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Parasitic Reactions in Nano-sized Silicon Anodes for Lithium-ion Batteries Han Gao, Lisong Xiao, Ingo Plümel, Gui-Liang Xu, Yang Ren, Xiaobing Zuo, Yuzi Liu, Christof Schulz, Hartmut Wiggers, Khalil Amine, and Zonghai Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04551 • Publication Date (Web): 08 Feb 2017 Downloaded from http://pubs.acs.org on February 9, 2017

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Parasitic Reactions in Nano-sized Silicon Anodes for Lithium-ion Batteries

Han Gao1, Lisong Xiao2, Ingo Plümel2, Gui-Liang Xu1, Yang Ren3, Xiaobing Zuo3, Yuzi, Liu4, Christof Schulz2, 5, Hartmut Wiggers2, 5, Khalil Amine1* and Zonghai Chen1* 1

Chemical Science and Engineering Division, Argonne National Laboratory, Lemont, IL, USA, 60439 2

Institute for Combustion and Gas Dynamics–Reactive Fluids (IVG), University of DuisburgEssen, Duisburg, Germany, 47057

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X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL, USA, 60439 4

Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL, USA, 60439 5

CENIDE, Center for Nanointegration Duisburg-Essen, Duisburg, Germany, 47057

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K. Amine: [email protected]

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Z. Chen: [email protected]

ACS Paragon Plus Environment

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Abstract When designing nano-Si electrodes for lithium-ion batteries, the detrimental effect of the cLi15Si4 phase formed upon full lithiation is often a concern. In this study, Si nanoparticles with controlled particle sizes and morphology were synthesized and parasitic reactions of the metastable c-Li15Si4 phase with the non-aqueous electrolyte was investigated. The use of smaller Si nanoparticles (~ 60 nm) and the addition of fluoroethylene carbonate additive played decisive roles in the parasitic reactions such that the c-Li15Si4 phase could disappear at the end of lithiation. This suppression of c-Li15Si4 improved cycle life of the nano-Si electrodes but with a little loss of specific capacity. Also the characteristic c-Li15Si4 peak in the dQ/dV plots can be used as an early-stage indicator to cell capacity fade during cycling. Our findings can contribute to the design guidelines of Si electrodes and allow us to quantify another factor to the performance of the Si electrodes. Key words: Lithium-ion battery; silicon nanoparticles; parasitic reaction; leakage current; interfacial reaction


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Table of Content

Parasitic Reactions 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 -4.5 -5.0

Suppression of c-Li15Si4

Si (130 nm) Si (60 nm)

c-Li15Si4 peak

80 60

-1

100

Peak intensity (mAhV )

120 Differential capacity (mAhV-1)

Static leakage current (µAmgSi-1)

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150 100 50 0 0

2

4

6

8 10

Cycle number

40

Si (130 nm) Si (60 nm)

20 0

0.0

0.2

0.4

0.6 +

Potential vs Li/Li (V)

0.8

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Potential vs Li/Li+ (V)


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Introduction Research on rechargeable lithium-ion batteries over the last few decades has aimed to provide high-performance energy storage solutions for the fast growing areas of consumer and industrial applications.1-4 New material developments and engineering optimizations are necessary to reach higher energy density, longer cycle life, and better rate capability for the next generation lithium-ion batteries. Nano-sized silicon (nano-Si) is one of the most promising anode materials with a high theoretical capacity of 3580 mAhg-1, which corresponds to a lithiated state of Li3.75Si.5, 6 This is almost 10 times larger than the specific capacity of commercial graphitic anode materials (i.e. 372 mAhg-1 for LiC6). Unlike bulk silicon, nano-Si has a lower probability of cracking and pulverization during lithiation and delithiation even with a volume change of nearly 300%.7-10 Numerous nano-Si materials, including 1D Si nanowires/nanofibers,11-13 2D Si nanosheets/nanowalls,14, 15 and 3D Si nanospheres/nanoparticles,16, 17 have been investigated and characterized. A strong dependence between the size of nano-Si and its cracking have been determined. For instance, a threshold size of 150 nm has been reported for Si nanoparticles (Si NPs).7 The movement of the two-phase boundary between the inner core of pristine Si and the outer shell of a-LixSi during lithiation causes the crack propagation in large-sized Si NPs.7 Although the cracking issue can be mitigated by using nano-Si smaller than the threshold size, current bare Si anodes can still suffer from quick capacity fade due to the high internal stresses associated with the formation of crystalline Li15Si4 (c-Li15Si4) phase upon full lithiation.18, 19 It is well known that the Li-Si phase diagram contains many intermediate compounds, including Li12Si7, Li7Si3, Li13Si4, and Li22Si5.20 However, these stable compounds are not formed during the electrochemical insertion of Li into Si.21 Instead, amorphous LixSi (a-LixSi) is formed


