Research Article pubs.acs.org/journal/ascecg
Electrolyte Optimization for Enhancing Electrochemical Performance of Antimony Sulfide/Graphene Anodes for Sodium-Ion Batteries− Carbonate-Based and Ionic Liquid Electrolytes Cheng-Yang Li,†,∇ Jagabandhu Patra,†,∇ Cheng-Hsien Yang,†,∇ Chuan-Ming Tseng,*,‡ Subhasish B. Majumder,§ Quan-Feng Dong,∥ and Jeng-Kuei Chang*,† †
Institute of Materials Science and Engineering, National Central University, 300 Jhong-Da Road, Taoyuan 32001, Taiwan Department of Materials Engineering, Ming Chi University of Technology, 84 Gungjuan Road, Taishan District, New Taipei City 24301, Taiwan § Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, West Bengal, India ∥ State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, 422, South Siming Raod, Xiamen 361005, Fujian, China ‡
S Supporting Information *
ABSTRACT: The electrolyte is a key component in determining the performance of sodium-ion batteries. A systematic study is conducted to optimize the electrolyte formulation for a Sb2S3/graphene anode, which is synthesized via a facile solvothermal method. The effects of solvent composition and fluoroethylene carbonate (FEC) additive on the electrochemical properties of the anode are examined. The propylene carbonate (PC)-based electrolyte with FEC can ensure the formation of a reliable solid-electrolyte interphase layer, resulting in superior charge−discharge performance, compared to that found in the ethylene carbonate (EC)/diethyl carbonate (DEC)-based electrolyte. At 60 °C, the carbonatebased electrolyte cannot function properly. At such an elevated temperature, however, the use of an N-propyl-Nmethylpyrrolidinium bis(fluorosulfonyl)imide ionic liquid electrolyte is highly promising, enabling the Sb2S3/graphene electrode to deliver a high reversible capacity of 760 mAh g−1 and retain 95% of its initial performance after 100 cycles. The present work demonstrates that the electrode sodiation/desodiation properties are dependent significantly on the electrolyte formulation, which should be optimized for various application demands and operating temperatures of batteries. KEYWORDS: Sodium-ion batteries, Electrolyte, Antimony sulfide, Ionic liquid, Additive
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INTRODUCTION The growing demand for lithium-ion batteries (LIBs) has caused concern about rising prices and the long-term availability of global lithium reserves.1,2 Sodium-ion batteries (NIBs) are potential alternatives to LIBs for large-scale energy storage applications due to the low cost and abundant availability of sodium precursors.3 The search for suitable anodes and cathodes with high capacity, good rate capability, and long lifespan for NIBs is currently the focus of much research.4−9 Graphite anodes, typically used for LIBs, are thermodynamically unfavorable for Na+ storage.10,11 The other carbonaceous anodes for NIBs, such as hard carbon, extended graphite, porous carbon, and graphene materials, generally possess specific capacities of only ∼300 mAh g−1.12−15 Therefore, antimony trisulfide (Sb2S3) has recently received much attention, because of its high reversibility,16,17 good reaction kinetics,18,19 and large theoretical capacity of 946 mAh g−1, based on both the conversion reaction (Sb2S3 + 6Na+ + 6e− → 2Sb + 3Na2S) and alloying reaction (2Sb + 6Na+ + 6e− → 2Na3Sb).16,20 Despite this potential, there are still many © 2017 American Chemical Society
challenges that must be addressed before this anode can be used in practical applications (e.g., Coulombic efficiency, cyclic stability, and elevated-temperature reliability). Several strategies have been utilized to improve the electrochemical properties of Sb2S3 anodes. Specifically, nanosizing the active material,19 performing surface coating,18,21 and introducing conducting nanocarbons17,22,23 have been used to promote material utilization, suppress polysulfide dissolution, increase electrode conductivity, and mitigate the strain generated during charging−discharging. Nevertheless, the electrode/electrolyte interface quality, which is strongly dependent on the electrolyte composition, is another crucial factor that governs the ultimate electrochemical sodiation/ desodiation performance. Unfortunately, to date, there have been no studies on electrolyte optimization for Sb2S3 anodes, and, thus, more work in this area is needed. Received: June 14, 2017 Revised: July 25, 2017 Published: July 26, 2017 8269
DOI: 10.1021/acssuschemeng.7b01939 ACS Sustainable Chem. Eng. 2017, 5, 8269−8276
Research Article
ACS Sustainable Chemistry & Engineering
importance of the electrolyte formulation, which affects the charge−discharge performance of the electrode to a great extent.
