Direct Visualization of Nucleation and Growth Processes of Solid

Jun 8, 2017 - ... of Molecular Nanostructure and Nanotechnology and Beijing National Laboratory for Molecular Science, CAS Research/Education Center f...
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Direct Visualization of Nucleation and Growth Processes of Solid Electrolyte Interphase Film Using in Situ Atomic Force Microscopy Yang Shi,†,‡ Hui-Juan Yan,†,‡ Rui Wen,*,†,‡ and Li-Jun Wan*,†,‡ †

Key Laboratory of Molecular Nanostructure and Nanotechnology and Beijing National Laboratory for Molecular Science, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100049, P.R. China ‡ University of the Chinese Academy of Sciences, Beijing 100190, P.R. China S Supporting Information *

ABSTRACT: An understanding of the formation mechanism of solid electrolyte interphase (SEI) film at the nanoscale is paramount because it is one of the key issues at interfaces in lithium-ion batteries (LIBs). Herein, we explored the nucleation, growth, and formation of SEI film on highly oriented pyrolytic graphite (HOPG) substrate in ionic liquid-based electrolytes 1butyl-1-methyl-pyrrolidinium bis(fluorosulfonyl)imide ([BMP]+[FSI]−) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([BMP]+[TFSI]−) by in situ atomic force microscopy (AFM) and found that the types of anions have significant influence on the structure of the formed SEI. In [BMP]+[FSI]− containing LiFSI, a compact and thin SEI film prefers to grow in the plane of HOPG substrate, while a rough and loose film tends to form in [BMP]+[TFSI]− containing LiTFSI. On the basis of in situ AFM observations, the relationship between the SEI structure and the electrochemical performance was clarified. KEYWORDS: in situ atomic force microscopy, Li-ion battery, solid electrolyte interphase, ionic liquid, interface

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INTRODUCTION It is well-known that electrochemical processes taking place at electrode/electrolyte interfaces play a crucial role in governing the performance of LIBs;1,2 for instance, formation of a stable solid electrolyte interface (SEI) film is important for stability and cyclability. Moreover, the influence of electrolyte on the interfacial structure and dynamics of SEI formation needs to be fundamentally understood. Ionic liquids (ILs) as electrolytes in LIBs have promising advantages due to their incombustibility, negligible vapor pressure in conjunction with remarkable ionic conductivity, and high thermal and electrochemical stability.3,4 I t h a s be e n r e p o r t e d t h a t I L s c o n t a i n in g b i s (trifluoromethanesulfonyl)imide ([TFSI]−), bis(fluorosulfonyl)imide ([FSI]−), and 1-butyl-1-methylpyrrolidinium ([BMP]+) are excellent candidates as electrolytes in LIBs.5,6 Unfortunately, although the interfacial processes in carbonate-solvent-based electrolyte are being widely studied for years,7−10 the direct data on the nanoscale structural evolution and interfacial dynamics in ILs by systematic in situ studies is still largely missing. To date, some in situ studies have been applied to explore the electrochemical reactions at the IL-based electrolyte/ electrode interface in LIBs.11 Schmidt et al. reported an in situ Raman spectroscopy study for the intercalation processes of an IL containing Li salt on graphite.12 Lahiri et al. discovered that [TFSI]− anions were less stable than [FSI]− anions via Raman spectroscopy.13 Mao et al. studied the surface processes in IL containing [FSI]− and [TFSI]− anions and Li salt influence using in situ scanning tunneling microscopy (STM) © 2017 American Chemical Society

on highly oriented pyrolytic graphite (HOPG) and gold electrodes. It was revealed that the surface decomposition of [FSI]− could induce forming an effective protective film to reduce the intercalation of cations and exfoliation processes of HOPG.14 However, insight into the early stage of SEI formation and the interfacial processes occurring on electrode in ILs is still in its infancy, and much effort must be devoted to its deep understanding. In the present work, the initial stages and dynamics of SEI formation at nanoscale are investigated by in situ AFM in [FSI]− and [TFSI]−-based ILs. In [FSI]− anionbased electrolyte, a thin and stable 2D SEI film is preferred to form on the substrate plane with nanoparticles (NPs) growing along the step edges, maintaining the lamellar structure on HOPG upon cycles. In contrast, in [TFSI]− anion-based electrolyte, it tends to form a rough and nonuniform SEI film without NPs protecting the edge sites, resulting in a poor electrochemical performance. The direct visualization of the nucleation and growth processes makes it possible to illustrate the SEI properties and its structure−reactivity correlations.

