Thermal Decomposition of the Solid Electrolyte Interphase (SEI) on

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Thermal decomposition of the Solid Electrolyte Interphase (SEI) on Silicon Electrodes for Lithium Ion Batteries Taeho Yoon, Mickdy S. Milien, Bharathy S Parimalam, and Brett L Lucht Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b00454 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 19, 2017

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Chemistry of Materials

Thermal decomposition of the Solid Electrolyte Interphase (SEI) on Silicon Electrodes for Lithium Ion Batteries Taeho Yoon, Mickdy S. Milien, Bharathy S. Parimalam, and Brett L. Lucht* Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881, USA

ABSTRACT: Thermal behavior of the solid electrolyte interphase (SEI) on a silicon electrode for Lithium ion batteries has been investigated by TGA. In order to provide a better understanding of the thermal decomposition of the SEI on silicon, the thermal decomposition behavior of independently synthesized lithium ethylene dicarbonate (LEDC) was investigated as a model SEI. The model SEI (LEDC) has three stages of thermal decomposition. Over the temperature range of 50~300oC, LEDC decomposes to evolve CO2 and C2H4 gasses leaving lithium propionate (CH3CH2CO2Li) and Li2CO3 as solid residues. The lithium propionate decomposes over the temperature range of 300~600oC to evolve pentanone leaving Li2CO3 as a residual solid. Finally, the Li2CO3 decomposes over 600oC to evolve CO2 leaving Li2O as a residual solid. A very similar thermal decomposition process is observed for the SEI generated on cycled silicon electrodes. However, two additional thermal decomposition reactions were observed characteristic of LixPOyFz at 300oC and the polyimide binder at 550oC. TGA measurements of Si electrodes after various numbers of cycles suggest that the LEDC on Si electrodes thermally decomposes during cycling to form lithium propionate and Li2CO3, resulting in increased complexity of the SEI.

1.

Introduction

decomposition of the SEI such as binder, electrolyte, and state of charge (SOC).7, 15-18 However, the mechanism and compositional changes which occur during SEI evolution remain unclear. In addition, the thermal decomposition of the SEI is accelerated by acidic or basic impurities further complicating an understanding of SEI evolution. Characterization of the decomposition products will provide important insight into the structure and function of the SEI. While lithium ethylene dicarbonate (LEDC) and lithium fluoride (LiF) have been reported to be the most common SEI components formed in ethylene carbonate/LiPF6 based electrolytes, many other components have been reported including; lithium alkyl carbonates (ROCO2Li), lithium carbonate (Li2CO3), lithium fluorophosphate (LixPOyFz), lithium oxide (Li2O), lithium oxalate (Li2C2O4), lithium alkoxides (LiOR), and polymeric/oligomeric materials.19-25 While the complexity of the SEI has been reported by many research groups, the source of the complexity is poorly understood. It is unclear if the complexity is derived from initial reduction reactions or subsequent thermal, electrochemical, or hydrolytic decomposition reactions of the initial reduction products.

Reductive decomposition of electrolyte in lithium ion batteries (LIBs) is inevitable since the working potentials of negative electrodes are below the electrochemical stability window of the electrolyte. The decomposition products precipitate on electrode surfaces resulting in an electronically insulating, but ionically conductive layer, known as the solid electrolyte interphase (SEI).1-4 The SEI behaves as a passivation film, which suppresses further reduction of the electrolyte while allowing lithium ion transport through the layer. However, the thermal/chemical stability of the SEI is questionable which can lead to calendar life and safety concerns. The exothermic decomposition of the SEI around 100~150oC has been reported to initiate thermal runaway in LIBs.5-8 In addition, thermal decomposition of the SEI components upon prolonged cycling at room (or moderately elevated) temperature results in changes in the composition of the SEI, altering the electrochemical behavior of the batteries.9-10 Thus, tremendous effort has been dedicated to understand the thermal behavior of the SEI.11-15 The thermal properties of the SEI have been investigated by measuring the heat transfer from SEI decomposition. Differential scanning calorimetry (DSC) and accelerating rate calorimetry (ARC) have been used to investigate the temperature and factors affecting the thermal

Thermogravimetric analysis (TGA) has been used to investigate the decomposition products of the SEI. The thermal decomposition temperature of SEI components

