Investigation of Solid Electrolyte Interphase Formed on Si

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Investigation of Solid Electrolyte Interphase (SEI) Formed on Si Nanoparticle Composite Electrodes Using Hyperpolarized 129Xe NMR Spectroscopy Yougang Mao, Naba K. Karan, Myeonghun Song, Russell Hopson, Pradeep R. Guduru, and Li-Qiong Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00250 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 3, 2017

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Investigation of Solid Electrolyte Interphase (SEI) Formed on Si Nanoparticle Composite Electrodes Using Hyperpolarized 129Xe NMR Spectroscopy Yougang Maoa, Naba K. Karanb, Myeonghun Songa, Russell Hopsona, Pradeep R. Gudurub, Li-Qiong Wang*a a

Department of Chemistry, Brown University, Providence, RI 02912

b

School of Engineering, Brown University, Providence, RI 02912

*Corresponding Author: [email protected]

Abstract:

Solid electrolyte interphase (SEI) plays an important role in determining electro-

chemical performances of Li-ion batteries. The ideal SEI layer protects the electrolyte from being further reduced on the electrode surface and allows Li ion diffusion in and out of electrodes without any consumption. However, degradation of the SEI layer over time, which contributes to the thickening of the SEI layer is a leading pathway for the gradual capacity fade. In this study, hyperpolarized (HP) 129Xe NMR technique was applied for the first time to probe changes in porosity and connectivity in Si nanoparticle composite electrodes as a result of the SEI formation. Nanopores are present in nanocomposite electrodes due to aggregation of the constituting nanoparticles. The connectivity among nanopores greatly affects the ion transport property of

the electrode materials, which has a substantial influence on the overall energy output of Li-ion batteries. In this work, information on thickness, uniformity of SEI layer and connectivity of the pores in the composite electrodes upon growing SEI was obtained from the analysis of temperature dependent HP

129

Xe NMR spectra. Such information is useful for gaining a better under-

standing of the degradation mechanism of SEI. This study demonstrates that HP 129Xe NMR is a potentially unique tool in probing the porosity and connectivity changes in porous practical electrodes during electrochemical cycling.

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TOC:

SEI


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INTRODUCTION Since their commercialization, Li-ion batteries have revolutionized the portable electronics in-

dustry landscape and now they are being considered for powering the electric vehicles. Significant advancements have been made in terms of increasing their energy density, safety and long-term cyclability owing to tremendous research efforts in the past few decades. Still, the performance of the state-of-the art Li-ion batteries falls short for vehicle electrification over practical driving ranges (~300 miles per single charge). Hence, further improvement of energy density of Li-ion batteries is of paramount importance for their commercially viable application in electric vehicles, which entails replacement of the existing electrodes (for example, graphite anode) with high capacity ones. In this regard, silicon is considered as one of the most promising candidates for an anode material in Li-ion batteries due to its high theoretical capacity, 3580 mAh/g (vs. 372 mAh/g of graphite).1 However, mechanical damage caused by colossal volumetric changes1, 2 during lithiation/delithiation cycles prevents its widespread practical application in Li-ion batteries. Many studies have focused on developing new types of Si electrode structures that can minimize the large volume changes during the electrochemical cycling. It was demonstrated that Si nanoparticle electrodes are better able to accommodate large cycling induced volumetric strain compared to their micron sized counterparts resulting in much improved electrochemical performances.3, 4 The practical capacity and cyclability of Si based electrodes were further enhanced by choosing an effective binder that holds the active particles and conductive super P carbon together and steadily adhere them onto the current collector.5-13 Solid electrolyte interphase (SEI) is believed to be one of the major factors in determining the electrochemical performance of Li-ion batteries. The ideal SEI layer protects the electrolyte from being further reduced on the electrode surface and allows Li ion diffusion in and out of the electrodes without any additional Li consumption. However, in reality, even though bulk of the SEI forms during the first


