A Bifunctional Redox Mediator Supported by an Anionic Surfactant for

Although the soluble redox mediator (RM) has been effectively applied in Li- ... bifunctional mediator, PTIO can not only get a low charge plateau, bu...
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A Bifunctional Redox Mediator Supported by an Anionic Surfactant for Long-Cycle Li-O2 Batteries Chengyang Xu, Guiyin Xu, Yadi Zhang, Shan Fang, Ping Nie, Langyuan Wu, and Xiaogang Zhang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00884 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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A Bifunctional Redox Mediator Supported by an Anionic Surfactant for Long-Cycle Li-O2 Batteries Chengyang Xu †, Guiyin Xu† , Yadi Zhang †, Shan Fang †, Ping Nie†, Langyuan Wu†, and Xiaogang Zhang*, † †

College of Material Science and Engineering & Jiangsu Key Laboratory of Materials and

Technology for Energy Conversion, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P.R. China. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

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ABSTRACT. Although the soluble redox mediator (RM) has been effectively applied in LiO2 batteries, parasitic reactions between the lithium anode and RM+ can result in poor cycle performance. Herein, we proposed a non-electroactive surfactant (sodium dodecyl sulfate, SDS) that could adsorb on the hydrophobic carbon surface and form a stable anionic layer on charge, which can effectively suppress the diffusion of oxidized RM+ and facilitate the charge transfer at the electrode-solution interface. To coordinate with SDS, a new RM named 2phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) was adopted due to its oxidation process following after in situ formation of the anionic layer. Moreover, as a bifunctional mediator, PTIO can not only get a low charge plateau, but also greatly enhance the discharge capacity when applied in Li-O2 batteries. The electrochemical results demonstrated that the cycling performance, energy efficiency and discharge capacity were significantly improved owing to the synergistic effect of PTIO and SDS.

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Nowadays, new energy storage systems have been extensively investigated due to the increasing demand for high energy density in various fields1-2. Among all of them, lithium oxygen (Li-O2) batteries have received intensive attention3-4. One of the major challenges in non-aqueous Li-O2 batteries is how to effectively decompose the insulated discharge product Li2O2, which often leads to high over-potentials and severe side reactions during charging5. Although solid-state catalysts have been widely applied to combat this challenge, the catalytic efficiency is limited by the insufficient sites for these catalysts in contact with the products Li2O26-8. On the contrary, redox mediators (RMs) can provide a mobile and sufficient contact interface to ensure the complete decomposition of Li2O2 and they are considered as a soluble catalyst in application to Li-O2 batteries9-10. During charging, RM molecules are firstly electro-oxidized into RM+ ions at the oxygen cathode, which immediately decompose Li2O2 chemically to release oxygen. Thus, the voltage of the charging process is reduced to the redox potential of RM/RM+ couple9, 11-14. So far, a series of soluble oxidation mediators, such as tetrathiafulvalene (TTF)15, 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO)16, lithium iodide17, and iron phthalocyanine (FePc)19, have been extensively investigated in Li-O2 batteries. As

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reported, the potential of oxygen evolution reaction (OER) is successfully reduced below 4.0 V by the incorporation of a selective oxidation mediator, suppressing undesirable side reactions and enhancing the round-trip efficiency9, 11-24. However, the usage of RM can also bring about a parasitic reaction caused by the migration of RM to the Li anode, which degrades the cycle performance of the cell14, 25. Therefore, the protection of the Li anode from the attack of RM is necessary for the application of RM in LiO2 batteries26-33. Zhou et al. reported a self-defense redox mediator of InI3 to form a stable indium layer on the Li metal surface and achieved a stable energy efficiency of 80% for 50 cycles26. Another group tried to prevent the migration of the RM to the Li anode by using a modified separator29. The deposition of an organic conductor TTF+Clx- on the cathode surface was also an effective approach that eliminates side reactions between the Li anode and TTF+ 3 ACS Paragon Plus Environment

