Polydopamine-treated three-dimensional carbon fiber-coated separator for achieving high-performance lithium metal batteries

Journal of Power Sources 430 (2019) 130–136

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Polydopamine-treated three-dimensional carbon fiber-coated separator for achieving high-performance lithium metal batteries Jeonghun Oh a, 1, Hearin Jo a, 1, Hongkyung Lee b, Hee-Tak Kim c, ***, Yong Min Lee d, **, Myung-Hyun Ryou a, * a

Department of Chemical and Biological Engineering, Hanbat National University, 125 Dongseo-daero, Yuseong-gu, Daejeon, 34158, Republic of Korea Energy and Environment Directorate, Pacific Northwest National Lab, 902 Battelle Boulevard, Richland, WA, 99354, United States Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, South Korea d Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 333 Techno Jungang-Daero, Daegu, 42988, Republic of Korea b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Vapor-grown carbon fiber Li metal electrode Polydopamine Li dendrite Dead li

The development of safe and high-performance lithium (Li) metal anodes has been a challenging issue that has not been addressed for decades. In this study, we have developed a thermally stable polydopamine-treated threedimensional (3D) carbon fiber-coated separator (P3D-CFS) using an economical and environment-friendly pro­ cess. P3D-CFS has a conductive coating layer that is used as a 3D hosting structure, which does not cause morphological changes in the Li metal anode. As a result, the unit cells (LiMn2O4/Li metal) employing P3D-CFS improve the cycle performance and rate capability compared to commercial polyethylene (PE) separators. P3DCFS maintained 83.1% of the initial discharge capacity at the 400th cycle, whereas bare PE maintains only 74.3% of the initial discharge capacity after the 250th cycle (C/2 ¼ 0.5 mA cm 2). P3D-CFS maintains 42.8% of the initial discharge capacity at a 7C rate (7 mA cm 2), whereas only 0.19% is maintained by bare PE under the same condition. Owing to the thermally stable properties of P3D-CFS, the open-circuit voltage of the unit cells (LiMn2O4/graphite) that employed P3D-CFS is maintained for over 60 min at 140 � C, whereas the unit cells that employed bare PE show a sudden voltage drop after only 3 min.

1. Introduction Although commercial lithium (Li)-ion batteries (LIBs) have been considered as a promising power source for consumer portable elec­ tronic devices and power tools since their development in 1991, they are approaching their limit in terms of their energy densities. Therefore, developing unprecedented energy-dense and highly safe battery systems are key to meeting the growth of large-scale devices such as electric vehicles and energy storage systems. Over four decades, Li metal has been one of the most promising anode material candidates owing to its highest theoretical specific capacity (3860 mAh g 1 or 2061 mAh cm 3) and lowest electrochemical potential ( 3.04 V versus the standard hydrogen electrode). However its use has been limited to practical

secondary batteries because of poor electrochemical performance and safety issues [1,2]. Over time, Li metal inevitably suffers from morphological changes, resulting in uncontrolled Li such as Li dendrites and dead Li. They cause safety problems due to an internal short circuit (i.e., Li dendrites) [3,4] and the formation of metallic nanopowder (i.e., dead Li) with inherent explosive or flammable characteristics [5]. In addition, they interfere with the electrochemical performance because of liquid-electrolyte consumption owing to the formation of solid-electrolyte interphase (SEI) on a newly exposed Li metal surface [6]. For the successful penetration of Li metal anodes to the secondary battery market, two main problems associated with Li metal must be solved: (1) stable electrochemical performance and (2) improved safety

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail address: [email protected] (M.-H. Ryou). 1 Both authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2019.05.003 Received 3 February 2019; Received in revised form 29 April 2019; Accepted 2 May 2019 Available online 16 May 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.

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dopamine solution (2 mg mL 1) was prepared using a mixture of tris buffer solution (pH 8.5), and a PD coating layer was applied by mixing the pristine VGCF powder into a dopamine solution. The mixture was shaken by hand for 1 min until a uniform suspension was formed. Then, the mixture was filtered using a vacuum suction machine and dried at room temperature for 24 h. To investigate the compositions of the modified surfaces, PD-treated VGCF was analyzed using XPS (MultiLab 2000, Ssk, Japan). The coating solution was prepared by mixing PD-treated VGCF, CMC, and DLSS at a constant amount in DI water (PD-coated VGCF/ CMC/DLSS/DI ¼ 0.3/0.05/0.04/4.45 by weight) through a vortex for 10 min [35]. The PE separator was coated by a doctor blade, followed by drying in a fume hood for 12 h at room temperature (25 � C). The thickness of the coating layer was 15 μm, and the basic physical prop­ erties of the separator, such as the Gurley number, are listed in Table S1 (Supporting Information).

