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Hierarchical hollow-structured anode for high-rate sodium-ion battery
Journal of Solid State Chemistry 283 (2020) 121159
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Hierarchical hollow-structured anode for high-rate sodium-ion battery Chuanqiang Wu, Yu Zhou, Changda Wang, Wen Zhu, Shiqing Ding, Shuangming Chen *, Li Song National Synchrotron Radiation Lab, University of Science and Technology of China, Hefei, Anhui, 230029, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: MoSe2 nanosheets XAFS High rate In-situ Raman Sodium-ion battery
Thanks to its larger surface and free volume, hollow nanostructure and nanomaterials are one kind of highly desirable electrodes for efficiently enhancing sodium-ion battery performance, especially at high rate. Herein, we demonstrate a design of hierarchical semi-open hollow structure, assembling with molybdenum selenide (MoSe2) nanosheets embedded on the nitrogen doped carbon matrix. The unique hollow structure can not only enable sufficient electrode/electrolyte interaction and fast electron transportation, but also suppress volume expansion during the insertion/extraction of sodium ion. As a direct outcome, the MoSe2/carbon anode exhibits excellent cycling and rate performance with a specific capacity of 431 mAh g1 at 200 mA g1, maintaining 87.7% capacity after 120 cycles. Notably, a discharge capacity of 199.1 mAh g1 can be achieved even at an ultra-high current density of 20 A g1, indicating quick Naþ storage ability. The results may open a new way to realize high-rate anode for ion battery applications.
1. Introduction Sodium-ion batteries (SIBs) have attracted researcher’s attention around the world in recent years, due to higher availability, environmental friendliness, and design flexibility. Another important reason is the cheaper price and rich abundance of sodium compared with lithium [1–4]. However, Naþ ions show a larger radius with 1.08 Å than Liþ ions, which greatly increases the diffusion barrier, causing the commercial anode material graphite not suitable for SIBs [5]. Therefore, it is imperative to explore suitable potential anode materials to improve the ability of ions transport and cycling as well as high rate performance [6]. Generally, inorganic nanostructures have important applications in many fields [7–17]. Recently, Molybdenum selenide (MoSe2), one of the member of transition metal dichalcogenides (TMDs) with typical two dimension layered structure, has been widely studied and considered as an ideal anode candidate for SIBs, since it has high theoretical capacity, large interlayer distance, as well as small band gap [18–20]. However, it still suffers from volume expansion during sodiation/desodiation, which will result in capacity attenuation. Other than this, MoSe2 also faces problems of low conductivity and aggregation of layers during cycling [21]. In order to overcome these disadvantages, extensive works have been done to improve the cycle and rate performance [22,23]. One approach is to composite MoSe2 with carbon materials, and MoSe2–C hybrids such as 3D C–MoSe2/reduced graphene oxide, MoSe2/MWCNT, MoSe2/porous carbon spheres have been reported and demonstrated
with enhanced performance for sodium storage. MoSe2 with different morphology has also been synthesized, such as fullerene-like MoSe2, MoSe2 nanoplates and so on [24–27]. Constructing hollow structures has been proven to be an efficient strategy to enhance rate performance, durability and specific capacity for both LIBs and SIBs. Qian group has synthesized hollow nanospheres of mesoporous Co9S8, the material shown outstanding electrochemical lithium storage as anode material. Qiao group has prepared Na2Ti3O7@N-doped carbon hollow spheres for SIBs displayed excellent rate performance. The excellent performance of hollow structures has been contributed to: First, hollow structures can offer the advantages of a large specific surface, short ion diffusion length, and a lot of active sites. Additionally, the voids in the shell can effectively buffer large volume expansion of the materials upon metal ions insertion/extraction [28–31]. Therefore it is promising to synthesis hollow MoSe2 structures for sodium storage. Herein, we designed a hierarchical hollow structure with MoSe2 nanosheets embedded on nitrogen doped carbon matrix. It’s interesting to note that those hollow structures are not completely close up and well dispersed. Employing this special structural materials as anode materials for sodium-ion batteries, the as prepared MoSe2@ nitrogen carbon semiopen hollow nanospheres (abbreviated as M@C S-HS) exhibit an excellent rate performance and durability that is distinct as compared to solid MoSe2@carbon nanospheres (abbreviated as S-M@C) or pristine compounds.
* Corresponding author. E-mail address: [email protected] (S. Chen). https://doi.org/10.1016/j.jssc.2019.121159 Received 14 October 2019; Received in revised form 11 December 2019; Accepted 29 December 2019 Available online 1 January 2020 0022-4596/© 2020 Elsevier Inc. All rights reserved.
