Stable Li Metal Anode with âIon-solvent-coordinatedâ Nonflammable

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Letter

Stable Li Metal Anode with “Ion-solvent-coordinated” Nonflammable Electrolyte for Safe Li Metal Battery 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., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02527 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

ACS Energy Letters

DOI: XXXX Article type: Letter

Stable Li Metal Anode with “Ion-solvent-coordinated” Nonflammable Electrolyte for Safe Li Metal Battery Lifen Xiao, *,† Ziqi Zeng, ‡ Xingwei Liu, ‡ Yongjin Fang, ‡ Xiaoyu Jiang, ‡ Yuyan Shao, § Lin Zhuang, ‡ Xinping Ai, ‡ Hanxi Yang, ‡ Yuliang Cao*,‡ and Jun Liu*,§

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, USA

§

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

1

ACS Paragon Plus Environment

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

ABSTRACT. 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 non-flammable electrolytes with wide salt to solvent ratio range and demonstrate that an ion-solvent-coordinated (ISC) non-flammable electrolyte—Lithium bis(fluorosulfonyl)imide (LiFSI)- triethyl phosphate (TEP) enables dendrite free Li metal plating/stripping on the Cu electrode with high CCE of 99.3% up to 350 cycles. The understanding of the fundamental electrochemistry tells us that the decrease of solvent activity in ISC electrolyte can improve reductive stability of the solvent on Li metal anode. The fundamental concept of ISC electrolyte developed in this work provides new perspectives for the development of high-stable and safe Li metal batteries.

2


Page 2 of 22

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

ACS Energy Letters

TOC GRAPHICS ISC Electrolyte

Smooth Li

Dilute Electrolyte

Dentritic Li

Decomposition product

3



Li 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 had been made in the last 60-80’s.3-5 However, the attempt for practical application had been 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 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 emergency of Li-ion batteries has largely diminished the development of Li metal batteries since the early 1990s. However, the energy density of Li-ion batteries shows limited room for improvement after three decades of development. Due to an increasing demand of advanced energy storage technologies to electrify transportation and large scale energy storage system, rechargeable Li metal batteries recently have again received much attention. Besides, Li metal anode enables the 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 4


Page 4 of 22

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

ACS Energy Letters

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 unable to build a robust SEI layer between Li anode and electrolyte. Therefore, constructing a high uniformity and stability SEI layer is essential for practical use of Li metal batteries. Many electrolyte additives are employed to facilitate the formation of stable SEI layer on Li metal anode, such as ﬂuoroethylene carbonate (FEC)9-10, LiNO310-11. Li metal anode with protective SEI layer exhibits a superior cycling performance in high-energy-density Li metal battery. Furthermore, unusual stability of Li anodes has been found in highly concentrated electrolyte with a variety of organic solvents12-15. For example, the Li metal anode exhibits high cycling coulombic efficiency (CCE) of 99.1% in 4 M LiFSI/DME electrolyte16. To improve the stability of carbonate electrolytes, 10 M LiFSI electrolyte was used to construct F-rich 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 be used in practical system. Although phosphate solvents are more reactive with Li metal anode due to the relatively lower Lowest Unoccupied Molecular Orbital (LUMO), the phosphate based electrolytes are found to be compatible with >4.0 V operation voltage.17 Solid electrolytes and ionic liquids are of particular interest since they can eliminate the safety issues such as flame or exploration, but they suffer from problems such as low 5



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 condition most of solvent molecules are coordinated with Li cations, which we call it as “ion-solvent-coordinated” (ISC) electrolyte here, can enable the utilization of nonflammable phosphate electrolytes (such as 1:2 Lithium bis(fluorosulfonyl)imide-triethyl phosphate (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 into graphite and improved reversibility of lithium metal anode. Moreover, the ISC electrolyte can move Li salt concentration outside the concentrated electrolyte region by tuning the molecular weight of solvent and salt (Equation S1), which is in favor of maintaining low viscosity and minimizing the cost of electrolyte.21 To further decrease salt concentration, a dilute high-concentration non-flammable electrolyte was proposed by containing 3.2 M LiFSI-TEP and bis(2,2,2-trifluoroethyl) ether (BTFE) diluter to form close to 1 M salt concentration and exhibit high CCE of 99%.22 Here we investigate more deeply the electrochemical reversibility of Li metal anode in wide MR range (1:5 ~ 1:1) and demonstrate that Li metal can be reversibly plated/stripped on a Cu current collector with extremely high CCE (99.3%) and long lifespan by using LiFSI-TEP (MR=1:1.5) ISC nonflammable electrolyte. This result demonstrates that rechargeable Li metal battery can also be constructed with high safety. 6