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. Although these methods were demonstrated to suppress parasitic reactions with the RM,

the loss of RM29 and the resistance of coating layers27 should also have been considered. Here, we develop a new method to form an anionic adsorption layer in situ on the cathode surface to restrict RM+ movement and promote the reaction kinetics of OER. Specifically, a non-electroactive surfactant (sodium dodecyl sulfate, SDS) was adopted to avoid additional reactions and SDS molecules were absorbed on the electrode surface by the attraction of opposite charges and the hydrophobic interaction of solvents34. An anionic layer of SDS can neutralize positive charges at the electrode and make the potential of zero charge (PZC) shift to the positive direction35, which brings about less repulsion and more attraction for RM+ ions36. Moreover, surfactant molecules can replace solvent molecules at the electrode-solution interface and effectively facilitate the charge transfer of redox reactions37. Therefore, RM+ ions can remain around the electrode surface due to a more positive PZC and the electrostatic attraction of SDS, preferably decomposing the discharge product and achieving excellent cycle performance. A new redox mediator named 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO), is proposed to coordinate with the surfactant SDS, for the reason that PTIO+ ions are generated just after in situ formation of the anionic layer. PTIO is a nitronyl nitroxide radical with high solubility and fast diffusion kinetics in the polar electrolyte38. The bifunctional effect in Li-O2 cells have been achieved by dual redox mediators39-40, or an oxidation mediator that can combine with the oxygen molecules19, 41. PTIO belongs to the latter and is proven to increase the discharge capacity and get a low charge plateaus in our study. Two redox reactions of PTIO are illustrated in Scheme 1, and reaction sites are both at the radical N-O bond, not at the dipolar N-O bond that may contribute to the dissolution of oxygen42. Thus, the synergistic effect of a bifunctional PTIO and the surfactant SDS not only significantly improves the energy efficiency and discharge capacity, but also simultaneously achieves a prolonged cycle life of Li-O2 batteries. 4 ACS Paragon Plus Environment

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Scheme 1. Redox couples of PTIO/PTIO+ and PTIO/PTIO-. Reaction sites for both couples are at the radical N-O bond.

Figure 1. CV curves of a glassy carbon electrode at a scan rate of 50 mV s-1 using DMSO solutions with different lithium salt and different concentrations of PTIO: (a) 0.1 M LiTFISI solution with 30 mM PTIO in Ar; (b) 0.1 M TBAP solution with 10 mM PTIO in Ar and O2; (c) a 0.1 M TBAP solution without and with 30 mM PTIO in O2; (d) 0.1 M TBAP solution with different concentrations of PTIO in O2. Basic properties of PTIO. The feasibility of PTIO as a redox mediator in Li-O2 batteries was determined firstly by cyclic voltammetry (CV) in a three electrode setup. The IR drop between working electrode and reference electrode in 0.1 M LiTFSI/DMSO was eliminated by the auto-compensation of the instrument. Redox properties of PTIO under Ar were 5 ACS Paragon Plus Environment

