Metal-organic-framework-derived N-doped hierarchically porous

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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 Seung-Keun Park, Jin Sung Park, and Yun Chan Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03104 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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ACS Applied Materials & Interfaces

Metal-organic-framework-derived N-doped hierarchically porous carbon polyhedrons anchored on crumpled graphene balls as efficient selenium hosts for high-performance lithiumselenium batteries

Seung-Keun Park, Jin-Sung Park and Yun Chan Kang*

Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea

* Corresponding author: [email protected] (Y.C. Kang)

KEYWORDS: crumpled graphene balls, metal-organic frameworks, spray pyrolysis, lithiumselenium batteries, hierarchically porous materials 1 ACS Paragon Plus Environment

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ABSTRACT Developing carbon scaffolds showing rational pore structures as cathode hosts is essential for achieving superior electrochemical performances of lithium-selenium (Li-Se) batteries. Hierarchically porous N-doped carbon polyhedrons anchored on crumpled graphene balls (NPC/CGBs) are synthesized by carbonizing a zeolitic imidazolate framework-8 (ZIF8)/CGB composite precursor, producing an unprecedented effective host matrix for highperformance Li-Se batteries. Mesoporous CGBs obtained by one-pot spray pyrolysis are used as a highly conductive matrix for uniform polyhedral ZIF-8 growth. During carbonization, ZIF-8 polyhedrons on mesoporous CGBs are converted into N-doped carbon polyhedrons showing abundant micropores, forming a high-surface-area, high-pore-volume hierarchically porous NPC/CGB composite whose small unique pores effectively confine Se during melt diffusion, thereby providing conductive electron pathways. Thus, the integrated NPC/CGBSe composite ensures high Se utilization originating from complete electrochemical reactions between Se and Li ions. The NPC/CGB-Se composite cathode exhibits high discharge capacities (998 and 462 mA h g-1 at the 1st and 1000th cycles, respectively, at a 0.5 C current density), good capacity retention (68%, calculated from the 3rd cycle), and excellent rate capability. A discharge capacity of 409 mA h g–1 is achieved even at an extremely high (15.0 C) current density.

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1.

Introduction

Recently, rechargeable lithium-selenium (Li-Se) batteries have attracted considerable attention because Se-based electrodes provide advantages over widely known lithium-sulfur (Li-S) ones, including comparable theoretical volumetric capacity (3253 mA h cm–3) and high electrical conductivity (1 × 10–3 S m–1), nearly 20 orders of magnitude than that of sulfur (5 × 10–28 S m–1).1-5 Despite such advantages, Li-Se batteries still face some significant challenges such as low bulk Se cathode utilization and sluggish ion/electron transport during cycling, resulting in poor electrochemical performance.6-10 To overcome these challenges, one of the most effective strategies is to confine Se into porous conductive matrixes showing large surface areas and unique pore structures.11-17 Combining Se with porous conductive matrixes improves cathode electrical conductivities, facilitates ion/electron transport, and restricts the size of Se nanoparticles, leading to enhanced electrochemical performances in Li-Se batteries. Graphene is an ideal conductive matrix for Se storage owing to its high surface area, excellent electrical conductivity, and unique physicochemical properties.18-19 However, owing to strong van der Waals interactions, graphene sheets can easily restack with each other or form irreversible agglomerates, which may greatly limit ion/electron transport as well as the accessible surface area for the electrolyte. Three-dimensional (3D) crumpled graphene balls (CGBs), produced by rapidly drying aerosolized droplets of graphene oxide (GO) nanosheets, are new carbon matrixes showing higher free volumes and surface areas than stacked graphene sheets, features that are favorable for effective Se storage within CGB matrixes.20-22 However, Se nanoparticles are easily agglomerated in CGB matrixes during melt diffusion because CGBs show open pore structures and few micropores (< 2 nm), resulting in electrochemical performance degradation. 3 ACS Paragon Plus Environment


