Visualization of Lithium Plating and Stripping via in Operando

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Visualization of Lithium Plating and Stripping via in Operando Transmission X-ray Microscopy Ju-hsiang Cheng, Addisu Alemayehu Assegie, Chen-Jui Huang, Ming-Hsien Lin, Alok Mani Tripathi, Chun-Chieh Wang, Mau-Tsu Tang, Yen-Fang Song, Wei-Nien Su, and Bing-Joe Hwang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01414 • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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The Journal of Physical Chemistry

Visualization of Lithium Plating and Stripping via in Operando Transmission X-ray Microscopy Ju-Hsiang Cheng†,⊥, Addisu Alemayehu Assegie§,⊥, Chen-Jui Huang†, Ming-Hsien Lin†,‡, Alok Mani Tripathi†, Chun-Chieh Wang∥, Mau-Tsu Tang∥, Yen-Fang Song∥, Wei-Nien Su§, Bing Joe Hwang* †, †

∥

Department of Chemical Engineering, National Taiwan University of Science and Technology,

Taipei 10607, Taiwan. §

Graduate Institute of Applied Science and Technology, National Taiwan University of Science

and Technology, Taipei 10607, Taiwan. ‡

Department of Chemical & Materials Engineering, Chung Cheng Institute of Technology,

National Defense University, Taoyuan, Taiwan. ∥

National Synchrotron Radiation Research Center (NSRRC), Hsinchu 30076, Taiwan.

Corresponding Author *[email protected] (B. J. Hwang)

1 Environment ACS Paragon Plus


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Abstract

Lithium dendrite growth dynamics on Cu surface is first visualized through a versatile and facile experimental cell by in operando transmission X-ray microscopy (TXM). Galvanostatic plating and stripping cycle(s) are applied on each cell. Upon plating/stripping process at ~ 1 mA cm-2, mossy lithium was clearly found growing and shrinking on the Cu surface as the applying time increases. It is interesting to note that the aspect ratio (height/width) of deposited lithium has increased with charge passed during plating, indicating a faster growing from the base. In addition, the dendritic or mossy lithium have been also observed when various high current densities (25 mA cm-2, 12.5 mA cm-2 and 6.3 mA cm-2) were applied in different cycle, showing a severe dendritic lithium formation that could be induced by inhomogeneous current distribution. The clear structure of dead lithium is found after the cycling, which also shows a lower efficiency and higher hazard when applying a higher current density. This work explores TXM as a useful tool for in operando dynamic visualization and quantitative measurement of lithium dendrite which is difficult to achieve with ex situ measurements and other microscopy techniques. The understanding of growth mechanism from TXM can be beneficial for the development of safe lithium ion and lithium metal batteries.


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1. Introduction

Lithium-ion battery is one of important energy storage systems for electronic devices and electrical vehicles due to its high energy and power density. However, in development of passing years, conventional lithium-ion battery comprised of graphite anode is unable to match the pressing demand of higher energy density device. As the first-studied anode material, lithium metal provides extremely high theoretical capacity (3860 mAh g-1) that far more than its graphite counterpart (372 mAh g-1), making lithium an attractive anode material. However, the problems of dendritic lithium growth and poor Coulombic efficiency hinder the commercialization of lithium metal batteries1-2. The former can lead to short circuit of cell which raises severe safety issue and the latter conducts the poor cyclability of the battery.

To overcome these problems, intensive efforts have been devoted by many research groups with various means of electrolyte modification with additive3, electrode surface coating4, new design of electrolyte5, use of solid electrolyte6 or the pulse charging strategy7. Whilst the suppression of dendritic lithium growth has been reported in these different strategies, the fundamental dendrite growth mechanism is not yet clear. In order to dynamically observe the lithium dendrite growth, many in situ techniques have been developed such as 7Li nuclear magnetic resonance (NMR)8-9, optical microscopy (OM)10, scanning electron microscopy (SEM)11, and transmission electron microscopy (TEM)12-13. The shape of dendrite and formation of dead lithium (represents the lithium which lost the electrical contact to electrode and not able to react, i.e. electrochemically inactive lithium) could be observed by these techniques. However, these techniques have some restrictions, such as indirect observation in NMR, high vacuum operation requirement and possible electron beam damage by SEM and TEM, and insufficient spatial resolution of OM.



