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Identification of the Solid Electrolyte Interface on Si/C Composite Anode with FEC as the additive Qi Li, Xiangsi Liu, Xiang Han, Yuxuan Xiang, Guiming Zhong, Jian Wang, Bizhu Zheng, Jigang Zhou, and Yong Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22221 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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ACS Applied Materials & Interfaces

Identification of the Solid Electrolyte Interface on Si/C Composite Anode with FEC as the Additive Qi Li†,‡, Xiangsi Liu†,‡, Xiang Han§, Yuxuan Xiang†, Guiming Zhong‖, Jian Wang⊥, Bizhu Zheng†, Jigang Zhou⊥,* and Yong Yang†,* †

Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory for

Physical Chemistry of Solid Surface, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China §

Fujian Provincial Key Laboratory of Semiconductors and Applications, Collaborative Innovation

Center for Optoelectronic Semiconductors and Efficient Devices, Department of Physics, Xiamen University, Xiamen 361005, P. R. China ‖

Xiamen Institute of Rare Earth Materials, Haixi institutes, Chinese Academy of Sciences, Xiamen

361024, P. R. China ⊥

Canadian Light Source, 44 Innovation Boulevard, Saskatoon, SK S7N 2V3, Canada

KEYWORDS: solid electrolyte interface, fluoroethylene carbonate, solid state NMR, XPS, XPEEM

ABSTRACT: Silicon-based anodes have potential to be used in the next-generation lithium ion batteries owing to its higher lithium storage capacity. However, the large volume change during charge/discharge process and the repeated formation of a new solid electrolyte interface (SEI) on

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the re-exposed Si surface should be overcome in order to achieve a better electrochemical performance. Fluoroethylene carbonate (FEC) has been widely used as electrolyte additive for Sibased anodes but the intrinsical mechanism in performance improvement is not clear yet. Here, we combined solid state NMR, XPS and X-PEEM to characterize the composition, structure and inhomogeneity of the SEI on Si/C composite anodes with or without FEC additive. Similar species are observed with two electrolytes but a denser SEI formed with FEC which could prevent the small molecules (i.e. LiPF6, P-O and Li-O species) penetrating to the surface of Si/C anode. The hydrolysis of LiPF6 leading to LixPOyFz and further to Li3PO4 could also be partially suppressed by the denser SEI formed with FEC. In addition, a large amount of LiF could protect the cracking and pulverization of Si particles. This study demonstrates a deeper understanding of the SEI formed with FEC which could be a guide for optimizing the Si-based anodes for lithium ion batteries.

1. INTRODUCTION The capacity of silicon-based anodes is about 10 times higher than commercialized graphite anodes, leading Si-based anodes to be one of the potential candidates for the next-generation lithium ion batteries.1,

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However, one challenge of the adoption of silicon-based anodes is

controlling the decomposition of electrolyte components on the surface of electrodes to form a passive solid electrolyte interface (SEI) during the electrochemical cycle.3-5 A stable SEI should keep electrolytes from decomposing and have a good Li+ conductivity. For instance, the SEI formed on graphite has a high stability and Li+ conductivity in the EC systems, which is one of the main reasons for the extremely high electrochemical reversibility of the graphite anodes.6 However, the SEI on the surface of Si anodes is unstable and easily broken due to the ~300% volume expansion during cycling7. Furthermore, the repeated volume expansion/contraction will