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EXPERIMENTAL SECTION

1-Butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide, 1-butyl-1methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, and lithium bis(fluorosulfonyl)imide were purchased from CoorsTek. Lithium bis(trifluoromethanesulfonyl)imide was purchased from Sigma. All of Received: April 21, 2017 Accepted: June 8, 2017 Published: June 8, 2017 22063

DOI: 10.1021/acsami.7b05613 ACS Appl. Mater. Interfaces 2017, 9, 22063−22067

Research Article

ACS Applied Materials & Interfaces these were dried under vacuum for 12 h to remove water. The water content in the IL was determined to be less than 20 ppm by Karl Fischer titration. Electrolytes were prepared by adding required amount of lithium salt in the neat ionic liquids. A freshly cleaved HOPG (ZYH-type Bruker Corp.) was used as the working electrode in an in situ fluid cell sealed by an 8 mm diameter O ring. The reference and counter electrodes were prepared by Li wires. In situ electrochemical AFM (Bruker Corp., Multimode 8 with a Nanoscope V controller) experiments were carried out in an argon filled glovebox (Mikrouna, Super 1220/750, H2O < 0.1 ppm, O2 < 0.1 ppm). The potential was controlled by Autolab PGSTAT302N (Metrohm Corp.). Peakforce tapping mode was applied during electrochemical (EC)AFM measurements with an insulating triangular silicon nitride AFM tip (Bruker Corp., k = 5 N m−1, f 0 = 150 kHz). In peakforce tapping, the probe periodically taps the sample, and the pN-level interaction force is measured directly by the deflection of the cantilever. Through superior force control, the feedback loop keeps the peak force constant. The HOPG was rinsed with DMC to remove the residual electrolyte on the surface before X-ray photoelectron spectroscopy (XPS) measurements. The XPS measurements were performed on an ESCALab220i-XL electron spectrometer (VG scientific) using 300 W Al Kα radiation. The base pressure was about 3 × 10−9 mbar, and the binding energies were referenced to the hydrocarbon C 1s peak at 284.8 eV. In the Li 1s high-resolution spectra, the peak at 55.7 eV represents the formation of Li−F. In the F 1s high-resolution spectra, the peak at 684.8 eV is assigned to Li−F. (Figure S3). X-ray diffraction (XRD) experiments were performed on an Empyrean instrument (PANalytical), and standard X-ray measurements were performed using a step size of 0.0260° at 20 s per step over a 2θ range of 22−30°. Cyclic voltammetric measurements were performed using 2032type coin cells with Li-metal foils as the counter electrodes and glass fibers (GF/D, Whatman) as separators. The scan rate was 1 mV/s for all measurements. Charge−discharge profiles were obtained using 2032-type coin cells on CT2001A (LANTHE Corp.). All potentials are reported referring to Li+/Li.

Figure 1. Cyclic voltammograms of the graphite electrode recorded at a scan rate of 1 mV/s in (a) [BMP]+[FSI]− containing 0.5 M LiFSI and (b) [BMP]+[TFSI]− containing 0.5 M LiTFSI.

