CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201402800
Synergistic Effects of Mixing Sulfone and Ionic Liquid as Safe Electrolytes for Lithium Sulfur Batteries Chen Liao,*[a] Bingkun Guo,[b] Xiao-Guang Sun,*[b] and Sheng Dai*[b, c] A strategy of mixing both an ionic liquid and sulfone is reported to give synergistic effects of reducing viscosity, increasing ionic conductivity, reducing polysulfide dissolution, and improving safety. The mixtures of ionic liquids and sulfones also show distinctly different physicochemical properties, including thermal properties and crystallization behavior. By using these electrolytes, lithium sulfur batteries assembled with lithium and mesoporous carbon composites show a reversible specific capacity of 1265 mAh g1 (second cycle) by using 40 % 1.0 m
lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in N-methyl-Npropylpyrrolidinium bis(trifluoromethylsulfonyl)imide with 60 % 1.0 m LiTFSI in methylisopropylsulfone in the first cycle. This capacity is slightly lower than that obtained in pure 1.0 m LiTFSI as the sulfone electrolyte; however, it exhibits excellent cycling stability and remains as high as 655 mAh g1 even after 50 cycles. This strategy provides a method to alleviate polysulfide dissolution and redox shuttle phenomena, at the same time, with improved ionic conductivity.
Introduction Lithium ion batteries (LIB) are common secondary batteries because of their relatively high specific energy density (150– 300 Wh kg1) compared with other rechargeable batteries, such as nickel–metal hydride batteries or nickel–cadmium batteries. Although the specific energy density of current LIBs with insertion oxide cathodes (such as the LiCoO2 cathode) is good for consumer electronics, it does not meet the requirements for either electric vehicles (EV) or smart grid storage. Alternative high-capacity cathode materials, such as sulfur and oxygen,[1, 2] hold promise for EV applications with a long driving range ( 320 km) because of their superior theoretical specific energy density (2600 and 11140 Wh kg1, respectively). Lithium sulfur (Li–S) batteries have attracted much attention because sulfur offers a high theoretical capacity of 1672 mAh g1. However, there are two main problems associated with using elemental sulfur as a cathode. First, the element sulfur is an intrinsically highly insulating material (5 1030 S cm1 at room temperature), and therefore, has a poor electric contact with the current collector. This problem is effectively addressed by developing a nanostructured carbon– sulfur cathode, which offers good electronic conductivity and better unitization of the active materials.[2] For example, Li–S [a] Dr. C. Liao Joint Center for Energy Storage Research Argonne National Laboratory 9700 S Cass Ave, Lemont, IL 60439 (USA) E-mail: [email protected] [b] Dr. B. Guo, Dr. X.-G. Sun, Dr. S. Dai Chemical Sciences Division, Oak Ridge National Laboratory One Bethel Valley Road, Oak Ridge, TN 37831 (United States) E-mail: [email protected] [c] Dr. S. Dai Department of Chemistry, University of Tennessee Knoxville 1420 Circle Dr., TN 37996 (USA) E-mail: [email protected]
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
batteries have been reported with a specific capacity of over 800 mAh g1 over 50 cycles.[3] The other problem is the severe dissolution of the polysulfide and the consequent redox shuttle effect observed for Li–S batteries. A good strategy to alleviate polysulfide dissolution is either to limit the diffusion rate of the lithium polysulfide or to utilize additives and protect the lithium metal anode through the formation of a protective solid–electrolyte interface (SEI) layer.[4] The current state of the art electrolyte for Li–S batteries is 1.0 m lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in a mixture of 1,2-dimethyloxyethane (DME) and 1,3-dioxlane (DOL; 1:1, v/v); however, polysulfide dissolution is severe in this mixture, despite the reported high specific capacity and high capacity retention.[5] Research efforts on the electrolyte development for Li–S batteries include the use of ionic liquids (IL),[6] sulfones,[7] and polymers.[8] The difficulty in developing new electrolytes for Li–S batteries lies in the high reactivity of the polysulfide anion toward functionalities such as ester, carbonyl, and carbonate groups; therefore, the selection of electrolytes for Li–S batteries is very limited. Mixtures of electrolytes have also been reported for Li–S batteries, for example, ILs can be mixed with ether solvents (e.g., poly(ethylene glycol) dimethyl ether (PEGDME)) to reduce the viscosity and improve the ionic conductivity.[6d] Recently, several groups reported Li–S batteries with a polysulfide solution as the catholyte, for example, Belharouak et al.[9a] developed an electrolyte consisting of 0.2 m lithium polysulfide (Li2Sx ; x = 4–8) and 0.5 m of LiNO3 in DME/ DOL (1:1 v/v).[9] Herein, we report the synergistic effect of mixing two efficient electrolytes for Li–S batteries: a sulfone electrolyte and an IL electrolyte.[10] The IL and sulfone were mixed at different volumetric ratios and physicochemical properties, such as thermal properties, ionic conductivity, and viscosity, were studied. Through our studies, it was demonstrated that the addition of ChemSusChem 0000, 00, 1 – 9
&1&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
Scheme 1. Structures of [MPPY][TFSI] and MIPS.
sulfone electrolyte (1.0 m LiTFSI in methylisopropylsulfone (MIPS); Scheme 1) to the IL electrolyte (1.0 m LiTFSI in Nmethyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([MPPY][TFSI]); Scheme 1) reduced both the viscosity and broke ion association clusters in the IL, and therefore, increased the ionic conductivity. The impedance of a symmetric Li j LiTFSI/IL j Li cell by utilizing the abovementioned mixtures as the electrolytes was also studied by means of electrochemical impedance spectroscopy (EIS), and it was found that the mixture containing 1.0 m LiTFSI in 40 % sulfone and 60 % IL showed the lowest impedance, in comparison with 1.0 m LiTFSI in pure sulfone or 1.0 m LiTFSI in pure IL. The electrochemical behavior of the Li–S batteries upon using a mesoporous carbon–sulfur (MC–S) composite and various electrolytes was also investigated. The presence of IL in the sulfone limited lithium polysulfide dissolution, as demonstrated by galvanostatic charge–discharge experiments. With the presence of 20 % IL in MIPS, the columbic efficiency decreased from 107 to 100 %, which suggested a significant decrease in polysulfide dissolution and suppression of the redox shuttle effect.
Results and Discussion Thermal properties The thermal stabilities of the ILs and sulfones, and mixtures of the two electrolytes were tested under a nitrogen atmosphere between 30 and 850 8C at a heating rate of 10 8C min1. As shown in Figure 1 a, the [MPPY][TFSI] ILs exhibit high thermal stabilities above 450 8C. The addition of 1 m LiTFSI into [MPPY] [TFSI] does not change the decomposition temperatures. However, the introduction of sulfone into the IL mixture lowers the stability because the sulfone has a lower thermal stability (less than 200 8C). Figure 1 b shows the TGA curve of a 1.0 m solution of LiTFSI in 20 % MIPS and 80 % [MPPY][TFSI]; the sulfone component was completely decomposed below 200 8C. Figure 2 compares the dynamic scanning calorimetry (DSC) profiles of electrolytes with different compositions of MIPS and [MPPY][TFSI] with and without 1 m LiTFSI. In Figure 2 a the DSC trace of pure [MPPY][TFSI] shows a melting point of 10.7 8C, whereas that of 1.0 m LiTFSI in [MPPY][TFSI] shows a crystallization temperature of 30 8C and a melting point of 8.8 8C. The melting point of the pure IL reported herein is very close 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. a) Thermogravimetric analysis (TGA) trace of the IL [MPPY][TFSI] with (g) and without LiTFSI (c). b) TGA of 1.0 m LiTFSI in 20 % MIPS and 80 % [MPPY][TFSI].
