Correlating Morphological Evolution of Li Electrodes with Degrading

Correlating Morphological Evolution of Li Electrodes with Degrading Electrochemical Performance of Li/LiCoO2 and Li/S Battery Systems: Investigated by Synchrotron X‑ray Phase Contrast Tomography Fu Sun,*,† Markus Osenberg,†,‡ Kang Dong,†,‡ Dong Zhou,†,‡ André Hilger,‡ Charl J. Jafta,§ Sebastian Risse,§ Yan Lu,§,⊥ Henning Markötter,† and Ingo Manke† †

Institute of Applied Materials, Helmholtz Centre Berlin for Materials and Energy, Hahn-Meitner-Platz 1, 14109 Berlin, Germany Institute of Material Science and Technologies, Technical University Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany § Institute of Soft Matter and Functional Materials, Helmholtz Centre Berlin for Materials and Energy, Hahn-Meitner-Platz 1, 14109 Berlin, Germany ⊥ Institute of Chemistry, University of Potsdam, Am Neuen Palais 10, House 9, 14469 Potsdam, Germany ‡

S Supporting Information *

ABSTRACT: Efficient Li utilization is generally considered to be a prerequisite for developing next-generation energy storage systems (ESSs). However, uncontrolled growth of Li microstructures (LmSs) during electrochemical cycling has prevented its practical commercialization. Herein, we attempt to understand the correlation of morphological evolution of Li electrodes with degrading electrochemical performances of Li/LiCoO2 and Li/S systems by synchrotron X-ray phase contrast tomography technique. It was found that the continuous transformation of the initial dense Li bulk to a porous lithium interface (PLI) structure intimately correlates with the gradually degrading overall cell performance of these two systems. Additionally, the formation mechanism of the PLI and its correlation with previously reported inwardly growing LmS and the lithium-reacted region have been intensively discussed. The information that we gain herein is complementary to previous investigations and may provide general insights into understanding of degradation mechanisms of Li metal anodes and also provide highly needed guidelines for effective design of reliable next-generation Li metal-based ESSs.

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not been achieved due to problematic issues that remain even after 5 decade of research, i.e., fast capacity decay, low cycle efficiency, and poor long-term cycle stability (attributed to low electronic conductivity of S and its reaction products, along with dissolution of lithium polysulfide (LPS) intermediates that continuously attack and corrode the Li anode during cycling).6,7 Pioneering work demonstrating that reversible capacity up to 1320 mAh/g can be attained through mesoporous carbonenhanced S utilization was reported by Nazar et al. in 2009.8 Since then, tremendous efforts have been dedicated toward

pcoming advanced portable electronic devices, electrical vehicle transportation applications, and grid scale electrical energy storage systems require a higher-level energy density than that available from the currently commercialized lithium ion battery (LIB) technology.1−3 This tremendously increasing demand for energy storage has sparked renewed interests in the S cathode (due to its overwhelmingly high theoretical specific capacity and energy density) and Li anode (due to the extra-high capacity and the lowest negative electrochemical potential).4 Additionally, the inexpensive, abundant, and nontoxic nature of S makes Li−S battery (LSB) technology the most promising candidate for the next-generation high-energy storage system.5 However, replacement of state-ofthe-art de/intercalation LIB technology by Li−S technology has © 2018 American Chemical Society

Received: December 12, 2017 Accepted: January 9, 2018 Published: January 9, 2018 356

DOI: 10.1021/acsenergylett.7b01254 ACS Energy Lett. 2018, 3, 356−365

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Cite This: ACS Energy Lett. 2018, 3, 356−365

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ACS Energy Letters

Figure 1. Photograph and schematic illustration of the customized electrochemical cell and illustration of the employed beamline setup. (a) Photograph of the fabricated cell; the enlarged picture in the green rectangle shows the interior of a blank cell, characterized by a laboratory X-ray setup. The scale bar is 1 mm. (b) Corresponding schematic representation of the cell consisting of a polyamide-imide housing (yellow), two screws (light gray), two sealing rings (pink), and a porous separator (white) sandwiched between two electrodes (blue and green). (c) Schematic representation of the experimental setup of the tomography station at the BAMline at BESSY II, Helmholtz-Zentrum Berlin, Germany.

