Ionic Liquid-Organic Solvent Mixture Based Polymer Gel Electrolyte

Article Cite This: J. Phys. Chem. C 2018, 122, 24788−24800

pubs.acs.org/JPCC

Ionic Liquid−Organic Solvent Mixture-Based Polymer Gel Electrolyte with High Lithium Concentration for Li-Ion Batteries Abhishek Lahiri,* Giridhar Pulletikurthi, Maryam Shapouri Ghazvini, Oliver Höfft, Guozhu Li, and Frank Endres* Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Strasse 6, 38678 Clausthal-Zellerfeld, Germany

J. Phys. Chem. C 2018.122:24788-24800. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 11/01/18. For personal use only.

S Supporting Information *

ABSTRACT: In this paper, we have investigated the performance and solid electrolyte interphase formation (SEI) of a hybrid polymer gel electrolyte based on poly(vinylidene fluoride)-co-hexafluoropropylene, 1-butyl-1methylpyrrolidinium bis(fluorosulfonyl)imide ([Py1,4]FSI), and lithium bis(fluorosulfonyl)imide (LiFSI) for all solidstate lithium-ion batteries. The effect of the addition of acetonitrile (AN) and different concentrations of LiFSI to such a polymer gel electrolyte was also examined. Compared to 1 M LiFSI in the ionic liquid−polymer electrolyte, the addition of 4 M LiFSI along with acetonitrile led to higher lithium storage capacity. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy were performed to analyze the SEI formation on a Ge electrode. The difference in the mechanical properties and SEI composition was observed upon changing the components of the polymer electrolyte. XPS analysis revealed a difference in the composition/thickness of the SEI layer, which varies upon increasing the Li-salt concentration and upon adding AN to [Py1,4]FSI. Such differences might have led to better Li storage capacity on using the concentrated IL−organic polymer electrolyte. Our study clearly shows that the addition of an organic solvent and high concentration of Li salt to the polymer gel electrolyte improves the performance of LIBs.

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INTRODUCTION Lithium-ion batteries (LIBs) have extensively been studied to meet the growing demand in energy storage devices. Therefore, recent investigations are being focused on developing better electrolyte components and electrode materials to develop safe and economical energy storage devices. The electrolyte is one of the critical components which can influence the performance of the lithium-ion batteries (LIBs). Secondary reactions often have shown to hamper the battery performance which results in a low Coulombic efficiency (CE) and consumption of the electrolyte. The formation of the solid electrolyte interphase (SEI) is an important aspect in LIBs and the behavior at the electrolyte/electrode interface becomes important at high current densities, which usually results in the formation of Li dendrites. Thus, studies have been performed to evaluate how the employed electrolyte influences the performance of LIBs. Several electrolytes have been tested for this purpose to improve the CE and stability of LIBs. One of the significant outcomes of such studies is the use of highly concentrated electrolytes (>2 M Li salt) which have gained a lot of attention recently. Concentrated electrolytes have been shown to suppress the formation of dendrites during lithium deposition/stripping, improve the stability of the SEI layer, and improve the thermal stability.1−3 Furthermore, concentrated electrolytes have an unusual solvation structure compared to their liquid counterparts. Usually concentrated electrolytes © 2018 American Chemical Society

were prepared using organic solvents and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).1 However, little has been investigated on using concentrated Li salts in ionic liquids, which are potential candidates for replacing the organic electrolytes. Yoon et al.4 studied the transport properties of LiFSI in N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([Py1,4]FSI). It was found that the conductivity of the electrolyte continuously decreased and the viscosity increased with an increase in the lithium salt concentration. However, between 2.4 and 3.2 M LiFSI, Walden plots showed a deviation compared to that at lower concentrations. Moreover, the decrease in the observed conductivity was not in accordance with the increase in viscosity, which implied that the conduction mechanism was complicated in the liquid electrolyte at such high concentrations. Gel polymer electrolytes (GPEs) have been investigated for lithium-ion batteries (LIBs) over the last 2 decades.5−9 They possess a few advantages over their liquid counterparts as they are flexible, noninflammable, and can also act as a separator.10−12 Another striking advantage is that the leakage and gas evolution during decomposition of the solvent can be avoided using polymer-based electrolytes. Polymer electrolytes have been employed for LIBs in conjunction with various Received: August 9, 2018 Revised: October 3, 2018 Published: October 5, 2018 24788

