Stable Li Metal Anode with “Ion–Solvent-Coordinated” Nonflammable

http://pubs.acs.org/journal/aelccp

Stable Li Metal Anode with “Ion−SolventCoordinated” Nonflammable Electrolyte for Safe Li Metal Batteries Lifen Xiao,*,† Ziqi Zeng,‡ Xingwei Liu,‡ Yongjin Fang,‡ Xiaoyu Jiang,‡ Yuyan Shao,§ Lin Zhuang,‡ Xinping Ai,‡ Hanxi Yang,‡ Yuliang Cao,*,‡ and Jun Liu*,§ ACS Energy Lett. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/23/19. For personal use only.

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State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China ‡ College of Chemistry and Molecular Sciences, Hubei Key Laboratory of Electrochemical Power Sources, Wuhan University, Wuhan 430072, China § Pacific Northwest National Laboratory, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: Li metal batteries have recently attracted extensive attention due to the pursuit of high-energy-density batteries for electric vehicles. However, safety and low cycling Coulombic efficiency (CCE) hinder the development of high-energy Li metal secondary batteries due to the high reactivity of Li metal with present electrolytes. Here, we investigate the reductive stability of nonflammable electrolytes with a wide salt to solvent ratio range and demonstrate that an ion−solvent-coordinated (ISC) nonflammable electrolyte−lithium bis(fluorosulfonyl)imide (LiFSI)-triethylphosphate (TEP) enables dendrite-free Li metal plating/stripping on the Cu electrode with a high CCE of 99.3% up to 350 cycles. Understanding of the fundamental electrochemistry tells us that the decrease of solvent activity in ISC electrolyte can improve the reductive stability of the solvent on Li metal anodes. The fundamental concept of ISC electrolyte developed in this work provides new perspectives for the development of highly stable and safe Li metal batteries.

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density of Li-ion batteries shows limited room for improvement after 3 decades of development. Due to an increasing demand of advanced energy storage technologies to electrify transportation and large-scale energy storage systems, rechargeable Li metal batteries recently have again received much attention. Besides, the Li metal anode enables utilization of Li-free cathodes such as metal fluorides and oxides and transition metal-free cathodes such as Li−S7 and Li−O28 batteries. However, the development of rechargeable Li metal batteries with high energy density would inevitably bring more serious safety hazards, especially if employing the currently used flammable carbonate electrolyte systems. An effective solution to battery safety is to use nonflammable electrolyte. Previous efforts of using nonflammable solvents such as organic phosphates were unsuccessful because the strong reactivity of phosphate solvents with Li led to the

i metal is regarded as the ultimate anode for rechargeable batteries because of its ultrahigh specific capacity of 3860 mAh g−1, very low redox potential of −3.04 V (vs SHE), and small gravimetric density of 0.534 g cm−3.1,2 Extensive studies on rechargeable Li metal batteries were carried out in the 1960−1980s.3−5 However, attempts for practical application were observed to be severely hindered by two major barriers. One is the growth of Li dendrite, and the other is the low Coulombic efficiency during repeated charging−discharging cycles. The outgrowth of Li dendrite protrudes from the anode surface to the cathode due to uneven deposition of Li, leading to internal short-circuiting to further cause serious safety issues, such as fires and explosions. The low cycling efficiency of the Li metal anodes consumes both the electrolyte and Li metal due to the high reactivity of Li metal toward electrolyte and reformation of the solid− electrolyte interface (SEI) film raised by the repeated cracking during Li plating and stripping, so as to rapidly increase the cell impedance and shorten the battery life.6 The emergence of Liion batteries has largely diminished the development of Li metal batteries since the early 1990s. However, the energy © XXXX American Chemical Society

Received: December 26, 2018 Accepted: January 16, 2019 Published: January 16, 2019 483

