Spectroscopic Characterization of the SEI Layer Formed on Lithium

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Spectroscopic Characterization of the SEI Layer Formed on Lithium Metal Electrodes in Phosphonium Bis(fluorosulfonyl)imide Ionic Liquid Electrolytes. Gaetan M.A Girard, Matthias Hilder, Nicolas Dupre, Dominique Guyomard, Donato Nucciarone, Kristina Whitbread, Serguei Zavorine, Michael Moser, Maria Forsyth, Douglas R. Macfarlane, and Patrick C. Howlett ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18183 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Applied Materials & Interfaces

Spectroscopic Characterization of the SEI Layer Formed on Lithium Metal Electrodes in Phosphonium Bis(fluorosulfonyl)imide Ionic Liquid Electrolytes. Gaetan M. A. Girarda, Matthias Hildera, Nicolas Dupreb, Dominique Guyomardb, Donato Nucciaronec, Kristina Whitbreadc, Serguei Zavorinec, Michael Moserc, Maria Forsytha, Douglas R. MacFarlaned and Patrick C. Howletta,* a

Institute for Frontier Materials (IFM), Deakin University, Waurn Ponds, Victoria 3216,

Australia. b

Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la

Houssinière, BP 32229, 44322 Nantes Cedex 3, France. c

Solvay Group, Niagara Falls, Canada L2H 6S5.

d

School of Chemistry, Monash University, Victoria 3800, Australia.

*E-mail: [email protected]

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KEYWORDS. FSI; ionic liquids; lithium; SEI layer; x-ray photoelectron spectroscopy


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ABSTRACT.

The chemical composition of the solid electrolyte interphase (SEI) layer formed on the surface of lithium metal electrodes cycled in phosphonium bis(fluorosulfonyl)imide ionic liquid (IL) electrolytes are characterised by magic angle spinning nuclear magnetic resonance (MAS NMR), X-ray photoelectron (XPS), Fourier-Transformed Infrared (FTIR) and electrochemical impedance (EIS) spectroscopy. A multi-phase layered structure is revealed which is shown to remain relatively unchanged during extended cycling (up to 250 cycles at 1.5 mA.cm-2, 3 mAh.cm-2, 50 ˚C). The main components detected by MAS NMR and XPS after several hundreds of cycles are LiF and breakdown products from the FSI- anion including Li2S. Similarities in chemical composition are observed in the case of the dilute (0.5 mol.kg-1 of Li salt in IL) and the highly concentrated (3.8 mol.kg-1 of Li salt in IL) electrolyte during cycling. The concentrated system is found to promote the formation of a thicker and more uniform SEI with larger amounts of reduced species from the anion. These SEI features are thought to facilitate more stable and efficient Li cycling and a reduced tendency for dendrite formation.


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1. Introduction It is well known that the solid electrolyte interphase (SEI) layer formed on lithium (Li) metal is critical to the overall battery performance.1,

2

Also the cycling efficiency of a Li

battery, defined as the ratio between the discharge and charge capacity, is highly dependent on the formation mechanism and stability of the SEI. While most studies have focused on SEI formation at the graphite electrode in the context of Li-ion batteries, SEI formation at the Li metal electrode is of critical importance in the development of next-generation metal based technologies such as Li-S and Li-air batteries.1 The standard electrolytes used in Li-ion batteries based on organic carbonate solvents are incompatible with the Li metal electrode as demonstrated by numerous studies which describe dendrite formation, ‘mossy’ Li deposition and the formation of ‘dead-lithium’ as key issues to be resolved.3, 4 Most approaches published in the literature have investigated new electrolyte components: new solvents,5-7 solvent mixtures8-12 or alternate Li salts.13-19 However, reports detailing SEI formation at the Li metal electrode in ionic liquid (IL) electrolytes remain relatively limited.2, 20-25

The research in this field has recently increased with the design of novel ILs with more

beneficial properties for SEI formation to prevent short circuits and thermal runaway (e.g. low vapor pressure, wide electrochemical window, thermal stability).2 Initial studies reported the use

of

ILs

based

on

the

bis(trifluoromethanesulfonyl)imide

(TFSI-)

and

bis(fluorosulfonyl)imide (FSI-) anions due to their superior ionic conductivity and their wide electrochemical window which facilitates reversible Li+ / Li0 deposition without any decomposition.19,

26

Basile et al.23 monitored the Li surface morphology as a function of

immersion time in a pyrrolidinium FSI IL and demonstrated that the SEI formed initially was compatible with the Li metal electrode by smoothing its surface. After 12 days of immersion a roughening of the Li surface was observed with the formation of coral crystal structures growing into grain boundaries. After 18 days of immersion these coral-like structures were not visible anymore and the surface had again become smooth, indicating a highly dynamic 4 ACS Paragon Plus Environment