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materials can be characterized by measuring weight losses upon heating via TGA. Furthermore, the thermal decomposition properties in combination with spectroscopic characterization tools can be used for material identification. For instance, the thermal decomposition of LEDC, lithium alkyl carboxylate (RCO2Li) and Li2CO3 occur over different temperature ranges (100~200oC, 400~500oC, and 600~900oC, respectively).19, 26-27 The thermal properties complement IR and XPS spectra which frequently suffer from overlapping peaks due to similar bonding environments. Despite these advantages, TGA has rarely been used to investigate the SEI due to technical limitations.8, 28-30 First, the quantity of SEI deposited on graphite anodes is too small to be analyzed by TGA. The SEI on graphite successfully passivates the surface to prevent further electrolyte reduction so that the mass of deposited SEI is minimized. In addition, micron-sized graphite with low surface area is generally used for commercial applications, further minimizing the mass of SEI formed. Second, conventional polymer binders, CMC, PAA, or PVDF thermally decomposes in the 100~500oC temperature range.31-33 Unfortunately, the thermal decomposition of SEI components occurs over a similar temperature range.19 Kang Xu and co-workers reported that LEDC thermally decomposes in the 100~300oC range, which coincides with binder decomposition. Third, interpretation of the TGA data is complicated by the complexity of the SEI components. Several components of the SEI as well as the binder will thermally decompose over a similar temperature range making assignments of the different thermal events difficult.

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source of many of the complex components previously reported in the SEI. 2.

Experimental

2.1. Materials Silicon nanoparticles (50 nm diameter) were purchased from Alfa Aesar. The silicon was mixed with conducting carbon (Super C) and binder solution to prepare a slurry. The binder solution, amic acid solution of poly(pyromellitic dianhydride-co-4,4’-oxydianiline), was purchased from Sigma Aldrich. All of the materials for the synthesis of the model SEI, lithium phosphates, and lithium alkyl carboxylates were purchased from Sigma Aldrich or Acros and used without further purification. Batterygrade ethylene carbonate (EC), diethyl carbonate (DEC), lithium hexafluorophosphate (LiPF6), and lithium difluorophosphate (LiPO2F2) were provided by a commercial supplier and used as received. 2.2. Synthesis Lithium ethylene dicarbonate (LEDC) was synthesized, as a model SEI, following a previously reported method.21 Naphthalene (4.0 g) and Li metal (0.215 g) were added to 50 mL of tetrahydrofuran (THF) to prepare lithium naphthalenide, a strong reducing agent. After stirring the solution overnight, 2.7 g of ethylene carbonate was added and allowed to stir overnight. All processes were carried out in a N2 glovebox. The product was collected by centrifugation, rinsed three times with dimethyl carbonate (DMC), and dried overnight, under vacuum. The purity of the LEDC is >95 % from a combination of IR and NMR spectroscopy. The synthetic procedures for the preparation of lithium carboxylates and lithium phosphates38 are available in the SI.

In this work, we have overcome the above-mentioned challenges by employing; silicon nanoparticles as an active material, a polyimide as a thermally stable binder, and independently synthesized LEDC as a reference material. The silicon electrode suffers from volume expansion/contraction during lithiation/de-lithiation accompanied by continuous decomposition of the electrolyte. Thick surface films are deposited consequently during prolonged cycling, in contrast to the graphite electrode. Furthermore, the large surface area of the nano-sized particles generates a sufficient quantity of SEI to allow TGA analysis.34-35 Polyimide, which has outstanding thermal stability (decomposition temperature: 500~600oC) as well as compatibility with silicon electrodes, was employed in order to avoid interference from the thermal decomposition of the binders.36 The independent generation of LEDC has been previously reported via chemical reduction of ethylene carbonate by the strong reducing agent, lithium naphthalenide.21, 37 The thermal decomposition of LEDC has been monitored/characterized by a combination of IR, XPS, TGA, and TGA-IR and the results have been compared to the SEI generated on silicon nanoparticles. This investigation provides both a better understanding of the thermal properties of the SEI deposited on the silicon electrode and significant insight into the

2.3. Electrode and coin cell preparation The Si electrodes were prepared by coating the slurry on Cu foil. The electrode was dried under vacuum at 25oC overnight. Dried electrodes were cured to convert the polyamide binder to polyimide by step heating at 110, 170, 250, and 350oC, holding 30 minutes at each step.36 The degree of imidization was identified by TGA profile, which shows that the polyamide was fully converted to polyimide (Figure S3). The electrode composition was fixed at 50:25:25 by weight ratio between Si, Super C, and polyimide binder. 2032-type coin cells containing Si and lithium electrodes were used for electrochemical testing. Polymeric separator and glass fiber were used as a binary separator. 1.2 M LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), 1:1 (v/v), was used as the electrolyte solution. 100 µL of the electrolyte was added to the coin cell, and the cells were cycled after 6 hours for wetting. 2.4. Electrochemical testing