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and when the lithium concentration in a-LixSi reaches a critical value of x = 3.75, the a-Li3.75Si undergoes a transformation to c-Li15Si4,21-23 which is not present in the equilibrium Li-Si phase diagram. This transformation to the metastable c-Li15Si4 has been confirmed by in-situ X-ray diffraction (XRD),21, 23 in-situ transmission electron microscopy (TEM),24, 25 in-situ nuclear magnetic resonance (NMR) spectroscopy,26, 27 and density functional theory (DFT) calculations.28 In general, this transformation occurs at potentials between 30 mV and 60 mV vs. Li/Li+. Although the detailed atomistic mechanism of the phase transformation to c-Li15Si4 is not fully understood, the amorphous phase and crystalline phase regions of lithium silicide during evolution and growth of c-Li15Si4 have been associated with high internal stresses.22 This can lead to particle cracking, poor electrical contact, and capacity fading. Recent studies have demonstrated that an external stress in Si thin film electrodes induced from a substrate can successfully suppress the formation of c-Li15Si4 at potentials lower than 50 mV.19 Similar phenomena have been observed in the case of amorphous Si alloys, where the external stress induced by an inactive matrix phase during the expansion and contraction of the active Si phase can suppress the formation of c-Li15Si4.29 However, the qualitative relationship between the formation of c-Li15Si4, the particle size of Si, and electrolyte additives has not been fully established yet. In this study, Si NPs with different particle size were synthesized in the gas phase by decomposition of monosilane in a hot-wall reactor.30 The particle sizes were controlled to be lower than the threshold size of 150 nm to minimize the influence of particle cracking. We used these Si NPs without any functionalizations or coatings to better isolate the effect of the particle size from other factors. Fluoroethylene carbonate (FEC), a common fluorinated electrolyte


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additive for Si-based anodes, was used in this study to understand its influence on the suppression of the c-Li15Si4. Hence, the objective is to investigate the relationship between the formation of c-Li15Si4 in respect of Si particle size and FEC additive upon full lithiation. Our findings can serve as design guidelines to achieve high-performance Si NP anodes for lithiumion batteries. Experimental Section Synthesis of Si NPs Si NPs were synthesized in N2 using a tubular hot-wall reactor (HWR, inner diameter 14 cm) and pure monosilane (SiH4, quality EHP, Air Liquide) as precursor. The experimental procedure was described in our previous work.31, 32 In brief, SiH4 gas was fed into the reactiontube through a concentric nozzle which is mounted at the top of the HWR. A coaxial H2 gas flow (quality 5.0, Air Liquide) was served as a sheath gas to prevent the decomposition of SiH4 on the reactor-wall. SiH4 starts to decompose (ca. 400°C) when passing the heating zone of the reactor, leading to the formation of solid Si NPs and H2 gas. These solid Si NPs were then collected and stored under N2. The crystallinity and the particle size of the Si nanoparticles are mainly determined by the reaction temperature and the residence time, which to some extend can be adjusted by the pressure. Generally, the lower the reaction temperature and pressure, the lower the crystallinity and the smaller the particle size. For the synthesis of the 3 samples used in this study, the main heating element and the pressure were set up as 800°C and 400 mbar for Si-A, 700°C and 400 mbar for Si-B, and 650°C and 300 mbar for Si-C, respectively. The production rate of Si nanoparticles by this method is around 0.5 to 1.0 kgh-1. Structure Characterizations


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Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images of the Si NPs were obtained from a JEOL JSM-7500 field emission scanning electron microscope and a JEOL JEM-2100F field emission electron microscope, respectively. The specific surface areas of Si nanoparticles were obtained by Brunauer-Emmett-Teller (BET) measurements, which were carried out at 77 K with a Quantachrome NOVA2200 analyzer. Small angle X-ray scattering (SAXS) and high energy Xray diffraction (HEXRD) experiments were carried out at the beamlines 12-ID-B and 11-ID-C of the Advanced Photon Source (APS) of Argonne National Laboratory, respectively, at room temperature. The 2D SAXS images, collected with a Pilatus 2M detectors (DECTRIS), were radially averaged to 1D profiles using computer programs at the beamline and presented as X-ray scattering intensities vs. momentum transfer (q=4πsin(theta)/λ), where theta is the Bragg angle and λ is the wavelength of the X-ray (0.886 Å in this experiment). The q range covers from 0.003 to 1.0 Å-1. The wavelength of X-ray used for HRXRD was pre-set to 0.11742 Å. A 2D Xray detector was used to collect the XRD profiles, which were then integrated into conventional 1D data (intensity vs. 2-theta) with a CeO2 calibration standard. Patterns of pair distribution function (PDF) were obtained with the GSAS-II software. X-ray photoelectron spectroscopy (XPS, VersaProbe II, Ulvac-Phi) with both monochromatic Al Kα light at 1486 eV and Mg Kα light at 1253 eV with an emission angle of 45° was used for surface characterizations of the Si NPs. Electrochemical Characterizations Si NP working electrodes were prepared by laminating a slurry onto copper foil substrates using a doctor blade. The slurry was composed of 60 wt.% active material (Si NPs), 20 wt.% conductive agent (C45 carbon black), and 20 wt.% lithiated polyacrylic acid (Li-PAA)


