Ionic LiquidâOrganic Solvent Mixture-Based Polymer Gel Electrolyte

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

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 J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07745 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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The Journal of Physical Chemistry

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, Frank Endres* Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Strasse 6, 38678 Clausthal-Zellerfeld, Germany Corresponding Authors: [email protected]; [email protected] Abstract In this paper, we have investigated the performance and solid electrolyte interphase formation (SEI)

of

a

hybrid

hexafluoropropylene

polymer-gel

(PVDF-HFP),

electrolyte

based

on

polyvinylidene

1-butyl-1-methylpyrrolidinium

fluoride-co-

bis(fluorosulfonyl)imide

([Py1,4]FSI), and lithium bis(fluorosulfonyl)imide (LiFSI) for all solid-state lithium ion battery. The effect of addition of acetonitrile (AN) and different concentrations of LiFSI to such polymergel electrolyte was also examined. Compared to 1M LiFSI in the ionic liquid-polymer electrolyte, addition of 4M LiFSI along with acetonitrile led to higher lithium storage capacity. X-ray photoelectron spectroscopy (XPS) and Atomic force microscopy (AFM) were used to analyse the SEI formation on a Ge electrode. A difference in the mechanical property and SEI compostion 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 with 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 addition of an organic solvent and high concentration of Li-salt to polymer gel electrolyte improves the performance of LIBs. 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 in order 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 1 ACS Paragon Plus Environment

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electrolyte. The formation of the solid-electrolyte interphase (SEI) is an important aspect in LIBs and the behaviour 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 (> 2M Li salt) which have gained 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 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 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 have been investigated for lithium ion batteries (LIBs) over the last two decades

5–9.

They possess a few advantages over their liquid counterparts as they are flexible,

non-inflammable and can also act as a separator

10–12.

Another striking advantage is that 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 anode materials in the past, however, these studies were carried out at temperatures > 60 °C 13. Polyethelene 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 PEO polymer containing 1M 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 2 ACS Paragon Plus Environment

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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 (0.05 C - 0.2 C) to improve the diffusion of Li ions. Ionic liquid based polymer electrolytes have recently gained a prominent research interest due to useful properties of these liquids

17,18.

They are non-inflammable, possess wide electrochemical

windows, low vapour pressure and high thermal stability to mention a few physicochemical properties. Negligible vapour pressures and non-inflammability of ILs are crucial properties to improve the safety of LIBs. Therefore, ionic liquids based gel polymer electrolytes (GPE) have received 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 temperature

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) 22.

19–

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 synthesised 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 electrodes

29,30

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 PVDF-HFP polymer-based electrolyte containing two different concentrations of LiFSI in [Py1,4]FSI (e.g. 1M and 4M). Furthermore, the addition of acetonitrile (AN) to the gel polymer electrolyte is also examined. Besides lithium ion battery performance, we also studied in detail 3 ACS Paragon Plus Environment


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. Experimental The Ionic liquid [Py1,4]FSI was purchased in the highest available quality from Solvionic (France) and was used after drying under vacuum at 100 oC to achieve a water content of below 10 ppm. LiFSI was bought from Fluorochem and was dried in vacuum at 100 oC 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 hours. The resultant mixture was subsequently mixed with 7.5 wt % PVDF-HFP/acetone solution. The weight ratio of the LiFSIionic 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 hours 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 50X objective with a laser excitation of 532 nm. Fourier transform infrared spectroscopy (VERTEX 70 V, Bruker Optics GmbH) with an attached attenuated total reflectance (ATR) module was used to characterise the polymer. The electrochemical window and the ionic conductivity of the polymer were also analyzed by sandwiching the polymer membrane between 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 equation 1. (Eq.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 preparation of the anode, Ge was electrodeposited on copper at -2.2 V vs. Pt for 30 minutes from 0.25 M GeCl4 in [Py1,4]TFSI. After electrodeposition, the remaining electrolyte was removed and the electrodeposited Ge was 4 ACS Paragon Plus Environment

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washed in the pure ionic liquid inside of 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 VersaStat 3 (Princeton Applied Research) potentiostat/galvanostat. X-ray diffraction patterns were recorded using a PANalytical Empyrean Diffractometer (Cabinet no. 9430 060 03002) with CuKα radiation. Thermogravimetric analysis (TGA) was performed in a TGA 850, Mettler-Toledo apparatus under Nitrogen atmosphere at a heating rate of 10 °C min1.

