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Improved lithium deposition on silver plated carbon fiber paper
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Nano Energy journal homepage: http://www.elsevier.com/locate/nanoen
Communication
Improved lithium deposition on silver plated carbon fiber paper Xiaoyun Li a, b, Gaojing Yang a, c, Simeng Zhang a, b, Zhaoxiang Wang a, b, c, *, Liquan Chen a a
Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China b College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing, 100190, China c School of Physical Sciences, University of Chinese Academy of Sciences, Beijing, 100190, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Lithium metal anode Carbon fiber Silver particles Lithium deposition Nucleation overpotential
Lithium metal is a promising anode material for its low redox potential and high theoretical specific capacity. However, issues such as dendritic growth, unstable interface and low coulombic efficiency hinder its practical applications. Construction of a porous current collector is supposed to be an effective way to address these issues and improve the cycling performances of the lithium metal battery. In this paper, commercial carbon fiber paper is used as a porous current collector for the lithium deposition, on which nanosized silver particles are plated beforehand by the silver mirror reaction. The lithiophilic silver particles alloy with the lithium and guide the subsequent lithium plating on the carbon fibers, effectively reducing the nucleation overpotential. Even at a current density of 0.5 mA cm 2, the overpotential of the lithium nucleation can still be as low as ~21 mV. The silver particles pinned on the carbon fibers ensure the stable electrochemical cycling of the lithium plating/ stripping. These prove the feasibility of using the silver-plated carbon fiber paper as a porous current collector of the lithium anode with high energy conversion efficiency and long cycling lifespan. This designing idea is of great significance for the development of the lithium metal anodes.
1. Introduction Lithium metal anode has attracted more attention than ever before for its low redox potential ( 3.04 V vs SHE) and ultra-high theoretical specific capacity (3860 mAh g 1) [1], with the boosting research trend in solid-state lithium batteries, lithium-sulfur batteries, lithium-air bat teries, etc. However, there are still many problems to be solved before these secondary lithium batteries can be commercialized, such as for mation and growth of the lithium dendrites and low coulombic effi ciency during the lithium plating/stripping process. Scientists have tried to address the issues of lithium dendrites and the stability of solid electrolyte interface (SEI) film by means of electrolyte additives [2,3], fabrication of artificial films [4,5], lithium salt modification [6,7] and porous current collectors [8–10]. Among these solutions, developing porous current collectors is an effective strategy because the porous current collector can reduce the local current density, improve the interface stability, and inhibit the generation of lithium dendrites. Carbon is one of the most commonly used current collectors for the lithium metal batteries for its low cost and stable structure [11]. Zuo et al. [12] used graphitized carbon fibers as a
three-dimensional current collector for the dendrite-free lithium depo sition. However, the wettability of the carbon materials to lithium is poor [13], increasing the lithium nucleation energy on a hetero surface. Yan et al. [14] reported that there are no nucleation barriers on metals exhibiting some solubility in lithium, such as Au, Ag, Zn and Mg. Therefore, lithophilic materials were used as the seed to guide the lithium deposition to reduce the nucleation overpotential. Yang et al. [13] utilized ultrafine silver nanoparticles prepared by rapid Joule heating for seeded lithium deposition. Zhang et al. [15] prepared coralloid silver-coated carbon fiber-based composite lithium anode by silver electroplating and molten lithium infusion for lithium metal bat teries. In addition, Xue et al. [16] enabled lithium-metal composite anodes with a hierarchical silver-nanowire-graphene host to keep ul trahigh rates and long-term cycling. The silver plated on the carbon substrate can effectively reduce the lithium plating overpotential and guide the lithium deposition. However, these strategies are complicated and the plated silver is not uniform or compact enough. In addition, the mechanism of the lithium plating/stripping on them has not been explored comprehensively. We hereby design a silver-plated carbon fiber paper (CP@Ag) as a
* Corresponding author. Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China. E-mail address: [email protected] (Z. Wang). https://doi.org/10.1016/j.nanoen.2019.104144 Received 2 September 2019; Received in revised form 24 September 2019; Accepted 27 September 2019 Available online 28 September 2019 2211-2855/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Xiaoyun Li, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104144
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porous current collector for the lithium plating and stripping. The size of the silver particles is controllable and the silver particles on the carbon substrate can effectively improve its affinity to lithium. The lithium nucleation overpotential is reduced from 35 mV on CP to 21 mV on CP@Ag at a current density of 0.5 mA cm 2 and the lithium is plated along the carbon fibers regularly in the CP@Ag cell. The CP@Ag cell exhibits stable cyclability at a current density of 0.5 mA cm 2 for about 1000 h.
actually composed of smooth carbon fibers of about 5 μm in diameter, forming a three-dimensional porous structure. The strong and sharp peak around 26� in the X-ray diffraction (XRD) pattern (Fig. 1d) repre sents the (002) plane of the graphitic carbon [12], indicating the high graphitization of the CP while the asymmetry of this diffraction dem onstrates the presence of some less graphitized carbon. The peaks around 1353 cm 1 and 1582 cm 1 in the Raman spectrum (Fig. 1e) are assigned to the D and G bands, respectively. The high intensity ratio (IG/ID) demonstrates the high graphitization of the CP [12], agreeing with the above XRD results. The surface of the carbon fiber becomes rough after silver plating (Fig. 1b and c). Most of the silver particles are only a few tens of nanometers in diameter, all pinned on the carbon fibers. The silver cladding is compact and uniform. The weak peaks at around 38� (2θ) and 44� in the XRD pattern of CP@Ag (inset of Fig. 1d) demonstrate the presence of the metallic silver [17] while their broadening proves the nanoscale characteristics of the silver particles. The peaks at 374.1 eV and 368.1 eV in the X-ray photoelectron spectroscopic (XPS) Ag3d spectrum (Fig. 1f) correspond to Ag3d3/2 and Ag3d5/2, respectively, indicating that the silver on CP@Ag exists as metallic silver [15]. The oxygen in the commercial CP (O1s peak at 531.9 eV; Fig. 1g) was introduced upon CP fabrication. The silver particles were not oxidized during the plating or drying process as no other O1s peaks or position shifting can be observed. The silver particles on CP@Ag-40 (Fig. 1c) are generally larger than those on the CP@Ag-10 sample (Fig. 1b). In addition, the coverage of the silver particles on CP@Ag-10 is lower than on CP@Ag-40. According to the ICP-AES results, the mass ratio of the commercial CP to the chemi cally deposited silver is 244.3 in CP@Ag-10 and 51.2 in CP@Ag-40, respectively. Therefore, the particle size and the coverage of the silver can be controlled by tuning the silver deposition temperature and time of the silver mirror reaction.
