Agglomeration Mechanism and a Protective Role of Al2O3 for

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An Agglomeration Mechanism and a Protective Role of Al2O3 for Prolonged Cycle Life of Si Anode in Lithium-ion Batteries Jaewook Shin, and EunAe Cho Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00145 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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Chemistry of Materials

An Agglomeration Mechanism and a Protective Role of Al2O3 for Prolonged Cycle Life of Si Anode in Lithium-ion Batteries Jaewook Shina,b and EunAe Choa,b*

a

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea. b

Advanced Battery Center, KAIST Institute for NanoCentury, Korea Advanced Institute of

Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea.

*Corresponding author Tel.: +82-42-350-3317 Email address: [email protected]

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Abstract Si, the high-capacity anode for Li-ion battery (LIB), has intrinsic 300% volume changes limiting its commercial application. The volume change leads to particle pulverization that result in loss of electrical contacts. Various nanostructures are proposed to avoid the pulverization, but the commercialization is still a distant future. Recently, Al2O3 has demonstrated its ability to enhance electrochemical cycling performance. However, a comprehensive mechanistic role of the Al2O3 has not been well understood. Here, we have combined electrochemical and chemical agitation test to propose two novel mechanisms: Si agglomeration and a protective role of the Al2O3. LiPF6, the common Li salt of the LIB electrolyte, decomposes and form HF that etches the native oxide layer then form labile Si-H surface. Because of the labile Si-H surfaces, the Si particles agglomerate during the volume changes. The Si agglomeration has a detrimental effect on the cycling performance associated with the loss of electrical contacts. On the other hand, in the presence of the Al2O3, the Al2O3 consumes the HF, protecting the native oxide layer that resists the agglomeration. Thus, the Si particles with Al2O3 are better dispersed. The Al2O3 allows the better Si dispersion during electrochemical cycles, resulting in improved capacity retention.


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1. Introduction Along with the advancements in Li-ion batteries (LIBs), developments in portable electronics and electric vehicles have grown consumer’s appetite for energy demanding applications.1 To keep pace with the energy demands, Si for the LIB anode material has been discussed to replace the current anode material, graphite. The Si has almost an order of magnitude higher gravimetric capacity (3579 mAh g-1 for Si/Li3.75Si) than that of the graphite (373 mAh g-1 for C6/LiC6).2 However, intrinsic challenges accompany the Si. They are largely classified into physical and chemical issues.3-5 First, physical issues arise from the fact that the Si particles undergo large volume changes (∼300%) throughout electrochemical charge (lithiation) and discharge (delithiation). These volume changes can lead to particle pulverization, electrode cracking, and electrical contact loss. Second, chemical issues arise from decomposition of electrolyte on the surface of the electrode and unstable formation of solid-electrolyte interphase (SEI). Unlike the graphite anode, the Si SEI grows continuously throughout the electrochemical cycles because new surfaces are repeatedly formed from the particle pulverization and electrode cracking. In addition, the Si anode forms non-uniform and thick SEI layer.6 As electrochemical cycles continue, these issues become exponentially severe, leading to the capacity decay. To solve the physical issues, Si particle size has been reduced to lessen the strain induced by the volume change.7-10 Through these studies, it is found that the Si particles pulverize upon lithiation only above a critical particle size, 150 nm.11 To solve the chemical issues, electrolyte additives have been employed.5,

12, 13

Among them, fluoroethylene

carbonate (FEC) has been shown to be effective in stabilizing the SEI formation.6 As such, some manufacturers have begun to blend a small amount of Si into the graphite anode.14 Nonetheless, the full replacement is still a challenge. In essence, further studies are necessary to take full advantage of the high capacity Si anode. Recently, growing number of the Si research papers are reporting back-end (postelectrode manufacturing) surface modification, such as atomic layered deposition (ALD)15-20 and molecular layered deposition (MLD).21-23 The ALD and the MLD can deposit various inorganic materials, especially Al2O3,16, 24-29 TiO2,18, 19, 30-32, TiN,17, 33, 34 ZnO,20 alucone,21, 22 and aluminum dioxybenzene,23 as a conformal coating on the Si anode electrode. These



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works have universally reported improved cycle performance. Particularly, He et al. report that the Al2O3 coating significantly reduced electrode cracks.16 However, there is still a debate over whether the benefit comes from the coating aspect of these works. On one hand, researchers argue that these inorganic coatings provide structural support for the electrode from cracking.16, 23 On the other hand, others are concerned that the integrity of the thin inorganic coating may be compromised during the 300% volume expansion. Although the benefit of these inorganic coatings, enhancing the physical properties, is indisputable, the comprehensive role of these inorganic coating materials is still unclear. To be able to take full advantage of these inorganic coating materials, it is vital to understand the role of these materials away from the coating aspect. To understand the role of these inorganic materials, spectroscopic characterization after the electrochemical cycles provides essential information. However, despite the benefit of these inorganic materials, the spectroscopic studies of these materials upon the cycling are scarce. Electron-based spectroscopies are difficult to be employed because the SEI and the lithiated Si particles are highly sensitive to the electron beam.35 X-ray based spectroscopic techniques can probe the electrode with minimum beam damage. However, because most elements involved in the Si anode are light elements, X-ray spectroscopic studies are largely limited to surface analyses. Furthermore, signals from the thick SEI overwhelms the electrode signals. Thus, understanding the chemical role of these inorganic materials has been incomplete. One method to spectrally observe the electrode materials without the SEI signal is agitating the Si powder and the inorganic material with the electrolyte.36 This simple soaking experiment has been recently gaining attention in the rechargeable battery studies including Si anode,36,

