Ionic Liquid Interphase: Chemical

Subscriber access provided by UNIV OF LOUISIANA

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Investigation of the Electrode/Ionic Liquid Interphase: Chemical Reactions of an Ionic Liquid and a Lithium Salt with Lithiated Graphite Probed by X-ray Photoelectron Spectroscopy Zhen Liu, Guozhu Li, Andriy Borodin, Xiaoxu Liu, Yao Li, and Frank Endres J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11549 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Investigation of the Electrode/Ionic Liquid Interphase: Chemical Reactions of an Ionic Liquid and a Lithium Salt with Lithiated Graphite Probed by X-ray Photoelectron Spectroscopy

Zhen Liu,1, * Guozhu Li,1 Andriy Borodin1, Xiaoxu Liu,2 Yao Li3,* and Frank Endres1,* 1Institute

of

Electrochemistry,

Clausthal

University

of

Technology,

Arnold-Sommerfeld-Strasse 6, 38678 Clausthal-Zellerfeld, Germany 2School

of Material Science and Engineering, Shanxi University of Science and

Technology, Xi’an, 710021, China 3Center

for Composite Materials and Structure, Harbin Institute of Technology,

Harbin, China E-mail: [email protected], [email protected], [email protected] ABSTRACT. The solid electrolyte interphase (SEI) plays an important role in the performance of a Li-ion battery. A detailed insight into the chemical reactions between the electrolyte and lithium at the electrode/electrolyte interphase is thus of great interest. In the present paper, two different sequences of stepwise nanometer-thick depositions of the ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide ([EMIm]FSI) and of lithium bis(fluorosulfonyl)imide (LiFSI) on Li-intercalated highly oriented pyrolytic graphite (HOPG) were employed to examine the decomposition products of [EMIm]+ cations and FSI− anions and their dependence on layer thicknesses and the annealing temperature by X-ray 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

photoelectron spectroscopy. The results indicate that the interaction of an IL adlayer with lithiated graphite leads to a stable passivation layer, which inhibits the deintercalation of Li. The passivation layer is composed of the decomposed products from both the [EMIm]+ cation and the FSI− anion, such as methyl-, ethyl-imidazole, LiF, Li2O, LiNSO2 and LiSO2F. Our work can provide valuable information on designing a suitable electrolyte to get a stable SEI layer for battery applications.

Introduction Ionic

liquids

with

the

bis(fluorosulfonyl)imide

(FSI−)

and

the

Bis(trifluoromethylsulfonyl)imide (TFSI−) anions have been reported as promising electrolytes for lithium-ion batteries (LIBs) due to their good electrochemical and physical properties such as low viscosity, high conductivity,

high chemical and

thermal stability, low corrosivity and large electrochemical windows.1-4 In addition, these ionic liquids allow highly reversible deposition-stripping of lithium at graphite electrodes.5-6 Yamagata et al. showed that a reversible charge–discharge behavior of a graphite electrode in an 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide ([EMIm]FSI) ionic liquid was achieved.7 lithium

bis(trifluoromethylsulfonyl)imide

Lahiri et al. reported the combination of and

1-butyl-1-methylpyrrolidinium

bis(fluorosulfonyl)imide (LiTFSI/[Py1,4]FSI) as a stable electrolyte for LIBs.8 The formation of a stable solid electrolyte interphase at the electrolyte/electrode interface was critical for the performance of LIBs.9-11 Some strategies including varying the concentration of the electrolyte,12-13 optimizing the structure of the IL,14-15 and the use 2


Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of additives16-18 have been reported in achieving a stable SEI and improving the performance of the battery. However, a detailed investigation of the composition and properties of the SEI remain rather limited. Basile et al. reported the interaction of lithium with 1-propyl-3-methylpyrrolidinium bis(fluorosulfonyl)imide ([Py1,3]FSI) containing different lithium salts using FTIR and XPS. The lithium salts have a great influence on the mechanism of SEI formation and on the composition of the SEI.9 The chemical reactions of lithium and [Py1,3]FSI were modeled using ab initio molecular dynamics simulations by Budi et al. and the possible breakdown products after days of exposure were also identified by XPS.19 We have previously studied the chemical reactions

of

1-octyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)

imide

([OMIm]TFSI) and of lithium with Si(111) surfaces on a molecular level by XPS. We can not only identify the possible breakdown products between an IL and a Li surface, but also follow their formation processes.20 Recently, the interaction of an IL monolayer

(1-butyl-1-methyl-pyrrolidinium

bis(trifluoromethylsulfonyl)imide,

[Py1,4]TFSI) with lithium on pristine and lithiated graphite and the intercalation and deintercalation of lithium at the IL-Graphite interface was studied by XPS and UPS.21-22 However, there is a dramatic difference in the composition of the SEI upon the addition of Li salts to the IL, which has significant implications for the performance of a battery. Aiming at a detailed atomic-scale understanding of the processes at the electrolyte/electrode interface, in the present paper, we focus on the interaction of the ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide ([EMIm]FSI) and 3



Page 4 of 28

of lithium bis(fluorosulfonyl)imide (LiFSI) with lithiated graphite surface by XPS. We have investigated their chemical reactions by stepwise evaporation of an IL and a lithium salt on lithiated graphite and monitored their decomposition process as well as the composition of the passivation layer by XPS.