Recently, metal-organic frameworks (MOFs), constructed by combining metallic ions and organic ligands, have been regarded as promising precursors for the preparation of porous carbon-based materials, owing to their unique features such as adjustable pore sizes, volumes, and shapes and large surface areas.23-31 Several researchers recently have reported MOFderived porous carbon matrixes showing numerous micropores as cathode hosts in Li-Se batteries.11,14,32 Li et al. demonstrated MOF-derived hollow hierarchically porous carbon spheres via a hydrothermal method and subsequent annealing.32 He et al. prepared 3D porous N-doped graphitic carbon/Co polyhedrons derived from MOFs and employed them as cathode hosts in Li-Se batteries.11 However, most of MOF-derived carbon materials show low degree of graphitization and many defects, leading to poor electrical conductivity. Thus, it is desirable to develop an effective approach to improve the conductivity of cathode hosts. Taking into account the disadvantages of MOF-derived carbon materials, combining such materials with other conductive matrixes, especially 3D porous-structured graphene, is expected to be a novel method of preparing advanced electrode materials for Li-Se batteries. However, synthesizing 3D-structured MOF-derived carbon/graphene composites still remains a great challenge; thus, they have been barely studied as cathode hosts in Li-Se batteries. Herein, we demonstrate the synthesis of MOF-derived N-doped porous carbon polyhedrons anchored on crumpled graphene balls (NPC/CGB) as effective host matrix for confining Se for Li-Se batteries. The mesoporous CGBs, obtained by facile spray pyrolysis, were used as the highly conductive substrate for the uniform deposition of polyhedral Znbased MOFs (ZIF-8). During carbonization, ZIF-8 polyhedrons on the mesoporous CGBs were transformed into N-doped porous carbon polyhedrons showing numerous micropores, resulting in the formation of a hierarchically porous NPC/CGB composite showing high pore volumes and surface areas. This rationally designed matrix facilitates electrolyte access through mesopores and effectively confines Se in small pores (micro/meso), and the CGB 4 ACS Paragon Plus Environment

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substrate offers electron conductive pathways during charging/discharging, which could improve the electrochemical performances of Li-Se batteries. Indeed, the Se-loaded NPCCGB composite exhibited excellent cycling and rate performances as a cathode material for Li-Se batteries.

2.

RESULTS AND DISCUSSION

Scheme 1. Formation mechanism of Se-nanoparticle-fillled hierarchically porous NPC/CGB composite.

Scheme 1 describes the formation mechanism of the Se-nanoparticle-filled hierarchically porous NPC/CGB composite. First, mesoporous CGBs were obtained by facile spray pyrolysis, which has been well documented in previous reports (Scheme 1-①).33-34 ZIF-8 polyhedrons were then well grown on the mesoporous CGB surfaces via a solution process (Scheme 1-②) in which van der Waals forces between mesoporous CGBs and organic 5 ACS Paragon Plus Environment


ligands resulted in the direct growth of the ZIF-8 polyhedrons, which were subsequently annealed at 900 °C under inert conditions to simultaneously transform the ZIF-8 polyhedrons into N-doped microporous carbon polyhedrons and volatilize Zn metal (Scheme 1-③), thereby forming NPC anchored on the mesoporous CGB substrate. Finally, the NPC/CGB-Se composite was prepared by Se melt-infiltration into NPC/CGB at 260 °C under an Ar atmosphere (Scheme 1-④).

