Self-Stabilized Solid Electrolyte Interface on a Host-Free Li-Metal

May 27, 2018 - High rate performance with a current density of 20 mA cm–2 and an ... transport at the anode/electrolyte interface are urgently neede...
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Article Cite This: Chem. Mater. 2018, 30, 4039−4047

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Self-Stabilized Solid Electrolyte Interface on a Host-Free Li-Metal Anode toward High Areal Capacity and Rate Utilization Zhenglin Hu,†,§ Shu Zhang,† Shanmu Dong,*,† Quan Li,‡,§ Guanglei Cui,*,† and Liquan Chen†,‡ †

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Qingdao Industrial Energy Storage Technology Institute, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, People’s Republic of China ‡ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: Lithium metal has long been regarded as a promising anode for its high energy density and low reduction potential. However, infinite volume change and undesired lithium dendrite and parasitic reactions still block the practical application of lithium-metal anodes, despite persistent research. To address these tough issues, a self-stabilized solid electrolyte interface (SEI) formed in lithium bis(trifluoromethane sulphonyl)imide-vinylene carbonate electrolyte with high ionic conductivity, mechanical strength, and compact structure is reported for the first time, delivering rapid lithium-ion diffusion and suppressed anode/electrolyte interfacial reactions. This excellent SEI contributes to homogeneous lithium plating/stripping and a constant voltage over 4000 h even under harsh conditions. High rate performance with a current density of 20 mA cm−2 and an areal capacity of 40 mA h cm−2 has also been achieved. The self-stabilized SEI provides a promising strategy to tackle the challenges raised by the intrinsic properties of a lithium-metal anode.

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INTRODUCTION Rechargeable batteries with high reversible capacity and rate performance are essential for the demands of electronic devices, electrical vehicles, and grid storage.1−4 For these purposes, promising lithium-metal secondary batteries have recently attracted broad attention due to the high theoretical specific capacity (3860 mA h g−1; 10 times that of a graphite anode), the low density (0.59 g cm−3), and the lowest negative electrochemical potential (−3.040 V vs the standard hydrogen electrode) of lithium metal.5−12 Thus, the realization of stable cycling for a lithium-metal anode with high capacity and rate performance is the ultimate goal for the practical application of lithium-metal cells.13−15 However, lithium plating/stripping around the lithium-metal anode/electrolyte interface in common liquid electrolytes (such as 1 M LiPF6 in EC/ DMC) always displays inhomogeneous deposition behavior under high capacity utilization, resulting in the risk of lithium dendritic growth. This can be assigned to the severe deterioration of native solid electrolyte interface (SEI) layers during cycling, arising from infinite volume change of lithium metal. The breakage and reconstruction of native SEI layers lead to continuous consumption of fresh lithium metal and low Coulombic efficiency. It is noted that homogeneous transport of lithium ions through the SEI layer can hardly be achieved under high current density due to the insufficient ionic © 2018 American Chemical Society

conductivity of the native SEI layer in commercial liquid electrolyte, which further deteriorates the uneven lithium distribution. Therefore, well-designed strategies to improve both stability and lithium-ion transport at the anode/electrolyte interface are urgently needed. To tackle these tough issues, it is rational to modify the component and structure of the SEI layer by engineering electrolyte contents,16,17 using varied additives,5,18−23 or constructing a new protective layer24,25 artificially. Admittedly, these protective effects generally endow the lithium-metal anode with dendrite-free performance to some extent. However, significantly destroyed lithium deposition is still a serious concern for coated lithium anodes especially under high current density and areal capacity (typically ≥4 mA cm−2 and 4 mA h cm−2),26 as these artificial or modified SEI layers can hardly satisfy both the high stability and high ionic conductivity simultaneously. According to Sand’s equation, lowering the local current density can be an effective strategy to enhance the duration of homogeneous lithium deposition (Sand’s time). Therefore, artificial conductive hosts with enlarged surface area for lithium plating/stripping have been widely reported.27,28 Received: February 16, 2018 Revised: May 16, 2018 Published: May 27, 2018 4039

