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reduced graphene oxide composite as high-performance anode material for sodium-ion batteries
Author’s Accepted Manuscript Amorphous CoS Nanoparticle/Reduced Graphene Oxide Composite as High-Performance Anode Material for Sodium-Ion Batteries Xiaoming Zhu, Xiaoyu Jiang, Xiaoling Liu, Lifen Xiao, Xinping Ai, Hanxi Yang, Yuliang Cao www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(17)30738-1 http://dx.doi.org/10.1016/j.ceramint.2017.04.128 CERI15108
To appear in: Ceramics International Received date: 21 March 2017 Revised date: 23 April 2017 Accepted date: 24 April 2017 Cite this article as: Xiaoming Zhu, Xiaoyu Jiang, Xiaoling Liu, Lifen Xiao, Xinping Ai, Hanxi Yang and Yuliang Cao, Amorphous CoS Nanoparticle/Reduced Graphene Oxide Composite as High-Performance Anode Material for Sodium-Ion Batteries, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.04.128 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Amorphous CoS Nanoparticle/Reduced Graphene Oxide Composite as High-Performance Anode Material for Sodium-Ion Batteries Xiaoming Zhua, Xiaoyu Jiangb, Xiaoling Liua, Lifen Xiaoc, Xinping Aib, Hanxi Yangb, Yuliang Caob* a
Hubei Collaboration Innovation Center of Non-power Nuclear Technology, School of Nuclear Technology & Chemistry and Biology, Hubei University of Science and Technology, Xianning 437100, (P. R. China) b College of Chemistry and Molecular Science, Hubei Key Laboratory of Electrochemical Power Sources, Wuhan University, Wuhan 430072, China c College of Chemistry, Central China Normal University, Wuhan 430079, China. *Corresponding author: Yuliang Cao, Tel: (+) 86-27-68754526, E-mail: [email protected]
Abstract Transition metal sulfides have been proved as promising candidates of anode materials for sodium-ion batteries (SIBs) due to their high sodium storage capacity, low cost and enhanced safety. In this study, the amorphous CoS nanoparticle/reduced graphene oxide (CoS/rGO) composite has been fabricated by a facile one-step electron beam radiation route to in situ decorate amorphous CoS nanoparticle on the rGO nanosheets. Benefiting from the small particle size (~2 nm), amorphous structure, and electronic conductive rGO nanosheets, the CoS/rGO nanocomposite exhibits high sodium storage capacity (440 mAh g-1 at 100 mA g-1), excellent cycling stability (277 mAh g-1 after 100 cycles at 200 mA g-1, 79.6% capacity retention) and high rate capability (149.5 mAh g-1 at 2 A g-1). The results provide a facile approach to fabricate promising amorphous and ultrafine metal sulfides for energy storage.
Key words: CoS nanoparticle; amorphous; reduced graphene oxide; sodium-ion battery; electron beam radiation
1. Introduction Renewable energy sources have attracted intense attention in the wake of the exhaustion of non-regenerative fossil fuels. Sodium-ion batteries (SIBs) have been considered to be intriguing candidates for next-generation low-cost energy storage systems instead of lithium-ion batteries (LIBs). [1-3] However, it is a more arduous challenge to find suitable materials that can accommodate Na+ and allow reversible Na+ insertion/deinsertion. [4, 5] So far, various alternative materials include carbon, phosphate hosts, metal oxides/sulfides and alloy based materials have been explored as potential anodes for SIBs.[5-10] Transition metal chalcogenides (such as SnS [9], SnS2 [11], FeS2 [12], NiS2 [13], MoS2 [14] and Sb2S3 [15]) have been widely studied due to their high theoretical capacity based on electrochemical conversion mechanisms. Besides, the weaker M-S bond of metal sulfides compared with M-O of metal oxides can be kinetically favorable for conversion reaction [16]. Among these materials, although cobalt sulfides (such as CoS, CoS2, Co3S4 and Co9S8) have been widely studied in energy storage due to their unique physical and chemical properties, most of research are focused on application in LIBs, dye-sensitized solar cells and supercapacitors [17-23]. Only few studies were reported for sodium-ion batteries. Shadike et al. demonstrated that CoS2–MWCNT nanocomposites exhibit a high reversible capacity of 770 mAh g-1 and retain 568 mAh g-1 after 100 cycles in ether-based electrolyte [24]. Zhou et al. reported the sodium storage performance of Co3S4@polyaniline nanotubes. The results indicated that the Co3S4@polyaniline nanotubes exhibited an initial discharge capacity of