of the battery system. Many efforts have been devoted to various methods based on electrolyte engineering (functional additives [7–9] and electrolyte composition modification [10–12]), interface engineer­ ing (protective layer [13–15] and artificial SEI [16–20]), stable host materials (containing pre-stored Li infused into nanoscale gaps in the form of molten Li [21–24]), and guided Li plating [surface patterning [13,14,25], three-dimensional (3D) current collector [26,27], and seeded growth to control the Li deposition [28,29]]. However, their main focus was to stabilize the Li metal anode surface for achieving extended cycle life; this only addressed the first of the Li metal problems listed above. Because ceramic-coated separators are considered to be the main elements to ensure safety of commercial LIBs [30,31], developing functional separators that are capable of improving the safety and per­ formance of the Li metal anode is necessary. Furthermore, from eco­ nomic and process points of view, the separator-development approach is more advantageous than direct Li metal surface modification because Li metal is sensitive to moisture, air, and solvents. For instance, Li metal must be treated in a sophisticated environment such as a glove box or drying room; however, this hampers process efficiency and increases process costs. In the current work, we addressed two requirements (i.e., improved cycle life and safety) of Li metal anodes by simply replacing the poly­ olefin separators with polydopamine (PD)-treated 3D carbon fibercoated separator (P3D-CFS). Another noticeable feature of P3D-CFS is that the process involved is economical and environment-friendly. The 3D electrical-conductive host structure of P3D-CFS inhibits Li dendrites and dead Li formation by forming a uniform current distri­ bution on the Li metal surface and applying an electric path to the iso­ lated dead Li [14,15,32,33]. Because of the existence of catechol and hydroxyl moieties of PD, the PD coating layer not only yields synergistic Li metal stabilization effects [33] but also enables an aquatic process to produce P3D-CFS. In contrast to the conventional surface-coating pro­ cess, which requires expensive and toxic organic solvents because of the hydrophobic properties of polyolefin microporous separators, PD allows for an aquatic process for P3D-CFS production. Furthermore, P3D-CFS shows highly improved thermal stability at high-temperature exposure (140 � C for 30 min) without shrinkage and improved wetting ability to polar liquid electrolytes. We thoroughly investigated the effects of P3D-CFS on the electrochemical properties of 2032-type coin half-cells (LiMn2O4/Li metal).

2.3. Electrode preparation The cathodes were prepared by casting NMP-based electrode slurry (LiMn2O4/Super-P Li®/PVdF ¼ 90/5/5 by weight) onto an aluminum foil (15 μm, Sam-A, Republic of Korea) using a doctor-blade technique. The coated slurry was dried in air at 130 � C for 1 h and roll-pressed using a gap-control-type roll-pressing machine (CLP-2025, CIS, Republic of Korea). The loading and thickness of the cathodes were controlled at ~10 mg cm 2 and ~55 μm, respectively. 2.4. Postmortem analysis of the P3D-CFS morphology Digital microscopy (VHX-900F, KEYENCE, Japan) and field emission SEM (FE-SEM, SU-5000, Hitachi, Japan) were used to investigate the morphological changes in P3D-CFS. 2.5. Electrochemical measurements To evaluate the electrochemical performance of the separator, the 2032 coin-type half-cells (LiMn2O4/Li metal) were assembled in a glove box filled with argon gas and then aged for 12 h before precycling. By using a charge/discharge cycler tester (PNE Solution, Republic of Korea), the unit cells were cycled between 3.0 and 4.3 V in CC mode for both charge and discharge at C/10 rate (0.1 mA cm 2) and stabilized for three cycles between 3.0 and 4.3 V in CC/constant voltage (CC/CV) and CC modes for charging and discharging, respectively, at a C/5 rate (0.2 mA cm 2) at 25 � C. To evaluate the rate capabilities, the discharge current densities of the unit cells were varied from C/2 to 15 C (C/2, 1, 2, 3, 5, 7, 10, 15, and C/2), and the charging current density was fixed at C/ 2. Furthermore, to evaluate the cycling performance, the cells were subjected to 400 and 200 cycles at a current density of C/2 (0.5 mA cm 2) and 2 C (2 mA cm 2) at 25 � C, respectively.

2. Experimental 2.1. Materials Vapor-grown carbon fiber (VGCF®-H, Showa Denko K.K., Japan), sodium carboxymethyl cellulose (CMC, WS-C, Dai-ichi Kogyo Seiyaku Co., Ltd.), dopamine hydrochloride (98%, Sigma Aldrich Co., Ltd.), Nmethyl-2-pyrrolidone (NMP, Sigma Aldrich Co., Ltd., Anhydrous, 99.5%), disodium laureth sulfosuccinate solution (DLSS, 28 wt% ASCO® DLSS, AK Chemtech Co., LTD.), and deionized (DI) water from a Milli-Q system (Millipore Co., LTD. >18.2 MΩ cm 1) were used as received without further purification. Li manganese oxide (LiMn2O4, Ilijin Ma­ terials Co., Republic of Korea), a conductive additive (Super-P Li®), Polyvinylidene fluoride (PVdF, KF-1300, Kureha Battery Materials Co., Japan), and Li metal foil (thickness ¼ 200 μm, Honjo Metal Co., Japan) were also used. A mixture of 1.15 M Li hexafluorophosphate (LiPF6) in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3/7 by volume) was purchased from Enchem (Republic of Korea) and used as electro­ lytes without further purification. Microporous PE membrane (PE, V20CFD, thickness ¼ 20 μm, Toray Battery Separator Film Co., Ltd., Japan) was used as separators.

