A High-Rate Ionic Liquid Lithium-O2

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A High-Rate Ionic Liquid Lithium-O Battery with LiOH Product Tao Zhang, and Zhaoyin Wen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00336 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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The Journal of Physical Chemistry

A High-Rate Ionic Liquid Lithium-O2 Battery with LiOH Product Tao Zhang, Zhao-Yin Wen* CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai, 200050, P. R. China AUTHOR INFORMATION Corresponding Author *[email protected]

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ABSTRACT

The open characteristic of lithium-O2 batteries brings out drying-out issue and explosion hazards. Ionic liquids (ILs) are reliable in terms of these issues because they have near-zero vapor pressure and are generally nonflammable. However, the rate capability of the Li-O2 batteries with ionic liquids is currently limited to 0.05-0.5 mA/cm2 owing to poor oxygen solubility, poor wettability with solid cathode, and high viscosity. Herein an ether-functionalized ammonium ionic liquid Li-O2 battery sustains repeated cycling at 5.0 mA/cm2 (80 oC), enhancing 1 order of magnitude in rate capability. The unprecedented high rate is attributed to the improved 3-phase interfacial properties (mainly O2 diffusion and wettability), LiOH-based cycling process at cathode and stable passivation film on Li anode. Our results demonstrate that a substantial improvement in rate capability can be achieved by coordinating cathode/IL electrolyte/anode interface and reaction mechanism. The high rate capability is encouraging a prospect for the operating conditions of electric vehicles under high power output (discharging) and rapid stateof-charge.


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Introduction Lithium-O2 batteries are unique among other electrochemical Li devices in that they are deliberately open to the atmosphere, breathing inexhaustibly ambient O2 and delivering theoretically high specific energy of 3505 Wh/kg, and energy density of 3435 Wh/L (based on Li2O2). The open characteristic has spurred great interest in their promising applications on electrified automotive propulsion with extended driving range, meanwhile brings out the dryingout issue arising from electrolyte evaporation, and the explosion hazards associated with the abundant O2 supply and the flammable electrolytes such as tetra(ethylene)glycol dimethyl ether (TEGDME) and dimethyl sulfoxide (DMSO).1-3 Ionic liquids (ILs) are advantageous in terms of these issues because they have near-zero vapor pressure and are generally nonflammable.4 The Li-O2 batteries with several kinds of ionic liquid electrolytes, mainly containing imidazolium-,5 pyrrolidinium-,6 piperidinium-based and ether-functionalized cations,7,8 have exhibited some favorable performance in cycling stability and energy efficiency. Nevertheless, the rate capability, which is a crucial requirement for propulsion applications, has remained too low in these ionic liquid Li-O2 batteries. The current densities delivered in the ionic liquid electrolytes are usually in the range of 0.05-0.5 mA/cm2.5-8 Work has focused on chemically modifying carbon air cathodes with ionic liquids to optimize the cathode/electrolyte interface.9-11 This approach has remarkable advantages on creating environmentally benign Li-air batteries and sealing carbon surface defects to suppress the side reactions. It can also improve the rate capability of the Li-air batteries slightly to the level of 0.2-1.2 mA/cm2.9 However, only the imidazolium-based ionic liquids were reported to be capable of modifying the carbon air cathodes, thereby limiting the further development of this approach. It is still imperative at


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present to increase the current density by at least 1 order of magnitude, that is, 5-10 mA/cm2, to raise the possibility that ionic liquid Li-O2 batteries may reach an expected rate capability on par with Li-ion or Li-sulfur technology.12,13 A number of issues prevent the rapid flow of current in the Li-O2 batteries with ionic liquid electrolytes, mainly including the high viscosity, and the poor wettability with the solid cathode surface. These shortcomings limit the motion of lithium ions, the transport of O2 and the transfer of electrons. As a non-polar molecule, the O2 solubility is mostly dependent on the polarity of the solvents. Some ionic liquids possess the polarity close to H2O or conventional organic media, however, most ionic liquids show the high viscosity, which hinders the mass transportation at the 3-phase electrochemical reaction interface. Although increasing temperature can directly lower the viscosity of ionic liquid electrolytes, it does not improve the rate capability in Li-O2 batteries significantly.5 Therefore, exploring the coordinated approaches towards these focal issues, not only viscosity, will be critical to revolutionize the rate capability in the ionic liquid Li-O2 batteries. No such study has been conducted so far. Here we prepared a new crosslinked network gel (CNG) consisting of single-walled carbon nanotubes (SWNTs) and etherfunctionalized ammonium ionic liquid of ([(C1OC2)C2C2C1N][NTf2]). The CNG was used as the oxygen cathode, assembling Li-O2 cells with the electrolyte containing the same ammonium ionic liquid. The ionic liquid Li-O2 battery was then tested in the temperature range 60-80 oC, achieving good cycleability at the current density as high as 5.0 mA/cm2. The 3-phase cathode reactions at the high current densities were investigated.