Figure 1. Morphologies, SAED patterns, and elemental mapping images of ZIF-8/CGB composite: (a) SEM image, (b) TEM image, (c and d) HR-TEM images, (e) SAED pattern, and (f) elemental mapping images. 6 ACS Paragon Plus Environment

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The changes in morphology and structure that had occurred in each process were characterized by various methods, as described in the Experimental Section. In Figure S1, the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of mesoporous CGB clearly show well-defined mesopores (indicated by arrows) and quasispherical morphology, which originated from the decomposition of graphene-nanosheetwrapped polystyrene (PS) beads. The high-resolution (HR) TEM image reveals stacked layers separated by interlayer distances of 0.33 nm, corresponding to the (002) plane of graphitic carbon. Although the overall sample morphologies remained unaltered after the growth of the ZIF-8 polyhedrons, the sample surfaces roughened owing to the ~50–100 nmdiameter ZIF-8 polyhedrons, as shown in Figure 1a-c. The inner CGB matrix can be obviously identified from the TEM and HR-TEM images (Figure 1c and d). The X-ray diffraction (XRD) pattern of the ZIF-8/CGB composite matched well that of previously studied ZIF-8/graphitic material composites (Figure S2). The selected area electron diffraction (SAED) pattern also reveals that the composite showed graphitic layers in the CGB matrix (Figure 1e). Energy dispersive X-ray spectroscopy (EDX) mapping images verified the uniform distribution of the ZIF-8 polyhedrons on the CGB matrix (Figure 1f). The N and Zn had originated from organic ligands and Zn ions in the ZIF-8 polyhedrons, respectively. The morphology and crystal structure of the hierarchically porous NPC/CGB composite prepared by carbonizing the ZIF-8/CGB composite in an Ar atmosphere is shown in Figure 2. The hierarchically porous NPC/CGB composite nanoparticles had perfectly retained the original quasi-spherical morphology of the ZIF-8/CGB composite, even after hightemperature annealing, indicating the robustness and thermal stability of the CGB matrix (Figure 2a and b). In the TEM images (Figure 2b and c), the NPC/CGB composite appeared darker than the CGB matrix, verifying NPC on the CGB matrix surface (Figure 2b and c). 7 ACS Paragon Plus Environment


Figure 2. Morphologies, SAED patterns, and elemental mapping images of hierarchically porous NPC/CGB composite: (a) SEM image, (b and c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images.

Notably, the NPC polyhedrons were distorted owing to the ZIF-8 polyhedron surface shrinkage during carbonization. As confirmed in the SEM images of ZIF-8 polyhedrons before and after annealing at 900 oC (Figure S3), NPC experienced surface shrinkage, but retained its original polyhedral shape well after calcination. The HR-TEM image (Figure 2d)


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Figure 3. Morphologies, SAED patterns, and elemental mapping images of NPC/CGB-Se: (a) SEM image, (b and c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images.

reveals micropores in NPC polyhedrons (indicated by arrows) and graphitic layers separated by 0.33 nm. The SAED pattern (Figure 2e) clearly showed two distinct rings corresponding to the (112) and (100) planes of graphitic carbon. In the XRD pattern of the NPC/CGB composite, although the corresponding peaks attributed to ZIF-8 polyhedrons had totally disappeared, broad peaks corresponding to graphitic carbon materials had appeared (Figure S2). The EDX mapping images shown in Figure 2f confirm the complete removal of Zn 9 ACS Paragon Plus Environment


