Wisdom from the Human Eye: A Synthetic Melanin Radical Scavenger

Article pubs.acs.org/cm

Wisdom from the Human Eye: A Synthetic Melanin Radical Scavenger for Improved Cycle Life of Li−O2 Battery Byung Gon Kim,†,§,∥ Sunjin Kim,‡,§,∥ Haeshin Lee,*,‡,§ and Jang Wook Choi*,†,§ †

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea § Center for Nature-inspired Technology (CNiT) in KAIST Institute NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: Li−O2 batteries are attractive systems because they can deliver much higher energy densities than those of conventional lithium-ion batteries by engaging light gas-phase oxygen as a cathode active material. However, the inevitable generation of residual superoxide radicals gives rise to irreversible side reactions and consequently causes severe capacity degradation over cycling. To address this chronic issue, herein, we have taken a lesson from the human eye. Analogous to Li−O2 batteries, the human eye is liable to attack by reactive oxygen species (ROS), from its lifetime exposure to sunlight. However, it protects itself from the ROS attack by using melanin as a radical scavenger. To mimic such a defense mechanism against radical attack, we included polydopamine (pD), which is one of the most common synthetic melanins, in the ether-based electrolyte. As an outcome of the superoxide radical scavenging by the pD additive, the irreversible side reaction products were alleviated significantly, resulting in superior cycling performance. The present investigation provides a message that simple treatments inspired by the human body or nature could be effective solutions to the problems in various energy devices.

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INTRODUCTION For timely advent of future lithium ion battery (LIB) applications, such as all electrified vehicles, the energy densities of LIBs are desirable to increase significantly. To this end, considerable efforts have been invested to develop novel electrode materials whose specific capacities can go beyond those of the classical intercalation-based ones. Among a variety of candidates along this direction, Li−O2 batteries have received discernible attention from the battery community, because the use of gas-phase oxygen molecules as a cathode active material allows the cells to hold 3−5 times higher practical energy densities, compared to those of the conventional LIBs.1−5 In the discharging process of Li−O2 batteries, Li ions and O2 molecules react to form Li2O2 as a main product, and the reaction goes reverse in the charging process.6−8 In-depth analyses have found that the discharging process necessarily yields superoxide radicals as reaction intermediates before formation of the final products. While superoxide radicals are key species in achieving the large capacities of Li−O2 cells, in fact, the residual superoxide radicals cause chemical degradation of nearly all cell components of electrolytes, binders, and carbonaceous electrodes, by parasitic reactions.7,9−14 The oxidative parasitic reactions are usually irreversible and therefore accumulate a variety of byproducts continuously © 2014 American Chemical Society

during cycling. Thus, the formation of the excessive superoxide radicals could impair the Coulombic efficiency in each cycle and subsequently long-term cycling performance.15−20 While searching for a solution to this chronic issue associated with the superoxide radical stress, we have noticed that the structural configuration of Li−O2 battery is analogous to that of the human eye (Figure 1a). The porous carbonaceous aircathode in Li−O2 battery directly contacts with gaseous oxygen, and, likewise, the cornea of the human eye is exposed to air. Both systems share the common next step, and that is the dissolution of oxygen molecules into the adjacent liquid media: the electrolyte, in the case of the Li−O2 battery,3 and the aqueous humor, in the case of the human eye.21 Subsequently, both systems generate reactive oxygen species (ROS). While the Li−O2 battery generates ROS from continuous conversion of the dissolved oxygen by discharging voltages, the eye generates ROS from lifetime exposure to the sunlight (i.e., UV irradiation).22 As a result, both systems could suffer from excessive generation of ROS, which is detrimental to the health of the overall systems.9,22,23 Nonetheless, the eye develops effective defense mechanisms to quench and/or scavenge the Received: May 1, 2014 Revised: August 5, 2014 Published: August 5, 2014 4757

dx.doi.org/10.1021/cm501578v | Chem. Mater. 2014, 26, 4757−4764

Chemistry of Materials

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exhibited markedly improved cycle life, compared to that of a control cell without such treatment.

