Demonstration of an Electrochemical Liquid Cell for Operando

Letter pubs.acs.org/NanoLett

Demonstration of an Electrochemical Liquid Cell for Operando Transmission Electron Microscopy Observation of the Lithiation/ Delithiation Behavior of Si Nanowire Battery Anodes Meng Gu,† Lucas R. Parent,‡ B. Layla Mehdi,§ Raymond R. Unocic,∥ Matthew T. McDowell,⊥ Robert L. Sacci,∥ Wu Xu,# Justin Grant Connell,○ Pinghong Xu,‡ Patricia Abellan,§ Xilin Chen,# Yaohui Zhang,#,∇ Daniel E. Perea,† James E. Evans,† Lincoln J. Lauhon,○ Ji-Guang Zhang,# Jun Liu,# Nigel D. Browning,§ Yi Cui,⊥,◆ Ilke Arslan,§ and Chong-Min Wang†,* Nano Lett. 2013.13:6106-6112. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/24/18. For personal use only.

†

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States Department of Chemical Engineering and Materials Science, University of California-Davis, One Shields Ave, Davis, California 95616, United States § Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ∥ Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ⊥ Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States # Energy and Environmental Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ∇ Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150001, China ○ Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States ◆ Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States ‡

S Supporting Information *

ABSTRACT: Over the past few years, in situ transmission electron microscopy (TEM) studies of lithium ion batteries using an open-cell configuration have helped us to gain fundamental insights into the structural and chemical evolution of the electrode materials in real time. In the standard opencell configuration, the electrolyte is either solid lithium oxide or an ionic liquid, which is point-contacted with the electrode. This cell design is inherently different from a real battery, where liquid electrolyte forms conformal contact with electrode materials. The knowledge learnt from open cells can deviate significantly from the real battery, calling for operando TEM technique with conformal liquid electrolyte contact. In this paper, we developed an operando TEM electrochemical liquid cell to meet this need, providing the configuration of a real battery and in a relevant liquid electrolyte. To demonstrate this novel technique, we studied the lithiation/delithiation behavior of single Si nanowires. Some of lithiation/delithation behaviors of Si obtained using the liquid cell are consistent with the results from the open-cell studies. However, we also discovered new insights different from the open cell configurationthe dynamics of the electrolyte and, potentially, a future quantitative characterization of the solid electrolyte interphase layer formation and structural and chemical evolution. KEYWORDS: Operando transmission electron microscopy, electrochemical liquid cell, Li-ion battery, Si anode, Si lithiation/delithiation

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driven by external energy. During the discharge process, Li+ ions flow back from the anode to the cathode, releasing the stored energy. The electrolyte encompasses both the anode and the

i-ion batteries are now indispensably used as energy storage devices for portable electronics and electric vehicles and also have started to enter the market of the renewable energies.1−10 The rechargeable batteries are comprised of three critical components: the cathode, anode, and electrolyte. In the charging process, the Li+ ions are extracted from the cathode, drift through the electrolyte ,and insert into the anode, which is a process © 2013 American Chemical Society

Received: September 11, 2013 Revised: November 4, 2013 Published: November 13, 2013 6106

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Figure 1. (a) Schematic drawing showing the experimental setup of the open-cell approach using ionic liquid as electrolyte; (b) schematic drawing showing the setup of the open cell approach using Li metal as lithium source and Li2O as solid electrolyte; (c) schematic drawing showing the setup of the liquid cell battery.

cathode, enabling the flow of Li+ ions between the two. All three components are directly involved in this dynamic ion transfer process during charge/discharge and undergo structural or chemical changes as a result. The rechargeable capacity and the battery life depends critically on the structural stability of the electrodes themselves, the electrolyte degradation rate, and the electrode−electrolyte interaction layerthe so-called “solid electrolyte interphase (SEI) layer”. One of the fundamental challenges for the battery research is the direct observation of the structural and chemical evolution of the components of the battery and how this directly correlates with the battery properties. Traditionally, observation of the structural and chemical evolution of the battery has mostly relied on ex-situ or post-mortem studies, which in many cases can provide key insights with respect to the structural changes of the battery materials but is lacking in dynamic information.11−13 Over the past few years, tremendous progress has been made toward developing methodologies for in situ direct observation of structural and chemical evolution of electrodes used for

lithium ion batteries.8,12−35 Most notably, the development of an in situ TEM cell that is based on an open-cell configuration using a single nanowire and either an ionic liquid or Li2O as the electrolyte. The fundamental designing concept of the open cell is schematically illustrated in Figure 1a for ionic liquid and Figure 1b for Li2O.18,22,24,29 Previous work carried out based on the open cell has provided insightful information on structural and chemical evolution of electrodes upon lithiation/delithiation. However, three typical deficiencies are associated with the open-cell configuration. First, for the open cell, the electrolyte is only in point contact with the electrode, which may inadvertently modify the diffusion pattern of Li ions in the electrode, and therefore, what has been obtained is not necessarily representative of the case where the electrode is fully immersed in the liquid electrolyte in a real battery. Second, for the case of using Li2O as the electrolyte, a large overpotential is normally applied to drive the Li ions into the electrode, which may change the kinetics and phase behaviors of solid-state electrode lithiation. Third, the use of the ionic liquid or Li2O electrolyte excludes some of the 6107

