Polyoxometalate as a Nature-Inspired Bifunctional Catalyst for Lithium

Jun 25, 2018 - Jun-Seo Lee† , Cheolmin Lee‡ , Jae-Yun Lee† , Jungki Ryu*‡ , and Won-Hee Ryu* ... Nature-inspired molecules present a family of...
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Polyoxometalate as a Nature-Inspired Bifunctional Catalyst for Lithium-Oxygen Batteries Jun-Seo Lee, Cheolmin Lee, Jae-yun Lee, Jungki Ryu, and Won-Hee Ryu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01103 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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

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ACS Catalysis

Polyoxometalate as a Nature-Inspired Bifunctional Catalyst for Lithium-Oxygen Batteries Jun-Seo Lee,1 Cheolmin Lee,2 Jae-Yun Lee,1 Jungki Ryu,2,* and Won-Hee Ryu1,* 1

Department of Chemical and Biological Engineering, Sookmyung Women’s University, 100

Cheongpa-ro 47-gil, Yongsan-gu, Seoul, 04310, Republic of Korea 2

Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National

Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan, 44919, Republic of Korea *Corresponding author. E-mail: [email protected] (Prof. Won-Hee Ryu), [email protected] (Prof. Jungki Ryu)

Abstract Nature-inspired molecules present a family of affordable, environmentally friendly catalysts to enable and enhance next-generation energy storage systems. In this study, we report the use of cobalt-based polyoxometalates (Co-POM) with an oxo-bridged tetracobalt active site, which is reminiscent of the natural oxygen-evolving complex, as an efficient and stable redox catalyst for Li-O2 batteries. Interestingly, Co-POM exhibits catalytic activity for both oxygen evolution and reduction reactions (OER and ORR, respectively) under a certain condition when it forms a stable dispersion of molecular aggregates, which can be controlled by types of electrolyte solvents and exposure to light. As a result of the optimized OER/ORR bifunctional activity, the Li-O2 cells facilitated by Co-POM redox reactions successfully achieve improved efficiency and a longer cycle life compared to reference cells. The reversibility of the Li-O2 reactions in the presence of the bifunctional Co-POM catalysts is confirmed by ex-situ characterizations. ACS Paragon Plus Environment

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Keywords: lithium-oxygen batteries, polyoxometalate, nature-inspired molecule, dispersion catalyst, water oxidation catalyst

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Introduction Development of an efficient and rechargeable energy storage device with high power and energy densities is a key technology for sustainable distribution and the use of energy in conjunction with existing and developing technologies on renewable energy and transportation.1-2 Research on conventional Li-ion batteries has approached its theoretical limit and has been limited to the simple replacement or moderate performance improvement of its components, such as cathode, anode, electrolyte, etc.3-4 For example, cathode materials have been extensively studied because of the intrinsic limitations of the currently available materials, such as (i) a lower capacity (i.e., LiCoO2: ~150 mAh/g) than anode materials (i.e., graphite: ~370 mAh/g, Si: ~2000 mAh/g) due to the limited number of Li-ion accommodation sites in the electrode, (ii) heavy weight, and (iii) high material cost due to the incorporation of large amounts of transition metals.5-6 To overcome such inherent problems, novel device architectures and candidate materials have been suggested recently.7-8 In particular, lithiumoxygen (Li-O2) batteries employing a lightweight and gaseous oxygen cathode have been spotlighted due to their exceptional high-energy density (practically 2~3 times higher than lithium-ion cells).9-12 Unlike Li-ion batteries relying on Li insertion chemistry, the Li-O2 batteries operate via the reversible oxygen reduction and evolution reactions (ORR and OER, respectively) on the electrode surface (2Li+ + O2 + 2e- ↔ 2Li2O2, Eo=2.96V vs. Li/Li+).13 As a result, they can accept more Li ions on a cathode and have much higher capacity (2 Li+ per O2 for Li-O2 cells and less than 1 Li+ per LiCoO2 for Li-ion cells).14 However, their poor rate capability and cyclability originating from the sluggish kinetics for the formation and decomposition of lithium oxide products (i.e., LiO2, Li2O2) and large overpotential for OER (> 1V) necessitate utilization of OER and ORR catalysts.15-16 Although decoration of the electrode with solid catalysts (e.g., noble metals and metal oxides) is found to facilitate the Li-O2 cell reactions, they are quickly deactivated upon cycling because of burial inside the continuously accumulating product residues.17-19 Incorporation of soluble redox molecules into an electrolyte solution has recently been considered an effective alternative strategy to address the aforementioned problems. Their redox properties enable ACS Paragon Plus Environment

