Influence of Polar Organic Solvents in an Ionic Liquid Containing

Research Article www.acsami.org

Influence of Polar Organic Solvents in an Ionic Liquid Containing Lithium Bis(fluorosulfonyl)amide: Effect on the Cation−Anion Interaction, Lithium Ion Battery Performance, and Solid Electrolyte Interphase Abhishek Lahiri,* Guozhu Li, Mark Olschewski, and Frank Endres* Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Strasse 6, 38678 Clausthal-Zellerfeld, Germany S Supporting Information *

ABSTRACT: Ionic liquid−organic solvent mixtures have recently been investigated as potential battery electrolytes. However, contradictory results with these mixtures have been shown for battery performance. In this manuscript, we studied the influence of the addition of polar organic solvents into the ionic liquid electrolyte 1 M lithium bis(fluorosulfonyl)amide (LiFSI)−1-butyl-1methylpyrrolidinium bis(fluorosulfonyl)amide ([Py1,4]FSI) and tested it for lithium ion battery applications. From infrared and Raman spectroscopy, clear changes in the lithium solvation and cation−anion interactions in the ionic liquid were observed on addition of organic solvents. From the lithiation/delithiation studies on electrodeposited Ge, the storage capacity for the ionic liquid−highly polar organic solvent (acetonitrile) mixture was found to be the highest at low C-rates (0.425 C) compared to using an ionic liquid alone and ionic liquid−less polar solvent (dimethyl carbonate) mixtures. Furthermore, XPS and AFM were used to evaluate the solid electrolyte interphase (SEI) and to correlate its stability with Li storage capacity. KEYWORDS: ionic liquids, lithium-ion battery, battery electrolytes, intermolecular interactions, solid−electrolyte interphase, atomic force microscopy, X-ray photoelectron spectroscopy

■

INTRODUCTION Ionic liquids consist of organic cations and anions with some interesting properties such as wide electrochemical windows, low vapor pressures, noninflammability, etc. Due to these properties, they have been applied in various research fields such as electrochemistry,1−3 separation technology,4,5 catalysis,6,7 biological applications,8,9 etc. Ionic liquids are also potential electrolytes for lithium-ion batteries.10−12 The main concern of organic electrolytes in LIBs is the safety issue, as they have rather high vapor pressure. Furthermore, they are flammable. Also, it has been shown that there are some chemical reactions between organic electrolytes and battery electrodes that affect the battery performance.13 However, compared to conventional organic electrolytes, ionic liquids possess higher viscosity and the diffusion of lithium ions is slow. The addition of organic solvents should result in a better battery performance. © 2016 American Chemical Society

In ionic liquids, the cation−anion interaction plays an important role and changes with variation of the ions. Extensive theoretical studies were done in order to understand the cation−anion interaction in ionic liquids which has been found to be between 300 and 400 kJ mol−1.14−17 Experimentally, these interactions have been studied using optical heterodynedetected Raman induced Kerr effect spectroscopy,18−20 THz spectroscopy,21−23 and low-energy neutron scattering.24 Fumino et al.25−27 showed using far-infrared spectroscopy (FIR) combined with density functional theory calculations (DFT) that it is possible to evaluate the cation−anion interactions in ionic liquids. Subsequently, they performed detailed studies by changing the anion species and the alkyl Received: October 7, 2016 Accepted: November 17, 2016 Published: November 17, 2016 34143

DOI: 10.1021/acsami.6b12751 ACS Appl. Mater. Interfaces 2016, 8, 34143−34150

Research Article

ACS Applied Materials & Interfaces

mixture of 50:50 vol % of concentrated H2SO4 and H2O2 (35%) followed by refluxing in distilled water. The electrochemical measurements were performed in an argonfilled glovebox with water and oxygen content of below 2 ppm (OMNI-LAB from Vacuum Atmospheres) by using a VersaStat II (Princeton Applied Research) potentiostat/galvanostat controlled by powerCV. For preparation of the anode, Ge was electrodeposited at −2.2 V vs Pt for 30 min from 0.25 M GeCl4 in [Py1,4]TFSI. After the electrodeposition was done, the remaining ionic liquid in the cell was removed and the electrodeposited germanium was washed in the pure ionic liquid inside the glovebox. For CV measurement, the Pt was replaced with Li and the lithiation/delithiation processes were performed at 1 mV s−1. For battery testing, half-cell setups were made with the electrodeposited material as the working electrode and lithium metal as the counter electrode. The two electrodes were separated with a Celgrad 2400 membrane, and the electrolyte used was either 1 M LiFSI/ [Py1,4]FSI, 1 M LiFSI/[Py1,4]FSI + 20 vol % AN or 1 M LiFSI/ [Py1,4]FSI + 20 vol % DMC. The galvanostatic charge−discharge cycles were performed using VersaStat 3 (Princeton Applied Research) potentiostat/galvanostat. For measurement of conductivity, samples were placed inside a sealed compartment with two platinum electrodes inside an Ar-filled glovebox. After thermal equilibration, the conductivity was measured using impedance spectroscopy. X-ray photoelectron spectra (XPS) were obtained using an ultrahigh vacuum (UHV) apparatus with a base pressure below 1 × 10−10 hPa. The sample was irradiated using the Al Kα line (photon energy of 1486.6 eV) of a nonmonochromatic X-ray source (Omicron DAR 400). Electrons emitted were detected by a hemispherical analyzer (Omicron EA125) under an angle of 45° to the surface normal with a calculated resolution of 0.83 eV for detail spectra and 2.07 eV for survey spectra. All XPS spectra were displayed as a function of the binding energy with respect to the Fermi level. Raman spectra were recorded with a Raman module (Ram 2) Bruker Vertex 70 V (Nd:YAG 1064 nm) with a Ge detector. For Raman analysis, the electrolyte was sealed in a glass capillary inside the glovebox and the spectra were obtained at an average of 250 scans with a resolution of 2 cm−1. Fourier transform infrared spectroscopy (VERTEX 70 V, Bruker Optics GmbH) was used to characterize the electrolyte. AFM experiments were perfomed with a Cypher S (Asylum Research) AFM inside a glovebox under argon atmosphere. Silicon SPM sensors from Nano World or Asylum Research were employed for all experiments presented in this study. The spring constant was 22.4 N/m which was determined from the built-in software.

