Multinuclear NMR Study of the Solid Electrolyte Interface Formed in

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Multinuclear NMR Study of the Solid Electrolyte Interface Formed in Lithium Metal Batteries Chuan Wan, Suochang Xu, Mary Y. Hu, Ruiguo Cao, Jiangfeng Qian, Zhaohai Qin, Jun Liu, Karl T. Mueller, Ji-Guang Zhang, and Jian Zhi Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15383 • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 5, 2017

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ACS Applied Materials & Interfaces

Multinuclear NMR Study of the Solid Electrolyte Interface Formed in Lithium Metal Batteries

Chuan Wan,a, c Suochang Xu,a Mary Y. Hu,a Ruiguo Cao,b Jiangfeng Qian,b Zhaohai Qin,c Jun Liu,b Karl T. Mueller,d Ji-Guang Zhang,b* Jian Zhi Hu,a*

a

Earth and Biological Science Directorate, Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, USA b

Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, USA c

d

College of Science, China Agricultural University, Beijing 100193, P. R. China

Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, USA

*

To whom the correspondence should be addressed:

E-mails: [email protected]; [email protected]

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ABSTRACT: The composition of the solid electrolyte interphase (SEI) layers formed in Cu|Li cells using lithium bis(fluorosulfonyi)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,2-dimethoxyethane (DME) electrolytes is determined by a multinuclear solid-state MAS NMR study at high magnetic field. It is found that the “dead” metallic Li is largely reduced in the SEI layers formed in a 4 M LiFSI-DME electrolyte system compared with those formed in a 1 M LiFSI-DME electrolyte system. This finding relates directly to the safety of Li metal batteries, as one of the main safety concerns for these batteries is associated with the “dead” metallic Li formed after long term cycling. It is also found that a large amount of LiF, which exhibits superior mechanical strength and good Li+ ionic conductivity, is observed in the SEI layer formed in the concentrated 4 M LiFSI-DME and 3 M LiTFSI-DME systems, but not in the diluted 1 M LiFSI-DME system. Quantitative 6Li MAS NMR results indicate that the SEI associated with the 4 M LiFSI-DME electrolyte is denser than those formed in the 1 M LiFSIDME and 3 M LiTFSI-DME systems. These studies reveal the fundamental mechanisms behind the excellent electrochemical performance associated with higher concentration LiFSI-DME electrolyte systems.

KEYWORDS: 6Li MAS NMR, high magnetic field, solid electrolyte interphase (SEI) layer, Li metal batteries, high concentration electrolytes


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1. INTRODUCTION During the last four decades, rechargeable lithium (Li)-ion batteries (LIBs) have become the dominant power source for portable electronic devices and electrical vehicles.1-2 While the continuous increase in energy consumption by consumer electronics and long rang electric vehicles (EVs) has driven a worldwide effort to further increase the energy density of LIBs, it is also realized that the energy density of the state of art LIBs will soon reach a theoretical limit. For example, the capacity of the graphite anode widely used in conventional LIBs is already near its specific capacity of 372 mAh g-1. Therefore, extensive efforts have been made to identify alternative anodes to replace carbonaceous anode materials.3 In this regards, Li metal is an ideal anode material due to its low density (0.534 g cm-3), lowest negative electrochemical potential (3.040 V vs. the standard hydrogen electrode) and extremely high theoretical special capacity (3860 mAh g-1), which is ten times as high as that of carbonaceous materials. Initial attempts to use Li metal as an anode in rechargeable LIBs can be traced back to the 1970s,4 but several seemingly insurmountable barriers, such as limited Columbic efficiency (CE) and dendritic Li growth during repeated Li deposition/stripping processes, have hindered their practical applications in rechargeable batteries.5-6 The cycling stability of a Li metal anode strongly depends on all side reactions with electrolyte components as Li is thermodynamically unstable when in contact with any organic solvents.6 Although the initial SEI layer formed on a Li metal surface immediately upon battery assembly can prevent the further degradation of a Li metal anode, the fresh Li metal surface formed during Li deposition/stripping processes will continue to react with electrolyte to form new SEI layers. These reactions will lead to a low CE and continuous consumption of Li metal and electrolytes.1, 7-8