capacity decay in both Li metal-based LIB and LSB technology. Unfortunately, a clear fundamental correlation among the inwardly growing LmS, the passivating LrR, and the overall electrochemical performance degradation mechanisms has not been established due to the fact that previous research frequently focused on a minor part of the broad and complex electrochemical evolutions of Li anode electrodes. Herein, synchrotron X-ray phase contrast tomography is employed to investigate the morphological evolution of Li electrodes in Li/LiCoO2 and Li/S cells assembled, respectively, with carbonate- and ether-based electrolyte. It first suggests that the interface of inwardly growing LmSs and the passivating LrRs can be denominated as a porous lithium interface (PLI) consisting of SEI-covering LmSs. In addition, its correlation with the morphological evolution of Li electrode and the electrochemical decay of the overall cell electrochemical performance as a function of cycle number are presented. The scientific understanding based on current systematic investigation has been used to elucidate underlying challenges of recently proposed advanced conceptual strategies for protecting Li electrodes. The currently presented unprecedented results and in-depth discussion may provide new insights into the underlying degradation mechanism of both Li−S technology and Li metalbased LIBs and may open up new design principles and opportunities for next-generation LMB systems. A photograph and corresponding illustration of the employed customized electrochemical cell are shown in Figure 1a,b, along

improving Li−S technology to commercialization, and the proposed strategies focus mostly on the S cathode,9−11 However, the Li metal anode in LSBs, which is directly involved in capacity decay and the shuttle effect, has attracted much less attention.12 Efficient Li metal utilization is a burdensome difficulty during the long history of LIB research, and eventually, implementation of Li metal in commercialized rechargeable battery systems failed.13 Nevertheless, Li metal as a counter and reference electrode for evaluation of new cathode and anode materials in half-cells has been widely used.14 Moreover, it has been generally accepted that successful implementation of Li metal is a prerequisite in nextgeneration battery technology.15 The research spotlight is recently falling on the Li anode, which seems to be a paramount limiting factor for Li metal batteries (LMBs, Li metal anodebased battery systems, including LSBs)16 as practical energy storage devices. Recently, it has been reported by Xiao et al. that the actual origin of onset of half-cell degradation and failure results from deterioration of the Li metal anode by forming an interface of an inwardly growing Li microstructure (LmS, such as dendrite, fiber, moss, etc.) covered by a solid electrolyte interface (SEI).14 Later on, Manthiram et al. also discovered that serious corrosion and passivation of Li metal and electrolyte decomposition are the main reasons for the failure of LSBs.17 They further identified the passivation layer on the surface of Li metal as the “lithium-reacted region” (LrR).18 These results are further corroborated by Zhang et al. and Chen et al., respectively.19,20 These investigations shed new light on fast 357


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Figure 2. Electrochemical performance and morphological evolution of Li electrodes in the Li/LiCoO2 cells. A1 shows a panorama view of the selected Li/LiCoO2-5 cell tomography data, representing all of the rest of the Li/LiCoO2 tomography data. The rest of the first column shows the electrochemical performances of all characterized cells. The second column shows selected slices of the reconstructed tomographic volumes (in both uncolored and colored labeling). All of the scale bars are 50 μm long. The third column shows the corresponding 3D representations of the second column after a combination of manual and automated phase filtering and color labeling. Detailed cycling procedures of all cells can be found in the Experimental Section.

types of electrochemical cells were investigated: a Li electrode paired with a LiCoO2/C composite electrode (employing carbonate-based electrolyte, five cells were characterized with the nomenclature Li/LiCoO2-n, n denotes the sample number)

with Figure 1c, a schematic illustration of the synchrotron X-ray imaging setup at BAMline of BESSY II, Berlin, Germany.21,22 Validation of the electrochemical performance of this customized cell can be found in our previous reports.23−26 Currently, two 358


