Correlating Morphological Evolution of Li Electrodes with Degrading

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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 ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01254 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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ACS Energy Letters

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 F. Sun, M. Osenberg, K. Dong, D. Zhou, H. Markötter, and I. Manke Institute of Applied Materials Helmholtz Centre Berlin for Materials and Energy Hahn-Meitner-Platz 1, 14109 Berlin, Germany

M. Osenberg, K. Dong, D. Zhou, A. Hilger Institute of Material Science and Technologies Technical University Berlin Strasse des 17. Juni 135, 10623 Berlin, Germany

C. J. Jafta, S. Risse, Y. Lu Institute of Soft Matter and Functional Materials Helmholtz Centre Berlin for Materials and Energy Hahn-Meitner-Platz 1, 14109 Berlin, Germany

Y. Lu Institute of Chemistry, University of Potsdam Am Neuen Palais 10, House 9 14469 Potsdam, Germany

ABSTRACT Efficient Li utilization is generally considered to be a prerequisite for developing next-generation energy storage systems (ESSs). However, uncontrolled growth of Li micro-structures (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 lithiumreacted region have been intensively discussed. The information we gain herein are complementary to previous investigations and may provide general insights into understanding of degradation mechanisms of Li-metal anode and also provide highly needed guidelines for effective design of reliable next-generation Li-metal based ESSs.

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Upcoming 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 LIB technology.1-3 This tremendously increasing demand for energy storage has sparked renewed interests in 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 non-toxic 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 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 Li anode during cycling).6-7 The pioneering work demonstrating that reversible capacity up to 1320 mAh/g can be attained through mesoporous carbon enhanced S utilization has been reported by Nazar et al. in 2009.8 Since then, tremendous efforts have been dedicated towards improving Li-S technology to commercialization and the proposed strategies focus mostly on S cathode,9-11 However, the Li metal anode in LSBs, which is directly involved in capacity decay and 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 next-generation battery technology.15 Research spotlight is recently falling on Li anode which seems to be a paramount limiting factor for Li metal batteries (LMBs, Li metal anode based 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 the deterioration of Li metal anode by forming an interface of inwardly-growing Li microstructure (LmS, such as dendrite, fiber, moss, etc.) covered by solid electrolyte interface (SEI).14 2 ACS Paragon Plus Environment

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ACS Energy Letters

Later on, Manthiram et al. have also discovered that serious corrosion and passivation of Li metal and electrolyte decomposition are main reasons for the failure of LSBs.17 They further identified the passivation layer on the surface of Li metal as “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 lights on fast 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 researches are frequently focusing on a minor part of the broad and complex electrochemical evolutions of Li anode electrode. 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 firstly suggests that the interface of inwardlygrowing LmSs and the passivating “LrRs” can be denominated as 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 metal based 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 and b, along with Figure 1c, a schematic illustration of the synchrotron X-ray imaging setup at BAMline of BESSY II, Berlin, Germany21-22. Validation of the electrochemical performance of this customized cell can be found in our previous reports.23-26 Currently, two types of electrochemical cells were investigated: a Li electrode paired with a LiCoO2/C composite electrode (employing carbonate-based electrolyte, 5 cells were characterized with the nomenclature Li/LiCoO2-n, n denotes sample number) and a Li electrode paired with monolithic carbon (C)27 soaked with chemically synthesized Li2S8 catholyte as active material (employing ether-based electrolyte, 6 cells were measured with the same nomenclature Li/S-n). Electrochemical performances of all cells are shown in the first column of Figure 2 and Figure 3. After cycling, all cells were characterized without prior disassembly. 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.

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Figure 1. Photograph and schematic illustration of the customized electrochemical cell and the 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 Xray setup. The scale bar is 1 mm. b) Corresponding schematic representation of the cell consisting of a polyamide-imide housing (yellow), two screw (light grey), two sealing rings (pink), 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

The first column of Figure 2 and Figure 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, Figure 3 A1 the Li/S-4 cell), respectively. The second column of Figure 2 and Figure 3 shows selected cross-sectional plane of the reconstructed Li/LiCoO2 and Li/S cells tomographic volumes, respectively. The third column of Figure 2 and Figure 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 4 ACS Paragon Plus Environment

