Revisiting Solid Electrolyte Interphase on the Carbonaceous

Electrodes Using Soft X-ray Absorption Spectroscopy. Yunok Kim. 1. , Dae Sik Kim. 2 ...... J. Power Sources 2002, 111, 193–209. (2) Verma, P.; Maire...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 29992−29999

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Revisiting Solid Electrolyte Interphase on the Carbonaceous Electrodes Using Soft X‑ray Absorption Spectroscopy Yunok Kim,† Dae Sik Kim,∥ Ji Hyun Um,† Jaesang Yoon,† Ji Man Kim,*,§,⊥ Hansu Kim,*,∥ and Won-Sub Yoon*,† †

Department of Energy Science, and §Department of Chemistry, Sungkyunkwan University, Suwon 440-756, South Korea Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea ⊥ School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China Downloaded via DURHAM UNIV on September 6, 2018 at 02:00:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.



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ABSTRACT: It is widely accepted that solid electrolyte interphase (SEI) layer of carbonaceous material is formed by irreversible decomposition reaction of an electrolyte, and acts as a passivation layer to prevent further decomposition of the electrolyte, ensuring reliable operation of a Li-ion battery. On the other hand, recent studies have reported that some transition metal oxide anode materials undergo reversible decomposition of an organic electrolyte during cycling, which is completely different from carbonaceous anode materials. In this work, we revisit the electrochemical reaction of an electrolyte that produces SEI layer on the surface of carbonaceous anode materials using soft Xray absorption spectroscopy. We discover that the reversible formation and decomposition of SEI layer are also able to occur on the carbonaceous materials in both Li- and Na-ion battery systems. These new findings on the unexpected behavior of SEI in the carbonaceous anode materials revealed by soft X-ray absorption spectroscopy would be highly helpful in more comprehensive understanding of the interfacial chemistry of carbonaceous anode materials in Li- and Na-ion batteries. KEYWORDS: reversible surface reaction, SEI layer, electrolyte decomposition, carbon anode, Li- and Na-ion batteries

1. INTRODUCTION In an electrochemical system, the importance of an interface between the electrode and electrolyte cannot be overemphasized. In particular, the interfacial electrochemical reaction and the resulting interfacial film, so-called solid electrolyte interphase (SEI), on the surface of electrode materials are highly correlated with the performance of Li-ion batteries (LIBs) such as their energy density, cycle performance, and safety.1−5 Carbonaceous anode materials have been mainly adopted in commercialized LIBs over other anode materials for LIBs because of their excellent capacity retention, good reliability, and relatively low and flat working potential. It is widely accepted that the irreversible electrochemical reaction of an electrolyte leads to form SEI on the surface of carbonaceous anode material upon the first discharge (Li insertion) process. Once SEI is properly formed with the help of well-adjusted electrolyte composition, the formed SEI layer prevents not only further decomposition of the electrolyte but also mechanical deformation of carbonaceous anode material mainly caused by solvent co-intercalation into graphene layer, thereby stabilizing the electrochemical reaction of carbonaceous anode materials with Li ion.5−7 This implies that SEI acts as a passivation film to ensure reversible electrochemical Li storage and removal reaction during cycling.2,6−9 On the other hand, recent reports © 2018 American Chemical Society

revealed that the reversible formation and decomposition of SEI layer are able to occur on the surface of some transition metal oxide anode materials even after the first cycle.10−15 Laruelle et al. first reported that reversible formation and decomposition of the gel-like polymer are caused by the electrochemical reaction of an electrolyte at the surface of electrode materials, which is contrary to the phenomena observed at carbonaceous anode materials. They stated that extra capacity beyond theoretical capacity of CoO based on the conversion reaction with lithium comes from this reversible electrochemical reaction of the electrolyte during cycling.15 To fill the gap between widely accepted cathodic breakdown mechanism of an organic electrolyte and the findings observed at the transition metal oxide anode, various analytical techniques have been employed for monitoring SEI formation on the transition metal oxide anode materials. Transmission electron microscopy (TEM), atomic force microscopy, and ellipsometry have been applied for obtaining the morphology of SEI layer. Chemical identification tools including X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy have provided information on Received: June 14, 2018 Accepted: August 8, 2018 Published: August 8, 2018 29992