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Transmission X-ray microscopy (TXM) is a powerful tool that operates under ambient pressure and atmosphere with good spatial resolution. It has been employed for several years to study the morphology transformation of anode and cathode materials, such as Sn14, sulfur15 and Li-rich compound16. Nevertheless, the lithium is a very light element compared to other anode/cathode materials and gives a small contrast under X-ray imaging; therefore it is rarely discussed by TXM until recent years. Harry et al. first reported the lithium dendrite underneath the lithium/polymer electrolyte interface and proposed a penetration mechanism17. Furthermore, the large-scale (~ 100 µm) 3D dendrite structure studied by micro X-ray computed tomography (micro-CT) has been demonstrated by Eastwood et al. and Sun et al. follows the growth of lithium microstructure and the separator breaking down resulted from lithium dendrite.18-19 All of them showed the ability of X-ray tomography/microscopy technique to observe the lithium formation either by ex situ or non-operating (in the same cell) measurements. However, a direct look on the lithium growth from nucleation to development of dendrite is not yet achieved by TXM up to now. In present work, in operando (under cell operation) TXM is the first time to carry out the high-spatial-resolution observation of real-time lithium dendrite growth by using in house developed in situ cell.

2. Experimental Section

An in house developed 2-electrode transparent polyethylene plastic cell is used to carry out for in operando measurement, as shown in Figure 1. The in situ plastic cell is assembled inside Arfilled glovebox with the O2 and H2O level lower than 1 ppm. A resin-insulated copper wire of diameter 100 µm is used as working electrode. The lithium metal was wrapped on a polished copper wire as the counter and reference electrode. Around 1 ~ 2 mL electrolyte of 1M LiPF6 in


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EC/DEC (v:v = 1:1) was filled into the plastic cell and then whole arrangement was sealed by heat sealer. In order to prevent the electrolyte leakage from the gap between plastic bag and wire electrode, the sealant which is commonly used in soft-pack battery is adopted to reinforce the cell sealing. TXM study utilized the beamline BL01B1 facility at National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The X-ray with photon energy at 8 keV penetrates in situ plastic cell and passes through a zone plate and phase ring to generate phase contrast images. The electrochemical test was simultaneously carried out by connecting a potentiostat (AutoLab M101) to the in situ plastic cell. The applied current is based on the surface of copper wire. Each image with 15 µm × 15 µm frame size is taken for 20 seconds to obtain a satisfactory image quality. Before each experiment, an image against the air was taken as a blank in order to make the background subtraction for the acquired images.

Figure 1. In operando TXM setups (a) in situ electrochemical cell scheme and (b) image of in situ plastic cell and (c) TXM instrument arrangement at synchrotron station measurement instrument.



