Effect of LiFSI Concentrations to Form Thickness and Modulus

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C: Energy Conversion and Storage; Energy and Charge Transport

Effect of LiFSI Concentrations to Form Thickness and Modulus Controlled SEI Layers on Lithium Metal Anodes Muqin Wang, Liyuan Huai, Guohong Hu, Shanshan Yang, Feihong Ren, Shuwei Wang, Zhenggang Zhang, Zhenlian Chen, Zhe Peng, Cai Shen, and Deyu Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02314 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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The Journal of Physical Chemistry

Effect of LiFSI Concentrations to Form Thickness and Modulus Controlled SEI Layers on Lithium Metal Anodes †‡

†

† §,1

Muqin Wang, , ,1 Liyuan Huai, ,1 Guohong Hu, ,

†

†§

Shanshan Yang, Feihong Ren, , Shuwei

Wang,† Zhenggang Zhang,† Zhenlian Chen,† Zhe Peng,†,* Cai Shen,†,* Deyu Wang†,*

† Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. ‡ University of Chinese Academy of Sciences, Beijing, 100049.

§Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, 215123, China.

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ABSTRACT: Improving the cyclic stability of lithium metal anodes is of particular importance for developing high energy density batteries. In this work, a remarkable funding shows that the control of lithium bis(fluorosulfonyl)imide (LiFSI) concentrations in electrolytes significantly alter the thickness and modulus of the related SEI layers, leading to varied cycling performances of Li metal anodes. In an electrolyte containing 2 M LiFSI, a SEI layer ~ 70 nm that is obviously thicker than those obtained in other concentrations, is observed through in-situ atomic force microscopy (AFM). In addition to the decomposition of FSI- anions that generates rigid lithium fluoride (LiF) as SEI component, the modulus of this thick SEI layer with high LiF content could be significantly strengthened to 10.7 GPa. Such a huge variation in SEI modulus, much higher than the threshold value of Li dendrite penetration, provides excellent performances of Li metal anodes with Coulombic efficiency higher than 99%. Our approach demonstrates that the FSIanions with appropriate concentration can significantly alter the SEI quality, establishing meaningful guideline for designing electrolyte formulation for stable lithium metal batteries.

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1. Introduction The need to advance electrode materials for high-energy batteries is made urgent by the expanding market, including portable electronics, electric vehicle and smart grid.1,2 Lithium metal anode that possesses ultrahigh capacity (3860 mAh g-1) and the lowest reduction potential (-3.04 V vs. standard hydrogen electrode) is undoubtedly the most promising candidate for its integration in next rechargeable lithium metal batteries (LMBs), that can potentially offer 2-3 times higher specific energies than that of the state-of-art lithium ion batteries (LIBs).3 However, uncontrolled Li plating/stripping intrinsically render an extremely instable interface of Li metal anode, facilitating dendritic growth and uneven breakthrough of the solid electrolyte interphase (SEI) layer, causing low Coulombic efficiency (CE) and even fire/explosion accidents triggered by the internal short-circuit.4

The SEI layers formed in conventional organic electrolytes are generally fragile and cannot withstand the drastic volume change of Li metal, while the as-generated breakthroughs often act as “hot spots” for subsequent Li deposition that accelerate the dendritic growth.4 Manipulating the Li/electrolyte interface for a more stable cycling of Li metal anodes is targeted as a common objective by the scientific community in recent years. Tremendous effects made by the use of artificial protective layer,5-11 Li host structure,12-16 3D high-area-surface matrix,17-21 and new formulation of electrolyte,22-31 have conducted to significant progress for the Li metal protection.

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Among the strategies devoted to Li metal protection, composing suitable electrolyte to improve the adaptability of SEI layer is eagerly pursued due to its convenience in practical applications.32 To address this issue, film-forming additives to form high quality SEI layer,22-26 Li salts with high ion conductivity,27-29 and stable solvents against electrochemical reduction,30,31 have been widely investigated by the scientific community.

A uniform SEI layer with high Li ion diffusivity could effectively regulate the initial deposits and subsequent Li plating/stripping morphology, meanwhile high mechanical strength of the SEI is also needed to withstand the large volume expansion of Li metal anode for high area capacities. Recently, lithium bis(fluorosulfonyl)imide (LiFSI) has been explored as an emerging salt to improve the cycling stability of Si and Li metal anodes.33-36 However, representative illustrations for the related SEI formations has not been well achieved in these electrolyte systems. We believe that a deepened understanding of the LiFSI-assisted SEI formation is helpful for designing and constructing durable SEI layer on lithium metal anodes.

