Structure of Spontaneously Formed Solid-Electrolyte Interphase on

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Structure of Spontaneously Formed Solid-Electrolyte Interphase on Lithiated Graphite Determined Using Small-Angle Neutron Scattering Robert L Sacci, Jose Leobardo Bañuelos, Gabriel M. Veith, Kenneth C Littrell, Yongqiang Cheng, Christoph U Wildgruber, Lacy L Jones, Anibal J. Ramirez-Cuesta, Gernot Rother, and Nancy J Dudney J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00215 • Publication Date (Web): 25 Mar 2015 Downloaded from http://pubs.acs.org on April 3, 2015

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The Journal of Physical Chemistry

Structure of Spontaneously Formed SolidElectrolyte Interphase on Lithiated Graphite Determined Using Small-Angle Neutron Scattering Robert L. Sacci,a* José Leobardo Bañuelos, ,b†* Gabriel M. Veith, a Ken C. Littrell, c Yongqiang. Q. Cheng,d Christoph. U. Wildgruber, d Lacy L. Jones, d Anibal J. Ramirez- Cuesta, d Gernot Rother, b and Nancy J. Dudney a* a

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN

37831, USA. E-mail: [email protected] b

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

c

High Flux Isotope Reactor, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

d

Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

Solid-electrolyte interphase, small-angle neutron scattering, inelastic neutron scattering, lithiated graphite, anodes

ACS Paragon Plus Environment

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ABSTRACT: We address the reactivity of lithiated graphite–anode material for Li-ion batteries with standard organic solvents used in batteries (ethylene carbonate and dimethyl carbonate) by following changes in neutron scattering. The reaction produces a nano-sized layer, the solidelectrolyte interphase (SEI), on the graphite particles and this method provides a different and perhaps simpler view of SEI formation than the usual electrochemically-driven reaction, which contains many reactions and products that are difficult to deconvolute. We probed the structure and chemistry of this layer using small-angle neutron scattering (SANS) and inelastic neutron scattering (INS). The SANS results show that the SEI filled 20–30 nm-sized pores, and the large difference in scattering between Li-graphite in contact with deuterated solvent vs. protiated solvent suggests that this "chemical" SEI is primarily organic in nature—i.e. it contained large amount of hydrogen. INS sensitivity towards hydrogen allowed us to selectively probe the SEI's chemistry and find similarities with oxygenated polymers such as polyethylene oxide.


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INTRODUCTION: One of the fundamental questions confronting battery research regards the mechanism of the solid-electrolyte interphase's (SEI) formation.1,2 The physicochemical properties of the SEI function to mitigate Li-ion transport and protect the electrolyte from the highly reducing anode.3 As such, control of the SEI's formation has important implications in the safety, irreversible capacity loss, energy density, and cost of a battery.4 Typically, the SEI is formed during the initial charging of a battery, and when formed, it protects the anode from reacting with the electrolyte while allowing Li ions to migrate to and from the anode.5 During this initial charging, three general reactions occur as depicted in Figure 1a: 1) electrochemical reduction of electrolyte; 2) Li intercalation; and 3) reduction of electrolyte by intercalated Li. It is unclear which of these reactions occur first, and how they follow from one another; that is SEI formation contains the proverbial "chicken and the egg" problem. The chemistry and structure of the electrochemically formed SEI have been studied using a wide variety of techniques including infrared spectroscopy, differential scanning calorimetry, quartz crystal microbalance, etc.2,3,6,7 These studies support the depiction of the SEI being a composite, multilayered structure with a dense inner layer composed Li2CO3 and LiF and a porous polymeric outer layer.8 Recent developments in novel in situ techniques such as electrochemical transmission electron microscopy9 have demonstrated the changes in the structure and thickness of the SEI during charge and discharge; these studies show that the SEI is not stable chemically nor structurally. The operando small angle neutron scattering (SANS) study by Bridges et al.10 on electrochemical charge/discharge cycling of lithium-intercalated graphite reported that the SEI formed along the surfaces of a mesoporous framework within the anode matrix. However, because of the complexity of the electrochemically formed SEI, the scattering data had to be fit