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rapid electron transfer from/to the electrode through their self-diffusion and subsequent redox reactions instead of a slow electron diffusion through the insulating lithium peroxide products. Excellent redox catalysts for Li-O2 cells should satisfy the following requirements: (i) high mobility in electrolyte; (ii) reversible redox properties for facile electron transfer; (iii) robust stability; and (iv) environmental friendliness. In this regard, various redox molecules have been explored, such as Li iodide, Li bromide, tetrathiafulvalene, Fe phthalocyanine, (2,2,6,6-tetramethylpiperdin-1-yl)oxyl, 2,5-di-tert-butyl-1,4benzoquinone, 5,10-dimethylphenazine, and heme.20-32 Although they are proven to be catalytically active for Li-O2 cells, they partially fulfill these criteria. In this context, polyoxometalates (POMs) are a promising molecular catalyst for Li-O2 cells. They can be readily synthesized from earth-abundant elements under mild conditions (i.e., aqueous solution and low temperature) and incorporate various oxo-bridged transition metal clusters. As a result, they display rich electrochemistry that can be finely tuned by controlling their composition and structure. POMs with a tetracobalt ([Co4(H2O)2(PW9O34)2]10-) (Co-POM) are especially reminiscent of a natural oxygenevolving complex in the photosystem II, where oxo-bridged manganese ions form a distorted cubane structure and catalyze water oxidation with a moderate overpotential of 0.3 ~ 0.4 V (Figure 1).33-36 Indeed, they have been spotlighted as a highly efficient and robust molecular water oxidation catalyst (WOCs). The oxo-bridged polynuclear active site in these POMs is known to readily accumulate the long-lived oxidation state and facilitate a slow four-electron process (2H2O → 4H+ + O2 + 4e-). In this regard, we anticipate the following advantages by incorporating POM catalysts in a Li-O2 system: (i) analogous oxidation characteristics between monovalent cations (H+ vs. Li+) and oxygen gas for improved catalytic activity; (ii) ready access to the cathode and reversible redox reactions by POMs (i.e., Co2+/Co3+) for facile catalytic charge transfer to the cathode; and (iii) efficient and reliable operation of Li-O2 batteries due to high stability of inorganic POM catalysts. Here we introduce a nature-inspired, catalytic system using Co-POM molecules for improved OER, ORR, and battery performance in Li-O2 cell. As illustrated in Figure 1, we revisit Co-POM molecules as an efficient WOC (i.e., OER catalyst) and prove its effects as both redox mediator and electrolyte ACS Paragon Plus Environment

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catalyst of Li-O2 batteries. We elucidate the electrochemical reaction mechanism of Co-POM in Li-O2 cells in comparison with a typical water oxidation system. In addition to the reported OER activity, CoPOM exhibits electrolyte- and dispersion state-dependent OER/ORR bifunctional catalytic activity. Interestingly, simple exposure of Co-POM to light results in the change in its dispersion state with a loss of the bifunctional catalytic activity. Optimization of these conditions leads to a significant performance improvement of the Li-O2 cell incorporating Co-POM molecules in terms of rate cycle performance. The reversible operation of Li-O2 cells and composition of discharge products in the presence of the CoPOM molecules are confirmed by ex-situ characterizations. We believe that our crossover approach can provide another hint for the breakthrough of challenging the Li-O2 battery system as well as novel insight into various forms of interdisciplinary studies.

Results & discussion Catalytic activity of Co-POM WOC was characterized by electrochemical and photocatalytic oxygen evolution tests. Cyclic voltammetry (CV) was carried out using a buffer solution (80 mM phosphate buffer, pH 8.0) in the presence and absence of dissolved Co-POM (Figure 2a). While the buffer alone exhibited a negligible current density, the dissolution of molecular Co-POM with the average hydrodynamic diameter of 2.54±0.96 nm (Figure S1) led to a significantly increased current density with a moderate onset potential for water oxidation of 1.62 V vs. reversible hydrogen electrode (RHE). As a result, there was a considerable decrease of the Tafel slope from 410 to 113 mV/decades through dissolution of Co-POM WOC (inset of Figure 2a). Given that water oxidation is carried out at a neutral pH using a dissolved WOC, the measured Tafel slope is quite impressive. Photocatalytic water oxidation activity was also evaluated to demonstrate its excellent catalytic performance in terms of charge transfer and oxygen evolution. Tris(bipyridine)ruthenium (Ru(bpy)32+), sodium persulfate, and Co-POM were used as a photosensitizer, electron scavenger, and WOC, respectively. It is noteworthy here that Ru(bpy)32+ becomes quickly deactivated in the absence of efficient hole scavengers under the visible light illumination.37 Thus, the linear increase in the photocatalytic production of molecular oxygen ACS Paragon Plus Environment

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indicates that Co-POM rapidly scavenged photogenerated holes from Ru(bpy)32+ and efficiently catalyzed OER (Figure 2b). The turnover frequency of Co-POM was calculated to be 9.0×102 O2 / POM · h or 3.6×103 electrons / POM · h, which is significantly higher than that of other Ru(bpy)32+-based photocatalytic water oxidation systems.37 These results demonstrate an excellent catalytic activity of CoPOM in various oxygen-evolving systems. Considering the excellent catalytic activity of Co-POM for water splitting, we employed the Co-POM molecule as a catalytic redox molecule for a Li-O2 battery. This crossover application should be distinct because electrochemical activity of Co-POM has been explored in an aqueous solution with a narrow potential window (~ 1 V), whereas battery systems operate in a non-aqueous electrolyte with a much wider

potential

window

(~

4

V).

