Multivalence-Ion Intercalation Enables Ultrahigh 1T Phase MoS 2 Nanoflowers to Enhanced Sodium-Storage Performance
2020; Chinese Chemical Society; Volume: 3; Issue: 5 Linguagem: Inglês
10.31635/ccschem.020.202000323
ISSN2096-5745
AutoresKun Ma, Yu Liu, Hao Jiang, Yanjie Hu, Rui Si, Honglai Liu, Chunzhong Li,
Tópico(s)Chemical Synthesis and Characterization
ResumoOpen AccessCCS ChemistryRESEARCH ARTICLE1 May 2021Multivalence-Ion Intercalation Enables Ultrahigh 1T Phase MoS2 Nanoflowers to Enhanced Sodium-Storage Performance Kun Ma†, Yu Liu†, Hao Jiang, Yanjie Hu, Rui Si, Honglai Liu and Chunzhong Li Kun Ma† Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Yu Liu† School of Chemical Engineering and Technology, Sun Yat-sen University, Guangzhou 510275 , Hao Jiang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Yanjie Hu Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Rui Si Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204 , Honglai Liu State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237. and Chunzhong Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237. https://doi.org/10.31635/ccschem.020.202000323 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Developing rapid charging and robust electrode materials for Na-ion batteries is of considerable significance in large-scale power electricity fields. Herein, the authors have proposed a multivalence-ion intercalation strategy to construct three-dimensional (3D) Co-MoS2 nanoflowers with tailorable 1T/2H phase and interlayer distance. The as-formed S–Co–S covalent bonds serve as "electric bridges" to accelerate interlayer charge transfer without 1T phase degeneration during sodiation and desodiation. Quantum density functional theory (QDFT) calculations further confirm that the optimal Co-MoS2 nanoflowers possess the highest Na adsorption energy with reduced ionic diffusion barrier. Consequently, they deliver a superior sodium-storage capacity of 351 mAh g−1 in 0.4–3.0 V even at 20 A g−1 without capacity fading at 5 A g−1 for 2000 cycles. The high electrochemical reversibility of the 1T phase in Co-MoS2, which accounts for such excellent performance, has been unveiled for the first time by in situ Raman spectra. This finding demonstrates important insights onto promoting two-dimensional (2D) nanomaterials toward rapid charging alkali-ion batteries. Download figure Download PowerPoint Introduction Rechargeable sodium-ion batteries (SIBs) have received extensive attention chiefly owing to their lower cost [nearly 40% of lithium-ion batteries (LIBs) and higher security (zero overdischarge)].1–3 Nevertheless, the bigger radius and weaker binding to substrate of Na+ (vs Li+) engender a very low specific capacity for the graphite anode of LIBs, that is, 35 mAh g−1 in carbonate-based electrolyte.4,5 The pivotal to drive the SIBs practical applications lies in the exploitation of electrode materials that can effectively and reversibly insert and extract Na+. Two-dimensional (2D) transition metal sulfides have been identified as a promising candidate due to their high theoretical specific capacity (e.g., 670 mAh g−1 for MoS2).6–8 In particular, the atomic thickness can give them almost maximum electrochemical active sites while the large interlayer space and edges can also contribute to tremendous extra pseudocapacitance and fast reaction kinetics.9–11 However, the stable 2H-MoS2 usually suffers from poor electrical conductivity with broad band gap (1.3–1.9 eV). And it is prone to stack, agglomerate, and collapse during repeated charge and discharge. Thus far, the most effective protocol to mitigate the above obstacles focuses on incorporating high conductive guest materials (e.g., conductive polymer, carbon, etc.) into the MoS2 interlayer by two primary approaches.12–14 One is to firstly exfoliate the existing MoS2 into single- or few-layered nanosheets by chemical or mechanical methods, and then decorate it with guest precursors on