Effect of LiFSI Concentrations To Form Thickness- and Modulus

Article Cite This: J. Phys. Chem. C 2018, 122, 9825−9834

pubs.acs.org/JPCC

Effect of LiFSI Concentrations To Form Thickness- and ModulusControlled 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*,† †

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China University of Chinese Academy of Sciences, Beijing 100049, China § Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China ‡

S Supporting Information *

ABSTRACT: Improving the cyclic stability of lithium metal anodes is of particular importance for developing high-energydensity batteries. In this work, a remarkable finding shows that the control of lithium bis(fluorosulfonyl)imide (LiFSI) concentrations in electrolytes significantly alters 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, an SEI layer of ∼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 an SEI component, the modulus of this thick SEI layer with a 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 FSI− anions with appropriate concentration can significantly alter the SEI quality, establishing a meaningful guideline for designing electrolyte formulation for stable lithium metal batteries. protective layer,5−11 Li host structure,12−16 3D high-areasurface matrix,17−21 and new formulation of electrolyte22−31 have contributed to significant progress for the Li metal protection. Among the strategies devoted to Li metal protection, composing a 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 a high-quality SEI layer,22−26 Li salts with high ion conductivity,27−29 and stable solvents against electrochemical reduction30,31 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 the 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 have not been

1. INTRODUCTION The need to advance electrode materials for high-energy batteries is made urgent by the expanding market, including portable electronics, electric vehicles, and smart grids.1,2 A 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 generation rechargeable lithium metal batteries (LMBs), which can potentially offer 2−3 times higher specific energies than that of the state-of-the-art lithium ion batteries (LIBs).3 However, uncontrolled Li plating/stripping intrinsically renders an extremely unstable interface of the 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 has been targeted as a common objective by the scientific community in recent years. Tremendous effects made by the use of artificial © 2018 American Chemical Society

Received: March 8, 2018 Revised: April 19, 2018 Published: April 24, 2018 9825

DOI: 10.1021/acs.jpcc.8b02314 J. Phys. Chem. C 2018, 122, 9825−9834

Article

The Journal of Physical Chemistry C

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 scraped patterns produced in the second CV cycles. (f) Young’s modulus mapping of the SEI layers formed during the third CV cycles.

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 the 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 a Li metal anode at high current density by the valuable control of salt concentration in the LiFSI-based electrolyte.

well-achieved in these electrolyte systems. We believe that a deepened understanding of the LiFSI-assisted SEI formation is helpful for designing and constructing a durable SEI layer on lithium metal anodes. On the basis of 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 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 of ∼70 nm, which was obviously thicker than those formed in other concentrations, has been obtained for the concentration of 2 M LiFSI. In addition to the presence of rigid lithium fluoride (LiF) as an SEI component derived from the FSI− decompositions, the thickest SEI layer formed in the 2 M LiFSI−DME electrolyte possessed the highest LiF

2. EXPERIMENTAL SECTION 2.1. In Situ AFM Measurements. LiFSI and LiTFSI were purchased from Guotai-Huarong New Chemical Materials Co., 9826


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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 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 climbingimage nudged elastic band (CI-NEB) method and verified by the existence of a single imaginary frequency based on the vibrational analysis.42,43

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 an 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 mm3) was cleaved with adhesive tape to obtain a flat basal plane. The electrolytes used were LiFSI- or LiTFSI-based electrolytes in different concentrations. The SEI layer was formed by cycling the LiHOPG cell at a scanning rate of 5 mV s−1 between 3.0 and 0 V. AFM topography was collected in ScanAsyst in Fluid mode using the SCANNASYST-FLUID+ tip (Bruker Corporation). After CV cycled, the SEI film was formed on an HOPG electrode. Contact mode was applied to scratch the surface using the same probe. The tip scratched the SEI film with a constant force of 300 nN in 2 × 2 μm2 and got in situ topography at the 5 × 5 μm2 scale by ScanAsyst in Fluid mode. The height profile was analyzed by the 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, using the 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 a Celgard separator film (diameter, 18 mm; thickness, 20 μm) in which an electrolyte amount of 70 μL was deposited. The same process was applied to assemble 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 × 105 Hz with a voltage perturbation of 5 mV. Ionic conductivities were measured by using a DDS-307A conductivity meter (LEICI, Shanghai INESA Co.) in a glovebox (MBRAUN, H2O ≤ 0.1 ppm, O2 ≤ 0.1 ppm). 2.3. Electrode Characterizations. After they were cycled, the Cu electrodes in Cu−Li cells were carefully disassembled in a glovebox. The Cu electrodes were rinsed with pure DMC to eliminate the 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 Xray 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 microscope. 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)