2. Experimental section The silver-plated CP@Ag sheets were prepared via the silver mirror reaction. The carbon fiber paper (CP; ~190 μm thick) was purchased from Toray Industries. Before the silver mirror reaction, the CP was immersed in the SnCl2 solution for 15 min. Another three stocks of so lution were prepared for the silver mirror reaction. Solution A was prepared by dissolving silver nitrate in 125 mL of water followed by adding 25% (concentration) ammonia in it dropwise. Some potassium hydroxide and the ammonia solution were added to 125 mL of water for Solution B. Solution C was obtained by adding 125 mL water and 2.5 g glucose in the concentrated sulfuric acid; the dissolution was accelerated by heating the mixture in an oil bath (100 � C). On starting the silver mirror reaction, Solution C was added into the mixture of Solution A and Solution B (1:1:1, v/v/v). The CP sheet was immersed into the mixture and stirred as the mixture started to become turbid at 40 � C. After 30 s, the CP sheet was rinsed with deionized water to remove the residual solution. CP@Ag was also similarly prepared in the above mixed solu tion at 10 � C for 60 s. These two samples were named CP@Ag-40 and CP@Ag-10, respectively. The CP@Ag sheets were stored in an oven at 80 � C for later use. Coin cells (CR2032) were assembled in an argon-filled glove box (MBraun Lab Master 130, the contents of both the O2 and H2O are below 0.1 ppm), with the vacuum-dried circular CP and CP@Ag disks (φ14) as the working electrode, fresh lithium foil (φ16) as the counter electrode, 1 mol L 1 lithium bis(trifluoromethane)sulfonamide (LiTFSI) dissolved in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 v/v) with 1 wt% LiNO3 additive as the electrolyte and glass fiber as the separator. The cell was further galvanostatically discharged for some content of lithium deposition after its potential reached 0.0 V (vs. Liþ/Li) on a Land CT2001A battery tester (Wuhan, China). After some electrochemical cycles, the electrode sheets were taken out of the cells, rinsed with DME and dried in the vacuum mini-chamber of the glove box prior to the physical characterization. The morphology of the CP, CP@Ag and deposited lithium was characterized on a scanning electron microscope (SEM, HITACHI S4800). All the samples were transferred with a specially designed device from the glove box to the vacuum chamber of the SEM instrument without exposure to air. The elemental composition and mapping were performed by energy dispersive spectroscopic (EDS, HORIBA EMAX) analysis on the SEM instrument at an acceleration voltage of 10 kV. The Raman spectra were recorded on a LabRAM HR Evolution Raman spectrometer (532 nm radiation) with a resolution of 2 cm 1. The X-ray photoelectron spectra (XPS) were recorded with Thermo Fisher ESCA LAB 250 Xi with monochromatic 150 W Al Kα radiation. The data were analyzed with program Advantage and the binding energies were referenced to the C1s line at 284.4 eV from contaminated carbon. The XRD patterns were collected in a reflection mode between 10� and 80� (2θ) using the Cu Kα (λ ¼ 1.5406 Å) radiation on a Bruker D8 Advance Diffractometer.
3.2. Electrochemical evaluation The lithium metal plating/stripping behaviors on CP and CP@Ag were galvanostatically tested at 0.5 mA cm 2 (Fig. 2a–c). The areal lithium plating capacity was set to 2 mAh cm 2. Some lithium is stored in CP and CP@Ag above 0.0 V because the lithium ions can react with silver to form Li-Ag alloys before insert into the graphitized carbon fibers (insets of Fig. 2a–c). The stage-like profile around 0.0 V in the inset of Fig. 2c is for the Li-Ag alloying process [18]. The Li-Ag alloying stage is not as distinct in CP@Ag-10 (inset of Fig. 2b) as in CP@Ag-40 (inset of Fig. 2c) because there is less silver and the crystallinity of the silver particles is lower in CP@Ag-10. The pre-lithiation and Li-Ag alloying improve the affinity of the silver-plated carbon substrate to the metallic lithium and decrease the nucleation overpotential. In addition, the po larization of the above cells become less severe with cycling (Fig. 2a–c). The lithium nucleation potential is 35 mV and the lithium depo sition potential is 26 mV on CP in the first cycle. In contrast, the lithium nucleation potential rises to 24 mV, 21 mV, and the lithium deposition potential increases to 20 mV, 18 mV on CP@Ag-10 and on CP@Ag-40, respectively, in the first cycle. According to the density functional theory (DFT) calculations [15], the binding energy of the lithium atom on the Ag (111) surface ( 2.07 eV) is lower than on the carbon surface (graphene; 1.14 eV). That means that the silver-plated CP@Ag is more lithiophilic than the naked CP; the silver particles can guide the lithium deposition on CP. In this way, the lithium nucleation overpotential is reduced. Moreover, both the lithium nucleation poten tial and deposition potential are higher on CP@Ag than on CP throughout the cycling. Therefore, the nucleation and deposition po tentials on the carbon fibers can be controlled by regulating the tem perature and the time of the silver mirror reaction. The coulombic efficiency is defined as the ratio of the lithium stripping capacity to the plating capacity. The initial coulombic effi ciency is 39.9%, 57.9% and 44.5% for the CP, CP@Ag-10 and CP@Ag-
3. Results and discussion 3.1. Physical characterization The SEM imaging (Fig. 1a–c) shows the morphology of the com mercial CP and the silver (Ag) plated CP (CP@Ag). The commercial CP is 2
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Fig. 1. Physical characterization of the commercial CP and the as-prepared CP@Ag. SEM images of (a) CP, (b) CP@Ag-10 and (c) CP@Ag-40, (d) XRD pattern, (e) Raman spectra, and (f) Ag3d and (g) O1s XPS spectra.