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2. Results and discussions
nanospheres do not have hollow structures. The phase and composite of M@C S-HS were confirmed by employing powder XRD and Raman analysis. Diffraction peaks of the calcined powders in Fig. 2a can be indexed to the hexagonal-phase MoSe2 according to the JCPDS 29–0914 [33]. Except for the peaks of MoSe2, peak at 26.35 belongs to carbon. As presented in Fig. 2b, Raman peaks at 239.02 cm1 and 279.6 cm1 are assigned to out of plane (A1 g) and in plane (E12g) mode of MoSe2 in M@C S-HS. Meanwhile, there are two strong peaks at 1335 cm1 and 1584 cm1, corresponding to D-band and G-band of carbon [34–38]. These results suggest that the as-prepared M@C S-HS is composed of MoSe2 and carbon. The content of MoSe2 in M@C S-HS was determined by TG analysis. As showing in Fig. 2c, the increase of weight before 320 C resulted from oxidization of MoSe2. Subsequent weight loss can be attributed to sublimation of SeO2 and oxidization of carbon at about 500 C [39]. Finally, the residual substance is MoO3 and the remaining weight is 39.88%. Accordingly, the weight percentage of MoSe2 can be calculated as about 70.34%. The N2 adsorption and desorption isotherms and pore size distribution in Fig. 2d reveal that M@C S-HS exhibits a specific surface area of 127.02 m2g-1 with typical mesoporous character [39–41]. This means that the semi-open hollow structure is more conducive to the contact between the electrolyte and the anode material. Synchrotron-based X-ray absorption fine structure (XAFS) which can investigate coordination surroundings and the local bond length of a given component as central atom was performed on the samples. Fig. S6 shows Normalized XANES spectra and Fig. 3a shows the oscillation curves of Mo K-edge in M@C S-HS, S-M@C and commercial MoSe2. It is clearly seen that the k3χ (k) oscillation curves of three sample have the similar oscillation shape, suggesting the Mo atoms of M@C S-HS have the similar local atomic arrangements comparing with S-M@C and commercial MoSe2. The Fourier transform (FT) profiles in the real space are shown in Fig. 3b. There are two main coordination peaks in the real space at about 2.14 and 2.93 Å, corresponding to Mo–Se and Mo–Mo bonds, respectively. It can be clearly seen, the intensities of the two peaks (represent coordination number) of M@C S-HS are obviously weaker than that of S-M@C and commercial MoSe2. Detailed fitting results in Fig. S7 and Table S1 illustrate the Mo–Se and Mo–Mo coordination numbers in M@C S-HS are 3.2 and 3.4 less than that of S-M@C and commercial MoSe2 which are 3.9, 4.1 and 6, 6, respectively. The smaller coordination number of MoSe2 in M@C S-HS can be attributed to more
The synthesis procedure of M@C S-HS is schematically illustrated in Fig. 1a. MoO3 microspheres consisted of small MoO3 nanospheres with size about 100 nm were synthesized via a simple solvothermal method. Fig. S1a shows the typical XRD pattern of the MoO3 precursor and Fig. S2a shows the SEM of the MoO3 microspheres. Then MoO3 microspheres as a reactive self-degraded template as well as additional Mo supply were hybrid with AHM and dopamine hydrochloride. After injecting the ammonia solution, Mo-polydopamine formed through polymerization of the Mo-dopamine complex. At the same time MoO3 microspheres dissolved after reaction for 10 min (Fig. S2b), and the small MoO3 nanospheres were served as templates of the hollow structure [32]. After reaction for 2 h, semi-open structures composite of Mo-dopamine with an orange color were obtained (Fig. S1b shows the XRD of the Mo-dopamine complex) and XRD peaks of MoO3 precursor cannot be observed. Finally, MoSe2 embedded on carbon matrix with semi-open hollow structures were achieved through a selenization process of the as resulting Mo-dopamine complex. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were employed to understand the morphology and micro-structure of samples. Fig. 1b and c shows the typical SEM images of M@C S-HS, in which uniform hollow spheres with sizes around 200 nm could be easily observed. Interestingly, most of those hollow spheres display a semi-open shape. The diameters of the voids in the shell are about 100 nm, which are in consistent with size of small MoO3 nanospheres. Besides, hierarchical nanosheets are also observed on the shell of hollow nanospheres. As shown as Fig. 1d, TEM image further demonstrates the nanospheres with hollow structures, consisting of numerous thin nanosheets. The SAED image shows that the M@C S-HS is composed of multicrystal (Fig. S3). HRTEM in Fig. 1e illustrates the nanosheets are formed by carbon matrix with MoSe2 nanosheets embedding. The interplanar spacing of the MoSe2 nanosheets is around 0.648 nm, corresponding to the (002) crystalplane of MoSe2. STEM-EDS with elemental mapping of the M@C S-HS further indicates the homogeneous distribution of the elements Mo, Se and C throughout the entire sample (Fig. S4 and Fig. 1f and g). The additional N elemental mapping suggests N-doping nature in as-prepared nanospheres. As the control samples, solid S-M@C nanospheres were also synthesized. Figs. S5a and 5b show the SEM and TEM images of S-M@C, obviously revealing these solid
Fig. 1. (a) Schematic illustration of the route for synthesis of M@C S-HS. Typical SEM images at low (b) and high (c) magnification of M@C S-HS. (d) TEM and (e) HRTEM, (f, g) STEM image and element mapping of M@C S-HS. 2
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Fig. 2. (a) XRD pattern, (b) Raman spectrum, (c) TGA curve of M@C S-HS and (d) N2 sorption isotherms and pore size distribution of M@C S-HS (the inset picture is image of pore size distribution) and S-M@C.