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Chemistry of Materials trolyte decomposition (SEI formation) and irreversible lithiation of the Si and binder. Electrochemical behavior of the polyimide binder has been previously reported.36 The polyimide showed abnormally large lithiation capacity under 0.5 V and delithiation over 1 V (vs. Li/Li+). Therefore, the lithiated polymer is barely delithiated under 0.7 V, which contributes to the low coulombic efficiency. (ii) The coulombic efficiency during the initial cycles is relatively low, but increases in latter cycles (from 96.0 to 98.5 %). Coulombic efficiency provides information about irreversible electrolyte reduction during the lithiation period and incomplete delithiation due to resistance increase during volume contraction.35, 39 The former is likely to make a major contribution for 50 nm-sized silicon because of its large surface area and relatively mitigated volume change.35 Hence, the low coulombic efficiency upon prolonged cycling indicates continuous electrolyte decomposition and SEI formation. However, the large surface area of nanoparticles and continuous SEI deposition affords superior TGA measurements due to the large quantity of SEI products deposited on the surface. However, the complexity of the SEI makes interpreting the TGA data difficult. Our approach is to conduct TGA of LEDC, the reduction product of EC, as a simplified model SEI, and then to apply the knowledge gained from the results to the actual SEI formed on Si electrodes.

Galvanostatic charge/discharge cycling was conducted using an Arbin BT2000 battery cycler at 25oC. All cells were cycled at a rate of C/20 for the 1st cycle to wet the electrode, activate Si particles, and generate an SEI. A constant current density of 1170 mA g-1 (C/3 rate, 0.76 mA cm-2) was applied to lithiate and delithiate cells for the remaining cycles. To compensate capacity loss during the lithiation period, the voltage was held at the end of charge until the current density decreases to 170 mA g-1 (C/20 rate, 0.11 mA cm-2). 2.5. Ex-situ analysis After cycling was completed, the silicon electrodes were extracted for ex-situ analysis. To avoid interference of residual electrolyte, the electrodes were rinsed by DMC and dried overnight, under vacuum. The electrodes were transferred under air-free conditions for IR and XPS measurements. A Bruker Tensor 27 was used to obtain the IR spectra. 256 scans with 4 cm-1 resolution were acquired in a N2 atmosphere glovebox. The XPS data were collected on a K-alpha spectrometer (Thermo Scientific). The X-ray spot size and pass energy were 400 µm and 50 eV, respectively. The cycled electrodes were scrubbed to detach powders from the Cu foil, in an Ar-filled glove box for TGA analysis. Since the adherence of the binder to the surrounding particles and Cu foil is reduced from repeated volume changes, the electrode material was easily removed from the current collector. Collected powders were transferred to the TGA instrument, SDT-Q600 Simultaneous Thermal Analyzer (TA Instruments Inc., USA). Samples were loaded into an alumina crucible and placed in a furnace quickly to minimize air exposure. The temperature was scanned from 50oC to 900oC at a rate of 20oC min-1 in nitrogen atmosphere. The samples were stabilized at 50oC to evaporate residual solvent before ramping temperature. 2.6. In-situ analysis TGA-IR was employed to analyze gaseous products of thermal decomposition. A sample was heated in the furnace of TG analyzer (TGA Q5000 V3.17 Build 265, TA Instruments Inc.) with N2 purging. The vented gas was routed into a Nicolet 6700 infrared spectrometer (FT-IR) by a heated transfer line. The temperature of the transfer line and gas cell in FT-IR were kept at 170°C. Samples were heated from 50°C to 650oC at 20°C/min. The IR spectra obtained by MCT/A detector (32 scans per each spectrum with 4 cm-1 resolution). 184 spectra were collected during an experiment. 3.

Results and discussion

3.1. Electrochemical performances and SEI deposition Cycleability and coulombic efficiency are presented in Figure 1. Silicon electrodes were cycled within the 0.7 – 0.005 V (vs. Li/Li+) range at a C/3 rate. Two characteristics should be noted; (i) Large irreversible capacity in the 1st cycle, thus low coulombic efficiency, is ascribed to elec-

Figure 1. Cycling performances of the silicon/Li half-cell. Cycleability is displayed in upper panel in which lithiation/delithiation capacities are presented by white and black circles respectively. The lower panel is the corresponding + Coulombic efficiency. Cut-off : 0.7~0.005 V (vs. Li/Li ), Cur-1 -2 rent : 1167 mA g (1/3 C-rate, 0.76 mA cm )

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sitions could occur for two different reasons; (i) the LEDC is impure and composed of 3 materials that thermally decompose over different temperature ranges, or (ii) the higher temperature thermal reactions are attributed to solid residues remaining after the 1st thermal reaction occurs at low temperature, 50~300oC. Considering that the model SEI has been characterized as LEDC, the latter is likely the case.