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Figure 2. (a) SEM image of the inner side of the biasing chip; (b) magnified view of the region labeled by the orange rectangle; (c) SEM image showing the welded Si NW electrode onto the Pt contact. Note that the Li location is labeled by the light blue color object in panel a.

Figure 3. In situ liquid-cell TEM observation of the lithiation of the Cu-coated Si (Cu−Si) NW. (a) TEM image showing the pristine state of the Cu−Si NW at 0 s; (b) core−shell formation of the Cu−Si NW during lithiation at 1658 s; (c) TEM image of the Cu−Si NW at 2462 s; (d) plotted width changes of the NW as a function of time. Note that, in all images from a to c, the Pt contact region is labeled by the black lines in the left of the image. The inset in panel c illustrating the cross sectional image after anisotropic swelling of the Si nanowire upon lithium insertion with maximum volume expansion along the ⟨110⟩ direction.

dynamic structural and chemical evolution of the electrodes and the SEI layer formation in batteries using real battery electrolytes. The conceptual setup for implementation of the in situ liquid cell battery to study the electrochemical reactions with normal liquid electrolyte is schematically illustrated in Figure 1c. The working electrode is a single Si nanowire, while the counter electrode is Li metal. This electrode geometry was implemented using a SiNx membrane deposited on Si chips as illustrated in Figure 2a−c. A SiNx membrane of ∼50 nm in thickness is used to seal the liquid, while still allowing transmission of the high energy

fundamental processes which only occur in real electrolytes and the battery operating conditions, such as the interaction between electrolyte and electrode and the SEI layer formation. To address the shortcomings of the open cell, in this work we describe a liquid-cell based approach for in situ or more precisely operando TEM studies of lithium ion batteries using a battery relevant liquid electrolyte and a lithium metal counter electrode. In this demonstration example, we use this new approach to observe the structural evolution of a single Si nanowire upon lithiation/ delithiation with controlled battery operating conditions. This present work opens the door for in situ TEM studies of both 6108

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expansion direction of Si upon lithiation as shown by the inset in Figure 3d).13,37 This nearly three times increase in diameter indicates a large volume expansion, which possibly includes partially the anisotropic Si volume expansion and partially the SEI layer formation. The diameter as a function of lithiation time is extracted from the in situ Movie S1 and is plotted in Figure 3d. The increase of the diameter is quicker at the beginning of the lithiation and slows down with the progression of the lithiation process. This phenomenon is related to the interface stress generated by the volume expansion, which limits the diffusion of Li ions further into the core. The slowing down of the lithiation after partial lithiation has also been consistently reported by earlier researchers.8,15,28 The delithiation process of a pure Si nanowire is shown in Figure 4. The delithiation process is performed by scanning the

