Multifunctional artificial solid electrolyte interphase layer for lithium metal anode in carbonate electrolyte

Multifunctional artificial solid electrolyte interphase layer for lithium metal anode in carbonate electrolyte

Solid State Ionics 344 (2020) 115095 Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi Mul...

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Solid State Ionics 344 (2020) 115095

Contents lists available at ScienceDirect

Solid State Ionics journal homepage: www.elsevier.com/locate/ssi

Multifunctional artificial solid electrolyte interphase layer for lithium metal anode in carbonate electrolyte

T

Qin Rana, Chongyu Hana, Anping Tanga, Hezhang Chena, Zilong Tanga, Kecheng Jiangb, ⁎ Yongjin Maic, Jinglun Wanga, a Key Laboratory of Theoretical Organic Chemistry and Functional Molecule, Ministry of Education, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China b TAFEL New Energy Technology Inc., Nanjing, Jiangsu 211113, China c School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Lithium metal anode Artificial solid electrolyte interphase Soaking method Allyl glycidyl ether

The practical application of Li metal in rechargeable batteries is seriously hindered by its irregular dendrite growth resulting in inferior cycling efficiency and poor safety. Herein, a multifunctional artificial solid electrolyte interphase (SEI) layer is constructed on lithium surface via chemical reactions between lithium metals and allyl glycidyl ether (AGE). The protected layer is mainly composed of cross-linked polyethylene chains and polyethylene oxide moieties, which significantly improved the cycling performance of Li|Li symmetric cells and high voltage Li|NCM523 (4.5 V) cells in the electrolyte of 1 M LiPF6 in EC/DMC/EMC (1:1:1, by wt.). Electrochemical impedance spectroscopy (EIS) and scanning electron microscope (SEM) measurements confirm that the protected layer could reduce the morphology changes and suppress the parasitic reactions of lithium anodes effectively. A probable mechanism based on polymerization and cross-linking reactions of AGE is proposed for the formation of artificial SEI layer. These results demonstrated that the multifunctional artificial solid electrolyte interphase (SEI) layer have considerable potential application to protect lithium metal anode.

1. Introduction As the growing demand for lithium-ion batteries (LIBs) in such applications as advanced consumer electronics, energy storage system, plug-in hybrid electric vehicles and electric vehicles, there is an urgent need for higher energy density devices [1,2]. Lithium metal anode is considered as the “Holy Grail” for high energy electrochemical energy storage devices because of its low reduction potential (−3.04 V vs standard hydrogen electrode) and high theoretical specific capacity (3860 mAh/g) [3,4]. However, the practical application of Li metal in rechargeable batteries is seriously hampered by its irregular dendrite growth because of the microscopic protrusions, non-uniform electric field distribution, as well as uneven supply of metal ion flux [5]. The past few years has witnessed the continuous efforts to suppress dendrite growth for achieving long-term stability of Li anodes [6–8], such as building artificial protection layer [9–11], optimizing the electrolyte formulation [12–14], adopting solid electrolytes [15,16], using functional separator [17,18], designing composite electrode [19,20] or three dimensional lithium-hosted materials [21–23], etc. Most recently, soaking-based method, which is an extremely simple,



effective and industrially applicable lithium metal pretreatment process, show remarkable progress for constructing a promising artificial protection layer [24,25]. For example, immersing the lithium metal electrodes in ionic liquid electrolytes based on N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([C3mPyr+][FSI−]) for 12 days could create a durable and Li+ permeable SEI film that allows safe charge-discharge cycling of commercially applicable Li|LiFSI/ [C3mPyr+][FSI−]|LiFePO4 cells for 1000 cycles with coulombic efficiencies > 99.5% [26]. Carbonate based electrolyte with high oxidation potential is attractive for high energy density batteries using high voltage cathode [27], while ethereal electrolytes solvent demonstrated to be more compatible with Li metal anodes [28–29]. It has been reported that cyclic ether, such as 1,3-dioxolane (DOL) [30] and 1,4-dioxane(DOA) [31], can be polymerized to form a flexible elastic SEI protective layer with short chain oligomers of poly(ethylene oxide) on the surface of Li metal that can accommodate the surface morphology changes of the anode upon cycling. What's more, different kind of polymerization methodologies for these ethereal solvents have been successfully developed, such as electrochemical reduction [32], electrochemical