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3. Results and Discussion A constant current plating/stripping has been performed at current density of ~ 1 mA cm-2 and process was recorded as shown in Figure 2 (the video can be found in the supporting information video_S1). The TXM image shows a good contrast between copper wire (black part) and electrolyte (bright part) (Figure 2(a)). This is due to different atomic number of electrolyte components and copper which interact differently to X-ray and, at the same time, the X-ray penetration distance in electrolyte is minimized to avoid additional X-ray absorption by the electrolyte. During initial plating process, the voltage decreases from OCV to 0 V up to the 680 s, and the surface of Cu remains free from any lithium deposition (Figure 2(b)). This is suggested to be the formation of solid electrolyte interphase (SEI) layer occurs only and no lithium deposition was observed at this stage. However, direct visualization of SEI layer is difficult due to very thin thickness (normally few nanometers) and poor contrast caused by similar atomic constituents as of electrolyte. On further lowering the potential from 680 s to 3340 s, the voltage first reached at -100 mV at 700 s and then gradually increased and stayed steady at -52 mV (The enlarged curve is shown in Figure S1). This change of plating curve can be explained by the different lithium growth behaviors, including the formation of nuclei on the heterogeneous Cu surface and the continuous plating on the lithium surface.20 At the beginning of deposition, the nucleation of lithium with higher kinetic barrier dominates the plating process, therefore a higher overpotential is observed. However, as the lithium nuclei formed, lithium plates easier on lithium than on Cu, thus a lower overpotential is obtained. It can be also supported by in operando TXM results. At the plating time 700 s, the separate lithium nuclei start appearing on the Cu electrode surface, and after the nuclei formed, the lithium grows based on the shape of nuclei and


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continuously increases in its size until overlaps to each other to form a 5~7 µm thick layer as shown in Figure 2(b)-(h). (Note that although the lithium has close atomic number with electrolyte, the lithium can be still observed due to the phase contrast mechanism). Considering the thickness of deposited lithium, it can be estimated by the plated capacity (0.74 mAh cm-2) and density of lithium (0.534 g cm-3), which is around 3.6 μm. This value lower than what observed in TXM around 5~7 μm implies the existence of voids and crevices between each mossy lithium and with an approximate porosity 30 ~ 50 %. Moreover, during the stripping process from 3350 s to 4750 s (Figure 2(i)-(l)), the voltage jumps positively to 56 mV and gradually increases as the stripping time accumulates. Finally a drastic voltage increase started at around 4620 s, which means no further active lithium can be extracted. The shrinkage in lithium layer is observed by TXM images, and it generally keeps the profile of the grain. At the end of the curve (Figure 2(k) to (l)), a blur layer with weak contrast remains on the Cu electrode surface. This floppy layer can be ascribed to the irreversible SEI and/or dead lithium, which lose the electrical contact with electrode surface or active lithium and shows no longer electrochemical active. Considering the low Coulombic efficiency of 41.9% obtained from plating/stripping cycle, the irreversible capacity can be attributed the formation of this remained structure.



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Figure 2. In operando TXM images of lithium plating (a)-(h) and stripping (i)-(l) and the bottom is the cycling curve of the in situ cell at the current density ~1 mAcm-2. All the images share the same scale bar in image (a), which is 2.5 µm.

In addition, the mossy lithium growth is extracted from the Figure 2 (the selected area and definition of lithium height, width and aspect ratio are shown in Figure S2 and Figure S3). The images of evolving lithium nuclei were traced during the covered timespan of plating/stripping and they are superimposed in Figure 3(a). At the beginning, a hemisphere lithium nucleus is observed at the electrode surface. As the lithium is plating, both the height (vertical growth) and the width (lateral growth) of deposited lithium increase with time. However, the aspect ratio defined as height to width of deposited lithium dynamically varies with time. Figure 3(b) shows the height, width and aspect ratio against the charge passed. It can be found that the growth rate in height is faster than in width upon plating/stripping process. The lateral growth can be


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ascribed to the growth from the surface of deposited lithium (the interface between deposited lithium and electrolyte), but the vertical growth is contributed from both the lithium surface and the base of the deposited lithium (the interface between deposited lithium and Cu substrate).21-22 Moreover, the aspect ratio increases with the charge passed during plating process. It indicates that the lithium growth is contributed more from the base compared to from the lithium surface, which give the images that lithium is extruded out of Cu surface, and it is suggested that the growth from the lithium surface could be impeded by either thicker SEI or poor electrical connection from the base to the surface. Upon stripping, the height of the plated lithium declines faster than the width of the plated lithium, as shown in Figure 3(b), suggesting that the dissolution from the base is also faster than that from the surface. Once the lithium on base is completely dissolved prior to the one on top, it can give rise to the formation of dead lithium, as observed in Figure 2(l).