open hollow structure, the sodium storage electrochemical test of M@C S-HS was performed. Fig. 4a depicts the typical CV curves of the first three cycles of M@C S-HS electrode at a scan rate of 0.2 mV s1 over a potential between 0.01 and 3 V. Two peaks at 0.95 V and 0.39 V can be observed in the first cathodic sweeping, the first one is ascribed to the intercalation of sodium ions into MoSe2 layers and lead to the formation of NaxMoSe2 [44]. Another peak at 0.39 V is assigned to the conversion from NaxMoSe2 to metallic Mo and Na2Se with the electrolyte decomposition to form a solid electrolyte interface (SEI) films [45]. Peaks appear at 1.68 V and 1.21 V in the following cycles are still controversial, and we will discuss the origin of these peaks in the subsequent part. The subsequent CV curves remain steady indicating high reversibility of the electrode. Fig. 4b shows the charge and discharge profiles for as-prepared electrode at a current density of 200 mA g1. The electrode displays the discharge and charge specific capacities of 563.8 and 431mAh g1 in the first cycle, and the coulombic efficiency is estimated as 76.4%. In the following cycles, the coulombic efficiency rises up to nearly 98%. As shown in Fig. 4c, the discharge capacity can still keep at 378.2 mAh g1
surface atoms compared to S-M@C and commercial MoSe2 because of the hollow structure. Wavelet transform (WT) was also used to analyse coordination surroundings of Mo in as prepared samples. As show in Fig. 3c-e, the WT contour plots of the three samples all display two intensity maximum at 7.5 Å1 and 11 Å1, corresponding to Mo–Se and Mo–Mo bonding, respectively. X-ray photoelectron spectroscopy (XPS) measurement was used to identify chemical composition and confirm the existence of N in carbon matrix of the M@C S-HS composites. The fullscan XPS spectrum of prepared M@C S-HS is shown in Fig. S8. Highresolution XPS of Mo, Se, C, N were detected, peaks located at 228.96 and 232.03eV in Fig. 3f can be assigned to Mo 3d5/2 and Mo 3d3/2 of Mo4þ [19,20]. Another two peaks in Fig. 3g at 54.6 and 55.6 eV belong to Se 3d5/2 and Se 3d3/2 of Se2 [22]. The spectrum of C 1s in Fig. 3h can be – C and divided into two peaks, peaks at 284.76 eV for sp2 bonded C– peaks at 285.98eV for C–N [42,43]. Peak of N–C also can be obtained in Fig. 3i for N1s. The XPS results confirm the formation of MoSe2 in M@C S-HS and the existence of N in carbon matrix. To evaluate the energy storage behavior of this hierarchical semi-
Fig. 3. (a) Synchrotron-radiation-based XAFS spectra show the Mo k-edge oscillation curves in k-space, (b) FT analysis in R-space and (c–e) Wavelet transform (WT) of M@C S-HS, S-M@C and commercial MoSe2. XPS spectra of M@C S-HS: (f) Mo 3d, (g) Se 3d (h) C 1s and (i) N 1s. 3
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Fig. 4. (a) CV curves at a scan rate of 0.2 mV s1 in the voltage range of 0.01–3 V and (b) discharge-charge curves at a current density of 200 mAg1 of M@C S-HS. (c) Cycling performance and at a current density of 200 mAg1 and (d) Rate cycling performance of M@C S-HS and S-M@C for sodium-ion batteries.
structure can provide a larger specific area than that of solid nanospheres and bring out more active sites. Meanwhile, it can also cause the M@C SHS electrode contact better with electrolyte, thus reduce the diffusion path of Naþ ions. (2) the unique hollow structure can prevent the volume change upon Naþ ions insertion/extraction. (3) nitrogen doped carbon matrix highly improves the conductivity of the hybrids which could offer fast kinetics for the intercalation/deintercalation of Naþ ion. (4) the aggregation and restack of MoSe2 layers could be effectively controlled due to restrict of the carbon layer. As we mentioned before, in the anodic scanning of the first cycle in Fig. 4a, there exists an oxidation peak at 1.68 V which is still controversial. Actually, different explanations have been proposed on this peak. For example, Jiang et al. attributed the anodic peak to oxidization of Mo into MoSe2 based on ex-situ XRD [25]. Qian and co-workers suggested that the peak derived from the conversion from Na2Se to Se by using ex-situ Raman technique [32]. Thus, it is still necessary to explore the storage mechanism of Naþ during the intercalation/extraction process. Hence, we used in-situ Raman and ex-situ XRD to investigate the Naþ intercalation/extraction mechanism. Figs. 5a and S11 shows the in situ cell for Raman device and the corresponding data. In particular, Fig. 5c displays in-situ Raman spectra at various discharge/charge states which have been marked on the charge/discharge curves (Fig. 5b) of the first cycle. It can be clearly seen that intensity of Raman peaks at 239 cm1 assigned to A1g mode of MoSe2 gradually weakens. At the same time, the