2.6. Battery-safety measurements The battery safety was evaluated by monitoring the OCV changes in the fully charged 2032 coin-type full cells (LiMn2O4/graphite) during the heat exposure (140 � C). The unit cells (LiMn2O4/graphite) were prepared by following the previous preparation procedure mentioned earlier. After the stabilization cycles, the unit cells were charged to 4.3 V at C/2 (CC/CV mode for charging and CC mode for discharging) at 25 � C and then exposed to 140 � C to monitor the OCV changes as a function of time. 3. Results and discussion

2.2. Preparation of P3D-CFS

A PD surface-coated vapor-grown carbon fiber (VGCF) (PD-VGCF) was prepared following the method used in our previous reports [33,34, 36]. After PD coating, the surface composition of PD-VGCF was analyzed

VGCF was surface-coated via a simple PD-coating method [34]. A 131

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using X-ray photoelectron spectroscopy (XPS) to confirm the existence of PD. Fig. 1a and b shows that PD-VGCF exhibited new oxygen (533.0 eV) and nitrogen (399.5 eV) peaks attributed to the oxygen and nitrogen moieties of PD (Fig. S1, Supporting Information) [34,37]. To investigate the effect of the PD surface coating on the dis­ persibility of VGCF in water, the same amounts (20 mg) of PD-VGCF and bare VGCF were immersed in vials filled with the same amount of water (10 mL). The PD-VGCF- and bare VGCF-containing vials were shaken for 2 min using a vortex mixer and stored for 1 min. Fig. 1c shows that the bare VGCF floated on the water surface, whereas the PD-VGCF was well dispersed in water. We speculate that the oxygen and nitrogen moieties of the PD converted the hydrophobic surface of VGCF to a hydrophilic surface [33,34,37]. P3D-CFS was prepared by a simple doctor-blade-coating method using an aqueous system. PD-VGCF was coated on polyolefin poly­ ethylene (PE) separators for preparing P3D-CFS, and bare VGCF was used as a control. Because bare VGCF exhibited poor dispersibility in a water medium (Fig. 1c), we selected a surfactant, i.e., disodium laureth sulfosuccinate (DLSS), to improve the dispersibility of carbon-based VGCF and the adhesion between the VGCF-based coating layer and polyolefin PE separator [35]. Although the presence of DLSS enabled doctor-blade coating of the PE separator, the final quality of the VGCF-coated PE separator remained non-uniform because of poor dis­ persibility (Fig. 2a). In contrast, P3D-CFS showed uniform coating re­ sults (Fig. 2b). To more specifically demonstrate the coating quality of the two systems, we observed the surface of the coated PE separators using a 3D digital microscope. The VGCF-coated PE separator showed a rough terrain (Fig. 2c and d), whereas P3D-CFS exhibited a smooth surface topography (Fig. 2d and f). The physical properties of P3D-CFS, such as the coating thickness, Gurley number, uptake amount, and ionic conductivity, are listed in Table S1 (Supporting Information). Because of the existence of a phys­ ical barrier, i.e., a coating layer, the Gurley number somewhat increased from 194.8 s per 100 mL (bare PE) to 212.1 s per 100 mL (P3D-CFS). Nevertheless, P3D-CFS showed higher ionic conductivity (1.075 mS cm 1) than bare PE (0.75 mS cm 1) because of the improved wetting ability resulting in the high uptake amount of liquid electro­ lytes. Furthermore, the electrical conductivity of P3D-CFS was measured using a four-point probe technique. P3D-CFS showed a good electrical conductivity of 13.0 � 0.463 S cm 1. The same amount of liquid electrolyte was dropped on the surface of both bare PE and P3D-CFS, and we monitored the surface using a digital camera (Fig. 3). P3D-CFS quickly absorbed the liquid droplets, whereas bare PE retained the shape of the liquid droplets. P3D-CFS showed a