Based on this consideration, we extensively assessed the chemical and physical properties of the SEI layers formed in LiFSI electrolytes based on 1,2-dimethoxyethane (DME) solvent. Using an in-situ atomic force microscopy (AFM), a remarkable funding shows that the control of LiFSI concentrations significantly alters the thickness and modulus of the related SEI layers. Among the investigated concentrations, a SEI layer ~ 70 nm, which was obviously thicker than those formed in other concentrations, has been obtained for the concentration of 2 M LiFSI. In addition 4 ACS Paragon Plus Environment

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to the presences of rigid lithium fluoride (LiF) as SEI component derived from the FSIdecompositions, the thickest SEI layer formed in the 2 M LiFSI-DME electrolyte possessed the highest LiF content and an outstanding Young’s modulus > 10 GPa, which largely exceeds the threshold value of Li dendrite penetration. Such a strong SEI layer not only provides tight isolation of Li metal from the electrolyte corrosion, but also enables fast Li diffusion for low polarization and uniform Li deposition. Due to the stable interphase layer on Li anode with uniform Li deposits, the CE is maintained higher than 99% in the Cu-Li cell for more than 250 cycles, at a current density of 0.5 mA cm-2. Impressively, stable cycling was also obtained even at a current density of 5 mA cm-2, proving the ability of cycling Li metal anode at high current density by the valuable control of salt concentration in LiFSI based electrolyte.

2. Experimental section 2.1 In-situ AFM measurements LiFSI and LiTFSI were purchased from Guotai-Huarong New Chemical Materials Co., Ltd. DME was purchased from Sigma-Aldrich Co. LLC. All the electrolyte constituents were used as received. In-situ AFM (Bruker Icon) experiments were performed in a glovebox (MBRAUN, H2O ≤ 0.1 ppm, O2 ≤ 0.1 ppm) at ambient temperature. The Li-HOPG cell was composed of HOPG electrode as working electrode (WE) and Li wire as counter and reference electrodes (CE and RE). HOPG (Bruker Corporation, ZYB grade, 12 × 12 × 2 mm) was cleaved with adhesive tape to obtain a flat basal plane. The electrolytes used were LiFSI or LiTFSI based electrolytes in different concentration. SEI layer was formed by cycling the Li-HOPG cell at a scanning rate of 5 ACS Paragon Plus Environment


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5 mV s-1 between 3.0 and 0 V. AFM topography was collected in ScanAsyst in Fluid mode using SCANNASYST-FLUID+ tip (Bruker Corporation). After CV cycled, the SEI film was formed on HOPG electrode. Contact mode was applied to scratch the surface using the same probe. The tip scratched SEI film with a constant force of 300 nN in 2 × 2 µm2, and got in-situ topography in 5 × 5 µm2 scale by ScanAsyst in Fluid mode. The height profile was analyzed by NanoScope Analysis program. HOPGs were removed from the Li-HOPG cells and rinsed with dimethylcarbonate (DMC) to remove residual electrolyte in an argon-filled glovebox. All the HOPG samples were dried in the glovebox to measure Young’s modulus of SEI films by PeakForce QNM mode. PeakForce QNM mode using TAP-525A tip (Bruker Corporation) to map Young’s modulus after a correction of modulus and deformation with standard samples (offered by Bruker Corporation).

2.2 Electrochemical measurements Cycling tests of Li plating/stripping were performed in Cu-Li cells using a battery testing system (LandCT2001 from LAND electronics Co, Ltd). Cu electrodes were used as working electrodes. Li foils were used as counter electrodes. Coin cells CR2032 were used for cell assembly, with Celgard separator film (diameter: 18 mm; thickness: 20 µm) in which an electrolyte amount of 70 µL was deposed. The same process was applied to assembly Li-Li cells. Voltage profiles and electrochemical impedance spectroscopy (EIS) spectra of the Li-Li cells were monitored using a potentiostat/galvanostat 1470E equipped with a frequency response analyzer 1455A from Solartron. The EIS measurements were performed in the frequency range from 1 × 10-1 to 1 × 6 ACS Paragon Plus Environment

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105 Hz with a voltage perturbation of 5 mV. Ionic conductivities were measured by using DDS307A conductivity meter (LEICI, Shanghai INESA Co.) in a glovebox (MBRAUN, H2O ≤ 0.1 ppm, O2 ≤ 0.1 ppm).