Page 6 of 22

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

ACS Energy Letters

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 oC for a week (Figure 1a). Great chemical stability of the Li metal foils in electrolyte is demonstrated with MR of 1:1.5 or higher as the surfaces of Li maintain smooth and shining. The electrochemical stability was measured by cyclic voltammetry (CV) using Pt microplate electrode (Figure 1b). It shows that all electrolytes are oxidation resistive over 5.0 V vs. Li/Li+. The plating and stripping peaks of Li metal become sharper with MR increasing, indicating 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 (ICE) in the 1:5, 1:3, 1:2, 1:1.5 and 1:1 electrolytes are 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 increase with MR increase and then slightly decrease in the 1:1 electrolyte due to the lowest ionic conductivity

and highest

viscosity. The polarization of the electrodes maintains identical after MR ≥ 1:2 (Figure 1c). Moreover, the nonflammable TEP electrolytes with MR≥ 1:2 have comparable conductivity (Figure S1) and viscosity for the applications of Li metal batteries with some highly concentrated electrolytes reported in previous literatures (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 7



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 1st cycle to > 99 % and remains 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. The 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 the plating/stripping curves exhibit similar electrochemical properties, implying that the electrode remains a stable interface structure after the formation of SEI film in the early cycles, only with slight increase of the electrochemical polarization. The super-high 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 very low CCE (< 40% over 12 cycles) (Figure S2a). With increasing MR, the CCEs improve greatly to 99 % for 1:2, 99.3 % for 1:1.5 and 99.2% for 1:1 as shown in Figure S2b, Figure 1d and Figure S2c, respectively.

8


Page 8 of 22

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

ACS Energy Letters

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 oC and storage time is 7 days. (b) The electrochemical windows of different MR electrolytes measured on Pt microelectrodes at a scanning rate of 50 mV s-1. (c) The 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. The (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.

The optical photoing and morphological imaging of the electrodes are a very efficient approach to directly observe the deposition of lithium metal. After the initial plating and stripping of lithium metal in the 1:5, 1:3, 1:2 and 1:1.5 electrolytes respectively, the Li|Cu coin cells were disassembled to collect the Cu foils for observation by digital camera and scanning electron microscopy (SEM) without 9



exposure to air (Figure 2 and S3). From the optical photos, the surface of the Cu foil after the initial plating in the 1:5 electrolyte shows no sign of Li metal (Figure 2a), indicating totally failed Li plating. When MR is 1:3, some part of the Cu foil presents a silver white color, showing inhomogeneous plating of Li metal (Figure S3a). When MRs are 1:2 and 1:1.5, uniform shining white layers are observed in Figure S3b and Figure 2b, indicating very successful Li plating is achieved. The optical images of the Cu electrodes after the first cycle stripping in the low MR electrolytes (1:5 and 1:3) (Figure 2c and S3c) show plenty of black residuals. As MR increases to 1:2 and 1:1.5, the residuals become less and the coloration becomes lighter with the Cu electrodes can be clearly observed (Figure S3d and Figure 2d), indicating that the plated lithium metal is almost completely stripped off and the quantity of the electrolyte decomposition product is much less than those in the low MR cases. Therefore, the optical visualization clearly demonstrates that the TEP solvent as well as salt in the electrolyte decomposition is largely depressed in the high MR electrolytes. The SEM image of the first Li plated Cu foil in the low MR of 1:5 electrolyte shows some ultra-small and non-conducting particles (Figure 2e), suggesting excess decomposition of the electrolyte. In the 1:3 electrolyte (Figure S3e), Li dendritic clusters can be found. As MRs are 1:2 and 1:1.5, Li blocks in nodule-like structure with round-shaped edges are plated (Figure S3f and Figure 2f). The dimension of the Li particles is around 1-2 µm in the 1:2 electrolyte (Figure S3f) and increases to several microns as MR is 1:1.5 (Figure 2f). The cross section of the Cu electrodes after the initial Li plating in the 1:5 electrolyte shows ultrasmall particles and a layer 10


