Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 2479−2489
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Flexible Artificial Solid Electrolyte Interphase Formed by 1,3Dioxolane Oxidation and Polymerization for Metallic Lithium Anodes Cheng Li, Qing Lan, Yifu Yang,* Huixia Shao, and Hui Zhan
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College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China ABSTRACT: Lithium−tin (Li−Sn) alloys are perfect substrate materials for anodes in high-energy density lithium metal secondary batteries. A new approach is proposed to further prevent the Li deposit on Li−Sn alloy substrates from reaction with electrolytes using an artificial solid electrolyte interphase (ASEI) based on electrochemical oxidation and polymerization of 1,3-dioxolane precursor with LiTFSI additive. This ASEI layer is flexible, stable, ion conductive, and electrically insulating, which can provide very stable cycling of Li−Sn alloy substrate anodes for Li deposition/ stripping with an average Coulombic efficiency of 98.4% at a current density of 1 mA cm−2. The Li−Sn alloy substrate is kept uniform and smooth without any dendrites and cracks after cycles. When the Li−Sn alloy substrate protected by ASEI is used as the anode of lithium−sulfur full cell, the cell shows much higher discharge capacity and better cycleability. This innovative and facile strategy of ASEI formation demonstrates a new and promising approach to the solution of the tough problems of Li dendrites in Li metal batteries. KEYWORDS: lithium metal anode, solid electrolyte interphase, Li−Sn alloy, DOL, lithium−sulfur batteries
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substrate surface and form NSEI.12−14 Unfortunately, NSEI is brittle which can be easily cracked, and cannot accommodate the morphology change of the anode because of the Li plating/ stripping on the Li−Sn alloy substrate. This leads to Li dendrite formation and to a loss of both Li and electrolyte solution to some extent due to the side reactions and the repair of the surface films, which further results in irreversible capacity fade, low CE, and even failure of the cell. Thus, designing a dense and flexible artificial solid electrolyte interphase (ASEI) layer upon the Li−Sn alloy should be a good strategy to overcome the most serious limitations of the NSEI. Most recently, examples of building an excellent ASEI for Li deposition show remarkable progress for Li metal anodes.15−28 For example, Chen et al.15 designed an ASEI layer based on in situ polymerization of ethyl α-cyanoacrylate precursor with LiNO3 additive. Later research by Noked et al.16 employed innovative approaches of self-healing electrochemical polymerization and atomic layer deposition and developed a hybrid organic/inorganic ASEI. Archer et al.17 created ASEI films by the in situ reaction of strong Lewis acid AlI3, Li metal, and aprotic liquid electrolytes. Recently, Huang et al.18 reported an ASEI consisting of a soft and compliant poly(vinylidene difluoride) (PVDF)−HFP organic matrix with LiF inorganic
INTRODUCTION Lithium metal is considered as an ideal anode material for rechargeable Li batteries and has been studied for over 40 years because of its high theoretical capacity (3860 mA h g−1), the lowest electrochemical potential, and low density.1,2 However, the growth and proliferation of Li dendrites caused by the inhomogeneous plating/stripping of Li and repeated breakage and repair of the naturally formed solid electrolyte interphase (abbreviate as NSEI) lead to the drying of electrolyte and serious corrosion of the bulk Li, which further result in increased irreversible capacity, low Coulombic efficiency (CE), and degeneration of Li metal batteries.3−6 One strategy to overcome the dendrite issue is replacing an organic liquid electrolyte with solid electrolytes.7 Another strategy is employing a non-Li substrate material instead of Li metal. Yang et al. demonstrated that Li−Zn8 and Li−Sn9−11 alloy substrate materials were admirable anodes to enable dense and dendrite-free Li deposition. Moreover, it is beneficial to save the Li resource because of the less use of Li element than full Li metal anode. Usually, the organic solvent is easy to be electrochemically reduced at the electrode/electrolyte interface when the electrode potential is below 1.0 V (vs Li/Li+). Therefore, when the Li−Sn alloy substrate is exposed to the electrolyte, immediate reactions occur between the freshly deposited Li metal on the Li−Sn alloy substrate and electrolyte species. The products by the side reactions of Li+ ions, anions, and organic solvents are insoluble and can be deposited on the Li−Sn alloy © 2018 American Chemical Society
Received: September 15, 2018 Accepted: December 17, 2018 Published: December 17, 2018 2479
DOI: 10.1021/acsami.8b16080 ACS Appl. Mater. Interfaces 2019, 11, 2479−2489
Research Article
ACS Applied Materials & Interfaces
Figure 1. LSV curves in the 0.5 mol L−1 LiTFSI/DOL electrolyte on the (a) Pt electrode, (b) Cu electrode, and (c) Sn electrode; LSV curves in the 0.5 mol L−1 LiClO4/DMC electrolyte on the (d) Pt electrode, (e) Cu electrode, and (f) Sn electrode. Potential scan rate: 1 mV s−1. (g) Digital photograph of the bare Sn metal and modified Sn (after the LSV test).