DOI: 10.1021/acs.chemmater.8b00722 Chem. Mater. 2018, 30, 4039−4047

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Figure 1. (a) Voltage profiles of lithium-metal plating/stripping in the Li|Li symmetrical cell cycled under a current density of 5 mA cm−2 and an areal capacity of 10 mA h cm−2 with 1 M LiTFSI-VC electrolytes. (b) Rate performance of the Li|Li symmetrical cell measured under different current densities and areal capacities with 1 M LiTFSI-VC. (c) Cycling performance and Coulombic efficiency of the LiFePO4|Li half cell with 1 M LiTFSI-VC electrolyte for 5000 cycles in the range 2.5−4.0 V. Deposition morphology of lithium metal to the copper foils using an in situ optical microscope in the Cu|Li half cells with the following different electrolytes: (d) 1 M LiTFSI-VC; (e) 1 M LiPF6-EC/DMC. (Current density for d and e: 10 mA cm−2. Plating times: 5, 10, 15, 20, 25, and 30 min.)

the absence of an additional artificial protective film or conductive hosts.

Unfortunately, the employment of these conductive hosts inevitably sacrifices the overall specific energy density of lithium-metal cells. Herein, we designed a novel combination of vinylene carbonate and lithium bis(trifluoromethanesulfonyl)imide salt (abbreviated as 1 M LiTFSI-VC) for the first time, which results in a self-stabilized and high ionic conductivity SEI on a lithium-foil anode. VC was usually used as the additive for graphite-based anodes in conventional lithium-ion cells.18,29−34 Because of the excellent film-forming effect, we employed VC as the major solvent for lithium-metal cells created in this study.35,36 Smooth lithium-plating/stripping curves for more than 4000 h with a high areal capacity of 10 mA h cm−2 at a high current density of 5 mA cm−2 were successfully achieved on untreated planar lithium foils, without the usage of additional conductive hosts or a three-dimensional-structured current collector. It was also the first demonstration of longterm cycling with constant and smooth polarization curves under such harsh conditions. The polarization curve still maintained its stability even under a very high current density of 20 mA cm−2 and an areal capacity of 40 mA h cm−2. By employing lithium plating and subsequently stripping on copper foils in Cu|Li cells, we characterized the SEI region in detail. To our surprise, the thickness of this SEI region became relatively constant after several initial cycles, indicating the subsequent parasitic reactions can be inhibited effectively by this as-formed SEI region. Hence, we named this SEI region a self-stabilized “SEI skeleton” for lithium metal. It was worth noting that this SEI skeleton possessed a superior ionic conductivity (7-fold greater than that formed in commercial LiPF6-EC/DMC electrolyte), a high Young’s modulus (average value: 34 GPa), and a compact structure, providing the lithiummetal anodes with extraordinary electrochemical properties in

