Graphitic Nanocarbon–Selenium Cathode with Favorable Rate

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Graphitic Nanocarbon-Selenium Cathode with Favorable Rate Capability for Li-Se Batteries Shuai-Feng Zhang, Wen-Peng Wang, Sen Xin, Huan Ye, Ya-Xia Yin, and Yu-Guo Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16708 • Publication Date (Web): 23 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017

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ACS Applied Materials & Interfaces

Graphitic Nanocarbon-Selenium Cathode with Favorable Rate Capability for Li-Se Batteries Shuai-Feng Zhang,1,2 Wen-Peng Wang,1,2 Sen Xin,1 Huan Ye,1,2 Ya-Xia Yin*,1,2 and Yu-Guo Guo*,1,2 1

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, and Beijing

National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, P. R. China. 2

School of Chemistry and Chemical Engineering, University of Chinese Academy of

Sciences, Beijing 100049, P. R. China.

KEYWORDS

Li-Se batteries, graphitic carbon, Se cathode, chain-like Se molecules, high-rate capability.

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ABSTRACT

A well-organized selenium/carbon nanosheets nanocomposite (Se/CNSs) is prepared by confining chain-like Sen molecules in hierarchically micro-mesoporous carbon nanosheets. A unique two-dimensional morphology and high graphitization degree of carbon nanosheets benefits fast Li+/e- access to the active Se, which guarantees a high utilization of Se during the (de)lithiation process. Besides, the chain-like Se molecules confined in the carbon matrix could alleviate the shuttle effect of polyselenides and promise a stable electrochemistry. Therefore, the resultant Se/CNSs delivers a highly reversible capacity, a long cycle life and favorable rate capabilities. Furthermore, a Li-Se pouch cell built from a metallic Li anode and the as-prepared Se/CNSs cathode exhibits an excellent electrochemical performance, demonstrating the potential of Se/CNSs in serving future energy storage devices with high energy density.

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1. Introduction

Commercial lithium-ion batteries (LIBs) with limitation in their theoretical energy output do not meet the ever-increasing demand on further elevating the energy density for future storage applications.1-4 Therefore, interests have been taken on Li-S batteries due to their high theoretical energy density of 2560 W h kg-1, which originates from the high electrode capacities (Li anode: 3860 mA h g-1, S cathode: 1672 mA h g-1) as yielded by the two-electron reaction between both electrodes.5-7 However, the S cathode in the Li-S battery suffers from the insulating nature of S and unavoidable dissolution of polysulfide intermediates during the cycling process.8 Carbon matrixes with high electric conductivity and optimized porous structure have been considered as the most promising ways to tackle the above problems with progress made in recent years.9-14 Unfortunately, limited cycling life and poor rate performance still inhibit the practical use of Li-S batteries.

Selenium, which belongs to the same group as S, has similar two-electron electrochemistry vs. Li. Meanwhile, the density of Se as high as 4.82 g cm-3 endows Li-Se batteries with high volumetric capacity of selenium (3253 mA h cm-3), which is close to that of S (3467 mA h cm-3).15-18 Apart from that, Se possesses much higher electric conductivity (10-5 S cm-1) than S (10-30 S cm-1). Therefore, Li-Se battery may be a promising candidate for both electric vehicles and electronic devices, where battery systems with high volumetric energy density are desired due to the limited space. However, polyselenide dissolution during cycling in ether-based electrolytes

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(which is similar to the polysulfides) leads to fast capacity fading, low Coulombic efficiency and poor cycling stability of Li-Se batteries.19-22 Much attempt has been made to suppress the dissolution of polyselenides.23-24 Confining Se in various carbon matrixes, such as mesoporous carbon25, micro-mesoporous carbon26-28 and graphene29-30, has been proved to be an effective strategy to retard the dissolution of polyselenides and improve the electric conductivity of cathode. Carbon matrix with diverse pore structures plays a critical role in determining the specific presence of Se molecules (e.g., cyclic Se8, chain-like Sen) and consequently accounting for different physical/chemical properties of Se. It has been found that chain-like Sen molecules are thermodynamically more stable and facilitate faster electronic conduction than the cyclic Se8 molecules.31-33 According to literature, nanoporous carbon matrixes are commonly prepared from two or more steps including the pyrolysis of carbon precursor and tuning the pore structure by chemical activation (e.g., KOH, CO2, H2O).9,