DOI: 10.1021/acsami.8b09939 ACS Appl. Mater. Interfaces 2018, 10, 29992−29999

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Figure 1. TEM images of (a) graphite, (b) soft, (c) hard carbon, and (d) XRD patterns, (e) Raman spectra, and (f) C K-edge soft XAS spectra of the graphite, soft, and hard carbon of the pristine particle.

report that a partial reversible electrochemical reaction of an organic electrolyte can occur even at the carbonaceous anode materials in Li- and Na-ion batteries (NIBs) using soft XAS studies on the three types of carbonaceous anode material, that is, graphite, soft, and hard carbon in the organic electrolytes used for Li- and Na-ion batteries.

the composition of SEI including organic and inorganic compounds.5 Recently, it was demonstrated that soft X-ray absorption spectroscopy (XAS) study can be a powerful tool to investigate the evolution of complex chemical species in SEI on the electrode materials with respect to the amount of Li ion stored. Li, C, and O K-edge soft XAS spectra on the carboncoated ZnFe2O4 anode material clearly showed that a partially reversible SEI mainly composed of lithium alkyl carbonate was formed as an uppermost layer in the surface film on the carboncoated ZnFe2O4 particles at the high level of depth of discharge condition.14 In particular, C K-edge soft XAS is a powerful tool for selectively analyzing these SEI layers on the electrode materials because soft XAS can probe the surface layer up to about 10 nm using the total electron yield (TEY) mode,16,17 since monitoring the evolution of the carbon species on the surface of electrode materials can bring deep insight into the dynamics of SEI during lithium storage and removal processes.2,18 This recent report motivated us to study SEI formed at the carbonaceous anode materials using soft XAS. Although one can reasonably assume the possibility of such a partial reversible reaction of an electrolyte on the surface of carbonaceous anode material, until now, there are few reports on the reversibility of electrolyte decomposition reaction on the carbon anode materials including graphite, soft, and hard carbon materials. In this work, we anticipated that soft XAS study on the carbonaceous anode materials not only provides further information on the reaction mechanisms of SEI formation at the surface of electrode materials in LIB systems but also gives us some clues to explain the marked difference between SEI formation reaction on the carbonaceous anode and that observed at the transition metal oxide materials. Herein, we

2. EXPERIMENTAL SECTION 2.1. Material Preparation. Commercially available graphite (SG6, POSCO chem. Tec.) and hard carbon (S(F), Kureha Co.) were used as electrode materials in this study. Soft carbon was prepared from the phenanthrene (SAMCHUN, 98%) with heat treatment at 900 °C for 3 h under N2 atmosphere with a heating rate of 10 °C m−1. 2.2. Structural Characterization. The high-resolution transmission electron microscopy was carried out using JEOL 2100F. The Xray diffraction (XRD) patterns of the graphite, soft, and hard carbon were acquired by Empyrean, PAN alytical with Cu Kα radiation. The Raman spectra were recorded using the 532 nm wavelength with an objective lens of 100× 0.9 NA. 2.3. Electrochemical Measurements. The slurries were manufactured by mixing the active materials as carbonaceous materials (90 wt %) and binder as poly(vinylidene fluoride) (KF 1100, Kureha Co.) dissolved in N-methylpyrrolidone. To prepare the working electrodes, obtained slurries were coated on a Cu foil as a current collector. The loading amount of the electrodes was fixed at about 2.5 mg cm−2, and then electrodes were dried at 80 °C for 30 min in a convection oven and heat-treated at 120 °C for overnight under vacuum. The Li-ion half cells were assembled using CR2032 cointype cells with lithium metal as the counter electrode, a porous polyethylene membrane as a separator, and an electrolyte of the 1 M LiPF6 and 0.3 M LiBF4 dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (3:7 v/v, Panax Etec Co., Ltd.) in an Ar-filled glovebox. To test the Na-ion cells, carbonaceous electrodes were incorporated into the CR2032 cointype cells with sodium metal as a 29993