Experimental section The ionic liquid [EMIm]FSI was obtained in the highest available quality from Io-Li-Tec (Germany) and was used after drying under vacuum at 100 °C to achieve a water content of below 2 ppm. The water content was determined by Karl-Fischer Titration. LiFSI (99.9%) was purchased from Sigma Aldrich. HOPG was purchased from SPI (ZYA, 10 mm × 10 mm × 1 mm), and it was initially treated using adhesive tape, fixed on a molybdenum sample plate with silver conductive paste, transferred into the UHV system and subsequently annealed at 600 K for at least 30 mins to remove residual surface contaminants. H-terminated silicon (H-Si(111)) was prepared as described in a previous paper.20 The H-Si(111) substrate was annealed at 700 K for 30 mins before using. X-ray Photoelectron Spectroscopy (XPS) measurements were performed with a Specs Phoibos 150 hemispherical analyzer using a Specs XR50 M monochromatic Al Kα source (1486.74 eV) with a base pressure of below 5 × 10−10 bar. The electron energy analyzer was used in a high energy resolution mode, with a pass energy (Epass) of 20 eV. Thin layers of [EMIm]FSI and of LiFSI (several nanometers) were produced by evaporation at 180 °C and 210 °C, respectively, under UHV in the 4


Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sample preparation chamber using a TCE-BSC (Kentax GmbH, Germany) evaporator. Lithium evaporation was performed by heating a lithium-metal alloy (AlfaVakuo AS-Li-010-3C) which is stored in a stainless steel tube at a current of 7.5 A in the sample preparation chamber with a pressure of ~5 × 10−9 bar. All spectra are displayed without scaling and shifting. Molybdenum was used as sample holder for the analysis. The definition of a monolayer (ML) is calibrated on H-Si(111) substrate, which was validated by XPS intensity loss of the Si 2p peak after deposition. The thickness of the film has been roughly estimated according to the description by Cremer et al.23 The thin films of the ILs sometimes form islands rather than a homogeneous film upon increasing the amount of ILs. This behavior is typical for ILs on many surfaces. Therefore, the amount of the IL and LiFSI on the surface was characterized by monolayers equivalent (MLE). It was calculated that one MLE of [EMIm]FSI was produced by evaporation at 180 °C for 6 mins and one MLE of LiFSI was produced by evaporation at 210 °C for 10 mins. One MLE of Li with a thickness of ~ 0.3 nm was obtained at a current of 7.5 A for 2 min (at a temperature of ~900 K).

Results and discussion It was reported that HOPG can be exfoliated into graphene by ILs at room temperature,24 while H-terminated Si(111) retained its structure after the treatment with ILs.25 Therefore, in order to better calculate the thickness of the thin films and to assign the peaks, a monolayer of [EMIm]FSI and of LiFSI were first vapor deposited 5



Page 6 of 28

on H-Si(111), respectively. Figure 1 shows the XP spectra of H-Si(111) and of thin films of [EMIm]FSI on H-Si(111). The freshly etched H-terminated Si(111) surface was quite clean and the XP spectrum exhibits two main peaks at 99.5 and 151.1 eV, which are assigned to Si 2p and Si 2s, respectively (Figure 1 red curves). The XP spectra of [EMIm]FSI (∼3 monolayers equivalent (MLE)) on a H-terminated Si(111) surface are also shown in Figure 1, black curves. In the F 1s region, the peak at a binding energy (BE) of 688.0 eV is assigned to fluorine in S-F bonds of the FSI− anion. The oxygen exhibits one signal at a BE of 533.1 eV. In the N 1s region, two peaks were observed. The peak at a BE of 402.3 eV originates from the imidazolium nitrogen atoms (Ncation) and the peak at a BE of 400.0 eV results from the FSI− nitrogen atoms (Nanion). The Nanion 1s peak shows an asymmetry toward lower binding energies, probably due to the formation of hydrogen bonds with H-Si (111) surface. The carbon signals at 287.0 and 284.9 eV are assigned to carbon atoms in the imidazolium ring and to carbons in the alkyl chain, respectively. The carbon atoms connected with nitrogen atoms appear at high binding energy. These assignments are in good agreement with previous reports.26

6


Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. XP spectra of [EMIm]FSI on H-terminated Si (111) obtained by vapor deposition of [EMIm]FSI with a thickness of ~ 3 nm under an ultra-high vacuum (black). Red curve: XPS of H-Si(111).

