Supplementary Materials for

Transcription

1 Supplementary Materials for Cycling Li-O 2 batteries via LiOH formation and decomposition Tao Liu, Michal Leskes, Wanjing Yu, Amy J. Moore, Lina Zhou, Paul M. Bayley, Gunwoo Kim, Clare P. Grey* This PDF file includes: Materials and Methods Supplementary Text Figs. S1 to S23 References (43 47) *Corresponding author. cpg27@cam.ac.uk Published 30 October 2015, Science 350, 530 (2015) DOI: /science.aac7730

2 * This file includes: Materials and Methods Additional Experiments and Discussion 1. SEM and Electrochemical Investigation of the Three Different Electrodes (Fig. S1 and S2) 2. Comparison of the Capacity Obtained for the I - /I - 3 couple (in Ar) with that Obtained for the Li-O 2 Cells (Fig. S3) 3. 7 Li NMR Characterization of the Discharge Products in the Presence of LiI and O 2 (Fig. S4) 4. 7 Li, 1 H NMR and SEM Characterization of the Discharge Products in the Absence of LiI (Fig.S5) 5. SEM Characterization of the Discharge Products Obtained with LiI (Fig. S6) 6. SEM and ssnmr Characterization of the Products with LiI in TEGDME (Fig. S7) 7. SEM Characterization of Electrodes after Multiple Cycles (Fig. S8) 8. Li-O 2 Battery Using an SP Carbon Electrode and LiI/DME (Fig. S9) 9. Electrochemistry of Samples for XRD and NMR in Fig. 2 (Fig. S10) 10. Li-O 2 Batteries Cycled at Higher Rates (Fig. S11) 11. Li-O 2 Cells Cycled in the Presence of High Concentrations of Water (Fig. S12) 12. Added Water Leads to the Formation of even Larger LiOH Crystals (Fig. S13) 13. Further Comments on the Specific Energy Density Calculations 14. Establishing the Discharge Mechanisms Sources of H in the Formed LiOH and the Role of Water (Fig. S14-18) LiOH/electron Molar Ratio during Discharge (Fig. S19) 15. Establishing the Charge Mechanisms (Fig. S20-21) 16. Electrochemistry Data and Fitted NMR Spectra for Figure S19 (Fig. S22-23)

3 Materials and Methods: Materials 1,2-Dimethoxyethane (DME) (Sigma Aldrich, 99.5%) and tetraethylene glycol dimethyl ether (TEGDME) (Sigma Aldrich, 99%) solvents were was refluxed with calcium hydride and distilled under a nitrogen atmosphere, and then stored over 4 Å molecular sieves under argon. The final water content of the solvents was measured to be below 10 ppm by Karl Fischer titration (Metrohm 899). Deuterated DME (Sigma Aldrich, D-10, 99.5 D%, 99% CP) was used as-received. Molecular sieves were washed with ethanol and acetone, dried overnight in an oven at 70 C and then at 275 C in vacuo for two days. Lithium bis(trifluoromethyl)sulfonylimide (LiTFSI) (3M Fluorad TM, HQ115) and LiI (Sigma-Aldrich, 99.9%) were dried at 160 C and 200 C, respectively, in vacuo for 12 hours before being used to prepare the electrolyte. Super P (SP) carbon black (~50 nm) and TiC nanoparticles (~40 nm) were purchased from Timcal and Skyspring nanomaterials respectively. All materials were stored and handled in an Ar glovebox with <0.1 ppm O 2 and <0.1 ppm H 2 O. Methods Electrode fabrication methods Mesoporous SP carbon electrodes were prepared from a mixture of 24 wt% SP carbon black, 38 wt% polyvinylidene fluoride (PVDF, copolymer) binder, and 38 wt% dibutylphthalate (DBP, Sigma-Aldrich) in acetone. The slurry was then spread into a self-supporting film and cut into ½ inch in diameter discs, which were washed with diethyl ether to remove the DBP. The resulting films were then annealed at 120 C in vacuo for 12 hours and transferred to the glovebox without exposure to air. The final carbon content in the electrodes is 39 wt%. Mesoporous TiC electrodes were prepared via a similar procedure, with carbon being replaced by TiC. A mixture of 48 wt% TiC, 8 wt% PVDF and 44 wt% DBP was used with acetone to make the slurry. The subsequent film making and drying method are exactly the same as used for fabricating SP carbon electrodes. Aqueous graphene oxide solution was synthesized by a modified Hummer s method (43). Briefly, concentrated H 2 SO 4 (96 ml) was added to a mixture of graphite flakes (2 g) and sodium nitrate (2 g), which was stirred at 0 C in a water/ice bath. KMnO 4 (12 g) was then gradually 2