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cycle, the SEI keeps growing in the subsequent cycles as well, albeit with a slower rate, which over time leads to gradual thickening of the SEI layer and performance failure. The mechanical stress, resulting from large volume changes in Si anodes during lithiation and delithiation, has a large negative impact on the stability of SEI layer and the integrity of the electrodes, which reduces the rechargeable capacity and the battery life. A combination of microscopy and spectroscopic techniques14-21 has been used to obtain information on the chemical compositions and morphology of SEI layer formed on silicon nanoparticle anodes. To this effect, TEM has been used extensively to get estimates on the SEI thickness on Si nanoparticles upon cycling. However, it is challenging for TEM to reliably discriminate the SEI layer from the lithiated Si nanoparticles. Previous TEM images14 have shown that after one cycle, the sharp edges of the fresh silicon nanoparticles were converted to a rough inhomogeneous layer due to the SEI formation. After many cycles, most silicon nanoparticles agglomerated into fused particles, thus making it difficult to reliably estimate the thickness of SEI layers formed on Si nanoparticle electrodes. Nanopores in Si electrodes, formed because of aggregation of silicon nanoparticles, often experience changes in porosity and connectivity upon SEI formation and cracking/agglomeration of nanoparticles due to the large volume changes during charge discharge cycles. The connectivity among nanopores greatly affects the ion transport property of the electrode materials, which has a substantial influence on the overall energy output of Li-ion batteries. Although many different experimental methods are now available for characterizing porous solids, reports addressing changes in connectivity of pores in nanoparticle electrode materials are scarce in the literature. Microscopic techniques, such as TEM are powerful for observing morphology changes in nanoparticles during electrochemical cycling, but it is difficult to probe the porosity and connectivity of the pores. Over the years,

129

Xe NMR has been de-

veloped into a powerful and robust method for studying porous solids.22, 23 The large chemical shift range of 129Xe is strongly dependent on both local environmental and chemical factors such as the composition of the matrix, nature and concentration of adsorbed molecules, and the shape and size of resident void spaces.23-28 The use of optical pumping approaches for the production of hyperpolarized (HP) 4 ACS Paragon Plus Environment

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xenon18 allows for a dramatic increase in the sensitivity of

129

Xe NMR up to a factor of 104, enabling

these pore characterization studies. Since spin polarized Xe gas percolates through the interconnected pores and samples the local pore environments, the unique advantage of HP 129Xe NMR technique compared with other experimental methods, such as small angle x-ray, neutron scattering, and gas absorption is that HP 129Xe MNR directly probes the connectivity between the pores in addition the pore spaces and surfaces. The present work demonstrates that

129

HP NMR is a unique tool for probing changes in porosity

and connectivity of the pores in nanoporous Si electrodes as a result of the SEI formation. Since HP 129

Xe NMR is extremely sensitive to any changes in porosity, experimentation and the interpretation of

the spectra are not straightforward. For the purpose of the demonstration, we choose a simple system that allows for the formation of SEI layer without cycling of the electrode in order to focus only on the changes of porosity arising from of the SEI growth. This simpler system also enables us to examine the thickness of SEI layers from the changes in porosity resulting only from the formation of SEI layer. The information on thickness, uniformity of SEI layer and connectivity of the pores in these electrodes upon growing SEI obtained from this study is expected to be useful for gaining a better understanding of the degradation mechanism of SEI.

EXPERIMENTAL SECTION

Sample Preparation. Silicon nanoparticles (~ 20 nm in diameter), conductive carbon super P and carboxymethyl cellulose (CMC), were purchased from Sigma Aldrich. For the preparation of silicon and carbon mixture (Si+C) with a weight ratio of 1:1, Si nanoparticles and super P carbon were mixed in ethanol and magnetically stirred for 4 hours, followed by sonication for 2 hrs. at room temperature. Next, the mixture was dried on a hotplate at 100°C for 15 hours with stirring. Final drying of the mixed powder was performed at 100°C for 9 hours in an oven. For the preparation of laminated composite Si electrodes, (Si+C+CMC), the dried powder mixture (silicon and carbon) was added into a 2 wt.% CMC 5 ACS Paragon Plus Environment