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illustrated in Figure 1a. Two reversible redox peaks at the potential of 2.1 and 3.7 V was demonstrated for couples of PTIO/PTIO- and PTIO/PTIO+. As for the CV tests under O2 atmosphere, we replaced the lithium salt of LiTFSI with the amine salt of TBAP, for the reason that reduced oxygen molecules in solutions could combine with Li+ to form LiO2 or Li2O215. The irreversible reduction current in a LiTFSI solution was observed clearly in Figure S1. Meanwhile, a well-defined redox peaks at the potential of 2.25 V in a TABP solution was present in Figure 1b, corresponding to the couple O2/O2-. In order to discuss the effect of PTIO on the electrochemical performance, we made a contrast analysis before and after adding PTIO. It can be seen from Figure 1c that the addition of 30 mM PTIO leads to the increased peak currents of the couple O2/O2- at the midpoint potentials of 2.25V. The peak currents of CV curves is relevant to the concentration at the same scan rate, which has been used to estimate the oxygen solubility with the RandlesSevick equation42-43. This implies a promotion of oxygen solubility with the addition of PTIO. For further confirmation, we contrasted the cathode currents of different systems by controlling the concentration of PTIO in O2-saturated solutions. Generally, more additives in solutions often cause the decrease of dissolved oxygen. On the contrary, when the concentration of PTIO is lower than 50 mM, the peak current of O2/O2- couple increases regularly with increasing concentrations of PTIO (Figure 1d). It is concluded that PTIO is related to the concentration of dissolved oxygen caused probably by the combination of PTIO with oxygen molecules. The dipolar N-O bond (N+→O-) of PTIO is a possible binding site for a strong inductive effect, since N-oxides44 and nitrate ions42 containing the dipolar N-O bond can increase the level of oxygen species in solutions. A similar phenomenon was demonstrated by Ryu et al. that heme could increase electrolyte oxygen solubility by a oxygen-complexing molecule (Heme(Fe2+)-O2), which works as a bifunctional redox mediator in Li-O2 cells41.

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Nevertheless, the direct evidence for the increase of oxygen solubility is more important to verify a bifunctional effect of PTIO. Oxygen solubility curves and the corresponding data list were obtained and shown in Figure 2. When the concentration of PTIO is lower than 50 mM, a significantly growth of oxygen solubility is observed, which is consistent with the result of CV curves above. However, an enhanced salting out effect lead to a moderating trend when the concentration of PTIO up to 100 mM, since the dissolved oxygen molecules can partly overflow from the solution. And the same result is present in the galvanostatic dischargecharge profiles of Li-O2 cells (Figure. S2). The obvious capacity growth is attributed to the oxygen solubility for cells containing 10 and 50 mM PTIO. From the results of CV analysis and oxygen solubility curves, it comes to a conclusion that soluble PTIO molecules contribute to the dissolution of oxygen and the salting out effect can not be deemed as a determinant until the concentration of PTIO more than 100 mM.

Figure 2. Oxygen solubility curves and the corresponding data list of a 0.1 M LiTFSI/DMSO solution containing no PTIO, 10 mM PTIO, 50 mM PTIO and 100mM PTIO.

The diffusion coefficient (D) of PTIO was obtained by linear sweep voltammetry experiments with a rotating disk electrode at different rotational speeds (ω; Figure S3a), and a value of 3.9×10-6 cm2 s-1 for PTIO was calculated with the plot limiting current vs ω1/2 7 ACS Paragon Plus Environment

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(Figure S3b). This is comparable to the diffusion coefficient of oxygen in TBAP-DMSO based electrolytes (Do = 4.4×10-6 cm2 s-1)45. High diffusion kinetics of a redox mediator is necessary for the mass transport to avoid the voltage polarization46.

Figure 3. (a) Full cycle of Li-O2 cells without and with 50 mM PTIO using a CNT film cathode at a current density of 0.1 mA cm-2. (b) Proposed mechanism for the electrochemical discharging of Li-O2 cells with PTIO. (c) First cycle of Li-O2 cells for 1000 mAh g-1. (d) Cycle performance of PTIO-containing Li-O2 cells for 500 mAh g-1. Effect of PTIO on the Li-O2 cell performance. The improved capacity and energy efficiency supported by PTIO is demonstrated in the galvanostatic discharge−charge profiles of Li-O2 cells. Considering the chemical stability and diffusion kinetics of PTIO, the electrochemical data was obtained in the electrolyte of 1 M LiTFSI/DMSO. A lithium foil pretreated with a 0.1 M solution of LiTFSI in propylene carbonate (PC) was served as the anode to prevent the reduction of oxygen molecules. A commercialized carbon nanotubes (CNTs) film was used as a free-standing cathode and purified by the method of nitric acid, which eliminated surface