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EXPERIMENTAL SECTION

Preparation of Synthetic Melanin. pD particles were synthesized by dopamine oxidation. 50 mM of dopamine hydrochloride (Aldrich, USA) was dissolved in 80% methanol (MeOH) (Daejeong, Korea), and 1.5 equiv. of sodium periodate (NaIO4) (Aldrich, USA) was added to the solution to oxidize dopamine. The dopamine solution was shaken for 12 h for sufficient oxidation. The synthesized pD particles were dialyzed with distilled water five times and freezedried. To examine the electrochemical stability of pD in a Li−O2 cell upon cycling, we prepared pD-coated polyethylene (pD-PE) separators for X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) characterizations because the pD particles are difficult to be collected separately from battery cells after cycling. The pD-PE separators were prepared by immersing the PE separators in the dopamine solution during its polymerization. For electrochemical testing of the pD-PE separators, the separators were added between the air-electrodes and glass fiber membranes in the Li− O2 cells. Superoxide Radical Scavenging Test. Before the electrolyte preparation, lithium bis(trifluoromethanesulfonyl) (LiTFSI) (Aldrich, USA) salt was dried under vacuum at 70 °C for 48 h, and tetraethylene glycol dimethyl ether (TEGDME) (Aldrich, USA) was stored in a bottle with 4 Å molecular sieves inside an argon-filled glovebox. The electrolyte was prepared by dissolving 1 M LiTFSI in TEGDME. To make the superoxide radical solution, 14.2 mg of potassium dioxide (KO2) (Aldrich, USA) and 52.9 mg of 18-crown-6 (Aldrich, USA) were added to 2 mL of the electrolyte. The solution was stirred for 4 h with light blocked for efficient radical generation. The solution was centrifuged at 13500 rpm for 10 min to remove unreacted reactants, and the supernatant was used for tests. To measure the radical generation and its concentration, nitroblue tetrazolium (NBT) (Aldrich, USA) was used. NBT is a widely used chemical for detecting radicals because the color of NBT changes to blue when it is reduced by radicals.31 The degree of NBT reduction can be quantified by ultraviolet−visible light (UV-vis) absorbance at 560 nm. Before testing radical scavenging capability of pD, the relation of color change of NBT and radical concentration was investigated. For this, superoxide radical solutions with three different concentrations were prepared by diluting the initial radical solution. The initial and diluted solutions are denoted as concentration 1, 1/2, and 1/4, respectively. Eighty microliters (80 μL) of 500 μM NBT solution was added to 200 μL of each radical solution. After 10 min, the color changes of the solutions were detected by UV-vis analyses. The relation of radical concentration and absorbance intensity turned out to be linear (see Figure S1a in the Supporting Information). The radical scavenging capabilities at various pD concentrations were measured using the same radical solution stated above. The different amounts of pD (0.5, 1.0, and 1.75 wt %) were added to the initial radical solution and the solutions were vortexed for 10 s for uniform dispersion of the pD particles. After the mixing, the samples were waited for 10 min for sufficient radical scavenging with light blocked. Subsequently, NBT was added. The solutions were centrifuged at 13500 rpm for 10 min to sink the pD particles. Then, the supernatants were collected and the absorbance values of the solutions were measured (see Figure S1b in the Supporting Information). The reduced radical concentrations were calculated using the equation in Figure S1a in the Supporting Information. The electron paramagnetic resonance (EPR) (Bruker EMX/plus, Germany) was also measured to evaluate the radical scavenging capability of pD. The initial radical solution was prepared based on the aforementioned process, and 0.5 wt % pD was added. EPR was measured under the following conditions: 20 K, 0.94 mW microwave power, 9.64 GHz microwave frequency, 100 kHz modulation frequency, and 10G Mod Amp. To investigate the radical scavenging of pD during electrochemical processes, cyclic voltammetry (CV) was performed by using a three-electrode beaker cell. A glassy carbon (GC) disk (Pine Instruments, Inc., USA) with 5 mm in

Figure 1. (a) Schematic illustrations comparing Li−O2 battery with the human eye. The analogous situations of both systems suggest an approach of using pD, a synthetic melanin, as a superoxide radical scavenger. (b) FT-IR spectra of dopamine (D, black), polydopamine (pD, red), and commercial melanin (M, blue). The inset shows the color difference between the samples.