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solution in water and stirred for 30 minutes. The slurry was cast on copper foil and dried under vacuum at 100°C for several hours. The weight ratio of (Si+C): CMC was kept fixed at 80:20 by weight. The (Si+C+CMC) sample was prepared by gently scrapping off the material from the laminated electrode and packed in a rotor for the NMR measurements. SEI was grown on the composite Si laminate electrodes (working electrode) using beaker cells with liquid electrolyte [1 M LiPF6 in 1:1:1 ethylene carbonate (EC)/diethyl carbonate (DEC)/dimethyl carbonate (DMC)] and lithium metal as a reference electrode. For beaker cell assembly, the laminated copper foil was cut into several circular electrodes (2 inch diameter). SEI was grown on the electrodes by holding them at a constant potential (0.5V vs. Li/Li+). Two SEI samples (SEI1 and SEI2) were prepared by holding the potential for 2 hrs. and 12 hrs., respectively. For comparison, a Si+C+CMC (soaked) sample was also made by soaking the laminated electrode in liquid electrolyte for 2 hrs without applying any potential. Afterwards, all samples were recovered from the beaker cell and thoroughly rinsed with DMC followed by natural drying for a few days in an Ar filled glove box. The dried samples were then gently scrapped off from the copper foil (current collector) and packed in rotors for the NMR measurements. All sample preparation including SEI formation, drying and packing in the rotors were done in an Ar filled glove box. Hyperpolarized

129

Xe NMR Measurements. HP

129

Xe NMR measurements were carried out

on a Bruker Avance DSX 300 spectrometer operating at 82.98 MHz (magnetic field 7.05 T) using a variable temperature static probe with a continuous flow (CF) of HP xenon. A single-pulse (SP) Blochdecay method was used and samples were loaded into 7 mm Zirconia rotors. The rotor cap with a smalldrilled hole allows HP

129

Xe gas to pass through small plastic tubing in to the samples. The HP

129

Xe

NMR experiments were performed using a home-made 129Xe polarizer which has a unique design based upon the study by Hersman and Saam groups.29, 30 Spectra were collected with a 1-µs

129

Xe pulse, and

a repetition delay of 0.5 second. A gas mixture of xenon-helium-nitrogen with a volume composition of 1%-66%-33% was used in all CF HP experiments. The flow rate was kept constant in the range of about 300 scc/min (gas flow 6 ACS Paragon Plus Environment

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normalized to standard conditions). In CF HP experiments, the HP xenon flow was delivered directly from the polarizer to the coil region of the NMR probe through 1.5 mm ID plastic tubing. Variable temperature NMR measurements were carried out in the 173 - 373 K range using a liquid N2 cooling assembly and temperature controller. All measured 129Xe NMR chemical shift data were referenced to the free xenon gas chemical shift at 0 ppm. RESULTS AND DISCUSSION Room Temperature HP

129

Xe NMR. A series of HP

129

Xe NMR measurements has been

carried out for Si nanocomposite electrode materials before and after the formation of SEI layer for studying the influence of SEI on porosity and connectivity of the pores in these nanocomposite electrodes. Figure 1 displays the CF HP

129

Xe NMR spectra taken at 293 K for (Si+C), (Si+C+CMC),

SEI1 and SEI2 samples. The spectra for all samples were taken under the identical condition in order to examine any changes in porosity and interconnectivity of the pores caused by the formation of SEI. 129

Xe NMR chemical shifts are very sensitive to the local environment of the material where

Xe is adsorbed mostly due to the large number of polarizable electrons. The observed

129

Xe NMR

chemical shift δobs for adsorbed xenon are the weighted averages among various environments sampled by xenon atoms within its characteristic diffusion distance: 24,31

