Probing Multiscale Transport and Inhomogeneity in ... - ACS Publications

Oct 17, 2016 - Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy, Telangana 502285, India. •S Supporting Informat...
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Probing Multiscale Transport and Inhomogeneity in a Lithium-Ion Pouch Cell Using In Situ Neutron Methods Hui Zhou,†,‡ Ke An,† Srikanth Allu,† Sreekanth Pannala,†,§ Jianlin Li,† Hassina Z. Bilheux,† Surendra K. Martha,†,∥ and Jagjit Nanda*,† †

Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States NECCES, State University of New York at Binghamton, Binghamton, New York 13902, United States § SABIC, Houston, Texas 77042, United States ∥ Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy, Telangana 502285, India ‡

S Supporting Information *

ABSTRACT: We demonstrate the lithiation process in graphitic anodes using in situ neutron radiography and diffraction in a single-layer pouch cell. The variation in neutron absorption contrast in graphite shows a direct correlation between the degree of lithiation and the discharge potential. The experimental neutron attenuation line profiles across the graphite electrode at various discharge times (potentials) were compared with lithium concentration profiles computed using a 3D electrochemical transport model. In conjunction with imaging/radiography, in situ neutron diffraction was carried out to obtain information about the local structural changes during various stages of lithiation in carbon. Combined in situ radiography and diffraction supported by 3D multiscale electrochemical modeling opens up a powerful nondestructive tool that can be utilized to understand the multiscale nature of lithium transport as well as observe various inhomogeneities at a cell level.

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degraded or postcycled electrodes has also provided important insights regarding the changes in microstructure and/or chemical composition at the surface and bulk.16−20 The majority of these techniques involves invasive procedures such as cutting open the cell and harvesting electrodes or utilizing nonstandard in situ cells for studying structural changes or transport where the electrode and cell geometries can deviate from those of a real working battery.3,4,6−8,17−20 Although insightful, such studies may not capture every detail of the cell chemistry or transport inhomogeneities intrinsic to a commercial battery cell. This study utilizes neutron imaging contrast to spatially observe lithium transport across thick graphite electrodes at various states of lithiation in a pouch cell configuration. In conjuction with in situ neutron diffraction, combined results provide the spatial map of lithium transport and distribution with accompanying chemical (structural) changes covering a length scale ranging from the nanometer to micron level. Neutron radiography/imaging profiles measure the change in

lithium-ion battery is a complex multiscale device covering a whole array of physical, chemical, and electrochemical processes covering multiple spatial and temporal scales.1 For example, electrochemical transport in a typical lithium-ion battery during charge and discharge involves both ion and electron transport across length scale ranging from the active primary particles (nanometers) to composite electrodes (∼100s of micron) and ultimately across the macroscopic cell.2 One of the key scientific challenges often is to observe such processes in situ across multiple length and time scales noninvasively to fundamentally understand key mechanisms that govern their performance such as transport, cycle life, and safety. This study reports a combined in situ neutron radiography and diffraction study of a lithium-ion cell enabled by multiscale 3D electrochemical modeling to probe transport and phase changes in graphite electrodes. In situ neutron approaches combined with multiscale modeling open up a powerful nondestructive tool that can be utilized to understand the multiscale nature of lithium transport as well as observe various inhomogeneities at electrode and cell levels. A number of in situ techniques have been recently developed for studying electrochemical transport during battery operation in order to get a fundamental understanding behind their capacity degradation and failure.3−15 In addition, ex situ study of © XXXX American Chemical Society

Received: August 12, 2016 Accepted: September 20, 2016

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DOI: 10.1021/acsenergylett.6b00353 ACS Energy Lett. 2016, 1, 981−986

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http://pubs.acs.org/journal/aelccp

