Electrolyte Optimization for Enhancing Electrochemical Performance

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Electrolyte optimization for enhancing electrochemical performance of antimony sulphide/graphene anodes for sodiumion 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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b01939 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 30, 2017

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ACS Sustainable Chemistry & Engineering

Electrolyte optimization for enhancing electrochemical performance of antimony sulphide/graphene anodes for sodium-ion batteries – carbonatebased and ionic liquid electrolytes

Cheng-Yang Lia,ǂ, Jagabandhu Patraa,ǂ, Cheng-Hsien Yanga,ǂ, Chuan-Ming Tsengb,*, Subhasish B. Majumderc, Quan-Feng Dongd, and Jeng-Kuei Changa,*

a

Institute of Materials Science and Engineering, National Central University, 300 Jhong-Da Road, Taoyuan 32001, Taiwan b

Department of Materials Engineering, Ming Chi University of Technology, 84 Gungjuan Road, Taishan Dist., New Taipei City 24301, Taiwan

c

Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, West Bengal, India

d

State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, 422, South Siming Raod, Xiamen, Fujian 361005, China

ǂ

These authors made equal contributions to this work.

*

Corresponding author. E-mail: [email protected] (Chuan-Ming Tseng) E-mail: [email protected] (Jeng-Kuei Chang)

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ACS Paragon Plus Environment


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Abstract The electrolyte is a key component in determining the performance of Na-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 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 carbonate-based 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 depend 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 Looking 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 a lot of 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 + 6 Na+ + 6e− → 2 Sb + 3 Na2S) and alloying reaction (2 Sb + 6 Na+ + 6e− → 2 Na3Sb).16,20 Despite this potential, there are still many challenges that need to 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 nano-carbons17,22,23 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. 3



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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), 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 less conducting 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. A systematic comparison between various electrolyte recipes for Sb2S3 is thus conducted in this study. 4


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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 non-flammability 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 the presented 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 columbic 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 carbonate-based and IL electrolytes are examined. The results point to the importance of the electrolyte formulation, which affects the charge–discharge performance of the electrode to a great extent.

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 5



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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 performance,21 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 grades 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. It is noted that 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 vacuum-dried 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 below 50 ppm. The ionic conductivity and viscosity of various electrolytes were measured using a TetraCon 325 conductivity meter and a Brookfield DV-I viscometer, respectively. To prepare the electrode, a slurry made of 70 wt% active material powder, 20 wt% carbon black, and 10 wt% poly(vinylidene fluoride) (PVDF) in N-methyl-2-pyrrolidone solution was mixed with ball milling and then pasted onto Cu foil. This electrode was 6


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vacuum-dried at 90 °C for 5 h, roll-pressed, and then punched to match the required dimensions of a CR2032 coin cell. The active material loading amount was typically 1–1.2 mg cm–2. Na foil and a glass fiber membrane were used as the counter electrode and separator, respectively. The assembly of the coin cells was performed in an argon-filled glove box (Innovation Technology Co. Ltd.), where both the moisture content and oxygen content were maintained at below 0.5 ppm.

Material and electrochemical characterizations. The crystallinity of the obtained Sb2S3/graphene composite was characterized using an X-ray diffractometer (Bruker, D8 Advance). The morphologies were inspected using field-emission scanning electron microscopy (SEM; FEI Inspect F50). High-resolution transmission electron microscopy (TEM; JEOL 2100F) was used to examine the material microstructures. Thermogravimetric analyses (TGA; Perkin-Elmer TGA7) were conducted under air with a heating rate of 5 °C min–1. Cyclic voltammetry (CV) was performed with a Biologic VSP-300 potentiostat in the range 0.01–2.0 V vs. Na/Na+ at a sweep rate of 0.1 mV s–1. The charge–discharge performance (such as capacity, rate capability, and cyclic stability) of the Sb2S3/graphene electrodes in various electrolytes was evaluated using a battery tester (Arbin, BT-2043). The coin cells were placed in a climatic chamber, where the temperature was controlled at 25 °C (±1 °C) or 60 °C (±1 °C). For each condition, at least five parallel cells were measured. The performance deviation was typically within 5%, and the reported data are the medians.

