Decomposition of Solid Electrolyte

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Insight into the Formation/Decomposition of Solid Electrolyte Interphase Films and Effects on the Electrochemical Properties of Sn/Graphene Anodes Mei Ma, Jiawei Yan, Chenglong Yu, and Shouwu Guo* Department of Electronic Engineering, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China

J. Phys. Chem. C Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/26/18. For personal use only.

S Supporting Information *

ABSTRACT: A robust solid electrolyte interphase film formed on the anode is of importance for the electrochemical performances of a lithium-ion battery. In this work, the formation and decomposition of the solid electrolyte interphase film on the Sn/graphene anode are studied through scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy measurements, which are performed systematically on the anode before and after the lithiation and the lithiation/delithiation cycling. It is demonstrated that at the first few lithiation/delithiation cycles, the solid electrolyte interphase film is formed and decomposed accordingly. However, with lithiation/delithiation cycling progressing, the solid electrolyte interphase film gets stable and stable, and a robust solid electrolyte interphase film is formed after 10 lithiation/ delithiation cycles, which is different from that on a bare Sn anode. The hyperfine X-ray photoelectron spectroscopy data show that the solid electrolyte interphase film should be initiated both on graphene and Sn, but the one formed on the graphene component is more stable than that on the Sn, which is enlarged with lithiation/delithiation progressing and covers the whole Sn/graphene anode after about 10 lithiation/delithiation cycles, forming a robust solid electrolyte interphase film.

1. INTRODUCTION As an anode material for lithium-ion batteries (LIBs), metallic tin (Sn) shows high theoretical specific capacity of 992 mA h g−1 and preferable voltage platform (0.38−0.66 V) for lithium ion insertion (lithiation) and extraction (delithiation).1−3 However, the large volume variation of the Sn anode during lithiation/delithiation weakens severely the electrochemical performances of the Sn anode and limits somehow its practical application.4,5 It was believed that during the lithiation, a solid electrolyte interphase (SEI) film was formed on the Sn anode through the reaction of the electrode materials with the electrolyte; on the other hand, owing to the mechanical stress and volume shrinking of Sn during the delithiation, the asformed SEI film fractures, resulting in the capacity and cycling stability fading of the Sn anode.6,7 To get rid of the volume variation, so far, a variety of Sn-based composites, such as nanosized Sn-based alloy, Sn−C, and Sn−carbon nanotubes composites have been prepared and tested as anodes for LIBs.8−13 It has been demonstrated that besides the suppression of volume variation, such as lithiation/delithiation rate capability and cycling stability of the composites can also be improved.14−16 More recently, the composites of Sn and graphene (Sn/graphene) with different compositions and morphologies have been designed and fabricated successfully as anodes for LIBs.17−24 Differing from the bare Sn and other Sn-based composites, it has been demonstrated that besides the enhanced electrochemical properties, the anodes made of © XXXX American Chemical Society

Sn/graphene composites have usually a thought-provoking broad delithiation plateau at the voltage of around 2.9 V (in a coin half-cell),25−29 and also show an ultrahigh initial Coulombic efficiency, which is comparable to that of commercial graphite anodes.25 However, the formation and decomposition of SEI films on the Sn/graphene anode during the lithiation/delithiation, the effects of the SEI film on the electrochemical performances, especially the origins of the aforementioned broad delithiation plateau, and the ultrahigh initial Coulombic efficiency of Sn/graphene anodes have not been fully elaborated, though a number of studies have been conducted on the electrochemical reaction mechanism4,20,30 and the interfacial kinetic process of Sn/graphene composites as anodes for LIBs.6,31−33 In the work, Sn/graphene composites with different Sn to graphene ratios (in weight) are prepared following a literature method.17 The morphology and structure of the as-prepared Sn/graphene composites are characterized using X-ray powder diffraction (XRD), Raman spectrum, field-emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) imaging. The comprehensive electrochemical performances of the Sn/graphene composite as the anode for LIBs were measured. More pronouncedly, after the Received: September 6, 2018 Revised: October 16, 2018 Published: October 16, 2018 A

DOI: 10.1021/acs.jpcc.8b08715 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. (a) FE-SEM images of bare Sn. (b,c) FE-SEM images, (d) TEM image, (e) Raman spectra, and (f) XRD pattern of the Sn/graphene-I composite.