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to a relatively large number of species, thus presenting ambiguities in extracting SEI thickness, structure, etc. Because of the complex nature of SEI formation we seek to bypass the two electrochemical events by decoupling the reactions in Figure 1a. This is done by first chemically lithiating graphite and then exposing it to a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC)—solvents typically used in Li-ion batteries—as depicted in Figure 1b. Since there are no Li-salts present in the solvent–that is, the solvent is free of LiPF6 or similar components–the system is relatively simple with only a few products may form. This approach, therefore, serves as a model for SEI growth by simplifying the chemistry thereby allowing us to focus on the differences in the porosity and surface area due to SEI formation. SANS and inelastic neutron scattering (INS) were used to quantify the structure and composition of the mesoscale SEI formation that occurs on graphite surfaces and inter-grain voids in the 1– 200 nm size range. SANS is a powerful technique in that it: 1) is not restricted to the study of simple model geometries; 2) provides the necessary contrast to study the formation of reaction products at an interface and in nanoscale pore spaces; and 3) has high penetration strength and is not limited by surface attenuation effects.11 Furthermore, as a bulk structural probe, SANS provides representative microstructural information over the entire sample instead of a small region. Because neutrons interact with the nucleus, SANS is sensitive to isotopic substitution, as well as to light elements including hydrogen. We exploit this fact to differentiate between an SEI composed primarily of Li-carbonate (hydrogen absent), or polymeric products (hydrogenbearing). Similarly INS is sensitive to vibrational modes involving hydrogens in the sample.12 Therefore, if the SEI layer is primarily organic, then bands from the layer will be visible over the


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graphite in the INS spectrum. This feature makes INS is the only vibrational technique that allows for such surface enhancement on black carbons. By coupling SANS and INS we find that the SEI is larger on LiC6 than on LiC12, and that there is little inorganic carbonate formed. The initially rough Li-graphite surfaces, which exhibit surface fractal scattering,13,14 become rougher, and exhibit nearly mass fractal scattering, due to the formation of an SEI that contains 1–2 nm-sized domains, i.e. the Li-graphite particles become interconnected. The formation of SEI seems to block access of solvent molecules to the pore surfaces as unfilled nanoscale spaces remain in the reacted samples. RESULTS: LITHIATION AND SOLVENT REACTION. High-energy ball-milling is used to increase the surface area of many materials15 and provide the energy to promote chemical reactions such as Li intercalation.16 Electron microscope images provided in the supplemental materials (Figure S1) show that the ball-milling procedure produces high-aspect-ratio lithiated carbon grains about 3 µm in thickness and between 8 and 18 µm in diameter.16 BET measurements show that the total surface area in the initial smooth nonporous graphite changes from 1.1 m2 g-1, to approximately 88 m2 g-1, with 10 m2 g-1 of the surface area corresponding to milling-formed micropores (Figure S2). This substantial increase in surface area and microporosity indicate that the grain surface becomes rough. The packing fraction of the samples is about 0.5 so that approximately half the empty volume consists of macropores and nanopores (Figure 3a). The macropore dimensions are on the same scale as the carbon particles and not directly observable by the SANS technique; however, nanopores, which are voids formed by the rough-surface contacts of the particle grains, are amenable to SANS structural characterization. Therefore, the