17

Nevertheless, the as-prepared

nanoporous pyrolytic carbons are usually amorphous and deliver poor electric conductivity. Therefore, it is highly desired to develop a facile method to synthesize nanoporous carbon materials with a high graphitization degree to host chain-like Sen molecules.32

Herein, we present a new type of Se/C cathode material by confining chain-like Sen into a highly graphitic carbon nanosheet (CNS), which is prepared by pyrolysis of potassium citrate by a low-cost and easy-handing method. A highly reversible

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capacity (almost equal to the theoretical capacity of Se), a long cycle life (1000 cycles at 0.5 C) and favorable rate capabilities (390 mA h g-1 at 10 C) of Li-Se battery are achieved by using the Se/CNSs cathode. The superior electrochemical performance is attributed to a rich content of micro-/mesopores and high graphitization degree of the carbon host. Furthermore, we have successfully realized a Li-Se pouch cell from the Se/CNSs cathode, which exhibits admirable cycling stability in carbonate-based electrolyte.

2. Experimental Section 2.1 Synthesis of Materials

Synthesis of CNSs. All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd and used directly without any further purification. In a typical synthesis, 10 g potassium citrate was grinded to powder and then poured into a nickel boat. The white power was decomposed at 600 oC in Ar atmosphere for 2 h and subsequently carbonized at 850 oC for another 2 h with a heating ramp of 5 oC min-1. After being cooled down to room temperature, the product was slowly added into diluted HCl and stirred for 1 h. Then the mixture was filtered, washed with the deionized water several times and dried at 80 oC for 12 h to obtain the carbon materials (denoted as CNSs-850). As a comparison, another carbon material was prepared under the same process except that the carbonization temperature was only 600 oC (denoted as CNSs-600).

Synthesis of Se/CNSs. The as-prepared CNSs-850 and Se powder with the weight 5



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ratio of (4:6) were mixed evenly and pressed into a slice at 10 MPa. After that, the Se/CNSs-850 was obtained by infusing Se into the carbon material under argon atmosphere at 240 oC for 12 h. The Se/CNSs-600 was also obtained in the same way for comparison.

2.2 Material Characterizations.

Transmission electron microscopy (TEM) images and EDXS elemental mapping images were carried out by using a Tecnai G2 F20 U-TWIN operated at 200 KV. SEM images of the as-prepared samples were taken by a JEOL 6710F field-emission scanning electron microscope (FE-SEM) operated at 10 kV. XRD measurements were collected by using Cu Kα radiation on a Rigaku D/max 2500 diffractometer. Nitrogen adsorption/desorption isotherms were obtained to analyze the pore structure of obtained materials by using an Autosorb-1 analyzer from Quantachrome Instruments at 77.3 K. A TG Netzsch 449F3 was used to perform thermogravimetric analysis (TGA). Raman spectroscopy was obtained by using a Digilab FTS3500 (bio-Rad) with a laser wave-length of 532 nm. X-ray photoelectron spectroscopy (XPS) was measured through a Thermos Scientific ESCA Lab 250Xi with 200 W monochromatic Al Kα radiation.

2.3 Electrochemical Characterizations.

A coin-type cell with 1M LiPF6 dissolved in EC/DMC/DEC (1:1:1, based on volume) as electrolyte was assembled in an argon-filled glove box with less than 0.1 ppm O2 and 0.1 ppm H2O and used to perform the electrochemical performance of 6


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Li-Se battery. The working electrode was prepared by coating the slurry consisting of Se/CNSs composite, Super P, NaCMC and SBR at a weight ratio of 80:10:5:5 on the pure aluminum (99.6 %, Goodfellow). The area density of obtained selenium electrode was about 2 mg cm-1. Lithium foil was used as counter electrode and reference electrode. Celgard separator was used as the isolation layer between the cathode and the anode. A Land electrochemical testing system was used to perform the electrochemical measurements of Li-Se batteries. The CV test was performed at the voltage range of 1.0-3.0 V with a scan rate of 0.05 mV s-1 on a Princeton PARSTAT MC 1000 multi-channel electrochemical workstation.