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Figure 3. Electrochemical performance and morphological evolution of Li electrodes of the Li/S cells. A1 shows a panorama view of the selected Li/S-4 cell tomography data, representing all other obtained Li/S tomography data. The rest of the first column shows the electrochemical performances of all characterized cells. The second column shows selected slices of the reconstructed tomographic volumes (in both uncolored and colored labeling). All of the scale bars are 50 μm long. The third column shows the corresponding 3D representations of the second column after a combination of manual and automated phase filtering and color labeling. Detailed cycling procedures of all cells can be found in the Experimental Section.

and a Li electrode paired with monolithic carbon (C)27 soaked with chemically synthesized Li2S8 catholyte as an active material (employing ether-based electrolyte, six cells were measured with

the same nomenclature Li/S-n). Electrochemical performances of all cells are shown in the first columns of Figures 2 and 3. After cycling, all cells were characterized without prior disassembly. 359


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Figure 4. Schematic illustration of the morphological evolution of PLI in a Li anode with regards to the overall cell electrochemical performances. (a) Illustration of the gradual growth of PLI: from left to right, a pristine state cell; Li anode undergoes the first plating during charge; Li anode undergoes the first stripping during discharge; Li anode undergoing the second plating during another charge; Li anode after the Nth stripping/ plating (N ≥ 2). (b,c) Electrochemical performances of the currently investigated Li/S-4 cell and Li/LiCoO2-4 cell.

discharge of the Li/LiCoO2-2 cell at 0.1 mA cm−2 for 4.7 h (Figure 2,D2), it can be seen from Figure 2,E2 that some voids were generated (white diamonds, due to partial dissolution of LmSs and/or the original dense Li bulk). The remaining electrochemically inactive or “dead” LmSs occupy the interface between the original Li bulk and the separator, forming a PLI.23 After the Li/LiCoO2-3 cell was subjected to five cycles (Figure 2,A3), a PLI of ∼23 μm thickness was formed, as shown in Figure 2,B3 (yellow arrow) and Figure 2,C3 (bright yellow). As the cycle number increased to 10 and 20 for the Li/LiCoO2-4 cell (Figure 2,A4) and the Li/LiCoO2-5 cell (Figure 2,A5), the thickness of the PLI increased, respectively, to ∼33 and ∼50 μm, as shown in Figure 2,B4,B5 (yellow arrow) and Figure 2,C4,C5 (bright yellow). Accompanying the increase of the PLI thickness, for example, the electrochemical capacity of the Li/LiCoO2-5 cell of the 20th discharge tremendously decreased to ∼2.3% of the first discharge one (red vs orange curve in Figure 2,A5). A similar trend was also observed for the Li/S cells. After the first cycle and second charge of the Li/S-2 cell (Figure 3,A2), a PLI with a thickness of ∼25 μm was formed, as shown in Figure 3,B2 (yellow arrow) and Figure 3,C2 (bright yellow). (For detailed morphological changes of the Li electrode during the first cycle in the Li/S cell, which has not been studied here due to the limited allocated beam time, readers can refer to Tonin et al.30) After the Li/S-3 cell was cycled for five cycles (Figure 3,A3), scenarios shown in Figure 3,B3,C3 were obtained: a PLI of ∼100 μm thickness was substituted for the originally dense solid Li bulk. A similar scenario but with a thicker PLI (∼210 μm, as shown in Figure 3,B4 (yellow arrow) and Figure 3,C4 (bright yellow)) was observed for the Li/S-4 cell after 10 cycles (Figure 3,A4). Last, as the cycle number further increased to 15 and 20