to those reported in the literature, which indicates the purity and low water content of the IL.[11] In Figure 2 b, the DSC trace of pure MIPS shows a crystallization point of 14.6 8C and a melting point of 15.8 8C, whereas 1.0 m LiTFSI in MIPS shows a crystallization temperature of 42.0 8C and a melting point of 0.6 8C. Introduction of 1 m LiTFSI into the [MPPY][TFSI] sulfone reduces both the crystallization and melting points. The addition of 1 m LiTFSI to mixtures of [MPPY][TSFI] and MIPS were also tested. In Figure 2 c, the DSC trace of 1.0 m LiTFSI in 80 % [MPPY][TFSI] and 20 % MIPS shows a crystallization point of 30 8C and a melting point of 8.8 8C, whereas 1.0 m LiTFSI/ MIPS in 20 % [MPPY][TFSI] and 80 % MIPS shows no clear crystallization and melting points. In conclusion, by mixing LiTFSI with [MPPY][TFSI] and MIPS, both the crystallization and melting points were significantly lowered and, at one point, the mixture showed a glass-like property with no clear glass transition and melting points (Figure 2 c, red line). Ionic conductivity and viscosity Different volumetric ratios (v/v) of 1.0 m LiTFSI in MIPS and 1.0 m LiTFSI in [MPPY][TFSI] electrolyte were used to prepare a variety of mixtures: 100 % sulfone electrolyte, 80 % sulfone electrolyte with 20 % IL electrolyte, 60 % sulfone electrolyte with 40 % IL electrolyte, 40 % sulfone electrolyte with 60 % IL electrolyte, 20 % sulfone electrolyte with 80 % IL electrolyte, and 100 % IL electrolyte. Figure 3 a shows the ionic conductivities as a function of the volumetric ratio of 1.0 m LiTFSI in [MPPY][TFSI]. The temperature dependence of the ionic conChemSusChem 0000, 00, 1 – 9
&2&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
Figure 3. a) Plot of ionic conductivity versus volumetric ratio for mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and 1.0 m LiTFSI in MIPS: c&c 23, c*c 30, c~c 40, c !c 50, c^c 56, c3c 64, c"c 83 8C. b) Temperature dependence of the ionic conductivity for mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and 1.0 m LiTFSI in MIPS: c&c 0, c*c 20, c~c 40, c !c 60, c^c 80, c3c 100 % IL.
Figure 2. DSC profiles of a) [MPPY][TFSI] with (c) and without LiTFSI (c); b) MIPS with (c) and without LiTFSI (c); and c) 1.0 m LiTFSI in 80 % [MPPY][TFSI] and 20 % MIPS (c), and 1.0 LiTFSI/MIPS in 20 % [MPPY] [TFSI] and 80 % MIPS (c).
ductivities of each sample was also monitored from 23 to 83 8C in an argon-filled glove box. Three distinctive features can be observed for the ionic conductivities. 1) The highest ionic conductivity was observed for the sample consisting of 40 % IL electrolyte and 60 % sulfone electrolyte. The 1.0 m solution of LiTFSI in pure sulfone electrolyte displayed the lowest ionic conductivity of all electrolytes because, unlike IL, sulfone does not have an intrinsic ionic conductivity. 2) Temperature has a greater effect on the ionic conductivity of the pure IL electrolyte. As shown in Figure 1 a, the 100 % IL electrolyte also displayed a lower ionic conductivity than any other electrolyte when temperature was lower than 50 8C; however, at higher temperatures, the ionic conductivity of the pure IL electrolyte increases much faster than the rest of the electrolytes. At 83 8C, the ionic conductivity of 1.0 m LiTFSI in [MPPY][TFSI] reaches 10.7 mS cm1, which is almost the same as the ionic 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
conductivity of the electrolyte consisting of 40 % IL electrolyte and 60 % sulfone electrolyte. 3) The function of ionic conductivity versus temperature is best described by the Vogel–Fulcher–Tamman equation [Eq. (1)]: s ¼ s 0 exp½B=ðTT 0 Þ
ð1Þ
in which s0 (S cm2) is a constant, B (K) is the pseudoactivation energy, and T0 (K) is the vanishing ion mobility temperature. As expected, upon increasing the temperature, the viscosity decreases (Figure 3 b), and thus, the ionic conductivity increases rapidly. The viscosity of 1.0 m LiTFSI in mixtures of ILs and sulfones are in the range of 66.7 to 319 cP at room temperature; a sharp increase in viscosity is observed after the IL electrolyte exceeds 50 % (v/v; Figure 4). As expected, the viscosity decreases with increasing temperature. Cyclic voltammetry Both IL and sulfone electrolytes can offer a wide electrochemical window region for the Li–S electrochemical reaction. It has been reported that the electrochemical windows of both 1.0 m LiTFSI in [MPPY][TFSI] and 1.0 m LiTFSI in MIPS on a platinum electrode exhibit an oxidation voltage of > 5.0 V versus Li/Li + . ChemSusChem 0000, 00, 1 – 9
&3&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
in the anodic scan were observed as major peaks for both IL and sulfone electrolytes, although distinctive multiple Li–Pt alloying peaks were also observed. Similar behavior was observed in the CV of the electrolyte consisting of different ratios of 1.0 m LiTFSI in [MPPY][TFSI] and 1.0 m LiTFSI in MIPS (Figure 5 a and d). Figure 5 b and c shows the CVs of two mixtures 1.0 m LiTFSI in [MPPY][TFSI] and MIPS at volumetric ratios of 20:80 and 80:20 on a platinum working electrode, with a similar Li–Pt alloy being observed in the range of 0.5 to 2 V. By comparing the CVs given in Figure 5 a–d, the current density of lithium deposition/stripping of 1.0 m LiTFSI in 100 % [MPPY] [TFSI] has the lowest value of about 4 mA cm2. The low curFigure 4. Temperature dependence of the viscosity of various mixtures of rent density in the 1.0 m solution of LiTFSI in 100 % [MPPY] 1.0 m LiTFSI in [MPPY][TFSI] and 1.0 m LiTFSI in MIPS: & 25, * 40, ~ 55, ! 70, [TFSI] is mostly attributed to the low lithium transference ^ 85 8C. number.[13] The electrochemical behavior of mixtures of electrolytes in the coin cells of Li–S cells was also examined by using the To test the possibility of repeated lithium deposition/dissoluMC–S electrode and various electrolytes. Figure 6 a and tion in different electrolytes, cyclic voltammograms (CVs) were b shows the CVs of 1.0 m LiTFSI in [MPPY][TFSI] and 1.0 m obtained in the voltage range of 3 to 0.5 V. We used platinum LiTFSI in MIPS on MC–S electrodes obtained at a scan rate of as a working electrode because it was previously reported for 0.1 mV s1 from 1 to 3 V. Two reversible peaks were observed the measurement of lithium plating/stripping for IL and sul[12] fone electrolytes. Figure 5 a and b shows the CVs of 1.0 m for the Li/S cell in 1.0 m LiTFSI in MIPS (Figure 6 a), which was LiTFSI in MIPS and 1.0 m LiTFSI in 80 % MIPS electrolyte on similar that reported for S in ordered MC–S electrodes.[14] The a platinum working electrode; the CVs were obtained in the two reversible peaks at 2.25 and 1.7 V were observed for 1.0 m voltage range of 3 to 0.5 V at a scan rate of 10 mV s1. Similar LiTFSI in MIPS; these corresponded to the two plateaus in the to previous reports, the lithium deposition peaks below 0 V in discharge profile (see Figure 9 a, below). An activation process the cathodic scan and the lithium stripping peaks below 0.5 V was observed as the electrochemical reactions between lithium and sulfur shifted to a higher voltage during the second and third cycles. On the contrary, the CV in Figure 6 d shows only one reversible broad peak at 1.95 V for 1.0 m LiTFSI in [MPPY][TFSI]. The broad feature of the reduction peak in electrolytes containing ILs reflects that the lithium ion encounters more polarization for 1.0 m LiTFSI in [MPPY] [TFSI]; this is attributed to higher viscosity and poorer charge transport. Figure 6 b and c shows the CVs of mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and MIPS at volumetric ratios of 20:80 and 80:20. The CVs in the mixture of 1.0 m LiTFSI in 80 % [MPPY][TFSI] and 20 % MIPS (Figure 6 c) resemble the CV of 1.0 m LiTFSI in pure [MPPY][TFSI] (Figure 6 d), whereas the CVs in the mixture of 1.0 m LiTFSI in 20 % [MPPY][TFSI] and 80 % MIPS (Figure 6 b) resembles the CV of 1.0 m LiTFSI in pure MIPS (FigFigure 5. CVs of electrolytes (c cycle 1, c cycle 2, c cycle 3) consisting of 1.0 m LiTFSI in a) [MPPY][TFSI], ure 6 a). b) 80 % MIPS and 20 % [MPPY][TFSI], c) 20 % MIPS and 80 % [MPPY][TFSI], and d) 1.0 m LiTFSI in [MPPY][TFSI] on a platinum electrode (surface area: 0.196 mm2) at a scan rate of 10 mV s1.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 9
&4&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