(CH2CH2OCH2O)nLi type. A surface film is thus formed by these short chain oligomers of poly(ethylene oxide) which is elastomeric and can accommodate the morphology change of the anode surface upon long-term cycling.5,33−35 In this work, we introduce a new countermeasure to enhance Li plating/ stripping cycle stability on the Li−Sn alloy substrate by facile electrochemical oxidation and polymerization of DOL. Unlike Lin and Zheng’s work,19,20 this polymer film is developed by anodic polarization at more positive potential than open circuit potential (OCP), making it different from the one formed by the reduction of DOL not only in its components, but also in its structure. Results show that the Li−Sn alloy substrate with this ASEI in the 1 mol L−1 LiTFSI/DOL−1,2-dimethoxyethane (DME) (1:1 v/v) electrolyte can provide a high average CE of 98.4% for Li plating/stripping, and effectively prevent Li from dendrite formation.
particles embedded in. Lin19 improved the Li−S battery performance through systematical pretreatment of the Li metal anode under controlled current. Also, Zheng20 created a nearperfect alkali metal anode surface based on electrochemical polishing as well as manipulation of electrolyte reduction processes. These strategies have presented a novel perspective to suppress side reactions and Li dendrite formation by forming an artificial protective layer on the Li metal. However, few reported publications about ASEI preparation on the nonLi substrate for Li metal plating/stripping can be found to our knowledge. Thus, designing flexible and stable polymer films on the Li−Sn alloy substrate should be a feasible route to construct the Li−Sn alloy substrate for Li anodes with stable cycling performance and high CE. 1,3-Dioxolane (DOL) is one of the commonly used electrolyte solvents demonstrated to be more compatible with Li metal anodes.29−32 As the Li metal anode is cycled in the DOL-based electrolyte, DOL is reduced on the Li metal surface to different alkoxy species, leading to anionic partial polymerization of DOL to form species of the LiO2480
DOI: 10.1021/acsami.8b16080 ACS Appl. Mater. Interfaces 2019, 11, 2479−2489
Research Article
ACS Applied Materials & Interfaces
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RESULTS AND DISCUSSION Formation of the DOL Polymer Films. It has been demonstrated that DOL is polymerized on the Li anode surface by the reduction reaction during Li plating/stripping, forming a NSEI on the Li surface.5 Although the NSEI layer with uncontrolled composition and morphology cannot offer a uniform and firm protective layer, this fact implies that electrochemical potential has influence on DOL polymerization. To find out the threshold potential for DOL polymerization, linear sweep voltammetry (LSV) measurement was performed on a three-electrode device which was made of transparent glass for convenient observation. Pt, Cu, and Sn disk electrodes were used as the working electrode and 0.5 mol L−1 LiTFSI/DOL was used as the electrolyte. The Sn electrode was prepared by deposition of the Sn metal on Cu foil with the method of ref 9. When the potential of the Pt working electrode was scanned negatively from OCP to 0 V (vs Li/Li+), no any cathodic peak was observed above the potential of Li deposition. Thus, a positive potential scan from OCP of the Pt working electrode was carried out at 1 mV s−1. As shown in Figure 1a, the electrochemical oxidation of DOL begins when the potential reaches 3.2 V, and the current increases as the potential changes to more positive values. It is clear that DOL solution is oxidized and polymerized on the Pt electrode surface at potential higher than 3.2 V. As the potential increases to 3.3 V, the current starts to drop and the current attains a small steady-state value at potential more positive than 3.5 V. It is because the polymer film is too thick to conduct the Li+ ion. On the basis of these results, it is inferred that an excellent polymer film with modest thickness and low interfacial resistance can be designed and prepared by taking potentiostatic control of the working electrode for appropriate time. The LSV test was also conducted with Cu and Sn electrodes. As shown in Figure 1b, the potential for the onset of the anodic current of the Cu electrode is 3.3 V. As the potential of Cu electrode increases from 3.3 to 3.6 V, the current also increases, and the current shows a sharp increase above 3.6 V. The current−potential curve on the Sn electrode is shown in Figure 1c. Two anodic peaks appear at potentials of 3.1 and 3.7 V, respectively. The current maintains a small steady-state value at potential more positive than 4.5 V. The reason for the small current value is also the DOL polymerization. To clarify the anodic processes occurring at the electrode/ electrolyte interface of Pt, Cu, and Sn electrodes during positive scanning of the electrode potential, LSV tests