Fig. 2. Comparison of the electrochemical performances of the cells with CP (a and d), CP@Ag-40 (b and e) and CP@Ag-10 (c and f) as the lithium deposition substrates. The voltage profiles of some selected cycles (a, b and c; the insets are for the voltage profiles around 0.0 V in the first discharge), the impedance spectrum of lithium deposition after some selected cycles (d, e and f) and the Coulombic efficiency on these substrates (insets of d, e, and f).
40, respectively, but sharply increase to 89.35%, 99.35%, and 94.93% in the 2nd cycle (insets of Fig. 2d–f). The low coulombic efficiency in the first cycle is attributed to the formation of the SEI film and the
occurrence of some other side reactions. In addition, some lithium ions are intercalated in the carbon fibers during discharge (above 0.0 V) but cannot be extracted during re-charge because of the low charge cut-off 3
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potential (0.1 V). It was reported that the Li-Ag alloy (up to AgLi12) cannot be completely de-alloyed until 0.25 V vs. Liþ/Li [18]. However, the charge cut-off potential was set to 0.1 V in order to highlight the distinct deposition behaviors of CP and CP@Ag in this work, leading to the incomplete de-intercalation of lithium from the carbon fiber and the incomplete de-alloying of the Li-Ag alloy. The coulombic efficiency of CP@Ag-10 in the first two cycles is higher than that of the other two samples because there are more defects in CP@Ag-10 than in other samples (Fig. 1e). The coulombic efficiency of the lithium stripping/ plating on CP, CP@Ag-10 and CP@Ag-40 maintains above 97.5%, 97.0% and 98.0%, respectively for over 110 cycles (insets of Fig. 2d–f and Fig. S1). The average coulombic efficiency of these cells is about 99% because the plated lithium can be stripped almost entirely during the charge process. The superiority of the silver plating can be observed even more clearly at a higher current density. Fig. S2 shows that the polarization on CP@Ag is smaller than on CP as the current density increases to 1 mA cm 2. Therefore, the CP@Ag cell is expected to have a higher energy conversion efficiency. All the cells can stably run for more than 140 cycles with coulombic efficiencies over 96%. The efficiency of CP@Ag-10 is lower because the lithium ions stored it cannot be extracted completely when the potential reaches 0.1 V (Fig. S2).
this stage (Fig. 3a). As the potential decreases to be negative, the lithium nucleation and deposition take place (Fig. 3b and c). The lithium nucleation starts at the intersections of the carbon fibers (Fig. 3b, Fig. S6a) because there are more defects there, beneficial for the lithium nucleation [19]. The sites with the oxygen-containing functional groups on the carbon fibers are probably another type of lithium deposition positions where the lithium prefers to nucleate due to their electrostatic attraction to the Liþ ions [20]. Heterogeneously distributed lithium particles are observed in the three-dimensional cavities of the carbon fibers when the deposition capacity reaches 2 mAh cm 2 in the CP cell (Fig. 3c), reflecting the low affinity of the carbon to the metallic lithium. The silver particles swell obviously due to the Li-Ag alloying as the CP@Ag-40 cell is discharged to around 0.0 V (Fig. 3d), agreeing with the stage-like potential profile (inset of Fig. 2b). The silver particles on the carbon fibers guide the lithium deposition afterwards (Fig. 3e, Fig. S6b). The metallic lithium is uniformly deposited on the silver-plated carbon fibers till the deposition capacity reaches 2 mAh cm 2. This indicates that the silver particles well facilitates the lithium deposition (Fig. 3f) and results in the increased lithium nucleation potential and lithium deposition potential (Fig. 2). Similar lithium deposition behavior is observed on CP@Ag-10 (Fig. S5). However, as the silver coverage on CP@Ag-10 is lower than on CP@Ag-40, the deposited lithium on CP@Ag-10 is not as compact or uniform as on CP@Ag-40. Corre spondingly, the lithium nucleation and deposition potentials on CP@Ag-10 are also lower than on CP@Ag-40. The morphology and the EDS mapping results of silver on CP@Ag-40 before and after 10 cycles of lithium plating/stripping are compared in Fig. 4. It is seen that the silver particles are still integrated, mostly adhered on the carbon fibers closely though the silver particles become larger and looser, suggesting that the volume variation due to Li-Ag alloying and de-alloying does not damage the adhesion of the silver particles on the fiber. The silver particles pinned on the carbon fibers can consistently induce the deposition of metallic lithium and reduce the overpotential effectively during cycling.