37

Li-S battery,38,

39

conversion type battery,40,

41

etc. Especially,

Philippe et al. have shown that the LiPF6 salt in the electrolyte decomposes to form HF which chemically etches the native oxide layer on the Si particles.36 Adopting this technique allows more in-depth analysis of the chemical state of the Si and the inorganic material without the influence of the SEI signal. In this study, we have chosen Al2O3 as the inorganic material because Al-based materials are the most commonly incorporated coating materials in the literature. This study aims to comprehend the role of Al2O3 in the Si anode. Si and Al2O3-incorporated Si (Si:Al2O3) samples were prepared, tested, and characterized. The Si:Al2O3 sample was prepared by


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simply mixing the Al2O3 powder with the Si powder as a composite additive to isolate the effect of the Al2O3 from the conformal coating. For the Si and Si:Al2O3 samples, electrochemical and agitation tests were performed and their morphologies and chemical species are analyzed. Through these analyses, we propose two novel mechanisms: Si agglomeration and a protective role of Al2O3. Si particle agglomeration is a detrimental phenomenon which can be commonly observed in the various Si anode reports but often not explicitly discussed.35 Nevertheless, the particle agglomeration can lead to loss of electrical contacts and capacity decay over prolonged cycles. We propose that the HF, the LiPF6 decomposition product, chemically etches the surface of Si, generating a labile Si-H surface. During cycling, the Si particles agglomerate because the labile Si-H surfaces come in contact. The Al2O3 added to the Si anode improves the cycling performance by consuming the HF. As a result, the Si surface is protected from forming the Si-H surfaces. Without the Si-H surfaces, the Si particles are less likely to agglomerate. The better distributed Si particles lead to better retention of electrical contact and ultimately electrochemical performance.

2. Experiments Si nanoparticle (Si, average particle size: 50 nm, Alfa Aesar), Super P (conductive carbon black, TIMCAL), Al2O3 nanoparticle (Al2O3, average particle size: 50 nm, Alfa Aesar), sodium carboxymethyl cellulose (CMC, DS = 0.9, Mw = 250,000, Sigma-Aldrich), Li metal disk (15.6 mm dia. x 0.45 mm thick, MTI Korea), 1 M LiPF6 in ethylene carbonate (EC) (StarLyte Electrolyte PanaXTech), 1 M LiPF6 in diethyl carbonate (DEC) (StarLyte Electrolyte PanaXTech), 1 M LiPF6 in ethylene carbonate:diethyl carbonate:fluoroethylene carbonate (EC:DEC:FEC) (45:45:10 v/v/v) (StarLyte Electrolyte PanaXTech), polyethylene separator (Tonen), and 2032 coin cell parts (Wellcos Corp.) were purchased and used without further purification. Si electrode fabrication: 0.1 g of CMC is dissolved in 2.0 ml of D.I. water by soaking overnight. Si electrode slurry was made by mixing 0.2 g of Si and 0.1 g of Super P and pouring the composite powder into the CMC solution. The Si:Al2O3 slurry was made by mixing 0.2 g of Si, 0.04 g of Al2O3, and 0.1 g of Super P and pouring the composite powder into the CMC solution. The slurries were vortex mixed, aged for a day, and vortex mixed again. The mixed slurries were coated on a Cu foil by slowly doctor blading with about 50



µm gap. The coated slurries were dried in ambient condition for an hour and further dried at 80 ºC under vacuum for 12 hours. The dried electrodes were cut into disks for the coin cell battery tests. The Si mass loading for both electrodes was about 0.7 mg cm-2. Electrolyte agitation experiment: Si:Al2O3 composite powder was prepared by mixing 50 mg of the Si and 10 mg of the Al2O3. 50 mg of the Si powder and the Si:Al2O3 composite powders were soaked in 5 ml EC:DEC electrolyte in a 10 ml glass vial. The mixtures were sealed and sonicated for 4 hours. Upon agitation, the powders were filtered, washed with copious amounts of DEC solvent three times, and dried under vacuum. Coin cell fabrication: The electrode disks were further dried in 80 ºC oven before entering Ar-filled glovebox (M.O. Tech). The EC:DEC electrolyte was prepared by mixing 1 M LiPF6 in EC and 1 M LiPF6 in DEC solutions in 1-to-1 volume ratio. The EC:DEC:FEC electrolyte was 1 M LiPF6 in EC:DEC:FEC solution. The prepared electrode, separator, Li metal, and the respective electrolyte were assembled in 2032 coin cell with a spring and spacer set. Electrochemical test: The electrochemical cycling tests were conducted via constant currentconstant voltage lithiation step and constant current delithiation. The constant voltage cut-off current was set at a C/40 rate. The first cycle was conducted with the C/20 rate constant current and the subsequent cycles were conducted with the C/10 rate constant current. The C rates were calculated based on the amount of active material (Si) and 3500 mAh g-1 capacity. The 1C rate achieves the 3500 mAh g-1 capacity in 1 hour. Upon 100 cycles, the coin cells were lithiated for further experiments and these electrodes are labeled as 100th lithiated electrodes. Electrochemical Impedance Spectroscopy (EIS) measurement: EIS measurement was conducted at the 100th lithiated electrodes. All the EIS measurements were obtained with 0.13 V open circuit voltage with 10 mV voltage amplitude. The frequency range was 100 KHz to 0.02 Hz. The measurements were collected by using Autolab PGSTAT302N with Nova 1.11 software. The measured spectra were fitted with ZIVE LAB software. Scanning Electron Microscopy (SEM) observations: The electrode and particle morphologies were observed by the SEM. The coin cells at 100th lithiated state were disassembled in the glovebox. The electrodes were separated, washed with copious amount of DEC solvent three times, and dried under vacuum for a week. The dried electrodes were taped on top of a flat SEM holder for the top view and on the side of a 90º SEM holder for the cross-section view.