δobs=Σi=1..n pi × δi ,

(1)

where δi is the chemical shift of xenon adsorbed on site i, pi is the fraction of xenon atoms in site i, and n represents the total number of sites. In porous materials without paramagnetic impurities and strong adsorption centers, the chemical shift in a site (δi) is a sum of three main contributions: δi =

δ0+ δS + δXe-Xe. In this formula, δ0 is the reference shift, which is usually set to 0 ppm, the chemical shift of xenon gas at zero pressure. The term δS is the contribution due to the interaction or collision 7 ACS Paragon Plus Environment

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of Xe atoms with the pore surfaces. In general, δS is characteristic of a given adsorption site and reflects both the chemical composition of the surface and also the geometry of the xenon environment in that particular site. The contribution δXe-Xe arises from Xe-Xe interactions and is dependent on xenon concentration. In this study, a very low partial pressure of xenon used in the flowing gas mixture (1% of total, or 10 mbar) excludes significant δXe-Xe contribution. Since the observed

129

Xe

NMR chemical shift δobs corresponds to the interaction or collision of Xe atoms with walls of varying pore environments, the information on the average pore size, non-uniformity and connectivity of the pores can be obtained from the analysis of values of the chemical shifts and the line widths of the 129

Xe NMR spectra. The non-uniformity of the pores refers to those pores with a distribution in pore

sizes and shapes, and the connectivity of the pores is related to how fast Xe can exchange between different types of pore environments. A sharp resonance peak at 0 ppm is observed in spectra in Fig. 1 for all samples. This peak is associated with the free Xe gas, and the peaks at larger chemical shifts correspond to xenon adsorbed within the pores of the material. A main resonance peak (~21 ppm) with a small shoulder at a lower chemical shift was observed for the (Si+C) sample, whereas a single resonance peak of 15.6 ppm at a relatively lower chemical shift was present for the (Si+C+CMC) sample. Presence of two resonance peaks suggests that there are at least two different pore environments for adsorbed Xe in the (Si+C) sample. In comparison, a single resonance peak observed for Xe adsorbed onto the laminated electrodes comprised of Si, C and CMC indicates the addition of CMC binders into the (Si+C) mixture generates more uniform pore distribution or better connected pores. The effect of binders on the porosity of the composite electrodes has been investigated using HP

129

Xe NMR in a previous study32

where cast electrodes with a different amount of CMC binders were examined without any electrochemical treatments. It was found that the addition of CMC changed the porosity in cast electrodes. Thus, it is not surprising to observe the porosity changes upon addition of the binder in this study.


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Obvious changes in chemical shifts for the adsorbed Xe resonances upon the formation of SEI are also clearly shown in Fig. 1, where the resonance peak at 15.6 ppm for laminated electrode of (Si+C+CMC) is shifted to lower values of 7-8 ppm for SEI containing samples. Significant changes in chemical shifts between samples before and after SEI formation indicate that HP

129

Xe NMR is very

sensitive to the growth of SEI and can be a potential tool for monitoring the growth of SEI layers during the electrochemical cycling. As illustrated in equation (1), a Xe atom that travels inside smaller pores has a higher probability to be adsorbed onto the surface site or has a shorter mean free path as compared with the Xe atom inside larger pores, giving rise to a higher chemical shift. Thus, in general, higher chemical shifts are associated with smaller pores when the type of the surface and the shape of the pores are similar. The large difference in chemical shifts between (Si+C) and (Si+C+CMC) samples is most likely associated with the different pore sizes or diameters of the pores since the pores are formed as a result of the aggregation of similar nanoparticles. The smaller chemical shift for the cast electrode of (Si+C+CMC) as compared with that of (Si+C) corresponds to larger pores formed between particles as a result of addition of the binders, in agreement with the previous study32 Furthermore, it will be shown later in the paper that the obvious reduction in chemical shifts for electrodes after the SEI formation is also related to the increase in pore sizes despite the difference in surface compositions for samples with and without SEI. Variable Temperature HP 129Xe NMR. Variations in 129Xe NMR spectra with temperature can be very sensitive to the dynamics of adsorbed xenon and subsequently to the morphology of the pore space.33, 34 A series of variable-temperature HP