much smaller contact angle (0� ) than the bare PE separator (22� ). Commercial polyolefin separators shrink during high-temperature exposure as they soften or melt because of the internal stress accumu­ lated during the stretching process involved during production [30,33, 35]. To investigate the effect of the coating layer that contains VGCF on the thermal shrinkage characteristics of PE separators, bare PE and P3D-CFS were cut into square samples (3 cm � 3 cm), and the dimen­ sional changes were monitored at high temperature (140 � C for 30 min). P3D-CFS maintained its original size, whereas the bare PE separator contracted to 38% of its original size (Fig. 4a, estimated using the pixel counting method. The effect of P3D-CFS on the safety of LIBs (LiMn2O4/separator/ graphite) was evaluated by monitoring the open-circuit voltage (OCV) changes during high-temperature exposure. Li metal-based half-cells cause serious safety problems during testing; thus, we used LIBs. For instance, some Li metal half-cells caused a fire during OCV monitoring due to high energy density, threatening the safety of the researchers and making equipment management difficult. Fully charged LIBs (4.3 V) were exposed to high temperature (140 � C), and the OCV was monitored as a function of time. The voltage of the LIB unit cell composed of bare PE suddenly dropped to 0 V after 3 min, whereas the LIB unit cell composed of P3D-CFS maintained the initial voltage for up to 1 h without change (Fig. 4b). Considering the thermal-shrinkage charac­ teristics of bare PE (Fig. 4a), we can infer that the sudden voltage drop of the LIB unit cell composed of bare PE was due to a direct short circuit ascribed to the separator shrinkage. To investigate the effects of P3D-CFS on the electrochemical prop­ erties such as cycle performance and rate capability of Li metal anodes, 2032 coin-type half-cells (LiMn2O4/separator/Li metal) were fabricated and evaluated. Unit cells composed of bare PE were used as a control. For simplicity, the unit cells composed of bare PE and P3D-CFS are denoted as bare PE and P3D-CFS, respectively. Bare PE and P3D-CFS exhibited similar coulombic efficiencies (CE), but the potential profile of each system was not superimposed because of the voltage drop during precycling (Fig. 5a), bare PE: charge capac­ ity ¼ 107.1 mAh g 1, discharge capacity ¼ 106.2 mAh g 1, and CE ¼ 99.1%; P3D-CFS: charge capacity ¼ 108.0 mAh g 1, discharge ca­ pacity ¼ 107.0 mAh g 1, and CE ¼ 99.1%]. The overpotential of bare PE was larger than that of P3D-CFS in both the charging and discharging processes. This larger overpotential of bare PE was reflected in the larger overall impedance after precycling (Fig. S2, Supporting Information) and the significantly poor rate capability of bare PE (Fig. 5b) compared with P3D-CFS. P3D-CFS maintained 42.8% of the initial discharge ca­ pacity at a 7C rate (at the 26th cycle, current density ¼ 7 mA cm 2), whereas PE maintained only 0.19% of the initial discharge capacity under the same condition. P3D-CFS showed a significantly improved cycle performance over bare PE (Fig. 5c and d). Under a mild operating condition (discharging condition ¼ C/2 ¼ 0.5 mA cm 2 and charging condition ¼ C/ 2 ¼ 0.5 mA cm 2), P3D-CFS maintained 83.1% of the initial discharge capacity at the 400th cycle (initial discharge capacity ¼ 108.33 mAh g 1 and discharge capacity at the 400th cycle ¼ 90.00 mAh g 1), whereas bare PE showed a sudden loss of cycle life after the 100th cycle, resulting in 74.3% (81.24 mAh g 1) retention of the initial discharge capacity after the 250th cycle. Under a harsh operating condition (discharging condi­ tion ¼ 2C ¼ 2 mA cm 2 and charging condition ¼ 2C ¼ 2 mA cm 2), the cycle performance gap between bare PE and P3D-CFS increased further. P3D-CFS maintained 75.7% of the initial discharge capacity at the 200th cycle (initial discharge capacity ¼ 100.92 mAh g 1 and discharge ca­ pacity at the 200th cycle ¼ 76.44 mAh g 1), whereas bare PE showed a sudden loss in the cycle life from the start of the operation, resulting in 20.0% (17.90 mAh g 1) retention of the initial discharge capacity after the 200th cycle. Considering this improved cycle performance, P3D-CFS exhibited a very stable CE compared with bare PE (Fig. S3, Supporting Information). To describe the origin of the performance improvement of P3D-CFS,

Fig. 1. XPS spectra of VGCF and PD-VGCF corresponding to (a) O 1 s and (b) N 1 s. (c) Digital camera images of the vials containing an aqueous solution of VGCF and PD-VGCF. 132

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Fig. 2. Digital camera images, 3D topographic images, and depth profiles corresponding to the red line shown in the 3D topographic images (in that order) of the VGCF-coated PE separator and P3D-CFS. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4. (a) Digital camera images of bare PE and P3D-CFS after hightemperature exposure (140 � C, 30 min, and sample size ¼ 3 cm � 3 cm). (b) OCV changes in the fully charged unit cells (LiMn2O4/separator/graphite) at high-temperature exposure (140 � C).