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process of formation. The commonly studied ILs for Li metal battery electrolytes include pyrrolidinium and imidazolium TFSI ILs,24, 27, 28 pyrrolidinium FSI ILs,23, 29-31 morpholinium FSI ILs32 and piperidinium TFSI and FSI ILs.32, 33 Most studies have used ILs with a nitrogenbased cation whereas there have only been limited studies comparing the electronic structures of ammonium and phosphonium-counterparts using XPS.34 An understanding of the mechanism of interaction between a phosphonium IL and the Li metal electrode is therefore important as novel phosphonium-based ILs with superior electrolyte properties have recently been described.35-38 Our recent studies revealed that a solution of 3.8 mol.kg-1 of LiFSI in P111i4FSI allowed repetitive plating / stripping of Li at up to 12 mAcm-2 and 6 mAhcm-2 for extended periods (over 450 plating / stripping cycles at 50 °C). The outstanding high cycling efficiency and cycling performance of the electrolyte on Li metal were attributed to the formation of a stable SEI at the Li metal electrode.39 In this study, the SEI was characterised by magic angle spinning nuclear magnetic resonance (MAS NMR), X-ray photoelectron (XPS), FourierTransformed Infrared (FTIR) and electrochemical impedance (EIS) spectroscopy to provide a better understanding of the SEI formation and composition. A comparison of the nature of the SEI formed in a ‘dilute’ IL electrolyte (0.5 mol.kg-1 of LiFSI in P111i4FSI) is also provided. This article represents the first report on the characterisation of the SEI formed on a Li metal surface in a small phosphonium FSI IL.

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2. Experimental Section 2.1

Materials

used.

The

ionic

liquid

trimethyl(isobutyl)phosphonium

bis(fluorosulfonyl)imide (P111i4FSI) was provided by Cytec Canada Inc. with > 99.5 % purity. The structure and purity were confirmed by 1H,

13

C,

19

F and

31

P NMR, MS and FTIR

spectroscopy. LiFSI (Nippon Shokubai, Japan) was used without further purification. The electrolytes were prepared by dissolving the desired amount of salt into the ionic liquid as previously reported.37 In the present work, the solution of 3.8 mol.kg-1 of LiFSI in P111i4FSI is referred to as the highly concentrated IL electrolyte (1:1.2 P+:Li+, i.e. 55 mol % Li+) whereas the solution of 0.5 mol.kg-1 of LiFSI in P111i4FSI is referred to as the dilute IL electrolyte (1:0.15 P+:Li+, i.e. 15 mol% Li+). Lithium foil (375 µm, 99.9 %) was obtained from Sigma Aldrich, and stored in an argon (Ar) filled glove box (< 1 ppm O2 and < 1 ppm H2O) with all the other chemicals. Lithium disks (11.5 mm diameter, ca. 1 cm2) were used for cell fabrication. 2.2 Electrochemical measurements. Li | Li symmetric cells (CR2032 type) were assembled inside an argon glove box as previously reported.39 The interfacial resistance evolution was monitored via electrochemical impedance spectroscopy (EIS) using a Multi Potentiostat VMP3 (Bio-Logic). The spectra were recorded between 5 mHz and 200 kHz with an amplitude of 10 mV. They were analysed with the EC-Lab software (Z Fit v. 10.39) and the impedance was reported within 2 % error. The obtained spectra were then fitted using equivalent electric circuit (EECs). Galvanostatic cycling tests of were carried out using a Multi Potentiostat VMP3 (Bio-Logic) at 50 °C ± 0.1 °C. The amount of charge applied during each cycle (1 cycle: 1 ‘plating’ + 1 ‘stripping’ process) was 3 mAh.cm-2 with a 375 µm thick Li electrode was employed. The current density for the Li metal plating/stripping was set to 1.5 mA.cm-2 (corresponding to 2 h ‘charge’ and 2 h ‘discharge’) with cutoff voltages of + 0.5 V and -0.5 V vs. Li/Li+.


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2.3 Fourier-Transformed Infrared spectroscopy (FTIR). Infrared spectra were acquired on a Perkin Elmer IR 101820 series spectrometer using the Spectrum (v.10.4) software. Ex situ attenuated total reflectance (ATR) spectra were obtained using a diamond ATR crystal in the mid-IR range (4000-450 cm-1). Cycled coin cells were disassembled to recover Li metal disks inside an argon glove box for further characterisation by FTIR and SEM. Prior to analysis, the electrodes were twice rinsed with dimethyl carbonate (DMC, 99 % Sigma Aldrich) to remove residual IL electrolyte and then dried under vacuum in the chamber of the glove box for 30 min. The rinsed and dried electrodes were hermetically transferred and loaded onto the sample stage after brief exposure to air. Firm pressure was applied to press the Li disk against the diamond window of the sample stage. All spectra were recorded with a 4 cm-1 resolution and 16 scans. 2.4 Scanning electron microscopy (SEM). Cycled electrodes were prepared in the same way as described in the previous paragraph for FTIR analysis. SEM images of cross sections of the Li electrodes were obtained with a JEOL JSM-IT300 at an accelerating voltage of 2 kV. Cross-sections of Li metal were prepared by cutting the Li electrodes with a surgical blade. The surfaces were transferred from the glove box to the SEM in a sealed stainless steel vessel filled with Argon. The vessel was introduced into the SEM via a purpose designed load-lock chamber for loading air-sensitive samples into the SEM chamber.39 2.5 Synchrotron soft X-ray photoelectron spectroscopy (SXR XPS). For all XPS experiments, careful precautions were taken in order to avoid moisture/air exposure of samples during transfer. Samples were transferred to the spectrometer via a hermetic transfer vacuum suitcase encapsulated in the glovebox and opened in the vacuum preparation chamber (at the Synchrotron facility). The binding energy scale was calibrated from the aliphatic hydrocarbon C 1s peak at 285.0 eV. The samples were mounted on a XPS sample stud using conductive carbon adhesive tape and placed in the vacuum suitcase. Soft X-ray photoelectron spectroscopic measurements were carried out at the Australian synchrotron facility where the 7 ACS Paragon Plus Environment