Page 10 of 22

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

ACS Energy Letters

of thin film (Figure 2i) while in the 1:1.5 electrolyte exhibits clearly the growth of large Li metal block (Figure 2j). The surface of the Cu electrode after the first stripping in the 1:5 electrolyte shows some small particles and dendritic structure are left (Figure 2g). In contrast, in the 1:1.5 electrolyte, the surface of the Cu electrode can be clearly observed while only few residues were detected (Figure 2h). Therefore, these observations illustrate that excessive TEP decomposition and dendritic Li growth occur in low MR electrolyte, while highly efficient SEI layer and non-dendritc Li plating happen in high MR electrolyte. Besides, the in-situ Li plating for 30 min more vividly demonstrates that only plenty of electrolyte decomposition products are recorded in the 1:5 electrolyte (Figure 2k), in a sharp contrast to the lithium metal growth in the 1:1.5 electrolyte (Figure 2l).

11



Figure 2. Optical and SEM images of the 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 1: 1.5 electrolyte. (k), (i) In situ optical microscopic images for the Li deposition on a Cu electrode at 0.15 μA for 30 min in the 1: 5 (k) and 1:1.5 (l) electrolyte.

Figure 3 shows the optical and SEM images of the Cu electrodes after 20 Li plating and stripping cycles in the 1:1.5 LiFSI-TEP electrolyte. A compact dark SEI layer is found to coat on the Cu electrodes both after stripping and plating (Figure 3a and b), which is different from the observations of the initial plating (a silver-color Li metal layer, Figure 2b) and striping (a thin and sparse dark SEI layer, Figure 2d). The SEM images for the plated and stripped Cu electrodes also display similar surface 12


Page 12 of 22

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

ACS Energy Letters

morphologies (Figure 3c and d). Li metal blocks are not found on the plated Cu electrode (Figure 3c), indicating that Li metal grows beneath the SEI layer. However, the cross-section images of the plated and stripped Cu electrodes exhibit obvious difference (Figure 3e and f). A deposition layer of ~15 μm can be found after plating (Figure 3e), which is reduced to ~10 μm after stripping (Figure 3f). Besides, some large particles can be observed in the deposition layer after plating (Figure 3e), but only uniform small particles present after stripping (Figure 3f). This indicates that Li metal grows beneath or in the crevice of the SEI layer while not on the surface, originating from non-conductive SEI film. Obviously, this growth model is advantageous to impede Li dendrite formation due to the protection or hindrance of the SEI film.

Figure 3. Optical and SEM images of the Li metal plating and stripping on a Cu electrode at the 20th cycle in the 1:1.5 LiFSI-TEP electrolyte. (a), (c), (e) plating. (b), (d), (f) stripping.

The EIS analysis further demonstrates the structural characteristics of the Li metal plating and stripping (Figure S4). It can be found that in the 1:1.5 electrolyte the 13



impedance of the initial Li plated electrode is much higher than those of Li stripped electrodes after the first, 10th and 100th cycle, implying that the Li stripped electrode shows only the resistance of the SEI layer due to the complete stripping of Li metal, while the plated electrode exhibits the whole resistance raised from the SEI layer and electrochemical reaction. After 10 cycles, the SEI layer resistance almost keeps unchanged, suggesting that very smooth electrolyte transport despite slight increase in thickness of the SEI layer as can be visualized in Figure 3. However, the impedance (420 Ω) of the initial plated electrode in the 1:5 electrolyte is much higher than that (120 Ω) in the 1:1.5 electrolyte. Moreover, much larger impedance (700 Ω) is detected after the initial stripping in the 1:5 electrolyte, indicating that more dense and nonconductive layer is formed on the Cu substance, in accordance with the optical and SEM observations (Figure 2c and g). The low and stable impedance characteristic in the high MR electrolyte is apparently ascribed to the enhanced tolerance of the electrolyte against decomposition. For a deep understanding of the compositions of the SEI layer, we carefully analyze the surface products on the Cu electrodes after the initial stripping by energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS), with the results shown in Tab. S3 and Figure S5. Apparently, the presence of P signal can only originate from the TEP solvent decomposition, and the F and S signals only root from the LiFSI salt decomposition. The surface products on the Cu electrodes in the low MR (1:5 and 1:3) electrolytes show high-content P and low-content F and S (Table S3), clearly mainly derived from the TEP decomposition. In contrast, the surface 14