owing to Zn volatilization during annealing. From the EDX quantitative analysis data (Figure S4), Zn element was also detected in trace amount in NPC/CGB composites, which also proves the removal of Zn element after thermal-treatment. The morphology and crystal structure of the hierarchically porous NPC/CGB composite obtained by melt-diffusion-induced Se loading (NPC/CGB-Se) are shown in Figure 3. Although the SEM images of the Se-loaded and unloaded NPC/CGB composites did not show any distinct differences (Figure 3a), the corresponding TEM images (Figure 3b and c) show that the NPC/CGB-Se composite surfaces had become smoother and that numerous micropores had disappeared from the carbon matrix owing to melted Se infiltration. The Se nanoparticles in the carbon matrix were clearly identified from the HR-TEM image (Figure 3d). The XRD and SAED patterns also show that the Se nanoparticles had infiltrated the carbon matrix well; that is, they show patterns only attributed to graphitic carbon materials (Figure S5 and 3e). The uniform Se distribution in the NPC/CGB matrix was also confirmed by EDX mapping images (Figure 3f). In contrast, the SEM and TEM images of the CGB-Se composite (Figure S6a-c) clearly show bulk Se agglomerations, owing to the lack of ample Se storage space and to open channels in the CGB matrix. The HR-TEM image in Figure S6d reveals bulk Se showing 0.378 nm crystal lattice fringes, corresponding to the (100) plane. The XRD and SAED patterns also confirm the large Se nanoparticles in the graphitic carbon materials (Figure S5 and S6e). The EDX mapping images shown in Figure S6f reveal uneven Se distribution, indicating that Se could easily infiltrate carbon matrixes showing small pores, thereby resulting in homogeneous Se nanoparticle distribution. The NPC/CGB-Se elemental composition was investigated by X-ray photoelectron spectroscopy (XPS, Figure 4). The XPS survey scan in Figure 4a shows that NPC/CGB-Se contained N, C, and Se. As shown in Figure 4b, the deconvoluted C 1s spectrum shows three peaks corresponding to sp2-hybridized C-C (284.5 eV), sp3-hybridized N-C/C-C (285.6 eV), 10 ACS Paragon Plus Environment

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and C=O (288.4 eV), indicating N-doped graphitic carbon material. The high-resolution N 1s spectrum in Figure 4c was fitted with three peaks located at 398.2, 400.2, and 401.4 eV, corresponding to pyridinic, pyrrolic, and graphitic N, respectively.35 N-doped carbon

Figure 4. XPS spectra of NPC/CGB-Se: (a) survey scan, (b) C 1s spectra, (c) N 1s spectra, and (d) Se 3d spectra.

matrixes can show enhanced electron conductivities.36 Particularly, pyrrolic N can induce numerous defects and active sites favorable for electrochemical reactions.37-38 The Se 3d spectrum (Figure 4d) clearly shows two major peaks correlating to the Se 3d5/2 and Se 3d3/2 states. A weak peak at 58.0 eV indicates that a small amount of SeO2 had originated from the partially oxidized surface Se. 11 ACS Paragon Plus Environment


The surface areas and detailed pore structures of unloaded and Se-loaded NPC/CGB and CGB were investigated by N2-sorption. As shown in Figure 5a, both unloaded samples show a typical IV isotherm with a hysteresis loop due to mesopores in the CGB matrix. Notably,

Figure 5. N2 sorption isotherms and BJH pore size distributions of (a and b) NPC/CGB and CGB, (c and d) and after selenium infiltration.

the NPC/CGB composite isotherm displays large uptake in the low P Po-1 range, which is attributed to the composite micropores. As confirmed in the Barrett–Joyner–Halenda (BJH) pore size distributions (Figure 5b), the sharp peak at ~4 nm in the NPC/CGB composite isotherm showed higher intensity, indicating that the small mesopores were more developed when the CGBs were combined with the NPC polyhedrons. The BET surface areas of 12 ACS Paragon Plus Environment

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NPC/CGB and CGB were 541.6 and 235.8 m2 g–1, respectively. The surface area of the Seloaded NPC/CGB composite had significantly decreased to 21.2 m 2 g -1 , and the corresponding BJH plots did not show any peaks, indicating that Se had fully infiltrated the available composite pores (Figure 5c and d). Meanwhile, CGB-Se showed a larger surface

Figure 6. (a and b) Raman spectra and (c) TG curves of NPC/CGB-Se and CGB-Se.

area than NPC/CGB owing to the agglomeration of Se nanoparticles generating empty mesopores in the CGB matrix.