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Figure 3. (a) Contour tracing of lithium dendrite growth by superimposition of the images extracted from Figure 2 in the time domain from 880 s to 3340 s for plating process and from 3340 s to 4320 s for stripping process, and (b) the evolution of dendrite height, width and aspect ratio together with plating-stripping curve from Figure 2.

Furthermore, the effect of applied current density on the dendrite formation is shown in Figure 4 (the video can be found in the supporting information video_S2). A current density of 25 mA cm-2, 12.5 mA cm-2 and 6.3 mA cm-2 were applied for the first (Figure 4(a)-(f)), second (Figure 4(g)-(l)) and third (Figure 4(m)-(r)) plating/stripping cycles, respectively. A dendritic porous structure is observed growing quickly in the first cycle, and as the current density was lowered down to 12.5 mA cm-2, layered-like lithium with frontier is observed as Figure 4(j). Afterwards, at current density of 6.3 mA cm-2, a similar layer was observed. (Note that the observing spot was changed but in the same in situ cell and electrode surface to better visualize the lithium growth). The capacity for the plating process is 0.75 mC, 0.47 mC, 0.32 mC and for the stripping process is 0.45 mC, 0.35 mC, 0.23 mC in the first, second and third cycles, respectively. Compared to the morphology of the lithium layer at 25 mA cm-2 and 12.5 mA cm-2, the capacity is about 1.6 times for the fully plating but the lithium volume for the 25 mA cm-2 plating is much larger than the 12.5 mA cm-2 plating. The Coulombic efficiency of each cycle is 59.6%, 74.1% and 72.6% at 25 mA cm-2, 12.5 mA cm-2 and 6.3 mA cm-2, respectively. The much lower efficiency in the first cycle relates to the SEI formation, and also the irreversible lithium dendritic structures caused by high current density.


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Figure 4. In situ TXM images of lithium plating for first (a)-(f), second (g)-(l) and third (m)-(r) plating/stripping cycle at 25 mA cm-2, 12.5 mA cm-2 and 6.3 mA cm-2. The black and red arrows in image (j) show the first-cycle dendritic lithium and the second-cycle mossy lithium, respectively. The blue arrow in image (p) shows the third-cycle mossy lithium. All the images share the same scale bar in image (a), which is 2.5 µm. The bottom is the cycling curve of voltage and current versus time.

Considering the morphology and volume difference, which is either dendritic lithium or mossy lithium shown in Figure 4(j) (the black and red arrow, respectively), it can be ascribed to the inhomogeneous/homogeneous deposition rate induced by various current density. As the model proposed in Figure 5, during the lithium plating, the continuous lithium-ion consumption is



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taking place on the electrode surface to be reduced to metallic lithium, which the reaction is subject to the kinetic limitations of the lithium reduction. This time domain promotes denser lithium microstructures formation in the form of mossy structure. The growth rate of surface lithium is generally the same and increases its thickness in layer-like growth, which named 2D growth. However, once the depletion of lithium ion occurs near the electrode surface, the reaction turns to depend on the diffusivity of lithium ion from the bulk solution. In this situation, the lithium metal will outreach faster as the dendritic form in order to obtain sufficient lithiumion concentration. This rapid lithium growth in height and with many branches was referred as 3D growth. As the result reported by Bai et al.23, the Sand’s equation was employed to estimate the critical time when the growth of mossy lithium turns to dendritic lithium, which named Sand’s time. Meanwhile, a clear voltage jump can be observed on the plating curve, indicating the resistance drastically increases while the dendritic structure occurs. However, in our case, the large voltage difference was not observed during first cycle plating process, indicating the plating time is still prior to the Sand’s time. The reason results in such morphology change could be due to the concentration gradient of lithium ion and nonuniformity of current distribution on the Cu surface. In addition, a higher contrast layer feature showed in Figure 4(p) (indicated by blue arrow) signifies a denser plating feature taking place in a lower applied current density. This observation shows up the current density can give a huge influence on its growth density even in the similar mossy-type lithium.