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structural changes caused by ball-milling and subsequent exposure to solvent may be directly probed by the SANS technique. After exposure to and reaction with the carbonate solution and subsequent vacuum removal of excess solution, the lithiated graphite showed a mass increase compared to the initial LiCx mass. LiC12 gained significantly more mass than LiC6 in both the deuterated and protiated samples; however, the mass increases cannot be fully assigned to SEI—i.e. some unreacted EC is still present (see supplemental material). LiC6 showed a large shift in the (002) diffraction peak from 22° to 25° as shown in the x-ray diffraction (XRD) patterns (Figure S3), which is sensitive to the Li-C stoichiometry. This indicates that a fraction of the intercalated Li reacted with the solvent and exited the graphite galleries. The reaction with LiC12 also caused a color change (purple to black) and (002) peaks from LiC24 (26°) and raw graphite appear (27°). The change in mass, coupled with the decrease in the gallery spacing, suggest the formation of an SEI. The pure graphite under identical solution exposure and removal showed no mass change and the SANS signal showed no change (Figure S5), indicating both that no reactions took place and complete solvent removal. SANS AND INS CHARACTERIZATION. The SANS plot in Figure 2a shows that when graphite is lithiated the scattering intensity, I(Q), decreases. We report I(Q) as a function of the absolute value of the scattering vector, or momentum transfer, defined as =

, where λ is

the neutron wavelength and θ is half the scattering angle. Therefore, the size in real-space of the observed features scales approximately as 1⁄ —i.e. large features appear at small Q, and vice versa. In Figure 2a, the SANS signal of the graphites originates from scattering by the graphiteair interface and the nanoscale pore spaces between the milled particles. For a two-phase


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system, I(Q) is proportional to the squared difference in the neutron scattering length density (NSLD) of the phases, ∆∗ —calculated from the sum of the coherent scattering lengths of all the atoms in a given phase. Therefore, changes in I(Q) can be correlated with a change in NSLD of one phase, or both. Because the scattering length of naturally abundant Li is negative, lithiation of graphite leads to a lower ∆∗ with air compared to pure graphite and thus a lower scattering intensity. When the LiCx is reacted with the EC:DMC mixture the intensity decreases within the Q range of 0.002 and 0.4 Å-1. Deuterating the solvent further decreases the scattering intensity since the SEI phase has a higher NSLD than air. In the current experiment, the effect of unfilled void spaces in the reacted samples must also be taken into account. The lithiated carbon (phase C), the reaction product phase (phase SEI), and the void spaces (phase 0) all combine to give a total scattered intensity which is weighed by the

∗ , and a volume fraction factor, , such square of the NSLD difference of the cross-terms, ,

that

∗ ∗ ∗ = !," !," + !,$%& !,%! + ' $%&," %!,"

(1)

In the range 0.02–0.1 Å-1, the signal decays as ∝ )* so that the slopes of the I(Q)–Q curves in a log-log plot (scattering law, p), are the same for the unreacted carbons, ca. 3.5, and decrease to ca. 3.0 when reacted with d-EC:DMC. Contrast variation shifts the contribution of the !,$%& and $%&," to the total intensity such that the ratio $%&," /!,$%& is greater for the LiCx + dEC:DMC sample. This means that the overall intensity in this range is dominated by the NSLD contrast between the carbons and the contents of the intergrain species, whereas the scattering law, p, is influenced by the interfacial contrasts, which transitions from the carbon–SEI interface dominating to the SEI–void interface upon deuteration. The lowering of the scattering intensity and the scattering law suggests that the SEI coats the LiCx and increases the surface roughness.


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The Porod plot in Figure 2b highlights subtle structural changes at the inter-grain pore scales of the LiCx–solvent reaction. Here, we normalize the I(Q) to the power law, * ,!- , of pristine LiCx-air system. Within the mid-Q range (0.005–0.1 Å-1) the scattering is slightly increased by hydrogen-bearing SEI and significantly decreases by the deuterated SEI layer, regardless of LiCx stoichiometry. At Q > 0.2 Å-1 the scattering increases for both protiated and deuterated SEI, corresponding to the formation of nanodomains along the SEI / LixC6 interface. The scattering increases due to overall sample composition changes involving deuterated SEI when Q < 0.01 Å-1; however, as this part of the SANS signal is convoluted with some degree of multiple scattering quantitative comparisons between the samples are not made in this region. Figure 2c shows the INS spectra of pure EC, LiC6, and LiC6 + EC/DMC, and LiC6 + EC/DMC after being washed three times with DMC and dried for 24 h. Note that the LiC6 background contributes little to the overall spectral intensities. That is, INS is more sensitive towards a nanometer-sized layer of hydrogen-bearing compounds than to 2 g worth of carbon.12,17 As expected from the XRD results, EC is clearly within the SEI layer of unwashed samples given the large peaks at 110 and 210 cm-1 being present in both neat EC and unwashed samples.18 Excessive washing of the reacted carbon with DMC removes the unreacted EC and reveals new peaks at 92 and 150 cm-1. These peaks are similar to those of polyethylene oxide (PEO),19 which is expected if the SEI is more polymeric in nature. The original EC peaks at 210 and 235 cm-1 are blue-shifted by 4 cm-1, as well as broadened, however these peaks can be assigned to an oxygenated polymer as well.19