To investigate the C and Se structural characteristics of NPC/CGB-Se and CGB-Se composites, Raman spectra were obtained, as shown in Figure 6a and b. The NPC/CGB-Se composite exhibited a broad band at 259 cm-1, which was attributed to the combination of chain-like Se (c-Se, 257 cm-1) and ring-like Se (r-Se, 267 cm-1) short-range ordered band modes.1,12,39 In addition, the Raman spectra did not show any peaks corresponding to trigonal Se (t-Se, 237 cm-1), indicating that the composites only contained short-range ordered Se. The Raman spectrum for the CGB-Se composite is similar to that for the NPC/CGB-Se composite except for the weak peak corresponding to t-Se, suggesting that the unique NPC/CGB composite pore structure had facilitated the formation of short-range ordered Se thereby resulting in high Se utilization and excellent electrochemical performance for Li-Se batteries. In Figure 6b, the Raman spectra show two major peaks at 1360 and 1590 cm-1, correlating with the D- and G-bands of defected graphitic carbon materials, respectively.40-42 The G-band is attributed to sp2-hybridized bonded C atoms whereas the D-band is assigned to the defects in carbon materials.43 The relative intensity ratio (ID/IG) of the two bands correlates with the degree of sample graphitization.44 The ID/IG values of NPC/CGB-Se and CGB-Se were calculated as 0.99 and 1.08, respectively. The lower ID/IG value for NPC/CGB-Se was due to graphitic layers produced in NPC and CGB during high-temperature annealing at 900 °C. The thermal behaviors of NPC/CGB-Se and CGB-Se were investigated by TGA under an inert Ar atmosphere (Figure 6c). The considerable weight losses of both samples occurred in the range 300–550 °C, which is attributed to Se volatilization within the carbon matrix. The weight loss of the NPC/CGB-Se composite occurred at a higher temperature than that of the CGB-Se composite owing to the well-defined porous structure in the NPC/CGB-Se composite. The Se nanoparticles confined to the small NPC/CGB pores required more energy to melt and diffuse, resulting in higher-temperature evaporation. The Se contents of NPC/CGB-Se and CGB-Se were both calculated as 60 wt.%. 14 ACS Paragon Plus Environment

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Figure 7. Electrochemical properties of NPC/CGB-Se and CGB-Se: (a and b) CV curves, (c) initial charge-discharge profiles, (d) cycle performances, and (e) rate performances.



To explore the electrochemical behaviors of NPC/CGB-Se and CGB-Se as cathode materials for Li-Se batteries, cyclic voltammograms (CVs) of the two samples were scanned at 0.1 mV s–1 in the range 1.0–3.0 V for the 1st and 5th cycles (Figure 7a and b). The voltammogram in Figure 7a, shows weak peaks at 2.1 and 2.3 V in the initial discharge profile, corresponding to the transformation of r-Se to c-Se, which disappeared after further cycling.1,12 The distinct reduction peak at 1.7 V was related to the formation of Li2Se by the electrochemical reaction between Se and Li+.39,45 Notably, the peak shifted to higher voltages in the following cycles, which was attributed to NPC/CGB-Se cathode deformation at the initial lithiation.46 In the first anodic sweep, the main peak appearing at 2.2 V was attributed to the conversion of Li2Se into Se nanoparticles. The CV of the CGB-Se cathode also showed a similar pattern except for the weak peak at 2.3 V in the initial discharge profile, which may be due to the irreversible multistep reaction between Se and Li+.14 It is noteworthy that the voltage gap between the main cathodic and anodic peaks of the NPC/CGB-Se cathode was much smaller than that between those of the CGB-Se cathode, indicating the lower resistance of the NPC/CGB-Se cathode. The initial charge/discharge curves of the NPC/CGB-Se and CGB-Se cathodes obtained at 0.5 C (1 C = 675 mA g–1) are shown in Figure 7c. The initial discharge profile of the NPC/CGB-Se cathode clearly shows a long plateau at about 1.8 V, corresponding to the formation of Li2Se, and two small plateaus at 2.1 and 2.3 V, attributed to the conversion of r-Se to c-Se, which are in good agreement with the CV data. In contrast, the initial discharge profile of the CGB-Se cathode did not show any clear plateaus owing to low Se utilization. The initial discharge capacities of the NPC/CGB-Se and CGB-Se cathodes measured at a current density of 0.5 C were 998 and 343 mA h g–1 and their corresponding initial Coulombic efficiencies were 66 and 19%, respectively. The high discharge capacity of NPC/CGB-Se cathode, which is beyond the theoretical value of 675 mA h g-1, can be ascribed to the high utilization of short-range ordered c-Se, which was formed from 16 ACS Paragon Plus Environment