were also carried in the 0.5 mol L−1 LiClO4/DMC electrolyte. As shown in Figure 1d, the onset of the anodic current occurs when the potential of the Pt electrode is near 5.5 V. Therefore, the upper potential limit of the electrochemical window of the LiClO4/DMC electrolyte is 5.5 V. From Figure 1e, as the positive scan begins from OCP of the Cu electrode, only nonfaradaic current is observed at the initial stage. When the electrode potential reaches 3.6 V the oxidation starts with current increase, and this current should correspond to the dissolution of the Cu electrode. By similar way, we can find that the Sn metal dissolves at potential higher than 2.8 V as shown in Figure 1f. Thus, the electrochemical reactions corresponding to the curves in Figure 1b,c can be analyzed. In Figure 1b, electrochemical oxidation of DOL occurs on the Cu electrode as the potential increases above 3.3 V. When the potential increases to 3.6 V, the Cu metal starts to dissolve so
as the current increases more quickly. As shown in Figure 1c, the dissolution of Sn metal occurs over the potential range from 2.8 to 3 V. As the potential increases above 3 V, electrochemical oxidation of DOL occurs and the polymer film is formed on the Sn electrode surface in the 0.5 mol L−1 LiTFSI/DOL electrolyte. The interfacial resistance increases with the formation of the polymer film, leading to lower ionic conductivity and decreased current. When the potential of the Sn electrode changes from 3.3 V to more positive potential, the Cu metal, which is under the Sn layer with a small portion exposed to the electrolyte, starts to dissolve so as the current increases again. But at the same time, the polymerization of DOL is still going on. As the potential is higher than 3.7 V, the polymerization of DOL become the dominant reaction and the dissolution of Sn and Cu is suppressed which behaves as the current decrease. After the LSV measurements, the threeelectrode devices were dissembled in the pure Ar-filled glovebox. As shown in Figure 1g, a clear and colorless polymer layer is seen coated on the silvery white Sn metal surface and no bare Cu metal can be found. Inductively coupled plasmaatomic emission spectroscopy measurement of the electrolyte was carried out for the analysis of the concentration of Sn and Cu ions before and after the LSV test for the Sn working electrode, and the results are shown in Table 1. Both the Table 1. Concentration of Sn and Cu Ions in the 0.5 mol L−1 LiTFSI/DOL Electrolyte (mol L−1) before LSV test after LSV test
Sn
Cu
7.4 × 10−5 4.9 × 10−4
2.6 × 10−6 5.9 × 10−5
concentrations of Sn and Cu increase after the LSV test. Therefore, there are a little Sn and Cu metal dissolved during the LSV test. However, the dissolved Sn and Cu mass is low as compared to the total mass of the Sn and Cu substrates, so the dissolution should not affect the function of these metal substrates. Preparation of the DOL Polymer Film. The polymerization of DOL by electrochemical oxidation has been experimentally confirmed by LSV. The further work is to prepare the polymer films as the ASEI on the chosen substrate electrode surface by potentiostatic control of the working electrode. After repeated attempts, it is found that an excellent polymer layer can be prepared on the Cu electrode by applying a voltage of 3.3 V for 800 s. The electrochemical impedance spectra (EIS) of Cu|Li cells with three-electrode configuration were measured. The working electrode was the Cu metal with and without the DOL polymer film, and both the reference and counter electrodes were Li metal. As shown in Figure 2a, EIS of the Cu electrode without the polymer layer is a sloping line. Because the EIS measurements were conducted under an OCP condition, the potential of the bare Cu electrode is more positive with respect to the reference. Under this condition, the charge-transfer reactions cannot occur and no ion conductive layer will be formed on the Cu surface. Therefore, there is only a sloping line on the Nyquist plot. EIS of the Cu electrode coated with the DOL polymer film consists of a semicircle and a sloping line. The semicircle in the high frequency range can only be caused by the polymer layer, but not the chargetransfer reactions. The frequency of the top point on the semicircle is 1171 Hz and the one for the inflection point is 2481
DOI: 10.1021/acsami.8b16080 ACS Appl. Mater. Interfaces 2019, 11, 2479−2489
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Figure 2. Nyquist plots of three-electrode cells with and without the polymer film on the (a) Cu metal surface and the (b) Li−Sn alloy surface in the 0.5 mol L−1 LiTFSI/DOL electrolyte. (c) Equivalent circuit. Top-view SEM images of (d) pristine Cu foil and (e) Cu foil coated with the DOL polymer film. Cross-section SEM images of the (f,g) Li−Sn alloy on Cu foil. (h) ASEI on the Li−Sn alloy substrate after 1 mA h cm−2 Li was deposited.