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usable photon energies range from 100 to 2500 eV.40 Photons were monochromatised by a Reixs plane grating monochromator based upon the SX700 monochromator design developed at the Berlin synchrotron BESSY (manufactured by FMB Berlin). The photoelectron kinetic energies (K.E) were measured using a Phoibos 100 / 150 analyser. Because of the low photon energies, measurements were conducted so that the same photoelectron K.E was used for all probe elements. Survey spectra were acquired at 20 eV pass energy and 0.5 energy step. High resolution region spectra were acquired at 10 eV pass energy and 0.05 energy step. For high resolution region spectra, the KE was selected at 100 eV above the binding energy (BE) corresponding to the same analysis depth for all spectra. No charge neutraliser was used during the measurements. The pressure in the analysis chamber was about 10−10 mbar. Instrument operation was performed using the XPS software (SpecsLab) available at the Australian Synchrotron. XPS data were analysed using the CASAXPS software (v. 2.3.13). Relative sensitivity factors (RSF) were taken from the Kratos Library and used to determine relative atomic percentages from survey and high-resolution scans of the most intense photoelectron peak for each element. Peak areas were measured after performing a two-point Shirley background subtraction. A Gaussian:Lorentzian algorithm (70:30 %) was used to fit the peaks to obtain quantitative results. The fit produces an estimated ± 10 % error in the atomic concentration determined for each peak. 2.6 Magic angle spinning NMR (MAS NMR). MAS NMR was used to obtain obtaining high resolution NMR data from solids.41, 42 All MAS-NMR experiments were carried out at the CNRS-IMN institute (Nantes, France). Li metal electrodes used for the 7Li,

31

P and

19

F

MAS NMR were removed from the coin cell inside the glove box. The deposit on the unrinsed Li electrode was scraped off the Li substrate with a stainless steel spatula. A 50:50 (wt %) mixture of deposit: alumina powder (Al2O3 AC300) was prepared for the NMR analysis and placed in a cylindrical 2.5 mm diameter zirconia rotor without being washed. To confirm the reproducibility of the preparation of cycled Li electrodes, Li surfaces extracted 8 ACS Paragon Plus Environment

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from replicate cells were analysed by 7Li and

19

F MAS NMR, the spectra obtained were

considered as identical, as illustrated in Figure S3 and S4. 7Li,

31

P and

19

F MAS NMR

experiments were carried out on a Bruker avance-500 spectrometer (B0 = 11.8T, Larmor frequencies ν0(7Li) = 194 MHz, ν0(31P) = 202 MHz, ν0(19F) = 470 MHz). MAS spectra were obtained by using a Bruker MAS probe. Spinning frequencies up to 25 kHz were used. 7Li and

31

P NMR spectra were acquired with a single pulse sequence and a recycle time of 30

seconds.

19

F NMR spectra were acquired using a Hahn echo sequence to discard the

significant contribution from the probe signal and a recycle time of 30 seconds.43 All the spectra were normalised taking into account the number of scans, the receiver gain and the mass of the sample. Integrated intensities were determined by using spectral simulation.



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3. Results and Discussion All surfaces analysed were plated Li metal surfaces extracted from Li | Li symmetric cells cycled for different numbers of plating / stripping steps, i.e. 1, 10, 50 or 250 cycles in the highly concentrated IL electrolyte (50 cycles only in the case of the dilute IL electrolyte) at j = 1.5 mA.cm-2 at 50 °C (3 mAh.cm-2).

3.1 EIS analysis of cycled Li surfaces: fitting The Li surface impedance was monitored after 1, 10 and 50 plating / stripping processes. The EIS spectra from a Li | Li symmetric cell cycled in the highly concentrated P111i4FSIbased electrolyte at 1, 10 and 50 cycles (Figure 1) are very similar indicating that the SEI structure and composition stabilises quickly under these conditions. The acquired impedance data were fitted with equivalent electric circuits (EECs) including the polymer electrolyte interphase (PEI) model developed by Thevenin and Muller44 and the layer model originally proposed by Aurbach et al.45-47 The circuits are presented in ESI, Table S1. The Nyquist and Bode (amplitude and phase angle) plots with fitted impedance spectra, for a Li | Li symmetric cell containing 3.8 mol.kg-1 of LiFSI in P111i4FSI electrolyte after 1 cycle, are presented in Figure S1. Fitting of the acquired impedance spectra was attempted using five different models detailed in the ESI, Tables S1 to S4. Along with the supporting information provided by the MAS NMR, FTIR and XPS analysis, the EIS data can be incorporated to validate a model for the structure of the SEI. The best fits were obtained when using layer models with CPEs: the EEC 1 (two-component layer model) gave the best fit of the impedance spectra before and after cycling, as previously described.39, 48 The addition of a Warburg diffusion component to the equivalent circuit (EEC 2) did not show any improvement as shown in Table S3. The fitted spectra show that the error in the fits usually comes from the contribution of the high-frequency EIS data. Our analysis here confirms that it is difficult to nominate one model in preference to the others on the basis 10 ACS Paragon Plus Environment