The effects of electrolyte composition on NIB performance have been investigated mainly for hard carbon (HC) anodes. Komaba et al. studied HC//Na cells with various electrolyte solvents comprising 1 M NaClO4.13 The propylene carbonate (PC) and ethylene carbonate (EC)/diethyl carbonate (DEC) solvents clearly outperformed EC/ethyl methyl carbonate (EMC), EC/dimethyl carbonate (DMC), and PC/vinylene carbonate (VC) solvents, in terms of cell stability. A comparative study was carried out by Ponrouch et al. on diverse electrolyte formulations with various salts (NaClO4, NaPF6, and NaTFSI) and solvents (PC, EC, DMC, DEC, dimethyl ether (DME), triglyme, and tetrahydrofuran) or their mixtures (EC/DMC, EC/DME, EC/PC, and EC/triglyme).24 They concluded that good Coulombic efficiencies were recorded in the cases of PC-, EC/PC-, and EC/DEC-based electrolytes, whereas EC/DMC- and EC/DME-based electrolytes showed poor performance. The other key component in an electrolyte is the additive. Adding fluoroethylene carbonate (FEC), which is a solid-electrolyte interphase (SEI) enhancer, to PC-based electrolytes has been found to effectively improve the cyclic stability of an HC electrode, whereas other additives, such as transdifluoroetyhene carbonate (DFEC), ethylene sulfite (ES), and VC, did not show any positive influences.25,26 However, the effects of FEC additive have been disputed, with research indicating that it resulted in the formation of a lessconducting SEI, compared to that produced in a FEC-free PC/ EC electrolyte.27 FEC also decreased HC capacity and Coulombic efficiency, suggesting that this additive is unnecessary.27 It should be noted that the optimal electrolyte formulation can be electrode-material-dependent. Nevertheless, data on the effects of the electrolyte on NIB electrodes other than HC are rather limited. PC- and EC/DEC-based electrolytes with and without FEC have been arbitrarily chosen in the literature to study the electrochemical performance of Sb2S3 anodes, while their influences are unknown. Therefore, a systematic comparison between various electrolyte recipes for Sb2S3 is conducted in this study. Besides conventional carbonate-based electrolytes, ionic liquid (IL) electrolytes, characterized by intrinsic conductivity, large electrochemical windows, excellent thermal stability, and designable physicochemical properties,28,29 have great potential for application in NIBs.30 The nonflammability and negligible volatility of ILs give them high safety and a low environmental impact, which are especially important for large-size NIBs. NaTFSI/CsTFSI and NaFSI/KFSI (TFSI = bis(trifluoromethanesulfonyl) imide; FSI = bis(fluorosulfonyl)imide) intermediate-temperature IL electrolytes (whose melting points are above room temperature) were first proposed by Nohira and Hagiwara et al. for NIBs.31,32 Room-temperature ILs based on imidazolium and pyrrolidinium cations were later developed33−37 and used for HC,38 TiO2,39 Na2Ti3O7,40 and phosphorus41 anodes. The compatibility of ILs with a Sb2S3 anode, which has never been explored, is addressed in this work. Herein, we conduct a detailed investigation on PC and EC/ DEC electrolytes (containing NaClO4 salt) with and without FEC to clarify their effects on the Coulombic efficiency, reversible capacity, rate capability, and cycle life of a Sb2S3/ graphene anode prepared using a facile one-pot solvothermal method. The above properties are also compared to those of an N-propyl-N-methylpyrrolidinium (PMP)−FSI IL electrolyte. The temperature effects (25 and 60 °C) for both the carbonatebased and IL electrolytes are examined. The results point to the
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EXPERIMENTAL SECTION