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EXPERIMENTAL SECTION

Materials. Lithium bis(trifluoromethanesulphonyl)imide (LiN(SO2CF3)2, LiTFSI, 3 M Inc.) salt was dried in a vacuum oven at 140 °C overnight in advance. Vinylene carbonate (VC) was purchased from the Sigma Corporation. LiTFSI and VC were mixed by mole to volume ratio, which was 1 mol per l L. Then, the mixture of LiTFSI and the VC solvent was stirred at room temperature for 12 h to obtain the electrolyte. All the experiments were performed in an argon-filled glovebox with less than 0.1 ppm O2 and 0.1 ppm H2O. Methods. LiFePO4 cathodes were prepared by grinding 80 wt % LiFePO4 powder mixed with 10 wt % Super P and 10 wt % polyvinylidene fluoride (PVDF; Sigma) which had been dissolved in N-methyl-2-pyrrolidone (NMP; Alfa) in advance. LiCoO2 cathodes were prepared by grinding 93 wt % LiCoO2 powder mixed with 4 wt % Super P and 3 wt % PVDF, which had been dissolved in NMP in advance. LiFePO4 and LiCoO2 cathodes were finally obtained by coating the as-prepared slurry onto Al foil, and then, the cathodes were dried at 120 °C for 24 h in a vacuum oven. Then, the LiFePO4 and LiCoO2 cathodes were punched into plates with a diameter of 12 mm. Lithium-metal foil (FMC-lithium, 250 μm) was received and stored in an argon-atmosphere glovebox. The lithium foil was punched into little wafers with a diameter of 16 mm as the anodes, and the coin cells were all assembled in a glovebox with an argon-gas environment. Electrochemical Measurements. Electrochemical impedance spectroscopy (EIS) was performed by an electrochemical working station (VMP-300) over the frequency range 0.1−106 Hz with a perturbation amplitude of 5 mV to better investigate the interfacial stability between lithium metal and different electrolytes. Electrochemical cycling tests in Li|Li symmetric cells, Cu|Li cells, and LiFePO4|Li cells were conducted in CR2032-type coin cells with LAND testing systems. All cells were assembled in an argon-filled glovebox with less than 0.1 ppm O2 and 0.1 ppm H2O. A cellulose membrane with a diameter of 16.5 mm was used as the separator. 4040

DOI: 10.1021/acs.chemmater.8b00722 Chem. Mater. 2018, 30, 4039−4047

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Figure 2. (a) Schematics of the lithium-deposition behaviors on the copper foils with 1 M LiTFSI-VC electrolyte and (b, c) corresponding SEM images before/after the first lithium-plating/stripping process. (d) Schematics of the lithium-deposition behaviors on the copper foils with 1 M LiPF6-EC/DMC electrolyte and (e, f) corresponding SEM images before/after the first lithium-plating/stripping process. (g) DEMS spectra of hydrogen production in the Cu|Li half cells with different electrolytes. (h) Young’s modulus distribution diagram for the SEI regions left on the copper foils after several initial cycles in different electrolytes. (The insets of c and f are the photographs of the SEI regions left on the copper foils after lithium stripping from 1 M LiTFSI-VC and 1 M LiPF6-EC/DMC electrolytes, respectively.) Characterization. Scanning electron microscopy (SEM; Hitachi S4800) was employed to detect the morphology information of lithium metal on the lithium-metal anodes or the copper foils. An in situ optical microscope from the Olympus Corporation was employed to obtain the depositional morphology of lithium metal with different electrolytes in real time so as to better study the interfacial stability. Conducting atomic force microscopy (C-AFM; Seiko, SPA-400) was employed to measure the electronic conductivity, the depositional morphology, and the Young’s modulus at the nanoscale. X-ray photoelectron spectroscopy (XPS) performed on a Thermo Scientific ESCA Lab 250Xi was used to collect the surface components. Fourier Transform IR (FTIR) spectroscopy was employed to confirm the reductive polymerization of the VC solvent on the surface of the lithium-metal anode after cycling. Secondary ion mass spectrometry (SIMS) was used to measure the components as the depth changes. Differential electrochemical mass spectrometry (DEMS) was employed to detect the hydrogen production in different electrolytes during the Li-plating/stripping process on the copper foils.