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of EIS fitting alone. The simplest charge transfer process would fit a single resistance / capacitance (RC) type equivalent circuit. The use of R / Q elements is relevant to the modeling of the SEI interface, suggesting the formation of several layers on an electrode surface (inner compact layers and outer diffuse layers). 49-51 The Li surface impedance was also monitored after 1, 10 and 50 plating / stripping processes in the dilute IL electrolyte and fitting results are presented in Table S4. The twoelement layer model also gave the best fit, as in the case of the highly concentrated IL electrolyte. As previously reported,39 the overall cell resistance determined by the fits over cycling was lower in the case of the low Li salt concentration (consecutively 55 Ω.cm-2 before cycling, 38 and 11 Ω.cm-2 after 1 and 50 plating / stripping processes) in comparison with the high Li salt concentration (consecutively 50 Ω.cm-2 before cycling, 40 and 23 Ω.cm-2 after 1 and 50 plating / stripping processes). These observations suggest the formation of a thinner, less resistive SEI over time in both cases.

3.2. FTIR analysis of cycled lithium surfaces FTIR39 measurements were conducted on Li metal electrodes cycled in both IL electrolytes after 1, 50 and 250 plating / stripping processes (50 cycles only in the case of the dilute IL electrolyte) and IR spectra are provided in Figure 2. Peak assignments have been previously reported.39 In general, the functional groups detected on the Li surface cycled in the dilute IL electrolyte were consistent with those detected in the case of the highly concentrated IL electrolyte. The IR analysis showed that similar species are present on both surfaces. Because of the thin nature of the SEI layer the intensity of the IR signals is very weak and only limited conclusions can be drawn from the normalised IR intensities. Three vibrational bands, νs(SNS), νa(SNS) and ν(SF), are modified when the number of cycles is increased: these bands correspond to the SNS bond of the FSI- anion. The νa(SNS) and ν(SF) bands of the FSI- anion are shifted by 20 cm-1 towards lower wavenumber (from 1 11 ACS Paragon Plus Environment


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to 50 cycles) and then by 13 cm-1 to higher wavenumber (from 50 to 250 cycles). The ν(SO2) band of the FSI- anion and bands associated with the P+ cation are not modified. Furthermore, with increasing cycle number a broadening of the νs(SNS) band at ca. 740 cm-1 is seen. These observations strongly suggest that the FSI- anion preferentially interacts/reacts with the Li metal surface leading to a distribution of environments and possible reaction products as will be discussed further below.

3.3. XPS analysis of cycled lithium surfaces In order to gain further understanding of the chemical species that compose the SEI layer during cycling in the highly concentrated IL electrolyte, XPS measurements were conducted after different numbers of plating / stripping processes (after 1, 50 and 250) and highresolution scans were acquired, Figures 3 and 4. Tables S5 and S6 provide a summary of XPS data and peak assignments for the different Li surfaces. Initial measurements revealed that chemical species from the IL itself were present on the surface of the sample in significant quantities, Figure 3. The results obtained from typical survey spectra comparing the atomic concentration of the chemical elements (F, O, N, C, S, P and Li) for plated Li surfaces (after 1, 50 and 250 plating / stripping processes) provide further details on the homogeneity of the film formed on the Li surface related to cycling performance (Table S5). Survey scans were conducted on two different spots on each surface and herein the average values are reported. The values of atomic concentrations for both spots were very close in the case of the highly concentrated IL electrolyte, suggesting a more uniform deposit on the Li surface, Figure 3a, in comparison with the dilute IL electrolyte, Figure 3b. While the survey spectra are dominated by carbon, oxygen and lithium, other chemical elements such as fluorine, nitrogen and sulfur are present in significant amounts on cycled Li surfaces, Figure 3c. As indicated by the FTIR measurements here (Figure 2) and in our previous report39 these elements are expected given the presence of strong FSI- based 12 ACS Paragon Plus Environment

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vibrations, in particular strong peaks denoting SO2 asymmetrical and symmetrical vibrations in addition to S-N stretching.39 These XPS spectra indicate that the SEI composition remains broadly similar throughout extensive cycling, comprised of similar overall proportions of each element.

3.3.1 Li 1s and F 1s spectra Figures 4a and 4b present the high resolution spectra of Li 1s and F 1s. The intensity of the Li 1s peaks is smaller than that of the prepared Li surface (Figure S2), which agrees with the observations from the SEM images, Figures 5a and 5b, that showed a more compact and thicker Li deposit forming on the bulk Li surface during cycling. After the 250 cycles the Li 1s spectra became broader and their amplitude slightly decreased. Given that the sampling depth of a photoelectron line is typically 5 to 10 nm,52 the SEI likely prevents ejection of photoelectrons from the underlying Li metal surface. The Li 1s peak envelops a broad domain where LiF and LiOH - Li2CO3 are known to reside at ca. 55 and 56 eV respectively.53 The F 1s spectra also show a peak at ca. 685 eV in all cases and this peak validates the presence of LiF on the surface. The results suggest that a layer of LiF was present on all surfaces. When looking at the amount of LiF on the surfaces (65 at. % at 1 cycle, 81 at. % at 50 cycles and 75 at. % at 250 cycles) the Li surfaces seem to quickly become dominated by LiF. A significant peak at ca. 688 eV was also detected on all surfaces corresponding to -SO2F.54 However, a quantitative comparison of the surfaces of 50 and 250 cycles reveals a decrease in the abundance of the -SO2F group while the intensity of the LiF peak increased from cycle 1 onwards (nearly doubled from 1 to 250 cycles). Similar observations as a function of etching time were reported for Li surfaces cycled in ammonium-based ILs for smaller amounts of charge passed (0.25 mA.cm-2 over 10 cycles).24