Corresponding author. E-mail address: [email protected] (J. Wang).

https://doi.org/10.1016/j.ssi.2019.115095 Received 15 August 2019; Received in revised form 18 September 2019; Accepted 9 October 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. The influence of soaking time on the cyclic performance of Li|Li symmetric cells in the electrolyte of 1 M LiPF6 in EC/DEC/EMC (1:1:1, by wt.).

polishing with a fixed stripping-plating procedure [33,34], electrochemical oxidation [35], or in situ chemical reactions [31] to control and optimize the formation of a smooth and uniform SEI layer. Based on the consideration of the cross-linked SEI film should be more stable than the short chain oligomers of poly(ethylene oxide), allyl glycidyl ether (AGE) as a functionalized cyclic ether is investigated as a cross-linking artificial SEI film forming agent for lithium metal anode in this work. The protected SEI layer, mainly composed of crosslinked polyethylene chains and polyethylene oxide moieties, demonstrated much better cycling performance for symmetric Li|Li cells and 4.5 V Li|NCM523 cells compared with those of the reference cells in carbonate based electrolyte. The designed SEI layer can better tolerate the volume change and suppress dendrite growth during cycling, which may greatly enhance its cycle life. Systematic electrochemical characterizations and surface morphology tests were carried out, and possible polymerization mechanism was proposed to understand how the artificial SEI formation and take effect for lithium metal anode. As far as we known, this is the first report that using multifunctional cyclic ether as better artificial SEI film forming agent for long-term stability of Li anodes in carbonate based electrolytes.

The typical load of active material is about 3 mg/cm2 for NCM523 electrode. 2.3. Measurements and characterizations Li|Li, AGE-Li|AGE-Li, Li|NCM523, and Li|Cu 2032-type coin cells were assembled in an argon-filled glove box (MBRAUN, H2O and O2 < 0.1 ppm), the electrolytes were fixed to be 100 μL per cell. Electrochemical cycling tests were performed on Neware-CT4008 testers at room temperature. In this study, three duplicated cells have been tested for each data on cycling performance. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested on Gamry Interface 1000E electrochemical workstation. The scanning rate of CV is 20 mV/s with the voltage range from the open circuit voltage to −0.5 V for Li|Cu cell. The EIS was conducted at open circuit voltage with 5 mV AC amplitude applied in a frequency window between 0.01 Hz-1 MHz. Attenuated total reflection flourier transformed infrared spectroscopy (ATR-FTIR) measurements were recorded on a Nicolet 6700 spectrometer (Thermo Fisher) from 4000 to 400 cm−1 at a resolution of 2 cm−1. The symmetric cells were disassembled in a glove box and the Li electrodes were rinsed with DMC for several times, and then dried for 6 h under reduced pressure for the surface analysis. Scanning electron microscope (JSM-6380LV) equipped with an energy dispersive spectroscope detector was used to inspect the surface morphology and chemical element of the electrodes.

2. Experimental 2.1. Materials and chemicals Lithium foil (15.8 × 0.6 mm) was purchased from Shenzhen Biyuan Electronics Co., China; Allyl glycidyl ether (AGE, from Macklin) was distilled with CaH2 before use, and its water content (17 ppm) was characterized by coulometric Karl Fischer titration; 1 M LiPF6 in EC/ DMC/EMC (1:1:1, by wt.) was commercially available from Zhangjiagang Guotai-Huarong Co., China; Celgard2400 microporous polypropylene membrane was used as separator; LiNi0.5Co0.2Mn0.3O2 (NCM523) was obtained from Tafel New Energy Technology Inc.

