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carbon composites with high sodium storage capacity
Nano Energy 58 (2019) 392–398
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Full paper
Direct conversion of metal-organic frameworks into selenium/selenide/ carbon composites with high sodium storage capacity ⁎
Xuming Yanga, Shuo Wangb, Denis Y.W. Yub, , Andrey L. Rogacha,
T
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a Department of Materials Science and Engineering, and Center for Functional Photonics (CFP), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong Special Administrative Region b School of Energy and Environment, and Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong Special Administrative Region
A R T I C LE I N FO
A B S T R A C T
Keywords: Sodium ion battery Selenium-based composites Cobalt selenide Metal-organic framework
Selenium (Se)-based materials for sodium (ion) batteries are currently attracting extensive attentions owing to their fast kinetics and excellent cyclability; at the same time, achieving high Se content, which is crucial to maintain the competitive edge over other kinds of electrode materials, still remains a challenge. We developed a confined annealing method which allows us to convert pristine metal-organic frameworks (MOFs) directly into selenium/selenide/carbon composites. It is a simultaneous process of carbonization, selenization and Se vapor deposition, and the combination of elemental Se and selenide results in a record-high Se content of 76 wt%, enhanced capacity and rate capability (490 and 384 mA h g−1 at 0.1 and 2.0 A g−1) exceeding most documented Se-based materials. The produced composites also exhibit excellent cycle stability (no decay for 700 cycles at 2 A g−1), which is correlated to dominant capacitive charge transport mode and the MOF-derived robust structure. Our work not only offers a proof of concept that Se content can be maximized by confining Se through both vapor deposition and chemical bonding with transition metals, but also demonstrates a general and green selenization approach without using any toxic or flammable chemicals. The introduced method will probably prevail for its wide applicability on various metal-containing precursors, and even be expanded to the fabrication of sulfur- and phosphor-based composites.
1. Introduction Capitalizing on the existing knowledge and established technologies from the lithium ion battery field, rechargeable sodium (ion) batteries have advanced tremendously in the past decade [1,2]. Beyond preliminary identification of electrode materials allowing sodium uptake and release, extensive research efforts are currently placed on enabling electrode materials which not only can be easily fabricated, but also excel in capacity, efficiency, cyclability, kinetics and all the other aspects considered for practical battery applications. Recently, inspiring results have been frequently reported on one particular type of materials (selenium and metal selenides) based on conversion reactions with selenium (Se) [3–9]. Several of these Se-based materials demonstrated an initial coulombic efficiency over 80%, and delivered superior rate performances and almost unfading capacity during long cycle testing [4,10,11]. For pure Se, the theoretical specific capacity is 678 mA h g−1, and the average sodiation potential versus sodium metal is around 1.5 V [7]. For the Se-based materials, the theoretical capacity depends
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on the weight ratio of Se, while the average sodiation potential barely changes when Se is composited with carbon hosts or alloyed with transition metals such as iron, cobalt, nickel and copper [4,12–14]. Given the large capacity and moderate sodiation potential, Se-based electrode materials, on one hand, can serve as anodes when matched with high-voltage cathodes like layered transitional metal oxides and polyanions [15]. On the other hand, they can also function as conversion-type cathodes and still offer a competitive energy and power density when paired with sodium metal anode. Compelling electrochemical performance, high initial coulombic efficiency as well as remarkable rate capability and cycle stability have been achieved for Se-based materials both on the basis of rationallystructured design and optimized electrolyte systems [4,11]. The introduction of an ether-based electrolyte (NaSO3CF3 in diethylene glycol dimethyl ether, also termed as “diglyme”) helped to improve initial coulombic efficiency and to significantly enhance cyclability. Still, rapid deterioration of capacity is observed in such ether-based electrolyte for bulk Se-based materials which were not subjected to specific
Corresponding authors. E-mail addresses: [email protected] (D.Y.W. Yu), [email protected] (A.L. Rogach).
https://doi.org/10.1016/j.nanoen.2019.01.064 Received 30 July 2018; Received in revised form 25 January 2019; Accepted 26 January 2019 Available online 28 January 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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2.2. Synthesis of Se-incorporated ZIF-67
treatments, for example compositing with carbon, to overcome expansion and dissolution issues [10,11]. Moreover, it is impractical to stabilize cycling by adding copious amount of carbon, as it will impair the competitiveness of Se-based electrodes in capacity against other candidate electrode materials. Thus, it is crucially important to develop facile synthetic methods to produce Se-based materials with high Se content, which can at the same time encapsulate Se-intermediates during discharge and charge processes. Selenium can be incorporated into carbon matrix via a melt-infusion method, but Se content is hardly controllable in this approach, and the Se loading of obtained composites is typically below 60 wt% [6,7,16,17]. Forming stoichiometric metal selenides is a more reliable way to immobilize Se, but metal selenides are commonly synthesized via solution methods using either toxic hydrazine (N2H4) or explosive sodium borohydride (NaBH4), and carbon components, which would reduce final Se content, are usually still required to extend cycle life [4,18]. Selenization of metal oxides or other metal-containing precursors is an alternative approach to obtain metal selenides, which have been accomplished in literature by immersing precursors in H2Se gas atmosphere or Se vapor under heating [10,19]. H2Se gas is extremely toxic and flammable, and the utilization rate of Se source is low, as large amount of H2Se gas or Se vapor is carried away before reacting with precursors. There is a strong desire in the scientific community to develop green route to prepare metal selenides specifically suitable for secondary ion batteries, where the usage of dangerous chemicals is avoided and waste of Se source is prevented. Herein, metal-organic framework (MOF), namely zeolitic imidazole framework-67, abbreviated as ZIF-67, is straightforwardly converted into Se/CoSe2/C composites after being annealed with Se powder in a sealed vessel. This method does not require the use of dangerous chemicals such as N2H4, NaBH4 and H2Se, and it maximizes the utilization of Se source due to the confined reaction space. The Se loading in the final products is also maximized, as CoSe2 has the highest Se content amongst all cobalt selenides, and there is extra elemental Se embedded within the framework. These two kinds of Se species accounts for 76 wt % of the total weight of the composite, which, to the best of our knowledge, is a record value for Se-based materials exploited in sodium (ion) batteries. The realized composite delivers specific capacity of 490 and 412 mA h g−1 at 0.1 and 1 A g−1 respectively in the abovementioned ether electrolyte, which stands out amongst existing transition metal selenides and Se/C composites [3–7,16,20]. Furthermore, high initial coulombic efficiency (84%) and long-term cycling without capacity decay are demonstrated. High Se content explains the superiority in capacity, and the excellent kinetics and durability originate from the supporting carbon frameworks derived from the ZIF precursors and the high percentage of capacitive response in total charge transfer during electrochemical processes. The introduced space-confined annealing method may become a preferred mean for selenization of various metal-containing precursors, and it is also applicable to the fabrication of metal sulfides and phosphides as well as sulfur/sulfide and phosphor/phosphide composites.
