure S1, S2 [35]. NH4ReO4 (0.3 g), CH4N2S (0.4 g) and HONH3Cl (0.2 g) were dissolved in 30 mL of deionized water, then the pre-prepared HMCSs were added into the solution. The precursor solution was transferred into a Teflon-lined stainless steel autoclave and maintained at 200 °C for 12 h. After that, collecting the prepared product, washing and drying at 60 °C. Finally, the NSFHCSs ReS2@C products were heated at 800 °C for 2h in Ar atmosphere. For comparison, the aggregated ReS2 flakes were obtained under the same hydrothermal conditions without adding HMCSs. In addition, all characterization and electrochemical tests are displayed in Supporting Information. 3. Results and Discussion 3.1 Characterization of NSFHCSs ReS2@C
4
Figure 1. Schematic illustration of the synthesis process and advantages of the
NSFHCSs ReS2@C composite. The synthetic approach and advantages of NSFHCSs ReS2@C hybrid structure are illustrated in Figure 1. Ultrathin ReS2 nanosheets are confined to grow inside HMCSs via a facile hydrothermal reaction. The morphologies of the ReS2@C nanocomposite are showed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The composites have the uniform spherical structure with a mean size of 420 nm. The ReS2 nanosheets are mainly encapsulated in the inner of carbon spheres to form the nanosheets filled hollow carbon spheres (NSFHCSs) ReS2@C (Figure 2a-d). Because the carbon shells with abundant mesoporous can serve as frameworks, part of ReS2 nanosheets are intimately embedded on the outer layer of the carbon shell. To investigate definite proof of the ReS2 nanosheets filled in the inner of hollow carbon spheres, ReS2@C composites are sliced by a microtome and the cross- section is observed by TEM. The ReS2 nanosheets are totally exposed after the microsection treatment (Figure 2e-f, Figure S3), which confirms that ReS2 nanosheets fill up the whole cavity of hollow carbon 5
sphere. The microsection process causes some defects in the filled area (marked with red boxes) of ReS2 nanosheets. The special NSFHCSs structure is further confirmed by EDX mapping analysis and line-scan spectrum. EDX mappings of NSFHCSs ReS2@C clearly show the homogeneous distribution of C, Re, and S elements (Figure 2g-j), and the line scan further confirms the spatial distribution of the C, Re and S along the lengthwise direction of the hollow carbon sphere (Figure 2k). The high-resolution TEM (HRTEM) image further proves that ultrathin nanosheets are embedded in the carbon shell (Figure 2l). As shown in Figure 2m, the number of layers in ReS2 nanosheets ranges from two to six layers, indicating typically ultrathin nanosheets structure. The lattice fringe spacing of ReS2 in ReS2@C is 0.63 nm, which is larger than that of natural ReS2 (0.62 nm), verifying the expanded interlayer spacing of ReS2 in ReS2@C (Figure 2n).
6
Figure 2. (a-b) TEM, (c-d) SEM images of NSFHCSs ReS2@C, (e-f) TEM images of NSFHCSs ReS2@C cross-section after microtoming, (g) Annular dark-field STEM image and the EDX elemental mappings of C (h), Re (i), S (j), (k) EDX line-scan profiles, and (l-n) HRTEM images of NSFHCSs ReS2@C, (o) The model of NSFHCSs ReS2@C structure. The mass proportion of ReS2 in ReS2@C composites can be determined to be 88.2 wt% by TGA (Figure S4). The specific surface area of NSFHCSs ReS2@C obtained through the N2 adsorption and desorption profile is as high as 261.7 m2 g-1 (Figure S5a). The adsorption model of ReS2@C is attributed to Type IV and the existence of a hysteresis loop is related to the mesoporous [36, 37]. The predominant micropore and mesoporous size ranged for ReS2@C are centered at approximately 1.9 and 3.7 nm, respectively (Figure S5b). These holey structures in the basal plane of the ReS2 sheets can remarkably reduce the transportation barrier and efficiently provide highway for diffusion. The large surface area and the porous structure are key to accelerating the ion/electron fast transmission through the electrolyte, and improving electrochemical performance of electrode material in LIBs and SIBs [38, 39]. For comparison, pure ReS2 nanosheets were prepared by hydrothermal method without HMCSs, the morphology of which is shown in Figure S6. Obviously, pure ReS2 nanosheets exhibit agglomeration, resulting in the specific surface area only 69.9 m2 g-1 (Figure S7).