ROS and free radicals.23−25 In detail, retinal pigment endothelium (RPE) in the eye performs the photoprotection role by the pigment called melanin, which is usually synthesized by oxidative polymerization of 3,4-dihydroxy-L-phenylalanine (DOPA), histidine, and cysteine.24 The melanin in the RPE absorbs and scavenges the ROS and free radicals in an extremely effective manner.22,23,26,27 Taking the lesson from such effective defense mechanism of the human eye, we hypothesized that addition of synthetic melanin might serve the protective roles against the excessive generation of free radicals and ROS in Li−O2 cells (Figure 1a). As the first proof of the concept, one of the most common synthetic melanins namely, polydopamine (pD)28was adopted as a radical scavenger in the present investigation. Contrary to melanins extracted from natural pigments, synthetic melanins are impurity-free, so that other chemical and biological factors can be excluded. Moreover, the current synthetic melanin, pD, can be prepared via a simple aqueous process.29,30 As an outcome of the pD addition in the electrolyte, the Li−O2 cell 4758



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Figure 2. Radical scavenging capability of pD. (a) The remaining radical concentration as a function of the pD concentration. The radical concentrations were determined by measuring the changes of the peak intensities at 560 nm in UV−vis spectra. (b) EPR spectra of the radical solution before (blue) and after (red) the pD addition. diameter, Ag/AgCl, and a Pt wire were used as the working, reference, and counter electrodes, respectively. For this testing, 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6 ) (Aldrich, USA) salt was dissolved in dimethyl sulfoxide (DMSO) (Aldrich, USA) because this electrolyte has been known to show exclusive O2−/ O2 redox reaction and therefore allowed us to focus on the scavenging capability of pD.32 The scan rate was 50 mV s−1 in the potential range of −1.3−0.0 V vs Ag/AgCl at room temperature. Li−O2 Battery Assembly and Electrochemical Tests. For preparation of the air-cathodes, multiwalled carbon nanotubes (MWNTs) (CNT Co., Ltd., Korea) and poly(vinylidene fluoride) (PVDF) (Aldrich, USA) binder were first dispersed in N-methyl-2pyrrolidone (NMP) (Aldrich, USA) in a weight ratio of 9:1. The slurry was then cast onto a 16-mm-diameter gas diffusion layer (GDL) (TGPH-090 carbon paper, Torray, Japan), which served as a current collector. CNTs were used as an electrode material, because they have been known to minimize side reactions and, thus, enhance the cyclability.17 The CNT loading density on the GDL was 0.5 mg cm−2. After the casting, the air-cathodes were dried at 70 °C for 24 h under vacuum to remove residual solvent. For electrochemical characterization, modified coin cells in which bottom covers have 20 holes (each with a diameter of 1 mm) were prepared to facilitate sufficient O2 gas flow. Each modified coin cell was composed of a Li metal anode (thickness = 600 μm, Honjo, Japan), a glass fiber membrane (GF/D, Whatman, USA) impregnated with the electrolyte, and the air-cathode. For radical scavenging effect, 0.4 mg pD (= 0.009 M of 5,6dihydroxyindole (DHI), the monomeric component of pD)29 was added to 300 μL of the electrolyte. All of the electrochemical tests were performed under 1 atm pure oxygen (99.999%) atmosphere using the electrolyte in which 1 M LiTFSI is dissolved in TEGDME. TEGDME was chosen because it has been found to be relatively stable against superoxide radical attack and thus generates minimal side reaction products other than Li2O2.11,16,33 TEGDME has also been known to form a more stable interface on the Li metal anode, compared to other solvents, including DMSO and acetonitrile.6,34 The Li−O2 cells were electrochemically tested under galvanostatic and CV modes using a battery cycler (WonAtech, WBCS3000, Korea) at room temperature inside a chamber. The galvanostatic tests were conducted at a current density of 200 mA g−1 with a fixed capacity of 600 or 1000 mAh g−1. The upper voltage cutoff was set to 4.7 V to avoid electrolyte decomposition.9,33 CV was carried out at a scan rate of 0.1 mV s−1 in the range of 2.0−4.5 V vs Li/Li+. Characterization of pD and the Air-Cathode. To confirm the formation of pD and also to compare with dopamine and commercial melanin (Aldrich, USA), FT-IR (Bruker, USA) characterization using KBr pellets was performed. The morphologies of the pD particles and air-cathodes were characterized by field-emission scanning electron microscopy (Sirion FE-SEM, FEI, USA). The elemental mapping of the pD particles was conducted using energy-dispersive X-ray spectroscopy (EDAX) (UHR-SEM, FEI, Magellan 400). The crystal structures or crystallinity of the air-cathodes were analyzed at different states of battery cycling by acquiring X-ray diffraction (XRD) patterns