Page 14 of 22

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

ACS Energy Letters

products on the Cu electrodes in the high MR (1:2, 1:1.5 and 1:1) electrolytes show high-content F and S corresponding to the LiFSI decomposition while no P content, indicating the TEP decomposition was suppressed to a large extent. Detailed surface analyses of the XPS spectra of the Cu electrode after 100 cycles in the 1:1.5 electrolyte are shown in Figure S5. The XPS spectra in the whole energy rang (Figure S5a) shows strong signals of F, N, S, O and C, while very weak signal of P, also indicating neglectable decomposition of TEP during the repeated plating/stripping cycles in the high MR electrolyte. In the F 1s spectra (Figure S5b), the peak at around 685 eV is attributed to LiF, while the peak at 688.2 eV most likely corresponds to LiFSI.24 The peaks at around 166.7 eV and 168.5 eV, 170 eV and 171.2 eV in the S 2p spectra (Figure S5c) are assigned to the S=O in LiSON (Li sulfur oxynitride),13 LiFSI,24 respectively. In the N 1s spectra (Figure S5d), the peaks at 398.5 and 400 eV can be assigned to S-N of LiSON22 and LiFSI.24 The strong signals of F, S and N demonstrate that in the high MR electrolyte most of the decomposition products are originated from the LiFSI salt while the reaction of the TEP solvent is largely impeded. The Li plating/stripping performance at 40 oC and 60 oC in the 1:1.5 electrolyte at a constant current density of 0.2 mA cm-2 was also measured with Li|Cu coin cells as shown in Figure S6. At 40 oC, the cell gives an ICE of 93.7 %, the CCE gradually increases and maintains at 98.7% thereafter (Figure S6b). At 60 oC, the ICE is 94.5%, then CCE reaches >99% after 30 cycles (Figure S6d). Therefore, this high MR LiFSI-TEP electrolyte displays stable chemical and electrochemical stability even at 15



high temperature and supports excellent Li plating and stripping performance. According to the above morphological and electrochemical investigations, it is concluded the high-efficient and stable Li plating/stripping in the high MR LiFSI-TEP electrolyte mainly originates from the suppression of the TEP solvent decomposition reaction. As is known that, in electrolyte the Li ions are all coordinated with the solvent molecules to form solvate. Based on the general coordinating number (n=4) of Li ion, a theoretic depletion of the free (uncoordinated) solvent molecules occurs in electrolyte with MR=1:4. Therefore, in low MR electrolyte, except a part of the TEP solvent molecules are coordinated with Li ions, there are plenty of free solvent molecules in the electrolyte, which are readily reactive with Li metal. With MR increasing, the free TEP becomes increasingly scarce with even 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 Equation 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, then becomes flat close to 0.01 when MR is  1:3. This leads to negative shift of the reduction potential of the solvent molecules, i.e., the stability of TEP against reduction enhances with MR increase. So, it can be known that low activity of TEP is due to the formation of the ISC structure in high MR electrolytes to decrease free TEP molecular, which is a main reason for improving the stability of the solvent. Meanwhile, the redox potential of Li/Li+ shifts towards more positive potential with increasing MR (Figure S7), suggesting lower 16


Page 16 of 22

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

ACS Energy Letters

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 (enhance 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, respectively. 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).

17



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

In summary, an ISC nonflammable LiFSI-TEP electrolyte has demonstrated high chemical and electrochemical stability, which is attributed to the enhanced reduction stability of TEP and reduced reactivity of metal Li resulting from the coordination of most of 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 battery with high reversibility and safety. Additionally, this study is conducted using coin cell configurations with more than adequate amount of electrolytes. The properties of Li plating and stripping need to be further verified in full cells using lean electrolytes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:XXXX. Experimental Section and additional characterization data AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (L.X.) * E-mail: [email protected] (Y.C.) * E-mail: [email protected] (J.L.) 18


Page 18 of 22

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

ACS Energy Letters

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (No. 2018YFB0104300), the National Science Foundation of China (Nos. 21673165, 21373155 and 21333007). J. Liu acknowledges the 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.