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Figures 4c and 4d present the high resolution C 1s and O 1s spectra respectively. The spectra indicate the presence of species associated with C-C / C-H at ca. 285 eV. On the C 1s spectra, the appearance of a ‘shoulder’ at ca. 286 eV at 50 and 250 cycles can be attributed to lithium carbonate (CO32-)53, 54 and Chetero at ca. 288-289 eV (e.g. C-P bond).34 The C 1s peak area is also becoming larger at 250 cycles. The overall C 1s peak shapes do not change for all samples, only the ratios vary. The O 1s spectra show a broad peak at ca. 532.0 eV and this peak corresponds to Li2O / Li2CO3.15, 23, 24, 53 This peak suggests the presence of Li2O and Li2CO3 species and these were reported to be present in other SEI formed in IL electrolytes23, 24 and carbonate electrolytes.8, 14

Minor peaks of unknown origin are also present at ca. 528 eV (less than 2 at. %).

3.3.3 N 1s, S 2p and P 2p spectra Figures 4e, 4f and 4g present the N 1s, S 2p and P 2p high resolution spectra. These elements were not present on the initially prepared Li surface (or were only present in very small quantities, < 0.5 at. %). The high-resolution spectra indicate that their presence is a result of the chemical interaction between the IL and the Li surface. The chemical species including the N and S elements that can be detected from these spectra are mainly attributed to the reaction of the FSI anion with the Li surface. The N 1s spectra show one broad peak that could overlap two distinct peaks attributed to the chemical environment of the element N if there is a cleavage of the anion, with a main component at ca. 399.9 eV.23,

24

The S 2p

spectra become very complex after 50 and 250 cycles with broad signals that contain at least two components (corresponding to spin-orbit doublets 2p1/2 and 2p3/2). Two to three signals are observed after 1 cycle, with a broad signal at ca. 166.4 eV that could contain two distinct peaks, whereas at least 5 components are observed at 50 and 250 cycles. The most intense peak (broad shouldered peak) observed at ca. 168.7 eV after 1 cycle is attributed to –SO2F species and is most likely an overlapping doublet.23,

54

After 50 and 250 cycles this peak 14


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becomes broader and shifts slightly to a higher binding energy at 169.4 eV after 50 cycles and 168.6 eV after 250 cycles. This peak is enveloping several components that could correspond to sulfur species such as sulfone or sulfite and breakdown products from the FSI anion.23, 24, 54 The FTIR spectra, Figure 2, confirmed the presence of O2S-N-SO2 species on the surface. A number of smaller peaks from 167 to 160 eV are observed at 50 and 250 cycles: a small component (at ca. 161.8 eV at 50 cycles and 161.1 eV at 250 cycles) appears in comparison to the S 2p spectrum at 1 cycle and this peak was assigned to lithium sulfide (Li2S).[23] However this component is present in small amounts (ca. 7 at. %). The P 2p spectra present one broad peak (two components 2p1/2 and 2p3/2) at 133.6 eV at 1 cycle, 133.8 eV at 50 cycles and 133.2 eV at 250 cycles. This peak is attributed to the phosphonium cation34, 54 and does not shift significantly with cycling, in agreement with the data obtained with FTIR spectroscopy. The FTIR spectra revealed the presence of –CH3 group (coming from the cation) on the surface after 1, 50 and 250 cycles which validates the hypothesis that the cation could be trapped within the SEI after several hundreds of cycles. The absence of a phosphate group (typically with a peak at ca. 132 – 133 eV),53,

54

a

phosphonate group (peak at 133.6 eV)[48, 49] or a phosphonic acid group (peak at 134.3 eV)53, 54

on the surface, indicates that the cation is stable and does not form these oxidised species

during cycling. These results also confirm that the nature of the interphase on the Li surfaces is very similar after 1, 50 and 250 cycles. When considering a quantitative analysis of the high-resolution XPS spectra, Figure 6, the amount of the LiF component on the F 1s spectra was smaller in the case of the dilute vs. highly concentrated IL electrolyte (ca. 50 % vs. > 69 %). The same trend was observed for the Li2O component on the O 1s spectra (64 % vs. 79 %), for the Li2CO3 component on the C 1s spectra (49 % vs. 60 %), for the Li2S component on the S 2p spectra (ca. 21 % vs. > 30 %) and for the LiF / Li2CO3 components on the Li 1s spectra. These observations indicate that the mechanism of SEI formation in the highly concentrated IL electrolyte results in larger 15 ACS Paragon Plus Environment


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amounts of more reduced chemical species coming from reduction of the FSI- anion suggesting that these reactions are favoured in the high concentration electrolyte.