The ZIF-67 crystals (100 mg) were vacuum-sealed in a glass vessel together with selenium powder (500 mg) and annealed at 600 °C for 9 h in a muffle furnace. After cooled down to room temperature, black powder was collected and denoted as Se-ZIF. 2.3. Materials characterization A field emission scanning electron microscope (FESEM, FEI Quanta FEG 450) and a transmission electron microscope (TEM, Philips CM 20) were employed to characterize the morphologies of the samples. A D2 PHASER X-ray diffractometer with Cu Kα (λ = 1.5406 Å) was used to collect the X-ray diffraction (XRD) patterns. Raman spectra were recorded on a Renishaw InVia Raman spectroscope using a 633 nm laser. X-ray photoelectron spectra (XPS) were recorded on a VG ESCALAB 220i-XL spectrometer equipped with a monochromatic Al Kα source. Nitrogen adsorption-desorption isotherms were collected using a Brunauer-Emmett-Teller Analyzer (Micromeritics, ASAP 2020). Thermogravimetric analysis (TGA) has been conducted separately in N2 and air with a heating rate of 5 °C min−1 from ambient temperature to 700 °C on a Mettler Toledo thermogravimetric analyzer. 2.4. Battery fabrication and electrochemical measurements Active materials, acetylene black and sodium carboxymethyl cellulose were mixed in an agate mortar with a weight ratio of 70:15:15. After the addition of an appropriate amount of water, the mixture turned into a viscous slurry under continuous grinding. The slurry was cast onto copper foil and dried at 80 °C for 30 min, then the foil was punched into small discs. After the active material loadings were identified, those circular electrode films were vacuum dried in a glass oven at 110 °C for 4 h and transferred into an Ar-filled glovebox. The mass loading of active materials varied from 0.8 to 1.2 mg cm−2, and the thickness of the electrode was 18 µm. 2032-type coin cells were assembled using the electrode films as working electrode, sodium metal foil as counter electrode, glass fiber as separator, and 1 M NaSO3CF3 in diethylene glycol dimethyl ether (DEGDME) as electrolyte. Galvanostatic tests were conducted on a Neware battery testing system. Cyclic voltammograms (CV) were collected on a multichannel VMP3 (Bio-Logic) electrochemical workstation. 3. Results and discussion With their intrinsic porous structure and metal ions uniformly distributed in organic networks through chemical coordination, MOFs are widely utilized as templates for fabricating electrode materials for electrochemical energy conversion [21,22]. This allows us to provide a specific comparison for the case of ZIF-67 template between the approach introduced in this work and the methods reported by others [23,24]. As schematically illustrated in Fig. 1, ZIF-67 can be processed into three kinds of Se-based composites. Route 1 is the space-confined annealing method we developed here to make Se/CoSe2/C composites. Here, untreated ZIF-67 crystals and Se powders are placed inside a sealed vessel without mixing, and Se vapor produced upon heating infiltrates into ZIF-67, alloys with cobalt ions in the frameworks, and deposits on the surface and pores. Meanwhile, ZIF-67 is carbonized to form a carbon matrix. Alternatively, selenization can also be realized by annealing mixed ZIF-67 and Se powders under flowing inert atmosphere in a tube furnace (Route 2). Following this route, the reported product is CoSe/C composite, because elemental Se is vaporized and removed under high temperature (> 700 °C), and CoSe is more thermally stable than CoSe2 [23]. ZIF-67 can also be used as a sacrificial template to prepare Se/C composites (Route 3) [24]. In that method, ZIF-67 is carbonized and then immersed into an acidic solution to etch cobalt away. After the removal of cobalt, the remaining carbon
2. Material and methods 2.1. Synthesis of ZIF-67crystals 5 mmol of Co(CH3COO)2·2H2O and 100 mmol of 2-methylimidazole were separately dissolved in 50 mL deionized water and mixed under magnetic stirring; the resultant purple solution was transferred into a Teflon lined stainless autoclave and subjected to hydrothermal treatment at 120 °C for 1 h. After it was cooled to room temperature, the purple precipitate was harvested by centrifugation, washed with ethanol and dried at 80 °C overnight.
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content of elemental Se is thus estimated to be 40 wt%, and since there is 18 wt% of the remaining Co3O4, this corresponds to a CoSe2 content of 49 wt%, and a remaining carbon content of 11 wt%. The overall Se content (from elemental Se and Se from CoSe2) is 76 wt% which, to the best of our knowledge, is a record value of Se-based materials applied in sodium (ion) batteries so far. The X-ray photoelectron spectroscopy (XPS) survey spectrum (Fig. 3d) implies that no impurities were introduced into the product during the Se vapor treatment. Chemical states of Co and Se are characterized via high-resolution Co 2p and Se 3d XPS spectra (Fig. 3e and f). The binding energy of Co 2p1/2 and 2p3/2 shifts from 795.7 and 780.2 eV (Fig. S4) to 793.4 and 778.5 eV after selenization, which is in agreement with literature values [10]. The binding energy of Co and Se of CoSe2 is close to that of metallic Co and elemental Se because the CoSe2 is not an ionic but an alloy compound. It is also noted that the carbonization of the organic component results in formation of nitrogen-doped carbon (Fig. S5), which is in favor of electrochemical performance due to enhanced conductivity [8]. From the nitrogen adsorption-desorption isotherms of ZIF-67 and Se-ZIF (Fig. S6), we found that the surface area drastically decreased from 820 to 3 m2 g−1, and the pore volume decreased from 0.44 to 0.014 cm3 g−1. Such a strong change is expected, as pristine metal-organic bonds are destroyed, the organic part is carbonized, the metal is bonded with Se, and the elemental Se is adsorbed into the composite, though the polyhedral morphology is roughly preserved. The sodium storage performance of the obtained Se-ZIF is evaluated with sodium foil as the counter electrode in the ether electrolyte mentioned above. Since carbonized ZIF-67 delivers a negligible capacity (< 25 mA h g−1, Fig. S7), all the electrochemical characteristics of Se-ZIF can be ascribed to the sodiation of Se species. The cyclic voltammograms (CV) at 0.1 mV s−1 are shown in Fig. 4a. Within the initial cathodic segment, two peaks located at 1.17 and 1.05 V are ascribed to step-wise conversion reactions of elemental Se and CoSe2 with sodium. After the initial cycle, the diffraction pattern of CoSe2 totally disappeared as shown in the ex situ XRD results (Fig. S8), and CV curve becomes complex: many peaks appear and keep changing. The CV curve cannot be fixed even after 12 cycles; such evolution has also been observed on various transition metal selenides tested in the ether electrolyte [4,10,11,31]. Despite the changing CV curves, the specific capacity of Se-ZIF is basically constant during the 100 cycles at 0.1 A g−1 (Fig. 4c). The discharge and charge capacity at the first cycle are 663 and 557 mA h g−1, and the coulombic efficiency is 84%. The reversible capacity drops to 510 mA h g−1 in the second cycle and stabilizes afterwards at around 490 mA h g−1. Voltage profiles recorded at the selected cycles are presented in Fig. 4b, and the evolution of discharge and charge plateaus are consistent with that of CV measurements. After 60 cycles, the profiles virtually overlap. CV curves of the cell after 100 cycles are shown in Fig. 3d; they completely overlap from cycle to cycle. We notice that the shape of the CV curves is identical with several previously reported Se-based materials (FeSe2, Fe7Se8, CoSe2), and with Se/graphene composite tested in the ether electrolyte [4,10,11,31]. The Raman spectrum of the electrode after 100 cycles is shown as Fig. 3e. The signal of CoSe2 (~190 cm−1) disappear completely, and the peak in the range of 260–280 cm−1 is ascribed to Se. It suggests that a gradual segregation of Co and Se occurs upon cycling, which can explain the observed evolution of the electrochemical data, and the fact that the Se/CoSe2/C composite shows the same final CV curve shape as transition metal selenides and Se/C composites do. Fortunately, cycle capacity is independent on the evolution of the voltage profiles, with almost no decay after 700 discharge/ charge cycles at 2 A g−1 (Fig. 4f). Such impressive cycle stability is frequently observed from Se-based materials tested in the ether electrolyte. As elucidated in the pioneering works introducing this kind of electrolyte, the compact solid electrolyte interface (SEI) film is able to protect the active material from side reactions [4]. The ex situ SEM image of the cycled electrode (Fig. S9) shows that polyhedral shape of the active material is largely preserved. We attribute the observed high
Fig. 1. Schematic illustration of the three routes of converting ZIF-67 into Sebased composites.