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The prepared SEM sample was coated with Os using HPC-1SW (Vacuum Device Inc., Japan) for 30 sec, three times. The electrolyte agitated powders were taped on top of a flat SEM holder and coated with Os with the similar procedure. The SEM images were obtained with Philips XL30 with ultra-high resolution (UHR) mode as an option. X-ray Photoelectron Spectroscopy (XPS) measurements: The chemical state at the surface of the electrodes and the agitated powders were obtained with XPS. The electrode samples were prepared the similar method as the SEM samples. The dried electrodes were taped on top of a vacuum sealed sample transfer vessel. The electrode samples were never exposed to air during the transfer nor the scan. The XPS system (K-Alpha+ ThermoFisher Scientific) utilized Al for the X-ray source. The survey scans were obtained with 400 µm spot size, 1.00 eV step size, 27.2 secs total scan time, and averaging two scans. The narrow scans were obtained with 400 µm spot size, 0.05 eV step size, 3 mins 20.5 secs total scan time, and averaging ten scans. Li 1s, O 1s, F 1s, C 1s, and Si 2p narrow scans were obtained for all samples. For the samples with Al2O3 content, additional Al 2p narrow scans are obtained. The etching was done with 2000 eV Ar cluster ion for 120 secs. The XPS results for the agitated powders were obtained with the similar condition. XPS fitting: The XPS spectra fits were performed with CasaXPS software (version 2.3.15, Casa Software Ltd.) to identify the atomic and chemical composition. All spectra were shifted relative to their respective carbon 1s sp3 (assigned as 284.8 eV) to compensate for any offsets occurred during the measurements. All the fitting followed a self-consistent method similar to a previous publication.13 All samples were assumed to be electronically insulating and fitted with linear backgrounds. The peak shapes were fitted with Voigt functions composed of 30% Lorentzian and 70% Gaussian functions. Initial peak fits were made of the spectra using the Levenberg-Marquardt least-squares algorithm and atoms with the same functionality were assumed to be stoichiometric.

3. Results 3.1. Electrochemically cycled Si electrodes Electrochemical performance To determine the effects of Al2O3 on electrochemical cycling performance of the Si



electrode, Al2O3 (9 wt%) was incorporated as a composite additive in the Si electrode (Si:Al2O3). As an electrolyte, a mixture of 1 M LiPF6 in EC:DEC was used with and without the addition of the electrolyte additive, FEC. The FEC is incorporated to stabilize the SEI formation. The first electrochemical cycling voltage profile of the Si and Si:Al2O3 electrodes are presented in Fig. 1a. From the first cycle profile, the first delithiation (discharge for anode) capacities and the coulombic efficiencies (C.E.s) are illustrated in Table 1. The Al2O3-added electrodes have smaller initial capacity most likely due to the insulating nature of Al2O3. However, these differences among the first cycle capacities are small. The FEC addition has little effect on the capacity, but more significantly, reduced the C.E. due to the irreversible decomposition of the FEC.13 Differential capacity (dQ dV-1) plot demonstrates that the Si reaction is the main contribution to the capacity (Fig. S1). All the voltage profiles, the capacities, and the C.E.s demonstrate the typical performance of the Si anode’s first lithiation and delithiation with and without the FEC electrolyte.13

Fig. 1: a) First cycle voltage profiles and b) cycle retentions (square symbol) with C.E. (circle symbol). c) EIS at the 100th lithiated state (symbol) with the respectively fitted spectra (line)


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and graphical representation of the fitting circuit model. d) The summed total resistive elements from the EIS fit.

Table 1. First electrochemical discharge capacities and C.E.s for Si and Si:Al2O3 electrodes. Electrolyte

Si

Si:Al2O3

Capacity (mAh g-1)

C.E. (%)

Capacity (mAh g-1)

C.E.