contamination with moisture/air, after lithiation and delithiation, the anode materials were transferred into a special chamber fitted to the XPS spectrometer in an argon-filled glovebox, then the XPS data were collected.3 The binding energies were calibrated using the C 1s peak at 284.82 eV, and core peaks were fitted using a Shirley background type. 2.3. Electrochemical Measurements. Electrochemical properties of as-obtained Sn and Sn/graphene composites as anodes were measured on coin half-cells, in which the working electrodes were prepared by mixing Sn/graphene composites or Sn with acetylene black and polyvinylidene fluoride (PVDF) at a weight ratio of 8:1:1 in 1-methyl-2-pyrrolidone (99.0%) solvent. The mixed slurry was fully grinded and then coated on a copper foil. After being dried in a vacuum oven at 110 °C for 12 h, the copper foil was cut into disks of ∼1 cm in diameter, which was used as the working electrode. The CR 2025 coin half-cells were assembled in an argon-filled glovebox, using metal lithium wafers as counter electrodes, Celgard 2325 membrane as the separator, and 1 M LiPF6 in ethylene carbonate/ethyl methyl carbonate/dimethyl carbonate (DMC) (1:1:1 in volume) as the electrolyte. The galvanostatic lithiation/delithiation properties of the working electrodes were tested on the LAND-CT2001A (Wuhan, China) system at various current densities. The cyclic voltammetry (CV) curves and EIS were measured using an Autolab electrochemical workstation (Metrohm, Switzerland).

lithiation and delithiation processes, the Sn/graphene composite electrodes are analyzed by FE-SEM, electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS), indicating that the formation/decomposition of the SEI film dominates the aforementioned board delithiation plateau and the Coulombic efficiency. It is also illustrated that after 10 lithiation/delithiation cycles, a robust SEI film can be formed on the surface of the Sn/graphene electrode, and the electrochemical performances get steady too, which is differing from the bare Sn anode and should be beneficial to the practical application of Sn/graphene as the anode.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Sn/Graphene Composite. Graphene oxide (GO) used in the work was synthesized through a modified Hummers method as reported previously.34 For preparation of Sn/graphene composites, in a typical experiment, 1 mmol of SnCl2·2H2O (Sinopharm Chemical Reagent Co., Ltd) was dissolved in 200 mL, 0.1 mg mL−1 of GO aqueous solution. Subsequently, NaBH4 (Sinopharm Chemical Reagent Co., Ltd) aqueous solution was dropwisely added in 5 times molar quantity excess with respect to SnCl2 and stirred for 2 h in an ice−water bath. The solid product was separated and washed with deionized water and dried at 50 °C in a vacuum oven, then annealed at 900 °C for 3 h in the Ar/H2 (5%) atmosphere. Finally, the Sn/graphene composite was obtained and named as Sn/graphene-I. By adjusting amounts of SnCl2·2H2O to 0.5 and 2 mmol, Sn/graphene-II and Sn/ graphene-III were prepared. For comparison, pure Sn was prepared under similar conditions. 2.2. Characterization. FE-SEM and TEM images of Sn and Sn/graphene composites were acquired on an Ultra 55 field-emission scanning electron microscope (Zeiss, Germany) operated at 5 kV and a FEI Talos F200X field emission transmission electron microscope (FEI, USA) worked at an accelerating voltage of 200 kV. The crystalline statue of Sn/ graphene composites was analyzed using a D8 ADVANCE diffractometer (Bruker, Germany) with Cu Kα irradiation (λ = 1.55406 Å) at the scanning rate of 6°/min. Raman spectra were obtained on a dispersive Raman microscope with the type of Senterra R200-L (Bruker, Germany). XPS spectra were collected on an AXIS Ultra DLD spectrometer (Kratos, Japan) with Al Kα radiation (hν = 1486.4 eV). To avoid the