The vapor deposition of intact LiFSI (~2 nm MLE) on a H-Si(111) surface is also evidenced by XPS, shown in Figure 2. The F 1s, O 1s and N 1s peaks, originating from the FSI− anions, have the same positions as those in [EMIm]FSI. The S 2p peak overlaps with Si 2s, which can not be distinguished. In addition, Li 1s was observed at a BE of 56.3 eV. Fig. 2 also shows a large reduction of silicon peaks when ~2 nm LiFSI is deposited on H-Si(111), while the silicon peaks reduce much less in Fig. 1 even with nominally ~3-nm IL deposited on H-Si(111). It seems that the IL does not completely wetting the silicon surface and rather forms islands. 7



Page 8 of 28

Figure 2. XP spectra of LiFSI on H-terminated Si (111) obtained by vapor deposition of LiFSI with a thickness of ~ 2 nm under an ultra-high vacuum.

We also characterized the interaction of Li with pristine HOPG by vapor deposition of lithium onto HOPG in an ultra-high vacuum system. Figure 3 shows the C 1s and Li 1s XP spectra of HOPG and of the lithium-intercalated HOPG. The C 1s peak of the lithium-intercalated HOPG in Figure 3 left panel reveals a shift by 0.3 eV to a higher BE compared to that of HOPG. Previous XPS studies on the interaction of lithium with HOPG indicated that the binding energy of the C 1s shifts due to charge transfer from lithium to the HOPG and the core level of C 1s in the lithium-intercalated HOPG is higher than that of HOPG.21, 27 It was reported that the small density of states in graphite around the Dirac point results in a considerable 8


Page 9 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


upshift of the Fermi energy with respect to the band structure, leading to a corresponding upshift of the binding energy of the C 1s core level.21,

28

Two main

peaks are observed in the Li 1s region as shown in Figure 3, right panel. The peak at ~56.4 eV is assigned to Li2O surface contaminations. The second Li 1s peak at 55.4 eV was attributed to intercalated Li.

Figure 3. XP spectra of the HOPG and of the lithium-intercalated HOPG after vapor deposition of 1, 2 and 3 MLE of Li in the C 1s and Li 1s regimes.

Interaction of [EMIm]FSI and of LiFSI with Li intercalated graphite In the first step, we studied the interaction of Li with [EMIm]FSI and with LiFSI on a HOPG surface by stepwise deposition of i) Li on HOPG, ii) an [EMIm]FSI monolayer on lithiated HOPG, iii) a LiFSI ultrathin layer on the top and iv) heating 9



Page 10 of 28

the sample. XP spectra were recorded after each deposition process and the representative spectra are shown in Figures 4 and 5. We first evaporated ~4 MLE of Li on HOPG. The XP spectra of Li on HOPG (Li/G) in the C 1s and in the Li 1s regimes are shown on the top of the panel. The interaction of an [EMIm]FSI ultrathin layer with the lithium-intercalated HOPG was monitored during the stepwise deposition of [EMIm]FSI. The C 1s, Li 1s and N 1s spectra are shown in Figure 4. The intensity of the graphite C 1s peak decreases upon increasing the amount of IL as a result of forming a passivation layer on HOPG. The small C 1s peaks at 286-287 eV are attributed to the Chetero and Calkyl of the imidazolium cation.22 In the Li 1s region, a peak shoulder is observed at ~ 56.7 eV with the deposition of 1 MLE of the IL (blue curve), which is located at a higher BE than that of metallic Li (55.1 eV) due to the reaction of Li with the IL. Upon further increasing the amount of the IL, the peak (at 56.7 eV) shifts to a higher BE and gradually increases its intensity. This process is accompanied by a decrease in the Li 1s peak at 55.1 eV. Finally, a broad peak at 58.1 eV was recorded at 3 MLE of [EMIm]FSI and the metallic Li 1s peak can hardly be seen. This broad peak seems to result from lithium compounds such as LiF, Li2O, LiNSO2 and LiSO2F. The N 1s spectra in Figure 4 are rather complicated as they include informations of the cation and the anion as well as their decomposition products. Our previous XPS results20 and recent simulations 29-31 show that both the imidazolium cation and the FSI− anion can be decomposed. At the very beginning, with the presence of 1 MLE of the IL, the N 1s spectrum shows a featureless curve. By increasing the amount of the IL, a peak at 10