EC:DEC

2673.8

88.0

2637.2

88.0

EC:DEC:FEC

2760.5

85.7

2567.1

85.1

While there are only slight differences among the first cycle delithiation capacities, in the prolonged cycles, there are vast differences among capacity retentions. The prolonged cycle discharge capacities and the respective C.E.s are plotted in Fig. 1b. The FEC addition improves both the capacity and the C.E.s in a good agreement with the previous reports.13 The Si:Al2O3 electrodes have consistently higher capacity than their counterpart, the Si electrodes. Furthermore, the Si:Al2O3 electrodes have similar C.E. compared to their counterpart but they seem to stabilize slightly faster. Between 10th and 30th cycles, the Si:Al2O3 electrodes have higher C.E.s than their counterpart. The 100th discharge capacities and C.E.s are listed in Table 2. In the same electrolyte condition, the Si:Al2O3 electrodes have the significantly higher capacity at the 100th cycle while the C.E.s are similar. The retention is lowest for the Si (EC:DEC) condition and the retention is highest for the Si:Al2O3 (EC:DEC:FEC) condition. The addition of Al2O3 and FEC, separately, improves capacity retention and combining the two additives produces the best performance.

Table 2. 100th electrochemical discharge capacities and C.E.s for Si and Si:Al2O3 electrodes. Electrolyte

Si -1

Si:Al2O3

Capacity (mAh g )

C.E. (%)

Capacity (mAh g-1)

C.E.

EC:DEC

726.7

99.0

1114.2

99.0

EC:DEC:FEC

1215.5

99.3

1670.5

99.3



To determine the electrochemical cause of such differences among capacity retentions, EIS is measured at the 100th lithiated state. Fig. 1c shows the experimental and model-fitted EIS Nyquist plots of the four samples Si and Si:Al2O3 in the electrolyte with and without the FEC. All four EIS plots have an equivalent series resistance (Rs) at the high frequency intercept, describing electronic conductivity of the electrodes, ionic conductivity of the electrolyte solution, and any electronic contact resistances associated with the hardware. All four EIS plots also have three semicircles at high frequency, medium frequency, and low frequency, describing three different charge transfer resistances of Li counter electrodeelectrolyte interface (R1), electrical double-layer on the SEI-electrolyte interface (R2), and Si-SEI (R3), respectively.42, 43 In the low-frequency region, the four EIS plots have Warburg impedance tail, describing Li diffusion into the Si surface.44 The EIS results are fitted with an equivalent series resistance (Rs), three RC circuit representing the charge transfer resistances with their respective pseudo-capacitances, and a Warburg element. All the resistances for the respective samples are presented in Fig. 1d. Consistent with the past reports, the FEC electrolyte lowers the total resistance because the FEC electrolyte additive facilitates the formation of a uniform SEI layer.35 On the other hand, the Al2O3 addition has a conflicting impedance result. Consistent with the capacity retention performance, the Si:Al2O3 (EC:DEC:FEC) electrode has the lowest resistance, even lower than the Si (EC:DEC:FEC) electrode. However, the Si:Al2O3 (EC:DEC) electrode has the highest resistance, even higher than the Si (EC:DEC) electrode. This is interesting because the Si:Al2O3 (EC:DEC) electrode has better capacity retention than the Si (EC:DEC) electrode while having the higher impedance. The Al2O3 is an insulator and it will contribute to the increased resistance. However, it does not explain the improved capacity retention.

Electrode morphology To be able to understand this phenomenon, the physical morphology of the cycled electrodes is observed using SEM. Electrode morphology at the 100th lithiated state is presented in Fig. 2. The top views demonstrate that after the 100 cycles, generally, the electrode surface has more cracks compared to their respective pristine electrode surfaces (Fig. S2). The pristine electrode surface has some cracks but they are sparsely distributed. After the cycles, there are ample amount of cracks in the electrode. The cracks are caused by the volume changes throughout the electrochemical cycles. They often lead to loss of


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electrical contacts that can cause loss of electrochemical capacity.9 The electrodes cycled in the FEC-containing electrolyte have narrower cracks meaning that the electrical contacts are better retained. However, qualitatively, the effect of Al2O3 addition on the electrode morphology is not very clear.

Fig. 2: a-d) Top view SEM images of the 100th lithiated electrodes. e-h) Cross-sectional SEM images of the 100th lithiated electrodes.

The cross-section views of the cycled electrodes are presented in Fig. 2 e-h. They show that all the electrodes have increased in thickness upon 100 cycles. The thicknesses are 35, 48, 30, and 30 µm for Si (EC:DEC), Si:Al2O3 (EC:DEC), Si (EC:DEC:FEC), and Si:Al2O3 (EC:DEC:FEC), respectively. The thickness of the pristine electrodes is about 4 µm thick (Fig. S3). Such large thickness increases are due to the combined effect of the Si volume expansion and the SEI formation.13 Consistent with the past reports, the FEC addition yields smaller electrode thickness after cycling.13 The wider cracks observed in the top view (Fig. 2 a-d) suggest that the electrodes cycled without the FEC have more void spaces and result in the larger thickness. The Al2O3 addition, on the other hand, yields larger electrode thickness. Especially, the Si:Al2O3 (EC:DEC) electrode has the largest electrode thickness. This large electrode thickness has contributed to the largest resistance for this electrode because the thicker electrodes have longer electron and ion diffusion pathway. The thickness of the electrodes explains the high resistance of the Si:Al2O3 (EC:DEC) electrode but the improved capacity retention by the Al2O3 addition is still difficult to explain in this scale of magnification.