Experimental


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SWNTs/IL gel cathode: Pristine SWNTs (5 mg, Hipco SuperPure) were dispersed in 0.6 mL ether-functionalized ammonium ionic liquid of N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl) ([(C1OC2)C2C2C1N][NTf2]) by ultrasonic dispersion for 30 minutes. The suspension was ground in an agate mortar for 20 minutes. With grinding, the viscosity of SWNTs/IL suspension gradually increased, and finally forming a sticky paste. The paste was then moved to a centrifuge tube and centrifuged at 9100 g (centrifugal force) for 3 hours. The upper excess ionic liquid was removed to get the lower black SWNTs/IL crosslinked gel. The typically high tangled SWNTs bundles were untangled upon being ground into the ether-functionalized ammonium ion-based [(C1OC2)C2C2C1N][NTf2] IL to form the gel by a possible specific interaction between the cation in IL and the π-electronic nanotube surface. The as-prepared gel was pasted finely onto a carbon paper (CP) current collector. The loading area was 0.25 cm2. The as-prepared electrode was dried at 80 oC in vacuum for 12 hours. The loading weight of SWNTs in the cathode was ≈ 0.5 mg/cm2. To compare the electrochemical performance of untangled SWNTs with entangled SWNTs, the similar suspension of SWNTs and [(C1OC2)C2C2C1N][NTf2] by ultrasonic dispersion was centrifuged directly at 9100 g without grinding to obtain the SWNTs/IL mixture. To compare the electrochemical property of the ether-functionalized ammonium ion-based ionic liquid Li-O2 battery with the imidazolium ion-based ionic liquid Li-O2 battery, the imidazolium ion-based ionic liquid of 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl) ([C2C1im][NTf2]) was used to prepare the SWNTs/IL gel cathode by the same gelation process as mentioned above. Ionic liquid electrolyte: The ether-functionalized ammonium ion-based ionic liquid electrolyte consists of Li salt of 0.5 M LiNTf2 in pure [(C1OC2)C2C2C1N][NTf2]. The imidazolium ion-


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based ionic liquid electrolyte consists of Li salt of 0.5 M LiNTf2 in pure [C2C1im][NTf2]. Water content in both the ionic liquid electrolytes is lower than 5 ppm. Li anode: The Li foil of thickness 0.2 mm was cut into a disc of 10 mm in diameter, and then pressed onto a stainless steel spacer in Li-O2 coin cells. Assembling Li-O2 coin cell: All the manipulations of cell assembling were carried out in an argon-filled high-integrity glovebox. CR2032-type coin cells with holes for O2 access were used as the holder. The amount of the ionic liquid electrolyte was 45 µL, immersed in a Waterman GF/C glass fiber separator. After assembling, the coin cells were put into a glass chamber with complete gas tightness and gas valves for the entrance and exit of oxygen, and then taken out from the argon-filled glovebox. Electrochemical measurement: Pure O2 was passed through the glass chamber to replace the argon gas completely, and then the gas valves were closed to prevent the exchange of outside air and inside pure O2. All the tests were carried out in the O2-tight glass chamber with an initial pressure of 1 atm. The glass chamber was placed in an incubator to control the measured temperature at 25-80 oC. The current density was constant in the range of 2.5-10.0 mA/cm2. The capacity was normalized by the weight of the SWNTs. The electrochemical tests were performed at 25-80 oC using Land charge/discharge machine and Autolab instruments. XRD analysis: X-ray powder diffraction (Rigaku) was used to analysis the discharge/charge products of the Li-O2 cells. After discharge and charging in pure O2, the cell was disassembled in glovebox, and then the gel cathode was sealed tightly for XRD measurements, attached with carbon paper current collector.