Page 11 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


401.5 eV starts to grow, which can be assigned to the cation decomposition into methyl- or ethyl-imidazole or the formation of a dimer by the interaction of two radicals as will be shown in the mechanism below. The decomposition of the FSI− anion to NSO2− also contributes to the N 1s peak at 401.5 eV. At 3 MLE of the IL, in addition to the peak at 401.5 eV, the spectrum shows peaks at 404.5 and 402.5 eV, which are related to the N atoms in the [EMIm]+ cation and in the FSI− anion, respectively. The F 1s, O 1s and S 2p spectra are shown in Figure 5. The F1s spectra show two peaks. The peak located at 687.5 eV is assigned to LiF and the peak at 690.8 eV is due to the FSI− anion. With less than 3 MLE of the IL, only LiF peaks were observed, which indicated that a defluorination of the FSI− anion occurs. In the O 1s region, at 1 MLE of the IL, a peak at 531.0 eV is found, which is attributed to LiO/LiOH.32 At 1.5 and 2 MLE of the IL, two peaks were observed. The peak at 533.6 eV may originate from NSO2− and LiSO2F compounds, formed by N−S bond cleavage. At 3 MLE of the IL, the peak shoulder at 535.7 eV is assigned to O 1s of the FSI− anions. Similar to the O 1s peak, four S 2p peaks are observed by stepwise deposition of the IL. The peaks at 162.7, 169.7, 171.4 and 173.0 eV are attributed to Li2S, NSO2− and LiSO2F compounds, Li coordinated FSI− and free FSI− anions, respectively.

However, all

peaks resulting FSI− anion are clearly shifted by ~2.6 eV toward higher BEs compared to those of neat [EMIm]FSI on H-Si(111) surface. Buchner et al. also found that both the Ncation and Nanion peaks of [Py1,4]TFSI shift by ∼1.5 eV to higher BEs upon postdeposition of 0.3 ML of Li on the IL covered HOPG.21 The formation of a 11



Page 12 of 28

passivation layer on the surface, formed by the reaction of the IL with Li also contributes to the shifts. In addition, the presence of Liδ+ is responsible for the decrease of the work function Φ and the observed BE shifts of the IL related states. 21 We evaporated ultrathin layers of LiFSI on the top of IL+Li/G surface and the XP spectra are shown in Figure 4 and Figure 5. In the second step, we have shown that intact molecularly adsorbed [EMIm]FSI ion pairs were present on the surface with 3 MLE of IL, indicating that lithium at the surface was consumed. Therefore, upon the deposition of LiFSI on the IL covered HOPG surface, the XP spectra of the Li 1s and the peaks resulting from the FSI− anions, i.e. N 1s, F 1s, O 1s and S 2p, do not shift their position but increase their intensities by increasing the loading of LiFSI. However, the C 1s and the peaks originating from the Li compounds such as LiF (687.5 eV), LiOH (531 eV), Li2S (162.7), LiNSO2 and LiSO2F (169.7) decrease their intensities as a result of the coverage of LiFSI layers on the surface. We focus on the influence of temperature on the XP spectra in the C 1s, Li 1s, N 1s, F 1s, O 1s and S 2p regimes. Upon heating, lithium will be released from the bulk and further react with IL and LiFSI at the surface. Obvious changes were observed upon heating as shown in Figures 4 and 5, bottom. The intensity of C 1s further decreases but the Li 1s peak increases, which indicates that more Li reacted with the IL or LiFSI. Upon further increasing the temperature to 373 K, the F 1s peak at 690.8 eV gradually lost its intensity and the peak at 687.5 eV (LiF) increases its intensity. The possible reactions could be: Li + FO2SNSO2F− → LiF + ·O2SNSO2F− 12


Page 13 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In addition, the N 1s, O 1s and S 2p of the FSI− anions downshift by ~1.1 eV to lower BEs, respectively. Li+ ions form complexes with the FSI− anions by coordination with O atoms upon dissolving of LiFSI in FSI− containing IL.33-34 The deposition of LiFSI on an [EMIM]FSI covered surface also leads to the formation of Li[FSI]x(x-1)− complexes. Previous Raman results clearly showed that two Raman peaks were observed by dissolving of metal salts (with FSI, TFSI or TFO anions) in ILs containing the same anions in the vibrational range of the anions.8, 35 The peak at a higher BE is due to metal coordinated anions and the peak at a lower BE is assigned to free anions. For example, the Raman spectrum of [EMIm]FSI shows a peak at 731 cm−1, which can be assigned to the νs(N−S) vibration of free FSI− anion. For LiFSI in [EMIm]FSI, a new peak at 744 cm−1 appears, which is assigned to FSI− anion bound to Li+ and the peak increases its intensity with the increase of the concentration of LiFSI.36 When the sample (LiFSI/[EMIm]FSI on Li/HOPG) is heated, a deintercalation of Li occurs, which alters the coordination environments of Li with FSI− anions. Therefore, the XP spectra shift upon heating.

13



Page 14 of 28

Figure 4. XP spectra of the HOPG after the deposition of various ultrathin layer equivalent (MLE) of i) Li, ii) [EMIm]FSI and iii) LiFSI and at different temperatures in the C 1s, Li 1s and N 1s regimes, respectively.

14


Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. XP spectra of the HOPG with the deposition of various monolayer equivalent (MEL) of Li, [EMIm]FSI and LiFSI and at different temperatures in the F 1s, O 1s and S 2p regime, respectively.