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ACS Energy Letters the optical density (OD) or contrast,21,22 and an attempt has been made for the first time to correlate the changes in OD upon lithiation to local lithium concentration profiles calculated using 3D electrochemical transport modeling.23 In addition, we provide a comparative analysis of electrode swelling measured by the neutron radiography technique with lattice dilation due to lithiation in graphite measured from neutron diffraction. The study is a proof-of-principle example that provides a precursor for the next-generation beamline at Oak Ridge National Laboratory that utilizes a spallation neutron source combining high spatial resolution imaging with phase and elemental capability (diffraction). This will no doubt herald a new era in the area of multimodal-multiscale imaging applied to energy storage and conversion systems. Neutron radiography was performed at the High Flux Isotope Reactor CG-1D beamline at Oak Ridge National Laboratory.24 The basic principle of neutron imaging and its application to the area of lithium-ion batteries were reported earlier by a number of groups.12,13,22,25,26 A brief overview is presented in the Supporting Information as well. Figure 1

Figure 2. (a) First cycle discharge profile of graphite versus time (minute) and capacity (mAh/g). (b) Corresponding normalized transmission radiographs at different stages of lithiation during the discharge process. The green dots are the time intervals at which we analyze the radiographs, as shown in (a).

in the graphite electrode for the equivalent discharge times indicated in Figure 2a. The data clearly show a gradual reduction of transmission in the two graphite electrodes during discharge due to an increasing lithium insertion inside of the graphitic layers, which proceeds as a function of voltage, a process normally referred to as staging.27 The end of discharge implies maximum neutron absorption or attenuation (blue area) due to high lithium absorption in the graphite. Comparing the lithium distribution at different time points during discharge, we observe nonuniformity along the electrode height (Y direction). This can happen for a number of reasons, including experimental artifacts and uneven thickness during cell fabrication. These effects are difficult to quantify with this through-plane setup but have been shown with in-plane study previously.25,28 In addition, nonuniform contact at the interface between the electrodes and separator could also contribute to the uneven lithium intercalation and diffusion, which cannot be confirmed due to limited spatial resolution of the technique. Figure 2b provides a direct observation of the lithiation progress in the graphite electrode at various discharge times. Even further, performing a line profile analysis of the radiographs, quantitative information about time-dependent lithium transport or lithium distribution during the lithiation progress can be obtained. Line profile analyses are performed across the cell in the X direction (see Figure 1b), as illustrated in Figure 3. The change of transmission of each pixel across the cell in the X direction can be obtained, which is related to the change of lithium concentration. In order to appreciate the experimental results illustrated in Figure 3, it is worthwhile to remember the cell geometry described in Figure 1b. The thick lithium metal (∼700 μm) is sandwiched between two graphite electrodes with a thin separator on each side. Data analysis was performed at two different cell heights (Figure 3a): the top (#1) and middle (#2). The two peaks seen in the Figure 3d,e correspond to the two graphite electrodes in the cell. Their intensity changes with discharge progress due to reduced transmissions with gradually increased lithium concentration during lithiation, as observed in the radiographs identified from 1 to 8 (see Figure 2 for different discharging voltages or times). The individual transmission profile indicates the nature of the spatial distribution of Li in the graphite

Figure 1. (a) Neutron radiography setup showing the source, the aperture, the cell in front of the detector, and a representative transmission radiograph of the pouch cell and (b) configuration of the pouch cell used for neutron radiography and diffraction. Details are described in the methods and Supporting Information.

shows the experimental radiography setup and the pouch cell geometry, with details provided in the methods discussion. Briefly, a collimated incident neutron beam passes through the pouch cell along the planar or edge direction as shown. The attenuated beam enters the neutron-sensitive detector, which digitally records the changes in neutron transmission, producing a 2D projection of the cell on the detector plane, resolving the internal electrode structure. The overall spatial resolution is determined by several experimental and instrumental factors as described in the methods discussion. The pouch cell has two graphite electrodes on both sides of lithium, designed to maximize the in situ diffraction signal from graphite. A discussion on neutron image contrast normalization, analysis, and factors contributing to measurement uncertainty used is provided in the Supporting Information and in a related study reported earlier by Siegel et al.22 Figure 2a shows the voltage profiles of the first discharge vs time in minutes, with the corresponding capacity in mAh/g shown. Eight normalized transmission radiographs equally spaced in time (as shown in Figure 2b) illustrate the change in Li content 982