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SEM observation and EDX analysis: The Li anodes after kept at 80 oC for 2 hours were washed by pure dimethoxyethane (DME) and then dried before SEM observation. To remove some residual glass fiber adhering on the surface, the Li anode was immersed in the pure DME solution for 6 hours. The residual glass fiber from the separator was removed carefully by a finepointed tweezers. The Nova Nano field emission and S-3400 SEM instruments were used for micrograph observation and energy dispersive X-ray (EDX) mapping analysis. Contact angle analysis: The contact angels of the ionic liquid electrolyte on the SWNTs/IL gel cathode and the SWNTs cathode were examined on AST Products with a model of Optima. FTIR analysis: KO2 and 18-crown-6 (crown ether) were added (1:1 molar ratio) to 1.5 mL of [(C1OC2)C2C2C1N][NTf2] ionic liquid and were stirred for 12 hours to ensure complete chelation of the KO2 by the crown ether. To simulate the discharge state at low (0.2 mA/cm2) and high (5.0 mA/cm2) rate, two kinds of KO2/crown ether solution at different O2- concentration (1 versus 25) were prepared. 0.1 M and 2.5 M solution of LiNTf2 in [(C1OC2)C2C2C1N][NTf2] ionic liquid were added into the KO2/crown ether solution to form the lithiated oxide LiO2, respectively. The solutions were stirred for 12 hours at 80 oC in an oxygen-filled glovebox. The reaction products were collected and then ground together with KBr and pressed into pellets under high pressure for Fourier transform infrared (FTIR) analysis. FTIR was obtained on a Bruker instrument of TENSOR 27 from 4000 to 2000 cm-1 with a resolution of 2 cm-1.

Results and Discussion


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Figure 1. Morphological and wettability change profiles of SWNT bundles upon gelation. (a) TEM image of as-received SWNTs. (b) TEM image of SWNTs/IL gel. (c) Contact angle measured for SWNTs cathode with IL electrolyte. (d) Contact angle measured for SWNTs/IL gel cathode with IL electrolyte. (e) Schematic illustration of the proposed IL-based lithium-O2 battery. Detailed configuration is described in supporting information. Figure 1a and 1b show the transmission electron microscopy (TEM) image of the SWNTs/IL gel, in contrast with as-received SWNTs. The heavily entangled SWNT bundles (Fig. 1a) were disengaged to form much finer bundles (Fig. 1b) upon being ground into the etherfunctionalized ionic liquid. This untangling effect is most likely due to the cation-π stacking interaction between the [(C1OC2)C2C2C1N] cation and the π-electronic surface of the carbon nanotube. Such noncovalent linkage has been reported to result in the gelation of SWNTs in the


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imidazolium-based ionic liquids.14,15 Herein, we reported, for the first time, on the similar crosslinked gelation of SWNTs in the ether-functionalized ammonium-based ionic liquid. The two kinds of CNG were compared in Fig. S1, in which the gel phase was estimated to entrap 6.4 × 1019 [(C1OC2)C2C2C1N][NTf2] molecules per 1 mg of SWNTs, comparable with the 6.2 × 1019 molecules for the imidazolium-based ionic liquid. The cation-π stacking interaction leads to a dramatic change in the wetting property of SWNTs with the IL electrolyte. The measured contact angle for the as-received SWNTs pressed on carbon paper was found to be 112o (Fig. 1c), representing the difficultly IL-wettable nature. In stark contrast, the contact angle for the SWNTs/IL CNG pasted on carbon paper was only 26o (Fig. 1d), much lower than 90o, showing strongly IL-wettability. Wetting of the SWNTs/IL gel surface by the ionic liquid depends upon the gel and liquid specific surface energy along with the nature of interaction at interfacial surfaces. The increased wettability suggests that the SWNTs/IL gel possesses higher surface free energy than the solid SWNTs, which is beneficial for the construction of an IL-based cathode/electrolyte interface with better affinity. With these structural and interfacial advantages in mind, we proposed an ionic liquid LiO2 battery for high discharge and charge rate capability. The schematic representation of the battery is illustrated in Fig. 1e, which can be characterized by three main aspects: 1) the SWNTs/IL gel cathode consists of 3-dimentional networks (Fig. 1b), in which the IL molecules anchor on the surface of the SWNTs, forming a thin ionic liquid film, which facilitates rapid O2 diffusion; 2) the SWNTs/IL gel cathode contains the same ether-functionalized ammoniumbased ionic liquid with the electrolyte, establishing a naturally elongating interface from the electrolyte (liquid state) to the cathode (gel state), which increases the interfacial wettability and reduces the amount of the electrolyte to only 45 µL; 3) the discharge and charge measurements


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are performed at temperatures up to 80 oC, which can reduce the viscosity of both the IL electrolyte and the SWNTs/IL gel cathode, permitting easy motion for lithium ions. The high temperatures can be tolerated by the ionic liquid due to its nonvolatile and nonflammable nature.