117 Hz, which are consistent with the EIS characteristics of interphase layers. Figure 2c shows the equivalent circuit corresponding to the impedance spectra. RO represents the Ohmic resistance of the cell, including the parts from the electrolyte, separator, and electrodes. The resistance and capacitance of the SEI layer are symbolized as RSEI and CSEI, respectively, and Cdl is the double-layer capacitance. Considering that no any charge-transfer reaction occurs during the EIS measurements, the equivalent circuit does not contain an element of charge-transfer resistance. Warburg impedance corresponds to Li+ ion diffusion through the electrode− electrolyte interphase. The Nyquist plots were fitted using Gamry Echem Analyst software, and the RSEI value of the DOL polymer film is calculated as 24.71 Ω for the Cu electrode. This is a further confirmation of the formation of the polymer film by DOL oxidation and polymerization. Figure 2b shows the EIS of the Li−Sn alloy electrode, the semicircle in high and medium frequency range for the bare Li−Sn alloy stands for NSEI during the preparation of the Li− Sn alloy substrate when the Sn electrode is gradually polarized to 0 V. The fitted results according to the equivalent circuit in Figure 2c show that the resistances of the SEI are 33.98 Ω for the bare Li−Sn alloy and 17.23 Ω for the modified Li−Sn alloy. Therefore, the modified Li−Sn alloy electrode shows a lower resistance of SEI. Scanning electron microscopy (SEM) observation was conducted to confirm the formation of the SEI film. As can
be seen in Figure 2d,e, the image of the pristine Cu foil displays a uniform surface with some small protuberances. When the surface of the Cu foil electrode is coated with the DOL polymer film, the surface of Cu foil is smooth and flat except for a few cracks. It can be seen from Figure 2f that the thicknesses of Cu foil and Li−Sn alloy are ∼3.7 and ∼9.6 μm, respectively. As shown in Figure 2g, the Li−Sn alloy is completely covered with the ASEI film, indicating a good contact of them. The final thickness of the DOL polymer film is ∼1.0 μm, as illustrated in Figure 2h. Electrochemical Performance. Cu|Li and Li−Sn|Li half cells with DOL polymer ASEI coated Cu and Li−Sn alloy electrodes were employed to investigate the electrochemical performance of the nonLi substrate Li anodes. The availability of Li during cycles is evaluated by CE which is calculated from the ratio of the amount of Li stripping from the substrate and the one of Li deposition in the same cycle. As shown in Figure 3(a1,b1), the Cu and Li−Sn alloy substrates coated with DOL polymer ASEI present a more stable cycling behavior and provide enhanced CEs of 98.5 and 98.4% in the LiTFSI/DOL electrolyte, respectively. However, it is unlikely to obtain a stable cycling of Li plating/stripping on the bare Cu electrode. The bare Li−Sn alloy substrate exhibits much better cycling performance than the bare Cu substrate, indicating that the Li−Sn alloy is more admirable than Cu. This result supports the report of Yang.9 2482
DOI: 10.1021/acsami.8b16080 ACS Appl. Mater. Interfaces 2019, 11, 2479−2489
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ACS Applied Materials & Interfaces
Figure 3. continued
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DOI: 10.1021/acsami.8b16080 ACS Appl. Mater. Interfaces 2019, 11, 2479−2489
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ACS Applied Materials & Interfaces
Figure 3. (a1−a3) Cycling performance of Cu electrodes in the 1 mol L−1 LiTFSI/DOL electrolyte; (a1) comparison of the CEs; (a2,a3) polarization of Li deposition/stripping on Cu substrates in different cycles. Cycle conditions: 0.4 mA cm−2 with an area capacity of 0.4 mA h cm−2. (b1−b3,c1−c3,d1−d3) Cycling performance of Li−Sn alloy electrodes in 1 mol L−1 LiTFSI/DOL, 1 mol L−1 LiTFSI/DOL−DME (1:1 v/v), and 1 mol L−1 LiTFSI/DOL−DME (1:1 v/v) and 2 wt % LiNO3 electrolyte, respectively; (b1−d1) comparison of the CEs; (b2,b3,c2,c3,d2,d3) polarization of Li deposition/stripping on Li−Sn alloy substrates in different cycles. Cycle conditions: 0.4 mA cm−2 with an area capacity of 0.4 mA h cm−2. (e1−e3) Cycling performance of Li−Sn alloy electrodes in the 1 mol L−1 LiTFSI/DOL−DME (1:1 v/v) and 2 wt % LiNO3 electrolyte; (e1) comparison of CEs; (e2,e3) polarization of Li deposition/stripping on Li−Sn alloy substrates in different cycles. Cycle conditions: 1 mA cm−2 with an area capacity of 1 mA h cm−2. The insets in (a2,a3,d1) show corresponding partial enlarged curves.