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ACS Energy Letters

Li/LiCoO2-1 cell and Li/S-1 cell, respectively. The Li electrode, the separator and the LiCoO2 (and carbon fiber) cathode are clearly discernable.28 In carbonate electrolyte assembled Li/LiCoO2-1 cell, the interface between Li electrode and separator is smooth (Figure 2 B1, C1). In contrast, there are already some LmSs appeared between Li electrode and separator in ether electrolyte assembled Li/S-1 cell (Figure 3 B1, C1).15, 29 In the following, correlation between the morphological evolution of 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 firstly charged at 0.16 mA cm-2 for 3.4 h (Figure 2 A2) and afterwards 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 Li bulk electrode due to Li plating. After the first discharge of 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 porous lithium interface (PLI).23 After Li/LiCoO2-3 cell has been subjected to 5 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 Li/LiCoO2-4 cell (Figure 2 A4) and Li/LiCoO2-5 cell (Figure 2 A5), the thickness of the PLI has increased respectively to ~33 µm 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 has tremendously decreased to ~2.3% of the 1st discharge one (red vs. orange curve in Figure 2 A5).




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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 the rest Li/LiCoO2 tomography data. The rest of the first column show the electrochemical performances of all characterized cells. The second column show selected slices of the reconstructed tomographic volumes (in both un-colored and colored labeling). All the scale bars are 50 µm long. The third column show 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.

A similar trend was also observed for the Li/S cells. After the first cycle and 2nd charge of Li/S-2 cell (Figure 3 A2), a thickness of ~25 µm PLI was formed, as shown in Figure 3 B2 (yellow arrow) and Figure 3 C2 (bright yellow). (For detailed morphological changes of Li electrode during the first cycle in 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 5 cycles (Figure 3 A3), scenarios shown in Figure 3 B3, C3 were obtained: ~100 µm thickness of PLI has substituted the originally dense solid Li bulk. Similar scenario but thicker thickness of PLI (of ~210 µm, as shown in Figure 3 B4 (yellow arrow) and Figure 3 C4 (bright yellow)) was observed for Li/S-4 cell after 10 cycles (Figure 3 A4). Lastly, as the cycle number further increased to 15 and 20 for Li/S-5 cell (Figure 3 A5) and Li/S-6 cell (Figure 3 A6), the thickness of the PLI has dramatically increased to ~240 µm 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 has steadily decreased to only ~19% of the 1st 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, to the continuously growing PLI.




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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 the other obtained Li/S tomography data. The rest of the first column show the electrochemical performances of all characterized cells. The second column show selected slices of the reconstructed tomographic volumes (in both un-colored and colored labeling). All the scale bars are 50 µm long. The third column show 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.

The liquid electrolyte in rechargeable LIBs functions as 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 electrolyte32 and the under developing 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 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 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 onetime-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 surface of Li metal, and the overall cell electrochemical performance have not been fully understood and established. Based on current investigation and our previous study of the morphological evolution of Li electrodes in Li symmetrical cells,23 the correlation among them is firstly proposed, as shown schematically in Figure 4. During charge (Li plating) process, Li will be extracted from the cathode material (here LiCoO2 or lithium polysulfide) to plate on the surface of originally dense Li bulk. The newly formed LmSs will be covered instantly by SEI. During subsequent discharge (Li stripping) process, partial of the previously plated Li and/or the original Li bulk will be dissolved (forming different sizes of cavities in Li electrode side) to react with the delithiated cathode material. At the same time, the electrochemically inactive or “dead” Li, generated due to 9 ACS Paragon Plus Environment


either the electron-insulating SEI covering or being electrically isolated, will accumulate on the surface of the original Li bulk, forming the currently observed porous interfacial structures. Resultantly, 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 during the following Li plating process. The gradually 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 non-reactive. We identify this steadily growing porous lithium interface between original Li bulk and separator as a function of cycle number as PLI. Reports on PLI evolution are very limited in battery research community due to the fact that it is hidden underneath Li17 and the technical challenge to nondestructively characterize and analyze them. Meanwhile, in correspondence with the gradually increase of the PLI, the electrochemical performance of the overall cell decreases steadily, as shown in Figure 4 b,c.

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: 10 ACS Paragon Plus Environment

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from left to right, a pristine state cell; Li anode undergoes the 1st plating during charge; Li anode undergoes the 1st stripping during discharge; Li anode undergoing the 2nd plating during another charge; Li anode after Nth stripping/plating (N≥2). b) and c), electrochemical performances of currently investigated Li/S-4 cell and Li/LiCoO2-4 cell.