The consumed Li metal and electrolyte components decompose to form a “passivation” film, 3 ACS Paragon Plus Environment


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i.e. the so called solid electrolyte interphase (SEI), on the surface of the electrode, which can protect the electrode from further corrosion. These “passivation” films also contain some “dead” Li metal particles that are electrically isolated and cannot participate in further battery operation. A “passivation” film that contains a large amount of isolated Li metal particles may become a safety hazard during mechanical damage to a battery. Therefore, it is necessary to minimize the amount of “dead” Li in the passivation layers.3-4, 9 The importance of the SEI lies in not only the protective function that it affords, but it also serves as a medium for Li+ diffusion.10-12 In general, the organic SEI components can still be subject to reduction and solvent penetration.13 The inorganic components (often accumulated next to the anode surface), on the other hand, exhibit better Li+ ionic conductivity and are stable against reduction, therefore they are responsible for the protective function of the SEI.13-19 As previously reported,11 an ideal SEI should have minimal electronic and maximal Li+ conductivity and be a compact layer adhering well to the anode with uniform morphology and composition, containing stable and insoluble passivating agents, and also contain minimal amount of “dead” Li particles. Obviously, these properties are determined by the composition of the SEIs, and thus it is critically important to understand the composition of SEIs formed in Li metal batteries, both as a function of time (cycling) and electrolyte composition. Various experimental methods such as electrical impedance spectroscopy (EIS),20 Fourier transform infrared (FTIR) spectroscopy,21-22 X-ray photoelectron spectroscopy (XPS),20, 23-24 and nuclear magnetic resonance (NMR) spectroscopy25-27 have been used to study the components of SEIs. NMR, an inherently quantitative and non-destructive method (with the ability of characterizing both amorphous and crystalline phases), is an ideal tool for investigating the compositions of SEI. For LIBs, the changing speciation and role of Li can be investigated by 4 ACS Paragon Plus Environment

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NMR. Both 7Li (I=3/2, 93% abundance) and 6Li (I=1, 7% abundance) have been utilized for investigating SEIs.28-33

6

Li has a natural abundance of 7%, a smaller quadrupole moment, and

generally much longer relaxation times than 7Li, making 6Li NMR a technique that can suffer from low sensitivity. However, for diamagnetic materials, a higher spectral resolution can be obtained with 6Li (compare to 7Li), thus offering significantly enhanced structural information. 34 In addition to 6Li and 7Li, the NMR-active nuclides

19

F,

13

C and 1H have been effectively

employed in studies of SEIs.35-36 In our previous work, a high performance Cu|Li cell, which can be cycled at high rates for more than 1000 cycles with high CE (average of 98.4%) and no increase in the cell impedance has been reported.37 For the SEI layer associated with this cell, previously reported scanning electron microscopy (SEM) and XPS studies indicated that the highly conductive and compact nature of the SEI provided highly stable voltage profiles and corrosion protection of the Li (or Cu) electrode in the highly concentrated electrolyte composed of lithium bis(fluorosulfonyi)imide (LiFSI) in 1,2-dimethoxyethane (DME).37 However, the molecular compositions of the SEI that play a vital role to the performance of the cell were not revealed, although XPS was used to determine the atomic distribution of the SEI (i.e., it contained Li, O, N, F, S, etc.). In this work, multinuclear (6Li,

19

F,

13

C and 1H) solid-state magic angle spinning (MAS) NMR

experiments were performed at a high magnetic field of 19.975 T in studies of the SEI layers carefully harvested from Cu electrodes. Air-tight rotors were utilized as the SEIs formed were quite air sensitive. A high magnetic field was used to improve the 6Li MAS NMR spectral resolution, as 6Li is a quadrupolar nuclide (I=1) and the line-broadening by the second order quadrupolar interaction is inversely proportional to the magnetic field strength, thus offering a better chance for detecting the various lithiated species in the SEI layer.38-39 Because cell 5 ACS Paragon Plus Environment