2.3 Electrodes characterizations After they were cycled, the Cu electrodes in Cu-Li cells were carefully disassembled in glovebox. The Cu electrodes were rinsed with pure DMC to eliminate residual trace of solvents and salts, and then they were stored in glovebox for further characterization. The microscopy analysis of the surface morphology was performed using scanning electron microscopy (SEM; FEI, QUANTA 250 FEG) and surface analysis was conducted with a PHI 3056 X-ray photoelectron spectrometer (XPS), which was excited by a Mg Kα radiation source at a constant power of 100 W (15 kV and 6.67 mA). For the in-situ and operando observation in the optical fixture, the images were recorded using a Leica DVM6 microscopy.

2.4 Modeling implementations All periodic plane wave pseudopotential density functional theory (DFT) calculations were carried using the Vienna ab initio simulation package (VASP) code.37,38 The electron-ion interactions were calculated by the projected augmented wave (PAW) method,39,40 and the exchange-correlation energies were described by the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional.41 The Li(110) surface was built by 3 × 6 7 ACS Paragon Plus Environment


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unit cells with four periodic atom layers, and a vacuum thickness of 15 Å was used to remove any interactions between slabs. The bottom two Li layers were fixed and the top two layers were fully relaxed. The plane wave basis set with the cutoff energy of 400 eV was used and geometries were relaxed until the forces on all atoms were less than 0.03 eV Å-1. The dipole correction along the z direction was applied to take into account the effect of the induced dipole moment. The Brillouin zone sampling was carried out using Monkhorst-Pack k-points of 3 × 2 × 1. The transition states (TSs) were well located with the climbing-image nudged elastic band (CI-NEB) method and verified by the existence of a single imaginary frequency based on the vibrational analysis.42,43

3. Results and discussion A systematic study of the SEI layers formed on highly oriented pyrolytic graphite (HOPG) substrates (Figure 1a) in various concentrations (1, 2 and 3 M) of LiFSI in DME was caught out using in-situ atomic force microscopy (AFM) coupled with electrochemical station. Cyclic voltammetry (CV) was used to provide the SEI formations, while topography and Young's modulus of the SEI layers were measured after each reductive scanning down to 0 V vs. Li/Li+, through appropriate techniques provided by the AFM. One should be mentioned that, after the formation of SEI layer in each CV cycle, a scrapping process using the AFM tip under a contact mode was performed to inform the SEI thickness.


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A dynamic structural evolution of the SEI layer during CV scanning from 3.0 to 0.28 V vs. Li/Li+ is shown in Figure 1b, for the electrolyte of 3 M LiFSI-DME. During the mapping, the AFM probe moved from the top to the bottom of the mapping zone then followed by its return to the initial position. In parallel, the CV scanning is kept for the reduction from 3.0 to 0.28 V vs. Li/Li+. As shown on the voltage range of 3.0 – 1.64 V vs. Li/Li+, no evident SEI layer is formed, consistent to the absence of reductive peak on the CV curves in Figure 1c. The formation of SEI layer mainly started below 1.64 V vs. Li/Li+, while the CV curves indicate that the formation of SEI layer in these LiFSI based electrolytes should take place ~ 1.2 V vs. Li/Li+. For the voltage range of 1.2 – 0.28V vs. Li/Li+, stable SEI layer with granular shape is observed for 3 M LiFSIDME. One should be mentioned that such a dynamic structural evolution could only be observed for the 1st CV cycle, and once the SEI layer is completely formed at 0 V vs. Li/Li+, no more changes could be observed for the subsequent CV cycles, indicating that the SEI layers should be stable for the initial cycling of Li metal anodes.

As shown in Figure 1c, changing the LiFSI concentration has no impact on the reduction potential of SEI formation that always started at ~ 1.2 V vs. Li/Li+. However, significant increases of reductive peak areas in initial cycles are observed for the 2 and 3 M concentrations compared to that of 1 M, indicating thicker SEI layers formed in these concentrations. The integral areas for the reductive peaks are shown in Figure 1d, the values for 2 and 3 M LiFSIDME are significantly higher than that of 1 M LiFSI-DME, indicating thicker SEI layer formations. The higher value of 3 M LiFSI-DME than that of 2 M LiFSI-DME should be due to 9 ACS Paragon Plus Environment


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the reparations of larger areas scrapped-off from the HOPG substrates during the scrapping process using the AFM tip in precedent reductive scanning (at 0 V vs. Li/Li+).