First, neither pure sulfone (1.0 m LiTFSI in MIPS) nor pure IL (1.0 m LiTFSI in [MPPY][TFSI]) were ideal electrolytes for the Li/ MC–S battery. As shown in Figure 8 a, for the pure sulfone electrolyte, the specific capacity for the first cycle is fairly high (1201 mAh g1), but dramatically drops to 577 mAh g1 after 20 cycles (Figure 8 a). For 1.0 m LiTFSI in [MPPY][TFSI] electrolyte (Figure 8 f), the starting specific capacity for the first cycle is only 281 mAh g1; however, the specific capacity quickly increases to 609 mAh g1 and stays at around 600 mAh g1 for the next 50 cycles. Second, suppression of polysulfide formation is observed by the introduction of 20 % (v/v) of the IL electrolyte into the sulfone. As shown in Figure 8 b–e, mixtures of 1.0 m LiTFSI in Figure 6. CVs of electrolytes (c cycle 1, c cycle 2, c cycle 3) consisting of 1.0 m LiTFSI in a) MIPS, b) 80 % [MPPY][TFSI] and MIPS have very MIPS and 20 % [MPPY][TFSI], c) 20 % MIPS and 80 % [MPPY][TFSI], and d) [MPPY][TFSI] on a MC–S electrode at promising performances in a scan rate of 0.1 mV s1. terms of specific capacity and cycling stability. In particular, by comparing the results in Reduction in interfacial resistance Figures 8 a and 4 b, the suppression of polysulfide formation is clear: by introducing 20 % (v/v) of the IL electrolyte into the The interfacial resistance depends, to a large extent, on the sulfone, the Coulombic efficiency decreases from 107 to 101 %. stability of the electrolytes towards lithium electrodes or The Coulombic efficiency of the Li–S cells remains at about whether a stable SEI can form to prevent further reactions. To 100 % for electrolytes with an IL content higher than 20 % (v/v; test the stability, the electrolytes were used to construct Li/Li Figure 8 b–f). This demonstrates the effective suppression of symmetric cells and the impedance spectra were monitored polysulfide formation through the addition of ILs. As previously with time. Typical impedance spectra (Figure 7 a) consist of reported, the IL has a low coordination ability toward the lithione semicircle at high frequency, which is mainly due to the um polysulfide salt because of the weak nature of the Lewis resistance of the surface layer, that is, the SEI film formed on acid/base pairs.[6c] The IL also increases the viscosity (Figure 4) the surface of the lithium electrodes (RSEI). The intercept at high frequency is the resistance of the bulk electrolyte (Rbulk), and provides slower charge transport, and therefore, suppresswhich remains nearly constant during the period of the stores the formation of lithium polysulfide in the Li–S batteries. age test. By monitoring the semicircle, the evolution of RSEI Third, mixing sulfone (1.0 m LiTFSI in MIPS) and IL (1.0 m LiTFSI in [MPPY][TFSI]) can increase both the specific capacity with time is shown in Figure 7 b. The value of RSEI remains fairly and capacity retention. A reversible capacity of 1265 mAh g1 low (< 800 W) when the ionic content is equal to or below 40 % (Figure 7 b, red, bright green, and blue), when the IL con(second cycle) is obtained for the first cycle when using 1.0 m tent exceeds 60 % (Figure 7 b, pink, dark green, and orange) LiTFSI in 40 % [MPPY][TFSI] and 60 % MIPS. This capacity is the value of RSEI increases substantially (> 1000 W). slightly lower than that obtained in the sulfone electrolyte; however, it exhibits excellent cycling stability, that is, the capacity is still as high as 655 mAh g1 even after 50 cycles (Figure 8 c). Better cycling stability in the IL electrolyte is also due Battery performance to its low coordination ability and high viscosity, which suppresses the solubility of lithium polysulfide in the electrolytes, The cycling performance of the Li/S cell containing various and thus, improves the cycle stability. mixture of 1.0 m LiTFSI in [MPPY][TFSI] and MIPS is shown in Indeed, a synergetic effect was observed in Li/MC–S batterFigure 8. The specific capacities were reported based on the ies when using a mixture of 1.0 m LiTFSI in [MPPY][TFSI] and weight of active sulfur. There are three noticeable trends in the MIPS. The mixture utilizes 1) the low coordination ability of the cycling performance. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 9
&5&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
Figure 7. a) Nyquist plots of the Li/MC–S half cell using 1.0 m LiTFSI in 100 % [MPPY][TFSI] after & 1, & 2, & 3, & 4, & 5, & 6, & 7, and & 8 h. b) RSEI of the Li/ MC–S half cell with various mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and MIPS: & 0, & 20, & 40, & 60, & 80, and & 100 % [MPPY][TFSI].
IL with low lithium polysulfide solubility, and therefore, significant suppression of lithium polysulfide formation, and 2) the low viscosity and impedance of the sulfone. As a result, a Li/ MC–S battery with a mixture of IL/sulfone showed a large capacity, high Coulombic efficiency, good cyclability, and suppressed internal shuttling compared with the Li–S battery. The specific capacity of the Li/MC–S batteries strongly depends on the solubility of the polysulfide and the viscosity of electrolytes. Recently, Novk and Urbonaite reported that the amount of electrolyte influenced the long-term performance of the Li–S batteries.[15] In our experiments, we used excess amounts of electrolytes. As shown in Figure 9, differences were observed in the voltage profile of the Li/MC–S cells at a current rate of 0.05C when using 1.0 m LiTFSI in different ratios of IL/ sulfone mixtures (only the ratio of the IL is shown in Figure 9 for simplicity). Similar to the previously discussed cyclic voltammetry results (Figure 6 a), there were two plateaus (2.3 and 2.08 V) in the discharge process in mixtures containing 0–80 % IL; however, only one plateau at around 2.15 V was observed in the discharge process for 1.0 m LiTFSI in 100 % IL (Figure 9 a). The single redox potential in Figure 9 a was attributed to the high viscosity and high polarization of the IL electrolyte. The specific discharge capacities of the first cycle followed the order 40 % IL > 0 % IL > 20 % IL 60 % IL > 80 % IL > 100 % IL (Figure 9 a). All cells could be charged to > 2.5 V with a steep increase, which indicated that no redox shuttle effect of the polysulfide solution was observed for our Li/MC–S cell systems 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org (Figure 9 b).[16] In electrolyte mixtures containing 0–60 % IL, the charge profile shows a small drop at the very beginning, as indicated in the inset of Figure 9 b. This voltage drop has been attributed to the chemical dissolution of insoluble Li2Sn (n < 2) with soluble polysulfide at the very beginning of charging process.[17] This dissolution process is less clear in mixtures of electrolytes containing 80 and 100 % IL, which indicates the prevention of polysulfide dissolution in mixtures containing a large amount of ILs. Impedance spectra were also recorded to understand chemistry at the interface in assembled coin cells of Li j electrolyte j MC–S batteries. As shown in Figure 10, only the value of RSEI in the Li/MC–S cell assembled with 100 % IL increased from 510 to 1000 Ws after 50 cycles at room temperature (Figure 10 a); the other cells with electrolytes containing 0–80 % IL showed no significant change (for electrolytes containing 0, 20, 40, 60, and 80 % IL; Figure 10 b provides a representative example). In the 100 % IL electrolyte, the SEI at the lithium metal anode increased with the number of cycles and protected the lithium metal from being attacked by the polysulfide. This demonstrates the essential role of the IL in the formation of a passivation film on the lithium surface. Although an increased SEI layer would hinder the transport of lithium ions and slow the kinetics of the battery, it provides excellent retention of the battery capacity. The RSEI value in electrolytes containing sulfones does not change much (Figure 10 b). Sulfones have much a lower viscosity and addition of the sulfone to the IL will decrease the formation of the passivation film, which explains the negligible RSEI changes.
Conclusions The optimum composition ratio for a mixture of IL and sulfone required consideration of both the effects of the suppression of lithium polysulfide formation and reduction of the impedance. Changing the composition ratio of IL and sulfone will change physicochemical properties, such as the thermal stability, the crystallization behavior, and the solubility and diffusion rate of lithium polysulfide. Our strategy of mixing an IL and sulfone provided a method to alleviate polysulfide dissolution and shuttle phenomena, and, at the same time, improved the ionic conductivity. The sulfone electrolyte displayed a much lower viscosity than that of the IL electrolyte; however, the ionic conductivity of the IL electrolyte was higher because of its intrinsic ionic nature. The addition of sulfone could prevent the formation of ion clusters in the IL by introducing a secondary nonionic solvent; therefore, reducing the viscosity for Li–S batteries.