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and high interfacial impedance. The performance of the lithium-metal cells with other salt/solvent combinations as well as with the VC additive (Figures S4 and S5) was also investigated. The experimental results revealed that only the combination of 1 M LiTFSI salt and the VC solvent endowed the lithium-metal anode with optimized performance. To highlight the lithium-plating/stripping pattern visually, Cu|Li half cells were assembled. As revealed in the in situ optical microscope test of the Cu|Li cells (Figure 1d, e), the lithium-metal surface in 1 M LiTFSI-VC solution displayed smooth morphology during the whole plating process, and no visible dendrites were detected even under a high current density of 10 mA cm−2. However, lithium metal exhibited heterogeneous deposition on the copper current collector with black spots at the beginning of plating from 1 M LiPF6-EC/ DMC electrolyte. Some visible protuberances appeared after discharging for 15 min and continued to grow into dendritic lithium. The SEI region in 1 M LiPF6-EC/DMC electrolyte was not stable enough and was easily destroyed with cracks and fractures. Fresh lithium metal around the broken area was exposed to the electrolyte later, leading to severe lithium consumption and the growth of dendritic lithium. Formation of the Self-Stabilized SEI Skeleton. The region on the copper foils after lithium plating and stripping was observed by SEM images with corresponding schematics (Figures 2, S6, and S7). For cells with 1 M LiTFSI-VC electrolyte, lithium metal preferred to grow along the horizontal direction (Figure 2b) as compared to the needlelike lithium growth in 1 M LiPF6-EC/DMC electrolyte with abundant protuberances appearing on the surface after the first plating process (Figure 2e). Laminar islands of lithium metal appeared after plating, and no dendrites come into being even under a high current density of 5 mA cm−2. Cross-view SEM images indicated that lithium-metal plating from 1 M LiTFSI-VC

RESULTS AND DISCUSSION

Electrochemical Performance. Cells cycled in 1 M LiTFSI-VC electrolyte exhibited a low polarization voltage (less than 40 mV) and an ultralong cycling life (over 4000 h) even under 5 mA cm−2 and 10 mA h cm−2 (Figure 1a). Li|Li symmetrical cells under decreased current density and areal capacity (Figure S1) also displayed more stable cycling curves than those in commercial 1 M LiPF6-EC/DMC electrolyte (easily got short-circuited). A smooth and compact surface for the lithium-metal anode was observed after 1000 cycles (Figure S3). The polarization curves were steady in each 10-cycle testing even at a very high rate of 20 mA cm−2 and an areal capacity of 40 mA h cm−2 (Figure 1b). On the other hand, besides the short-circuit, morphology deformation and surface exfoliation of the lithium-metal anode with loose and porous structure existed in cells with 1 M LiPF6-EC/DMC electrolyte merely for 50 cycles (Figure S3), leading to severe polarization 4041

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Figure 3. SEM images of the skeleton structure on copper foils after 1, 5, 10, 20, and 50 cycles in the Cu|Li cells with (a) 1 M LiTFSI-VC and (b) 1 M LiPF6-EC/DMC. All the Cu|Li half cells were discharged for 5 mA h cm−2 and then charged to 2 V at a current density of 2.5 mA cm−2 in each cycle. (c) Coulombic efficiency of the Cu|Li half cells under a current density of 2.5 mA cm−2 and an areal capacity of 5 mA h cm−2 with different electrolytes. AFM images of the (d) response current and (e) surface morphology for the self-stabilized SEI skeleton after lithium plating/stripping in 1 M LiTFSI-VC electrolyte under a current density of 2.5 mA cm−2 and an areal capacity of 5 mA h cm−2 for 50 cycles.

Figure 4. Nyquist spectra of the Li|Li symmetric cells with different electrolytes (a) before and (b) after the first cycle under a current density of 2.5 mA cm−2 and an areal capacity of 5 mA h cm−2. (c, d) Corresponding enlarged view and data fitting of impedance spectra (b). Schematics of resistance contributions in the Li|Li symmetrical cells (e) before and (f) after cycling with 1 M LiTFSI-VC electrolyte (RLE,bulk: the bulk resistance of liquid electrolyte; RSEI,bulk: the bulk resistance of the SEI layer; Rcharge transfer: the charge transfer resistance, including RAE/SE and RLE/SE.). (g) In-situ EIS spectra of bare lithium electrodes with 1 M LiTFSI-VC electrolytes in the Li|Li symmetric cells for different plating times (from 30 to 240 min) under a current density of 2.5 mA cm−2. (h) Enlarged images in the dotted boxes of panel g.

electrolyte possessed a more compact structure with a thickness of about 115 μm after plating for 50 mA h (Figure S8). After

complete stripping of lithium metal from the copper foils in 1 M LiTFSI-VC, a smooth and compact region left on the copper 4042