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performance is correlated with lithium salt identity and concentration within the electrolyte,37, 40 to gain further insight into the differences between the SEIs formed in different electrolytes the SEIs formed in two different concentration (1M and 4 M) of LiFSI-DME and 3 M LiTFSI-DME electrolyte were analyzed in this study. 2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparations Lithium fluoride (LiF), lithium hydroxide monohydrate (LiOH•H2O), lithium oxide (Li2O), lithium sulfide (Li2S) and lithium carbonate (Li2CO3) were obtained from Sigma-Aldrich Co. Ltd.. 1,2-Dimethoxyethane (DME) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were obtained from BASF Corporation. Lithium bis(fluorosulfonyl)imide (LiFSI) was obtained from Nippon Shokubai Co., Ltd. All of the chemicals were used as received. Li foil and Cu foil were purchased from MTI Corporation and All Foils, Inc., respectively. The SEI sample from a Cu electrode of a Cu|Li cell cycled at 1.0 mA•cm-2 for 200 cycles using 4.0 mol/L (4 M) LiFSIDME electrolyte is identified as sample A; the SEI sample from a cell cycled at 1.0 mA•cm-2 for 200 cycles using 1.0 mol/L (1 M) LiFSI-DME electrolyte is identified as sample B; the SEI sample from a cell cycled at 1.0 mA•cm-2 for 200 cycles using 3.0 mol/L (3 M) LiTFSI-DME electrolyte is identified as sample C. The materials and samples were stored and handled in an MBraun LABmaster glove box with Ar atmosphere (< 1 ppm O2 and < 1 ppm H2O). Coin cells (Cu|Li) were assembled in the glove box with a copper foil as the working electrode and a single piece of lithium foil used as both the counter and reference electrodes.

A

polyethylene membrane was used as the separator, while Cu foil served as the substrate for Li metal deposition. The Cu foil was washed by immersing it in 1M HCl for 10 min, followed by 6 ACS Paragon Plus Environment

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rinsing separately with deionized water and acetone three times. The Cu foil was quickly dried in a vacuum oven at room temperature and then transferred into an Ar-filled glovebox. To standardize the testing, 75 µl of electrolyte was used in each coin cell. The current density for the Li metal plating/stripping was set to 1 mA cm-2 using a Lanhe battery testing station at room temperature. The plating of Li was capacity controlled by setting the deposition time to 1 h for a capacity of 1.0 mAh cm−2, while the stripping process was voltage controlled by setting the upper cutoff voltage to 0.5 V vs. Li/Li+. The effective area of the Cu foil for Li deposition was 2.11 cm2 (diameter 1.64 cm). The SEI layers formed in each Cu|Li cell were harvested by carefully scraping the SEI layer from the Cu electrodes after disassembling the cells that were in a state of full charge (i.e., all Li was stripped from the Cu electrode) in the Ar glovebox without scraping the Cu metal. The scraped off layers were washed using the solvent of electrolyte, i.e. DME for five times, and then dried in a vacuum chamber attached to the glovebox for 12 hours. The main purpose of the washing/drying process is to remove the residual salt and solvent left in the SEI layer to prevent their interference in the subsequent NMR measurement. Because LiFSI is highly solvable in DME, all of residual salt will be removed from SEI layer after they were washed five times in DME and all DME solvent will be removed by the drying process. Considering the technical challenges of only removing the soluble parts embedded in SEI layer without losing any insoluble part of SEI layer

41

, the procedure of DME washing and the subsequent vacuum

chamber drying were kept the same from sample to sample so that the results can be quantitatively compared. The dried powder samples were then loaded into sealed 4 mm MAS rotors. We found that significant pressure builds up by spinning the sample rotor containing dried SEI samples inside the high magnetic field of 19.97 T, causing ejection of the end cap that 7 ACS Paragon Plus Environment


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seals the MAS rotor. To avoid this problem, the dried SEI samples were soaked with DME using approximately 1:1 weight ratio of SEI powder samples and DME before sealing the rotor. DME can absorb the heat generated during the rotating process and avoid pressure build up during the MAS experiments. 2.2. NMR Measurements 6

Li,

19

F,

13

C and 1H MAS NMR experiments were performed at 20°C on a Varian-Inova 850

MHz NMR spectrometer, operating at a magnetic field of 19.975 T, with a commercial 4 mm pencil type MAS probe. The 6Li MAS spectra were acquired at 125.050 MHz using a single solid π/4 pulse with a pulse width of 2 µs and a recycle delay time of 10 s.