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binder. After laminating, the electrodes were first dried at 70 ºC in air overnight and subsequently punched (15 mm in diameter). All punched electrodes were vacuum-dried at ca. 130 ºC overnight prior to cell assembly. The Si mass loading on each electrode was ca. 0.8 to 0.9 mg. Discharge-charge tests were conducted on CR2032-type half-cells with a lithium metal reference/counter electrode and a Si NP working electrode on a MACCOR series 4000 battery tester at room temperature (ca. 25 ºC). The electrolytes used were 1.2 M LiPF6 in a solvent mixture of ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7 by weight) with and without 10 wt.% FEC. Celgard 2325 was used as the separator. A home-build high-precision leakage current measuring system (based on Keithley 2401 source meters) was used to investigate the reactions between the Si NP working electrodes and the electrolytes after formation. The working electrodes were held at each specific potential for 20 hours to reach an equilibrium state at 30 ºC. Results and Discussions Structure properties (e.g. morphology, particle size, initial crystallinity, and etc.) of the three Si NP samples were first examined. The microstructures of the three samples were characterized by SEM (Figures 1a to c). They all have an aggregated morphology consisting of roughly spherical primary particles with a narrow distribution of the primary particle size. They show diameters of about 130 nm, 90 nm, and 60 nm, respectively for Si-A, Si-B, and Si-C. SAXS was also employed as an indirect method to confirm the primary particle size of these Si NPs. It can provide a more reliable particle size comparison from the statistical point of view. Figure 1d shows the measured SAXS scattering intensity for all three Si NPs. The particle size of


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the Si NPs increased in the order of Si-CSi-B>Si-C in the electrolyte with FEC additive. The reason causing this variation in capacity of the three Si NPs will be discussed later in more detail. In addition, the position of potential plateau of initial lithiation (around 0.15 V) decreased in the order of Si-C>Si-B>Si-A. This was resulted from the difference in the initial crystallinity


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of the Si NPs. The lower potential plateau of Si-A was due to the larger activation energy associated with the Si-Si bond breakage in the crystalline region. On the other hand, the amorphous sample (Si-C) was more open than Si-A and Si-B such that Li could penetrate the Si matrix more readily. This gives a complete lithiation at a slightly higher potential than the other two Si NP samples. Upon initial delithiation, a potential plateau could be observed for Si-A and Si-B at ca. 0.45 V vs. Li/Li+, which indicates the formation of c-Li15Si4 phase at the end of their initial lithiation. Si-C, on the other hand, did not show any sign of c-Li15Si4 formation. Figure 2b shows the same potential profiles of the Si NP/Li half cells but in a FEC-free electrolyte. A similar trend in the specific capacity values was observed: Si-A exhibited the highest specific capacity (~3170 mAhg-1) while Si-C had the lowest (~2950 mAhg-1). Si-A also exhibited the lowest potential plateau of initial lithiation while Si-C displayed the highest potential plateau of initial lithiation. The evidence of the c-Li15Si4 phase formation in Si-A and Si-B could also be observed in Figure 2b, as revealed by the potential plateau of initial delithiation. Yet again, cLi15Si4 was not formed in Si-C in the first cycle. Since the three Si NPs had different initial crystallinity, the potential profiles of the Si NP/Li half cells after 2nd cycle were compared in Figures 2c and d to understand the outcome of the variation in the initial crystallinity. The same trend in the capacity difference between the three Si NPs was observed in Figures 2c and d. Due to the additional reductive decomposition of FEC, the 1st cycle coulombic efficiency (CE) decreased with the addition of FEC (see Figure S3). However this difference in CE became much less for the 2nd cycle. Among the three Si NPs, the smaller-sized Si-C showed lower CE because of its smaller particle size and more electrochemically active surface area. Therefore more reductive reactions can occur during the lithiation process compared to the cells using larger-sized Si NPs.


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It has been demonstrated in the literature that crystalline Si is converted into amorphous Si after the first lithiation-delithiation cycle.21, 27 In our case, all three Si NP samples in the 2nd discharge-charge cycle exhibited overlapping lithiation curves with two plateaus: (i) a highpotential lithiation plateau at ca. 0.25 V; and (ii) a low-potential lithiation plateau at ca. 0.1 V. These correspond to the diffusion of Li into regions which have primarily Si neighbors and diffusion of Li into regions which have primarily Li neighbors, respectively.33 The overlapping lithiation curves in the 2nd cycle suggest that all three Si NPs were dominated by amorphous materials after the 1st discharge-charge cycle. Upon the 2nd delithiation in Figures 2c and d, both Si-A and Si-B showed the potential plateau at ca. 0.45 V vs. Li/Li+, which was the same potential plateau observed during the 1st delithiation process (Figures 2a and b). This suggests a small amount of c-Li15Si4 phase still formed at the end of 2nd lithiation process for these two Si NPs. Oppositely Si-C did not show any significant formation of the c-Li15Si4 phase even after the 2nd cycle. To further confirm the amorphous nature of all three Si NPs after the 1st cycle, both the pristine electrodes and cycled electrodes were characterized with HEXRD. Figure 3 shows the ex-situ XRD patterns of as-prepared Si NP electrodes (Figure 3a) as well as the electrodes after 1st discharge-charge cycle at 1.5 V vs. Li/Li+ (Figure 3b) in the electrolyte with FEC additive. In Figure 3a, the c-Si peak intensities decreases in the order of electrodes made of Si-A, Si-B, and Si-C. However after one discharge-charge cycle, no c-Si peaks can be identified (Figure 3b). This supports our results from Figure 2 that all Si NPs turned into amorphous-dominating materials after the initial lithiation and delithiation, thus the initial crystallinity of the Si NPs may not play a critical role in the suppression of c-Li15Si4 after the first cycle.