578.8 mAh g-1 and decrease to 252.5 mAh g-1 after 100 cycles at 200 mA g-1 in propylene carbonate (PC) electrolyte [16]. The comparisons of the electrochemical performances of CoS/rGO in different electrolytes were also reported by Peng et al. Among the electrolytes,
diethylene glycol dimethylether (DEGDME) with NaCF3SO3 showed a smaller voltage polarization and larger capacity retention than that in carbonate-based electrolyte (ethylene carbonate/diethyl carbonate and propylene carbonate) [25]. Herein, we demonstrate a facile one-step electron beam assisted solution reaction to in situ decorate CoS amorphous nanoparticles on rGO nanosheets and disclose its electrochemical performances as SIBs anode materials in the commonly used carbonate-based electrolyte. The solution reaction assisted by the electron beam radiation has a fast reaction rate, which is advantageous for forming amorphous and nanosized particles with narrow size distribution [26, 27]. The amorphous CoS nanoparticles facilitate to enhancing their electrochemical kinetics, improving the sodium ion diffusion, decreasing the structural stress/strain and narrowing the potential hysteresis [28-30]. In addition, the graphene network can enhance the electronic conductivity to improve the capacity and rate capability, as well as buffer the volume change to prevent them from aggregation during cycling [31]. These unique features endow the CoS/rGO nanocomposite with high sodium storage capacity (440 mAh g-1 at 100 mA g-1), excellent cycling stability (277 mAh g-1 after 100 cycles at 200 mA g-1, 79.6% capacity retention) and high rate capability (149.5 mAh g-1 at 2 A g-1). 2. EXPERIMENTAL METHODS 2.1 Synthesis of CoS/rGO nanocomposites All the chemicals are analytical grade and used as purchased. Graphene oxide (GO)
suspension was obtained from Shandong Yuhuang New Energy Technology Co., Ltd. In a typical procedure, 2.5 g Co(CH3COO)2·4H2O (Sinopharm Chemical) and 3.16 g Na2S2O3 (Aladdin) was dissolved in 375 mL deionized water (18 MΩ cm-1, ULUPURE, Chengdu, China). Then 10 mL GO suspension (10 mg mL-1) was added into the solution under stirring. A few isopropanol (2 mL) was added into the resulting mixture as a radical scavenger. The mixture was sonicated for 15 min and then transferred into a PE package to radiate using a 1 MeV electron beam accelerator (Wasik Associates, USA) at a dose of 250 KGy at a dose rate of 50 KGy per pass (1 Gy = 1 J Kg -1). The resulting products were washed with deionized water for several times and then freeze-drying (SCIENTZ-10N, Ningbo, China). In the contrast experiment, the bare rGO and bare CoS were prepared by electron beam radiation under the same conditions. 2.2 Materials characterization X-ray diffraction (XRD) was performed on a Shimazdu, model LabX XRD-6000 instrument using Cu Kα radiation. Raman spectroscopy was performed with a laser micro-Raman spectrometer (Renishaw in Via, Renishaw, 532 nm excitation wavelength). X-ray photoelectron spectroscopy (XPS) was conducted by using a Thermo Fisher ESCALAB 250Xi with an Al Kα X-ray source. Thermogravimetric analysis (TGA) was carried out (Thermo analysis instrument Q 500, Burlington) with a heating rate of 10 °C min-1 in air. Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed by a field-emission scanning electron microscope (FE-SEM, ZEISS Merlin Compact VP, Germany; EDS, Oxford Instruments Link ISIS). Transmission electron microscope (TEM) was performed using a field-emission transmission electron microscope (JEM-2100 HR). 2.3 Electrochemical measurement
The electrode was prepared by mixing the CoS/rGO (or bare CoS, rGO) with Super P, polytetrafluoroethylene (PTFE) and sodium carboxymethylcellulose (CMC) in deionized water (8:1:0.5:0.5, weight ratio). The obtained slurry was pasted on a Cu foil followed by drying in a vacuum oven at 100 °C for 12 h. The 2016-type coin cells with sodium foil as the counter electrode, 1 M NaClO4 in ethylene carbonate/diethyl carbonate (1:1, by volume) containing 2 vol% fluoroethylene carbonate as the electrolyte were assembled in argon-filled glove box. The cells were galvanostatically cycled on a LAND cycler (Wuhan LAND Electronics Co., China) between 0 V and 3 V. Cyclic voltammetry (CV) measurements and electrochemical impendence spectroscopy (EIS) tests were carried out with Autolab PGSTAT128N (Eco Chemie, Netherlands). Cyclic voltammetry measurements were carried out at a scan rate of 0.1 mV s-1 between 0 and 3 V (versus Na/Na+). The electrochemical impendence spectroscopy (EIS) were tests
over the
frequency range from 100 kHz to 0.01 Hz with amplitude of 10 mV.