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 gradually inwardly-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 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 build-up of PLI.42 However, fundamental explanation of the formation of “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 both in carbonate-based LIBs14, 43 and ether-based LSBs.17, 19 In addition, it has been reported that the electrolyte of LSBs can be totally decomposed during longterm cycling.17, 19 The gradually 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 LSB is corroborated by the formed voids in 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 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 commercialization.44 For example, interfacial engineering strategy of developing various surface patterns on Li metal13, 45 or employing Li 11 ACS Paragon Plus Environment


spheres46 as anode may alter preferential locations of Li plating/stripping process,47 yet the inherent nature of LmS formation is not fundamentally eliminated. Using various artificial protection layers, including, e.g., through decomposing many kinds of additives such as organic and inorganic compounds by forming passivation layer,48 to stabilize 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 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 electrodes52 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 Novel strategy of selfhealing electrostatic shield (SHES) proposed by Zhang et al. seems a promising solution to prevent dendrite growth in LMBs.54 But soon they found that self-aligned, highly compacted Li nanorods were generated instead.55 Similarly, the 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 to the currently observed LmSs in Li/S cells (Figure 3 B4-B6). Recently the study interest has turned to the emerging solid state ceramic/polymer (or composites of both) electrolyte57-58 and highly concentrated electrolyte59-60 for restraining LmSs growth. Nevertheless, these strategies need more exploring since, for example, the LmSs growth can still occur underneath/through the solid state electrolyte.61-66 To sum up, according to 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 SEIcovering LmSs, the generation of the LmS composing PLI structure will be intrinsic and unavoidable due to its high reactive nature.67 It is therefore suggested that, contrast to conventional wisdom of forming 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 the employing of solid metal electrode to a room-temperature liquid alloy may also open up new opportunities to practically accomplish an 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 great challenge to the established use of Coulombic efficiency for evaluating Li plating/stripping efficiency. 1), the observed morphological evolution of Li electrodes embodies the derivative findings of NMR measurement charactering Li electrode. It has been demonstrated that the NMR spectroscopy is capable of detecting the growth of LmS70 and monitoring the accumulation of LmSs during electrochemical cycling.71 Following study by Wang et al.72 further discovered 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 12 ACS Paragon Plus Environment

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ACS Energy Letters

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 Li anode supports, to some extent, the multilayer structure of the Li-electrolyte interface proposed by Aurbach et al.74 They have attributed a 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 consumptions 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. Based on our previous research23, 26 and current findings, the electrochemically plated Li cannot entirely be stripped during 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 to be plated/reacted. This process inherently contradicts the foundation of Coulombic efficiency 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 Coulombic efficiency 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 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 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 property and morphological structure evolution of SEI/PLI may lead to new approaches to stabilize the long-term 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 the cryo-electron 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 the practical commercialization of Li metal for the next-generation energy storage systems.83 Without ground-breaking 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 13 ACS Paragon Plus Environment


protection and even battery design,85 the current investigation suggests that the combination of facilities capable of accessing to interior electrode evolutions non-destructively and threedimensionally with exterior electrochemical performance characterization may be essential for understanding the underlying causes that dominate the decaying mechanisms in battery research.86

Experimental Section: detailed information of material, battery preparation, electrochemical measurement, tomography measurement and data processing is available in Supporting Information.

Author Information Corresponding Author *E-maill: [email protected]; [email protected] ORCID Fu Sun: 0000-0001-6787-6988 Charl J. Jafta: 0000-0002-9773-6799 Present address: Oak Ridge National Laboratory, Oak Ridge, TN, United States Notes The authors declare no competing financial interests. Acknowledgements 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. Supporting Information Available: detailed information of material, battery preparation, electrochemical measurement, tomography measurement and data processing. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Zhou, D.; Jia, H.; Rana, J.; Placke, T.; Scherb, T.; Kloepsch, R.; Schumacher, G.; Winter, M.; Banhart, J. Local Structural Changes of Nano-Crystalline ZnFe2O4 During Lithiation and DeLithiation Studied by X-Ray Absorption Spectroscopy. Electrochim. Acta 2017, 246, 699-706.


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ACS Energy Letters

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Correlating Morphological Evolution of Li Electrodes with Degrading

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