Self-stabilized solid electrolyte interface on host-free Li metal anode

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Self-stabilized solid electrolyte interface on host-free Li metal anode towards high areal capacity and rate utilization Zhenglin Hu, Shu Zhang, Shanmu Dong, Quan Li, Guanglei Cui, and Liquan Chen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00722 • Publication Date (Web): 27 May 2018 Downloaded from http://pubs.acs.org on May 27, 2018

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Chemistry of Materials

Self-stabilized solid electrolyte interface on host-free Li metal anode towards high areal capacity and rate utilization Zhenglin Hu†,§, Shu Zhang†, Shanmu Dong*,†, Quan Li‡,§, Guanglei Cui*,† and Liquan Chen†,‡ †

Qingdao Industrial Energy Storage Technology Institute, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P. R. China

‡

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China §

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

ABSTRACT: Lithium metal has long been regarded as a promising anode for its high energy density and low reduction potential. However, infinite volume change, undesired lithium dendrite and parasitic reactions still block the practical application of lithium metal anodes, despite persistent researches. To addressing these tough issues, a self-stabilized 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 constant voltage over 4000 hours 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 lithium metal anode.

INTRODUCTION Rechargeable batteries with high reversible capacity and rate performance are essential for the demands in 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, ten times that of graphite anode), low density (0.59 g cm-3) and the lowest negative electrochemical potential (-3.040 V vs. the standard hydrogen electrode) of metal lithium.5-12 Thus the realization of stable cycling for lithium metal anode with high capacity and rate performance is the ultimate goal for the practical application of lithium metal cells.13-15 However, Li plating/stripping around the lithium metal anode/electrolyte interface in the 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 SEI layers during cycling, arising from infinite volume change of lithium metal. The breakage and reconstruction of native SEI layers leads 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 conductivity of native SEI layer in commercial liquid electrolyte, which further deteriorates the uneven lithium distribution. Therefore, well-designed strategies to improve both stability and Li-ions transport at the anode/electrolyte interface are urgently needed. To tackle these tough issues, it is rational to modify the component and structure of SEI layer by engineering

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electrolyte contents,16-17 using varied additives5,

18-23

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or constructing new protective layer24-25 artificially.

Admittedly, these protective effects generally endow lithium metal anode with dendrite-free performance to some extent. However, significantly destroyed lithium deposition are still 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 down 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 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 lithium foil anode. VC was usually used as the additive of graphite-based anodes in conventional Li-ion cells18, 29-34. Due to the excellent film-forming effect, we employed VC as the major solvent of lithium metal cells creatively in this study35-36. Smooth lithium plating/stripping curves for more than 4000 hours with high areal capacity of 10 mA h cm-2 at high current density of 5 mA cm-2 were successfully achieved on untreated planar lithium foils, without the usage of additional conductive hosts or 3D structured current collector. It was also the first demonstration of long-term cycling with constant and smooth polarization curves under such harsh condition. The polarization curve still maintained its stability even under a very high current density of 20 mA cm-2 and areal capacity of 40 mA h cm-2. By employing Li plating and subsequently stripping on Cu foils in Cu|Li cells, we detailedly characterized the SEI region. To our surprise, the thickness of this SEI region became relatively constant after the initial several cycles, indicating the subsequent parasitic reactions can be inhibited effectively by this as-formed SEI region. Hence, we named this SEI region as a self-stabilized “SEI skeleton” for lithium metal. It was worth noting that this SEI skeleton possessed superior ionic conductivity (sevenfold than that formed in commercial LiPF6-EC/DMC electrolyte), high Young`s modulus (average value: 34 GPa) and compact structure, providing lithium metal anodes with extraordinary electrochemical properties in the absence of additional artificial protective film or conductive hosts.