X-ray Photoelectron spectra (XPS) were obtained using an ultrahigh vacuum (UHV) apparatus

with a base pressure below 1x10-10 hPa. The sample was transferred from the glove box to XPS using a specialised transfer chamber. The sample was irradiated using the Al Kα line (photon energy of 1486.6 eV) of a non-monochromatic X-ray source (Omicron DAR 400). Electrons emitted were detected by a hemispherical analyser (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. AFM experiments were performed with a Cypher S AFM (Asylum Research) inside of a glovebox under argon atmosphere. Silicon SPM-sensors with a spring constant of 16.6 N/m from Nano World or Asylum research were employed. Results and Discussion The polymer membranes were characterized using XRD, SEM, and TGA as shown in the figures S1, S2, and S3 of Supporting Information, respectively. From XRD (Fig. S1), it is evident that PVDF-HFP shows a partial crystallinity with peaks arising at 18.1°, 20° and 39 °, and can be related to α-PVDF

20.

On 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 on addition of 1M LiFSI and 1M LiFSI+AN in the polymer electrolyte whereas on addition of 4M LiFSI and 4M LiFSI+AN, only a peak broadening is observed. The disappearance of crystallinity and 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 5 ACS Paragon Plus Environment


the polymer is shown in figure S2. PVDF-HFP shows a rough structure in figure S2a indicating some crystallinity. With 1M LiFSI (fig S2b) and in presence of acetonitrile (fig S2c), the microstructure becomes smoother. However, some indication of particles are still seen. On addition of IL+4M LiFSI, the microstructure shows the formation of some defects with some roughness (figure S2d). In comparison, the presence of IL+ 4M 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 are compared in figure S3. The PVDF-HFP polymer is stable up to 380 °C whereas on addition of [Py1,4]FSI, the thermal stability reduces to 280 °C (red line, fig S1) due to the decomposition of the ionic liquid. The addition of 20 vol% acetonitrile to the ionic liquid does not change the thermal decomposition temperature. With 1M LiFSI in 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 4M 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 4M 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 a ionic liquidorganic 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 electrochemical window of the polymer electrolyte was found to be 5.1 V (Fig. S4). The performance of polymer electrolytes in LIBs were tested using Ge as anode at room temperature. The CV of the polymer electrolyte on electrodeposited germanium is shown in Fig. 1. The electrodeposition of germanium has been explained elsewhere in literature 30. In Fig. 1a, a peak at 1.75 V is observed in the first cycle of the CV for the polymer electrolyte with 1M LiFSI+[Py1,4]FSI, which can be due to the formation of a SEI layer. An increase in 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 Fig 1a, the addition of acetonitrile to the polymer electrolyte shows only an intercalation and deintercalation process at 0.5 V. On 6 ACS Paragon Plus Environment

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increasing the Li salt concentration to 4M (Fig. 1c), it is evident in the first cycle that the formation of SEI layer and lithiation process commences at ~2 V and 0.7 V vs. Li/Li+, respectively. Here, the delithiation peak occurs at 0.75 V in the 1st, 5th and 10th cycles.

Fig. 1. CVs of a polymer electrolytes at 25 °C containing (a) 1M LiFSI+[Py1,4]FSI on electrodeposited Ge (b) 1M LiFSI+[Py1,4]FSI+20 vol% aceteonitrile on Ge (c) 4M LiFSI+[Py1,4]FSI on electrodeposited Ge, and (d) 4M LiFSI+[Py1,4]FSI+20 vol% acetonitrile on Ge. A slight increase in the current during lithiation and delithiation processes also occurs compared to the polymer electrolyte containing 1M LiFSI. On addition of acetonitrile to PVDF-HFP + 4 M LiFSI+[Py1,4]FSI (Fig. 1d), the lithiation and delithiation occur at 0.6 V vs Li/Li+. Furthermore, no peaks have been observed between 2.0 V and 1.0 V for both polymer electrolytes containing different concentrations of LiFSI upon adding acetonitrile to them (Figs. 1b and d). On comparing the CVs in Fig.1, it is evident that the current observed during the lithiation and delithiation processes for the polymer electrolyte (4M LiFSI+ [Py1,4]FSI+20 vol% AN) is two times higher compared to the polymer electrolyte with 1M LiFSI+ [Py1,4]FSI as well as 4M LiFSI+ [Py1,4]FSI. 7 ACS Paragon Plus Environment