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NPC/CGB composite with a well-developed porous structure. The degree of Se utilization is directly related to the capacity value of electrode, thus the formation of short-range ordered Se results in high initial capacity of NPC/CGB-Se cathode. Also, the irreversible formation of solid electrolyte interphases (SEIs) on the surface of electrode during the initial cycle are responsible for the excess initial capacity. In contrast, the low initial discharge capacity of the CGB-Se cathode was due to the barely utilizable bulk Se nanoparticles that had formed during melt diffusion. The low Coulombic efficiency of CGB-Se was mainly attributed to irreversible Li storage in carbon materials. The cycling performances and Coulombic efficiencies of the NPC/CGB-Se and CGB-Se cathodes measured at a current density of 0.5 C are presented in Figure 7d. The initial capacity losses and low Coulombic efficiencies of electrodes can be ascribed to electrolyte decomposition and to the irreversible formation of solid electrolyte interphases (SEIs), which have been commonly reported in previous studies.6,47 The Coulombic efficiency of the NPC/CGB-Se cathode quickly stabilized during the early few cycles and increased above 99% after the 5th cycle. In contrast, that of the CGBSe cathode reached over 98% after 77 cycles owing to the low utilization and conductivity of bulk Se. The discharge capacity of the NPC/CGB-Se cathode after the 1000th cycle was 462 mA h g–1, and the capacity retention from the 10th cycle was 81.4%. The discharge capacity of the CGB-Se cathode after the 1000th cycle, on the other hand, was only 51 mA h g–1. The CGB-Se capacity slightly but gradually increased during cycling owing to the formation of ultrafine Se nanoparticles during the activation of bulk Se. NPC/CGB-Se cathode also showed stable cycling performances even at high current densities of 1.0 and 2.0 C. The electrode delivered discharge capacities of 544 and 515 mA h g-1 after 100 cycles at current densities of 1.0 and 2.0 C, respectively (Figure S7). The rate capabilities of the NPC/CGB-Se and CGB-Se cathodes are displayed in Figure 7e. The discharge capacities of NPC/CGB-Se at current densities of 0.5, 1.0, 2.0, 5.0, 7.0, 10.0, 17 ACS Paragon Plus Environment


and 15.0 C were 564, 552, 527, 487, 442, and 409 mA h g–1, respectively. It is worth mentioning that a satisfactory capacity of 409 mA h g-1 was achieved even at a high current density of 15.0 C. Furthermore, the composites regained most of their discharge capacities when the current density returned to 0.5 C. The CGB-Se cathode discharge capacity fell below 5 mA h g-1 at a relatively low current density of 5.0 C. In consideration of the requirements for scale-up fabrication, NPC/CGB-Se cathode with higher areal Se loading was also prepared. Although the Se loading increased to 1.8 mg cm-2, the discharge capacity of NPC/CGB-Se cathode at 0.5 C still maintained at 380 mA h g-1 after 90 cycles (Figure S8). Owing to effective confinement of Se within NPC/CGB matrix, the cathode with higher areal Se loading showed good cycling performance. To explain the role of CGB in the composite, NPC-Se cathode was tested under same conditions with other cathodes (Figure S9). As shown in Figure S9a, NPC-Se cathode delivered specific capacity of 374 mA h g-1 after 80 cycles, which is lower than NPC/CGB-Se composites. Furthermore, the specific capacity rapidly decreased when tested at high current densities (Figure S9b). These results clearly showed that the combination of porous NPC and conductive CGB enhances the electrochemical performance. The mesoporous CGBs play a role as the conductive backbone to maintain the structural integrity as well as offer conductive pathway for fast electron transfer during cycling. The superior electrochemical performance of the NPC/CGB-Se composite as a cathode material for Li-Se batteries was compared to those of other carbon-based cathode materials previously reported in the literature, and the detailed comparisons are summarized in Table S1. The outstanding cycling stability and excellent rate capability of the NPC/CGB-Se cathode were attributed to its unique structural features, which offer the following advantages. The highly conductive CGB matrix ensured fast electron transport, particularly during high