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In order to investigate the suppression of c-Li15Si4 in Si-C as well as to differentiate the effect of the FEC additive, the Si NP/Li half cells with and without FEC were cycled from 1.5 to 0 V vs. Li/Li+ at 0.1 C for 10 cycles (Figures S4 to S6). The resulting differential capacity (dQ/dV) plots for the delithiation potentials are shown in Figure 4. Two different peaks could be identified in the dQ/dV plots, they were assigned to the delithiation of amorphous LixSi (at a lower potential) and the delithiation of c-Li15Si4 (at a higher potential). We focused on the second peak at 0.45 V vs. Li/Li+ because it is the characteristic peak corresponding to the formation of cLi15Si4.23, 34, 35 Figures 4a to c and Figures 4d to e show the peak evolution for the three Si NPs in the electrolytes with and without FEC, respectively. Also shown in the insets are the measured characteristic peak intensities of c-Li15Si4 as a function of cycle number. Since the mass loadings of Si were almost identical, the measured peak intensity can be directly used as an indicator for the extent of the formation of c-Li15Si4. Generally, the peak intensity at 0.45 V vs. Li/Li+ slowly increased with the cycle number, indicating a higher amount of c-Li15Si4 phase being formed. Comparing the electrolyte with FEC to the one without, the FEC had a positive effect by greatly reducing the formation of c-Li15Si4 (Figures 4a to c). Among the three Si NPs, Si-C showed the least formation of c-Li15Si4 (Figures 4c and f) while Si-A showed the greatest (Figures 4a and d). This trend is in a good agreement with the results observed in Figure 2. Together with the FEC additive, Si-C demonstrated very stable behavior with virtually no increase in the c-Li15Si4 peak intensity (Figure 4c), indicating a successful suppression of the formation of the c-Li15Si4 phase. In fact, Si-C showed stable capacity with no sign of c-Li15Si4 after 20 discharge-charge cycles at 0.1 C (Figure S7). To our knowledge, this phenomenon has not been reported before.


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To further confirm this observation, HRTEM analysis was performed on the lithiated SiA without FEC additive and lithiated Si-C with FEC additive. These two samples were chosen because they represent two extreme conditions for the formation of c-Li15Si4 (see Figures 4c and 4d). Both samples were discharged to 0 V vs. Li/Li+ before taking the TEM images. As shown in Figure 5a, a significant amount of polycrystalline Li15Si4 was formed at the end of lithiation in the lithiated Si-A without FEC, showing the crystallization of a-Li3.75Si to c-Li15Si4. In contrast, the lithiated Si-C with FEC (Figure 5b) was amorphous without any formation of c-Li15Si4. Figures 5c and 5d are electron diffraction patterns confirming the presence of c-Li15Si4 in Si-A (Figure 5a) and the absence of c-Li15Si4 in Si-C (Figure 5b). Since electron diffraction patterns are obtained at localized areas, fast Fourier transform (FFT) images (see insets of Figures 5a and 5b) are also included represent a more statistically meaningful observation. Indeed, c-Li15Si4 is formed in Si-A (without FEC) samples but not in Si-C (with FEC) samples. In order to understand the observed c-Li15Si4 and its effect on cycling, the cycling performances of the Si NP electrodes are shown in Figure 6. In both the FEC-containing and the FEC-free electrolytes, all cells showed similar specific capacity at 0.1 C (first 10 cycles). Although decreases in the specific capacity values were observed by increasing the C-rate to 0.5, all three Si NPs still showed very similar capacity at this C-rate. In fact, these Si NP electrodes showed very similar rate performance in the FEC-containing electrolyte (Figure S8a) but serious capacity loss was observed in the FEC-free electrolyte (Figure S8b). The results shown in Figures 6 and S8 clearly indicate that: (i) Si-C had much better cycling performance than the other two Si NPs; and (ii) the use of FEC additive improved the capacity retention. The trend in capacity fade observed here was well correlated with the formation of c-Li15Si4. Even though no c-Li15Si4 can be formed during higher rate cycling at 0.5 C due to cell polarization (Figure S9),


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we believe the worse cyclability of Si-A and Si-B were still caused by the high internal stresses associated with the formation of c-Li15Si4 phase during the first 10 cycles at 0.1 C. In Figure 6b, although all three cells showed rapid reduction in capacity over time, still Si-C demonstrated a superior performance than the other two Si NPs. Both results from Figure 4 and Figure 6 suggest the formation of c-Li15Si4 has a detrimental effect on the cyclability of the Si NP cells and that the c-Li15Si4 peak in the dQ/dV plots can be used as an early-stage indicator to the capacity fade during cycling. It is reported that c-Li15Si4 formation can be suppressed by stress-potential coupling during cycling of Si thin films with the presence of substrates.36, 37 This include compressive stress from its attachment during lithiation (i.e. expansion) and tensile stress from its attachment during delithiation (i.e. contraction). However this cannot explain our observations in the composite electrodes. Even though the adhesion strength and stress at break of the stiffer PAA family are superior to other commonly used binders,38 such as carboxymethylcellulose (CMC) and poly(vinylidene fluoride) (PVDF), the stress values induced from Li-PAA during the expansion and contraction of the Si phase are too small to suppress the formation of c-Li15Si4 by stress-potential coupling. Therefore, in our case, the full lithiation of Si NPs (i.e. discharge down to 0 V vs. Li/Li+) always leads to a-Li3.75Si (as seen by the consistent discharge capacity values shown in Figure 2), which should transform to c-Li15Si4. Here we suspect the metastable cLi15Si4 phase formed in our Si NPs is rather reactive toward the alkyl carbonate based electrolytes such that it could disappear during a parasitic reaction, especially in the case of Si-C with the FEC additive. An earlier in-situ NMR study by Key et al. has suggested a spontaneous reaction would occur between the lithium silicide with the non-aqueous electrolyte.27 Another way to study the


