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Improved Cycling Performance of Lithium−Oxygen Cells by Use of a Lithium Electrode Protected with Conductive Polymer and Aluminum Fluoride Jae-Hong Kim,† Hyun-Sik Woo,† Won Keun Kim,‡ Kyoung Han Ryu,‡ and Dong-Won Kim*,† †
Department of Chemical Engineering, Hanyang University, Seoul 133-791, Korea R&D Division, Hyundai Motor Company, Gyeonggi-do 437-815, Korea
‡
S Supporting Information *
ABSTRACT: Lithium−oxygen batteries have attracted great attention for advanced energy storage systems because of their high specific energy. The enhancement of the interfacial stability of lithium negative electrodes is one of the many technical challenges toward high safety and long life lithium−oxygen batteries due to their high reactivity toward organic electrolytes and the lithium dendrite growth during the repeated cycling. Herein, we demonstrate that the protective layer comprising conductive polymer and AlF3 particles on lithium metal stabilized the lithium electrode by effectively reducing the reductive decomposition of the liquid electrolyte and suppressing the growth of lithium dendrite. As a result, the cycling performance of a lithium−oxygen cell assembled with a surface-modified lithium electrode was remarkably improved as compared to a cell with a pristine lithium electrode. KEYWORDS: protected lithium electrode, lithium dendrite, lithium−oxygen cell, conductive polymer, aluminum fluoride, cycling performance
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INTRODUCTION Rechargeable lithium−oxygen battery employing organic electrolyte has attracted much attention as a next-generation energy storage device due to its high specific energy.1−5 In its most common configuration, the lithium−oxygen battery uses lithium metal as the negative electrode, an organic solution as the electrolyte, and a porous carbon as the positive electrode. It has many technical challenges regarding both the electrodes and the electrolytes for its practical implementation, one of which is the low interfacial stability of the lithium negative electrode.6 The formation and growth of lithium dendrite due to inhomogeneous lithium deposition during cycling can lead to cell failure and unpredictable events such as short circuits or local overheating. Lithium metal is also unstable against nearly all chemical species and reacts with organic solvents and salts in liquid electrolytes. In addition, the continuous formation and growth of the solid electrolyte interphase (SEI) layer on the lithium electrode leads to an increase of interfacial resistances and significant capacity fading with repeated cycling. 7 Especially, the parasitic reactions of the liquid electrolyte with lithium metal in the lithium−oxygen batteries accelerate the electrolyte depletion with continuous evaporation of volatile solvents in the porous carbon positive electrode exposed to air, which eventually leads to battery failure. Therefore, it is of great urgency to improve the cycling stability of lithium negative electrode to develop safe lithium−oxygen batteries with good cycling stability. © 2016 American Chemical Society
To improve the cycling stability of the lithium metal-based batteries, the lithium metal surface has been modified and stabilized by chemical treatment,8,9 silane-based coatings,10−14 functional additives,15−19 glass ceramic electrolytes,20−22 polymer-based materials,23−31 and carbon coatings.32,33 In our previous study, we found that the cycling performance of the Li/LiCoO2 cell could be improved by the surface modification of lithium metal using poly(3,4-ethylenedioxythiophene)-copoly(ethylene glycol) (PEDOT-co-PEG).34 PEDOT-co-PEG can conduct Li+ ions through the polymer when impregnated with liquid electrolyte,34−36 and it strongly adheres to the lithium metal surface without detachment. These are unique characteristics that make PEDOT-co-PEG a useful material in the formation of a protective layer on the lithium metal surface. A large number of research groups have also reported that aluminum fluoride (AlF3) could play a role as a functional additive to suppress the electrolyte decomposition.37−42 Therefore, it would be good to combine them to stabilize the lithium negative electrode in the lithium−oxygen cell. On the basis of the above background, in this work, the lithium metal surface has been modified by forming the protective layer with PEDOT-co-PEG and AlF3 inorganic particles (Figure 1) for applications as a negative electrode in Received: August 19, 2016 Accepted: November 9, 2016 Published: November 9, 2016 32300
DOI: 10.1021/acsami.6b10419 ACS Appl. Mater. Interfaces 2016, 8, 32300−32306
Research Article
ACS Applied Materials & Interfaces
photoelectron spectroscopy (XPS) (Thermo VG Scientific ESCA 2000) was carried out to investigate the chemical composition of the surface layer formed on lithium electrode after the repeated cycles. XRD patterns of the carbon positive electrodes were obtained via a powder X-ray diffractometer (XRD, D/MAX RINT 2000) utilizing a Cu Kα radiation source after charge and discharge cycles.