The effect of DME in the electrolyte on the bare Li−Sn alloy electrode can be seen from the comparison of Figure 3(b1,c1). The CE of Li−Sn|Li cell with the bare Li−Sn alloy electrode in the LiTFSI/DOL−DME (1:1 v/v) electrolyte is about 82.0% with fluctuation, which is much lower than the value of 93.0% of the cell with the LiTFSI/DOL electrolyte. This result clearly manifests that DME in the electrolyte has caused inferior cycling performance of the Li−Sn|Li cell. However, as the Li− Sn alloy substrate was coated with the ASEI, the cell provides stable cycling and the CE reaches 97.1% in the same LiTFSI/ DOL−DME electrolyte. This is a further demonstration for the effectiveness of the ASEI for the protection of Li anodes.
LiNO3 additive has been proved to be effective in improving NSEI characteristics on the Li metal anode in the LiTFSI/ DOL−DME electrolyte. Here the influence of LiNO3 addition in the electrolyte is also studied, and the results are given in Figure 3d,e. As shown in Figure 3(d1), Li plating/stripping on the bare Li−Sn substrate electrode in the LiTFSI/DOL−DME (1:1 v/v) and 2 wt % LiNO3 electrolyte has gained an average value of 98.1%, which is much higher than the one in the electrolyte without LiNO3 addition. It proves that the cells with LiNO3 additive in the electrolyte provide more excellent Li plating/stripping cycling performance, which is consistent with the results of the published articles.36−40 Further detection shows that the bare Li−Sn alloy surface is covered 2484
DOI: 10.1021/acsami.8b16080 ACS Appl. Mater. Interfaces 2019, 11, 2479−2489
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ACS Applied Materials & Interfaces by multicomponent products, including Li alkylcarbonate and LiNxOy, produced by the reaction among Li, LiNO3, and solvents.38 These reaction products deposit on the substrate and form a flatter and homogeneous surface film than that formed in the electrolyte without LiNO3 addition, insuring better ionic conductivity and homogeneous current density during repeated Li plating/stripping on the surface. Therefore, the topographic change of the substrate can be reduced and less Li dendrites and dead Li will be formed upon cycling. As a result, the Li−Sn|Li cell with LiNO3 as additive in electrolyte shows better cycling performance. The unsatisfactory thing is that though the LiNO3 addition in the electrolyte increases the CE of the bare Li−Sn alloy substrate electrode, the CE behaves fluctuant. Under the current of 1 mA cm−2, the modified Li− Sn alloy electrode also delivers an enhanced CE of 98.4%. Although the bare Li−Sn alloy in the same electrolyte exhibits a much poor CE of 80.7%. In general, these results indicate that the DOL polymer film formed by the novel method can significantly enhance the reversibility of the Li plating/ stripping, block undesirable interface reaction, and slow down the accumulation rate of SEI on the electrode surface. Ultimately, the cycling stability of Li plating/stripping on the Cu and Li−Sn alloy substrate electrodes can be improved significantly. The charge−discharge profiles of Cu|Li and Li−Sn|Li cells are shown in Figure 3(a2,a3,b2,b3). The sum of the overpotential of Li deposition and oxidation can be defined as voltage hysteresis, and it can be used as a parameter to characterize the reaction dynamics. After long-term cycling, the Cu|Li cells with the bare and modified Cu electrode showed voltage hysteresis of 45 and 22 mV, respectively, and the voltage hysteresis of the Li−Sn|Li cells with bare and modified Li−Sn substrate electrodes are 42 and 24 mV, respectively. Therefore, all the modified substrate electrodes exhibit lower and stable polarization upon cycling because of the lower resistance of the ASEI. To assess the effects of cycling on the morphologies of Li− Sn alloy substrate, the Li−Sn|Li cells after 100 cycles were dissembled. Figure 4 compares the scanning electron microscopy (SEM) images of the bare and modified Li−Sn alloy substrates. In Figure 4(a1,b1), the bare Li−Sn alloy substrate displays loose structure with mosslike Li dendrites. An additional amount of electrolyte is consumed and “dead Li” accumulates upon long-term cycling due to unsatisfactory cycling performance. As seen in Figure 3(b1,c1), the corresponding CEs behave fluctuant. One can see in Figure 4(c1) that the bare Li−Sn alloy substrate is covered by dendritic and dead Li even though the electrode was cycled in the LiNO3-added electrolyte. However, as shown in Figure 4(a2,b2,c2), the modified Li−Sn alloy substrate exhibits improved morphology and appears more uniform and compact interface without any significant cracks or dendrites. The improvement of surface stability is surely caused by the protective ASEI film. In other words, the ASEI formed by DOL oxidation and polymerization is more effective in enhancing the cycleability and stability of Li plating/stripping than the addition of DME and LiNO3 in the basic LiTFSI/DOL electrolyte. Compositional Analysis of the Polymer Films. The functional groups in the ASEI films on the electrode surface without cycling was analyzed with Fourier transform infrared spectrometry (FTIR). As illustrated in Figure 5, the reflection peaks at 2877 and 2924 cm−1 are assigned to the symmetric
Figure 4. SEM images of Li−Sn alloy substrates after 100 cycles in different electrolytes. (a1,a2) 1 mol L−1 LiTFSI/DOL electrolyte; (b1,b2) 1 mol L−1 LiTFSI/DOL−DME (1:1 v/v) electrolyte; (c1,c2) 1 mol L−1 LiTFSI/DOL−DME (1:1 v/v) and 2 wt % LiNO3 electrolyte. (a1−c1) bare Li−Sn alloy substrate; (a2−c2) modified Li−Sn alloy substrate. Cycling condition: 0.4 mA cm−2 with an area capacity of 0.4 mA h cm−2.
