Dendrite-Free Li Metal Anode for Rechargeable Li-SO2 Batteries

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Surfaces, Interfaces, and Applications 2

Dendrite-Free Li Metal Anode for Rechargeable Li-SO Batteries Employing Surface Modification with NaAlCl-2SO Electrolyte 4

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Juhye Song, Jaehwan Chun, Ayoung Kim, Hojae Jung, Hyun Jong Kim, Young-Jun Kim, Goojin Jeong, and Hansu Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08731 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 20, 2018

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ACS Applied Materials & Interfaces

Dendrite-Free Li Metal Anode for Rechargeable LiSO2 Batteries Employing Surface Modification with NaAlCl4-2SO2 Electrolyte Juhye Song, † Jaehwan Chun, ‡ Ayoung Kim, † Hojae Jung, † Hyun Jong Kim, † Young-Jun Kim,*, § Goojin Jeong,*, ‡ and Hansu Kim*,†

†

Department of Energy Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea

‡

Advanced Batteries Research Center, Korea Electronic Technology Institute, 68 Yatap-dong, Bundang-gu, Seongnam 13509, Republic of Korea

§

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 16419, Republic of Korea

Corresponding Author *E-mail: [email protected]; [email protected]; [email protected] KEYWORDS: lithium metal, dendrite-free, surface modification, inorganic electrolyte, lithium rechargeable battery

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Dendritic growth of a Li metal anode during cycling is one of major issues to be addressed for practical application of Li metal rechargeable batteries. Herein, we demonstrate that surface modification of Li metal with Na-containing SO2 electrolyte can be an effective way to prevent dendritic Li growth during cell operation. The surface-modified Li metal anode exhibited no dendritic deposits even under a high areal capacity (5 mA h cm-2) and a high current density (3 mA cm-2), while the unmodified anode showed typical filamentary Li deposition. The surfacemodified Li metal anode also demonstrated significantly enhanced electrochemical performance, which could be attributed to the newly-formed Na-containing inorganic surface layer that exhibits uniform and dense properties. Consequently, surface modification with a Na-containing SO2 inorganic electrolyte is suggested as one of the most effective ways to realize a highly stable Li metal anode with dendrite-free Li deposition for Li metal-based rechargeable batteries.


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1. INTRODUCTION The lithium metal anode is an ideal candidate for high energy Li secondary batteries because it possesses the lowest electrochemical potential (-3.04 V vs. standard hydrogen electrode) and highest theoretical capacity (3,860 mA h g-1) among various anode materials.1-3 However, dendritic Li deposition during charging and its high chemical reactivity with electrolyte hinder its practical application.4-7 To solve these problems, various efforts have been made, and some significant progress has been achieved. Zhang et al.8 and Qian et al.9 demonstrated that a high salt concentration in electrolyte provides a stable chemical environment for cycle stability and a high Coulombic efficiency of a Li metal electrode without dendritic growth of Li. Stark et al.10 reported that dendritic growth of Li can be suppressed by the addition of a small amount of Na+-containing ionic liquid in the Li ionic liquid for Li battery applications. Recently, Pang et al.11 reported that a surface-modified Li metal electrode using a designed electrolyte additive can form an amorphous Li3PS4 surface layer that improved the interfacial stability of Li metal, thus showing long-life dendrite-free Li plating. Many approaches such as artificial protective layer employing organic/inorganic composite film12 and Cu3N/SBR composite,13 pre-SEI layer formation via the electroplating method,14 and AlCl3 electrolyte additive15 have been also proposed for suppressing the dendritic growth of Li. These research findings provide important clues to develop an effective approach to Li metal stabilization for successful commercialization of Li metal-based rechargeable batteries. We recently introduced rechargeable Na-SO216-18 and Li-SO219 battery systems employing SO2based inorganic electrolyte as another candidates for post lithium ion batteries. The merits of these batteries are based on some promising physical and electrochemical properties of the SO2-based inorganic electrolyte including (i) non-flammability, (ii) high ionic conductivity (~ 10-1 S cm-1),