Detailed assembly and cycling routines of the investigated cells as well as tomography data normalization, reconstruction, and 3D presentation procedures are given in the Experimental Section. The first column of Figures 2 and 3 shows the electrochemical performances of the investigated Li/LiCoO2 and Li/S cells (except Figure 2,A1 and Figure 3,A1, being a representation of an overview of all obtained Li/LiCoO2 and Li/S tomography data: Figure 2,A1 being the Li/LiCoO2-5 cell and Figure 3,A1 the Li/ S-4 cell), respectively. The second column of Figures 2 and 3 shows the selected cross-sectional plane of the reconstructed Li/ LiCoO2 and Li/S cells’ tomographic volumes, respectively. The third column of Figures 2 and 3 shows the corresponding 3D presentations of the second column after a combination of phase filtering and color labeling. Figure 2,B1 and Figure 3,B1 show the cross-sectional views of the uncycled Li/LiCoO2-1 cell and Li/S1 cell, respectively. The Li electrode, the separator, and the LiCoO2 (and carbon fiber) cathode are clearly discernible.28 In a carbonate electrolyte assembled Li/LiCoO2-1 cell, the interface between the Li electrode and separator is smooth (Figure 2,B1,C1). In contrast, there are already some LmSs appearing between the Li electrode and separator in the ether electrolyte assembled Li/S-1 cell (Figure 3,B1,C1).15,29 In the following, correlation between the morphological evolution of the Li electrode in different cells (after different electrochemical conditions) with the corresponding cell’s electrochemical performance as a function of cycle number is presented. The Li/LiCoO2-2 cell was first charged at 0.16 mA cm−2 for 3.4 h (Figure 2,A2), and afterward, a tomography was performed. As can be seen from Figure 2,B2 (white triangle) and Figure 2,C2 (bright yellow), a small amount of LmSs were generated on the surface of a Li bulk electrode due to Li plating. After the first 360


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during the following Li plating process. The gradual dissolution of the original dense Li bulk and the subsequent accumulation of the electrochemically plated LmSs synergistically lead to the observed interfacial changes in both carbonate electrolyte assembled Li/LiCoO2 cells and ether electrolyte assembled Li/ S cells. This scenario fundamentally explains the origin of the inwardly growing LmSs interface, and it also reveals that the accumulated LmSs (or the LrRs) are mostly electrochemically nonreactive. We identify this steadily growing PLI between the original Li bulk and separator as a function of cycle number as PLI. Reports on PLI evolution are very limited in the battery research community due to the fact that it is hidden underneath of Li17 and the technical challenge to nondestructively characterize and analyze them. Meanwhile, in correspondence with the gradual increase of the PLI, the electrochemical performance of the overall cell decreases steadily, as shown in Figure 4b,c. Previous investigations attribute the growth of PLI to serious Li metal corrosion by continuous electrolyte and/or LPS attack.14,17,20,40 However, the combination of our previous investigation23 and current findings suggest that it is actually the electrochemical dissolution (Li stripping) of the initial Li bulk and the following accumulation of electrochemical inactive or dead LmSs that both contribute to the gradual inward growing PLI. The continuously growing PLI will inevitably lead to the overall cell performance degradation. On one hand, the buildup of PLI will unavoidably increase the charge-transfer resistance in terms of a decreased diffusion of Li+ ions during battery cycling. It has been demonstrated by using impedance spectroscopy that a thicker PLI leads to a higher internal resistance.14,20 For example, this intimate correlation has been corroborated by Zhang et al., who found that an infinite increase of resistance evolves as a result of growing PLI (the “thick crust” in their report) characterized by SEM during repeated cycling.41 Furthermore, Chen et al. have also conducted rigorous study of correlation among the PLI (the “compact interphase layer” in their report), the effective diffusion coefficient of Li ions within the PLI, and the overall cell voltage profile, concluding that the rapid capacity fade and failure in the Li metal full cell batteries is caused by the buildup of PLI.42 However, fundamental explanation of the formation of the thick crust and compact interphase layer is lacking in their reports. On the other hand, the gradually growing PLI continuously consumes both original Li bulk and electrolyte. Consistent with current findings, the continuously growing PLI during prolonged cycling has been reported in both carbonatebased LIBs14,43 and ether-based LSBs.17,19 In addition, it has been reported that the electrolyte of LSBs can be totally decomposed during long-term cycling.17,19 The gradual deficiency of liquid electrolyte also results in the increase of the internal resistance and leads to the final failure of the cell. The depletion of ether electrolyte in the LSB is corroborated by the formed voids in the monolithic C/Li2S8 cathode (white arrows in Figure 3,B6). The current investigation is complementary to previous reports about failure mechanisms in LMBs (including LSBs), and for the first time, it fundamentally correlates the internal morphological evolution of the Li electrode with the external overall cell electrochemical performance. In addition, it represents a major step forward in understanding the fundamental degradation mechanism of LMBs (including LSB) and can perfectly explain the reason why recently proposed advanced conceptual strategies of Li protection cannot result in long-term cycling (1000−3000 cycles) stability required by