As seen, both the [EMIm]+ cation and the FSI− anion reacted with Li. The decomposition strongly depends on the ratios between Li, [EMIm]+ cation and FSI− anion. Therefore, in the second step, the interaction of Li with [EMIm]FSI and with LiFSI on a HOPG surface was studied in a reversal procedure by stepwise deposition of i) Li on HOPG, ii) a LiFSI ultrathin layer on lithiated HOPG, iii) an [EMIm]FSI ultrathin layer on the top and iv) heating the sample. The XPS spectra are shown in Figures 6 and 7. Each time we use the same HOPG substrate and it was treated using adhesive tape. The C 1s shoulder in Fig. 6 may result from C-hetero or contaminations on the surface of HOPG due to continuous exfoliation. However, after 15



Page 16 of 28

the deposition of 4 monolayer equivalent Li, the peak shoulder almost disappeared and showed very similar curves (Figs. 4 and 6). The following experiments of the deposition of [EMIm]FSI and LiFSi on Li covered HOPG should not be influenced by this C 1s shoulder. In Figure 6, broad Li 1s peaks centered at 58 and 56.7 eV are observed and their intensities increase upon increasing the amount of LiFSI, accompanied by the expense of the metallic Li 1s peak at 55.1 eV. The deposition of 1.5 MLE LiFSI on lithium intercalated HOPG results in a complete breakdown of the FSI− anion and no intact LiFSI ion pair could be observed. The decomposition of the FSI− anion occurs via a defluorination reaction and the cleavage of the N−S and S=O bonds. However, the decomposition mechanism of the FSI− anion in LiFSI and in [EMIm]FSI are different as evidenced by comparing the F 1s, O 1s and S 2p peaks in Figures 5 and 7. In the F 1s region (Figure 7 left panel), the F 1s peak shifts by 0.3 eV from 687.3 to 687.6 eV upon the increase of LiFSI from 0.3 MLE to 1.5 MLE. In the O 1s region, the peak shifts by 0.4 eV from 530.5 to 530.9 eV, which is assigned to LiO or LiOH and the peak shifts by 1 eV from 534.1 to 535.1 eV, which may originate from intermediates or radical ions, such as ·SO2F and ·NSO2. However, all these peaks (F 1s and O 1s) are located at the same positions in Figure 5 top of the panel in the case of the deposition of [EMIm]FSI on lithium intercalated HOPG. In the S 2p regions in Figure 7, four peaks were observed. The peaks at 171.4, 169.5 and 162.8 eV have similar positions as those of [EMIm]FSI on lithium intercalated HOPG in Figure 5. The S 2p peak at 172.7 eV (Figure 7) has a lower BE than of free FSI− anion which is located at 173 eV (Figure 5). 16


Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


After vapor deposition of an [EMIm]FSI monolayer on the top surface, obviously two peaks centered at 404.6 and 402.3 eV are observed in the XP spectra in the N 1s region (Figure 6), which are attributed to the nitrogen atoms of the [EMIm]+ cation and of the FSI− anion. In Figure 7, in the O 1s region, only two peaks are recorded, one centered at 534.8 eV and the other one centered at 530.5 eV. The formation of Li[FSI]x(x-1)− complexes is responsible for the O 1s peak at 534.8 eV, while the O 1s peak of free FSI− anion is located at 535 eV. Upon increasing temperature, the N 1s peaks shift to 400.7 and to 399 eV, indicating a breakdown of the [EMIm]+ cation. In the first procedure (the deposition of LiFSI on IL covered Li/G), a passivation layer formed on the surface and intact LiFSI and IL ion pairs can be found on the top as evidenced by the F1s, O 1s and S 2p peaks in Figure 5. Both the [EMIm]+ cation and the FSI− anion of the IL interact with Li and form a stable passivation layer on the surface, which inhibits the deintercalation of Li from HOPG. However, in the reversal procedure (the deposition of IL on LiFSI covered Li/G), no intact ion pairs were found. Lithium compounds (Li2O, Li2S, and LiF) and organic radical ions (·SO2F and ·NSO2) were formed on the surface upon the deposition of LiFSI on Li/G. These products continue to react with further deposited IL. At the same time, the deintercalation of lithium from HOPG occurs, changing the coordinating environments of the FSI− anion. These results indicate that the passivation layer formed by the reaction of Li with LiFSI is not stable.

17



Page 18 of 28

Figure 6. XP spectra of the HOPG after the deposition of various monolayer equivalent (MLE) of i) Li, ii) LiFSI and iii) [EMIm]FSI and at different temperatures in the C 1s, Li 1s and N 1s regimes, respectively.

18


Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. XP spectra of the HOPG after the deposition of various monolayer equivalent (MLE) of i) Li, ii) LiFSI and iii) [EMIm]FSI and at different temperatures in the F 1s, O 1s and S 2p regimes, respectively.