DOI: 10.1021/acs.jpcc.8b07745 J. Phys. Chem. C 2018, 122, 24788−24800

Article

The Journal of Physical Chemistry C

drying under vacuum at 100 °C to achieve a water content of below 10 ppm. LiFSI was bought from Fluorochem and was dried in a vacuum at 100 °C prior to use. Acetonitrile (99.8%) was obtained from Sigma-Aldrich. Polymer gel membranes were prepared by a solution method. First, different concentrations of LiFSI were dissolved in [Py1,4]FSI or in [Py1,4]FSI/acetonitrile (AN). The solution was then stirred for 2 h. The resulting mixture was subsequently mixed with 7.5 wt % PVDF-HFP/acetone solution. The weight ratio of the LiFSI−ionic liquid mixture to PVDF-HFP mixture was set at 7:3. These two mixtures (PVDF-HFP and LiFSI−IL) were mixed and stirred for another 2 h to obtain a homogenous solution. Finally, the solution was cast over a glass Petri dish, and acetone was subsequently evaporated under vacuum to produce the gel polymer electrolyte film. All preparation steps were carried out in an argon filled glove-box, in which the samples were stored until further use. Raman spectra were recorded by a Bruker Senterra Raman microscope using 50× objective with a laser excitation of 532 nm. Fourier transform infrared spectroscopy (FTIR, VERTEX 70 V, Bruker Optics GmbH) with an attached attenuated total reflectance module was performed to characterize the polymer. The electrochemical window and the ionic conductivity of the polymer were also analyzed by sandwiching the polymer membrane between the two symmetric stainless-steel electrodes. The ionic conductivity was measured using impedance spectroscopy. The resistance was determined from the impedance plot and the ionic conductivity was calculated using eq 1

anode materials in the past, however, these studies were carried out at temperatures >60 °C.13 Poly(ethylene oxide) (PEO)based polymer electrolytes containing lithium salts showed a reasonable reversibility on a carbon electrode at 60 °C.13 Coowar et al. showed that a reversible capacity of 290 mAh g−1 could be obtained with the PEO polymer containing 1 M lithium bis(trifluoromethylsulfonyl)imide at 90 °C.14 The possibility of using a polymer-based electrolyte for graphite, SiO/graphite, and silicon was also reported at 60 °C.15,16 These electrodes exhibited a Coulombic efficiency of ∼80%. The major issue associated with polymer electrolytes is that they usually have low ionic conductivities at room temperature. Therefore, battery tests were conducted at elevated temperatures and at low charge−discharge rates (0.05C−0.2C) to improve the diffusion of Li ions. Ionic liquid-based polymer electrolytes have recently gained a prominent research interest due to the useful properties of these liquids.17,18 They are noninflammable, possess wide electrochemical windows, low vapor pressure, and high thermal stability, to mention a few physicochemical properties. Negligible vapor pressures and noninflammability of ILs are the crucial properties to improve the safety of LIBs. Therefore, ionic liquid-based gel polymer electrolytes (GPE) have received a lot of attention. In this regard, PEO-based polymer electrolytes with various ionic liquids containing lithium salts have been prepared and tested on various materials at elevated temperatures.21 However, these PEO-based polymers suffer from quite low ionic conductivities (e.g., (0.7−5) × 10−4 S cm−1) at room temperature.18 In comparison to PEO polymers, poly((vinylidene fluoride)-co-hexafluoropropylene) (PVDF-HFP) copolymers containing lithium salts and ionic liquid mixtures showed higher conductivities (e.g., in the range of 10−3 S cm−1).19−22 Kim et al. reported an ionic conductivity of 1.1 × 10−2 S cm−1 at 60 °C for an ionic liquid−PVDF composite electrolyte based on a morpholinium salt.21 Furthermore, understanding the processes at the electrode/ electrolyte interface is crucial to improve the cycle life of LIBs. However, the formation of a solid electrolyte interphase (SEI) with polymer-based electrolytes has been scarcely reported.23,24 Germanium is a high capacity anode material with a theoretical Li storage capacity of 1600 mAh g−1 due to the formation of Li4.4Ge.25−27 Various Ge nanostructures have been synthesized using vacuum techniques and employed as anodes in LIBs. Low-dimensional and porous structures have particularly shown good battery performance.28 However, little has been investigated regarding the SEI layer formation on Ge electrodes29,30 and no studies have been reported with Ge electrodes in combination with polymer electrolytes. Here, we investigated and compared the lithium-ion battery performance of a Ge electrode using the PVDF-HFP polymerbased electrolyte containing two different concentrations of LiFSI in [Py1,4]FSI (e.g., 1 and 4 M). Furthermore, the addition of acetonitrile (AN) to the gel polymer electrolyte is also examined. Besides lithium-ion battery performance, we also studied in detail about the SEI formation on Ge electrodes using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Furthermore, the mechanical properties of the SEI layer are reported.

σ=

1 L × Rb A

(1)

Rb is the bulk resistance obtained from the impedance plot, L and A are the thickness and cross-sectional area of the sample, respectively. For the preparation of the anode, Ge was electrodeposited on copper at −2.2 V vs Pt for 30 min from 0.25 M GeCl4 in [Py1,4]TFSI. After electrodeposition, the remaining electrolyte was removed and the electrodeposited Ge was washed in the pure ionic liquid inside the glove box. For battery tests, half-cells were made with the electrodeposited Ge as a working electrode and a lithium foil as a counter electrode, which were separated by the polymer electrolyte. The cyclic voltammetry (CV) and galvanostatic charge−discharge cycles were performed using a VersaStat 3 (Princeton Applied Research) potentiostat/galvanostat. X-ray diffraction (XRD) patterns were recorded using a PANalytical Empyrean Diffractometer (Cabinet no. 9430 060 03002) with Cu Kα radiation. Thermogravimetric analysis (TGA) was performed in a TGA 850, Mettler-Toledo apparatus under a nitrogen atmosphere at a heating rate of 10 °C min−1. X-ray photoelectron spectra (XPS) were obtained using an ultrahigh vacuum apparatus with a base pressure below 1 × 10−10 hPa. The sample was transferred from the glove box to XPS using a specialized transfer chamber. 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° normal to the surface with a resolution of 0.83 eV for detail spectra and 2.07 eV for survey spectra, respectively. All XPS spectra were displayed as a function of the binding energy with respect to the Fermi level.