4 added and the mixture was continuously stirred at 0 C for 90 minutes. The reaction temperature was subsequently raised and kept at 35 C for 2 hours, after which 80 ml deionized water was slowly added to the suspension. Additional water (200 ml) and H 2 O 2 (30%, 10 ml) were introduced. At this point, a suspension of graphite oxides was obtained. This graphite oxides suspension was allowed to settle down and the clear solution at the top was repeatedly removed and replaced with deionized water until the suspension became neutral. The resulting slurry of graphite oxide was subjected to many ultrasonication and centrifugation cycles until no sediment was found at the bottom of the centrifuge tube. A well-dispersed aqueous graphene oxide solution was finally synthesized. To fabricate reduced graphene oxide (rgo) electrodes, the obtained graphene oxide solution was first concentrated by annealing it in a vial at C to form a viscous gel that has a graphene oxide concentration of ~10 mg/ml. The gel was cast onto a stainless steel (ss) mesh (Advent Research Materials) using a volumetric pipette and then frozen and stored in a vial in liquid N 2. The graphene oxide electrodes on ss-meshes were freeze-dried for 12 hours in vacuo and then subjected to pyrolysis in a furnace under Ar at 550 C for 2 hours, to obtain rgo electrodes. These electrodes were further dried at 150 C in vacuo before being used to make batteries. The masses of rgo electrodes were carefully measured by comparing the masses of a few bare meshes with their respective masses after the rgo electrodes had been coated, an average value being taken for a specific batch of electrodes. Li-O 2 cell assembly and electrochemical measurements All Li-O 2 cells investigated in this work are based on a Swagelok design. They were assembled by stacking a disc of lithium foil (0.38 mm thickness, Sigma-Aldrich), 2 pieces of borosilicate glass fibre separators (Whatman) soaked with electrolyte, and an O 2 positive electrode (SP, TiC or rgo). The electrolytes used in this study include 0.25 M LiTFSI/DME or 0.25 M LiTFSI/TEGDME with/without the addition of 0.05 M LiI. A 0.5 cm 2 hole was drilled through the current collector, so that the positive electrode can readily access O 2. The assembled Swagelok cell was then placed in a 150 ml Li-O 2 glass chamber, the two electrodes being electrically connected to two tungsten feedthroughs. Pure O 2 was purged through the chamber via two Young s taps for 25 minutes, and the cell was then rested for 10 hours before cycling. The volume of TEGDME electrolyte used for a cell was typically 0.2 ml. DME electrolyte is 3