and (iii) versatile use as an electroactive catholyte or solely as an ion-conducting electrolyte.17-20 For a Li-SO2 battery, remarkable improvement of battery performance could be also achieved by exploiting various carbon nanostructured materials for the cathode materials.19 However, a Li metal anode in the Li-SO2 systems still suffers from the dendritic growth of Li metal during cycling.20-22 Not only was scattered needle-like Li deposited, but cracks on the surface of Li metal were also observed.23 Therefore, these problems should be resolved for successful application of Li-SO2 batteries. We previously reported that highly concentrated Na+-conducting inorganic electrolyte provided a compact and dense SEI layer for the Na metal anode, leading to the suppression of dendritic growth of Na and improved electrochemical performance.16 This result implies that there is a close relationship between the surface layer on the metallic anode and its deposition morphology. Inspired by this result, we try to pretreat Li metal surface by immersing it in the NaAlCl 4-2SO2 electrolyte, expecting similar phenomena on Li metal electrode. We herein report the unique interfacial properties of Li metal electrode controlled by a Na-containing, SO2-based inorganic electrolyte, showing non-dendritic Li deposition during cycling. The surface-modified Li metal electrode showed improved electrochemical performance in Li/Li symmetric cells even under a high areal capacity (5 mA h cm-2) and high current density (3 mA cm-2) compared to the unmodified electrode. From these findings, surface modification of Li metal with the Nacontaining inorganic electrolyte is suggested as an effective method for practical application of Li metal for high energy Li metal-based rechargeable batteries.

2. EXPERIMENTAL SECTION


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Electrolyte synthesis. The NaAlCl4-2SO2 and LiAlCl4-3SO2 inorganic electrolytes were prepared according to the methods presented in our previous reports, respectively.16,19 Preparation of the surface-modified Li metal electrode. To obtain the NaAlCl4-2SO2modified Li metal electrode, fresh Li metal foil was immersed in NaAlCl4-2SO2 electrolyte for 1 day. To remove the residual electrolyte, the modified Li metal was washed with SOCl2 (Thionyl chloride, >99.5%, Daejung Chemicals) for a short time and vacuum-dried at room temperature for 12 h. For the LiAlCl4-3SO2-modified Li meal electrode, we proceeded in the same way using a LiAlCl4-3SO2 electrolyte. Electrochemical measurement. Beaker-type Li/Li symmetric cells with a LiAlCl4-3SO2 inorganic electrolyte were fabricated to observe the morphology of Li electrodeposits for the unmodified Li metal electrode and NaAlCl4-2SO2-modified Li metal electrode with respect to various current densities and areal capacities. For the short-circuit test, 2032 coin-type Li/Li symmetric cells using a LiAlCl4-3SO2 inorganic electrolyte were assembled. Galvanostatic polarization was conducted for the short-circuit test under a current density of 3 mA cm−2 and an areal capacity of the working Li metal electrode of 85.6 mA h cm−2. The coin-type Li/Li symmetric cells were also employed for cycle test. We evaluated the cycling performance of Li/Li symmetric cells under the following conditions: an areal capacity of the working Li metal electrode of 85.6 mA h cm−2 and a fixed capacity of 5 mA h cm−2 (17% of the initial capacity of Li). The current density for Li deposition/stripping was 3 mA cm−2. To check the electrochemical performance of 2032 coin-type Li-SO2 cells employing the unmodified Li metal or the surface-modified Li metal electrode, carbon electrodes were fabricated using the mixture of 90% Ketjenblack (KB, EC600JD) and 10% polytetrafluoroethylene (PTFE, Sigma-Aldrich), which was coated on Ni mesh as a current collector. The electrodes were used after vacuum drying at 200 °C for 12 h in oven. In