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Figure 3. Line profile analyses for the eight selected images from Figure 2 at two different cell heights in the Y direction: (a) position of the two line profiles used for transmission as a function of the X direction studies; (b) neutron image of position #1 at the discharge state of point 4; (c) neutron image of position #2 at the discharge state of point 4; (d) line profile #1 transmission plot; and (e) line profile #2 transmission plot.

incident neutron beam path. The uneven curvature of the electrode thickness is clearly noticed along the Y direction. The result further highlights the significance of this method in detecting the changes in the internal cell geometry and uniformity noninvasively. To further quantify the state of lithiation in the graphite electrode with the experimentally observed neutron radiography, we modeled lithium transport in graphite using similar electrode/cell dimensions and parameters. Both spatial and line profiles of lithium concentration as a function of voltage or state of charge (SOC) were calculated using a 3D electrochemical transport model, AMPERES, developed at Oak Ridge National Laboratory.23,29 The Li/LiyC6 cell sandwich was modeled with galvanostatic discharge boundary conditions. A 3D volume averaged formulation was used to capture the transport of lithium in graphite across the cell. The solid-phase particles were assumed to be spherical with radii much smaller than the thickness of the electrodes. Consistent with this assumption and to reduce computational complexity, the Duhamel approximation was used as a solution profile in the radial direction of these spherical particles.23 The physical domain consisted of a lithium foil negative electrode, a porous separator, and a porous carbon positive electrode with an electrolyte. We model the half-cell, that is, two regions of a separator and positive electrode. The conservation equation of the lithium-ion concentration ce within the electrolyte solution is given by

electrode at a given moment in time. The peak of the transmission profile gradually decreases with lithiation, implying lower neutron transmission as a function of the formation of LixCy ions, which could reach a maximum at the end of the discharge, LiC6. In addition, at later times in the discharge process, the profiles shift toward the bulk of the electrode from the Li metal/separator toward the other edge, closer to the current collector. It is interesting to compare the transmission values for two specific cell height locations (Y axis) using the line profiles as a function of discharge time. The transmission intensities are different, indicating a nonuniform Li distribution in the Y−Z plane. In addition, the change of transmission during the discharge steps is distinct for each profile for the two graphite electrodes even at the same discharge state, indicating a nonuniform rate of Li intercalation/diffusion. It could be plausible that the observed variation of transmission profiles is partly contributed by extrinsic factors such as uneven bending of electrode layers during cell fabrication that could lead to smearing of lithium metal into the graphite electrode or vice versa. Furthermore, any local stress or uneven pressure can distort the electrode, lithium metal layers, and the corresponding local thickness. To illustrate this aspect further, the line profile analysis showing the average transmission values integrated along the Z direction for regions of interest (marked by blue rectangles) at two different cell height locations is shown in Figure 3a. Any loss of contact, defects, or voids along the Y−Z plane can significantly affect the rate of lithium transport and hence affect the local lithium content per electrode volume. For the lithium metal sandwiched between two graphite electrodes, the relative change of Li concentration (during discharge) compared to the overall content of lithium is negligible; therefore, the normalized transmission should remain relatively unchanged in the thick lithium region. Nevertheless, profile #2 exhibits a somewhat different profile shape where there appear to be changes in transmission in the lithium region as well. At present, the exact reason for such observation is unknown, but such variation can be caused by the curved or nonuniform electrode thickness along the Z direction (perpendicular to the X−Y plane), where the graphite electrode could partly overlap with the Li metal along the

1 − t+0 Li ∂(εece) − ∇·(εeDeeff (εe)∇ce) = j ∂t F

The potential in the solution phase is referenced to the lithium electrode and is defined by conservation of charge ∇·(εeK eff (εe)∇ϕe) + ∇·(εeKDeff (εe)∇ ln ce) = −j Li

