carbon-rich core–shell composites as cathode materials for rechargeable lithium–selenium batteries

carbon-rich core–shell composites as cathode materials for rechargeable lithium–selenium batteries

Journal of Power Sources 279 (2015) 88e93 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/loca...

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Journal of Power Sources 279 (2015) 88e93

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Selenium/carbon-rich coreeshell composites as cathode materials for rechargeable lithiumeselenium batteries Zhian Zhang a, *, Xing Yang a, Zaiping Guo b, Yaohui Qu a, Jie Li a, Yanqing Lai a a b

School of Metallurgy and Environment, Central South University, Changsha 410083, China Institute for Superconducting & Electronic Materials, University of Wollongong, NSW 2522, Australia

h i g h l i g h t s  The Se/C coreeshell composites are synthesized by a one-step solution route.  The Se/C composites exhibit good electrochemical properties than pristine selenium.  The Se/C composites would be a promising cathode material for LieSe batteries.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 September 2014 Received in revised form 10 November 2014 Accepted 1 January 2015 Available online 2 January 2015

Selenium/carbon-rich (Se/carbon-rich) coreeshell composites are prepared by a one-step hydrothermal synthesis method as a cathode for rechargeable lithium batteries. The Se/carbon-rich composites are characterized and examined by X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetry (TGA) measurements. It is demonstrated from galvanostatic discharge/charge process that the Se/carbon-rich composites exhibit the discharge capacity of 558 mA h g1 in the first cycle and maintain capacity of 181 mA h g1 after 80 cycles at a rate of 0.5C, which is better than that of the pristine selenium. The result demonstrates that the unique coreeshell structure is effective in suppressing the dissolution of polyselenides into the electrolyte and in maintaining high utilization of the active materials during the charge/discharge process. It provides a new selenium-based cathode material for rechargeable lithium batteries. © 2015 Elsevier B.V. All rights reserved.

Keywords: Core-shell composites Hydrothermal synthesis method Polyselenides Selenium-based cathode material Rechargeable lithium batteries

1. Introduction The rapidly developing market for the emerging plug-in hybrid vehicle and mobile electronics has prompted the urgent need for rechargeable batteries with high energy density. Despite the numerous advantages, the overall energy density of lithium ion batteries is limited by the low capacity of current cathode materials [1e3]. Therefore, cathode materials with high specific capacity have been extensively investigated. Among them, Lithiumesulfur (LieS) battery has attracted extensive attention in the last two decades due to high theoretical capacity (1675 mA h g1) of sulfur cathode. However, the sulfur cathode still suffers from some issues [4e6]: (1) sulfur has low electronic conductivity; (2) high-order polysulfide intermediates are soluble in organic electrolytes; (3) sulfur undergoes large volume change during lithiation/delithiation. As a

* Corresponding author. E-mail address: [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.jpowsour.2015.01.001 0378-7753/© 2015 Elsevier B.V. All rights reserved.

congener of sulfur, selenium as a potential cathode material was studied in lithium batteries. Selenium has a theoretical gravimetric capacity of 675 mA h g1 based on molecular weight of 78.86 g mol1, which is lower than sulfur (1672 mA h g1). However, its theoretical volumetric capacity of 3253 mA h cm3 based on 4.82 g cm3 is comparable to that of sulfur (1672 mA h g1). Moreover, selenium has higher electrical conductivity, approximately 20 orders of magnitude greater than sulphur [7e12]. The advantages of selenium make it an alternative promising cathode material in high-energy density lithium batteries for specific applications. However, similar to sulfur cathodes, one of the biggest challenges to selenium is the dissolution issue of high-order polyselenides, resulting in a rapid capacity decrease, low Coulombic efficiency [9]. To address these issues, many experiments have been made, which focus on enhancing the electrical conductivity of the cathode and suppressing the loss of soluble polyselenide intermediates during cycling. Various conductive porous carbons [9,10,13e19],

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porous metal oxide [20], conducting polymers [12,21], and graphenes [12,22] have been used as host materials for selenium cathode to improve the electrochemical performance of LieSe batteries. Abouimrane et al. [7] conducted pioneering work on the use of Se and SeSx as a cathode material for rechargeable Li-ion and Na-ion batteries. Wang et al. [9] synthesized the seleniumimpregnated carbon composites containing 30 wt% selenium by infusing Se into mesoporous carbon at a temperature of 600  C under vacuum, exhibiting good electrochemical performances. Kundu et al. [12] coated conducting polymer polypyrrole and graphene on selenium nanofiber to improve the electrochemical performance and minimize the polarization between charging and discharging. Most of these method employed by heating treatment to confine Se into the pores of absorbing materials, thus these composites can improve electrical conductivity and trap some polyselenides during cycling. However, these cathodes were prepared by cumbersome synthetic processes. To make the selenium encapsulated porous structure via the templates, multiple synthetic steps are needed. In this study, we report the selenium/carbon-rich coreeshell composites prepared by a one-step hydrothermal synthesis method. The outer carbon shell not only ensures high electronic conductivity of the composite, but provides a nanoscale capsule to confine selenium and polyselenides. As a cathode for rechargeable lithium batteries, it exhibits good cycling performance and high Coulombic efficiency.