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Figure 3. (a) XRD patterns, (b) Raman spectra, (c) XPS spectra of NSFHCSs ReS2@C, (d, e, f) high resolution spectrum of Re 4f, S 2p, and C 1s of the NSFHCSs ReS2@C, respectively. The compositions of as-prepared samples were investigated by X-ray diffraction (XRD) and Raman spectra. The diffraction peaks of samples can be consistent with JCPDS card No. 89-0341 for pure triclinic ReS2 (Figure 3a) [22, 40]. Compared with pure ReS2 nanosheets, the (002) diffraction peak of ReS2 in ReS2@C is broadened with low intensity, indicating the formation of fewer-layered ReS2 nanosheets [24, 41]. The XRD result is consistent with HRTEM images (Figure 2m, n). As shown in Figure 3b, the peaks at approximately 150 and 158 cm-1 are indicative of the in-plane (Eg) vibrational modes of ReS2, and the signals at approximately 209 cm-1 are attributed to the out-of-plane (Ag) vibrational modes of ReS2 [17, 20]. Meanwhile, the Raman spectra also display several additional peaks in the range of 100-400 cm-1 due to the unique asymmetry in the distorted 1T structure for ReS2. Moreover, two strong peaks at 1350 and 1590 cm-1 are assigned to the D-band and G-band of HMCSs in ReS2@C [42, 43]. The survey of X-ray photoelectron spectroscopy (XPS) spectrum shows the elemental composition and 8
chemical states of ReS2@C composites. Figure 3c verifies the presence of C, Re, and S elements in ReS2@C. Figure 3d shows the high-resolution spectrum of Re 4f, in which the peaks at 43.8 and 41.4 eV are characteristic of Re 4f5/2 and Re 4f7/2 of Re4+, respectively [20, 24]. The two S 2p peaks located at 161.9 and 163.1 eV are associated with S 2p3/2 and S 2p1/2 binding energies, severally (Figure 3e). The C 1s peaks at 284.7, 286.6 and 289.1 eV are ascribed to C-C, C-O and C=O, respectively (Figure 3f). The XPS result confirms the existence of ReS2 and carbon spheres, corresponding to the XRD and Raman results.
3.2 Electrochemical Performances of ReS2@C as Anode Materials for Lithium-ion Batteries
9
Figure 4. Electrochemical performances of NSFHCSs ReS2@C and pure ReS2 nanosheets as anode materials for LIBs: (a, b) CV profiles over a voltage range of 0.005-3.0 V at a scan rate of 0.1 mV s-1; (c, d) charge/discharge profiles at 1 A g-1; (e) cycling performance at 0.1 A g-1; (f) cycling performance of NSFHCSs ReS2@C at 1 A g-1; (g) rate performances; (h) rate capability of ReS2-based electrodes for LIBs. The ReS2@C material was first evaluated as LIB anode material. Figure 4a shows the cyclic voltammograms (CVs) for the first three cycles of the ReS2@C electrode. Two dominant reduction peaks can be clearly seen at 1.3 and 0.7 V in the first negative scan. When the reduction peak is at 1.3 V, lithium ions are inserted into the structure of ReS2 to form LixReS2 (ReS2 + x Li+ + x e- → LixReS2) [43, 44]. The followed distinct peak located at 0.7 V is ascribed to the reduction of LixReS2 to Li2S and metallic Re (LixReS2 + (4-x) Li+ + (4-x) e- → Re + 2 Li2S), accompanying with the irreversible formation of the solid electrolyte interface (SEI) [45, 46]. In the next anodic sweeps, one tiny peak at 1.8 V is attributed to the conversion of Li2S+Re gradually to LixReS2. The obvious peak at 2.4 V indicates the formation of ReS2 [21, 44]. Additionally, it is shown that the anodic peak of ReS2@C has a larger integral area and a better coincidence of the subsequent curves, whereas the anodic peak of pure ReS2 significantly weakens with the number of scanning increase (Figure 4b). Thus, ReS2@C architecture presents a much better stability and a higher charge-discharge specific capacity than pure ReS2 nanosheets. The charge-discharge voltage profiles of the ReS2@C electrode for the 1st, 2nd, 3rd, 5th and 10th cycles were examined at a current density of 1 A g-1. As illustrated in Figure 4c, the first discharge and charge capacities can reach 1546 and 1157 mA h g-1, respectively. The initial Coulombic efficiency (CE) is approximate 74%, which is 10