19

F MAS spectra

were acquired at 799.491 MHz using a single π/4 pulse with a pulse width of 4.5 µs and a recycle delay of 10 s.

1

H MAS spectra were acquired at 849.731 MHz using a single π/2 pulse with a

pulse width of 6.5 µs and a recycle delay of 15 s.

13

C MAS spectra were acquired at 213.672

MHz using a single π/2 pulse with a pulse width of 3 µs and a recycle delay of 15 s. 6Li, 19F, 13C and 1H NMR chemical shifts were referenced to 1 M LiCl aqueous solution (0 ppm), CFCl3 using secondary reference sodium trifluoroacetate (CF3COONa, -79.3 ppm), tetramethylsilane (TMS) using secondary reference adamantane (1.85 ppm) and tetramethylsilane (TMS) using secondary reference adamantane (37.85 ppm), respectively. All the SEI samples were packed with a 1:1 weight ratio of SEI:DME into an air-tight MAS rotor in the Ar glove box for better spinning stability during the experimental period as explained above.

The Nuts program

(v.2012, Acorn NMR Inc., Las Positas, CA, USA) was used to simulate the spectral resonances and thereby to deconvolute the peaks for analysis.


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Quantitative peak areas for the deconvoluted peaks in the 6Li MAS NMR spectra of various samples were obtained using the following strategy. All experimental conditions, including the matching and tuning conditions of the RF circuit of the NMR probe, were kept identical from sample to sample. Therefore, the absolute peak area relative to the peak area from a known standard is directly proportional to the sample weight and the number of time-domain acquisition scans that were co-added for each sample. The absolute peak area for each deconvoluted peak was obtained by simulating the spectrum of each sample using the Nuts program. The peak area obtained was then normalized based on the weight of samples and number of accumulation scans. The normalized peak area for each product can then be quantitatively compared between the samples because the normalized peak area is proportional to the amount of each individual product in the SEI samples.



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Figure 1. 6Li MAS NMR spectra of the SEI samples collected from the Cu electrodes of different Cu|Li cells using electrolytes containing (a) 4 M LiFSI-DME (with 6144 scans, sample A), (b) 1 M LiFSI-DME (with 16046 scans, sample B) and (c) 3 M LiTFSI-DME (with 4192 scans, sample C). All spectra were acquired with a recycle delay time of 10 s. The spinning side bands are indicated by red asterisks. The line broadening used for processing the data was 20 Hz. The red numbers in the brackets indicate the corresponding normalized peak areas. 3. RESULTS AND DISCUSSION The single pulse 6Li MAS NMR spectra of the SEIs extracted from the Cu electrode of the various Cu|Li cells using 4 M (sample A), 1 M (sample B) LiFSI-DME and 3 M LiTFSI-DME (sample C) electrolyte cycled for 200 cycles are shown in Figure 1. The peak at around 264 ppm is due to metallic Li in the SEI layer. 42 Note that the relative intensities of the metallic Li in each spectrum remains the same during the entire experiment period, indicating that the rotor is well


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sealed, i.e., any reaction with components of air (O2, H2O and N2) are avoided. There are two probable sources for the metallic Li.

One is the dead Li particles or broken dendritic Li

nanofibers inside the SEI layer on the Cu electrode that is unable to cycle back to the Lielectrode during the final delithiation process. The other possible source is the residual metallic Li deposited on the Cu electrode surface between the SEI layer and the outer surface of the Cu electrode. The latter case is unlikely because the Li adhered directly to the Cu should be fully stripped off at a voltage of 0.5 V vs Li/Li+. Indeed, the voltage cycling profile (Figure S1 of the Supporting Information) reveals that the voltage increases sharply at the end of the charging process, indicating complete stripping of the Li from the Cu substrate. Therefore the peak at 264 ppm is associated with the dead metallic Li trapped inside the SEI layer, including any broken Li dendrites.32 In order to quantitatively compare the amounts of the various Li species between sample A and sample B, the peak area of the 264 ppm peak associated with sample B is normalized to 1.00 in units of per mg per scan (see Table S1 in the Supporting Information). The results show that sample A contains less metallic Li (0.61) but significantly more lithiated species (including organic and inorganic compounds) (0.80) compared with the case of sample B (1.00 for metallic Li and 0.21 for lithiated species, see Figure 1). A decreased amount of metallic Li in the SEI correlates to enhanced cycling performance, while more lithiated species in the SEI indicates more anion components and thus a denser layer for the concentrated electrolyte case, consistent with the results from SEM studies of this high performance Cu|Li cell system using 4 M LiFSI/DME as electrolyte.37 Notably, the observation of metallic Li in the SEI layer is an important and unique result obtainable through the use of NMR methods. Previous SEM and XPS studies were unable to observe the dead Li in the SEI layer.37



