Understanding limited reversible capacity of SiO electrode during 1st

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Understanding limited reversible capacity of SiO electrode during 1 cycle and its effect on initial coulombic efficiency st

Geunho Choi, Jeonghan Kim, and Byoungwoo Kang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01057 • Publication Date (Web): 02 Aug 2019 Downloaded from pubs.acs.org on August 3, 2019

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Chemistry of Materials

Understanding limited reversible capacity of SiO electrode during 1st cycle and its effect on initial coulombic efficiency Geunho Choi‡, Jeonghan Kim‡, and Byoungwoo Kang* Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea ABSTRACT: We tried to understand the reversible capacity of SiO during 1st cycle and its effect on the poor initial coulombic efficiency (ICE). Several SiO samples that have slightly different microstructures were prepared by solid-state reaction. They have similar irreversible capacity but have different reversible capacity during the 1st cycle. As a result, the ICEs of the samples increase as their reversible capacities increase. The limited reversible capacity in SiO is originated from the degree of the lithiation in 1st discharge process that can be caused by the microstructures. Given that Si in SiO is embedded in SiO2 matrix, the microstructure can induce strong compressive stress to Si, especially during the lithiation. The induced compressive stress can decrease the thermodynamic redox potential of Si. As a result, Si in SiO is not fully lithiated to c-Li3.75Si indicating lower electrochemical activity of Si in SiO compared to Si nanoparticles or thin-film electrode. The findings suggest that the low reversible capacity of in SiO can be increased by controlling stress-released microstructure and then improve ICE in addition to superior capacity retention.

1. Introduction Silicon is a promising anode material for lithium ion batteries because it has about ten times higher theoretical capacity (Li22Si5, 4200 mAh g-1) than that of graphite (LiC6, 372 mAh g-1), and a low operating voltage of 0.1 ~ 0.2 V.1 However, the charge/discharge electrochemical reaction of Si with Li suffers from large volumetric changes of about 300 ~ 400%, which can lose contacts between Si particles and conducting agents; then a thick solid electrolyte interface (SEI) can be formed on the newly-exposed surface via electrolyte decomposition, and severe polarization can be induced.2 As a result, the achievable capacity of Si severely degrades over cycles.3 To overcome these problems, silicon monoxide (SiOx, x~1) has been proposed because SiO has a disproportionated structure in which amorphous Si is surrounded by amorphous SiO2 matrix in atomic level4 and this unique structure of SiO can help to mitigate severe volume change of Si during cycles resulting in the improved capacity retention.5 Even though SiO can significantly improve capacity retention during cycles, SiO can suffer from poor initial coulombic efficiency that can be related to its reactive siliconoxygen component and its microstructure. In the microstructure of SiO, both Si and silicon oxides can react electrochemically with Li. The Si reacts with Li to form lithium-silicon alloys (LixSi) and the silicon oxide reacts with Li to form Li-Si-O products. Formation of the Li-Si alloy is electrochemically reversible, whereas formation of Li-Si-O products is irreversible in the operating voltage range, so large irreversible capacity loss (ICL) occurs during the 1st charge/discharge cycle.6, 7 Although the oxide matrix can effectively accommodate large volume change and thereby can improve capacity retention during cycles, the ICL from these irreversible products leads to degradation of reversible capacity and poor initial coulombic efficiency (ICE) that can be described by the following equation:

ICE =

Reversible Capacity Reversible Capacity + Irreversible Capacity Loss

(1)