Co-POM

was

dispersed

in

1

M

lithium

bis(trifluoromethane)sulfonamide (LiTFSI) in an ether-based electrolyte, and CV analysis was carried out to investigate its electrochemical redox features in the Li-O2 cell (Figures 2c-f). We chose two different ether-based solvents: diethylene glycol dimethyl ether (DEGDME) and tetraethylene glycol dimethyl ether (TEGDME). Ethers with a different chain-length and dissociation properties have been widely used as an electrolyte solvent in Li-O2 cells (insets of Figure 2c and 2d).38-40 It is noteworthy here that the catalytic formation and decomposition of discharge products highly depend on the types of solvent species29 since the solvation of catalysts can have a significant influence on their accessibility to the nearby oxygen electrodes. Both cases using LiTFSI+DEGDME and LiTFSI+TEGDME without CoPOM show a sloping curve in the cathodic region until 2.0 V and a broad peak in the anodic region, corresponding to the formation and decomposition of Li2O2 products, respectively (Figure 2c and 2d). This result is identified as a typical charge-discharge reaction in Li-O2 cells, as reported.20, 41 A peak related to cathodic reaction in the case of LiTFSI+DEGDME appears at a higher voltage than that of LiTFSI+TEGDME, indicating that the former shows faster kinetics for ORR. While no additional peaks appeared for the LiTFSI+DEGDME electrolyte, additional peaks were clearly observed after the addition of Co-POM (i.e., LiTFSI+TEGDME+Co-POM): a cathodic peak at 2.46 V and anodic peaks at 2.75, 3.34, and 4.34 V (Figure 2e and 2f). The shift of the cathodic peak to a higher voltage (2.11 V → ACS Paragon Plus Environment

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2.46 V) means improved ORR kinetics upon discharge of Li-O2 cells. The additional anodic peaks are attributed to more effective OER by Co-POM. This result demonstrates the OER/ORR bifunctional activity of Co-POM, which depends on the type of electrolyte. For Li-O2 cells, reversible decomposition of lithium oxide products (i.e., LiO2, Li2O2) is difficult due to the low conductivity of the products and the large overpotential for OER. To measure the degree of overpotential for OER, we present galvanostatic intermittent titration technique (GITT) curves obtained during the first charge cycle (Figure 2g and 2h). The overpotential value associated with cell resistance is determined from the difference between the constant current voltage (applied current on) and open circuit voltage (applied current off) at each point during charging. The cell containing dispersed Co-POM exhibits significantly lower overpotential (0.6 V for DEGDME and 0.45 V for TEGDME) upon charging, compared to the pristine cell without Co-POM (0.7 V for DEGDME and 0.85 V for TEGDME). The results support the CV analysis that Co-POM effectively catalyzes Li-O2 cell reactions in TEGDME rather than DEGDME. The TEGDME possess two times longer molecular chain than DEGDME, indicating that the TEGDME effectively surrounds a large area of the POM molecule compared to that of DEGDME (insets of Figure 2c and 2d). In this regard, delocalized charge regions in TEGDME could make stronger molecular interactions with Co-POM for enabling catalytic activity. To further verify solvent effects with different chain lengths, Figures S7 and S8 show CV and initial charge/discharge curves obtained from dimethyl ether (DME), DEGDME, and TEGDME with and without Co-POM. From the electrochemical results, we confirmed that electrochemical performance of Li-O2 cells including shorter ether chain such as DME and TEGDME become deteriorated when Co-POM is included, unlike TEGDME electrolyte case. Although further detailed reasons for solvent effects should be investigated as a future work, it is implied that the catalytic activity of Co-POM can be selectively determined, even under the same etherbased solvents. We evaluated the electrochemical performance of Li-O2 cells with the Co-POM catalyst to verify its beneficial effect for practical application (Figure 3). As a control, we prepared a pristine multi-walled carbon nanotube (MWCNT) oxygen electrode without any catalysts. Figures 3a and 3b present the first ACS Paragon Plus Environment