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Figure 1. (a) AFM image of a pristine HOPG substrate; (b) Dynamic structural evolution of the SEI layer during the CV scanning; (c) CV curves and (d) Integral areas for the reductive peaks of the LiFSI-DME electrolytes with different concentrations of LiFSI; (e) AFM images of the SEI layers and their scrapped patterns produced in the 2nd CV cycles; (f) Young’s modulus mapping of the SEI layers formed in the 3rd CV cycles.

Similar rod-like morphology has been observed for all the concentration in the initially formed SEI layers (Figure S1), however, consistent with the reductive peak areas in CV, thicker layers of 38-40 nm were scrapped off from the SEI layers formed in 2 and 3 M LiFSI-DME, while it was only ~ 25 nm for that in 1 M LiFSI-DME. In the 2nd CV cycles, newly formed SEI layers have thicker thickness in comparison to their initial states of the 1st cycle, ~ 32, 70 and 62 nm for 1, 2 and 3 M LiFSI-DME (Figure 1e). Especially, for 2 M LiFSI-DME, the SEI morphology likely changed into smaller but more agglomerated particles. In the 3rd CV cycle, a stable thickness ~ 75 nm was kept for 2 M LiFSI-DME, while they were 32 and 48 nm for 1 and 3 M LiFSI-DME (Figure S2). The thickness variations were probably related to evolving chemical compositions in these SEI layers upon cycling, for which a detailed XPS study has been caught out and will be discussed in next part of the manuscript. It must be noted that, the mapping of Young's modulus on these SEI layers after the 3rd CV cycles showed quite different mechanical strengths. As shown in Figure 1f, an impressive high modulus of 10.7 GPa was obtained for the SEI layer formed in 2 M LiFSI-DME, while they are separately 3.5 and 4.2 GPa in 1 and 3 M LiFSI-DME. Since 6 GPa is a critical modulus value for a protective layer to inhibit the Li 11 ACS Paragon Plus Environment


dendrite growth,44 it seems that only the concentration of 2 M LiFSI in DME is able to provide suitable SEI layer for stable cycling of Li metal anodes.

To further understand the noteworthy correlation between the Young’s modulus and chemical compositions of the SEI layers formed in LiFSI-DME electrolytes, repeated Li plating/stripping process were applied to Cu electrodes in Cu-Li cells to make evolving the SEI layers. Systematically, XPS measurements were caught out to study the surface chemistry of the SEI layers. As shown in Figure 2a-c for the F 1s spectra, a persistent presence of LiF was kept for the SEI layer formed in 2 M LiFSI-DME from the 5th to 50th cycles, while Figure 2d shows that the atomic concentration of F 1s related to LiF keeps almost constant ~ 5% upon cycling. For 1 and 3 M LiFSI-DME, the concentrations of F 1s related to LiF gradually decreased with cycling proceeding, separately lowered to ~1.5 and 2 % at the 50th cycle.

It has proven that a LiF-rich SEI layer could improve Li ion diffusivity and mechanical strength to suppress SEI breakthrough by Li dendrites and corrosion reaction with organic electrolytes.45-48 Therefore, the high LiF content in the SEI layer for 2 M LiFSI-DME is a good insight for its protection of Li metal anodes. As shown by the decomposition processes of LiFSI investigated by our density functional theory (DFT) calculations (Figure 2e), the inorganic LiFSI could be easily dissolved in Li+ and FSI- anions, thus the dissociation reactions should be considered from the co-adsorption of Li+ + FSI- on Li surfaces. As shown in Figure 2e, FSI- and Li+ anions are separately adsorbed at the hollow site with two oxygen atoms and the bridge site 12 ACS Paragon Plus Environment

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in the co-adsorption structure of Li+ + FSI-. The S-F bond breaks via transition state 1 (TS1) to reach the final state of F(SO2)2N + LiF. At TS1, the S-F bond length is slightly elongated to 1.67 Å from the initial state of 1.59 Å. In the final state, both F(SO2)2N and LiF are adsorbed at the hollow site. This reaction needs to overcome a small energy barrier of 0.14 eV and is exothermic by 6.60 eV.