DOI: 10.1021/acsenergylett.8b02527 ACS Energy Lett. 2019, 4, 483−488

Letter

Cite This: ACS Energy Lett. 2019, 4, 483−488

Letter

ACS Energy Letters inablity to build a robust SEI layer between the Li anode and electrolyte. Therefore, constructing a highly uniform and stabile SEI layer is essential for practical use of Li metal batteries. Many electrolyte additives are employed to facilitate the formation of a stable SEI layer on a Li metal anode, such as fluoroethylene carbonate (FEC)9,10 and LiNO3.10,11 A Li metal anode with a protective SEI layer exhibits superior cycling performance in high-energy-density Li metal batteries. Furthermore, unusual stability of Li anodes has been found in highly concentrated electrolyte with a variety of organic solvents.12−15 For example, the Li metal anode exhibits high cycling Coulombic efficiency (CCE) of 99.1% in 4 M LiFSI/ DME electrolyte.16 To improve the stability of carbonate electrolytes, 10 M LiFSI electrolyte was used to construct Frich interphases, leading to high Li plating/stripping CE of ∼99.3%.15 However, the low flash point and boiling point of ether- and carbonate-type solvents make it unsafe to use in practical systems. Although phosphate solvents are more reactive with a Li metal anode due to the relatively lower lowest unoccupied molecular orbital (LUMO), phosphatebased electrolytes are found to be compatible with a >4.0 V operation voltage.17 Solid electrolytes and ionic liquids are of particular interest because they can eliminate safety issues such as flame or exploration, but they suffer from problems such as low conductivity, which greatly restricts their application.18−20 Recently, we found that the molar ratio (MR) of salt to solvent, rather than the molar concentration of electrolyte, is the key factor for providing highly stable electrode/electrolyte interfaces.21 Under high MR conditions, most solvent molecules are coordinated with Li cations, which we call “ion−solvent-coordinated” (ISC) electrolytes here, and can enable utilization of nonflammable phosphate electrolytes (such as 1:2 lithium bis(fluorosulfonyl)imide-triethylphosphate (LiFSI-TEP) (∼2.2 mol L−1) to construct safe commercial Li-ion batteries by effectively suppressing the reactivity of solvent molecules and realizing reversible Li interaction in graphite and improved reversibility of the lithium metal anode. Moreover, the ISC electrolyte can move the Li salt concentration outside of the concentrated electrolyte region by tuning the molecular weight of the solvent and salt (eq S1), which is in favor of maintaining low viscosity and minimizing the cost of the electrolyte.21 To further decrease salt concentration, a dilute high-concentration nonflammable electrolyte was proposed containing 3.2 M LiFSI-TEP and a bis(2,2,2-trifluoroethyl)ether (BTFE) diluter to form close to a 1 M salt concentration and exhibit a high CCE of 99%.22 Here we investigate more deeply the electrochemical reversibility of Li metal anode in a wide MR range (1:5−1:1) and demonstrate that Li metal can be reversibly plated/stripped on a Cu current collector with an extremely high CCE (99.3%) and long lifespan by using LiFSI-TEP (MR = 1:1.5) ISC nonflammable electrolyte. This result demonstrates that a rechargeable Li metal battery can also be constructed with high safety. The chemical stability of the nonflammable electrolyte toward Li metal was evaluated by storing Li foils in nonflammable electrolytes with various MRs of LiFSI salt to TEP solvent from 1:5 to 1:1 under 60 °C for 1 week (Figure 1a). Great chemical stability of the Li metal foils in electrolyte was demonstrated with a MR of 1:1.5 or higher as the surfaces of Li remained smooth and shiny. The electrochemical stability was measured by cyclic voltammetry (CV) using a Pt microplate electrode (Figure 1b). It showed that all electro-

Figure 1. Dependence of the physicochemical and electrochemical properties on MRs of the LiFSI-TEP electrolytes. (a) Photos of the Li foils in different MR electrolytes. The storage temperature is 60 °C, and the storage time is 7 days. (b) Electrochemical windows of different MR electrolytes measured on Pt microelectrodes at a scanning rate of 50 mV s−1. (c) Initial charging−discharging curve of the Li|Cu cells in different MR electrolytes at a current density of 0.1 mA cm−2. Pink line: 1:5. Yellow line: 1:3. Red line: 1:2. Green line: 1:1.5. Blue line: 1:1. (d) Coulombic efficiency and (e) Li metal plating/stripping profile of the Li|Cu cell cycled at 0.1 mA cm−2 in the first cycle and then 0.2 mA cm−2 in 1:1.5 electrolyte; the deposition capacity was 1 mAh cm−2. Inset: Charge−discharge curves of the Li|Cu cell at selected cycles. The inset plots of (e) are the expanded views from the bottom plot.