Experimental Section Preparation of electrolytes and Li–S cells The IL, [MPPY][TFSI], was synthesized as described in previously.[12] MIPS was purchased from TCI America and was distilled under vacuum after being heated at reflux over CaH2 under argon overnight. It was freeze dried for 4 days under high vacuum (< 0.008 mbar; 1 bar = 1 105 Pa) and then stored over 4 molecuChemSusChem 0000, 00, 1 – 9
&6&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
Figure 8. Typical cycling performance (& charge, * discharge) and Coulombic efficiency (&) of the Li–S cells with mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and MIPS at different volumetric ratios: a) a) MIPS, b) 80 % MIPS and 20 % [MPPY][TFSI], c) 60 % MIPS and 40 % [MPPY][TFSI], d) 40 % MIPS and 60 % [MPPY][TFSI], e) 20 % MIPS and 80 % [MPPY][TFSI], and f) [MPPY][TFSI].
lar sieves (freshly activated at 400 8C for 4 h) in an argon-filled glove box. The water contents of [MPPY][TFSI] and MIPS were less than 45 ppm, as detected by Karl–Fisher titration. LiTFSI from 3M was dried at 140 8C under high vacuum for 48 h before being added to the IL to make the electrolyte solution, 1.0 m LiTFSI in [MPPY][TFSI], inside an argon-filled glove box. Mesoporous carbon particles were synthesized by carbonization of a nanostructured polymeric composite that was obtained by self-assembly of a block copolymer (i.e., Pluronic F127) and phenol resin (i.e., phloroglucinol–formaldehyde) under acidic conditions by a soft-template method.[18] MC–S composite electrodes were prepared by following a previously reported procedure.[19] The sulfur loading was 0.5–0.8 mg cm2. The weight percentages of sulfur in the MC–S composites were measured by means of TGA on a TA 2950 instrument. The samples were loaded onto platinum sample pans and heated to 700 8C at 10 8C min1 under a nitrogen atmosphere. The sulfur loading was about 45 %, as demonstrated by TGA. Batteries were assembled as 2032-type coin cells inside an argon-filled glove 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
box by using a MC–S composite electrode as the cathode, lithium metal as the counter electrode, Celgard 3401 as the separator, and the mixed electrolyte as the electrolyte.
Electrochemical measurements Cyclic voltammetry for the coin cell was performed by using MC–S as the working electrode and lithium metal as both the counter and reference electrodes on a Gamry Instrument under different scan rates from 1 to 3 V. The Li–S batteries were tested on an Arbin BT2000 cycling station under different current densities between 1.0 and 3.0 V at room temperature. For the impedance study, a symmetric cell, Li j LiTFSI/IL j Li, was fabricated by using Celgard 3401 as the separator and the impedance spectroscopy was monitored as a function of time with a Gamry Instrument to study the interfacial stability toward lithium metal anode of the IL electrolytes. Linear sweep voltammetry was performed with a three ChemSusChem 0000, 00, 1 – 9
&7&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org electrode cell (a Pt working electrode with a surface area of 0.00196 cm2 as the working electrode and two lithium reference electrodes) in the first cathodic scan.
Acknowledgements This research was supported by the U.S. Department of Energy’s Office of Basic Energy Science, Division of Materials Sciences and Engineering. C.L. was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Keywords: electrochemistry · ionic liquids · lithium–sulfur batteries · polymers · sulfones
Figure 9. a) Galvanostatic discharge curves of the MC–S cell at a rate of 0.05C in electrolytes consisting of mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and MIPS at 20 8C: c 0, c 20, c 40, c 60, c 80, and c 100 % [MPPY][TFSI]. b) Galvanostatic charge curves of the MC–S cell at a rate of 0.05C in electrolytes consisting of mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and MIPS at 20 8C: c 0, c 20, c 40, c 60, c 80, and c 100 % [MPPY][TFSI].
[1] a) P. G. Bruce, S. A. Freunberger, L. J. Hardwick, J. M. Tarascon, Nat. Mater. 2012, 11, 172 – 172; b) J. M. Tarascon, M. Armand, Nature 2001, 414, 359 – 367. [2] X. Ji, K. T. Lee, L. F. Nazar, Nat. Mater. 2009, 8, 500 – 506. [3] W. Li, G. Zheng, Y. Yang, Z. W. Seh, N. Liu, Y. Cui, Proc. Natl. Acad. Sci. USA 2013, 110, 7148 – 7153. [4] E. S. Shin, K. Kim, S. H. Oh, W. I. Cho, Chem. Commun. 2013, 49, 2004 – 2006. [5] Y. Mikhaylik, I. Kovalev, R. Schock, K. Kumaresan, J. Xu, J. Affinito in Battery/Energy Technology, Vol. 25 (Eds.: Z. Ogumi, N. J. Dudney, S. R. Narayanan), 2010, pp. 23 – 34. [6] a) L. Wang, H. R. Byon, J. Power Sources 2013, 236, 207 – 214; b) L. Suo, Y.-S. Hu, H. Li, M. Armand, L. Chen, Nat. Commun. 2013, 4, 1481; c) N. Tachikawa, K. Yamauchi, E. Takashima, J. W. Park, K. Dokko, M. Watanabe, Chem. Commun. 2011, 47, 8157 – 8159; d) J. H. Shin, E. J. Cairns, J. Power Sources 2008, 177, 537 – 545. [7] B. Guo, X.-G. Sun, G. M. Veith, Z. Bi, S. M. Mahurin, C. Liao, C. Bridges, M. P. Paranthaman, S. Dai, Adv. Energy Mater. 2013, 3, 708 – 712. [8] Y. Yang, M. T. McDowell, A. Jackson, J. J. Cha, S. S. Hong, Y. Cui, Nano Lett. 2010, 10, 1486 – 1491. [9] a) R. Xu, I. Belharouak, J. C. M. Li, X. Zhang, I. Bloom, J. BareÇo, Adv. Energy Mater. 2013, 3, 833 – 838; b) S. S. Zhang, J. A. Read, J. Power Sources 2012, 200, 77 – 82. [10] B. Guo, T. Ben, Z. Bi, G. M. Veith, X.-G. Sun, S. Qiu, S. Dai, Chem. Commun. 2013, 49, 4905 – 4907. [11] H. Sakaebe, H. Matsumoto, Electrochem. Commun. 2003, 5, 594 – 598. [12] X.-G. Sun, S. Dai, Electrochim. Acta 2010, 55, 4618 – 4626. [13] C. Liao, N. Shao, K. S. Han, X.-G. Sun, D.-E. Jiang, E. W. Hagaman, S. Dai, Phys. Chem. Chem. Phys. 2011, 13, 21503 – 21510. [14] X. Liang, Z. Wen, Y. Liu, H. Zhang, L. Huang, J. Jin, J. Power Sources 2011, 196, 3655 – 3658. [15] S. Urbonaite, P. Novk, J. Power Sources 2014, 249, 497 – 502. [16] L. Hu, Z. Xue, K. Amine, Z. Zhang, Journal of The Electrochemical Society 2014, 161, A1777 – A1781. [17] S. S. Zhang, D. T. Tran, J. Power Sources 2012, 211, 169 – 172. [18] B. Guo, X. Wang, P. F. Fulvio, M. Chi, S. M. Mahurin, X.-G. Sun, S. Dai, Adv. Mater. 2011, 23, 4661 – 4666. [19] X.-G. Sun, X. Wang, R. T. Mayes, S. Dai, ChemSusChem 2012, 5, 2079 – 2085. Received: August 6, 2014 Published online on && &&, 0000
Figure 10. Impedance spectroscopy of a MC–S electrode recorded before (&) and after 50 cycles (*): a) 1.0 m LiTFSI in 100 % [MPPY][TFSI] and b) 1.0 m LiTFSI in 40 % [MPPY][TFSI] and 60 % MIPS.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 9
&8&
These are not the final page numbers! ÞÞ
FULL PAPERS Optimum composition: Mixtures of an ionic liquid and sulfone show distinctly different physicochemical properties from either of the pure components, including thermal properties and crystallization behavior. Lithium sulfur batteries that are assembled from lithium and a mesoporous carbon composite have a good reversible capacity and excellent cycling stability.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
C. Liao,* B. Guo, X.-G. Sun,* S. Dai* && – && Synergistic Effects of Mixing Sulfone and Ionic Liquid as Safe Electrolytes for Lithium Sulfur Batteries
ChemSusChem 0000, 00, 1 – 9
&9&
These are not the final page numbers! ÞÞ
Synergistic Effects of Mixing Sulfone and Ionic Liquid as Safe Electrolytes for Lithium Sulfur Batteries Chen Liao,*[a] Bingkun Guo,[b] Xiao-Guang Sun,*[b] and Sheng Dai*[b, c] A strategy of mixing both an ionic liquid and sulfone is reported to give synergistic effects of reducing viscosity, increasing ionic conductivity, reducing polysulfide dissolution, and improving safety. The mixtures of ionic liquids and sulfones also show distinctly different physicochemical properties, including thermal properties and crystallization behavior. By using these electrolytes, lithium sulfur batteries assembled with lithium and mesoporous carbon composites show a reversible specific capacity of 1265 mAh g1 (second cycle) by using 40 % 1.0 m
lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in N-methyl-Npropylpyrrolidinium bis(trifluoromethylsulfonyl)imide with 60 % 1.0 m LiTFSI in methylisopropylsulfone in the first cycle. This capacity is slightly lower than that obtained in pure 1.0 m LiTFSI as the sulfone electrolyte; however, it exhibits excellent cycling stability and remains as high as 655 mAh g1 even after 50 cycles. This strategy provides a method to alleviate polysulfide dissolution and redox shuttle phenomena, at the same time, with improved ionic conductivity.