3. Results and discussion To obtain an optimized artificial SEI film, the effect of different soaking time on the cycle performance of Li|Li symmetric cell was conducted with a current density of 1 mA/cm2 and a capacity of 1 mAh/ cm2. As shown in Fig. 1, the Li electrode with AGE treatment for 0.5 min, 2 min, 5 min, 10 min, 15 min and 30 min can be stably cycled under the polarization potential of 0.1 V for 78 cycles,106 cycles, 88 cycles, 82 cycles, 84 cycle, and 77 cycles, respectively. It can be seen that the soaking time influence the performance of Li symmetric cell significantly, which may related to the thickness of the artificial SEI film. Subsequently, the cycle performances of Li|Li symmetric cells with/ without AGE-treated Li electrode in the electrolyte of 1 M LiPF6 in EC/ DEC/EMC (1:1:1, by wt.) were compared under the same conditions (Fig. 2). The Li|Li symmetric cell with the Li electrode soaked in AGE

2.2. Electrode preparation For the preparation of AGE-Li electrode, the lithium sheet was immersed in AGE solution for a certain period of time, then taken out and volatilized naturally at room temperature. Working electrodes were prepared by mixing NCM523, Super P and polyvinylidene fluoride at a weight ratio of 80:10:10 in N-methyl-2-pyrrolidone, then the slurry was spread onto aluminum foil and dried overnight at 80 °C under vacuum. 2

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Fig. 2. Comparison study voltage profiles (a) and voltage hysteresis (b, c) of symmetric Li cells with or without AGE soaked Li electrode.

1 mA/cm2 and a capacity of 1 mAh/cm2 (Fig. 4). The typical Nyquist plots of the EIS measurement of the cells were similar with one big depressed semicircle and a straight line in the low frequency region, it has been widely recognized that the high frequency semicircle was attributed to the lithium transport through the SEI film of lithium electrode. For the cell without AGE treatment, the radius of the semicircle for Li|Li cell decreased throughout its first thirty cycles followed by the radius increments remarkably, which is consistent with the activation process of the charge-discharge curves. In contrast, the radius of the semicircle for AGE-Li|AGE-Li cell decreased gradually during the test number of cycles (Fig. 4b), which is also consistent with the variation of the value of polarization potential as shown in Fig. 2c. Notably, AGE-Li|AGE-Li cell displayed smaller radius of the semicircle compared to that of Li|Li cell before cycling, which may be ascribed to the lithiophilic poly(ethylene oxide) moieties formed on the surface of Li anode after AGE treatment. In our duplicated experiments, it was confirmed that AGE soaking method effectively lowered the impedance of the Li|Li symmetric cells, which means that higher Li+ ion migration rate and better kinetic characters can be achieved. This could be the reason why superior cyclic performance observed for the AGE-Li|AGELi cells. To investigate the morphology change of Li electrode after stripping and plating, SEM analysis was carried out for Li electrode from top view and cross-section view after certain cycles (Fig. 5). After 38 cycles with a capacity of 1 mAh/cm2 (to a thickness of 4.8 μm) at a current density of 1 mA/cm2, the Li electrode without AGE treatment become porous and rough moss and showed evident cracks on the surface from the top view (Fig. 5a). Meanwhile, the total thickness of lithium electrode expanded to 662 μm could be observed from the side view (Fig. 5b), wherein the thickness of the loose layer of the lithium electrode is 339 μm, indicating that continuous SEI film formation and thickness expansion of the lithium electrode during cycle process. Even after 137 cycles under the same cycling condition, the Li electrode with AGE treatment exhibited a smooth and dense surface morphology and the SEI film had a very similar volume expansion compared with the

Fig. 3. Cycling performance of AGE-Li|AGE-Li symmetric cells (soaked in AGE for 5 min) under different current density with the capacity of 1 mAh/cm2.

for 2 min can stably run for > 300 h, while the cell without AGE treatment only steady cycled for 80 h, after which the voltage begins to increase gradually (Fig. 2a). Simultaneously, the AGE-Li|AGE-Li cell exhibited small hysteresis voltage of 0.19 [email protected] cycles with low growth rate (Fig. 2c), while the polarization voltage of the comparison Li|Li cell increased rapidly from 0.3 [email protected] cycles to 0.78 [email protected] cycles (Fig. 2b), which may relate to the continuous SEI break down and electrolyte decomposition. What's more, AGE-Li|AGE-Li symmetric cell under the current density of 0.5 mA/cm2 with the capacity of 1mAh/ cm2 can be stably cycled > 400 h even with the AGE soaking time prolonged for 5 min (Fig. 3). Therefore, an effective SEI could be formed on the surface of Li metal by the simple strategy of immersing treatment with AGE. The EIS measurements of Li|Li cells with/without soaking Li electrodes in AGE were tested after different cycles with a current density of 3