the surface.15–17 After restacking them together and the following chemical reaction, the guest has been generated in the MoS2 interlayer, as shown in Figure 1a. Another method is to in situ introduce guest precursors into the MoS2 interlayer with functional inner surface by the chemical forces and the subsequent 2D space confined reaction, as shown in Figure 1b.18–20 Despite considerable progress, the MoS2 interlayer totally filled with guest materials ineluctably affects Na+ rapid diffusion. And, the electron transfer capability in the MoS2 interlayer is still unsatisfactory. Fortunately, 1T-MoS2 usually possesses broad interlayer space with conductive nature (10–100 S cm−1).21–23 The phase transformation can fundamentally overcome the slow electrons transfer of 2H-MoS2. However, the metastable 1T-MoS2 is very easy to spontaneously convert into the more stable 2H phase. Therefore, it is very critical but a challenge to achieve stable and high proportion 1T phase MoS2 with fast interlayer electron transfer capability. Figure 1 | Design concept, synthesis, and characterization of the Co-MoS2 nanoflowers. (a–c) The conventional protocols and new concept on overcoming the drawbacks of MoS2 electrode materials. (d) Illustration of morphological change and interlayer distance of the Co-MoS2 samples with increasing the amount of Co2+. (e–g) The corresponding SEM images of the three samples. Download figure Download PowerPoint In the discharge process of secondary batteries, the intercalation of cations (e.g., Li+, Na+, Mg2+, etc.) can greatly promote 1T phase formation in MoS2-based electrode materials.24–26 Inspired by these observations, we implement a multivalence-ion intercalation enabling S–Co–S covalent bonds pillared strategy to precisely control MoS2 1T/2H phase and interlayer distance, as shown in Figure 1c. The as-formed S–Co–S covalent bonds can regulate maximum interlayer space to 1.10 nm, with 1T phase as high as 74%; the bonds furthermore act as bridges for rapid electron transfer between MoS2 interlayers. Meanwhile, introducing Co ions triggers the self-assembly of MoS2 nanosheets into three-dimensional (3D) nanoflowers, effectively avoiding the stacking and restacking while enhancing ionic conductivity. Quantum density functional theory (QDFT) calculations further verify that the S–Co–S covalent bonds pillared 1T-MoS2 has a highest Na+ adsorption energy and a dramatically reduced diffusion barrier. Consequently, the optimal Co-MoS2 nanoflowers deliver the best sodium-storage capacity of 351 mAh g−1 in 0.4–3.0 V even at 20 A g−1 (only 1.0 min) without capacity fading at 5 A g−1 for 2000 cycles. The in situ Raman spectra unveiled for the first time the high reversibility of the 1T phase in Co-MoS2 during sodiation/desodiation, which accounts for the excellent sodium-storage performance. Experimental Methods Synthesis of the Co-MoS2 hybrids Typically, 1.0 g of Na2MoO4 and 1.2 g of thiourea were firstly dissolved in a solution containing 15 mL of water and 15 mL of ethanol. Subsequently, 5 mL of HCl solution (2.0 M) was used to regulate the solution pH to ∼1.0. Then, 0.1 g of CoCl2·6H2O was added into above mixture under vigorous stirring for 1 h. After that, the mixture was placed into a 50 mL autoclave and heated in an electronic oven at 180 °C for 24 h. The final product was obtained by filtration, rinsing, and drying the precipitate. The Co content in the Cox-MoS2 sample can be tuned by changing the CoCl2·6H2O concentration. As a control, the corresponding pure MoS2 sample was also produced under the same conditions only without adding CoCl2·6H2O in the synthesis process. Characterization The powder X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance X-ray diffractometer (Cu Kα radiation, λ = 0.154 nm). Scanning electron microscopy (SEM; S-4800; Hitachi, Tokyo, Japan) and field emission transmission electron microscopy (FETEM; JEOL-2100F; JEOL, Tokyo, Japan) were applied to investigate