Figure 2. Rate and impedance property of the ether-functionalized ammonium ionic liquid Li-O2 cell with the SWNTs/IL gel cathode. (a) Discharge/charge curves controlled by cut-off voltages of 2.0 V and 3.8 V. Grey line: solid SWNTs cathode. The tests were carried out in pure O2. (b) Arrhenius plots of the ohmic and interfacial resistance. The impedance spectra at 60-80 oC are shown in the inset.


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Figure 2a exhibits a series of discharge/charge behavior resulting from variations in the temperature and rate conditions for the proposed Li-O2 cell. At 60 oC, the cell can be discharged smoothly at a relatively high rate of 2.5 mA/cm2, delivering a large capacity of 3310 mAh/g, which is over eight times of 375 mAh/g for the Li-O2 cell with solid SWNTs cathode and the same IL electrolyte (see Fig. S2). The large capacity was reversed on charging at the same rate with the cut-off voltages of 2.0 V and 3.8 V. This result demonstrates that a substantial improvement in rate capability can be achieved on the level over 1.0 mA/cm2 through the coordinated approaches, mainly including cathode structure, interfacial wettability and proper temperature. To see the limitation of the rate capability, the current density was increased twice to 5.0 mA/cm2, however, the discharge capacity decreased significantly below 1000 mAh/g. We then increased the measured temperature to 70 and 80 oC, respectively. The ionic liquid Li-O2 cells clearly show a trend in increase of specific capacity as the measured temperature increases. In particular, when operating at 80 oC, the discharge capacity even reached 1290 mAh/g (over 1000 mAh/g) at an unprecedented current density of 7.5 mA/cm2. From the electrochemical point of view, the interface resistance, which is susceptible to temperature, is also an important factor related to the improved rate capability. Figure 2b shows the Arrhenius plots as a function of temperature for the ionic liquid Li-O2 battery. The interfacial ionic conductivity increased with a uniform slope as increasing temperature from 25 to 60 oC (impedance spectra shown in Fig. S3). After that, a slight volcano dependence was observed in the range of 60-80 oC. Meanwhile, the ohmic electron conductivity was almost invariable with temperature in the range of 50-80 oC due to the high electro-conductivity of the SWNTs. These results rule out the interfacial resistance as the reason for the improved rate capability at 60, 70 and 80 oC. Generally the 3-phase reaction chemistry has a rate-limiting step which affects


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reaction kinetics and hence the rate capability. Combing all the above results, it is evident that improving the cathode structure and regulating the interfacial wettability are the keys to increasing the rate capability of the ionic liquid Li-O2 battery in terms of cell structure.

Figure 3. Cycling performance and discharge/charge mechanism of the ionic liquid Li-O2 cell. (a) Cycling profiles under a capacity limitation of 1000 mAh/g at 80 oC and 5.0 mA/cm2. The tests were carried out in pure O2. (b) XRD patterns of the SWNTs/IL gel cathode after discharged at


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80 oC and different current densities. Blue: 0.2 mA/cm2; Orange: 1.0 mA/cm2; Green: 5.0 mA/cm2 after the 1st discharge and charge. (c) FTIR spectra of the reaction products derived from metastable superoxide and Li salt under different concentration (1 versus 25 to simulate the low and high rate of 0.2 and 5.0 mA/cm2). The ionic liquid Li-O2 cell was then cycled at a high rate of 5.0 mA/cm2, with a fixed capacity of 1000 mAh/g under 80 oC. Figure 3a displays the cycling performance until the 25th cycle, in which the discharge voltage has reached the cut-off voltage of 2.0 V, and the charge voltage approaches the upper bound of 3.8 V. Although some recently reported Li-O2 batteries with ionic liquid electrolytes exhibited the cycleability to tens of cycles or the recharge voltage below 3.8 V at ambient temperature,6,16 they did not present the current densities over 0.05 mA/cm2. In addition, Das have reported on the Li-O2 batteries with ionic liquid electrolytes under 25-150 oC,5 and Park on the batteries with ether-based electrolytes under -10-70 oC,17 but the current densities used in these studies were no more than 0.5 mA/cm2. The results obtained herein were obtained on just the SWNTs/IL gel and catalyst-free cathode and should thus be considered quite preliminary. The much higher rate performance, however, are encouraging a prospect for the operating conditions of electric vehicles under high power output (discharging) and rapid state-of-charge, as illustrated in Fig. 3a. Powder X-ray diffraction (XRD) patterns of the discharge and charge products for the ionic liquid Li-O2 battery at 5.0 mA/cm2 are presented in Fig. 3b. It is unexpected that the discharge product is identified as the crystalline LiOH (PDF no. 85-0736). This is inconsistent with the previous studies on the Li-O2 batteries with ionic liquid electrolytes, in which the crystalline Li2O2 have been indicated as the discharge product.6,9,16,17 To shed light on the deviation, the current density was adjusted down to 1.0 and 0.2 mA/cm2, while still keeping the