3. RESULTS AND DISCUSSION The morphologies of the as-prepared Sn/graphene composites were characterized using FE-SEM and TEM and compared with bare Sn particles prepared under the same condition. Figure 1b,c shows selectively the SEM images of Sn/grapheneI differing from the bare Sn particles (Figure 1a), and in the composite a wrinkled ultrathin film was coated uniformly on the surface of Sn nanoparticles (Figure 1b−d). To illustrate the thin film composition, the Raman spectrum of the composite was collected and is shown in Figure 1e. The peaks appeared at 1334, 1575, 2663, and 2908 cm−1 in the Raman spectrum are well fitted to the D, G, 2D, and 2G characteristic peaks of graphene (more specifically the chemically reduced GO).35 On the other hand, the Sn nanoparticles seem anchored uniformly on the graphene sheets, implying that the nucleation of Sn nanoparticles was initiated on graphene sheets. The average size of Sn nanoparticles in the composites is smaller than 50 B


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Figure 2. (a) Galvanostatic lithiation/delithiation curves of Sn/graphene composites, and bare Sn electrodes at current densities from 0.1 to 10 A g−1, (b) cycling performance of the Sn/graphene-I electrode at a current density of 0.1 A g−1 for 200 cycles, (c) galvanostatic lithiation/delithiation voltage profiles of the initial several cycles of Sn/graphene-I from 0.01 to 3 V, measured at 0.1 A g−1, and (d) CV curves of the first several lithiation/delithiation cycles of the Sn/graphene-I electrode at a scanning rate of 0.5 mV s−1 from 0.01 to 3 V.

that the lithiation plateaus of the Sn/graphene-I composite electrode at 0.25−0.86 V are similar to those of the bare Sn that are related to the alloying and dealloying reactions between Sn and Li ions,38 and expectedly, the lithiation plateaus of graphene occurred from 0.01 to 0.25 V were detected for the composite electrode.17,19,39,40 The delithiation plateaus of the Sn/graphene-I composite electrode at 0.2, 0.54, 0.73, 0.82, and 0.88 V represent the extraction of lithium ions from graphene and from LixSn alloys, respectively.17,38 Besides, a broad delithiation plateau at around 2.95 V and an ultrahigh initial Coulombic efficiency of 99.19% were detected from the Sn/graphene-I composite during the first few lithiation/ delithiation cycles, which were observed by other research groups, too.25,41 Obviously, the broad plateau is not from the bare Sn anode. As demonstrated in Figure S4, the more the graphene sheets coated on the Sn, the larger the plateau and the higher the corresponding initial Coulombic efficiency. The initial Coulombic efficiency of the Sn/graphene-II composite, Sn/graphene-III composite, and pure Sn are 89.26, 81.99, and 65.43%, respectively. As illustrated in Figure 2d, the CV curve of Sn/graphene-I acquired during the first lithiation/ delithiation cycle show an oxidation peak at around 2.95 V too. However, starting from the second lithiation/delithiation cycle, the broad delithiation plateau and the oxidation peak at 2.95 V decayed gradually and disappeared finally. Considering the electrolyte used in this work has a stable electrochemical window within 4 V,42,43 meaning the peak at around 2.95 V should not be simply related to the oxidation of the electrolyte. Additionally, the XRD data of the Sn/graphene-I electrode before and after the first lithiation/delithiation cycle (Figure S5) are almost identical, excluding the formation of novel crystalline materials. To elaborate the reasons causing the aforementioned abnormal electrochemical performance of Sn/graphene composite electrodes during first several lithiation/delithiation cycles, the interfacial electrochemical kinetic process and structural variations of Sn/graphene-I electrodes were studied after lithiation and delithiation. To do that, two coin half-cells,