EXPERIMENTAL SECTION Materials. Lithium bis(trifluoromethanesulphonyl)imide (LiN(SO2CF3)2, LiTFSI, 3M Inc.) salt was dried in a vacuum oven at 140 °C overnight in advance. Vinylene carbonate was purchased from Sigma Corporation. LiTFSI and VC were mixed by ratio of mole number to volume, which is 1 mol per l litre. Then the mixture of LiTFSI and VC solvent was stirred at room temperature for 12 hours to obtain the electrolyte. All the experiments were performed in an argon-filled glove box with less than 0.1 ppm oxygen 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 has 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% Polyvinylidene Fluoride (PVDF, Sigma) which has been dissolved in N-methyl- 2-pyrrolidone (NMP, Alfa) 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 hours 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 glove box. 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 glove box with an argon gas environment.


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Electrochemical measurements. Electrochemical impedance spectroscopy (EIS) was tested by an electrochemical working station (VMP-300) over a frequency range of 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 glove box with less than 0.1 ppm oxygen and 0.1 ppm H2O. Cellulose membrane with a diameter of 16.5 mm was used as the separator. Characterization. Scanning electron microscope (SEM, Hitachi S-4800) was employed to detect the morphology information of lithium metal on lithium anodes or copper foils. In-situ optical microscope from 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 microscope (C-AFM, Seiko, SPA-400) was employed to measure the electronic conductivity, depositional morphology and Young`s modulus in nano-scale. X-ray photoelectron spectroscopy (XPS) was used to collect the surface components which was performed on a Thermo Scientific ESCA Lab 250Xi. Fourier Transform infrared spectroscopy (FTIR) was employed to prove the reductive polymerization of VC solvent on the surface of lithium metal anode after cycling. Secondary ion mass spectroscopy (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 Li plating/stripping process on copper foils.

RESULTS AND DISCUSSION Electrochemical performance Cells cycled in 1 M LiTFSI-VC electrolyte exhibited low polarization voltage (less than 40 mV) and ultra-long cycling life (over 4000 hours) 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 stable cycling curves than those in commercial 1 M LiPF6-EC/DMC electrolyte (easily got short-circuited). Smooth and compact surface of lithium metal anode was observed after 1000 cycles (Figure S3). The polarization curves kept steady in each ten-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 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 and high interfacial impedance. Performance of lithium metal cells with other salt/solvent combinations as well as the VC additive (Figure S4, S5) was also investigated. Experimental result revealed that only the combination of 1 M LiTFSI salt and VC solvent endowed lithium metal anode with optimized performance.



Figure 1. (a) Voltage profiles of Li metal plating/stripping in a Li|Li symmetrical cell cycled under a current density of -2

-2

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

To highlight the Li plating/stripping pattern visually, Cu|Li half cells was assembled. As revealed in the in-situ optical microscope test of Cu|Li cells (Figure 1d, e)，surface of lithium metal 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, metal lithium exhibited heterogeneous deposition on 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 minutes and continued to grow into dendritic lithium. The SEI region in 1 M LiPF6-EC/DMC electrolyte was not stable enough and easily got 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 self-stabilized SEI skeleton The region on Cu foils after Li plating and stripping was observed by SEM images with corresponding schematics (Figure 2, S6, S7). For cells with 1 M LiTFSI-VC electrolyte, lithium metal preferred to grow along horizontal direction (Figure 2b) compared with the needle-like Li growth in 1 M LiPF6-EC/DMC electrolyte with abundant protuberance appeared 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 high current density of 5 mA cm-2. Cross-view SEM images indicated that lithium metal plating from 1 M LiTFSI-VC electrolyte possessed more compact structure with a thickness of about 115 μm after plating for 50 mA h (Figure S8). After complete stripping of metal lithium from copper foils in 1 M LiTFSI-VC, a smooth and compact region left on the Cu foil was observed and mainly occupied by SEI (Figure 2c). The inset of Figure 2c also revealed the


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compact skeleton formed in 1 M LiTFSI-VC, compared to the inhomogeneous SEI region in 1 M LiPF6-EC/DMC electrolyte (Figure 2f).

