Selenium Encapsulated into MetalâOrganic Frameworks Derived N

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Selenium Encapsulated into Metal-Organic Frameworks Derived Ndoped Porous Carbon Polyhedrons as Cathode for Na-Se Batteries Qiuju Xu, Ting Liu, Yi Li, Linyu Hu, Chunlong Dai, Youquan Zhang, Yan Li, Dingyu Liu, and Maowen Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14380 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

Selenium Encapsulated into Metal-Organic Frameworks Derived N-doped Porous Carbon Polyhedrons as Cathode for Na-Se Batteries Qiuju Xu,†,‡ Ting Liu,†,‡ Yi Li,†,‡ Linyu Hu,†,‡ Chunlong Dai,†,‡ Youquan Zhang,†,‡ Yan Li,§ Dingyu Liu,†,‡ and Maowen Xu†,‡,*

† Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, P.R. China ‡ Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Chongqing 400715, P.R. China § College of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, P.R. China * E-mail:[email protected] (M. W. Xu)

ABSTRACT: The substitution of Se for S as cathode for rechargeable batteries which confining selenium in porous carbon attracts much attention as a potential research for energy storage systems. To date, there are no reports about Metal-Organic Frameworks to use for Na-Se batteries. Herein, MOFs derived nitrogen-doped porous carbon polyhedrons (NPCPs) have been obtained via facile synthesis and annealing treatment. Se is encapsulated into the mesopores of carbon polyhedrons homogeneously by melt-diffusion process to form Se/NPCPs composite, using as cathode for advanced Na-Se batteries. Se/NPCPs cathode exhibits excellent rate capabilities of 351.6 and 307.8 at 0.5C and 2C, respectively, along with the good

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cycling performance with high Coulombic efficiency of 99.7 % and slow decay rate of 0.05 % per cycle after 1000 cycles at 2C, which resulting from that the NPCPs holds the unique porous structure to accommodate volumetric expansion of Se during discharge-charge processes. Nitrogen doping could enhance the electrical conductivity of carbon matrix and facilitate rapid charge transfer.

KEYWORDS:

metal

organic

frameworks,

nitrogen-doped

porous

carbon

polyhedrons, Se-based material, volumetric expansion, Na-Se batteries

1.INTRODUCTION Recently, Na-ion batteries have been highlighted for renewable energy grid storage and electric vehicles due to the abundance and low cost of sodium.1,2 The traditional high-temperature Na-S batteries have been developed for large-scale renewable energy storage because of the outstanding advantages including high theoretical specific energy density (760 W h kg−1), high-energy efficiency (approaching 100%), low material cost (sufficiency of S and Na in the earth’s crust) and long cycle life.3 Although the high-temperature Na-S batteries have already been manufactured commercially, their high operation temperature (>300°C) is the major defect. In addition, the use of β-alumina solid electrolyte and the high operating temperature increase the cost of battery manufacturing, maintenance and safety concerns.4 Therefore, it would be promising to explore novel cathode materials for Na-ion batteries, which can be operated at room temperature. Selenium has intrinsic electrical conductivity (1 × 10−3 S m−1), which is much higher than that of sulfur (5 × 10−28 S m−1) due to the semiconductor properties of Se, so it may offer higher capacity utilization and better rate capability.5 And its theoretical volumetric capacity density (3253 mA h cm−3) is as high as that of sulfur (3467 mA h cm−3).6,7 Se possesses a further higher reaction activity with Na at room