3.3. Evolution of interface and morphology The electrochemical impedance spectrum (EIS) of the CP and CP@Ag cells is shown in Fig. 2d–f and explains the difference of the lithium plating and stripping behaviors on different substrates (Fig. 2). Table S1 indicates that all the cells have similar and stable ohmic resistances (RS) but the RS of CP@Ag is only half that of the naked-CP cell, due to the existence of the thick conducting silver. The interface resistances (R1) of the two CP@Ag cells are lower than that of the naked-CP cell and do not change obviously with cycling. This means that the SEI film on the CP@Ag cell is more stable and is responsible for the lower polarization on CP@Ag, leading to higher cycling stability and smaller overpotential. The charge transfer resistance (R2) of the CP@Ag cell is smaller than that of the CP cell, suggesting that the lithium ions can be reduced to metallic lithium on CP@Ag more easily. These explain the reduced nucleation overpotential and increased lithium deposition potential in the CP@Ag cell. Table S1 also shows that the charge transfer resistance (R2) of all the cells decreases with cycling, leading to the reduced overpotential. The lithium plating/stripping process was recorded with the SEM imaging (Fig. 3). The lithium ions are intercalated in the carbon fibers when the CP cell is discharged to 0.0 V. Metallic lithium is not visible at
4. Conclusions In summary, nanosized silver particles were chemically deposited on commercial fiber paper (CP@Ag) to reduce the nucleation overpotential of lithium and improve its plating/stripping cycling performances on the carbon fiber. The size and the coverage of the silver particles on the fi bers can be controlled by regulating the reaction temperature and time. Silver plating on the carbon fiber enhances its affinity to the deposited lithium and thereby increases the lithium nucleation and deposition potentials when the silver-plated CP was used as the porous current
Fig. 3. SEM images of lithium plating on CP (a, b and c) and on CP@Ag-40 (d, e and f): (a, and d) discharge to 0.0 V, (b and e) plating 0.25 mAh cm plating 2 mAh cm 2. 4
2
and (c and f)
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the National Key Development Program of China (Grant No. 2015CB251100). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.nanoen.2019.104144. References [1] X.B. Cheng, R. Zhang, C.Z. Zhao, Q. Zhang, Chem. Rev. 117 (2017) 10403–10473. [2] F. Ding, W. Xu, G.L. Graff, J. Zhang, M.L. Sushko, X. Chen, Y. Shao, M. H. Engelhard, Z. Nie, J. Xiao, X. Liu, P.V. Sushko, J. Liu, J.-G. Zhang, J. Am. Chem. Soc. 135 (2013) 4450–4456. [3] W. Li, H. Yao, K. Yan, G. Zheng, Z. Liang, Y.M. Chiang, Y. Cui, Nat. Commun. 6 (2015) 7436. [4] N.W. Li, Y.X. Yin, C.P. Yang, Y.G. Guo, Adv. Mater. 28 (2016) 1853–1858. [5] Q.C. Liu, J.J. Xu, S. Yuan, Z.W. Chang, D. Xu, Y.B. Yin, L. Li, H.X. Zhong, Y.S. Jiang, J.M. Yan, X.B. Zhang, Adv. Mater. 27 (2015) 5241–5247. [6] J. Zheng, J.A. Lochala, A. Kwok, Z.D. Deng, J. Xiao, Adv. Sci. 4 (2017) 1700032. [7] G. Yang, Y. Li, S. Liu, S. Zhang, Z. Wang, L. Chen, Energy Storage Mater. (2019). htt ps://doi.org/10.1021/acs.nanolett.8b04376. [8] Y. Zhang, B. Liu, E. Hitz, W. Luo, Y. Yao, Y. Li, J. Dai, C. Chen, Y. Wang, C. Yang, H. Li, L. Hu, Nano Res. 10 (2017) 1356–1365. [9] Y. Li, J. Jiao, J. Bi, X. Wang, Z. Wang, L. Chen, Nano Energy 32 (2017) 241–246. [10] G. Yang, Y. Li, Y. Tong, J. Qiu, S. Liu, S. Zhang, Z. Guan, B. Xu, Z. Wang, L. Chen, Nano Lett. 19 (2018) 494–499. [11] G. Yang, S. Zhang, Y. Tong, X. Li, Z. Wang, L. Chen, Carbon 155 (2019) 9–15. [12] T.T. Zuo, X.W. Wu, C.P. Yang, Y.X. Yin, H. Ye, N.W. Li, Y.G. Guo, Adv. Mater. 29 (2017) 170039. [13] C. Yang, Y. Yao, S. He, H. Xie, E. Hitz, L. Hu, Adv. Mater. 29 (2017) 1702714. [14] K. Yan, Z. Lu, H.-W. Lee, F. Xiong, P.-C. Hsu, Y. Li, J. Zhao, S. Chu, Y. Cui, Nat. Energy 1 (2016) 16010. [15] R. Zhang, X. Chen, X. Shen, X.-Q. Zhang, X.-R. Chen, X.-B. Cheng, C. Yan, C.Z. Zhao, Q. Zhang, Joule 2 (2018) 764–777. [16] P. Xue, S. Liu, X. Shi, C. Sun, C. Lai, Y. Zhou, D. Sui, Y. Chen, J. Liang, Adv. Mater. 30 (2018) 1804165. [17] Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P.A. van Aken, J. Maier, Adv. Mater. 22 (2010) 2247–2250. [18] G. Taillades, J. Sarradin, J. Power Sources 125 (2004) 199–205. [19] S.-i.T. Jun-ichi Yamaki, Katsuya Hayashi, Keiichi Saito, Yasue Nemoto, Masayasu Arakawa, J. Power Sources 74 (1998) 219–227. [20] Q. Wang, C. Yang, J. Yang, K. Wu, L. Qi, H. Tang, Z. Zhang, W. Liu, H. Zhou, Energy Storage Mater 15 (2018) 249–256.
Fig. 4. The SEM imaging (a and c) and EDS mapping (b and d) for silver on CP@Ag-40 before (a and b) and after (c and d) 10 cycles.