this work, the electrodes possess the mass loading of 5.2-5.5 mg cm-2 corresponding to an areal capacity of 7.9-8.2 mA h cm-2. We identified the cycling capacity and overpotential behavior of Li-SO2 cells at a rate of 2 C discharge and 0.5 C charge (1 C = 1500 mA g-1) given voltage range of 2.9 V-3.9 V (vs. Li/Li+). We used a glass microfiber filter (GC50, Advantec) as a separator when assembling the 2032 coin-type cells. Characterization. Morphological observations of the Li metal electrodes after cycling and after immersion in LiAlCl4-3SO2 and NaAlCl4-2SO2 inorganic electrolytes were carried out using a field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7000F). The crystalline states in the surface layer of the Li metal were identified by ex situ X-ray diffraction (XRD, Rint-2000, Rigaku) in the 2θ range of 25-90o. For further characterization of the chemical states of the surface of the Li metal, X-ray photoelectron spectroscopy (XPS, Thermo Scientific Sigma Probe) was employed. All XPS spectra were shifted by the same values based on the C 1s peak at 284.8 eV. For all ex situ analyses, Li metal electrodes were obtained from the cycled cells and washed with SOCl2 (>99.5%, Daejung Chemicals) in an Ar-filled glove box. After vacuum drying for 24 h, the prepared Li metal electrodes were carefully loaded onto holder followed by sealed in an Ar-filled container that prevents exposure of the Li metal to the air. The sample-loaded holder was immediately transferred to the vacuum-chamber of analytical instruments from the Ar-filled container within a few seconds in the moisture-controlled dry room.

3. RESULTS AND DISCUSSION Figure 1a shows SEM images of Li deposits on the Li metal electrode in LiAlCl4-3SO2 electrolyte obtained under various areal capacity and current density conditions. Interestingly, a polygonal Li deposit was observed at a relatively low areal capacity and current density, which is


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similar to the behavior of Na deposition in NaAlCl4-2SO2 electrolyte.16 Such a well-defined polygonal deposit seems to be a unique nature of alkali metal anode, such as Li and Na, in a highly concentrated SO2-based inorganic electrolyte.16 However, filamentary Li deposits started to be observed as the areal capacity and current density increased, which is regarded as typical behavior of Li deposition in a Li metal anode system6,24,25 (also see Supporting Information Figure S3 corresponding to the low magnification images of Figure 1a). Generally speaking, the formation of Li dendrite is suppressed at a relatively low current density, and this is mainly because the SEI layer on the Li metal is stably maintained during cycling. However, at high current density, Li metal undergoes uneven and severe volume changes during repeated Li deposition/dissolution. This leads to cracks of the surface film and exposure of the active Li surface, which should be controlled for practical application of a Li metal anode.6,26 To suppress Li dendrite formation in the LiAlCl4-3SO2 electrolyte, we tried to modify the Li metal surface by immersing it in a Na-containing SO2 electrolyte, i.e., NaAlCl4-2SO2, expecting that the surface of the NaAlCl4-2SO2-modified Li metal would become chemically and mechanically stable like a Na metal electrode in the same NaAlCl4-2SO2 electrolyte.16 Figure 1b exhibits the morphology of the Li deposit on the surface-modified Li metal electrode in LiAlCl43SO2 electrolyte. Remarkably, no dendritic Li deposits were observed under any of the cycling conditions even including the high areal capacity of 5 mA h cm-2 and current density of 3 mA cm2

(also see Supporting Information Figure S4 corresponding to the low magnification images of

Figure 1b). These noticeable morphological changes are obvious in the corresponding crosssectional images where the surface-modified Li electrode produces flat and dendrite-free Li growth (Figure 1c).