chain of the imidazolium cation to understand the intermolecular forces between the ions.28 Comparatively fewer studies have been reported on the interaction between the cations and anions in ionic liquids in the presence of other solvents.29−31 Furthermore, the influence of organic solvents on the interaction of ion pairs in aprotic ionic liquids has not been evaluated by FIR. Some theoretical studies have shown that both polar and nonpolar molecular solvents behave differently in ionic liquids.32−34 Recent experimental studies showed that the addition of polar solvents such as acetonitrile increases the conductivity of the ionic liquid.35 Addition of organic solvents has also been shown to improve the battery performance compared to pure ionic liquids, as they decrease the viscosity and increase the ionic conductivity.36−40 Furthermore, the electrolyte remains noninflammable. Kühnel et al.41 showed that for a mixture of propylene carbonate and 1butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]TFSI), the battery performance at low charge− discharge rates was similar to that of pure propylene carbonate (PC). However, with increase in the charge−discharge rate, the performance of the battery with the mixed electrolyte decreased compared to the organic electrolyte. In comparison to PC, the addition of a mixture of ethylene carbonate (EC):dimethyl carbonate (DEC) in [Py1,4]TFSI did not show any improvement in the battery performance. The same authors further showed that on addition of [Py1,4]TFSI in a ternary molecular electrolyte of EC:DMC:diethyl carbonate (DEC), the battery performance decreased with charge−discharge cycles.42 The authors claimed that the change in the solid electrolyte interphase (SEI) might have caused a decrease in battery performance. However, Theivaprakasam et al.43 showed that the electrolyte mixture of [Py1,4]TFSI with 1 M LiPF6:EC:DEC had a better thermal stability as well as battery performance compared to the EC:DEC based organic electrolyte. In this manuscript, we compare the influence of addition of polar (acetonitrile) and less polar (dimethyl carbonate) organic solvents into 1-butyl-1-met hylpyr rolidinium bis(fluorosulfonyl)amide ([Py1,4]FSI) containing 1 M lithium bis(fluorosulfonyl)amide (LiFSI). Using Raman and infrared spectroscopy, the changes in the ionic liquid and lithium solvation were studied. The electrolyte was also used for lithiation/delithiation studies with electrodeposited germanium in the presence and absence of organic electrolytes in the ionic liquid. We also studied how the organic solvents in the ionic liquid influenced the solid electrolyte interphase (SEI) on the germanium electrode and evaluated the SEI layer using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS).

■

■

RESULTS AND DISCUSSION The cyclic voltammetry (CV) results of 1 M LiFSI/[Py1,4]FSI, 1 M LiFSI/[Py1,4]FSI with 20 vol % acetonitrile (AN) and 20 vol % dimethyl carbonate (DMC) on electrodeposited germanium are compared in Figure 1. The electrodeposition of germanium was performed on a copper substrate from 0.25 M GeCl4/[Py1,4]TFSI and has been shown elsewhere.44 From Figure 1a, in the first CV cycle, a peak at 1.5 V is seen in the cathodic regime and an increase in negative current is observed at 0.5 V. These two peaks correspond to the formation of the SEI layer and to the intercalation of Li+ ion in Ge, respectively.44 In the anodic regime, a peak at ∼0.7 V is seen which can be related to the deintercalation of Li+ ions. The fifth CV cycle only shows an intercalation process commencing at ∼0.6 V and a deintercalation peak at ∼1.0 V. Compared to Figure 1a, on addition of polar solvent (AN) in the ionic liquid (Figure 1b), broad peaks at ∼1.7 and ∼1.55 V are seen in the first CV cycle which can be related to the formation of an SEI layer. The intercalation peak occurs at 0.5 V, and the deintercalation process occurs at 0.65 V. The fifth cycle only