3. RESULTS AND DISCUSSION Figure 1 shows a simplified schematic illustration for the formation of CoS/rGO nanocomposites. In the electron beam radiation process, water radiolysis yields three major primary transient species, eaq-, HO· and H·(equation 1, the numbers are the radiation chemical yields in units of µmol J-1), which eaq- and H· have strong reducibility while HO· has strong oxidizing ability [27]. Isopropanol is added into the aqueous solution to scavenge the HO/H radicals, leaving behind the eaq- in the solution so as to react with the thiosulfate to released S2homogeneously. Then the produced S2- react with Co2+ to form CoS nanoparticles, which tightly anchored onto graphene (equation 2-4) [32]. Meanwhile, the graphene oxide is also reduced by the
eaq- and isopropanol radicals (equation 5) [33]. As a result, such fast reaction for CoS can efficiently suppress the growth of crystalline CoS so as to facilitate the formation of amorphous and ultra-tiny CoS, which in situ tightly anchored onto the surface of rGO nanosheets. radiation H2O 0.26eaq 0.06H 0.27 OH+0.045H2 0.07H2 O2 0.26H3O
(CH3 )2CHOH+ OH/ H (CH3 )2C OH+H2O/H2 eaq S2O32 S SO32
S2 + Co2 CoS
(1)
(2)
(3)
(4)
(CH ) COH/e
3 2 aq GO rGO
(5)
The XRD patterns of the CoS and CoS/rGO are shown in Figure 2a. No obvious diffraction peaks can be discerned, indicating the amorphous nature of the materials. The graphene content is estimated to be 25% according to the TG analysis. Figure 2b displays the Raman spectra of the CoS and CoS/rGO. Both of the materials exhibit the peaks at about 467, 511 and 663 cm-1 correspond to the Eg, F2g and A1g modes of CoS, respectively [25]. As for the CoS/rGO, there are two additional Raman peaks occurring at 1349 cm-1 (D band) and 1586 cm-1 (G band), corresponding to the characteristic of the graphene. Figure 2c gives XPS spectrum of the CoS/rGO. As expected, Co, S, O and C elements are detected. The C1s XPS spectra of GO and CoS/rGO are presented in Figure 2d. The peak of rGO attributed to C-OH (286.4 eV) is attenuated and the O=C-OH (288.3 eV) peak almost eliminated, indicating loss of oxygen and reduction of GO [34]. It is noted that a slight shift to lower binding energy of C-OH is observed for CoS/rGO, indicating some sample charging in the spectra for GO [35]. In the high-resolution Co 2p XPS spectra (Figure 2e and 2f), the peaks at 779.2 and 794.2 eV can be attributed to Co3+ while the peaks at
781.2 and 797.1 eV are ascribed to Co2+.[36, 37] The relative intensity and area of the main 2P3/2 line indicate the majority of Co2+. The presence of a small number of Co3+ could correspond to partial oxidation of CoS due to the exposure in air.[25, 38] SEM images on the CoS and CoS/rGO are shown in Figure 3a-c. Figure 3a displays the typical CoS has spherical-like morphology with size of around 40 nm. However, in the presence of rGO, the CoS on the rGO can hardly be observed and the rGO nanosheets exhibit only rough surface (Figure 3b). To ensure the presence of CoS on the rGO, the EDS mapping images of CoS/rGO were used to confirms the presence and homogeneous distribution of Co, O, C elements in the composites (Figure 3c), indicating that the size of the CoS particles is very tiny. It proves that rGO nanosheets play a vital role in the nuclei formation and growth of CoS nanoparticles, which is possibly due to the confining effect of GO [39]. The TEM image (Figure 3d) of the bare CoS shows that the morphology and size of the CoS is consistent with SEM images (Figure 3a). For the CoS/rGO, the TEM images of the CoS/rGO reveal that the CoS nanoparticles with the size of about 2 nm homogenously disperse on the surface of rGO (Figure 3e, 3f). The selected-area electron diffraction (SAED) patterns shown in inset of Figure 3d and Figure 3e indicate the amorphous nature of both bare CoS and CoS/rGO, which is in agreement with the XRD results. Figure 4a and b show the cyclic voltammogram (CV) curves for CoS and CoS/rGO electrodes at scan rate of 0.1 mV s-1 in the potential range of 0-3 V. Two reduction peaks at 1.4 V and 0.9 V are observed for both electrodes, which are associated with the multi-electron reaction mechanism similar with LIBs [24, 25]. The reduction peak at 1.4 V can be related to the initial insertion of sodium forming NaxCoS, and the peak at 0.9 V might originate from the conversion reaction to form Na2S and Co. For the initial anodic process of CoS electrode, the oxidation peak
at 1.85 V can be assigned to the reverse conversion reaction of Co with Na2S. In the subsequent cycles, the oxidation peak shifts to 2.0 V, suggesting high electrochemical polarization, similar to the previous work [17]. However, for the initial anodic process of the CoS/rGO, there are three peaks at 1.15 V, 1.61 V and 2.09 V (Figure 4b). The peak at 1.15 V could be assigned to desodiation of rGO while the peaks at 1.61 V and 2.09 V belong to the reaction: Co + Na2S → NaxCoS + (2 − x)Na + (2 − x)e, NaxCoS → CoS + xNa+ + xe, respectively.