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cause the cracking and pulverization of Si particles and then a new silicon surface will be exposed to the electrolyte, leading to continuous growing of the electronically insulated SEI during electrochemical process, resulting in electrochemical detachment of Si particles and continuous electrolytes consumption, thus to a rapid decay of electrochemical capacity. The formation of a stable SEI on the surface of Si-based anodes by adding fluoroethylene carbonate (FEC) additive is one of the most widely used strategies for improving the electrochemical performance of Si-based anodes, by which the cycling stability could be significantly improved without complex design of material structures.8-11 Choi et al. attributed the improvement of electrochemical performance to the formation of LiF and Si-Fx groups on the surface of Si anode by the decomposition of FEC,12 which had been confirmed by many other researchers recently.13, 14 However, other studies showed that the polycarbonate or polymer-like layer formed on the surface was the main factor for improving the performance of Si-based anodes.8, 11, 15 The reasons for these controversies are that the growth of the SEI is affected by many factors, including current density,16 electrochemical window,17 binders18 and even the cleaning and transfer process19 before characterization. The SEI is generally considered to consist of an inner inorganic layer (i.e. Li2CO3, LiF, etc.) closing to the electrodes and an outer organic layer (LEDC, polycarbonate, etc.) closing to electrolytes.20-22 Tasaki et al. found that different SEI components had different solubility and the organic salt components were more soluble in the electrolytes.23 Schroder et al. found that F- decomposed from FEC and/or LiPF6 could react with the oxide layer on the surface of Si, thus changing the surface structure of Si-based material.24 Shen et al. showed that the SEI formed in the EC system was thinner and more dispersed, while the SEI formed in the FEC system was thicker and denser.25 Huang et al. used in-situ electrochemical AFM to image the surface structure of Si/C anode during the first cycle. For the


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EC/DMC and the FEC/DMC systems, the volume of micron particles continually changed and SEI layer grew with the charge and discharge. However, the effect of FEC need to be further explored in the long term cycles.26 In this paper, the binder-free porous Si/C composite materials were synthesized by chemically etching of micro-sized Si particles and then mixing with cyclic polyacrylonitrile (c-PAN), which showed good electrochemical behaviors27 and could be processed by industrial methods, thus demonstrating the capability for future commercialization. Solid state nuclear magnetic resonance (ssNMR), X-ray Photoelectron Spectroscopy (XPS) and X-ray Photoemission Electron Microscopy (X-PEEM) techniques were employed to investigate the composition, structure and inhomogeneity of the SEI on Si/C composite anode at multiple elemental sites (i.e. Si, F, O, etc.) in order to get a deep insight of the decomposition mechanism of FEC additive in stabilizing SEI on Si anodes. 2. Experimental Section 2.1 Preparation of Materials. Porous Si was synthesized by a scalable Cu-assisted chemical etching method reported before.27 The as-prepared porous Si was ball-milled in a c-PAN solution (10 wt% in dimethylformamide (DMF), Si:c-PAN = 1:1), then the mixed slurry was drop-cast onto Cu foil and dried naturally. After that, the electrodes were heated to 700 ̊C for 6 h under Ar/H2 atmosphere, then transferred to glove box for battery assembly. The mass ratio of Si:C is about 7:3.28 2.2 Electrochemical Measurements. The batteries were fabricated with Li foil as anode, 1 M LiPF6 in EC/DMC with or without 10 vol% FEC as electrolyte (labeled as FEC and EC/DMC, respectively), and Si/C composite material as cathode. Galvanostatic electrochemical


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measurements were conducted on a LAND CT-2001A (Wuhan, China) battery test system at 25 ̊C. Batteries were hold at 2 V for 12 h in the last cycle and disassembled immediately after electrochemical measurements, then electrodes were softly washed three times by dimethyl carbonate (DMC) for the following characterization. 2.3 Characterization of Materials. Solid state NMR measurements were performed on a Bruker Avance III 400 MHz NMR spectrometer with a 1.3 mm probe under spinning rates from 50 kHz to 60 kHz. Samples were loaded in the Ar-filled glove box. The 19F, 31P and 7Li shift are referenced to LiF (-204 ppm), ADP (-1 ppm) and LiF (-1 ppm) powders, respectively. Solution NMR were performed on a Bruker Avance II 400 MHz NMR spectrometer. X-ray photoelectron spectroscopy (XPS) was carried out on a PHI 5000 Versa Probe III spectrometer (ULVAC-PHI, Japan) and etching experiments were performed on the sample surface using argon ion beam gun operating at 25.1 W. The binding energy scale was calibrated by the C 1s peak at 284.8 eV. An airtight specimen holder was used to avoid moisture and air contamination during sample transfer. X-ray photoemission electron microscopy (X-PEEM) measurements were performed at the SM beamline of Canadian Light Source (CLS). The monochromatic X-ray beam was focused using an ellipsoidal mirror to ~20 μm spot on the sample in PEEM and incident at a grazing angle of 16º. The sample was biased at -20 kV with respect to the PEEM objective lens and the base pressure of the PEEM chamber was maintained at ~10-9 Torr. Image stacks (sequences) for a specific field of view (FOV) at the Si, O, and F K-edges were measured. The acquired X-PEEM data were analyzed using aXis2000 (http://unicorn.mcmaster.ca/aXis2000.html). 3. Results and discussion 3.1 Electrochemical performance. Figure S1 shows the electrochemical performance of Si/C composite anode for lithium ion batteries with two electrolytes. The discharge capacity decays