To study the effect of Al2O3 additive, particle level morphology is further investigated. The particle morphology after 100 cycles is presented in Fig. 3. The average particle sizes are 545, 301, 353, and 265 nm for Si (EC:DEC), Si:Al2O3 (EC:DEC), Si (EC:DEC:FEC), and Si:Al2O3 (EC:DEC:FEC), respectively. Compared to the pristine particles (∼50 nm in size, Fig. S3), the particle size has increased significantly. Such large increases, from 50 nm to 265~545 nm, cannot be attributed only to the 300% volume expansion of Si and formation of the thick SEI layer. It is likely that the particles agglomerated during the electrochemical cycles. Since large particles can readily pulverize during cycles, electrical contacts will be lost. These increases in particle size significantly contribute to the loss of capacity retention as shown in Fig. 1b. The Si (EC:DEC) has the largest particle size, corresponding to the fastest capacity decay. The FEC addition yields smaller particle size. However, more significantly, the Al2O3 addition is more effective in yielding the smaller particle. Since the Al2O3 additive hinders agglomeration of Si particles, it improves capacity retention.

Fig. 3: a-d) SEM images of the 100th lithiated particles. e) Size distribution of the 100th lithiated particles.

Surface chemistry To investigate the chemical influence of the Al2O3 on the SEI growth, XPS technique is utilized for Si and Si:Al2O3 cycled in EC:DEC and EC:DEC:FEC. Since the XPS is a surface-sensitive technique, electrode materials covered with thick (>10 nm) SEI layers are


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difficult to observe.13 To be able to observe the electrode signals, the outermost surface is etched using ion beam. Then, XPS results provide information on the SEI layer as well as the electrode material surface. Before and after the ion beam etching, survey scans for the electrodes are collected, as presented in Fig. S4. Before the etching, in all cases, a significant amount of C, O, and F signals are observed, mostly delivering information on the chemical composition of the SEI layer (Fig. S4). Neither Si nor Al signals, the electrode materials, are detected. After the etching, Si signal is clearly detected as shown in Fig. S4. Those results reflect that before the etching, the XPS spectra show upper surface (outer SEI layer) and after the etching, the XPS spectra show lower surface (electrode materials and inner SEI layer).

Narrow scans of Li 1s, O 1s, F 1s, C 1s, Si 2p, and Al 2p spectra for Si and Si:Al2O3 are measured before and after the etching (Fig. 4 and S5). The Li 1s spectra show LiOx and LiF species (Fig. 4). For both samples, without the FEC, LiOx signal is dominant at the upper surface and the LiF signal is dominant at the lower surface. With the FEC, LiF is dominant throughout the upper and lower surface. In addition, according to the survey, etching the surface increases the amount of F 1s signal (Fig. S4), in which LiF is the dominant species (Fig. S5). While both LiOx and LiF are SEI components, LiOx is a polymeric decomposition product of EC, DEC, and FEC organic molecules and LiF is a decomposition product of LiPF6 and FEC.13 These results suggest that EC, DEC, and FEC decompose on the upper surface while the LiPF6 and FEC decompose on the inner surface, closer to the electrode material. The EC, DEC, and FEC molecules decompose mainly due to the reduction reaction at the surface of the Si electrode.45 The LiPF6 decomposes due to a trace amount of H2O molecules in the electrolyte.36 The Al2O3 addition, on the other hand, shows slight changes in the amount of LiOx and LiF formation but no significant changes in the species formation.



Fig. 4: Li 1s and O 1s XPS spectra of the 100th lithiated Si and Si:Al2O3 electrodes cycled in EC:DEC or EC:DEC:FEC electrolyte, before and after etching.

The O 1s spectra show five species: C-O-C, O=C, O-C, LixO, and R-O-CO2 (Fig. 4). The LixO species appears at the lower surface of Si and Si:Al2O3 in both electrolytes, suggesting that the LixO species is different from the LiOx identified in the Li 1s region. The LixO species is an inorganic lithium oxide species rather than the polymeric Li salt.13 The RO-CO2 species appears only in the FEC-cycled samples because the FEC decomposition is different from that of the EC. The exact elementary steps for FEC decomposition are still debatable but two possible routes, so-called “de-fluorination” and “ring opening,” commonly produce the R-O-CO2 species.6 Presence of CO3 species from the C 1s region for the FECcycled electrodes confirms the presence of R-O-CO2 species (Fig. S5). It should be noted that R-O-CO2 and CO3 species are observed only on the upper surface of the FEC-cycled samples and the higher amount of LiF is present at the lower surface. These results suggest that the Fion is released from FEC on Si electrode surface first and polymeric deposition occurs later. Between the two debatable decomposition routes, the “de-fluorination” route, in which the FACS Paragon Plus Environment