3.2. Thermal decomposition of the model SEI The model SEI was synthesized, and the thermal decomposition behavior was monitored by TGA in order to help understand the thermal decomposition of the SEI on silicon electrodes. EC has been reduced by a strong reducing agent, lithium naphthalenide, whose standard redox potential is 0.3 V (vs. Li/Li+),40 which is below the reduction potential observed for EC on graphite electrodes, 1 V vs. Li/Li+.41-42 The reduction product is a reasonable model for the SEI since several previous investigations have reported that the SEI formed in EC-based electrolytes is dominated by EC reduction products, in particular, LEDC.4, 20-25, 43 Characterization of the model SEI from the reduction with lithium naphthalenide has been previously reported as LEDC.21, 37 The IR spectra of the synthesized LEDC and surface of silicon electrodes after 100 cycles covered with an SEI are depicted in Figure 2. The IR spectra have very similar peak positions and similar relative peak ratios. Peaks at 1653, 1400, 1315, 1100 and 825 cm-1 are characteristic of LEDC.19-20, 37 The primary difference between the two IR spectra is observed in the 1400~1600 cm1 range, which displays an intense absorbance for the cycled Si electrode, but not for the synthesized LEDC. The absorption in the 1400~1600 cm-1 region can be attributed to the Li2CO3 whose IR absorption bands are at 1490, 1433 and 875 cm-1 and lithium carboxylates in the range of 1550~1600 cm-1.20, 27, 44 Interestingly, there is no IR absorption observed in the 1750-1800 cm-1 region where poly or oligo ethylene carbonate is observed, suggesting either the absence or low concentration of these species.45 Further discussion about formation mechanisms and generation and evolution of Li2CO3 and carboxylates on the Si electrode surface upon cycling are discussed below.

Figure 3. TGA and DTG curves of reduced EC (LEDC). Temo -1 perature scan rate: 20 C min

In order to better understand the TGA profile, the residual products of LEDC were collected after being heated to 300oC and 600oC at a rate of 20oC min-1 in the TGA furnace and analyzed by IR and XPS (Figures 4 and 5). The characteristic IR absorptions of LEDC completely disappear after being heated to 300oC, which indicates that the weight loss in the 50~300oC range (~25 %) is associated with the decomposition of LEDC. The peaks at 860 cm-1 and 1430 cm-1 are attributed to Li2CO3, which is consistent with a previous report that one of the LEDC decomposition products is Li2CO3.19 An additional absorption is observed at 1577 cm-1 characteristic of lithium carboxylates.27 Interestingly there is no evidence for the generation of the lithium salt of ethylene glycol since there are no absorptions characteristic of C-H (~2900 cm-1) or C-O (~1100 cm-1). Upon being heated to 600oC, the peak at 1577 cm-1 in the IR spectrum disappears leaving only the absorptions characteristic of Li2CO3. The IR spectra and DTG curves of representative RCO2Li salts are provided in Figure S1. The IR spectra in Figure 4 coupled with the thermal decompositions depicted in Figure 3 suggests that RCO2Li is one of the thermal decomposition products of LEDC.

Figure 2. IR spectra obtained from the model SEI that is chemically synthesized (upper) and Si electrode after 100 cycles (lower).

The TGA results displayed in Figure 3 provide thermal decomposition of LEDC. There are three primary thermal events (50~250oC, 300~500oC, and 700~900oC) observed in the TGA profile. The three stages of thermal decompo-

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Gaseous products of the thermal decomposition of LEDC were analyzed by TGA-IR. The gases from the TGA furnace were routed to an FT-IR with N2 carrier gas. The temperature of the TGA furnace was stabilized at 50oC to evaporate DMC or THF and then ramped to 650oC at a rate of 20oC min-1. Figure 6a presents the Gram-Schmidt Absorbance over the temperature range. The GramSchmidt Absorbance is the integration of a spectrum over full wave length, which provides semi-quantitative information on evolved gases. The intensity of the GramSchmidt Absorbance is not proportional to the amount of gas when it is a mixed gas since different gasses have different IR intensities. Despite the limitation of GramSchmidt Absorbance, the gas evolution (Fig. 6a) and weight loss of LEDC (Fig. 3) are well correlated. Selected IR spectra of evolved gases at different temperatures are depicted in Figure 6b. At 120oC, set of the peak at 10001100 cm-1 and a broad peak at 2900 cm-1 are characteristic of ethylene (C2H4).47 As the temperature is increased to 200oC C2H4 evolution is continued, and CO2 evolution begins as evidenced by peaks at 700, 2300, 3600 cm-1.47 The C2H4 and CO2 are attributed to the LEDC decomposition as shown in Fig. 4, which supports the thermal decomposition mechanism; LEDC Li2CO3 + C2H4 + CO2 + “O”.19 At this time it is unclear if O2 gas is evolved or if Li2O is generated. Interestingly, the mass lost at this time corresponds to only 25 %, not the 44 % expected for the reaction above. Gas evolution at the range of 300~600oC is correlated with thermal decomposition of one of the solid products from LEDC decomposition. The IR absorptions which are observed in this temperature range include peaks at 750, 1750, 2950, 3050 cm-1. The peak intensities are weaker than those of CO2 or C2H4. The best match of the IR spectrum is a ketone, 3pentanone.47 3-pentanone is a thermal decomposition product of lithium propionate (2RCO2Li Li2CO3 + R(CO)R).27

Figure 4. IR spectra obtained from; (i) reduced EC (LEDC), o (ii) collected powder after heating up to 300 C, and (iii) o 600 C in a TGA furnace.