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 performed using in situ atomic force microscopy (AFM) coupled with an 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 mentioned that, after the formation of SEI layer in each CV cycle, a scraping process using the AFM tip under a contact mode was performed to inform the SEI thickness. 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, which was 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 over the voltage range of 3.0−1.64 V vs Li/Li+, no evident SEI layer is formed, consistent with the absence of a reductive peak on the CV curves in Figure 1c. The formation of the SEI layer mainly started below 1.64 V vs Li/Li+, while the CV curves indicate that the formation of the SEI layer in these LiFSI-based electrolytes should take place ∼1.2 V vs Li/Li+. For the voltage range 1.2−0.28 V vs Li/Li+, the stable SEI layer with granular shape is observed for 3 M LiFSI−DME. It should be mentioned that such a dynamic structural evolution could only be observed for the first 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 LiFSI−DME are significantly higher than that of 1 M LiFSI−DME, indicating thicker SEI layer formations. The higher value for 3 M LiFSI−DME than that for 2 M LiFSI−DME should be due to the reparations of larger areas scraped-off from the HOPG substrates during the 9827


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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, and 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.

for that in 1 M LiFSI−DME. In the second CV cycles, newly formed SEI layers have thicker thicknesses in comparison to their initial states of the first cycle, ∼32, 70, and 62 nm for 1, 2, and 3 M LiFSI−DME (Figure 1e). In particular, for 2 M LiFSI−DME, the SEI morphology likely changed into smaller but more agglomerated particles. In the third CV cycle, a stable thickness of ∼75 nm was maintained for 2 M LiFSI−DME,

scraping process using the AFM tip in the preceding reductive scanning (at 0 V vs Li/Li+). A similar rod-like morphology has been observed for all the concentrations in the initially formed SEI layers (Figure S1); however, consistent with the reductive peak areas in CV, thicker layers of 38−40 nm were scraped off from the SEI layers formed in 2 and 3 M LiFSI−DME, while it was only ∼25 nm 9828


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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. Discharge−charge 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.

to LiF remains 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 been proven that a LiF-rich SEI layer could improve Li ion diffusivity and mechanical strength to suppress the SEI breakthrough by Li dendrites and corrosion reaction with organic electrolytes.45−49 Therefore, the high LiF content in the SEI layer for 2 M LiFSI−DME is a good insight for the 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 coadsorption 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 in the coadsorption 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

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 performed, which will be discussed in the next part of this paper. It must be noted that the mapping of Young’s modulus on these SEI layers after the third CV cycles showed quite different mechanical strengths. As shown in Figure 1f, an impressively 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 dendrite growth,44 it seems that only the concentration of 2 M LiFSI in DME is able to provide a 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 processes were applied to Cu electrodes in Cu−Li cells to continue evolving the SEI layers. Systematically, XPS measurements were performed 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 retained 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 9829


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cycling proceeding, indicating massive side reactions of Li surface with solvent. Even taking into account −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 the 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 the Li metal anode. Due to the abundant content of LiF, the thick SEI layer with a high modulus formed in 2 M LiFSI−DME could greatly isolate the uniformed Li deposits from the electrolyte, and stabilize the Li/ electrolyte interface for subsequent cycles. Although the SEI layer formed in 2 M LiFSI−DME is 2 times thicker than that of 1 M LiFSI−DME (Figure 1e), and the ionic conductivity of 2 M LiFSI−DME (6.16 mS cm−1, Figure S4) is lower than that of 1 M LiFSI−DME (6.54 mS cm−1, Figure S4), their voltage hystereses are still similar at ∼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. 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 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 nonuniform SEI layer (Figure S3), but also viscous and compact Li deposits (Figure S5a,b), causing drastic polarization for both Li plating and stripping processes (the voltage hysteresis ∼195 mV at the 100th cycle, Figure 3b). In a 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 the Li metal in the LiFSIbased 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 is 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 cycles are severely fluctuating, indicating their inability to enable Li metal cycling at high current density. Li−Li symmetrical cells coupled with electrochemical impedance spectroscopy (EIS) have been investigated to study the detailed internal impedance changes in the LiFSIbased electrolytes (Figure 3d). The voltage hysteresis of Li−Li cells gives direct insights into the internal impedance, including