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cooled to room temperature, the final product was collected by centrifugation and washed with distilled water and ethanol several times. Finally, the products were dried in vacuum at 80  C for 12 h. The experiment treats sodium selenite with ascorbic acid to obtain Se [Eq. (1)]. Ascorbic acid was employed as both reductant and carbon source to form a carbon shell.

Na2 SeO3 þ 2C6 H8 O6 þ 4Hþ /2C6 H7 NaO6 þ Se þ 3H2 O

(1)

2.2. Materials characterization The overall morphology of the samples was examined with scanning electron microscope (SEM, Sirion 200) and transmission electron microscopy (TEM, Tecnai G2 20ST). The elements on the surface of the samples were identified by energy-dispersive X-ray spectroscopy (EDX). Powder X-ray diffraction (XRD, Rigaku3014) using Cu Ka radiation was employed to identify the crystalline phase of Se/carbon-rich coreeshell composites. Thermogravimetric analysis (TGA, SDTQ600) was conducted in determining the selenium content in the composites.

2. Experimental section 2.1. Material synthesis The synthesis process for Se/carbon-rich coreeshell composites is illustrated in Fig. 1. All the reagents were analytical grade. 0.07 g of sodium selenite (Sigma, 99.9% metals basis) was dissolved in 40 ml of ultrapure water containing 0.08 g of CTAB (Aladdin, 99.9% metals basis). Then 0.8 g of ascorbic acid (Aladdin, 99.9% metals basis) was added to the solution under vigorous stirring. The colour of the dispersion changed from colourless to red suggesting formation of the amorphous selenium. Then the mixture was transferred into a Teflon-lined autoclave with 60 ml capacity, followed by hydrothermal treatment at 200  C for 24 h in an oven. After

Fig. 1. Schematic illustration for the preparation process of Se/carbon-rich coreeshell composites.

Fig. 2. (a) X-ray diffraction patterns and (b) Raman spectra of the Se/carbon-rich coreeshell composites and the pristine selenium.

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2.3. Electrochemical measurements The Se/carbon-rich coreeshell composites were mixed with carbon black and sodium alginate binder to form the slurry at the weight ratio of 8:1:1. The slurries of the cathodes were coated on an aluminium foil in the thickness of 20 mm, dried at 60  C overnight, and then the cathodes were cut into pellets with a diameter of 1.0 cm and dried for 12 h in a vacuum oven at 60  C. The mass of aluminium foil with a diameter of 1.0 cm about 4.25 mg, and the mass of the Se/carbon-rich coreeshell composites on the pellet about 7.42 mg. The same method was used to fabricate the selenium cathode for comparison. All capacities in this study were calculated based on selenium mass. The electrochemical characterization was carried out using a CR2025 type coin cell with a lithium metal sheet as the counter and reference electrode and a Celgard 2400 film as separator. The Se/carbon-rich coreeshell composite cathodes were used as working electrode. The electrolyte used was 1 M bis(trifluoromethane) sulfonamide lithium salt (Sigma Aldrich) in a mixed solvent of 1,3-dioxolane (Acros Organics) and 1,2-dimethoxyethane (Acros Organics) with a volume ratio of 1: 1. The test cells were assembled in an argon-filled glove box (Universal 2440/750) in which oxygen and water contents were less than 1 ppm. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted using the PARSTAT 2273 electrochemical measurement system. CV tests were performed at a scan rate of 0.2 mV s1 in the voltage range of 1.0e3.0 V. The galvanostatic charge/discharge tests were carried out at a constant current density of 337.5 mA g1 (0.5C) in the potential range of 1.0e3.0 V under a LAND CT2001A

Fig. 3. SEM micrograph (a,b), TEM micrograph (c,d) and elemental mapping images (e) of the Se/carbon-rich coreeshell composites in the border area enclosed by the square in the TEM micrograph (d).