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DISCUSSION. The electrochemical reduction of EC in DMC in the presence of Li-salts has been well-studied using electrochemical quartz crystal microscopy (EQCM), infrared and Raman spectroscopy.7 The reduction potential of EC, as written in Eqs. (2–4), is around 1.2 V vs. Li/Li+ and is thought to be the major contributor of the SEI.2,8,20 Zhuang et al.6,21 compared the infrared adsorption of synthesized Li2(CH2OCO2)2 with the electrochemical reduction of EC and found strong evidence for it being the primary product. If the intercalated Li has access to surface sites, it will act as a reducing agent (E = 0.05 V vs. Li/Li+) on EC to form carbonate, ethylene gas, and intermediate radical species that can polymerize via:7,20,22

)

CH CO' + e) 23 CH CH OCO ∙ CH CH OCO) e) 23 CH g + CO) + ' ) ) 2 ∙ CH CH OCO 23 CH CH OCO

(2) (3) (4)

Note that DMC is known to reduce to form lithium methoxide and Li2CO3, however, since EC is a major component of the SEI, we assume its reaction is more favorable.20 Given that the XRD diffraction profiles are indicative of the Li-C stoichiometry, we can approximate the extent of the reaction, i.e., the reaction between LiC6/12 and EC is of the form:

2 LiC9 + : + ;< CH CO' 23 LiC

+ : − ;< Li CO' + ; Li CH CH CO

(5)

LiC + : − ;< Li CO' + ; Li CH CH CO

(6)

2 LiC + : + ;< CH CO' 23

where 0 ≤ x ≤ 0.5, (disregarding ethylene gas production). Therefore, we expect the surface layer to be a mixture of Li2CO3 and Li2(CH2CH2CO3)2, with the latter representing the polymeric or hydrogen-bearing component of the SEI. The NSLD of these compounds along with the graphites are given in Table 1. The NSLD difference between deuterated and protiated EC, along


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with XRD results, are used to ascertain the relative amounts of carbonate and polymer composing the SEI. Attempts to obtain chemical information using optical spectroscopy were unsuccessful as the graphite dominates signals from infrared adsorption and Raman spectroscopy. However, inelastic neutron scattering (INS) is sensitive to vibrational modes involving hydrogen and possesses no selection rules.12 Therefore, if the layer were primarily organic then bands from the layer would be visible even above a 2 g carbon background.12,17 The major result from the INS spectra in Figure 2c shows that the SEI is polymeric in nature. The red-shifting (5 cm-1) and broadening of the original EC peaks at 110 and 210 cm-1 can be due to either EC trapped within the SEI or the SEI itself.18 If the peaks were due to EC, then major vibrations at 680 and 880 cm-1, corresponding to H2C scissoring and asymmetric stretching, respectively, would still be present.18 These peaks are not found and the shape of the spectra more closely corresponds to a linear alkyl-oxide chain,19 which supports the polymeric SEI interpretation. However, care must be taken as the hydrogen containing vibrational bands in Li2(EC)2 overlap with to those of EC, which again, overlap to those of an extensive polymer like PEO (see supplemental materials). This is because vibrations associated with ethylene groups (–CH2–CH2–) tend to appear around 100 and 200 cm-1. In this regard, it is difficult to come to a definitive chemical identification based solely on INS, however, hydrogen isotopic substitution in the SANS measurements helps to discern between these components. Filling of the close-contact inter-grain voids by the SEI is observed as a decrease in I(Q) near Q = 0.014 Å-1 (arrow in Figure 2a). We take the signal here to be proportional to the square of the NSLD difference between the Li-graphite substrate and the contents of the pore space, i.e.,