19



Page 20 of 28

Figure 8. Reaction mechanism of Li with [EMIm]+ cation and FSI− anion in an ultra-high vacuum on HOPG (see also the references29-30 ).

We have found experimental evidence to support the previous proposed reaction mechanisms,29-30 which is summarized in Figure 8. The [EMIm]+ cation decomposes into methyl-imidazole, ethyl-imidazole or an EMIm radical upon the interaction with Li. Ring opening reactions could also occur at the very beginning with an excess amount of Li. The interaction of two EMIm radicals leads to the formation of a dimer, a disproportionation reaction, and cage-like structure. The decomposition of the FSI− 20


Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


anion by Li is rather complicated. The reactions lead to defluorination, N−S and S=O bond cleavage. Several intermediates and radicals could be also formed. Furthermore, the FSI− anions in LiFSI and in [EMIm]FSI have different reaction mechanisms with Li, as evidenced by the XP spectra in Figures 5 and 7. The consumption of Li by [EMIm]+ cations and the formation of a passivation layer reduce the relative ratios of Li to FSI− anions, resulting in a relatively gentle breakdown of the FSI− anions (pathway a and b). However, the exposure of LiFSI to Li leads to a deep decomposition (pathway c and d).

Conclusion In order to get a better understanding on the electrode/electrolyte interphase in Li-ion batteries, we have investigated the interaction of monolayers of the IL [EMIm]FSI and of LiFSI with Li on HOPG. Their chemical reactions and the composition of the passivation layer were monitored by XPS. Li was first vapor deposited on HOPG. The intercalation of Li into HOPG was evidenced by the shifts of the Fermi energy of the C 1s peak. A metallic Li 1s peak was also found on HOPG by the deposition of 3 monolayer equivalent of Li (less than 1 nm thick). Monolayers of [EMIm]FSI and of LiFSI were stepwise deposited on lithiated HOPG, respectively. The decomposition of both the [EMIm]+ cation and of the FSI− anion by Li was monitored by XPS. For the FSI− anion, the reactions undergo defluorination, N−S and S=O bond cleavage. For the [EMIm]+ cation, the methyl or the ethyl group can be detached by lithium, formation of methyl-, ethyl-imidazole or radicals. The deposition 21



Page 22 of 28

procedure (evaporation of [EMIm]FSI on LiFSI covered Li/HOPG or of LiFSI on [EMIm]FSI covered Li/HOPG) has a great influence on the reaction mechanism and the composition of the passivation layer. The results also indicated that Li can be stabilized when an IL adlayer is present at the interface. In addition, the formation of a stable interfacial layer by the reaction of IL with Li can inhibit the decomposition of LiFSI on the top. Acknowledgment We would like to thank a joint project funded by National Natural Science Foundation of China (NSFC) and Deutsche Forschungsgemeinschaft (DFG) (EN 370/28-1) and the National Natural Science Foundation of China (No.51761135123) References 1.

Matsumoto, H.; Sakaebe, H.; Tatsumi, K.; Kikuta, M.; Ishiko, E.; Kono, M. Fast

Cycling of Li/LiCoO2 Cell with Low-Viscosity Ionic Liquids Based on Bis(fluorosulfonyl)imide [FSI]−. J. Power Sources 2006, 160, 1308-1313. 2.

Kühnel, R.-S.; Lübke, M.; Winter, M.; Passerini, S.; Balducci, A. Suppression of

Aluminum Current Collector Corrosion in Ionic Liquid Containing Electrolytes. J. Power Sources 2012, 214, 178-184. 3.

Menne, S.; Vogl, T.; Balducci, A. The Synthesis and Electrochemical

Characterization of Bis(fluorosulfonyl)imide-Based Protic Ionic Liquids. Chem. Commun. 2015, 51, 3656-3659. 4.

Ishikawa, M.; Sugimoto, T.; Kikuta, M.; Ishiko, E.; Kono, M. Pure Ionic Liquid

Electrolytes Compatible with a Graphitized Carbon Negative Electrode in 22


Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Rechargeable Lithium-ion Batteries. J. Power Sources 2006, 162, 658-662. 5.

Best, A. S.; Bhatt, A. I.; Hollenkamp, A. F. Ionic Liquids with the

Bis(fluorosulfonyl)imide Anion: Electrochemical Properties and Applications in Battery Technology. J. Electrochem. Soc. 2010, 157, A903-A911. 6.

Towada, J.; Karouji, T.; Sato, H.; Kadoma, Y.; Shimada, K.; Ui, K. Charge–

Discharge

Characteristics

of

Natural

Graphite

Electrode

in

N,

N-diethyl-N-Methyl-N-(2-methoxyethyl)ammonium Bis(trifluoromethylsulfonyl)amide Containing Lithium Ion for Lithium-Ion Secondary Batteries. J. Power Sources 2015, 275, 50-54. 7.

Yamagata, M.; Matsui, Y.; Sugimoto, T.; Kikuta, M.; Higashizaki, T.; Kono, M.;

Ishikawa,

M.