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rapidly with the EC/DMC electrolyte, which only ~250 mAh/g of capacity is maintained after 50 cycles, leading to the failure of batteries. However, cycling stability improves significantly after adding FEC additive, of which discharge capacity reaches ~1750 mAh/g after 50 cycles and coulombic efficiency reaches above 98% after 5 cycles. Electrochemical measurements show that side reactions decrease thus result in better electrochemical behavior. In order to understand the working mechanism of FEC, we employed ssNMR, XPS and X-PEEM to investigate the SEI on Si/C composite anodes after 1 cycle, 30 and 50 cycles (labeled as C1, C30, C50, respectively) by multiple elements. 3.2 Solid state NMR. Solid state NMR is a powerful tool to characterize the SEI owing to its non-destructive, quantitative and amorphous phase detection.29 The total amount of components from the SEI is involved in the NMR spectra. In this part, 7Li,

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F and

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P MAS NMR were

employed to investigate the main inorganic components in the SEI. 3.2.1 19F MAS NMR. 19F NMR has relatively high sensitivity. In these two electrolytes, the Fcontaining species in the SEI are formed from the decomposition of LiPF6 and/or FEC. Figure 1 shows similar F-containing species form but the amount of each component varies significantly with two electrolytes. The resonance at ca. -204 ppm shows an asymmetrical lineshape, which was also appeared in the 19F NMR spectra of Jin et al.,15 but the authors did not mention it. The spin I = 1/2 of 19F nucleus indicates no asymmetric broadening of the resonance theoretically. Thus, a deconvolution shows that the asymmetrical resonance at ca. -204 ppm consists of two resonances at -204 ppm and -195 ppm. The resonance at -204 ppm is the characteristic signal of LiF, while at -195 ppm is temporarily assigned to HF.30 Both of them increase with electrochemical cycling with two electrolytes, however, the amount of LiF and HF is much higher with FEC additive. The resonance at -72 ppm is assigned to LiPF6, which has a F-P split with a J-coupling constant JF-P =


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710 Hz due to the direct connection between F and P atoms. Although all electrodes were washed three times by DMC before the testing, residual LiPF6 still could not be removed completely and increases with cycling, which we believe that LiPF6 may interact with some components in the SEI. Furthermore, the amount of LiPF6 with the EC/DMC electrolyte is much higher than that with FEC, which will be further discussed below. The resonances at -84 ppm and -145 ppm are assigned to PO2F2- and PO3F2-, respectively. These two components generate by partial hydrolysis of LiPF6 from the following equations: LiPF6 + 2H2O → LiPO2F2 + 4HF (Equ 1) LiPO2F2 + LiF + H2O → Li2PO3F + 2HF (Equ 2) The resonance at -121 ppm is assigned to SiOxFy, which might be formed by F- attacking the oxide layer SiOx on the Si particles and not show regular changes with cycling. The low content of SiOxFy might not affect the SEI and will be discussed elsewhere.


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Figure 1. Normalized 19F MAS NMR spectra of Si/C composite electrodes with two electrolytes after different cycles. The spinning sidebands are marked as asterisks (*). To further verify the above assignments, solution

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F NMR with a higher resolution was

employed to detect the decomposed production of 2 mL EC/DMC/FEC electrolyte with 0.05 mL H2O for incomplete hydrolysis. The

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F signals split conforms with the n+1 rule. As shown in

Figure S2, the doublet at -71.0 and -72.9 ppm are assigned to LiPF6, the doublet at -81.5 and -84.0 ppm are assigned to LiPO2F2, the singlet at -121.6 ppm is assigned to FEC, the singlet at -148.6 ppm is assigned to Li2PO3F, and the singlet at -188.3 ppm is assigned to HF. These solution 19F NMR results confirm the assignments of different F-containing species in the solid state 19F NMR spectra above. 3.2.2 31P MAS NMR. The P-containing species all come from electrolyte salt LiPF6, thus the decomposition of LiPF6 could be easily analyzed by

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P NMR as shown in Figure 2. The

resonances at 5.7 and -16.7 ppm are assigned to PO2F2- and PO3F2-, respectively, which appear in C1 samples with two electrolytes. In C30/C50 samples, the PO2F2- and PO3F2- signals almost totally disappear and a new signal at 2 ppm appears, besides the intensity of this new signal of the EC/DMC samples is much higher than that of the FEC samples.