we can deduce that the P3D-CFS stabilizes the Li metal anodes, thereby leading to improved performance. Postmortem characterization was conducted to clarify the effect of P3D-CFS on the electrochemical performance improvement of the Li metal anodes. After precycling, the unit cells (LiMn2O4/separator/Li metal) composed of bare PE and P3D-CFS were disassembled. The separators were gently removed from the Li metal anode, and the Li metal and separator sides were both observed. In the case of the unit cells composed of bare PE, the area sandwiched by the LiMn2O4 cathode has a dark gray and black color with a rough texture (Fig. 6a), whereas the surface of the bare PE-faced Li metal was clean (Fig. 6b). This obser­ vation was not surprising because the morphological changes in the Li metal due to the formation of uncontrolled Li metal such as Li dendrite and mossy Li were reported in previous studies [1,6,9,14,25,33,38]. In

Fig. 3. Digital camera images and contact angle images of the bare PE and P3DCFS after dropping droplets of liquid electrolyte.

Li/Li symmetric cells were fabricated, and galvanostatic polarization experiments (Li plating/stripping) were conducted by applying a con­ stant current (CC) (1.0 mA cm 2). Thus, the effect of the Li metal anodes can be independently evaluated while excluding the effect of LiMn2O4 on cell performance. The Li/Li symmetric cells that employed bare PE separators showed higher overpotential compared with those that employed P3D-CFS (Fig. S4, Supporting Information). Also, bare PE separators showed more Li dendrite compared with those employed P3D-CFS at selective galvanostatic cycling (Fig. S5, Supporting Infor­ mation). Considering the improved cycle performance, rate capability, and galvanostatic polarization results of the P3D–CFS–containing cells, 133

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Fig. 5. (a) Voltage profiles during precycling. (b) Rate capabilities and cycle performance under a (c) mild operating condition (discharging condition ¼ C/ 2 ¼ 0.5 mA cm 2 and charging condition ¼ C/2 ¼ 0.5 mA cm 2) and (d) harsh operating condition (discharging condition ¼ 2C ¼ 2 mA cm 2 and charging condi­ tion ¼ 2C ¼ 2 mA cm 2) of the unit cell (LiMn2O4/separator/Li metal).

contrast, the unit cells composed of P3D-CFS showed a silver-colored Li metal surface with a smooth texture (Fig. 6c). Interestingly, the area where the P3D-CFS faced the Li metal transformed into a silver color. To clarify the Li deposition process and the origin of the silver ma­ terial in P3D-CFS, the surface and cross-section of the P3D-CFS were observed using scanning electron microscopy (SEM). During the Li plating, no positional preferences of Li deposition for P3D-CFS were present, which was well consistent with the uniform distribution of the Li plating reported in previous conductive 3D hosting material studies [26,27,32,39]. Powdered Li metal was uniformly formed throughout the interior of P3D-CFS as well as on the surface facing the Li metal electrode (Fig. 7). According to the “lightning-rod theory [15,40],ˮ the regions of high curvature, i.e., the end of VGCF in our experiment, acted as charge centers with high charge densities and attracted Li ions during the plating process to form uniform Li plating throughout the P3D-CFS. From the results, we can infer that the silver material observed on the surface of the P3D-CFS after precycling is a Li metal. In conclusion, the Li metal formed dendrites and dead Li during repeated plating/stripping, which in turn inhibited the cycle perfor­ mance of the Li metal (Fig. 8a – c). In contrast, Li ions were plated inside the P3D-CFS, rather than on the surface of the Li metal, and the surface of the Li metal remained flat and shiny during prolonged cycles (Fig. 8d and e). P3D-CFS prevented generation of dead Li because it can supply electrons to isolated Li through cobwebbed VGCF. As a result, plated Li in the P3D-CFS, which should have become dead Li, was used as active Li metal source during further plating/stripping (Fig. 8f).

Fig. 6. Digital camera images of the disassembled unit cells (LiMn2O4/sepa­ rator/Li metal) after precycling corresponding to that shown in Fig. 5(a). (a) and (c) Li metal side and (b) and (d) separator side of the bare PE and P3D-CFS, respectively. 134

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Fig. 7. SEM images of the surface and cross section of P3D-CFS (a) before Li plating (pristine P3D-CFS) and after (b) 10 min and (c) 30 min (þ1.0 mA cm Li plating.

2

)

Fig. 8. Schematic figures showing the Li metal anodes of the unit cells composed of (a–c) bare PE and (d–f) P3D-CFS during Li plating and stripping.