High-Performance

Graphite

Negative

Electrode

in

a

Bis(fluorosulfonyl)imide-based Ionic Liquid. J. Power Sources 2013, 227, 60-64. 8.

Lahiri,

A.;

Schubert,

T.

J.

S.;

Iliev,

B.;

Endres,

F.

LiTFSI

in

1-butyl-1-methylpyrrolidinium Bis(fluorosulfonyl)amide: A Possible Electrolyte for Ionic Liquid Based Lithium Ion Batteries. Phys. Chem. Chem. Phys. 2015, 17, 11161-11164. 9.

Basile, A.; Bhatt, A. I.; O’Mullane, A. P. Stabilizing Lithium Metal Using Ionic

Liquids for Long-Lived Batteries. Nature Commun. 2016, 7, ncomms11794. 10. Grande, L.; von Zamory, J.; Koch, S. L.; Kalhoff, J.; Paillard, E.; Passerini, S. Homogeneous Lithium Electrodeposition with Pyrrolidinium-based Ionic Liquid Electrolytes. ACS Appl. Mater. Interfaces 2015, 7, 5950-5958. 11. Nádherná,

M.;

Reiter,

J.;

Moškon,

J.;

Dominko,

R.

Lithium 23



Page 24 of 28

Bis(fluorosulfonyl)imide–Pyr14TFSI Ionic Liquid Electrolyte Compatible with Graphite. J. Power Sources 2011, 196, 7700-7706. 12. Liu, C.; Ma, X.; Xu, F.; Zheng, L.; Zhang, H,; Feng, W,; Huang, X,; Armand, M.; Nie,

J.;

Chen,

H.;

et

al.

Ionic

Liquid

Bis(fluorosulfonyl)imide/N-methyl-N-propylpiperidinium

Electrolyte

of

Lithium

Bis(fluorosulfonyl)imide

for Li/Natural Graphite Cells: Effect of Concentration of Lithium Salt on the Physicochemical and Electrochemical Properties. Electrochim. Acta 2014, 149, 370-385. 13. Zhang, H.; Qu, W.; Chen, N.; Huang, Y.; Li, L.; Wu, F.; Chen, R. Ionic Liquid Electrolyte with Highly Concentrated LiTFSI for Lithium Metal Batteries. Electrochim. Acta 2018, 285, 78-85. 14. Al-Masri, D.; Yunis, R.; Hollenkamp, A. F.; Pringle, J. M. A Symmetrical Ionic Liquid/Li Salt System for Rapid Ion Transport and Stable Lithium Electrochemistry. Chem. Commun. 2018, 54, 3660-3663. 15. Salem, N.; Zavorine, S.; Nucciarone, D.; Whitbread, K.; Moser, M.; Abu-Lebdeh, Y. Physical and Electrochemical Properties of Some Phosphonium-Based Ionic Liquids and the Performance of Their Electrolytes in Lithium-Ion Batteries. J. Electrochem. Soc. 2017, 164, H5202-H5209. 16. Yoo, D.-J.; Kim, K. J.; Choi, J. W. The Synergistic Effect of Cation and Anion of an Ionic Liquid Additive for Lithium Metal Anodes. Adv. Energy Mater. 2018, 8, 1702744. 17. Yang, B.; Li, C.; Zhou, J.; Liu, J.; Zhang, Q. Pyrrolidinium-based Ionic Liquid 24


Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electrolyte with Organic Additive and LiTFSI for High-Safety Lithium-ion Batteries. Electrochim. Acta 2014, 148, 39-45. 18. Deshpande, A.; Kariyawasam, L.; Dutta, P.; Banerjee, S. Enhancement of Lithium Ion Mobility in Ionic Liquid Electrolytes in Presence of Additives. J. Phys. Chem. C 2013, 117, 25343-25351. 19. Budi, A.; Basile, A.; Opletal, G.; Hollenkamp, A. F.; Best, A. S.; Rees, R. J.; Bhatt, A. I.; O’Mullane, A. P.; Russo, S. P. Study of the Initial Stage of Solid Electrolyte Interphase Formation Upon Chemical Reaction of Lithium Metal and N-methyl-N-propyl-pyrrolidinium-Bis(fluorosulfonyl)imide. J. Phys. Chem. C 2012, 116, 19789-19797. 20. Liu, Z.; Li, G.; Borodin, A.; Liu, X.; Li, Y.; Endres, F. Interactions between Lithium, an Ionic Liquid, and Si(111) Surfaces Studied by X-Ray Photoelectron Spectroscopy. J. Phys. Chem. Lett. 2018, 9, 4673-4678. 21. Buchner, F.; Forster-Tonigold, K.; Kim, J.; Adler, C.; Bansmann, J.; Groß, A.; Behm, R. J. Experimental and Computational Study on the Interaction of an Ionic Liquid Monolayer with Lithium on Pristine and Lithiated Graphite. J. Phys. Chem. C 2018, 122, 18968-18981. 22. Buchner, F.; Kim, J.; Adler, C.; Bozorgchenani, M.; Bansmann, J.; Behm, R. J. Intercalation and Deintercalation of Lithium at the Ionic Liquid–Graphite(0001) Interface. J. Phys. Chem. Lett. 2017, 8, 5804-5809. 23. Cremer, T.; Stark, M.; Deyko, A.; Steinrück, H. P.; Maier, F. Liquid/Solid Interface of Ultrathin Ionic Liquid Films: [C1C1im][Tf2N] and [C8C1im][Tf2N] on 25