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Figure 2. Normalized 31P MAS NMR spectra of Si/C composite electrodes with two electrolytes after different cycles. Combining with the 19F NMR results above, it could be deduced that the LixPOyFz species which are formed from the hydrolysis of LiPF6 (Equ 1, 2) in the first cycle may undergo further hydrolysis to form Li3PO4 by the following equation (the LixPOyFz species almost disappear in while a new signal appears in

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F NMR

P NMR of the C30/C50 samples, indicating this new specie

contains P but no F element): Li2PO3F + LiF + H2O → Li3PO4 + 2HF

(Equ 3)

Note that the amount of P-containing species of the FEC C30 sample is lower than that of the FEC C1 sample, indicating the P-containing species may interact weakly with other SEI components and could be partially dissolved in the electrolyte. Furthermore, the amount of Pcontaining species of the FEC C30/C50 samples are much lower than that of the EC/DMC C30/C50 samples, indicating the SEI formed with FEC could partially suppress the continuously decomposition of LiPF6 during long-term cycling.


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To further verify the above assignment, solution

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P NMR was also employed for LiPF6 in

acetonitrile solution with 0.05 mL H2O for incomplete hydrolysis. As shown in Figure S3,

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P

signals split also conforms to the n+1 rule. The seven peaks centered at -144 ppm are assigned to LiPF6, the triplet centered at -17.5 ppm are assigned to LiPO2F2, the doublet at -4.5 and -10 ppm are assigned to Li2PO3F, and the singlet at 0.7 ppm is assigned to Li3PO4. The solution 31P NMR results demonstrate the hydrolysis of LiPF6, which also confirm the assignments of different Pcontaining species in the above ssNMR results. 3.2.3 7Li MAS NMR. Since Li-containing species are diamagnetic, the chemical shifts of different Li-containing salts are too close (usually within 5 ppm) to distinguish in 7Li MAS NMR. Thus, the 7Li signals represent all Li-containing salts in the SEI, which mainly include inorganic salts (i.e. LiF, Li2CO3, etc.) and semi-organic salts (i.e. ROCO2Li, ROLi, etc.). As shown in Figure S4, resonance at ca. 0 ppm represents the total amount of Li salts in the SEI. Table S1 shows integrated area of resonance at ca.0 ppm. A small amount of Li salts forms of the EC/DMC C1 sample but increases rapidly during cycling. However, a large amount of Li salts forms of the FEC C1 sample but remains relatively stable along the cycling, indicating the SEI formed with FEC could reduce the occurrence of side reactions, which is beneficial to improve the electrochemical performance. Furthermore, a small shoulder resonance appears at ca. 10 ppm of the FEC C50 sample, which is temporarily assigned as LixSi.31 Since all samples were hold at 2 V for 12 h before disassembly for totally extraction of Li, the presence of LixSi might be that the repeated volume expansion/contraction of Si particles during cycle leads to electrochemical detachment of LixSi and failure for delithiation. No LixSi signal is observed of the EC/DMC samples might be that the rapidly decaying capacity leads to smaller volume change than that of the FEC samples, resulting in less detachment of LixSi thus no LixSi signal in 7Li NMR after long-term cycling.


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3.3 XPS. XPS could characterize the existing elements, their valence and existing forms. The non-destructive detecting depth is ~10 nm under the instrument and experimental conditions used in this study. Furthermore, the detecting depth could reach ~50 nm by Ar+ etching, which supplies more composition and distribution information of the SEI at different depth. 3.3.1 Si 2p. As shown in Figure S5a and S5b, the Si substance (99.3 eV17) and the oxide layer of SiO2 (103.8 eV17) are observed in the pristine sample, of which the thickness of the oxide layer is about 1 nm;27 the Si-O bond on the surface could be partially reduced during the sintering process in Ar/H2 and combines with N in the c-PAN to form the Si-N bond (101.8 eV27). All samples show no Si signals after cycling (except for the weak Si signals detected of the EC/DMC C1 sample), indicating the SEI formed after cycling covers the pristine Si.