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functional groups to avoid side reactions with PTIO. Firstly, Li-O2 cells without and with 50 mM PTIO were examined under O2 without capacity limitation at a current density of 0.1 mA cm-2 (about 400 mA g-1). As shown in Figure 3a, the maximal discharge capacity of Li-O2 cells increases from 2900 mAh g-1 (without PTIO) to 7000 mAh g-1 (with 50 mM PTIO). The highly improved capacity simply comes from the change of oxygen solubility in the eletrolyte. The rise of oxygen concentration can increase the discharge capacity by promoting the mass transport of O2 in the electrolyte47, which is potentially related to the first reduction step on discharge (O2 + e- →O2-). Most studies in state-of-the-art Li-O2 cells focus on the kinetics of oxygen reduction reaction (ORR) through various catalysts and electrode materials. However, the mass transport of O2 is also essential for the practical application48. As the proposed mechanism shown in Figure 3b. PTIO can maintain a balance between O2 provision and consumption, and keep a good level of oxygen solubility in the electrolyte, which contributes to the practical growth of discharge capacity. Simultaneously, with the incorporation of PTIO, the morphology of discharge products change from the large particles to ordered crystallites, which is closely related to the stability and solubility of oxygen species enhanced by PTIO (Figure S4). As a bifunctional mediator, PTIO can not only play a role in the discharging process, but also provide a low charge plateaus of 3.7 V and achieve a round-trip efficiency of 74%. A clear comparison for the overvoltage of the charging process is present in Figure 3c and the same result for TEGDME electrolyte has been observed in Figure S5. As illustrated, a overpotential decrement of 0.5 V are certainly achieved by PTIO and this fits with the catalytic mechanism of dissolved redox mediators. During charging, PTIO molecules are firstly elctrooxidized into PTIO+ ions, then combines with the discharge product Li2O2 generating O2 by the chemical oxidation. Thus, the potential of PTIO/PTIO+ couple determines the voltage plateaus of charging profiles. Although PTIO has the catalysis as expected, parasitic reactions caused by the diffusion of PTIO are not negligible. The loss of PTIO mediator and packed 9 ACS Paragon Plus Environment

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products on the Li anode surface result in the instability of charging voltage and the worsening of cycling performance (Figure 3d). The confirmatory experiments conducted by us is for checking the Li anode of PTIO-containing Li-O2 cells after 20 cycles by FTIR and EDX measurements (Figure S6). The highlighted peaks at 1380 cm-1, 1464 cm-1, and 1622 cm-1 in FTIR spectrum are responding to the N-O, C=N and C=C bonds of PTIO respectively49, and there is obvious distribution of N and O elements on the Li anode surface according to the result of the EDX observation. Therefore, the existence of parasitic reactions between the Li anode and PTIO+ was definitely confirmed due to the severe capacity decay and specific resultants on the surface of Li anode.

Figure 4. (a, b) The mechanism of particles distribution on the electrode surface during charging: (a) without SDS; (b) with SDS. (c) Differential capacitance curves of CNT film in contact with solutions of 0.1 M LiTFSI/DMSO with different SDS concentrations below CMC. (d) Complex plane impedance plots of glassy carbon in contact with solutions of 10 mM PTIO without and with 5 mM SDS at three dc potentials from 3.5 to 3.7 V. Design and verification of an anionic adsorption layer of SDS. Nowadays in Li-O2 battery, most studies are inclined to the protection of the lithium anode to prevent a shuttle effect of 10 ACS Paragon Plus Environment