the surface morphology and microstructure. High-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images were recorded on a JEOL JEM-ARF200F (JEOL, Tokyo, Japan) electron microscope (200 kV) with a spherical aberration corrector. Raman and X-ray photoelectron spectroscopy (XPS) spectra were investigated on a Bruker RFS 100/S (Bruker, Karlsruhe, Germany) spectrometer and PHI 5000 VersaProbe (ULVAC-PHI, Inc., Chigasaki, Japan), respectively. UV–Vis absorption spectra were carried out with a PerkinElmer Lambda 950 (PerkinElmer, Waltham, Massachusetts, USA) UV/Vis/near-infrared (NIR) spectrometer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on a NETZSCH STA409PC analyzer (NETZSCH-Gerätebau GmbH, Sayre, Germany) under flowing N2 atmosphere. The Mo K-edge X-ray absorption fine structure (XAFS) measurements were made at the 14W1 beamline in Shanghai Synchrotron Radiation Facility (SSRF; Shanghai, China). The XAFS data were collected under fluorescence mode with a Si (111) monochromator and Lytle-type ion chamber, where the calibrated energy was based on the Mo K-edge of pure Mo foil. The data extraction and fitting were performed by using Athena and Artemis codes (Shanghai Synchrotron Radiation Facility, Shanghai, China). Electrochemical measurements Electrochemical measurements were performed using coin-type 2025 cells, which were assembled in an argon-filled glovebox. The working electrode was prepared by mixing active materials, carbon ECP, and poly(vinyl difluoride) (PVDF; 5%) with a weight ratio of 8∶1∶1, and then pasted on pure Cu foil. The loading mass of the active material is around 1.0 mg cm−2. Pure Na foil was used as counter electrode, and the separator was Whatman GF/D (Tsingtao, Shandong, China). The electrolyte was composed of 1.0 M NaPF6 in dimethoxyethane (DME). Cyclic voltammetry (CV) tests were performed on an Autolab PGSTAT302N at different sweep speeds from 0.2 to 10 mV s−1. Galvanostatic charge/discharge experiments were implemented by a LAND CT2001A (Wuhan Batrui Technology Co. LTD, Wuhan, Hubei, China) battery tester in the thermotank (25 °C) within 0.4–3 V. The galvanostatic intermittent titration technique (GITT) measurements were also executed on the above battery tester with a test pulse of charging and discharging at 50 mA g−1 for 15 min and rest for 1 h. DFT calculation All QDFT calculations were implemented in the Dmol3 module of Material Studio software (Accelrys, San Diego, California, USA). The exchange correlation functional is approximated by the Perdew–Burke–Ernzerhof (PBE) method with polarized spin; the one body wave function is expanded by the Double Numerical plus d-functions 3.5 basis set; the self-consistent iteration is considered to converge when the change on energy lower than 2.72 × 10−5 eV; the system is considered to be in equilibrium when the change of energy, maximum force, and maximum displacement is lower than 2.72 × 10−4 eV, 0.545 eV/nm, and 5 × 10−4 nm, respectively. The QDFT calculation is performed in periodic boxes 0.95 nm × 0.73 nm × 0.64 nm and 0.95 nm × 0.73 nm × 1.1 nm for the 1T-MoS2-0.64 and 1T-MoS2-1.1, respectively. For 2H-MoS2, the corresponding box sizes are 0.99 nm × 0.99 nm × 0.64 nm and 0.99 nm × 0.99 nm × 1.1 nm, respectively. The Co-MoS2 model is built with a box size of 0.95 nm × 0.73 nm × 1.1 nm. The adsorption energy (Ea) of a Na atom in the materials is calculated by: E a = E M o S 2 + N a − E M o S 2 − E N a E a = E C o − M o S 2 + N a − E C o − M o S 2 − E N a where E M o S 2 + N a , E M o S 2 , E N a , E C o − M o S 2 + N a, and E C o − M o S 2 are the energies of Na-absorbed MoS2, pure MoS2, pure metal Na, Na-absorbed Co-MoS2, and Co-MoS2, respectively. In situ Raman measurement In situ Raman spectra were performed in a sealed two-electrode system and recorded on the Renishaw inVia (Renishaw, Gloucestershire, England) microspectrometer with a 532-nm laser. The two-electrode system was assembled in a glovebox. The