Figure 2. (a) Schematics of Li deposition behaviors on Cu foils with 1 M LiTFSI-VC electrolyte and (b, c) the corresponding SEM images before/after the first Li plating/stripping process. (d) Schematics of Li deposition behaviors on Cu foils with 1 M LiPF6-EC/DMC electrolyte and (e, f) the corresponding SEM images before/after the first Li plating/stripping process. (g) DEMS spectra of hydrogen production in Cu|Li half cells with different electrolytes. (h) Young`s modulus distribution diagram for the SEI regions left on copper foils after the initial several cycles in different electrolytes. (The inset of c, f are the photographs of SEI regions left on Cu foils after Li stripping from 1 M LiTFSI-VC and 1 M LiPF6-EC/DMC electrolytes respectively.)

DEMS spectra (Figure 2g) revealed the hydrogen evolution process for lithium metal cycled in 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. It suggested that the parasitic reactions was suppressed drastically, indicating an effective separation of lithium metal from electrolyte by the SEI skeleton with 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 than that of SEI formed in commercial electrolyte (also depicted in Figure S9). Thus, the excellent compactness and stability accommodated severe volume change and hindered the consumption of metal lithium effectively. Thickness change of the SEI region was also measured by SEM test after Li stripping from Cu 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 at the initial several 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 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 LiPF6-EC/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 LiTFSI-VC electrolyte delivered superior electrochemical performance. 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 capacity of 5 mA h cm-2 also exhibited significantly



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enhanced and stable Coulombic efficiency (over 97%) with constant polarization (Figure 3c, S12) compared to that in 1 M LiPF6-EC/DMC solution (lower than 85% and dropped drastically).

Figure 3. SEM images of the skeleton structure on copper foils after 1, 5, 10, 20 and 50 cycles in Cu|Li cells with: (a) 1 M -2

LiTFSI-VC and (b) 1 M LiPF6-EC/DMC. All Cu|Li half cells were discharged for 5 mA h cm and then charged to 2V at -2

a current density of 2.5 mA cm in each cycle. (c) Coulombic efficiency of Cu|Li half cells under a current density of -2

-2

2.5 mA cm and an areal capacity of 5 mA h cm with different electrolytes. AFM images of the (d) response current and (e) surface morphology for the self-stabilized SEI skeleton after lithium plating/striping 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.

In general, SEI regions at the anode/electrolyte interface were regarded as electronic insulator. However, mild response current was observed by conducting atomic force microscope (C-AFM) spectra (Figure 3d, S13) as well, which reflects the homogeneity of 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 response current compared with the SEI in 1 M LiPF6-EC/DMC, which proves its uniform components on surface and facilitates compact and smooth deposition of lithium metal from the initial stage of Li plating. AFM test also proved 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 self-stabilized SEI skeleton In addition to the compact and tough structure, ionic conductivity of this self-stabilized SEI skeleton also played a vital role for the enhanced plating/stripping behaviors in 1 M LiTFSI-VC electrolyte. To confirm this point, electrochemical impedance spectroscopy (EIS) test of Li|Li symmetric cells was employed (Figure 4) before and after the first cycle. The as-prepared cells possessed initial impedance values 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 effective physical barrier during the storage time of 7 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 lithium metal anode after plating. Three arcs located in different areas (the fitting lines were displayed in Figure S15) appeared after the 1st cycling, and the corresponding fmax values (the maximum


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frequency of its responded arc) were marked in Figure 4c and 4d. Previous researches 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. LE, SE and AE are the abbreviation of liquid electrolyte, solid electrolyte and anode respectively).45 Resistance contributions around the interface between lithium metal and the electrolyte in Li|Li symmetrical cell before and after cycling with 1 M LiTFSI-VC electrolyte were also schematically depicted in Figure 4e and 4f. Cells possessed a SEI bulk impedance about 1 Ω in 1 M LiTFSI-VC electrolyte, which was much smaller than cells in 1 M LiPF6-EC/DMC electrolyte (about 13 Ω). To get the detailed ionic conductivity of SEI layer based on Figure 4c, d, the following equation was used 46-50