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The same strategy can be used for comparing the SEIs using different electrolyte formulations. Figure 1C shows the 6Li MAS NMR spectrum acquired using the same experimental conditions on sample C (residual left on Cu substrate of Cu|Li cell after 200 cycles in 3 M LiTFSI-DME electrolyte and final delithiation). The normalized amount of metallic Li (0.9) and the lithiated species (0.63) in sample C both lie between the amounts found for Sample A and Sample B. The electrochemical performance of the battery for sample C indeed lies between Sample A and Sample B (see Figure S1 of the Supporting Information), where the plating/stripping Coulombic efficiency (CE) (ratio of charge recovered from stripping and the required for plating of Li metal) achieves a value of 99.3% for the 4 M LiFSI (Sample A), 98.2% for the 3 M LiTFSI (Sample C) and 88.9% for the 1 M LiFSI (Sample B) systems. Thus, these results suggest that the Cu|Li cell using highly concentrated LiFSI-DME electrolyte can efficiently form a denser SEI layer on a Cu electrode comprising relatively less metallic Li and more lithiated species in the SEI than those from either a dilute electrolyte or a LiTFSI-DME electrolyte. Identification of the lithiated species within the SEI is of major importance for understanding both the physical and chemical properties of the layer. The insets in Figure 1 show the spectral range between -10 and 10 ppm to highlight the peaks corresponding to the lithiated species for the three samples. For sample A in Figure 1a, a sharp peak located at about -1.8 ppm is clearly observed along with two broad peaks located at about -0.25 and 1.4 ppm. For sample B in Figure 1b, three main peaks and a shoulder peak are observed, where a sharp peak located at about -1.8 ppm, two broad peaks located at about 0 and 1.0 ppm, and an additional shoulder peak features at about 2.9 ppm are observed. For sample C in Figure 1c, the lithiated species are dominated by an overall narrower resonance, where two shoulder peaks at 2.5 and 1.0 ppm and a


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major peak centered at about 0.6 ppm are seen, indicating the SEI layers from the different samples contain different lithiated species both in quantity and in speciation.

Figure 2. 6Li MAS NMR spectra of standard samples. (a) LiFSI, spun at 11200 Hz, with recycle delay 150 s and 492 scans, (b) LiTFSI, with 48 scans, (c) LiF, spun at 11200 Hz, with recycle delay 150 s and 400 scans, (d) Li2CO3, with 892 scans, (e) LiOH•H2O, spun at 11100 Hz, with recycle delay 80 s and 30 scans, (f) Li2S, with 2232 scans, (g) Li2O, with 800 scans. The spectra of (b, d, f, g) were acquired with 10 kHz spin speed and 80 s recycle delay. The line broadenings used for processing the data were 20 Hz for (a, b, c, e) and 10 Hz for (d, f, g).

To perform spectral assignments for the various lithiated peaks observed in the spectra of Figure 1, the 6Li MAS NMR spectra of LiFSI, LiTFSI, LiF, Li2CO3, LiOH•H2O, Li2S and Li2O – all of which are possible lithiated species 7, 14, 17, 43 in the SEI layers of a Li-ion battery - were acquired at a magnetic field of 19.97 T. As shown in Figure 2, the line widths of these standards are substantially different due to the different electric-field-gradient (EFG) environments around the



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Li nucleus. 44 Through comparisons between Figure 2 and Figure 1, the sharp peaks centered at 1.8 ppm for sample A and B are very close to the -1.7 ppm sharp peak of LiFSI; the -0.25 and 0 ppm peaks are close to those of Li2CO3 (0.06 ppm), CH3OLi, (around 0 ppm; ref. 29) and Li2O2 (0 ppm based on our previous results