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rate cycling, resulting in enhanced rate capability. The unique NPC/CGB composite pore structure facilitated the formation of short-range ordered c-Se and r-Se, thereby enabling high

Figure 8. Nyquist plots of NPC/CGB-Se and CGB-Se (a) before cycling and (b) after 50 cycles.

Se utilization. The composite mesopores provided easily accessible electrolyte pathways, resulting in fast ion diffusion/transport deep inside electrodes. Nyquist plots of the NPC/GCB-Se and CGB-Se assembled coin cells were obtained before cycling and after the 50th cycle, as shown in Figure 8. The plots show semicircles in the medium-frequency range, which are used to determine electrode charge transfer resistance (Rct).48-51 NPC/GCB-Se showed lower Rct values than CGB-Se. The enhanced electron transfer kinetics of NPC/GCB-Se can be attributed to its unique hierarchically porous structure, which enabled high utilization of short-range ordered Se. The Rct of CGB-Se was higher than that of NPC/GCB-Se after 50 cycles owing to the low bulk Se activation in CGBSe. Both cycled samples showed increased Rct, presumably because the SEI layer formation had increased the resistance.52 Also, the Rct value of CGB-Se was higher than that of NPC-Se, suggesting the presence of bulky Se in the composites increased charge transfer resistance of 19 ACS Paragon Plus Environment


electrode (Figure S9c and d). NPC/GCB-Se showed a low Rct even after 50 cycles, demonstrating its high structural integrity during cycling. The SEM images of the NPC/GCBSe electrode material after 1000 cycles, as shown in Figure S10, further support its high structural integrity during cycling. The composite had retained its original morphology well, as shown in the SEM images in Figure 3.

3.

CONCLUSIONS

In summary, an NPC/CGB composite consisting of MOF-derived N-doped porous carbon polyhedrons and spray-pyrolysis-derived mesoporous CGBs has been prepared for the first time and has been utilized as the conductive matrix for the uniform growth of polyhedral MOFs. The unique composite pore structure not only provided sufficient space to accommodate Se during melt diffusion but also facilitated electrolyte penetration deep inside the electrodes, resulting in high Se utilization and fast ion/electron transport during cycling. The NPC/CGB-Se composite exhibited superior electrochemical performances such as long cycle life and excellent rate capability when used as the cathode material for Li-Se batteries. It is expected that this novel hierarchically pore-structured carbon host might be extended to other energy-related applications such as Li-S batteries, supercapacitors, and so on.

4.

EXPERIMENTAL SECTION

4.1. Sample Preparation. Mesoporous CGB was prepared via facile spray pyrolysis using 1 L of aqueous spray solution containing GO nanosheets (10 mg mL-1) and polystyrene nanobeads (~100 nm, 30.0 g). The temperature of the tubular quartz reactor used for the spray pyrolysis and the feeding rate of the carrier gas (N2) were maintained at 700 °C and 10 L min1

, respectively. To prepare the ZIF-8/CGB composites, 0.05 g of CGB and 0.732 g of 20 ACS Paragon Plus Environment