lytes are oxidation-resistive over 5.0 V vs Li/Li+. The plating and stripping peaks of Li metal became sharper with increasing MR, indicating a more reversible electrochemical reaction. Figure 1c displays the initial plating/stripping curves of Li metal on Cu electrodes using Li|Cu coin cells in the electrolytes with various MRs at a constant current density of 0.1 mA cm−2. The initial Coulombic efficiencies (ICEs) in the 1:5, 1:3, 1:2, 1:1.5, and 1:1 electrolytes were found to be 18, 84, 94.8, 95.6, and 92.1%, respectively, in good accordance with the CV results (Figure 1b). The ICEs gradually increased with increasing MR and then slightly decreased in the 1:1 electrolyte due to the lowest ionic conductivity and highest viscosity. The polarization of the electrodes remained identical after MR ≥ 1:2 (Figure 1c). Moreover, the nonflammable TEP electrolytes with MR ≥ 1:2 had comparable conductivity (Figure S1) and viscosity for the applications of Li metal batteries with some highly concentrated electrolytes reported in previous literature (Table S1). According to the electrochemical properties, the 1:1.5 electrolyte is optimal due to the high ICE of 95.6% and low polarization (Figure 1c). The high ICE (95.6%) is even comparable to that in ether (97.1%),16 indicative of low side reaction of the electrolyte and high electrochemical reversibility. The cycling performance of the Li|Cu coin cell in the 1:1.5 LiFSI-TEP electrolyte is shown in Figure 1d. The CCE of the cell increases gradually from 95.6% at the first cycle to >99% 484

DOI: 10.1021/acsenergylett.8b02527 ACS Energy Lett. 2019, 4, 483−488

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ACS Energy Letters

Figure 2. Optical and SEM images of Li metal plating and stripping on a Cu electrode in LiFSI-TEP electrolytes. (a,e,i) Plating and (c,g) stripping in 1:5 electrolyte. (b,f,j) Plating and (d,h) stripping in 1:1.5 electrolyte. (k,i) In situ optical microscopic images for Li deposition on a Cu electrode at 0.15 μA for 30 min in 1:5 (k) and 1:1.5 (l) electrolytes.

and retains an average CE of 99.3% up to 350 cycles, which is higher than those in other electrolytes (Table S2), such as carbonate (95%),23 ether (98%),16 and ionic liquid (97%).20 Further details of the cycling plating/stripping curves are displayed in Figure 1e and the inset. In the early, middle, and late stages of the cycling process, all of the plating/stripping curves exhibit similar electrochemical properties, implying that the electrode remains a stable interface structure after the formation of a SEI film in the early cycles, only with a slight increase of the electrochemical polarization. The superhigh CCE (99.3%) should be attributed to the strong suppression of the reductive decomposition of the TEP solvent due to the absence of free TEP molecules in the ISC electrolyte.21 This conclusion is also verified by the plating/stripping cycling curves in various MR electrolytes (Figure S2). In low MR electrolyte (1:3), the Li|Cu coin cell can hardly be cycled, presenting a very low CCE (99% after 30 cycles (Figure S6d). Therefore, this high MR LiFSITEP electrolyte displays stable chemical and electrochemical stability even at high temperature and supports excellent Li plating and stripping performance. According to the above morphological and electrochemical investigations, it is concluded the highly efficient and stable Li plating/stripping in the high MR LiFSI-TEP electrolyte mainly originates from suppression of the TEP solvent decomposition reaction. As known, in electrolyte, the Li ions are all coordinated with the solvent molecules to form a solvate. On the basis of the general coordinating number (n = 4) of the Li ion, a theoretical depletion of the free (uncoordinated) solvent molecules occurs in electrolyte with MR = 1:4. Therefore, in low MR electrolyte, except for a part of the TEP solvent molecules coordinated with Li ions, there are plenty of free solvent molecules in the electrolyte that are readily reactive with Li metal. With increasing MR, the free TEP becomes increasingly scarce with even an FSI anion also participating in the coordination to form an ISC electrolyte. From the electrochemical perspective, the reduction potential of the TEP solvent follows the Nernst equation, as shown in eq S2. The activity variation of TEP molecules was measured through modified vapor pressure, as shown in Figure S7. With increasing MR, the activity of the free TEP first dramatically decreases and then becomes flat close to 0.01 when MR is ≥1:3. This leads to a negative shift of the reduction potential of the solvent molecules, i.e., the stability of TEP against reduction is enhanced with MR increase. Therefore, it can be known that the low activity of TEP is due to the formation of the ISC structure in high MR electrolytes decreasing free TEP molecules, which is a main reason for improving the stability of the solvent. Meanwhile, the redox potential of Li/ Li+ shifts toward a more positive potential with increasing MR (Figure S7), suggesting lower reactive activity of metal Li with TEP. Therefore, in the ISC electrolyte, two effects enable the accomplishment of reversible Li metal plating and stripping: one is that the depletion of the free solvent molecules reduces the reduction potential (enhances the reduction tolerance) of TEP; another is that the Li+ ions coordinated tightly by the solvent molecules and anions enhance the redox potential of Li/Li+ so as to decrease the reactive activity of metal Li. Figure 4 shows the schematic illustration of the electrochemical processes in low and high MR electrolytes. In low MR (≤1:3) electrolytes, the decomposition of low stable free TEP molecules occurs preferentially and continuously to produce nonconductive deposits, and Li metal fails to plate (Figure 4a). In high MR (≥1:2) electrolytes, most of the solvent molecules are coordinated with Li ions, leading to largely enhanced reduction stability, which facilitates Li metal plating and growth on the Cu electrode (Figure 4b). In summary, an ISC nonflammable LiFSI-TEP electrolyte has demonstrated high chemical and electrochemical stability,