A Li-Se pouch cell was fabricated by using the laminated structure with 4 cathodes and 5 anodes. The slurry consisting of Se/CNSs-850, SP, NaCMC and SBR with the weight ratio of 93:3:1.5:2.5 was coated evenly on the both sides of the Al current collector. The area density of the obtained cathode was about 5.7 mg cm-2. The electrolyte used for pouch cell stayed the same. The electrochemical measurements were performed using the land electrochemical testing system with the current range from 1 mA to 5 A.

3. Results and Discussion

The CNSs were synthesized by one-step method without extra activation agent (Figure 1). According to the thermogravimetric analysis (Figure S1), the potassium citrate was first heated to 600 oC for 2 h under Ar atmosphere to completely decompose to C, H2O (g) and K2CO3. After that, the mixture was further carbonized 7



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at 850 oC for 2 h to obtain micro-mesoporous carbon materials (denoted as CNSs-850) by the self-generated K2CO3 activation.34-36 After heating the mixture of Se and CNSs-850 at 240 oC in Ar atmosphere, Se was infiltrated into the micropores of CNSs-850 driven by the capillary force to obtain Se/C composites (abbreviated as Se/CNSs-850).37 For comparison, the Se/CNSs-600 was prepared with the similar process except that the carbon matrix was prepared by only carbonization at 600 oC for 2 h.

SEM and TEM characterizations were employed to evaluate the morphology of the CNSs-850 and Se/CNSs-850. Figure 2a and Figure 2c show the two-dimensional nanosheet structure of CNSs-850. The CNSs-600 also has the similar structure (Figure S2a). The thickness of CNSs-850 is less than 100 nm, which is favor of Li+ diffusion and electrolyte infiltration to the electrode. Meanwhile, the width of 3-8 µm can effectively prevent the agglomeration during the electrode fabrication procedure. Furthermore, one can clearly see from HRTEM image (Figure 2d) that CNSs-850 have abundant nanopores and the spacing of aligned lattice fringes in the CNSs is about 0.34 nm, in accordance with the (002) plane of the sp2 carbon, indicating its high electric conductivity. After loading selenium, no selenium residues can be observed on the surface of carbon materials (Figure 2b, Figure S3a-b).38 The EDX elemental mappings (Figure 2e) demonstrate that the Se is uniformly distributed in the carbon materials, which is consistent with the results of the SEM images of Se/CNSs-850. Meanwhile, no obvious change of morphology feature about Se/CNSs-600 is achieved after loading Se into CNSs-600 (Figure S2b). 8


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XRD patterns and Raman spectroscopy are used to further probe the structure features of Se/CNSs composites. XRD patterns of selenium power, Se/CNSs-850, Se/CNSs-600 are shown in Figure 3a. After the heating process, the diffraction peaks of crystalline Se in Se/CNSs-850 disappear, indicating the selenium is in an amorphous state and completely dispersed into nanoporous carbon materials.20 However, the crystal diffraction peaks of Se in Se/CNSs-600 is clearly observed, demonstrating some crystal selenium still exists. According to the Figure 2b, the peaks of Se in Se/CNSs in Raman spectra is blueshifted to about 256.3 cm-1 in comparison with crystal Se and the peak intensity of Se in Se/CNSs-850 is lower than that of Se/CNSs-600, which may be attributed to the change of existing form of Se molecules.39-40 In addition, the peaks at ~1350 cm-1 are related to the defected and disordered carbon and the peaks at ~1580 cm-1 are related to the sp2 hybrid carbon atoms.41 The intensity ratio of ID/IG in Se/CNSs-850 is about 0.625 and lower than that of Se/CNSs-600, revealing that the CNSs-850 have high graphitization degree and excellent electric conductivity. This result is also proved by the 2D peeks at ~2700 cm-1 in the Se/CNSs-850.

