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Lithium Transport in an Amorphous LixSi Anode Investigated by Quasi-elastic Neutron Scattering Robert L Sacci, Michelle L Lehmann, Souleymane Omar Diallo, Yongqiang Q. Cheng, Luke L. Daemen, James F. Browning, Mathieu Doucet, Nancy J. Dudney, and Gabriel M. Veith J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 29, 2017

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The Journal of Physical Chemistry

Lithium Transport in an Amorphous LixSi Anode Investigated by Quasi-elastic Neutron Scattering Robert L. Sacci, †* Michelle L. Lehmann, † Souleymane O. Diallo, ‡ Yongqiang Q. Cheng, ‡ Luke L. Daemen, ‡ James F. Browning, ‡ Mathieu Doucet, § Nancy J. Dudney, † Gabriel M. Veith † †

Materials Science and Technology Division, ‡ Chemical and Engineering Materials Division,

and § Neutron Data Analysis and Visualization Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

ABSTRACT: We demonstrate the room temperature mechanochemical synthesis of highly defective LixSi anode materials and characterization of the Li transport. We probed the Li+ selfdiffusion using quasi-elastic neutron scattering (QENS) to measure the Li self-diffusion in the alloy. Li diffusion was found to be significantly greater (3.0 x 10-6 cm2 s-1) than previously measured crystalline and electrochemically made Li-Si alloys; the energy of activation was determined to be 0.20 eV (19 kJ mol-1). Amorphous Li-Si structures are known to have superior Li diffusion to their crystalline counterparts; therefore, the isolation and stabilization of defective Li-Si structures may improve the utility of Si anodes for Li-ion batteries.

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INTRODUCTION Silicon has become a viable candidate to replace graphite as the anode of choice for Li-ion batteries due to its low cost and high-energy capacity (4200 mAh g-1, an order of magnitude greater than graphite)1,2. However, its commercial use has been limited by slow lithium diffusion, excessive volume expansions, and particle cracking3-8. In order to develop means to improve Li transport and diffusion in silicon-based anodes, the mechanism and dynamics of Li transport has been studied using various electroanalytical9-13 and nuclear magnetic resonance (NMR) techniques14-16. These reports show a wide range of chemical diffusion coefficients and energy of activations, which may be due to the varying structures, electrical conductivity of the electrode material, or electrolyte environment. In this preliminary report, we expand the use of QENS to study Li-self diffusion in high capacity anode materials. QENS has been utilized for over three decades to study diffusion and molecular dynamics in solids, plastics, and confined fluids17. While most systems studied by QENS involve probing the dynamics of the protons–1H has one of the largest incoherent neutron scattering cross-section (iNSC) of any elemental isotope commonly found in battery materials–there have been reports that follow Li and Na diffusion in various ionic conductors.18-20 Zabel et al.21,22 looked at Li transport in LiC6, the typical anode material for commercial Li-ion batteries, in the early 80's and found that the 2D Li diffusion proceeded in a jump-like fashion. Here, we use QENS to probe Li diffusion in Li15Si4, a major phase formed during electrochemical charge and discharge of Si-based Li-ion anodes. We expect any QENS signal to be unequivocally attributed to Li diffusion given that it is far more mobile than Si and the iNSC of Si is near zero while that of 7Li is 0.78 barn 17. Diffusion


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will be measured in terms of self-diffusion, i.e., Li performs a random walk through the Si lattice or amorphous structure17,23. As discussed later, this is intrinsically different than Li diffusion as measured electrochemically but similar to that probed using NMR 17. We show that high-energy ball milling, used to make these materials, does result in some crystalline features of Li15Si4 independent of reaction stoichiometry. Therefore, the energy of activation of Li diffusion will be a combination of the amorphous phase and a small crystalline phase. EXPERIMENTAL High-energy ball milling of lithium ribbon (99.99%, Alfa Aesar) and silicon powder (99.9985%, Alfa Aesar) with graphite coated ZrO2 balls within high-density polyethylene (HDPE) bottles in stoichiometric quantities (Li15Si4) chemically lithiated the amorphous silicon. This method has been shown to produce high purity lithium compounds with graphite24 and can be used as a step in treating the surfaces of Li-ion anode materials25. As the reaction is highly exothermic and the product a strong reducing agent, the lithium was added in six aliquots with milling for 90 min between each stage. X-ray diffractometer (X'Pert Pro, PANalytical) used to evaluate the extent of lithiation and subsequent crystallization was operated at 40 kV and 40 mA. The product was sealed between two Mylar sheets using Torr Seal®. Differential scanning calorimetry (DSC, NETZSCH DSC 214 Polyma) sample were gently packed (minimizing surface contact with the Al) and sealed in Al cans in an Ar-filled glove box and the reference was a similar Al can filled with Ar(g). The temperature waveform was set to 10 K min-1 ramp from 300 to 625 K with a 5 min hold before a -10 K min-1 controlled cooling ramp back to 300 K. A second temperature cycle was taken after a 1 h hold at 300 K. We note here that Li is known to react with Al; however, given the fact that the Li is within the Si and the melting point of the alloy is never reached, we doubt reaction with Al—the Al container remained in pristine condition.