Page 26 of 28

Au(111). Langmuir 2011, 27, 3662-3671. 24. Elbourne, A.; McLean, B.; Voïtchovsky, K.; Warr, G. G.; Atkin, R. Molecular Resolution in Situ Imaging of Spontaneous Graphene Exfoliation. J. Phys. Chem. Lett. 2016, 7, 3118-3122. 25. Watanabe, K.; Maruyama, S.; Matsumoto, Y. P-Si(111):H/Ionic Liquid Interface Investigated through a Combination of Electrochemical Measurements and Reflection High Energy Electron Diffraction Surface Analysis in Vacuum. Chem. Phys. Lett. 2016, 655-656, 6-10. 26. Olschewski, M.; Gustus, R.; Höfft, O.; Lahiri, A.; Endres, F. Monochromatic X-Ray Photoelectron Spectroscopy Study of Three Different Ionic Liquids in Interaction with Lithium-Decorated Copper Surfaces. J. Phys. Chem. C 2017, 121, 2675-2682. 27. Momose, H.; Honbo, H.; Takeuchi, S.; Nishimura, K.; Horiba, T.; Muranaka, Y.; Kozono, Y.; Miyadera, H. X-Ray Photoelectron Spectroscopy Analyses of Lithium Intercalation and Alloying Reactions on Graphite Electrodes. J. Power Sources 1997, 68, 208-211. 28. Ulrike, A. S.; Petrović, M.; Gerber, T.; Martínez-Galera, A. J.; Grånäs, E; Arman, M. A.; Herbig, C.; Schnadt, J.; Kralj, M.; Jan Knudsen, J. Core Level Shifts of Intercalated Graphene. 2D Mater. 2017, 4, 015013. 29. Shkrob,

I.

A.;

Marin,

T.

W.;

Zhu,

Y.;

Abraham,

D.

P.

Why

Bis(fluorosulfonyl)imide Is a “Magic Anion” for Electrochemistry. J. Phys. Chem. C 2014, 118, 19661-19671. 26


Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30. Haskins, J. B.; Yildirim, H.; Bauschlicher, C. W.; Lawson, J. W. Decomposition of Ionic Liquids at Lithium Interfaces. 2. Gas Phase Computations. J. Phys. Chem. C 2017, 121, 28235-28248. 31. Kroon, M. C.; Buijs, W.; Peters, C. J.; Witkamp, G.-J. Decomposition of Ionic Liquids in Electrochemical Processing. Green Chem. 2006, 8, 241-245. 32. Dupin, J.-C.; Gonbeau, D.; Vinatier, P.; Levasseur, A. Systematic XPS Studies of Metal Oxides, Hydroxides and Peroxides. Phys. Chem. Chem. Phys. 2000, 2, 1319-1324. 33. Solano, C. J. F.; Jeremias, S.; Paillard, E.; Beljonne, D.; Lazzaroni, R. A Joint Theoretical/Experimental Study of the Structure, Dynamics, and Li+ Transport in Bis([tri]fluoro[methane]sulfonyl)imide TFSI-Based Ionic Liquids. J. Chem. Physics 2013, 139, 034502. 34. Kerner, M.; Plylahan, N.; Scheers, J.; Johansson, P. Ionic Liquid Based Lithium Battery Electrolytes: Fundamental Benefits of Utilising Both TFSI and FSI Anions? Phys. Chem. Chem. Phys. 2015, 17, 19569-19581. 35. Liu, Z.; Zein El Abedin, S.; Endres, F. Raman and FTIR Spectroscopic Studies of 1-ethyl-3-methylimidazolium Trifluoromethylsulfonate, Its Mixtures with Water and the Solvation of Zinc Ions. ChemPhysChem 2015, 16, 970-977. 36. Fujii, K.; Hamano, H.; Doi, H.; Song, X.; Tsuzuki, S.; Hayamizu, K.; Seki, S.; Kameda, Y.; Dokko, K.; Watanabe, M.; et al. Unusual Li+ Ion Solvation Structure in Bis(fluorosulfonyl)amide Based Ionic Liquid. J. Phys. Chem. C 2013, 117, 19314-19324. 27



Page 28 of 28

TOC GRAPHICS

28


Ionic Liquid Interphase: Chemical

Recommend Documents