[40] In the subsequent cycles, these peaks are almost overlapped, indicating highly reversibility of the CoS/rGO compared to the bare CoS (Figure 4a). The differences in CV curves between the CoS/rGO and bare CoS could be due to the introduction of rGO to not only induce the formation of the extra tiny particles of CoS, but also enhance the electron conductivity, which improve the electrochemical reversibility of CoS/rGO electrode. Figure 4c and d show the first three charge-discharge curves of the CoS and CoS/rGO electrodes at 100 mA g-1, respectively. Both electrodes show slight potential plateaus located at 1.4 V and 0.9 V during discharging. In the charge process, the CoS electrode exhibits a potential plateau at 1.9 V in first cycle and shifts to higher potential in the following cycle. For the CoS/rGO, two successive charge plateaus are almost fixed at 1.7 and 2.1 V for first three cycles, indicating good electrochemical reversibility of the CoS/rGO. The results were well matched to the CV curves (Figure 4a and b). The CoS electrode displays an initial discharge/charge capacities of 539.1 and 345.5 mAh g-1 respectively, corresponding to a coulombic efficiency of 64.1%. In contrast, the initial discharge/charge capacities for the CoS/rGO are 643.7 and 440.1 respectively, showing a coulombic efficiency of 68.4%, higher in both capacity and efficiency than those of the CoS. These results indicate that the incorporation of the rGO enhances the electrochemical
utilization of the composites due to the improvement of electron conductivity and decrease of the particle size. The cycling performance of the rGO, CoS and CoS/rGO is shown in Figure 5a. The capacity values are calculated based on the total mass of the composite. Both the rGO and CoS electrodes exhibit lower electrochemical performance than that of CoS/rGO. The rGO shows low initial charge capacity (133.5 mAh g-1) and poor coulombic efficiency (25.4%). For the CoS electrode, the specific capacity decays to 150 mAh g-1 at the 40th cycle and then rapidly reduces to 10 mAh g-1. However, the CoS/rGO electrode shows a reversible capacity of 277 mAh g-1 at 100th cycle with the coulombic efficiency of 99.5%, corresponding to 79.6% of the initial charge capacity. Figure 5b compares the rate capability of the CoS and CoS/rGO. The CoS electrode delivers a reversible capacity of 345.5, 202.4, 146.1, 104.6 and 57.4 mAh g-1 at current rates of 100, 200, 500, 1000 and 2000 mA g-1, respectively, which are considerably lower than the CoS/rGO electrode (440.1, 379.1, 325.8, 280.1, 149.5 mAh g-1) at the same rate. When the current density is reduced back to 100 mA g-1, the capacity of the CoS/rGO can be also recovered to 356.7 mAh g-1. The good cycling stability and rate capability of the CoS/rGO electrode can be ascribed to uniformly distributed tiny CoS particles in the conductive rGO network. The ultrafine CoS particles can mitigate the volume change and improve reaction kinetics, while the rGO network provides high conductive support and restraints particle pulverization during cycling. Electrochemical impedance spectroscopy was conducted to clarify the improved electrochemical performance of the CoS/rGO composite. Figure 6 shows the Nyquist plots of the CoS and CoS/rGO after 20th cycle. Based on the equivalent circuit model in the inset of Figure 6, the charge-transfer resistance (Rct) of the CoS and CoS/rGO electrode is 239 Ω and 26 Ω,
respectively. It is obvious that the Rct of CoS/rGO is much lower than that of the CoS, indicating a fast reaction kinetics owing to combination with the conductive rGO [41]. 4. Conclusions In summary, amorphous CoS nanoparticles were anchored on the surface of graphene nanosheets by a facile one-step electron beam radiation route as anode material for SIBs. The electrochemical tests showed the CoS/rGO nanocomposite exhibits a high reversible capacity, cycling stability and superior rate performance. The improvement of the electrochemical properties is due to the extremely tiny CoS particle (~ 2nm) and electron conductive rGO network. In addition, the electron beam radiation route is also a sample and convenient method for preparation of amorphous, ultrafine and uniformly distributed metal sulfides for energy storage, biomaterial and catalyst applications.
Acknowledgements The authors gratefully acknowledge the financial support by the 2011 Program of Hubei Province, the National Key Basic Research Program of China (No. 2015CB251100) and National Science Foundation of China (No. 21673165, 21373155 and 21333007), Natural Science Foundation of Hubei Province, China (Grant No. 2015CFC774), Program for New Century Excellent Talents in University (NCET-12-0419) and Hubei National Funds for Distinguished Young Scholars (2014CFA038).
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Figure 1. Schematic illustration of the formation of the CoS/rGO nanocomposites. Figure 2. (a) XRD patterns and (b) Raman spectra of the CoS and CoS/rGO samples, (c) XPS spectrum of CoS/rGO, (d) C1s high-resolution spectra of the CoS/G and GO. Co 2p high-resolution spectrum of (e) the CoS and (f) the CoS/rGO. Figure 3. SEM images of the (a) CoS particles and (b) CoS/rGO nanocomposites. (c) The element mapping images of the CoS/rGO nanocomposites. TEM images of the (d) CoS particles and (e, f) CoS/rGO nanocomposites. The insets are corresponding SAED pattern. Figure 4. CV curves of the (a) CoS and (b) CoS/rGO electrodes and the discharge/charge profiles of (c) the CoS and (d) CoS/rGO electrodes. Figure 5. (a) Cycling performance of the rGO (50 mA g-1), CoS and CoS/rGO (200 mA g-1) and (b) rate performance of the CoS and CoS/rGO electrodes. Figure 6. Electrochemical impedance spectra of the (a) CoS and (b) CoS/rGO electrodes after 20th cycle. The inset is the equivalent circuit.