To examine the effect of the morphological changes of Li deposits on the electrochemical performance of the Li metal anode, we carried out short-circuit and cycling tests using Li/Li symmetric cells. As shown in Figure 2a, the NaAlCl4-2SO2-modified Li/Li symmetric cell did not show any short-circuit behavior for more than 27 h, corresponding to 96% of the required time (28.5 h) for complete stripping of Li metal electrode. On the other hand, the unmodified Li/Li symmetric cell showed a drastic voltage drop to 0.0 V (vs. Li/Li+) after 20 h. The short-circuit of the unmodified Li electrodes might result from filamentary growth of Li deposits.26,27 As schematically illustrated (Figure 2b-d), for the expected phenomena of Li growth during the above short-circuit tests, we speculate that the surface layer of the NaAlCl4-2SO2-modified Li electrode provides a stable chemical and mechanical environment during repeated Li deposition/dissolution, thereby suppressing filamentary growth of Li. We also found another positive effect of the surface modification, in concerning the voltage-delay issue that is normally observed in Li-SO2 primary battery system.28,29 While observed the relatively large polarization of potential at the start in the unmodified Li/Li cell, the surface-modified Li/Li cell exhibits less polarization after the storage, which is strongly related to the improved physicochemical nature of surface film achieved by the surface modification. We also performed charge/discharge cycling tests using symmetric Li/Li cells for a given condition of 300 cycles for 1000 h, and the results are shown in Figure 2e. The current density for Li stripping and deposition is 3 mA cm-2, and the areal capacity for the reaction is 5 mA h cm-2, which corresponds to 17% of the total capacity of the working Li metal electrode. In the early stage of cycling, a slightly large initial overpotential was observed for the surface-modified Li/Li cell. As cycling continued, however, the surface-modified Li/Li cell exhibited reduced and stabilized polarization behavior for 1,000 h, while the unmodified Li/Li cell showed a steadily


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increased overpotential during cycling. This improvement of cycling performance could be attributed to the non-dendritic growth of Li deposits as well as highly stable interface on the Li metal pre-modified with NaAlCl4-2SO2 electrolyte. To understand the improved electrochemical performance and morphology of the surfacemodified Li metal anode, we investigated the interfacial properties of the Li metal using various analytical tools including SEM, EDS, and XPS. Figure 3 shows the SEM-EDS results for the surface of Li metal after immersion in LiAlCl4-3SO2 and NaAlCl4-2SO2 electrolytes. The Li metal immersed in LiAlCl4-3SO2 possessed an irregular surface layer with poorly defined crystals (Figure 3a). The corresponding EDS mapping in Figure 3b-d revealed that sulfur, oxygen, and chlorine were unevenly distributed on the Li metal surface. We also verified the presence of crystalline LiCl in the surface film through XRD analysis (see Supporting Information Figure S5a). In agreement with previous reports,30-32 these results are associated with a surface layer consisting of sulfur-oxy compounds and LiCl. On the contrary, the surface morphology of Li metal after immersion in the NaAlCl4-2SO2 electrolyte exhibited a uniform and dense layer, as shown in Figure 3e. The elemental EDS mapping results (Figure 3f-h) showed that sulfur, oxygen, and chlorine elements were uniformly distributed throughout its surface layer. We also identified the presence of crystalline NaCl in the surface layer by XRD analysis (Figure S5b). Additionally, we verified that the surface layers on Li metal electrodes after immersion in LiAlCl 4-3SO2 or NaAlCl4-2SO2 electrolytes show about 700~800 nm of thickness as shown in Figure S6. In order to understand the chemical state of the surface layer, XPS analysis was conducted along with qualitative depth profile analysis. The panels in Figure 4a show the Li 1s, Na 1s, Cl 2p, S 2p, and O 1s XPS spectra for the Li surfaces after storage in the LiAlCl4-3SO2 electrolyte. The Li 1s and O 1s XPS spectra show that various lithium compounds were formed compared to the native