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EXPERIMENTAL SECTION The ionic liquid [Py1,4]FSI was purchased in the highest available quality from Solvionic (France) and was used after 24789


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Figure 1. CVs of a polymer electrolytes at 25 °C containing (a) 1 M LiFSI + [Py1,4]FSI on electrodeposited Ge, (b) 1 M LiFSI + [Py1,4]FSI + 20 vol % acetonitrile on Ge (c) 4 M LiFSI + [Py1,4]FSI on electrodeposited Ge, and (d) 4 M LiFSI + [Py1,4]FSI + 20 vol % acetonitrile on Ge.

related to α-PVDF.20 Upon the addition of IL, IL + LiFSI and IL + LiFSI + AN, the peak at 39° disappears. A slight shift in the peak at 20° along with peak broadening occurs upon the addition of 1 M LiFSI and 1 M LiFSI + AN in the polymer electrolyte, whereas upon the addition of 4 M LiFSI and 4 M LiFSI + AN, only a peak broadening is observed. The disappearance of crystallinity and the appearance of no new peaks indicate that the high concentration LiFSI salt completely dissolves in the ionic liquid and does not precipitate during the polymer synthesis. The microstructure of the polymer is shown in Figure S2. PVDF-HFP shows a rough structure in Figure S2a indicating some crystallinity. With 1 M LiFSI (Figure S2b) and in the presence of acetonitrile (Figure S2c), the microstructure becomes smoother. However, some indications of particles are still seen. Upon the addition of IL + 4 M LiFSI, the microstructure shows the formation of some defects with some roughness (Figure S2d). In comparison, the presence of IL + 4 M LiFSI + AN (Figure S2e), the microstructure is almost featureless indicating that a homogenous polymer is formed. The thermal stability of the polymer was tested using TGA and is shown in Figure S3. The TGA of the polymer containing [Py1,4]FSI and LiFSI with and without acetonitrile is compared in Figure S3. The PVDF-HFP polymer is stable up to 380 °C, whereas upon the addition of [Py1,4]FSI, the thermal stability reduces to 280 °C (red line, Figure S1) due to the decomposition of the ionic liquid. The addition of 20 vol %

AFM experiments were performed with a Cypher S AFM (Asylum Research) inside a glove-box under an argon atmosphere. Silicon SPM-sensors with a spring constant of 16.6 N m−1 from Nano World or Asylum research were employed.

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RESULTS AND DISCUSSION The polymer membranes were characterized using XRD, scanning electron microscopy, and TGA as shown in Figures S1−S3 of the Supporting Information, respectively. From XRD (Figure S1), it is evident that PVDF-HFP shows a partial crystallinity with peaks arising at 18.1, 20, and 39°, and can be Table 1. Ionic Conductivity of Polymer Electrolytes Containing Ionic Liquid with Two Different Concentrations of LiFSI at 25 °C polymer PVDF-HFP PVDF-HFP PVDF-HFP PVDF-HFP PVDF-HFP PVDF-HFP PVDF-HFP

+ + + + + + +

[Py1,4]FSI [Py1,4]FSI + 20 vol % AN 1 M LiFSI + [Py1,4]FSI 1 M LiFSI + [Py1,4]FSI + 20 vol % AN 2 M LiFSI + [Py1,4]FSI + 20 vol % AN 4 M LiFSI + [Py1,4]FSI + 20 vol % AN 4 M LiFSI + [Py1,4]FSI

conductivity (mS cm−1) 0.2 0.4 1.9 2.3 6 8.2 1.5 24790


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Figure 2. (a) IR spectra of the PVDF-HFP polymer electrolyte containing [Py1,4]FSI (black line), 1 M LiFSI + [Py1,4]FSI (red line), 1 M LiFSI + [Py1,4]FSI + 20 vol % AN (blue line), 4 M LiFSI + [Py1,4]FSI (pink line), and 4 M LiFSI + [Py1,4]FSI + 20 vol % AN (green line) between 400 and 1700 cm−1 wavenumbers. (b) Comparison of Raman spectra of [Py1,4]FSI, 1 M LiFSI−[Py1,4]FSI, 1 M LiFSI + [Py1,4]FSI + 20 vol % AN, 4 M LiFSI + [Py1,4]FSI, and 4 M LiFSI + [Py1,4]FSI + 20 vol % AN in the region between 680 and 800 cm−1, (c) comparison of the Raman spectra in the region of 40 and 200 cm−1 and (d) Raman spectra in the region between 2800 and 3200 cm−1.