EXPERIMENTAL SECTION

1-Butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide ([Py1,4]FSI) was purchased in the highest available quality from Solvionic and were used after drying under vacuum at 100 °C to remove the water content to below 2 ppm. GeCl4 (99.999%) and LiFSI were purchased from Alfa Aesar and Fluorochem, respectively. Acetonitrile (99.8%) and dimethyl carbonate (>99%) were purchased from Sigma-Aldrich. The working electrode in the experiment was a copper plate. Prior to the experiments, the copper plate was cleaned in isopropanol and acetone to remove surface contaminations. Platinum wires were used as reference and counter electrodes. The electrochemical cell was a Teflon cell which was clamped over a Teflon-covered Viton O-ring onto the substrate, yielding a geometric surface area of 0.3 cm2. Prior to the experiments, the Teflon cell and the O-ring were cleaned in a 34144


Research Article


Figure 1. First and fifth CV cycles of (a) 1 M LiFSI/ [Py1,4]FSI on electrodeposited Ge, (b) 1 M LiFSI/ [Py1,4]FSI + 20 vol % AN on electrodeposited Ge, and (c) 1 M LiFSI/ [Py1,4]FSI + 20 vol % DMC on electrodeposited Ge.

Figure 2. (a) Comparison of IR spectra of pure IL with LiFSI in the presence of AN and DMC. (b) Raman spectra between 680 and 800 cm−1 of IL with LiFSI in the presence of AN and DMC (c) FIR spectra of IL with LiFSI in the presence of AN and DMC.

shows intercalation and deintercalation processes at ∼0.7 V and ∼0.75 V, respectively. The magnitude of the current is much higher in the presence of AN compared to the IL electrolyte which can be related to a change in conductivity as shown later in the manuscript. Figure 1c shows the influence of the addition of a less polar solvent (DMC) to the ionic liquid. A peak at 1.5 V is evident in Figure 1c for the first CV cycle in the cathodic regime and can be related to the formation of an SEI layer, whereas a small rise in negative current related to the intercalation of Li occurs at ∼0.45 V. A small deintercalation process is also seen at ∼0.65 V. However, in the fifth cycle, the intercalation and deintercalation processes take place at 0.45 and 0.65 V, respectively. Furthermore, it is evident that the magnitude of current is much lower compared to only IL and IL + AN electrolytes. Thus, it is clear from the CV cycles in Figure 1 that the intercalation and deintercalation processes as well as the formation of an SEI layer are influenced in the presence of polar solvents in the ionic liquid. To further study the influence of molecular solvents, infrared and Raman spectroscopy were performed on the electrolytes. Figure 2a compares the IR spectra between 400 and 1500 cm−1. On addition of LiFSI to the IL, slight changes are seen in the νsSNS, νasSNS, and νSF at 729 and 827 cm−1. Changes even occur at 565 cm−1 for the δaSO2, and zoomed in spectra are shown in Figure S1 in Supporting Information. These changes in the ionic liquid in the presence of LiFSI relate to the coordination of Li with the IL forming [Li(FSI)3 ]2− and are consistent with the data shown previously.45 On addition of dimethyl carbonate (blue line, Figure 2a), changes in the νsSNS, νasSNS, and νSF are also seen. Furthermore, additional peaks are observed which correspond to DMC. This indicates that addition of DMC changes the lithium solvation in the ionic liquid and there is some immiscibility in the ionic liquid. In comparison, the addition of acetonitrile does not show additional peaks. Changes in the lithium solvation can be seen based on the changes in the νsSNS, νasSNS and νSF in Figure 2a and the change in δaSO2 in Figure S1. In order to better understand the lithium solvation, Raman spectroscopy was performed. The region between 680 and 800

cm−1 (Figure 2b) is the most significant region wherein changes in the lithium solvation can be clearly noticed. The complete Raman spectra are shown in Figure S2. In Figure 2b it is evident that on addition of LiFSI to the IL, a shift of 5 cm−1 from 725 to 730 cm−1 occurs and the broadness of the peak increases (Figure 2b, red line). On addition of DMC (Figure 2b, blue line), a decrease in the 730 cm−1 peak occurs and a new peak at 741 cm−1 is observed which clearly suggests that Li solvation in the ionic liquid changes. In comparison, on addition of AN, a peak shift of 3 cm−1 to higher wavenumbers occurs and the peak broadness decreases, which suggests a slight change in the lithium ion solvation. Thus, the Raman results indicate that less polar solvents affect the lithium solvation significantly compared to highly polar solvents. FIR has been shown as a useful region to study the cation− anion interaction in ILs.25−28 The FIR spectra of the IL + LiFSI in the presence of the molecular solvents are shown in Figure 2c. It is evident that on addition of LiFSI, there is some decrease in the intensity of the cation−anion interaction of the IL which can be related to the solvation of Li+ ions in the IL. On addition of DMC, no change in the cation−anion interaction is observed. However, on addition of AN, the peak intensity related to the cation−anion interaction almost disappears, which indicates that the acetonitrile interacts with the ionic liquid and affects the cation−anion interactions significantly. The change in the cation−anion interaction combined with the different lithium solvation in the ionic liquid on addition of molecular solvents might have led to a different CV and measured current in Figure 1. The conductivity of the electrolyte was therefore studied and is shown in Table 1. [Py1,4]FSI showed a conductivity of 7.8 mS Table 1. Ionic Conductivity of Ionic Liquids Containing 1 M LiFSI with Various Molecular Solvents at 26 °C