4. Conclusion

Acknowledgment

In this study, we have developed thermally stable and electrically conductive P3D-CFS for Li metal anodes. Hydrophilic PD surface treat­ ment enabled an economical and environment-friendly aqueous process for P3D-CFS production. In addition, the improved wetting ability of P3D-CFS provided improved ionic conductivity compared with bare PE; thus, the unit cell composed of P3D-CFS showed improved rate capa­ bilities. P3D-CFS not only improved the cycle performance of Li metal by controlling uncontrolled Li during plating but also improved the safety of Li metal without changing the separator dimensions at hightemperature exposure. P3D-CFS possesses an electrically conductive VGCF coating layer, which is preferred over the Li metal surfaces when Li ions are plated. As a result, the surface Li metal remained clean without uncontrolled Li. In the unit cells composed of P3D-CFS, the electrically isolated Li formed during plating was no longer dead Li. Instead, it was regenerated as an active Li metal source during repeated cycles. Considering the thin coating thickness and cycle performance improvement of the P3D-CFS compared with those of the previous studies (Table S2, Supporting Information), the P3D-CFS can be a very competitive material.

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No.2018M3A7B4071066). This work was also supported by the Industrial Technology Innovation Project (No. 20001370) of Korea Evaluation Institute of Industrial Technology. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.05.003. References [1] K. Xu, Nonaqueous liquid electrolytes for lithium-based rechargeable batteries, Chem. Rev. 104 (10) (2004) 4303–4418. https://pubs.acs.org/doi/abs/10.1021/ cr030203g. [2] H. Zhang, H. Zhao, M.A. Khan, W. Zou, J. Xu, L. Zhang, J. Zhang, Recent progress in advanced electrode materials, separators and electrolytes for lithium batteries, J. Mater. Chem. 6 (42) (2018) 20564–20620. http://lps3.pubs.rsc.org.libra.kaist. ac.kr/en/content/articlelanding/2018/ta/c8ta05336g/unauth#!divAbstract. [3] K. Liu, P. Bai, M.Z. Bazant, C.-A. Wang, J. Li, A soft non-porous separator and its effectiveness in stabilizing Li metal anodes cycling at 10 mA cm 2 observed in situ in a capillary cell, J. Mater. Chem. 5 (9) (2017) 4300–4307. https://pubs.rsc. org/en/content/articlelanding/2017/ta/c7ta00069c/unauth#!divAbstract.