Poor ICE is caused mainly by ICL that is a result of irreversible formation of Li-Si-O products during the 1st discharge.6 To increase ICE by decreasing the ICL, several approaches such as a pre-lithiation process or using electrochemically-inactive matrix have been suggested.8, 9 The previous studies only focus on the ICL to improve poor ICE of SiO. However, given the above equation, ICE also is affected by the reversible capacity in addition to the ICL. Increasing the reversible capacity of SiO can improve ICE. If the reaction of Si with Li is completely reversible, the reversible capacity can be maximized and then ICE can be increased and effectively controlled by the minimizing ICL. In this study, we tried to understand the reversible capacity of SiO in 1st cycle and its effect on ICE, and the reason why the reversible capacity in SiO is limited. Several SiO samples that have slightly different microstructures were prepared by solid-state reaction synthesis. XRD, TEM, and Raman spectroscopy were used to characterize the microstructures and bulk structures of the samples. Even though the samples have different microstructures originated from different heat treatments, they had very similar irreversible capacities but achieved different reversible capacities. The degree of lithiation of Si in SiO affects the reversible capacities. The higher degree of lithiation is, the higher reversible capacity is. Therefore, ICEs of the samples increase with the increase in the degree of lithiation. However, the Si in SiO cannot be fully lithiated up to crystalline Li3.75Si (c-Li3.75Si) during the 1st discharge. Lithiation of Si that was surrounded by the silicon oxide matrix in SiO can induce strong compressive stress partly due to the reaction of silicon oxide matrix with Li and the expansion of the volume with lithiation process. This

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induced compressive stress can lower its redox potential below the cut-off voltage and then the lithiation of Si is not fully achieved resulting in suppressed the formation of Li-rich silicon-alloy phases. This limited reversible capacity could be related to the microstructure. Even though the microstructure of SiO suppresses volumetric changes and thereby it can achieve good capacity retention, it can limit further lithiation that can lower the reversible capacity and then lower ICE. Controlling microstructures of SiO can increase reversible capacity during the 1st cycle, and thereby increase the ICE. 2. Experiment 2.1 Sample Preparation SiO (Aldrich, 325 mesh) powders were heated at 800 °C, 900 °C, or 1000 °C for 5 h in Ar. Then, to reduce the sizes of particles, the samples were treated by planetary ball milling at 500 RPM for 2 h in acetone to achieve similar particle size among the samples. 2.2. Characterization of material properties The morphology and particle size of the samples were characterized using a scanning electron microscope (SEM) (Philips electron optics, XL30S). X-ray diffraction (XRD) patterns were obtained at Cu-Kα line using a Rigaku D/MAX2500/PC. The high-resolution TEM (HRTEM) image and selected area electron diffraction (SAED) were observed using transmission electron microscope (TEM, JEM-2100F, JEOL). Confocal Raman spectroscopy (laser excitation wavelength λ = 532 nm, power 1 mW, resolution 1 μm, integration time 10 s, confocal mode, Alpha 300R, Witec, Ulm, Germany) was used to detect the Si in the SiO sample and to characterize the microstructure of the samples. The instrument is equipped with a microscope with a focal spot size of 500 nm. NMR data was acquired with 400MHz Solid stat NMR spectrometer (AVANCE III HD, Bruker, Germany) at KBSI Western Seoul center. Magic angle spinning (MAS) 7Li NMR frequency of 155.55 MHz equipped with a 4 mm MAS probe was performed. For this, single-pulse excitation was used in combination with a recycle delay of 5 s. The sample rotation rate was 14 kHz. MAS 29Si NMR frequency of 79.51 MHz was performed. For this, single-pulse excitation was used in combination with a recycle delay of 20 s. The sample rotation rate was 15.5 kHz. 2.3. Preparation of the electrode and electrochemical measurements Active materials SiO (unheated/heat-treated), Super P (Timcal) and PAA (poly(acrylic acid)) as a binder were mixed in a weight ratio of 7:2:1. The resulting slurry was cast on Cu foil using a doctor blade that has a uniform thickness. Pure Li metal was used as a counter-electrode. The electrolyte was 1 M LiPF6 solution in a 1:1 (V/V) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (Panax). The separator was polypropylene film (Celgard 2400). Electrochemical measurements were performed by using Swagelok-type cells, which were assembled in a glove box in Ar atmosphere that held < 1 ppm H2O. For the electrochemical test, the cells were galvanostatically charged and discharged at C/10 rate between 0.01 and 2 V at room temperature (1C rate = 1,400 mA·g-1) using a Maccor Series 4000. In additional constant voltage test, the constant voltage was 10 mV until the current reached < C/200 or for 200 hours. For the NMR, the cell was galvanostatically charged at C/20 rate with the voltage hold at 10 mV for 100 h.