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charge/discharge profiles of MWCNT electrodes cycled between 4.5 and 2.3 V at a current density of 100 mA g-1carbon in a LiTFSI+DEGDME and LiTFSI+TEGDME electrolyte, respectively, with and without Co-POM, respectively. The pristine cell comprising LiTFSI+DEGDME exhibits a higher discharge capacity value (6972 mAh g-1carbon) than that (3090 mAh g-1carbon) using LiTFSI+TEGDME.4243

However, coulombic efficiency was slightly better with the LiTFSI+TEGDME electrolyte (95.2 %)

than with LiTFSI+DEGDME (93.8%). After the addition of Co-POM, coulombic efficiency reached almost 100% for both cases. While the capacity of the cell utilizing LiTFSI+DEGDME+Co-POM decreased about two-fold, using LiTFSI+TEGDME+Co-POM significantly increased from 3090 mAh g1

carbon

to 4066 mAh g-1carbon. Catalytic redox reactions of Co-POM only occur in LiTFSI+TEGDME, not

LiTFSI+DEGDME. This finding implies that the catalytic activity of a soluble Co-POM catalyst highly depends on the type of solvent medium. We focus on the effectiveness of Co-POM as an electrolyte catalyst in this study, and additional studies regarding the effects of other solvents will be conducted in future work. We also show the cycle stability of the cells with and without Co-POM in different solvents between 4.5 and 2.3 V at a current density of 100 mA g-1carbon (Figures 3c-f). Consistent with CV and charge/discharge results, Co-POM more effectively works in LiTFSI+TEGDME rather than in LiTFSI+DEGDME. Although LiTFSI+DEGDME delivers superb capacity value itself, it is not further catalyzed by a Co-POM molecule, and charge/discharge profiles are continuously changed with a higher voltage gap during cycling (Figures 3c and 3d). In the case of LiTFSI+TEGDME without Co-POM, discharge capacity is suddenly degraded after the 24th cycle, and the terminal charge voltage quickly reached the top of the voltage window of 4.5 V (Figure 3e). In contrast, a cell with Co-POM exhibited a stable charge-discharge behavior for 40 cycles and a terminal charge voltage of only 4.2 V, confirming that the Co-POM molecule effectively catalyzes both ORR and OER and improves the efficiency and cycle life of Li-O2 cells (Figure 3f). Unexpectedly, we found that the dispersion-state and its catalytic activity in Li-O2 cells can be controlled by light exposure. The photographs of the electrolytes in various combinations are shown in Figure 4a. LiTFSI+DEGDME and LiTFSI+TEGDME electrolytes are transparent. After the addition of ACS Paragon Plus Environment

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Co-POM into the electrolyte, Co-POM is dispersed in the solvent and is rarely dissolved in the electrolyte (opaque solution) (Figure S2). However, there was no Co-POM sediment on the bottom, even after a few days, meaning that the Co-POM molecules were well-dispersed rather than solubilized. Initially, the electrolyte solution and POMs were kept under dark conditions to avoid any potential degradation. After the exposure to even room light, interestingly, the LiTFSI+TEGDME+Co-POM solution became transparent and violet colored. In contrast, LiTFSI+DEGDME+Co-POM remained unchanged except for a color change from white to violet (Figure 4a). To evaluate the catalytic behavior of the electrolytes with Co-POM before and after exposure to room light, we presented GITT curves obtained during the first charge cycle in different electrolytes (Figure 4b and 4c). Then both cells using LiTFSI+DEGDME+Co-POM and LiTFSI+TEGDME+Co-POM electrolytes were deteriorated by exposure to room light. However, the degradation trend is different for each electrolyte. The overpotential value of the cell using LiTFSI+DEGDME+Co-POM after light exposure suddenly increased in the entire charging region (0.7 V → 0.9 V). On the other hand, the degree of overpotentials of the cell using LiTFSI+DEGDME+Co-POM was almost the same before and after light exposure, and absolute values for Von and Voff shifted to higher values after light exposure. However, absolute voltages at Ion and Ioff shifted to higher values even at the beginning of charging, and these are maintained during charging. These results demonstrate that the Co-POMs can lose their catalytic function by light exposure and consequent dissociation in a solution. We further investigated the electrochemical properties of CoPOM before and after light exposure (Figure S3). Redox peaks related to Co-POM were diminished after light exposure and became similar to a pristine electrolyte without Co-POM. As a result, cell performance was deteriorated by light exposure of Co-POM, and the charge-discharge behavior was consequently changed into a pristine electrolyte case, which is the same for the CV results (Figure S4). To further explore the chemical status of Co-POM molecule after light exposure and its subsequent dissolution

in

electrolyte,

we

performed

ultraviolet-visible

absorption

measurement

of

LiTFSI+TEGDME+Co-POM electrolytes with and without light exposure. For electrolytes without light exposure, there was no significant signal due to severe scattering of light by Co-POM dispersion (Figure ACS Paragon Plus Environment