PII: DOI: Reference:
S0272-8842(17)30738-1 http://dx.doi.org/10.1016/j.ceramint.2017.04.128 CERI15108
To appear in: Ceramics International Received date: 21 March 2017 Revised date: 23 April 2017 Accepted date: 24 April 2017 Cite this article as: Xiaoming Zhu, Xiaoyu Jiang, Xiaoling Liu, Lifen Xiao, Xinping Ai, Hanxi Yang and Yuliang Cao, Amorphous CoS Nanoparticle/Reduced Graphene Oxide Composite as High-Performance Anode Material for Sodium-Ion Batteries, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.04.128 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Amorphous CoS Nanoparticle/Reduced Graphene Oxide Composite as High-Performance Anode Material for Sodium-Ion Batteries Xiaoming Zhua, Xiaoyu Jiangb, Xiaoling Liua, Lifen Xiaoc, Xinping Aib, Hanxi Yangb, Yuliang Caob* a
Hubei Collaboration Innovation Center of Non-power Nuclear Technology, School of Nuclear Technology & Chemistry and Biology, Hubei University of Science and Technology, Xianning 437100, (P. R. China) b College of Chemistry and Molecular Science, Hubei Key Laboratory of Electrochemical Power Sources, Wuhan University, Wuhan 430072, China c College of Chemistry, Central China Normal University, Wuhan 430079, China. *Corresponding author: Yuliang Cao, Tel: (+) 86-27-68754526, E-mail: [email protected]
Abstract Transition metal sulfides have been proved as promising candidates of anode materials for sodium-ion batteries (SIBs) due to their high sodium storage capacity, low cost and enhanced safety. In this study, the amorphous CoS nanoparticle/reduced graphene oxide (CoS/rGO) composite has been fabricated by a facile one-step electron beam radiation route to in situ decorate amorphous CoS nanoparticle on the rGO nanosheets. Benefiting from the small particle size (~2 nm), amorphous structure, and electronic conductive rGO nanosheets, the CoS/rGO nanocomposite exhibits high sodium storage capacity (440 mAh g-1 at 100 mA g-1), excellent cycling stability (277 mAh g-1 after 100 cycles at 200 mA g-1, 79.6% capacity retention) and high rate capability (149.5 mAh g-1 at 2 A g-1). The results provide a facile approach to fabricate promising amorphous and ultrafine metal sulfides for energy storage.
Key words: CoS nanoparticle; amorphous; reduced graphene oxide; sodium-ion battery; electron beam radiation
1. Introduction Renewable energy sources have attracted intense attention in the wake of the exhaustion of non-regenerative fossil fuels. Sodium-ion batteries (SIBs) have been considered to be intriguing candidates for next-generation low-cost energy storage systems instead of lithium-ion batteries (LIBs). [1-3] However, it is a more arduous challenge to find suitable materials that can accommodate Na+ and allow reversible Na+ insertion/deinsertion. [4, 5] So far, various alternative materials include carbon, phosphate hosts, metal oxides/sulfides and alloy based materials have been explored as potential anodes for SIBs.[5-10] Transition metal chalcogenides (such as SnS [9], SnS2 [11], FeS2 [12], NiS2 [13], MoS2 [14] and Sb2S3 [15]) have been widely studied due to their high theoretical capacity based on electrochemical conversion mechanisms. Besides, the weaker M-S bond of metal sulfides compared with M-O of metal oxides can be kinetically favorable for conversion reaction [16]. Among these materials, although cobalt sulfides (such as CoS, CoS2, Co3S4 and Co9S8) have been widely studied in energy storage due to their unique physical and chemical properties, most of research are focused on application in LIBs, dye-sensitized solar cells and supercapacitors [17-23]. Only few studies were reported for sodium-ion batteries. Shadike et al. demonstrated that CoS2–MWCNT nanocomposites exhibit a high reversible capacity of 770 mAh g-1 and retain 568 mAh g-1 after 100 cycles in ether-based electrolyte [24]. Zhou et al. reported the sodium storage performance of Co3S4@polyaniline nanotubes. The results indicated that the Co3S4@polyaniline nanotubes exhibited an initial discharge capacity of
578.8 mAh g-1 and decrease to 252.5 mAh g-1 after 100 cycles at 200 mA g-1 in propylene carbonate (PC) electrolyte [16]. The comparisons of the electrochemical performances of CoS/rGO in different electrolytes were also reported by Peng et al. Among the electrolytes,