34145

electrolyte

conductivity (mS cm−1)

[Py1,4]FSI 1 M LiFSI-[Py1,4]FSI 1 M LiFSI-[Py1,4]FSI + 20 vol % AN 1 M LiFSI-[Py1,4]FSI + 20 vol % DMC

7.8 6.6 14 8.5


Research Article

ACS Applied Materials & Interfaces cm−1 and is comparable with that reported previously.46 On addition of 1 M LiFSI, the conductivity decreases to 6.6 mS cm−1. With the addition of AN, the conductivity increases which can be related to the change in the cation−anion interaction as seen from the FIR study. In comparison, a small increase in conductivity is observed on addition of DMC, which might be related to a change in lithium solvation. The electrolytes were also used to test lithium battery half cells with electrodeposited germanium as anode and lithium metal as counter electrode. Figure 3a compares the charge− discharge curves in three different electrolytes at 0.425 C.

Figure 4. (a) Deflection image of the SEI layer formed on Ge electrode after 50 charge−discharge cycles in 1 M LiFSI-[Py1,4]FSI. (b) Force map of the area shown in (a). (c) Force−separation profile of the brown spot shown in (b). (d) Force−separation profile of the black spot shown in (b).

deposited Ge shows a spherical morphology (Figure S3a) and the force−separation profile shows a direct contact (Figure S3b). Compared to electrodeposited Ge, from the deflection image in Figure 4a, a layered structure is seen. Figure 4b shows the force maps obtained over the area of 1 × 1 μm2. The force− separation profile of the black and brown region is shown in Figure 4c and Figure 4d, respectively. The force separation profile in Figure 4c of the brown spot shows a “push-through” force of about 60 nN at a separation distance of 20 nm. This thickness and push-through force correspond to the SEI layer. In the darker region (Figure 4d), the SEI layer was about 75 nm thick with a higher push through force of about 260 nN. Therefore, from the force map in Figure 4b it can be said that the SEI formed is inhomogeneous and the thickness varies at different places. The darker regions show a thicker SEI layer, whereas the brighter region shows a thin SEI layer. Some bright spots showed direct contact with no separation distance, which might be due to impenetrable SEI layer. On addition of acetonitrile into the ionic liquid, the AFM image shows a rough and porous structure (Figure 5a). The force map in Figure 5b again shows bright and dark regions that correspond to the change in thickness and probably composition of the SEI layer. The force−separation profiles of two spots are shown in Figure 5c and Figure 5d. The SEI layer thickness is found to be between 50 and 100 nm and is much thicker compared to using only the ionic liquid electrolyte. In the force−separation profiles in Figures 5c, two “push through” forces are observed at 14.3 and 4.2 nm. In Figure 5d, the “push through” forces occur at 58.5 and 11 nm. These “push through” forces might correspond to formation of a layered SEI structure. Such a layered SEI structure has recently been observed on a silicon electrode with organic electrolytes.47 On addition of DMC to the ionic liquid, the AFM image in Figure 6a shows the formation of large clusters of about 500 nm in size. The force map of the SEI layer is shown in Figure 6b which clearly shows that the SEI layer is inhomogeneous. The force−separation curves of the two spots are shown in Figure 6c and Figure 6d from which it is evident that the thickness of

Figure 3. (a) Charge−discharge cycle at 0.425 C in three different electrolytes. Charge−discharge cycles at different C rates (b) in 1 M LiFSI-[Py1,4]FSI, (c) in 1 M LiFSI-[Py1,4]FSI + 20 vol % AN, (d) in 1 M LiFSI-[Py1,4]FSI + 20 vol % DMC. The first cycle was performed at 0.05 C to form a stable SEI.

It is evident from Figure 3a that the maximum capacity obtained during lithiation and delithiation was on using 1 M LiFSI-[Py1,4]FSI + 20 vol % AN as electrolyte. The capacity storage during the lithiation process on using 1 M LiFSI[Py1,4]FSI and 1 M LiFSI-[Py1,4]FSI + 20 vol % DMC is similar, whereas the specific capacity during delithiation for DMC was found to be lowest. The charge−discharge processes over 50 cycles at various C-rates using three different electrolytes are shown in Figure 3b−d. It is evident comparing the charge−discharge curves that at the 0.425 C rate, the ionic liquid electrolyte containing acetonitrile has the highest specific capacity of about 600 mAh g−1. The Coulombic efficiency was also found to be the highest. On increasing the charge− discharge rate to 0.85 C, both IL electrolyte and IL + AN electrolyte containing LiFSI show similar capacity storage. However, the capacity storage for IL + AN decreased by about 33%. In comparison, IL with DMC showed very low capacity storage. With higher C-rate of 1.7 C, it is clear that the IL with LiFSI shows better specific capacity and IL + DMC shows the lowest specific capacity. Thus, it is evident that at low charge− discharge rates, the mixture of more polar solvent with IL is a better electrolyte, whereas at higher charge−discharge rates, IL based electrolytes outperform the mixed electrolytes. The marked difference in Figure 3 suggests that the solid electrolyte interphase (SEI) formed is different, which affects the battery performance. Therefore, the SEI layer was evaluated using AFM and XPS. Figure 4 shows the deflection image, force map, and two force profiles of the SEI layer formed on electrodeposited Ge on using 1 M LiFSI-[Py1,4]FSI. Electro34146


Research Article


Figure 7. XPS survey spectra of Ge anode surfaces after cycling in the three different electrolytes.