Introduction Lithium ion batteries (LIB) are common secondary batteries because of their relatively high specific energy density (150– 300 Wh kg1) compared with other rechargeable batteries, such as nickel–metal hydride batteries or nickel–cadmium batteries. Although the specific energy density of current LIBs with insertion oxide cathodes (such as the LiCoO2 cathode) is good for consumer electronics, it does not meet the requirements for either electric vehicles (EV) or smart grid storage. Alternative high-capacity cathode materials, such as sulfur and oxygen,[1, 2] hold promise for EV applications with a long driving range ( 320 km) because of their superior theoretical specific energy density (2600 and 11140 Wh kg1, respectively). Lithium sulfur (Li–S) batteries have attracted much attention because sulfur offers a high theoretical capacity of 1672 mAh g1. However, there are two main problems associated with using elemental sulfur as a cathode. First, the element sulfur is an intrinsically highly insulating material (5 1030 S cm1 at room temperature), and therefore, has a poor electric contact with the current collector. This problem is effectively addressed by developing a nanostructured carbon– sulfur cathode, which offers good electronic conductivity and better unitization of the active materials.[2] For example, Li–S [a] Dr. C. Liao Joint Center for Energy Storage Research Argonne National Laboratory 9700 S Cass Ave, Lemont, IL 60439 (USA) E-mail: [email protected] [b] Dr. B. Guo, Dr. X.-G. Sun, Dr. S. Dai Chemical Sciences Division, Oak Ridge National Laboratory One Bethel Valley Road, Oak Ridge, TN 37831 (United States) E-mail: [email protected] [c] Dr. S. Dai Department of Chemistry, University of Tennessee Knoxville 1420 Circle Dr., TN 37996 (USA) E-mail: [email protected]
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
batteries have been reported with a specific capacity of over 800 mAh g1 over 50 cycles.[3] The other problem is the severe dissolution of the polysulfide and the consequent redox shuttle effect observed for Li–S batteries. A good strategy to alleviate polysulfide dissolution is either to limit the diffusion rate of the lithium polysulfide or to utilize additives and protect the lithium metal anode through the formation of a protective solid–electrolyte interface (SEI) layer.[4] The current state of the art electrolyte for Li–S batteries is 1.0 m lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in a mixture of 1,2-dimethyloxyethane (DME) and 1,3-dioxlane (DOL; 1:1, v/v); however, polysulfide dissolution is severe in this mixture, despite the reported high specific capacity and high capacity retention.[5] Research efforts on the electrolyte development for Li–S batteries include the use of ionic liquids (IL),[6] sulfones,[7] and polymers.[8] The difficulty in developing new electrolytes for Li–S batteries lies in the high reactivity of the polysulfide anion toward functionalities such as ester, carbonyl, and carbonate groups; therefore, the selection of electrolytes for Li–S batteries is very limited. Mixtures of electrolytes have also been reported for Li–S batteries, for example, ILs can be mixed with ether solvents (e.g., poly(ethylene glycol) dimethyl ether (PEGDME)) to reduce the viscosity and improve the ionic conductivity.[6d] Recently, several groups reported Li–S batteries with a polysulfide solution as the catholyte, for example, Belharouak et al.[9a] developed an electrolyte consisting of 0.2 m lithium polysulfide (Li2Sx ; x = 4–8) and 0.5 m of LiNO3 in DME/ DOL (1:1 v/v).[9] Herein, we report the synergistic effect of mixing two efficient electrolytes for Li–S batteries: a sulfone electrolyte and an IL electrolyte.[10] The IL and sulfone were mixed at different volumetric ratios and physicochemical properties, such as thermal properties, ionic conductivity, and viscosity, were studied. Through our studies, it was demonstrated that the addition of ChemSusChem 0000, 00, 1 – 9
&1&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
Scheme 1. Structures of [MPPY][TFSI] and MIPS.
sulfone electrolyte (1.0 m LiTFSI in methylisopropylsulfone (MIPS); Scheme 1) to the IL electrolyte (1.0 m LiTFSI in Nmethyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([MPPY][TFSI]); Scheme 1) reduced both the viscosity and broke ion association clusters in the IL, and therefore, increased the ionic conductivity. The impedance of a symmetric Li j LiTFSI/IL j Li cell by utilizing the abovementioned mixtures as the electrolytes was also studied by means of electrochemical impedance spectroscopy (EIS), and it was found that the mixture containing 1.0 m LiTFSI in 40 % sulfone and 60 % IL showed the lowest impedance, in comparison with 1.0 m LiTFSI in pure sulfone or 1.0 m LiTFSI in pure IL. The electrochemical behavior of the Li–S batteries upon using a mesoporous carbon–sulfur (MC–S) composite and various electrolytes was also investigated. The presence of IL in the sulfone limited lithium polysulfide dissolution, as demonstrated by galvanostatic charge–discharge experiments. With the presence of 20 % IL in MIPS, the columbic efficiency decreased from 107 to 100 %, which suggested a significant decrease in polysulfide dissolution and suppression of the redox shuttle effect.
Results and Discussion Thermal properties The thermal stabilities of the ILs and sulfones, and mixtures of the two electrolytes were tested under a nitrogen atmosphere between 30 and 850 8C at a heating rate of 10 8C min1. As shown in Figure 1 a, the [MPPY][TFSI] ILs exhibit high thermal stabilities above 450 8C. The addition of 1 m LiTFSI into [MPPY] [TFSI] does not change the decomposition temperatures. However, the introduction of sulfone into the IL mixture lowers the stability because the sulfone has a lower thermal stability (less than 200 8C). Figure 1 b shows the TGA curve of a 1.0 m solution of LiTFSI in 20 % MIPS and 80 % [MPPY][TFSI]; the sulfone component was completely decomposed below 200 8C. Figure 2 compares the dynamic scanning calorimetry (DSC) profiles of electrolytes with different compositions of MIPS and [MPPY][TFSI] with and without 1 m LiTFSI. In Figure 2 a the DSC trace of pure [MPPY][TFSI] shows a melting point of 10.7 8C, whereas that of 1.0 m LiTFSI in [MPPY][TFSI] shows a crystallization temperature of 30 8C and a melting point of 8.8 8C. The melting point of the pure IL reported herein is very close 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. a) Thermogravimetric analysis (TGA) trace of the IL [MPPY][TFSI] with (g) and without LiTFSI (c). b) TGA of 1.0 m LiTFSI in 20 % MIPS and 80 % [MPPY][TFSI].