diethylene glycol dimethylether (DEGDME) with NaCF3SO3 showed a smaller voltage polarization and larger capacity retention than that in carbonate-based electrolyte (ethylene carbonate/diethyl carbonate and propylene carbonate) [25]. Herein, we demonstrate a facile one-step electron beam assisted solution reaction to in situ decorate CoS amorphous nanoparticles on rGO nanosheets and disclose its electrochemical performances as SIBs anode materials in the commonly used carbonate-based electrolyte. The solution reaction assisted by the electron beam radiation has a fast reaction rate, which is advantageous for forming amorphous and nanosized particles with narrow size distribution [26, 27]. The amorphous CoS nanoparticles facilitate to enhancing their electrochemical kinetics, improving the sodium ion diffusion, decreasing the structural stress/strain and narrowing the potential hysteresis [28-30]. In addition, the graphene network can enhance the electronic conductivity to improve the capacity and rate capability, as well as buffer the volume change to prevent them from aggregation during cycling [31]. These unique features endow the CoS/rGO nanocomposite with high sodium storage capacity (440 mAh g-1 at 100 mA g-1), excellent cycling stability (277 mAh g-1 after 100 cycles at 200 mA g-1, 79.6% capacity retention) and high rate capability (149.5 mAh g-1 at 2 A g-1). 2. EXPERIMENTAL METHODS 2.1 Synthesis of CoS/rGO nanocomposites All the chemicals are analytical grade and used as purchased. Graphene oxide (GO)
suspension was obtained from Shandong Yuhuang New Energy Technology Co., Ltd. In a typical procedure, 2.5 g Co(CH3COO)2·4H2O (Sinopharm Chemical) and 3.16 g Na2S2O3 (Aladdin) was dissolved in 375 mL deionized water (18 MΩ cm-1, ULUPURE, Chengdu, China). Then 10 mL GO suspension (10 mg mL-1) was added into the solution under stirring. A few isopropanol (2 mL) was added into the resulting mixture as a radical scavenger. The mixture was sonicated for 15 min and then transferred into a PE package to radiate using a 1 MeV electron beam accelerator (Wasik Associates, USA) at a dose of 250 KGy at a dose rate of 50 KGy per pass (1 Gy = 1 J Kg -1). The resulting products were washed with deionized water for several times and then freeze-drying (SCIENTZ-10N, Ningbo, China). In the contrast experiment, the bare rGO and bare CoS were prepared by electron beam radiation under the same conditions. 2.2 Materials characterization X-ray diffraction (XRD) was performed on a Shimazdu, model LabX XRD-6000 instrument using Cu Kα radiation. Raman spectroscopy was performed with a laser micro-Raman spectrometer (Renishaw in Via, Renishaw, 532 nm excitation wavelength). X-ray photoelectron spectroscopy (XPS) was conducted by using a Thermo Fisher ESCALAB 250Xi with an Al Kα X-ray source. Thermogravimetric analysis (TGA) was carried out (Thermo analysis instrument Q 500, Burlington) with a heating rate of 10 °C min-1 in air. Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed by a field-emission scanning electron microscope (FE-SEM, ZEISS Merlin Compact VP, Germany; EDS, Oxford Instruments Link ISIS). Transmission electron microscope (TEM) was performed using a field-emission transmission electron microscope (JEM-2100 HR). 2.3 Electrochemical measurement
The electrode was prepared by mixing the CoS/rGO (or bare CoS, rGO) with Super P, polytetrafluoroethylene (PTFE) and sodium carboxymethylcellulose (CMC) in deionized water (8:1:0.5:0.5, weight ratio). The obtained slurry was pasted on a Cu foil followed by drying in a vacuum oven at 100 °C for 12 h. The 2016-type coin cells with sodium foil as the counter electrode, 1 M NaClO4 in ethylene carbonate/diethyl carbonate (1:1, by volume) containing 2 vol% fluoroethylene carbonate as the electrolyte were assembled in argon-filled glove box. The cells were galvanostatically cycled on a LAND cycler (Wuhan LAND Electronics Co., China) between 0 V and 3 V. Cyclic voltammetry (CV) measurements and electrochemical impendence spectroscopy (EIS) tests were carried out with Autolab PGSTAT128N (Eco Chemie, Netherlands). Cyclic voltammetry measurements were carried out at a scan rate of 0.1 mV s-1 between 0 and 3 V (versus Na/Na+). The electrochemical impendence spectroscopy (EIS) were tests
over the
frequency range from 100 kHz to 0.01 Hz with amplitude of 10 mV.