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Figure 5. The scheme of different growth types related to dendritic (3D) and mossy (2D) lithium at high and low current density.

A larger view of electrode surface is shown in Figure 6 via the mosaic acquisition mode, where several single frames have been acquired and stitched together into one image. High contrast region in each frame of raster TXM images is due to the background shift in composite image frame. This flaw of TXM images remains limited only to boundary region of frame which do not affect the studied object c.a. lithium microstructure in present case. Figure 6(a) shows the surface of copper wire before the lithium plating/stripping. Figure 6(b) shows an overview on the Cu surface after the first cycle plating/stripping. A porous-like loose structure is found on the Cu surface, as the observation from the Figure 4(c) and (d). This structure can be assigned as dead lithium, which the root of it is disconnected from the electrode surface, since it cannot be further extracted out as the observation from the stripping curve. Interestingly, after 3 cycles, as seen in Figure 6(c), it seems a gap appears between the remained dead lithium and electrode surface. 13 Environment ACS Paragon Plus


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The possible scenario accounting for this observation can be the continuous deposition of lithium from the electrode surface but not the top of lithium during cycling.

Figure 6. TXM mosaic images for the various applied current density shown in Figure 4, (a) before cycle (b) after one-cycle at 25 mA cm-2 and (c) after three-cycle plating/stripping at 25 mA cm-2, 12.5 mA cm-2 and 6.3 mA cm-2 for each cycle. The scale bar of all images is 10 µm.

A scheme of proposed forming mechanism for the Figure 6 is shown in Figure 7. The dendritic structure formed after the first cycle and was out of connection to the electrode surface. As the lithium plating in the second cycle, the new lithium grew directly from the electrode surface and 14 Environment ACS Paragon Plus

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this denser structure pushed out the dendritic dead lithium formed in the first cycle out of the Cu electrode surface. Moreover, as the lithium stripping proceeded in the second cycle, the dendritic structure did not come back to the electrode surface due to the newly formed SEI. Consequently, this gave rise to the formation of dead lithium structure. Although a higher reversibility is shown in the second cycle, it still contributes small amount of irreversible SEI/dead lithium. This residual structure formed in the second cycles is lesser than that in the first cycle and gives a lower contrast during TXM imaging. Similar scenario took place in the third cycle. Therefore, a clear gap can be visualized in the Figure 6(c). In addition, a relatively high contrast spot can be observed in the Figure 6(b) with red line circled, and this spot can be found being above the gap in Figure 6(c). This observation provides another proof that the dead lithium structure is pushed out by the next cycles, which also agrees with the result from in situ OM10 and in situ TEM12-13.

Figure 7. The proposed mechanism of dead lithium movement during the cycling, which is drew from Figure 4. The applied current of first, second, third cycles are 25 mA cm-2, 12.5 mA cm-2 and 6.3 mA cm-2, respectively.



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Another support for the explanation of dead lithium is shown in Figure 8 and video_S3. A impurity, which resembled the SEI-isolated and electrochemically inactive dead lithium, was placed on the Cu electrode surface before the cycle. During the lithium plating process, the impurity was lift up by the growing lithium underneath, but remained in the nearby location after the stripping process. This result suggests that once the dead lithium is formed, even when the plating/stripping current density is lowered down in the next cycle to create a denser and flatter surface, the dead lithium is still not able to be eliminated. Furthermore, during the cycling, the dead lithium could gradually agglomerate out of surface, result in uneven lithium surface, deform the separator and then raise a higher risk of short-circuit as the observation by X-ray tomography.19 Therefore, more efforts are required to circumvent the formation of dead lithium including surface modification electrode, optimization of the electrolyte composition and so on. Herein, our in operando TXM technique provides an excellent way to further understand the growth behavior of lithium and prevention of it.