to those reported in the literature, which indicates the purity and low water content of the IL.[11] In Figure 2 b, the DSC trace of pure MIPS shows a crystallization point of 14.6 8C and a melting point of 15.8 8C, whereas 1.0 m LiTFSI in MIPS shows a crystallization temperature of 42.0 8C and a melting point of 0.6 8C. Introduction of 1 m LiTFSI into the [MPPY][TFSI] sulfone reduces both the crystallization and melting points. The addition of 1 m LiTFSI to mixtures of [MPPY][TSFI] and MIPS were also tested. In Figure 2 c, the DSC trace of 1.0 m LiTFSI in 80 % [MPPY][TFSI] and 20 % MIPS shows a crystallization point of 30 8C and a melting point of 8.8 8C, whereas 1.0 m LiTFSI/ MIPS in 20 % [MPPY][TFSI] and 80 % MIPS shows no clear crystallization and melting points. In conclusion, by mixing LiTFSI with [MPPY][TFSI] and MIPS, both the crystallization and melting points were significantly lowered and, at one point, the mixture showed a glass-like property with no clear glass transition and melting points (Figure 2 c, red line). Ionic conductivity and viscosity Different volumetric ratios (v/v) of 1.0 m LiTFSI in MIPS and 1.0 m LiTFSI in [MPPY][TFSI] electrolyte were used to prepare a variety of mixtures: 100 % sulfone electrolyte, 80 % sulfone electrolyte with 20 % IL electrolyte, 60 % sulfone electrolyte with 40 % IL electrolyte, 40 % sulfone electrolyte with 60 % IL electrolyte, 20 % sulfone electrolyte with 80 % IL electrolyte, and 100 % IL electrolyte. Figure 3 a shows the ionic conductivities as a function of the volumetric ratio of 1.0 m LiTFSI in [MPPY][TFSI]. The temperature dependence of the ionic conChemSusChem 0000, 00, 1 – 9
&2&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
Figure 3. a) Plot of ionic conductivity versus volumetric ratio for mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and 1.0 m LiTFSI in MIPS: c&c 23, c*c 30, c~c 40, c !c 50, c^c 56, c3c 64, c"c 83 8C. b) Temperature dependence of the ionic conductivity for mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and 1.0 m LiTFSI in MIPS: c&c 0, c*c 20, c~c 40, c !c 60, c^c 80, c3c 100 % IL.
Figure 2. DSC profiles of a) [MPPY][TFSI] with (c) and without LiTFSI (c); b) MIPS with (c) and without LiTFSI (c); and c) 1.0 m LiTFSI in 80 % [MPPY][TFSI] and 20 % MIPS (c), and 1.0 LiTFSI/MIPS in 20 % [MPPY] [TFSI] and 80 % MIPS (c).
ductivities of each sample was also monitored from 23 to 83 8C in an argon-filled glove box. Three distinctive features can be observed for the ionic conductivities. 1) The highest ionic conductivity was observed for the sample consisting of 40 % IL electrolyte and 60 % sulfone electrolyte. The 1.0 m solution of LiTFSI in pure sulfone electrolyte displayed the lowest ionic conductivity of all electrolytes because, unlike IL, sulfone does not have an intrinsic ionic conductivity. 2) Temperature has a greater effect on the ionic conductivity of the pure IL electrolyte. As shown in Figure 1 a, the 100 % IL electrolyte also displayed a lower ionic conductivity than any other electrolyte when temperature was lower than 50 8C; however, at higher temperatures, the ionic conductivity of the pure IL electrolyte increases much faster than the rest of the electrolytes. At 83 8C, the ionic conductivity of 1.0 m LiTFSI in [MPPY][TFSI] reaches 10.7 mS cm1, which is almost the same as the ionic 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
conductivity of the electrolyte consisting of 40 % IL electrolyte and 60 % sulfone electrolyte. 3) The function of ionic conductivity versus temperature is best described by the Vogel–Fulcher–Tamman equation [Eq. (1)]: s ¼ s 0 exp½B=ðTT 0 Þ
ð1Þ
in which s0 (S cm2) is a constant, B (K) is the pseudoactivation energy, and T0 (K) is the vanishing ion mobility temperature. As expected, upon increasing the temperature, the viscosity decreases (Figure 3 b), and thus, the ionic conductivity increases rapidly. The viscosity of 1.0 m LiTFSI in mixtures of ILs and sulfones are in the range of 66.7 to 319 cP at room temperature; a sharp increase in viscosity is observed after the IL electrolyte exceeds 50 % (v/v; Figure 4). As expected, the viscosity decreases with increasing temperature. Cyclic voltammetry Both IL and sulfone electrolytes can offer a wide electrochemical window region for the Li–S electrochemical reaction. It has been reported that the electrochemical windows of both 1.0 m LiTFSI in [MPPY][TFSI] and 1.0 m LiTFSI in MIPS on a platinum electrode exhibit an oxidation voltage of > 5.0 V versus Li/Li + . ChemSusChem 0000, 00, 1 – 9
&3&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
in the anodic scan were observed as major peaks for both IL and sulfone electrolytes, although distinctive multiple Li–Pt alloying peaks were also observed. Similar behavior was observed in the CV of the electrolyte consisting of different ratios of 1.0 m LiTFSI in [MPPY][TFSI] and 1.0 m LiTFSI in MIPS (Figure 5 a and d). Figure 5 b and c shows the CVs of two mixtures 1.0 m LiTFSI in [MPPY][TFSI] and MIPS at volumetric ratios of 20:80 and 80:20 on a platinum working electrode, with a similar Li–Pt alloy being observed in the range of 0.5 to 2 V. By comparing the CVs given in Figure 5 a–d, the current density of lithium deposition/stripping of 1.0 m LiTFSI in 100 % [MPPY] [TFSI] has the lowest value of about 4 mA cm2. The low curFigure 4. Temperature dependence of the viscosity of various mixtures of rent density in the 1.0 m solution of LiTFSI in 100 % [MPPY] 1.0 m LiTFSI in [MPPY][TFSI] and 1.0 m LiTFSI in MIPS: & 25, * 40, ~ 55, ! 70, [TFSI] is mostly attributed to the low lithium transference ^ 85 8C. number.[13] The electrochemical behavior of mixtures of electrolytes in the coin cells of Li–S cells was also examined by using the To test the possibility of repeated lithium deposition/dissoluMC–S electrode and various electrolytes. Figure 6 a and tion in different electrolytes, cyclic voltammograms (CVs) were b shows the CVs of 1.0 m LiTFSI in [MPPY][TFSI] and 1.0 m obtained in the voltage range of 3 to 0.5 V. We used platinum LiTFSI in MIPS on MC–S electrodes obtained at a scan rate of as a working electrode because it was previously reported for 0.1 mV s1 from 1 to 3 V. Two reversible peaks were observed the measurement of lithium plating/stripping for IL and sul[12] fone electrolytes. Figure 5 a and b shows the CVs of 1.0 m for the Li/S cell in 1.0 m LiTFSI in MIPS (Figure 6 a), which was LiTFSI in MIPS and 1.0 m LiTFSI in 80 % MIPS electrolyte on similar that reported for S in ordered MC–S electrodes.[14] The a platinum working electrode; the CVs were obtained in the two reversible peaks at 2.25 and 1.7 V were observed for 1.0 m voltage range of 3 to 0.5 V at a scan rate of 10 mV s1. Similar LiTFSI in MIPS; these corresponded to the two plateaus in the to previous reports, the lithium deposition peaks below 0 V in discharge profile (see Figure 9 a, below). An activation process the cathodic scan and the lithium stripping peaks below 0.5 V was observed as the electrochemical reactions between lithium and sulfur shifted to a higher voltage during the second and third cycles. On the contrary, the CV in Figure 6 d shows only one reversible broad peak at 1.95 V for 1.0 m LiTFSI in [MPPY][TFSI]. The broad feature of the reduction peak in electrolytes containing ILs reflects that the lithium ion encounters more polarization for 1.0 m LiTFSI in [MPPY] [TFSI]; this is attributed to higher viscosity and poorer charge transport. Figure 6 b and c shows the CVs of mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and MIPS at volumetric ratios of 20:80 and 80:20. The CVs in the mixture of 1.0 m LiTFSI in 80 % [MPPY][TFSI] and 20 % MIPS (Figure 6 c) resemble the CV of 1.0 m LiTFSI in pure [MPPY][TFSI] (Figure 6 d), whereas the CVs in the mixture of 1.0 m LiTFSI in 20 % [MPPY][TFSI] and 80 % MIPS (Figure 6 b) resembles the CV of 1.0 m LiTFSI in pure MIPS (FigFigure 5. CVs of electrolytes (c cycle 1, c cycle 2, c cycle 3) consisting of 1.0 m LiTFSI in a) [MPPY][TFSI], ure 6 a). b) 80 % MIPS and 20 % [MPPY][TFSI], c) 20 % MIPS and 80 % [MPPY][TFSI], and d) 1.0 m LiTFSI in [MPPY][TFSI] on a platinum electrode (surface area: 0.196 mm2) at a scan rate of 10 mV s1.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 9
&4&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