3. RESULTS AND DISCUSSION Figure 1 shows a simplified schematic illustration for the formation of CoS/rGO nanocomposites. In the electron beam radiation process, water radiolysis yields three major primary transient species, eaq-, HO· and H·(equation 1, the numbers are the radiation chemical yields in units of µmol J-1), which eaq- and H· have strong reducibility while HO· has strong oxidizing ability [27]. Isopropanol is added into the aqueous solution to scavenge the HO/H radicals, leaving behind the eaq- in the solution so as to react with the thiosulfate to released S2homogeneously. Then the produced S2- react with Co2+ to form CoS nanoparticles, which tightly anchored onto graphene (equation 2-4) [32]. Meanwhile, the graphene oxide is also reduced by the
eaq- and isopropanol radicals (equation 5) [33]. As a result, such fast reaction for CoS can efficiently suppress the growth of crystalline CoS so as to facilitate the formation of amorphous and ultra-tiny CoS, which in situ tightly anchored onto the surface of rGO nanosheets. radiation H2O 0.26eaq 0.06H 0.27 OH+0.045H2 0.07H2 O2 0.26H3O
(CH3 )2CHOH+ OH/ H (CH3 )2C OH+H2O/H2 eaq S2O32 S SO32
S2 + Co2 CoS
(1)
(2)
(3)
(4)
(CH ) COH/e
3 2 aq GO rGO
(5)
The XRD patterns of the CoS and CoS/rGO are shown in Figure 2a. No obvious diffraction peaks can be discerned, indicating the amorphous nature of the materials. The graphene content is estimated to be 25% according to the TG analysis. Figure 2b displays the Raman spectra of the CoS and CoS/rGO. Both of the materials exhibit the peaks at about 467, 511 and 663 cm-1 correspond to the Eg, F2g and A1g modes of CoS, respectively [25]. As for the CoS/rGO, there are two additional Raman peaks occurring at 1349 cm-1 (D band) and 1586 cm-1 (G band), corresponding to the characteristic of the graphene. Figure 2c gives XPS spectrum of the CoS/rGO. As expected, Co, S, O and C elements are detected. The C1s XPS spectra of GO and CoS/rGO are presented in Figure 2d. The peak of rGO attributed to C-OH (286.4 eV) is attenuated and the O=C-OH (288.3 eV) peak almost eliminated, indicating loss of oxygen and reduction of GO [34]. It is noted that a slight shift to lower binding energy of C-OH is observed for CoS/rGO, indicating some sample charging in the spectra for GO [35]. In the high-resolution Co 2p XPS spectra (Figure 2e and 2f), the peaks at 779.2 and 794.2 eV can be attributed to Co3+ while the peaks at
781.2 and 797.1 eV are ascribed to Co2+.[36, 37] The relative intensity and area of the main 2P3/2 line indicate the majority of Co2+. The presence of a small number of Co3+ could correspond to partial oxidation of CoS due to the exposure in air.[25, 38] SEM images on the CoS and CoS/rGO are shown in Figure 3a-c. Figure 3a displays the typical CoS has spherical-like morphology with size of around 40 nm. However, in the presence of rGO, the CoS on the rGO can hardly be observed and the rGO nanosheets exhibit only rough surface (Figure 3b). To ensure the presence of CoS on the rGO, the EDS mapping images of CoS/rGO were used to confirms the presence and homogeneous distribution of Co, O, C elements in the composites (Figure 3c), indicating that the size of the CoS particles is very tiny. It proves that rGO nanosheets play a vital role in the nuclei formation and growth of CoS nanoparticles, which is possibly due to the confining effect of GO [39]. The TEM image (Figure 3d) of the bare CoS shows that the morphology and size of the CoS is consistent with SEM images (Figure 3a). For the CoS/rGO, the TEM images of the CoS/rGO reveal that the CoS nanoparticles with the size of about 2 nm homogenously disperse on the surface of rGO (Figure 3e, 3f). The selected-area electron diffraction (SAED) patterns shown in inset of Figure 3d and Figure 3e indicate the amorphous nature of both bare CoS and CoS/rGO, which is in agreement with the XRD results. Figure 4a and b show the cyclic voltammogram (CV) curves for CoS and CoS/rGO electrodes at scan rate of 0.1 mV s-1 in the potential range of 0-3 V. Two reduction peaks at 1.4 V and 0.9 V are observed for both electrodes, which are associated with the multi-electron reaction mechanism similar with LIBs [24, 25]. The reduction peak at 1.4 V can be related to the initial insertion of sodium forming NaxCoS, and the peak at 0.9 V might originate from the conversion reaction to form Na2S and Co. For the initial anodic process of CoS electrode, the oxidation peak
at 1.85 V can be assigned to the reverse conversion reaction of Co with Na2S. In the subsequent cycles, the oxidation peak shifts to 2.0 V, suggesting high electrochemical polarization, similar to the previous work [17]. However, for the initial anodic process of the CoS/rGO, there are three peaks at 1.15 V, 1.61 V and 2.09 V (Figure 4b). The peak at 1.15 V could be assigned to desodiation of rGO while the peaks at 1.61 V and 2.09 V belong to the reaction: Co + Na2S → NaxCoS + (2 − x)Na + (2 − x)e, NaxCoS → CoS + xNa+ + xe, respectively.