explained elsewhere in the literature.30 In Figure 1a, a peak at 1.75 V is observed in the first cycle of the CV for the polymer electrolyte with 1 M LiFSI + [Py1,4]FSI, which can be due to the formation of a SEI layer. An increase in the negative current is observed at ∼0.5 V, which is related to the lithiation of Ge. A delithiation peak is observed at 0.6 V in the anodic scan. In comparison to Figure 1a, the addition of acetonitrile to the polymer electrolyte shows only an intercalation and deintercalation process at 0.5 V. Upon increasing the Li salt concentration to 4 M (Figure 1c), it is evident in the first cycle that the formation of SEI layer and lithiation process commences at ∼2 and 0.7 V vs Li/Li+, respectively. Here, the delithiation peak occurs at 0.75 V in the 1st, 5th, and 10th cycles. A slight increase in the current during lithiation and delithiation processes also occurs compared to the polymer electrolyte containing 1 M LiFSI. Upon the addition of acetonitrile to PVDF-HFP + 4 M LiFSI + [Py1,4]FSI (Figure 1d), the lithiation and delithiation occur at 0.6 V vs Li/Li+. Furthermore, no peaks have been observed between 2.0 and 1.0 V for both polymer electrolytes containing different concentrations of LiFSI upon adding acetonitrile to them (Figure 1b,d). On comparing the CVs in Figure 1, it is evident

acetonitrile to the ionic liquid does not change the thermal decomposition temperature. With 1 M LiFSI in the presence and absence of acetonitrile, almost no weight loss occurs until 165 °C after which about 10% weight loss occurs until 265 °C. However, when 4 M LiFSI is added to the ionic liquid a weight loss of 4% takes place between room temperature and 100 °C, and can be related to some remaining trapped acetone. However, the polymer remains stable until 265 °C. In the presence of acetonitrile and 4 M LiFSI, an initial weight loss of about 8% takes place and the decomposition of the polymer commences at 265 °C. Thus, from the TGA results it can be said that the polymer electrolyte containing an ionic liquid− organic solvent mixture with LiFSI is stable until a temperature of 265 °C. The electrochemical window of the polymer was also analyzed by sandwiching the polymer membrane between symmetric stainless-steel electrodes. Figure S4 shows the cyclic voltammetry of the polymer membrane from which it is clear that the polymer electrolyte has an electrochemical window of 5.1 V. The performance of polymer electrolytes in LIBs was tested using Ge as the anode at room temperature. The CV of the polymer electrolyte on electrodeposited germanium is shown in Figure 1. The electrodeposition of germanium has been 24791


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Figure 3. Comparison of charge−discharge profiles for 50 cycles on electrodeposited Ge at (a) 0.375C in 1 M LiFSI + [Py1,4]FSI and 1 M LiFSI + [Py1,4]FSI + 20 vol % acetonitrile polymer electrolytes. (b) 4 M LiFSI + [Py1,4]FSI and 4 M LiFSI + [Py1,4]FSI + 20 vol % AN polymer electrolytes at various current densities. (c) 4 M LiFSI + [Py1,4]FSI and 4 M LiFSI + [Py1,4]FSI + 20 vol % AN polymer electrolytes at 0.375C.

imide-based PVDF-HFP polymer electrolyte containing LiTFSI.20 To better understand the influence of acetonitrile in the IL− polymer electrolyte, Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy were performed. The FTIR spectra of the ionic liquid with 1 and 4 M LiFSI in the presence and absence of acetonitrile are compared in Figure 2a. The changes in the anion vibrational modes can be seen between 400 and 1700 cm−1. Here, the prominent changes are observed at 737 cm−1, which is related to the symmetric bending mode of SNS bonds. Upon the addition of 1 M LiFSI, a blue shift of about 3 cm−1 occurs which shifts to 747 cm−1 upon the addition of 4 M LiFSI (∼10 cm−1). The peak broadening and the blue shift can be related to the coordination of LiFSI with [Py1,4]FSI. Such blue shifts of ∼2 and 5 cm−1 were also observed for the νasSNS + νSF peak at 829 cm−1 upon the addition of 1 and 4 M LiFSI, respectively. Furthermore, shifts of about 5 cm−1 to higher wavenumbers occur for νsSO2 peaks at 1215 and 1173 cm−1. These results are consistent with the previously reported data for LiFSI in [Py1,4]FSI, and support the interaction of Li+ with [Py1,4]FSI.33 No recognizable changes have been observed between 2800 and 3100 cm−1 wavenumbers, which are related to [Py1,4]+ upon adding LiFSI to the polymer electrolyte (Figure S5). However, from the IR spectra, no clear changes in lithium