DOI: 10.1021/acs.chemmater.8b00722 Chem. Mater. 2018, 30, 4039−4047

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Figure 5. (a) XPS spectra of C 1s, F 1s, N 1s, and Li 1s for the SEI skeleton fetched from the Cu|Li half cells after 50 cycles with 1 M LiTFSI-VC electrolyte. (b) XPS etching spectra of C 1s, F 1s, and Li 1s of the SEI skeleton in the Cu|Li cells with 1 M LiTFSI-VC electrolyte after 50 cycles. The etching depths for each spectrum were 2, 4, 6, 8, and 10 nm, respectively. (c) SIMS spectra of the elements O and F for the SEI skeleton in the Cu|Li cells with 1 M LiTFSI-VC electrolyte after 50 cycles. The etching time is 600 s. All cells were cycled under 2.5 mA cm−2 for 5 mA h cm−2 in each plating and stripping process. (d) Schematic of the self-stabilized SEI skeleton formed on the surface of the lithium-metal anode with 1 M LiTFSIVC electrolyte.

The thickness change of the SEI region was also measured by SEM after lithium stripping from the copper foils for different cycles (Figure 3a, b). We found that the SEI region in 1 M LiTFSI-VC electrolyte reached a thickness of around 50 μm after several initial cycles and then maintained this value (the blue dotted line in Figure S10) during the whole testing process, suggesting the formation of a self-stabilized SEI skeleton. This skeleton can serve as an in-situ-formed “host”, accommodating the severe volume change with excellent structural stability during a long cycling process (further demonstrated by SEM images in Figure S11). On the contrary, huge fluctuation of the SEI region was observed in 1 M LiPF6EC/DMC electrolyte after the first cycle (with unmeasurable thickness), and the severe consumption of fresh lithium metal contributed to its continuously growing thickness (nearly 400 μm). As a result, the self-stabilized SEI skeleton in 1 M LiTFSIVC electrolyte delivered superior electrochemical performance. The Cu|Li cell cycled in 1 M LiTFSI-VC electrolyte for 50 times under a current density of 2.5 mA cm−2 and an areal

foil was observed and mainly occupied by the SEI (Figure 2c). The inset of Figure 2c also revealed the compact skeleton formed in 1 M LiTFSI-VC as compared to the inhomogeneous SEI region in 1 M LiPF6-EC/DMC electrolyte (Figure 2f). DEMS spectra (Figure 2g) revealed the hydrogen-evolution process for lithium metal cycled in the Cu|Li half cells. The flux of hydrogen in 1 M LiTFSI-VC electrolyte was significantly lower than that in 1 M LiPF6-EC/DMC electrolyte during the plating process. This result suggested that the parasitic reactions were suppressed drastically, indicating an effective separation of lithium metal from the electrolyte by the SEI skeleton with a compact structure.37−40 As shown in Figure 2h, AFM images revealed the high mechanical strength of this SEI as the average Young’s modulus value of 34 GPa was twice that of the SEI formed in the commercial electrolyte (also depicted in Figure S9). Thus, the excellent compactness and stability accommodated a severe volume change and hindered the consumption of lithium metal effectively. 4043