45

). To further confirm the existences of the carbon-

containing Li2CO3 and proton-containing CH3OLi, sample A was further subjected to 13C and 1H NMR analyses. For the 13C spectrum (see Figure S2 in the Supporting Information), no peak in the vicinity of 180 ppm for Li2CO3 was obtained, indicating no observable Li2CO3 in the sample; while two classes of peaks for DME at 72 and 58 ppm were observed. For the 1H spectrum (see Figure S3), two broad peaks are located at 3.6 and 3.5 ppm, which is mainly due to the solvent DME and/or the existence of CH3OLi

29

; and a broad peak centered at around -1.2 ppm is

observed, which can be assigned to LiOH by comparing with literature reports (a broad peak in the range -1.0 to -1.5 ppm) 46-47. Thus, the degradation species Li2O2 and CH3OLi may exist in the SEI layers. The peaks at 1.4 and 1.0 ppm are close to the 1.3 ppm peak of LiOH•H2O, where the literature 6Li chemical shift value of dehydrated LiOH is about 1.0 ppm.46 The shoulder peak at 2.5 ppm in Figure 1c appears close in shift value to the peak from Li2S, and the 2.9 ppm feature in Figure 1b has the same chemical shift as the resonance from Li2O. The peak with a center at 0.6 ppm in Figure 1c, however, cannot be assigned as readily, but could arise from a combination of two to three peaks as is evident from the detailed deconvolution below. Overall, resonances from LiFSI and LiOH are assigned in both sample A and sample B; Li2O is clearly present in sample B. Sample C evidently contains Li2S and LiOH. Furthermore, the existence of Li2O2 and CH3OLi in these samples are possible. However, it is difficult to separate the peaks associated with LiF in the three samples as the 6Li NMR peak of LiF overlaps with the broad signals of SEI spectra between -1.8 and 0 ppm.


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Figure 3. 19F MAS NMR spectra of (a) background, spun at 11.2 kHz, (b) SEI from 4 M LiFSIDME electrolyte (sample A), spun at 10.0 kHz, (c) SEI from 1 M LiFSI-DME electrolyte (sample B), spun at 7.2 kHz, (d) SEI from 3 M LiTFSI-DME-electrolyte (sample C), spun at 8.1 kHz, (e) LiF standard, spun at 11.2 kHz, (f) LiTFSI standard, spun at 11.2 Hz, (g) LiFSI standard, spun at 10.5 kHz. 128 scans were accumulated with 10 s recycle delay for all of the samples. The spinning side bands are indicated by the red asterisks. The line broadening used for processing the data was 50 Hz.

In order to further assign the 6Li signals corresponding to LiF, LiFSI and LiTFSI in the SEIs, 19F NMR was applied to identify these fluorine-containing components. Due to the much larger chemical shift range than that found for 6Li,

19

F NMR is more sensitive to local structure



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changes, offering a sensitive tool to differentiate the various fluorine containing compounds in the SEI. Figure 3 compares the

19

F MAS NMR spectra of the SEIs from 4 M LiFSI-DME

electrolyte (sample A), 3 M LiTFSI-DME electrolyte (sample B), 1 M LiFSI-DME electrolyte (sample C) and standard samples of LiF, LiTFSI, LiFSI, as well as the experimental background. It is apparent that the center band at -207.0 ppm is from LiF (note that the center band location was identified by varying the sample spinning rate), and the peak at 50.4 ppm in Figure 3b for sample A is from LiFSI. The peak at -130.7 ppm is the background signal. Thus, the existences of LiF and LiFSI have been further confirmed in Sample A. For sample B and sample C, the 50 ppm peak of LiFSI was observed, but the -207 ppm of LiF was not observed for Sample B (see Figure 3c). Several unassigned peaks emerged at -75 to -80 ppm, which are generally considered to arise from the degradation products of -SO2CF3, -COCF3, -CF3, or -CF2 groups. These observations indicate that the existence of LiF in sample B is unlikely or with quantity too low to be observed, and some Li+ species associated with -SO2CF3, -COCF3, -CF3, or -CF2 groups may be the additional fluorine-containing components. Figure 3d for sample C comprises the signal of background and a peak centered at -80.9 ppm, which is assigned to LiTFSI. A broad peak located at about 207 ppm is also observed, which can be assigned to LiF. In summary, the solutes (LiFSI and LiTFSI) of the electrolytes were clearly detected in their corresponding SEIs, while only samples A and C contain LiF with sufficient quantity to be observed. Combining the results obtained from 6Li,