Figure 1. Materials characterizations of four SiO samples prepared at different experimental conditions. (a) SEM images, (b) XRD patterns of the samples. Dotted line emphasizes the shift of the hump that corresponds to amorphous SiO, (c) HRTEM image and SAED (inset) of the SiO-1000 sample, and (d) Raman spectra of Si in the samples (a-Si: amorphous Si, cSi: crystalline Si).

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Chemistry of Materials

3. Results and Discussion SiO samples with various microstructures To understand the electrochemical behaviors of SiO, we prepared several samples at different experimental conditions. The samples were heated at 800, 900, or 1000 °C under Ar atmosphere (samples were labelled as SiO-n, where n is the treatment temperature). All samples such as an unheated sample and the heat-treated SiO samples were ballmilled with planetary miller to eliminate the effect of the particle size by making samples similar in size, 1 ~ 3 μm in all samples (Fig. 1a). As a consequence of the different heat treatments, the samples had different XRD patterns (Fig. 1b). The XRD pattern of the unheated sample shows a large hump indicating that the SiO is amorphous. The XRD patterns of the heattreated SiO samples show peaks of crystalline Si (c-Si) at 2θ = 28°, 48°, 56° that were clarified and increased as the heattreatment temperature increased. As the temperature increased from 800 °C, the center of the hump in the XRD shifted to a lower angle, indicating that Si and SiO2 start to be disproportionate. Thermodynamically metastable SiO starts to disproportionate to form Si and SiO2 (SiO → 0.5·Si + 0.5·SiO2) at high temperature.10 The SiO-800 sample does not have several peaks of crystalline Si but has much broadening hump compared to the unheated SiO sample indicating that the disproportionated reaction gets started but is not completed. In Fig. 1c, the HRTEM image of the SiO-1000 sample shows a nano-crystallite phase embedded in the amorphous material. Noteworthy, the selected area electron diffraction (SAED) pattern (inset) show only Si diffraction ring pattern with the diffuse ring of the amorphous SiO2 phase. This indicates that the SiO-1000 sample has the disproportionated microstructure, which is nano-crystallites Si embedded in amorphous SiO2 matrix, as reported in the literature.4, 10-18 This structure could be more easily confirmed by comparing bright field and dark field image by STEM (Supporting information, Fig. S1). Also, the SiO-900 sample shows similar characteristics with the SiO-1000 sample; a nanocrystallite Si is embedded in the amorphous SiO2 matrix (Supporting information, Fig. S2). These microstructural changes caused by different degree of the disproportionation at different heating temperatures can be explained in detail by Raman spectroscopy, which can distinguish between amorphous Si (a-Si) and crystalline Si (cSi) of Si in the samples.19 The Raman peak of the unheated sample confirms that it has only amorphous Si in the SiO2 matrix (Fig. 1d). Raman spectra also clearly show that c-Si forms at temperatures ≥ 900 °C. These results suggest that the a-Si is transformed to c-Si at 900 °C, and that this transformation further underwent at 1000 °C. The XRD, TEM, and Raman analyses indicate that the heat-treated SiO samples have slightly different microstructure from the unheated sample. The degree of the change in the microstructure can be recognized by the amount of Si and SiO2 that are disproportionated from the SiO and crystalline size of Si. This information can be estimated by comparing the Si peak from XRD (Fig. 1b), TEM images (Fig. 1c and Supporting Information Fig. S1, S2), and from the Raman spectrum (Fig. 1d).20 In the unheated sample, the a-Si and a-SiO2 could be uniformly distributed at the atomic level, whereas in the heated samples, a-Si start to be crystallized and could be separated from the matrix amorphous SiO2. The microstructures of SiO can be

increasingly separated into c-Si and matrix a-SiO2 as the treatment temperature increased. Electrochemical properties of SiO at 1st cycle To understand the electrochemical reaction of SiO, its microstructure should be properly understood. Based on TEM data in Fig. 1c, we assume that 1 mol SiO is composed of 0.5 mol Si and 0.5 mol SiO2, with the Si embedded in SiO2 matrix.4 Furthermore, according to the well-known previous studies we assume that Si can be lithiated up to c-Li3.75Si (Eq. 2),21 and that reactive SiO2 component can be lithiated to form c-Li3.75Si and inactive Li4SiO4 (Eq. 3).6 This is further supported by Supporting information Table S1, S2. (2)