Figure 5. FTIR spectrogram of polymer films.
and asymmetric stretching vibration of C−H, and the ones at 1359 and 1465 cm−1 are caused by the bending vibration of C−H. The peak corresponding to symmetric stretching vibration of C−O−C appears at 1106 cm−1. Therefore, it can be inferred that the dominant components of the ASEI film is the oligomers of polydioxolane such as −CH2(CH2OCH2)nCH2−.33 The flexibility of the ASEI layer should be attributed to these functional groups, which further block the undesirable interface reactions and enhance the CE and cycle stability of Li plating/stripping. The chemical composition of the ASEI and NSEI films on the chosen substrate electrodes was further analyzed by X-ray photoelectron spectroscopy (XPS). It is obvious in Figure 6a, the C 1s spectra of both ASEI and NSEI show a maximum binding energy at 284.8 eV, corresponding to oligomer of polydioxolane, such as LiO−(CH2CH2)n−OLi. The energy peak appeared at 286.2 eV should originate from the oligomer LiO−CH2(CH2OCH2)nCH2−OLi, which is correspondent to 2485
DOI: 10.1021/acsami.8b16080 ACS Appl. Mater. Interfaces 2019, 11, 2479−2489
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strength.41 The calculation with joint density functional theory demonstrated that Li+ diffusion at the electrode/electrolyte interface could be facilitated by LiF,42 which could further improve the uniformity of the Li plating and suppress the formation of the Li dendrite. Based on these data, it is reasonable to deduce that the ASEI layer contains higher content of oligomers of polydioxolane than NSEI, leading to a more flexible protective layer on the substrate. When Li is deposited and stripped on the substrate, the flexibility makes the surface layer more adaptive to the volume change of the electrode more easily, thus the uniform Li deposition is promoted. Meanwhile, more content of LiF in the ASEI layer can also improve the mechanical strength and Li+ conductivity, which favors the formation of dendrite-free Li plating. Application in Li−Sulfur Full Cells. The feasibility of the facile approach toward building a flexible and stable ASEI layer by DOL oxidation and polymerization on the Li−Sn alloy substrate have been demonstrated through our research and this substrate material can be applied in Li−S full cells. LiTFSI/DOL−DME was used as the electrolyte in Li−sulfur (S|Li) batteries. Sulfur−carbon (S−C) compound was used as the cathode, and the Li saturated Li−Sn alloy substrate was used as the anode with an area of 1.539 cm2, upon which totally 4 mA h of Li metal was plated prior to the cell assembly [in such a case, the anode was symbolized as Li(Li−Sn)]. CR2016-type S|Li(Li−Sn) testing coin cells were assembled in the Ar-filled glovebox. In the test cells, 1.806 mA h and 1.722 mA h S−C cathodes were matched with the modified and bare Li(Li−Sn) anodes, respectively. The cells were tested with galvanostatic discharge/charge on a LAND-CT 2001A battery test system (LANHE, China) in 1.5−3.0 V at 0.2 C (1 C = 1672 mA g−1). Figure 7 shows the relationship of the specific capacity and CE of the cells with cycle number. One can see from Figure 7a that the cell with the modified Li(Li−Sn) anode shows higher discharge capacity than the one with the bare Li(Li−Sn) anode during the whole 120 cycles. In Figure
Figure 6. High-resolution XPS spectra of (a) C 1s and (b) F 1s after lithium stripping from the Cu electrode after 10 cycles in the 1 mol L−1 LiTFSI/DOL−DME (1:1 v/v) and 2 wt % LiNO3 electrolyte.