much higher than that of the pure ReS2 (775 and 493 mA h g-1, and 63% CE as shown in Figure 4d). The capacity loss of ReS2@C electrode is around 26% mainly due to the electrolyte decomposition and the inevitable formation of SEI films. After the first cycle, it is observed that the discharge curves almost overlap, demonstrating that the ReS2@C electrode offers good cycling stability. Figure 4e exhibits the galvanostatic cycle behaviors of ReS2@C and pure ReS2 nanosheets at 0.1 A g-1, together with the CE of ReS2@C electrode. For ReS2@C, the discharge capacities reach 928 and 719 mA h g-1 at the initial 2nd cycle and after 100th cycles, respectively. The initial capacity fading is supposed to be owing to formation SEI films [46, 47]. Moreover, although the initial CE of ReS2@C is only 74%, it significantly raises to 97% in the 3rd cycles and maintains above 99% after 10th cycles, proving convenient Li+ insertion/extraction and effective electron diffuse in the ReS2@C electrode [31]. Conversely, at the same current density, the capacity of pure ReS2 nanosheets decays along with cycling processes (Figure 4e). Encouraged by the stable circulation, the ultralong cycling performance of the ReS2@C electrode is also tested at a high rate of 1 A g-1 (Figure 4f). Very strikingly, a specific capacity of ReS2@C electrode still remains at 578 mA h g-1 even after 1000 cycles. The NSFHCSs ReS2@C provide enough buffer space to alleviate the volume effect of the ReS2 nanosheets and prevent the exfoliation of ReS2 from the carbon spheres, which are beneficial to the improvement of reversible capacity and cycle stability of ReS2@C electrode. Figure 4g presents the outstanding rate performance of the ReS2@C electrode. 11
Following the current densities raise from 0.1 to 0.2, 0.5, 1, 2, 5, and 8 A g-1, the ReS2@C electrode displays the corresponding reversible capacities of 849, 806, 703, 619, 540, 438 and 365 mA h g-1, respectively. In comparison, these values are superior to pure ReS2 nanosheets and HMCSs (Figure S8a). Notably, as the current rate reverses to 0.1 A g-1, the ReS2@C electrode can still regains a reversible capacity near 838 mA h g-1. This excellent rate capability of our ReS2@C electrode is superior to previous literature reports (as shown in Figure 4h and Table S1), which is related to the well-defined heterogeneous interfaces providing large specific surface area, excellent chemical/mechanical stability, and good structural flexibility [44]. The electrochemical impedance spectra (EIS) is adapted to explain the electrode kinetics. The ReS2@C electrode exhibits small charge-transfer resistance (Rct), which proves that the interface resistance is reduced and the rapid transfer of electron/ion is promoted (Figure 5a). We also investigated the diffusion behavior by the slope of Zʹ vs ω-1/2 (Figure 5b) [48]. The diffusion coefficient (DLi) values of the electrodes are obtained by Warburg coefficient σ and the corresponding values are exhibited in Table S2, suggesting that this ReS2@C electrode could significantly enhance conductivity and kinetic behavior to a certain extent [49, 50]. Meanwhile, to better understand the electrochemical kinetics, CV curves were also tested at different scan rates to calculate the contribution from diffusion-controlled and capacitive effects (Figure 5c). As shown in Figure 5d, the b values are determined by the slope of the curve log (і) and log (ν) described by following equations [50, 51]: i = a νb
(1) 12
log (i)=log (a) + b log (ν)
(2)
i: current density; v: sweep rate; a, b: constants. When the b value is approximate to 1, indicating that capacitive process plays an important role in electrochemical reaction, and if the b is 0.5, it is diffusion-controlled behavior. The b values of two redox peaks (R and O) are 0.81 and 0.83, respectively, indicating that the capacitive behavior plays a major role in capacity contribution of ReS2@C electrode. By analyzing the relationship between sweep speed and current, the contribution of capacitive behavior and diffusion-controlled at a given voltage can be quantitatively distinguished. As following the equations: i = k1ν + k2ν0.5
(3)
i/ν0.5 = k1ν0.5 + k2
(4)
i: current density at a given voltage; v: sweep rate; capacitive behavior (k1ν); diffusion-controlled (k2ν0.5); k1 and k2 are variable parameters. As shown in Figure 5e, the contribution percentages of the capacitive behavior are 56.8%, 62.3%, 66.8%, 71.2%, 74.2%, 77.1% at the scan rates of 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0 mV s-1, respectively. From the above result, the capacitive behavior dominates the majority of the whole capacity of ReS2@C electrode, which is the reason for its excellent high-rate capability. The ratio of capacitive behavior increases with the sweep rate, and the capacitive contribution reaches 74.2% at 0.8 mV s-1 (Figure 5f). The results prove that the porous structure and void space of the ReS2@C electrode with high conductivity facilitate charge and ion transportation.
