Effect of Cation Structure on Electrochemical Behavior of Lithium in

Research Article pubs.acs.org/journal/ascecg

Effect of Cation Structure on Electrochemical Behavior of Lithium in [NTf2]‑based Ionic Liquids Bonita Dilasari, Yeojin Jung, Gunha Kim, and Kyungjung Kwon* Department of Energy & Mineral Resources Engineering, Sejong University, Seoul 143-747, Republic of Korea ABSTRACT: Room temperature ionic liquids (RTILs) containing bis(trifluoromethylsulfonyl)imide [NTf2]− anion are attractive electrolytes due to their preferable properties such as extremely wide electrochemical stability window and superior ionic conductivity. We study electrochemical behavior of [NTf2]-based RTILs with four types of cations, 1-butyl-1-methylpyrrolidinium [BMPyr]+, 1-butyl-2,3-dimethylimidazolium [BDMIm]+, 1-ethyl-3-methylimidazolium [EMIm]+, and 1-butyl-3-methylpyridinium [BMPy]+ in neat conditions and in the presence of lithium salts. Cyclic voltammetry shows bulk lithium reduction peaks in all RTILs, and surface characterization confirms the presence of Li metal and LiOH on the electrodeposits. The least stable [BMPy] [NTf2] shows a large cathodic peak at a potential more positive than the bulk lithium reduction peak indicating RTIL decomposition, whereas the other RTILs do not have apparent decomposition peaks. KEYWORDS: Lithium, Room temperature ionic liquids, Cyclic voltammetry, Electrodeposition, Bis(trifluoromethylsulfonyl)imide

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INTRODUCTION Research activity on lithium is still fascinating in this era of pursuing a reliable energy storage and conversion technology. Owing to its featured properties such as electropositivity and lightness, lithium has been exploited for energy storage system research since 1970s.1 Various battery systems utilize lithium as they can achieve high energy density, such as lithium−metal battery, lithium-ion battery, lithium−air battery, and lithium− sulfur battery.1−3 A variety of research approaches have been applied to these systems and improvements to battery performance have been updated steadily until now. Advanced research on lithium in battery systems will continuously be a hot issue because a global demand for batteries with high energy density, long life cycle, excellent safety, and low cost material keeps increasing. Because of its highly negative reduction potential, the electrochemical behavior of lithium can only be studied in nonaqueous electrolytes. Room temperature ionic liquids (RTILs) are promising electrolytes that can facilitate reduction and oxidation reactions of lithium. Some prominent properties of RTILs such as high electrochemical stability, low volatility, and inflammability offer a sufficient quality as electrolytes for lithium metal and lithium-ion cells.4 Among a vast variation of RTIL systems, RTILs containing bis(trifluoromethylsulfonyl)imide [NTf2]− anion are the most preferable due to their extremely wide electrochemical stability window and superior ionic conductivity. One of the reasons for their preferable properties is a charge delocalization phenomenon,5 which creates a weak ion−ion interaction and makes them have a high mobility. Application of [NTf2]-based electrolytes in lithium batteries has been reported to perform high cycling efficiency.6−8 © 2015 American Chemical Society

The electrochemical behavior of RTILs containing lithium salt is very unique. An addition of lithium salt Li[NTf2] to [NTf2]-based RTILs results in decreased ionic conductivity due to an increase in viscosity.9,10 On the other hand, an addition of lithium salt could also enhance cathodic stability of the RTILs.11,12 Although some publications already covered electrochemical behavior of Li/Li+ couple in the [NTf2]based RTILs,11,13−16 more comprehensive data about lithium redox reactions in RTIL systems are required considering the potential role of lithium in battery technology. We investigate the effect of cation in the [NTf2]-based RTILs on cathodic stability of the RTILs, lithium redox behavior and surface composition on lithium deposits. This study can provide reliable data to complement currently available data of lithium redox reactions in the [NTf2]-based RTILs, which are one of the most studied RTILs for lithium battery applications. We carried out a series of electrochemical measurements and surface characterizations in the [NTf2]-based RTILs with four different cations: 1-butyl-1-methylpyrrolidinium [BMPyr]+, 1butyl-2,3-dimethylimidazolium [BDMIm]+, 1-ethyl-3-methylimidazolium [EMIm] + , and 1-butyl-3-methylpyridinium [BMPy]+.