Similarly, in the solid phase, we have conservation of solidphase concentration ∂(εscs,avg) ∂t

− ∇·(εsDseff (εs)∇cs,avg) = −

j Li F

and conservation of charge 983

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(attenuation) profiles at specific discharge times. The transmission profiles are inverted to match with the calculated lithium concentration profiles, providing a correlation between the spatial variation of local lithium concentration and neutron absorption contrast at a particular state of lithiation across the electrode thickness. We notice good agreement between the modeled Li concentration at different SOCs and the neutron attenuation profile in the graphite electrode regions located at the end regions (0−0.1 and 0.4−0.5 mm, Figure 4). The experimental absorption profiles at a particular state of discharge appear to be broader than the calculated concentration. This is understandable because the modeled Li concentration profiles are not convoluted with any broadening parameters or function to take into account various experimental uncertainties such as a diverging beam profile, sample−beam interactions, and cell geometry effects. The presence of liquid electrolyte in the Li metal and separator will also contribute to the finite absorption contrast, leading to smearing of the neutron attenuation profile apart from other broadening effects, as described above. The computed spatial maps showing the lithium distribution across the graphite electrode are shown in the Supporting Information (Figure S4) with details. It is important at this point to mention that while the imaging/radiography method is based on changes in the OD of the material medium, the electrochemical transport model calculates the local changes in lithium concentration based on diffusion. Although indirect, both are sensitive to the local variation and transport of lithium at comparable length scales. This novel aspect is discussed for the first time in this study. In situ neutron radiography as illustrated in the previous section can profile a spatial contrast of the degree of lithiation over the entire volume of the cell/electrode, but it does not provide chemical or phase information. Here we carried out in situ neutron diffraction to investigate phase transformation as well as the changes in lattice dimensions as lithium is intercalated in graphite. An in situ diffraction experiment was carried out on the same pouch cell geometry with an exact electrode thickness and geometry to compare with radiography results. Graphite is alternated layers of graphene in an ABABAB stacking sequence.30 Lithiation in graphite will cause an increase of interplanar spacing between graphene layers, to subsequently form LiC24, LiC12, and LiC6 phases with different stacking sequences between graphene and ordered Li layers.27,31,32 As shown in Figure 5a, the d spacing of graphite (004) increased as soon as the current was applied, and after about 1/3 of the theoretical Li was intercalated, another strong diffraction peak appeared at around 1.76 Å, which is characteristic of LiC12(004).33 This phase exists through the following whole discharge process but with gradually decreased intensity and the concomitant appearance of a LiC6(002) peak at around 1.85 Å34 at the end of discharge. As for the LiC24 phase, there is a weak peak at around 1.73 Å, a characteristic of LiC24(006)35 that can be observed in the discharge process, which is more apparent in the charge process. The increased lattice spacing contributes to the electrode expansion, which can be determined by neutron radiography as well through measurements of the thickness change of the graphite electrode. Due to limited spatial resolution, it is difficult for us to distinguish the graphite electrode from the cell. If we assume that all other components of the cell remain constant and only lithiation to graphite causes cell expansion, we can use the change of cell thickness to evaluate the electrode

and the chemical kinetics at the electrode/electrolyte interface is given by the Butler−Volmer equation ⎡ ⎛ αajF ⎞ ⎛ αcjF ⎞⎤ inj = i0⎢exp⎜ ηj⎟ − exp⎜ η ⎟⎥ ⎝ RT j⎠⎦ ⎣ ⎝ RT ⎠

To computationally well pose this half-cell simulation without the anode region, three boundary conditions are necessary iapp ∂c Deeff e = F ∂x x = 0 σ eff ϕe

∂ϕs ∂x

x=0

= iapp x=L

=0

Various materials parameters such as density, porosity, electrolyte composition, and electrode thickness are provided as electrochemical transport model input for a meaningful comparison of the imaging results. Details regarding the modeling parameters and input are described in the Supporting Information. On the basis of this model, we calculated the line profiles of the solid-state lithium concentration (in mol/cm3) as well as 2D spatial profiles as a function of discharge time, as shown in Figure 4. Any inhomogeneity in the cell fabrication such as