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temperature, making it a prospective choice for ambient applications. As a result, Se has been regarded as the promising cathode for Na-ion batteries. Similar to S, Se-based cathode also sustain large volume expansion, resulting in low coulombic efficiency and fast capacity fading.5 Confinement of selenium in porous carbon and the structure of nanofibrous and nanoporous selenium have been used to overcome such difficulty.8 Combined Se with carbon material can prominently improve the cycle performance, Coulombic efficiency and selenium utilization because of the increasing electronic and ionic conductivities.9,10 Luo et al.11 prepared selenium-impregnated carbon composites by infusing Se into mesoporous carbon at high temperature under vacuum as cathode for Li-Se batteries and Na-Se batteries; Yu’s group12 developed selenium/porous carbon nanofibers composite electrode to buffer the shuttle reaction of intermediate polyselenides during cycling processes; Wang et al.13 reported that organic selenide fiber was prepared by electrospinning technique and delivered high capacities; Goodenough’s group14 successfully loaded Se molecules into the microporous slits of carbons to give a long cycling life; Ding et al.15 created a cathode based on capsuling selenium into cellulose-derived carbon nanosheets. Among all above strategies, diverse carbon matrices derived porous carbon were developed to encapsulate Se as cathode for Na-Se batteries. Metal Organic Frameworks (MOFs) have attracted extensive attention because of its uniquely porous structure, large specific surface area and microstructural controllability, showing various applications in C2H2 storage, hydrogen evolution, CO2 adsorption and electrocatalytic water splitting, etc. Many porous carbon materials derived from MOFs with abundant pores were used for confining S or Se and then as cathode for Li-S batteries16,17, Na-S batteries18,19 and Li-Se batteries20,21. However, up to now, there are no reports about MOFs derived porous carbon to infuse Se as cathode for Na-Se batteries. Herein, we prepared nitrogen-doped porous carbon polyhedrons (NPCPs) derived from ZIF-67 (a subclass of MOFs) for the loading of Se. Such NPCPs hold abundant



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mesopores with diameter >2 nm and the unique porous structure makes it possible for Se to be firmly confined in the mesopores by melting-diffusion progress. Compared with other porous carbon, the NPCPs don’t need KOH activation to create more pores12 and possess larger surface area and pore volume than other mesoporous carbon11. Not only could the hollow structure offer continuous electron transportation path, but also it can buffer volume changes.22,23 When used as cathode for Na-Se batteries firstly, Se/NPCPs composite shows superior electrochemical performance.

2.EXPERIMENTAL SECTION Preparation of Se/NPCPs composite. In a typical preparation, 2.5 mmol of Co(NO3)2·6H2O was dissolved in 50mL of methanol to form a clear red solution under stirring, which was subsequently poured into the other solution made from 25 mmol of 2-methylimidazole (C4H6N2) dissolves in 50 mL of methanol under stirring for 5 min, and then being kept still for 24 h at ambient temperature. Then obtained precipitates were collected by filtration, washed with ethanol several times and dried at 60°C for 12 h. Finally purple ZIF-67 was obtained. Then ZIF-67 was heated at 700°C for 3 h under Ar atmosphere with a heating rate of 5°C min-1. To remove Co species, the black product after carbonization was immersed into HCl solution. After filtration and washed with deionized water, ZIF-67 was successfully turned into NPCPs. Then pristine Se powder and NPCPs were grinded mixedly in agate mortar with weight ratio of 2:1. Subsequently, the mixture was transferred to tube furnace and heated at 260°C for 20 h under argon atmosphere to obtain Se/NPCPs composite. Materials

characterization.

Crystallographic

structure

of

samples

were

investigated by X-ray diffractometer (Maxima-X XRD-7000) and Cu K-alpha radiation. Raman patterns were acquired by using Invia Refl (Renishaw, UK). The morphologies and microstructures of products were inspected by using field-emission


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scanning electron microscopy (FESEM, JSM-7800F), transition electron microscope (TEM, JEM-2100) and STEM (Tecnai-G2-F30). The BET specific surface areas and pore size distributions were tested by Quadrasorb evo 2QDS-MP-30 (Quantachrome Instruments, USA). The weight percent of Se was determined by thermogravimetric analyzer (TGA, Q50). Moreover, X-ray photoelectron spectroscopy (XPS) characterization was analyzed by ESCALAB 250Xi electron spectrometer. Electrochemical measurements. The cathode electrode was prepared by mixing active material (Se/NPCPs composite), acetylene black (AB) and sodium alginate ((C6H7O6Na)n, SA) water-soluble binder with weight ratio of 80:10:10 in agate mortar. Then the slurry was covered on aluminum foil and dried overnight at 60°C under vacuum. There was ~2.0 mg cm-2 active material (~0.8 mg cm-2 Se) loaded on the electrode and the specific capacity was calculated based on the mass of Se (1C equals to 675 mA g-1). The Se/AB cathode was made by the same method. CR2032 coin cells were used and assembled in Ar-filled glove box. In coin cells, we used Se/NPCPs composite and Se/AB as cathode, Celgard 2400 as the separator, and a disk of sodium foil as reference electrode. The electrolyte was 1M NaClO4 dissolved in mixed solvent of EC-DEC with 3 wt% FEC. The amount of the electrolyte in cell is 45µL. Cyclic voltammetry (CV) test was conducted by Arbin Instruments testing System. And the galvanostatic discharge-charge test was achieved on LAND instrument testing system. Electrochemical impedance spectroscopy (EIS) test was carried on CHI 600c electrochemical workstation.