performed according to the above electrochemical measurements. To observe the initial stage of SEI formation, EC-AFM was used to monitor the morphologies of HOPG at different potentials in [BMP]+[FSI]− containing 0.5 M LiFSI. In situ images were acquired along the CV trace via a negativedirection sweep from open circuit potential (OCP). At OCP, atomically flat terrace and step edges are visible on the clean HOPG (Figure 2a). When the potential approaches to 0.9 V, NPs (30 nm in height, Figure S2b) form along the step edges of HOPG, as indicated by the yellow arrows (Figure 2b), which can be associated with initial decomposition of [FSI]− ions in the presence of Li ions.18,19 With the potential negatively shifting to 0.6 V (Figure 2c), the NPs along the step edges grow gradually and combine with each other, preferring to align in a row. Followed by the nucleation on the edge sites, NP nuclei (6 nm in height, Figure S2d) begin to emerge on the terraces of HOPG. Figure 2d shows an in situ AFM image at 0.6 V with scan size of 300 nm, in which the NPs on terrace can be seen manifestly. The growth of the nuclei on the terraces of HOPG reaches up to about 30 nm in height on holding the potential at 0.6 V (Figure S2d). When the potential is close to 0.56 V, interestingly, thin films with thicknesses of about 3 nm (Figure S2f) start to grow around the edge of every nuclei, forming a yolk structure (Figure 2e). With time increasing at 0.56 V (Figure 2f and g), by keeping the inner core (nuclei) unchanged, the outer shell (thin film) around the initially distributed nuclei has 2D growth (growth rate is ca. 2.4 nm/ min) and gradually combines together to form a uniform and compact film covering the flat terrace of HOPG. The observed interfacial dynamics is directly corresponding to the nucleation and growth process of SEI film formation. The final morphology of the SEI layer can be seen clearly on HOPG at 0 V (Figure 2h): well aligned NPs adsorb along the step edge, and the smooth thin film within embedded nuclei covers all over the terrace with roughness of 5.9 nm. To understand the chemical component of these NPs, XPS was performed on

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RESULTS AND DISCUSSION The electrochemical behaviors of graphite electrode in [FSI]−based ([BMP]+[FSI]− containing 0.5 M LiFSI) and [TFSI]−based ([BMP]+[TFSI]− containing 0.5 M LiTFSI) electrolyte were investigated using cyclic voltammetry (CV), as shown in Figure 1. The cathodic peak C1 at 1.0 V in Figure 1a may be related to the surface decomposition of [FSI]− anions.15,16 Peak C2 and C3 probably correspond to the formation of SEI film: peak C2 at 0.6 V involves the nucleation, and peak C3 at 0.5 V involves the growth process of SEI layer (it was drawn based on the later in situ AFM results). The intercalation of Li ion commences just below 0.5 V and reaches a peak at ca. 0 V (C4). However, no deintercalation peak can be observed in the first cycle. In the second cycle, only the intercalation (C4) and deintercalation processes at 0.2 V (A4) are observed. It should be noted that there is a partial decomposition of [BMP]+ cations at peak C4. The CV of [BMP]+[TFSI]− containing 0.5 M LiTFSI on graphite is shown in Figure 1b. The pair of peaks at 0.4/1.0 V (C′/A′) correspond to the intercalation and deintercalation of [BMP]+ cations.17 The CV of graphite in the IL with binary anions of both [FSI] − and [TFSI] − ([BMP]+[TFSI]− containing 0.5 M LiFSI) is presented in Figure S1, which shows similar SEI formation and insertion of Li+ in the first cycle without extraction in the second cycle. It is evident that the electrochemical properties related to formation of the SEI layer are obviously different in the three electrolytes. To understand the interfacial processes at nanoscale and structure−reactivity correlations, in situ AFM was further 22064

DOI: 10.1021/acsami.7b05613 ACS Appl. Mater. Interfaces 2017, 9, 22063−22067

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ACS Applied Materials & Interfaces

Figure 3. In situ AFM images of HOPG in [BMP]+[TFSI]− containing 0.5 M LiFSI at (a) OCP and (b) 0.95, (c) 0.69, (d) 0.57, (e) 0.16, and (f) 0 V (4 min for one image scan).

that in [FSI]−-based electrolytes. Interestingly, when potential is polarized to 0.57 V, the shell (outer film) has 3D growth to form an island-like structure differing from 2D formation in [FSI]− anions (Figure 3d). As a result of 3D growth, a stacked lamellar structure is formed at 0.16 V (Figure 3e), and a porous film composed of those irregular lamella can be observed at 0 V (Figure 3f) with roughness of 36.4 nm. In situ AFM was further performed on HOPG in [BMP]+[TFSI]− containing 0.5 M LiTFSI to understand the interfacial process in pure [TFSI]− anions. The in situ AFM images are shown in Figure 4. The HOPG appears clean with a few steps at OCP (Figure 4a). Until 0.90 V, nucleation of NPs is yet formed at the step edges, which is different from the process involved in the presence of [FSI]− anions (Figure 4b). When the potential is shifted to 0.55 V, the nucleation process was observed on the terrace of HOPG, and the step edges became brightly highlighted, as shown in Figure 4c. It should be noted that the similar highlighted bright steps can be seen in pure IL of [BMP]+[TFSI]− (Figure S4b), indicating a slash intercalation of the [BMP]+ cations into the sites under step edge. Moreover, visible cracks in optical images of HOPG (Figure S5) also showed drastic curling of the edges. When the potential was reduced to 0.4 V, a large number of island structures forms on the terrace of HOPG via 3D growth (Figure 4d), similar to the phenomena in the binary anions (Figure 3d). With the growth going on at 0.4 V, a nonuniform and quite loose SEI film finally covers the HOPG surface, as