that the current observed during the lithiation and delithiation processes for the polymer electrolyte (4 M LiFSI + [Py1,4]FSI + 20 vol % AN) is 2 times higher compared to the polymer electrolyte with 1 M LiFSI + [Py1,4]FSI as well as 4 M LiFSI + [Py1,4]FSI. This indicates that at higher concentrations of the Li salt and acetonitrile the diffusivity of Li+ improves. Theoretical studies have shown that the addition of acetonitrile improves the ionic conductivity of ionic liquids.31,32 The measured ionic conductivities of the polymer containing ionic liquid with various concentrations of LiFSI are shown in Table 1. It is evident from Table 1 that upon addition of acetonitrile to [Py1,4]FSI, the conductivity of the polymer increases 2 times. The addition of 1 M LiFSI increases the ionic conductivity to 1.9 mS cm−1, whereas the addition of acetonitrile to 1 M LiFSI/IL polymer further improves the conductivity slightly to 2.3 mS cm−1. The addition of 4 M LiFSI with acetonitrile in the polymer electrolyte increases the ionic conductivity to 8.2 mS cm−1. In comparison, the polymer containing 4 M LiFSI without acetonitrile shows a conductivity of only 1.5 mS cm−1. This clearly shows that acetonitrile improves the ionic conductivity of the polymer electrolyte. The measured conductivity for 4 M LiFSI−[Py1,4]FSI + 20 vol % AN was found to be nearly 4 times higher than that obtained for 1butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)24792


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Figure 4. (a) Deflection image of the SEI layer formed on a Ge electrode after 50 charge−discharge cycles in a polymer electrolyte of 1 M LiFSI− [Py1,4]FSI. (b) Force map of the area shown in (a), which was taken over an area of 2 μm × 2 μm and obtained by taking 426 points. (c) The force−separation profile of the black spot shown in (b). (d) The force−separation profile of the brown spot shown in (b).

be expected that a similar complex is formed in our case for the FSI anion too. Earlier studies have shown that the intermolecular contributions can be identified in the low frequency Raman region.35,36 Figure 2c shows the Raman spectra between 40 and 200 cm−1 from which a decrease in the peak intensity at 77 cm−1 is seen upon the addition of 1 and 4 M LiFSI (red and pink lines) in the PVDF-HFP/ionic liquid polymer. Furthermore, a decrease in the peak intensity at 165 cm−1 occurs. Upon the addition of acetonitrile to the polymer electrolyte containing 1 and 4 M LiFSI (Figure 2c, blue line and green line), a shift to lower wavenumber by 10 cm−1 is noted for 77 cm−1 and the second peak around 165 cm−1 almost disappears, signifying that the intermolecular contributions changed upon adding acetonitrile. In the case of imidazolium ions, the contribution from the cation was identified between 150 and 200 cm−1 and was related to the out-of-plane bending mode of CH3CH2−(N).35,36 Therefore, it appears from Figure 2c that for [Py1,4]+ cation, the butyl and methyl groups bound to nitrogen are affected upon the addition of Li salt and acetonitrile. The peak at 77 cm−1 was assigned to the collective motion of cations involved in the

solvation were observed in the presence and absence of acetonitrile, and therefore, Raman spectroscopy was performed. Figure 2b shows the most significant region of the Raman spectra of the ionic liquid/PVDF-HFP, wherein upon the addition of metal salts significant changes take place. The complete Raman spectra are shown in the Supporting Information, Figure S6. A strong peak is observed at 726 cm−1 for [Py1,4]FSI, which shifts to higher wavenumbers with a decrease in peak intensity upon the addition of 1 M LiFSI. The decrease in the peak intensity and peak shift can be related to the formation of [Li(FSI)3]2− as reported in the literature.34 Upon the addition of 20 vol % acetonitrile, a further lowering of peak intensity and a shift of 3 cm−1 to a higher wavenumber is observed. This indicates that acetonitrile affects the Li+ complexation in the polymer electrolyte. Upon increasing the concentration of LiFSI to 4 M along with the addition of 20 vol % acetonitrile in the polymer electrolyte, further decrease in peak intensity and peak shift to higher wavenumbers are observed. Seo et al.32 showed that acetonitrile interacts with the ionic liquid containing LiTFSI and forms a (AN)n−Li− TFSI complex using quantum chemical calculations, density functional theory, and molecular dynamics simulations. It can 24793


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Figure 5. (a) Deflection image of the SEI layer formed on the Ge electrode after 50 charge−discharge cycles in a polymer electrolyte of 1 M LiFSI−[Py1,4]FSI + 20 vol % AN. (b) Force map of the area shown in (a). (c) The force−separation profile of the black spot shown in Figure 7b. (d) The force−separation profile of the brown spot shown in (b).

decreases to 267 mAh g−1 at a higher discharge rate (ca. 1.5C). In comparison, the specific capacity for the polymer electrolyte in the presence of acetonitrile (Figure 3b) shows an increased storage capacity of 648 mAh g−1 at 0.375C and 310 mAh g−1 at 1.5C. Furthermore, from Figure 3b, it is evident that over the 50 cycles, the storage capacity of the polymer electrolyte with acetonitrile was about 20−30% higher at all applied current densities compared to the ones without acetonitrile in the polymer electrolytes. The lithiation/delithiation processes were also examined on germanium for 50 cycles at 0.375C. Figure 3c compares the charge−discharge curves for 50 cycles at 0.375C with 4 M LiFSI + [Py1,4]FSI and 4 M LiFSI + [Py1,4]FSI + 20 vol % AN polymer electrolytes. The increase in Li storage capacity in germanium was found to be about 12% higher in the polymer electrolyte containing acetonitrile. The capacity retention for 50 cycles at 0.375C was found to be 89% after 50 cycles for the organic IL−polymer electrolyte, whereas the capacity retention decreased to 84% for the IL−polymer electrolyte. Furthermore, the charge−discharge profiles in Figure 3c are comparable with 1.1 M LiPF6 in the ethylene carbonate/diethyl carbonate mixture.37