Results and discussion The morphology of the obtained graphene nanosheets, examined using TEM, is shown in Figure S1 (a). The atomic force microscopy data in Figure S1 (b) confirm a sheet thickness of about 4 nm, corresponding to 3–5 carbon layers. Figure 1 (a) shows the SEM image of the 7



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Sb2S3/graphene composite prepared using the solvothermal method. Sb2S3 rods had an average diameter of ~200 nm and a length of up to the micron scale were observed. The TEM micrograph in Figure 1 (b) shows that the surface of the rod is coated with a continuous carbon layer of ~5 nm in the thickness. Figure 1 (c) shows a high-resolution TEM image of the Sb2S3 rod. The indicated lattice spacing of 1.9 Å is attributed to the (001) plane distance of an orthorhombic structure (space group: Pbnm (no.62)).47,48 The corresponding selected area electron diffraction (SAED) pattern, taken along the [100] projection of the Sb2S3 single crystal, is exhibited in Figure 1 (d). Figure S2 shows the X-ray diffraction pattern of the composite. All the diffraction peaks matched the standard orthorhombic Sb2S3 (JCPDS No. 42-1393), which is consistent with the SAED data. The absence of carbon-related signals indicates the low-crystallinity of the carbon coating layer and graphene nanosheets. There was no detectable impurity phase, suggesting that the obtained Sb2S3/graphene is of high quality and purity. The TGA data in Figure S3 confirm that the carbon coating and graphene each contribute ~5 wt% to the composite. Optimization of the Sb2S3 to graphene ratio is beyond the scope of this work and can be an interesting topic for future study. The electrochemical characteristics of the Sb2S3/graphene electrode in various electrolytes were investigated using CV. Figure 2 (a) shows the voltammograms recorded in the PC/FEC electrolyte. In the first cathodic scan, the first reduction peak centered at ~1.1 V was associated with the intercalation of Na+ into the Sb2S3 layered structure17 and formation of SEI film.49 The second reduction around 0.73 V can be ascribed to further sodiation of the produced NaxSb2S3 intermediate phase,18 resulting in the formation of Sb nanoparticles embedded in Na2S.17,18 Afterwards, an alloying reaction of Sb with additional Na+ forming Na3Sb occurred at approximately 0.25 V.16,49 In the following positive scan, the dealloying and reconversion reaction peaks were located at 0.8 V and 1.3 V, respectively. The CV curves recorded in the PMP–FSI IL electrolyte (Figure 2 (b)) show similar electrochemical 8


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behavior. The CV data for other electrolytes are shown in Figure S4. As summarized in Table 1, the first-cycle coulombic efficiencies of the electrodes in PC, PC/FEC, EC/DEC, EC/DEC/FEC, and IL electrolytes were 52%, 60%, 48%, 55%, and 65%, respectively. The efficiency loss was attributed to both the electrolyte decomposition (and SEI formation) and a partial trapping of Na+ in the bulk Sb2S3. Clearly, the PC-based electrolytes outperform the EC/DEC-based electrolytes in terms of the initial coulombic efficiencies. It was also found that the FEC additive led to formation of superior SEI layers that can suppress extensive decomposition of the electrolytes, increasing the efficiencies. Of note, the highest efficiency was found for the IL electrolyte. This suggests the greater passivation ability of the SEI, which was derived from the IL anion decomposition.50,51 We believe that the efficiencies can be further improved by optimizing the binders and the electrode microstructures.52,53 Nevertheless, the results clearly indicate that the electrolyte composition indeed plays a role in determining the SEI properties. As shown in Figures 2 (a) and (b), in the second CV cycle, the 1.1 V cathodic peak disappeared. This can be explained by formation of amorphous Sb2S3 (and thus no characteristic intercalation peak can be observed) after one sodiation– desodiation cycle18 and the existence of a surface protective film on the electrode. The CV behavior became steady in the subsequent scans, and an efficiency value of >95% was obtained at the third cycle. Figure 2 (c) shows the charge–discharge curves of the Sb2S3/graphene electrodes with various electrolytes recorded at a current density of 50 mA g–1 after three CV conditioning cycles. The conditioning cycles (either using CV or low-rate charge/discharge) are generally needed for battery electrodes to stabilize the SEI and increase the coulombic efficiency. The sloping curves are coincident with the CV results of the overlapping redox peaks. The measured discharge (desodiation) capacities were 710, 710, 560, 580, and 660 mAh g–1, respectively, with the PC, PC/FEC, EC/DEC, EC/DEC/FEC, and IL electrolytes. While the 9