Synthesis of Sb2S3/Graphene Composite. Graphene nanosheets were prepared using a modified Staudenmaier method.42 Detailed procedures can be found in earlier papers.43,44 The fabrication of the Sb2S3/graphene composite was conducted using a solvothermal method. Briefly, 0.015 mol of L-cysteine and 0.005 mol of SbCl3 were dissolved in 35 mL of ethylene glycol, which contained 25 wt % glucose and 5 wt % graphene. This solution was then transferred to a 50 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 8 h. The glucose was used to create a carbon coating. The Sb2S3/ graphene with a carbon coating layer has been reported to possess superior electrochemical performance21 and, thus, is used in this study. The resulting powder was repeatedly washed with anhydrous ethanol and collected via centrifugation. All the chemicals were of analytical grade and used without further purification. Electrolyte Preparation and Cell Assembly. PC and EC/DEC (1:1 by volume) solvents with and without FEC (5% by volume) were prepared with 1 M NaClO4 salt. Note that the use of NaFSI salt in carbonate electrolytes easily causes Al current collector corrosion problems at the cathode sides.45 The NaClO4 is the most commonly used salt for carbonate electrolytes in the literature. Accordingly, NaClO4 is adopted in this study. PMP−FSI IL was prepared and purified by following a published procedure.46 The IL was washed with dichloromethane, filtered to remove precipitates, and then vacuumdried at 100 °C for 12 h before use. 1 M NaFSI (99.7%, Solvionic) was dissolved into the IL to provide Na+ conduction. The mixture was continuously stirred by a magnetic paddle for 24 h to ensure uniformity. The water content of all electrolytes, measured using a Karl Fisher titrator, was 99.5%. This reflects the excellent compatibility between the Sb2S/graphene electrode and the PMP−FSI IL electrolyte, leading to the exceptional reversibility and cyclic stability. Figure 3 compares the morphologies of the electrodes after being cycled in the PC/ FEC and IL electrolytes. With the former electrolyte, the electrode active material became agglomerated and a relatively thick surface SEI film formed, resulting in a reduction of accessible reaction sites. In contrast, the electrode cycled in the IL electrolyte showed a minor morphology change with thinner SEI coverage. The high integrity of the electrode ensured that its electrochemical properties were highly preserved. Figure 4a shows the TGA data of the PC/FEC and PMP− FSI IL electrolytes. The carbonate electrolyte began to show a weight loss below 100 °C, at which point the organic solvent was highly volatile. In contrast, the IL electrolyte exhibited a decomposition temperature as high as ∼400 °C. Although the TGA analysis is a dynamic test that could overestimate the thermal stability limit of the electrolytes, these results clearly reveal that the IL electrolyte is promising for elevated-
Figure 3. SEM micrographs of (a) as-prepared Sb2S3/graphene electrode and the electrodes after first charge−discharge cycle performed in (b) PC/FEC and (c) PMP−FSI IL electrolytes at 25 °C.
Figure 4. (a) TGA data of PC/FEC and PMP−FSI IL electrolytes. Flammability tests of (b) PC/FEC electrolyte and (c) PMP−FSI IL electrolyte.
temperature applications. The flammability of the two electrolytes is compared in Figure 4b. Glass fiber papers were used to absorb the electrolytes and then tested with an electric Bunsen burner under air. While the carbonate electrolyte violently burned, the IL electrolyte was not ignited, indicating the higher safety of the latter electrolyte. The electrochemical properties of the Sb2S3/graphene electrodes were further examined in the PC/FEC and IL electrolytes at 60 °C. The resulting charge−discharge curves at various rates are shown in Figures 5a and 5b. Figure 6 summarizes the temperature effects for the two electrolytes. As shown in Figure 6a, the maximum capacity of the electrode measured in the PC/FEC electrolyte decreased as the temperature rose, whereas that for the IL electrolyte increased 8273
DOI: 10.1021/acssuschemeng.7b01939 ACS Sustainable Chem. Eng. 2017, 5, 8269−8276
Research Article
ACS Sustainable Chemistry & Engineering
Figure 5. Charge−discharge curves of Sb2S3/graphene electrodes recorded in (a) PC/FEC and (b) PMP−FSI IL electrolytes at 60 °C. (c) EIS spectra and (d) cyclic stability of Sb2S3/graphene electrodes measured in the aforementioned two electrolytes at 60 °C.