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The QENS measurements were taken at the Backscattering Silicon Spectrometer (BASIS), located at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL)26. The instrument's inverted geometry uses a wide angular coverage of large Si(111) crystal arrays to Bragg-select the final energy (2.08 meV) of the neutrons scattering off the sample out of a broad wavelength band of incoming neutrons. This configuration makes it possible to achieve: 1) superior energy resolution (FWHM = 3.5 µeV); 2) accurately determine the scattering momentum, Q, and energy, E, transferred to the sample using the angular position of the crystals; and 3) the time it takes each neutron to travel from the moderator to the detector. The instrument covers a useful dynamics range from -100 to +100 µeV, and a momentum space from 0.3–2 Å-1. The Li compound (3.6 g) was loaded and sealed between two Al plates with an indium gasket. The plates defined the sample thickness to be 1 mm and the gasket inhibited investigations above 375 K. These containers were then inserted into a close-cycle-refrigerator and QENS measurements were taken from 200 to 370 K. This includes standard diagnostic elastic scans as a function of temperature to determine the proper temperature range for Li diffusion that falls within the spectrometer observable window and subsequent high statistical quality QENS data at two temperatures of 300 and 370 K (2.5 h each). The temperature range here is not expected to be enough to react with the container, also no evidence of reaction seen.


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a Crystallization peaks

No peaks on repeated sweeps

b)

Figure 1. (a) differential scanning calorimetry, which shows crystallization peaks, and (b) x-ray diffractogram with Rietveld refinements of the milled Li15Si4 alloy highlighting the anti-site requirement in fitting the pattern. Large peak at 26.4° is from the mylar foil. Inset shows VESTA rendering of “amorphous” Li15Si4 with 95% confidence ellipsoids.

The inelastic neutron scattering (INS) measurements were taken at VISION (BL-16B) located at SNS27. The exact same sample and holder used in the QENS measurement was used here without any modification, though the temperature of the measurement was 5 K. RESULTS and DISCUSSION Li forms a series of crystalline line compounds with Si upon heating to liquid phase and cooling thereafter; however, at ambient conditions Li can diffuse into silicon upon contact producing


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amorphous silicides28. In situ TEM29-31 and XRD of mechano-29,32 and electrochemical33,34 alloying of Li and Si have shown that crystalline phases only occur at high Li loading, that is once the Li stoichiometry is greater than Li15Si4. Therefore, to obtain crystalline line compounds, annealing of amorphous alloys between 420-520 K must be done

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. To show this, a DSC

spectrum (Figure 1a) was recorded and clear crystallization peaks occurred between the mentioned range. This transition was irreversible because once the crystalline phase was formed the only other phase transition was the melting point, which is ~970 K – beyond the DSC can’s limit 35.

Figure 2: Observed quasielastic neutron response (QENS) for Li15Si4 at 370 K (red) and 300 K (green) and at momentum transfer 0.9 Å-1. The instrument resolution (blue) is shown for comparison. Inset shows the piecewise fitting results from Eq. 1.