129

Xe NMR spectra for samples: (Si+C), (Si+C+CMC)

(soaked), SEI1 and SEI2 are displayed in Fig. 2, where the bottom spectrum was recorded at 173 K. The temperature was raised in 20 K increments. For all samples, it is clearly observed that the NMR peaks associated with the adsorbed Xe move toward the higher chemical shifts with decreasing temperature. The slower exchange between the gas phase and adsorbed Xe at reduced temperatures results in larger


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observed chemical shifts. In addition, for all samples, most peak widths of adsorbed Xe resonances become broader at lower temperatures, in agreement with the observed trend in a previous study.35 The reduction in Xe gas exchange at low temperatures allows the Xe to access non-uniform pore environments. In the absence of chemical shift anisotropy, low temperature line-widths are related to pore non-uniformity, while at temperatures ≥ 300 K where fast exchange of Xe is present, the line widths are representative of both pore non-uniformity and interconnectivity among the different pores and channels since Xe can fast exchange among different interconnected pores or channels at higher temperatures. A higher degree of connectivity between non-uniform pores results in a narrower line width with increasing temperature for adsorbed Xe due to the fast exchange that averages out the effects of pore non-uniformity. Based on a previous study where large line widths were associated with chemical shift anisotropy,36 the much narrower line widths observed in this study suggests negligible effects of chemical shift anisotropy present in these Si nanocomposite electrodes. Consequently, the comparative variable temperature 129Xe NMR line width analysis conducted here provides an excellent gauge of pore non-uniformity and connectivity. As evidenced in Figure 2, two overlapping 129Xe NMR peaks are present at all temperatures for the Si+C material indicating the existence of two types of pores that are not well connected since the fast exchange Xe atoms at higher temperature did not average out the effect of two different pore environments. These two types of the pores are most likely from two different domains of pores. By contrast, a single 129Xe NMR peak is observed at all temperatures for samples with CMC. The presence of two overlapping peaks in the (Si+C) sample and a single narrow peak in the samples with CMC indicates that pores are better connected in samples with CMC. The higher connectivity in CMC samples is a result of a better mixing of Si and C nanoparticles with aid of CMC binders in preparing cast electrodes. Figure 2 also shows that the line widths of the HP

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Xe spectra for (Si+C) and (Si+C+CMC)

samples are broader than those of SEI1 and SEI2 at higher temperatures, indicating that the formation of 10 ACS Paragon Plus Environment

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SEI has enhanced the connectivity of the pores, most likely due to the enlarged pores or channels as a result of SEI formation. As shown in Fig. 2, electrode samples with and without SEI all experience line broadening with decreasing temperature, indicating the non-uniformity of the pores.

The non-

uniformity is most likely caused by many factors including the non-uniform particle size, the addition of the CMC binder and the non-uniform SEI layers. Therefore, this study shows that temperature dependent HP 129Xe NMR is capable of providing information not only on the non-uniformity of the pores but also the connectivity among different pore environments. Our previous study32 showed that pore structures were partially collapsed at low temperatures for the electrode materials with a higher CMC content, demonstrating the potential usefulness of variable-temperature HP 129Xe NMR for examining the integrity of the electrode. The integrated areas of the NMR signals associated with adsorbed Xe on surfaces with respect to those of free Xe gas were found to decrease with decreasing temperature due to the significant reduction in the number of nanopores as a result of partially collapsed pore structures at low temperatures for the nanocomposite electrodes with higher CMC content.32 However, the opposite trend in the integrated peak areas observed in this study for Si nanocomposite electrodes with or without SEI indicates that pore structures in all samples, before or after SEI growth, are stable at low temperature and the SEI growth did not affect the integrity of the electrodes at low temperatures. Parameters Derived from the Variable Temperature HP 129Xe NMR. Figure 3 displays the temperature dependence of adsorbed 129Xe chemical shifts for (Si+C), soaked (S+C+CMC), SEI1 and SEI2 samples. Two distinct chemical shift curves are clearly displayed in Fig. 3 for (Si+C), confirming the co-existence of two types of the pores in the (Si+C) mixture. Whereas similar curves observed for SEI1 and SEI2 indicate that SEI1 and SEI2 have similar pore structures and sizes. Physical parameters related to the Xe adsorption properties in the composite electrodes can be obtained from the temperature-dependence of chemical shift data. Variations in the 11 ACS Paragon Plus Environment