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side reaction between lithium silicide and the non-aqueous electrolyte is to monitor the static leakage current at different potentials. These static leakage current values can be used as a quantitative indicator of the rate of the parasitic reactions. By using our home-built high precision leakage current measurement system, the static leakage current measured on half cells are solely contributed from reactions occurring on the working electrode (i.e. Si NP electrodes in this case).39 Figure 7 shows the static leakage current measured as a function of the potential. The cells were held at each potential for 20 hours to reach an equilibrium state. The inset of Figure 7a shows a typical current relaxation curve with a non-linear curve fit in order to extract the static leakage current. In Figure 7, the absolute values of the leakage current increased with decreasing potential in both FEC-containing and FEC-free electrolytes, indicating an increase in the rate of the parasitic reactions. Since the static leakage current would follow the Tafel relationship with lowering potential if the parasitic reactions were electrochemical processes in nature, the peak parasitic reactions observed at potentials of 0.25 and 0.1 V vs. Li/Li+ suggest a change in the reaction mode from electrochemical to chemical processes at these two potentials. These two potentials correspond to the two potential plateaus in the discharge-charge curves during lithiation (Figures 2c and d). It is speculated here that the new reaction mode could be related to the formation of a more negative oxidation-state Si on the particle surface and generation of new species on the electrode surface during the diffusion of Li. More importantly, a significant difference between the three Si NPs can be differentiated by further reducing the potential to 0.05 V vs. Li/Li+. At this potential, Si-C showed much higher static leakage current than the other two Si NPs in both FEC-containing and FEC-free electrolytes, suggesting faster parasitic reactions. Since c-Li15Si4 phase is already formed at this


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low potential, this observation can serve as an evidence of our suggestion such that more cLi15Si4 reacted with the electrolyte in the case of Si-C (e.g. Li3.75Si → Li3.75-δSi + δLi+). In both electrolyte systems, Si-C exhibited higher leakage current at 0.05 V due to the smaller particle size of Si-C and its larger surface area (Table S1), providing more active electrochemical surface area of the composite electrode. Unlike in the case of Si-C, where the interfacial reaction continuously consume c-Li15Si4, the reactions between the electrolyte and the c-Li15Si4 formed in Si-A and Si-B were slower. Thus, more c-Li15Si4 were present in those two electrodes as shown in Figure 4. Also revealed by Figure 7 is the reason behind the difference in the specific capacity of the three Si-NPs. The parasitic reactions between the lithium silicide with the non-aqueous electrolyte would remove Li, leading to a reduction in capacity. Thus the faster parasitic reactions in the case of Si-C had lower specific capacity. This indeed agrees with the trend obtained from Figure 2. A rough estimation based on the difference in the lithiation capacity of Si-A and Si-C (~200 mAhg-1) gave a 0.2 Li removal per Li3.75Si, which suggests a lithiated state of Li3.55Si for Si-C at the end of discharge. Comparing the electrolyte with FEC (Figure 7a) and the one without FEC (Figure 7b), SiC with FEC demonstrated slightly higher static leakage current at 0.05 V than the one without FEC additive. We attribute this to the differences in the solid electrolyte interphase (SEI). In general, the SEI formed by the FEC-free electrolyte is primarily composed of lithium oxides and lithium alkoxides while the SEI formed by the FEC-containing electrolyte consists of lithium fluoride and polyene-compounds.40-43 Due to the more stable nature of the SEI formed in the FEC-containing electrolyte, the passivation film is thinner and more compact. In contrast, the SEI formed by the FEC-free electrolyte is much thicker, resulting in higher impedance (Figure


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S10). This can obstruct the reaction between the c-Li15Si4 and the electrolyte, leading to lower leakage current. It can also be observed in Figure 7 that the parasitic reactions occur at a slower rate at 0.2 V vs. Li/Li+. This implies the cycling performance of the Si NPs should be improved at this cutoff potential. Although the specific capacity of the Si NPs will not be fully extracted, Si NP/Li cells were still cycled from 1.5 to 0.2 V vs. Li/Li+ as a proof of concept. Figure 8 shows the cycling performance of the three Si NPs with the 0.2 V cutoff potential in both FEC-containing and FEC-free electrolytes. The first 11 cycles were obtained under a C-rate of 0.1 while the rest were cycled at 0.5 C. At 0.1 C, the capacity values were ca. 1500 mAhg-1, which implies a Si lithiated state of Li1.6Si (with a theoretical capacity of 1527 mAhg-1). Similar to the results obtained in Figure 6, all cells showed a decrease in capacity with the increase in current density. However they exhibited a much improved cyclability with only a slightly capacity loss when comparing the values at 20th cycle and 200th cycle (Figure 8a). All three Si NPs showed similar capacity retention, which was opposite to the trend observed in Figure 6 (where the cutoff potential = 0 V). This is because no c-Li15Si4 was formed in all samples and they all exhibited similar static leakage current values (see Figure 7). Assuming the lithiated state of Li1.6Si at 0.2 V, this still gives a theoretical volume expansion of ca. 120%. Therefore, the addition of FEC became critical to form stable SEIs as demonstrated by the much worse cycling performance of the cells in Figure 8b compared to the ones shown in Figure 8a. Rapid capacity fade occurred owning to the non-stable SEI, which continuously consume Li even though they showed the similar low static leakage current as in the FEC-containing electrolyte (Figure 7).