3.75Li + Si = Li3.75Si SiO2 +2Li +

3.75 2

1

1

Li = 2Li4SiO4 + 2Li3.75Si

(3)

Figure 2. Experimental electrochemical property and NMR measurement of Si and Li of the unheated SiO. (a) Voltage profile at C/10 and (b) differential capacity plot of (a) with respect to voltage (dQ/dV) for the unheated SiO. (c) 7Li/29Si NMR spectroscopy of fully lithiated SiO electrode with the voltage hold at 10 mV for 100 h.

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According to equations 2 and 3, we calculated lithiation capacity, delithiation capacity, and ICL which are 2318.1 mAhg-1, 1710.1 mAhg-1, and 608 mAhg-1, respectively yielding the initial coulombic efficiency, 73.8%. However, the result (Fig. 2a) of the electrochemical experiment on the unheated SiO sample shows that the lithiation capacity is 2135.5 mAhg-1 and the delithiation capacity is 1522.3 mAhg-1 leading to the irreversible capacity of 613.2 mAhg-1. As a consequence, the ICE is 71.3%, which is lower than the calculated ICE of 73.8%. The lowering of the ICE of SiO compared to the calculated ICE is already reported in previous results that had much lower ICE.22, 23 Thus, the low ICE in the experimental result compared to the calculated one can be ascribed to a low reversible capacity rather than the irreversible capacity; i.e., the Si in the SiO could be not fully lithiated, or fully delithiated, or both. The differential capacity plot with respect to voltage (Fig. 2b) clearly indicates that a fully lithiated phase c-Li3.75Si was not observed in the SiO sample even though in Si nanoparticles or thin film Si electrode, c-Li3.75Si is easily recognized by the peak of the redox reaction at ~ 450 mV in the charge.24-26 The experimental results confirmed that SiO could be not fully lithiated up to c-Li3.75Si, but only up to Li3.5Si (300 mV) and Li2.0Si (500 mV), which can be seen by vertical dotted line in the dQ/dV curve (Fig. 2b).25,26 Figure 2c shows the 7Li/29Si NMR result of fully lithiated SiO with the voltage hold at 10mV for 100 h. In 7Li NMR, different chemical shifts can be related to different local Li environments: 10 ~ 20 ppm for a-Li3.5Si, 0 ~ 10 ppm for aLixSi (x < 2.0), 0 ppm for SEI, and 0 ~ 13 ppm for c-Li3.75Si.6,

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We confirm that there is only the a-Li3.5Si chemical shift peak and c-Li3.75Si does not appear even with fully lithiated SiO. This is consistent with the absence of the c-Li3.75Si reaction in the electrochemical data (Fig 2b). Furthermore, 29Si NMR in Fig. 2c clearly shows the formation of Li SiO 4 4 at ~ -68 ppm chemical shift as an irreversible product in SiO. We could not detect other irreversible products such as Li2O. Therefore, NMR and electrochemical results can show that the theoretical capacity calculation based on Li4SiO4 as an irreversible products is reasonable. Although the irreversible capacity by the formation of the SEI is not precisely considered,5 the irreversible capacity compared to the total capacity will be not very high because the samples have micro-sized particles that have low surface area. Therefore, we recalculated the capacity with the assumption that Si is lithiated to Li3.5Si by modifying Eq. (2) and Eq. (3) with by replacing Li3.75Si with Li3.5Si. The resulting discharge capacity was 2203 mAhg-1, charge capacity was 1596 mAhg-1, and ICE became 72.4%. The irreversible capacity, 608 mAhg-1 was not changed because the irreversible reaction is only related to composition and fraction of SiO2 discharge oxides (Supporting Information Table S1, S2). However, the experimental result of SiO is still lower than the calculated lithiation capacity obtained from the assumption that Si could be lithiated to Li3.5Si, not Li3.75Si indicating that the reversible capacity of the SiO is not fully used in experiments. 27