The porous structures of the CNSs and Se/CNSs composite are investigated with N2 adsorption/desorption isotherms. According to Figure 3c, CNSs-850 has the type IV isotherm, implying the massive micro-/mesopores existed in the carbon materials. Those pores in CNSs-850 are derived from the decomposition of the precursor before 600 oC and the activation of K2CO3 at the carbonization process of 850 oC. The specific BET area is 1358 cm2 g-1 and the total pore volume is about 0.631 cm3 g-1, in 9



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which the micro-porous volume contribution is 0.502 cm3 g-1 (calculated from the Density Functional Theory method).42-44 In comparison, the CNSs-600 only has microporous structure and its pore volume (0.3 cm3 g-1) is lower than that of CNSs-850. This result suggests that CNSs-850 could host more Se than CNSs-600. Figure 3d shows that the pore size distribution of CNSs-850 is concentrated on nanopores of 0.8, 1.2 and 2 nm. These micropores have been proved to confine Se molecules in chain-like structure from previous report.18, 33 After loading Se, the pore volume of Se/CNSs-850 is reduced to 0.03 cm3 g-1, demonstrating that Se is completely infiltrated into the nanopores of CNSs (Figure S4). Thermogravimetric analysis is used to determine the Se ratio of the composite. Figure S5 shows the percentage of Se in the Se/CNSs-850 is about 60 %.45 And Se/CNSs-600 is prepared with the same loading amount of Se for comparison.

Coin cells (2032) were fabricated to evaluate the electrochemical performances of Li-Se batteries consisting of Se/CNSs-850 composites as the cathode and Li foil as the anode. The cyclic voltammograms of Se/CNSs-850 was measured between 1.0 V and 3.0 V at the rate of 0.05 mV s-1 (Figure S6). At the initial discharge-charge process, one pair of redox peaks is observed around 1.75/2.2 V, corresponding to the highly reversible conversion of chain-like Sen to Li2Se. In the subsequent cycles, sharp redox peaks almost overlap together, demonstrating that the composites have excellent cycling stability. Figure 4a shows the typical galvanostatic discharge/charge curves (GDC) of Se/CNSs-850 at 0.1 C (1C = 675 mA g-1), which is coincided with the cyclic voltammetry analysis. At the initial cycle, Se/CNSs-850 exhibits the initial 10


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specific capacity of nearly 900 mA h g-1 based on the mass of Se in Se/CNSs-850 composite, which is much higher than the theoretical capacity of Se and consistent with previous reports.40 Possible reason might come from the formation of a solid electrolyte interface (SEI) film on the surface of Se/CNSs-850 and the reversible lithiation of highly graphitic carbon. The detailed reason will be discussed later. Upon 2nd cycle, the specific discharge capacity is almost equal to theoretical capacity of Se , implying that chain-like Sen in micropores has high electrochemical activity. The cycling performance of Se/CNSs-850 is shown in Figure 4b. After 100 cycles, the reversible capacity retains about 600 mA h g-1, indicating the superior cycling stability. In addition, the Coulombic efficiency after the first cycle is nearly 100 %, confirming the complete alleviation of shuttle effect. After disassembling the cell, the morphology of Se/CNSs-850 still keeps the same, proving its superior structure stability (Figure S7). To evaluate its long cycle performance, the Se/CNSs-850 was also performed at the current of 0.5 C for 1000 cycles. The reversible capacity still remains about 376 mA h g-1 and the fading rate is as low as 0.044 % per cycle, proving the excellent cycling stability of Se/CNSs-850. As the comparison, the Se/CNSs-600 has an obvious voltage profile platform at 2.4-2.2 V at the first discharge process (Figure S8a), which may be attributed to the side reaction between crystal Se molecules on the surface of carbon material and carbonate-based electrolytes. During the sequential cycles, the reversible capacity of Se/CNSs-600 about 400 mA h g-1, lower than that of Se/CNSs-850, is achieved because of fewer nanopores in the CNSs-600 (Figure S8b). 11