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FEC did not substantially affect the capacity at such a low rate, the PC-based electrolytes were clearly more suitable than the EC/DEC-based electrolyte for the Sb2S3 electrode. The electrode rate capability in various electrolytes was further examined, and the results are shown in Figure 2 (d). When the charge–discharge rate increased to 1500 mA g–1, the obtained capacities in the same electrolytes decreased to 340, 285, 55, 150, and 240 mAh g–1, respectively, corresponding to 48%, 40%, 10%, 26%, and 36% retentions compared to the capacities at 50 mA g–1 (as summarized in Table 1). The conductivity and viscosity values of the electrolytes are listed in Table 2. There are obviously other overwhelming factors (rather than conductivity and viscosity) that determined the discharge performance. Figure 2 (e) shows the electrochemical impedance spectroscopy (EIS) data of the electrodes in various electrodes. The Nyquist spectra are composed of a semicircle at high frequency and a sloping line at low frequency, which can be characterized by the equivalent circuit shown in the figure inset, where Re, Rct, CPE, and W are the electrolyte resistance, interfacial charge transfer resistance, interfacial constant phase element, and Warburg impedance associated with Na+ diffusion in the electrode, respectively.54 It is found that the Rct values, which are related to the EIS semicircle diameters, are much higher than the Rs values, and are approximately 200, 300, 1650, 900, and 550 Ω, respectively, with the PC, PC/FEC, EC/DEC, EC/DEC/FEC, and IL electrolytes. This trend is consistent with the electrode rate capability shown in Figure 2 (d) (i.e., the higher the Rct value, the lower the high-rate performance). The lower coulombic efficiencies for the EC/DEC-based electrolytes (as described previously) suggest the existence of less effective SEI layers with inferior passivation ability, which could lead to continuous accumulation of electrolyte decomposition products on the electrodes, resulting in higher Rct values than those found for the PC-based electrolytes. Although addition of FEC (an SEI-enhancing additive) is helpful, the EC/DEC-based electrolytes are considered unfavorable for the Sb2S3 electrodes. In fact, 10


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the PC-derived SEI is more appropriate. Nevertheless, in this case, the FEC additive increased the Rct at the electrode, and thus adversely influenced the high-rate performance. This finding is in line with the data in the literature for an HC anode in a PC/EC electrolyte, which indicated that FEC can reduce the ionic conductivity of the derived SEI film, increasing charge–discharge polarization.27 Our results indicate that the effects of FEC can be electrolyte-composition-dependent (beneficial for the EC/DEC electrolyte, but detrimental for the PC electrolyte when the electrode rate capability is concerned). In the IL electrolyte, even though the ionic conductivity is relatively low (see Table 2), an appropriate capacity and rate capability were obtained. This can be explained by the moderate Rct value of the IL cell (since Rct, rather than Re, is the dominant factor). The cyclic stability of the Sb2S3/graphene electrodes in various electrolytes was also evaluated (at 100 mA g–1), and the data (after three CV conditioning cycles) are shown in Figure 2 (f). The electrodes retained 48%, 83%, 40%, and 69% of their initial discharge capacities after 100 charge–discharge cycles with the PC, PC/FEC, EC/DEC, and EC/DEC/FEC electrolytes, with the saturated coulombic efficiencies of 96.0%, 99.5%, 95.0%, and 98.0%, respectively. The EC/DEC-derived SEI showed inadequate passivation. The growing SEI during repeated charging/discharging not only continuously consumed Na+ and the electrolyte, but also increased the interface resistance, leading to the rapid fall in capacity. The addition of FEC led to the formation of a more robust SEI film. As a result, the electrode degradations (in both EC/DEC- and PC-based electrolytes) were suppressed. Earlier research proposed that the FEC-derived long-chain flexible polycarbonates in the SEI film can accommodate active material volume expansion during charging/discharging and limit undesired side interactions with the electrolyte, increasing electrode cyclability.55 Furthermore, the FEC can decompose into VC and fluoride ions.56. These fluoride ions, instead of those coming from the PVDF binder, preferentially reacted with Na+ to form NaF, 11