local structure.35 The decrease in the intensity and peak shift in the presence of acetonitrile indicates that the collective motion of the cation is affected. This can be seen at higher frequencies of 2800−3200 cm−1 in Figure 2d, wherein the νCH2 peak of the [Py1,4]+ cation shows a slight shift to higher wavenumbers upon the addition of acetonitrile with 4 M LiFSI. Thus, from the Raman and IR spectra in Figure 2, it is evident that the LiFSI concentration and the addition of acetonitrile affects the Li+ solvation and probably influences the cation−anion interaction in the polymer electrolytes. The charge−discharge performance of the polymer electrolyte was also investigated on Ge electrodes. Figure 3a compares the charge−discharge profiles for polymer electrolytes containing 1 M LiFSI + [Py1,4]FSI and 1 M LiFSI + [Py1,4]FSI + 20 vol % acetonitrile on Ge at a 0.375C rate. An average specific capacity of about 350 mAh g−1 was obtained on using 1 M LiFSI + [Py1,4]FSI, whereas upon the addition of 20 vol % acetonitrile the capacity is improved to 400 mAh g−1. On further increasing the concentration of LiFSI to 4 M in the polymer electrolyte, an improvement in the storage capacity is obtained as seen in Figure 3b. It is evident that at 0.375C, a capacity of 480 mAh g−1 is obtained, which 24794


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Figure 6. (a) Deflection image of the SEI layer formed on the Ge electrode after 50 charge−discharge cycles in a polymer electrolyte of 4 M LiFSI−[Py1,4]FSI. (b) Force map of the area shown in (a). (c) The force−separation profile of the brown spot shown in (b). (d) The force− separation profile of the black spot shown in (b).

is required to rupture the layer. Compared to the liquid electrolyte with 1 M LiFSI,30 no “push through” forces are observed, which indicates the formation of a more stable SEI layer upon using the polymer electrolyte. This clearly indicates a change in the interfacial processes at the polymer electrolyte/ solid interface. Furthermore, from the force−separation map in Figure 4b, it is evident that the SEI layer is inhomogeneous. In comparison, upon the addition of 20 vol % AN to the polymer electrolyte containing 1 M LiFSI, the morphology of the SEI layer appears to be different (Figure 5a). A smoother surface consisting of large islands is observed. The force map in Figure 5b again shows bright and dark regions which correspond to an inhomogeneous SEI layer formation. Figure 5c shows the force−separation curve of the black spot wherein an increase in force is observed from 75 nm. The increase in force occurs in two regions. In the first region, it increases slowly to ∼200 nN and an almost exponential increase is seen to 800 nN below 25 nm. This suggests that the composition varies along the depth of the SEI layer. The brown spot in Figure 5d shows an increase in force to 575 nN from 32 nm. Comparing Figures 4 and 5, it is evident that in the presence of acetonitrile, a slightly thicker SEI layer with different mechanical properties is formed, and might have led to an improved capacity as seen in Figure 3a.

Thus, the addition of a high concentration of Li salt and acetonitrile to the polymer electrolyte improves the Li storage capacity in germanium. This can be attributed to an increase in the lithiation/delithiation of Ge due to higher Li+ concentration in the polymer electrolyte and acetonitrile which enhances the mass transport of Li+ species. To further understand the influence of the LiFSI concentration and the role of acetonitrile, we have analyzed the polymer/electrode interface after 50 charge−discharge cycles using AFM. Figure 4 shows the AFM image of the SEI layer, the force maps, and two force−separation curves of Ge after cycling in the polymer electrolyte containing 1 M LiFSI + [Py1,4]FSI. The AFM image shows the formation of large clusters of ∼500 nm in size (Figure 4a). From the force map in Figure 4b, dark and bright regions can be observed which is due to the difference in the force required to rupture the SEI layer.30,38 The force−separation curve of a black spot and brown spot are shown in Figure 4c,d, respectively. In Figure 4c, an increase in force is observed from 70 nm up to about 400 nN before direct contact with the substrate which corresponds to the formation of a SEI layer. In comparison to the black spot in Figure 4c, the brown spot shows a different response (Figure 4d) wherein an increase in force is observed from 25 nm and a force of 175 nN 24795


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Figure 7. (a) Deflection image of the SEI layer formed on the Ge electrode after 50 charge−discharge cycles in a polymer electrolyte of 4 M LiFSI−[Py1,4]FSI + 20 vol % AN. (b) Force map of the area shown in (a). (c) The force−separation profile of the brown spot shown in (b). (d) The force−separation profile of the black spot shown in (b).