This is further supported by the XPS spectra in Figure 5. The C1s spectrum of the model SEI was deconvoluted into 4 peaks, C-C/C-H bond at 284.8 eV, C-O at 286.8 eV, OC=O at 288.8 eV, and CO3 at 290.1 eV.9, 14, 22, 46 The C-O peak at 286.8 eV in the C 1s XPS spectrum is significantly decreased after heating to 300oC consistent with the decomposition of LEDC upon heating at 300oC (Figure 5). A similar decrease is observed for C-O IR absorption at (1100 cm-1, Figure 4). On the other hand, the characteristic peaks of RCO2Li at 284.8 and 288.7 eV (Figure S2) and Li2CO3 at 290.1 eV are retained.20, 46 The O1s spectra provided in Figure 5 provide additional support. The O 1s peak characteristic of C-O bonds at 533.5 eV is significantly diminished after heating 300oC, while the O=C peak at 531.8 eV remains, consistent with the C1s spectra.46 The XPS spectrum obtained after 300oC heating is very similar to that of RCO2Li except for the Li2CO3 peak in the C1s spectrum, implying that the material is composed of a mixture of RCO2Li and Li2CO3 which is consistent with the IR results (Figure 4). In summary, the TGA results suggest two solid products of the thermal decomposition of LEDC (Figure 3), RCO2Li, and Li2CO3.

Figure 5. XPS spectra obtained from; (i) model SEI, (ii) colo lected powder after heating up to 300 C. Atomic concentrations of carbon, oxygen, and lithium are in an inset table.

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550oC the evolution of CO2 is observed consistent with the thermal decomposition of Li2CO3.26

Figure 7. Contour map of IR spectra obtained during heating o TGA furnace from 50 to 650 C. Temperature ramping rate : o 20 C, sampling interval : 12.19 sec. Figure 6. Gases evolved in TGA furnace during heating were analyzed by FTIR. (a) The quantitative information of the gases is presented by Gram-Schmidt Absorbance. (b) IR spectra of gases evolved at different temperatures were selected. The temperatures presented in (a) and (b) are that of TGA o furnace. IR measurement cell was maintained at 170 C.

3.3. Thermal decomposition of SEI on silicon electrode The SEI formed on Si electrodes has similar IR absorptions to LEDC shown in Figure 2. The XPS spectra of carbon, oxygen, and lithium support the similarity between LEDC and the SEI on the Si electrode (Figure S4). In addition to similar peak shapes, the relative atomic composition of C 1s, O 1s, and Li 1s are also comparable, which supports that EC reduction products are the primary components of outer SEI on Si electrodes after 100 cycles. Note that fluorine and phosphorous compounds contribute to 3 % of the outer SEI. While the outer SEI is dominated by LEDC, the inner SEI has a greater concentration of inorganic species including Li2CO3, LiF and LixSiOy.50-53 Based on the TGA results obtained from the model SEI, we expect the TGA of the SEI on Si electrodes to have thermal decompositions characteristic of LEDC, so the TGA curves will include evidence of RCO2Li and Li2CO3. The TGA profiles of cycled silicon electrodes are displayed in Figure 8a. The silicon electrode collected after 60 cycles has a similar profile to that of LEDC (Figure 3); thermal decomposition of LEDC (50~200oC), RCO2Li (400~500oC), and Li2CO3 above 600oC. After the weight loss in the 50~200oC range, the IR spectrum clearly displays the disappearance of the absorptions associated with LEDC (Figure 8b). The characteristic absorptions of RCO2Li at 1588 cm-1 and Li2CO3 at 1420 cm-1 dominate the IR spectrum of the cycled Si collected after heating at 220oC. The Li2CO3 decomposition in the SEI begins at a lower temperature than that for the independently prepared LEDC (Figure 8a). This can be attributed to high content of carbon in the Si electrode. Carbon has been known to lead to carbothermal reduction, altering the