Figure 4. Schematic illustrations of the Li metal plating on a Cu electrode in low MR (a) and high MR (ISC) (b) electrolytes.

which is attributed to the enhanced reduction stability of TEP and reduced reactivity of metal Li resulting from the coordination of most TEP molecules with Li+ ions. Successful Li metal plating/stripping has been achieved with high CCE (>99.3%) and cycling stability for a wide temperature range. This result demonstrates a viability to construct rechargeable Li metal batteries with high reversibility and safety. Additionally, this study is conducted using coin cell configurations with a more than adequate amount of electrolytes. The properties of Li plating and stripping need to be further verified in full cells using lean electrolytes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b02527. Experimental Section and additional characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.X.). *E-mail: [email protected] (Y.C.). *E-mail: [email protected] (J.L.). ORCID

Yongjin Fang: 0000-0002-8988-525X Lin Zhuang: 0000-0002-5642-6735 Xinping Ai: 0000-0002-8280-0866 Yuliang Cao: 0000-0001-6092-5652 487

DOI: 10.1021/acsenergylett.8b02527 ACS Energy Lett. 2019, 4, 483−488

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ACS Energy Letters

(18) Zhou, D.; Liu, R.; He, Y.-B.; Li, F.; Liu, M.; Li, B.; Yang, Q.-H.; Cai, Q.; Kang, F. SiO2 Hollow Nanosphere-Based Composite Solid Electrolyte for Lithium Metal Batteries to Suppress Lithium Dendrite Growth and Enhance Cycle Life. Adv. Energy Mater. 2016, 6, 1502214. (19) Guo, Q.; Han, Y.; Wang, H.; Hong, X.; Zheng, C.; Liu, S.; Xie, K. Safer Lithium Metal Battery Based on Advanced Ionic Liquid Gel Polymer Nonflammable Electrolytes. RSC Adv. 2016, 6, 101638− 101644. (20) Basile, A.; Bhatt, A. I.; O’Mullane, A. P. Stabilizing Lithium Metal Using Ionic Liquids for Long-Lived Batteries. Nat. Commun. 2016, 7, 11794. (21) Zeng, Z.; Murugesan, V.; Han, K. S.; Jiang, X.; Cao, Y.; Xiao, L.; Ai, X.; Yang, H.; Zhang, J.-G.; Sushko, M. L.; et al. Non-Flammable Electrolytes with High Salt-to-Solvent Ratios for Li-ion and Li-Metal Batteries. Nat. Energy 2018, 3, 674−681. (22) Chen, S.; Zheng, J.; Yu, L.; Ren, X.; Engelhard, M. H.; Niu, C.; Lee, H.; Xu, W.; Xiao, J.; Liu, J.; et al. High-Efficiency Lithium Metal Batteries with Fire-Retardant Electrolytes. Joule 2018, 2, 1548−1558. (23) Liu, Y.; Liu, Q.; Xin, L.; Liu, Y.; Yang, F.; Stach, E. A.; Xie, J. Making Li-metal Electrodes Rechargeable by Controlling the Dendrite Growth Direction. Nat. Energy 2017, 2, 17083. (24) Kim, H.; Wu, F.; Lee, J. T.; Nitta, N.; Lin, H.-T.; Oschatz, M.; Cho, W. I.; Kaskel, S.; Borodin, O.; Yushin, G. In Situ Formation of Protective Coatings on Sulfur Cathodes in Lithium Batteries with LiFSI-Based Organic Electrolytes. Adv. Energy Mater. 2015, 5, 1401792.

Jun Liu: 0000-0001-8663-7771 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (No. 2018YFB0104300) and the National Science Foundation of China (Nos. 21673165, 21373155, and 21333007). J.L. acknowledges support from the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award KC020105-FWP12152 for providing oversight of the scientific directions of this project and for detailed study of the reaction mechanisms.

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DOI: 10.1021/acsenergylett.8b02527 ACS Energy Lett. 2019, 4, 483−488