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measured temperature of 80 oC. The LiOH peaks were still observed at 1.0 mA/cm2. However, when decreasing to 0.2 mA/cm2, the current density comparable with that used in the previous studies,6,9,16,17 the peaks agreement with crystalline Li2O2 (PDF no. 73-1640) occurred. This finding suggests that the cycling mechanism of the ionic liquid Li-O2 battery has changed into LiOH formation and decomposition with the increasing current densities. Typically, LiOH is the cathodic product in aqueous Li-air systems via a four-electron reaction (4Li+ + O2 + H2O + 4e- → 4LiOH), where the O-O bond must be broken during oxygen reduction (discharge). In this study, the bond-breaking reaction is unlikely to be triggered by simply varying the current density. Figure 3c shows a simulating probe of the interaction of O2with the ammonium IL electrolyte. The metastable superoxide was generated from the complexing reaction of KO2 with 18-crown-6 (crown ether) in the ammonium IL solution. The LiOH product was detected when the LiNTf2 salt was added into the IL solution with high concentration O2- at 80 oC. According to the Hard Soft Acid Base theory of Pearson,18 the large [(C1OC2)C2C2C1N]+ ammonium cation, which have soft acidity, would stabilize the electrogenerated soft base O2-, forming [(C1OC2)C2C2C1N]+…O2- ion pairs.19 At high rate, Li+ ions and O2- accumulate massively on the 3-phase interface. Due to the different Lewis acidity of competing cations ([(C1OC2)C2C2C1N]+ versus Li+), LiO2 is the predominant product because of the high charge density of Li+, as identified by the simulating probe. Owing to the high nucleophilicity of the LiO2,20 the ammonium IL electrolyte may become reactive to form the LiOH product. Note that the discharge/charge overpotentials in Fig. 3a increased gradually as cycling, which should be related to the consumption of the [(C1OC2)C2C2C1N]+ cations. Herein the high concentration LiO2 was confirmed as the reason for the LiOH formation at high rate on discharge, but the detailed reactive process remains to be seen.


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Figure 4. Comparison of the high-temperature stability for two kinds of ionic liquid Li-O2 batteries. (a) Variations of the cell OCV on the rest time in pure oxygen. (b) Analysis of Li metal surface, including SEM and EDX, after keeping at 80 oC for the Li-O2 cell with etherfunctionalized ammonium ionic liquid electrolyte and (c) with imidazolium ionic liquid electrolyte. The images of the Li metal and separator are also shown. In addition to the ether-functionalized ammonium ionic liquid Li-O2 battery, an imidazolium ionic liquid Li-air system has been reported previously.9 Here the rate capability of the Li-O2 battery with the [C2C1im][NTf2] ionic liquid was also investigated. The imidazolium-


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based Li-O2 battery is capable of giving discharge capacity up to 1000 mAh/g at 7.5 mA/cm2 and 80 oC likewise (see Fig. S4). However, unlike the ether-functionalized ammonium ionic liquid Li-O2 battery, the open circuit voltage (OCV) of the imidazolium system was unstable at 80 oC, as shown in Fig. 4a. Further investigations in Fig. 4b indicated that a passivation film consisting of whisker composites was formed on the surface of Li metal in the ether-functionalized ammonium ionic liquid Li-O2 battery. The surface morphology can be observed more clearly at a higher magnification (Fig. S5). From energy dispersive X-ray (EDX) mapping analysis, the whisker composites contained mainly the Li oxides and LiF. On the contrary, no passivation film containing Li oxides was detected in the imidazolium ionic liquid Li-O2 cell (Fig. 4c), where the surface morphology of the Li metal was similar with that of the pristine Li metal (Fig. S6). The [C2C1im][NTf2] reacted continuously with Li metal at 80 oC (Fig. S5), due to the presence of the acidic proton at the C-2 position.21 Thus, Our attempt to high-rate Li-O2 batteries leads to two findings; (1) the cycling mechanism at air cathode changes from Li2O2 to LiOH formation and decomposition with the increasing current densities, and (2) a passivation film consisting of whisker Li oxides and LiF is formed on the surface of Li anode in the ether-functionalized ammonium ionic liquid electrolyte, which is the prerequisite for obtaining the stable rate capability. The ether-functionalized ammonium ionic liquid Li-O2 battery is prominent with regard to both the benefits of high rate capability and high-temperature stability. In summary, the ether-functionalized ammonium ionic liquid disengaged the heavily entangled SWNT bundles, forming a new cross-linked network gel. The ionic liquid Li-O2 battery with the ammonium-based gel cathode exhibited high rate capability, delivering discharge capacity of 1290 mAh/g at 7.5 mA/cm2, and sustaining repeated cycling at 5.0 mA/cm2 for 25 cycles (80 oC). The unprecedented high rate is attributed to the improved 3-phase interface