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 paper. 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 TFSI− anions; thus, the dissociation reactions have been taken into account from the coadsorption of Li+ + TFSI− on Li surfaces. Starting from the initial state of TFSI− at the hollow site and Li+ at the bridge site, one of the C−S bond scissions occurs via TS2 to form the coadsorbed 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 needed 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 coadsorption state is not achieved, indicating that the C−S bond scission is dominant during the decomposition of TFSI−. Therefore, these results agreed with the experimental observation that many CF3 species were formed at the Li surface in the LiTFSI-based electrolyte while LiF was the main component of LiFSI decomposition products on the Li surface. Consistently, due to the lack of LiF component, a poor quality SEI layer which 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 an electrolyte as shown later in this work. It should be noted that the measurement of Young’s modulus of such an SEI layer is not meaningful since large discontinuities on the SEI layer expose the substrate (HOPG) toward the AFM tip during a complete scanning, and the obtained modulus is only a mean value for 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 modeling, the formation of LiF originates from the reaction of splitting a F ion on the Li surface after the cleavage of the S−F bond in the FSI− anion; 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 been proven to have beneficial effects for fast Li ion transfer by their presence 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 the 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 the Li surface to form a thick and LiF-rich layer, which sustainably protects the Li surface by hindering a side reaction with solvent, as shown by the remaining 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 9830


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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 first Li plating, the first Li stripping, and the 20th Li stripping in 1−3 M LiFSI−DME, with scale bar = 100 μm. SEM images of Li deposits on Cu substrates in (d) 1 M, (e) 2 M, and (f) 3 M LiFSI− DME.

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 at ∼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 unstable 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 a salt concentration that was too high during the drying up of solvent, which simultaneously caused low ionic conductivity and viscous Li/electrolyte interfaces, and then resulted in very sluggish Li ion transfers. From the Li−Li cell tests, this 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 into 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 consists 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 process. As shown in Figure 4c, significant concentration-controlled morphologies were observed for 1− 3 M LiFSI−DME. After the first 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 a 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). We have figured out that the Li deposit thicknesses during the first plating process, as shown in Figure 4c, are ∼61 μm for 9831


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direct the formation of an SEI layer of ∼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 a 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 TEGDME, an optimal LiFSI concentration could still be obtained for stable Li metal anodes, demonstrating the universal applicability of the LiFSI concentration regulation strategy. On the basis of the findings of this work, 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.

the layers formed in 1 and 2 M LiFSI−DME while the thickness is ∼38 μm in 3 M LiFSI−DME. The more compact Li deposits in the higher LiFSI concentration have also been reported in a 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 though they are significantly more compact than that observed in a conventional carbonate electrolyte (Figure S8). 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 demonstrated above, the thickness and LiF content of the SEI layer do 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 first Li stripping, all the Li deposits were virtually decomposed, leaving thin transient zones (surrounded by yellow lines) that we consider the formation of SEI layers. A clearer zone was observed for 2 M 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, on the basis of 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 LiFSI is slightly different while shifting the solvents from DME to TEGDME (probably due to the different solvation of LiFSI in these two solvents), and the CE obtained in 1.5 M LiFSI−TEGDME 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 the solvent system, an optimal concentration of 1 M has been observed for the LiFSI. On the basis of these results, we could conclude that the regulation of Li salt anions and their concentrations should also have a prominent impact on the SEI quality.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b02314. AFM images of the SEI layers and their scraped patterns formed in LiFSI- and LiTFSI-based electrolytes at different CV cycles; optical and SEM images of Li deposits formed in LiPF6−EC:DMC and LiTFSI−DME electrolytes; ionic conducitivies of LiFSI-based electrolytes; discharge−charge profile of the 50th cycle in the 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; and cycling performances of Cu−Li cells for the LiFSI−TEGDME electrolytes with various LiFSI concentrations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Zhenlian Chen: 0000-0002-2160-0705 Zhe Peng: 0000-0002-9675-6296 Cai Shen: 0000-0001-5825-4028 Deyu Wang: 0000-0002-8364-026X Author Contributions ∥

M.W., L.H., and G.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Zhejiang Provincial Natural Science Foundation of China (Grants LQ17E020004 and LQ18E020004), National Natural Science Foundation of China (NSFC) for the research fund for International Young Scientists (Grant 51650110490), China Postdoctoral Science Foundation funded project (Grant 2016M601985), and the National Key Research and Development program (Grant 2016YFB0100106), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant XDA09010403).

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 alters the thickness and modulus of the related SEI layers. An optimal concentration of 2 M LiFSI in DME could 9832


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Effect of LiFSI Concentrations To Form Thickness- and Modulus

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