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4d). On the other hand, a relatively small peak near 500 nm and a relatively strong peak near 570 nm were identified, corresponding to typical POM absorption (Figure 4e). Two peaks were associated with the chemical state of Co ions centered in Co-POM molecules. The small peak at a lower wavelength of 500 nm was related to a high spin state of Co-POM (Co3+), and that at a higher wavelength (~570 nm) was related to a low spin state of Co-POM (Co2+).44 This observation supports our finding that the dispersion state of POM in an electrolyte is only available for providing catalytic activity rather than a soluble or dissociated state of a Co ion center before Li-O2 cell reaction. Most relevant studies have reported redox activities of soluble catalysts for Li-O2 batteries.20, 28 We first demonstrated that dispersed molecule in an electrolyte can effectively function as a catalyst. Catalyst decoration strategies have been approached through two different typical methods: (1) immobilization of solid catalysts on an oxygen electrode and (2) incorporation of a soluble catalyst in an electrolyte.18 . In this work, we offer another way to develop a catalytic system for Li-O2 batteries by employing dispersed catalyst molecules. In addition, catalytic redox properties can be turned off by photo-induced dissociation of POM aggregates and their consequent solvation in an ether-based electrolyte. Considering that the artificial switching and control of Co-POM activity could be tuned off, we could develop an interesting concept called “in-situ switch-off catalyst” in a Li-O2 cell system by controlling the light emission into an electrolyte whenever we want. Ex-situ characterizations were carried out to elucidate the structure of discharge products formed on the electrode surface and to confirm the reversibility of Li-O2 cells employing Co-POM catalysts (Figure 5). We present X-ray photoelectron spectra (XPS) of the discharged and charged electrodes collected in the C 1s, O 1s, and Li 1s regions, respectively (Figure 5a-c). C-O and C-C peaks originating from the MWCNT oxygen electrode substrate were found in the spectra of both electrodes (Figure 5a). The strong C-O peak was attributed to a trapped TEGDME electrolyte.18 CO32- peak was observed for both electrodes, corresponding to partial contamination of discharge products during sample transfer in air.45 The inevitable formation of CO32- on an electrode surface after discharge and charge has been reported as a transformation of lithium oxide products (i.e., LiO2, Li2O2) by reacting with a carbon substrate ACS Paragon Plus Environment

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(MWCNT).46 We also detected CF3 peak at 293 eV for both electrodes regarding residual LiTFSI salt. To verify the reversibility of product formation and decomposition reactions, XPS spectra for O 1s and Li 1s were observed (Figure 5b and 5c). In the O 1s, the pristine electrode shows two peaks at 533 eV (C-O) and 530.7 eV (O2-, OH-) related to the MWCNT substrate and H2O (or hydroxide). The feature for O2- and OH- resulted from minor surface contamination of MWCNT through exposure to ambient air.47 After discharge, a broad peak was deconvoluted into two peaks centered at ~533.4 and ~531.7 eV, corresponding to C-O and Li2O2, respectively.45 The shoulder peak related to Li2O2 was diminished after charge. Similarly, two deconvoluted peaks corresponding to LiTFSI salt (56 eV) and Li2O2 (55 eV), respectively, were found in the case of spectra collected from Li 1s. The intensity of the peak for Li2O2 decreased after charge.48 These results mean that the discharge products were reversibly decomposed upon charging when assisted by a Co-POM catalyst. Surface morphologies of the MWCNT electrodes at different electrochemical states were shown (Figure S5). After discharge, lithium oxide products (i.e., LiO2, Li2O2) were precipitated and covered the electrode surface. The products were reversibly decomposed after the following charge. From the magnified SEM images collected after full discharge, typical products composed of particulate and amorphous shapes were fully covered on the electrode (Figure 5d). It has been elucidated that the morphology transforms from planar to particulate structures during discharge reaction because amorphous LiO2 is formed at the beginning, and the further reduction to Li2O2 (particular shape) by disproportionation occurs.18,

49-50

There is no XRD peak in terms of

lithium-oxide species on discharged and charged electrodes, which means that the products remained in an amorphous phase rather than a crystalline structure, as reported (Figures 5e and S6).51-52 Our observation regarding discharge products concludes that the addition of Co-POM does not affect the product features. To further elucidate the fact that Co-POM only facilitates catalytic reaction and is not incorporated as a side product by self-decomposition, we show the XPS spectra of the discharged and charged electrodes collected in the Co 2p (Figure 5f). We could not find any Co peak from both discharged and charged electrodes, demonstrating stability of the Co-POM catalyst against a Li-O2 cell environment. We first report that Co-POMs can be utilized as a dispersed catalytic agent and ACS Paragon Plus Environment

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successfully confirm their effective catalytic reaction in a Li-O2 cell. This allows for a new cross-linking study of nature-inspired water oxidation catalysts and the emerging energy storage system.