Figure 3. Electrochemical properties of the SiO samples prepared at different temperatures. (a) Voltage curves at the 1st cycle of the samples. Vertical line in (a) indicates the end of delithiation capacity in the samples, (b) Delithiation capacities (right axis; black closed circle: reversible capacity), irreversible capacities (right axis; wine open circle), and ICE (left axis; red closed square) of the samples, (c) Differential capacity plot with respect to voltage (dQ/dV) of 1st charge process in the samples, and capacities contributed by regions I (0.2 ~ 0.4 V) and II (0.4 ~ 0.6 V).

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Chemistry of Materials

Electrochemical properties of the SiO samples Electrochemical cell tests were carried out in the unheated SiO and the heated SiO samples. During the 1st cycle, the samples show quite different lithiation (discharge) capacities but surprisingly similar irreversible capacity loss of about 600 mAhg-1 (Fig. 3a) even though the microstructure of the samples are different (Fig. 1). Similar results were observed in a previous report on the disproportionated SiO.10, 27 Considering that the irreversible capacity can mainly originate from reactive SiO2 component in SiO, the observed similarity in the irreversible capacity indicates that the portion of reactive SiO2 component in the samples is not very different between the samples even though they underwent different heat-treatment under Ar atmosphere. This observation suggests that the heat treatment mainly affects the microstructure of the samples such as crystallinity, particle size, and degree of the disproportionated reaction in SiO but barely affects the amount of the reactive SiO2 component that could be related to oxygen. Fig. 3 shows that the samples have quite different reversible capacity, which can be related to the activity of Si and its microstructure. Furthermore, the coulombic efficiency of the samples significantly depends on the reversible capacity because of similar irreversible capacity loss in the samples; the SiO-800 sample has the highest reversible capacity leading to the highest ICE, whereas the SiO-900 sample has the lowest reversible capacity leading to the lowest ICE (Fig. 3b). We can distinguish the difference in the reversible capacity of each sample in the dQ/dV-voltage profile. (Fig. 3c) The entire range of the dQ/dV for all samples is plotted in Supporting information Fig. S3. The electrochemical reactions of Si in the charge process occurred in two regions, I (0.2 ~ 0.4 V) and II (0.4 ~ 0.6 V). In region I, Li3.5Si can be mainly delithiated whereas in region II, Li2.0Si can be mainly delithiated.25, 26 It should be noted that all SiO samples did not have the peak of the redox reaction at ~ 450 mV in the charge process indicting that the fully lithiated phase, c-Li3.75Si in all SiO samples was not observed. The SiO-800 and unheated SiO sample have higher reversible capacity than the other samples because they have large contribution of capacity from region I reaction compared to that from region II. This result indicates that the SiO-800 and unheated SiO sample can achieve further lithiation during the 1st discharge leading to large portion of Li3.5Si formation. The SiO-800 sample has higher reversible capacity than unheated SiO. The capacity of the Si-800 sample is almost the theoretical capacity obtained by the assumption that Si is lithiated to Li3.5Si leading to the highest ICE among the samples. The difference in the microstructure of the samples can lead to different achievable reversible capacity when they have similar irreversible reaction. Heat treatment of the SiO samples at high temperature can change their microstructure via different degree of the disproportionated reaction. As the discharge capacity increases, the reversible capacity increases resulting in the increase of the ICE. As a result, ICEs of the samples increase linearly with the increase in the reversible capacity when the irreversible capacity was not changed (Fig. 3b). However, none of the samples achieved the fullylithiated phase (c-Li3.75Si),24-26 which can yield the highest reversible capacity and then the highest ICE of SiO when the irreversible capacity is not changed.