electrons for imaging. A step-by-step illustration of the cell assembling process is illustrated in Figure S1 in the Supporting Information. As shown in Figure 2a, the biasing chip has six Pt electrodes. The Pt electrodes extend from the SiNx window to the edge of the chip, therefore allowing the connection of the electrode to the outside circuit. A single or multiple Si nanowires can be mounted on one of the Pt electrodes using focused ion beam (FIB) manipulation and Pt deposition welding, as illustrated in the image of Figure 2b and c and Figure S2 in the Supporting Information. The welded Si NWs extend to the electron-transparent SiNx membrane region to enable the imaging of the nanowire in electron transmission mode. A small piece of Li metal was loaded on a separate Pt electrode to serve as counter electrode as illustrated in Figure 2a. A droplet of 1.0 M of lithium perchlorate, LiClO4-containing mixed ethylene carbonate (EC) and dimethyl carbonate (DMC) electrolytes (3:7, v/v), was applied to the top surface of the SiNx membrane (fully immersing all of the Pt electrodes, Li metal, and Si NWs), and a blank chip with a SiNx membrane facing down was placed over the biasing chip to seal the liquid electrolyte. The sealing is completed based on a three-O-ring technique, and the whole device is implanted on a biasing in situ TEM liquid holder (Hummingbird Scientific, Lacey, WA, USA). The assembly process was completed in argon filled glovebox to avoid atmospheric degradation of the electrolyte/electrodes. The assembled cell in cross-section is schematically illustrated in Figure 1c. The viewing window dimensions are 50 μm × 50 μm. The normal thickness of liquid layer is 500−1000 nm. However, after loading the cell into TEM column, the membrane will bulge outward due to the pressure differences. We do not measure the thickness of the liquid layer in the TEM column. The liquid electrochemical cell is connected to an electrochemical testing station (Bio-Logic, Knoxville, TN) fitted with the ultralow current capability, therefore allowing us to test the single nanowire battery in both potentiostatic and galvanostatic modes. Two types of Si nanowires (Si NWs) were used for this study. The first one is an as-grown crystalline Si NWs. The second one is a crystalline Si NW that was half-side coated with thin layer of Cu. The thickness of the Cu coating layer is ∼25−32 nm as determined by using electron energy loss spectroscopy (EELS). Details of the measurement procedure are described in the Supporting Information (Figure S3). The coated thin Cu layer serves dual functions: (1) to provide mechanical support to the Si NW during cycling, therefore potentially extending the lifetime of the NWs in Li-ion batteries; (2) to increase the electrical conductivity of the Si NWs that can again potentially enhance the performance of these NWs in Li-ion batteries.36 The lithiation of the Cu coated crystalline Si NW in the liquid cell is performed by holding the voltage of the Cu−Si anode to ∼0.03 V range. The structural evolution of the nanowire upon lithiation is illustrated by the captured video frames as shown in Figure 3a−c and the Supporting Information (Movie S1). The pristine Cu−Si NW shows an overall diameter of ∼100 nm revealed by the dark contrast in Figure 3a. Following the lithation, the dark diffraction contrast region is measured to be ∼80 nm, which is the width of the Cu coating on the Si nanowire. This is because the Cu does not lithiate and maintains its crystalline during the electrochemical lithiation of Si. The lithiation of the Si nanowire immersed in the liquid electrolyte progresses in the core−shell fashion. The total diameter of the wire changes from 100 to 298 nm at 1658 s and to 391 nm at 2462 s. Based on the projected radial dimension increase, this indicates that the radial direction of this wire is along the ⟨110⟩ direction (maximum volume

Figure 4. Delithiation process captured using liquid cell battery. (a) Current vs voltage plot of the delithiation process; (b−d) STEM Z-contrast image and bright field images of the nanowire at different states of the delithiation. (The left side of each panel in b−d shows the HAADF Z-contrast image, and the right side shows the corresponding bright field STEM image acquired simultaneously.) Note that the white arrows indicate the deposited Pt markers for reference.

voltage from 0 to 0.65 V at an increment of 0.3 mV/s, and the current vs voltage curve is plotted in Figure 4a. The image shown in Figure 4b−d represents a pair of dark-field (left) and brightfield (right) scanning transmission electron microscopy (STEM) images. The white arrows in the dark-field image indicate the Pt markers, which were intentionally deposited on the nanowire using electron beam deposition in the FIB SEM. The Pt markers show a higher Z-contrast in the dark field images, highlighting the positions of the wire in the liquid cell. The diameter of the lithiated Si NW is 195 nm as shown in Figure 4b. The diameter of the NW shrank when the voltage scanned to 0.45 V as shown in 6109