collector of the lithium metal anode. The lithium nucleation and depo sition potentials can be increased to 21 mV and 18 mV, respectively, at a current density of 0.5 mA cm 2. The strong adhesion of the plated silver ensures the cycling stability of the lithium plating and stripping while the reduction of the polarization improves the high energy con version efficiency of the cell. These prove the feasibility of using CP@Ag as a current collector of the lithium anode with high energy conversion efficiency and long cycling lifespan. Clearly some other cheaper metals that can form alloys with lithium such as zinc (Zn) and tin (Sn), can also be good alternatives to silver. This designing idea is of great significance for the development of lithium metal anodes. Acknowledgments We acknowledge the financial support of this work by the National Natural Science Foundation of China (NSFC Grant No. 51372268) and
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Contents lists available at ScienceDirect
Nano Energy journal homepage: http://www.elsevier.com/locate/nanoen
Communication
Improved lithium deposition on silver plated carbon fiber paper Xiaoyun Li a, b, Gaojing Yang a, c, Simeng Zhang a, b, Zhaoxiang Wang a, b, c, *, Liquan Chen a a
Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China b College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing, 100190, China c School of Physical Sciences, University of Chinese Academy of Sciences, Beijing, 100190, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Lithium metal anode Carbon fiber Silver particles Lithium deposition Nucleation overpotential
Lithium metal is a promising anode material for its low redox potential and high theoretical specific capacity. However, issues such as dendritic growth, unstable interface and low coulombic efficiency hinder its practical applications. Construction of a porous current collector is supposed to be an effective way to address these issues and improve the cycling performances of the lithium metal battery. In this paper, commercial carbon fiber paper is used as a porous current collector for the lithium deposition, on which nanosized silver particles are plated beforehand by the silver mirror reaction. The lithiophilic silver particles alloy with the lithium and guide the subsequent lithium plating on the carbon fibers, effectively reducing the nucleation overpotential. Even at a current density of 0.5 mA cm 2, the overpotential of the lithium nucleation can still be as low as ~21 mV. The silver particles pinned on the carbon fibers ensure the stable electrochemical cycling of the lithium plating/ stripping. These prove the feasibility of using the silver-plated carbon fiber paper as a porous current collector of the lithium anode with high energy conversion efficiency and long cycling lifespan. This designing idea is of great significance for the development of the lithium metal anodes.
1. Introduction Lithium metal anode has attracted more attention than ever before for its low redox potential ( 3.04 V vs SHE) and ultra-high theoretical specific capacity (3860 mAh g 1) [1], with the boosting research trend in solid-state lithium batteries, lithium-sulfur batteries, lithium-air bat teries, etc. However, there are still many problems to be solved before these secondary lithium batteries can be commercialized, such as for mation and growth of the lithium dendrites and low coulombic effi ciency during the lithium plating/stripping process. Scientists have tried to address the issues of lithium dendrites and the stability of solid electrolyte interface (SEI) film by means of electrolyte additives [2,3], fabrication of artificial films [4,5], lithium salt modification [6,7] and porous current collectors [8–10]. Among these solutions, developing porous current collectors is an effective strategy because the porous current collector can reduce the local current density, improve the interface stability, and inhibit the generation of lithium dendrites. Carbon is one of the most commonly used current collectors for the lithium metal batteries for its low cost and stable structure [11]. Zuo et al. [12] used graphitized carbon fibers as a
three-dimensional current collector for the dendrite-free lithium depo sition. However, the wettability of the carbon materials to lithium is poor [13], increasing the lithium nucleation energy on a hetero surface. Yan et al. [14] reported that there are no nucleation barriers on metals exhibiting some solubility in lithium, such as Au, Ag, Zn and Mg. Therefore, lithophilic materials were used as the seed to guide the lithium deposition to reduce the nucleation overpotential. Yang et al. [13] utilized ultrafine silver nanoparticles prepared by rapid Joule heating for seeded lithium deposition. Zhang et al. [15] prepared coralloid silver-coated carbon fiber-based composite lithium anode by silver electroplating and molten lithium infusion for lithium metal bat teries. In addition, Xue et al. [16] enabled lithium-metal composite anodes with a hierarchical silver-nanowire-graphene host to keep ul trahigh rates and long-term cycling. The silver plated on the carbon substrate can effectively reduce the lithium plating overpotential and guide the lithium deposition. However, these strategies are complicated and the plated silver is not uniform or compact enough. In addition, the mechanism of the lithium plating/stripping on them has not been explored comprehensively. We hereby design a silver-plated carbon fiber paper (CP@Ag) as a
* Corresponding author. Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China. E-mail address: [email protected] (Z. Wang). https://doi.org/10.1016/j.nanoen.2019.104144 Received 2 September 2019; Received in revised form 24 September 2019; Accepted 27 September 2019 Available online 28 September 2019 2211-2855/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Xiaoyun Li, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104144
X. Li et al.
Nano Energy xxx (xxxx) xxx
porous current collector for the lithium plating and stripping. The size of the silver particles is controllable and the silver particles on the carbon substrate can effectively improve its affinity to lithium. The lithium nucleation overpotential is reduced from 35 mV on CP to 21 mV on CP@Ag at a current density of 0.5 mA cm 2 and the lithium is plated along the carbon fibers regularly in the CP@Ag cell. The CP@Ag cell exhibits stable cyclability at a current density of 0.5 mA cm 2 for about 1000 h.
actually composed of smooth carbon fibers of about 5 μm in diameter, forming a three-dimensional porous structure. The strong and sharp peak around 26� in the X-ray diffraction (XRD) pattern (Fig. 1d) repre sents the (002) plane of the graphitic carbon [12], indicating the high graphitization of the CP while the asymmetry of this diffraction dem onstrates the presence of some less graphitized carbon. The peaks around 1353 cm 1 and 1582 cm 1 in the Raman spectrum (Fig. 1e) are assigned to the D and G bands, respectively. The high intensity ratio (IG/ID) demonstrates the high graphitization of the CP [12], agreeing with the above XRD results. The surface of the carbon fiber becomes rough after silver plating (Fig. 1b and c). Most of the silver particles are only a few tens of nanometers in diameter, all pinned on the carbon fibers. The silver cladding is compact and uniform. The weak peaks at around 38� (2θ) and 44� in the XRD pattern of CP@Ag (inset of Fig. 1d) demonstrate the presence of the metallic silver [17] while their broadening proves the nanoscale characteristics of the silver particles. The peaks at 374.1 eV and 368.1 eV in the X-ray photoelectron spectroscopic (XPS) Ag3d spectrum (Fig. 1f) correspond to Ag3d3/2 and Ag3d5/2, respectively, indicating that the silver on CP@Ag exists as metallic silver [15]. The oxygen in the commercial CP (O1s peak at 531.9 eV; Fig. 1g) was introduced upon CP fabrication. The silver particles were not oxidized during the plating or drying process as no other O1s peaks or position shifting can be observed. The silver particles on CP@Ag-40 (Fig. 1c) are generally larger than those on the CP@Ag-10 sample (Fig. 1b). In addition, the coverage of the silver particles on CP@Ag-10 is lower than on CP@Ag-40. According to the ICP-AES results, the mass ratio of the commercial CP to the chemi cally deposited silver is 244.3 in CP@Ag-10 and 51.2 in CP@Ag-40, respectively. Therefore, the particle size and the coverage of the silver can be controlled by tuning the silver deposition temperature and time of the silver mirror reaction.