over the entire surface and might have led to an improved Li storage. Upon the addition of acetonitrile to the polymer electrolyte with 4 M LiFSI−[Py1,4]FSI, the changes in the SEI layer can be seen in the AFM deflection image (Figure 7a). The force map in Figure 7b again indicates an inhomogeneous nature of the SEI layer. The force−separation curve of a brown spot revealed that the SEI layer is 50 nm thick (Figure 7c). The force− separation curve of the black spot is shown in Figure 7d. An increase in force of about 50 nN is observed from a distance of ∼60 to 35 nm after which the force increases to about 400 nN before contacting the substrate. The increase in force in the two regions indicates that the composition of the SEI layer has changed along the depth of the sample. However, unlike in Figure 6d, no plastic deformation is observed, which means that the SEI layer formed in the presence of AN will have a higher yield strength. Therefore, Ge electrode will not undergo stress during lithiation/delithiation which might have led to an improved charge−discharge cycling. To further investigate the SEI layers after cycling, XPS was performed. The XPS survey spectra of Ge after 50 charge− discharge cycles are compared in Figure S7. In both the spectra (Figure S7a,b), dominant peaks of F and O are evident.

Upon increasing the concentration of LiFSI to 4 M in the polymer, the SEI layer formed shows a rough surface made up of islands in the range of 300−500 nm as seen in the AFM deflection image (Figure 6a). From the force map in Figure 6b, dark and bright regions are still seen, which are similar to those in Figures 4 and 5 indicating an inhomogeneous nature of the SEI layer. The force−separation curves of a brown and a black spot are shown in Figure 6c,d, respectively. In Figure 6c, an increase in force to ∼600 nN is observed from 43 nm before contacting the substrate. In comparison, the black spot in Figure 6d shows a different force−separation profile, wherein an increase in force is observed from 75 nm which plateaus at 40 nm. A further increase in force is seen again at ∼10 nm. The initial increase in force can be related to an elastic deformation of the SEI layer followed by a plastic deformation.39 It has been shown that plastic deformation in the SEI layer would generate a stress in the electrode due to volume expansion during lithiation leading to the formation of cracks in the electrode.40 This indicates that with lithiation/ delithiation processes, the Ge electrode might have undergone stress. However, compared to the 1 M LiFSI polymer electrolyte (Figure 4), it appears that for a higher concentration, a thicker and more uniform SEI layer is formed 24796


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Figure 8. XPS detailed spectra of an electrodeposited Ge anode surface after 50 charge−discharge cycles in PVDF-HFP containing 1 M LiFSI− [Py1,4]FSI at different sputtering intervals (a) Ge 3d, (b) F 1s, and (c) Li 1s. XPS detailed spectra of an electrodeposited Ge anode surface after 50 charge−discharge cycles in PVDF-HFP containing 1 M LiFSI−[Py1,4]FSI + 20 vol % AN at different sputtering intervals (d) Ge 3d, (e) F 1s (f) Li 1s.

published results for a liquid electrolyte,29,30 the formation of LixGe is not seen at lower binding energies which suggest that no lithium is found in Ge after 50 charge−discharge cycles on using the polymer electrolyte and complete delithiation has been achieved during discharging. For F 1s spectra in Figure 8b, the peak at 685 eV can be assigned to LiF39 and a prominent peak at 688.5 eV is related to the FSI− anion. With sputtering, the LiF peak dominates the spectra and no FSI anion peak is seen. Figure 8c shows a peak at 55.6 eV, which can be deconvoluted to two peaks. These

Besides, Ge, N, C, S, and Li can also be discerned. The detailed spectra of Ge, F, and Li after 50 charge−discharge cycles with PVDF-HFP containing 1 M LiFSI−[Py1,4]FSI are shown in Figure 8. The peaks are deconvoluted with Gaussian functions and shown in the same figure. After 10 min of Ar+ ion sputtering the sample, Figure 8a shows peaks/shoulders at 29.5, 30.5, and 32.5 eV, which can be related to elemental Ge, GeOx, and GeO2, respectively.29 Here, the presence of both elemental Ge and GeO2 in the layer indicates that Ge has been oxidized along the depth of the sample. However, compared to 24797


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Thus, based on the post analysis of the Ge electrode after cycling using gel polymer electrolytes (GPEs), we can say that the addition of a high concentration of Li salt (LiFSI) and acetonitrile to the GPE improves the mechanical stability of the SEI layer. The AFM results indicate that the SEI layer is thicker and more flexible. Furthermore, surface analysis after cycling (XPS analysis) indicates that the addition of high concentration of Li salt (LiFSI) and acetonitrile to the GPE prevents the oxidation of Ge electrodes, which can be a reason for improved specific capacity.