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= 0.014 Å) = B⁄CD- × ∆∗

∗ ∗ = B⁄CD- × FDG − *HIJ

(6)

where A is a constant determined from the pristine sample scattering, and CD- is the carbon mass of the LixC6 substrate used for normalization. Since the NSLD of the deuterated Li-EC species is close to that of LixC6 (Table 1), ∆∗ is expected to be small for a primarily organic SEI and the scattering signal resulting from deuterated vs. protiated components in the inter-grain pore spaces will show a greater decrease with respect to the empty Li-graphite. The large difference between the scattering from the deuterated and protiated products and the INS spectra suggest that the SEI is primarily organic in nature. In other words, formation of Li2CO3 is less favorable or kinetically slower than polymer formation. ∗ Within the nanopore space of KLM , the average NSLD is treated as consisting of three phases,

namely a polymeric EC phase, solid EC, and empty space, such that ∗ ∗ ∗ ∗ KLM = KN%! + O%! + ' MPKN

(7)

where the are normalized volume fractions such that ∑ = 1. The choice of the three phases is guided by the SANS and INS data (Figure 2), as well as the XRD results in Figure S2. Because the LiCx + EC/DMC samples showed no XRD signals from the Li2CO3 phase, we were constrained the fitting of the volume fraction in Eq. (6) to the normalized scattering intensity using Eq. (7). The results are provided in Table 2. The increased mass gain between the protiated and deuterated samples results from excess EC in the deuterated sample. In the protiated samples, LiC6 showed more SEI growth than LiC12, which suggests that the driving force for EC reduction is greater for LiC6 than LiC12 due to the greater amount of Li present.


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We follow the changes in the pore volume and specific surface areas of the particles within the nanopores by fitting the deuterated samples' scattering data using PRINSAS,23 a program that fits a polydisperse distribution of spheres to the scattering data, as illustrated in Figures 3a–d and discussed in detail in the supplemental materials. The model is intended for two-phase systems– e.g. carbon and air (Figure 3b)–and the SEI forms a third phase (Figure 3c); therefore, qualitative information can be obtained only by comparison of the LiCx-SEI with the pristine LiCx. Because the NSLD of the deuterated electrolyte is close to the value of the carbon, we treat the SEI/Carbon as a single phase in order to identify the pore scales at which changes occur. As discussed above, the material filling the inter-grain pores consists mainly of Li2(EC)2 along with some unreacted solid EC. From the fit, we obtained pore volume distributions and specific surface area (SSA) for the samples as shown in Figures 3e and 3f, respectively. The SSA calculated for pristine LiCx was ~40 m2 g-1, which is near the N2-BET measurement of milled graphite (88 m2 g-1). Gas adsorption gives a higher value since the probe (N2 molecule) is smaller than the minimum r in the calculation, and can access finer surface details yielding a higher total surface area. The size scale of porosity changes upon exposure to d-EC/DMC. The distributions of both the total pore spaces and SSA decrease by 1–2 orders of magnitude in the reacted samples with the largest observed relative decrease at about r = 10–15 nm, showing that SEI fills the inter-particle pores. There is a peak in the pore size distribution between 1 and 2 nm, which is on the same length scale of the SEI nanodomains within the SEI phase. This feature is indicative of the SEI layer causing increased particle roughness at this length scale. The largest decrease in pore volume distribution and SSA is seen with the LiC12 system, which shows that the voids of LiC12 become filled to a greater extent than LiC6. However, only 20% of the filling phase is polymeric