Conclusion In summary, we report the effective catalytic effects of Co-POM as a nature-inspired and abundant molecular catalyst toward the decomposition and oxidation reaction of lithium oxide products (i.e., LiO2, Li2O2) in a Li-O2 cell. The Co-POM, known as a water oxidation catalyst, exhibited excellent OER/ORR bifunctional catalytic performance for Li-O2 batteries. We found that the Co-POMs selectively exhibit redox properties in the longer ether-chain environment of the TEGDME solvent and not in the DEGDME electrolyte. The Li-O2 cell employing Co-POM molecules achieves a reduced overpotential and longer cycle performance compared with the control. Interestingly, the Co-POMs dispersed in an electrolyte become soluble when exposed to room light and consequently lose bifunctional catalytic activity. We also verify the reversible formation and decomposition of discharge products on the oxygen electrode by ex-situ studies. POM molecules effectively catalyze the reactions without self-decomposition during cell operation. This work provides several viewpoints: (i) POM catalysts used in water oxidation can be effectively utilized for crossover application in a Li-O2 battery system; (ii) a class of POM molecules should be dispersed in an electrolyte, instead of using dissolution, to offer the catalytic activities; and (iii) the redox characteristics of POM molecules can be tuned off by light emission, enabling the catalyst to be switched off. We provide an interesting pathway to develop efficient and robust catalyst materials for next-generation energy storages by introducing nature-inspired molecules.

Associated content Supporting Information Available: Experimental procedure, dynamic light scattering analysis, photographs of electrolytes with and without catalyst, additional electrochemical data (charge/discharge

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curves and CV data), ex situ surface morphologies, ex situ XRD results,. This material is available free of charge via the Internet at http://pubs.acs.org. Note The authors declare no competing financial interest. Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2016R1C1B2011442). This work was also supported by the 2018 Research Fund (1.180014.01) of UNIST (Ulsan National Institute of Science and Technology).