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RM. Thus, the resistance of coating layers and the loss of RM are still an challenge. Here, we attempt to design an anionic layer of the surfactant SDS in situ on the carbon surface to solve these problems. The particles distribution on the electrode surface was schematically shown in Figure 4a, b. In the absence of SDS, the repulsive interaction of positive charges at the electrode drives unreacted PTIO+ ions away, and TFSI- or DMSO particles make no difference. In the presence of SDS, TFSI- and DMSO particles were replaced by the surfactant SDS with a strong adsorption, and an anionic layer promote the charge neutralization on the electrode surface. Thus, a decreased repulsive interaction of positive charges and the electrostatic attraction by SDS restrict PTIO+ ions around the carbon surface. And this hypothesis is definitely proven as following. The adsorption of the surfactant SDS on a CNT film electrode is researched by differential capacitance curves, which is obtained by EIS measurements with Brug’s formula50 and the detail is present in the supporting information (Figure S7). As illustrated in Figure 4c, with the increasing concentrations of SDS below its critical micelle concentration (CMC), the capacitance of double-electric layer is more lower, which means the adsorbance of the surfactant SDS is higher51. Moreover, the potential at minimum capacitance means the charge neutralization on the electrode surface, often called a potential of zero charge (PZC)35, 52. Therefore, the adsorption layer of SDS made a PZC shift to the positive direction as illustrated, and the maximum of PZC is located at 3.6 V closed to the redox potential of PTIO when the concentration of SDS is up to 5 mM. That is to say, when PTIO is oxidized at the electrode, the anionic layer of SDS is ready to suppress the diffusion of PTIO+. Simultaneously, a increased capacitance below 3.0 V indicates that the anionic layer would disappear on the carbon surface during the discharging process. As mentioned above, the resistance of the adsorption layer is important to the charge transfer of PTIO oxidation reaction. Therefore, EIS measurements were performed at the interface between glassy carbon and solutions of 10 mM PTIO without and with SDS at three dc potentials from 3.5 to 3.7 V. In the absence of SDS, the impedance spectra at the potential 11 ACS Paragon Plus Environment

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of 3.5 V deviates slightly from the vertical line, representing an ideal capacitor with no parallel resistance (Figure 4d). An increase of the curvature of plots appears at 3.6 and 3.7 V, which suggests for the contribution of a charge transfer resistance. With the incorporation of SDS, the deviation of impedance plots is more evident at each potential and plots of -Z′′ vs. Z′ tend to the formation of semicircles. The curvature variation of plots suggests that a SDS layer promote the charge transfer process of PTIO/PTIO+ couple. For the further argument on this point, impedance data in Fig. 4d are fitted to the equivalent circuit in Figure S8 by means of complex nonlinear least squares (CNLS). The results from the CNLS fitting are list in Table S1. The charge transfer resistance Rct decreased regularly with the addition of SDS, which is consistent with the analysis of impedance plots. In addition, the current of CV plots with SDS grows obviously (Figure S9) owing to the enhanced reaction kinetics.

Figure 5. First cycle of Li-O2 cells without and with additives at 0.1 mA cm-2 for the capacity of: (a) 2000 mAh g-1; (b) 1000mAh g-1. (c) Cycle performance of PTIO-containing Li-O2 cells with 5 mM SDS for 500 mAh g-1. (d) Cyclability of PTIO-containing Li-O2 cells without and with SDS for 500 mAh g-1. 12 ACS Paragon Plus Environment

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A synergistic effect of PTIO and SDS on the Li-O2 cell performance. From the verification above, it is confirmed that an anionic layer of SDS can be absorbed in situ on the carbon surface and is beneficial to the oxidation of PTIO with a decreased charge transfer resistance. Simultaneously, since solvent molecules in the electric double layer were replaced, a more sufficient contact interface between PTIO+ and Li2O2 was obtained, which could facilitate the reaction kinetics of the decomposition of Li2O2. These conclusions were further proven by the results of electrochemical performance. When using the surfactant SDS in Li-O2 cells, the charge plateaus is stable and lower on the basis of the catalytic effect of PTIO (Figure 5a,b). This means that an anionic layer of SDS further improved the catalytic efficiency of PTIO due to a enhanced reaction kinetics. Meanwhile, the ability of SDS to suppress the diffusion of PTIO is also indicated by the improvement of cycling performance. Compared with SDS-free Li-O2 cells (Figure 3d), the energy efficiency on charge is stable in SDS-containing Li-O2 cells (Figure 5c), with the charging potentials gathering around the PTIO/PTIO+ redox potential. Thus, the cycling performance is greatly improved without capacity fade after cycling for 150 cycles (Figure 5d). According to these electrochemical benefits above, it is believed that our design of an anionic adsorption layer is resonable and feasible in Li-O2 batteries. Nevertheless, the inhibition of SDS for the shuttle effect of PTIO requires further analysis and discussion in the next section. Surface analysis of Li anodes after charging. Detecting the surface state of the Li anode during cycling is crucial for the determination of the shuttle reactions caused by PTIO. Moreover, the variation of resultants on the Li anode surface provides a powerful proof for the inhibitory migration process of PTIO in SDS-containing Li-O2 cells26, 29. Figure 6a, b present the XPS spectra of Li anodes after first cycle in Li-O2 cells. The Li 1s spectrum in Figure 6a, exhibits a main peak at 54.9 eV for Li metal and a peak over 55.5 eV for the byproducts. The content variation of by-products suggests a difference made by SDS. Then the resultants caused by soluble PTIO is identified by the O 1s and N 1s spectra for the existence 13 ACS Paragon Plus Environment