working electrode was a mixture of active materials and Carbon ECP pasted on Cu foam. The loading mass of the active materials was around 1.5 mg cm−2. The counter electrode was pure Na foil, and the separator was Whatman GF/D. The electrolyte was 1.0 M NaPF6 in DME. After aging for 5 h, the galvanostatic charge/discharge curves were carried out at 0.2 mA g−1 within 0.4–3.0 V. Results and Discussion Synthesis and characterization of the Co-MoS2 nanoflowers A simple hydrothermal approach has been applied to synthesize Coo-intercalated MoS2 electrode materials, as illustrated in Figure 1d. It can be observed that the morphologies evolve from MoS2 nanosheets into Co-MoS2 nanoflowers with the increase of Co2+ concentration. Meanwhile, the corresponding interlayer space has been enlarged to a maximum value of 1.10 nm. The SEM images of the three samples are provided in Figures 1e–1g, that is, pure MoS2 nanosheets (0% Co), Co-MoS2 aggregates (3.9 wt % Co, named as Co-MoS2-L), and Co-MoS2 nanoflowers (6.2 wt % Co), respectively. These results demonstrate that Co ions promote the MoS2 nanosheets to assemble into Co-MoS2 nanoflowers. This is mainly because Co2+ can serve as the "joining sites" to interlink adjacent nanosheets during the MoS2 growth,27 yielding the nanoflower-structured Co-MoS2. The Co content in Co-MoS2 samples is estimated by the inductively coupled plasma mass spectrometry (ICP-MS) result. In Figure 1g, the uniform Co-MoS2 nanoflowers are composed of many interconnected nanosheets with diameters of ∼500 nm. Such an impressive architecture has been reckoned to ensure high ion conductivity and alleviate the stacking/restacking and agglomeration of nanosheets during the repeated charge and discharge processes. Evolution of microstructure and interlayer spacing in Co-MoS2 samples The Co-MoS2 nanoflowers have been further characterized by FETEM. Figure 2a exhibits a representative flower-like structure with obvious brightness contrast, implying the existence of abundant pores created by the assembly of many nanosheets. The corresponding high-resolution TEM image is provided in Figure 2b, giving a nanosheet subunit thickness of about below 10 nm. The interlayer distance is broadened to 1.10 from 0.80 nm of the pure MoS2 nanosheets ( Supporting Information Figure S1). There are no Co-contained nanoparticles on the surface of MoS2 nanosheets. As shown in the transmission electron microscopy-energy dispersive spectrometer (TEM-EDS) mapping (Figure 2c), the homogeneous distribution of Mo, Co, and S elements indicates that strong chemical bonds are formed in the Co-MoS2 nanoflowers. We further give the X-ray diffraction (XRD) patterns of the pure CoS2, the pure MoS2 nanosheets, and the Co-MoS2 samples with Co content from 3.9 to 9.7 wt %, as shown in Figure 2d. It is conspicuous that the typical (002) peak at 11.0° gradually shifts to 9.3° and 8.0°, corresponding to enlarged interlayer distances of 0.95 and 1.10 nm, respectively. Notably, two new peaks located at 11.7° (#2) and 20.2° (#3) appear for the Co-MoS2 nanoflowers, which are indexed to the characteristic peaks of tetragonal 1T-MoS2.21,28 1T-MoS2 will be discussed in detail later. The results are identical to the above TEM observations, indicating that Co ions have been successfully incorporated into the MoS2 interlayer in the form of S–Co–S covalent bonds, and promote the phase transformation without any observable impurities (e.g., CoS2). When the Co content further increases to 9.7 wt %, there is no change for the (002) peak position, but CoS2 diffraction peaks appear. The thermal stability of the Co-MoS2 nanoflowers is also investigated by XRD analysis with the pure MoS2 nanosheets as a control. After annealing at 300 °C for 0.5 h in Ar, the Co-MoS2 nanoflowers maintain an unchanged (002) peak position at 8.0° (Figure 2e). We can also find that the broad #2 peak disappears and a new strong peak arises at 13.6°. This is because the thermal