Figure 4. Comparison of the calculated lithium concentration as a function of electrode thickness (red curves) to the experimental line profiles of the attenuation (blue curves) at discharge times of 25, 80, and 110 min.

variable porosity in the electrode is difficult to quantify and is neglected for the purpose of this study; hence, uniform properties of the materials and thickness is assumed. Also, the Li intercalation reaction is modeled as a single-step process; thus, different phases of LixCy cannot be identified in the modeling results. In the line profiles of Figure 4, we observe that as the discharge continues the Li concentration gradually increases with formation of peaks near the separator/electrode interface and then shifts toward the bulk of the electrode toward the end of the discharge as time proceeds. Superimposed (blue lines) on this are the neutron transmission 984

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transport model simulated using a real electrochemical parameter and pouch cell geometry. In conjunction with in situ radiography, we also carried out in situ neutron diffraction for a similar pouch cell configuration with the intent to observe the phase changes as lithium gets intercalated in graphite forming LixCy, finally leading to LiC6. Although in situ neutron radiography of pouch cells is at an early stage of development in terms of desired spatial resolution, this study demonstrates a proof-of-principle method where imaging/radiography combined with diffraction can emerge as a powerful tool for studying degradation and inhomogeneities in electrochemical devices such as lithium-ion batteries nondestructively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00353. Experimental methods; basic concept of neutron imaging; data normalization and measurement uncertainty of neutron imaging; schematic illustration of the setup of in situ neutron diffraction (Figure S3); and electrochemical modeling of lithium transport in graphite (PDF)

Figure 5. (a) In situ neutron diffraction pattern during the first discharge and charge. The voltage (current) is plotted in the side panel to the left (right) of the diffraction data. The acronyms used in the figure are OCV, open-circuit voltage; CCD, constant current discharge; and CCC, constant current charge. (b) The thickness change of the cell from line profile analysis (black block) and the change in the intergraphite layer distance from neutron diffraction (blue circle) during the discharge process. (c) The thickness change of the cell vs the change of the intergraphene layer distance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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expansion. To determine the thickness of the cell, a line profile analysis needs to be performed on the raw data of the neutron image, and the fwhm of the transmission profile can be used to estimate it. The thickness change of the cell with discharge is shown in Figure 5b. The lattice parameters can be determined with the diffraction pattern by Rietveld refinement, and the distance of the intergraphene layers is calculated based on the crystal structural models of each phase. The relative change of the intergraphene layer distance during electrochemical discharge is also shown in Figure 5b. The thickness change with lithiation determined by neutron radiography (black block) is compared to the intergraphene layer distance change from neutron diffraction (blue circle) in Figure 5c. Strong correlation between the two techniques can be directly observed, although the magnitude of the thickness change is higher than the d spacing change because in the lithiation process, the cell expansion not only comes from the lattice expansion of graphite electrode but is also caused by other factors, such as gassing inside of the cell25,28,36 and the formation of the SEI film.37,38 In summary, this work demonstrates in situ neutron radiography of the lithiation process in graphite during discharge comprised of lithium metal and graphite electrodes in a pouch cell format. Despite the current limited spatial resolution of this method, we are able to clearly correlate the changes in neutron imaging contrast to the state of lithiation in situ in a practical pouch cell configuration. The neutron absorption clearly increases as more lithium is intercalated inside of the graphitic planes, resulting in more attenuation of the neutron beam. The neutron attenuation profile at different discharge times (or SOCs) qualitatively agrees with lithium concentration profiles calculated using a 3D electrochemical

ACKNOWLEDGMENTS This research used resources at the High Flux Isotope Reactor and Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. J.N., H.Z., S.A., S.P., and S.M. acknowledge support from the Office of Vehicle Technology, EERE, DOE. The authors thank Drs. Andrew Payzant and Thomas Proffen for critical reading of the manuscript and valuable comments.

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REFERENCES

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