using an X-ray diffractometer (micro XRD, Rigaku, Japan) with Cu Kα (λ = 0.15406 nm). The chemical compositions and bonding characteristics of the pD particles, the air-cathodes, and the pD−PE separators were analyzed by FT-IR and XPS (Sigma probe, Thermo VG Scientific, England) with a Mg Kα line as the X-ray source. The C 1s peak at 284.8 eV was used as reference. For analyses of the aircathodes after electrochemical tests, the cells were disassembled in an argon-filled glovebox, and the air-cathodes were washed with 1,2dimethoxyethane (DME) (Aldrich, USA) and dried under vacuum for 24 h. For all the characterizations, moisture and air contamination were avoided by using an airtight sample box during sample transfer.

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RESULTS AND DISCUSSION As described above, the pD particles were prepared through dopamine oxidation by employing sodium periodate as an oxidizing agent. The formation of pD was first confirmed by its distinct FT-IR spectrum, compared to that of dopamine (Figure 1b). Dopamine hydrochloride (black line) exhibited a broad and strong band at 3500−3000 cm−1, corresponding to the O−H and NH2 stretching. The three small peaks at 2640, 2531, and 2429 cm−1 represent the N−H stretching mode.35,36 Upon polymerization, because of the intramolecular cyclization of dopamine, the three N−H stretching peaks of dopamine disappeared. Instead, peaks at 1616 and 1514 cm−1 that were assigned to the indole/indoline structure of pD newly appeared.37,38 In addition, a commercial melanin (blue line) showed almost the same spectrum as that of pD, implying the structural similarity between both compounds.30 XPS (Figure S2 in the Supporting Information), SEM (Figure S3 in the Supporting Information), and EDAX (Figure S4 in the Supporting Information) results also confirmed the synthesis of pD and are in good agreement with those previously reported.30,39 To investigate the radical scavenging capability of pD, the NBT reduction method was used. Superoxide radicals were generated by the reaction between KO2 and crown ether,11,40 and the UV-vis spectral change of the NBT solution was monitored. The superoxide radical concentration can be quantified by the linear relationship between the superoxide radical concentration and the UV-vis absorbance of the NBT solution (see Figure S1a in the Supporting Information).31 When tested at different pD concentrations (see Figure 2a and Figure S1b in the Supporting Information), the absorbance intensity of the NBT solution decreased with increasing pD concentration, verifying the quantitative radical scavenging capability of pD. EPR characterization further supported the radical scavenging capability of pD (Figure 2b). The original superoxide 4759



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Figure 3. (a) Cyclic voltammograms of the bare and pD cells under 1 atm of pure Ar and O2 (99.999%) when measured at 0.1 mV s−1 in the potential range of 2.0−4.5 V. (b) The first and third voltage profiles of the bare and pD cells when measured at 200 mA g−1 with a fixed capacity of 600 mAh g−1. C 1s XPS spectra of the air-electrodes after the first discharge for the (c) bare and (d) pD cells when measured at a current density of 200 mA g−1 with a fixed capacity of 1000 mAh g−1.

reactions involving oxygen molecules. Interestingly, when galvanostatically tested for the deep charge operation (Figure S5b in the Supporting Information), the bare cell showed a different behavior from that of the CV data. That is, the bare cell showed a similar recharge profile to that of the pD cell, unlike the CV result. This discrepancy between the CV and galvanostatic data can be understood in a way that during the galvanostatic recharge of the bare cell, side products formed during discharge can be decomposed or other side products can be newly formed/decomposed at high potential range,12,42 both of which turn out as recharge capacity comparable to that of the pD cell. By contrast, in the CV measurement driven by voltage sweep, such electrochemical reactions related to side products may not be feasible, making the recharge capacity of the bare cell much smaller than that of the pD cell. In addition, it is noted that the scavenging effect of pD holds true only to a small portion of radicals, and the majority of radicals still participate in the main reaction generating Li2O2. The small difference in the full discharge capacity (Figure S5a in the Supporting Information) between the bare and pD cells is one evidence supporting such view. To confirm further, CV measurements were conducted to characterize the reaction, O2 + e− ↔ O2• in the presence of pD, using 0.1 M TBAPF6 as salt (Figure S6a in the Supporting Information). TBAPF6, instead of Li salt, was used32,43 to focus exclusively on the reversibility of the given redox reaction. When tested at different pD concentrations, the reversibility, defined by the integrated area ratio between cathodic and anodic peaks, decreases with the pD concentration, because of the increased number of pD that scavenges radicals. However, the ratios are mostly ≳0.9 in all ranges of pD concentration (see the caption of Figure S6a in the Supporting Information for values), reconfirming that the radical scavenging of pD is a relatively minor event.