frameworks are collected from the suspension, dried, intensively mixed with Se powder, and heated at an elevated temperature (generally 260 °C) over the melting point of Se to allow enough time for the Se melt to completely infuse into the pores. Within this melt-infusion processing, the weight ratio of carbon frameworks and Se powder should be optimized by multiple trials, with each trial involving time consuming mixing and heating, electrode fabrication and battery testing. As compared with Routes 1 and 2, Route 3 is rather tedious, and cobalt is not utilized but sacrificed; still, this melt-infusion method remains the most prevailing one to load Se (also sulfur and phosphor) into MOFs [25–28]. We will show in the following how our suggested method (Route 1 in Fig. 1) not only greatly simplifies the synthesis process, but also increases the Se content by confining Se in the resulting composites both via vapor deposition on the frameworks and through the chemical bonding with cobalt. To follow the changes of the precursor materials after the spaceconfined annealing treatment in Se vapor, several characterization techniques were employed to compare the pristine ZIF-67 and the ZIFderived Se/CoSe2/C composite (which we denote as Se-ZIF further on). Scanning electron microscopy (SEM) images collected at different magnifications are displayed in Fig. 2. The ZIF-67 crystals whose structure has been confirmed by the X-ray diffraction (XRD) pattern shown in Fig. S1 are dodecahedra with uniform size (~1.3 µm) and clean surface. Their shape and size are barely changed after annealing with Se, but the surface slightly collapsed due to the carbonization of the organic ligands, and some cavities are observed (Fig. 2d). The energy dispersive X-ray (EDX) elemental mapping (Fig. S2) indicates uniform distribution of Se, Co and C in the composite, and the intense Se peak in the EDX spectrum (Fig. S3) suggests a high Se content. The XRD pattern of Se-ZIF (Fig. 3a) reveals a crystalline form of cobalt selenide (CoSe2, PDF#89-2002), which has the highest Se/Co atomic ratio amongst various Co-Se compounds including CoSe, Co0.85Se, Co3Se4, Co8Se9, etc.. The Raman spectrum (Fig. 3b) features a sharp peak at 189 cm−1 corresponding to Se-Se stretching mode of CoSe2, and a broad peak at 250 cm−1 corresponding to Se-Se vibrational mode of elemental Se chains which are amorphous in nature [29,30]. The relative content of these two Se species are quantified by thermogravimetric analysis (TGA), as presented in Fig. 3c. The weight loss in N2 atmosphere originates from the vaporization of elemental Se, and the remaining weight in air is attributed to Co3O4 after annealing. The 394
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Fig. 2. SEM images of (a, b) ZIF-67 and (c, d) Se-ZIF at different magnifications.
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Fig. 3. (a) XRD pattern, (b) Raman spectrum, (c) TGA curves, (d) XPS survey spectrum, (e) core-level Co 2p and (f) Se 3d XPS spectra of the Se-ZIF composite. 395
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specific capacity of 490, 457, 431, 412, 384 and 228 mA h g−1, respectively. The capacity retention is as high as 78% when the current is increased from 0.1 to 2 A g−1. As displayed in Fig. 5b, the decrease in the discharge plateaus and the increase in the charge plateaus are small. In Fig. 5c, we offer a comparison of the rate performances among SeZIF, Se/C composites and transition metal selenides which have been previously applied in sodium (ion) batteries [3–6,11–13,20,33]. Given Se-ZIF is a Se/CoSe2/C composite, a more comprehensive comparison
cycle stability of Se-ZIF to the existence of the robust ZIF-derived framework and the stable SEI formed in the ether electrolyte. The SEI film is thin, so that irreversible capacity caused by SEI formation and the energy barrier of sodium ion transportation across SEI films are low [32]. That is why high initial coulombic efficiency and excellent rate capability are also frequently observed in this electrolyte. The rate performance of Se-ZIF is shown in Fig. 5a. At the rates of 0.1, 0.2, 0.5, 1, 2 and 5 A g−1, the composite reaches a reversible
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Fig. 5. (a) Rate performance, (b) voltage profiles at various rates, (c) comparison with reported Se/C and transition metal selenide, (d) CV curves at various scan rates, (e) peak current dependence on scan rate and (f) visualized pseudocapacitive contribution at 0.4 mV s−1 of the Se-ZIF electrode. 396
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with Se/C and cobalt selenides is also provided (Table S1). The delivered capacity of Se-ZIF is amongst the best, and the superiority over the reported Se-based materials is mainly attributed to the high Se weight ratio (76 wt%). It exceeds various pure transition metal selenides (FeSe2: 74%; CoSe2: 73%; NiSe2: 73%; CuSe2: 71%; ZnSe: 55%; MoSe2: 62% weight percentage of Se, respectively), and the advantage would be even larger when these selenides are composited with carbon. To achieve a better understanding of the exceptional rate capability of Se-ZIF electrodes, the dependence of CV peak current on the scan rate, and the charge transport mode (capacitive versus diffusion-limited) are analyzed. The CV curves measured at 0.1, 0.2, 0.4, 0.8 and 1.6 mV s−1 after long cycles are shown in Fig. 5d. The peak currents of the six labeled peaks (P1-P6) are plotted versus scan rate with both xand y-axis given in logarithmic scale. In such presentation, the electrochemical reaction is purely diffusion-limited (slow) when the slope equals 0.5, and it is capacitive (rapid) when the slope equals 1. In the case of Se-ZIF, the slopes calculated by linear fitting for P1-P6 peaks are 0.92, 0.75, 0.99, 0.69, 0.73 and 0.94, respectively. This implies that reactions corresponding to these redox peaks are controlled by both modes, with half of them (P1, P3 and P6) being close to pure capacitive mode. To determine the percentage of the two modes, the CV curves are analyzed according to the equation: i(V) = k1v + k2v1/2, where i(V) is the current at a fixed potential (V), v is the scan rate, k1 and k2 are coefficients to be resolved by fitting [34]. The pseudocapacitive contribution at 0.4 mV s−1 is presented as a shaded area inside the CV curve (Fig. 5f), and its percentage is quantified as the area fraction of the shaded region. The overall pseudocapacitive contribution within the cut-off voltage range reaches 77%, and the dominant capacitive reaction mode agrees with the fast kinetics.