the FTIR results. Furthermore, the peak at 287.5 eV should originate from CH3CH2OCH2OLi (DOL reduction product).5 Therefore, the ASEI contains more LiO− CH2(CH2OCH2)nCH2−OLi and CH3CH2OCH2OLi components, which contributed to a flexible protective film. As shown in Figure 6b, the peaks at 685.5 and 689.2 eV can be attributed to the C−F bond (−CF3 and −CF2) and LiF, respectively. The ASEI layer contains more inorganic crystalline components of LiF, which makes the ASEI much stronger in mechanical
Figure 7. (a) Cycling performance of S|Li(Li−Sn) batteries with ASEI and NSEI layers on the Li−Sn alloy substrates in the 1 mol L−1 LiTFSI/ DOL−DME (1:1 v/v) electrolyte. (b) Relation of CE with cycle number. (c) Voltage profiles of the S|Li(Li−Sn) cells at 0.2 C. The black and red curves stand for the battery with bare Li(Li−Sn) and the one with modified Li(Li−Sn), respectively. 2486
DOI: 10.1021/acsami.8b16080 ACS Appl. Mater. Interfaces 2019, 11, 2479−2489
Research Article
ACS Applied Materials & Interfaces
substrate by a three-electrode device. The cell had a Sn working electrode, Li-plate counter electrode, and Li-rod reference electrode. 0.5 mol L−1 LiTFSI/DOL electrolyte was prepared in advance and 2 mL of the electrolyte was employed in each three-electrode device to standardize the testing. The cells were assembled in a glovebox filled with ultrapure Ar gas, and the O2 and H2O contents were below 0.1 ppm. The Sn metal was alloyed with Li after application of 0.1 mA cm−2 constant cathodic current to the working electrode powered by a LAND CT2001A test system. When the potential of working electrode was moved from the OCP value to 0 V, the Li−Sn alloy substrate was thus prepared. Preparation of ASEI on the Li−Sn Alloy Substrate Electrode. The three-electrode device was also used to prepare ASEI on the Li− Sn alloy substrate. The Sn substrate was employed as the working electrode, and the Li metal was used as the counter electrode and reference electrode. 0.5 mol L−1 LiTFSI/DOL was used as the electrolyte. A potentiostatic method was conducted with a CHI 660a workstation (Shanghai Chenhua Instruments Limited, China). By applying a potential of 3.0 V on the Sn working electrode for 800 s, a suitable polymer layer was formed on the surface of Sn electrode. Then, the Sn metal was alloyed with Li using the same approach as the one for Li−Sn alloy preparation. Thus, the Li−Sn alloy substrate coated with ASEI was prepared. Preparation of the Sulfur Cathode. A mixture of sulfur and Ketjenblack carbon (mass ratio of 7:3) was calcined at 155 °C for 10 h. The cathode was prepared by mixing the sulfur/carbon composite and PVDF in the weight ratio of 85:15. Then, the homogeneous slurry was coated on Al foil, dried under vacuum at 48 °C for 3 h, and another 10 h at 58 °C in a vacuum oven. After that, the electrode was punched into round discs with an area of 1.131 cm2. The average active material mass loading of electrode was 1.0 mg cm−2. Structure Characterization. The morphology of the Li−Sn alloy substrates after prolonged cycling was characterized by SEM (Zeiss Merlin Compact SEM, Carl Zeiss AG) operated at 5.0 kV. FTIR analysis was applied to detect the surface film on anodes, and the spectra was collected with a resolution of 2.0 cm−1 using a Nicolet 5700 FTIR spectrometer (Thermo Fisher Scientific, U.S.A.). XPS (Thermo scientific ESCALAB250Xi, U.S.A.) was used to detect the surface composition. Before testing, the residual electrolytes on electrodes were removed by rinsing with the DOL solvent and then they were dried in Ar-filled glovebox. Electrochemical Measurements. LSV measurements of the three-electrode test cells were carried out with the CHI 660a workstation. A Pt disk with a diameter of 0.3 mm was used as the working electrode, and the Li metal was used as both the counter and the reference electrodes. Linear potential sweeping was applied on the substrate electrode from OCP to 6 V at a scan rate of 1 mV s−1. A two-electrode cell configuration using a standard CR2016 coin-cell was adopted to evaluate the cycling performance of various anode samples, and the positive electrodes were Li−Sn alloy substrates punched into 12.0 mm disks. Li foil with a diameter of 15.6 mm was employed as the negative electrode, and Celgard 2326 (polypropylene membrane) was employed as the separator. The Li−Sn alloy disks were rinsed with DOL−DME (1:1 v/v) and dried in an Ar atmosphere before assembling the cells. As a cell is tested as half cells, the term “discharge” is related to the Li deposition on the substrate electrode, and the term “charge” is related to Li stripping from the substrate. During each cycle, the Li metal was plated on the substrate at a preset current for 1 h and then oxidized till the upper potential limit of 0.05 V (vs Li/Li+). The low cut-off voltage for charging of the Li−Sn|Li half cells was set to prevent the Li−Sn alloy substrate from de-alloying of Li.