Figure 2. In situ AFM images of HOPG in [BMP]+[FSI]− containing 0.5 M LiFSI at (a) OCP and (b) 0.90, (c) 0.60, (d) 0.60 (4 min after c), (e) 0.56, (f) 0.56 (8 min after e), (g) 0.56 (8 min after f), and (h) 0 V (4 min for one image scan).

the sample with only NPs on the step edges (Figure S3). It is proposed that the NPs are ascribed to the inorganic LiF, and the thin film around NPs may be composed of the organic compounds of reaction product of the electrolyte decomposition and Li ions. Figure 3 shows the in situ AFM images in [BMP]+[TFSI]− containing 0.5 M LiFSI with binary anions of both [FSI]− and [TFSI]−. At OCP, atomically flat terrace and step edges are visible on the HOPG (Figure 3a). In comparison to the process in [FSI]− anions, only a few NPs are formed at the step edge of the HOPG upon applying a potential of 0.95 V (Figure 3b). The sizes of the NPs are around 3 nm in height (Figure S2h). When the potential approaches 0.69 V, the NPs start to form on the terraces of HOPG, and then thin film grows around to form a yolk structure (Figure 3c). This initial stage is similar to 22065

DOI: 10.1021/acsami.7b05613 ACS Appl. Mater. Interfaces 2017, 9, 22063−22067

Research Article

ACS Applied Materials & Interfaces

Figure 5. Schematic diagram of the SEI formation process (a−c) and charge−discharge profiles of graphite electrode at 0.2 C in (a′) [BMP] + [FSI]− containing 0.5 M LiFSI, (b′) [BMP] + [TFSI] − containing 0.5 M LiFSI, and (c′) [BMP]+[TFSI]− containing 0.5 M LiTFSI. Figure 4. In situ AFM images of HOPG in [BMP]+[TFSI]− containing 0.5 M LiTFSI at (a) OCP and (b) 0.90, (c) 0.55, (d) 0.40, (e) 0.40 (8.5 min after d), and (f) 0.40 V (8.5 min after e). (4.25 min for one image scan).

efficiency (Figure 5a′). It is obvious that the platform of charge/discharge at 0.1/0.2 V is corresponded to the insertion/ extraction of Li ions. In [BMP]+[TFSI]− containing 0.5 M LiFSI, the less [FSI]− anions result in the formation of fewer NPs at the step edges of HOPG (Figure 5b2), followed with the nucleation process on the terrace (Figure 5b3). In contrast, due to the 3D growth of the film, a porous and rough SEI layer finally covered the electrode surface (Figures 5b4 and 5). The charge−discharge profile of the binary anions demonstrates a serious capacity loss and a poor Coulombic efficiency (Figure 5b′). When it comes to [BMP]+[TFSI]− containing 0.5 M LiTFSI, no formation of NPs occurs along the step edges in the absence of [FSI]− anions, which results in curling of the HOPG layer due to the intercalation of [BMP]+ cations (Figures 5c2− 4). The segregated and loose SEI layer formed on the terrace may still make HOPG surface exposed into electrolyte (Figure 5c5). Coherently, the platform of charge/discharge at 0.4/1.0 V corresponded to the intercalation/deintercalation of [BMP]+ ions instead of Li ions (Figure 5c′). The poor Coulombic efficiency and capacity fading is a consequence of the loose and segregated SEI film and the serious [BMP] + cations intercalations. On the basis of the above analysis, the proposed mechanism of the structural evolution of SEI formation in three electrolytes is coincident with the discharge capacity and Coulombic efficiency comparison, as shown in Figure S7.