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Mitlin, Selenium impregnated monolithic carbons as free-standing cathodes for high volumetric energy lithium and sodium metal batteries, Adv. Energy Mater. 8 (2017) 170198. [18] F. Zhang, C. Xia, J. Zhu, B. Ahmed, H. Liang, D.B. Velusamy, U. Schwingenschlögl, H.N. Alshareef, SnSe2 2D anodes for advanced sodium ion batteries, Adv. Energy Mater. 6 (2016) 1601188. [19] Y.J. Hong, J.H. Kim, Y. Chan Kang, Sodium-ion storage performance of hierarchically structured (Co1/3Fe2/3)Se2 nanofibers with fiber-in-tube nanostructures, J. Mater. Chem. A 4 (2016) 15471–15477. [20] F. Niu, J. Yang, N. Wang, D. Zhang, W. Fan, J. Yang, Y. Qian, MoSe2‐Covered N,P‐Doped Carbon Nanosheets as a long-life and high-rate anode material for sodium-ion batteries, Adv. Funct. Mater. 27 (2017) 1700522. [21] G. Zou, H. Hou, P. Ge, Z. Huang, G. Zhao, D. Yin, X. Ji, Metal-organic frameworkderived materials for sodium energy storage, Small 14 (2018) 1702648. [22] G. Maurin, C. Serre, A. Cooper, G. Ferey, The new age of MOFs and of their porousrelated solids, Chem. Soc. Rev. 46 (2017) 3104–3107. [23] Y. Zhang, A. Pan, L. Ding, Z. Zhou, Y. Wang, S. Niu, S. Liang, G. Cao, Nitrogendoped yolk-shell-structured CoSe/C dodecahedra for high-performance sodium ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 3624–3633. [24] Q. Xu, T. Liu, Y. Li, L. Hu, C. Dai, Y. Zhang, Y. Li, D. Liu, M. Xu, Selenium encapsulated into metal–organic frameworks derived N-doped porous carbon polyhedrons as cathode for Na-Se batteries, ACS Appl. Mater. Interfaces 9 (2017) 41339–41346. [25] S.-K. Park, J.-S. Park, Y.C. Kang, Selenium-infiltrated metal-organic frameworkderived porous carbon nanofibers comprising interconnected bimodal pores for LiSe batteries with high capacity and rate performance, J. Mater. Chem. A 6 (2018) 1028–1036. [26] X. Cao, C. Tan, M. Sindoro, H. Zhang, Hybrid micro-/nano-structures derived from metal-organic frameworks: preparation and applications in energy storage and conversion, Chem. Soc. Rev. 46 (2017) 2660–2677. [27] W.H. Li, S.H. Hu, X.Y. Luo, Z.L. Li, X.Z. Sun, M.S. Li, F.F. Liu, Y. Yu, Confined amorphous red phosphorus in MOF-derived N-doped microporous carbon as a superior anode for sodium-ion battery, Adv. Mater. 29 (2017) 1605820. [28] S.-K. Park, J.-S. Park, Y.C. Kang, Metal-organic-framework-derived N-doped hierarchically porous carbon polyhedrons anchored on crumpled graphene balls as efficient selenium hosts for high-performance lithium-selenium batteries, ACS Appl. Mater. Interfaces 10 (2018) 16531–16540. [29] S.N. Yannopoulos, K.S. Andrikopoulos, Raman scattering study on structural and dynamical features of noncrystalline selenium, J. Chem. Phys. 121 (2004) 4747–4758. [30] D. Kong, H. Wang, Z. Lu, Y. Cui, CoSe2 nanoparticles grown on carbon fiber paper: an efficient and stable electrocatalyst for hydrogen evolution reaction, J. Am. Chem. Soc. 136 (2014) 4897–4900. [31] X. Yang, J. Wang, S. Wang, H. Wang, O. Tomanec, C. Zhi, R. Zboril, D.Y.W. Yu, A. Rogach, Vapor-infiltration approach toward selenium/reduced graphene oxide composites enabling stable and high-capacity sodium storage, ACS Nano 12 (2018)
4. Conclusions In summary, a Se/CoSe2/C composite fabricated from ZIF-67 via space-confined annealing with Se powders possesses a high sodium storage capacity, remarkable stability and rate capability, which are amongst the best between all Se-based electrode materials reported so far. Such superior performance is attributed to the high Se content (76 wt%), the ZIF-derived robust framework, and large pseudocapacitive contributions. We demonstrated the feasibility of increasing Se content, which is critical for Se-based materials, by combining vapor deposition and chemical bonding with metals. The introduced spaceconfined annealing method is simple, effective, and applicable for many other kinds of metal-containing precursors, so it has a potential to become a popular fabrication approach towards Se-based electrode materials in the future. In particular, this method can be easily modified to fabricate sulfur- and phosphor-based materials by replacing Se with sulfur and phosphor. Acknowledgements This work was supported by City University of Hong Kong (SRG project 7005080) and by the Research Grant Council of Hong Kong S.A.R. (project CityU 21202014). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.nanoen.2019.01.064. References [1] J.Y. Hwang, S.T. Myung, Y.K. Sun, Sodium-Ion batteries: present and future, Chem. Soc. Rev. 46 (2017) 3529–3614. [2] C. Delmas, Sodium and sodium-ion batteries: 50 years of research, Adv. Energy Mater. 8 (2018) 1703137. [3] K. Zhang, M. Park, L. Zhou, G.H. Lee, W. Li, Y.M. Kang, J. Chen, Urchin-like CoSe2 as a high-performance anode material for sodium-ion batteries, Adv. Funct. Mater.
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Dr. Denis Y. W. Yu is an Associate Professor at the School of Energy and Environment, City University of Hong Kong. He received his Ph.D. in Applied Physics from the School of Engineering and Applied Sciences at Harvard University in 2003. He then worked as an engineer at SANYO Electric Co. Ltd. in Japan, developing cathode and anode materials for Li-ion batteries. Afterwards, he led the battery activities at the Energy Research Institute at Nanyang Technological University and TUM CREATE Centre for Electromobility in Singapore as a senior scientist. His research interests include fabrication, development and characterization of materials and electrodes for energy storage applications.