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21. Lim, H. D.; Song, H.; Kim, J.; Gwon, H.; Bae, Y.; Park, K. Y.; Hong, J.; Kim, H.; Kim, T.; Kim, Y. H.; Lepro, X.; Ovalle-Robles, R.; Baughman, R. H.; Kang, K., Superior Rechargeability and Efficiency of Lithium-Oxygen Batteries: Hierarchical Air Electrode Architecture Combined with a Soluble Catalyst. Angew Chem Int Edit 2014, 53, 3926-3931. 22. Yu, M. Z.; Ren, X. D.; Ma, L.; Wu, Y. Y., Integrating a redox-coupled dye-sensitized photoelectrode into a lithium-oxygen battery for photoassisted charging. Nat Commun 2014, 5, 5111, 16. 23. Gao, X. W.; Chen, Y. H.; Johnson, L. R.; Jovanov, Z. P.; Bruce, P. G., A rechargeable lithiumoxygen battery with dual mediators stabilizing the carbon cathode. Nat Energy 2017, 2, 17118, 1-7. 24. Kwak, W. J.; Hirshberg, D.; Sharon, D.; Afri, M.; Frimer, A. A.; Jung, H. G.; Aurbach, D.; Sun, Y. K., Li-O-2 cells with LiBr as an electrolyte and a redox mediator. Energ Environ Sci 2016, 9, 23342345. 25. Chen, Y. H.; Freunberger, S. A.; Peng, Z. Q.; Fontaine, O.; Bruce, P. G., Charging a Li-O2 battery using a redox mediator. Nat Chem 2013, 5, 489-494. 26. Feng, N. N.; He, P.; Zhou, H. S., Enabling Catalytic Oxidation of Li2O2 at the Liquid-Solid Interface: The Evolution of an Aprotic Li-O-2 Battery. Chemsuschem 2015, 8, 600-602. 27. Zhu, Y. G.; Wang, X. Z.; Jia, C. K.; Yang, J.; Wang, Q., Redox-Mediated ORR and OER Reactions: Redox Flow Lithium Oxygen Batteries Enabled with a Pair of Soluble Redox Catalysts. ACS Catal 2016, 6, 6191-6197. 28. Park, J. B.; Lee, S. H.; Jung, H. G.; Aurbach, D.; Sun, Y. K., Redox Mediators for Li-O-2 Batteries: Status and Perspectives. Adv Mater 2018, 30, 1704162, 1-13. 29. Bergner, B. J.; Schurmann, A.; Peppler, K.; Garsuch, A.; Janek, J., TEMPO: A Mobile Catalyst for Rechargeable Li-O-2 Batteries. J Am Chem Soc 2014, 136, 15054-15064. 30. Gao, X. W.; Chen, Y. H.; Johnson, L.; Bruce, P. G., Promoting solution phase discharge in Li-O2 batteries containing weakly solvating electrolyte solutions. Nature Materials 2016, 15, 882-888. 31. Lim, H. D.; Lee, B.; Zheng, Y.; Hong, J.; Kim, J.; Gwon, H.; Ko, Y.; Lee, M.; Cho, K.; Kang, K., Rational design of redox mediators for advanced Li-O-2 batteries. Nat Energy 2016, 1, 16066. 32. Li, Y.; Dong, S.; Chen, B.; Lu, C.; Liu, K.; Zhang, Z.; Du, H.; Wang, X.; Chen, X.; Zhou, X.; Cui, G., Li–O2 Cell with LiI(3-hydroxypropionitrile)2 as a Redox Mediator: Insight into the Working Mechanism of I– during Charge in Anhydrous Systems. The Journal of Physical Chemistry Letters 2017, 8, 4218-4225. 33. Yin, Q.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L., A fast soluble carbon-free molecular water oxidation catalyst based on abundant metals. Science 2010, 328, 342-345. 34. Huang, Z.; Luo, Z.; Geletii, Y. V.; Vickers, J. W.; Yin, Q.; Wu, D.; Hou, Y.; Ding, Y.; Song, J.; Musaev, D. G.; Hill, C. L.; Lian, T., Efficient Light-Driven Carbon-Free Cobalt-Based Molecular Catalyst for Water Oxidation. J Am Chem Soc 2011, 133, 2068-2071. 35. Geletii, Y. V.; Botar, B.; Kögerler, P.; Hillesheim, D. A.; Musaev, D. G.; Hill, C. L., An AllInorganic, Stable, and Highly Active Tetraruthenium Homogeneous Catalyst for Water Oxidation. Angewandte Chemie International Edition 2008, 47, 3896-3899. 36. Geletii, Y. V.; Besson, C.; Hou, Y.; Yin, Q.; Musaev, D. G.; Quiñonero, D.; Cao, R.; Hardcastle, K. I.; Proust, A.; Kögerler, P.; Hill, C. L., Structural, Physicochemical, and Reactivity Properties of an All-Inorganic, Highly Active Tetraruthenium Homogeneous Catalyst for Water Oxidation. J Am Chem Soc 2009, 131, 17360-17370. 37. Limburg, B.; Bouwman, E.; Bonnet, S., Rate and Stability of Photocatalytic Water Oxidation using [Ru(bpy)3]2+ as Photosensitizer. Acs Catal 2016, 6, 5273-5284. 38. Laoire, C. O.; Mukerjee, S.; Abraham, K. M.; Plichta, E. J.; Hendrickson, M. A., Influence of Nonaqueous Solvents on the Electrochemistry of Oxygen in the Rechargeable Lithium-Air Battery. J Phys Chem C 2010, 114, 9178-9186.