Li metal surface (see Supporting Information Figure S7), and new peaks of Cl 2p and S 2p were generated. Li reacts with SO2 during storage in a LiAlCl4-3SO2 electrolyte, forming Li2S (54.5 eV)33 with various species including Li2O (53.5 eV),34 LiOH/Li2CO3 (55.3 eV),34 and Li2SO4 (55.7 eV)35 in the Li 1s XPS spectra. We also found that the LiCl (56.2 eV) peak became stronger with increasing Ar ion etching time, and the LiCl-related peak in Cl 2p3/2 spectra could be detected at 198.8 eV in Figure 4a.36 It was confirmed that crystalline LiCl exists in the surface film of the LiAlCl4-3SO2-modified Li metal electrode. The S 2p XPS spectra in Figure 4a show that lithium sulfide (Li-S, ~162.1 eV)37 was generated at the beginning of the reaction of Li metal in contact with SO2 in the electrolyte, and therefore the intensity increased as the Ar ion etching time increased.38 Many lithium sulfur-oxy compounds including Li2SO4 (169.5 eV), Li2SnO6 (168.2 eV), and Li2S2O4/Li2SO3 (166.3-166.6 eV) were also observed,31 which were formed by further reactions between primary products and SO2 in the electrolyte38 (Supporting Information Figure S8 shows C 1s and Al 2p XPS spectra). The panels in Figure 4b show the Li 1s, Na 1s, Cl 2p, S 2p, and O 1s XPS spectra for the Li surfaces after storage in a NaAlCl4-2SO2 electrolyte. Similar to the surface of the LiAlCl4-3SO2modified Li metal electrode, various lithium compounds were observed in the surface film of the NaAlCl4-2SO2-modified Li metal electrode, but a LiCl peak was not observed, as shown in the Li 1s XPS spectra. The peaks at 1,071.6 eV for Na 1s and 198.5 eV for Cl 2p3/2 revealed that NaCl was generated on the NaAlCl4-2SO2-modified Li metal electrode.39 Various Na compounds including NaOH (1,073.1 eV),40 Na2O (1,072.5 eV),39 NaxSy (~1,072 eV),41 Na-O-S (1,071.21,071.6 eV),42 and Na2SO4 (1,071.2 eV)42 were also observed at Na 1s. In the S 2p XPS spectra, various sulfur-oxy compounds (S-O bonds, oxidation state of S species) that may be assigned as metal sulfates (Li2SO4 and Na2SO4, ~169 eV43), Na-tetrathionate (Na2S2-S2O6, 168.8 eV),31 Na-


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thiosulfate (Na2S-SO3, 167.5 eV),31 metal dithionite (Li2S2O4 and Na2S2O4, 166.3-166.4 eV),31 and Na-sulfite (Na2SO3, 166.2 eV)31 were detected on the surface-modified Li electrode. The peak intensities of metal sulfides such as Li-S and Na-S (160.1 eV)44 increased with increasing etching time, similar to S 2p shown in Figure 4b. It was also confirmed that such metal sulfides came from the reaction of Li metal and NaAlCl4-2SO2 electrolyte38 (Supporting Information Figure S9 shows C 1s and Al 2p XPS spectra). Figure 4b shows that Li and Na sulfur-oxy compounds complicatedly coexisted in the surface film of the NaAlCl4-2SO2-modified Li metal electrode. In a previous study, we found that smooth and compact passivation film composed of NaCl with various sodium sulfur-oxy compounds were formed on the surface of Na metal electrode.16 From the above XPS results, we found that the surface-modified Li metal with the NaAlCl4-2SO2 inorganic electrolyte also possesses a similar surface structure composed of Na-containing compounds such as Na metal electrode in the NaAlCl4-2SO2 electrolyte (the possible chemical reaction equation between Li metal and electrolytes is displayed in the Supporting Information). From the results achieved so far, we speculate that the irregular and porous surface layer observed in the LiAlCl4-3SO2-immersed Li electrode easily cracks due to volume expansion and mechanical stress caused by repeated Li electrodeposition/dissolution. In addition, cracking at the defects of surface films causes active sites at the Li metal electrode, which induces filamentary Li growth. On the contrary, the relatively uniform and dense surface film of the NaAlCl 4-2SO2modified Li electrode, which consists of inorganic Li and Na sulfur-oxy compounds with fine NaCl crystals, could act as a favorable protective layer supplying sufficient mechanical rigidity and homogeneous current distribution to suppress the growth of Li filaments.