3.4 MAS NMR characterisation Ex-situ Magic Angle Spinning Nuclear Magnetic Resonance spectroscopy (MAS NMR) was used to characterise the nature of the SEI layer that resulted from the interaction of the IL electrolyte and the Li metal substrate during cycling.55 This technique is becoming increasingly reported in the context of characterisation of the SEI layer in Li batteries.56-58 Li electrodes were cycled at j = 1.5 mA.cm-2 at 50˚C in 3.8 mol.kg-1 of LiFSI in P111i4FSI and the surface was scraped and NMR samples were prepared as reported in the experimental section before measuring the 7Li,

31

P and

19

F NMR spectra. The

31

P NMR spectra are illustrated in

Figure 7a and show one sharp resonance at 24 ppm, which confirms that the cation resonance is attributed to the intact phosphonium cation itself which is incorporated, unaltered, into the film as suggested by the FTIR and XPS data above.59, 60 Figure 7b presents the 7Li NMR spectra from the SEI deposit. The small signal at 260 ppm is assigned to Li metal which always presents a small quantity when the surface is removed.61 All the other resonances are around 0 ppm and correspond to diamagnetic species. These broad overlapping resonances were fitted and the results are presented in Figure 7c. There is a sharp resonance at -1.3 ppm that corresponds to LiF.61,

62

The broadest resonance arises at

approximately 2.8 ppm and may correspond to Li2O.62 The intermediate signal (0.5 ppm) could correspond to other diamagnetic species (such as Li2CO3 or organic lithiated species that could contain the CHF=CR2, CF=CF2 or RCH2F groups).62 Overall the 7Li NMR spectra show that LiF is the best defined and most easily assigned lithiated SEI component in good agreement with the XPS and FTIR analysis. The 19F NMR spectrum is shown in Figure 7d. The signal at - 205 ppm corresponds to LiF, which is in agreement with the signal detected at -1.3 ppm on the 7Li spectrum (Figure 2).61, 62 16 ACS Paragon Plus Environment

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Several broad resonances can be observed between -140 and -170 ppm. In this region, the resonances are typically assigned to the following groups: CHF = CR2, CF = CF2 and RCH2F where R can be attributed from ethyl- to butyl- groups.61,

63

These species could also be

lithiated compounds. Clearly there is a decomposition reaction of the FSI- anion which results in a mixture of compounds that make up the SEI layer.56

3.5 Comparative study between plated and stripped Li metal surface Further investigation of nanoscale structure and deposit composition was required to better understand the role of the stripped surface in supporting high efficiency, high rate cycling by comparing stripped and plated Li surfaces after 50 plating / stripping processes at j = 1.5 mA.cm-2 (q = 3 mAh.cm-2) in 3.8 mol.kg-1 of LiFSI in P111i4FSI. SEM analysis revealed the presence of a sparse deposit on the stripped surface, Figure 5c, with a morphological appearance similar to that of the plated surface.39 A dynamic property of the SEI supporting a ‘breathing mode of Li cycling’ was hypothesised according to FTIR analysis, with stretching vibrations corresponding to the anion and cation being less intense in the case of the stripped surface.39 Differences in the XPS survey spectra were observed (Table S7, ESI): higher content of oxygen (20.6 vs. 17.5 at.%) and lower content of Li (25.6 vs. 34.7 at.%) were detected in the case of the stripped surface. When looking at high-resolution spectra (Table S8), the main differences seem to reside in the S 2p spectra with a smaller number of apparent components in the case of the stripped surface. The results from the stripped surface suggest that either, the SEI layer is dissolving and reforming with cycling, or, more likely, that the charged surface has a higher surface area due to the nanostructured Li metal deposit which would lead to a higher surface area of SEI. As a hypothesis to summarise the findings, the plated surface has more metal and covering sulfur species compared to the stripped surface. The surface morphology is changing which reflects the different measured composition.



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3.6 Proposed mechanism of SEI formation at lithium metal electrodes The combination of MAS NMR, FTIR, XPS and EIS techniques gives us a better insight into SEI formation in the P111i4FSI IL electrolytes at the Li metal interface. These complimentary techniques used to obtain a detailed picture of the SEI were highly consistent in general. Based on the spectroscopic evidence and EIS analysis discussed above, the chemical nature of SEI components was revealed as follows. The model herein proposed indicates a layered structure formed after the first plating / stripping process. This model suggests the presence of: •

an inner layer dominated by LiF and Li2O, chemical species related to the FSI- anion (e.g. Li2S) and anion related functional groups (-SO2F and -N-SO2)

•

species related to the P+ cation, most likely the intact cation, present in an outer layer close to the electrolyte interface.

A mechanism of SEI formation in the highly concentrated IL electrolyte is proposed in Figure 8. A legend indicating the name of chemical species is also provided. This model was chosen based on the conclusions derived from the different spectroscopic analysis as well as previous reports available in the literature, as explained below. It is important to note that this is our hypothetical mechanism based on the evidence available. Reduction products of TFSI/FSI based electrolytes have previously been proposed by Aurbach et al,8 Howlett et al,24 Basile et al.23 and Tułodziecki et al.64 In the literature several mechanisms of SEI formation by the FSI- anion reduction have been proposed. Basile65 proposed a single electron reduction pathway leading to the formation of anions via the intermediate radical species: ∙ ∙ + → → +

(Equation 1)

In this mechanism the FSI- anion is dissociated and forms a radical. The F- anion could then react with Li+ to form LiF, as detected in the XPS spectra described above. Previously, in the


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case of the TFSI- anion, Howlett et al.24 proposed a mechanism involving cleavage of the S-N bond: ∙ ∙ + → → +