DOI: 10.1021/acs.chemmater.8b00722 Chem. Mater. 2018, 30, 4039−4047

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Chemistry of Materials capacity of 5 mA h cm−2 also exhibited significantly enhanced and stable Coulombic efficiency (over 97%) with constant polarization (Figures 3c and S12) compared to that in 1 M LiPF6-EC/DMC solution (lower than 85% and dropped drastically). In general, the SEI regions at the anode/electrolyte interface were regarded as an electronic insulator. However, a mild response current was observed by C-AFM spectra (Figures 3d and S13) as well, which reflects the homogeneity of the surface components of the SEI regions after cycling, impacting the subsequent deposition morphology of lithium metal.41 The SEI skeleton formed in 1 M LiTFSI-VC shows homogeneous distribution of the response current as compared with the SEI in 1 M LiPF 6 -EC/DMC, which confirms its uniform components on the surface and facilitates compact and smooth deposition of lithium metal from the initial stage of lithium plating. AFM also confirmed the smooth and flat surface of this SEI skeleton with a fluctuation less than 270 nm in a scanning area of 5 μm × 5 μm (Figure 3e). Superior Ionic Conductivity of the Self-Stabilized SEI Skeleton. In addition to the compact and tough structure, the ionic conductivity of this self-stabilized SEI skeleton also played a vital role in the enhanced plating/stripping behaviors in 1 M LiTFSI-VC electrolyte. To confirm this point, EIS of the Li|Li symmetric cells was performed (Figure 4) before and after the first cycle. The as-prepared cells possessed an initial impedance value of 40 Ω in 1 M LiTFSI-VC electrolyte (much lower than that in 1 M LiPF6-EC/DMC solution), which can be mainly associated with the charge transfer as reported before.42−44 We found that the SEI layer formed in 1 M LiTFSI-VC electrolyte also exhibited excellent stability with low interface impedance (around 40 Ω) and provided lithium metal with an effective physical barrier during the storage time of seven days (Figure S14). It is worth noting that the total impedance decreased greatly for cells after the first cycle (Figure 4a, b), which may be attributed to the enlarged surface area of the lithium-metal anode after plating. Three arcs located in different areas (the fitting lines were displayed in Figure S15) appeared after the first cycling, and the corresponding f max values (the maximum frequency of its responded arc) were marked in Figure 4c, d. Previous research indicated that the first semicircle in the Nyquist plots reflected the effect caused by the SEI layer (RSEI,bulk) and the other two arcs were related to the charge transfer resistance (RLE/SE and RAE/SE; liquid electrolyte (LE), solid electrolyte (SE), and anode (AE)).45 Resistance contributions around the interface between lithium metal and the electrolyte in the Li|Li symmetrical cell before and after cycling with 1 M LiTFSI-VC electrolyte were also schematically depicted in Figure 4e, f. The cells possessed a SEI bulk impedance of about 1 Ω in 1 M LiTFSI-VC electrolyte, which was much smaller than that of cells in 1 M LiPF6-EC/DMC electrolyte (about 13 Ω). To get the detailed ionic conductivity of the SEI layer based on Figure 4c, d, the following equation was used:46−50