for the Li/S-5 cell (Figure 3,A5) and the Li/S-6 cell (Figure 3,A6), the thickness of the PLI dramatically increased to ∼240 and ∼280 μm, respectively, as shown in Figure 3,B5,B6 (yellow arrow) and Figure 3,C5,C6 (bright yellow). In correspondence with the increase of the thickness of PLI, for example, the electrochemical capacity of the Li/S-6 cell of the 20th discharge steadily decreased to only ∼19% of the first discharge one (red vs orange curve in Figure 3,A6). Combining the morphological evolution of Li electrodes in both Li/LiCoO2 and Li/S cells with the corresponding overall cell’s electrochemical performances, one can conclude that the gradual electrochemical capacity decay does correlate, at least to some extent, with the continuously growing PLI. The liquid electrolyte in rechargeable LIBs functions as a matrix to transport Li ions during cycles and usually consists of organic solvents and lithium salts.31 It must be carefully chosen to withstand the redox environment at both electrodes and the involved voltage range without decomposition or degradation. Currently, the commercialized LIBs employ carbonate-based electrolyte,32 and the underdeveloped LSBs employ ether-based electrolyte.33 However, due to its highly reactive nature, Li metal is thermodynamically unstable in most organic electrolytes, and it reacts instantly with them, forming a SEI layer on the Li surface.34 The composition of SEI formed in carbonate-based electrolyte contains various decompositions of solvents and lithium salts.35 The existence of LPS in ether-based electrolyte further complicates the composition of the SEI in LSBs.36 Although compositionally different, the SEI layer is generally believed to be a Li ion conductor but an electron insulator, limiting further electrolyte decomposition.37 More investigations have proved that the SEI-covering LmSs are electrochemically inactive during following cycles, resulting in the well-known “dead Li”.12,38 On the other hand, unlike a one-time-only electrodeposition process of other metals (such as Zn, Cu, Ag, etc.), Li metal in rechargeable LMBs needs to be plated and stripped repeatedly during charge/discharge processes.39 Therefore, the generated LmSs during each cycle will accumulate on the surface of Li metal and ultimately lead to many potentially hazardous problems.14,39 Unfortunately, the correlation among the electrochemically inactive or dead LmSs, the accumulated LmSs (or the previously observed interface of inwardly growing LmSs/passivating LrRs) on the surface of Li metal, and the overall cell electrochemical performance have not been fully understood and established. On the basis of the current investigation and our previous study of the morphological evolution of Li electrodes in Li symmetrical cells,23 the correlation among them is first proposed, as shown schematically in Figure 4. During the charge (Li plating) process, Li will be extracted from the cathode material (here LiCoO2 or lithium polysulfide) to plate on the surface of the originally dense Li bulk. The newly formed LmSs will be covered instantly by SEI. During the subsequent discharge (Li stripping) process, part of the previously plated Li and/or the original Li bulk will be dissolved (forming different sizes of cavities on the Li electrode side) to react with the delithiated cathode material. At the same time, the electrochemically inactive or dead Li, generated due to either the electroninsulating SEI covering or being electrically isolated, will accumulate on the surface of the original Li bulk, forming the currently observed porous interfacial structures. As a result, with the increase of cycle number, more original dense Li bulk will be dissolved, forming large quantities of cavities/voids, which will be occupied by the accumulation of electrochemically inactive LmSs 361