[40] In the subsequent cycles, these peaks are almost overlapped, indicating highly reversibility of the CoS/rGO compared to the bare CoS (Figure 4a). The differences in CV curves between the CoS/rGO and bare CoS could be due to the introduction of rGO to not only induce the formation of the extra tiny particles of CoS, but also enhance the electron conductivity, which improve the electrochemical reversibility of CoS/rGO electrode. Figure 4c and d show the first three charge-discharge curves of the CoS and CoS/rGO electrodes at 100 mA g-1, respectively. Both electrodes show slight potential plateaus located at 1.4 V and 0.9 V during discharging. In the charge process, the CoS electrode exhibits a potential plateau at 1.9 V in first cycle and shifts to higher potential in the following cycle. For the CoS/rGO, two successive charge plateaus are almost fixed at 1.7 and 2.1 V for first three cycles, indicating good electrochemical reversibility of the CoS/rGO. The results were well matched to the CV curves (Figure 4a and b). The CoS electrode displays an initial discharge/charge capacities of 539.1 and 345.5 mAh g-1 respectively, corresponding to a coulombic efficiency of 64.1%. In contrast, the initial discharge/charge capacities for the CoS/rGO are 643.7 and 440.1 respectively, showing a coulombic efficiency of 68.4%, higher in both capacity and efficiency than those of the CoS. These results indicate that the incorporation of the rGO enhances the electrochemical
utilization of the composites due to the improvement of electron conductivity and decrease of the particle size. The cycling performance of the rGO, CoS and CoS/rGO is shown in Figure 5a. The capacity values are calculated based on the total mass of the composite. Both the rGO and CoS electrodes exhibit lower electrochemical performance than that of CoS/rGO. The rGO shows low initial charge capacity (133.5 mAh g-1) and poor coulombic efficiency (25.4%). For the CoS electrode, the specific capacity decays to 150 mAh g-1 at the 40th cycle and then rapidly reduces to 10 mAh g-1. However, the CoS/rGO electrode shows a reversible capacity of 277 mAh g-1 at 100th cycle with the coulombic efficiency of 99.5%, corresponding to 79.6% of the initial charge capacity. Figure 5b compares the rate capability of the CoS and CoS/rGO. The CoS electrode delivers a reversible capacity of 345.5, 202.4, 146.1, 104.6 and 57.4 mAh g-1 at current rates of 100, 200, 500, 1000 and 2000 mA g-1, respectively, which are considerably lower than the CoS/rGO electrode (440.1, 379.1, 325.8, 280.1, 149.5 mAh g-1) at the same rate. When the current density is reduced back to 100 mA g-1, the capacity of the CoS/rGO can be also recovered to 356.7 mAh g-1. The good cycling stability and rate capability of the CoS/rGO electrode can be ascribed to uniformly distributed tiny CoS particles in the conductive rGO network. The ultrafine CoS particles can mitigate the volume change and improve reaction kinetics, while the rGO network provides high conductive support and restraints particle pulverization during cycling. Electrochemical impedance spectroscopy was conducted to clarify the improved electrochemical performance of the CoS/rGO composite. Figure 6 shows the Nyquist plots of the CoS and CoS/rGO after 20th cycle. Based on the equivalent circuit model in the inset of Figure 6, the charge-transfer resistance (Rct) of the CoS and CoS/rGO electrode is 239 Ω and 26 Ω,
respectively. It is obvious that the Rct of CoS/rGO is much lower than that of the CoS, indicating a fast reaction kinetics owing to combination with the conductive rGO [41]. 4. Conclusions In summary, amorphous CoS nanoparticles were anchored on the surface of graphene nanosheets by a facile one-step electron beam radiation route as anode material for SIBs. The electrochemical tests showed the CoS/rGO nanocomposite exhibits a high reversible capacity, cycling stability and superior rate performance. The improvement of the electrochemical properties is due to the extremely tiny CoS particle (~ 2nm) and electron conductive rGO network. In addition, the electron beam radiation route is also a sample and convenient method for preparation of amorphous, ultrafine and uniformly distributed metal sulfides for energy storage, biomaterial and catalyst applications.
Acknowledgements The authors gratefully acknowledge the financial support by the 2011 Program of Hubei Province, the National Key Basic Research Program of China (No. 2015CB251100) and National Science Foundation of China (No. 21673165, 21373155 and 21333007), Natural Science Foundation of Hubei Province, China (Grant No. 2015CFC774), Program for New Century Excellent Talents in University (NCET-12-0419) and Hubei National Funds for Distinguished Young Scholars (2014CFA038).
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Figure 1. Schematic illustration of the formation of the CoS/rGO nanocomposites. Figure 2. (a) XRD patterns and (b) Raman spectra of the CoS and CoS/rGO samples, (c) XPS spectrum of CoS/rGO, (d) C1s high-resolution spectra of the CoS/G and GO. Co 2p high-resolution spectrum of (e) the CoS and (f) the CoS/rGO. Figure 3. SEM images of the (a) CoS particles and (b) CoS/rGO nanocomposites. (c) The element mapping images of the CoS/rGO nanocomposites. TEM images of the (d) CoS particles and (e, f) CoS/rGO nanocomposites. The insets are corresponding SAED pattern. Figure 4. CV curves of the (a) CoS and (b) CoS/rGO electrodes and the discharge/charge profiles of (c) the CoS and (d) CoS/rGO electrodes. Figure 5. (a) Cycling performance of the rGO (50 mA g-1), CoS and CoS/rGO (200 mA g-1) and (b) rate performance of the CoS and CoS/rGO electrodes. Figure 6. Electrochemical impedance spectra of the (a) CoS and (b) CoS/rGO electrodes after 20th cycle. The inset is the equivalent circuit.