nm, a little bit smaller than that of bare Sn nanoparticles prepared under the same condition, implying that the graphene sheets confined the growth and blocked the agglomeration of Sn nanoparticles.28 The XRD patterns of the composites are depicted in Figure 1f, which are consistent with that of the crystalline Sn (JCPDS card no. 04-0673), but no diffraction peak of graphite appeared, indicating that graphene sheets dispersed well in the composites, which is fully in agreement with FE-SEM and TEM images. The approximate mass fraction of graphene in Sn/graphene-I composite is 3.65% estimated from the thermogravimetric analysis (TGA) data (Figure S1). The electrochemical properties of as-obtained Sn/graphene composites and Sn as anodes for LIBs were studied on the coin half-cells and are shown in Figure 2. In contrast to the bare Sn, in Figure 2a, the specific capacity and the lithiation/ delithiation rate capability of Sn/graphene-I and II composites have been improved dramatically, but not for Sn/graphene-III. Even at a high current density of 5 A g−1, the Sn/graphene composites still manifest specific capacities, for example, the specific capacity of Sn/graphene-I can reach 310 mA h g−1, but it is closed to zero for the bare Sn anode. The reason is that with the increase of the content of the Sn nanoparticles within the composite, the graphene sheets seem not fully cover the Sn nanoparticles surface, as shown in Figure S2. Additionally, after 200 lithiation/delithiation cycles at 0.1 A g−1 (Figure 2b), the specific capacity and Coulombic efficiency of the Sn/grapheneI composite can retain at about 805 mA h g−1 and 98%, respectively, which are much higher than those of Sn (Figure S3). These data imply that the graphene sheets in the composite play key roles in accelerating the electron and lithium ion transportation among the Sn nanoparticles that contribute to the lithiation/delithiation rate capability, suppressing the excessive pulverization and reducing the volume variation of the Sn nanoparticles, which are beneficial to the capacity retaining and cycling stability.36,37 Further, if looking at closely the electrochemical performances during the first lithiation/delithiation cycle (Figure 2c), it can be found C


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Figure 3. FE-SEM images of Sn/graphene-I electrodes (a) before using, (b) after lithiation only, and (c) after one lithiation/delithiation cycle; (d− f) the corresponding EIS spectra of Sn/graphene-I electrodes acquired in the frequency range of 0.1 Hz to 100 kHz, and insets are the corresponding equivalent circuits.

Table 1. Rct and RSEI Values of Sn/Graphene-I Electrodes before and after Lithiation and Delithiation Sn/graphene-I electrodes

RSEI (Ω)

estimated error (%)

Rct (Ω)

estimated error (%)a

before lithiation after lithiation only after one lithiation/delithiation cycle

92.9

0.338

87.7 44.0 72.1

0.780 0.961 0.703

a

Errors are given by the NOVA software accompanied with the Autolab electrochemical workstation (Metrohm, Switzerland).

Figure 4. XPS spectra of C 1s, O 1s, F 1s, and Sn 3d of Sn/graphene-I electrodes before using (a1−d1), after the first lithiation process (a2−d2), and after the first lithiation/delithiation cycle (a3−d3).

one is after lithiation only and the other one is after one lithiation/delithiation cycle at 0.1 A g−1, were disassembled in

an argon-filled glovebox, and the Sn/graphene-I was separated by rinsing with DMC to remove the residual LiPF6. The FED


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acetylene black, C−H, C−O, and CO show up again accompanying with the drastic decreases of peaks of C−H and CO32−, suggesting the hydrocarbon and Li2CO3 components in the SEI film were decomposed. In contrast, the peak at 291.00 eV from PVDF is always detectable which might be due to the fact that the binder is barely covered by the SEI film.3 For O 1s XPS, before using (Figure 4b1), one peak at 531.21 eV related to Sn−O and two peaks at 533.98 and 532.55 eV from C−O, and CO of graphene were detected. After lithiation, Figure 4b2, a strong O 1s XPS peak at 531.72 eV assigned to Li2CO3 was observed,50 confirming further that the as-formed SEI film contains Li2CO3. Besides, a weak O 1s peak appeared at 533.10 eV that may be attributed to the lithium alkyl carbonates, another possible component of the SEI film.52 Comparably, after one lithiation/delithiation cycle, Figure 4b3, the O 1s peak from Li2CO3 decreases sharply accompanying with the reappearance of peaks of Sn−O and Sn−O−C, proving again the Li2CO3 component in the SEI film was decomposed during the delithiation process. Interestingly, as depicted in Figure 4c1, before using, the F 1s XPS peak can be assigned to only PVDF. After lithiation, Figure 4c2, besides the one from PVDF, a weak F 1s peak related to LiF was detected. After one lithiation/delithiation cycle, Figure 4c3, the F 1s peak of LiF gets stronger. These data indicate that the LiF is one of the stable components in the SEI film,53 differing from the hydrocarbon and Li2CO3, which were decomposed during the delithiation process. Figure 4d1−3 exhibits the Sn 3d XPS spectra. After lithiation, almost no Sn 3d XPS peaks can be detected (Figure 4d2), indicating that the surface of the electrode was covered densely by the SEI film,54 which is consistent with the FE-SEM results (Figure 3). Unlike the irreversible buildup of the SEI film on the Sn-based alloy anode,51 after the first delithiation (Figure 4d2), the Sn 3d core peaks of the Sn/graphene-I anode reappears owing to the decomposition of the SEI film. Getting together, the results indicate clearly that during the lithiation, a SEI film containing hydrocarbon compound, Li2CO3, and LiF, was formed, but the hydrocarbon compound and Li2CO3 components are decomposed through the following delithiation. The formation and decomposition of the SEI films during subsequent lithiation and delithiation processes were also studied through XPS measurements. Figure S6 and Table S2 selectively present XPS spectra of C 1s, O 1s, F 1s, and Sn 3d and as-detected atomic ratios of C, O, F, and Sn elements of Sn/graphene-I after the second and fifth lithiation, and the second and fifth lithiation/delithiation cycles. As depicted in Figure S6a1, after the second lithiation, the XPS peaks of C 1s from CO32− (289.90 eV) and C−H (284.82 eV) increase dramatically in comparing with that of the C 1s peaks after the first lithiation/delithiation cycle (Figure 4a3), suggesting that the SEI film was formed again on the surface of the electrode. However, after the second lithiation/delithiation (Figure S6a2), the SEI film seems decomposed again too. Actually, as illustrated in Figure S6a3a4, the formation and decomposition of the SEI film happens even during the fifth lithiation/ delithiation cycle. However, with the increase of the lithiation/ delithiation cycling time, as exhibited in Table S2, the difference of the contents of C−H and CO32− detected on the Sn/graphene-I electrode after the lithiation and the lithiation/delithiation cycle is smeared gradually, indicating the hydrocarbon species and the Li2CO3 component in the SEI film were decomposed partially during delithiation processes. This is confirmed further by the FE-SEM (Figure S7) and EIS