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Figure 3. In-depth analysis of XPS Si 2p spectra of Si/C composite electrodes after (a) 1 cycle, (b) 30 cycles with the EC/DMC electrolyte and after (c) 1 cycle, (d) 30 cycles with FEC additive. In order to obtain the depth distribution information of the SEI, C1/C30 samples were etched by Ar+ and each etching time was 2 minutes with the etching speed of ~5 nm/min. As shown in Figure 3a, the Si, SiO2 and Si-N signals appear and increase with increasing etching time of the EC/DMC C1 sample. The variations of each signals of the EC/DMC C30 sample in Figure 3b are similar to that of the EC/DMC C1 sample. However, the much lower intensity indicates the SEI is not completely penetrated during above etching time. The FEC samples also show a series of Sicontaining signals after etching, as shown in Figure 3c and 3d, with no SiO2 but LixSiOy (100.8 eV) signal appears, which may be generated by F- from the decomposition of FEC attacking the oxide layer SiO2 to form LixSiOy.24 The LixSi signal (97.8 eV17) appears after etching of the FEC C30 sample, which also conforms to 7Li MAS NMR. In order to analyze the thickness of the SEI, we integrated the area of substance Si signal at 99.3 eV in Si 2p spectra after etching, and assumed that Ar+ etching had penetrated the entire SEI if the integrated area varied less than 10%. As shown in Table S2, the thickness of the SEI of both C1 samples are estimated as ~30 nm according to the etching speed of 5 nm/min. However, the Si signal of the EC/DMC C30 sample is still very low and the area varies more than 10% after etching, indicating the thickness of the SEI should be thicker than the total etching depth of ~50 nm. In contrast, the thickness of SEI is ~40 nm of the FEC C30 sample. These thickness analysis shows that more side reactions happen and electrolytes continuously decompose to form SEI, leading to a much thicker SEI with the EC/DMC electrolyte, while the SEI formed with FEC could segregate the electrolytes from the Si surface and its thickness remains stable during long-term electrochemical cycling.


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3.3.2 F 1s. As shown in Figure S5c and S5d, LiF (684.8 eV11) and LixPOyFz (~687 eV32) are observed of both C1 samples. The amount of LiF increases of both C30 samples but decreases of C50 samples, which is inconsistent with the increasing amount of LiF after 50 cycles in 19F NMR. It might be that more organic SEI components generate in the outer side which covered the LiF signal in XPS F 1s spectra. As shown in Figure 4, the amount of LiF and LixPOyFz reduces of all samples after Ar+ etching. The integrated area of LiF with different etching times are used to analyze the distribution information of LiF in the SEI, as shown in Table S3. The results show that the amount of LiF of the EC/DMC C30 sample increases by 6-10 times comparing that of the EC/DMC C1 sample at the same etching time. However, the amount of LiF of the FEC C30 sample only increases by 1-3 times than that of the FEC C1 sample. Considering that the F-containing species are formed by decomposition of LiPF6 with the EC/DMC electrolyte, a large increasing amount of LiF after longterm cycling indicates a large amount of decomposition of LiPF6 and then a lower concentration of the Li salt in the electrolyte, which may be one of the main reasons for rapidly decaying capacity. Instead, LiF is derived from the decomposition of FEC and/or LiPF6 after adding FEC. Comparing the amount of LiF with the thickness of the SEI, we find that the LiFinner:LiFouter ratio are ~0.38 of both C1 samples. However, it is less than 0.53 of the EC/DMC C30 sample, while it reaches 0.75 of the FEC C30 sample, indicating that FEC could significantly average the distribution of LiF from inner to outer side of the SEI, and this uniform LiF layer may protect the volume expansion/contraction of Si/C composite electrodes.