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RESULTS AND DISCUSSION Figure 2a and b presents the FE-SEM images of the lithium electrodes before and after surface modification. As can be seen
Figure 1. Schematic illustration for suppressing lithium dendrite growth and electrolyte decomposition by the protective layer composed of PEDOT-co-PEG and AlF3 inorganic particles.
the lithium−oxygen cells. The results presented herein demonstrated that the use of a surface-modified lithium electrode could effectively suppress the dendrite growth and the irreversible decomposition of the oxygen-dissolving liquid electrolyte. Because of the beneficial effects of the protected lithium electrode, the lithium−oxygen cell with surfacemodified lithium electrode exhibited an improved cycling performance as compared to the cell with a pristine lithium electrode.
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EXPERIMENTAL SECTION
Electrode Preparation and Cell Assembly. A proper amount of aluminum fluoride (20 wt % relative to PEDOT-co-PEG) was uniformly dispersed in PEDOT-co-PEG solution by sonication for 3 h, as reported earlier.42 Lithium metal foil (Honjo Metal Co. Ltd.) was used as a pristine lithium electrode by pressing onto a copper current collector. The protected lithium electrode was prepared by spincoating the solution onto the pristine lithium electrode. The carbon positive electrode was prepared by mixing Ketjen black and poly(vinylidene fluoride) with a weight ratio of 90:10 in N-methyl pyrrolidone (NMP) and casting onto a gas diffusion layer.43 A Swagelok-type lithium−oxygen cell was assembled with surfacemodified lithium negative electrode, glass fiber membrane (GF/D, Whatman) impregnated with liquid electrolyte, and carbon positive electrode with a carbon loading of 1.0 mg cm−2, as reported in our previous works. 43−45 A solution of 1.0 M lithium bis(trifluoromethane) sulfonylimide (LiTFSI) dissolved in tetra(ethylene glycol) dimethyl ether (TEGDME) was used as the liquid electrolyte. Characterization and Measurements. A field emission scanning electron microscope (FE-SEM, JEOL KSM-6300) was used to investigate the morphologies of the lithium electrodes. Energy dispersive X-ray spectroscopy (EDS) was carried out to examine the elemental distribution in the protective layer formed on the lithium electrode. Electrochemical impedance spectroscopy of the symmetrical Li/oxygen-saturated electrolyte/Li cells was performed from 10 mHz to 100 kHz at 25 °C.31 The oxygen-saturated electrolyte was prepared by bubbling high-purity oxygen gas into liquid electrolyte overnight. Galvanostatic lithium plating/stripping measurements of symmetric Li/oxygen-saturated electrolyte/Li cells were performed at current densities of 0.05−0.8 mA and 25 °C. The charge and discharge cycling tests of the lithium−oxygen cells placed in a chamber filled with highpurity oxygen gas were performed at a constant current rate of 200 mA g−1 in the voltage range of 2.0−5.0 V at 25 °C.43−45 Impedance behavior of the lithium−oxygen cells was investigated by electrochemical impedance spectroscopy using a three-electrode cell (El-cell GmbH, Germany).46 Surface morphologies of the lithium electrodes in the cells were analyzed by FE-SEM after repeated cycling.31 X-ray
Figure 2. SEM images of (a) the pristine lithium electrode and (b) the surface-modified lithium electrode. EDS mapping images of (c) O, (d) S, (e) Al, and (f) F on the surface-modified lithium electrode with PEDOT-co-PEG and AlF3.