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Table 1. Ionic conductivitis of polymer electrolytes containing ionic liquid with two different concentrations of LiFSI at 25°C. Conductivity (mS cm-1)

Polymer PVDF-HFP+[Py1,4]FSI

0.2

PVDF-HFP+[Py1,4]FSI+20 vol% AN

0.4

PVDF-HFP+1M LiFSI+ [Py1,4]FSI

1.9

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

2.3


6


8.2

PVDF-HFP+4M LiFSI+ [Py1,4]FSI

1.5

This indicates that at higher concentration of the Li salt and acetonitrile improves the diffusivity of Li+. 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 on addition of acetonitrile to [Py1,4]FSI, the conductivity of the polymer increases two times. The addition of 1M LiFSI increases the ionic conductivity to 1.9 mS cm-1, whereas the addition of acetonitrile to 1M LiFSI/IL polymer, further improves the conductivity slightly to 2.3 mS cm-1. The addition of 4M LiFSI with acetonitrile in the polymer electrolyte increases the ionic conductivity to 8.2 mS cm-1. In comparison, the polymer containing 4M 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 4M LiFSI-[Py1,4]FSI+20 vol% AN was found to be nearly four times

higher

than

that

bis(trifluoromethanesulfonyl)imide

obtained

(BMIMTFSI)

for based

1-butyl-3-methylimidazolium PVDF-HFP

polymer

electrolyte

containing LiTFSI 20. In order 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 1M and 4M LiFSI in presence and absence of acetonitrile are compared in Fig. 2a. The changes in the anion vibrational modes can be seen between 400 and 8 ACS Paragon Plus Environment

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1700 cm-1. Here, the prominent changes are observed at 737 cm-1, which is related to the symmetric bending mode of SNS bonds. On addition of 1M LiFSI, a blue shift of about 3 cm-1 occur which shifts to 747 cm-1 on addition of 4M LiFSI (~ 10 cm-1). The peak broadening and the blue shift can be related to coordination of LiFSI with [Py1,4]FSI. Such blue shifts of ~2 and 5 cm-1 were also observed for νasSNS+νSF peak at 829 cm-1 on addition of 1M and 4M 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 previously reported data for LiFSI in [Py1,4]FSI, and supports 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 (Fig. S5). However, from the IR spectra, no clear changes in the lithium solvation were observed in the presence and absence of acetonitrile, and therefore Raman spectroscopy was performed. Fig. 2b shows the most significant region of the Raman spectra of the ionic liquid/PVDF-HFP, wherein on the addition of metal salts significant changes take place. The complete Raman spectra are shown in the supporting information Fig. 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 addition of 1M LiFSI. The decrease in the peak intensity and peak shift can be related to the formation of [Li(FSI)3]2- as reported in literature

34.

On

addition of 20 vol% acetonitrile, a further lowering of peak intensity and a shift of 3 cm-1 to higher wavenumber is observed. This indicates that acetonitrile affects the Li+ complexation in the polymer electrolyte. On increasing the concentration of LiFSI to 4M and also on addition of 20 vol% acetronitrile in the polymer electrolyte, further decrease in peak intensity and peak shift to higher wavenumbers are observed. Seo et al

32

showed using quantum chemical calculations

(QC), density functional theory (DFT), and molecular dynamics (MD) simulations that acetonitrile interacts with the ionic liquid containing LiTFSI and forms a (AN)n-Li-TFSI complex. It can 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.

Fig. 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 on addition of 1M and 4M LiFSI (red and pink lines) in the PVDF-HFP/ionic liquid polymer.

9 ACS Paragon Plus Environment


Figure 2. (a) IR spectra of PVDF-HFP polymer electrolyte containing [Py1,4]FSI (black line), 1M LiFSI+[Py1,4]FSI (red line), 1M LiFSI+[Py1,4]FSI+20 vol% AN (blue line), 4M LiFSI+[Py1,4]FSI (pink line) and 4M 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, 1M LiFSI-[Py1,4]FSI, 1M LiFSI+ [Py1,4]FSI+20 vol% AN, 4M LiFSI+ [Py1,4]FSI and 4M 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. Furthermore, a decrease in the peak intensity at 165 cm-1 occurs. On addition of acetonitrile to the polymer electrolyte containing 1M and 4M LiFSI (Fig. 2c, blue line and green line), a shift to lower wavenumber by 10 cm-1 is noted for the 77 cm-1 and the second peak around 165 cm-1 almost disappears, signifying that the intermolecular contributons 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 the CH3CH2-(N). 35,36