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Li-EC in LiC12 whereas LiC6 contains 77% (Table 2), therefore the pores of the LiC12 samples are filled with SEI and unreacted EC. Again, this coincides with LiC6 being more reactive than LiC12. For r > 50 nm, LiC6 shows little change and LiC12 shows an increase in the surface area. An increase in the SSA agrees with the observation that the surfaces become rougher (as observed from the surface fractal scattering) after exposure to the electrolyte. For a more quantitative assessment, data collected at lower Q-values and with less multiple scattering than the present experiment are necessary. The SANS and fitting results are in agreement with various post mortem analyses of the SEI formation in Li-ion batteries by electron microscopy8,20 and qualitatively similar to the SANS results of Bridges et al.10 A number of in situ experiments show that the SEI can be thicker, upwards to 100 nm before "collapsing" when formed at high overpotentials and in the presence of excess electrolyte.9,24,25 With that said, a dense and thick SEI is desirable, as it would provide better the anode protection without sacrificing Li-transport rates.20 In conclusion, we have shown how neutron scattering techniques can be used to investigate the solid electrolyte interphase that forms on battery materials. While the SEI is a complicated system, we have attempted to simplify the system by chemically lithiating graphite, increasing its surface area, and then exposing it to select solvents in order to construct the SEI piecewise. Ball milling the graphites produced a nanotextured interface; solvent filling and SEI growth along this interface was readily observed using SANS. Starting with the assumption that the SEI was formed from the sole reaction with EC to produce a combination of Li2CO3 and Li2(EC)2, we were able to fit the resulting SANS data to show that a hydrogen-bearing layer coats the lithiated particles and fills inter-grain voids. That is, the chemical reaction between lithiated graphite and organic solvent preferentially produces a more organic SEI than the dense inorganic SEI


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typically found in electrochemical cells containing Li-salts and fluorides. As a consequence of the increased surface area, INS could be utilized in order to obtain chemical information on the SEI. We showed that the organic component of the SEI had similar vibrational features to polyethylene oxide and that EC binds more strongly with the surface of lithiated graphite due to the formation of high surface area SEI than to raw graphite.

Methods The prelithiated graphite samples were prepared by milling mesophase graphite (MPG-A, Pred Materials) and stabilized lithium powder (SLMP, FMC) together in stoichiometric ratio using zirconia balls (5 mm dia.) for 90 min. The fresh products (0.3 g) were then soaked in deuterated or protiated EC/DMC solution, (3:7 wt.%) for 3 days. Excess solvent was removed in vacuo for 5 h at 1 x 10-4 Torr. A raw milled graphite control experiment was performed and we found no evidence of scattering due to residual EC, i.e. EC is fully sublimed under these conditions. The sample holder was a CR2023 stainless steel (SS) coin cell (Figure S4 in supporting material) as per Bridges et al.10 Between 0.15 and 0.3 g of the powdered sampled was packed into an aluminum spacer ring. A SS disk and spring were placed atop the sample and the cell was hermetically sealed with a coin cell crimper. All sample coin cells, including an empty one for background subtractions, were loaded on a multi-sample translation stage at the General-Purpose Small-Angle Neutron Scattering Diffractometer (GP-SANS) located on beam line CG-2 at the High Flux Isotope Reactor (HFIR), Oak Ridge National Laboratory.26 Measurements were carried out in transmission geometry and an 8 mm-diameter beam size was chosen to avoid scattering from the spring, spacer, and gasket along the edges of the sample cell, as shown in


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Figure S4. Further descriptions of the SANS experiment, data reduction, and modeling, along with the synthesis of the LixC6 material are provided in the supplemental materials (SM).16 INS was conducted on beam line 16B (VISION) at the Spallation Neutron Source (SNS, ORNL);27 measurements were made on a ~2 g packed powder sample loaded into a vanadium cylindrical vessel then cooled to 5 K. More details are provided in SM.


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a)

b)

Figure 1. Schematic of the different strategies for forming an SEI along with the types of reactions involved. The electrochemical process (a) incorporates two distinct electrolyte reduction mechanisms: an electron enters the system and reduces an adsorbed molecule/ion (green layer) and intercalated Li exits the lattice and reduces an adsorbed molecule/ion (blue layer). These processes occur parallel and will form a complicated passivation layer (cyan) The chemical strategy (b) utilized in this report, only allows the chemical reaction of intercalated Li with ethylene carbonate or dimethyl carbonate.