19

F,

13

C, and 1H MAS NMR studies, it become clear

that LiFSI, LiF, Li2O2 (and/or CH3OLi), LiOH and Li2O exist in sample A, while LiF is not observable in sample B, and LiTFSI, LiOH, Li2S and LiF are possible components in sample C. To extract quantitative information regarding the lithiated species from the 6Li NMR spectra, deconvolutions have been performed on the SEI spectra and the results are shown in Figure 4. A 16 ACS Paragon Plus Environment

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minimum of six individual peaks are required to deconvolute the spectra for sample A and sample B, while five peaks are needed for sample C. Values for the observed chemical shifts, as well as the relative ratios of the integrated signal intensities, are summarized in Table 1. The integrated peak intensities (normalized by unit weight and unit number of scans) are listed in Table S2 of the Supporting Information. Since more peaks are obtained from the spectral deconvolution than via visual examination, the assignments are open for review and revision.



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Figure 4. The expanded plots of Figure 1 between -20 and 20 ppm and the deconvolutions of the 6 Li NMR spectra for the SEIs harvested from Cu|Li batteries with (a) 4 M LiFSI-DME (sample A), (b) 1 M LiFSI-DME (sample B) and (c) 3 M LiTFSI-DME (sample C) as electrolyte.

As discussed above for sample A, the sharp peak at -1.8 ppm (line width 38 Hz) is attributed to LiFSI. The broad peak at -0.9 ppm (252 Hz) is ascribed to LiF. The broad peak at -0.1 ppm (177 Hz) is assigned to Li2O2 and/or CH3OLi, since no

13

C NMR signal from Li2CO3 was

observed. The peaks at 1.3 ppm (201 Hz), 2.4 ppm (227 Hz) and 2.8 ppm (529 Hz) are assigned to LiOH, Li2S, and Li2O, respectively.

For sample B, the spectrum has very similar

deconvoluted peaks as sample A. For this example, the peaks at -1.9 (56 Hz), 0 (201 Hz), 1.3 (184 Hz), 2.4 (221 Hz) and 2.9 (263 Hz) ppm correspond well with the standard peaks of LiFSI, Li2O2 (and/or CH3OLi), LiOH, Li2S and Li2O, respectively. Considering the absence of the 19F NMR signal for LiF in sample B, the -1.1 ppm (244 Hz) peak is now assigned to lithiated species associated with -SO2CF3, -COCF3, -CF3 or –CF2 groups. For sample C, the peaks are assigned as follows: the peak at 2.4 ppm to Li2S, the peak at 1.0 ppm to LiOH, the peak at 0 ppm to Li2O2 (and/or CH3OLi), the peak at -1.0 ppm to LiF and the peak at -1.2 ppm to LiTFSI (see Figure 4c). It should be pointed out that the broader line widths associated with these deconvoluted peaks when compared to those of the corresponding standard (crystalline) samples are due to the less crystalline nature of the Li species formed in SEIs.

Table 1. Decomposition details of 6Li NMR spectra of the SEI samples. Standard samples

Chemical shifts (ppm) of standard samples

Li2O

2.9

Chemical Shift (Line Width) and Peak Ratio Sample A

Sample B

Sample C

2.8 (529 Hz) 7±2%

2.9 (263 Hz) 26±2%

-


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a

Li2S

2.4

2.4 (227 Hz) 25±2%

2.4 (221 Hz) 9±2%

2.4 (166 Hz) 11±2%

LiOH•H2O

1.3

1.3 (201 Hz) 24±1%

1.3 (184 Hz) 23±1%

1.0 (169 Hz) 35±2%

Li2O2/CH3OLi

0

-0.2 (177 Hz) 23±1%

0 (201 Hz) 30 ±1%

0 (195 Hz) 36±2%

LiF

-1.0

-0.9 (252 Hz) 17±1%

-1.1 (244 Hz) 10±1%a

-1.0 (277 Hz) 5±2%

LiFSI

-1.7, -2.2

-1.8 (38 Hz) 5±0.5%

-1.9 (56 Hz) 3±0.5%

-

LiTFSI

-1.2

-

-

-1.2 (521 Hz) 12±2%

Lithiated species associated with -SO2CF3, -COCF3, -CF3 or –CF2 groups.