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Conclusion When designing nano-Si electrodes for lithium-ion batteries, the formation of the cLi15Si4 phase upon full lithiation, resulting in undesired capacity fade, is often a concern. In this study, we have demonstrated that the formation of the c-Li15Si4 has a detrimental effect on the cyclability of the Si NP cells and the c-Li15Si4 peak in the dQ/dV plots can be used as an earlystage indicator to the capacity fade during cycling. In addition, the metastable c-Li15Si4 phase formed in our Si NPs is rather reactive toward the non-aqueous electrolytes such that it could disappear during a parasitic reaction. Although a small amount of capacity was lost due to this reaction, the suppression of the c-Li15Si4 formation improved cycling performance of the cells. The Si NPs with smaller particle sizes as well as the addition of FEC played critical roles in the parasitic reaction. Among the three Si NPs, the smaller-sized Si-C exhibited the fastest parasitic reaction at 0.05 V vs. Li/Li+. Therefore even the c-Li15Si4 is formed during the lithiation of Si-C, it disappeared at the end of the discharge. This finding allows us to quantify another factor to the performance degradation of the Si electrode and provides a baseline of designing highperformance nano-Si electrodes. Acknowledgements Research at the Argonne National Laboratory was funded by U.S. Department of Energy (DOE), Vehicle Technologies Office. Support from Tien Duong of the U.S. DOE’s Office of Vehicle Technologies Program is gratefully acknowledged. Use of the resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC0206CH11357. The authors thank Prof. Dr. M. Winterer from University of Duisburg-Essen (UDE)


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for the access to the SEM and Dr. U. Hagemann from Interdisciplinary Center for Analytics on the Nanoscale (ICAN), UDE for the XPS characterization and the fruitful discussion on the measurement results. L. Xiao acknowledges the financial support of "Programm zur Förderung des exzellenten wissenschaftlichen Nachwuchses" from UDE. H. Gao would also like to acknowledge the NSERC Canada Postdoctoral Fellowships Program. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org: enlarged plots of PDF peaks of the Si nanoparticles, XPS spectra of the three Si NPs, 1st and 2nd cycle coulombic efficiency of Si/Li half cells, discharge-charge profiles of the Si/Li half cells, rate performance of the three Si NP electrodes, evolution of differential capacity plot of the Si/Li half cells, Nyquist plots of Si/Li half cells, and BET surface area of the three Si NPs. References 1. Tarascon, J. M.; Armand, M. Nature 2001, 414, 359-367. 2. Mukherjee, R.; Krishnan, R.; Lu, T.-M.; Koratkar, N. Nano Energy 2012, 1, 518-533. 3. Evarts, E. C. Nature 2015, 526, S93-S95. 4. Scrosati, B. Electrochim. Acta 2000, 45, 2461-2466. 5. Hassoun, J.; Scrosati, B. J. Electrochem. Soc. 2015, 162, A2582-A2588. 6. Luo, F.; Liu, B.; Zheng, J.; Chu, G.; Zhong, K.; Li, H.; Huang, X.; Chen, L. J. Electrochem. Soc. 2015, 162, A2509-A2528. 7. Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y. ACS Nano 2012, 6, 1522-1531. 8. McDowell, M. T.; Ryu, I.; Lee, S. W.; Wang, C.; Nix, W. D.; Cui, Y. Adv. Mater. 2012, 24, 6034-6041. 9. Ryu, I.; Choi, J. W.; Cui, Y.; Nix, W. D. J. Mech. Phys. Solids 2011, 59, 1717-1730. 10. Li, J.; Dozier, A. K.; Li, Y.; Yang, F.; Cheng, Y.-T. J. Electrochem. Soc. 2011, 158, A689-A694. 11. Cui, L.-F.; Ruffo, R.; Chan, C. K.; Peng, H.; Cui, Y. Nano Lett. 2009, 9, 491-495. 12. Zhou, G. W.; Li, H.; Sun, H. P.; Yu, D. P.; Wang, Y. Q.; Huang, X. J.; Chen, L. Q.; Zhang, Z. Appl. Phys. Lett. 1999, 75, 2447-2449.