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(b)

(a)

(c)

Figure 2. (a) Scattering curves of graphite, LiC6, and LiC6 + d-EC. Inset table summarized scattering power law of all samples showing that the formed SEI roughens the surface of the graphite particles; b) SANS curves normalized by * ,!- highlight changes at the inter-grain


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pore scales. The intensity at Q < 0.01 Å-1 reflect the change in the carbon/SEI system composition in comparison to the surrounding macrospaces. The region Q ~ 0.014 Å-1 is used to provide a quantitative measure of the amount of polymeric SEI, residual EC, and remaining empty pore space given in Table 2. At Q > 0.2 Å-1, the increased scattering is due to ≈ 1 nm size domains within the newly developed phase on the surface of the carbon substrate; and c) Inelastic neutron scattering of EC, LiC6 + EC/DMC, and washed LiC6 + EC/DMC. Stars (*) indicate major peaks from ascribed to EC18 and crosses (†) correspond to peak locations of polyethylene oxide-type vibrations.


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(e)

(f)

Figure 3. (a–d) depict the regions that are probed by SANS. The SEI causes agglomeration of the carbon particles and decreases the empty volume within the nanopores. PRINSAS software23 fits the empty space in the inter-grain voids with a general distribution of spheres to produce volume-normalized pore size distributions (e) and the specific surface area (SSA) (f) of the initial LiC6 and LiC12 compared with those reacted with d-EC/DMC. The SSA values for r near 0.4 nm agree with the BET–obtained SSA.


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Table 1. Neutron scattering length densities of lithiated carbons and deuterated (d)/protiated EC used in this study, along with possible lithium carbonate and polymeric SEI products.

Table 2. Percent composition of products of LixC6 + EC/DMC and the mass gain. The deuterated samples display large EC diffraction peaks that agree with the increase Mass Gain %. The ∗ volume fractions of phases 1, 2 and 3 that give the average pore NSLD, *HIJ , are displayed. The

polymeric EC fraction, c1, is lower for LiC12 than LiC6, corresponding to less SEI formation.


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AUTHOR INFORMATION Corresponding Author * To whom correspondence should be addressed. Email: [email protected]. Tel: +1 865 241 5135 (RLS); [email protected] +44 (0) 1235 445923 (JLB). Author Present Address † ISIS Facility R3 UG15, STFC Rutherford Appleton Laboratory, Harwell, Didcot, Oxon, OX11 0QX, UK. E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript and contributed equally. ACKNOWLEDGMENT The experiments and authors (RLS, JLB, GR, and NJD) were supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (BES-DOE), and as part of a user proposal by Oak Ridge National Laboratory's Spallation Neutron Source (YQ, UC, LLJ, AJRC) and High Flux Isotope Reactor (KCL), which are sponsored by the Scientific User Facilities Division, BES-DOE. Additional experimental support for G.M. Veith was provided by Materials Science and Engineering Division of the U.S. BES-DOE.


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ASSOCIATED CONTENT Supporting Information. Details regarding the synthesis of the lithiated graphite, SANS and INS experiment and SANS modeling. SEM image of the milled carbons along with particle size distribution (S1); N2 and CO2 gas adsorption isotherms curves obtained from N2(g) and CO2(g) demonstrating micropore formation from milling (S2); XRD and representative Rietveld refinement of products showing the amount of material reacted (S3); SANS of carbons along with control reaction between graphite and EC/DMC (S4); Calculated volume fractions of the SEI and carbons (S5). This material is available free of charge via the Internet at http://pubs.acs.org.

ABBREVIATIONS SANS, small-angle neutron scattering; SEI, solid-electrolyte interphase; EC, ethylene carbonate; DMC, dimethyl carbonate; PEO, polyethylene oxide; INS, inelastic neutron scattering; SSA, specific surface area.

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Structure of Spontaneously Formed Solid-Electrolyte Interphase on

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