treatment process causes the generation of partial 2H-MoS2. As for the pure MoS2 nanosheets, an obvious peak shift from 11.0° to 13.1° happens due to the removal of small molecules in the interlayer (e.g., thioureas).29,30 This observation indicates that the S–Co–S covalent bonds can solidly pillar the MoS2 interlayer. Nevertheless, the S–Co–S covalent bonds have broken when annealing at 500 °C for 0.5 h. Only one peak at 14.1° can be observed for both samples. To further verify this viewpoint, the TGA/DSC curves of the two samples are also provided in Figure 2f. The endothermic peak at ∼280 °C is mainly attributed to elimination of the inserted small molecules. Furthermore, the emergence of another peak at 430 °C only for the Co-MoS2 nanoflowers may be rooted in the dissociation of the covalent bonds, which is in good agreement with the above XRD characterization. Notably, we also tried many other transition metals (e.g., Fe2+, Ni2+, and Cu2+) to modify MoS2 ( Supporting Information Figure S2). Specifically, introducing Fe/Cu ions severely damaged the crystal structure of MoS2, accompanied by the generation of many impurities. In Ni-MoS2, there are no observable effects on interlayer spacing and phase composition of MoS2 compared with pure MoS2. Based on above results, we choose the Co-MoS2 as the target for further research. Figure 2 | Evolution of microstructure and interlayer spacing in Co-MoS2 samples. (a) High-magnification, (b) high-resoluton TEM images, and (c) TEM-EDS mapping of the Co-MoS2 nanoflowers [inset of (b) showing the interlayer spacing along the red line]. (d) XRD patterns of the pure CoS2, the MoS2 nanosheets, and the Co-MoS2 with different Co content. (e) XRD patterns after annealing at different temperatures and (f) TGA/DSC curves of the MoS2 nanosheets and Co-MoS2 nanoflowers. Download figure Download PowerPoint Regulation of phase composition in Co-MoS2 samples The Raman spectra and XPS were performed to reveal the chemical and electronic structure of the as-prepared samples. As for the two Co-MoS2 samples, the peaks at 299 and 398 cm−1 in Raman spectra (Figure 3a) are ascribed to the libration and stretch modes of S–Co–S covalent bonds,31 which are also proved by the XPS spectra ( Supporting Information Figure S3). Besides, another four peaks at 147, 237, 276, and 337 cm−1 can be indexed to typical J1, J2, E1g, and J3 modes of 1T-MoS2, respectively.21,28 In contrast, the pure MoS2 control only shows two characteristic peaks at 381 and 407 cm−1 of the 2H phase.32 Another direct evidence is that two absorption bands at 614 and 669 nm of 2H-MoS2 are absent in UV–Vis spectra of the two Co-MoS2 samples ( Supporting Information Figure S4).22 The 1T phase transformation will greatly enhance the electrical conductivity. Meanwhile, the S–Co–S covalence pillared MoS2 can effectively suppress the restacking and 1T phase degeneration. The 1T phase content in the Co-MoS2 samples has been estimated by high-resolution Mo 3d XPS spectra (Figure 3b), showing an obviously increasing trend with the addition of Co2+. For example, the 1T-MoS2 can reach as high as 74% for the Co-MoS2 nanoflowers. The equivalent high-resolution S 2p spectra give a similar viewpoint ( Supporting Information Figure S5). The electrical conductivity of three samples is also evaluated by the four-point probe measurement. The Co-MoS2 nanoflowers deliver an excellent conductivity of 4.3 S cm−1 owing to their high 1T phase content, which is greatly higher than Co-MoS2-L (1.1 S cm−1) and MoS2 nanosheets (0.02 S cm−1). In addition, as displayed in Supporting Information Figure S6, the Co-MoS2 nanoflowers also possess a larger Brunner-Emmet-Teller (BET) surface area of 93.8 m2 g−1 with a richer pore-size distribution compared with MoS2 nanosheets (35.2 m2 g−1), which can expose more Na+-storage sites and achieve favorable electrolyte infiltration. To further understand the electronic state and coordination environment at atomic