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The rate capability of Se/CNSs-850 is further investigated at various rates from 0.1 C to 10 C (Figure 4c-d). The specific capacity is reduced from 700 mA h g-1 to 390 mA h g-1 when the current density increases from 0.1 C to 10 C, suggesting its superior rate capability. One slope voltage plateau at about 1.9 V still maintains and no obvious polarization is observed even at 10 C, validating the exceptional stability and conductivity of Se/CNSs-850. When the current recovers to 0.5 C, the Se/CNSs-850 still deliver the reversible capacity as high as 600 mA h g-1, which corresponds to 89 % of the theoretical capacity (Figure 4d). These results manifest the remarkable rate capability and abuse tolerance of Se/CNSs-850.46 The interface resistance of Se/C electrode is measured through the Electrochemical impedance spectroscopy (EIS). According to Figure S9a, the charge-transfer resistance (Rct) of the pristine Se/CNSs-850 cell is obviously smaller than that of Se/CNSs-600, proving the fast kinetic process of Se/CNSs-850 (Figure S9b). In addition, the interface resistance of Se/CNSs-850 cell does not change significantly even after 100 cycles, demonstrating the high interface stability of Se/CNSs-850. These results are also confirmed by the excellent electrochemical performances of Se/CNSs-850.

X-ray photoelectron spectroscopy was employed to further clarify the chemical composition change of Se/CNSs-850 during the discharge and charge process. Before discharge process, two peaks at about 284.8 and 286.5 eV are observed in the C1s spectra (Figure 5a), corresponding to the C-C and C-O respectively.18 After the discharge process, three new peaks at 290.2, 288.6 and 283.9 eV are generated and ascribed to the C-F, -COOLi and Cn-Li.47 The C-F and -COOLi generated during 12


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discharge process may be resulted from the decomposition of the electrolytes to from solid electrolyte interface (SEI) film on the surface of Se/CNSs-850 and the Cn-Li might come from the lithiation of sp2 hybrid carbon. After the subsequent charge process, the peaks of C-F, -COOLi still remain, but the peaks of Cn-Li disappear. Those results indicate that the irreversible decomposition of electrolytes and the reversible lithiation of highly graphitic carbon occur during the first discharge/charge process.48 As a result, Li-Se batteries show low Coulombic efficiency of 77.7% (Figure 4a) and slightly high capacity during the initial discharge/charge process. Similar phenomenon is also observed for pure CNSs-850 electrode (Figure S10). Furthermore, no obvious change of the peaks of C-F and -COOLi is observed at the 2nd cycle (Figure S11), demonstrating that the stable SEI layer has generated after first cycle. This is coincided with the high Coulombic efficiency about almost 100 % after the first cycle in GDC curves of Se/CNSs-850. In the spectra of Se (Figure 4b), two peaks at 55.4 and 56.2 eV correspond to the Se3d5/2 and Se3d3/2 respectively before cycling.18, 33, 49 After the discharge process, the binding energy of Se3d5/2 and Se3d3/2 is reduced to 55.1 and 55.9 eV due to the lithiation of Se. After the subsequent charge process, the binding energy is converted back to 55.4 and 56.2 eV respectively, indicating the excellent reversibility of chain-like Sen molecules in Se/CNSs-850.33

To further verify the practical application of our Se/CNSs composite, we prepared the Se/CNSs-850 through our easy-handing one-step method and assembled a Li-Se pouch cell with laminated structure (Figure 6a). The electrochemical performance of Se/CNSs-850 prepared in large quantity is first tested in coin cells at 13



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0.1 C. The reversible capacity of about 650 mA h g-1 is achieved, implying the excellent homogeneity and stability of Se/CNSs synthesis (Figure S12). In Figure 6b, the pouch cell exhibits a reversible capacity about 620 mA h at 0.1 C and the Coulombic efficiency is close to 100 % after the first cycle. After 20 cycles, the reversible capacity still retains about 600 mA h, indicating the good cycling stability (Figure 6c).

The Se/CNSs-850 with remarkable electrochemical performance can be attributed to the unique features of CNSs: (1) The micro-mesoporous structure of CNSs can load and confine Se into chain-like formation, thus suppressing the shuttle effect during the electrochemical process in carbonate-based electrolyte and promising high electrochemical activity and excellent cycling stability of Li-Se batteries. (2) The carbon materials with abundant hybrid sp2 carbon can effectively improve the electric conductivity of electrode and achieve high rate capability of Li-Se batteries. (3) The two dimensional and interlinked structure can shorten Li+ ion diffusion distance and facilitate the electrolyte infiltration. Therefore, the two-dimensional carbon nanosheets with abundant sp2 hybrid carbon prepared at 850 o

C may be the ideal substrate materials for Li-Se batteries.