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Figure 4. In-depth analysis of XPS F 1s spectra of Si/C composite electrodes after (a) 1 cycle, (b) 30 cycles with the EC/DMC electrolyte and after (c) 1 cycle, (d) 30 cycles with FEC additive. 3.3.3 P 2p. As shown in Figure S6a and S6b, P-O signals (133.8 eV33) are observed of both C1 samples and decrease of the C30/C50 samples with two electrolytes, while the LiPF6 signal (137 eV33) only appears of EC/DMC C30/C50 samples. The intensity of P-O signal of the FEC samples is lower than that of the EC/DMC samples, indicating less hydrolysis of LiPF6 with FEC additive,34 which is consistent with 31P NMR results.


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Figure 5. In-depth analysis of XPS P 2p spectra of Si/C composite electrodes after (a) 1 cycle, (b) 30 cycles with the EC/DMC electrolyte and after (c) 1 cycle, (d) 30 cycles with FEC additive. Figure 5 shows the P 2p spectra after Ar+ etching, which are quite different with two electrolytes. The intensity of P-O signal decreases of both C1 samples with increasing etching time, indicating the hydrolysis of LiPF6 tends to happen on the outer side of the SEI. However, the intensity of PO signal increases with etching time of the EC/DMC C30 sample. Since the hydrolysis reaction occurs on the outer side of the SEI, the increasing P-O signal intensity in the inner side of the SEI indicates that the P-O species formed in the outer side could migrate to the inner side with the EC/DMC electrolyte. The migration of P-O species demonstrates a loose and porous structure of the SEI, therefore the small molecules like P-O species could gradually migrate into the inner side after long-term cycling. The SEI formed is denser with FEC, thus the P-O species could not migrate to the inner side of the SEI.


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3.3.4 O 1s. The O-containing species in the SEI are mainly derived from the electrochemical decomposition of EC, DMC and/or FEC, with a small part of the oxide layer SiO2 as well as a trace amount of H2O in the electrolyte. As shown in Figure S6c and S6d, the C-O and C=O signals of the pristine sample are from partial oxidation of the carbon surface, which the intensity are relatively low and could be ignored in subsequent analysis. The O 1s spectra show a broad peak after cycling, which could be assigned to C-O (532.4 eV), C=O (531.1 eV), P-O (530 eV) and LiO (528.6 eV, including Li2O and ROLi) signals according to XPS P 2p and the previous literature analysis.34, 35 The peak lineshape is different from the C1 samples to C30 samples, but varies a little from the C30 to C50 samples with two electrolytes, indicating the SEI formed in the first cycle is unstable, of which the composition and structure continue to evolve in the following cycles. Figure 6 shows O 1s spectra of cycled samples after Ar+ etching. The amount of P-O signal is fitted on the basis of P 2p spectra. The intensity of Li-O signal is rather low but increases significantly after first etching and then decreases except for the EC/DMC C30 sample, in which the Li-O signal continuously increases with etching time. The distribution in the different depth indicates that the Li-O species are more likely to be formed in the outer side of the SEI, which a simply DMC washing process could clean it away due to the weak interaction with other SEI components. Furthermore, it could migrate to the inner side of the loose and porous SEI formed with the EC/DMC electrolyte. In addition, the C-O and C=O signals from organic species increase significantly of the EC/DMC C30 sample. Since organic species are relatively large (i.e. LEDC, LMC, etc.), it is hard to migrate thus indicating electrochemical reductions tend to occur in the inner side of the SEI. In contrast, the distribution of organic species of the FEC C30 sample is


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similar with the FEC C1 sample, indicating a relative stable organic SEI structures formed in the first cycle.

Figure 6. In-depth analysis of XPS O 1s spectra of Si/C composite electrodes after (a) 1 cycle, (b) 30 cycles with the EC/DMC electrolyte and after (c) 1 cycle, (d) 30 cycles with FEC additive. 3.4 X-PEEM. X-PEEM have been employed in characterizing CEI components and interfacial inhomogeneity on commercialized LiCoO236 and LiFePO437 already. Benefited by imaging surface with tens of nanometer spatial resolution and combining with XANES, morphology and composition information could be obtained simultaneously at the same region.38 In this study, XPEEM was employed to characterize the micro-structure and chemical composition for cycled Si/C electrodes, in order to reveal the effects of FEC additive during cycling and understand the mechanism of surface reaction at high spatial resolution.