after 120 cycles, with 87.7% capacity remained. In contrast, the S-M@C electrode shows obviously lower capacity and poorer cycling stability, with the severe capacity fading from the beginning (the discharge and charge specific capacities are 550.2 and 358.6 mAh g1 in the first cycle). More importantly, M@C S-HS shows a superior rate capability compared to S-M@C shown in Fig. 4d. Even at an ultra-high current density of 20 A g1, the as prepared hollow structure still can deliver a discharge capacity of 199.1 mAh g1, exhibiting a better rate performance than that of previous works in Table S2. It means the quick charging and discharging behavior, while maintaining a high capacity. The electrochemical impedance spectra (EIS) of both M@C S-HS and S-M@C were conducted to further confirm the superior kinetics of M@C S-HS. Fig. S9 shows that the samples exhibit the similar geometric shapes with a semicircle and a straight line which represent the charge-transfer process and typical Warburg impedance, respectively. The diameter of semicircle of M@C S-HS is obviously smaller than that of S-M@C, suggesting a lower charge-transfer resistance of the semi-open hollow structure. Apart from this, the impedance slope of the M@C S-HS is higher than that of S-M@C, demonstrating a higher mobility of Naþ in M@C S-HS electrode (Table S3). At the same time, we measured the I–V characteristics of the M@C S-HS, indicating that the material has good conductivity (Fig. S10). Based on above, it could be concluded that the superior cycling performance and rate capability of the M@C S-HS electrode is contributed to the following reasons: (1) the design of semi-open hollow
Fig. 5. (a) In situ cell for Raman device, (b) Discharge and charge curves, (c) in-situ Raman spectra of M@C S-HS electrode of the first cycle. 4
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width of the peak gradually widens indicate the formation of NaxMoSe2 and crystalline of MoSe2 reduced due to the intercalation of Naþ into MoSe2. When the electrode was discharged to 0.5 V and 0.1 V, two peaks at 160 cm1 and 220.8 cm1 which can be indexed to Na2Se started to emerge and became strong. As the charge process proceeds, Raman peaks of Na2Se gradually decays, meanwhile peak at 236.46 cm1 belonging to Se shows up and becomes strong [46]. This result suggests the intercalation of Naþ into MoSe2 result in the formation of NaxMoSe2 before the MoSe2 discharged to 0.7 V. Subsequently, NaxMoSe2 converted to Mo and Na2Se. What is more, Se rather than MoSe2 generated during the charge process [34]. As shown as CV curves, the peak at 1.21 V in second discharge can be attributed to the reduction of Se to Na2Se. After that, the subsequent sweeps repeat the same peaks, indicating the good stability of anode materials. Ex-situ XRD data shown in Fig. S12 also demonstrates that peaks of Na2Se and MoOx (derived from rapid oxidation of Mo in the air during XRD test) can be detected during the discharge process. When the electrode was charged back from 0.01 to 3.0 V, only peaks of MoOx can be observed also implying that MoSe2 is not existence anymore after the full charge. Therefore, the sodium storage mechanism of M@C S-HS electrode in the voltage range of 0.01–3.0 V can be shown as follows: First discharge process:
Notes
MoSe2 þ xNaþ þ xe → NaxMoSe2
(1)
Appendix A. Supplementary data
NaxMoSe2 þ (4-x)Naþ þ (4-x) e → Mo þ 2Na2Se
(2)
Supplementary data to this article can be found online at https://do i.org/10.1016/j.jssc.2019.121159.
(3)
References
The authors declare no competing financial interest.
Declaration of competing interest 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. Acknowledgment This work is financially supported by NSFC (11574280, 11605201, 21727801, U1632151), the Fundamental Research Funds for the Central Universities (Grant No. WK2310000074), and the USTC start-up fund. We thank the Shanghai synchrotron Radiation Facility (14W1 endstation, SSRF), the Heifei Synchrotron Radiation Facility (Photoemission and MCD Endstations, NSRL), and the USTC Center for Micro and Nanoscale Research and Fabrication for helps in characterizations.
First charge process: Na2Se → 2Naþ þ 2e þ Se Following discharge and charge process: (discharge) Se þ 2Naþ þ 2e → Na2Se
(4)
(charge) Na2Se→ 2Naþ þ 2e þ Se
(5)
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3. Conclusion In summary, a hierarchical semi-open hollow structure with MoSe2 nanosheets embedded on nitrogen doped carbon matrix has been successfully synthesized. The unique hollow structure can provide a larger specific area and effectively buffer large volume expansion upon Naþ ions insertion/extraction. When tested as anode material for Na-ion batteries, the M@C S-HS electrodes exhibit an obviously better rate performance and durability in contrast to solid MoSe2 nanospheres. More importantly, the hybrids still can deliver a specific capacity of 199.1 mAh g1 tested at an ultra-high current density of 20 A g1. The Naþ intercalation/extraction mechanism has also been verified by our in-situ Raman and ex-situ XRD results, suggesting the formation of Se substance during the first charge process. This work not only provides a new hollow structure but also deepens the understanding of sodium storage mechanism of MoSe2, which will bring out new insights into designing promising anode material in sodium ion battery. The storage mechanism of those transition metal chalcogenides in batteries can be further studied by means of in-situ synchrotron radiation X-ray diffraction spectroscopy and in situ TEM technology in future. Author contributions L. Song and S. Chen conceived the research and designed the project. C.W. and S.D. performed most of the experiments. C.W and W.Z contributed to transmission electron microscopy characterizations. C.W and Y.Z analyzed the data and co-wrote the paper. All authors discussed the results and commented on the manuscript.