radical solution (blue line) exhibited a broad range of a oscillating peak due to large g-anisotropy originating from coupling between the spin magnetic moment and the orbital magnetic moment of superoxide radical,41 indicating that a substantial amount of superoxide radicals exists in the solution. Upon the addition of 0.5 wt % of pD, the peak intensity decreased drastically (red line), because of the oxidation of the oxygen radicals by the radical scavenging capability of pD. The radical scavenging ability of pD turned out to be effective, even in one cycle of CV tests (Figure 3a). When scanned at a rate of 0.1 mV s−1, the cell containing pD (denoted as pD cell) exhibited a slightly smaller main cathodic peak near 2.3 V, compared with that of the cell without pD (denoted as bare cell), because, in the pD cell, pD scavenges both active and residual superoxide radicals. Consistent with the CV data, the galvanostatic curve of the pD cell showed ∼250 mAh g−1 smaller specific capacity than that of the bare cell when measured at 200 mA g−1 (see Figure S5a in the Supporting Information). Thus, the pD additive could sacrifice the discharge capacity to some extent in deep-discharge operations. On the contrary, the anodic curves displayed an opposite trend that the current densities of the pD cell were higher than those of the bare cell in the potential range of 3.6− 4.3 V. This opposite trend is attributed to more efficient decomposition of discharging products in the case of pD cell, because irreversible side products from superoxide attack significantly decreased, which is also supported by the following XPS and FT-IR results. In the same context, the CV data of the pD cell exhibited enhanced reversibility when the integrated areas of the anodic and cathodic peaks are compared, once again by assistance of the pD additive that alleviated superoxide radical attack and thus irreversible side reactions. The control experiment testing the pD cell in Ar atmosphere (oxygen not accessible) showed almost no tangible current densities (Figure 3a, inset), indicating that the observed capacities are from the 4760



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Figure 4. (a) Cycling performance of the bare and pD cells when measured at 200 mA g−1 with a constant capacity of 600 mAh g−1. (b) XRD spectra and (c) SEM images of the air-electrode of the pD cell at the pristine, first discharged, and first charged states. (d) XPS spectra (C 1s and F 1s bands), (e) FT-IR spectra, and (f) SEM images of the air-electrodes after 10 cycles for the bare and pD cells. All of the samples whose data are shown in panels b−f were cycled at a current density of 200 mA g−1 with a fixed capacity of 1000 mAh g−1.

Figure 3b depicts the first and third discharge/charge voltage profiles of the bare and pD cells when measured at a current density of 200 mA g−1 with a constant specific capacity of 600 mAh g−1. We conducted fixed capacity tests to unify the capacity effect between both cells and rather focus solely on the radical scavenging effect. All of the capacities and current densities in this study are based on the mass of carbon only. While both samples showed almost the same discharging profiles near 2.7 V, the pD cell exhibited markedly lower charging potentials compared to those of the bare counterpart. In more detail, the charging potentials in the beginning period were quite similar between both samples, but started to behave different from the midcharging region. This observation is ascribed to the fact that, in the pD cell, the reduced superoxide attack by the pD additive produced smaller amounts of high potential products like Li2CO3 (ca. 3.86 V)10 with inferior reversibility, leading to the smaller overpotentials from the midpotential region.12,42,44 Instead, the pD cell generated more reversible Li2O2 with the lower equilibrium potential (ca. 2.96 V)3 as a dominant product during the discharging process. However, as the cycling progresses, the voltage profile of the pD cell also increased. To elucidate the role of the pD additive, XPS analyses were conducted for the samples after the first and third discharge. The deconvoluted C 1s spectrum of the bare cell after the first discharge (Figure 3c) showed the three grown peaks at 286.7, 289, and 289.7 eV, assigned to ether/alkoxides, carboxylates, and carbonates, respectively.34,45 These peaks are associated with a combination of the discharging products, C−O containing decomposition products, RCOOLi, and Li2CO3, all of which are related to the electrolyte decomposition facilitated by the superoxide attack.9,12,45 The XPS spectrum before cycling is presented in Figure S7 in the Supporting Information for comparison. Remarkably, the pD cell exhibited much smaller peaks related to the formation of such discharging products after the first discharge operated in the same condition (Figure 3d), verifying the significantly decreased side reactions by the radical scavenging effect of pD. The