We further checked on the crystalline nature of the milled product using XRD—Figure 1b. The pattern shows Bragg reflections that closely match that of Li15Si4 (96-400-1837, COD). The relative peak intensities did not match the reported phase; in fact, the only way to arrive at a reasonable fit to the XRD data was to have two phases with different Si occupancy at the Si sites within Li15Si4. The Si occupancy value for the major phase (85 mole%) was 0.7 and that of the


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more pure phase was 0.98 (10 mole%). The amorphous Si composes the remaining 5% of the powder sample. Analyzing the full-width-half-max of the Bragg peaks using provided and estimation of the crystallite size 36. The starting amorphous Si powder was found to have 0.35% micro-strain have 120 nm crystallites. The high occupancy and low occupancy phases had 0.10 and 1.1% micro-strain, and 300 and 22 nm crystallite sizes, respectively. The order of magnitude difference in the low occupancy’s strain and crystallite size compared with the high occupancy’s and starting Si powder’s and its composing and estimated 90% of the sample suggested that the product of high energy milling is a highly disordered form of the line compound. The irreversible crystallization peaks in the DSC support this. In order to maintain this amorphous nature, which more closely resembles electrochemically relevant compounds, we did not anneal the product. Figure 2 shows the quasielastic scattering of Li15Si4 at 300 and 370 K. The total signal is a combination of elastic, (), and QENS signal along with a background term, (). The QENS signal was modeled using a single Lorenzian function with Λ() as it half-width-at-halfmaximum (HWHM). The total scattering is therefore of the form

(, ) = () + (1 − )

1 Λ() ⨂() + () + Λ()

(1)


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Figure 3: Inelastic neutron backscattering spectrum of Li15Si4 showing any protons present in the sample. The difference spectrum is a combination of the phonons from Li15Si4 and polyethylene (< 1 mass%) from the milling vessel. where () is the instrument resolution function. QENS spectra taken below 250 K showed no change in peak width, and therefore the spectra at 200 K is taken to be the measurement (). The temperature dependency of Λ() provides information on the hopping frequency. Spanning a Q range and fitting a suitable diffusion model can allow for extraction of the diffusion coefficient and energy of activation. As seen in Fig. 2 the QENS signal broadens as temperature increases, thereby showing that measurement of self-diffusion in this material is possible. As stated before, silicon has little-to-no incoherent scattering; however, since protons bear a large incoherent scattering cross-section, care must be taken to control the amount of hydrogen in the system. Figure 3 shows the inelastic neutron scattering spectrum of Li15Si4. The spectrum shows no indicative peaks of hydrogen compounds such as LiH37, H2O38 or known SixOyHz39 compounds. It should be noted that peaks do appear at 720 cm-1 and between 1300–1600 cm-1 above the background. These peaks cannot be ascribed to crystalline Li15Si4; rather, these peaks


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are from a ~1 wt% contamination of polyethylene originating from the milling bottle40. The dynamics of the system as measured by QENS do not match that of polyethylene41,42. Also, unreacted metallic Li may have been present in the sample which would affect the QENS measurement. 7Li NMR of the sample showed no Knight peaks at 270 ppm which corresponding to metallic lithium43,44. Therefore the QENS response was equivalently from the Li within the LiSi alloy. The dependency of Λ on Q and temperature is shown in Figure 4a. All the QENS broadening data could be fit with a single Lorenzian function and likewise we fitted the HWHM to a single isotopic jump-diffusion model for amorphous material45:

Λ() =

ℏ 1 −

(2)

where D is the 3D diffusion coefficient and the time between jumps. The jump length, l, can be calculated using = √6#. Eq. 2 provides an adequate fit to the Λ profiles. The diffusion of the Li was found to increase with temperature as expected by the Einstein-Stokes diffusion expression (Eq. 3, Figure 4b); D increased from 3.0 to 13.5 × 10-6 cm2 s-1 upon increasing the temperature from to 370 K with an energy of activation of 0.20 ± 0.3 eV. The jump length and hopping time, τ, changes with temperature from 2.3 Å and 30 ps (300 K) to 5.1 Å and 33 ps (370 K), respectively. The rendering of the crystal structure (inset in Figure 1b) highlights the the fact that most of the Li is not bonded with Si and is expected to be able to move relatively freely especially when the Si sub-lattice shows disorder.