σ = 2πfmax ε0εr

in 1 M LiTFSI-VC exhibited about a 7-fold higher ionic conductivity than that in 1 M LiPF6-EC/DMC electrolyte. During the lithium-plating test for 4 h with 1 M LiTFSI-VC electrolyte, the first arc related to the SEI bulk impedance was nearly constant (about 1 Ω) (Figures 4g, h and S16) under a current density of 2.5 mA cm−2. This result suggested that the high ionic conductivity of this SEI layer in 1 M LiTFSI-VC electrolyte facilitated fast lithium-ion transport and limited the continuous increase of voltage polarization, guaranteeing a stable performance and an ultralong cycling life of the lithiummetal anode. Because the high ionic conductivity and structure stability are closely related to the components of the SEI skeleton layer, XPS, SIMS, and FTIR spectrocopy were performed to better elucidate the component distribution and the internal structure of this self-stabilized SEI skeleton formed in 1 M LiTFSI-VC solution (Figures 5 and S17−S20). XPS etching technology was introduced with etching depths of 2, 4, 6, 8, and 10 nm, respectively. Figure 5a revealed the SEI skeleton in 1 M LiTFSI-VC mainly consisted of LiN3 (55.0 eV, Li 1s; 398.5 eV, N 1s), LiF (55.7 eV, Li 1s; 685.2 eV, F 1s), CH (284.8 eV, C 1s), CC (285.2 eV, C 1s), CO (288.6 eV, C 1s), and C F (289.9 eV, C 1s). Curves of C 1s, F 1s, N 1s, and Li 1s (Figure S17) confirmed the excellent stability of this SEI skeleton formed in 1 M LiTFSI-VC electrolyte without the formation of new species during the whole testing process. On the contrary, severe destruction of the SEI layer in 1 M LiPF6EC/DMC was clearly confirmed by the appearance of several new reaction products (Figure S18). XPS spectra also indicated that the SEI skeleton possessed a high carbon content on the surface but a low amount of carbon was detected inside, which may be assigned to the reductive polymerization of the VC solvent at the initial stage. The amounts of the elements F and Li were relatively stable throughout the whole SEI skeleton (Figure 5b). To better prove the polymerization of the VC solvent at the outermost shell of this skeleton, FTIR spectroscopy was performed on the skeleton fetched from the Cu|Li symmetric cell with 1 M LiTFSI-VC electrolyte after cycling and the pure VC solvent (Figure S19). Peaks characteristic of lithium alkyl carbonates (1640 cm−1) and Li 2 CO 3 (1440 cm −1 ) were observed. Peaks at 1802 cm−1(νCO), 1059 cm−1 (νCO), and 860 cm−1 (δOCO2) were attributed to ROCO2Li or (CH2OCO2Li)2 species. Peaks at 1780 and 1150 cm−1 were assigned to the formation of poly(vinylene carbonate). According to the XPS and FTIR spectra, the self-stabilized SEI skeleton in 1 M LiTFSI-VC mainly consists of an organic shell while the inside is mainly composed of inorganic compounds. This unique structure can accommodate the volume change and suppress the dendritic growth of lithium metal effectively, ensuring the superb electrochemical performance. Ideally, the SEI layer continuously grows by electron/ionparticipated reactions and stops with sufficient thickness (less than 10 nm) to prevent further electron tunneling.41 Thus, the SEI layer should be a uniform, thin layer with considerable ionic conductivity, allowing electron−ion combination on the lithium-metal surface during the electrochemical process.24,51 However, the growth of the SEI beyond the tunneling-allowed thickness is commonly observed with an elusive formation mechanism. Both the radical species in the liquid electrolytes and the breakage/repair process during cycling have been proposed as the reason for this growth.7,52 The maximum thickness of the SEI region has never been claimed. Thus, it is

(1)

where σ represents the ionic conductivity of the in-situ-formed SEI skeleton and ε0 and εr are the permittivities of free space and the SEI layer (ε0 = 8.9 × 10−12 F/m and εr = 10.0 F/m based on a previous report), respectively.46,47,50 The ionic conductivity σ1 (for the SEI layer formed in 1 M LiPF6-EC/ DMC) was 2.16 × 10−5 S/m and σ2 (for the SEI formed in 1 M LiTFSI-VC) was 1.39 × 10−4 S/m. Therefore, the SEI skeleton 4044

DOI: 10.1021/acs.chemmater.8b00722 Chem. Mater. 2018, 30, 4039−4047

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CONCLUSIONS In summary, we demonstrated a self-stabilized SEI formed in 1 M LiTFSI-VC electrolyte with high ion conductivity, high mechanical strength, and compact structure, leading to extraordinary electrochemical performance on a bare lithiummetal anode without the usage of artificial SEI layers or conductive hosts, even under large current density and high areal capacity. Tough issues such as severe parasitic reactions and dendritic lithium in conventional liquid electrolytes have been inhibited dramatically with the aid of this self-stabilized SEI. This contribution highlighted the importance of the salt/ solvent design for lithium-anode protection. Furthermore, these results suggested the possibility of applying ultrathin lithium foils (≤10 μm) for lithium-metal batteries, which could maximize the energy density of lithium-metal batteries.