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Hierarchical hollow-structured anode for high-rate sodium-ion battery Chuanqiang Wu, Yu Zhou, Changda Wang, Wen Zhu, Shiqing Ding, Shuangming Chen *, Li Song National Synchrotron Radiation Lab, University of Science and Technology of China, Hefei, Anhui, 230029, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: MoSe2 nanosheets XAFS High rate In-situ Raman Sodium-ion battery
Thanks to its larger surface and free volume, hollow nanostructure and nanomaterials are one kind of highly desirable electrodes for efficiently enhancing sodium-ion battery performance, especially at high rate. Herein, we demonstrate a design of hierarchical semi-open hollow structure, assembling with molybdenum selenide (MoSe2) nanosheets embedded on the nitrogen doped carbon matrix. The unique hollow structure can not only enable sufficient electrode/electrolyte interaction and fast electron transportation, but also suppress volume expansion during the insertion/extraction of sodium ion. As a direct outcome, the MoSe2/carbon anode exhibits excellent cycling and rate performance with a specific capacity of 431 mAh g1 at 200 mA g1, maintaining 87.7% capacity after 120 cycles. Notably, a discharge capacity of 199.1 mAh g1 can be achieved even at an ultra-high current density of 20 A g1, indicating quick Naþ storage ability. The results may open a new way to realize high-rate anode for ion battery applications.
1. Introduction Sodium-ion batteries (SIBs) have attracted researcher’s attention around the world in recent years, due to higher availability, environmental friendliness, and design flexibility. Another important reason is the cheaper price and rich abundance of sodium compared with lithium [1–4]. However, Naþ ions show a larger radius with 1.08 Å than Liþ ions, which greatly increases the diffusion barrier, causing the commercial anode material graphite not suitable for SIBs [5]. Therefore, it is imperative to explore suitable potential anode materials to improve the ability of ions transport and cycling as well as high rate performance [6]. Generally, inorganic nanostructures have important applications in many fields [7–17]. Recently, Molybdenum selenide (MoSe2), one of the member of transition metal dichalcogenides (TMDs) with typical two dimension layered structure, has been widely studied and considered as an ideal anode candidate for SIBs, since it has high theoretical capacity, large interlayer distance, as well as small band gap [18–20]. However, it still suffers from volume expansion during sodiation/desodiation, which will result in capacity attenuation. Other than this, MoSe2 also faces problems of low conductivity and aggregation of layers during cycling [21]. In order to overcome these disadvantages, extensive works have been done to improve the cycle and rate performance [22,23]. One approach is to composite MoSe2 with carbon materials, and MoSe2–C hybrids such as 3D C–MoSe2/reduced graphene oxide, MoSe2/MWCNT, MoSe2/porous carbon spheres have been reported and demonstrated
with enhanced performance for sodium storage. MoSe2 with different morphology has also been synthesized, such as fullerene-like MoSe2, MoSe2 nanoplates and so on [24–27]. Constructing hollow structures has been proven to be an efficient strategy to enhance rate performance, durability and specific capacity for both LIBs and SIBs. Qian group has synthesized hollow nanospheres of mesoporous Co9S8, the material shown outstanding electrochemical lithium storage as anode material. Qiao group has prepared Na2Ti3O7@N-doped carbon hollow spheres for SIBs displayed excellent rate performance. The excellent performance of hollow structures has been contributed to: First, hollow structures can offer the advantages of a large specific surface, short ion diffusion length, and a lot of active sites. Additionally, the voids in the shell can effectively buffer large volume expansion of the materials upon metal ions insertion/extraction [28–31]. Therefore it is promising to synthesis hollow MoSe2 structures for sodium storage. Herein, we designed a hierarchical hollow structure with MoSe2 nanosheets embedded on nitrogen doped carbon matrix. It’s interesting to note that those hollow structures are not completely close up and well dispersed. Employing this special structural materials as anode materials for sodium-ion batteries, the as prepared MoSe2@ nitrogen carbon semiopen hollow nanospheres (abbreviated as M@C S-HS) exhibit an excellent rate performance and durability that is distinct as compared to solid MoSe2@carbon nanospheres (abbreviated as S-M@C) or pristine compounds.
* Corresponding author. E-mail address: [email protected] (S. Chen). https://doi.org/10.1016/j.jssc.2019.121159 Received 14 October 2019; Received in revised form 11 December 2019; Accepted 29 December 2019 Available online 1 January 2020 0022-4596/© 2020 Elsevier Inc. All rights reserved.