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ion is released first, better describes the current results. The differences in the SEI composition due to the FEC addition are consistent with the previous report.13 The FEC additive clearly changes the formation of the chemical species on the electrode surface. These differences are known to improve the Li+ ion diffusion in the FEC cycled electrodes.13 The Al2O3 additive, on the other hand, does not change the formation of the chemical species (Fig. 4, S4 and S5). To gain better insight, more quantitative analysis of the XPS results is conducted. The SEI components can be largely divided into organic species such as the polymeric Li salts and inorganic species such as LixO and LiF. For the SEI on the Si anode, the inorganic species are known to be favorable because the inorganic species allow faster Li+ diffusion compared to the organic species.46, 47 Due to the enhanced Li+ diffusion, the increased ratio of inorganic species is often associated with the improved Si electrode performance. In addition to the SEI species, upon etching, electrode material signals are observed including C-Li species, Si species, and Al species (Fig. S4 and S5). Individual identification and the composition of the XPS species are summarized in Table S1 - S8. The quantitative amounts of organic SEI, inorganic SEI, and electrode material, are compared (Fig. 5). The etching, the FEC, and the Al2O3 all decrease the amount of the organic species and increases the amount of the inorganic species. Since LiPF6 and FEC decompositions, producing the inorganic SEI species, occur at the lower surface, the etching and the FEC addition will increase the amount of inorganic SEI species. Furthermore, the FEC and the Al2O3 decrease the amount of the electrode species. The decrease in the electrode signal by the FEC addition can be explained by the top-view SEM images of the electrode (Fig. 2 a-d). The top-view show flatter surface for EC:DEC cycled electrodes and furrowed surface for EC:DEC:FEC cycled electrodes. This difference in the electrode morphology suggests that the SEI from EC:DEC electrolyte is formed non-uniformly and fills the pores in the composite electrode. Whereas the SEI from EC:DEC:FEC electrolyte is formed more uniformly and pores are better retained. Consequently, non-uniform SEI will appear more in a two-dimensional projection of the electrode surface. The decrease in the electrode signal by the Al2O3 addition can be explained by the particle size (Fig. 3). The larger particles have less SEI owing to the smaller surface area. The simplified schematic illustration of this phenomenon is presented in Fig. 5d.



Fig. 5: a) The composition of the upper surface species in 100th lithiated electrodes (before etching). b) The composition of the lower surface species in 100th lithiated electrodes (after etching). c) Schematic illustration of the SEI layer on the 100th lithiated electrodes and XPS sampling depth.

It is clear that cycling the Si anode results in agglomerated particles. This type of agglomeration leads to loss of electrical contact and capacity decay. The Al2O3 additive hinders this particle agglomeration and improves the particle distribution. Thus, the Al2O3 additive prolongs the cycle life of the Si anode. However, the comprehensive role of Al2O3 is difficult to understand because of low Al 2p signal (Fig. S5). The SEI signals overwhelm the electrode signals even after etching. To be able to analyze the role of Al2O3, the interaction among the Si, Al2O3, and electrolyte must be studied without the SEI.

3.2. Agitated Si and Si:Al2O3 powders in electrolyte Particle morphology The thick SEI layer formed during the electrochemical cycling hinders the comprehensive study for the role of the Al2O3. Based on the morphology and chemical species studies on the cycled electrode, the Al2O3 does not alter the SEI species. Thus, further studies are conducted without the formation of the SEI layer from electrochemical cycling.


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To simulate the Si particles interacting with each other in the LIB cell, Si and Si:Al2O3 powders were soaked and agitated in the 1 M LiPF6 EC:DEC electrolyte. The morphology of the sample powders before and after the agitation is shown in Fig. 6. The pristine Si powder is about 50 nm nanoparticles. Upon agitation in the electrolyte, the Si particle size increases significantly. Even without the volume expansion from the lithiation nor the SEI formation, the particle size increases. For the Si:Al2O3 powder, upon the agitation, the particle size has remained similar. Compared to that of the agitated Si powder, agitated Si:Al2O3 powder particle size is significantly smaller. Similar to the particles after electrochemical cycles, as observed in Fig. 3, the Si particles upon agitation have agglomerated and the Al2O3 hinders such agglomeration. Furthermore, depending on the duration of the agitation the agglomeration of Si particles progressively become more severe in both size and frequency (Fig.S6). These consistent results confirm that the agitation experiment is appropriate for simulating the agglomeration phenomenon.

Fig. 6: SEM images of the a) pristine Si particle, b) agitated Si particle, c) pristine Si:Al2O3



composite powder, and d) agitated Si:Al2O3 composite powder.

Particle surface composition Since this agitation experiment can simulate the Si particle agglomeration phenomenon, to be able to comprehend the chemical interaction between the solid particles and the electrolyte, the chemical composition is studied with XPS. The surface composition of the agitated particles is presented in Fig. 7. Si 2p region spectrum for the agitated Si show the presence of Si-Si, SiOxFy, Si-OH, and Si-H species. The LiPF6 salt is known to decompose and release HF.48-50 HF can etch the surface of the Si native oxide layer.51, 52 The presence of Si-H species suggests that the Si native oxide layer undergoes such HF etching. The SiOxFy and Si-OH species are intermediate species during the etching process. The presence of intermediate species suggests that the etching process is incomplete. However, Si-H species clearly suggest that complete etching is done on some surfaces. The Si-H surface is known to be an undesired for the Si anode performance.10 The Si-H surfaced Si electrode, compared to the Si native oxide surfaced Si electrode, has higher initial capacity but significantly deteriorating capacity retention. More importantly, the Si-H surface is highly labile that it often leads to particle agglomeration.53


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Fig. 7: a) Si 2p XPS spectrum (inset: zoomed in the spectrum) and b) F 1s XPS spectrum of the agitated Si powder. c) Si 2p XPS spectrum (inset: zoomed in the spectrum) and d) F 1s XPS spectrum (inset: Al 2p spectrum) of the agitated Si:Al2O3 powder. .