peaks can be related to the formation of LiOx and Li2S/LiF whose binding energies differ within ±0.1 eV. With sputtering, the peak intensity of Li 1s increases which suggests that a relatively thick SEI layer has formed on the Ge electrode. Upon the addition of acetonitrile in the polymer electrolyte with 1 M LiFSI, the Ge 3d spectra in Figure 8d show a significant difference compared to that of the Ge 3d spectra shown in Figure 8a. A higher intensity for the peak related to elemental Ge is observed along with GeO2. With sputtering, the elemental Ge peak intensity increases and a slight shift in the GeO2 to lower binding energy (∼1 eV) is observed, indicating that the complete oxidation of Ge did not take place along the depth of the sample. Comparing the Ge 3d spectra with that in Figure 8a, it is evident that the presence of acetonitrile lowers the oxidation of Ge during cycling. The F 1s spectra (Figure 8e) show a dominant peak corresponding to the formation of LiF. Besides, a small peak corresponding to the FSI− is also similar to that observed in Figure 8b. The Li 1s spectra (Figure 8f) correspond to the formation of LiOx and Li2S/LiF as observed in Figure 8c. The XPS survey spectra and detailed spectra of Ge after 50 charge−discharge cycles with 4 M LiFSI−[Py1,4]FSI and 4 M LiFSI−[Py1,4]FSI + 20 vol % AN polymer electrolytes are compared in Figures S8. The survey spectra show (Figure S8a,b) dominant peaks of F and O, and smaller peaks of Ge, N, C, S, and Li. The detailed spectra of Ge, F, and Li after 50 charge−discharge cycles with PVDF-HFP containing 4 M LiFSI−[Py1,4]FSI is shown in Figure S9. Ge and oxides of Ge can be observed from the detailed spectra of Ge (Figure S9a). In the F 1s spectra, two peaks corresponding to LiF and the FSI− anion are observed (Figure S9b).40 Here, the LiF peak dominates the spectra with a decrease in the intensity of the FSI anion after sputtering. The Li 1s spectra in Figure S9c show peaks related to the formation of LiOx and Li2S/LiF, which seems to be identical with the ones with lower LiFSI concentration. Upon the addition of AN to 4 M LiFSI− [Py1,4]FSI, the XPS spectra of Ge show a lowering of the GeO2 component (Figure S9d). This is further substantiated from the O 1s detailed spectra in Figure S10. The O 1s spectra of Ge cycled with a low concentration of LiFSI (1 M) and with the 4 M LiFSI polymer electrolyte show the formation of LiOx/ LiOH, Li2CO3, and GeO2 (Figure S10a−d). At a higher LiFSI concentration with acetonitrile (Figure S10d), a prominent peak of LiOx is observed whereas Li2CO3 disappears. This suggests that the composition of the SEI layer has changed upon using 4 M LiFSI and acetonitrile to the polymer. Figure S9e shows the detailed spectra of F 1s, wherein peaks related to LiF and FSI anions are observed. However, with sputtering, it is clear that the F 1s related to the ionic liquid considerably decreases and a strong peak of LiF is seen. The Li 1s in Figure S9f shows the formation of LiOx, Li2S, and LiF. Thus, comparing the AFM and XPS (Figures 4−8 and S7−S10), it appears that the SEI layer differs considerably at low (1 M) and high concentrations (4 M) with and without acetonitrile in the polymer electrolyte. The composition and thickness as well as the mechanical properties of the SEI layer varies with the LiFSI concentration and upon the addition of acetonitrile to the polymer electrolyte. Furthermore, it appears that the higher Li concentration and acetonitrile in the electrolyte limits the oxidation of the Ge anode during cycling. These factors might have assisted to improve the specific capacity with concentrated electrolyte containing organic solvent as seen in Figure 3.

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CONCLUSIONS In this paper, we have presented an ionic liquid−organic solvent mixture-based polymer electrolyte with both low and high lithium-ion concentrations. Conductivity measurements showed that the conductivity increases with the increase in concentration from 1 to 4 M and upon the addition of acetonitrile to the polymer electrolyte. From IR and Raman spectroscopic studies, it was observed that an increase in concentration and the presence of acetonitrile changes the lithium-ion solvation, which might have resulted in an increased conductivity. For the IL−polymer electrolyte with 4 M LiFSI and acetonitrile, an increased Li storage capacity in Ge has been obtained compared to the IL−polymer electrolyte with a lower content of LiFSI (e.g., 1 M). AFM results showed that the mechanical properties as well as the SEI layer thickness change upon increasing the LiFSI concentration and upon the addition of acetonitrile in [Py1,4]FSI. XPS analysis of the SEI layer showed that compared to low concentration LiFSI, the concentrated electrolyte and the addition of AN to the polymer electrolyte prevents the oxidation of germanium, which led to an improved battery performance. Thus, the current study clearly indicates that a concentrated polymer electrolyte based on the ionic liquid−organic solvent mixture can be considered as a potential and safe (solid state) electrolyte for lithium-ion batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b07745. Raman, IR, AFM, and XPS spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.L.). *E-mail: [email protected] (F.E.). ORCID

Abhishek Lahiri: 0000-0001-8264-9169 Giridhar Pulletikurthi: 0000-0002-3588-493X Oliver Höfft: 0000-0002-1313-3166 Frank Endres: 0000-0002-5719-7241 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Karin Bode, Institute of Inorganic Chemistry (Prof. A. Adam) for help with Raman and IR measurements. The authors would also like to thank Ulrike Koecher, Institute of Technical Chemistry, for TGA measurements. 24798


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