All spectra obtained during heating are displayed as a contour map in Figure 7. The IR spectra are plotted as a function of temperature. The peak intensities are represented by color, which the brighter is more intensive. Under 100oC, three peaks are observed at 1280, 1450, and 1750 cm-1 characteristic of residual DMC solvent from LEDC rinsing. The evolution of C2H4 with characteristic peaks are at 1000 and 2900 cm-1 begins at ~50oC and continues to evolve to 250oC. CO2 evolution begins at 150oC and continues to 250oC, as evidenced by absorptions at 650, 2300, and 3650 cm-1. The evolution of C2H4 and CO2 correlate well with LEDC decomposition as observed in the solid state IR spectra in Figure 3. However, it is interesting that ethylene evolution clearly begins at ~90oC (2 min at 20oC min-1) before CO2 evolution suggesting that an intermediate, lithium peroxodicarbonate (Li2C2O6), is generated. The previously reported K2C2O6 decomposes at ~140 oC to generate CO2, and ½ O2.48 We propose an analogous decomposition thermal decomposition of Li2C2O6 to generate Li2CO3, CO2 and ½ O2.49 However, O2 gas is not observed since O2 is not IR active. New absorptions, characteristic of 3-pentanone, are observed at 700, 1750, and 2950 cm-1 in the temperature range of 350~500oC. As the temperature is further increased above

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decomposition kinetics and products, while thermal decomposition of pure Li2CO3 is known to begin above 800oC.26, 54 The carbon and Li2CO3 mixture decomposes into Li2O and CO in the 700~950oC range, whereas pure Li2CO3 begins to decompose over 800oC generating, Li2O and CO2.

rapid decomposition. Thermal decomposition mechanism of the binder has not been clarified. But the state of charge (SOC) of PI, the catalytic effect of other components in the electrodes or exothermic/endothermic heat transfer might influence the decomposition kinetics or mechanisms of the binder. Other commonly used binders for silicon anodes, such as PAA or CMC, may affect the SEI formation and evolution reactions.56, 57 3.4. SEI evolution upon cycling and compositional complexity The DTG curves of Si electrodes collected after various numbers of cycles are provided in Figure 9. The increased peak intensity observed in the 50~250oC range, corresponding to LEDC decomposition, with increased cycling is consistent with the continuous reduction of the electrolyte to thicken the SEI and the observed low coulombic efficiency. It was determined above in Figure 3 that LEDC decomposes to generate RCO2Li and Li2CO3. Similar peaks characteristic of RCO2Li and Li2CO3 are observed in the DTG profiles in Figure 9. After 30th cycles, the LEDC in the SEI decomposes into RCO2Li (~450oC), and Li2CO3 (>600oC). However, it is notable that the intensity ratios between LEDC, RCO2Li, and Li2CO3 change as a function of cycling. In particular, the peaks characteristic of RCO2Li and Li2CO3 have greater increases in intensity between the 30th and 50th cycles than that observed for LEDC. This suggests that not all the RCO2Li and Li2CO3 are products of LEDC decomposition during the TGA experiment. Rather, the RCO2Li and Li2CO3 were present in the SEI prior to the TGA measurement. This is consistent with LEDC thermally decomposing during prolonged cycling to generate RCO2Li, and Li2CO3 in the SEI. The LEDC decomposition could be accelerated by the surface of the lithiated silicon electrode or acidic decomposition products of the LiPF6 electrolyte. In addition, CO2, one of gas products of the LEDC decomposition, could be reduced on the surface of Si electrode to generate Li2C2O4, HCO2Li, or Li2CO3.58 The chemical structure of SEI formed on silicon upon initial formation cycles has been previously reported to be primarily composed of LEDC, LiF and LixSiOy.21-22, 52 In contrast, Figure 7 suggests the presence of LixPOyFz, RCO2Li and Li2CO3 in the SEI on Si electrodes after 50-cycles, which is consistent with previous reports for silicon electrodes which have more cycling.50 The TGA data suggests that the accelerated thermal decomposition of LEDC on the surface of the Si electrode increases the complexity of SEI and likely contributes to inconsistencies of the SEI composition from different research groups.

Figure 8. (a) TGA and DTG curves of 60-cycled silicon eleco -1 trode. Temperature scan rate : 20 C min . (b) IR spectra obtained from; (i) silicon electrode after 60 cycles, (ii) collected o powder after 220 C.

Since the thermal decomposition observed between 250oC and 350oC does not result from LEDC (Figure 3), the thermal decomposition of LixPOyFz, a decomposition product LiPF6 which has frequently been reported as a component of the SEI, has been investigated.55 Independently obtained or prepared lithium phosphate derivatives undergo thermal decomposition around 300oC (Figure S5).55 Based on the thermal decomposition temperature and comparison to the model SEI, the weight loss in the 250~350oC range results from the thermal decomposition of LixPOyFz. The sharp and prominent peak in the 500~550oC range is attributed to a binder decomposition based on the temperature (Figure S3). The constant weight loss observed in the 500~550oC range for electrodes collected after various numbers of cycles further supports binder decomposition (Figure 9). The PI binder in the fresh electrode decomposes over a broad range (500~700oC, Figure S3), while the decompositions in cycled electrodes were observed at lower temperatures (500~550oC), indicative of

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tion of the SEI during long-term cycling is depicted in Figure 10b.