: σ = 2πfmaxε0εr

(1)

Figure 4. Nyquist spectra of Li|Li symmetric cells with different electrolytes (a) before and (b) after the first cycle -2

-2

under a current density of 2.5 mA cm and an areal capacity of 5 mA h cm . (c, d) Corresponding enlarged view and data fitting of impedance spectra (b). Schematics of resistance contributions in 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 SEI layer; Rcharge transfer: the charge transfer resistance, including RAE/SE and RLE/SE.). (g) In-situ EIS test of bare Li electrodes with 1 M LiTFSI-VC electrolytes in Li|Li symmetric cells for different plating time (from 30 minutes -2

to 240 minutes) under a current density of 2.5 mA cm . (h) The enlarged image in the dotted box of (g).

Where σ represents the ionic conductivity of the in-situ formed SEI skeleton. The ε0 and εr are the permittivity of free space and SEI layer (ε0 = 8.9×10-12 F/m and εr = 10.0 F/m based on previous report).46-47, 50 The ionic conductivity σ1 (for SEI layer formed in 1 M LiPF6-EC/DMC) was 2.16×10-5 S/m and σ2 (for SEI formed in 1 M LiTFSI-VC) was 1.39×10-4 S/m. Therefore, the SEI skeleton in 1 M LiTFSI-VC exhibited about seven-fold higher ionic conductivity than that in 1 M LiPF6-EC/DMC electrolyte. During the Li plating test for 4 hours with 1 M LiTFSI-VC electrolyte, the first arc related to the SEI bulk impedance was nearly constant (about 1 Ω) (Figure 4g, h, S16) under a current density of 2.5 mA cm-2. It 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



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polarization, guaranteeing stable performance and ultra-long cycling life of lithium metal anode.

Figure 5. (a) XPS spectra of C 1s, F 1s, N 1s and Li 1s for SEI skeleton fetched from 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 Cu|Li cells with 1 M LiTFSI-VC electrolyte after 50 cycles. The etching depth for each spectrum was 2 nm, 4 nm, 6 nm, 8 nm and 10 nm, respectively. (c) SIMS spectra of element O and F for the SEI skeleton in Cu|Li cells with 1 M LiTFSI-VC electrolyte -2

-2

after 50 cycles. The etching time is 600 seconds. All cells were cycled under 2.5 mA cm for 5 mA h cm in each plating and stripping process. (d) Schematic of the self-stabilized SEI skeleton formed on the surface of lithium metal anode with 1 M LiTFSI-VC electrolyte.

Since the high ionic conductivity and structure stability are closely related to the components of the SEI skeleton layer, XPS, SIMS and FTIR tests were employed to better elucidate the component distribution and the internal structure of this self-stabilized SEI skeleton formed in 1 M LiTFSI-VC solution (Figure 5, S17-S20). XPS etching technology was introduced with etching depth for 2, 4, 6, 8 and 10 nm respectively. Figure 5a revealed