in Figure 2a, the pristine lithium electrode without a protective layer exhibited a smooth surface. After PEDOT-co-PEG and AlF3 particles were coated onto the lithium metal, the surface of lithium electrode showed a homogeneous morphology with uniform surface coverage by a thin protective layer. The protective layer was firmly adhered to lithium metal without detachment. The ionic conductivity of the protective layer impregnated with liquid electrolyte was measured as 2.8 × 10−3 S cm−1. This result implies that the protective layer can effectively conduct Li+ ions from the electrolyte solution to lithium metal and vice versa during the electrochemical operation of lithium−oxygen cell. The thickness of the protective coating layer was approximately 1.0 μm. To confirm the surface elements in the protective layer formed on lithium metal surface, the EDS mapping images of various elements were obtained. As shown in Figure 2c−f, all of the elements arising from PEDOT-co-PEG (O and S) and AlF3 particles (Al and F) are evenly distributed on the surface of the protective layer formed on the lithium electrode, indicating that the 32301
DOI: 10.1021/acsami.6b10419 ACS Appl. Mater. Interfaces 2016, 8, 32300−32306
Research Article
ACS Applied Materials & Interfaces
Figure 3. Time evolution of electrochemical impedance spectra of the symmetrical Li/oxygen-saturated liquid electrolyte/Li cells at 25 °C. (a) Pristine lithium electrode and (b) surface-modified lithium electrode.
Figure 4. Voltage profiles of the symmetrical Li/oxygen-saturated liquid electrolyte/Li cells assembled with (a) the pristine and (b) the surfacemodified lithium electrodes with increasing current density from 0.05 to 0.8 mA cm−2.
The interfacial behavior between lithium electrode and oxygen-dissolving liquid electrolyte was investigated by subjecting galvanostatic lithium plating/stripping cycles to the symmetrical Li/liquid electrolyte/Li cells. The cells were stabilized under an open circuit condition for 24 h before cycling. Figure 4 shows the voltage profiles corresponding to the galvanostatic lithium stripping and deposition, which were obtained at different current densities. The cell with the pristine lithium electrode showed high polarization, particularly at high current densities, which can be ascribed to the increase in overpotential caused by irregular deposition and stripping of lithium during cycles. In this cell, the lithium metal surface is directly exposed to the liquid electrolyte, and the continuous growth of lithium dendrites can occur on the lithium electrode during cycling, which results in the continuous formation of a new SEI layer and gradual consumption of the electrolyte solution.7,50 The thickened SEI layer and the depletion of the electrolyte could impose high resistance to the stripping and deposition of lithium during repeated cycles, which increases the overpotential and produces unstable voltage profiles in the cell, especially at high current densities. In contrast, more stable and lower voltage profiles could be obtained in the cell with surface-modified lithium electrode. The protective layer on the lithium metal played a positive role as a stable SEI with low resistance and high uniformity, which contributed to the even current distribution and, consequently, stable and low voltage responses. These results suggest that the protective layer could effectively reduce the interfacial resistances by suppressing the dendrite growth and improve the interfacial stability by
uniform surface layer composed of PEDOT-co-PEG and AlF3 particles is formed on the lithium metal. Figure 3 shows the electrochemical impedance spectra of symmetrical Li/liquid electrolyte/Li cells, as a function of storage time. The measurements were performed in symmetric two-electrode cells employing an oxygen-saturated electrolyte. All of the spectra were composed of two overlapped semicircles. The semicircle at high frequency is related to an SEI layer resistance (RSEI) and its capacitance, while the low frequency semicircle can be attributed to the charge transfer resistance (Rct) and double layer capacitance.47−49 The pristine lithium electrode-based cell showed a gradual increase of RSEI with time. This result suggests that the resistive layer is continuously grown on the pristine lithium electrode due to the highly reactive nature of lithium metal toward oxygendissolving electrolyte solution. Formation of the resistive layer on the lithium electrode retards the charge transfer reaction at the electrode and electrolyte interface, which results in an increase of Rct. On the other hand, the semicircles observed in the cell with the protected lithium electrode did not significantly expand with time. That is, RSEI remained almost constant, and Rct slightly increased initially and eventually stabilized after 2 days. These results suggest that the protective layer effectively improves the interfacial stability of lithium electrode, because it can restrict access of the oxygen-dissolving electrolyte to the lithium metal surface. Even after 10 days, the interfacial resistances remained at low values, confirming that the interface between surface-modified lithium electrode and oxygen-dissolving liquid electrolyte may indeed be very stable. 32302
DOI: 10.1021/acsami.6b10419 ACS Appl. Mater. Interfaces 2016, 8, 32300−32306
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Figure 5. (a) Voltage profiles of the lithium−oxygen cell with PEDOT-co-PEG/AlF3-coated lithium electrode at 0.2 mA cm−2 and 25 °C. (b) Cycle performance of the lithium−oxygen cells with different lithium electrodes at a limited capacity of 1000 mAh g−1.