smaller peak intensities were preserved even after the third discharge (Figure S8 in the Supporting Information), suggesting the persistent antioxidant effect of pD. However, these peaks slightly increased compared to those of the first discharge case, presumably because of Li2O2-induced electrolyte instability12,42 and carbon decomposition over 3.5 V.12,13 The radical scavenging effect of pD turned out directly in the superior cycling performance (Figure 4a). When the capacity was fixed to 600 mAh g−1, the bare cell began to drop its original capacity abruptly near the 60th cycle, whereas the pD cell fully retained the capacity over 100 cycles, again due to the alleviated side reactions by the radical scavenging of pD. The more sustainable operation of the pD cell was reflected in the more persistent voltage profiles (see Figure S9 in the Supporting Information), which are in sharp contrast against those of the bare cell that exhibited increased overpotential because of the successive side reactions. The effect of the pD addition was valid when the fixed capacity increased to 1000 mAh g−1 (see Figure S10 in the Supporting Information). The contribution of pD to the improved reversibility in each cycle was verified by series of analytical results. The XRD patterns obtained at different states of the first cycle (Figure 4b) indicate that Li2O2 was formed (see the star marks) and decomposed in a reversible manner.46 The Li 1s peak change in the XPS spectra also supported the reversible process with Li2O2 (see Figure S11 in the Supporting Information).34 Similarly, the SEM images (Figure 4c) taken at the same states of the first cycle are in the same line toward the reversibility. The air-cathode showed particles attributed to Li2O2 with dimensions around 200−300 nm at the end of discharge, but those particles disappeared after the subsequent charge. The improved reversibility by the pD additive was confirmed further by similar analytical results after 10 cycles. First, in the XPS analyses, the pD cell exhibited the decreased intensities for the aforementioned C 1s peaks at 286.7, 289, and 289.7 eV, compared to those of the bare cell (Figure 4d). Similarly, the F 1s spectrum of the pD cell showed a smaller LiF peak at 685.2 eV than that of the bare cell. The formation of the LiF peak is 4761



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Figure 5. XRD spectra of the air-electrode in (a) bare and (b) pD cells during 100 cycles. The red boxes include the (100) and (101) peaks of Li2O2. FT-IR spectra of the air-electrode in (c) bare and (d) pD cells during 100 cycles. All of the samples whose data are shown in panels a−d were cycled at a current density of 200 mA g−1 with a constant capacity of 600 mAh g−1.

Figure 6. C 1s XPS spectra of discharged and charged electrodes of the bare and pD cells after different discharge/charge cycles when measured at 200 mA g−1 with a constant capacity of 600 mAh g−1.

the SEM characterization (Figure 4f) also supported the distinct reversibility between both cells. The SEM image of the pD cell after 10 cycles clearly showed the pristine morphology of CNTs, whereas the SEM image of the bare cell showed CNTs covered by a substantial amount of the side products.50 The distinct trend between both cells was well maintained in the extended numbers of cycling. In XRD characterization, the

attributed to the decomposition of the binder and Li salt by the superoxide radical attack.34,47−50 Second, the FT-IR results (Figure 4e) constitute a consistent picture. The spectrum of the pD cell showed negligible peaks at the wavenumbers corresponding to the side products alongside the signals related to the electrolyte, whereas the spectrum of the bare cell exhibited the peaks assigned to CH3CO2Li and Li2CO3. Finally, 4762