2. Experimental section The silver-plated CP@Ag sheets were prepared via the silver mirror reaction. The carbon fiber paper (CP; ~190 μm thick) was purchased from Toray Industries. Before the silver mirror reaction, the CP was immersed in the SnCl2 solution for 15 min. Another three stocks of so lution were prepared for the silver mirror reaction. Solution A was prepared by dissolving silver nitrate in 125 mL of water followed by adding 25% (concentration) ammonia in it dropwise. Some potassium hydroxide and the ammonia solution were added to 125 mL of water for Solution B. Solution C was obtained by adding 125 mL water and 2.5 g glucose in the concentrated sulfuric acid; the dissolution was accelerated by heating the mixture in an oil bath (100 � C). On starting the silver mirror reaction, Solution C was added into the mixture of Solution A and Solution B (1:1:1, v/v/v). The CP sheet was immersed into the mixture and stirred as the mixture started to become turbid at 40 � C. After 30 s, the CP sheet was rinsed with deionized water to remove the residual solution. CP@Ag was also similarly prepared in the above mixed solu tion at 10 � C for 60 s. These two samples were named CP@Ag-40 and CP@Ag-10, respectively. The CP@Ag sheets were stored in an oven at 80 � C for later use. Coin cells (CR2032) were assembled in an argon-filled glove box (MBraun Lab Master 130, the contents of both the O2 and H2O are below 0.1 ppm), with the vacuum-dried circular CP and CP@Ag disks (φ14) as the working electrode, fresh lithium foil (φ16) as the counter electrode, 1 mol L 1 lithium bis(trifluoromethane)sulfonamide (LiTFSI) dissolved in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 v/v) with 1 wt% LiNO3 additive as the electrolyte and glass fiber as the separator. The cell was further galvanostatically discharged for some content of lithium deposition after its potential reached 0.0 V (vs. Liþ/Li) on a Land CT2001A battery tester (Wuhan, China). After some electrochemical cycles, the electrode sheets were taken out of the cells, rinsed with DME and dried in the vacuum mini-chamber of the glove box prior to the physical characterization. The morphology of the CP, CP@Ag and deposited lithium was characterized on a scanning electron microscope (SEM, HITACHI S4800). All the samples were transferred with a specially designed device from the glove box to the vacuum chamber of the SEM instrument without exposure to air. The elemental composition and mapping were performed by energy dispersive spectroscopic (EDS, HORIBA EMAX) analysis on the SEM instrument at an acceleration voltage of 10 kV. The Raman spectra were recorded on a LabRAM HR Evolution Raman spectrometer (532 nm radiation) with a resolution of 2 cm 1. The X-ray photoelectron spectra (XPS) were recorded with Thermo Fisher ESCA LAB 250 Xi with monochromatic 150 W Al Kα radiation. The data were analyzed with program Advantage and the binding energies were referenced to the C1s line at 284.4 eV from contaminated carbon. The XRD patterns were collected in a reflection mode between 10� and 80� (2θ) using the Cu Kα (λ ¼ 1.5406 Å) radiation on a Bruker D8 Advance Diffractometer.
3.2. Electrochemical evaluation The lithium metal plating/stripping behaviors on CP and CP@Ag were galvanostatically tested at 0.5 mA cm 2 (Fig. 2a–c). The areal lithium plating capacity was set to 2 mAh cm 2. Some lithium is stored in CP and CP@Ag above 0.0 V because the lithium ions can react with silver to form Li-Ag alloys before insert into the graphitized carbon fibers (insets of Fig. 2a–c). The stage-like profile around 0.0 V in the inset of Fig. 2c is for the Li-Ag alloying process [18]. The Li-Ag alloying stage is not as distinct in CP@Ag-10 (inset of Fig. 2b) as in CP@Ag-40 (inset of Fig. 2c) because there is less silver and the crystallinity of the silver particles is lower in CP@Ag-10. The pre-lithiation and Li-Ag alloying improve the affinity of the silver-plated carbon substrate to the metallic lithium and decrease the nucleation overpotential. In addition, the po larization of the above cells become less severe with cycling (Fig. 2a–c). The lithium nucleation potential is 35 mV and the lithium depo sition potential is 26 mV on CP in the first cycle. In contrast, the lithium nucleation potential rises to 24 mV, 21 mV, and the lithium deposition potential increases to 20 mV, 18 mV on CP@Ag-10 and on CP@Ag-40, respectively, in the first cycle. According to the density functional theory (DFT) calculations [15], the binding energy of the lithium atom on the Ag (111) surface ( 2.07 eV) is lower than on the carbon surface (graphene; 1.14 eV). That means that the silver-plated CP@Ag is more lithiophilic than the naked CP; the silver particles can guide the lithium deposition on CP. In this way, the lithium nucleation overpotential is reduced. Moreover, both the lithium nucleation poten tial and deposition potential are higher on CP@Ag than on CP throughout the cycling. Therefore, the nucleation and deposition po tentials on the carbon fibers can be controlled by regulating the tem perature and the time of the silver mirror reaction. The coulombic efficiency is defined as the ratio of the lithium stripping capacity to the plating capacity. The initial coulombic effi ciency is 39.9%, 57.9% and 44.5% for the CP, CP@Ag-10 and CP@Ag-
3. Results and discussion 3.1. Physical characterization The SEM imaging (Fig. 1a–c) shows the morphology of the com mercial CP and the silver (Ag) plated CP (CP@Ag). The commercial CP is 2
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Fig. 1. Physical characterization of the commercial CP and the as-prepared CP@Ag. SEM images of (a) CP, (b) CP@Ag-10 and (c) CP@Ag-40, (d) XRD pattern, (e) Raman spectra, and (f) Ag3d and (g) O1s XPS spectra.