3. REDULTS AND DISCUSSION



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Figure 1. XRD patterns (a); Raman spectrums (b) of pristine Se, NPCPs and Se/NPCPs composite.

Figure 1a reveals XRD patterns of pristine Se and products. For pristine Se, diffraction peaks correspond with trigonal crystalline (JCPDS 06-0362). Broad diffraction peak of NPCPs around 26o exhibits characteristic of graphitized carbon21, and the low-intensity peaks located at 45o, 76.5o are residue Co, which accord with the peaks 44.2o (111), 75.9o (220) of Co (JCPDS 15-0806). After the heat treatment of Se and NPCPs, both diffraction peaks of carbon and Se are observed distinctly, indicating that Se exists in carbon matrix. In addition, the XRD pattern of ZIF-67 is displayed in Figure S1 and all diffraction peaks correspond well with previous research.24 The structure characteristics of pristine Se, NPCPs and Se/NPCPs composite are further researched by Raman spectroscopy in Figure 1b. Both Raman spectra of NPCPs and Se/NPCPs composite show carbon peaks at 1320 cm-1 (amorphous structure, D band) and 1580 cm-1 (graphitized structure, G band). In particular, the G band with lower intensity demonstrates the partially graphitized nature of NPCPs.25 For pristine Se, the peaks at 144 and 460 cm-1 represent Se12 with a ring structure


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while the peak at 235 cm-1 is attributed to chain-structured Se.11 However, those three peaks can not be observed while a peak of ~260 cm-1 exists in Se/NPCPs composite, the only peak explains Se8 ring.15 It can be concluded the selenium exists in Se/NPCPs composite, and the Se8 ring has appropriate sizes and dimensions to fit into the porous carbon easily.

Figure 2. FESEM images (a), (b), (c) and STEM elemental mapping images of C (d), N (e) and Se (f); XPS spectrum of C 1s (g), N 1s (h) and Se 3d (i) of Se/NPCPs composite.

Figure S2 shows the morphologies of ZIF-67 and NPCPs. The particles of ZIF-67 hold polyhedral shapes as well as uniform distribution and size of ~250 nm with smooth surfaces. After carbonization, porous carbon material inherits original polyhedral uniform morphologies and dimensions with shrinkage while the smooth surfaces become distinctly rough. Compared with NPCPs, Se/NPCPs composite



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maintains homogeneous dispersion in Figure 2a and holds few bulk Se on the surface in Figure 2b and c, accounting for the successful encapsulation of Se inside the NPCPs. The Se content in Se/NPCPs composite was 48.5wt%, which measured by TGA in Figure S3. Meanwhile, HAADF-STEM image and corresponding element mappings of C, N and Se are showed in Fig. 2d–f, which not only clearly shows uniform distribution of Se, but also verify homogeneous presence of effective N in the carbon matrix. And FESEM element mapping images also degree with that. (Figure S4) The XPS analysis was tested to research chemical valence state and component of Se/NPCPs composite. Figure S5 reveals the survey spectra, which testifying the existence of C, N and Se. The peaks of C 1s spectrum at 284.5, 285.6, 286.7 and 288.1 eV, correspond to C-C, C=N, C-N and C=O, respectively, as shown in Figure 2g.6,26,27 The existence of N element can be verified by the N 1s spectra in Figure 2h. The spectra shows the different oxidation states of nitrogen atoms, including pyrrolic N at 399.3eV, graphitic N at 400.9 eV, and oxidized N at 402.6 eV.28 The doped nitrogen can improve the electronic conductivity of porous carbon. In particular, pyrrolic N is more favorable because it can create numerous extrinsic defects and active sites.28 Figure 2i displays the spectra of Se 3d, two peaks at 55.9 and 55.1 eV correspond to Se 3d3/2 and Se 3d5/2, which attributing to spin-orbit interaction.21,29 Furthermore, the peak at 58.8eV fits with Se-O bonding.30


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Figure 3. TEM and HRTEM images of NPCPs (a), (c), (e) and Se/NPCPs composite (b), (d), (f); Adsorption-desorption isotherms (g), pore size distributions (h) of NPCPs and Se/NPCPs composite; Schematic diagram (i) of the preparation process of Se/NPCPs composite.