insufficient to analyze the SEI skeleton structure merely by XPS and FTIR spectroscopy. We used SIMS to get a depth profile with microscale thickness. The test was performed for different SEI regions formed on copper foils after lithium stripping (Figures 5c and S20). The experimental result displayed various element distributions with the increase of etching depth at the micrometer scale. The SEI skeleton in 1 M LiTFSI-VC electrolyte exhibited a stable element content during the whole test process of 600 s, which confirmed the uniform composition in this skeleton, facilitating homogeneous ionic conductivity and lithium deposition. The high content of the elements C and O at the initial etching stage can be assigned to the polymerization of the VC solvent on the surface, and the corresponding schematic of this self-stabilized SEI skeleton was also depicted (Figure 5d). However, from the lithium profile of the SEI in LiPF6-EC/DMC electrolyte, a distinct peak near the surface was observed while the amounts of C, F, and O decreased drastically from the surface. This result suggested that the SEI region in the commercial electrolyte was mainly distributed at the outermost surface and a large amount of “dead lithium” was enveloped inside, which gives rise to the low Coulombic efficiency and the short cycle life. The high ionic conductivity, mechanical stability, and compact structure of the SEI skeleton in 1 M LiTFSI-VC electrolyte endowed the lithium-metal anode with homogeneous deposition behavior and ultralong cycling life. To further highlight the merits of the SEI skeleton formed on the lithiummetal anode, LiFePO4|Li half cells were assembled with 1 M LiTFSI-VC electrolyte (Figures 1c and S21). It was worth mentioning that the capacity retention of the LiFePO4|Li cell was still over 85% after 5000 cycles with a polarization voltage of less than 300 mV and a Coulombic efficiency of nearly 100%. SEM images revealed the smooth and compact surface of lithium metal in 1 M LiTFSI-VC electrolyte after cycling. Our work reveals that the LiTFSI-VC combined electrolyte enables host-free lithium-metal anodes with excellent performance in the absence of additional artificial modification. Suppressed parasitic reactions and dendritic lithium are successfully achieved in lithium-metal battery systems. The following outstanding properties of the SEI skeleton formed in 1 M LiTFSI-VC electrolyte have been highlighted: (1) Self-stabilized performance induced by self-limited growth and mechanical strength. The lithium-metal anode reacted with 1 M LiTFSI-VC electrolyte at the initial stage, forming an in-situ SEI skeleton during the first several cycles. However, the subsequent reactions were effectively suppressed when the SEI skeleton reached a stable thickness and compactness. AFM and DEMS results revealed that the unique SEI skeleton possessed high mechanical strength (average Young’s modulus value: 34 GPa) and suppressed the side reactions effectively. FTIR and XPS results confirmed the existence of poly(vinylene carbonate) at the outermost layer of the skeleton, which could give rise to its self-stabilized behavior. (2) Excellent ionic conductivity. Detailed analysis of the Nyquist spectra for the Li|Li symmetric cells with different electrolytes before and after the first cycling reveals that the in-situ SEI skeleton in 1 M LiTFSI-VC electrolyte possesses a nearly 7-fold greater ionic conductivity than that in the 1 M LiPF6-EC/DMC electrolyte.

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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.chemmater.8b00722. Interfacial stability of lithium metal with different electrolytes; different morphologies of lithium deposition on the copper current collector; excellent performance of the self-stabilized SEI skeleton; and investigation of interfacial components and stability (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guanglei Cui: 0000-0002-8008-7673 Author Contributions

G. Cui and Z. Hu proposed the concepts. S. Dong designed the experiments. Z. Hu carried out the experiments and performed the analysis. S. Dong and Z. Hu wrote the manuscript with help from all coauthors, and all authors contributed to interpretation of the data. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Qingdao Key Lab of Solar Energy Utilization and Energy Storage Technology, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, People’s Republic of China for fruitful help. This original research was supported by funding from the “135” Projects Fund of the CAS-QIBEBT Director Innovation Foundation, the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDA09010105), and the Youth Innovation Promotion Association of CAS (2016193).

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REFERENCES

(1) Tarascon, J.-M.; Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414, 359−367. (2) Ma, L.; Hendrickson, K. E.; Wei, S.; Archer, L. A. Nanomaterials: Science and applications in the lithium−sulfur battery. Nano Today 2015, 10, 315−338.

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DOI: 10.1021/acs.chemmater.8b00722 Chem. Mater. 2018, 30, 4039−4047

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DOI: 10.1021/acs.chemmater.8b00722 Chem. Mater. 2018, 30, 4039−4047