Consequently, the thickest SEI layer formed in 2 M LiFSI-DME should possess the highest LiF amount, compared to those obtained in other concentrations, leading to the remarkable Young’s modulus of ~ 10 Gpa, and the outstanding ability to provide stable Li plating/striping cycling that we will show later in this manuscript.



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Figure 2. XPS spectra of F 1s, S 2p, N 1s and C 1s of the SEI layers formed in 1, 2, 3 M LiFSI-DME and 1 M LiTFSI-DME, for (a) 5, (b) 20 and (c) 50 cycles; (d) Atomic concentrations of F 1s, S 2p, N 1s and C 1s for the SEI layers formed in the investigated electrolytes at different cycle numbers; Potential energy diagram and structures for decomposition product formations of (e) LiFSI and (f) LiTFSI on Li(110) surface. 14 ACS Paragon Plus Environment

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In contrast to the LiFSI-based electrolyte, the SEI layer formed in an electrolyte of 1 M LiTFSI-DME only showed -CF3 components. As shown by the decomposition processes of LiTFSI on Li surfaces (Figure 2f), the organic LiTFSI could be dissolved in Li+ and TFSIanions, thus the dissociation reactions have been taken into account from the co-adsorption of Li+ + TFSI- on Li surfaces. Starting from the initial state of TFSI- at hollow site and Li+ at the bridge site, one of the C-S bond scission occurs via TS2 to form the co-adsorbed structure of CF3(SO2)2N* + CF3* + Li+. At TS2, the C-S bond is elongated to 2.08 Å from the initial state of 1.90 Å. This reaction need to overcome an energy barrier of 0.30 eV and is exothermic by 4.15 Å. The C-F bond scission of TFSI- was also considered to react with Li+ to form LiF, however, the final state of the co-adsorption state is not achieved, indicating that the C-S bond scission is dominant during the decomposition of TFSI-. Therefore, these results are agreed with the experiment observation that much CF3 were formed at the Li surface in LiTFSI based electrolyte while LiF was the main component of LiFSI decomposition products on Li surface.

Consistently, due to the lack of LiF component, poor quality SEI layer consisted of isolated small grain particles was formed in LiTFSI-DME (Figure S3a), with thinner thickness than those formed in LiFSI based electrolyte (Figure S3b), and the Li plating/stripping cycling is extremely unstable in such electrolyte as shown later in this work. It should be noted that the measurement of Young’s modulus of such a SEI layer is not meaningful since large discontinuities on the SEI



layer expose the substrate (HOPG) towards the AFM tip during a complete scanning, and the obtained modulus is only a means value of the SEI layer and the exposed HOPG zone.

Besides LiF, other SEI components including polythionate complex, Li2SO3, Li2N2O2 and Li3N are also observed on the S 2p and N 1s spectra (Figure 2a-c). According to the reaction pathway of the FSI- anion break down proposed by our modelling, the formation of LiF originates from the reaction of splitting F ion on Li surface after the cleavage of S-F bond in FSIanion, meanwhile the S-N bond in the residual fragment F(SO2)2N could be further decomposed to form S and N containing species. Consistently, the highest atomic concentrations of S 2p and N 1s contents also belonged to the SEI layer formed in 2 M LiFSI-DME (Figure 2d), and it is worth noting that the related components of polythionate complex, Li2SO3 and Li3N have also proven beneficial effects for fast Li ion transfer by their presences in SEI layers.50-52

The C 1s signal is normally related to the reduction of solvent molecules, as a result, similar reduction products are observed for the investigated electrolytes (Figure 2a-c), due to the use of single solvent of DME. However, the atomic contents of C 1s are totally different (Figure 2d). The low content of C 1s for the electrolyte of 2 M LiFSI-DME indicates a preferential decomposition of LiFSI instead of DME at Li surface to form a thick and LiF-rich layer, which sustainably protects the Li surface by hindering side reaction with solvent, as shown by the remained low content of C 1s upon cycling. In sharp contrast, especially for the SEI formed in 1 M LiTFSI-DME, the content of C 1s rapidly increases with cycling proceeding, indicating 16 ACS Paragon Plus Environment

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massive side reactions of Li surface with solvent. Even taking into account of -CF3 derived from the decomposition of LiTFSI (~1-2 % in atomic concentration), the residual content of C 1s is still significantly higher than other cases. The latter is consistently related to the absence of LiF content and poor quality of the SEI layer formed in the 1 M LiTFSI-DME electrolyte.