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13. McDowell, M. T.; Lee, S. W.; Ryu, I.; Wu, H.; Nix, W. D.; Choi, J. W.; Cui, Y. Nano Lett. 2011, 11, 4018-4025. 14. Kim, W.-S.; Hwa, Y.; Shin, J.-H.; Yang, M.; Sohn, H.-J.; Hong, S.-H. Nanoscale 2014, 6, 4297-4302. 15. Wan, J.; Kaplan, A. F.; Zheng, J.; Han, X.; Chen, Y.; Weadock, N. J.; Faenza, N.; Lacey, S.; Li, T.; Guo, J.; Hu, L. J. Mater. Chem. A 2014, 2, 6051-6057. 16. Li, H.; Huang, X.; Chen, L.; Wu, Z.; Liang, Y. Electrochem. Solid-State Lett. 1999, 2, 547-549. 17. Su, X.; Wu, Q.; Li, J.; Xiao, X.; Lott, A.; Lu, W.; Sheldon, B. W.; Wu, J. Adv. Energy Mater. 2014, 4, 1300882. 18. Rhodes, K.; Dudney, N.; Lara-Curzio, E.; Daniel, C. J. Electrochem. Soc. 2010, 157, A1354-A1360. 19. Iaboni, D. S. M.; Obrovac, M. N. J. Electrochem. Soc. 2016, 163, A255-A261. 20. van der Marel, C.; Vinke, G. J. B.; van der Lugt, W. Solid State Commun. 1985, 54, 917919. 21. Li, J.; Dahn, J. R. J. Electrochem. Soc. 2007, 154, A156-A161. 22. Obrovac, M. N.; Christensen, L. Electrochem. Solid-State Lett. 2004, 7, A93-A96. 23. Hatchard, T. D.; Dahn, J. R. J. Electrochem. Soc. 2004, 151, A838-A842. 24. Liu, X. H.; Zhang, L. Q.; Zhong, L.; Liu, Y.; Zheng, H.; Wang, J. W.; Cho, J.-H.; Dayeh, S. A.; Picraux, S. T.; Sullivan, J. P.; Mao, S. X.; Ye, Z. Z.; Huang, J. Y. Nano Lett. 2011, 11, 2251-2258. 25. Liu, X. H.; Zheng, H.; Zhong, L.; Huang, S.; Karki, K.; Zhang, L. Q.; Liu, Y.; Kushima, A.; Liang, W. T.; Wang, J. W.; Cho, J.-H.; Epstein, E.; Dayeh, S. A.; Picraux, S. T.; Zhu, T.; Li, J.; Sullivan, J. P.; Cumings, J.; Wang, C.; Mao, S. X.; Ye, Z. Z.; Zhang, S.; Huang, J. Y. Nano Lett. 2011, 11, 3312-3318. 26. Key, B.; Bhattacharyya, R.; Morcrette, M.; Seznéc, V.; Tarascon, J.-M.; Grey, C. P. J. Am. Chem. Soc. 2009, 131, 9239-9249. 27. Key, B.; Morcrette, M.; Tarascon, J.-M.; Grey, C. P. J. Am. Chem. Soc. 2011, 133, 503512. 28. Gu, M.; Wang, Z.; Connell, J. G.; Perea, D. E.; Lauhon, L. J.; Gao, F.; Wang, C. ACS Nano 2013, 7, 6303-6309. 29. Du, Z.; Hatchard, T. D.; Dunlap, R. A.; Obrovac, M. N. J. Electrochem. Soc. 2015, 162, A1858-A1863. 30. Hülser, T.; Schnurre, S. M.; Wiggers, H.; Schulz, C. KONA Powder Part. J. 2011, 29, 191-207. 31. Xiao, L.; Sehlleier, Y. H.; Dobrowolny, S.; Orthner, H.; Mahlendorf, F.; Heinzel, A.; Schulz, C.; Wiggers, H. ChemElectroChem 2015, 2, 1983-1990. 32. Sehlleier, Y. H.; Dobrowolny, S.; Plümel, I.; Xiao, L.; Mahlendorf, F.; Heinzel, A.; Schulz, C.; Wiggers, H. J. Appl. Electrochem. 2016, 46, 229-239. 33. Li, J.; Smith, A.; Sanderson, R. J.; Hatchard, T. D.; Dunlap, R. A.; Dahn, J. R. J. Electrochem. Soc. 2009, 156, A283-A288. 34. Chevrier, V. L.; Liu, L.; Le, D. B.; Lund, J.; Molla, B.; Reimer, K.; Krause, L. J.; Jensen, L. D.; Figgemeier, E.; Eberman, K. W. J. Electrochem. Soc. 2014, 161, A783-A791. 35. Obrovac, M. N.; Krause, L. J. J. Electrochem. Soc. 2007, 154, A103-A108. 36. Sethuraman, V. A.; Srinivasan, V.; Bower, A. F.; Guduru, P. R. J. Electrochem. Soc. 2010, 157, A1253-A1261.


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37. Sethuraman, V. A.; Chon, M. J.; Shimshak, M.; Srinivasan, V.; Guduru, P. R. J. Power Sources 2010, 195, 5062-5066. 38. Magasinski, A.; Zdyrko, B.; Kovalenko, I.; Hertzberg, B.; Burtovyy, R.; Huebner, C. F.; Fuller, T. F.; Luzinov, I.; Yushin, G. ACS Appl. Mater. Interfaces 2010, 2, 3004-3010. 39. Zeng, X.; Xu, G.-L.; Li, Y.; Luo, X.; Maglia, F.; Bauer, C.; Lux, S. F.; Paschos, O.; Kim, S.-J.; Lamp, P.; Lu, J.; Amine, K.; Chen, Z. ACS Appl. Mater. Interfaces 2016, 8, 3446-3451. 40. Nakai, H.; Kubota, T.; Kita, A.; Kawashima, A. J. Electrochem. Soc. 2011, 158, A798A801. 41. Etacheri, V.; Haik, O.; Goffer, Y.; Roberts, G. A.; Stefan, I. C.; Fasching, R.; Aurbach, D. Langmuir 2012, 28, 965-976. 42. Schroder, K.; Alvarado, J.; Yersak, T. A.; Li, J.; Dudney, N.; Webb, L. J.; Meng, Y. S.; Stevenson, K. J. Chem. Mater. 2015, 27, 5531-5542. 43. Choi, N.-S.; Yew, K. H.; Lee, K. Y.; Sung, M.; Kim, H.; Kim, S.-S. J. Power Sources 2006, 161, 1254-1259.