SEM images of Sn/graphene-I composites before and after lithiation only, and also after one lithiation/delithiation cycle are acquired. As shown in Figure 3a, the surface of Sn/ graphene-I before using is smooth and nothing seems covered on. In contrast, after lithiation, the surface of Sn/graphene-I is coated uniformly with a dense film, Figure 3b, hinting the formation of an SEI film.3 Interestingly, after finishing one cycle of lithiation/delithiation, the surface morphology of the Sn/graphene-I electrode (Figure 3c) is almost the same as that of the one before using (Figure 3a), indicating that the SEI film was most probably decomposed. It is well known that the formation and decomposition of the SEI film can also affect the electrochemical kinetic properties of anode.31,44,45 Thus, the corresponding EIS of Sn/graphene-I electrodes before and after lithiation only and one cycle of lithiation/delithiation were collected and shown in Figure 3d−f. The transfer resistances (Rct) at the surface of the electrodes were derived from the depressed semicircle at the intermediate frequency range in EIS through equivalent circuit fitting (the insets in Figure 3d−f), and the detailed data are shown in Table 1. Comparably, the Rct for the electrode before and after one cycle of lithiation/delithiation is 87.7 and 72.1 Ω, respectively, very closing. This implies that there should assume similar surface status meaning no SEI film left over after one cycle of lithiation/delithiation, which is in agreement with the FE-SEM results. For the electrode after lithiation only, there are two depressed semicircles detected in EIS, as displayed in Figure 3e. The corresponding Rct derived from the semicircle at the intermediate frequency range, and the resistance related to the semicircle at the high frequency range, are 44.0, and 92.9 Ω, respectively. The later one should be assigned to the resistance of the SEI film.46−48 These results lead us to believe that the formation of the SEI during lithiation, but it is decomposed after the delithiation that might be related to the oxidation peak at 2.95 V on CV and the broad delithiation plateau formation. To get insight into the detailed formation and decomposition procedure of the SEI film on the Sn/graphene-I electrode during first several lithiation and delithiation processes, XPS data of the electrodes before and after lithiation, and after one cycle lithiation/delithiation were acquired (Figure 4). The detailed binding energy values and the atomic ratios (%) of C, O, F, and Sn elements are summarized in Table S1. For the electrode before using, the C 1s XPS data, Figure 4a1, is composed of five peaks at 284.75, 284.82, 286.35, 288.50, and 291.00 eV, which can be assigned to the carbon atoms from acetylene black (C-H, C-O, C=O), graphene (C-C/C=C, C-O, C=O),34 and PVDF (C-H, C-F) used as the binder during the electrode preparation.49 For the electrode after lithiation, as depicted in Figure 4a2, the C 1s peak (at 284.75 eV) correlated to carbon atoms of acetylene black disappeared, the intensities of peaks of the carbon atoms of graphene and PVDF decreased, and the intensity of the peak from C−H at 284.82 eV increased dramatically, indicating the formation of the SEI film on the Sn/graphene electrode (in agreement with the FE-SEM imaging result, Figure 3b), and the hydrocarbon species should be one of the dominated components of the SEI film. Meanwhile, a new peak appeared at 289.90 eV corresponding to CO32− (Li2CO3), revealing that Li2CO3 should be another main component of the SEI film, which is similar to the SEI films formed on other Sn-based anodes.50,51 Remarkably, after finishing one cycle of lithiation/ delithiation, Figure 4a3 shows that the C 1s peaks from E