REFERENCES (1) Whittingham, M. S. History, Evolution, and Future Status of Energy Storage. Proc. IEEE 2012, 100, 1518-1534. (2) Aurbach, D.; Cohen, Y. The Application of Atomic Force Microscopy for the Study of Li Deposition Processes. J. Electrochem. Soc. 1996, 143, 3525-3532. (3) Brandt, K. Historical Development of Secondary Lithium Batteries. Solid State Ionics 1994, 69, 173-183. (4) Tarascon, J.-M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359-368. (5) Satter, R. Effects of Light-Dark Cycles. Science (Washington, DC, U. S.) 1976, 192, 1226. (6) Zhang, J.; Xu, W.; Henderson, W. A. Li Metal Anodes and Rechargeable Lithium Metal Batteries; Springer International Publishing, Cham, Switzerland; 2017. (7) Ji, X.; Lee, K. T.; Nazar, L. F. A Highly Ordered Nanostructured Carbon–Sulphur Cathode for Lithium–Sulphur Batteries. Nat. Mater. 2009, 8, 500. (8) Lee, J.-S.; Tai Kim, S.; Cao, R.; Choi, N.-S.; Liu, M.; Lee, K. T.; Cho, J. Metal–Air Batteries with High Energy Density: Li–Air versus Zn–Air. Adv. Energy Mater. 2011, 1, 34-50. (9) Zhang, X.-Q.; Cheng, X.-B.; Chen, X.; Yan, C.; Zhang, Q. Fluoroethylene Carbonate Additives to Render Uniform Li Deposits in Lithium Metal Batteries. Adv. 19



Funct. Mater. 2017, 27, 1605989. (10) Zhang, X.-Q.; Chen, X.; Cheng, X.-B.; Li, B.-Q.; Shen, X.; Yan, C.; Huang, J.-Q.; Zhang, Q. Highly Stable Lithium Metal Batteries Enabled by Regulating the Solvation of Lithium Ions in Nonaqueous Electrolytes. Angew. Chem. Int. Ed. 2018, 57, 5301-5305. (11) Yan, C.; Yao, Y.-X.; Chen, X.; Cheng, X.-B.; Zhang, X.-Q.; Huang, J.-Q.; Zhang, Q. Lithium Nitrate Solvation Chemistry in Carbonate Electrolyte Sustains High-Voltage Lithium Metal Batteries. Angew. Chem. Int. Ed. 2018, 130, 14251-14255. (12) Sodeyama, K.; Yamada, Y.; Aikawa, K.; Yamada, A.; Tateyama, Y. Sacrificial Anion Reduction Mechanism for Electrochemical Stability Improvement in Highly Concentrated Li-Salt Electrolyte. J. Phys. Chem. C 2014, 118, 14091-14097. (13) Wang, J.; Yamada, Y.; Sodeyama, K.; Watanabe, E.; Takada, K.; Tateyama, Y.; Yamada, A. Fire-extinguishing organic electrolytes for safe batteries. Nat. Energy 2018, 3, 22-29. (14) Jiao, S.; Ren, X.; Cao, R.; Engelhard, M. H.; Liu, Y.; Hu, D.; Mei, D.; Zheng, J.; Zhao, W.; Li, Q.; et al. Stable Cycling of High-Voltage Lithium Metal Batteries in Ether Electrolytes. Nat. Energy 2018, 3, 739-746. (15) Fan, X.; Chen, L.; Ji, X.; Deng, T.; Hou, S.; Chen, J.; Zheng, J.; Wang, F.; Jiang, J.; Xu, K.; ea al. Highly Fluorinated Interphases Enable High-Voltage Li-Metal Batteries. Chem 2018, 4, 174-185. (16) Qian, J.; Henderson, W. A.; Xu, W.; Bhattacharya, P.; Engelhard, M.; Borodin, O.; Zhang, J. G. High Rate and Stable Cycling of Lithium Metal Anode. Nat. Commun. 2015, 6, 6362. (17) Burrow, P. D.; Gallup, G. A.; Modelli, A. Are There π* Shape Resonances in Electron Scattering from Phosphate Groups? J. Phys. Chem. A 2008, 112, 4106-4113. (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 20


Page 20 of 22

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

ACS Energy Letters

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.

21



282x190mm (150 x 150 DPI)


Page 22 of 22

Stable Li Metal Anode with âIon-solvent-coordinatedâ Nonflammable

Recommend Documents

Stable Li Metal Anode with âIon-solvent-coordinatedâ Nonflammable