The cycling performances of Cu-Li coin cells are shown in Figure 3a. At 0.5 mA cm-2 for a fixed areal capacity of 1 mAh cm-2, the 2 M LiFSI-DME electrolyte showed outstanding stability of Li plating/stripping with a CE > 99% for more than 250 cycles. As previously mentioned, the SEI quality must play a crucial role for the cycling of Li metal anode. Due to the abundant content of LiF, the thick SEI layer with high modulus formed in 2 M LiFSI-DME could greatly isolate the uniformed Li deposits from the electrolyte, and stabilize Li/electrolyte interface for subsequent cycles. Although the SEI layer formed in 2 M LiFSI-DME is two times thicker than that of 1 M LiFSI-DME (Figure 1e), and the ionic conductivity of 2 M LiFSI-DME (6.16 mS cm1

, Figure S4) is lower than that of 1 M LiFSI-DME (6.54 mS cm-1, Figure S4), their voltage

hysteresis are still similar ~ 45 mV from the discharge-charge profile of the 100th cycle (Figure 3b). These results prove the high Li ion diffusivity of the SEI layer formed in 2 LiFSI-DME, which is a dominant factor for the cell polarization. For 3 M LiFSI-DME, the hysteresis increased to 85 mV at the 100th cycle, which might be a result of the low ionic conductivity (5.58 mS cm-1, Figure S4) and the increased interfacial impedance of the SEI layer upon cycling.



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Figure 3. (a) Cycling performances of Cu-Li cells for the investigated electrolytes at current densities of 0.5 and 5 mA cm-2 for a fixed areal capacity of 1 mAh cm-2; Dischargecharge profiles of the 100th cycle for the investigated electrolytes in conditions of (b) 0.5 mA cm-2 and (c) 5 mA cm-2, for a fixed areal capacity of 1 mAh cm-2; (d) voltage profiles with in-situ recorded EIS spectra of Li-Li symmetrical cells using 1, 2 and 3 M LiFSI-DME electrolytes, at a fixed current density of 0.5 mA cm-2 with a stripping/plating capacity of 1 mA h cm-2.


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It should be noted that the cycling stability of Li metal anodes in 1 M LiTFSI-DME was far less than those obtained in LiFSI based electrolytes. The CE rapidly dropped below 25% after 10 cycles at 0.5 mA cm-2 and 1 mAh cm-2 (Figure 3a). It seems that the high content of organic species in the SEI layer formed in 1 M LiTFSI-DME, such as -CF3 (Figure 2a-c), is a main cause for the poor performances. It caused not only a non-uniform SEI layer (Figure S3), but also viscous and compact Li deposits (Figure S5a and b), causing drastic polarization for both Li plating and stripping process (the voltage hysteresis ~ 195 mV at the 100th cycle, Figure 3b).

In practical application of Li metal batteries, the current density has to be handled higher than 3 mA cm-2. In this work, a high current density of 5 mA cm-2 has been applied in Cu-Li cells to assess the cycling stability of Li metal in the LiFSI-based electrolyte. As expected, the best performances still belong to those obtained in 2 M LiFSI-DME, with an average CE ~ 96% over 150 cycles (Figure 3a). In comparison, the cycling in 1 M LiTFSI-DME and 1 M LiFSI-DME are unfeasible, while the CE for 3 M LiFSI-DME is ~ 90% for less than 150 cycles. At the 100th cycle, the voltage hysteresis is only ~ 170 mV for 2 M LiFSI-DME, significantly lower than those achieved in other electrolytes (Figure 3c and Figure S6). Two typical carbonate and ether based electrolytes, i.e. 1 M LiPF6-EC:DEC with 5% vol. FEC and 1 M LiTFSI-DME:DOL with 0.2 M LiNO3, have also been tested. As shown in Figure S7, the cycling are severely fluctuating, indicating their inability to enable Li metal cycling at high current density.



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Li-Li symmetrical cells coupled with electrochemical impedance spectroscopy (EIS) have been investigated to study the detailed internal impedance changes in the LiFSI based electrolytes (Figure 3d). The voltage hysteresis of Li-Li cells give direct insights of the internal impedance, including the bulk impedance of the electrolyte (the high frequency intercept impedance) and the interfacial impedance related to the passivation state of Li surfaces (the depressed semicircle).