13
Figure 5. (a) Nyquist plots and applied equivalent circuit model (inset), and (b) graph of the Z´plotted against ω-1/2 at low frequency region for the NSFHCSs ReS2@C and pure ReS2; (c) CV curves of NSFHCSs ReS2@C at different scan rates; (d) log (i) versus log (v) plots at each redox peak; (e) capacitive contribution ratio at various sweep rates; (f) capacitive contribution at 0.8 mV s-1. In order to systematically study the enhanced of lithium storage performance, the first principle calculations were implemented to investigate the adsorption behavior of Li on (002) crystal plane of ReS2 with and without carbon. Through calculation, we found that Li preferentially adsorbs on the Re top site (TRe) with adsorption energy at -0.406 eV, relative to the S top site (TS) with adsorption energy at -0.388 eV (Figure 6a-b). Li adsorbs on hollow site of pure carbon with the adsorption energy at -0.538 eV (Figure 6c) [52, 53]. The strong adsorption of Li located on top of the Re atom is also related to the coordination of three nearest neighbor S atoms with negative charge. Compared to ReS2, the adsorption strength of Li at the heterogeneous interfaces between carbon and ReS2 is greatly enhanced, and the adsorption energy at the two sites are -0.906 eV (TRe) and -0.840 eV (TS), respectively (Figure 6d-e). And Li also prefers to adsorb on the Re top site (TRe) of ReS2@C composite. The above 14
theoretical results indicate that the synergistic effect of heterogeneous interface between ReS2 and carbon significantly enhances the adsorption energy of Li, promotes the stability of Li in the intercalation process and improves electrochemical performance of ReS2@C hybrid [52, 54].
Figure 6. Ball-and-stick models for the most stable configurations and binding energies of Li adsorption via DFT calculation. ReS2 layer (a) TRe site and (b) TS site, (c) graphene layer (hollow site), ReS2@C (d) TRe site and (e) TS site.
15
Figure 7.（a-f) HRTEM images, (g) SAED pattern, (h-k) The elemental mapping for NSFHCSs ReS2@C after the 1000th discharge cycles. (l) Schematic illustration of the lithiation/delithiation process.
After the 1000th discharge cycles, the structure and morphology of NSFHCSs ReS2@C electrode were explored by TEM (Figure 7a, S9, S10), in which ReS2@C composites have sustained the intact sphere structure. Moreover, the internal ReS2 layers have broken into nanograins around 8 nm, which are uniformly distributed within the carbon framework (Figure 7b). Enough room in HMCSs can accommodate the volume change, avoiding the structure and morphology collapse of the ReS2@C. In HRTEM (Figure 7c-d), the lattice fringes with an interlayer distance of 0.33 and 0.23 nm corresponding to the d-spacing of Li2S (111) and Re (101), respectively [14]. The SAED can be indexed as the mixture of Li2S and Re (Figure 7g). It reveals the lithiation reaction of ReS2 nanosheets involve the conversion from ReS2 phase to Re 16
and Li2S, which is consistent with the CV. Meanwhile, it is clearly seen that an uneven SEI film is formed on the surface of the nanospheres (Figure 7e), which stabilizes the electrode and improves the migration kinetics of Li leading to the prolonged cycle life[46, 55]. Although, ReS2 nanosheets are broken into nanograins, HMCSs intercept the escape of them, which aggregate at the edge of the inner shell due to the confinement effect (Figure 7f, inside the yellow dotted line). Furthermore, the corresponding EDX elemental mappings also demonstrate the presence of C, Re and S elements after the cycle (Figure 7h-k, S11). The internal reaction mechanism is presented as shown Figure 7l.