4. Conclusions

In summary, we have achieved a scalable synthesis of a novel Se/C composite cathode material at a low cost. Among Se/CNSs composite, chain-like Sen molecule is confined in the micro-/mesopores carbon nanosheets with highly graphitic degree, 14


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which effectively eliminate the shuttle of polyselenides and exhibit superior electrochemical performance in Li-Se batteries. The optimal Se/C composite (Se/CNSs-850) shows a reversible capacity as high as 672 mA h g-1 (almost the theoretical capacity of Se), remains about 376 mA h g-1 after 1000 cycles at 0.5 C and is able to remain a reversible capacity of ~390 mA h g-1 at a high rate of 10 C, which strongly supports the favorable electrochemical stability and rate capability of Se/CNSs cathode material. Furthermore, a Li-Se pouch-cell is built based on the above Se/C composite cathode. We believe such a carbon matrix host with high graphitization degree and abundant micro-/mesopores is promising for building electrode materials with large specific capacities to enable high-energy storage systems.

ASSOCIATED CONTENT

Supporting Information

Supporting Information Available: TGA measurement of potassium citrate, SEM images of CNSs-600 and Se/CNSs-600, TEM image and HRTEM image of Se/CNSs-850, N2 adsorption/desorption of the Se/CNSs-850, TGA curve of Se/CNSs-850, CV profiles of Se/CNSs-850, SEM image of Se/CNSs-850 after 100 cycles,

Electrochemical

performances

of

Se/CNSs-600,

EIS

of

Se/CNSs,

Discharge/charge curves of CNSs-850 without loading Se, XPS spectra of the as-prepared Se/CNSs-850 for C 1s region during the 2nd cycle, Discharge/charge curves of Se/CNSs-850 prepared in quantity at the current of 0.1 C. 15



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AUTHOR INFORMATION Corresponding Author

* Tel/Fax: (+86)-10-82617069, E-mail: [email protected] (Y.G.G.); [email protected] (Y.X.Y.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the Ministry of Science and Technology of the People’s Republic of China (Grant No. 2016YFA0202500), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA09010300), the National Natural Science Foundation of China (Grant Nos. 51225204, U1301244, and 21127901) and the Chinese Academy of Sciences.

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Figure 1. Schematic illustration showing the preparation process of the Se/CNSs composite.

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Figure 2. Morphological characterization of the as-prepared CNSs-850 and Se/CNSs-850 composites. (a) SEM images of CNSs-850 and (b) Se/CNSs-850; (c) TEM images and (d) HRTEM images of CNSs-850. (e) Images by scanning transmission electron microscopy (STEM) and corresponding elemental mappings of bare Se and C.

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Figure 3. (a) XRD patterns and (b) Raman spectra of Se powder, Se/CNSs-600 and Se/CNSs-850. (c) N2 adsorption/desorption isotherms and (d) pore-size distribution curves of CNSs-600 and CNSs-850 obtained by the DFT method.

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Figure 4. Electrochemical performance of the Se/CNSs-850 cathode in a Li-Se battery. (a) Typical discharge/charge profiles and (b) cycling performance of Se/CNSs-850 at 0.1 C. (c) Discharge/charge voltage profiles and (d) cycling performance at different C rates. (e) Long cycling performance and Coulombic efficiency at 0.5 C. The specific capacity values are based on the mass of Se in composites.

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Figure 5. XPS spectra of the Se/CNSs-850 cathode. (a) C 1s spectra and (b) Se 3d spectra of the pristine cathode before discharge, after initial discharge to 1 V and after initial charge back to 3 V.

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Figure 6. Electrochemical performance of a Li-Se pouch cell based on the Se/CNSs-850 cathode. (a) Optical image showing the pouch cell is powering a light emitting diode (LED). (b) Voltage profiles and (c) cycling performance at 0.1 C.

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TOC

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Graphitic Nanocarbon–Selenium Cathode with Favorable Rate

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