chargeedischarge system. All experiments were conducted at room temperature. 3. Results and discussion Fig. 2(a) shows the characteristic XRD pattern of the pristine selenium and the Se/carbon-rich coreeshell composites, respectively. It can be seen that the X-ray diffraction peak of the Se/ carbon-rich coreeshell composites around 23.5 (100), 29.7 (101), 41.3 (110), 43.6 (102), 45.4 (111), 51.8 (201), 55.7 (112), and 61.5 (202) are in good agreement with the standard spectrum of selenium (JCPDS, card no: 06-0362) [9e11,21]. However, apart from diffraction peak of the Se/carbon-rich coreeshell composites, no obvious peaks for carbon or graphite can be observed. This suggests that the carbon exists mostly in amorphous state due to the process of hydrothermal synthesis at low temperature [21,23]. Fig. 2(b) shows the Raman spectrum of the pristine selenium and the Se/carbon-rich coreeshell composites. A strong peak located at 234 cm1 can be observed in the Raman spectrum of the pristine selenium, which is a characteristic signature of trigonal selenium and can be assigned to the stretching vibration of helical selenium chains (A1 mode). The peak at 140 cm1 corresponds to the transverse optical phonon mode (E mode) [24,25]. The Raman spectrum of the products shows two obvious peak at 1372 cm1 (D band) and 1600 cm1 (G band), which corresponds to the vibrations of the carbon atoms of disordered amorphous carbon and crystalline graphite [26]. A weak characteristic resonance peak of t-Se at

Fig. 4. SEM (a,b) micrograph of the as-synthesized samples without selenium core; EDX spectrum of (c,d) the Se/carbon-rich coreeshell composites and the assynthesized samples without selenium core.

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262 cm1 also can be observed, which assigned to the ringstructured Se8 [9]. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis were used to investigate the microstructure of the as-synthesized samples. Fig. 3(a) and (b) presents the SEM image of the as-synthesized samples. The general overview SEM image of the as-synthesized samples shows exclusively one-dimensional (1D) structure with a micrometer length distribution and diameter of 500e700 nm. Fig. 3(b) is a high magnification SEM image which demonstrates the structural character of the coreeshell structures with an inner core and an outer shell. Fig. 3(c) presents the TEM image of the as-synthesized samples with the distinct dark/light contrast radial direction. It indicates a difference of phase composition of the coreeshell structures, which is in good agreement with the results of SEM. As shown in Fig. 3(d), elemental mapping was employed in the border area surrounded by the square. As shown in Fig. 3(e), the carbon was mainly distributed in the shell region and the selenium was only distributed in the core region. It can be further confirmed that the as-synthesized samples possess an inner selenium core and an outer carbon shell. The outer carbon shell can enhance the conductivity of the coreeshell composites, and a thicker selenium core indicates that the Se/carbon-rich coreeshell composites can load enough active materials. As reported that selenium is easily soluble in N2H4$H2O [27,28], the as-synthesized composites were soaked in hydrazine monohydrate (N2H4$H2O, 85%) for 4 h to eliminate the selenium core. Fig. 4(a) and (b) present the SEM images of as-synthesized samples after soaking, demonstrating that as-synthesized samples soaked by N2H4$H2O still keep previous morphology. Fig. 4(c) and (d) show the EDX spectrum of Se/carbon-rich coreeshell composites and assynthesized samples without selenium core, respectively, revealing that the selenium core of the samples disappeared after soaking treatment. As shown in Fig. 5, the Se content in the Se/carbon-rich coreeshell composites was determined to be 43.2 wt% by thermogravimetric analysis. In order to understand the reduction/oxidation reactions in LieSe batteries, the CV curves of the Se/carbon-rich composite cathode was recorded at the scan rate of 0.2 mV s1 in the potential range 1.0e3.0 V as shown in Fig. 6. It can be seen that the Se/ carbon-rich composite cathode have two obvious cathodic peaks and one anodic peak, which are consistent with the results using the similar ether-based electrolyte [8,20,29].

Fig. 5. TGA curve of the Se/carbon-rich coreeshell composites and as-synthesized samples without selenium core under N2 atmosphere.

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Fig. 6. Cyclic voltammogram profile of the Se/carbon-rich coreeshell composites at a scan rate of 0.2 mV s1 in the voltage range 1.0e3.0 V.