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Fig. 4. EIS measurements of Li|Li cells with/without soaking Li electrodes in AGE after different cycles.

originate from Li2CO3, Li2O, or LiOH formed in the massive production of lithium foil or EDS sample preparation process. For ATR-FTIR analysis (Fig. 6b), the distinct peak of AGE at 2925 cm−1 (υC=C), 1646 cm−1 (υC=C), 922 cm−1 (υC=C), 3068 cm−1(υC-H of epoxide), and 995 cm−1 (υC-O of epoxide) were disappeared on the surface of Li electrode after soaking for 55 h. Instead, new band at 2898 cm−1 (υC-H) and 1456 cm−1 (υC-C) probably belongs to the feature peaks of the polymerization product of CH2 = CH-, the intensive peak at 1102 cm−1 (υCO) may ascribe to the ring-opening cross-link of epoxide moieties. The IR bands of Li2CO3 at 1719 cm−1, 1523 cm−1, 843 cm−1 are also observed for FT-IR analysis. Apparently, AGE is involved in the formation of an artificial SEI film on the surface of Li electrode with a unique

reference Li|Li cell after 38 cycles, as shown in Fig. 5c and Fig. 5d. Combined with the EIS results, it can be deduced that the surface of the lithium electrode treated by AGE soaking may form a lithiophilic and elastic SEI film facilitating Li+ transportation and suppressing volume expansion. To verify the artificial film formation on the surface of lithium electrode after immersing treatment, EDS and ATR-FTIR analysis were conducted (Fig. 6). In EDS measurement (Fig. 6a), the carbon signal at 0.27 keV with an element weight of 35.6% related to oxygen atoms is observed on the surface of lithium metal electrode with the AGE immersing time of 80 h; a carbon intensity of 12.1% has also been detect for the pristine lithium electrode without AGE treatment, which may

Fig. 5. Top view and cross-section view SEM images of (a, b) Li metal after 38 cycles, (c, d) AGE-Li metal after 137 cycle in the electrolyte of 1 M LiPF6-EC/EMC/DEC with a capacity of 1 mAh/cm2 at a current density of 1 mA/cm2. 4

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Fig. 6. (a) EDS and (b) ATR-FTIR analysis of the SEI film on the surface of lithium metal.

chemical structure, which is attributed to the significantly improvement of the cycle performances of Li|Li cells. To understand the reduction ability of AGE, the energy values of the lowest unoccupied molecular orbital (LUMO) for ethylene oxide(EO), 1,3-dioxlane(DOL), tetrahydrofuran (THF), 1,4-dioxane(DOA), tetrahydropyran(THP) and allyl glycidyl ether (AGE) were calculated by Chem3D Ultra Software using a molecular orbital package (MOPAC), which is a general purpose semi-empirical molecular orbital program for the study of chemical reactions involving molecules, ions, and linear polymers. Table 1 shows the calculated LUMO energies of the cyclic ether compounds, which is generally associated with the anodic reactivity. According to the calculated results of the LUMO energy, EO possesses the highest reduction activity among the mono‑oxygen-substituted cyclic ethers (LUMOTHP > LUMOTHF > LUMOEO), which may relate to its chemical structure with highly strained three membered ring. Compared with the mono‑oxygen-substituted THF and THP, the introduce of the second oxygen atom into the cyclic ether results in increasing their reduction activity correspondingly (LUMOTHF > LUMODOL, LUMOTHP > LUMODOA). Deduced from the LUMO energy level of LUMODOA > LUMOEO > LUMODOL, EO may also be reduced to form elastic oligomers of poly(ethylene oxide) similar to DOL and DOA when used as electrolyte solvent for lithium metal anode. It is to be pointed out that AGE as a functionalized analogue of EO, the introduction of allyl group leads to the reduction of the LUMO energy remarkably (LUMOEO, 2.71 ev > LUMOAGE, 1.14 ev). These calculated data are well supported by the experimental results of reductive decomposition potentials for AGE as additive in the electrolyte of 1 M LiPF6 in EC/DEC/EMC(1:1:1, by wt.). The reduction potential of AGE was verified by cyclic voltammetry test as shown in Fig. 7. By adding AGE into the blank electrolyte, a new reduction peak at the voltage around 1.07 V has been observed and the intensity of the peak increased evidently as the adding amount of AGE increasing from 1 vol % to 5 vol%. It is worth to be mentioned that the lithium plating/ stripping current density increased significantly as with 5 vol% AGE addition. Based on the calculated date and the CV results, the higher