Therefore, it appears from Fig.2c that for [Py1,4]+ cation, the butyl and methyl groups bound 10 ACS Paragon Plus Environment

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to nitrogen are affected on addition of Li salt and acetonitrile. The peak at 77 cm-1 was assigned to the collective motion of cations involved in the local structure 35. The decrease in the intensity and peak shift in 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 Fig. 2d, wherein the νCH2 peak of the [Py1,4]+ cation shows a slight shift to higher wavenumbers on addition of acetonitrile with 4M LiFSI. Thus, from the Raman and IR spectra in Fig. 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. Fig.3a compares the charge-discharge profiles for polymer electrolytes containing 1M LiFSI+[Py1,4]FSI and 1M LiFSI+[Py1,4]FSI+20 vol% acetonitrile on Ge at 0.375 C rate. An average specific capacity of about 350 mAh g-1 was obtained on using 1M LiFSI+[Py1,4]FSI whereas on addition of 20 vol% acetonitrile the capacity is improved to 400 mAh g-1. On further increasing the concentration of LiFSI to 4M in the polymer electrolyte, an improvement in storage capacity is obtained as seen in Fig. 3b. It is evident that at 0.375 C, a capacity of 480 mAh g-1 is obtained, which decreases to 267 mAh g-1 at a higher discharge rate (ca. 1.5 C). In comparison, the specific capacity for the polymer electrolyte in presence of acetonitrile (Fig. 3b) shows an increased storage capacity of 648 mAh g-1 at 0.375 C and 310 mAh g-1 at 1.5 C. Furthermore from Fig.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 a germanium for 50 cycles at 0.375 C. Fig. 3c compares the chargedischarge curves for 50 cycles at 0.375 C with 4M LiFSI+[Py1,4]FSI and 4M 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 0.375 C was found to be 89% after 50 cycles for the organic-IL polymer electrolyte whereas the capacity retention decreased to 84% for polymer electrolyte. Furthermore, the charge-discharge profiles in Fig. 3c are comparable with 1.1 M LiPF6 in ethylene carbonate:diethyl carbonate mixture. 37



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Figure 3. Comparison of charge-discharge profiles for 50 cycles on electrodeposited Ge at (a) 0.375 C in 1M LiFSI+[Py1,4]FSI and 1M LiFSI+[Py1,4]FSI+20 vol% acetonitrile

polymer

electrolyte (b) 4M LiFSI+[Py1,4]FSI and 4M LiFSI+[Py1,4]FSI+20 vol% AN polymer electrolytes at various current densities (c) 4M LiFSI+[Py1,4]FSI and 4M LiFSI+[Py1,4]FSI+20 vol% AN polymer electrolytes at 0.375 C Thus, the addition of 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 LiFSI concentration and the role of acetonitrile, we have analyzed the polymer/electrode interface after 50 charge-discharge cycles using AFM. Fig.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 1M LiFSI+[Py1,4]FSI. 12 ACS Paragon Plus Environment

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Fig.4 (a) Deflection image of the SEI layer formed on a Ge electrode after 50 charge-discharge cycles in a polymer electrolyte of 1M LiFSI-[Py1,4]FSI (b) Force map of the area shown in Fig 4a, which was taken over an area of 2 µm×2 µm and obtained by taking 426 points. (c) Forceseparation profile of the black spot shown in Fig. 4b (d) Force-separation profile of the brown spot shown in Fig. 4b. The AFM image shows the formation of large clusters of ~ 500 nm in size (Fig.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 Fig. 4c and 4d, 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 Fig.4c, the brown spot shows a different response (Fig 4d) wherein an increase in force is observed from 25 nm and a force of 175 nN is required to rupture the layer. Compared to liquid electrolyte with 1M LiFSI 30,

no ‘push through’ forces are observed, which indicates the formation of a more stable SEI

layer on 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 Fig. 4b, it is evident that the SEI layer is inhomogeneous.



In comparison, on addition of 20 vol% AN to the polymer electrolyte containing 1M LiFSI, the morphology of the SEI layer appears to be different (Fig. 5a). A smoother surface consisting of large islands is observed. The force map in Fig. 5b shows again 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 rise in force consists of two regions. In the first region it rises slowly to ~ 200 nN and an almost exponential rise 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 Fig. 5d shows a rise in force to 575 nN from 32 nm. Comparing Figs. 4 and 5, it is evident that in the presence of acetonitrile, a slightly thicker SEI layer with different mechanical property is formed, and might have led to an improved capacity as seen in Fig.3a. On 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 (Fig 6a). From the force map in Fig. 6b, dark and bright regions are still seen, which are similar to those in Figs. 4 and 5 indicating an inhomogeneous nature of the SEI layer. The forceseparation curves of a brown and a black spot are shown in Fig. 6c and 6d, respectively. In Fig.6c, an increase in force to ~ 600 nN is observed from 43 nm before contacting the substrate. In comparison the black spot in Fig. 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.