scale, XAFS measurement was conducted. Figure 3c displays the Mo K-edge X-ray absorption near-edge structure (XANES) spectra of the pure MoS2 nanosheets and the Co-MoS2 nanoflowers. The S–Co–S bond intercalation obviously decreases the peak intensity at 20,025 eV and results in the peak disappearance at 20,092 eV compared with the pure MoS2 nanosheets, further confirming the 1T phase dominated Co-MoS2 nanoflowers.33,34 Notably, the redshift of the rising edge for Co-MoS2 nanoflowers indicates strong electronic coupling between Co and MoS2 by means of S–Co–S covalent bonds, which may cause structural deformation of the MoS2 layer. The accompanied defects can be observed in Supporting Information Figure S7. According to the Mo K-edge oscillation function k3χ(k) profiles ( Supporting Information Figure S8), the corresponding Fourier transform (FT) curves are provided in Figure 3d. The Co-MoS2 nanoflowers show a smaller radial distance (2.75 vs 3.16 Å of MoS2) in Mo–Mo peak, suggesting a shortened Mo–Mo bond length.33 Meanwhile, the drastically decreased peak intensities for both Mo–S and Mo–Mo bonds indicate the reduction of coordination number of Mo atoms, which is mainly because Co shares sulfur atoms with Mo. The detailed structure parameters are also provided in Supporting Information Table S1. The S–Co–S covalence pillared MoS2 nanoflowers with high 1T phase content are endowed with fast electron/ion dual-conducting capability and high structural stability. Figure 3 | Regulation of phase composition in Co-MoS2 samples. (a) Raman spectra of the pure CoS2, the MoS2 nanosheets, the Co-MoS2-L, and the Co-MoS2 nanoflowers. (b) High-resolution Mo 3d spectra of the MoS2 nanosheets, the Co-MoS2-L, and the Co-MoS2 nanoflowers. (c) Mo K-edge XANES spectra and (d) the Fourier transform spectra of the Mo K-edge extended X-ray absorption fine structure (EXAFS) for the MoS2 nanosheets and Co-MoS2 nanoflowers. Download figure Download PowerPoint Electrochemical properties for SIBs To highlight the effects of 1T-MoS2 on sodium-storage performance, the 2H-MoS2 with similar morphology has been synthesized as a control according to the previous report ( Supporting Information Figure S9).35 It is well accepted that the conversion reaction during charge and discharge generally induces recrystallization and internal stresses, resulting in serious coarsening and pulverization of intermediate products.11 It has been extensively documented that the conversion reaction of MoS2 for SIBs usually occurs below 0.4 V.15,36 In this study, the voltage range of 0.4–3.0 V was adopted to avoid the conversion reaction as much as possible, although it will be at the expense of partial specific capacity. Figure 4a shows the first three CV profiles of the Co-MoS2 nanoflowers between 0.4 and 3 V at 0.2 mV s−1. In the first cycle, only one reduction peak at ∼1.5 V can be detected, which is related to the reduction of residual thioureas.18,37 Interestingly, the typical reduction peak at ∼1.0 V belonging to the Na+ insertion triggered phase transformation from 2H to 1T cannot be observed, but appears in the control sample ( Supporting Information Figure S10) and other 2H-MoS2 electrode materials.20,21 This phenomenon indicates negligible phase transformation during Na+ insertion, further verifying abundant 1T phase in the Co-MoS2 nanoflowers. The oxidation peaks at 1.53, 1.73, and 2.09 V are derived from Na+ extraction. Interestingly, the S–Co–S covalent bonds can be recovered during sodiation/desodiation, and the related analysis and discussion have been displayed in Supporting Information Figure S11. Supporting Information Figure S12 gives the sodium-storage performances of the Co-MoS2 samples with different 1T/2H ratio. The Co-MoS2 nanoflowers exhibit the highest specific capacity at various rates. The first Coulombic efficiency (CE) is 81.6% with a discharge capacity as high as 554 mAh g−1 ( Supporting Information Figure S13), which rapidly