(Equation 2)

The authors also reported, with XPS etching experiments, the formation of an inner layer dominated by LiF on the Li surface. Recently, Basile65 reported that this pathway was also favourable in the FSI- system where one S-N bond breaks due to reduction of the sulphur atom and leads to the formation of two species, ∙ and , as illustrated in the insert mechanism Figure 8. These two species can then be reduced to form ⋅ and via the following equations: (Equation 3)

∙ + → + ∙ + → ⋅ +

(Equation 4)

The MAS NMR, FTIR and XPS analysis presented above indeed suggests the presence of these two species, ⋅ and , bound to the Li surface as lithium salts. The FTIR analysis also revealed the presence of Li-SNS bending vibrations; this implies the existence of fluoride free fragments of the FSI- anions that can be formed via the following reaction: ∙ + → +

(Equation 5)

Thus a number of pathways are possible which all lead to the formation of species detected to be part of the SEI according to the spectroscopic characterisation. From these analyses, the following conclusions can be postulated: (i) The SEI formed by chemical reaction of Li metal with P111i4FSI is composed of chemical compounds containing fluorine, oxygen and sulfur which are the products of the FSI- anion decomposition. The following compounds were identified: lithium fluoride (LiF), lithium oxide (Li2O), lithium sulfide (Li2S, LiSO2, Li2NSO2F) and possibly lithium carbonate (Li2CO3). (ii) The SEI formed on the Li metal surface presents a layered structure.



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(iii)There is no evidence showing chemical reduction of the cation (at the studied temperature). If any reaction occurs, breakdown products from the cation could result from a pyrolysis reaction (via Hofmann elimination) and stay trapped within the SEI formed structure. Dissolution of these products into the electrolyte could also be considered. (iv) The reduction reaction of the FSI- anion is the critical step for SEI formation. The cleavage of the FSI- anion at its nitrogen center seems to happen almost instantly upon interaction with the lithium metal. This cleavage releases the –SO2F functional group that itself undergoes a further reaction that releases fluoride anions. The Fanions react instantly with the Li surface to form a stable compound during cycling, LiF. After 1 cycle the compounds LiF and LiSO2F are already present on the Li surface. In the case of the dilute IL electrolyte the spectroscopic data revealed evidence of a similar mechanism of SEI formation to that of the highly concentrated IL electrolyte. The main differences confirmed by XPS and EIS revealed the formation of a thinner SEI with otherwise near identical chemical species, supported by SEM analysis, Figure 5d. Similarities in chemical composition of the plated and stripped Li electrodes in both low and high Li salt concentration cases was also observed. In the case of the highly concentrated IL electrolyte, by looking at differences in Li surfaces after 1, 50 and 250 plating / stripping processes, the study revealed the formation of a chemically stable SEI after the first cycle. SEI composition and thickness were found to be stable with increasing number of cycles.


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4. Conclusion This first report on the characterisation of the SEI formed on a Li metal surface in a small phosphonium FSI IL helps establish guidelines for selecting, designing and discovering materials for Li metal protection. The choice of IL and Li salt concentration in an electrolyte are crucial to allow effective protection of the Li metal electrode. The SEI formed in P111i4FSI-based electrolytes was characterised using MAS NMR, FTIR, XPS and EIS techniques. This study provides an understanding on the composition and structure of the SEI formed in these electrolytes. Observations from the various techniques indicate an evolution of the SEI composition with cycling. This study shows the stability and evolution of the surface up to 500 hours of continuous cycling and longer duration has previously been reported by us.39 Further study and examination of longer duration cycled surfaces is warranted. We also found that the chemical nature of the SEI formed on the Li metal surface was dependent on the Li salt concentration. Differences in chemical composition were observed between the SEI formed in the dilute IL electrolyte and the highly concentrated IL electrolyte during cycling. However, the main differences were found to be the formation of a thicker SEI with larger amounts of reduced species from the anion in the case of the high Li salt concentration. Moreover, multiple site sampling measurements using FTIR, XPS, MAS NMR and SEM generally indicated a more uniform SEI in the high Li salt concentration system. This correlates with the differences observed in the cycling properties and deposit morphologies exhibited in our previous studies. Herein we proposed one reduction pathway possible to describe the mechanism of LiFSI / IL mixture breakdown leading to SEI formation on Li metal in the highly concentrated IL electrolyte. Equivalent circuit modeling of the EIS data suggested a layered structure for the SEI on Li metal. Significant quantities of chemical species associated with the FSI- anion were formed on the surface film whereas the cation appeared to be inactive. The major components of the SEI after several hundreds of cycles were LiF and Li2O (LiOH) and 21 ACS Paragon Plus Environment


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breakdown products from the FSI- anion including Li2S. The SEI formed using P111i4FSIbased electrolyte exhibits a complex chemical composition which is stable over several hundreds of cycles.