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allowing PVDF to maintain its integrity. The VC further decomposed to release CO2, which reacted electrochemically with Na+ to form Na2CO3, an SEI constituent that is favorable for passivation.55,56 Consequently, the PC/FEC electrolyte led to an electrode cyclic stability that is superior to those seen for the other carbonate-based electrolytes. Notably, in the IL electrolyte the electrode retained as much as 96% of the initial capacity after 100 cycles, with the coulombic efficiency stabilizing at >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 4 (a) 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-temperature applications. The flammability of the two electrolytes is compared in Figure 4 (b). 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. 12


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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 5 (a) and (b). Figure 6 summarizes the temperature effects for the two electrolytes. As shown in Figure 6 (a), the maximum capacity of the electrode measured in the PC/FEC electrolyte decreased as the temperature rose, whereas that for the IL electrolyte increased 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 5 (c) vs. ~550 Ω in Figure 2 (e)), which also boost the electrode high-rate performance in the IL electrolyte, as shown in Figure 6 (b). It was found that the rate capability significantly decayed at 60 °C with the PC/FEC electrolyte (Figure 6 (b)). 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 5 (c)), which hindered Na+ migration across the interface. This is associated with the relatively low coulombic efficiency found for the PC/FEC electrolyte (Figure 5 (d)), which can lead to growth of the SEI. Figure 5 (d) 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 6 (c)) due to the low thermal stability of the electrolyte and the SEI generated.57,58 In contrast, the non-volatility and chemical benignity of the IL electrolyte contributed to the good cell stability at 60 °C. Of note, the IL is non-flammable 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 13



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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.

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 when cyclic stability and safety of batteries are the key concerns.

Associated content Supporting information 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.

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Author information Corresponding author E-mail: [email protected] (Chuan-Ming Tseng) Address: 84 Gungjuan Road, Taishan Dist., New Taipei City 24301, Taiwan E-mail: [email protected] (Jeng-Kuei Chang) Address: 300 Jhong-Da Road, National Central University, Taoyuan 32001, Taiwan

Author contribution Cheng-Yang Li, Jagabandhu Patra, Cheng-Hsien Yang ǂ

These authors contributed equally.

Acknowledgements The financial support provided for this work by the Ministry of Science and Technology (MOST) of Taiwan and by the Key Project of NSFC U1305246 is gratefully appreciated.

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16


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List of Figures Figure 1. (a) SEM image, (b) TEM image, (c) high-resolution TEM lattice image, and (d) SAED pattern of synthesized Sb2S3/graphene sample. Figure 2. CV curves of Sb2S3/graphene electrodes recorded in (a) PC/FEC and (b) PMP-FSI electrolyte. (c) Charge–discharge curves at 50 mA g–1, (d) rate capability, (e) EIS spectra, and (f) cyclic stability of Sb2S3/graphene electrodes measured in various electrolytes at 25 °C. 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. 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 above two electrolytes at 60 °C. 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.

24


Page 24 of 33

Page 25 of 33

For Table of Contents Use only

∼1.9Å (001)

Capacity (m Ah/g)

1000

b* ⊗

c*

Sb2S3/graphene

Electrolyte

Efficiency (% )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 750 75

500 250 0

25

50

75

Cycle number

50 100

Synopsis Electrolyte formulations are for the first time systematically optimized for a Sb2S3/graphene electrode, which is a potential anode for sodium-ion batteries.

25



Page 26 of 33

1 2 3 Table 1. Electrochemical properties of Sb2S3/graphene electrodes measured in various electrolytes 4 5 at 25 and 60 °C. 6 7 8 9 10 PC PC/FEC EC/DEC EC/DEC/FEC PMP-FSI PC/FEC PMP-FSI 11 12 Electrolytes (25 °C ) (25 °C ) (25 °C ) (25 °C ) (25 °C ) (60 °C ) (60 °C ) 13 14 15 52 60 48 55 65 50 65 16 First-cycle efficiency (%) 17 710 710 560 580 660 610 760 capacity 18 Maximum –1 (mAh g ) 19 20 48 40 10 26 36 19 55 21 High-rate retention (%) 22 48 83 40 69 96 43 95 23 Cyclic stability (%) 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Ionic conductivity and viscosity of various electrolytes at 25 and 60 °C

Temperature 25 °C

60 °C

Conductivity (mS cm–1)

Viscosity (cP)

PC

6.5

6.7

PC/FEC

6.0

7.0

EC/DEC

6.4

3.0

EC/DEC/FEC

6.2

3.2

PMP-FSI

2.0

27.4

PC/FEC

8.4

2.4

PMP-FSI

7.0

4.3

Electrolyte



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 28 of 33

(b) 5 nm

2 µm

5 nm

(c)

(d) (001)

(020) (011) (022)

b* 

c*

5Å

[100] orth.