TEM images further reveal the morphology and unique porous structure of NPCPs.(Figure 3a and c) TEM images of Se/NPCPs composite are showed in Figure 3b and d, the carbon matrix keeps original morphology after Se encapsulation. In terms of Figure 3a, polyhedrons with particle size ~250nm can be observed clearly under low magnification, and those small black spots are residual Co particles and Figure S6 shows the amount of Co is 3.49 wt%. Hollow carbon matrix is filled with Se after heating treatment at 260°C, as shown in Figure 3b. Figure 3c and d further prove distinctions of porous structure before and after Se encapsulation, it can be observed easily that the carbon matrix NPCPs hold mesopores while Se exists in porous carbon after its encapsulation. Figure 3e and f display the HRTEM of NPCPs and Se/NPCPs composite. In Figure 3e, the disordered fringes denote the graphite



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carbon in NPCPs, and lattice fringes in Figure 3f can be assigned to (101) lattice interplanar spacing of Se, demonstrating existence of Se inside Se/NPCPs composite. To further research the porous features, BET specific surface areas and pore size distributions are both exhibited in Figure 3g and h. Figure 3g illustrates the BET specific surface areas before and after Se encapsulation. As shown in Figure S7, the BET surface area of NPCPs reach up to 568.207 m2 g-1 which can provide sufficient internal space for Se loading. For comparison, Se/NPCPs composite only holds specific surface area of 19.648 m2 g-1. The obvious decline demonstrates that Se was successfully encapsulated into the internal space of porous carbon matrix, which is consistent with TEM results. Furthermore, the decrease of pore volume (Figure S7) can also prove this. Figure 3h displays the pore size distributions of NPCPs and Se/NPCPs composite. It is observed that NPCPs consist of abundant mesopores with pore diameter ˃ 2nm, and the hysteresis loop at N2 pressures (P/P0 = 0.4 ~ 0.95) in Figure 3g indicates the presence of mesopores.31,32 After encapsulation of Se, the amount of mesopores performs sharp reduction, demonstrating that Se was successfully encapsulated into the mesopores. The pore volume of the NPCPs is 0.403 cm3 g-1 (Figure S7), which corresponds to a theoretical Se loading of ~63 wt % (calculated based on the density of Se, 4.26 g cm 3).14 −

Based on TEM and BET results, the schematic diagram that exhibits transform course from ZIF-67 to eventual Se/NPCPs composite vividly is shown in Figure 3i. Obtained purple precursor was transformed to nitrogen-doped porous carbon polyhedrons by annealing at 700°C for 3h, while the carbon matrix holds a mass of mesopores. Subsequently, Se particles were successfully encapsulated inside the hollow area of NPCPs via melting-diffusion process at 260°C for 20h. Therefore, the unique carbon matrix provides a conductive framework for uniform Se dispersion.


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Figure 4. (a) CV curves of Se/NPCPs composite; (b) Discharge-charge profiles of Se/NPCPs composite at 0.5C; (c) Rate capability of Se/NPCPs composite; (d) Discharge-charge profiles of Se/NPCPs composite at different current densities.

To explore the redox reactions during discharge-charge processes, CV curves of Se/NPCPs composite were obtained at scan rate of 0.1 mV s-1 between 2.5 and 0.5 V, as shown in Figure 4a. There are two reduction peaks and one oxidation peak in 1st cycle, indicating two-step phase change reaction to form Na2Se and single-phase transformation reaction from Na2Se to Se. The reduction peaks at ~1.8 V and ~1.25 V correspond to reactions of Se and Na: step one, Se turns into the intermediate phase Na2Sen (n≥4); step two, Na2Sen turns into final Na2Se phase. The single oxidation peak at ~2.0 V indicates that direct phase change from Na2Se to Se.5,6 Notably, the reduction peak emerged at ~1.8 V almost disappears, and the reduction peak emerged at ~1.25 V shifts to ~1.5 V in subsequent cycles, which could be attributed to the electrochemical activation process in the first discharge process.33 In terms of the oxidation peak, it shifts to ~1.95 V subsequently. The coincident peaks after 2nd cycle demonstrate the good electrochemical stability of the electrode.