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Figure 7. (a) Si, (c) F, and (e) O X-PEEM chemical mappings and (b) Si, (d) F, and (f) O K-edge XANES of the same sample region of the FEC C30 sample. The red sample regions refer to small Si particles, while the green regions indicate the large Si particles in (a), or chemically distinct regions in (c) and (e). Figure 7 and S7 show the Si, F, O X-PEEM chemical mapping and XANES at Si, F, O K-edge from selected regions of both C30 samples. The blurred image with dispersed particles is shown of the EC/DMC C30 sample (Figure S7a), which might be caused by a poor conductivity due to a thick insulating SEI. The low intensity and quality especially in Si K-edge and O K-edge XANES in Figure S7b and S7f support such assumption. Since Si K-edge XANES could detect 30-60 nm depth with TEY (total electron yield) mode39, only the presence of SiO2/LiSiOx signal at ~1846.8 eV40 (Figure S7b) indicates that a thicker SEI (>60 nm) may covered the whole surface thus no pristine Si is detected. In contrast, small and large Si particles (shown in red and green) can be


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chemically mapped out in the Si X-PEEM image (Figure 7a) of the FEC C30 sample. The overlapped Si K-edge XANES with the presence of Si peak at 1840.8 eV40 and SiO2/LiSiOx peak (Figure 7b) indicates the same composition in both small and large Si particles and also a thinner SEI formed with the FEC additive. It also hints a denser SEI exists in both large and small Si particles (which might be formed due to early cracking of large Si particle). The dispersed Si particles of the EC/DMC C30 sample and existing large Si particles of the FEC C30 sample in Si X-PEEM mappings indicate that the SEI formed with FEC could partially protect the Si particles from cracking and pulverization during cycling. The F X-PEEM image is also unclear with the EC/DMC C30 sample (Figure S7c), but more clear with FEC C30 sample (Figure 7c), in which red and green regions are both composed of LiF (~692 eV and ~701 eV41) and small amount of species at ~696 eV which could be assigned to LixPOyFz at F K-edge XANES (Figure 7d and S7d). It is noted that the red region in this mapping has higher F concentration shown as a larger edge jump than green region, and the red region mostly corresponds to large Si particles. In addition, a decreasing intensity at ~692 eV of the EC/DMC C30 sample comparing to that of the FEC C30 sample (Figure 7d and S7d) indicates less LiF formed with the EC/DMC electrolyte. The O X-PEEM image (Figure S7e) is brighter of the EC/DMC C30 sample than Si/F X-PEEM images (Figure S7a and S7c), indicating more O-containing species in the outer side of the SEI, but a severely distorted O K-edge XANES (Figure S7f) due to low oxygen signal and/or low conductivity made it hard to analyze their composition. After adding FEC, the chemistry difference between large and small Si particles has been further studied by O K-edge XANES extracted from X-PEEM (Figure 7e). The XANES at both regions shows dominated Li 2CO3 feature including C=O π* feature at ~534 eV and C=O σ* at ~540 eV.42 The enhanced ~540 eV peak in the green


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region may be due to enrichment of lithium oxide.42 Furthermore, the spatial distribution of these two representative oxygen species are mapped out in Figure 7e in which Li2CO3 is shown in red and mixture of Li2CO3 with possible lithium oxide is shown in green.

Figure 8. Si/O/F ternary elemental composition map. Red region represents F, green region represents O and blue region represents Si, while the overlapping region satisfies RGB rules. Chemically distinct sample region 1 and 2 are outlined in the figure. In addition, the visualization of component distribution of the FEC C30 sample was also achieved using color-coded composite maps of different combinations of individual elements as shown in Figure 8 for Si, O and F. It clearly shows that the surface of Si particles is covered by F and O species, but divided into two kinds of distinct regions. Region 1 is large Si particles inferred from the coexistence of Si and F but with very little O species present as indicated from the overall purple color. In contrast, region 2 is rougher surface with O species enriched but very low F amounts. Since F and O species are mainly contributed by LiF and organic stuff, respectively, this elemental composition map results show that the surface of large Si particles has more LiF than that of the small Si particles after cycling with FEC, indicating LiF could protect the integrity of Si particles and suppress the cracking and pulverization during cycling, thus resulting in better electrochemical performance.