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of N-O bonds. The Li anode for XPS measurements was not pretreated with the LiTFSI/PC solution, avoiding the disturbance of elements. As illustrated in Figure 6b, the peak at 532.6 eV in O 1s spectrum, corresponds to the N-O bond only coming from the nitroso-group of PTIO53. The peak at 531 eV probably indicates the presence of lithium oxides, since Li metal is not protected with pretreatment. The existence of N-O bonds indicates the by-product caused by PTIO, and a decline in the content of N-O bonds supports for the inhibitory effect of SDS. The same result is obtained in N 1s spectrum, and the content of N element decreases in SDS-containing Li-O2 cells (Figure S10).

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Figure 6. In XPS spectra (a, b), black lines for primary data and circles for the fits. (a) Highresolution Li 1s spectrum of the Li anode after first cycle in PTIO-containing Li-O2 cells without SDS (top) and with SDS (bottom); (b) O 1s spectrum of the same Li anode as (a) without SDS (bottom) and with SDS (bottom); (c) the XRD pattern of the Li anode after first cycle without SDS (centre), with SDS (top) and after first discharging process (contrast sample, bottom) . Another strong evidence was provided by the XRD patterns of Li anodes after first cycle (Figure 6c). The contrast sample refers to the Li anode after first discharging process, which eliminates the influence of products on discharge. From the XRD pattern of a contrast sample, the signal peaks of Li metal and LiOH are detected29, thus LiOH can’t be a criterion for side products of the charging process. Compared with XRD patterns of the contrast sample, new peaks between 30º and 40º appeared in the pattern of a SDS-free Li anode and was not detected in the SDS-containing Li anode. For the determination of these side products, we put the lithium metal into a solution full of PTIO+ ions under Ar atmosphere and the XRD pattern of resultants corresponds to that of the SDS-free Li anode at three new peaks (Figure S11). Therefore, it is concluded that an anionic layer of SDS can contribute to the inhibitory migration of PTIO to the Li anode and the excellent cycling performance of SDS-containing Li-O2 cells can be explained by the results of surface analysis.

A new nitronyl nitroxide radical, PTIO, is selected as a redox mediator due to its fast diffusion kinetics and suitable oxidation potential. Moreover, the ability to improve the oxygen solubility is significantly important for PTIO to be a bifunctional catalyst. The application of dissolved PTIO in Li-O2 cells, not only brings about a low charge plateau of 3.7 V, but also improves the discharge capacity from 2900 mAh g-1 to 7000 mAh g-1. An anionic adsorption layer of SDS is proven to in situ form on the cathode surface just before the oxidation of PTIO, which can efficiently suppress the migration of PTIO+ to the Li anode and facilitate the reaction kinetics of OER. When using the adsorption layer of SDS in Li-O2 cells, it can further enhance the catalytic efficiency of PTIO, and simultaneously improve the 15 ACS Paragon Plus Environment

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cycling performance without capacity fade for 150 cycles compared with the SDS-free cells (30 cycles). Thus, a synergistic effect of PTIO and SDS can effectively improve the discharge capacity, reduce the charge over-potentials and achieve a prolonged cycle life with a stable energy efficiency in Li-O2 batteries.