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Xe chemical

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shifts with temperature can be fitted to extract parameters related to the adsorption properties using a model based upon Henry’s Law, as described previously.33 In the fast exchange approximation with weak adsorption, the temperature dependence of the observed chemical shifts, δ for arbitrary pores can be expressed as:

= (1 +

√

∆

), =

(2)

Where Vg is the free volume inside porous materials, T is the temperature, S is a specific surface area, K0 is the pre-exponential term of Henry’s constant K defined in eq. 11 from a previous study,33 R is the universal gas constant, ∆Hads is the heat of adsorption and δs is the component of the observed 129

Xe chemical shift characteristic of the interaction between xenon and the surface. Under our exper-

imental conditions with a low Xe partial pressure of about 10 mbar, δs can be approximated by the observed chemical shift of xenon at the lowest observed temperatures before xenon starts to condense (~180K in the experiments). Although Equation (2) contains 3 variables, in reality only two parameters, ∆H and Vg/S, have pronounced variation and effect on the fit. The situation is further simplified as the possible range of δs is very well defined from the low temperature measurements. Thus, a strongly constrained δs effectively reduces the situation to a two-parameter fit. We also note that the ranges over which the other variables in the equation can change, are, in fact, also well constrained. Considering the rather broad temperature range studied, we believe the results of the fits are sufficiently reliable. The applicability and limitations of Equation (2) for analysis of the temperature dependence of the 129Xe chemical shift has been discussed previously.33, 34,37-40 As shown in Fig. 3, the experiment data are fitted well using the Henry's equation, which suggests that adsorbed Xe atoms under our experimental conditions follow the Hendry’s law adsorption behavior at temperatures as low as 173 K. Based on a typical Henry's constant K (atoms m-2 Torr-1) at 298 K from the previous study33 and the surface coverage of one adsorbed Xe atom of 18 Å2, we estimated that less than 0.5% total pore surface is covered by the adsorbed Xe atoms under the Xe partial pressure of 12 ACS Paragon Plus Environment

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7.6 Torr under our experimental condition. At lowest temperature of 173 K, the adsorbed Xe coverage is about twice that of 298 K but is still less than 1%. Therefore, the contribution of δXe-Xe is negligible even at the lowest temperature in this study due to the low adsorbed Xe coverage. Table I lists the ∆Hads and δs values obtained from the fits of these variable temperature chemical shift curves. All samples have similar heats of adsorption of 15-18 KJ/mol, indicating typical physical adsorption of Xe on all surfaces. The values of δs among samples with similar chemical compositions and surface geometries should be similar since δs is characteristic of the surface of the pores. Table 1 shows that the two peaks of the adsorbed

129

Xe for Si+C (1) and Si+C (2) have similar characteristic

chemical shifts of 110 ppm and 109 ppm, respectively due to the same surface chemical compositions and the geometry of the pores for Si+C (1) and Si+C (2) resonance peaks. However, when the sample (Si+C) was mixed with CMC and coated with SEI, δs decrease from 110-109 ppm to 91-95 ppm. It is understandable to observe differences in δs values among these samples. However, it is surprising to see similar δs values for samples with and without SEI despite different surface chemical compositions between these two samples. However, similar δs values were also observed in the previous study41 for mesoporous silica coated with and without ammonia borane inside the mesoporous channels. Thus, the trend of the δs values cannot be predicted simply by changes in the chemical compositions because δs depends not only on the surface chemical compositions but also on the geometry and shape of the pores including the defect sites. Using empirical chemical shift-pore size correlations developed to fit inorganic systems such as MCMs and zeolites, 33,34 the average pore diameters can be estimated based on the experimental values of the observed chemical shift and the characteristic δs obtained from the temperature dependent data. The pore sizes reported in Table I should be regarded with caution, as there may be an unaccounted scaling factor due to differences in the chemical composition. However, the result on the