DOI: 10.1021/acsami.8b09939 ACS Appl. Mater. Interfaces 2018, 10, 29992−29999

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Figure 2. Electrochemical profiles and C K-edge soft XAS spectra of (a) graphite, (b) soft, and (c) hard carbon during cycling in Li-ion batteries. counter electrode, a glass fiber (GF/F; Whatman) as a separator, and Na-ion-conducting electrolytes. The 1 M sodium salt of NaPF6 was dissolved in 3:7 EC/DEC (a carbonate-based electrolyte) and diethylene glycol dimethyl ether (an ether-based electrolyte). These electrolytes were stirred at 60 °C for 24 h and kept with molecular sieves (beads type, 4 Å (8−12 mesh), Sigma-Aldrich) for removing residual H2O. These Li- and Na-ion cells were galvanostatically discharged and charged in a constant current of 100 mA g−1 within a voltage window of 0.001−3 V (vs Li/Li+) or (vs Na/Na+) at room temperature. Cyclic voltammetric (CV) measurements were conducted with a sweep rate of 0.5 mV s−1 within a voltage window of 0.001−3 V (vs Li/Li+) during 10 cycles. 2.4. C K-Edge Soft XAS. C K-edge soft XAS spectra were recorded at the 10D XAS KIST beamline at Pohang Light Source-II (PLS-II) in Korea. To record the C K-edge soft XAS spectra of the pure active carbon materials, the electrode was prepared without a carbon conductor. Cells in different states of charge and discharge were disassembled, and electrodes were washed with DEC solution in the Arfilled glovebox for eliminating the contamination in the air atmosphere. Then, the prepared electrodes were measured on TEY mode under a base pressure of 3 × 10−10 Torr and 0.01 eV resolution.

The Raman spectra of three samples exhibit the two intense peaks corresponding to the D and G band of the carbon materials at around 1350 and 1590 cm−1, which correspond to Raman band for the disordered carbon and graphite, respectively. The intensity ratio of the two peaks (ID/IG) is 0.559 for graphite, 1.076 for soft carbon, and 1.088 for hard carbon. Due to the low graphitization of the soft and hard carbon, the intensity ratios of these carbons are higher than graphite, and the hard carbon showed more disordered nature compared to soft carbon.19−22 Figure 1f represents C K-edge soft XAS spectra of the graphite, soft, and hard carbon particles. TEY mode of the obtained soft XAS spectra provides us the detailed information on the surface of each particle ranging from 1 to 10 nm. It is well known that sp2 and sp3 bonds in the carbon framework show two transitions: the absorption edge observed at 285.5 eV corresponds to the transition of C 1s to π* and the edge observed at 292.2 eV comes from the transition of C 1s to σ*.17,23,24 Figure 1f clearly shows these two electron transitions in all of the carbonaceous materials. Note that, in the spectra for soft and hard carbon, there is a unique hump observed between 288 and 290 eV, which can be attributed to the structural defects of disordered carbon.21 A small peak observed at 287.6 eV for hard carbon particles corresponds to C−H bond, suggesting that hydrogen atoms exist as impurity at the surface of hard carbon particles because of a dangling bond from the structural disorder.25−28 Figure 2a shows the C K-edge soft XAS spectra with the corresponding voltage profile of the graphite electrode for the first cycle and the second discharge step. Each dot in the voltage

3. RESULTS AND DISCUSSION Figure 1a−c shows the TEM images of the graphite, soft, and hard carbon, respectively. Graphite has the highly well crystalized graphene layer, as shown in Figure 1a. The graphene layers of the soft and hard carbon are disordered, as shown in Figure 1b,c. Figure 1d indicates XRD patterns of the graphite, soft, and hard carbon particles. Soft and hard carbon showed broad Bragg peaks at around 23° (for (002) plane) and 44° (for (100) plane) with low intensity because of their low crystallinity. 29994