Supporting Information Available: < Experimental section; CV curves; LSV curves ; Complex plane impedance plots; The fitting results of impedance data; Galvanostatic discharge-charge profiles; FTIR spectra; TEM image and mapping images; XPS spectra; XRD patterns. >

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Program on Key Basic Research Project of China (973 Program, no. 2014CB239701), Natural Science Foundation of China (no. 51372116, 51672128), C. Xu acknowledges Funding for Graduate Innovation Center in NUAA (no. kfjj20170616).

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(38) Duan, W.; Vemuri, R. S.; Milshtein, J. D.; Laramie, S.; Dmello, R. D.; Huang, J.; Zhang, L.; Hu, D.; Vijayakumar, M.; Wang, W. et al. A symmetric organic-based nonaqueous redox flow battery and its state of charge diagnostics by FTIR. J. Mater. Chem. A 2016, 4, 54485456. (39) Gao, X.; Chen, Y.; 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. (40) Zhu, Y. G.; Jia, C.; Yang, J.; Pan, F.; Huang, Q.; Wang, Q. Dual redox catalysts for oxygen reduction and evolution reactions: towards a redox flow Li-O2 battery. Chem. Commun. 2015, 51, 9451-9454. (41) Ryu, W. H.; Gittleson, F. S.; Thomsen, J. M.; Li, J.; Schwab, M. J.; Brudvig, G. W.; Taylor, A. D. Heme biomolecule as redox mediator and oxygen shuttle for efficient charging of lithium-oxygen batteries. Nat. Commun. 2016, 7, 12925. (42) Khan, A.; Lu, X.; Aldous, L.; Zhao, C. Oxygen reduction reaction in room temperature protic ionic liquids. J. Phys. Chem. C 2013, 117, 18334-18342. (43) Nissim, R.; Batchelor-McAuley, C.; Compton, R. G. Measuring oxygen solubility in micelles. ChemElectroChem 2016, 3, 105-109. (44) Yasa, S. R.; Kaki, S. S.; Poornachandra, Y.; Kumar, C. G.; Penumarthy, V. Synthesis, characterization, antimicrobial and biofilm inhibitory activities of new N-oxide esters. Med. Chem. Res. 2017, 26, 1689-1696. (45) Muhammad, H.; Tahiri, I. A.; Muhammad, M.; Masood, Z.; Versiani, M. A.; Khaliq, O.; Latif, M.; Hanif, M. A comprehensive heterogeneous electron transfer rate constant evaluation of dissolved oxygen in DMSO at glassy carbon electrode measured by different electrochemical methods. J. Electroanal. Chem. 2016, 775, 157-162. (46) Xia, C.; Black, R.; Fernandes, R.; Adams, B.; Nazar, L. F. The critical role of phasetransfer catalysis in aprotic sodium oxygen batteries. Nat. Chem. 2015, 7, 496-501. (47) Read, J.; Mutolo, K.; Ervin, M.; Behl, W.; Wolfenstine, J.; Driedger, A.; Foster, D. Oxygen transport properties of organic electrolytes and performance of lithium/oxygen battery. J. Electrochem. Soc. 2003, 150, A1351. (48) Wan, H.; Mao, Y.; Liu, Z.; Bai, Q.; Peng, Z.; Bao, J.; Wu, G.; Liu, Y.; Wang, D.; Xie, J. Influence of enhanced O2 provision on the discharge performance of Li-air batteries by incorporating fluoroether. ChemSusChem 2017, 10, 1385-1389. (49) Benzon, K. B.; Varghese, H. T.; Panicker, C. Y.; Pradhan, K.; Tiwary, B. K.; Nanda, A. K.; Alsenoy, C. V. Spectroscopic and theoretical characterization of 2-(4-methoxyphenyl)4,5-dimethyl-1H-imidazole 3-oxide. Spectrochim. Acta Part A 2015, 151, 965-79. (50) Hammons, J. A.; Ilavsky, J. Surface Pb nanoparticle aggregation, coalescence and differential capacitance in a deep eutectic solvent using a simultaneous sample-rotated small angle X-ray scattering and electrochemical methods approach. Electrochim. Acta 2017, 228, 462-473. (51) Anastopoulos, A. G.; Papoutsis, A. D.; Papaderakis, A. A. Differential capacitance and electrochemical impedance study of surfactant adsorption on polycrystalline Ni electrode. J. Solid. State. Electrochem. 2015, 19, 2369-2377. (52) Gugala-Fekner, D.; Nieszporek, J.; Sienko, D. Adsorption of anionic surfactant at the electrode-NaClO4 solution interface. Monatsh. Chem. 2015, 146, 541-545. (53) Plaksin, P. M.; Sharma, J.; Bulusu, S.; Adams, G. F. X-ray photoelectron spectra of N,N-dimethylnitramine and N,N-dimethylnitrosamine and their interpretation in terms of molecular orbital calculations. J. Electron Spectrosc. Relat. Phenom. 1975, 6, 429-50.