Ln (#) = −

& + Ln (#* ) '( )

(3)


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The diffusion coefficients and the energy of activation obtained through fitting agree with other reports. Kuhn et al.14,15,46 found that Li diffusion mechanism through Li12Si7 contained least three different pathways: each containing energy of activations of 0.55, 0.32, and 0.18 eV, respectively. The NMR measurement provides jump rates and if we assume their modeled jumping distance of 3.0 Å, their calculated diffusion coefficient for crystalline Li-Si would be 8.4 × 10-8 cm2 s-1, near the temperatures studied here. Tritsaris et al.47,48 found agreement with

a

b)

-1

EA = 0.20 eV (19 kJ mol )

Figure 4: (a) the overall Q-dependence of the half-width-at-half-maximum (Λ, solid symbols) of the QENS signal fitted to Equation 2 (solid lines); (b) resulting diffusion coefficients plotted against 1000 T-1.


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these values using density functional theory calculations. Other groups have used electroanalytical techniques to measure chemical Li diffusion as a function of crystalline phases. Li et al.8 showed that the Li diffusion rate in the electrochemically formed amorphous Li-Si materials is lower than the same stoichiometry during delithiation: 1.5 vs. 4.5 × 10-13 cm2 s-1, they also showed no obvious effect of state of charge (Li concentration) on the diffusion coefficient. The cause of this was speculated to be related to either highpressure/stress conditions or a byproduct of irreversible dynamics. Kulova et al.13 measured similar diffusion rates using cyclic voltammetry and impedance spectroscopy, and found the energy of activation was 0.44 eV. Yoshimura et al.49 obtained a chemical diffusion rate of ~10-9 cm2 s-1 for a vacuum deposited thin film, while a crystalline sample was along the order of 1011

cm2 s-1. They attributed the two order of magnitude difference to the less rigid and dense

nature of the vacuum deposited sample. Wang's group used in situ TEM to show that electronic properties of the Si have a dramatic effect upon the electroanalytical measurement of Li diffusion, as well as the particles' propensity for crystallization29-31. Therefore, the broad range in the electrochemical diffusion coefficients may have to be interpreted in terms of structure and electrical conductivity of the Si being investigated. Fedorov et al.50 used molecular dynamics (MD) simulations to model chemical diffusion and found agreement with Yoshimura and coworkers' results, thereby supporting the fact that Li transport through amorphous, disordered, and possibly strained silicon is significantly faster than well-ordered crystalline Si. We speculate that this would extend to self-diffusion, which would show a decrease in the energy barrier required for a jump and a possible increase in the jump length in amorphous Li-Si materials51. Wen and Huggins10 found that Li self-diffusion in alloys is sensitive to the crystalline phase, with the self-diffusion near Li15Si4 being ~2.9 x 10-7 cm2 s-1.


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Our measured values for amorphous Li-Si are within the two orders of magnitude difference between amorphous and crystalline silicon found by Yoshimura et al. and Fedorov et al. While a more complete QENS study coupled with structural investigation such as x-ray or neutron pair distribution analysis is required, our results are a start in understanding the dynamics of Li in amorphous high energy density anode materials. CONCLUSION In summary, we showed that diffusion mechanisms within Li, and quite possibly Na, energy storage anode materials might be readily studied using QENS. This is true provided that the host material is composed of low incoherent neutron scatterers, e.g., Sb, Pb, and Si. The energy of activation of Li self-diffusion in amorphous lithium silicide was found to be 0.20 eV, which is in excellent agreement with those calculated from MD simulations, and measured using NMR. We know that Li diffusion in Si seems to be sensitive to stoichiometric, concentration, and probing technique; therefore, future work will focus on comparing different Li concentrations in amorphous Li-Si compounds with their crystalline counterparts. AUTHOR INFORMATION Corresponding Author * corresponding authors: [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.


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ACKNOWLEDGMENT Research supported by the Office of Basic Energy Science (BES-DOE) Division of Materials Science and Engineering (RLS, MLL, NJD, GMV – synthesis, characterization) and as part of a user proposal by Oak Ridge National Laboratory's Spallation Neutron Source (SNS), which is sponsored by the Scientific User Facilities Division, BES-DOE (SOD, YQC, LLD, JFB, MD).

This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-publicaccess-plan).

ABBREVIATIONS QENS, quasi-elastic neutron scattering; DSC, differential scanning claorimetry; XRD, x-ray diffraction REFERENCES 1 2

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Lithium Transport in an Amorphous Li x Si Anode ... - ACS Publications

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