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bare cell lost the (100) and (101) peaks assigned51 to Li2O2 after 30 cycles, whereas the pD cell preserved them until 50 cycles (see Figures 5a and 5b), which is related to the side products inhibiting the nucleation and crystallization of Li2O2. In the FT-IR and XPS characterizations (Figures 5c and 5d and 6), the pD cell showed weaker signals corresponding to carbonates and carboxylates than those of the bare cell at different cycle numbers, although even the pD cell had a tendency of moderately growing signals for these side products with cycling. The pD additive also turned out to contribute to the stability of Li metal anode. It has been known that the reduced TEGDME at the Li metal could convert the crossover oxygen to superoxide radicals, which can generate side products, such as LiOH and Li2CO3, by reaction with the Li metal.15,52 Also, water could be generated from the electrolyte and binder decomposition at the air-cathode and diffuse to the Li metal anode due to poor miscibility of water with the organic electrolyte.9,49 Similarly to the crossover oxygen, the crossover water can react with the Li metal, producing the aforementioned side products. As a symptom of such side products, the Li metal is subject to color change from silver to black.52−54 However, the pD cell maintained pretty much the original color after one cycle in contrast with the bare cell (see Figure S12 in the Supporting Information) via the radical scavenging effect of pD. In addition, the stability of pD in a Li−O2 cell was tested upon cycling. For this, pD was coated onto polyethylene (PE) separators, and the separators were characterized after 10 cycles using XPS and FT-IR (see Figure S13 and S14 in the Supporting Information). Those spectroscopic data showed the same spectra with those before cycling, which indicates stable characteristic of pD during successive cycling in the Li−O2 environment. It can be noted that, as in the case of melanin in the human eye, the radical scavenging of pD may not be fully reversible. Even in such condition, however, the radical scavenging effect could be quite substantial for a decent number of cycles for the following reasons. First, melanin is designed to capture ROS in a quasi-permanent manner based on the reaction between indoles: once pD-[DHI]n scavenges a radical, it reacts with other scavenged DHI, pD-[DHI]m, to form a new DHI, which can scavenge one additional radical. 24,55 Hence, upon scavenging of radicals and subsequent formation of a covalent bond, pD neither contains nor releases radicals. The reaction process can be summarized as below.

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CONCLUSION In summary, we have used polydopamine (pD), which is one of the most representative synthetic melanins, as a superoxide radical scavenger to adopt the lesson from the iris for the emerging Li−O2 battery technology. The radical scavenging capability of the synthetic melanin alleviated various side reactions that produce irreversible products, resulting in far superior cycling performance. The current approach is immediately applicable to a wide range of metal-air batteries suffering from similar superoxide radical attack, since the procedure is extraordinarily simple and versatile. Moreover, other numerous radical-scavenging analogues existing in nature or living organisms could fulfill similar functions toward improving the cycling stability of Li−O2 cells and could thus be interesting research subjects. Also, based on the promise of the current investigation, it would be a valuable research direction to pursue more sustainable radical scavengers that could be regenerated by external stimuli and additional chemical reactions.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray photoelectron spectroscopy (XPS) data, field-emission scanning electron microscopy (FE-SEM) images, energydispersive X-ray spectroscopy (EDAX) data, UV-vis spectra data, Fourier transform infrared spectroscopy (FT-IR) data, and additional electrochemical data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H. Lee). *E-mail: [email protected] (J. W. Choi). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.W.C. acknowledges the financial support by National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST, NRF-2010-C1AAA001-0029031 and NRF-2012-R1A2A1A01011970). H.L. acknowledges National Research Foundation of Korea (NRF) Midcareer Scientist grant (2014002855). B.G.K. acknowledges NRF2013-Global PhD Fellowship Program. The authors appreciate Korea Basic Science Institute (KBSI) for EPR measurements.

pD−[DHI]n + R• → pD−[DHI]n • + R pD−[DHI]m + R• → pD−[DHI]m • + R pD−[DHI]n • + pD−[DHI]m • → pD−[DHI]m −[ DHI]n

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→ pD−[DHI]m + n

Second, as discussed above, the radical scavenging of pD in our system is applied to a limited portion of generated radicals, and the majority of radicals participate in the main cell reaction (2Li + O2 ↔ Li2O2). Hence, despite the irreversible character of pD in scavenging radicals, its influence under the given cell condition could be effective for a reasonably large number of cycles, as indicated by the CV result in Figure S6b in the Supporting Information.

REFERENCES

(1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Nat. Mater. 2012, 11, 19. (2) Lee, J.-S.; Tai Kim, S.; Cao, R.; Choi, N.-S.; Liu, M.; Lee, K. T.; Cho, J. Adv. Energy Mater. 2011, 1, 34. (3) Shao, Y.; Ding, F.; Xiao, J.; Zhang, J.; Xu, W.; Park, S.; Zhang, J.G.; Wang, Y.; Liu, J. Adv. Funct. Mater. 2013, 23, 987. (4) Zhang, T.; Imanishi, N.; Shimonishi, Y.; Hirano, A.; Takeda, Y.; Yamamoto, O.; Sammes, N. Chem. Commun. 2010, 46, 1661.

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