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Figures and captions

Scheme 1 Redox couples of PTIO/PTIO+ and PTIO/PTIO-. Reaction sites for both couples are at the radical N-O bond.

Figure 1. CV curves of a glassy carbon electrode at a scan rate of 50 mV s-1 using DMSO solutions with different lithium salt and different concentrations of PTIO: (a) 0.1 M LiTFISI solution with 30 mM PTIO in Ar ; (b) 0.1 M TBAP solution with 10 mM PTIO in Ar and O2; (c) a 0.1 M TBAP solution without and with 30 mM PTIO in O2; (d) 0.1 M TBAP solution with different concentrations of PTIO in O2.

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Figure 2. Oxygen solubility curves and the corresponding data list of a 0.1 M LiTFSI/DMSO solution containing no PTIO, 10 mM PTIO, 50 mM PTIO and 100mM PTIO.

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Figure 3. (a) Full cycle of Li-O2 cells without and with 50 mM PTIO using a CNT film cathode at a current density of 0.1 mA cm-2. (b) Proposed mechanism for the electrochemical discharging of Li-O2 cells with PTIO. (c) First cycle of Li-O2 cells for 1000 mAh g-1. (d) Cycle performance of PTIO-containing Li-O2 cells for 500 mAh g-1.

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Figure 4. (a, b) The mechanism of particles distribution on the electrode surface during charging: (a) without SDS; (b) with SDS. (c) Differential capacitance curves of CNT film in contact with solutions of 0.1 M LiTFSI/DMSO with different SDS concentrations below CMC. (d) Complex plane impedance plots of glassy carbon in contact with solutions of 10 mM PTIO without and with 5 mM SDS at three dc potentials from 3.5 to 3.7 V.

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Figure 5. First cycle of Li-O2 cells without and with additives at 0.1 mA cm-2 for the capacity of: (a) 2000 mAh g-1; (b) 1000mAh g-1. (c) Cycle performance of PTIO-containing Li-O2 cells with 5 mM SDS for 500 mAh g-1. (d) Cyclability of PTIO-containing Li-O2 cells without and with SDS for 500 mAh g-1.

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Figure 6. In XPS spectra (a, b), black lines for primary data and circles for the fits. (a) Highresolution Li 1s spectrum of the Li anode after first cycle in PTIO-containing Li-O2 cells without SDS (top) and with SDS (bottom); (b) O 1s spectrum of the same Li anode as (a) without SDS (bottom) and with SDS (bottom); (c) the XRD pattern of the Li anode after first cycle without SDS (centre), with SDS (top) and after first discharging process (contrast sample, bottom) . 25 ACS Paragon Plus Environment