Fig. 7(a) displays the discharge/charge curves in the 1st, 2nd, 10th, 30th, 60th and 80th cycles of the Se/carbon-rich coreeshell composites in the voltage range between 1.0 and 3.0 V at the current density of 337.5 mA g1 (0.5C). Consistent with the results reported by Abouimrane et al. [7,30], for the cell with ether-based electrolyte, there are plateaus at around 2.2 and 2.0 V in the first discharge, which correspond to the reduction of Se to high-order polyselenides Li2Sen (n  4) and further reduction to Li2Se2 and Li2Se [8,9,30]. The discharge/charge curves of the Se/carbon-rich coreeshell composites with stable potential plateaus confirm that the composite cathode possesses high reversibility and good stability. Fig. 7(b) shows the discharge/charge curves in the 1st, 10th and 30th of the Se/carbon-rich coreeshell composites and the pristine selenium. Note that the difference between charge and discharge voltages of the Se/carbon-rich coreeshell composites is smaller than that for the pristine selenium cathode. The smaller electrochemical polarization demonstrates a better conductivity of the Se/carbon-rich coreeshell composite cathode. As presented in Fig. 7(c), the Se/carbon-rich coreeshell composites exhibit discharge capacity of 558 mA h g1 in the first cycle and maintain capacity of 181 mA h g1 after 80 cycles at a rate of 0.5C. Different from previous report [11,21], the pristine selenium displays an initial specific discharge capacity of 321 mA h g1 and retain 84 mA h g1 after 80 cycles. There are several factors accounting for the difference. Firstly, carbonate-based electrolyte can positively affect electrochemical performance compared with the ether-based electrolyte [30]. Secondly, a different current density (0.5C) was employed in our work. Moreover, the composite cathode presents a higher Coulombic efficiency. These indicate that the unique coreeshell structure of the composites is effective in suppressing the dissolution of polyselenides into the electrolyte and in maintaining high utilization of the active materials during the charge/discharge process. Fig. 7(d) displays the rate capability of selenium/carbon coreeshell composites and the pristine selenium. The cells were tested with different C-rates at 0.5C, 1C, 2C, and 3C for 5 cycles each and then the durability test was continued at 0.5C for 5 cycles. After 15 cycles at a current density of 3C (2025 mAh g1), the Se/carbonrich composite cathode shows a rate reversible capacity of 155 mAh g1 better than the pristine selenium of 75 mAh g1. After 20 cycles, as the current density returns to 0.5C, the reversible specific capacity of Se/carbon-rich composite cathode can remains at values of 190 mAh g1, which is double of the pristine selenium. Fig. 8 displays the impedance plots for the Se/carbon-rich

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Fig. 7. (a) Discharge/charge curves in the 1st, 2nd, 10th, 30th, 60th and 80th cycles of the Se/carbon-rich coreeshell composites; (b) Discharge/charge curves in the 1st, 10th and 30th of the Se/carbon-rich coreeshell composites and the pristine selenium; (c) Cyclability and Coulombic efficiency of the Se/carbon-rich coreeshell composites and the pristine selenium at the current density of 337.5 mA g1; (d) The rate capability of Se/carbon-rich coreeshell composites and the pristine selenium (All capacities in this study were calculated based on selenium mass).

composite cathode, and the (b) Se/carbon-rich coreeshell composite cathode after cycling at 0.5C for 50 cycles. The Se/carbon-rich coreeshell composite cathode still maintains a relatively structural integrity, indicating that the electrochemical process has limited impact on the cathode structure during cycling. Moreover, the fibrous structure of Se/carbon-rich composite is beneficial for constructing a carbon network, which provided electronic conduction pathways and worked as mechanical support. Therefore, the Se/carbon-rich coreeshell composite is a promising seleniumbased cathode material for rechargeable lithium batteries. 4. Conclusion

Fig. 8. Impedance plots and equivalent circuit for the Se/carbon-rich coreeshell composites and the pristine selenium before cycling.

coreeshell composites and the pristine selenium before cycling. All of the impedance spectra have the similar feature: a depressed semicircle at medium-to-high frequency and an inclined line at low-frequency, which is similar to the previous literature [8,20,29]. Moreover, an equivalent circuit was used to fit the spectra of the batteries, which is displayed in Fig. 8. According to the equivalent circuit, a quantitative analysis of the spectra was conducted. The Se/ carbon-rich coreeshell composites exhibited a much lower charge transfer resistance than that of pristine selenium (36.07 U cm2 vs. 87.89 U cm2), which can be attribute to the better conductivity of the Se/carbon-rich coreeshell composite cathode. Fig. 9 shows SEM images of (a) Se/carbon-rich coreeshell

In summary, we have successfully synthesized Se/carbon-rich coreeshell composites by a one-step hydrothermal synthesis method as a cathode for rechargeable lithium batteries. It exhibit discharge capacity of 558 mA h g1 in the first cycle and maintain capacity of 181 mA h g1 after 80 cycles at a rate of 0.5C, which is due to the good electronic conductivity and the unique structure of the Se/carbon-rich coreeshell composites to confine selenium and

Fig. 9. SEM images of (a) Se/carbon-rich composite cathodes before cycling, the (b) Se/ carbon-rich composite cathodes after cycling at 0.5 rate for 50 cycles.

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polyselenides. It provides a new Se-based cathode material for rechargeable lithium batteries. Acknowledgement The authors thank the financial support of the Teacher Research Fund of Central South University (2013JSJJ027). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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