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Fig.5. (a) Deflection image of the SEI layer formed on Ge electrode after 50 charge-discharge cycles in a polymer electrolyte of 1M LiFSI-[Py1,4]FSI+20 vol% AN (b) Force map of the area shown in fig 5a. (c) Force-separation profile of the black spot shown in figure 7b (d) Forceseparation profile of the brown spot shown in fig 5b. 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 formation of cracks in the electrode40. This indicates that with lithiation/delithiation processes, the Ge electrode might have undergone stress. However, compared to the 1M LiFSI polymer electrolyte (Fig. 4), it appears that for higher concentration, a thicker and more uniform SEI layer is formed over the entire surface and might have led to an improved Li storage.



Figure 6. (a) Deflection image of the SEI layer formed on the Ge electrode after 50 chargedischarge cycles in a polymer electrolyte of 4M LiFSI-[Py1,4]FSI (b) Force map of the area shown in Fig 6a. (c) Force-separation profile of the brown spot shown in figure 6b (d) Forceseparation profile of the black spot shown in Fig. 6b. On addition of acetonitrile to the polymer electrolyte with 4M LiFSI-[Py1,4]FSI, the changes in the SEI layer can be seen in the AFM deflection image (Fig. 7a). The force map in Fig. 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 (Fig.7c). The force separation curve of the black spot is shown in Fig. 7d. An increase in force of about 50 nN is observed from a distance of ~ 60 nm 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 Fig. 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.


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Figure 7. (a) Deflection image of the SEI layer formed on Ge electrode after 50 charge-discharge cycles in a polymer electrolyte of 4M LiFSI-[Py1,4]FSI+20 vol% AN (b) Force map of the area shown in Fig 7a. (c) Force-separation profile of the brown spot shown in Fig. 7b (d) Forceseparation profile of the black spot shown in Fig. 7b. 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 Fig. S7. In both of the spectra (Fig. S7a and Fig. S7b), dominant peaks of F and O are evident. 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 1M LiFSI-[Py1,4]FSI are shown in Fig. 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 published results for a liquid electrolyte

29,30,

formation of LixGe is not seen at lower binding

energies which suggest that no lithium is found in the Ge after 50 charge-discharge cycles on using the polymer electrolyte and complete delithiation has been achieved during discharging. 17 ACS Paragon Plus Environment


Figure. 8. XPS detailed spectra of an electrodeposited Ge anode surface after 50 charge-discharge cycles in PVDF-HFP containing 1M LiFSI-[Py1,4]FSI at different sputtering intervals (a) Ge 3d, (b) F 1s (c) Li 1s XPS detailed spectra of an electrodeposited Ge anode surface after 50 chargedischarge cycles in PVDF-HFP containing PVDF-HFP containing 1M LiFSI-[Py1,4]FSI+20 vol% AN at different sputtering intervals (d) Ge 3d, (e) F 1s (f) Li 1s 18 ACS Paragon Plus Environment