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2. Results and discussions
nanospheres do not have hollow structures. The phase and composite of M@C S-HS were confirmed by employing powder XRD and Raman analysis. Diffraction peaks of the calcined powders in Fig. 2a can be indexed to the hexagonal-phase MoSe2 according to the JCPDS 29–0914 [33]. Except for the peaks of MoSe2, peak at 26.35 belongs to carbon. As presented in Fig. 2b, Raman peaks at 239.02 cm1 and 279.6 cm1 are assigned to out of plane (A1 g) and in plane (E12g) mode of MoSe2 in M@C S-HS. Meanwhile, there are two strong peaks at 1335 cm1 and 1584 cm1, corresponding to D-band and G-band of carbon [34–38]. These results suggest that the as-prepared M@C S-HS is composed of MoSe2 and carbon. The content of MoSe2 in M@C S-HS was determined by TG analysis. As showing in Fig. 2c, the increase of weight before 320 C resulted from oxidization of MoSe2. Subsequent weight loss can be attributed to sublimation of SeO2 and oxidization of carbon at about 500 C [39]. Finally, the residual substance is MoO3 and the remaining weight is 39.88%. Accordingly, the weight percentage of MoSe2 can be calculated as about 70.34%. The N2 adsorption and desorption isotherms and pore size distribution in Fig. 2d reveal that M@C S-HS exhibits a specific surface area of 127.02 m2g-1 with typical mesoporous character [39–41]. This means that the semi-open hollow structure is more conducive to the contact between the electrolyte and the anode material. Synchrotron-based X-ray absorption fine structure (XAFS) which can investigate coordination surroundings and the local bond length of a given component as central atom was performed on the samples. Fig. S6 shows Normalized XANES spectra and Fig. 3a shows the oscillation curves of Mo K-edge in M@C S-HS, S-M@C and commercial MoSe2. It is clearly seen that the k3χ (k) oscillation curves of three sample have the similar oscillation shape, suggesting the Mo atoms of M@C S-HS have the similar local atomic arrangements comparing with S-M@C and commercial MoSe2. The Fourier transform (FT) profiles in the real space are shown in Fig. 3b. There are two main coordination peaks in the real space at about 2.14 and 2.93 Å, corresponding to Mo–Se and Mo–Mo bonds, respectively. It can be clearly seen, the intensities of the two peaks (represent coordination number) of M@C S-HS are obviously weaker than that of S-M@C and commercial MoSe2. Detailed fitting results in Fig. S7 and Table S1 illustrate the Mo–Se and Mo–Mo coordination numbers in M@C S-HS are 3.2 and 3.4 less than that of S-M@C and commercial MoSe2 which are 3.9, 4.1 and 6, 6, respectively. The smaller coordination number of MoSe2 in M@C S-HS can be attributed to more
The synthesis procedure of M@C S-HS is schematically illustrated in Fig. 1a. MoO3 microspheres consisted of small MoO3 nanospheres with size about 100 nm were synthesized via a simple solvothermal method. Fig. S1a shows the typical XRD pattern of the MoO3 precursor and Fig. S2a shows the SEM of the MoO3 microspheres. Then MoO3 microspheres as a reactive self-degraded template as well as additional Mo supply were hybrid with AHM and dopamine hydrochloride. After injecting the ammonia solution, Mo-polydopamine formed through polymerization of the Mo-dopamine complex. At the same time MoO3 microspheres dissolved after reaction for 10 min (Fig. S2b), and the small MoO3 nanospheres were served as templates of the hollow structure [32]. After reaction for 2 h, semi-open structures composite of Mo-dopamine with an orange color were obtained (Fig. S1b shows the XRD of the Mo-dopamine complex) and XRD peaks of MoO3 precursor cannot be observed. Finally, MoSe2 embedded on carbon matrix with semi-open hollow structures were achieved through a selenization process of the as resulting Mo-dopamine complex. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were employed to understand the morphology and micro-structure of samples. Fig. 1b and c shows the typical SEM images of M@C S-HS, in which uniform hollow spheres with sizes around 200 nm could be easily observed. Interestingly, most of those hollow spheres display a semi-open shape. The diameters of the voids in the shell are about 100 nm, which are in consistent with size of small MoO3 nanospheres. Besides, hierarchical nanosheets are also observed on the shell of hollow nanospheres. As shown as Fig. 1d, TEM image further demonstrates the nanospheres with hollow structures, consisting of numerous thin nanosheets. The SAED image shows that the M@C S-HS is composed of multicrystal (Fig. S3). HRTEM in Fig. 1e illustrates the nanosheets are formed by carbon matrix with MoSe2 nanosheets embedding. The interplanar spacing of the MoSe2 nanosheets is around 0.648 nm, corresponding to the (002) crystalplane of MoSe2. STEM-EDS with elemental mapping of the M@C S-HS further indicates the homogeneous distribution of the elements Mo, Se and C throughout the entire sample (Fig. S4 and Fig. 1f and g). The additional N elemental mapping suggests N-doping nature in as-prepared nanospheres. As the control samples, solid S-M@C nanospheres were also synthesized. Figs. S5a and 5b show the SEM and TEM images of S-M@C, obviously revealing these solid
Fig. 1. (a) Schematic illustration of the route for synthesis of M@C S-HS. Typical SEM images at low (b) and high (c) magnification of M@C S-HS. (d) TEM and (e) HRTEM, (f, g) STEM image and element mapping of M@C S-HS. 2
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Fig. 2. (a) XRD pattern, (b) Raman spectrum, (c) TGA curve of M@C S-HS and (d) N2 sorption isotherms and pore size distribution of M@C S-HS (the inset picture is image of pore size distribution) and S-M@C.