First, neither pure sulfone (1.0 m LiTFSI in MIPS) nor pure IL (1.0 m LiTFSI in [MPPY][TFSI]) were ideal electrolytes for the Li/ MC–S battery. As shown in Figure 8 a, for the pure sulfone electrolyte, the specific capacity for the first cycle is fairly high (1201 mAh g1), but dramatically drops to 577 mAh g1 after 20 cycles (Figure 8 a). For 1.0 m LiTFSI in [MPPY][TFSI] electrolyte (Figure 8 f), the starting specific capacity for the first cycle is only 281 mAh g1; however, the specific capacity quickly increases to 609 mAh g1 and stays at around 600 mAh g1 for the next 50 cycles. Second, suppression of polysulfide formation is observed by the introduction of 20 % (v/v) of the IL electrolyte into the sulfone. As shown in Figure 8 b–e, mixtures of 1.0 m LiTFSI in Figure 6. CVs of electrolytes (c cycle 1, c cycle 2, c cycle 3) consisting of 1.0 m LiTFSI in a) MIPS, b) 80 % [MPPY][TFSI] and MIPS have very MIPS and 20 % [MPPY][TFSI], c) 20 % MIPS and 80 % [MPPY][TFSI], and d) [MPPY][TFSI] on a MC–S electrode at promising performances in a scan rate of 0.1 mV s1. terms of specific capacity and cycling stability. In particular, by comparing the results in Reduction in interfacial resistance Figures 8 a and 4 b, the suppression of polysulfide formation is clear: by introducing 20 % (v/v) of the IL electrolyte into the The interfacial resistance depends, to a large extent, on the sulfone, the Coulombic efficiency decreases from 107 to 101 %. stability of the electrolytes towards lithium electrodes or The Coulombic efficiency of the Li–S cells remains at about whether a stable SEI can form to prevent further reactions. To 100 % for electrolytes with an IL content higher than 20 % (v/v; test the stability, the electrolytes were used to construct Li/Li Figure 8 b–f). This demonstrates the effective suppression of symmetric cells and the impedance spectra were monitored polysulfide formation through the addition of ILs. As previously with time. Typical impedance spectra (Figure 7 a) consist of reported, the IL has a low coordination ability toward the lithione semicircle at high frequency, which is mainly due to the um polysulfide salt because of the weak nature of the Lewis resistance of the surface layer, that is, the SEI film formed on acid/base pairs.[6c] The IL also increases the viscosity (Figure 4) the surface of the lithium electrodes (RSEI). The intercept at high frequency is the resistance of the bulk electrolyte (Rbulk), and provides slower charge transport, and therefore, suppresswhich remains nearly constant during the period of the stores the formation of lithium polysulfide in the Li–S batteries. age test. By monitoring the semicircle, the evolution of RSEI Third, mixing sulfone (1.0 m LiTFSI in MIPS) and IL (1.0 m LiTFSI in [MPPY][TFSI]) can increase both the specific capacity with time is shown in Figure 7 b. The value of RSEI remains fairly and capacity retention. A reversible capacity of 1265 mAh g1 low (< 800 W) when the ionic content is equal to or below 40 % (Figure 7 b, red, bright green, and blue), when the IL con(second cycle) is obtained for the first cycle when using 1.0 m tent exceeds 60 % (Figure 7 b, pink, dark green, and orange) LiTFSI in 40 % [MPPY][TFSI] and 60 % MIPS. This capacity is the value of RSEI increases substantially (> 1000 W). slightly lower than that obtained in the sulfone electrolyte; however, it exhibits excellent cycling stability, that is, the capacity is still as high as 655 mAh g1 even after 50 cycles (Figure 8 c). Better cycling stability in the IL electrolyte is also due Battery performance to its low coordination ability and high viscosity, which suppresses the solubility of lithium polysulfide in the electrolytes, The cycling performance of the Li/S cell containing various and thus, improves the cycle stability. mixture of 1.0 m LiTFSI in [MPPY][TFSI] and MIPS is shown in Indeed, a synergetic effect was observed in Li/MC–S batterFigure 8. The specific capacities were reported based on the ies when using a mixture of 1.0 m LiTFSI in [MPPY][TFSI] and weight of active sulfur. There are three noticeable trends in the MIPS. The mixture utilizes 1) the low coordination ability of the cycling performance. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 9
&5&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
Figure 7. a) Nyquist plots of the Li/MC–S half cell using 1.0 m LiTFSI in 100 % [MPPY][TFSI] after & 1, & 2, & 3, & 4, & 5, & 6, & 7, and & 8 h. b) RSEI of the Li/ MC–S half cell with various mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and MIPS: & 0, & 20, & 40, & 60, & 80, and & 100 % [MPPY][TFSI].
IL with low lithium polysulfide solubility, and therefore, significant suppression of lithium polysulfide formation, and 2) the low viscosity and impedance of the sulfone. As a result, a Li/ MC–S battery with a mixture of IL/sulfone showed a large capacity, high Coulombic efficiency, good cyclability, and suppressed internal shuttling compared with the Li–S battery. The specific capacity of the Li/MC–S batteries strongly depends on the solubility of the polysulfide and the viscosity of electrolytes. Recently, Novk and Urbonaite reported that the amount of electrolyte influenced the long-term performance of the Li–S batteries.[15] In our experiments, we used excess amounts of electrolytes. As shown in Figure 9, differences were observed in the voltage profile of the Li/MC–S cells at a current rate of 0.05C when using 1.0 m LiTFSI in different ratios of IL/ sulfone mixtures (only the ratio of the IL is shown in Figure 9 for simplicity). Similar to the previously discussed cyclic voltammetry results (Figure 6 a), there were two plateaus (2.3 and 2.08 V) in the discharge process in mixtures containing 0–80 % IL; however, only one plateau at around 2.15 V was observed in the discharge process for 1.0 m LiTFSI in 100 % IL (Figure 9 a). The single redox potential in Figure 9 a was attributed to the high viscosity and high polarization of the IL electrolyte. The specific discharge capacities of the first cycle followed the order 40 % IL > 0 % IL > 20 % IL 60 % IL > 80 % IL > 100 % IL (Figure 9 a). All cells could be charged to > 2.5 V with a steep increase, which indicated that no redox shuttle effect of the polysulfide solution was observed for our Li/MC–S cell systems 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org (Figure 9 b).[16] In electrolyte mixtures containing 0–60 % IL, the charge profile shows a small drop at the very beginning, as indicated in the inset of Figure 9 b. This voltage drop has been attributed to the chemical dissolution of insoluble Li2Sn (n < 2) with soluble polysulfide at the very beginning of charging process.[17] This dissolution process is less clear in mixtures of electrolytes containing 80 and 100 % IL, which indicates the prevention of polysulfide dissolution in mixtures containing a large amount of ILs. Impedance spectra were also recorded to understand chemistry at the interface in assembled coin cells of Li j electrolyte j MC–S batteries. As shown in Figure 10, only the value of RSEI in the Li/MC–S cell assembled with 100 % IL increased from 510 to 1000 Ws after 50 cycles at room temperature (Figure 10 a); the other cells with electrolytes containing 0–80 % IL showed no significant change (for electrolytes containing 0, 20, 40, 60, and 80 % IL; Figure 10 b provides a representative example). In the 100 % IL electrolyte, the SEI at the lithium metal anode increased with the number of cycles and protected the lithium metal from being attacked by the polysulfide. This demonstrates the essential role of the IL in the formation of a passivation film on the lithium surface. Although an increased SEI layer would hinder the transport of lithium ions and slow the kinetics of the battery, it provides excellent retention of the battery capacity. The RSEI value in electrolytes containing sulfones does not change much (Figure 10 b). Sulfones have much a lower viscosity and addition of the sulfone to the IL will decrease the formation of the passivation film, which explains the negligible RSEI changes.
Conclusions The optimum composition ratio for a mixture of IL and sulfone required consideration of both the effects of the suppression of lithium polysulfide formation and reduction of the impedance. Changing the composition ratio of IL and sulfone will change physicochemical properties, such as the thermal stability, the crystallization behavior, and the solubility and diffusion rate of lithium polysulfide. Our strategy of mixing an IL and sulfone provided a method to alleviate polysulfide dissolution and shuttle phenomena, and, at the same time, improved the ionic conductivity. The sulfone electrolyte displayed a much lower viscosity than that of the IL electrolyte; however, the ionic conductivity of the IL electrolyte was higher because of its intrinsic ionic nature. The addition of sulfone could prevent the formation of ion clusters in the IL by introducing a secondary nonionic solvent; therefore, reducing the viscosity for Li–S batteries.