3.3 Electrochemical Performances of ReS2@C as Anode Materials for Sodium-ion Batteries
Figure 8. Electrochemical performances of NSFHCSs ReS2@C and pure ReS2 nanosheets as anode materials for SIBs: (a, b) CV profiles over a voltage range of 0.005-3.0 V at a scan rate of 0.1 mV s-1; (c, d) charge/discharge profiles at 0.2 A g-1; (e) rate performances (0.1 to 2 A g-1); (f) Cycling performance at 0.2 A g-1; (g) Nyquist plots and applied equivalent circuit model (inset). In addition, the sodium storage behavior of this electrode was investigated as well. Figure 8a displays the CV curves for the initial three cycles. In the first 17
reduction process, the reduction peaks at 1.1, 0.61, and 0.11 V correspond to the multistep conversion reaction between the Na+ and ReS2 and the formation of metallic Re nanocrystals and Na2S (ReS2 + x Na+ + x e- → NaxReS2, NaxReS2 + (4-x) Na+ + (4-x) e- → Re + 2 Na2S). In the following reverse anodic scan, all the positive-going CV curves appear three oxidation peaks associated with a stepwise desodiation process [46, 56, 57]. The CV profiles overlap well from the second scan, suggesting that the ReS2@C nanocomposite has excellent cycling stability for SIBs. The CV of pure ReS2 nanosheets is shown in Figure 8b. The galvanostatic discharge-charge curves of ReS2@C and ReS2 at 0.2 A g-1 are shown in Figure 8c and Figure 8d, respectively. The initial discharge capacity loss in SIBs is related to the irreversible formation of SEI films and the electrolyte decomposition. From the second cycle, highly consistent discharge-charge curves of ReS2@C are obtained, which corroborates remarkable cycling stability. Figure 8e presents the rate performance of ReS2@C electrode at different current densities from 0.1 to 2 A g-1. High reversible capacities of 388, 363, 323, 283, 261, 225 and 190 mA h g-1 could be remained, corresponding current densities are 0.1, 0.2, 0.4, 0.8, 1, 1.5 and 2 A g-1, respectively. As the current density retunes to 0.1 A g-1, the capacity could still be recovered to 375 mA h g-1. Figure 8f shows the cycling performance at 0.2 A g-1. The ReS2@C electrode exhibits high stability within 400 cycles accompanied by the capacity fade in the first 10 cycles. Alike LIBs, highly conductive porous carbon and favorable buffer void of ReS2@C electrode can accommodate the mechanical stresses induced by volume change, thus improving the 18
stability of the electrode. In contrast, pure ReS2 nanosheets electrode specific capacity drops dramatically from 420 to 115 mA h g-1 within 60 cycles. The cycling stability of the reported ReS2-based in Table S3. The mechanical stability of NSFHCSs ReS2@C is clearly superior to that of pure ReS2. Additionally, as shown in Figure 8g, the Nyquist plot of NSFHCSs ReS2@C in SIBs also shows similar characteristics to LIBs. However, it is worth noting that the Rct value of NSFHCSs ReS2@C in SIBs is much larger than that in LIBs at the same experimental condition. Regarding the LIBs, the impedance values of SIBs are larger due to the much bigger size of Na ion leading to the sluggish sodium ion diffusion kinetics [43, 48]. Hence, the excellent electrochemical performance of NSFHCSs ReS2@C for both lithium and sodium storage can be attributed to its unique structure and morphology: (1) The few-layer ReS2 nanosheets with an enlarged interlayer spacing could reduce the barriers of Li+/Na+ mobility and increase more active sites. (2) The ultrathin ReS2 nanosheets fully filled in the cavity of hollow carbon sphere might increase the space use ratio and the interspace among ReS2 nanosheets can buffer the volume expansion. (3) The well-defined heterogeneous interfaces could enhance the kinetics of Li+/Na+ transport by shortening the Li+/Na+ diffusion distance. (4) The incorporation of porous carbon framework might improve the intrinsic conductivity and maintain the structural stability and integrity. 4. Conclusion In summary, we designed a unique hybrid nanostructure that ultrathin ReS2 nanosheets are confined in hollow mesoporous carbon nanospheres to form NSFHCSs 19
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Conflict of Interest Statement
The authors declare no competing financial interest.