As shown in Figure 3d, at the beginning of each test (from 4 to 40 h, shown on the EIS spectra), all the cells obeyed a reduction of interfacial impedance that is a so-called “activation process” during which SEI layers formed on the Li foils up to a steady state. Over a cycling of 200 h, the voltage hysteresis was stably maintained ~ 30 mV in 2 M LiFSI-DME, and its impedance curves almost remained constant from 80 to 200 h (~3 and 1.8 Ω for the bulk and interfacial impedances). In contrast, the cycling in 1 M LiFSI is clearly fluctuating and the polarization progressively enlarged, namely due to the increase of bulk impedance caused by electrolyte decomposition at the instable Li/electrolyte interfaces (15, 32 and 42 Ω at 80, 120 and 160 h). For 3 M LiFSI-DME, a sudden increase of the hysteresis (~ 300 mV) has occurred after 150 h, whereas the bulk and interfacial impedances were separately raised to 48.5 and 43 Ω at 160 h. The latter was probably due to the achievement of too high salt concentration during the drying up of solvent, which simultaneously caused low ionic conductivity and viscous Li/electrolyte interfaces, then resulted in very sluggish Li ion transfers.


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From the Li-Li cell tests, it clearly demonstrates the unique cycling stability of Li metal anodes in the electrolyte of 2 M LiFSI-DME. According to the results obtained in this work, the stable cycling must be a result of the stabilized Li/electrolyte interface, enabled by the thick SEI layer enriched in LiF content with high mechanical stability and ion diffusivity. To give direct insight of the interfacial morphologies correlated to their electrochemical performances, the accumulations of side products in different electrolytes have been pursued by using a specially designed operando optical fixture, as shown in Figure 4a. This fixture is consisted of an electrically connected quartz chamber to allow direct optical observation of Li plating/stripping on a Cu substrate. The obtained stable voltage profiles of Li plating/stripping processes prove the reliability of our operando fixture (Figure 4b).

The optimal images were recorded at the end of each plating or stripping processes. As shown in Figure 4c, significant concentration-controlled morphologies were observed for 1-3 M LiFSIDME. After the 1st plating process (2.5 mAh cm-2 at a current density of 1.25 mA cm-2), dendritic deposits were still observed in 1 M LiFSI-DME while it progressively changed into granular and compact shapes with concentration increasing, especially in 3 M LiFSI-DME. The SEM images also confirmed this trend at smaller scale (Figure 4d-f). Among these electrolytes, clearly separated deposits were found in 1 M LiFSI-DME (Figure 4d), meanwhile a very compact layer was observed in 3 M LiFSI-DME (Figure 4f). It should be mentioned that, the growth of Li dendrites was much more extensive in carbonate-based electrolyte (Figure S8).



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We have figured out the Li deposit thicknesses during the first plating process, as shown in Figure 4c, which are ~ 61 µm for the layers formed in 1 and 2 M LiFSI-DME while it’s ~ 38 µm in 3 M LiFSI-DME. The more compact Li deposits in higher LiFSI concentration has also been reported in previous work.36 It should be mentioned that, according to the Li plating amount of 2.5 mAh cm-2, the theoretical thickness of the deposited Li should be ~ 12 µm, while all the Li layers observed herein are thicker than the theoretical value, indicating that they are still porous, even they are significantly more compact than that observed in conventional carbonate electrolyte (Figure S8).


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Figure 4. (a) Schematic representation of the operando fixture for optical observation of Li plating/stripping on Cu substrates; (b) Typical voltage profiles during Li plating/stripping processes on Cu substrates; (c) Digital photos of the 1st Li plating, the 1st Li stripping, and the 20th Li stripping in 1-3 M LiFSI-DME, scale bare: 100 µm; SEM images of Li deposits on Cu substrates in (d) 1 M, (e) 2 M, (f) 3 M LiFSI-DME.



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The common phenomenon of diffusion-controlled charge transfer caused by the ion concentration gradient at the electrode surface is the main cause of dendrite growth during electroplating, while dendrite mitigation realized by using high Li salt concentration electrolyte has been implemented in previous works.27,36 We also observe the consistent morphology change by varying the LiFSI concentration in this work (Figure 4c-f). However, as above demonstrated, the thickness and LiF content of the SEI layer does not increase with the LiFSI concentration in a monotone way, the best compromise between the SEI quality and the morphology of Li deposits should be located at a moderate concentration. As shown in Figure 4c, after the 1st Li stripping, all the Li deposits were virtually decomposed, leaving thin transient zones (surrounded by yellow lines) that we consider as the formation of SEI layers. A clearer zone was observed for 2 LiFSI-DME, probably due to the higher content of inorganic species such as LiF in this SEI layer. Consistently, after 20 plating/stripping cycles, the lowest content of side products was found on the Cu substrate in 2 M LiFSI-DME, indicating that the LiF enriched thick SEI layer has exhibited a prominent effect to inhibit the side reactions during Li plating/stripping.