F 1s spectrum for the agitated Si has an asymmetric signal with at least two species (Fig. 7b). According to the respective survey, O 1s, and C 1s spectra, only possible coordination for the F species is the Si-F species (Fig. S7). Therefore, two species included in Fig. 7c are assigned to surface Si-F and bulk Si-F species.54-56 The surface Si-F represents SiF bonds on the outermost surface and has more covalent bonding environment. On the other hand, the bulk Si-F represents Si-F bonds in the grain boundaries of the agglomerated Si particles. These bulk Si-F bonds are in ionic bonding environment. In other words, the ionic F- ions have more Si cation coordination compared to the covalent F- ion. This difference in coordination environment causes the significant binding energy (B.E.) shift to the lower energy.57 The presence of the Si-H surface and the ionic Si-F species supports and confirms



the agglomeration of Si particles. The HF causes Si agglomeration by forming the labile Si-H surface. On the other hand, the Al2O3 addition hinders the Si agglomeration as observed in Fig. 3. To understand the role of Al2O3, XPS measurement is also obtained for the agitated Si:Al2O3 powder. Si 2p spectrum for the agitated Si:Al2O3 show only SiOxFy species without the labile Si-H (Fig. 7c), demonstrating that the native Si-oxide layer is undergoing HF etching as well. However, the absence of the labile Si-H surface hinders the Si agglomeration. F 1s and Al 2p spectra for the agitated Si:Al2O3 show a change in chemical state of the Al2O3 (Fig. 7d). According to the Al 2p spectrum, O2- of the Al2O3 is replaced with F-. AlF3 species signal dominates in the Al 2p region.58 In the F 1s spectrum, although the result shows almost symmetric signal, according to the Si 2p and Al 2p region spectra, there are at least two fluorinated species. Since the survey, O 1s, and C 1s spectra do not suggest any additional fluorinated species (Fig. S8), the F 1s signal is assigned with Al-F and surface (covalent) Si-F signals. The Al-F signal is assigned because the Al-F signal is observed in the Al 2p spectrum. The surface (covalent) SiF species is present because the SiOxFy species is found in the Si 2p region. More importantly, unlike the agitated Si F 1s spectrum, no bulk (ionic) Si-F species is observed. This is consistent with the well dispersed Si particles. Without the agglomeration, no bulk Si-F species will be present. The surface Si-F and Al-F in F 1s region have similar B.E. However since the presence of both species are indisputable, Si-F and Al-F species are labeled with 688.31 eV and 688.48 eV peak positions, respectively. The HF etches the Si surface, forming the labile Si-H surface. This surface leads to the Si agglomeration. However, when Al2O3 is present, the HF reacts with the Al2O3 instead of etching the Si native oxide layer. The Al2O3 consumes the HF and hinders the labile Si-H surface formation. This set of XPS results on the agitated particles are confirmed with transmission electron microscopy (TEM) images. The Si and Si:Al2O3 agitated particles are prepared into TEM samples by focused ion beam (FIB). The agglomerated Si particle in the agitated Si sample is completely reconstructed and does not resemble the original morphology (Fig.S9ab). The agglomerated Si particle is mainly composed of Si and F atoms (Fig.S9c-f). On the other hand, the agglomerated Si:Al2O3 sample demonstrates a well dispersed set of particles with Si particles well retaining the original morphology (Fig.S10a-b). The F atoms in the Si:Al2O3 sample are mostly concentrated with Al atoms (Fig.S10c-g). This set of images clearly demonstrates that the HF reaction takes place. The HF reaction occurs with Si and


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leads to agglomeration. With Al2O3, the HF reaction mainly occurs with Al2O3 instead of Si leaving Si well dispersed.

4. Discussions Based on the results presented in this study, a comprehensive mechanism of Si agglomeration and the protective role of Al2O3 are illustrated in Fig. 8. In a typical LIB, the electrolyte contains LiPF6 salt. However, LiPF6 is known to decompose and form HF (Fig. 8a), which can etch the Si native oxide layer. Upon HF-etching, the Si-H layer is formed on the Si surface, replacing the native oxide layer (Fig. 8b). This Si-H layer is labile and can lead to agglomeration. A typical Si electrode is prepared with well dispersed Si particles (Fig. 8c). From the LiPF6 decomposition, HF forms and during electrochemical lithiation, the Si particles expand in size. The HF converts Si surface into the labile Si-H surface and the particle expansion causes particles to come in contact with each other. During the contact, the Si-H surfaces bind to each other. During delithiation, the particles contract in size. Due to the bound Si surfaces, the particles agglomerate. When HF etching continues and the electrochemical cycle repeats, this agglomeration becomes more severe and ultimately leads to loss of electrical contacts among active Si particles. On the other hand, in Si:Al2O3 electrode (Fig. 8d), when the LiPF6 decomposition produces HF, this is consumed by Al2O3, yielding AlF3. As a consequence, the Al2O3 protects the Si native oxide layer. Even if the particles come in contact with each other during particle expansion, particles can be separated. This is the main role of the Al2O3 composite additive for prolonging the Si anode cycle life.