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EXPERIMENTAL SECTION

Four RTILs used in this study, [BMPyr] [NTf2], [BDMIm] [NTf2], [EMIm] [NTf2], and [BMPy] [NTf2], were purchased from C-TRI Special Issue: Ionic Liquids at the Interface of Chemistry and Engineering Received: September 1, 2015 Revised: October 20, 2015 Published: October 28, 2015 491

DOI: 10.1021/acssuschemeng.5b00987 ACS Sustainable Chem. Eng. 2016, 4, 491−496

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic structures of cations and anion used in this study. (halide content 5−8 ppm; water content ∼90 ppm). The schematic structures of cations and anion in the tested RTILs are presented in Figure 1. All RTILs were dried in a vacuum oven at 100 °C for 24 h before used to reduce the water content up to ∼20 ppm. Dried lithium salt (Li[NTf2], Sigma-Aldrich, 99.95%) was added to the RTILs to obtain a concentration of 0.5 M. Cyclic voltammetry (CV) measurements were performed with a WonATech (WPG 100) potentiostat. A three-electrode system, which consisted of Au foil (0.025 mm in thickness) as a working electrode, Pt wire (0.5 mm in diameter) as a quasi-reference electrode, and coiled Pt wire (0.25 mm in diameter) as a counter electrode, was adopted. The working electrode was polished with alumina 0.05 μm on an alumina polishing pad, whereas the counter and reference electrodes were polished with a SiC paper 1500 grid. Polished electrodes were cleaned in a piranha solution (mixture of H2SO4 and H2O2 with ratio 1:1) and dried prior to use. The scan rate for CV was set to 10 mV s−1. Ferrocene (SigmaAldrich, 98%) was used as an internal reference, and all potentials reported in the paper were converted to ferrocene/ferrocenium couple basis (vs Fc/Fc+). Lithium was potentiostatically electrodeposited in each RTIL for 30 min on Au foil substrate for surface characterization. The potentials in the potentiostatic electrodeposition were −3.5 V for [BMPyr] [NTf2], and −3.9 V for [BDMIm] [NTf2], [EMIm] [NTf2], and [BMPy] [NTf2]. All experiments were carried out in nitrogen atmosphere inside a glovebox (Korea Kiyon, oxygen and moisture content less than 1 ppm). Surface composition of electrodeposits was examined by a X-ray photoelectron spectroscopy (XPS) instrument (Theta Probe by Thermo Fisher Scientific Co.) with monochromatic Al Kα radiation. Survey spectra were acquired at 200 eV pass energy and each element spectra were acquired at 100 eV pass energy. Peak fitting was performed with XPSPEAK41 software.

Figure 2. CVs recorded on Au electrode for each RTIL in neat condition.

RESULTS AND DISCUSSION Electrochemical Measurements. Figure 2 shows a comparison of cathodic stability limit, where a cathodic current starts to increase rapidly, in the tested RTIILs in neat condition. [BMPyr] [NTf2] exhibited cathodic stability up to −3.0 V, the most negative limit among the other tested RTILs, followed by [BDMIm] [NTf2] up to −2.6 V and [EMIm] [NTf2] up to −2.2 V. [BMPy] [NTf2] showed the least negative cathodic limit, about −1.5 V. From this comparison, it is clear that the cation structure significantly affects the electrochemical properties of the RTILs with the same anion. [BMPyr] [NTf2] is one of RTILs having the widest electrochemical window and it is

presumed that the configuration of [BMPyr]+ cation somehow leads to the stability of the cation against reduction.17 Meanwhile, Suarez et al.18 proposed a mechanism for 1-nbutyl-3-methylimidazolium cation reduction, which is assumed to be preceded by the reduction of acidic single H proton from C2−H bonding. This mechanism can be applied to [BMPy]+ and [EMIm]+ in consideration of the structures of the cations as shown in Figure 1. [BMPy]+ and [EMIm]+ do have C2−H bonds; therefore, these cations are more acidic and easier to be reduced compared to the other cations. On the other hand, [BDMIm]+ and [BMPyr]+ cations that have C2−(CH3) bond and H−C2−H bond, respectively, are less acidic and tend to be

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DOI: 10.1021/acssuschemeng.5b00987 ACS Sustainable Chem. Eng. 2016, 4, 491−496

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ACS Sustainable Chemistry & Engineering reduced at more negative potentials. Another logical reason for the less reactive properties of the [BMPyr]+ cation compared to the others could be the absence of double bond in [BMPyr]+. Figure 3 shows CV measurements of [BMPyr] [NTf2] containing 0.5 M Li[NTf2] salt in the first, second, and third

Figure 5. CV of Au electrode in [EMIm] [NTf2] containing 0.5 M Li[NTf2].