Figure 5. (a) Deflection image of the SEI layer formed on Ge electrode after 50 charge−discharge cycles in 1 M LiFSI-[Py1,4]FSI + 20 vol % AN. (b) Force map of the area shown in (a). (c, d) Force− separation profile of the brown spots shown in (b).

and oxygen are observed. Besides, contributions from carbon, chlorine, fluorine, nitrogen, and sulfur are also seen, which can be related to the formation of the SEI layer and some residual ionic liquid. The chlorine peak comes from some remaining 0.25 M GeCl4−ionic liquid electrolyte that was used for electrodeposition of germanium prior to cycling. On evaluation of the normalized peak areas of the elements of the SEI layer (excluding Ge, Cu, Cl) in the survey spectra, further information can be drawn. The stoichiometry of the different elements is shown in a bar chart in Figure 8. When

Figure 6. (a) Deflection image of the SEI layer formed on Ge electrode after 50 charge−discharge cycles in 1 M LiFSI-[Py1,4]FSI + 20 vol % DMC. (b) Force map of the area shown in (a). (c, d) Force− separation profile of the brown spots shown in (b).

the SEI layer is about 50 nm. However, compared to the force− separation curves in Figures 4 and 5, no clear “push through” forces are observed which indicates that the SEI layer does not form a layered structure. Interestingly, some attraction forces are observed which might be due to the presence of a semisolid SEI layer48 or to some liquid phase remaining during the cleaning process. Thus, from the AFM studies, it is evident that with the addition of the more polar organic solvent, a thicker SEI layer is formed whereas on the addition of less polar solvent, a thin SEI layer is formed. To understand the composition of the SEI layer with and without the addition of organic solvents, XPS was performed on the samples. Before analysis, residual ionic liquid from the sample surfaces was removed by sputtering the sample with Ar+ ions, accelerated to a kinetic energy of 1 keV for 5 min. Figure 7 compares the XPS survey spectra of the cycled Ge anode in different electrolytes. Dominant peaks of copper, germanium,

Figure 8. Stoichiometry of all XPS peaks except those assigned to the substrate and Ge-anode surface.

using the pure IL or the IL + DMC mixture as electrolytes, the SEI mainly is composed of carbon, oxygen, and fluorine in approximately equal amounts. In addition S 2p of about 9% is found on both electrode surfaces. For ionic liquid and acetonitrile mixtures, the SEI is dominated by carbon and oxygen. In addition about 10% of nitrogen and about 6% of fluorine were found on the electrode surface. The presence of nitrogen and the high amount of carbon indicates that besides IL decomposed products, some decomposed products of acetonitrile are also involved in the SEI layer. 34147


Research Article


Figure 9. XPS Ge 2p3/2, F 1s, and O 1s detail spectra of Ge anode surfaces after cycling.

contributions of Li2S to the SEI structure could be observed from the survey spectra and in a previous work.44

Figure 9 shows the XPS detail spectra of Ge 2p, F 1s, and O 1s for all three samples. Since a broad structure between 284 and 287 eV BE can be assigned to contributions of C−C, C−N, and C−O bindings for all three samples, the C 1s detail spectrum is taken as binding energy reference and is shown in Supporting Information Figure S4. In Figure S4, peaks around 289 and 291 eV are found for all the samples, which can be assigned to lithium carbonate. The most prominent species in the Ge 3d spectrum in Figure 9 can be attributed to the Ge anode material. The Ge 3d spectrum also shows a clear shoulder between 1216 and 1217 eV on using IL + AN and IL + DMC electrolytes, which can be related to the formation of LixGe1−x. In a previous work we observed similar structures after intercalation of lithium to the Ge anode, which disappeared during the deintercalation process.44 This indicates that a residual amount of lithium still might be stored in the germanium anode after 50 charge− discharge cycles for the mixture of organic solvent and ionic liquid electrolytes. This might be the reason for a decrease in specific capacity for both IL + AN (at higher C-rates) and IL + DMC mixtures in Figure 3c and Figure 3d, respectively. Therefore, further optimization of the SEI layer and composition is needed in order to decrease the capacity loss. In the F 1s spectra, the peak between 684 and 686.5 eV can be assigned to Li−F49 and is the major component for each of the samples. A small peak at 688.5 eV is also observed and can be associated with the FSI anion. The FSI component was found to be smallest for the pure ionic liquid electrolyte, which may be due to the formation of a denser layered structure of the SEI as seen from the AFM image in Figure 4a. In addition, the O 1s spectra in Figure 9 shows rather broad peaks between 531 and 533 eV, which can be assigned to either lithium carbonate or FSI anion structures. However, for the Ge anodes with IL and the mixture of IL and DMC as electrolytes, a shoulder around 530 eV can be observed, which is attributed to LiOH or Li2O2.42,50 Due to the weak signal/noise ratio of N 1s and S 2p, detail spectra were not shown here, although