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(c) Number cycles

3.8 mol.kg-1 of P111i4FSI Rcell χ2

LiFSI χ/ √N

in 0.5 mol.kg-1 P111i4FSI Rcell χ2

LiFSI

in

χ/ √N

Before cycling

50

1.8 x 10-2 1.9 x 10-2 55

1.7 x 10-2 3.1 x 10-1

1

40

3.3 x 10-2 2.4 x 10-2 37

4.1 x 10-2 2.2 x 10-1

10

45

7.2 x 10-3 1.1 x 10-2 38

5.1 x 10-3 1.1 x 10-1

50

23

0.8 x 10-3 3.1 x 10-3 12

1.1 x 10-5 6.4 x 10-3

(d)

Figure 1. Nyquist plots of the impedance spectra of Li | Li cells after 1, 10 and 50 cycles (3 mAh.cm-2, j = 1.5 mA.cm-2 at 50 °C) with different electrolytes (a) 3.8 mol.kg-1 LiFSI in P111i4FSI, (b) 0.5 mol.kg-1 LiFSI in P111i4FSI; (c) cell resistance Rcell (± 1 Ω) and fitting results for a two-component layer model after 1, 10 and 50 cycles with both electrolytes (same cycling conditions) and (d) equivalent electric circuit (EEC) for a two-component layer model.



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Figure 2. Low-frequency portion FTIR spectra of SEI films of plated Li surface in (a) 3.8 mol.kg-1 of LiFSI in P111i4FSI after 1, 50 and 250 cycles at j = 1.5 mA.cm-2 at 50˚C, reproduced from reference [39] and (b) 0.5 mol.kg-1 of LiFSI in P111i4FSI after 50 cycles at j = 1.5 mA.cm-2 at 50˚C after rinsing with DMC. The prepared film is a pristine Li surface brushed and rinsed with hexane.

Figure 3. Chemical composition (atomic %) summary determined from the survey spectra for two spot areas (1 and 2) of a plated Li surface after 50 plating / stripping processes in (a) the highly concentrated IL electrolyte; (b) dilute IL electrolyte and (c) after 1, 50 and 250 cycles at j = 1.5 mA.cm-2 at 50˚C in the highly concentrated IL electrolyte.


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Figure 4. High-resolution photoelectron spectra (a) Li 1s; (b) F 1s; (c) C 1s; (d) O 1s; (e) N 1s; (f) S 2p and (g) P 2p for the plated Li surfaces after 1, 50 and 250 cycles at j = 1.5 mA.cm2

at 50˚C in 3.8 mol.kg-1 of LiFSI in P111i4FSI. 25 ACS Paragon Plus Environment


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(b)

(a)

10µm

50µm (c)

(d)

10µm

10µm

Figure 5. SEM images (EHT = 2.00 kV) of (a) and (b) the plated Li electrode; (c) the stripped Li electrode after 50 cycles at j = 1.5 mA.cm-2 (q = 3 mAh.cm-2), in 3.8 mol.kg-1 of LiFSI in P111i4FSI at 50 ˚C after rinsing with DMC; (d) plated Li electrode in 0.5 mol.kg-1 of LiFSI in P111i4FSI cycled in same conditions. Figure (d) was reproduced with permission from ref. 39


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Figure 6. High-resolution photoelectron spectra (a) Li 1s; (b) F 1s; (c) C 1s; (d) O 1s; (e) N 1s; (f) S 2p and (g) P 2p for the plated Li surfaces after 50 cycles at j = 1.5 mA.cm-2 at 50˚C in 0.5 and 3.8 mol.kg-1 of LiFSI in P111i4FSI. 27 ACS Paragon Plus Environment


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Figure 7. (a) 31P NMR spectra; (b) 7Li NMR spectra; (c) 7Li NMR spectra with fitted peaks (500 MHz, 25 kHz spinning speed) and (d) 19F NMR spectra with fitted peaks (500 MHz, 23 kHz spinning speed) for the 50:50 (wt %) mixture of SEI deposit: alumina powder (Al2O3 AC300). The SEI deposit was scraped off the plated Li electrode after 50 cycles at j = 1.5 mA.cm-2 at 50˚C in 3.8 mol.kg-1 of LiFSI in P111i4FSI. 28 ACS Paragon Plus Environment

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Figure 8. Schematic representation of the SEI formed at the Li surface in 3.8 mol.kg-1 of LiFSI in P111i4FSI electrolyte. The insert window displays the chosen reduction pathway of the FSIanion (bottom right).


29


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Detailed description of the EIS fitting results, XPS data and peak assignments and NMR spectra carried out for the lithium surfaces employed in the comparison between the two ionic liquid electrolytes. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to thank the ARC (Australian Research Council) and Solvay Group for funding this research as part of a Linkage Project LP120200181. MAS-NMR measurements were undertaken at CNRS-IMN, Nantes, France. XPS measurements were undertaken on the


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Soft X-ray spectroscopy beamline at the Australian Synchrotron, Victoria, Australia. We acknowledge the Australian Synchrotron and Dr. Bruce Cowie for operations support. Prof. Maria Forsyth and Prof. Douglas R. MacFarlane are also grateful to the ARC for their Australian Laureate Fellowships.

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Table of contents

A Spectroscopic Characterisation of The SEI Layer Formed on Lithium Metal Electrodes in Phosphonium Bis(fluorosulfonyl)imide Ionic Liquid Electrolytes.

Gaetan M. A. Girard, Matthias Hilder, Nicolas Dupre, Dominique Guyomard, Donato Nucciarone, Kristina Whitbread, Serguei Zavorine, Michael Moser, Maria Forsyth, Douglas R. MacFarlane and Patrick C. Howlett

ToC figure 36 mm high × 60 mm broad


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Spectroscopic Characterization of the SEI Layer Formed on Lithium

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