Figure 9. DTG curves of silicon electrodes that collected after various length of cycling.

3.5. Thermal decomposition mechanism of SEI The real-time gas analysis (TGA-IR) combined with post-mortem characterization of solid products (XPS and IR) affords a strong understanding of the decomposition paths or the primary component of the SEI, LEDC. The thermal decomposition mechanism of LEDC, a model for the SEI, is summarized in Figure 10a. The thermal decomposition of independently prepared LEDC (4 mg) begins with C2H4 evolution from ~50oC. Loss of C2H4 from LEDC implies the generation of a transiently stable (several minutes) intermediate, Li2C2O6. As the temperature exceeds 1500C, CO2 evolution is observed along with additional C2H4 loss. The change in gas evolution suggests the existence of two competing decomposition mechanisms. The first is attributed to the Li2C2O6 decomposition, yielding Li2CO3 (1.14 mg). A competing reaction yields CH3CH2CO2Li (1.86 mg). The presence of the ethyl group directly bound to the carbonyl carbon implies a possible C2H4 insertion into a CO2 fragment or direct reaction between C2H4 with Li2C2O6. While the insertion of C2H4 into CO2 is unusual, related transition metal mediated insertions of alkenes into CO2 have been previously reported.59 However, at this time the mechanism for formation of CH3CH2CH2CO2Li remains elusive. As the temperature enters the 300~600oC range, CH3CH2CO2Li decomposes to Li2CO3 and 3-pentanone.27 Li2CO3 (2 mg) decomposes yielding Li2O (0.8 mg) and CO2 gas above 600oC. The retained mass of the solids and evolved masses of the gasses quantitatively agree with the proposed mechanism.

Figure 10. Illustrations of (a) thermal decomposition mechanism of LEDC and (b) compositional change of SEI during cycling on Si electrode.

4.

Conclusions

TGA has been employed to investigate the thermal decomposition behavior of the SEI deposited on the silicon electrode. The large surface area and repeated volume change upon cycling of silicon nanoparticles lead to continuous electrolyte reduction generating a sufficient quantity of SEI for TGA analysis. The polyimide binder, which thermally decomposes over 500oC, was employed to avoid overlap of the thermal decomposition of the SEI components and the binder. Thermal decomposition temperatures of the SEI were evaluated by scanning the temperature from 25 to 900oC in N2 atmosphere. The solid residue was characterized at various stages of thermal decomposition by XPS and IR. The independently prepared reduction product of EC, LEDC, thermally decomposes to generate RCO2Li and Li2CO3. The SEI deposited on silicon electrode contains DTG peaks consistent with the decomposition of LEDC, RCO2Li and Li2CO3, but also includes peaks characteristic of LixPOyFz around 300oC and the polyimide binder around 550oC. TGA analysis obtained at various stages of cycling suggests that the LEDC on the silicon electrode thermally decomposes during cycling accumulating the thermal decomposition products in the SEI on the electrode surface. The thermal decomposition could be accelerated during prolonged

The SEI on Si electrode is dominated by LEDC in initial cycles.22, 52 Our data suggests that the LEDC initially formed on the Si electrode decomposes through a similar path to the thermal decomposition of the independently prepared LEDC to generate RCO2Li and Li2CO3. The high concentration of RCO2Li and Li2CO3 in the SEI after prolonged cycling suggests that the lithiated silicon surface or LiPF6 electrolyte is catalyzing the thermal decomposition of LEDC upon cycling. The change in the composi-