7397–7405. [32] J. Zhang, D.-W. Wang, W. Lv, S. Zhang, Q. Liang, D. Zheng, F. Kang, Q.-H. Yang, Achieving superb sodium storage performance on carbon anodes through an etherderived solid electrolyte interphase, Energy Environ. Sci. 10 (2017) 370–376. [33] X. Yang, J. Zhang, Z. Wang, H. Wang, C. Zhi, D.Y.W. Yu, A.L. Rogach, Carbonsupported nickel selenide hollow nanowires as advanced anode materials for sodium-ion batteries, Small 14 (2018) 1702669. [34] T. Brezesinski, J. Wang, S.H. Tolbert, B. Dunn, Ordered mesoporous [alpha]-MoO3 with iso-oriented nanocrystalline walls for thin-film pseudocapacitors, Nat. Mater. 9 (2010) 146–151.
Mr. Xuming Yang received his Bachelor degree in Applied Chemistry in 2013 and Master degree in Physical Chemistry in 2016 from Central South University, China. Since 2016, he is working on his Ph.D. under the supervision of Prof. Andrey Rogach at City University of Hong Kong. His research is focused on materials for electrochemical energy storage and conversion.
Prof. Andrey L. Rogach is a Chair Professor of Photonics Materials and the Founding Director of the Centre for Functional Photonics at City University of Hong Kong. He received his Diploma in Chemistry and Ph.D. in Physical Chemistry from the Belarusian State University in Minsk (Belarus), and worked as a staff scientist at the Institute of Physical Chemistry of the University of Hamburg (1995–2002), and at the Department of Physics of the Ludwig-Maximilians-University of Munich (2002–2009), Germany, where he completed his Habilitation in experimental physics. His research focuses on synthesis, assembly and optical spectroscopy of nanomaterials, and their use for energy and optoelectronic applications. Andrey Rogach is an Associate Editor of ACS Nano.
Mr. Shuo Wang received his Bachelor degree from Zhengzhou University in 2012 and his master degree from City University of Hong Kong in 2015. After graduation, he joined Dr. Denis Yu's group as a Ph.D. student. His research interests include the fabrication and experimental mechanism exploration of electrode materials for lithium and sodium ion batteries.
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Full paper
Direct conversion of metal-organic frameworks into selenium/selenide/ carbon composites with high sodium storage capacity ⁎
Xuming Yanga, Shuo Wangb, Denis Y.W. Yub, , Andrey L. Rogacha,
T
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a Department of Materials Science and Engineering, and Center for Functional Photonics (CFP), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong Special Administrative Region b School of Energy and Environment, and Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong Special Administrative Region
A R T I C LE I N FO
A B S T R A C T
Keywords: Sodium ion battery Selenium-based composites Cobalt selenide Metal-organic framework
Selenium (Se)-based materials for sodium (ion) batteries are currently attracting extensive attentions owing to their fast kinetics and excellent cyclability; at the same time, achieving high Se content, which is crucial to maintain the competitive edge over other kinds of electrode materials, still remains a challenge. We developed a confined annealing method which allows us to convert pristine metal-organic frameworks (MOFs) directly into selenium/selenide/carbon composites. It is a simultaneous process of carbonization, selenization and Se vapor deposition, and the combination of elemental Se and selenide results in a record-high Se content of 76 wt%, enhanced capacity and rate capability (490 and 384 mA h g−1 at 0.1 and 2.0 A g−1) exceeding most documented Se-based materials. The produced composites also exhibit excellent cycle stability (no decay for 700 cycles at 2 A g−1), which is correlated to dominant capacitive charge transport mode and the MOF-derived robust structure. Our work not only offers a proof of concept that Se content can be maximized by confining Se through both vapor deposition and chemical bonding with transition metals, but also demonstrates a general and green selenization approach without using any toxic or flammable chemicals. The introduced method will probably prevail for its wide applicability on various metal-containing precursors, and even be expanded to the fabrication of sulfur- and phosphor-based composites.
1. Introduction Capitalizing on the existing knowledge and established technologies from the lithium ion battery field, rechargeable sodium (ion) batteries have advanced tremendously in the past decade [1,2]. Beyond preliminary identification of electrode materials allowing sodium uptake and release, extensive research efforts are currently placed on enabling electrode materials which not only can be easily fabricated, but also excel in capacity, efficiency, cyclability, kinetics and all the other aspects considered for practical battery applications. Recently, inspiring results have been frequently reported on one particular type of materials (selenium and metal selenides) based on conversion reactions with selenium (Se) [3–9]. Several of these Se-based materials demonstrated an initial coulombic efficiency over 80%, and delivered superior rate performances and almost unfading capacity during long cycle testing [4,10,11]. For pure Se, the theoretical specific capacity is 678 mA h g−1, and the average sodiation potential versus sodium metal is around 1.5 V [7]. For the Se-based materials, the theoretical capacity depends
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on the weight ratio of Se, while the average sodiation potential barely changes when Se is composited with carbon hosts or alloyed with transition metals such as iron, cobalt, nickel and copper [4,12–14]. Given the large capacity and moderate sodiation potential, Se-based electrode materials, on one hand, can serve as anodes when matched with high-voltage cathodes like layered transitional metal oxides and polyanions [15]. On the other hand, they can also function as conversion-type cathodes and still offer a competitive energy and power density when paired with sodium metal anode. Compelling electrochemical performance, high initial coulombic efficiency as well as remarkable rate capability and cycle stability have been achieved for Se-based materials both on the basis of rationallystructured design and optimized electrolyte systems [4,11]. The introduction of an ether-based electrolyte (NaSO3CF3 in diethylene glycol dimethyl ether, also termed as “diglyme”) helped to improve initial coulombic efficiency and to significantly enhance cyclability. Still, rapid deterioration of capacity is observed in such ether-based electrolyte for bulk Se-based materials which were not subjected to specific
Corresponding authors. E-mail addresses: [email protected] (D.Y.W. Yu), [email protected] (A.L. Rogach).