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Figure 8. The plating/stripping with the existence of surface impurity. All the images share the same scale bar in image (a), which is 2.5 µm.

4. Conclusions We have shown first time the ability of TXM to differentiate the various lithium structures under in operando conditions with plastic cell design and study the dynamic lithium growth mechanism. The formation and dissolution of mossy lithium is clear observed and reveals that the height of mossy lithium grows/shrinks greater than its width, during the plating/stripping process. Furthermore, the effect of current density on the lithium shape is evidenced, which mossy and dendritic lithium are found the growing tendency under lower and higher current density, respectively. Moreover, the remained lithium, which called “dead lithium”, was clearly observed after high-current-density cycling, suggesting that growth of dendritic lithium structure contributes more than the mossy structure to form a dead lithium structure. This demonstrated setup for TXM imaging technique with good resolution is relatively easy comparing to the other measurement techniques and is able to fit versatile experiment studies such as electrode surface coating or effect of electrolyte composition. Although only in operando 2D imaging is reported in this work due to the longer acquisition time interval for the 3D tomography (few minutes to tens of minutes), the more improvement are under developing and could further advance our understanding on the lithium dendrite growth mechanism and its prevention in the near future.

Associated Content Supporting Information



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The enlarged plating/stripping curve of Figure 2 and in operando TXM images are shown from Figure S1 to S3. The videos of Figure 2, Figure 4 and Figure 8 are showed from video S1 to S3.

Author Information Corresponding Author *E-mail: [email protected]. Tel: +886 2 27376624. Notes ⊥ These authors contributed equally. The authors declare no competing financial interest.

Acknowledgements The financial support from the Ministry of Science and Technology (MoST) (MOST 105-3113E-011-001, MOST 105-ET-E-011-004-ET, MOST 103-2221-E-011-156-MY3, MOST 1042923-M-011-002-MY3), the Ministry of Economic Affairs (MoEA) (101-EC-17-A-08-S1-183), the Top University Projects (100H45140), the Global Networking Talent 3.0 Plan (NTUST 104DI005) from the Ministry of Education of Taiwan, as well as the facilities of support from National Taiwan University of Science and Technology (NTUST) and National Synchrotron Radiation Research Center (NSRRC) are acknowledged. The authors thank Ms. Hsueh-chi Wang for many helps on the measurements. Special thanks to Mr. Hsin-fu Huang for his support in fabricating the setup. References 1. Xu, W.; Wang, J.; Ding, F.; Chen, X.; Nasybulin, E.; Zhang, Y.; Zhang, J.-G., Lithium Metal Anodes for Rechargeable Batteries. Energy & Environmental Science 2014, 7, 513-537.