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Journal of Power Sources 430 (2019) 130–136 [23] D. Lin, Y. Liu, Z. Liang, H.-W. Lee, J. Sun, H. Wang, K. Yan, J. Xie, Y. Cui, Layered reduced graphene oxide with nanoscale interlayer gaps as a stable host for lithium metal anodes, Nat. Nanotechnol. 11 (7) (2016) 626. https://www.nature.com/art icles/nnano.2016.32. [24] Z. Liang, D. Lin, J. Zhao, Z. Lu, Y. Liu, C. Liu, Y. Lu, H. Wang, K. Yan, X. Tao, Composite lithium metal anode by melt infusion of lithium into a 3D conducting scaffold with lithiophilic coating, P. Natl. Acad. Sci. 113 (11) (2016) 2862–2867. https://www.pnas.org/content/113/11/2862/. [25] M.H. Ryou, Y.M. Lee, Y. Lee, M. Winter, P. Bieker, Mechanical surface modification of lithium metal: towards improved Li metal anode performance by directed Li plating, Adv. Funct. Mater. 25 (6) (2015) 834–841. https://onlinelibrary.wiley.co m/doi/full/10.1002/adfm.201402953. [26] C.-P. Yang, Y.-X. Yin, S.-F. Zhang, N.-W. Li, Y.-G. Guo, Accommodating lithium into 3D current collectors with a submicron skeleton towards long-life lithium metal anodes, Nat. Commun. 6 (2015) 8058. https://www.nature.com/articles/n comms9058. [27] S. Matsuda, Y. Kubo, K. Uosaki, S. Nakanishi, Lithium-metal deposition/dissolution within internal space of CNT 3D matrix results in prolonged cycle of lithium-metal negative electrode, Carbon 119 (2017) 119–123. https://www.sciencedirect.com/ science/article/pii/S0008622317303937. [28] K. Yan, Z. Lu, H.-W. Lee, F. Xiong, P.-C. Hsu, Y. Li, J. Zhao, S. Chu, Y. Cui, Selective deposition and stable encapsulation of lithium through heterogeneous seeded growth, Nat. Energy 1 (3) (2016) 16010. https://www.nature.com/articles/nene rgy201610. [29] T.-T. Zuo, Y.-X. Yin, S.-H. Wang, P.-F. Wang, X. Yang, J. Liu, C.-P. Yang, Y.-G. Guo, Trapping lithium into hollow silica microspheres with a carbon nanotube core for dendrite-free lithium metal anodes, Nano Lett. 18 (1) (2017) 297–301. https ://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b04136. [30] S.S. Zhang, A review on the separators of liquid electrolyte Li-ion batteries, J. Power Sources 164 (1) (2007) 351–364. https://www.sciencedirect.com/scienc e/article/pii/S0378775306022452. [31] H. Lee, M. Yanilmaz, O. Toprakci, K. Fu, X. Zhang, A review of recent developments in membrane separators for rechargeable lithium-ion batteries, Energy Environ. Sci. 7 (12) (2014) 3857–3886. https://pubs.rsc. org/en/content/articlelanding/2014/ee/c4ee01432d/unauth#!divAbstract. [32] H.-K. Kang, S.-G. Woo, J.-H. Kim, S.-R. Lee, Y.-J. Kim, Conductive porous carbon film as a lithium metal storage medium, Electrochim. Acta 176 (2015) 172–178. https://www.sciencedirect.com/science/article/pii/S0013468615300566. [33] M.H. Ryou, D.J. Lee, J.N. Lee, Y.M. Lee, J.K. Park, J.W. Choi, Excellent cycle life of lithium-metal anodes in lithium-ion batteries with mussel-inspired polydopaminecoated separators, Adv. Energy Mater. 2 (6) (2012) 645–650. https://onlinelibrary. wiley.com/doi/full/10.1002/aenm.201100687. [34] M.H. Ryou, Y.M. Lee, J.K. Park, J.W. Choi, Mussel-inspired polydopamine-treated polyethylene separators for high-power Li-ion batteries, Adv. Mater. 23 (27) (2011) 3066–3070. https://onlinelibrary.wiley.com/doi/full/10.1002/adma. 201100303. [35] H. Jeon, D. Yeon, T. Lee, J. Park, M.-H. Ryou, Y.M. Lee, A water-based Al2O3 ceramic coating for polyethylene-based microporous separators for lithium-ion batteries, J. Power Sources 315 (2016) 161–168. https://www.sciencedirect.com/ science/article/pii/S0378775316302373. [36] D. Song, D. Jung, I. Cho, M.H. Ryou, Y.M. Lee, Mussel-Inspired polydopaminefunctionalized super-P as a conductive additive for high-performance silicon anodes, Adv. Mater. Interf. 3 (17) (2016) 1600270. https://onlinelibrary.wiley. com/doi/abs/10.1002/admi.201600270. [37] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Mussel-inspired surface chemistry for multifunctional coatings, Science 318 (5849) (2007) 426–430. http://science.sciencemag.org/content/318/5849/426.full. [38] D. Lin, Y. Liu, Y. Cui, Reviving the lithium metal anode for high-energy batteries, Nat. Nanotechnol. 12 (3) (2017) 194–206. https://www.nature.com/articles/nnan o.2017.16. [39] Y. Yang, J. Xiong, J. Zeng, J. Huang, J. Zhao, VGCF 3D conducting host coating on glass fiber filters for lithium metal anodes, Chem. Commun. 54 (10) (2018) 1178–1181. https://pubs.rsc.org/ru/content/articlelandi ng/2018/cc/c7cc07828e/unauth#!divAbstract. [40] S.H. Wang, Y.X. Yin, T.T. Zuo, W. Dong, J.Y. Li, J.L. Shi, C.H. Zhang, N.W. Li, C. J. Li, Y.G. Guo, Stable Li metal anodes via regulating lithium plating/stripping in vertically aligned microchannels, Adv. Mater. 29 (40) (2017) 1703729. https://o nlinelibrary.wiley.com/doi/full/10.1002/adma.201703729.