Fig. 7. CV curves of the electrolyte with or without AGE addition for Li|Cu cells.

reductive decomposition potential of 1.07 V (vs. Li+/Li) for AGE makes it as promising film forming reagent for lithium metal anodes. Based on the above analysis, we proposed a reaction mechanism on the artificial SEI film formation reaction on lithium metal surface, as can be seen in Fig. 8. Firstly, in the present of the highly reductive species (Li metal) or alkaline compounds (such as LiOH, Li2CO3, Li2O), the chemical induced polymerization reaction may be taken place at the position of CH2 = CH– or epoxide moiety. Then, under the electrochemical reduction condition, cross-linking reaction might be happen to form a network film with elastic (polyethylene, red line in Fig. 8) and Li+ conductive (polyethylene oxide, blue line in Fig. 8) characteristics, which is a desirable structure for protecting lithium electrode for large volume expansion and Li+ conduction during plating and stripping

Table 1 Calculated LUMO energy of cyclic ether. Compound

Structure

LUMO/ev

Compound

Tetrahydropyran (THP)

3.18

1,4-Dioxane (DOA)

2.83

Tetrahydrofuran (THF)

3.11

1,3-Dixolane (DOL)

2.47

Ethylene oxide (EO)

2.71

Allyl glycidyl ether (AGE)

1.14

5

Structure

LUMO/ev

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Fig. 8. Possible reaction mechanism to form network film on lithium metal anode.

soaking treatment. With the optimized immersing time of 2 min, the AGE pretreated Li electrode demonstrated stable cyclability for AGELi|AGE-Li cells over 300 h with a capacity of 1 mAh/cm2 (to a thickness of 4.8 μm) at a current density of 1 mA/cm2 and superior capacity retention for 4.5 V AGE-Li|NMC523 cells in the electrolyte of 1 M LiPF6 in EC/DMC/EMC (1:1:1, by wt.). Probable reaction mechanism was proposed through polymerization and cross-linking reaction, producing multifunctional SEI with elastic polyethylene chains and Li+ conductive polyethylene oxide moieties. This strategy for surface modification of lithium metal anode is of great promising to benefit next generation lithium metal rechargeable batteries and beyond. Further investigation and manipulation of the cross-linking using multifunctional cyclic ether are crucial to the success of lithium metal anode protection, and it will be a continuing focus of our research. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 9. Cycling performance of 4.5 V Li|NCM523 cells with/without Li electrodes soaked in AGE using LiPF6-carbaonate based electrolyte.

Acknowledgments

process. It is well known that the LiPF6-carbaonate based electrolyte can endure high oxidative potential compared with ether based electrolyte of 1 M LiTFSI in DOL/DME (1:1, by wt.). Herein, the cycling performance of 4.5 V Li|NCM523 cell with/without Li electrodes soaked in AGE in the electrolyte of 1 M LiPF6 in EC/DMC/EMC (1:1:1, by wt.) were collected as shown in Fig. 9. Both the cells exhibited high initial specific capacity of 187 mAh/g with the initial coulombic efficiency of 87% for Li|NCM523 and 83% for AGE-Li|NCM523, respectively. The low initial coulombic efficiency of AGE-Li|NCM523 possibly related to the cross-linking reaction to form a network film on Li metal surface, as described in Fig. 8. It turned out that the cell with AGE treatment displayed an improved capacity retention of 15% compared with that of Li|Li cell after 47 cycles at the current rate of 0.2 C. Further work on the electrolyte formulation should be carried out to obtain a better cycling stability.

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