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ACS Energy Letters commercialization.44 For example, the interfacial engineering strategy of developing various surface patterns on Li metal13,45 or employing Li spheres46 as the anode may alter preferential locations of the Li plating/stripping process,47 yet the inherent nature of LmS formation is not fundamentally eliminated. Using various artificial protection layers, including, e.g., decomposing many kinds of additives such as organic and inorganic compounds by forming a passivation layer,48 to stabilize the Li surface has been reported to improve the Coulombic efficiency (CE) of Li electrodes.5,49 However, it has been demonstrated that the rapid consumption and decomposition of additives during long-term cycling undermines their effectiveness in protecting the Li electrode50 and that Li dissolution and deposition during stripping and plating will still occur beneath the artificial protection layer.51 Approaches of employing different negative current collectors, Li composite electrodes,52 or heterogeneous metal seeds (concisely speaking, Li “hosting” materials) to preferentially guide the deposition location of LmS are proposed without focusing on the elimination of them.53 The novel strategy of a self-healing electrostatic shield (SHES) proposed by Zhang et al. seems to be a promising solution to prevent dendrite growth in LMBs.54 However, soon they found that self-aligned, highly compacted Li nanorods were generated instead.55 Similarly, a deposited Li columnar structure has also been reported by Huang et al. in a carbonate−ether mixed electrolyte.56 Unfortunately, these deposited Li columnar structures share great similarities with the currently observed LmSs in Li/S cells (Figure 3,B4−B6). Recently, research interest has turned to the emerging solid-state ceramic/polymer (or composites of both) electrolyte57,58 and highly concentrated electrolyte59,60 for restraining LmS growth. Nevertheless, these strategies need more exploration because, for example, LmS growth can still occur underneath/through the solid-state electrolyte.61−66 To sum up, according to the current investigation, without ground-breaking novel electrolyte that allows newly electrochemically plated lithium to, crystallographically speaking, layer-by-layer homoepitaxially grow onto the initial Li bulk instead of forming electrochemically inactive SEI-covering LmSs, the generation of the LmS composing the PLI structure will be intrinsic and unavoidable due to its high reactive nature.67 It is therefore suggested that, in contrast to conventional wisdom of forming a SEI passivation layer, developing novel electrolytes that are fully thermodynamically stable with Li may fundamentally eliminate the growth of PLI.68 Alternatively, recent demonstration of the essential change from employing a solid metal electrode for a room-temperature liquid alloy may also open up new opportunities to practically accomplish a LmS-free electrodeposition.69 The current results also substantiate the reports of LmSs characterized by nuclear magnetic resonance (NMR) spectroscopy, provide new fundamental insights into the data interpretation of impedance spectroscopy characterization, and pose a great challenge to the established use of CE for evaluating Li plating/stripping efficiency. (1) The observed morphological evolution of Li electrodes embodies the derivative findings of NMR measurement characterizing the Li electrode. It has been demonstrated that NMR spectroscopy is capable of detecting the growth of LmS70 and monitoring the accumulation of LmSs during electrochemical cycling.71 The study by Wang et al.72 further showed that the bulk Li metal NMR signal intensity continuously decreased during subsequent cycles, while the LmS NMR signal intensity kept steadily increasing. Our findings agree well with their results in a way that the concrete morphological