The following results are readily obtained by evaluating the quantitative results in Table 1. (1) samples A and B contain similar decomposed lithiated species except LiF; (2) For the comparison between SEIs from different concentrations of LiFSI-DME electrolyte, the contents of LiFSI and Li2S in sample A are significantly higher than those in sample B, while the content of Li2O in sample B is much higher than that of sample A, and LiF exists in sample A but not Sample B; (3) For the comparison between SEIs from different types of electrolyte (LiFSI and LiTFSI), sample C (the one with LiTFSI) contains fewer lithiated species than samples A and B, i.e. Li2O is absent in sample C, and a significant smaller amount of LiF, both in relative ratio and absolute peak area, than sample A (see Table 1 and Table S2 in the Supporting Information). It has been reported previously that there are more FSI- anions in the first solvation shell of Li+ ions in Cu|Li batteries under highly concentrated (4M LiFSI-DME) conditions compared with the coordination environment of Li+ when using a 1M LiFSI-DME electrolyte

40

. This is

consistent with the higher content of LiFSI, LiF and Li2S observed in sample A and can be explained by the fact that Li+ ions in highly concentrated 4 M LiFSI-DME electrolyte have more



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chance to react and co-deposit with FSI- group, where LiF is a reaction product and the LiFSI is the co-deposited product in the SEI layer. Because LiF exhibits good ionic conductivity and mechanical stability, increased amount of Li halides in the SEI (especially LiF) can lead to significant improvement in the CE of Cu|Li cells using 4 M LiFSI-DME electrolyte as reported previously.14-17 4. CONCLUSIONS The fundamental mechanism that leads to excellent electrochemical performance of Cu|Li cells using high concentration LiFSI-DME electrolytes has been investigated. The SEI samples from Cu|Li cells with two different concentrations of LiFSI-DME electrolytes (1 M and 4 M) were studied by multinuclear (6Li, 19F, 13C and 1H) MAS NMR experiments. Several important results are obtained in this work. The SEI layer associated with higher concentration (4 M) of LiFSIDME contains a relatively smaller amount of “dead” metallic Li than the cases with dilute electrolytes (1 M LiFSI-DME and 3 M LiTFSI-DME). This finding is critical as less “dead” metallic Li present in the SEI layers can significantly improve the safety of Li metal batteries after long term cycling. The SEI associated with the 4 M LiFSI-DME electrolyte is denser than those formed in the 1 M LiFSI-DME and 3 M LiTFSI-DME systems. LiFSI, LiF, Li2O2 (and/or CH3OLi), LiOH, Li2S and Li2O are found in the SEI samples obtained in the cells utilizing the 4 M LiFSI-DME electrolyte. The amount of LiF, a lithiated compound with superior mechanical strength and good Li+ ionic conductivity, observed in concentrated 4 M LiFSI/DME system is not detectable in the 1 M LiFSI/DME system. The SEI associated with the 3 M LiTFSI-DME electrolyte contains fewer lithiated species than the LiFSI-DME systems, i.e., with the absence of Li2O and significantly decreased amounts of Li2S and LiF. These


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studies reveal the fundamental mechanisms behind the excellent electrochemical performance associated with higher concentration LiFSI-DME electrolyte systems.

ASSOCIATED CONTENT Supporting Information Li plating/stripping profiles; peak intensities;

13

C and 1H MAS NMR spectra; additional

19

F

MAS NMR spectra; and deconvoluted peak intensities. AUTHOR INFORMATION Corresponding Authors * Tel: (509) 371-6544; fax: (509) 371-6546; e-mail: [email protected]. * Tel: (509) 372-6515; e-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES). The NMR sample preparations were supported by funding from the U.S. 21 ACS Paragon Plus Environment


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