Figure 5a shows the cycle performance of Li-SO2 cells employing the unmodified Li metal or the surface-modified Li metal electrode. Both Li-SO2 cells presented similar capacity retention during cycling, but different reversible capacity every each cycle; the surface-modified Li-SO2 cell shows higher capacities than those of the unmodified cell by ~100 mA h g-1. In this regard, we examined the voltage profiles of each cell, as displayed in Figure 5b and c. The polarization of voltage of the surface-modified Li-SO2 cells are less than those of the unmodified Li-SO2 cells. This different overpotential may be from the different surface property of Li metal anode. To summarize, the surface-modified Li metal shows highly stable surface layer, and thus, the surfacemodified Li-SO2 full cell presents relatively lower polarization and higher reversible capacity during cycling.

4. CONCLUSION We demonstrated that surface modification of a Li metal electrode with NaAlCl4-2SO2 electrolyte produced a mechanically and chemically stable surface film. The formation of a uniform, dense, electronically insulating, and chemically stable Li- and Na-containing inorganic surface film on the Li metal results in dendrite-free Li deposition at currents up to 3 mA cm-2 with an areal capacity of 5 mA h cm-2. Stable long-term Li deposition/dissolution of symmetric cells over 1,000 h without short-circuiting was also demonstrated. Our findings provide a simple and efficient way to stabilize Li metal, basically changing the deposition behavior from dendritic to non-dendritic. Further optimization of the chemical and mechanical stabilities of Li surface film is possible by exploring pre-modification with other SO2-based inorganic electrolytes.


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Figure 1. SEM images of the Li deposit for the (a) unmodified Li metal electrode and (b) NaAlCl42SO2-modified Li metal electrode in a LiAlCl4-3SO2 inorganic electrolyte after three cycles under various areal capacity and current density conditions. (c) Cross-sectional SEM images of the Li deposit obtained under a high areal capacity (5 mA h cm-2) and current density (3 mA cm-2). The red box shows the unmodified Li metal electrode, and the blue box shows the NaAlCl4-2SO2modified Li metal electrode. Additionally, Supporting Information Figure S1 and S2 shows SEM images of the fresh Li metal electrode and the morphological behavior of Li electrodeposition/dissolution at a low areal capacity (0.5 mA cm-2) and current density (1.5 mA h cm-2), respectively.



Figure 2. Electrochmical performance of Li electrodeposition/dissolution. (a) The voltage profiles of Li/Li symmetric cells obtained during short-circuit tests. (b) A schematic illustraction of the short-circuit test showing the morphological behavior of the Li counter electrode employing (c) the unmodified Li metal or (d) the NaAlCl4-2SO2-modified Li metal electrode. (e) Charge/discharge cycle tests of Li/Li symmetric cells in a LiAlCl4-3SO2 inorganic electrolyte.


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Figure 3. The SEM-EDS analysis of Li metal electrodes after immersion in SO2-based inorganic electrolyte for 1 day. (a) SEM image and the corresponding (b) S (green dot), (c) O (yellow dot), and (d) Cl (cyan dot) elemental EDS mappings for the Li metal immersed in LiAlCl4-3SO2. (e) SEM image and the corresponding (f) S (green dot), (g) O (yellow dot), and (h) Cl (cyan dot) elemental EDS mappings for the Li metal immersed in NaAlCl4-2SO2 electrolyte.



Figure 4. XPS analysis of the surface layer of Li metal electrode after immersion in (a) LiAlCl43SO2 and (b) NaAlCl4-2SO2 electrolytes for 1 day.