Fig. 2. Comparison of the electrochemical performances of the cells with CP (a and d), CP@Ag-40 (b and e) and CP@Ag-10 (c and f) as the lithium deposition substrates. The voltage profiles of some selected cycles (a, b and c; the insets are for the voltage profiles around 0.0 V in the first discharge), the impedance spectrum of lithium deposition after some selected cycles (d, e and f) and the Coulombic efficiency on these substrates (insets of d, e, and f).
40, respectively, but sharply increase to 89.35%, 99.35%, and 94.93% in the 2nd cycle (insets of Fig. 2d–f). The low coulombic efficiency in the first cycle is attributed to the formation of the SEI film and the
occurrence of some other side reactions. In addition, some lithium ions are intercalated in the carbon fibers during discharge (above 0.0 V) but cannot be extracted during re-charge because of the low charge cut-off 3
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potential (0.1 V). It was reported that the Li-Ag alloy (up to AgLi12) cannot be completely de-alloyed until 0.25 V vs. Liþ/Li [18]. However, the charge cut-off potential was set to 0.1 V in order to highlight the distinct deposition behaviors of CP and CP@Ag in this work, leading to the incomplete de-intercalation of lithium from the carbon fiber and the incomplete de-alloying of the Li-Ag alloy. The coulombic efficiency of CP@Ag-10 in the first two cycles is higher than that of the other two samples because there are more defects in CP@Ag-10 than in other samples (Fig. 1e). The coulombic efficiency of the lithium stripping/ plating on CP, CP@Ag-10 and CP@Ag-40 maintains above 97.5%, 97.0% and 98.0%, respectively for over 110 cycles (insets of Fig. 2d–f and Fig. S1). The average coulombic efficiency of these cells is about 99% because the plated lithium can be stripped almost entirely during the charge process. The superiority of the silver plating can be observed even more clearly at a higher current density. Fig. S2 shows that the polarization on CP@Ag is smaller than on CP as the current density increases to 1 mA cm 2. Therefore, the CP@Ag cell is expected to have a higher energy conversion efficiency. All the cells can stably run for more than 140 cycles with coulombic efficiencies over 96%. The efficiency of CP@Ag-10 is lower because the lithium ions stored it cannot be extracted completely when the potential reaches 0.1 V (Fig. S2).
this stage (Fig. 3a). As the potential decreases to be negative, the lithium nucleation and deposition take place (Fig. 3b and c). The lithium nucleation starts at the intersections of the carbon fibers (Fig. 3b, Fig. S6a) because there are more defects there, beneficial for the lithium nucleation [19]. The sites with the oxygen-containing functional groups on the carbon fibers are probably another type of lithium deposition positions where the lithium prefers to nucleate due to their electrostatic attraction to the Liþ ions [20]. Heterogeneously distributed lithium particles are observed in the three-dimensional cavities of the carbon fibers when the deposition capacity reaches 2 mAh cm 2 in the CP cell (Fig. 3c), reflecting the low affinity of the carbon to the metallic lithium. The silver particles swell obviously due to the Li-Ag alloying as the CP@Ag-40 cell is discharged to around 0.0 V (Fig. 3d), agreeing with the stage-like potential profile (inset of Fig. 2b). The silver particles on the carbon fibers guide the lithium deposition afterwards (Fig. 3e, Fig. S6b). The metallic lithium is uniformly deposited on the silver-plated carbon fibers till the deposition capacity reaches 2 mAh cm 2. This indicates that the silver particles well facilitates the lithium deposition (Fig. 3f) and results in the increased lithium nucleation potential and lithium deposition potential (Fig. 2). Similar lithium deposition behavior is observed on CP@Ag-10 (Fig. S5). However, as the silver coverage on CP@Ag-10 is lower than on CP@Ag-40, the deposited lithium on CP@Ag-10 is not as compact or uniform as on CP@Ag-40. Corre spondingly, the lithium nucleation and deposition potentials on CP@Ag-10 are also lower than on CP@Ag-40. The morphology and the EDS mapping results of silver on CP@Ag-40 before and after 10 cycles of lithium plating/stripping are compared in Fig. 4. It is seen that the silver particles are still integrated, mostly adhered on the carbon fibers closely though the silver particles become larger and looser, suggesting that the volume variation due to Li-Ag alloying and de-alloying does not damage the adhesion of the silver particles on the fiber. The silver particles pinned on the carbon fibers can consistently induce the deposition of metallic lithium and reduce the overpotential effectively during cycling.