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2. Aurbach, D., A Short Review of Failure Mechanisms of Lithium Metal and Lithiated Graphite Anodes in Liquid Electrolyte Solutions. Solid State Ionics 2002, 148, 405-416. 3. Ding, F., et al., Dendrite-Free Lithium Deposition Via Self-Healing Electrostatic Shield Mechanism. Journal of the American Chemical Society 2013, 135, 4450-4456. 4. Liu, Y.; Lin, D.; Liang, Z.; Zhao, J.; Yan, K.; Cui, Y., Lithium-Coated Polymeric Matrix as a Minimum Volume-Change and Dendrite-Free Lithium Metal Anode. Nature communications 2016, 7, 10992. 5. Suo, L.; Hu, Y.-S.; Li, H.; Armand, M.; Chen, L., A New Class of Solvent-in-Salt Electrolyte for High-Energy Rechargeable Metallic Lithium Batteries. Nature communications 2013, 4, 1481. 6. Zhou, D.; Liu, R.; He, Y.-B.; Li, F.; Liu, M.; Li, B.; Yang, Q.-H.; Cai, Q.; Kang, F., SiO2 Hollow Nanosphere-Based Composite Solid Electrolyte for Lithium Metal Batteries to Suppress Lithium Dendrite Growth and Enhance Cycle Life. Advanced Energy Materials 2016, 6, 1502214. 7. Mayers, M. Z.; Kaminski, J. W.; Miller, T. F., Suppression of Dendrite Formation Via Pulse Charging in Rechargeable Lithium Metal Batteries. The Journal of Physical Chemistry C 2012, 116, 26214-26221. 8. Bhattacharyya, R.; Key, B.; Chen, H.; Best, A. S.; Hollenkamp, A. F.; Grey, C. P., In Situ NMR Observation of the Formation of Metallic Lithium Microstructures in Lithium Batteries. Nature materials 2010, 9, 504-510. 9. Chang, H. J.; Ilott, A. J.; Trease, N. M.; Mohammadi, M.; Jerschow, A.; Grey, C. P., Correlating Microstructural Lithium Metal Growth with Electrolyte Salt Depletion in Lithium Batteries Using7li Mri. Journal of the American Chemical Society 2015, 137, 15209-15216. 10. Steiger, J.; Kramer, D.; Mönig, R., Microscopic Observations of the Formation, Growth and Shrinkage of Lithium Moss During Electrodeposition and Dissolution. Electrochimica Acta 2014, 136, 529-536. 11. Motoyama, M.; Ejiri, M.; Iriyama, Y., Modeling the Nucleation and Growth of Li at Metal Current Collector/LiPON Interfaces. Journal of The Electrochemical Society 2015, 162, A7067-A7071. 12. Leenheer, A. J.; Jungjohann, K. L.; Zavadil, K. R.; Sullivan, J. P.; Harris, C. T., Lithium Electrodeposition Dynamics in Aprotic Electrolyte Observed in Situ Via Transmission Electron Microscopy. ACS nano 2015, 9, 4379-4389. 13. Mehdi, B. L., et al., Observation and Quantification of Nanoscale Processes in Lithium Batteries by Operando Electrochemical (S)TEM. Nano letters 2015, 15, 2168-73. 14. Chao, S.-C.; Yen, Y.-C.; Song, Y.-F.; Chen, Y.-M.; Wu, H.-C.; Wu, N.-L., A Study on the Interior Microstructures of Working Sn Particle Electrode of Li-Ion Batteries by in Situ XRay Transmission Microscopy. Electrochemistry Communications 2010, 12, 234-237. 15. Nelson, J.; Misra, S.; Yang, Y.; Jackson, A.; Liu, Y.; Wang, H.; Dai, H.; Andrews, J. C.; Cui, Y.; Toney, M. F., In Operando X-Ray Diffraction and Transmission X-Ray Microscopy of Lithium Sulfur Batteries. Journal of the American Chemical Society 2012, 134, 6337-6343. 16. Chen, C. J.; Pang, W. K.; Mori, T.; Peterson, V. K.; Sharma, N.; Lee, P. H.; Wu, S. H.; Wang, C. C.; Song, Y. F.; Liu, R. S., The Origin of Capacity Fade in the Li2MnO3·LiMO2 (M = Li, Ni, Co, Mn) Microsphere Positive Electrode: An Operando Neutron Diffraction and Transmission X-Ray Microscopy Study. Journal of the American Chemical Society 2016, 138, 8824-8833.