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Figure 5. (a) Cycle performance of Li-SO2 cells employing the unmodified Li metal or the surfacemodified Li metal electrode. Galvanostatic voltage profiles of the unmodified Li-SO2 cell and the surface-modified Li-SO2 cell (corresponding to (a)) at different cycle number of (b) 50th, and (c) 100th. Note that the charge rate is 0.5 C and discharge rate is 2 C (1 C = 1500 mA g-1) with the voltage range from 2.9 V to 3.9 V (vs. Li/Li+).



ASSOCIATED CONTENT Supporting Information SEM observation of the Li deposit for the cycled Li metal electrode after immersion in LiAlCl43SO2 and NaAlCl4-2SO2 electrolytes; Cross-sectional SEM observation of the Li metal electrodes after immersion in electrolytes; SEM observation and XPS spectra of fresh Li metal surface; XPS and XRD analysis of Li metal surface after immersion in electrolytes; Chemical reaction schemes between Li metal and LiAlCl4-3SO2 or NaAlCl4-2SO2 electrolyte (PDF)

AUTHOR INFORMATION Corresponding Author * (H. K.) E-mail: [email protected] * (G. J.) E-mail: [email protected] * (Y.-J. K.) E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Samsung Research Funding Center for Future Technology (SRFCMA1501-07).


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Table of Contents Graphics



82x44mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 1. SEM images of the Li deposit for the (a) unmodified Li metal electrode and (b) NaAlCl4-2SO2modified Li metal electrode in a LiAlCl4-3SO2 inorganic electrolyte after three cycles under various areal capacity and current density conditions. (c) Cross-sectional SEM images of the Li deposit obtained under a high areal capacity (5 mA h cm-2) and current density (3 mA cm-2). The red box shows the unmodified Li metal electrode, and the blue box shows the NaAlCl4-2SO2-modified Li metal electrode. Additionally, Supporting Information Figure S1 and S2 shows SEM images of the fresh Li metal electrode and the morphological behavior of Li electrodeposition/dissolution at a low areal capacity (0.5 mA cm-2) and current density (1.5 mA h cm-2), respectively. 177x130mm (300 x 300 DPI)



Figure 2. Electrochmical performance of Li electrodeposition/dissolution. (a) The voltage profiles of Li/Li symmetric cells obtained during short-circuit tests. (b) A schematic illustraction of the short-circuit test showing the morphological behavior of the Li counter electrode employing (c) the unmodified Li metal or (d) the NaAlCl4-2SO2-modified Li metal electrode. (e) Charge/discharge cycle tests of Li/Li symmetric cells in a LiAlCl4-3SO2 inorganic electrolyte. 177x132mm (300 x 300 DPI)


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Figure 3. The SEM-EDS analysis of Li metal electrodes after immersion in SO2-based inorganic electrolyte for 1 day. (a) SEM image and the corresponding (b) S (green dot), (c) O (yellow dot), and (d) Cl (cyan dot) elemental EDS mappings for the Li metal immersed in LiAlCl4-3SO2. (e) SEM image and the corresponding (f) S (green dot), (g) O (yellow dot), and (h) Cl (cyan dot) elemental EDS mappings for the Li metal immersed in NaAlCl4-2SO2 electrolyte. 170x80mm (300 x 300 DPI)



Figure 4. XPS analysis of the surface layer of Li metal electrode after immersion in (a) LiAlCl4-3SO2 and (b) NaAlCl4-2SO2 electrolytes for 1 day. 170x111mm (300 x 300 DPI)


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Figure 5. (a) Cycle performance of Li-SO2 cells employing the unmodified Li metal or the surface-modified Li metal electrode. Galvanostatic voltage profiles of the unmodified Li-SO2 cell and the surface-modified Li-SO2 cell (corresponding to (a)) at different cycle number of (b) 50th, and (c) 100th. Note that the charge rate is 0.5 C and discharge rate is 2 C (1 C = 1500 mA g-1) with the voltage range from 2.9 V to 3.9 V (vs. Li/Li+). 170x138mm (300 x 300 DPI)


Dendrite-Free Li Metal Anode for Rechargeable Li-SO2 Batteries

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