■

CONCLUSIONS In this paper we have compared the influence of organic solvents in the ionic liquid containing LiFSI for battery applications. From IR and Raman spectroscopy, it was shown that the more polar organic solvent influences the cation−anion interaction in the ionic liquid as well as the conductivity of the electrolyte. From the charge−discharge analysis with the Ge anode, it was found that IL + AN gave the highest specific capacity at low C-rate, whereas on increasing the C-rate, the ionic liquid electrolyte was found to have a lower capacity loss. AFM analysis showed that the SEI layer for IL electrolyte had a layered morphology whereas a porous and clustered structure was formed for IL + AN and IL + DMC, respectively. From XPS, the composition of the SEI layer in presence and absence of organic solvents was found to be different. Furthermore, XPS results clearly showed that there is an accumulation of lithium in Ge anode during the charge−discharge processes on addition of organic solvents to the ionic liquid electrolyte. Thus, it appears that although IL + highly polar organic solvent is a good battery electrolyte, further optimization of the electrolyte composition is needed in order to obtain a stable SEI layer and limit the accumulation of lithium in Ge anode.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12751.

■

Raman, IR, AFM, and XPS spectra (PDF)

AUTHOR INFORMATION

Corresponding Authors

*A.L.: e-mail, [email protected]. *F.E.: e-mail, [email protected]. 34148


Research Article

ACS Applied Materials & Interfaces ORCID

Isoelectronic Neutral Binary Solution. J. Phys. Chem. A 2005, 109, 9388−9392. (19) Hyun, B.-R.; Dzyuba, S. V.; Bartsch, R. A.; Quitevis, E. L. Intermolecular Dynamics of Room-Temperature Ionic Liquids: Femtosecond Optical Kerr Effect Measurements on 1-Alkyl-3methylimidazolium Bis((trifluoromethyl)sulfonyl)amides. J. Phys. Chem. A 2002, 106, 7579−7585. (20) Xiao, D.; Rajian, J. R.; Li, S.; Bartsch, R. A.; Quitevis, E. L. Additivity in the Optical Kerr Effect Spectra of Binary Ionic Liquid Mixtures: Implications for Nanostructural Organization. J. Phys. Chem. B 2006, 110, 16174−16178. (21) Callender, R. H.; Dyer, R. B.; Gilmanshin, R.; Woodruff, W. H. Fast Events in Protein Folding: The Time Evolution of Primary Processes. Annu. Rev. Phys. Chem. 1998, 49, 173−202. (22) Leitner, D. M.; Havenith, M.; Gruebele, M. Biomolecule LargeAmplitude Motion and Solvation Dynamics: Modeling and Probes from THz to X-rays. Int. Rev. Phys. Chem. 2006, 25, 553−582. (23) Plusquellic, D. F.; Siegrist, K.; Heilweil, E. J.; Esenturk, O. Applications of Terahertz Spectroscopy in Biosystems. ChemPhysChem 2007, 8, 2412−2431. (24) Hudson, B. S. Inelastic Neutron Scattering: A Tool in Molecular Vibrational Spectroscopy and a Test of ab Initio Methods. J. Phys. Chem. A 2001, 105, 3949−3960. (25) Fumino, K.; Wulf, A.; Ludwig, R. The Cation-Anion Interaction in Ionic Liquids Probed by Far-Infrared Spectroscopy. Angew. Chem., Int. Ed. 2008, 47, 3830−3834. (26) Fumino, K.; Fossog, V.; Wittler, K.; Hempelmann, R.; Ludwig, R. Dissecting Anion-Cation Interaction Energies in Protic Ionic Liquids. Angew. Chem., Int. Ed. 2013, 52, 2368−2372. (27) Fumino, K.; Ludwig, R. Analyzing the Interaction Energies between Cation and Anion in Ionic Liquids: The Subtle Balance between Coulomb Forces and Hydrogen Bonding. J. Mol. Liq. 2014, 192, 94−102. (28) Fumino, K.; Reimann, S.; Ludwig, R. Probing Molecular Interaction in Ionic Liquids by Low Frequency Spectroscopy: Coulomb Energy, Hydrogen Bonding and Dispersion Forces. Phys. Chem. Chem. Phys. 2014, 16, 21903−21929. (29) Dominguez-Vidal, A.; Kaun, N.; Ayora-Cañada, M. J.; Lendl, B. Probing Intermolecular Interactions in Water/Ionic Liquid Mixtures by Far-Infrared Spectroscopy. J. Phys. Chem. B 2007, 111, 4446−4452. (30) Stange, P.; Fumino, K.; Ludwig, R. Ion Speciation of Protic Ionic Liquids in Water: Transition from Contact to Solvent-Separated Ion Pairs. Angew. Chem., Int. Ed. 2013, 52, 2990−2994. (31) Fumino, K.; Stange, P.; Fossog, V.; Hempelmann, R.; Ludwig, R. Equilibrium of Contact and Solvent-Separated Ion Pairs in Mixtures of Protic Ionic Liquids and Molecular Solvents Controlled by Polarity. Angew. Chem., Int. Ed. 2013, 52, 12439−12442. (32) Chaban, V. V.; Voroshylova, I. V.; Kalugin, O. N.; Prezhdo, O. V. Acetonitrile Boosts Conductivity of Imidazolium Ionic Liquids. J. Phys. Chem. B 2012, 116, 7719−7727. (33) Pádua, A. A. H.; Costa Gomes, M. F.; Canongia Lopes, J. N. A. Molecular Solutes in Ionic Liquids: A Structural Perspective. Acc. Chem. Res. 2007, 40, 1087−1096. (34) Canongia Lopes, J. N. A.; Costa Gomes, M. F.; Padua, A. A. H. Nonpolar, Polar, and Associating Solutes in Ionic Liquids. J. Phys. Chem. B 2006, 110, 16816−16818. (35) Ueno, K.; Murai, J.; Ikeda, K.; Tsuzuki, S.; Tsuchiya, M.; Tatara, R.; Mandai, T.; Umebayashi, Y.; Dokko, K.; Watanabe, M. Li+ ion Solvation and Ionic Transport in Lithium Solvate Ionic Liquids Diluted by Molecular Solvents. J. Phys. Chem. C 2016, 120, 15792− 15802. (36) Kalhoff, J.; Eshetu, G. G.; Bresser, D.; Passerini, S. Safer Electrolytes for Lithium-ion Baterries: State of Art and Perspectives. ChemSusChem 2015, 8, 2154−2175. (37) Xu, K. Electrolytes and Interphases in Li-ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503−11618. (38) Menne, S.; Kühnel, R. S.; Balducci, A. The Influence of the Electrochemical and Thermal Stability of Mixtures of Ionic Liquid and