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became sharper and stronger with an increase of depth of discharge. However, it should be noted that peaks B and C did not completely disappear even after the full charge state, indicating that some chemical species associated with peaks B and C did not undergo further chemical/electrochemical reaction upon further steps after their formation by the electrochemical reaction of an electrolyte at the first discharge reaction. Overall, soft XAS spectra of the soft carbon anode showed that reversible decomposition reaction of SEI partially occurred on the surface of soft carbon materials during cycling. Soft XAS spectra for hard carbon showed a similar tendency to soft carbon but slight difference in the changes of the intensity (Figure 2c), meaning that irreversible formation of SEI on the hard carbon particles is more severe than graphite and soft carbon particles. To more clearly see the change of SEI on the carbonaceous anode materials during cycling, we compared the recorded C Kedge soft XAS spectra for graphite, soft, and hard carbon at the end of discharge and charge during two cycles (Figure 2, bottom). The peaks in the C K-edge spectra reflecting carbonaceous materials (i.e., peaks A and E) did not show any change in their shape and intensity on the whole electrochemical reactions. On the other hand, their surface layer underwent reversible changes with respect to depth of discharge and charge for three types of carbonaceous anode materials. We, however, also found that there is a distinguishable difference in the change of ex situ soft XAS spectra during cycling between graphite and disordered carbon materials. While the peaks B, C, D, and F observed in the spectra of graphite particles showed completely reversible behavior during cycling, the same peaks for soft and hard carbon anode maintained with the changes of their intensity with respect to depth of discharge and charge. These results indicate that the decomposition of SEI layer at the disordered carbon electrodes just partially occurred. Considering that major peaks such as peaks D and F showed reversible variations upon discharge and charge process, major components of SEI on the carbon surface would show partially reversible electrochemical behavior during cycling. On the other hand, the minor carbon containing components in SEI, such as other organic compounds containing C−H and −COOH functional groups, are irreversibly formed on the disordered carbon materials irrespective of types of carbonaceous anode materials. There may be several reasons to account for the difference in the degree of reversible reaction of an electrolyte on the carbonaceous anode materials. The crystallinity of carbonaceous materials would hold the key for the reversibility of SEI formation reaction on their surface during cycling. As mentioned earlier in Figure 1f, compared to graphite, soft and hard carbon have much larger defects. The hump peak between C−H σ* and C σ* peaks observed in the C K-edge soft XAS spectra of soft and hard carbon particles might be related to the structural defects of disordered carbon, such as C−H bond and dangling bonds, which would affect the formation/decomposition behavior of SEI layer on disordered carbon. This imperfection in the disordered carbon materials could cause more vigorous electrolyte decomposition at the first cycle, which left either thicker or nonhomogeneous SEI film on the soft and hard carbon particles. Moreover, the presence of the C−H bond observed only on the hard carbon surface of a pristine particle would be one of the main reasons for the difference in the degree of electrolyte decomposition, thus resulting in more irreversible behavior of SEI on the hard carbon anode compared to soft carbon.

profile denotes the sampling points during cycling. As earlier noted, peaks A and E correspond to the electron transitions of C 1s → π* and C 1s → σ* located on the absorption energies at 285.5 and 292 eV, respectively.17,23,24 The peaks B, C, and D observed at 287.6, 288.7, and 290.6 eV are attributed to the C− H σ*, −COOH π*, and −CO π* transitions, respectively. Peak F observed at around 300.8 eV can be assigned as the peak coming from CO32− species.26,27 New evolved peaks, that is, peaks B, C, D, and F, appeared at the beginning of the first discharge process. It is well known that SEI on the carbonaceous anode materials mainly consists of lithium alkyl carbonate and Li2CO3 with relatively small amount of other organic compounds.2 We found that the observed peaks B, C, D, and F well reflected the aforementioned major components of SEI formed on the surface of a graphite particle.2,3,18 Peaks D and F may correspond to Li2CO3 and lithium alkyl carbonate, and peaks B and C come from other organic compounds containing C−H and −COOH functional groups, respectively.26,29 In the forthcoming deep discharge and the initial stage of the charge process, we could not detect any notable change in soft XAS spectra, implying that other chemical species to form SEI layer were no longer formed even at further insertion and extraction of Li ions into/from the graphite particles. According to the widely accepted reaction mechanism of SEI formation on the carbonaceous anode materials, it is no wonder that we cannot detect the changes in the composition of SEI after the first discharge. Unexpectedly, we found that soft XAS spectra recorded after the completion of the first cycle did not show any peak corresponding to main components of SEI layer, revealing that most of SEI layer formed at the first cycle abruptly disappeared after full charge up to 3 V (vs Li/Li+). At the initial state of second discharge, the peaks to show SEI formation started to evolve again and then this newly formed SEI maintained during discharge without any change in the soft XAS spectra. These results indicated the possibility that reversible decomposition and reformation of SEI layer can occur even on the graphite anode for the initial two cycles. The C K-edge soft XAS spectra of graphite at the different cutoff voltages were recorded, as shown in Figure S1. The voltage profiles with cutoff voltages of ∼1, 2, and 3 V during charge process are, respectively, shown in Figure S1a,b,c, and their corresponding XAS spectra are, respectively, shown in Figure S1d,e,f. While SEI layer on a graphite surface with cutoff voltages below 3 V maintained during cycling, the abrupt disappearance of SEI layer was observed in the graphite electrode charged up to 3 V. While the carbon coated-ZnFe2O4 anode showed a gradual decrease in the peak intensity of soft XAS spectra with an increase in the amount of Li ions extracted, the graphite anode showed an abrupt disappearance when the potential of graphite reached more than 2.5 V. To corroborate our findings on the unexpected behavior of SEI on the graphite anode, we conducted the same soft XAS measurement on other types of carbonaceous anode materials: soft and hard carbon materials (Figure 2b,c). As shown in Figure 2b, soft XAS spectra of the soft carbon electrode showed more gradual changes during discharge and charge. Upon discharge, new peaks associated with SEI formation, such as peaks D and F, appeared, and the shape of these peaks became sharper with an increase of depth of discharge. Similar to graphite, upon charge process, these peaks in the soft XAS spectra gradually diminished with an increase of the amount of Li ions extracted from the lithiated soft carbon anode. Soft XAS peaks related to SEI appeared again at the forthcoming second discharge and 29995