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For F 1s spectra in Fig. 8b, the peak at 685 eV can be assigned to LiF 39 and a prominent peak at 688.5 eV is related to FSI- anion. With sputtering, the LiF peak dominates the spectra and no FSI anion peak is seen. Fig. 8c shows a peak at 55.6 eV, which can be deconvoluted to two peaks. These 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. On addition of acetonitrile in the polymer electrolyte with 1M LiFSI, the Ge 3d spectra in Figure 8d shows a significant difference to that of the Ge 3d spectra shown in Fig.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 (~ 1eV) is observed indicating that complete oxidation of Ge did not take place along the depth of the sample. Comparing the Ge 3d spectra with that in Fig. 8a, it is evident that the presence of acetonitrile lowers the oxidation of Ge during cycling. The F1s spectra (Fig. 8e) show a dominant peak corresponding to the formation of LiF. Besides, a small peak corresponding to the FSI- is also seen similar to that observed in Fig. 8b. The Li 1s spectra (Fig. 8f) correspond to the formation of LiOx and Li2S/LiF as observed in Fig. 8c. The XPS survey spectra and detailed spectra of Ge after 50 charge-discharge cycles with 4M LiFSI-[Py1,4]FSI and 4M LiFSI-[Py1,4]FSI+20 vol% AN polymer electrolytes are compared in Figures. S8. The survey spectra shows (Fig. S8a and Fig. S8b) 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 chargedischarge cycles with PVDF-HFP containing 4M LiFSI-[Py1,4]FSI is shown in Fig. 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 (Fig. 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 Fig S9c show peaks related to the formation of LiOx, Li2S/LiF, which seems to be identical with the ones with lower LiFSI concentration. On addition of AN to 4M LiFSI[Py1,4]FSI, the XPS spectra of Ge shows a loweing of GeO2 component (fig S9d). This is further substantiated from the O 1s detailed spectra in Fig. S10. The O1s spectra of Ge cycled with low concentration of LiFSI (1M) and with 4M LiFSI polymer electrolyte shows the formation of LiOx/LiOH, Li2CO3 and GeO2 (Fig S10a, S10b, S10c, S10d). At higher LiFSI concentration with acetonitrile (Fig S10d), a prominent peak of LiOx is observed whereas Li2CO3 disappears. This 19 ACS Paragon Plus Environment


suggests that the composition of the SEI layer has changed on using 4M LiFSI and acetonitrile to the polymer. Fig. S9e shows the detailed spectra of F 1s wherein peaks related to LiF and FSI anion are observed. However, with sputtering, it is clear that the F1s related to the ionic liquid considerably decreases and a strong peak of LiF is seen. The Li 1s in Fig. S9f shows the formation of LiOx, Li2S, and LiF. Thus, comparing the AFM and XPS (figures 4-8 and S7 to S10), it appears that the SEI layer differs considerably at low (1M) and at high concentration (4M) 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 on the addition of acetonitrile to the polymer electrolyte. Furthermore, it appears that 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 Fig. 3. Thus, based on the post analysis of the Ge electrode after cycling using gel polymer electrolytes (GPEs), we can say that the addition of 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 analayis) 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. 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 increase in concentration from 1M to 4M and on 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 IL-polymer electrolyte with 4M LiFSI and acetonitrile, an increased Li storage capacity in Ge has been obtained compared to IL-polymer electrolyte with lower content of LiFSI (eg. 1M). AFM results showed that the mechanical properties as well as SEI layer thickness change on increasing the LiFSI concentration and on 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 20 ACS Paragon Plus Environment

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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 ionic liquid-organic solvent mixture can be considered as a potential and safe (solid state) electrolyte for lithium ion batteries. Acknowledgement The authors would like to thank Mrs. Karin Bode, Institute of Inorganic Chemistry (Prof. A. Adam) for help with Raman and IR measurements. The authors would also like to thank Mrs. Ulrike Koecher, Institute of Technical Chemistry, for TGA measurements. Supporting Information Supporting information contains Raman, IR, AFM and XPS spectra References 1. Yamada, Y.; Yamada, A. Review—Superconcentrated Electrolytes for Lithium Batteries. J. Electrochem. Soc. 2015, 162, A2406-A2423. 2. Qian, J.; Henderson, W. A.; Xu, W.; Bhattacharya, P.; Engelhard, M.; Borodin, O.; Zhang, J.-G. High Rate and Stable Cycling of Lithium Metal Anode. Nature Commun. 2015, 6, 6362. 3. Yamada, Y.; Furukawa, K.; Sodeyama, K.; Kikuchi, K.; Yaegashi, M.; Tateyama, Y.; Yamada, A. Unusual Stability of Acetonitrile-Based Superconcentrated Electrolytes for Fast-Charging Lithium-Ion Batteries. J. Am. Chem. Soc. 2014, 136, 5039–5046. 4. Yoon, H.; Best, A. S.; Forsyth, M.; MacFarlane, D. R.; Howlett, P. C. Physical Properties of High Li-ion content N-Propyl-N-Methylpyrrolidinium Bis(fluorosulfonyl)imide Based Ionic Liquid Electrolytes. Phys. Chem. Chem. Phys. 2015, 17, 4656–4663. 5. Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587–603. 6. Fergus, J. W. Ceramic and Polymeric Solid Electrolytes for Lithium-Ion Batteries. J. Power Sources 2010, 195, 4554–4569. 21 ACS Paragon Plus Environment


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