Figure 6. SEM images of the lithium electrodes disassembled from the lithium−oxygen cells after 30 cycles. (a) Pristine lithium electrode and (b) surface-modified lithium electrode.
inhibiting deleterious reactions of the electrolyte solution with the lithium metal, as discussed in Figure 3. The full discharge behaviors of the lithium−oxygen cells assembled with different lithium electrodes were investigated and compared at different current densities. It should be noted that the carbon electrode was prepared without any other catalysts. As depicted in Figure S1, the use of protected lithium electrode gave slightly higher discharge capacities for all current densities. The buildup of insoluble discharge products in a fully discharged cell may lead to permanent choking of porous carbon electrodes after a few cycles.51,52 To maintain the facile electrochemical reactions at the carbon positive electrode, the capacity utilization was limited to 1000 mAh g−1. Figure 5a shows the voltage profiles of the lithium−oxygen cell with PEDOT-co-PEG/AlF3-coated lithium electrode with the repeated cycling, which were obtained at 0.2 mA cm−2 (200 mA g−1, 0.2C). The discharge and charge curves corresponding to the formation and dissolution of Li2O2 were observed during the repeated cycles. Although the overpotential slightly increased with cycling, the cell did not show any capacity fading up to at least 70 cycles. In contrast, the pristine lithiumbased cell cycled up to 20 cycles, after which the discharge capacity gradually decreased, as shown in Figure S2. The difference in cycling behavior between the two cells may be associated with the lithium electrode, as the other cell components, except the lithium electrode, are the same in the two cells. The obvious difference in cycling stability of the cells using different lithium electrodes is shown in Figure 5b. It shows that the use of the protected lithium electrodes significantly improves the cycling stability of the lithium− oxygen cell. As mentioned earlier, the early failure of the cell
with a pristine lithium electrode can be ascribed to dendrite formation and exhaustion of the electrolyte solution during repeated cycling. Moreover, lithium dendrites can be detached from the lithium negative electrode during the repeated cycling, and the isolated lithium metal can lead to side reactions with the oxygen-dissolving liquid electrolyte due to its highly reactive nature, severely degrading the cycling performance. In contrast, the cell assembled with PEDOT-co-PEG/AlF3coated lithium electrode exhibited a stable cycling performance with no capacity fading up to the 75th cycle. Such improved capacity retention can be ascribed to the enhanced interfacial stability of the lithium electrode with a protective layer. The cell with PEDOT-co-PEG/AlF3-coated lithium electrode showed more stable cycling behavior than the cell with lithium electrode modified by only PEDOT-co-PEG, confirming a beneficial effect of AlF3 to suppress the electrolyte decomposition.37−42 The electrochemical characteristics of the lithium−oxygen cells exposed to oxygen atmosphere were investigated by electrochemical impedance spectroscopy. It is worth reminding that all of the impedance measurements were performed using the cells with three-electrode configuration. As shown in Figures S3 and S4, all of the spectra showed a depressed semicircle that can be assigned to the interfacial resistance.53 Both electrolyte resistance and interfacial resistance increased with cycling, irrespective of type of lithium electrodes. A continuous increase in the electrolyte resistance can be ascribed to loss of organic solvent in open atmosphere conditions. When comparing the interfacial resistance at lithium anodes, the surface-modified lithium electrode exhibited more stable and lower interfacial resistance than the pristine lithium electrode 32303
DOI: 10.1021/acsami.6b10419 ACS Appl. Mater. Interfaces 2016, 8, 32300−32306
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ACS Applied Materials & Interfaces
Figure 7. F 1s XPS spectra of (a) the pristine lithium electrode and (b) the surface-modified lithium electrode after 30 cycles.
fluoride as compared to the pristine lithium electrode. Lithium fluoride is formed via the reductive decomposition of LiTFSI salt on the surface of electrode.58,59 When the lithium fluoride forms on the lithium negative electrode, the charge transfer reaction between the electrode and electrolyte may be hindered, because it is not conducting for electrons and Li+ ions. It results in the increase of the charge transfer resistance, as depicted in Figure S3. The suppression of the electrolyte decomposition at the protected lithium electrode can be attributed to blocking of the electron transfer from the lithium metal to the electrolyte. The above-discussed results demonstrate that the protective layer comprising PEDOT-co-PEG and AlF3 on the lithium electrode could effectively reduce the electrolyte decomposition as well as suppress the dendrite growth, which provides more stable cycling behavior in the lithium−oxygen cells.