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Figure 3. Electrochemical profiles and C K-edge soft XAS spectra of (a) graphite, (b) soft, and (c) hard carbon during cycling in Na-ion batteries.

To further verify unexpected reversible reaction of SEI on the carbonaceous anode materials, we extended soft XAS study to a Na-ion battery (NIB) system. We conducted the electrochemical insertion and removal of Na ion into/from graphite in the ether-based organic electrolyte because of the inactivity of graphite with Na ion in the carbonate-based organic electrolyte,30,31 while the carbonate-based organic electrolyte was employed in the disordered carbon anodes. We found that there is little difference in the changes of SEI of graphite between formed in the electrolyte for LIBs and formed in the electrolyte for NIBs. The evolution of peaks B, C, D, and F showed SEI formation on the graphite surface mainly composed of sodium alkyl carbonate and Na2CO3 and other organic compounds (Figure 3).32,33 We also found that SEI on the surface of graphite showed reversible formation and decomposition in the etherbased Na-ion-conducting electrolyte. After full charge up to 3 V, SEI layer of graphite entirely disappeared, which exactly coincides with the pristine graphite surface (Figure 3a). Soft and hard carbon in the organic-carbonate-based Na-ionconducting electrolyte also showed almost the similar behavior to that observed in the carbonate-based Li-ion-conducting organic electrolyte (Figure 3b,c). These results showed that the same phenomenon, that is, partial reversible decomposition of SEI during cycling, commonly occurs on the surface of disordered carbon anodes in Li- and Na-ion batteries. While graphite showed a complete decomposition of SEI layer after full charge process, disordered carbon anodes did not return to their pristine state (Figure 3, bottom). In addition, peaks A and E coming from carbonaceous anode materials themselves could not be detected after the first full discharge. This implies that SEI

formed on the disordered carbon anodes in the Na-ionconducting electrolyte might be thicker than that in the Liion-conducting electrolyte, as previously reported.12 Although the results presented here are quite different from the widely accepted behavior of SEI on the carbonaceous anode materials, we think that our findings can further elaborate the previously established explanation on SEI layer formed at the surface of anode materials in Li- and Na-ion batteries. We observed complete disappearance of SEI layer at the graphite anode after full charge up to 3.0 V; however, the decomposition of SEI layer for disordered carbon occurred partially. To correlate these findings with the electrochemical behavior of carbonaceous anode materials, we compared their differential capacity plots (DCP) and cyclic voltammogram (CV). The distinct peak around 0.8 V at the 1st discharge is known to be originated from the irreversible formation of SEI layer on the graphite surface.34,35 When the graphite anode is operated below 2.5 V, this peak appears only at around 0.8 V during the 1st discharge, as shown in Figure S1a,b. However, as shown in Figure 4a, the hump peak of around 0.8 V at the cathodic part was detected continuously over the cycles because of the reformation of SEI. Moreover, DCP for the graphite anode shows the distinctive peak at around 2.5 V (vs Li/Li+) upon charge, suggesting the presence of certain electrochemical oxidation during charge and discharge process. A similar peak could be also observed at the CV of the graphite anode (Figure 4b). These results suggested that there is another electrochemical oxidation reaction at high potential of about 2.5 V. Recently, Hu et al. reported similar electrochemical behavior of highly oriented pyrolytic graphite (HOPG), showing anodic CV 29996