open hollow structure, the sodium storage electrochemical test of M@C S-HS was performed. Fig. 4a depicts the typical CV curves of the first three cycles of M@C S-HS electrode at a scan rate of 0.2 mV s1 over a potential between 0.01 and 3 V. Two peaks at 0.95 V and 0.39 V can be observed in the first cathodic sweeping, the first one is ascribed to the intercalation of sodium ions into MoSe2 layers and lead to the formation of NaxMoSe2 [44]. Another peak at 0.39 V is assigned to the conversion from NaxMoSe2 to metallic Mo and Na2Se with the electrolyte decomposition to form a solid electrolyte interface (SEI) films [45]. Peaks appear at 1.68 V and 1.21 V in the following cycles are still controversial, and we will discuss the origin of these peaks in the subsequent part. The subsequent CV curves remain steady indicating high reversibility of the electrode. Fig. 4b shows the charge and discharge profiles for as-prepared electrode at a current density of 200 mA g1. The electrode displays the discharge and charge specific capacities of 563.8 and 431mAh g1 in the first cycle, and the coulombic efficiency is estimated as 76.4%. In the following cycles, the coulombic efficiency rises up to nearly 98%. As shown in Fig. 4c, the discharge capacity can still keep at 378.2 mAh g1
surface atoms compared to S-M@C and commercial MoSe2 because of the hollow structure. Wavelet transform (WT) was also used to analyse coordination surroundings of Mo in as prepared samples. As show in Fig. 3c-e, the WT contour plots of the three samples all display two intensity maximum at 7.5 Å1 and 11 Å1, corresponding to Mo–Se and Mo–Mo bonding, respectively. X-ray photoelectron spectroscopy (XPS) measurement was used to identify chemical composition and confirm the existence of N in carbon matrix of the M@C S-HS composites. The fullscan XPS spectrum of prepared M@C S-HS is shown in Fig. S8. Highresolution XPS of Mo, Se, C, N were detected, peaks located at 228.96 and 232.03eV in Fig. 3f can be assigned to Mo 3d5/2 and Mo 3d3/2 of Mo4þ [19,20]. Another two peaks in Fig. 3g at 54.6 and 55.6 eV belong to Se 3d5/2 and Se 3d3/2 of Se2 [22]. The spectrum of C 1s in Fig. 3h can be – C and divided into two peaks, peaks at 284.76 eV for sp2 bonded C– peaks at 285.98eV for C–N [42,43]. Peak of N–C also can be obtained in Fig. 3i for N1s. The XPS results confirm the formation of MoSe2 in M@C S-HS and the existence of N in carbon matrix. To evaluate the energy storage behavior of this hierarchical semi-
Fig. 3. (a) Synchrotron-radiation-based XAFS spectra show the Mo k-edge oscillation curves in k-space, (b) FT analysis in R-space and (c–e) Wavelet transform (WT) of M@C S-HS, S-M@C and commercial MoSe2. XPS spectra of M@C S-HS: (f) Mo 3d, (g) Se 3d (h) C 1s and (i) N 1s. 3
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Fig. 4. (a) CV curves at a scan rate of 0.2 mV s1 in the voltage range of 0.01–3 V and (b) discharge-charge curves at a current density of 200 mAg1 of M@C S-HS. (c) Cycling performance and at a current density of 200 mAg1 and (d) Rate cycling performance of M@C S-HS and S-M@C for sodium-ion batteries.
structure can provide a larger specific area than that of solid nanospheres and bring out more active sites. Meanwhile, it can also cause the M@C SHS electrode contact better with electrolyte, thus reduce the diffusion path of Naþ ions. (2) the unique hollow structure can prevent the volume change upon Naþ ions insertion/extraction. (3) nitrogen doped carbon matrix highly improves the conductivity of the hybrids which could offer fast kinetics for the intercalation/deintercalation of Naþ ion. (4) the aggregation and restack of MoSe2 layers could be effectively controlled due to restrict of the carbon layer. As we mentioned before, in the anodic scanning of the first cycle in Fig. 4a, there exists an oxidation peak at 1.68 V which is still controversial. Actually, different explanations have been proposed on this peak. For example, Jiang et al. attributed the anodic peak to oxidization of Mo into MoSe2 based on ex-situ XRD [25]. Qian and co-workers suggested that the peak derived from the conversion from Na2Se to Se by using ex-situ Raman technique [32]. Thus, it is still necessary to explore the storage mechanism of Naþ during the intercalation/extraction process. Hence, we used in-situ Raman and ex-situ XRD to investigate the Naþ intercalation/extraction mechanism. Figs. 5a and S11 shows the in situ cell for Raman device and the corresponding data. In particular, Fig. 5c displays in-situ Raman spectra at various discharge/charge states which have been marked on the charge/discharge curves (Fig. 5b) of the first cycle. It can be clearly seen that intensity of Raman peaks at 239 cm1 assigned to A1g mode of MoSe2 gradually weakens. At the same time, the