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3.5 The SEI formed with two electrolytes. According to above results, we briefly summarize the composition, structure and inhomogeneity of the SEI on the Si/C electrodes and how FEC affect the SEI. The components in the SEI are mainly derived from the electrochemical reductions of electrolyte EC, DMC and/or FEC additive and the hydrolysis of LiPF6 salt. The electrochemical reduction decomposition tends to occur in the inner side of the SEI due to the participation of electrons (CO and C=O organic groups in the inner side are higher than that in the outer side in XPS O 1s), while the hydrolysis tends to occur in the outer side of the SEI (P-O species in the outer side are higher than it in the inner side in the XPS P 2p). In the components part, the types of species in the SEI would not be affected by FEC, but the amount of most species has changed. A large amount of LiPF6 hydrolyzes to LixPOyFz species at the first cycle and further hydrolyzes to Li3PO4 (31P NMR), while the total amount of Li salts increases significantly after cycling (7Li NMR) with the EC/DMC electrolyte. In contrast, a large amount of LiF generates in the first cycle and increases after cycling (19F NMR), but the amount of P-containing and Li-containing species would not change significantly (31P/7Li NMR) with FEC, indicating LiF is mainly formed from decomposition of FEC rather than hydrolysis of LiPF6. In the structural part, the SEI grows significantly with the EC/DMC electrolyte after cycling (Ar+ etching with 10 minutes could not penetrate the SEI in XPS Si 2p), which is loose and porous (PO and Li-O species could migrate to the inner side of the SEI according to XPS P 2p and O 1s as well as a large amount of LiPF6 is trapped in the SEI in 19F NMR) and the ratio of C-O and C=O groups in the inner and outer side changes (XPS O 1s) during long-term cycling. In contrast, the SEI formed with FEC almost maintains the same thickness after cycling (XPS Si 2p), which is


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denser and more compact (small molecules like P-O and Li-O species formed in the outer side could not migrate to inner side of the SEI according to XPS P 2p and O 1s) while the ratio of C-O and C=O groups in the inner and outer side almost maintains the same (XPS O 1s). In addition, DMC could wash Li-O species (Li-O signal increases significantly after first Ar+ etching and then decreases in XPS O 1s) but hardly other species (i.e. LiF, Li2CO3 and C-O, C=O organic stuff) away. In the morphology part, most of Si particles are cracked after cycling with the EC/DMC electrolyte, but maintain more complete with a smoother surface with FEC additive (Si X-PEEM image). In addition, the amount of LiF is higher on the large Si particles, indicating LiF could prevent cracking and pulverization of the Si particles during long-term cycling. 4. Conclusions In this paper, ss NMR, XPS and X-PEEM techniques were employed to investigate composition, structure and inhomogeneity of the SEI on Si/C composite electrodes with or without FEC additive. We find that the hydrolysis of LiPF6 generated to LixPOyFz first and further to form Li3PO4 during the subsequent cycles. A large amount of LiF from the decomposition of FEC could cover the surface of Si particles, which partially suppresses the volume change during the cycling. The SEI formed with the EC/DMC electrolyte is loose and porous, which could capture small molecules such as LiPF6, P-O and Li-O species in the inner side. In contrast, the SEI formed with FEC is dense and compact, meanwhile the distribution of each components in different depth is more uniform and could prevent the diffusion and migration of small molecules into the inner layer as well as the hydrolysis of LiPF6 during cycling. With the help of these advanced characterization techniques, we could better understand the role of FEC additive and why the SEI is better with FEC on Si-based anodes for lithium ion batteries.


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ASSOCIATED AMOUNT Supporting Information. Electrochemical performance, solution 19F, 31P and solid state 7Li MAS NMR spectra, XPS Si 2p, F 1s, P 2p, O 1s spectra and X-PEEM of the EC/DMC C30 sample. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions ‡

Q. Li and X. Liu contributed equally.

Funding Sources National Key Research and Development Program of China National Natural Science Foundation of China Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by National Key Research and Development Program of China (grant no. 2018YFB0104400 and 2018YFB0905400) and National Natural Science Foundation of China (grant no. 21233004, 21428303, and 21621091). The Canadian Light Source


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C Composite

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