evolution of Li metal (and LmS) coincides with the obtained derivative development of Li metal (and LmS) signal peaks. (2) Electrochemical impedance spectroscopy (EIS) has been plagued with accuracy when it comes to finding equivalent circuits to investigate the charge-transfer processes in LIBs.73 The currently observed PLI on top of the Li anode supports, to some extent, the multilayer structure of the Li−electrolyte interface proposed by Aurbach et al.74 They have attributed the very high capacitances calculated by the fitting procedure to a porous, high-area interface, and they speculated that the Li surface is dynamic as a function of time (cycle number) based on the change in resistance.75 Their assumptions are experimentally substantiated by the current observation of the continuously growing PLI as a function of cycle number. The currently observed PLI may guide future impedance data interpretation in finding more relevant equivalent circuits to analyze the internal resistance.76 (3) The gradually growing PLI, in addition, suggests that the LmS components are actually electrochemically inactive or dead during each cycle. On the basis of our previous research23,26 and current findings, the electrochemically plated Li cannot entirely be stripped during the Li stripping process. Instead, it is the original Li bulk that undergoes significant dissolving to compensate for the depletion of Li ions in the electrolyte used for plating/reacting. This process inherently contradicts the foundation of CE evaluation, which assumes that it is the electrochemically plated Li formed in the previous Li plating process that is stripped during the following Li stripping process. The growing PLI may fundamentally explain the classic scenario where the CE remains steadily close to 100% while continuously ceaseless capacity decay occurs in cycling LMBs.14,77 Apart from that, the current investigation also suggests that fundamental understanding of the Li electrode on the cycle stability in LMBs (including LSBs) is crucial to further improve the cell performance. Especially more investigations are urgently required to fully understand the formation and electrochemical property of the SEI during LmS formation as well as the chemical and electrochemical behavior of PLI during cell cycling.78 The profound understanding of the composition, chemical and electrochemical properties, and morphological structure evolution of SEI/PLI may lead to new approaches to stabilize the longterm cycling stability of Li metal for practical applications.79 For example, phenomenologically different LmSs grown under various electrolyte environments have been shown by Shi et al.80 In addition, the preferential crystallographic growth direction observed by Li et al.81 and the totally different amorphous crystallization nature of electrochemically deposited Li by Wang et al.,82 both resulting from cryoelectron microscopy, further challenge our understanding of the growth mechanism of the LmSs. Thus, more scientific explorations should be ignited from chemistry, electrodeposition, physics, as well as engineering communities to achieve practical commercialization of Li metal for next-generation energy storage systems.83 Without groundbreaking technology, the utilization of Li metal in LSBs may be replaced by other more suitable anode materials84 as it was replaced by carbonaceous anode in LIBs in 1991, again. Although significant challenges still remain for successful commercialization of LMB systems via simultaneously and harmoniously improving cathode material utilization, electrolyte stability, separator multifunction, Li anode protection, and even battery design,85 the current investigation suggests that the combination of facilities capable of accessing interior electrode evolutions nondestructively and three-dimensionally with exterior electro362