The reversible capacity increases by further lithiation of SiO during the discharge The reversible capacity of SiO can be related to the degree of lithiation in the discharge process. To further increase the reversible capacity, the sample was discharged in constant current (CC) and constant current - constant voltage (CC-CV) modes. Constant voltage (CV) of 10 mV was applied in the SiO-1000 sample to further form lithium-rich silicon-alloy phases with high capacity (Fig. 4). During CC mode, the SiO1000 sample was lithiated to 1958.9 mAhg-1 and delithiated to 1352.1 mAhg-1. However, in CC-CV mode, the sample was lithiated to 2216.3 mAhg-1 and delithiated to 1581.9 mAhg-1 (Fig. 4a). Application of different discharge condition such as CC-CV mode increased the reversible capacity and thereby increased ICE from 69.0 % to 71.4 %. Given that the irreversible capacity in CC-CV mode is quite similar to that in other samples or CC mode (Fig. 4a), only the reversible capacity increases and thereby the ICE can increase. Furthermore, the differential capacity plot with respect to voltage (dQ/dV, Fig. 4b) shows that the increase in the reversible capacity in CC-CV mode is mainly a result of increase in the Li3.5Si (300 mV) reaction (Region II reaction in Fig. 3c) rather than in the Li2.0Si (500 mV) reaction (Region I reaction in Fig. 3c). Thus, further lithiation that is induced by constant 10 mV can increase the reversible capacity, and thereby the ICE of SiO. By using CC and CC-CV experiment, the kinetic of lithiation reaction in SiO can be different; the reversible lithiation reaction in SiO sample can be kinetically limited but the irreversible lithiation reaction can be not. This reversible capacity shortage caused by the kinetic can be one of the factors that can degrade initial coulombic efficiency in SiO.

Figure 4. Comparison of electrochemical properties of the SiO samples at different discharge conditions. (a) Voltage curves and (b) Differential capacity plot with respect to voltage (dQ/dV) of the SiO-1000 sample at constant-current (CC) mode and constant current-constant voltage (CC-CV) mode (constant voltage 10 mV until the current reaches < C/200), (c) VoltageTime curve and its Voltage-Capacity curve (inset) and (d) Differential capacity plot with respect to voltage (dQ/dV) of the unheated SiO sample at constant current and voltage hold at 10 mV for 200 h. Black: unheated SiO sample; red: delithiation reaction of Si to illustrate c-Li3.75Si reaction as a reference peak

Fully-lithiated phase, c-Li3.75Si (450 mV in delithiation process) was not formed in the SiO during further lithiation even with constant voltage at 10 mV, even though this cLi3.75Si phase can form easily in the Si nanoparticles or thin