Finally, the LiFSI regulation strategy has also been implemented in another solvent, the tetraethylene glycol dimethyl ether (TEGDME). Assuming that the effect of LiFSI regulation is based on the concentration of salt anions, an optimal LiFSI concentration should still exist by changing the solvent. As shown in Figure S9a, based on the solvent of TEGDME, an optimal concentration of 1.5 M LiFSI is observed. A high CE ~ 98.1% was maintained for more than 150 cycles in the optimal 1.5 M LiFSI-TEGDME electrolyte. Although the optimal concentration of 24 ACS Paragon Plus Environment

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LiFSI is slightly different while shifting the solvents form DME to TEGDME (probably due to the different solvation of LiFSI in these two solvents), and the CE obtained in 1.5 M LiFSITEGDME is lower than that of 2 M LiFSI-DME (probably due to the higher reactivity of TEGDME than DME with Li metal), the general applicability of the LiFSI regulation strategy is still validated. Similar concentration controlled stabilities of Li metal cycling have also been observed for carbonate based electrolytes with various LiFSI concentrations (Figure S9b). Using EC:DEC (1:1) with 5% vol. FEC as solvent system, an optimal concentration of 1 M has been observed for the LiFSI. Based on these results, we could conclude that the regulation of Li salt anions and their concentrations should also occupy a prominent impact on the SEI quality.

4. Conclusion We have studied in detail the effect of LiFSI concentrations for improving the cycling stability of Li metal anodes. A remarkable finding revealed that the control of LiFSI concentrations significantly alter the thickness and modulus of the related SEI layers. An optimal concentration of 2 M LiFSI in DME could conduct to the formation of a SEI layer ~ 70 nm. In addition to the SEI component of rigid LiF derived from the FSI- decompositions, this SEI layers possessed the highest LiF content and Young’s modulus higher than 10 GPa, providing tight isolation of Li metal from the electrolyte corrosion, and Li diffusion for low polarization and uniform Li deposition. Consequently, the Li plating/stripping stability is significantly improved in this electrolyte, with Coulombic efficiencies higher than 99% for more than 250 cycles at 0.5 mA cm-2 and 96% for more than 150 cycles at 5 mA cm-2. In similarity, by changing the solvent in 25 ACS Paragon Plus Environment


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TEGDME, an optimal LiFSI concentration could still be obtained for stable Li metal anodes, demonstrating the universal applicability of the LiFSI concentration regulation strategy. Based on the finding of this wok, we demonstrate the importance of salt anion selection as well as the salt concentration regulation, which could significantly alter the quality of SEI layer and render the crucial effect on the cycling stability of Li metal anodes.

ASSOCIATED CONTENT

Supporting Information.

AFM images of the SEI layers and their scrapped patterns formed in LiFSI and LiTFSI based electrolytes at different CV cycles, optical and SEM images of Li deposits formed in LiPF6EC:DMC and LiTFSI-DME electrolytes, ionic conducitivies of LiFSI based electrolytes, discharge-charge profile of the 50th cycle in Cu-Li cell for 1 M LiTFSI-DME, cycling performances of Cu-Li cells for the electrolytes of 1 M LiPF6-EC:DEC (1:1) with 5% vol. FEC and 1 M LiTFSI-DME:DOL (1:1) with 0.2 M LiNO3, cycling performances of Cu-Li cells for the LiFSI-TEGDME electrolytes with various LiFSI concentrations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 1

These authors contribute equally to this work.


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Corresponding Author *E-mail: (Z. P.) [email protected]; (C. S.) [email protected]; (D. W.) [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Zhejiang Provincial Natural Science Foundation of China (Grant No. LQ17E020004 and LQ18E020004), National Natural Science Foundation of China (NSFC) for the research fund for International Young Scientists (Grant No. 51650110490), China Postdoctoral Science Foundation funded project (Grant No. 2016M601985), and the National key research and development program (Grant No. 2016YFB0100106), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA09010403).

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TOC Graphic


Effect of LiFSI Concentrations to Form Thickness and Modulus

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