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properties (mainly O2 diffusion and wettability), LiOH-based cycling process at cathode and stable passivation film on Li anode. It was found that the discharge product of the ionic liquid LiO2 battery changed from the conventional Li2O2 to the LiOH when operated at high rate. This change is ascribable to the different Lewis acidity of competing cations (Li+ versus [(C1OC2)C2C2C1N]+), which facilitates the nucleophilic reaction of high concentration LiO2 on the ammonium IL electrolyte with increasing current densities. The detailed process may be related to the hydrogen abstraction of O2- for the [(C1OC2)C2C2C1N]+ cation, forming the intermediates of HO2, H2O2 and H2O,22 but this remains to be seen. It is relevant here to recall the recent studies that demonstrated the efficient cycleability based on LiOH product through H2O addition or H2O circulation in nonaqueous Li-O2 batteries.23,24 The reactivity of the [(C1OC2)C2C2C1N]+ cations at high rate may be suppressed by the introduction of tiny H2O and the proper catalysts, promising on further improvements in cycleability. Currently, we are further investigating the influence of LiOH formation in ionic liquids including, the influence of redox mediators, the role of catalysts, and the stabilizing agents for ILs. ASSOCIATED CONTENT Supporting Information. Molecular Formula of [(C1OC2)C2C2C1N][NTf2] ionic liquid, formation process of the gel-air cathode, discharge/charge curves of the solid SWNTs cathodes at 60, 70 and 80 oC, change of impedance spectra of the ether-functionalized ammonium ionic liquid Li-O2 cells in the temperature range of 25-60 oC, rate property of the imidazolium ionic liquid Li-O2 cell, magnification of SEM images corresponding to Fig. 4, and the surface morphology of the pristine Li metal.


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AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] Author Contributions T. Z. and Z. W. conceived and designed the experiments. T. Z. performed the experiment. T. Z. and Z. W. carried out the data analysis. T. Z. wrote the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported financially by the “Hundred Talents” program of the Chinese Academy of Sciences, as well as by the National Natural Science Foundation of China under Grant No. 51432010, 51672299, and the Key Fundamental Research Project of Shanghai (14JC1493000). REFERENCES (1) Peng, Z.; Freunberger, S. A.; Chen, Y.; Bruce, P. G. A Reversible and Higher-Rate Li-O2 Battery. Science 2012, 337, 563-566. (2) Jung, H-G.; Hassoun, J.; Park, J-B.; Sun, Y-K.; Scrosati, B. An Improved HighPerformance Lithium-Air Battery. Nat. Chem. 2012, 4, 579-585. (3) Li, F.; Zhang, T.; Zhou, H. Challenges of Non-Aqueous Li-O2 Batteries: Electrolytes, Catalysts, and Anodes. Energy Environ. Sci. 2013, 6, 1125-1141. (4) Hallett, J. P.; Welton, T. Room-Temperature Ionic Liquid: Solvents for Synthesis and Catalysis. 2. Chem. Rev. 2011, 111, 3508-3576. (5) Das, S. K.; Xu, S.; Emwas, A-H.; Lu, Y. Y.; Srivastava, S.; Archer, L. A. High Energy Lithium-Oxygen Batteries – Transport Barriers and Thermodynamics. Energy Environ. Sci. 2012, 5, 8927-8931.


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TOC GRAPHICS


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Figure 1 104x130mm (300 x 300 DPI)



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Figure 2 133x209mm (300 x 300 DPI)


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Figure 3 84x160mm (300 x 300 DPI)



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Figure 4 133x123mm (300 x 300 DPI)


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TOC 29x10mm (300 x 300 DPI)


A High-Rate Ionic Liquid Lithium-O2

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