Experimental Section Preparation of electrolytes and Li–S cells The IL, [MPPY][TFSI], was synthesized as described in previously.[12] MIPS was purchased from TCI America and was distilled under vacuum after being heated at reflux over CaH2 under argon overnight. It was freeze dried for 4 days under high vacuum (< 0.008 mbar; 1 bar = 1 105 Pa) and then stored over 4 molecuChemSusChem 0000, 00, 1 – 9
&6&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
Figure 8. Typical cycling performance (& charge, * discharge) and Coulombic efficiency (&) of the Li–S cells with mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and MIPS at different volumetric ratios: a) a) MIPS, b) 80 % MIPS and 20 % [MPPY][TFSI], c) 60 % MIPS and 40 % [MPPY][TFSI], d) 40 % MIPS and 60 % [MPPY][TFSI], e) 20 % MIPS and 80 % [MPPY][TFSI], and f) [MPPY][TFSI].
lar sieves (freshly activated at 400 8C for 4 h) in an argon-filled glove box. The water contents of [MPPY][TFSI] and MIPS were less than 45 ppm, as detected by Karl–Fisher titration. LiTFSI from 3M was dried at 140 8C under high vacuum for 48 h before being added to the IL to make the electrolyte solution, 1.0 m LiTFSI in [MPPY][TFSI], inside an argon-filled glove box. Mesoporous carbon particles were synthesized by carbonization of a nanostructured polymeric composite that was obtained by self-assembly of a block copolymer (i.e., Pluronic F127) and phenol resin (i.e., phloroglucinol–formaldehyde) under acidic conditions by a soft-template method.[18] MC–S composite electrodes were prepared by following a previously reported procedure.[19] The sulfur loading was 0.5–0.8 mg cm2. The weight percentages of sulfur in the MC–S composites were measured by means of TGA on a TA 2950 instrument. The samples were loaded onto platinum sample pans and heated to 700 8C at 10 8C min1 under a nitrogen atmosphere. The sulfur loading was about 45 %, as demonstrated by TGA. Batteries were assembled as 2032-type coin cells inside an argon-filled glove 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
box by using a MC–S composite electrode as the cathode, lithium metal as the counter electrode, Celgard 3401 as the separator, and the mixed electrolyte as the electrolyte.
Electrochemical measurements Cyclic voltammetry for the coin cell was performed by using MC–S as the working electrode and lithium metal as both the counter and reference electrodes on a Gamry Instrument under different scan rates from 1 to 3 V. The Li–S batteries were tested on an Arbin BT2000 cycling station under different current densities between 1.0 and 3.0 V at room temperature. For the impedance study, a symmetric cell, Li j LiTFSI/IL j Li, was fabricated by using Celgard 3401 as the separator and the impedance spectroscopy was monitored as a function of time with a Gamry Instrument to study the interfacial stability toward lithium metal anode of the IL electrolytes. Linear sweep voltammetry was performed with a three ChemSusChem 0000, 00, 1 – 9
&7&
These are not the final page numbers! ÞÞ
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org electrode cell (a Pt working electrode with a surface area of 0.00196 cm2 as the working electrode and two lithium reference electrodes) in the first cathodic scan.
Acknowledgements This research was supported by the U.S. Department of Energy’s Office of Basic Energy Science, Division of Materials Sciences and Engineering. C.L. was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Keywords: electrochemistry · ionic liquids · lithium–sulfur batteries · polymers · sulfones
Figure 9. a) Galvanostatic discharge curves of the MC–S cell at a rate of 0.05C in electrolytes consisting of mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and MIPS at 20 8C: c 0, c 20, c 40, c 60, c 80, and c 100 % [MPPY][TFSI]. b) Galvanostatic charge curves of the MC–S cell at a rate of 0.05C in electrolytes consisting of mixtures of 1.0 m LiTFSI in [MPPY][TFSI] and MIPS at 20 8C: c 0, c 20, c 40, c 60, c 80, and c 100 % [MPPY][TFSI].
[1] a) P. G. Bruce, S. A. Freunberger, L. J. Hardwick, J. M. Tarascon, Nat. Mater. 2012, 11, 172 – 172; b) J. M. Tarascon, M. Armand, Nature 2001, 414, 359 – 367. [2] X. Ji, K. T. Lee, L. F. Nazar, Nat. Mater. 2009, 8, 500 – 506. [3] W. Li, G. Zheng, Y. Yang, Z. W. Seh, N. Liu, Y. Cui, Proc. Natl. Acad. Sci. USA 2013, 110, 7148 – 7153. [4] E. S. Shin, K. Kim, S. H. Oh, W. I. Cho, Chem. Commun. 2013, 49, 2004 – 2006. [5] Y. Mikhaylik, I. Kovalev, R. Schock, K. Kumaresan, J. Xu, J. Affinito in Battery/Energy Technology, Vol. 25 (Eds.: Z. Ogumi, N. J. Dudney, S. R. Narayanan), 2010, pp. 23 – 34. [6] a) L. Wang, H. R. Byon, J. Power Sources 2013, 236, 207 – 214; b) L. Suo, Y.-S. Hu, H. Li, M. Armand, L. Chen, Nat. Commun. 2013, 4, 1481; c) N. Tachikawa, K. Yamauchi, E. Takashima, J. W. Park, K. Dokko, M. Watanabe, Chem. Commun. 2011, 47, 8157 – 8159; d) J. H. Shin, E. J. Cairns, J. Power Sources 2008, 177, 537 – 545. [7] B. Guo, X.-G. Sun, G. M. Veith, Z. Bi, S. M. Mahurin, C. Liao, C. Bridges, M. P. Paranthaman, S. Dai, Adv. Energy Mater. 2013, 3, 708 – 712. [8] Y. Yang, M. T. McDowell, A. Jackson, J. J. Cha, S. S. Hong, Y. Cui, Nano Lett. 2010, 10, 1486 – 1491. [9] a) R. Xu, I. Belharouak, J. C. M. Li, X. Zhang, I. Bloom, J. BareÇo, Adv. Energy Mater. 2013, 3, 833 – 838; b) S. S. Zhang, J. A. Read, J. Power Sources 2012, 200, 77 – 82. [10] B. Guo, T. Ben, Z. Bi, G. M. Veith, X.-G. Sun, S. Qiu, S. Dai, Chem. Commun. 2013, 49, 4905 – 4907. [11] H. Sakaebe, H. Matsumoto, Electrochem. Commun. 2003, 5, 594 – 598. [12] X.-G. Sun, S. Dai, Electrochim. Acta 2010, 55, 4618 – 4626. [13] C. Liao, N. Shao, K. S. Han, X.-G. Sun, D.-E. Jiang, E. W. Hagaman, S. Dai, Phys. Chem. Chem. Phys. 2011, 13, 21503 – 21510. [14] X. Liang, Z. Wen, Y. Liu, H. Zhang, L. Huang, J. Jin, J. Power Sources 2011, 196, 3655 – 3658. [15] S. Urbonaite, P. Novk, J. Power Sources 2014, 249, 497 – 502. [16] L. Hu, Z. Xue, K. Amine, Z. Zhang, Journal of The Electrochemical Society 2014, 161, A1777 – A1781. [17] S. S. Zhang, D. T. Tran, J. Power Sources 2012, 211, 169 – 172. [18] B. Guo, X. Wang, P. F. Fulvio, M. Chi, S. M. Mahurin, X.-G. Sun, S. Dai, Adv. Mater. 2011, 23, 4661 – 4666. [19] X.-G. Sun, X. Wang, R. T. Mayes, S. Dai, ChemSusChem 2012, 5, 2079 – 2085. Received: August 6, 2014 Published online on && &&, 0000
Figure 10. Impedance spectroscopy of a MC–S electrode recorded before (&) and after 50 cycles (*): a) 1.0 m LiTFSI in 100 % [MPPY][TFSI] and b) 1.0 m LiTFSI in 40 % [MPPY][TFSI] and 60 % MIPS.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 9
&8&
These are not the final page numbers! ÞÞ
FULL PAPERS Optimum composition: Mixtures of an ionic liquid and sulfone show distinctly different physicochemical properties from either of the pure components, including thermal properties and crystallization behavior. Lithium sulfur batteries that are assembled from lithium and a mesoporous carbon composite have a good reversible capacity and excellent cycling stability.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
C. Liao,* B. Guo, X.-G. Sun,* S. Dai* && – && Synergistic Effects of Mixing Sulfone and Ionic Liquid as Safe Electrolytes for Lithium Sulfur Batteries
ChemSusChem 0000, 00, 1 – 9
&9&
These are not the final page numbers! ÞÞ