Figure 1. (a) SEM image, (b) TEM image, (c) high-resolution TEM lattice image, and (d) SAED pattern of synthesized Sb2S3/graphene sample.


Page 29 of 33

0.9

(a)

1st cycle 2nd cycle 3rd cycle

0.6 0.3 0.0 -0.3

Current (mA)

0.3 0.0 -0.3

+

1.5 1.0 0.5 0.0 0

200

400

600

800

Specific capacity (mAh/g) 2000

(e)

1500

CPE Re

1000

Rct

W

500 0 0

500

1000 1500 2000 2500

2.0

+

Potential (V vs. Na/Na )

(d)

800 600 400

PC PC/FEC EC/DEC EC/DEC/FEC PMP-FSI

200 0 0

5

10

15

20

25

30

Number of cycle 1000


1.5

1500 mA/g


1.0

1000 mA/g

(c)

1000

0.5

500 mA/g

2.0

0.0

2.0

250 mA/g

1.5

100 mA/g

1.0

Specific capacity (mAh/g)

0.5


0.0

Potential (V vs. Na/Na ) +

0.6

-0.9

-0.9


1st cycle 2nd cycle 3rd cycle

-0.6

-0.6

-Z" (ohm)

(b)

50 mA/g

Current (mA)

0.9

110

(f)

100

800

90

600

80 400 200

70 PC PC/FEC EC/DEC

0 0

Z' (ohm)

20

60

EC/DEC/FEC PMP-FSI

40

60

Cycle number

80

50 100

Figure 2. CV curves of Sb2S3/graphene electrodes recorded in (a) PC/FEC and (b) PMP-FSI electrolyte. (c) Charge–discharge curves at 50 mA g–1, (d) rate capability, (e) EIS spectra, and (f) cyclic stability of Sb2S3/graphene electrodes measured in various electrolytes at 25 C.


Coulombic efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

10 µm

(b)

10 µm

(c)

10 µm

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.


Page 30 of 33

Page 31 of 33

120 100

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

PC/FEC PMP-FSI

(b)

80 60 40 20 0 100

200

300

400

500

o

Temperature ( C)

(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.


+

(a)

1.6 50 mA/g 100 mA/g 250 mA/g 500 mA/g 1000 mA/g 1500 mA/g

1.2 0.8 0.4 0.0 0


+

2.0

200

400

600

800

2.0 (b) 1.6

0.8 0.4 0.0 0

CPE Re

Rct

1000

PC/FEC PMP-FSI

W

500

0 0

500

1000

1500

2000

200

400

600

800

Specific capacity (mAh/g) Specific capacity (mAh/g)

-Z" (Ohm)

(c)

50 mA/g 100 mA/g 250 mA/g 500 mA/g 1000 mA/g 1500 mA/g

1.2

Specific capacity (mAh/g) 1500

Page 32 of 33

1000

110

(d)

100

800

90

600

80

PC/FEC PMP-FSI

400

70

200 0 0

Z' (Ohm)

60 20

40

60

Cycle number

80

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 above two electrolytes at 60 °C.


50 100

Coulombic efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1000

(a)

High rate retention (%)


PC/FEC PMP-FSI

800 600 400 200 0

25 OC

100

(b)

60 40 20 0

25 OC

100

(c)

60 OC

Temperature ( OC )

Temperature ( OC ) 120

PC/FEC PMP-FSI

80

60 OC

Cyclic stability (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Page 33 of 33

PC/FEC PMP-FSI

80 60 40 20 0

25 OC

60 OC

Temperature ( OC )

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.


Electrolyte Optimization for Enhancing Electrochemical Performance

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