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comparison of two similar samples is sufficiently reliable. For a given model, the resonance with a larger chemical shift often corresponds to a smaller pore size. Pore size, Interconnectivity and Electrode Performance. Table 1 shows a larger pore observed for samples containing SEI, consistent with the decrease in chemical shifts upon the growth of SEI (Figure 1). Results from Table 1 indicate that the growth of SEI on the composite electrode surface increases the size of the pores. Although the δs values did not show a clear trend among samples with and without SEI, the chemical shift reduction observed in Fig. 1 is related to the increase in pore sizes as a result of SEI formation. The pore diameters were estimated based on the experimental observed chemical shift and the characteristic δs obtained from the temperature dependent data. However, the observed chemical shift δ weighs more than the characteristic δs in determining the pore size.33 Figure 4 presents a schematic illustration for explaining the changes in pore spacing as a result of the growth of SEI. However, it should be read with caution as it represents a simplified model only for the sake of illustration. Fig. 4 clearly shows that the size of the pores made of the aggregation of nanoparticles increases as the particle size increases due to the formation of SEI layers onto these particles. Therefore, the reduction in 129Xe NMR chemical shifts observed for SEI samples in Fig. 1 is related to the increase of the pore size as a result of SEI formation. However, in contrast with the expectation that the amount of SEI formed on sample SEI2 is higher than that on sample SEI1, the similar chemical shifts and line widths observed for SEI1 and SEI2 samples indicate that pore environments are similar in both samples, suggesting that the growth of SEI layers nearly stopped within first two hours of the initial potential hold. It is interesting to observe in Fig. 1 a slightly smaller chemical shift for SEI2 sample than for SEI1 sample (the small difference of < 1 ppm in chemical shifts was observed consistently in repeating measurements). One possible explanation is that a small amount of newly formed SEI layers may be dissolved or peeled off after longer time soaking in the electrolyte solution. Based on the pore diameters given in Table I, the thickness of SEI layers can be estimated as14 ACS Paragon Plus Environment

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suming hexagonal tightly packed spheres (shown in Fig. 4). It is important to notice that upon the initial SEI growth for 2 hours, we observed an increase of 36 nm in diameter of nanoparticles, (or thickness of SEI of 18 nm) resulted from the growth of SEI layers on Si nanoparticles. Since the particle may not be fully in hexagonal tight packing, the reported thickness of the SEI is an upper limit of estimation. More interestingly, there is no obvious change in the pore diameter after additional 10 hours of constant potential holding, indicating the faster SEI growth rate in the initial 2 hrs. at a constant potential hold. Thus HP 129Xe NMR can be used to probe the growth rate of SEI formation. HP 129Xe NMR data given in this study shows that the 129Xe chemical shift is sensitive not only to pore structures but also the interconnectivity among the nanopores. As described in a previous study, 32 the increased CMC content can strongly connect the nanoparticles and conductive super P carbon together and increase the electron conductivity of electrodes. However, SEI may reduce the role of CMC to connect the particles because it increases the space between the particles and therefore reduces the concentration of CMC chains among the particles. Although we have successfully demonstrated that HP 129Xe NMR can be used to probe changes in porosity and connectivity of the pores in nanoporous Si electrodes as a result of SEI formation, there are still challenges for the future application of HP