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Figure 4. DCPs of (a) graphite, (c) soft, and (e) hard carbon in Li-ion batteries within a voltage window of 0.001−3 V at a current density of 100 mA g−1 during 10 cycles, and CV curves of (b) graphite, (d) soft, and (f) hard carbon within a voltage window of 0.001−3 V at a sweep rate of 0.5 mV s−1 during 10 cycles in Li-ion batteries.

evidence to show the possibility that there is another chemistry, in particular, reverse reaction of SEI on the surface of carbonaceous anode materials in both Li- and Na-ionconducting electrolytes, which has been already observed at the transition metal oxide anode materials in the Li-ionconducting electrolyte. We also found that the observed reversible reaction of SEI is highly dependent on the types of carbonaceous anode materials. Graphite showed completely reversible formation and decomposition of SEI layer when the charging potential reaches more than 2.5 V, while disordered carbon showed partially irreversible SEI layer formation in both Li- and Na-ion battery systems. It should be noted that the complete disappearance of SEI could be found only at the graphite when charged at high potential of more than 2.5 V (vs Li/Li+). With a practical standpoint of view, it is not possible to charge the anode up to such a high potential because of overdischarge of the cell, which causes another abuse condition

peak at the same potential in 0.5 M lithium bis(tirfluoromethanesulfonyl)imide salt: N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide ionic liquid electrolyte. They also attributed such an anodic peak to the corresponding oxidation process of desorption of reoxidation of the product generated in the cathodic reaction of HOPG.36 On the other hand, DCPs and CVs of disordered carbon did not show such a marked peak because the surface reaction occurred throughout the whole potential range during discharge and charge process, not at a fixed potential (Figure 4c−f). Figure 5 illustrates the schematic of SEI formation and decomposition mechanism on graphite and disordered carbon on the basis of our soft XAS study. Until now, it is widely accepted that SEI is formed through irreversible electrochemical reaction of an organic electrolyte at the first discharge process and does not undergo any electrochemical reaction during cycling. However, soft XAS study in this work provided clear 29997

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Figure 5. Schematic illustration of SEI layer formation and decomposition mechanisms of (a) graphite and (b) disordered carbon.

such as dissolution of Cu current collector and overcharge of cathode material. Nevertheless, we firmly believe that these unexpected results presented here can give insights for more elaborate understanding of the interfacial electrochemical reaction between the electrolyte and carbonaceous anode materials for Li- and Na-ion batteries.



4. CONCLUSIONS Reversible interfacial electrochemical reaction of the electrolyte on carbonaceous anode materials was clearly observed via synchrotron-based C K-edge soft XAS. Some of SEI layer formed on the carbonaceous anode materials at the initial discharge is reversibly decomposed upon charge process and then formed again on the surface of carbonaceous anode materials, which is an entirely different behavior from what has been known so far. The degree of reversibility in this electrochemical reaction of an electrolyte is highly dependent of the types of carbonaceous materials. We firmly believe that our findings can help us take a step forward toward comprehensive understanding of the exact nature of SEI layer on the carbonaceous anode materials with detailed reaction mechanism, thus guiding us to make more correct material designs for electrode and electrolyte used in Li- and Na-ion battery systems.



Electrochemical curves of the graphite anode at different voltage ranges and C K-edge soft XAS spectra at the end of discharge and charge with different working voltages (Figure S1); DCP of the graphite anode at different working voltages (Figure S2) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.M.K.). *E-mail: [email protected] (H.K.). *E-mail: [email protected] (W.S.Y.). ORCID

Ji Man Kim: 0000-0003-0860-4880 Hansu Kim: 0000-0001-9658-1687 Won-Sub Yoon: 0000-0002-6922-2088 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2017R1A4A1015770).



ASSOCIATED CONTENT

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b09939. 29998

DOI: 10.1021/acsami.8b09939 ACS Appl. Mater. Interfaces 2018, 10, 29992−29999

Research Article

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DOI: 10.1021/acsami.8b09939 ACS Appl. Mater. Interfaces 2018, 10, 29992−29999