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The galvanostatic discharge-charge test was conducted at 0.5C within the voltage window of 2.5 ~ 0.5 V in Figure 4b and the voltage platforms consist with the peak positions in CV curves. The Na-Se batteries can deliver initial discharge capacity of 976.1 mA h g-1, with 437.3 mA h g-1 being reversible. The reason of excellent capacity is high conductivity of carbon and electrochemical reaction with homogeneous Se, and irreversible capacity in first cycle may be attributed to SEI (solid electrolyte interface) formation and some irreversible capturing of Na within carbon matrix at high potentials.15 Besides, the capacity fading of 2nd cycle may be attributed to the unavoidable polyselenides dissolution within electrolyte.The profiles of 2nd, 5th, 10th, 20th and 50th cycles almost overlap, suggesting stable electrochemical performance. The Se/NPCPs composite electrode shows excellent rate capability, as exhibited in Figure 4c. It delivers discharge capacity of 392.2, 351.6, 327.7, 307.8 and 282.3 mA h g-1 at 0.2C, 0.5C, 1C, 2C and 3C, respectively. The capacity regains 377.8 mA h g-1 when returns to 0.2C after operating at various rates, exhibiting an excellent rate performance of Se/NPCPs composite. The decreasing discharge capacity of Se/NPCPs composite at 0.2C may be caused by incomplete utilization of Se which attached on polyhedrons surface. The coulombic efficiency increased gradually, which could be ascribed to the the activation process of Se. Further enhancing current density to 0.5C, 1C, 2C and 3C, discharge capacity of 345, 324.6, 302.2 and 276.6 mA h g-1 is remained after 20, 30, 40 and 50 cycles, respectively.

This excellent rate

capability of Se/NPCPs is significant and meaningful for the practical application of Na-Se batteries. Figure 4d exhibits discharge-charge voltage profiles of Se/NPCPs cathode at different current rates (0.2, 0.5, 1, 2 and 3C) within the voltage window of 2.5 ~ 0.5V. Discharge and charge curves overlap at current rates below 1C, indicating tiny polarization and excellent rate capability during electrochemical reaction. It can be seen that both discharge capacity and discharge plateau decline at higher current rates above 1C due to higher ohmic and kinetic overpotentials on this occasion.16 However,


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inclined platforms can still be observed. To tune the amount of Se, we used different Se/NPCPs ratios (that is, 13:6 and 7:3) during the heating treatment, the Se content calculated from the TGA curves for the Se/NPCPs composite was ~55 and 60 wt%, respectively. (Figure S8a, b) The capacity of the composite with ~55 wt% Se is lower and decreases rapidly than which is ~50 wt%, however, the performance of ~60 wt% Se is rather worse. (Figure S8e). It may be caused by that the samples hold numerous bulk Se on the surface clearly in Figure S8c and d, which resulting in poor conductivity and capacity fading.

Figure 5. (a) Cycling performance and Coulombic efficiency of Se/NPCPs composite electrode at 2C, (b) Nyquist plots of the battery with the fresh Se/NPCPs composite cathode and after cycling. (c) FESEM images of Se/NPCPs composite after cycling.

Besides excellent rate capability, Se/NPCPs composite shows good cycling performance as well, which is illustrated in Figure 5a. The enhancement of specific capacity before 34th cycle could be attributed to the activation process of Se.34 The decreasing capacity may be ascribed to incomplete utilization of Se. When it reaches 1000th cycle, a discharge capacity of 161.4 mA h g-1 still retains with Coulombic efficiency stays around 99.7%. Additionally, the capacity decline of per cycle is