increases to 98.7% in the following two cycles. Figure 4b shows the capacity retention of the two samples at 0.2–20 A g−1. The Co-MoS2 nanoflowers deliver an ultrahigh reversible capacity of 452 mAh g−1 at 0.2 A g−1 and 351 mAh g−1 at 20 A g−1, which is much higher than the MoS2 nanoflowers (189 mAh g−1 at 0.2 A g−1 and 135 mAh g−1 at 20 A g−1) and the MoS2 nanosheets in Supporting Information Figure S14 (232 mAh g−1 at 0.2 A g−1 and 156 mAh g−1 at 20 A g−1). To our knowledge, this superior charge ability is the best reported for MoS2-based anode materials. An overall comparison is provided in Supporting Information Table S2. Furthermore, the Co-MoS2 nanoflowers can be quickly charged to 85% within about 1.0 min using a constant current of 0.2 A g−1 ( Supporting Information Figure S15), exhibiting a great potential in developing electric vehicles and smart grids. More significantly, the Co-MoS2 nanoflowers always keep an unchanged discharge capacity of 410 mAh g−1 at 5 A g−1 through 2000 cycles (continuous operation for 166 h), as shown in Figure 4c, manifesting a long cycle life. Furthermore, the TEM result in Supporting Information Figure S16 shows that the cycled Co-MoS2 maintains the expanded interlayer distance of ∼1.08 nm even after 2000 cycles, implying that the S–Co–S covalent bonds can tightly pillar MoS2 interlayers. Figure 4 | Electrochemical investigation for SIBs. (a) The initial three CV curves of the Co-MoS2 nanoflowers at 0.2 mV s−1. (b) Rate performances of the MoS2 nanoflowers and the Co-MoS2 nanoflowers. (c) Capacity retention of the Co-MoS2 nanoflowers at 5 A g−1 for 2000 cycles (continuous operation for 166 h). (d) Relationships between peak currents and sweep rate. (e) Normalized capacitive and diffusion-controlled contributions to capacity at different sweep rates of the Co-MoS2 nanoflowers. (f) Na-ion diffusion coefficient of the Co-MoS2 nanoflowers and the MoS2 nanoflowers under discharge and charge states. Download figure Download PowerPoint Investigation for sodium-storage principle in Co-MoS2 nanoflowers The sodium-storage mechanism in the Co-MoS2 nanoflowers has been investigated in detail to clarify the improvement principle of electrochemical performance. The CV curves at 1–10 mV s−1 are presented in Supporting Information Figure S17a. All the curves maintain a similar contour without big shifts of redox peak positions, implying low polarization during Na+ insertion and extraction. The corresponding relationship between the sweep rate (v) and peak current (i) is also provided in Figure 4d based on a power law of log i = b log v + log a,38,39 in which slope b values close to 0.5 and 1.0 represent a diffusion-controlled and a capacitance-dominated ionic storage process, respectively. It can be seen that the Co-MoS2 nanoflowers possess much higher b values for both anodic (0.92) and cathodic (0.96) peaks than those of MoS2 nanoflowers in Supporting Information Figure S18, showing almost total surface-controlled behaviors with fast reaction kinetics. The capacitive contribution can be quantified by separating current value i at a fixed potential into the capacitive-dominated process (k1v) and the intercalation-controlled process (k2v1/2), respectively, according to the equation of i = k1v + k2v1/2).9,40 As shown in Figure 4e, the capacitive contribution ratios for the Co-MoS2 nanoflowers at various sweep rates are calculated, demonstrating an increasing tendency from 89.7% of the total charge at 2 mV s−1 to a maximum of 93.5% at 10 mV s−1. The CV curve at 10 mV s−1 with a typical calculated capacitive contribution in the shaded region is also provided in Supporting Information Figure S17b. Meanwhile, the Co-MoS2 nanoflowers have a much lower charge-transfer resistance of 59 Ω compared with the 2H-MoS2 nanoflowers (189 Ω), as shown in Supporting Information Figure S19. We further evaluate the ion diffusion kinetics of th
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