Figure 3. CV of Au electrode in [BMPyr] [NTf2] containing 0.5 M Li[NTf2].

cycles. During the first cycle, two small bumps, marked as C1 and C2, in cathodic scan were observed at −2 and −2.7 V. The C1 peak may be attributed to the formation of lithium monolayer at a potential more positive than the reduction of bulk lithium, called as underpotential deposition (UPD),11,19 whereas the C2 peak is attributed to partial decomposition of [BMPyr]+ cation and cathodic breakdown of [NTf2]− anion that might precede or concurrently occur with the bulk lithium reduction.12 In accordance with the previous reports,11,12 we also found that the addition of lithium salt resulted in a typical cathodic stability extension in [BMPyr] [NTf2] as well as in the other tested RTILs as shown in Figures 4, 5, and 6. This

Figure 6. CV of Au electrode in [BMPy] [NTf2] containing 0.5 M Li[NTf2].

the alteration of electrode surface characteristic by the UPD reaction as well.11 A rapid increase in cathodic current at −3.2 V is postulated to be due to bulk lithium reduction. This reaction reached the peak C3 at −3.5 V, followed by a decrease in current. The scan direction was immediately reversed when the current was about to start increasing again to avoid the possible decomposition of the RTIL. During the anodic scan, two peaks were observed. The larger peak A1 at −2.2 V is attributed to reoxidation of electrodeposited lithium, whereas the smaller peak A2 at −1.6 V is attributed to reoxidation of cation species that are previously reduced at C2. Unlike the first cycle, the lithium UPD peak was not observed in the second and the third cycles. The reason is because the nature of the electrode surface has changed after the first cycle, and a thin black layer indeed remained on the surface after incomplete stripping of the electrodeposited lithium. The only small bump observed at −2.2 V (second cycle) and −2.1 V (third cycle) during the cathodic scan is analogous to the C2 peak in the first cycle. Bulk lithium reduction occurs at more positive potentials in the second and the third cycles compared to the first cycle, indicating less overpotential required for the reduction once the thin layer was formed on the substrate. CV measurements in [BDMIm] [NTf2] containing 0.5 M Li[NTf2] shown in Figure 4 exhibits similar features to the [BMPyr] [NTf2] case. Two small cathodic peaks, C1 (−2 V) and C2 (−2.6 V), were observed, followed by a higher peak, C3. Anodic scan also resulted in two consecutive peaks A1 and A2 at −2 and −1.75 V, respectively. The subsequent cycles only

Figure 4. CV of Au electrode in [BDMIm] [NTf2] containing 0.5 M Li[NTf2].

phenomenon has been observed so far only in RTILs containing lithium salts, and is unlikely to occur in cases of other metal salts. The unique characteristic of lithium saltcontaining RTILs is assumed to be due to the formation of a surface layer at a certain potential, which modifies the nature of the working electrode surface. The formation of surface layers, known as a solid electrolyte interphase (SEI), on lithium metal has been investigated actively. The SEI, consisting of a native film and reduction products of the [NTf2]− anion,20 is only penetrable for the lithium ions and prevents the further reduction of cation species of RTILs. Aside from the SEI formation, the cathodic stability limit could be extended due to 493

DOI: 10.1021/acssuschemeng.5b00987 ACS Sustainable Chem. Eng. 2016, 4, 491−496

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ACS Sustainable Chemistry & Engineering exhibited one small cathodic peak analogous to the C2 peak in the first cycle, and anodic peaks were obtained with slightly higher currents than the first cycle. The third CV result, which is shown in Figure 5, presents the potential scan results of Au electrode in [EMIm] [NTf2] electrolyte. A similar UPD peak was also observed at −2 V as in [BMPyr] [NTf2] and [BDMIm] [NTf2]. The cathodic current abruptly increased at −3.4 V and when it reached −3 mA cm−2, the increment was slowed down as if it reached a peak. The short current plateau was immediately overlapped by more rapid current increase that should be related to further decomposition of the RTIL species. Reverse scan resulted in an anodic peak A1 of which current was lower than the other RTILs. A smaller anodic peak was observed between −1 and 0 V, which may be attributed to the oxidation of reduced [EMIm]+ species. As aforementioned, cathodic peaks at more positive potentials appeared in the following cycles due to lower activation energy for the lithium reduction after the first cycle. A different cathodic behavior was observed in [BMPy] [NTf2] containing 0.5 M Li[NTf2] as shown in Figure 6. In the first cycle, a cathodic current began to increase dramatically at −1.8 V and reached a peak at −2.5 V, then descended back toward zero current. From −3.3 to −4 V, the cathodic current increased again and formed a higher cathodic peak, which was attributed to the bulk lithium reduction. Considering that the cathodic stability limit (−1.5 V) of neat [BMPy] [NTf2] was the most positive among the tested RTILs, it is reasonable to assume that the first cathodic peak C1 was related to the reduction of [BMPy]+. A small bump detected in the second and the third cycles at −2.4 V was seemingly similar to C1, but only a small amount of current flowed due to a different nature of the electrode surface after the first electrodeposition of lithium. Meanwhile, anodic peaks were shifted to more positive potentials indicating higher overpotential needed to strip electrodeposited layers that formed in the second and the third cycles. We also examined the dependency of reduction and oxidation behavior on lithium salt concentration. As depicted in Figure 7, current densities in both reduction and oxidation