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Figure 1. (a-c) SEM images, (d) small-angle X-ray scattering curves, (e) high energy X-ray diffraction patterns, and (f) pair distribution functions of the Si-A, Si-B, and Si-C nanoparticles.


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2.0

2.0

Potential vs Li/Li+ (V)

Si-A Si-B Si-C

1.5

1.0

0.5

FEC-free 1st cycle

(b) Potential vs Li/Li+ (V)

With FEC 1st cycle

(a)

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Specific capacity (mAhg

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(d) Potential vs Li/Li+ (V)

With FEC 2nd cycle

(c) Potential vs Li/Li+ (V)

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Si-A Si-B Si-C

1.5

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1000

2000

3000

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Figure 2. Potential profiles of Si NP/Li half cells using Si-A, Si-B, and Si-C electrodes in both FEC-containing and FEC-free electrolytes at 0.1 C after 1st discharge-charge cycle (a, b) and 2nd discharge-charge cycle (c, d).


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2

3

Intensity (a.u.)

(311)

(220)

Si-A Si-B Si-C

(422)

(b)

(111)

(a) Intensity (a.u.)

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Si-A Si-B Si-C

Si (ICDD: 00-027-1402)

as-prepared 4

5

6

2

2-theta (deg.)

3

after 1st cycle 4

5

6

2-Theta (deg.)

Figure 3. X-ray diffraction patterns of (a) as-prepared Si NP electrodes; and (b) electrodes after 1st discharge-charge cycle at 1.5 V vs. Li/Li+ in the electrolyte with FEC additive.


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50 0 0

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Differential capacity (mAhV )

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(a)



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60 50 40 30 20 10 0 0

2

4

6

8 10

Cycle number

0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 +


0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Potential vs Li/Li+ (V)

Figure 4. Evolution of differential capacity (dQ/dV) plots of the Si NP/Li half cells during cycling under 0.1 C rate with Si-A, Si-B, and Si-C electrodes in both FEC-containing (a-c) and FEC-free (d-f) electrolytes (insets show the intensity of the c-Li15Si4 peak at 0.45 V vs. Li/Li+ as a function of cycle number).


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Figure 5. High-resolution transmission electron microscopic images of (a) lithiated Si-A without FEC additive and (b) lithiated Si-C with FEC additive (both samples were discharged to 0 V vs. Li/Li+); and electron diffraction patterns from (c) lithiated Si-A without FEC additive and (d) lithiated Si-C with FEC additive (the insets show the fast Fourier transform images of the microscopic images).


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4000 3500 3000 2500 2000 1500 1000 500 0

0.1C 0.5C

Si-A Si-B Si-C

(a) With FEC 0


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10

20

30

4000 3500 0.1C 0.5C 3000 2500 2000 1500 1000 500 (b) FEC-free 0 0 10 20 30

40 50 60 Cycle number

70

80

90

100

Si-A Si-B Si-C

40 50 60 Cycle number

70

80

90

100

Figure 6. The cyclability of the three Si NP anodes in (a) FEC-containing and (b) FEC-free electrolytes from 1.5 to 0 V vs. Li/Li+ (first 10 cycles are tested under a C-rate of 0.1 while the rest cycles are under a C-rate of 0.5).


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1

-


0.0 (a) With FEC -0.5 Si-A -1.0 Si-B -1.5 Si-C -2.0 0 -20 2 -2.5 R =0.993 -40 -3.0 -60 -80 -3.5 y=A*exp(-x/t)+y0 -100 leakage current -4.0 -120 curve fit -140 -4.5 2 4 6 8 10 12 14 16 18 20 Time (hr) -5.0 0.0 0.2 0.4 0.6 0.8 Current (µAmgSi )

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0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 -4.5 -5.0

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(b) FEC-free Si-A Si-B Si-C

0.0


0.2

0.4

0.6

0.8


Figure 7. Variation of the static leakage current density as a function of the lower cutoff potential for the Si NP/Li half cells at 25 ºC in (a) FEC-containing and (b) FEC-free electrolytes (the inset shows a typical current relaxation curve collected to extract the static leakage current).


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4000 3500 3000 2500 2000 1500 1000 500 0

(a) With FEC

0 Specific capacity (mAhgSi-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4000 3500 3000 2500 2000 1500 1000 500 0

20

40

60

Si-A Si-B Si-C

80 100 120 140 160 180 200 Cycle number

(b) FEC-free

0

20

40

60

Si-A Si-B Si-C

80 100 120 140 160 180 200 Cycle number

Figure 8. The cyclability of the three Si NP anodes in (a) fluorinated and (b) non-fluorinated electrolytes from 1.5 to 0.2 V vs. Li/Li+ (first formation cycle is from 1.5 to 0 V vs. Li/Li+ at 0.1 C, then 10 more cycles are tested under a C-rate of 0.1 while the rest cycles are under a C-rate of 0.5).


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Parasitic Reactions in Nanosized Silicon Anodes ... - ACS Publications

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