Abhishek Lahiri: 0000-0001-8264-9169 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Karin Bode, Institute of Inorganic Chemistry (Prof. A. Adam), for help with Raman and IR measurements.

■

REFERENCES

(1) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-ion Batteries: A Review. Energy Environ. Sci. 2011, 4, 3243−3262. (2) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-liquid Materials for the Electrochemical Challenges of the Future. Nat. Mater. 2009, 8, 621−629. (3) MacFarlane, D. R.; Tachikawa, N.; Forsyth, M.; Pringle, J. M.; Howlett, P. C.; Elliott, G. D.; Davis, J. H.; Watanabe, M.; Simon, P.; Angell, C. A. Energy Applications of Ionic Liquids. Energy Environ. Sci. 2014, 7, 232−250. (4) Berthod, A.; Ruiz-Á ngel, M. J.; Carda-Broch, S. Ionic liquids in Separation Techniques. J. Chromatogr. A 2008, 1184, 6−18. (5) Zhao, H.; Xia, S.; Ma, P. Use of Ionic Liquids as ‘Green’ Solvents for Extractions. J. Chem. Technol. Biotechnol. 2005, 80, 1089−1096. (6) Olivier-Bourbigou, H.; Magna, L.; Morvan, D. Ionic liquids and Catalysis: Recent Progress from Knowledge to Applications. Appl. Catal., A 2010, 373, 1−56. (7) Zhang, Q.; Zhang, S.; Deng, Y. Recent Advances in Ionic Liquid Catalysis. Green Chem. 2011, 13, 2619−2637. (8) Shiddiky, M. J. A.; Torriero, A. A. J. Application of Ionic Liquids in Electrochemical Sensing Systems. Biosens. Bioelectron. 2011, 26, 1775−1787. (9) Marrucho, I. M.; Branco, L. C.; Rebelo, L. P. N. Ionic Liquids in Pharmaceutical Applications. Annu. Rev. Chem. Biomol. Eng. 2014, 5, 527−546. (10) Eshetu, G. G.; Armand, M.; Scrosati, B.; Passerini, S. Energy Storage Materials Synthesized from Ionic Liquids. Angew. Chem., Int. Ed. 2014, 53, 13342−13359. (11) Mun, J.; Jung, Y. S.; Yim, T.; Lee, H. Y.; Kim, H.-J.; Kim, Y. G.; Oh, S. M. Electrochemical Stability of bis(trifluoromethanesulfonyl)amide-based Ionic Liquids at Elevated Temperature as a Solvent for a Titanium Oxide Bronze Electrode. J. Power Sources 2009, 194, 1068− 1074. ́ (12) Lewandowski, A.; Swiderska-Mocek, A. Ionic Liquids as Electrolytes for Li-ion Batteries- An Overview of Electrochemical Studies. J. Power Sources 2009, 194, 601−609. (13) Philippe, B.; Dedryvere, R.; Gorgoi, M.; Rensmo, H.; Gonbeau, D.; Edström, K. Improved Performances of Nanosilicon Electrodes Using the Salt LiFSI: A Photoelectron Spectroscopy Study. J. Am. Chem. Soc. 2013, 135, 9829−9842. (14) Paulechka, Y. U.; Kabo, G. J.; Blokhin, A. V.; Vydrov, O. A.; Magee, J. W.; Frenkel, M. Thermodynamic Properties of 1-Butyl-3methylimidazolium Hexafluorophosphate in the Ideal Gas State. J. Chem. Eng. Data 2003, 48, 457−462. (15) Tsuzuki, S.; Tokuda, H.; Hayamizu, K.; Watanabe, M. Magnitude and Directionality of Interaction in Ion Pairs of Ionic Liquids: Relationship with Ionic Conductivity. J. Phys. Chem. B 2005, 109, 16474−16481. (16) Liu, Z.; Huang, S.; Wang, W. A Refined Force Field for Molecular Simulation of Imidazolium-Based Ionic Liquids. J. Phys. Chem. B 2004, 108, 12978−129898. (17) Turner, E. A.; Pye, C. C.; Singer, R. D. Use of ab Initio Calculations toward the Rational Design of Room Temperature Ionic Liquids. J. Phys. Chem. A 2003, 107, 2277−2288. (18) Shirota, H.; Castner, E. W., Jr. Physical Properties and Intermolecular Dynamics of an Ionic Liquid Compared with its 34149