Table 1. Coulombic Efficiency with a Variation of Li[NTf2] Concentration Li[NTf2] concentration (M)

reduction peak charge density (mC/cm2)

oxidation peak charge density (mC/cm2)

Coulombic efficiency (%)

0.1 0.2 0.5

22.9 66.5 100.3

3.9 25.3 48.4

16.9 38.0 48.2

more reversible lithium dissolution/deposition behavior. Because the addition of lithium salt to neat RTILs lowers the cathodic stability limit of RTILs as confirmed in this study, a higher lithium salt concentration could facilitate the separation of lithium deposition and cathodic decomposition of RTILs. Therefore, the highest lithium salt concentration (0.5 M) case would include the least Coulombic charge from side reactions such as the decomposition of RTILs, resulting in the most reversible dissolution/deposition behavior. Surface Characterizations. XPS spectra of Li 1s on the substrate surface after the potentiostatic electrodeposition in each RTIL is shown in Figure 8. A Li metal peak at 54.6 eV and a LiOH peak at 54.9 eV were well fitted to the Li 1s spectra of all RTILs,21 except for the [BMPy] [NTf2] case that was preferably fitted with a Li2O peak at 54.0 eV instead of LiOH.20 The appearance of Li2O is believed to be a reduction product of the electrolyte.20,22 This is consistent with the electrochemical measurement results that identified [BMPy] [NTf2] as the least stable RTIL among the tested RTILs. The formation of LiOH may originate from water impurities in the electrolyte or the reaction of lithium with moisture in the ambient air during the preparation of characterization process. In addition, slightly different spectra were also observed in [BDMIm] [NTf2]. The most significant peak was at 55.3 eV, which is contributed to Li2CO3/LiOH compounds suspected from contamination in glovebox atmosphere.20 It is noteworthy that Li metal peak with the highest intensity was obtained on the sample from [BMPyr] [NTf2], which was the most stable RTIL.

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CONCLUSIONS We have evaluated electrochemical behavior of lithium in [NTf2]-based RTILs with four different types of cations ([BMPyr]+, [BDMIm]+, [EMIm]+, and [BMPy]+) containing Li[NTf2]. Electrochemical measurements revealed the cathodic stability of the tested RTILs followed the decreasing order of [BMPyr] [NTf2] > [BDMIm] [NTf2] > [EMIm] [NTf2] > [BMPy] [NTf2]. The addition of Li[NTf2] widened the cathodic stability region in all tested RTILs. Cathodic scan exhibited a small bump attributed to lithium UPD at a potential of −2 V in [BMPyr] [NTf2], [BDMIm] [NTf2], and [EMIm] [NTf2] containing 0.5 M Li[NTf2], followed by a bulk lithium reduction peak at potentials more negative than cathodic stability limit of the neat RTILs. Different reduction behavior was observed in the case of [BMPy] [NTf2], which showed a large cathodic peak at −2.5 V indicating RTIL decomposition. The structure of cation exerted a strong influence on the cathodic stability of the RTIL, thus affecting the lithium electrodeposition as well. XPS analysis of electrodeposited surface showed a Li2O peak as the most significant peak obtained in [BMPy] [NTf2] + 0.5 M Li[NTf2], whereas the other RTILs showed Li metal and LiOH peaks. The highest Li metal intensity was obtained from the sample electrodeposited in the most stable [BMPyr] [NTf2].

Figure 7. CV of Au electrode in [BMPyr] [NTf2] with a variation of Li[NTf2] concentration.

peaks were proportional to Li[NTf2] concentration. We calculated the charge density of lithium reduction and oxidation peaks and compared it to obtain Coulombic (dissolution/ deposition) efficiency as shown in Table 1. The potential range of −3.3 to −3.9 V and −2.9 to −1.4 V for reduction and oxidation peaks, respectively, was set for charge calculation. It was evident that a higher lithium salt concentration provides 494

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Figure 8. Li 1s spectra of electrodeposited surface in [BMPyr] [NTf2] (a), [BDMIm] [NTf2] (b), [EMIm] [NTf2] (c), and [BMPy] [NTf2] (d) containing 0.5 M Li[NTf2].

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AUTHOR INFORMATION

Corresponding Author

*K. Kwon. Tel.: +82-2-3408-3947. Fax: +82-2-3408-3671. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013R1A1A2010795).

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