https://doi.org/10.1016/j.nanoen.2019.01.064 Received 30 July 2018; Received in revised form 25 January 2019; Accepted 26 January 2019 Available online 28 January 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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2.2. Synthesis of Se-incorporated ZIF-67
treatments, for example compositing with carbon, to overcome expansion and dissolution issues [10,11]. Moreover, it is impractical to stabilize cycling by adding copious amount of carbon, as it will impair the competitiveness of Se-based electrodes in capacity against other candidate electrode materials. Thus, it is crucially important to develop facile synthetic methods to produce Se-based materials with high Se content, which can at the same time encapsulate Se-intermediates during discharge and charge processes. Selenium can be incorporated into carbon matrix via a melt-infusion method, but Se content is hardly controllable in this approach, and the Se loading of obtained composites is typically below 60 wt% [6,7,16,17]. Forming stoichiometric metal selenides is a more reliable way to immobilize Se, but metal selenides are commonly synthesized via solution methods using either toxic hydrazine (N2H4) or explosive sodium borohydride (NaBH4), and carbon components, which would reduce final Se content, are usually still required to extend cycle life [4,18]. Selenization of metal oxides or other metal-containing precursors is an alternative approach to obtain metal selenides, which have been accomplished in literature by immersing precursors in H2Se gas atmosphere or Se vapor under heating [10,19]. H2Se gas is extremely toxic and flammable, and the utilization rate of Se source is low, as large amount of H2Se gas or Se vapor is carried away before reacting with precursors. There is a strong desire in the scientific community to develop green route to prepare metal selenides specifically suitable for secondary ion batteries, where the usage of dangerous chemicals is avoided and waste of Se source is prevented. Herein, metal-organic framework (MOF), namely zeolitic imidazole framework-67, abbreviated as ZIF-67, is straightforwardly converted into Se/CoSe2/C composites after being annealed with Se powder in a sealed vessel. This method does not require the use of dangerous chemicals such as N2H4, NaBH4 and H2Se, and it maximizes the utilization of Se source due to the confined reaction space. The Se loading in the final products is also maximized, as CoSe2 has the highest Se content amongst all cobalt selenides, and there is extra elemental Se embedded within the framework. These two kinds of Se species accounts for 76 wt % of the total weight of the composite, which, to the best of our knowledge, is a record value for Se-based materials exploited in sodium (ion) batteries. The realized composite delivers specific capacity of 490 and 412 mA h g−1 at 0.1 and 1 A g−1 respectively in the abovementioned ether electrolyte, which stands out amongst existing transition metal selenides and Se/C composites [3–7,16,20]. Furthermore, high initial coulombic efficiency (84%) and long-term cycling without capacity decay are demonstrated. High Se content explains the superiority in capacity, and the excellent kinetics and durability originate from the supporting carbon frameworks derived from the ZIF precursors and the high percentage of capacitive response in total charge transfer during electrochemical processes. The introduced space-confined annealing method may become a preferred mean for selenization of various metal-containing precursors, and it is also applicable to the fabrication of metal sulfides and phosphides as well as sulfur/sulfide and phosphor/phosphide composites.
The ZIF-67 crystals (100 mg) were vacuum-sealed in a glass vessel together with selenium powder (500 mg) and annealed at 600 °C for 9 h in a muffle furnace. After cooled down to room temperature, black powder was collected and denoted as Se-ZIF. 2.3. Materials characterization A field emission scanning electron microscope (FESEM, FEI Quanta FEG 450) and a transmission electron microscope (TEM, Philips CM 20) were employed to characterize the morphologies of the samples. A D2 PHASER X-ray diffractometer with Cu Kα (λ = 1.5406 Å) was used to collect the X-ray diffraction (XRD) patterns. Raman spectra were recorded on a Renishaw InVia Raman spectroscope using a 633 nm laser. X-ray photoelectron spectra (XPS) were recorded on a VG ESCALAB 220i-XL spectrometer equipped with a monochromatic Al Kα source. Nitrogen adsorption-desorption isotherms were collected using a Brunauer-Emmett-Teller Analyzer (Micromeritics, ASAP 2020). Thermogravimetric analysis (TGA) has been conducted separately in N2 and air with a heating rate of 5 °C min−1 from ambient temperature to 700 °C on a Mettler Toledo thermogravimetric analyzer. 2.4. Battery fabrication and electrochemical measurements Active materials, acetylene black and sodium carboxymethyl cellulose were mixed in an agate mortar with a weight ratio of 70:15:15. After the addition of an appropriate amount of water, the mixture turned into a viscous slurry under continuous grinding. The slurry was cast onto copper foil and dried at 80 °C for 30 min, then the foil was punched into small discs. After the active material loadings were identified, those circular electrode films were vacuum dried in a glass oven at 110 °C for 4 h and transferred into an Ar-filled glovebox. The mass loading of active materials varied from 0.8 to 1.2 mg cm−2, and the thickness of the electrode was 18 µm. 2032-type coin cells were assembled using the electrode films as working electrode, sodium metal foil as counter electrode, glass fiber as separator, and 1 M NaSO3CF3 in diethylene glycol dimethyl ether (DEGDME) as electrolyte. Galvanostatic tests were conducted on a Neware battery testing system. Cyclic voltammograms (CV) were collected on a multichannel VMP3 (Bio-Logic) electrochemical workstation. 3. Results and discussion With their intrinsic porous structure and metal ions uniformly distributed in organic networks through chemical coordination, MOFs are widely utilized as templates for fabricating electrode materials for electrochemical energy conversion [21,22]. This allows us to provide a specific comparison for the case of ZIF-67 template between the approach introduced in this work and the methods reported by others [23,24]. As schematically illustrated in Fig. 1, ZIF-67 can be processed into three kinds of Se-based composites. Route 1 is the space-confined annealing method we developed here to make Se/CoSe2/C composites. Here, untreated ZIF-67 crystals and Se powders are placed inside a sealed vessel without mixing, and Se vapor produced upon heating infiltrates into ZIF-67, alloys with cobalt ions in the frameworks, and deposits on the surface and pores. Meanwhile, ZIF-67 is carbonized to form a carbon matrix. Alternatively, selenization can also be realized by annealing mixed ZIF-67 and Se powders under flowing inert atmosphere in a tube furnace (Route 2). Following this route, the reported product is CoSe/C composite, because elemental Se is vaporized and removed under high temperature (> 700 °C), and CoSe is more thermally stable than CoSe2 [23]. ZIF-67 can also be used as a sacrificial template to prepare Se/C composites (Route 3) [24]. In that method, ZIF-67 is carbonized and then immersed into an acidic solution to etch cobalt away. After the removal of cobalt, the remaining carbon
2. Material and methods 2.1. Synthesis of ZIF-67crystals 5 mmol of Co(CH3COO)2·2H2O and 100 mmol of 2-methylimidazole were separately dissolved in 50 mL deionized water and mixed under magnetic stirring; the resultant purple solution was transferred into a Teflon lined stainless autoclave and subjected to hydrothermal treatment at 120 °C for 1 h. After it was cooled to room temperature, the purple precipitate was harvested by centrifugation, washed with ethanol and dried at 80 °C overnight.