Research Article

ACS Applied Materials & Interfaces Organic Carbonate on the Performance of High Power Lithium-ion Batteries. Electrochim. Acta 2013, 90, 641−648. (39) Evans, T.; Olson, J.; Bhat, V.; Lee, S.-H. Effect of Organic Solvent Addition to PyR13FSI+ LiFSI Electrolytes on Aluminum Oxidation and Rate Performance of Li(Ni1/3Mn1/3Co1/3)O2 Cathodes. J. Power Sources 2014, 265, 132−139. (40) Vogl, T.; Menne, S.; Balducci, A. Mixtures of Protic Ionic Liquids and Propylene Carbonate as Advanced Electrolytes for Lithium-ion Batteries. Phys. Chem. Chem. Phys. 2014, 16, 25014− 25023. (41) Kühnel, R.-S.; Böckenfeld, N.; Passerini, S.; Winter, M.; Balducci, A. Mixtures of Ionic Liquid and Organic Carbonate as Electrolyte with Improved Safety and Performance for Rechargeable Lithium Batteries. Electrochim. Acta 2011, 56, 4092−4099. (42) Lombardo, L.; Brutti, S.; Navarra, M. A.; Panero, S.; Reale, P. Mixtures of ionic liquid- alkylcarbonates as electrolytes for safe lithium-ion batteries. J. Power Sources 2013, 227, 8−14. (43) Theivaprakasam, S.; MacFarlane, D. R.; Mitra, S. Electrochemical Studies of N-Methyl-N-Propyl Pyrrolidinium bis(trifluoromethanesulfonyl)imide Ionic Liquid Mixtures with Conventional Electrolytes in LiFePO4/Li Cells. Electrochim. Acta 2015, 180, 737−745. (44) Lahiri, A.; Borisenko, N.; Borodin, A.; Olschewski, M.; Endres, F. Characterization of the Solid Electrolyte Interface during Lithiation/ Delithiation of Germanium in an Ionic Liquid. Phys. Chem. Chem. Phys. 2016, 18, 5630−5637. (45) Huang, J.; Hollenkamp, A. F. Thermal Behavior of Ionic Liquids Containing the FSI Anion and the Li+ Cation. J. Phys. Chem. C 2010, 114, 21840−21847. (46) Yoon, H.; Best, A. S.; Forsyth, M.; MacFarlane, D. R.; Howlett, P. C. Physical Properties of High Li-ion Content N-propyl-Nmethylpyrrolidinium bis(trifluorosulfonyl)amide Based Ionic Liquid Electrolytes. Phys. Chem. Chem. Phys. 2015, 17, 4656−4663. (47) Kuznetsov, V.; Zinn, A.-H.; Zampardi, G.; Borhani-Haghighi, S.; La Mantia, F.; Ludwig, A.; Schuhmann, W.; Ventosa, E. Wet Nanoindentation of the Solid Electrolyte Interphase on Thin Film Si Substrate. ACS Appl. Mater. Interfaces 2015, 7, 23554−23563. (48) Lahiri, A.; Olschewski, M.; Gustus, R.; Borisenko, N.; Endres, F. Surface Modification of Battery Electrodes via Electroless Deposition with Improved Performance for Na-ion Batteries. Phys. Chem. Chem. Phys. 2016, 18, 14782−14786. (49) Kowalczyk, S.; McFeely, F.; Ley, L.; Pollak, R.; Shirley, D. X-ray Photoemission Studies of the Alkali Halides. Phys. Rev. B 1974, 9, 3573−3581. (50) Wu, Q.-H.; Thissen, A.; Jaegermann, W. Photoelectron Spectroscopic Study of Li Oxides on Li Over-deposited V2O5 Thin Film Surfaces. Appl. Surf. Sci. 2005, 250, 57−62.

34150


Influence of Polar Organic Solvents in an Ionic Liquid Containing

Recommend Documents