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Zn(NO3)2·6H2O were dispersed in 30 mL of methanol by sonication, and 1.6 g of 2methylimidazole (Sigma, 99%) dissolved in 40 mL of methanol was rapidly added. The mixture was subsequently allowed to react without heating or stirring for 6 h. The obtained ZIF-8/CGB composite was washed with ethanol and was annealed at 900 °C under an Ar atmosphere for 2 h, yielding the NPC/CGB composite. NPC polyhedrons were also obtained by same treatment without CGB. To obtain the NPC/CGB-Se composite, the NPC/CGB composite was well mixed with Se nanoparticles (Sigma, 99.99%) in a mass ratio of 4:6 and was then annealed at 260 °C for 12 h under an Ar atmosphere in a tube furnace. 4.2. Characterization Techniques. The morphological characteristics of the samples were analyzed by scanning electron microscopy (SEM, VEGA3) and transmission electron microscopy (TEM, JEM-2100F). The phase and structural properties were examined using Xray diffraction spectroscopy (XRD, X’Pert PRO with Cu Kα radiation, λ = 1.5418 Å) at the Korea Basic Science Institute (Daegu). The chemical properties of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ScientificTM K-AlphaTM). Raman spectroscopy (Jobin Yvon LabRam HR800) excited by a 632.8-nm He/Ne laser at room temperature was used to determine the structural properties of the Se-infiltrated samples. The sample surface areas and porosities were analyzed using the Brunauer–Emmett–Teller (BET) method with high-purity N2. The amount of infiltrated Se was determined by performing thermogravimetric analysis (TGA, Pyris 1 Thermogravimetric Analyzer, Perkin Elmer) in the range 25–700 °C at 10 °C min−1 under an Ar atmosphere. 4.3. Electrochemical Measurements. The electrodes were prepared by the slurry process. The slurry was prepared by mixing the electrode material, Super P® carbon black, and sodium carboxymethyl cellulose (CMC) in a mass ratio of 7:2:1. Lithium metal chips and porous polypropylene films were used as the counter electrodes and separators, respectively. 1 M LiPF6 dissolved in a mixture of ethylene carbonate/diethyl carbonate (EC/DEC; 1:1 v/v) was 21 ACS Paragon Plus Environment


used as the electrolyte. The electrochemical properties of the electrodes were analyzed in the range 1.0–3.0 V at various current densities. Cyclic voltammograms (CVs) were scanned at 0.1 mV s-1. The cathode diameter was 14 mm, and the Se mass loading was approximately 0.4 mg cm-2.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. SEM and TEM images of mesoporous CGB microspheres; XRD patterns of CGB, ZIF8/CGB, and NPC/CGB microspheres; SEM images of ZIF-8 polyhedrons before and after annealing at 900 oC; EDX spectrum and elemental composition table of NPC/CGC composites; XRD patterns of NPC/CGB-Se and CGB-Se; SEM and TEM images of CGB-Se; Cycle performances of NPC/CGB-Se at current densities of 1.0 and 2.0 C; Cycle performance of NPC/CGB-Se with higher areal Se loading at current densities of 0.5 C; Electrochemical properties of NPC-Se; SEM images of NPC/CGB-Se after 1000 cycles; Table comparing electrochemical performances of various nanostructured materials used as cathode materials for lithium-selenium batteries reported in the previous literatures.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017M1A2A2087577 and NRF-2017R1A4A1014806). 22 ACS Paragon Plus Environment

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ToC figure Hierarchically porous N-doped carbon polyhedrons anchored on crumpled graphene balls are synthesized as effective Se hosts for high-performance lithium-selenium batteries. This rationally designed matrix facilitates electrolyte access through mesopores and effectively confines Se in small pores, and the CGB substrate offers electron conductive pathways during charging/discharging, which could improve the electrochemical performances of Li-Se batteries.


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Metal-organic-framework-derived N-doped hierarchically porous

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