Fig. 8: Schematic illustration of the Si particle agglomeration with and without the Al2O3 additive. a) HF formation. b) Si-H formation. c) Si agglomeration in the Si anode. d) Protective role of Al2O3 in segregating Si particles in the Si anode.

The role of Al2O3 is clear. The Al2O3 consumes the HF and protects the physical morphology of the Si particles. It is interesting to note that the addition of both Al2O3 and FEC additives have shown to demonstrate the best performance. This is because while the Al2O3 additive mainly affects the physical morphology of the Si particles and the FEC additive affects the chemical species. The intrinsic challenge for the Si anode is the combination of physical and chemical issues. The FEC additive is an effective additive for stabilizing the chemical decomposition of the electrolyte molecules and the Al2O3 additive is an effective additive for stabilizing the physical deformation of the Si particles. These two additives do not interfere with each other but rather have the synergistic role of solving the two challenges of the Si anode. Lastly, the findings in this work can significantly reduce the cost of Si electrode manufacturing. Both the ALD and MLD require high-cost apparatuses with vacuum and temperature elevation. Furthermore, the precursor gases are often expensive. In addition to the high cost, the common binder material for the Si anode, cellulose-derived polymers are not stable at the elevated temperatures.59 If the ALD or MLD coating materials can serve their purpose by simply being mixed in as the composite electrode, the manufacturing cost can be greatly reduced.


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5. Conclusion A comprehensive mechanism of the Si agglomeration phenomenon and the protective role of Al2O3 additive are summarized by analyzing physical and chemical changes of the Si electrodes after prolonged cycles and the Si powders after agitating in the electrolyte solution. On the electrode level, we have demonstrated that the Si particles agglomerate significantly. However, the Al2O3 additive helps to alleviate the agglomeration. Interestingly, the Al2O3 additive alleviates the agglomeration without affecting the chemical species of the SEI layer. The benefit of Al2O3 addition is mainly in the physical morphology of the Si particles. By adopting the simple agitation experiment between the Si and Si:Al2O3 powders with the electrolyte, more in-depth surface analysis is enabled. In a typical Si anode, the Si native oxide layer is etched by HF and forms the labile Si-H surface. This labile surface binds with other particles during cycles and leads to loss of electrical contacts in the electrode. On the other hand, Al2O3 protects the Si native oxide layer by consuming HF. Al2O3 sacrifices itself and yields AlF3 species. Because Si particles remain covered by the native oxide layer, the particles remain well dispersed. The Al2O3 preserves the physical integrity of the Si particles. Along with the FEC additive, stabilizing the electrolyte decomposition, these two additives complement each other. When they are incorporated together, the synergistically improved electrochemical performance is achieved. The Si agglomeration is a phenomenon that can be seen in a number of previous Si anode studies. However, studies nor discussions on such phenomenon have been scarce. Nevertheless, the Si agglomeration in the LIB electrode has a detrimental effect on the cycle performance because it leads to the loss of electrical connections. Here, we demonstrate that the Al2O3 additive has the ability to protect the Si from agglomerating. In the past literature, Al2O3 has been incorporated in the Si anode as coating materials. However, these techniques require high manufacturing cost. Here, the benefit of incorporating Al2O3 has been demonstrated by the simple composite addition without the extra modification step. The current Si anode research focuses on the particle morphology at the initial state and the chemical composition of the SEI. This work proposes comprehensive mechanism on the particle morphology change after the electrochemical cycles by combining the information on the electrochemically cycled electrode and the electrolyte agitated particle. The Si agglomeration mechanism, the protective Al2O3 mechanism, and the agitation experiment demonstrated in this study will allow further discoveries of oxide, nitrides, sulfides, etc.,



better optimized for the HF consumption.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. dQ dV-1 plot of the first cycle voltage profile, SEM images of the pristine electrodes and agitated powders, survey, F 1s, C 1s, Si 2p, and Al 2p XPS spectra of cycled electrodes and/or agitated powders, tables of XPS composition, TEM images and EDS mapping of the agitated particles as well as respective experimental procedures.

Acknowledgements The authors would like to thank the characterization facility in Department of Materials Science and Engineering at KAIST, KAIST Analysis Center for Research Advancement, and National Nanofab Center for granting assess to their equipment. This work was supported by the National Research Foundation of Korea (NRF) (2018006908) and the Agency for Defense Development (ADD) (UD170092GD).

Supporting Information Supporting Information is available free of charge on the ACS Publications website or from the author.

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Table of Contents/Abstract Graphic for An Agglomeration Mechanism and a Protective Role of Al2O3 for Prolonged Cycle Life of Si Anode in Lithium-ion Batteries Jaewook Shina,b and EunAe Choa,b*

a

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea. b

Advanced Battery Center, KAIST Institute for NanoCentury, Korea Advanced Institute of

Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea.


Agglomeration Mechanism and a Protective Role of Al2O3 for

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