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file electrode.24-26 Nonetheless, reversible capacity of Si in SiO was increased using constant voltage at 10 mV because of the increase in the reaction of Li3.5Si formation. In CV mode, only Si reactivity in SiO increased; this observation indicates that the Si in SiO is not fully lithiated. Given that Si in SiO is surrounded by SiO2 oxide matrix, the limited lithiation activity may be related to the microstructure of the SiO; i.e., embedded Si in the SiO2 matrix in SiO can affect the lithiation reactivity of Si in SiO. Sufficient reaction time to observe the c-Li3.75Si phase in SiO sample To electrochemically form crystalline lithium silicide (cLi3.75Si) in SiO, the unheated sample was discharged with constant current and constant voltage at 10 mV for 200 h (Fig. 4c, d). Considering that the c-Li3.75Si phase in Si is easily formed at 50 mV during the lithiation,25 we assume that cLi3.75Si phase could be formed electrochemically if the sample were held for sufficiently long time at 10 mV. However, the SiO sample did not show any characteristic voltage peak (450 mV during delithiation) of c-Li3.75Si in the dQ/dV curve (Fig. 4d) even though the sample was discharged with very prolonged 10 mV hold. Also, we performed electrochemical test with the electrode that has 40 wt% of Super P as a conducting agent (Super P) (SiO : Super P : PAA = 50 : 40 : 10) to minimize the effect of the ohmic polarization on the cLi3.75Si reaction. The electrochemical properties (Supporting information, Fig. S4) clearly shows that the c-Li3.75Si reaction is not observed even though the ohmic drop is minimized. Therefore, the absence of c-Li3.75Si phase in the SiO suggests that complete lithiation of Si in SiO is not easily achieved due to thermodynamic factors rather than kinetic factors. The Si in SiO is not fully exploited causing limited reversible capacity. As a result, the limited activity of Si in SiO can decrease its ICE. The stress induced by the lithiation process and its effect on the redox potential To understand the reason why c-Li3.75Si phase does not form in SiO samples even with the prolonged voltage hold, we consider a thermodynamic factor that can affect the formation of c-Li3.75Si in SiO. Previous studies on thin-film Si electrodes demonstrate that the stress induced by the lithiation of Si due to large volume expansion is coupled with its redox potential.28 Based on the Larche-Cahn potential that governs the relationship of the Li chemical potential with stress in the Li-Si solid solution,29 in experimental result a compressive stress of 1 GPa can decrease the redox lithiation potential by 100 ~ 120 mV, whereas that of tensile stress can increase. Considering that Si is embedded in the SiO2 matrix in SiO as we observed in the TEM data (Fig. 1c), the lithiation can induce different volume changes in Si and the matrix, and cause the increase in the stress. The Raman peak position of c-Si and a-Si shifts to high wavenumber when they are under compressive stress, and to low wavenumber when they are under tensile stress (c-Si: ∆ω (𝑐𝑚 ―1) = ―4.4σ (GPa), a-Si: ∆ω (𝑐𝑚 ―1) = ―510 ± 120ϵ and ∆ω: peak shift, σ: stress ϵ: strain).28, 30, 31 Raman measurements were carried out to

understand the difference in the stress in the SiO samples. Raman peak shift of the pristine samples (Fig. 1d and Table 1) was observed and this shift could be ascribed to the changes in the microstructure of the samples. Compared to the unheated SiO sample, the SiO-900 sample showed the highest

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peak shift to high wavenumber; this shift indicates that the Si can be under compressive stress. However, the SiO-800 sample showed peak shift to lower wavenumber than unheated SiO sample; this shift indicates less compressive stress than the SiO-900 sample. Considering the effect of induced stress on the redox potential in Fig. 5a, the lithiation potential of Si in the SiO-900 sample may be further lowered due to strong compressive stress compared to the SiO-800 sample. If the reaction potential falls below the cut-off voltage, high-capacity lithiated phases such as Li3.5Si or cLi3.75Si cannot form in the SiO-900 sample. In an electrochemical cell test of SiO, a cut-off voltage of 10 mV is usually used to avoid plating of Li metal. In contrast, relatively lower compressive stress in the SiO-800 sample than in the SiO-900 sample can enable to form high-capacity lithiated phases such as Li3.5Si, not c-Li3.75Si (Fig. 3c). Given that the Raman peak position of c-Si and a-Si is also affected by the particle size via the size-dependent Raman shift, which the smaller the particle size is the higher the Raman shift is, the Raman shift should increase as the heating temperature increases because the crystallite size of Si increases with the temperature.32 However, the SiO samples show different trend from the prediction of the size-dependent Raman shift. This suggests that the effect of stress can be dominant in the SiO samples. Therefore, the Raman peak shift in the samples could be ascribed to the stress applied to Si that can be induced by different microstructures. Table 1.