(Figure S3). On the other hand, the interfacial resistances at carbon cathodes showed almost similar behavior with the repeated cycling (Figure S4). These results suggest that the improvement of the cycling stability of the lithium−oxygen cell with protected lithium electrode mainly arises from the enhanced interfacial stability of the lithium negative electrode in the cell. Figure S5 presents XRD patterns of the carbon positive electrodes in the cell, whose charge/discharge states are given in the figure. After the first discharge cycle, it shows the characteristic peaks corresponding to crystalline Li2O2,52,54 confirming the formation of Li2O2 during the discharge process. After recharging, no characteristic Li2O2 peaks could be observed, indicating reversible decomposition of Li2O2 via the oxygen evolution reaction in this cell. However, the peak intensity of Li2O2 was decreased after the 10th discharge cycle, indicating that there was not much Li2O2 formed as after the first discharge cycle and the common side reactions still occurred at the carbon cathode. The morphologies of the lithium electrodes were examined after repeated cycling. The pristine lithium electrode exhibited dendritic features with particulate morphologies, as shown in Figure 6a. The current may be localized at the tip of the dendrites on the electrode, and the dendrite tips thus experienced increased deposition rates due to the inhomogeneous electric field, which in turn promoted rapid dendrite growth.55,56 Increasing the surface area and surface roughness due to dendrite formation also accelerated the decomposition of the liquid electrolyte, and thus continuous depletion of the electrolyte occurs with thickening of the SEI layer during cycling. As a result, such dendritic morphologies result in a decrease in the discharge capacity with cycling and eventually led to cell failure, as discussed in Figure 5b. In contrast, the lithium electrode with protective layer had rather smooth and uniform morphologies that evenly spread over the lithium electrode. These results indicate that the protective layer effectively suppressed the dendrite growth by facilitating the uniform distribution of Li+ ions over the entire lithium electrode and by acting as a physical barrier that inhibited dendrite growth. The surface composition of the lithium negative electrodes was characterized via F 1s XPS spectra after 30 cycles, as shown in Figure 7. The main peak observed at 689.0 eV corresponds to the TFSI anion, and the peak at 684.7 eV can be assigned to lithium fluoride.57 The peak corresponding to aluminum fluoride can be observed in the surface-modified lithium electrode. It is noticeable that the surface-modified lithium electrode exhibited much lower peak intensity for lithium
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CONCLUSIONS In this study, the thin conductive layer comprised of the PEDOT-co-PEG and AlF3 particles was assessed as a protective layer on lithium metal to improve the interfacial stability of the lithium electrode in contact with an oxygen-dissolving liquid electrolyte. Both the electrolyte decomposition and the dendrite growth could be effectively suppressed in the lithium−oxygen cell employing the surface-modified lithium electrode during charge and discharge cycles, consequently resulting in a remarkable improvement in the cycling stability of the cell. On the basis of the the obtained results, the lithium electrode with protective layer can be promising as a negative electrode in the lithium−oxygen batteries with enhanced cycling stability.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10419. Initial discharge curves of the Li−O2 cells with different lithium electrodes, charge and discharge curves of the Li−O2 cell with a pristine lithium electrode, AC impedance spectra of lithium anodes and carbon cathodes in the lithium−oxygen cells with different lithium electrodes, and XRD diffraction patterns of the carbon positive electrodes with repeated cycling (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 32304
DOI: 10.1021/acsami.6b10419 ACS Appl. Mater. Interfaces 2016, 8, 32300−32306
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Hyundai Motor Co. REFERENCES
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DOI: 10.1021/acsami.6b10419 ACS Appl. Mater. Interfaces 2016, 8, 32300−32306