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39. McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Girishkumar, G.; Luntz, A. C., Solvents' Critical Role in Nonaqueous Lithium-Oxygen Battery Electrochemistry. J Phys Chem Lett 2011, 2, 1161-1166. 40. Gittleson, F. S.; Jones, R. E.; Ward, D. K.; Foster, M. E., Oxygen solubility and transport in Liair battery electrolytes: establishing criteria and strategies for electrolyte design. Energ Environ Sci 2017, 10, 1167-1179. 41. Li, F. J.; Zhang, T.; Zhou, H. S., Challenges of non-aqueous Li-O-2 batteries: electrolytes, catalysts, and anodes. Energ Environ Sci 2013, 6, 1125-1141. 42. Carbone, L.; Gobet, M.; Peng, J.; Devany, M.; Scrosati, B.; Greenbaum, S.; Hassoun, J., Comparative Study of Ether-Based Electrolytes for Application in Lithium-Sulfur Battery. Acs Appl Mater Inter 2015, 7, 13859-13865. 43. Sharon, D.; Hirshberg, D.; Afri, M.; Frimer, A. A.; Aurbach, D., The importance of solvent selection in Li–O2 cells. Chem Commun 2017, 53, 3269-3272. 44. Evangelisti, F.; Car, P. E.; Blacque, O.; Patzke, G. R., Photocatalytic water oxidation with cobalt-containing tungstobismutates: tuning the metal core. Catal Sci Technol 2013, 3, 3117-3129. 45. Gittleson, F. S.; Ryu, W. H.; Schwab, M.; Tong, X.; Taylor, A. D., Pt and Pd catalyzed oxidation of Li2O2 and DMSO during Li-O-2 battery charging. Chem Commun 2016, 52, 6605-6608. 46. McCloskey, B. D.; Speidel, A.; Scheffler, R.; Miller, D. C.; Viswanathan, V.; Hummelshoj, J. S.; Norskov, J. K.; Luntz, A. C., Twin Problems of Interfacial Carbonate Formation in Nonaqueous Li-O-2 Batteries. J Phys Chem Lett 2012, 3, 997-1001. 47. Ryu, W. H.; Wilson, H.; Sohn, S.; Li, J. Y.; Tong, X.; Shaulsky, E.; Schroers, J.; Elimelech, M.; Taylor, A. D., Heterogeneous WSX/WO3 Thorn-Bush Nanofiber Electrodes for Sodium-Ion Batteries. Acs Nano 2016, 10, 3257-3266. 48. Zhu, Y. G.; Jia, C. K.; Yang, J.; Pan, F.; Huang, Q. Z.; Wang, Q., Dual redox catalysts for oxygen reduction and evolution reactions: towards a redox flow Li-O-2 battery. Chem Commun 2015, 51, 9451-9454. 49. Gittleson, F. S.; Ryu, W. H.; Taylor, A. D., Operando Observation of the Gold-Electrolyte Interface in Li-O-2 Batteries. Acs Appl Mater Inter 2014, 6, 19017-19025. 50. Griffith, L. D.; Sleightholme, A. E. S.; Mansfield, J. F.; Siegel, D. J.; Monroe, C. W., Correlating Li/O-2 Cell Capacity and Product Morphology with Discharge Current. Acs Appl Mater Inter 2015, 7, 7670-7678. 51. Lyu, Z. Y.; Zhou, Y.; Dai, W. R.; Cui, X. H.; Lai, M.; Wang, L.; Huo, F. W.; Huang, W.; Hu, Z.; Chen, W., Recent advances in understanding of the mechanism and control of Li2O2 formation in aprotic Li-O-2 batteries. Chem Soc Rev 2017, 46, 6046-6072. 52. Kwabi, D. G.; Tulodziecki, M.; Pour, N.; Itkis, D. M.; Thompson, C. V.; Shao-Horn, Y., Controlling Solution-Mediated Reaction Mechanisms of Oxygen Reduction Using Potential and Solvent for Aprotic Lithium-Oxygen Batteries. J Phys Chem Lett 2016, 7, 1204-1212.

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Figure 1. Schematic illustration of applying Co-POM molecule as catalyst in Li-O2 battery system

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Figure 2. (a) Electrochemical catalytic behavior of Co-POM molecule in water-splitting system. (b) Oxygen evolution test of Co-POM molecule under light illumination. Cyclic voltammetry (CV) curves of Li-O2 cell (c) without Co-POM (e) with Co-POM in DEGDME electrolyte (d) without Co-POM (f) with Co-POM in TEGDME electrolyte. Overpotential of Li-O2 cells obtained by Galvanostatic Intermittent Titration Test (GITT) (g) in DEGDME electrolyte (h) in TEGDME electrolyte

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Figure 3. Cell performance test in Co-POM-containing electrolyte. Initial charge/discharge curves of the MWCNT electrode in (a) 1 M LiTFSI + DEGDME and 1 M LiTFSI + DEGDME + Co-POM solutions and (b) 1 M LiTFSI + TEGDME and 1 M LiTFSI + TEGDME + Co-POM solutions in a voltage window between 4.5 and 2.3 V at a current density of 100 mA g-1carbon. Cycle tests were performed under a specific capacity limit of 600 mA h g-1carbon between 4.5 and 2.3 V at a current density of 100 mA g-1carbon in conditions of (c) LiTFSI + DEGDME (d) LiTFSI + DEGDME + Co-POM (e) LiTFSI + TEGDME (f) LiTFSI + TEGDME + Co-POM. ACS Paragon Plus Environment

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Figure 4. Light-absorbing effect of dispersed Co-POM electrolyte. (a) Observable color change of electrolyte under light. Cell overpotential obtained by Galvanostatic Intermittent Titration test (GITT) from Li-O2 cells with (b) LiTFSI + DEGDME + Co-POM with and without light absorbance (c) LiTFSI + DEGDME + Co-POM with and without light absorbance. UV-vis spectrum of (d) LiTFSI + DEGDME + Co-POM (e) light-absorbed LiTFSI + DEGDME + Co-POM

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Figure 5. Ex-situ measurements on oxygen electrodes. Ex-situ X-ray photoelectron spectra obtained from pristine, discharged, and charged electrodes in the (a) C 1s, (b) O 1s, (c) Li 1s; (d) magnified SEM image of discharge products on oxygen electrode (e) Ex-situ X-ray diffraction peak obtained from pristine, discharged, and charged oxygen electrodes. (f) Ex-situ X-ray photoelectron spectra obtained from discharged and charged electrodes collected in the Co 2p.

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A table of contents (TOC)

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