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Figure 5. C 1s, O 1s, F 1s, and Sn 3d XPS spectra acquired from Sn/graphene-I electrodes after the 10th lithiation (a1−d1) and 10 lithiation/ delithiation cycle (a2−d2).

electrochemical performances and also the exploration of their practical application of the Sn-based LIB anode materials.

(Table S4) results. This is meaning, on the other hand, the SEI film may get stable after certain lithiation/delithiation cycles. Fortunately, as shown in Figure 5 and Table S3, XPS spectra of C 1s, O 1s, F 1s, and Sn 3d and as-detected atomic ratios of C, O and F of Sn/graphene-I after the 10th lithiation and delithiation are almost the same, demonstrating the formation of a robust SEI film, consistent with their FE-SEM and EIS results (Figure S7 and Table S4). The core XPS peaks of C 1s, O 1s, and F 1s are assigned to hydrocarbon species, Li2CO3, lithium alkyl carbonates, and LiF, respectively, which should be the main components of the as-formed SEI film. Besides, as illustrated in Figures 4, 5, and S6, the Sn 3d XPS peak can be clearly detected with almost no intensity fluctuation after first lithiation (Figure 4d2), implying that the SEI film seems formed and decomposed mainly on the graphene. The reason is that the Sn nanoparticles within the Sn/graphene composites are covered by the graphene sheets (Figure 1). The formation/ decomposition of the SEI film account for, first, the fluctuation of the Coulombic efficiencies of the Sn/graphene anode during the first few lithiation/delithiation cycles, which are 99.19, 69.79, 88.72, 91.89, 94.05, and 96.47% for the first, second, third, fourth, fifth, and 10th cycle, respectively. Second, the fact that a robust SEI film formed after more than 10 lithiation/ delithiation cycles can also explain why the specific capacity of the coin cells with the Sn/graphene anode get constant after almost 10 cycles (Figure 2b).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b08715. TGA curves of the Sn/graphene-I composite, FE-SEM images of the Sn/graphene-II and Sn/graphene-III composite, cycling performance of the Sn anode, galvanostatic lithiation/delithiation voltage profiles of the first cycle of Sn/graphene-II and Sn/graphene-III, and Sn anodes, XRD patterns of Sn/graphene-I electrodes before and after the first lithiation/delithiation cycle, XPS data of Sn/graphene-I electrodes after the first several lithiation and lithiation/delithiation cycles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shouwu Guo: 0000-0002-2219-7700 Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS In summary, the formation/decomposition of the SEI film on the Sn/graphene composite anode during lithiation and delithiation were studied systematically. It was demonstrated that during the first few lithiation and delithiation, the SEI film was formed and decomposed partially, and a robust SEI film could be formed after more than 10 lithiation/delithiation cycles that contains hydrocarbon species, Li2CO3, lithium alkyl carbonates, and LiF. It was found that the formation and decomposition of the SEI films happened mainly on the graphene. It was illustrated also that the formation/ decomposition of the SEI film dominated the Coulombic efficiencies, the cycling stability of the Sn/graphene anode during the first few lithiation/delithiation cycles. The results should be beneficial for the understanding fundamentally the

ACKNOWLEDGMENTS The work was financially supported by the National “973 Program” of China (nos. 2014CB260411 and 2015CB931801) and the National Science Foundation of China (no. 11374205).

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REFERENCES

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Decomposition of Solid Electrolyte

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