shown in Figures 4e and f. It is noted that it is hard to capture the interfacial morphology below the potential of 0.4 V due to the serious roughness during the slashing surface reaction. Ex situ XRD measurement (Figure S6) was further carried out to test the HOPG structure. It is shown that the pristine HOPG (002) peak at 2θ = 26.55° largely decreased after charging in [BMP]+[TFSI]− containing 0.5 M LiTFSI, while two new peaks at 28.42 and 25.02° appear, which confirm the structure degeneration of the graphite layer and formation of graphite intercalation compounds (GICs) caused by intercalation of [BMP]+ cations.20 Thus, on the basis of the above in situ AFM results, a schematic diagram displaying the various SEI film formation processes in [FSI]− and [TFSI]−-based IL electrolytes is proposed in Figure 5. In [BMP]+[FSI]− containing 0.5 M LiFSI, NPs are initially formed at the step edges (Figure 5a2), followed by nucleation on the terrace and a sequent 2D growth of a stable SEI film (Figures 5a3−5). Meanwhile, the NPs gradually grow and align well along the step edge to prevent the intercalation of [BMP]+ cations into the graphite layers, which is in good agreement with the previous STM investigation.14 The high quality of uniform and stable SEI film formed in [FSI]− anions is directly related to its excellent properties, including promoting the Li-ion conductivity21 and preventing the [BMP]+ cations intercalation. It is further substantiated with the charge−discharge profiles in the [FSI]−-based electrolyte, which show excellent cyclability and Coulombic

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CONCLUSION In summary, we provided deep insight into the interfacial process of SEI film on HOPG in [TFSI]− and [FSI]−-based ILs by in situ AFM. The SEI films in [FSI]−-based ILs nucleate from forming NPs at the edge, which contribute to maintenance of the graphite layer structure. The following 22066

DOI: 10.1021/acsami.7b05613 ACS Appl. Mater. Interfaces 2017, 9, 22063−22067

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ACS Applied Materials & Interfaces