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merely 0.05%. Compared with other cathode materials11-14,23,29, our work manifest lower capacity decay rate. (Figure S9) The good cycling performance could be ascribed to the unique hollow structure of NPCPs material. Nitrogen doping could enhance the electrical conductivity and facilitate rapid charge transfer.35 To prove the stability when the Se reacts with Na+ more completely, the cycling performance of lower current density is demonstrated in Figure S10. It exhibits a reversible capacity of 366.3 mA h g-1 after 100 cycles at 0.2C. To research the kinetics of electrode process, Nyquist plots before cycling and after 1000 cycles at 2C are tested and displayed in Figure 5b. The Rct (212Ω) before cycling suggests higher impedance due to the lower electrochemical contact.35 And the Rct decreases by around 88% after 1000 cycles, indicating that the NPCPs matrix acts could enhance charge-transfer ability of Na-Se batteries. For comparison, electrochemical performance of Se/AB was investigated and displayed in Figure S11, in which the capacity decreases rapidly to 50 mA h g-1 until 70th cycle, which is inferior to Se/NPCPs composite. To further investigate the structural stability of the Se/NPCPs composite, FESEM analysis was tested on the cycled electrode after 1000 cycles at 2C in Figure 5c. The morphology of Se/NPCPs composite maintained well after overlong cycling, indicating that the unique structure of NPCPs can effectively buffer the volume change. The pore volume of the NPCPs and Se/NPCPs is 0.403 and 0.030 cm3 g-1, respectively. (Figure S7) The pore volume of Na2Se is < 0.2974 cm3 g-1 (calculated based on the density of Na2Se, 2.58 g cm 3), −

which showing that the mesopores is enough to accommodate the discharge products Na2Se. Therefore, NPCPs holds the unique porous structure to accommodate volumetric expansion of Se upon sodication. The XPS analysis after cycling was tested in Figure S12, which testifying the existence of Na, C, N and Se. The peaks shift compared with Figure 2h, indicative of a chemical interaction that results in electron transfer,36 which showing that the nitrogen effect during electrochemical process.


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In Figure S13a, the peaks at 780.7 and 796.6 eV correspond to the Co 2p3/2 and Co 2p1/2, respectively. The energy difference of

∆E=15.9

eV between the two peaks

implies the Co species exist as Co2+ ion.28 It can be concluded that the reason of weight loss at ~550℃ in Figure S3 is the existence of CoSe2 (Figure S13b). The capacity of CoSe2 can be neglectable, because its capacity contribution is very low (Figure S13c).

4.CONCLUSIONS MOFs-derived nitrogen-doped porous carbon polyhedrons were prepared via facile synthesis and further annealing, and then Se/NPCPs composite was gained by melting-diffusion process. The Se/NPCPs composite electrode of Na-Se batteries exhibits excellent rate capability, outstanding cycling performance with a capacity decay as small as 0.05% per cycle. The unique hollow structure of NPCPs can buffer volume changes of Se, which resulting in good electrochemical performance. Our work provides a novel kind of Se-based cathode, the application of MOFs offers a new approach for the development of remarkable Na-Se batteries.

Supporting Information XRD pattern of ZIF-67 precursor; FESEM images of ZIF-67 and NPCPs; TGA result of Se/NPCPs composite; EDS elemental mappings of Se/NPCPs composite; XPS survey spectrum of Se/NPCPs composite; EDS line scanning of Se/NPCPs composite; Comparison of BET performance between NPCPs and Se/NPCPs composite; TGA and FESEM results of Se content of ~55 wt% and ~60 wt%, Cycling performance of Se/NPCPs composite with different content of Se at 0.5C; The comparisons of decay rate per cycle with others; Cycling performance of Se/NPCPs composite at 0.2C; Cycling performance of Se/AB composite at current density of 675 mA g-1; XPS results of survey and N 1s spectrum of Se/NPCPs composite after cycling; XPS



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spectra of Co 2p of Se/NPCPs composite, TGA result of CoSe2 and cycling performance of CoSe2 at current density of 337.5 mA g-1. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (M. W. Xu) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is financially supported by grants from the National Natural Science Foundation of China (No. 21773188), Basic and frontier research project of Chongqing (cstc2015jcyjA50031) and Fundamental Research Funds for the Central Universities (XDJK2017A002，XDJK2017B048) and Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011). Yan Li is supported by the National Science Foundation of China (Grant No. 51402019) and the Beijing Natural Science Foundation (Grant No. 2152011).

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