Raman peak shift of the samples Raman shift (cm-1)

Sample

Amorphous Si (a-Si)

Crystal Si (c-Si)

SiO

463.2

-

SiO-800

462.1

-

SiO-900

477.9

513.5

SiO-1000

476.1

508.3

Thus, stress induced by the microstructures of SiO can affect the electrochemical activity of Si embedded in oxide matrix, and thereby leads to different lithiation behaviors that can strongly affect the reversible capacity of SiO. In our experimental results, c-Li3.75Si phase did not form in any of the SiO samples. Similar results have been observed previously.6, 18, 23 Strong compressive stress that can be caused by the microstructure in a pristine material can have an negative influence on the electrochemical activity of Si and then can decrease reversible capacity and ICE. This is further supported by Supporting information Fig. S3 and Fig S5. Furthermore, in a SiO material, the SiO2 that constitutes the matrix reacts irreversibly to form lithium silicates during the 1st lithiation, prior to the lithiation of Si.6, 33 The Si surrounded by the lithiated oxide matrix could experience muchincreased compressive stress due to the volume expansion of the matrix.6 As lithiation of Si proceeds, LixSi can be formed starting from the surface; this reaction causes further volume expansion on the surface, so the compressive stress on unreacted Si in the core can increase due to the volume expansion of LixSi on the surface. This strong compressive stress can lower the thermodynamic redox potential of Si far below the cut-off voltage (Fig. 5b). As a result, the formation of c-Li3.75Si phase can be suppressed due as a result of the

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Chemistry of Materials

decrease in its redox potential resulting in the limited reversible capacity. To increase the reversible capacity of SiO by formation of c-Li3.75Si, the stress that can be induced by matrix or by the lithiation process should be released. The microstructure in SiO can help to mitigate the large volume change and thereby

increase capacity retention, but the induced stress can cause limited reversible capacity and then low ICE. Therefore, increase in ICE and capacity retention requires a controlled microstructure that has a buffer matrix for volume expansion and simultaneously decreases induced stress on Si.

Figure 5. Schematic diagrams of the relationship between stress induced by the microstructure and redox potential of Si in the SiO. (a) The relationship between Raman shift, stress, and Si redox potential. (b) Schematic diagram of the evolution of stress in Si in SiO during the lithiaton

4. Conclusion We tried to understand the origin of the limited reversible capacity of SiO with respect to its microstructure and its effect on the ICE during the 1st cycle by using several SiO samples that had different microstructures. The samples had different reversible capacity of Si in SiO but achieved similar irreversible capacity even though they have different microstructures. As a result, the ICEs of the sample linearly increases according to the reversible capacity. The further increase in the reversible capacity can increase ICE of SiO. We found out that the reversible capacity in SiO is related to the formation of high-capacity lithiated silicon phases such as Li3.5Si and c-Li3.75Si in 1st discharge process. Even though the prolonged voltage hold in discharge process was applied to SiO electrode or the electrode with high amount of a conducting agent, fully-lithiated c-Li3.75Si did not form at room temperature indicating limited reversible capacity of SiO. The microstructure of SiO, in which Si is embedded in the SiO2 matrix as observed in TEM image, can cause the limited reversible capacity because it can affect the degree of lithiation in 1st discharge. The Si in SiO can be in the state of strong compressive stress induced by this microstructure of SiO and this stress can get increased as the lithiation progresses. As a result, the redox potential of the lithiation for c-Li3.75Si can be lowered below the cut-off voltage resulting in the suppressed formation of c-Li3.75Si in SiO. This suppression can limit the reversible capacity of SiO, and yield low ICE. We suggest that control of the microstructure in SiO can increase reversible capacity by releasing induced stress

and can improve the initial coulombic efficiency of the SiO electrode.

ASSOCIATED CONTENT Supporting Information. TEM images of the SiO samples and its electrochemical reactions based on different discharged oxide products.

AUTHOR INFORMATION Corresponding Author * B. Kang, E-mail address: [email protected]

Author Contributions ‡G. Choi and J. Kim contributed equally to this work

ACKNOWLEDGMENT This work was supported by the Fundamental R&D Program for Technology of World Premier Materials (WPM) funded by the Ministry of Knowledge Economy (grant no. 10037918). This work was also partially supported by Brain Korea 21 PLUS Project for Center for Creative Industrial Materials (F14SN02D1707) and POSTECH Basic Science Research Institute Grant. This research was also partially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2017R1A4A1015811)

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