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content of elemental Se is thus estimated to be 40 wt%, and since there is 18 wt% of the remaining Co3O4, this corresponds to a CoSe2 content of 49 wt%, and a remaining carbon content of 11 wt%. The overall Se content (from elemental Se and Se from CoSe2) is 76 wt% which, to the best of our knowledge, is a record value of Se-based materials applied in sodium (ion) batteries so far. The X-ray photoelectron spectroscopy (XPS) survey spectrum (Fig. 3d) implies that no impurities were introduced into the product during the Se vapor treatment. Chemical states of Co and Se are characterized via high-resolution Co 2p and Se 3d XPS spectra (Fig. 3e and f). The binding energy of Co 2p1/2 and 2p3/2 shifts from 795.7 and 780.2 eV (Fig. S4) to 793.4 and 778.5 eV after selenization, which is in agreement with literature values [10]. The binding energy of Co and Se of CoSe2 is close to that of metallic Co and elemental Se because the CoSe2 is not an ionic but an alloy compound. It is also noted that the carbonization of the organic component results in formation of nitrogen-doped carbon (Fig. S5), which is in favor of electrochemical performance due to enhanced conductivity [8]. From the nitrogen adsorption-desorption isotherms of ZIF-67 and Se-ZIF (Fig. S6), we found that the surface area drastically decreased from 820 to 3 m2 g−1, and the pore volume decreased from 0.44 to 0.014 cm3 g−1. Such a strong change is expected, as pristine metal-organic bonds are destroyed, the organic part is carbonized, the metal is bonded with Se, and the elemental Se is adsorbed into the composite, though the polyhedral morphology is roughly preserved. The sodium storage performance of the obtained Se-ZIF is evaluated with sodium foil as the counter electrode in the ether electrolyte mentioned above. Since carbonized ZIF-67 delivers a negligible capacity (< 25 mA h g−1, Fig. S7), all the electrochemical characteristics of Se-ZIF can be ascribed to the sodiation of Se species. The cyclic voltammograms (CV) at 0.1 mV s−1 are shown in Fig. 4a. Within the initial cathodic segment, two peaks located at 1.17 and 1.05 V are ascribed to step-wise conversion reactions of elemental Se and CoSe2 with sodium. After the initial cycle, the diffraction pattern of CoSe2 totally disappeared as shown in the ex situ XRD results (Fig. S8), and CV curve becomes complex: many peaks appear and keep changing. The CV curve cannot be fixed even after 12 cycles; such evolution has also been observed on various transition metal selenides tested in the ether electrolyte [4,10,11,31]. Despite the changing CV curves, the specific capacity of Se-ZIF is basically constant during the 100 cycles at 0.1 A g−1 (Fig. 4c). The discharge and charge capacity at the first cycle are 663 and 557 mA h g−1, and the coulombic efficiency is 84%. The reversible capacity drops to 510 mA h g−1 in the second cycle and stabilizes afterwards at around 490 mA h g−1. Voltage profiles recorded at the selected cycles are presented in Fig. 4b, and the evolution of discharge and charge plateaus are consistent with that of CV measurements. After 60 cycles, the profiles virtually overlap. CV curves of the cell after 100 cycles are shown in Fig. 3d; they completely overlap from cycle to cycle. We notice that the shape of the CV curves is identical with several previously reported Se-based materials (FeSe2, Fe7Se8, CoSe2), and with Se/graphene composite tested in the ether electrolyte [4,10,11,31]. The Raman spectrum of the electrode after 100 cycles is shown as Fig. 3e. The signal of CoSe2 (~190 cm−1) disappear completely, and the peak in the range of 260–280 cm−1 is ascribed to Se. It suggests that a gradual segregation of Co and Se occurs upon cycling, which can explain the observed evolution of the electrochemical data, and the fact that the Se/CoSe2/C composite shows the same final CV curve shape as transition metal selenides and Se/C composites do. Fortunately, cycle capacity is independent on the evolution of the voltage profiles, with almost no decay after 700 discharge/ charge cycles at 2 A g−1 (Fig. 4f). Such impressive cycle stability is frequently observed from Se-based materials tested in the ether electrolyte. As elucidated in the pioneering works introducing this kind of electrolyte, the compact solid electrolyte interface (SEI) film is able to protect the active material from side reactions [4]. The ex situ SEM image of the cycled electrode (Fig. S9) shows that polyhedral shape of the active material is largely preserved. We attribute the observed high
Fig. 1. Schematic illustration of the three routes of converting ZIF-67 into Sebased composites.
frameworks are collected from the suspension, dried, intensively mixed with Se powder, and heated at an elevated temperature (generally 260 °C) over the melting point of Se to allow enough time for the Se melt to completely infuse into the pores. Within this melt-infusion processing, the weight ratio of carbon frameworks and Se powder should be optimized by multiple trials, with each trial involving time consuming mixing and heating, electrode fabrication and battery testing. As compared with Routes 1 and 2, Route 3 is rather tedious, and cobalt is not utilized but sacrificed; still, this melt-infusion method remains the most prevailing one to load Se (also sulfur and phosphor) into MOFs [25–28]. We will show in the following how our suggested method (Route 1 in Fig. 1) not only greatly simplifies the synthesis process, but also increases the Se content by confining Se in the resulting composites both via vapor deposition on the frameworks and through the chemical bonding with cobalt. To follow the changes of the precursor materials after the spaceconfined annealing treatment in Se vapor, several characterization techniques were employed to compare the pristine ZIF-67 and the ZIFderived Se/CoSe2/C composite (which we denote as Se-ZIF further on). Scanning electron microscopy (SEM) images collected at different magnifications are displayed in Fig. 2. The ZIF-67 crystals whose structure has been confirmed by the X-ray diffraction (XRD) pattern shown in Fig. S1 are dodecahedra with uniform size (~1.3 µm) and clean surface. Their shape and size are barely changed after annealing with Se, but the surface slightly collapsed due to the carbonization of the organic ligands, and some cavities are observed (Fig. 2d). The energy dispersive X-ray (EDX) elemental mapping (Fig. S2) indicates uniform distribution of Se, Co and C in the composite, and the intense Se peak in the EDX spectrum (Fig. S3) suggests a high Se content. The XRD pattern of Se-ZIF (Fig. 3a) reveals a crystalline form of cobalt selenide (CoSe2, PDF#89-2002), which has the highest Se/Co atomic ratio amongst various Co-Se compounds including CoSe, Co0.85Se, Co3Se4, Co8Se9, etc.. The Raman spectrum (Fig. 3b) features a sharp peak at 189 cm−1 corresponding to Se-Se stretching mode of CoSe2, and a broad peak at 250 cm−1 corresponding to Se-Se vibrational mode of elemental Se chains which are amorphous in nature [29,30]. The relative content of these two Se species are quantified by thermogravimetric analysis (TGA), as presented in Fig. 3c. The weight loss in N2 atmosphere originates from the vaporization of elemental Se, and the remaining weight in air is attributed to Co3O4 after annealing. The 394
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Fig. 2. SEM images of (a, b) ZIF-67 and (c, d) Se-ZIF at different magnifications.