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Chemistry of Materials Reactions Controlling the Thermal Stability of Graphite Anodes. Electrochim. Acta 2002, 47, 1885-1898. 8. Watanabe, I.; Yamaki, J.-i., Thermalgravimetry–Mass Spectrometry Studies on the Thermal Stability of Graphite Anodes with Electrolyte in Lithium-Ion Battery. J. Power Sources 2006, 153, 402-404. 9. Park, H.; Yoon, T.; Mun, J.; Ryu, J. H.; Kim, J. J.; Oh, S. M., A Comparative Study on Thermal Stability of Two Solid Electrolyte Interphase (SEI) Films on Graphite Negative Electrode. J. Electrochem. Soc. 2013, 160, A1539-A1543. 10. Park, H.; Yoon, T.; Kim, Y.; Lee, J. G.; Kim, J.; Kim, H.s.; Ryu, J. H.; Kim, J. J.; Oh, S. M., Thermal Behavior of Solid Electrolyte Interphase Films Deposited on Graphite Electrodes with Different States-of-Charge. J. Electrochem. Soc. 2015, 162, A892-A896. 11. Du Pasquier, A.; Disma, F.; Bowmer, T.; Gozdz, A. S.; Amatucci, G.; Tarascon, J. M., Differential Scanning Calorimetry Study of the Reactivity of Carbon Anodes in Plastic Li-Ion Batteries. J. Electrochem. Soc. 1998, 145, 472-477. 12. Yamaki, J. I.; Takatsuji, H.; Kawamura, T.; Egashira, M., Thermal Stability of Graphite Anode with Electrolyte in LithiumIon Cells. Solid State Ionics 2002, 148, 241-245. 13. Choi, N. S.; Profatilova, I. A.; Kim, S. S.; Song, E. H., Thermal Reactions of Lithiated Graphite Anode in LiPF6-Based Electrolyte. Thermochim. Acta 2008, 480, 10-14. 14. Profatilova, I. A.; Kim, S.-S.; Choi, N.-S., Enhanced Thermal Properties of the Solid Electrolyte Interphase Formed on Graphite in an Electrolyte with Fluoroethylene Carbonate. Electrochim. Acta 2009, 54, 4445-4450. 15. Ryou, M.-H.; Lee, J.-N.; Lee, D. J.; Kim, W.-K.; Jeong, Y. K.; Choi, J. W.; Park, J.-K.; Lee, Y. M., Effects of Lithium Salts on Thermal Stabilities of Lithium Alkyl Carbonates in Sei Layer. Electrochim. Acta 2012, 83, 259-263. 16. Richard, M. N.; Dahn, J. R., Accelerating Rate Calorimetry Study on the Thermal Stability of Lithium Intercalated Graphite in Electrolyte. I. Experimental. J. Electrochem. Soc. 1999, 146, 2068-2077. 17. Richard, M. N.; Dahn, J. R., Accelerating Rate Calorimetry Studies of the Effect of Binder Type on the Thermal Stability of a Lithiated Mesocarbon Microbead Material in Electrolyte. J. Power Sources 1999, 83, 71-74. 18. Profatilova, I. A.; Stock, C.; Schmitz, A.; Passerini, S.; Winter, M., Enhanced Thermal Stability of a Lithiated NanoSilicon Electrode by Fluoroethylene Carbonate and Vinylene Carbonate. J. Power Sources 2013, 222, 140-149. 19. Xu, K.; Zhuang, G. V.; Allen, J. L.; Lee, U.; Zhang, S. S.; Ross, P. N.; Jow, T. R., Syntheses and Characterization of Lithium Alkyl Mono- and Dicarbonates as Components of Surface Films in Li-Ion Batteries. J. Phys. Chem. B 2006, 110, 7708-7719. 20. Verma, P.; Maire, P.; Novák, P., A Review of the Features and Analyses of the Solid Electrolyte Interphase in Li-Ion Batteries. Electrochim. Acta 2010, 55, 6332-6341. 21. Nie, M.; Chalasani, D.; Abraham, D. P.; Chen, Y.; Bose, A.; Lucht, B. L., Lithium Ion Battery Graphite Solid Electrolyte Interphase Revealed by Microscopy and Spectroscopy. J. Phys. Chem. C 2013, 117, 1257-1267. 22. Nie, M.; Abraham, D. P.; Chen, Y.; Bose, A.; Lucht, B. L., Silicon Solid Electrolyte Interphase (SEI) of Lithium Ion Battery Characterized by Microscopy and Spectroscopy. J. Phys. Chem. C 2013, 117, 13403-13412. 23. Xu, K., Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chemical Reviews 2014, 114, 11503-11618. 24. Gauthier, M.; Carney, T. J.; Grimaud, A.; Giordano, L.; Pour, N.; Chang, H.-H.; Fenning, D. P.; Lux, S. F.; Paschos, O.;

cycling or storage due to the low decomposition temperature of LEDC, repeated volume change causing damages on SEI (that is, LEDC), reactivity of the LiPF6 salt, or the surface reactivity of the lithiated silicon electrode. While the dominant component of the initial SEI is LEDC, from the reduction of EC, along with LiF and LixPFyOz from the reduction of LiPF6, the thermal decomposition of LEDC during cycling increases the complexity of SEI. In addition, the inherent thermal instability of LEDC suggests that in order to develop lithium ion batteries with long life, electrolyte formulations should be used which limit the presence of EC reduction products on the anode surface. Alternative solvents or additives which generate initial reduction products with superior stability but comparable or better lithium ion conductivity would be preferable.

ASSOCIATED CONTENT Supporting Information. Material synthesis, DTG, IR, and XPS of representative lithium alkyl carboxylates and phosphates, TGA and DTG of fresh Si electrode, and XPS of Si electrode surface after cycling This material is available free of charge via the Internet at http://pubs.acs.org.

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

ACKNOWLEDGMENT The authors gratefully acknowledge funding from Department of Energy Office of Basic Energy Sciences EPSCoR Implementation award (DE-SC0007074)

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