relatively low Coulombic efficiency found for the PC/FEC electrolyte (Figure 5d), which can lead to growth of the SEI. Figure 5d also shows that the organic−electrolyte and IL− electrolyte cells retained 43% and 95% of their initial capacities, respectively, after 100 charge−discharge cycles at 60 °C. The durability of the former cell clearly decayed, compared to that at 25 °C (see Figure 6c), because of the low thermal stability of the electrolyte and the SEI generated.57,58 In contrast, the nonvolatility and chemical benignity of the IL electrolyte contributed to the good cell stability at 60 °C. Of note, the IL is nonflammable and highly safe (see Figure 4). Clearly, at such an elevated operation temperature, the IL is a promising electrolyte for the Sb2S3/graphene electrode. Unsatisfactory cyclic stability has long been a critical concern for alloy/ conversion-type electrodes. This study confirms that the use of an IL electrolyte can effectively mitigate this problem for a Sb2S3 anode, even for applications at 60 °C. Although the IL electrolyte is relatively expensive at the current stage, large-scale production will reduce the cost. In addition, development of cost-effective anions and cations for ILs is already underway.
Figure 6. Effects of temperature on (a) reversible capacity at 50 mA g−1, (b) capacity retained ratios at 1500 mA g−1 (compared to values at 50 mA g−1), and (c) cyclic stability of Sb2S3/graphene electrodes measured in PC/FEC and PMP−FSI IL electrolytes.
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CONCLUSIONS
The present study found that the electrolyte formulation strongly determined the charge−discharge properties of the Sb2S3/graphene electrode. The PC-based electrolytes were more compatible than the EC/DEC-based electrolytes with the Sb2S3 electrode. Incorporation of FEC in the PC electrolyte can enhance surface passivation and improve cyclic stability of the electrode, but reduce the rate capability to some extent. However, this PC/FEC electrolyte cannot properly function at an elevated temperature. The SEI formed in the IL is effective to guarantee high electrode durability upon repeated cycling. At 60 °C, reversible capacities of 760 and 420 mAh g−1 were obtained at 50 and 1500 mA g−1, respectively, in the IL electrolyte. Moreover, 95% of the initial performance can be retained after 100 sodiation/desodiation cycles. Electrolyte engineering is crucial to achieve high-performance NIBs. In the current, an IL electrolyte is shown to be promising, especially
from 660 mAh g−1 at 25 °C to 760 mAh g−1 at 60 °C. The latter is associated with the increased ionic conductivity (see Table 2) and Na+ transference number36 and the reduced Rct at the elevated temperature (i.e., ∼300 Ω in Figure 5c vs ∼550 Ω in Figure 2e), which also boost the electrode high-rate performance in the IL electrolyte, as shown in Figure 6b. It was found that the rate capability significantly decayed at 60 °C with the PC/FEC electrolyte (Figure 6b). Since the electrolyte conductivity (see Table 2) and Na+ diffusion in the electrode should increase with increasing temperature, the deterioration in performance is attributed to the formation of a more resistive film at the electrode (see Figure 5c), which hindered Na+ migration across the interface. This is associated with the 8274
DOI: 10.1021/acssuschemeng.7b01939 ACS Sustainable Chem. Eng. 2017, 5, 8269−8276
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ACS Sustainable Chemistry & Engineering
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when cyclic stability and safety of batteries are the key concerns.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01939. TEM and AFM data of graphene nanosheets, XRD data of Sb2S3/graphene, TGA data of various samples, CV curves of Sb2S3/graphene electrodes recorded in various electrolytes (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (C.-M. Tseng). *E-mail:
[email protected] (J.-K. Chang). ORCID
Quan-Feng Dong: 0000-0002-4886-3361 Jeng-Kuei Chang: 0000-0002-8359-5817 Author Contributions ∇
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support provided for this work by the Ministry of Science and Technology (MOST) of Taiwan and by the Key Project of NSFC (No. U1305246) is gratefully appreciated.
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REFERENCES
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DOI: 10.1021/acssuschemeng.7b01939 ACS Sustainable Chem. Eng. 2017, 5, 8269−8276