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Fig. 4. (a) CV of initial 12 loops at 0.1 mV s−1; (b) voltage profiles of selected cycles at 0.1 A g−1; (c) cycle performance at 0.1 A g−1; (d) CV at 0.1 mV s−1 and (e) Raman spectrum after 100 cycles; (f) cycle performance at 2 A g−1 of the Se-ZIF electrode.
specific capacity of 490, 457, 431, 412, 384 and 228 mA h g−1, respectively. The capacity retention is as high as 78% when the current is increased from 0.1 to 2 A g−1. As displayed in Fig. 5b, the decrease in the discharge plateaus and the increase in the charge plateaus are small. In Fig. 5c, we offer a comparison of the rate performances among SeZIF, Se/C composites and transition metal selenides which have been previously applied in sodium (ion) batteries [3–6,11–13,20,33]. Given Se-ZIF is a Se/CoSe2/C composite, a more comprehensive comparison
cycle stability of Se-ZIF to the existence of the robust ZIF-derived framework and the stable SEI formed in the ether electrolyte. The SEI film is thin, so that irreversible capacity caused by SEI formation and the energy barrier of sodium ion transportation across SEI films are low [32]. That is why high initial coulombic efficiency and excellent rate capability are also frequently observed in this electrolyte. The rate performance of Se-ZIF is shown in Fig. 5a. At the rates of 0.1, 0.2, 0.5, 1, 2 and 5 A g−1, the composite reaches a reversible
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Fig. 5. (a) Rate performance, (b) voltage profiles at various rates, (c) comparison with reported Se/C and transition metal selenide, (d) CV curves at various scan rates, (e) peak current dependence on scan rate and (f) visualized pseudocapacitive contribution at 0.4 mV s−1 of the Se-ZIF electrode. 396
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with Se/C and cobalt selenides is also provided (Table S1). The delivered capacity of Se-ZIF is amongst the best, and the superiority over the reported Se-based materials is mainly attributed to the high Se weight ratio (76 wt%). It exceeds various pure transition metal selenides (FeSe2: 74%; CoSe2: 73%; NiSe2: 73%; CuSe2: 71%; ZnSe: 55%; MoSe2: 62% weight percentage of Se, respectively), and the advantage would be even larger when these selenides are composited with carbon. To achieve a better understanding of the exceptional rate capability of Se-ZIF electrodes, the dependence of CV peak current on the scan rate, and the charge transport mode (capacitive versus diffusion-limited) are analyzed. The CV curves measured at 0.1, 0.2, 0.4, 0.8 and 1.6 mV s−1 after long cycles are shown in Fig. 5d. The peak currents of the six labeled peaks (P1-P6) are plotted versus scan rate with both xand y-axis given in logarithmic scale. In such presentation, the electrochemical reaction is purely diffusion-limited (slow) when the slope equals 0.5, and it is capacitive (rapid) when the slope equals 1. In the case of Se-ZIF, the slopes calculated by linear fitting for P1-P6 peaks are 0.92, 0.75, 0.99, 0.69, 0.73 and 0.94, respectively. This implies that reactions corresponding to these redox peaks are controlled by both modes, with half of them (P1, P3 and P6) being close to pure capacitive mode. To determine the percentage of the two modes, the CV curves are analyzed according to the equation: i(V) = k1v + k2v1/2, where i(V) is the current at a fixed potential (V), v is the scan rate, k1 and k2 are coefficients to be resolved by fitting [34]. The pseudocapacitive contribution at 0.4 mV s−1 is presented as a shaded area inside the CV curve (Fig. 5f), and its percentage is quantified as the area fraction of the shaded region. The overall pseudocapacitive contribution within the cut-off voltage range reaches 77%, and the dominant capacitive reaction mode agrees with the fast kinetics.
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4. Conclusions In summary, a Se/CoSe2/C composite fabricated from ZIF-67 via space-confined annealing with Se powders possesses a high sodium storage capacity, remarkable stability and rate capability, which are amongst the best between all Se-based electrode materials reported so far. Such superior performance is attributed to the high Se content (76 wt%), the ZIF-derived robust framework, and large pseudocapacitive contributions. We demonstrated the feasibility of increasing Se content, which is critical for Se-based materials, by combining vapor deposition and chemical bonding with metals. The introduced spaceconfined annealing method is simple, effective, and applicable for many other kinds of metal-containing precursors, so it has a potential to become a popular fabrication approach towards Se-based electrode materials in the future. In particular, this method can be easily modified to fabricate sulfur- and phosphor-based materials by replacing Se with sulfur and phosphor. Acknowledgements This work was supported by City University of Hong Kong (SRG project 7005080) and by the Research Grant Council of Hong Kong S.A.R. (project CityU 21202014). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.nanoen.2019.01.064. References [1] J.Y. Hwang, S.T. Myung, Y.K. Sun, Sodium-Ion batteries: present and future, Chem. Soc. Rev. 46 (2017) 3529–3614. [2] C. Delmas, Sodium and sodium-ion batteries: 50 years of research, Adv. Energy Mater. 8 (2018) 1703137. [3] K. Zhang, M. Park, L. Zhou, G.H. Lee, W. Li, Y.M. Kang, J. Chen, Urchin-like CoSe2 as a high-performance anode material for sodium-ion batteries, Adv. Funct. Mater.
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Dr. Denis Y. W. Yu is an Associate Professor at the School of Energy and Environment, City University of Hong Kong. He received his Ph.D. in Applied Physics from the School of Engineering and Applied Sciences at Harvard University in 2003. He then worked as an engineer at SANYO Electric Co. Ltd. in Japan, developing cathode and anode materials for Li-ion batteries. Afterwards, he led the battery activities at the Energy Research Institute at Nanyang Technological University and TUM CREATE Centre for Electromobility in Singapore as a senior scientist. His research interests include fabrication, development and characterization of materials and electrodes for energy storage applications.
7397–7405. [32] J. Zhang, D.-W. Wang, W. Lv, S. Zhang, Q. Liang, D. Zheng, F. Kang, Q.-H. Yang, Achieving superb sodium storage performance on carbon anodes through an etherderived solid electrolyte interphase, Energy Environ. Sci. 10 (2017) 370–376. [33] X. Yang, J. Zhang, Z. Wang, H. Wang, C. Zhi, D.Y.W. Yu, A.L. Rogach, Carbonsupported nickel selenide hollow nanowires as advanced anode materials for sodium-ion batteries, Small 14 (2018) 1702669. [34] T. Brezesinski, J. Wang, S.H. Tolbert, B. Dunn, Ordered mesoporous [alpha]-MoO3 with iso-oriented nanocrystalline walls for thin-film pseudocapacitors, Nat. Mater. 9 (2010) 146–151.
Mr. Xuming Yang received his Bachelor degree in Applied Chemistry in 2013 and Master degree in Physical Chemistry in 2016 from Central South University, China. Since 2016, he is working on his Ph.D. under the supervision of Prof. Andrey Rogach at City University of Hong Kong. His research is focused on materials for electrochemical energy storage and conversion.
Prof. Andrey L. Rogach is a Chair Professor of Photonics Materials and the Founding Director of the Centre for Functional Photonics at City University of Hong Kong. He received his Diploma in Chemistry and Ph.D. in Physical Chemistry from the Belarusian State University in Minsk (Belarus), and worked as a staff scientist at the Institute of Physical Chemistry of the University of Hamburg (1995–2002), and at the Department of Physics of the Ludwig-Maximilians-University of Munich (2002–2009), Germany, where he completed his Habilitation in experimental physics. His research focuses on synthesis, assembly and optical spectroscopy of nanomaterials, and their use for energy and optoelectronic applications. Andrey Rogach is an Associate Editor of ACS Nano.
Mr. Shuo Wang received his Bachelor degree from Zhengzhou University in 2012 and his master degree from City University of Hong Kong in 2015. After graduation, he joined Dr. Denis Yu's group as a Ph.D. student. His research interests include the fabrication and experimental mechanism exploration of electrode materials for lithium and sodium ion batteries.
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