129

Xe NMR for the in-situ measurements of the po-

rosity during electrochemical cycling. The advantage of the HP

129

Xe NMR technique is its extremely

high sensitivity to any changes in porosity, offering unique and useful information on the growth of SEI and the thickness of SEI. On the other hand, the high sensitivity of Xe atom also comes with difficulties in terms of the quantification and interpretation of the

129

Xe NMR data. Thus, it requires a systematic

study to obtain useful information since HP 129Xe MNR is not simple and straightforward for a complex multi component system. Nevertheless, the unique information obtained from the HP

129

Xe NMR on

the porosity and connectivity in nanoporous materials is critical for a better understanding of the chemical and mechanical degradation of the porous electrodes, complementary to other techniques. Our future study is to examine the changes in SEI layers and porosity of electrode upon electrochemical cy15 ACS Paragon Plus Environment

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cling and to assess the feasibility of the HP 129Xe NMR for the in-situ measurements.

CONCLUSIONS In this study, hyperpolarized (HP)

129

Xe NMR has been successfully applied to obtain infor-

mation on the porosity and connectivity change as a result of SEI formation on multi-component nanocomposite electrodes. Variable temperature dependent HP 129Xe NMR spectra were used to probe the thickness and uniformity of SEI layers as well as connectivity of the pores in silicon nanoparticle electrodes upon growing SEI. It was observed that the SEI growth rate is time dependent: ~18 nm of SEI layer was formed on Si nanoparticle surface in the first 2 hours of SEI growth by constant potential hold and no significant SEI growth was observed in the subsequent 10 hours of potential hold. Furthermore, the low temperature data show that pore structure in all samples before and after SEI growth stay intact at low temperature and SEI growth did not affect the integrity of the electrode at low temperature. Our study demonstrates that HP

129

Xe NMR is a potentially powerful technique in

probing the porosity and connectivity changes in practical porous electrodes during electrochemical cycling, which complements other techniques. The future study will explore the porosity upon electrochemical cycling. The information obtained from the 129Xe NMR study is useful for gaining a better understanding of the degradation mechanism of SEI.

ACKNOWLEDGEMENT HP

129

Xe NMR work was supported by the EPSCoR Implementation Grant DE-SC0007074,

Office of Basic Energy Sciences, United States Department of Energy (U.S. DOE). The authors are grateful for the Advance Grant HRD-0548311 for purchasing parts of the equipment.


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Table I ∆Hads, δs and D values are listed based on the variable temperature chemical shift data 19 ACS Paragon Plus Environment

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Si Nanoparticle Electrode

∆Hads (KJ/mol)

δs (ppm)

Ds (nm)

Si + C (1)

14.8±0.2

110±1.3

10

Si +C (2)

15.3±0.1

109±0.5

13

Si+C+CMC (soaked)

16.6±0.6

91±1.5

12

SEI

18.4±0.3

93±1.3

24

The parameters for SEI is the average value of SEI 1 and SEI 2 based on the similar temperature dependent chemical shift curves for SEI1 and SEI2 samples.

Figure Captions


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Figure 1 CF HP 129Xe NMR spectra taken at 293 K for samples of Si+C, Si+C+CMC, SEI1 and SEI2. The peaks at 0 ppm are cut off due to the vertical expansion of the spectra. Figure 2 Temperature dependent HP

129

Xe NMR spectra taken for differently prepared Si nanoparticle

electrodes. Figure 3 Temperature dependent chemical shift plots for Si+C, Si+C+CMC (soaked), SEI1 and SEI2 materials. The dots are experimental data and the dashed lines are fitted data along with the error bars. Figure 4

Schematic illustration of the formation of SEI on nanoparticles.


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Figure 1


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Figure 2

23

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100

Si+C (1) Si+C (2) Si+C+CMC SEI1 SEI2

80

Chemical Shift (ppm)

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60

40

20

0

150

200

250

300

350

400

Temperature (K)

Figure 3

24


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SEI

Figure 4

25


Investigation of Solid Electrolyte Interphase Formed on Si

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