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the SEI skeleton in 1 M LiTFSI-VC mainly consisted of LiN3 (55.0 eV, Li 1s and 398.5 eV, N 1s), LiF (55.7 eV, Li 1s and 685.2 eV, F1s), C-H (284.8 eV, C1s), C-C (285.2 eV, C1s), C=O (288.6 eV, C1s) and C-F (289.9 eV, C1s). Curves of C 1s, F 1s, N 1s and Li 1s (Figure S17) proved 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 LiPF6-EC/DMC was clearly confirmed by the appearance of several new reaction products (Figure S18). XPS spectra also indicated that the SEI skeleton possessed high carbon content on surface but low amount of carbon was detected inside, which may be assigned to the reductive polymerization of VC solvent at the initial stage. The amount of element F and Li was relatively stable throughout the whole SEI skeleton (Figure 5b). To better prove the polymerization of VC solvent at the outermost shell of this skeleton, FTIR test was carried out on the skeleton fetched from 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 Li2CO3 (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 cm-1 and 1150 cm-1 were assigned to the formation of poly(vinylene carbonate). According to the spectra of XPS and FTIR, the self-stabilized SEI skeleton in 1 M LiTFSI-VC mainly consists of 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, SEI layer continuously grows by electron/ion participated reactions and stops with sufficient thickness (less than 10 nm) to prevent the electron tunneling further.41 Thus SEI layer should be a uniform, thin layer with considerable ionic conductivity, allowing electron-ion combination on lithium metal surface during electrochemical process.24, 51 However, the growth of SEI beyond tunneling-allowed thickness are commonly observed with elusive formation mechanism. Both the radical species in liquid electrolytes and the breakage/repair process during cycling have been proposed as the reason for this growth.7, 52 The maximum thickness of SEI region has never been claimed. Thus, it is insufficient to analyze the SEI skeleton structure merely by XPS and FTIR. We used SIMS to get a depth profile with thickness of micro-scale. The test was performed for different SEI regions formed on copper foils after Li stripping (Figure 5c and S20). Experimental result displayed various element distribution with the increase of etching depth in micrometer-scale. SEI skeleton in 1 M LiTFSI-VC electrolyte exhibited stable element content during the whole test process of 600 seconds, which proved the uniform composition in this skeleton, facilitating homogeneous ionic conductivity and lithium deposition. The high content of element C and O at the initial etching stage can be assigned to the polymerization of VC solvent on surface and the corresponding schematic of this self-stabilized SEI skeleton was also depicted (Figure 5d). However, from the Li profile of SEI in LiPF6-EC/DMC electrolyte, a distinct peak near the surface was observed while the amount of C, F and O decreased drastically from the surface. This result suggested that the SEI region in commercial electrolyte mainly distributed at the outermost surface and a large amount of “dead lithium” was enveloped inside, which give rise to the low Coulombic efficiency and short cycle life. The high ionic conductivity, mechanical stability and compact structure of the SEI skeleton in 1 M LiTFSI-VC electrolyte endowed lithium anode with homogeneous deposition behavior and ultra-long cycling life. To further highlight the merits of the SEI skeleton formed on lithium metal anode, LiFePO4|Li half cells were assembled with 1 M LiTFSI-VC electrolyte (Figure 1c and S21). It was worth mentioning that the capacity retention of LiFePO4|Li cell was still over 85% after 5000 cycles with a polarization voltage 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. 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. 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 reaches to 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 tests confirmed the existence of poly(vinylene carbonate) at 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 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 nearly seven-fold ionic conductivity than that in 1 M LiPF6-EC/DMC electrolyte.

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 bare lithium metal 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 salt/solvent design for lithium anode protection. Furthermore, these results suggested the possibility of applying ultra-thin lithium foils (≤10 μm) for lithium metal batteries, which could maximize the energy density of lithium metal batteries.

ASSOCIATED CONTENT Supporting Information Available: Interfacial stability of lithium metal with different electrolytes. Different morphology of lithium deposition on Cu current collector. Excellent performance of the self-stabilized SEI skeleton. Investigation of interfacial components and stability (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected]

Author Contributions Guanglei Cui and Zhenglin Hu proposed the concepts. Shanmu Dong designed the experiments. Zhenglin Hu carried out the experiments and performed the analysis. Shanmu Dong and Zhenglin Hu wrote the manuscript with help from all co-authors and all authors contributed to interpretation of the data.

Notes We thank Qingdao Key Lab of Solar Energy Utilization and Energy Storage Technology, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P. R. China, for fruitful help. The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This original research was supported by funding from "135" Projects Fund of 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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Self-stabilized solid electrolyte interface on host-free Li metal anode

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