after 120 cycles, with 87.7% capacity remained. In contrast, the S-M@C electrode shows obviously lower capacity and poorer cycling stability, with the severe capacity fading from the beginning (the discharge and charge specific capacities are 550.2 and 358.6 mAh g1 in the first cycle). More importantly, M@C S-HS shows a superior rate capability compared to S-M@C shown in Fig. 4d. Even at an ultra-high current density of 20 A g1, the as prepared hollow structure still can deliver a discharge capacity of 199.1 mAh g1, exhibiting a better rate performance than that of previous works in Table S2. It means the quick charging and discharging behavior, while maintaining a high capacity. The electrochemical impedance spectra (EIS) of both M@C S-HS and S-M@C were conducted to further confirm the superior kinetics of M@C S-HS. Fig. S9 shows that the samples exhibit the similar geometric shapes with a semicircle and a straight line which represent the charge-transfer process and typical Warburg impedance, respectively. The diameter of semicircle of M@C S-HS is obviously smaller than that of S-M@C, suggesting a lower charge-transfer resistance of the semi-open hollow structure. Apart from this, the impedance slope of the M@C S-HS is higher than that of S-M@C, demonstrating a higher mobility of Naþ in M@C S-HS electrode (Table S3). At the same time, we measured the I–V characteristics of the M@C S-HS, indicating that the material has good conductivity (Fig. S10). Based on above, it could be concluded that the superior cycling performance and rate capability of the M@C S-HS electrode is contributed to the following reasons: (1) the design of semi-open hollow
Fig. 5. (a) In situ cell for Raman device, (b) Discharge and charge curves, (c) in-situ Raman spectra of M@C S-HS electrode of the first cycle. 4
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width of the peak gradually widens indicate the formation of NaxMoSe2 and crystalline of MoSe2 reduced due to the intercalation of Naþ into MoSe2. When the electrode was discharged to 0.5 V and 0.1 V, two peaks at 160 cm1 and 220.8 cm1 which can be indexed to Na2Se started to emerge and became strong. As the charge process proceeds, Raman peaks of Na2Se gradually decays, meanwhile peak at 236.46 cm1 belonging to Se shows up and becomes strong [46]. This result suggests the intercalation of Naþ into MoSe2 result in the formation of NaxMoSe2 before the MoSe2 discharged to 0.7 V. Subsequently, NaxMoSe2 converted to Mo and Na2Se. What is more, Se rather than MoSe2 generated during the charge process [34]. As shown as CV curves, the peak at 1.21 V in second discharge can be attributed to the reduction of Se to Na2Se. After that, the subsequent sweeps repeat the same peaks, indicating the good stability of anode materials. Ex-situ XRD data shown in Fig. S12 also demonstrates that peaks of Na2Se and MoOx (derived from rapid oxidation of Mo in the air during XRD test) can be detected during the discharge process. When the electrode was charged back from 0.01 to 3.0 V, only peaks of MoOx can be observed also implying that MoSe2 is not existence anymore after the full charge. Therefore, the sodium storage mechanism of M@C S-HS electrode in the voltage range of 0.01–3.0 V can be shown as follows: First discharge process:
Notes
MoSe2 þ xNaþ þ xe → NaxMoSe2
(1)
Appendix A. Supplementary data
NaxMoSe2 þ (4-x)Naþ þ (4-x) e → Mo þ 2Na2Se
(2)
Supplementary data to this article can be found online at https://do i.org/10.1016/j.jssc.2019.121159.
(3)
References
The authors declare no competing financial interest.
Declaration of competing interest 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. Acknowledgment This work is financially supported by NSFC (11574280, 11605201, 21727801, U1632151), the Fundamental Research Funds for the Central Universities (Grant No. WK2310000074), and the USTC start-up fund. We thank the Shanghai synchrotron Radiation Facility (14W1 endstation, SSRF), the Heifei Synchrotron Radiation Facility (Photoemission and MCD Endstations, NSRL), and the USTC Center for Micro and Nanoscale Research and Fabrication for helps in characterizations.
First charge process: Na2Se → 2Naþ þ 2e þ Se Following discharge and charge process: (discharge) Se þ 2Naþ þ 2e → Na2Se
(4)
(charge) Na2Se→ 2Naþ þ 2e þ Se
(5)
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3. Conclusion In summary, a hierarchical semi-open hollow structure with MoSe2 nanosheets embedded on nitrogen doped carbon matrix has been successfully synthesized. The unique hollow structure can provide a larger specific area and effectively buffer large volume expansion upon Naþ ions insertion/extraction. When tested as anode material for Na-ion batteries, the M@C S-HS electrodes exhibit an obviously better rate performance and durability in contrast to solid MoSe2 nanospheres. More importantly, the hybrids still can deliver a specific capacity of 199.1 mAh g1 tested at an ultra-high current density of 20 A g1. The Naþ intercalation/extraction mechanism has also been verified by our in-situ Raman and ex-situ XRD results, suggesting the formation of Se substance during the first charge process. This work not only provides a new hollow structure but also deepens the understanding of sodium storage mechanism of MoSe2, which will bring out new insights into designing promising anode material in sodium ion battery. The storage mechanism of those transition metal chalcogenides in batteries can be further studied by means of in-situ synchrotron radiation X-ray diffraction spectroscopy and in situ TEM technology in future. Author contributions L. Song and S. Chen conceived the research and designed the project. C.W. and S.D. performed most of the experiments. C.W and W.Z contributed to transmission electron microscopy characterizations. C.W and Y.Z analyzed the data and co-wrote the paper. All authors discussed the results and commented on the manuscript.
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