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observed based on the liquid cell provides a global view of the response of the whole single NW with lithium insertion. On the other hand, for the open-cell configuration the lithium ion source is only in contact with the end of the Si nanowire, which uniquely leads to the sequential lithiation process of the nanowire in only one direction as shown by the TEM image in Figure 5b. Despite this one-dimensional contact, it is apparent that the Si NW is lithiated in a core−shell fashion but also shows tapering features at the lithiation front as illustrated by the TEM image shown in Figure 5b. The core−shell lithiation process in the open cell is related to the fast diffusion of Li on the surface of Si NW. The fast surface diffusion of lithium leads to the covering of the nanowire by lithium, which is potentially equivalent to immersing the nanowire in the liquid electrolyte (as in the case for liquid cell). Further, in terms of imaging spatial resolution and spectroscopy capability, the open cell offers the advantage of high spatial resolution that cannot be matched by the case of using liquid-cell approach.21 This is representatively illustrated in Figure 5c for the case of lithiation of the Si NW. The STEM image and Li and Si maps obtained using electron energy loss spectroscopy (EELS) consistently reveal the formation of LixSi phase and distribution of the inserted lithium ions during lithiation of Si NW. The above qualitative comparison clearly indicates that, in the sense of the end production of lithiation, the open cell and liquid cell show consistent results. However, related to the differences of the cell setup, the dynamic structural evolution of the materials in response to the lithium insertion obtained by the open cell may differ from that obtained by the liquid cell. The open-cell configuration may indeed have the propensity of producing artifacts, especially for the lithium diffusion pattern. Therefore, it appears that in situ TEM based on the open cell is best-suited for exploring the intrinsic structural and chemical evolution of electrode materials upon lithium ion insertion and extraction. The in situ liquid electrochemical cell can give global dynamic structural evolution of the whole nanowire, which is essential for understanding the behavior of the materials in the electrode. Most importantly, the liquid cell approach provides the possibility for direct observation of the evolution of the interface across the electrolyte and electrode, but with reduced spatial resolution. Although we have successfully demonstrated the charge and discharge of the single Si NW in the liquid electrolyte, we also identified challenges for perfecting the performance of the liquid cell that enables the capturing of information related to the behavior of electrode materials during charge/discharge of battery, such as the formation of the SEI layer and its structural and chemical evolution. For the case of the Si NW as illustrated in Figure 3, we realized that, following the initial lithiation, it is rather challenging to discriminate the SEI layer from the amorphous Li x Si in the liquid cell imaging condition. In addition, to allow the visible observation of the SEI layer, the electron beam effects need to be calibrated, and the electron dose needs to drop down below the damage threshold of the electrolytes. Therefore, improvement of liquid cell design to enhance the spatial resolution appears to be very important. This can be done by optimizing the thickness of the liquid layer and control of electron dose. In summary, using a novel in situ liquid cell battery platform with a real electrolyte, we have directly observed lithiation and delithiation of fully submerged Si NW electrodes during electrochemical testing. This liquid-cell nanobattery approach provides indispensable complementary information to the widely applied open-cell approach. The complete electrochemical process can be more fully understood by combining open-cell

Figure 4c. The diameter kept decreasing as lithium ions were extracted. In the end, the diameter shrank to ∼92 nm in the final delithiated state as shown in Figure 3d at 0.65 V. The large volume change as measured based on the reduction in diameter from 195 to 92 nm indicates that most of the Li ions were extracted during this process. The whole delithiation process is shown in Movie S2 in the Supporting Information. It is beneficial to compare the lithiation behavior observed using the in situ liquid electrochemical cell with that obtained based on the open-cell configuration, and this can be done by comparing the lithiation behavior of a single crystalline Si NW as collectively illustrated in Figure 5. For the case of the liquid

Figure 5. (a) Schematic drawing of the lithium insertion process in the liquid cell in all directions; (b) schematic drawing of the lithium diffusion and insertion, which starts in only one directionthe TEM image in the right side of panel b shows the reaction front and the asformed LixSi shell and Si core; (c) STEM, Si, Li, and overlaid Si and Li EELS maps showing the distribution of Si and lithium during the lithiation.

electrochemical cell as shown in Figure 5a, the Si NW is fully immersed in the liquid electrolyte, which allows the insertion of lithium ions into Si from all possible directions at the same time. The lithiation of the single nanowire proceeds in a core−shell mode with a uniform shell thickness along the axial direction of the whole nanowire (see Figure 3). Therefore, what has been 6110

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and liquid-cell battery TEM techniques. The open-cell approach provides important information regarding the composition, phase transformation, and atomic resolution structural changes of the electrode itself, allowing high-resolution microscopy to be obtained. On the other hand, the liquid cell allows the usage of any form of liquid electrolyte that is relevant to real battery and full emersion of the electrodes. Therefore, the liquid cell has tremendous potential for the study of the electrolyte−electrode interaction: the SEI formation and growth kinetics. To successfully and quantitatively measure the SEI layer using this liquidcell battery approach, optimized cell development and electron dose calibrations are underway. Our results using this in situ liquid electrochemical cell open a new avenue for the study of the SEI formation in all of the electrochemical reactions inside a working battery cell.

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ASSOCIATED CONTENT

S Supporting Information *

(1) Movie S1 and Movie S2 showing the lithiation and delithiation of Si nanowire in liquid electrolyte; (2) method; (3) additional figures to illustrate the details on assembling the cells, SEM shows the welding of the Si NW on chips by FIB, and measurement of the Cu thickness. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

M.G. and L.R.P. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work at PNNL and Stanford was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The development of the electrochemical liquid cell is supported by Chemical Imaging Initiative at Pacific Northwest National Laboratory (PNNL) and Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 18769 under the Batteries for Advanced Transportation Technologies (BATT) program. The work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the Department of Energy under Contract DE-AC05-76RLO1830. The work in Oak Ridge is supported by the Fluid Interface Reactions Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Office of Basic Energy Sciences (BES)-DOE (RRU). Work at Northwestern University was supported by NSF DMR-1006069. We appreciate the help of Norman Salmon and Daan Hein Alsem of Hummingbird Scientific for assistance regarding the use of the liquid holder.

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