[4] J. Song, Y. Wang, C.C. Wan, Review of gel-type polymer electrolytes for lithium-ion batteries, J. Power Sources 77 (2) (1999) 183–197. https://www.sciencedirect.co m/science/article/pii/S0378775398001931. [5] G. Derrien, J. Hassoun, S. Panero, B. Scrosati, Nanostructured Sn–C composite as an advanced anode material in high-performance Lithium-ion batteries, Adv. Mater. 19 (17) (2007) 2336–2340. https://onlinelibrary.wiley.com/doi/abs/10. 1002/adma.200700748. [6] D. Aurbach, E. Zinigrad, Y. Cohen, H. Teller, A short review of failure mechanisms of lithium metal and lithiated graphite anodes in liquid electrolyte solutions, Solid State Ionics 148 (3–4) (2002) 405–416. https://www.sciencedirect.com/science/ article/pii/S0167273802000802. [7] F. Ding, W. Xu, G.L. Graff, J. Zhang, M.L. Sushko, X. Chen, Y. Shao, M. H. Engelhard, Z. Nie, J. Xiao, Dendrite-free lithium deposition via self-healing electrostatic shield mechanism, J. Am. Chem. Soc. 135 (11) (2013) 4450–4456. https://pubs.acs.org/doi/abs/10.1021/ja312241y. [8] W. Li, H. Yao, K. Yan, G. Zheng, Z. Liang, Y.-M. Chiang, Y. Cui, The synergetic effect of lithium polysulfide and lithium nitrate to prevent lithium dendrite growth, Nat. Commun. 6 (2015) 7436. https://www.nature.com/articles/ncomms8436. [9] X.Q. Zhang, X.B. Cheng, X. Chen, C. Yan, Q. Zhang, Fluoroethylene carbonate additives to render uniform Li deposits in lithium metal batteries, Adv. Funct. Mater. 27 (10) (2017) 1605989. https://onlinelibrary.wiley.com/doi/full/10. 1002/adfm.201605989. [10] J. Qian, W.A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin, J.G. Zhang, High rate and stable cycling of lithium metal anode, Nat. Commun. 6 (2015) 6362. https://www.nature.com/articles/ncomms7362. [11] J. Zheng, M.H. Engelhard, D. Mei, S. Jiao, B.J. Polzin, J.-G. Zhang, W. Xu, Electrolyte additive enabled fast charging and stable cycling lithium metal batteries, Nat. Energy 2 (3) (2017) 17012. https://www.nature.com/articles/nene rgy201712. [12] X. Fan, L. Chen, O. Borodin, X. Ji, J. Chen, S. Hou, T. Deng, J. Zheng, C. Yang, S.C. Liou, Non-flammable electrolyte enables Li-metal batteries with aggressive cathode chemistries, Nat. Nanotechnol. 13 (2018) 715–722. https://www.nature. com/articles/s41565-018-0183-2. [13] S. Kim, J. Park, A. Friesen, H. Lee, Y.M. Lee, M.-H. Ryou, Composite protection layers for dendrite-suppressing non-granular micro-patterned lithium metal anodes, Electrochim. Acta 282 (2018) 343–350. https://www.sciencedirect.com/ science/article/pii/S0013468618311356. [14] J. Park, J. Jeong, Y. Lee, M. Oh, M.H. Ryou, Y.M. Lee, Micro-Patterned lithium metal anodes with suppressed dendrite formation for post lithium-ion batteries, Adv. Mater. Interf. 3 (11) (2016) 1600140. https://onlinelibrary.wiley.com/d oi/abs/10.1002/admi.201600140. [15] H. Jo, D. Song, Y.-C. Jeong, Y.M. Lee, M.-H. Ryou, Study on dead-Li suppression mechanism of Li-hosting vapor-grown-carbon-nanofiber-based protective layer for Li metal anodes, J. Power Sources 409 (2019) 132–138. https://www.sciencedirec t.com/science/article/pii/S0378775318310309. [16] K. Yan, H.-W. Lee, T. Gao, G. Zheng, H. Yao, H. Wang, Z. Lu, Y. Zhou, Z. Liang, Z. Liu, Ultrathin two-dimensional atomic crystals as stable interfacial layer for improvement of lithium metal anode, Nano Lett. 14 (10) (2014) 6016–6022. https://pubs.acs.org/doi/abs/10.1021/nl503125u. [17] N.W. Li, Y.X. Yin, C.P. Yang, Y.G. Guo, An artificial solid electrolyte interphase layer for stable lithium metal anodes, Adv. Mater. 28 (9) (2016) 1853–1858. http s://onlinelibrary.wiley.com/doi/full/10.1002/adma.201504526. [18] S. Choudhury, C.T.-C. Wan, W.I. Al Sadat, Z. Tu, S. Lau, M.J. Zachman, L. F. Kourkoutis, L.A. Archer, Designer interphases for the lithium-oxygen electrochemical cell, Sci. Adv. 3 (4) (2017) e1602809. http://advances.scien cemag.org/content/3/4/e1602809.full. [19] R. Xu, X.Q. Zhang, X.B. Cheng, H.J. Peng, C.Z. Zhao, C. Yan, J.Q. Huang, Artificial soft–rigid protective layer for dendrite-free lithium metal anode, Adv. Funct. Mater. 28 (8) (2018) 1705838. https://onlinelibrary.wiley.com/doi/full/10. 1002/adfm.201705838. [20] X. Liang, Q. Pang, I.R. Kochetkov, M.S. Sempere, H. Huang, X. Sun, L.F. Nazar, A facile surface chemistry route to a stabilized lithium metal anode, Nat. Energy 2 (9) (2017) 17119. https://www.nature.com/articles/nenergy2017119. [21] X.-B. Cheng, H.-J. Peng, J.-Q. Huang, R. Zhang, C.-Z. Zhao, Q. Zhang, Dual-phase lithium metal anode containing a polysulfide-induced solid electrolyte interphase and nanostructured graphene framework for lithium–sulfur batteries, ACS Nano 9 (6) (2015) 6373–6382. https://pubs.acs.org/doi/abs/10.1021/acsnano.5b01990. [22] Y. Liu, D. Lin, Z. Liang, J. Zhao, K. Yan, Y. Cui, Lithium-coated polymeric matrix as a minimum volume-change and dendrite-free lithium metal anode, Nat. Commun. 7 (2016) 10992. https://www.nature.com/articles/ncomms10992.

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