(7) Liu, X. R.; Wang, D.; Wan, L. J. Progress of Electrode/Electrolyte Interfacial Investigation of Li-Ion Batteries via In Situ Scanning Probe Microscopy. Science Bulletin 2015, 60 (9), 839−849. (8) Deng, X.; Liu, X. R.; Yan, H. J.; Wang, D.; Wan, L. J. Morphology and Modulus Evolution of Graphite Anode in Lithium Ion Battery: An In Situ AFM Investigation. Sci. China: Chem. 2014, 57 (1), 178−183. (9) Liu, X. R.; Wang, L.; Wan, L. J.; Wang, D. In Situ Observation of Electrolyte-Concentration-Dependent Solid Electrolyte Interphase on Graphite in Dimethyl Sulfoxide. ACS Appl. Mater. Interfaces 2015, 7 (18), 9573−9580. (10) Zheng, J.; Zheng, H.; Wang, R.; Ben, L.; Lu, W.; Chen, L.; Chen, L.; Li, H. 3D Visualization of Inhomogeneous Multi-Layered Structure and Young’s Modulus of the Solid Electrolyte Interphase (SEI) on Silicon Anodes for Lithium Ion Batteries. Phys. Chem. Chem. Phys. 2014, 16 (26), 13229−13238. (11) Chen, C. Y.; Sano, T.; Tsuda, T.; Ui, K.; Oshima, Y.; Yamagata, M.; Ishikawa, M.; Haruta, M.; Doi, T.; Inaba, M.; Kuwabata, S. In situ Scanning Electron Microscopy of Silicon Anode Reactions in LithiumIon Batteries during Charge/Discharge Processes. Sci. Rep. 2016, 6, 36153−36161. (12) Baranchugov, V.; Markevich, E.; Salitra, G.; Aurbach, D.; Semrau, G.; Schmidt, M. A. In Situ Raman Spectroscopy Study of Different Kinds of Graphite Electrodes in Ionic Liquid Electrolytes. J. Electrochem. Soc. 2008, 155 (3), A217−A227. (13) Lahiri, A.; Schubert, T. J. S.; Iliev, B.; Endres, F. LiTFSI in 1butyl-1-methylpyrrolidinium Bis(fluorosulfonyl)amide: A Possible Electrolyte for Ionic Liquid Based Lithium Ion Batteries. Phys. Chem. Chem. Phys. 2015, 17, 11161−11164. (14) Hu, X.; Chen, C.; Yan, J.; Mao, B. W. Electrochemical and Insitu Scanning Tunneling Microscopy Studies of Bis(fluorosulfonyl)imide and Bis(trifluoromethanesulfonyl)imide Based Ionic Liquids on Graphite and Gold Electrodes and Lithium Salt Influence. J. Power Sources 2015, 293, 187−195. (15) Appetecchi, G. B.; Montanino, M.; Balducci, A.; Lux, S. F.; Winterb, M.; Passerini, S. Lithium Insertion in Graphite from Ternary Ionic Liquid-Lithium Salt Electrolytes. Electrochemical Characterization of the Electrolytes. J. Power Sources 2009, 192 (2), 599−605. (16) Liu, C.; Ma, X.; Xu, F.; Zheng, L.; Zhang, H.; Feng, W.; Huang, X.; Armand, M.; Nie, J.; Chen, H.; Zhou, Z. Ionic Liquid Electrolyte of Lithium Bis(fluorosulfonyl)imide/N-methyl-N-propylpiperidinium Bis(fluorosulfonyl)imide for Li/Natural Graphite Cells: Effect of Concentration of Lithium Salt on The Physicochemical and Electrochemical Properties. Electrochim. Acta 2014, 149, 370−385. (17) Martha, S. K.; Markevich, E.; Burgel, V.; Salitra, G.; Zinigrad, E.; Markovsky, B.; Sclar, H.; Pramovich, Z.; Heik, O.; Aurbach, D.; Exnar, I.; Buqa, H.; Drezen, T.; Semrau, G.; Schmidt, M.; Kovacheva, D.; Saliyski, N. A Short Review on Surface Chemical Aspects of Li Batteries: A Key for A Good Performance. J. Power Sources 2009, 189 (1), 288−296. (18) Shkrob, I. A.; Marin, T. W.; Zhu, Y.; Abraham, D. P. Why Bis(fluorosulfonyl)imide Is A “Magic Anion” for Electrochemistry. J. Phys. Chem. C 2014, 118 (34), 19661−19671. (19) Howlett, P. C.; Brack, N.; Hollenkamp, A. F.; Forsyth, M.; MacFarlane, D. R. Characterization of the Lithium Surface in NMethyl-N-alkylpyrrolidinium Bis(trifluoromethanesulfonyl)amide Room-Temperature Ionic Liquid Electrolytes. J. Electrochem. Soc. 2006, 153 (3), A595−A606. (20) Sutto, T. E.; Trulove, P. C.; De Long, H. C. Direct X-Ray Diffraction Evidence for Imidazolium Intercalation into Graphite from an Ionic Liquid. Electrochem. Solid-State Lett. 2003, 6 (3), A50−A52. (21) Qian, J.; Henderson, W. A.; Xu, W.; Bhattacharya, P.; Engelhard, M.; Borodin, O.; Zhang, J. G. High Rate and Stable Cycling of Lithium Metal Anode. Nat. Commun. 2015, 6, 6362−6370.

nucleation of NPs on the terraces and a uniform 2D growth process guarantee a compact SEI film. In contrast, no NPs grow along the step edges in [TFSI]−-based ILs in the absence of [FSI]− anions, which directly cause the intercalation of [BMP]+ cations and induce curling and exfoliation of the graphite layer. Furthermore, the lower Li-ion diffusion of [TFSI]−-based ILs favors a 3D growth mode, which in turn results in formation of weak SEI films on the surface. We propose that the initial growth mode plays a crucial role in the structure and properties of the SEI layer. A 2D growth mode helps to form a compact SEI and remains stable in the following cycles, which ensures excellent battery performances, while a 3D growth process tends to form a loose and unstable film, leading to a severe capacity loss and a poor Coulombic efficiency.

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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/acsami.7b05613. Cyclic voltammograms of graphite electrode recorded in [BMP]+[TFSI]− containing 0.5 M LiFSI, detailed AFM images with cross section profiles, and XPS spectra of the HOPG polarized to 1 V in [BMP]+[FSI]− containing 0.5 M LiFSI (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Li-Jun Wan: 0000-0002-0656-0936 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key R&D Program of China (Grant 2016YFA0202500), “Hundred Talents Program” from Chinese Academy of Sciences and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant XDB1202100).

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REFERENCES

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DOI: 10.1021/acsami.7b05613 ACS Appl. Mater. Interfaces 2017, 9, 22063−22067