7b, the average CEs of the full cells with the modified and bare Li(Li−Sn) anodes are 92.7 and 82.5%, respectively. This result surely demonstrates that the ASEI layer on the Li(Li−Sn) anode has improved the cycling performance of Li plating/ stripping. The ASEI layer on the anode surface has hindered the soluble polysulfide intermediates from being reduced chemically to form lower order polysulfide, which is easy to diffuse back to the cathode and to be re-oxidized there.43−46 With the presence of ASEI layer on the anode surface, the polysulfide shuttle effect is effectively alleviated, and two charge/discharge plateaus of S|Li(Li−Sn) cells with bare Li(Li−Sn) and modified Li(Li−Sn) alloy anodes become obvious as shown in Figure 7c. The high and low discharge voltage plateaus correspond to the reduction of sulfur to longchain polysulfide and the further reduction of long-chain polysulfide to Li2S, respectively.
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CONCLUSIONS It has been shown that the Li−Sn alloy substrate is one of the hopeful anode substrate materials to replace Li metal in Li secondary batteries, and this substrate can promote the realization of an applicable Li metal battery technology. In this work, we demonstrated an effective and facile electrochemical strategy to overcome the most serious obstacles of NSEI and to suppress Li dendrites growth by adopting the electrochemical oxidation and polymerization of DOL to construct an ASEI film on substrate electrodes, for example the Li−Sn alloy substrate. The ASEI film contained oligomers of polydioxolane, such as LiO−(CH2CH2)n−OLi and LiO− CH2(CH2OCH2)nCH2−OLi, and these components could construct a flexible ASEI layer. The ASEI possesses the features of dense, uniform, low film resistance, stable, ion conductive, and electrically insulating. The Li−Sn alloy substrate anode with this novel ASEI provided much better cycling performance for Li plating/stripping with high stability and CE. Furthermore, the ASEI films could enhance the cycling performance of S|Li(Li−Sn) full cells with higher cycling capacity and stability. Our fabrication strategy for preparing ASEI provides a new approach for non-Li substrate protection by an innovative electrochemical potential-controlled method. We believe that the ASEI film proposed in this work can not only be applied to the Li−Sn alloy substrate, but also offer new opportunities for other non-Li substrate materials.
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EXPERIMENTAL SECTION
Preparation of the Sn Substrate Precursor. Sn was electrochemically deposited onto Cu foil (99.8%, HF-Kejing Materials Technology Co., Ltd.) from an aqueous solution in a two-electrode cell. Cu foil was used as the cathode and Sn plate (99.9%, Sinopharm Chemical Reagent Co., Ltd.) was used as the anode. After acid pretreatment for the Cu foil, the tin layer was electrodeposited on the Cu foil using the method in ref 9. 10 mA cm−2 constant current was applied to the Cu foil electrode for 4 min to form a Sn layer on the Cu foil, which then formed the Sn substrate precursor. Finally, the Sn substrate was dried at 50 °C for 4 h in a vacuum oven. Preparation of the Li−Sn Alloy Substrate. If the Sn electrode is polarized to a sufficiently low potential in a Li+-containing liquid organic electrolyte, the Li+ can be reduced electrochemically on the electrode surface, forming well-defined LixSny intermetallic phases with the Sn metal at room temperature. As the potential of the Sn electrode decreases, the molar ratio of Li in LixSny increases. The saturated Li22Sn5 intermetallic phase can be formed when the potential of Sn electrode falls to 0 V. Thus, a readily accessible electrochemical route was utilized to prepare the Li−Sn alloy
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Cheng Li: 0000-0002-8496-5090 Yifu Yang: 0000-0002-4949-4502 2487
DOI: 10.1021/acsami.8b16080 ACS Appl. Mater. Interfaces 2019, 11, 2479−2489
Research Article
ACS Applied Materials & Interfaces
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Hui Zhan: 0000-0002-8878-7885 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.Y., C.L., and H.S. greatly appreciated the financial support from the National Natural Science Foundation of China (No. 21233004). H.Z. and Q.L. would express their sincere thanks for the financial support of the National Key Research and Development Project (2016YFB0100400) and the National Natural Science Foundation of China (No. 21875172).
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