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(8) Ji, X.; Lee, K. T.; Nazar, L. F. A Highly Ordered Nanostructured Carbon-Sulphur Cathode for Lithium-Sulphur Batteries. Nat. Mater. 2009, 8, 500−506. (9) Sun, Q.; He, B.; Zhang, X.-Q.; Lu, A.-H. Engineering of Hollow Core−Shell Interlinked Carbon Spheres for Highly Stable Lithium− Sulfur Batteries. ACS Nano 2015, 9, 8504−8513. (10) Chen, X.; Peng, H.-J.; Zhang, R.; Hou, T.-Z.; Huang, J.-Q.; Li, B.; Zhang, Q. An Analogous Periodic Law for Strong Anchoring of Polysulfides on Polar Hosts in Lithium Sulfur Batteries: S- or Li-Binding on First-Row Transition-Metal Sulfides? ACS Energy Lett. 2017, 2, 795− 801. (11) Li, M.; Carter, R.; Douglas, A.; Oakes, L.; Pint, C. L. Sulfur VaporInfiltrated 3D Carbon Nanotube Foam for Binder-Free High Areal Capacity Lithium−Sulfur Battery Composite Cathodes. ACS Nano 2017, 11, 4877−4884. (12) Cao, R.; Xu, W.; Lv, D.; Xiao, J.; Zhang, J.-G. Anodes for Rechargeable Lithium-Sulfur Batteries. Adv. Energy Mater. 2015, 5, 1402273−1402296. (13) Park, J.; Jeong, J.; Lee, Y.; Oh, M.; Ryou, M.-H.; Lee, Y. M. MicroPatterned Lithium Metal Anodes with Suppressed Dendrite Formation for Post Lithium-Ion Batteries. Adv. Mater. Interfaces 2016, 3, 1600140− 1600149. (14) Lu, D.; Shao, Y.; Lozano, T.; Bennett, W. D.; Graff, G. L.; Polzin, B.; Zhang, J.; Engelhard, M. H.; Saenz, N. T.; Henderson, W. A.; et al. Failure Mechanism for Fast-Charged Lithium Metal Batteries with Liquid Electrolytes. Adv. Energy Mater. 2015, 5, 1400993−1401000. (15) 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. Nat. Mater. 2014, 13, 69− 73. (16) Li, W.; Zheng, H.; Chu, G.; Luo, F.; Zheng, J.; Xiao, D.; Li, X.; Gu, L.; Li, H.; Wei, X.; et al. Effect of Electrochemical Dissolution and Deposition Order on Lithium Dendrite Formation: A Top View Investigation. Faraday Discuss. 2014, 176, 109−124. (17) Qie, L.; Zu, C.; Manthiram, A. A High Energy Lithium-Sulfur Battery with Ultrahigh-Loading Lithium Polysulfide Cathode and Its Failure Mechanism. Adv. Energy Mater. 2016, 6, 1502459−1502466. (18) Zu, C.; Dolocan, A.; Xiao, P.; Stauffer, S.; Henkelman, G.; Manthiram, A. Breaking Down the Crystallinity: The Path for Advanced Lithium Batteries. Adv. Energy Mater. 2016, 6, 1501933−1501942. (19) Cheng, X.-B.; Yan, C.; Huang, J.-Q.; Li, P.; Zhu, L.; Zhao, L.; Zhang, Y.; Zhu, W.; Yang, S.-T.; Zhang, Q. The Gap between Long Lifespan Li-S Coin and Pouch Cells: The Importance of Lithium Metal Anode Protection. Energy Storage Mater. 2017, 6, 18−25. (20) Han, Y.; Duan, X.; Li, Y.; Huang, L.; Zhu, D.; Chen, Y. Effects of Sulfur Loading on the Corrosion Behaviors of Metal Lithium Anode in Lithium−Sulfur Batteries. Mater. Res. Bull. 2015, 68, 160−165. (21) Sun, F.; Markötter, H.; Zhou, D.; Alrwashdeh, S. S. S.; Hilger, A.; Kardjilov, N.; Manke, I.; Banhart, J. In Situ Radiographic Investigation of (De)Lithiation Mechanisms in a Tin-Electrode Lithium-Ion Battery. ChemSusChem 2016, 9, 946−950. (22) Zielke, L.; Sun, F.; Markötter, H.; Hilger, A.; Moroni, R.; Zengerle, R.; Thiele, S.; Banhart, J.; Manke, I. Synchrotron X-Ray Tomographic Study of a Silicon Electrode before and after Discharge and the Effect of Cavities on Particle Fracturing. ChemElectroChem 2016, 3, 1170−1177. (23) 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. (24) Sun, F.; Markötter, H.; Manke, I.; Hilger, A.; Kardjilov, N.; Banhart, J. Three-Dimensional Visualization of Gas Evolution and Channel Formation inside a Lithium-Ion Battery. ACS Appl. Mater. Interfaces 2016, 8, 7156−7164. (25) Sun, F.; Markötter, H.; Dong, K.; Manke, I.; Hilger, A.; Kardjilov, N.; Banhart, J. Investigation of Failure Mechanisms in Silicon Based Half Cells During the First Cycle by Micro X-Ray Tomography and Radiography. J. Power Sources 2016, 321, 174−184.

chemical performance characterization may be essential for understanding the underlying causes that dominate the decaying mechanisms in battery research.86

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EXPERIMENTAL SECTION Detailed information on material, battery preparation, electrochemical measurement, tomography measurement, and data processing is available in the Supporting Information.

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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.7b01254. Detailed information on material, battery preparation, electrochemical measurement, tomography measurement, and data processing (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. ORCID

Fu Sun: 0000-0001-6787-6988 Present Address

C.J.J.: Oak Ridge National Laboratory, Oak Ridge, TN, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Heinrich Riesemeier, the beamline scientist at BESSY II, for his valuable assistance and engineer Norbert Beck for fabricating the beamline battery. This work is sponsored by the Helmholtz Association and the China Scholarship Council.

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REFERENCES

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Correlating Morphological Evolution of Li Electrodes with Degrading

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