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17. Harry, K. J.; Hallinan, D. T.; Parkinson, D. Y.; MacDowell, A. A.; Balsara, N. P., Detection of Subsurface Structures Underneath Dendrites Formed on Cycled Lithium Metal Electrodes. Nature materials 2014, 13, 69-73. 18. Eastwood, D. S., et al., Three-Dimensional Characterization of Electrodeposited Lithium Microstructures Using Synchrotron X-Ray Phase Contrast Imaging. Chemical communications 2015, 51, 266-268. 19. Sun, F.; Zielke, L.; Markötter, H.; Hilger, A.; Zhou, D.; Moroni, R.; Zengerle, R.; Thiele, S.; Banhart, J.; Manke, I., Morphological Evolution of Electrochemically Plated/Stripped Lithium Microstructures Investigated by Synchrotron X-Ray Phase Contrast Tomography. ACS nano 2016, 10, 7990-7997. 20. Wood, K. N.; Kazyak, E.; Chadwick, A. F.; Chen, K.-H.; Zhang, J.-G.; Thornton, K.; Dasgupta, N. P., Dendrites and Pits: Untangling the Complex Behavior of Lithium Metal Anodes through Operando Video Microscopy. ACS Central Science 2016, 2, 790-801. 21. Steiger, J.; Kramer, D.; Mönig, R., Mechanisms of Dendritic Growth Investigated by in Situ Light Microscopy During Electrodeposition and Dissolution of Lithium. Journal of Power Sources 2014, 261, 112-119. 22. Stark, J. K.; Ding, Y.; Kohl, P. A., Nucleation of Electrodeposited Lithium Metal: Dendritic Growth and the Effect of Co-Deposited Sodium. Journal of the Electrochemical Society 2013, 160, D337-D342. 23. Bai, P.; Li, J.; Brushett, F. R.; Bazant, M. Z., Transition of Lithium Growth Mechanisms in Liquid Electrolytes. Energy Environ. Sci. 2016, 9, 3221-3229.


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Figure 1. In operando TXM setups (a) in situ electrochemical cell scheme and (b) image of in situ plastic cell and (c) TXM instrument arrangement at synchrotron station measurement instrument. Figure 1 167x115mm (300 x 300 DPI)

ACS Paragon Plus Environment


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Figure 2. In operando TXM images of lithium plating (a)-(h) and stripping (i)-(l) and the bottom is the cycling curve of the in situ cell at the current density ~1 mA/cm2. All the images share the same scale bar in image (a), which is 2.5 µm. Figure 2 161x176mm (300 x 300 DPI)


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Figure 3. (a) Contour tracing of lithium dendrite growth by superimposition of the images extracted from Figure 2 in the time domain from 880 s to 3340 s for plating process and from 3340 s to 4320 s for stripping process, and (b) the evolution of dendrite height, width and aspect ratio together with plating-stripping curve from Figure 2. Figure 3 194x112mm (300 x 300 DPI)



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Figure 4. In situ TXM images of lithium plating for first (a)-(f), second (g)-(l) and third (m)-(r) plating/stripping cycle at 25 mA cm-2, 12.5 mA cm-2 and 6.3 mA cm-2. The black and red arrows in image (j) show the first-cycle dendritic lithium and the second-cycle mossy lithium, respectively. The blue arrow in image (p) shows the third-cycle mossy lithium. All the images share the same scale bar in image (a), which is 2.5 µm. The bottom is the cycling curve of voltage and current versus time. Figure 4 186x170mm (300 x 300 DPI)


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Figure 5. The scheme of different growth types related to dendritic (3D) and mossy (2D) lithium at high and low current density. Figure 5 186x119mm (300 x 300 DPI)



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Figure 6. TXM mosaic images for the various applied current density shown in Figure 4, (a) before cycle (b) after one-cycle at 25 mA cm-2 and (c) after three-cycle plating/stripping at 25 mA cm-2, 12.5 mA cm-2 and 6.3 mA cm-2 for each cycle. The scale bar of all images is 10 µm. Figure 6 112x183mm (300 x 300 DPI)


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Figure 7. The proposed mechanism of dead lithium movement during the cycling, which is drew from Figure 4. The applied current of first, second, third cycles are 25 mA cm-2, 12.5 mA cm-2 and 6.3 mA cm-2, respectively. Figure 7 178x97mm (300 x 300 DPI)



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Figure 8. The plating/stripping with the existence of surface impurity. All the images share the same scale bar in image (a), which is 2.5 µm. Figure 8 185x103mm (300 x 300 DPI)


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130x87mm (300 x 300 DPI)


Visualization of Lithium Plating and Stripping via in Operando

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