3.3. Evolution of interface and morphology The electrochemical impedance spectrum (EIS) of the CP and CP@Ag cells is shown in Fig. 2d–f and explains the difference of the lithium plating and stripping behaviors on different substrates (Fig. 2). Table S1 indicates that all the cells have similar and stable ohmic resistances (RS) but the RS of CP@Ag is only half that of the naked-CP cell, due to the existence of the thick conducting silver. The interface resistances (R1) of the two CP@Ag cells are lower than that of the naked-CP cell and do not change obviously with cycling. This means that the SEI film on the CP@Ag cell is more stable and is responsible for the lower polarization on CP@Ag, leading to higher cycling stability and smaller overpotential. The charge transfer resistance (R2) of the CP@Ag cell is smaller than that of the CP cell, suggesting that the lithium ions can be reduced to metallic lithium on CP@Ag more easily. These explain the reduced nucleation overpotential and increased lithium deposition potential in the CP@Ag cell. Table S1 also shows that the charge transfer resistance (R2) of all the cells decreases with cycling, leading to the reduced overpotential. The lithium plating/stripping process was recorded with the SEM imaging (Fig. 3). The lithium ions are intercalated in the carbon fibers when the CP cell is discharged to 0.0 V. Metallic lithium is not visible at
4. Conclusions In summary, nanosized silver particles were chemically deposited on commercial fiber paper (CP@Ag) to reduce the nucleation overpotential of lithium and improve its plating/stripping cycling performances on the carbon fiber. The size and the coverage of the silver particles on the fi bers can be controlled by regulating the reaction temperature and time. Silver plating on the carbon fiber enhances its affinity to the deposited lithium and thereby increases the lithium nucleation and deposition potentials when the silver-plated CP was used as the porous current
Fig. 3. SEM images of lithium plating on CP (a, b and c) and on CP@Ag-40 (d, e and f): (a, and d) discharge to 0.0 V, (b and e) plating 0.25 mAh cm plating 2 mAh cm 2. 4
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the National Key Development Program of China (Grant No. 2015CB251100). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.nanoen.2019.104144. References [1] X.B. Cheng, R. Zhang, C.Z. Zhao, Q. Zhang, Chem. Rev. 117 (2017) 10403–10473. [2] F. Ding, W. Xu, G.L. Graff, J. Zhang, M.L. Sushko, X. Chen, Y. Shao, M. H. Engelhard, Z. Nie, J. Xiao, X. Liu, P.V. Sushko, J. Liu, J.-G. Zhang, J. Am. Chem. Soc. 135 (2013) 4450–4456. [3] W. Li, H. Yao, K. Yan, G. Zheng, Z. Liang, Y.M. Chiang, Y. Cui, Nat. Commun. 6 (2015) 7436. [4] N.W. Li, Y.X. Yin, C.P. Yang, Y.G. Guo, Adv. Mater. 28 (2016) 1853–1858. [5] Q.C. Liu, J.J. Xu, S. Yuan, Z.W. Chang, D. Xu, Y.B. Yin, L. Li, H.X. Zhong, Y.S. Jiang, J.M. Yan, X.B. Zhang, Adv. Mater. 27 (2015) 5241–5247. [6] J. Zheng, J.A. Lochala, A. Kwok, Z.D. Deng, J. Xiao, Adv. Sci. 4 (2017) 1700032. [7] G. Yang, Y. Li, S. Liu, S. Zhang, Z. Wang, L. Chen, Energy Storage Mater. (2019). htt ps://doi.org/10.1021/acs.nanolett.8b04376. [8] Y. Zhang, B. Liu, E. Hitz, W. Luo, Y. Yao, Y. Li, J. Dai, C. Chen, Y. Wang, C. Yang, H. Li, L. Hu, Nano Res. 10 (2017) 1356–1365. [9] Y. Li, J. Jiao, J. Bi, X. Wang, Z. Wang, L. Chen, Nano Energy 32 (2017) 241–246. [10] G. Yang, Y. Li, Y. Tong, J. Qiu, S. Liu, S. Zhang, Z. Guan, B. Xu, Z. Wang, L. Chen, Nano Lett. 19 (2018) 494–499. [11] G. Yang, S. Zhang, Y. Tong, X. Li, Z. Wang, L. Chen, Carbon 155 (2019) 9–15. [12] T.T. Zuo, X.W. Wu, C.P. Yang, Y.X. Yin, H. Ye, N.W. Li, Y.G. Guo, Adv. Mater. 29 (2017) 170039. [13] C. Yang, Y. Yao, S. He, H. Xie, E. Hitz, L. Hu, Adv. Mater. 29 (2017) 1702714. [14] K. Yan, Z. Lu, H.-W. Lee, F. Xiong, P.-C. Hsu, Y. Li, J. Zhao, S. Chu, Y. Cui, Nat. Energy 1 (2016) 16010. [15] R. Zhang, X. Chen, X. Shen, X.-Q. Zhang, X.-R. Chen, X.-B. Cheng, C. Yan, C.Z. Zhao, Q. Zhang, Joule 2 (2018) 764–777. [16] P. Xue, S. Liu, X. Shi, C. Sun, C. Lai, Y. Zhou, D. Sui, Y. Chen, J. Liang, Adv. Mater. 30 (2018) 1804165. [17] Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P.A. van Aken, J. Maier, Adv. Mater. 22 (2010) 2247–2250. [18] G. Taillades, J. Sarradin, J. Power Sources 125 (2004) 199–205. [19] S.-i.T. Jun-ichi Yamaki, Katsuya Hayashi, Keiichi Saito, Yasue Nemoto, Masayasu Arakawa, J. Power Sources 74 (1998) 219–227. [20] Q. Wang, C. Yang, J. Yang, K. Wu, L. Qi, H. Tang, Z. Zhang, W. Liu, H. Zhou, Energy Storage Mater 15 (2018) 249–256.
Fig. 4. The SEM imaging (a and c) and EDS mapping (b and d) for silver on CP@Ag-40 before (a and b) and after (c and d) 10 cycles.
collector of the lithium metal anode. The lithium nucleation and depo sition potentials can be increased to 21 mV and 18 mV, respectively, at a current density of 0.5 mA cm 2. The strong adhesion of the plated silver ensures the cycling stability of the lithium plating and stripping while the reduction of the polarization improves the high energy con version efficiency of the cell. These prove the feasibility of using CP@Ag as a current collector of the lithium anode with high energy conversion efficiency and long cycling lifespan. Clearly some other cheaper metals that can form alloys with lithium such as zinc (Zn) and tin (Sn), can also be good alternatives to silver. This designing idea is of great significance for the development of lithium metal anodes. Acknowledgments We acknowledge the financial support of this work by the National Natural Science Foundation of China (NSFC Grant No. 51372268) and
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