5 more volatile and was found to evaporate rapidly during the O 2 purge and to be absorbed into the viton rubber coating of the electrical cables (this latter problem occurs due to the current in-house design of our Li-O 2 cells and can be avoided using better cell designs). Consequently, ml DME electrolyte was used for a cell. The electrode loading in this work ranged from 0.01 to 0.15 mg and the thickness of the rgo electrodes varied from 30 to 200 μm. For Li-O 2 cells subjected to more prolonged cycling, thinner electrodes (30-50 μm) were used. The cycling rate was quoted based on the mass of carbon in an electrode. For example, 5 A/g c rate (Fig. 4B) of a 0.01 mg electrode is equivalent to cycling the cell at a current of 50 ua, giving a rate per unit area of 0.1 ma/cm 2. The electrochemical measurements (Galvanostatic discharge/charge, cyclic voltammetry) were conducted using either an Arbin battery cycler or a Biologic VMP. All potentials are referenced against Li/Li +. Electrode characterization For all batteries, the characterization of electrodes involved first disassembling the cell, rinsing the O 2 positive electrode twice in dry acetonitrile (<1 ppm H 2 O), each time with 2 ml CH 3 CN for 30 minutes. The washed electrodes were then dried in vacuo overnight for further characterization. X-ray diffraction (XRD) measurements were performed using a Panalytical Empyrean diffractometer operated in a reflection mode, with Cu Kα1 radiation (λ= Å). The cycled electrode was sandwiched between two Kapton polyimide films in an air tight sample holder. Scanning electron microscopic (SEM) images were recorded with a Hitachi S-5500 in lens field emission electron microscope. The electrode samples were hermetically sealed during transfer to the electron microscope. Once the seal was opened, the sample was loaded into the high vacuum SEM chamber within 10 seconds. All solid-state NMR (ssnmr) spectra were acquired on either a 16.4 T Bruker Avance III or an 11.7 T Bruker Avance III spectrometer using 1.3 mm HX probes. A rotor synchronized Hahn-echo pulse sequence was used to acquire 1 H magic angle spinning (MAS) spectra with a spinning speed of 60 khz (unless stated otherwise), and an rf field strength of 125 khz. A one-pulse sequence was used to acquire 7 Li NMR spectra under MAS and static conditions, with an rf field strength of 167 khz. 1 H and 7 Li shifts were externally referenced to solid adamantane at 1.8 ppm and lithium carbonate at 0 ppm, respectively. The same receiver gain, number of 4

6 scans and recycle delay values (optimized values of between s were used) were employed to measure the electrodes (from the same batch) to allow quantitative comparison between spectra. Additional Experiments and Discussion 1. SEM and Electrochemical Investigation of the Three Different Electrodes In this work, the electrochemical performance of cells made of the three types of electrodes discussed on the main text were compared to evaluate the magnitude of various contributions to the charge overpotential, such as catalytic effects of different electrode materials, concentration polarization due to the diffusion of electrode active species and ohmic loss caused by the insulating discharge product. The rgo electrodes, figure S1 (c), clearly contain much larger pore sizes and pore volumes than SP electrodes (e), which leads to a lower tortuosity and thus more efficient diffusion of the active species within the electrolyte (Li +, solvated O 2, mediators etc.) in rgo than in SP. Therefore the smaller overpotential for rgo electrode than SP is ascribed, at least in part, to the interconnecting macroporous framework. SP carbon (e) and TiC (f) electrodes are comprised of particles of similar sizes (~50 nm) and are made by the same fabrication method. They both have similar mesoporous electrode structures. The difference in the electrochemical performance between SP and TiC electrodes (Fig. 1A, main text) is tentatively attributed to the difference in their intrinsic catalytic activities. It is important to stress that it is difficult to separate unambiguously the different contributions to the overpotential (e.g., the activation barrier of the reaction, ohmic loss, diffusion of active species). When comparing the SP with TiC electrodes, for example, it is difficult to ensure that the electrical resistance and surface areas of the electrodes are identical even if the pore structure is similar. Thus, the current method of comparison (Fig. 1) gives a qualitative estimation of the various origins for the charge overpotentials rather than a quantitative evaluation. 5

7 Fig. S1 SEM images showing hierarchically macroporous rgo electrodes at various magnifications (a-d), mesoporous Super P (SP) carbon (e) and mesoporous TiC electrodes (f). 6

8 As can be seen on in figure S2 (a), rgo and SP carbon electrodes exhibit good stability within the voltage window V; gradually rising cathodic and anodic currents were observed out of this voltage range. TiC is a less inert electrode material in LiTFSI/DME: rapidly rising cathodic and anodic currents were observed below 2.5 and 3.75 V, respectively. Figures S2 (b) and (c) illustrate that rgo, SP and TiC electrode all reversibly cycle LiI (3I - I e - ). In figure S2 (b), the separation between the redox peaks of I - - / I 3 in TEGDME-based electrolyte (blue curve) is wider than that in DME-based electrolyte (red curve). This is probably associated with the higher viscosity of TEGDME and hence a slower diffusion of mediators in this electrolyte. Fig. S2 (a) Cyclic voltammograms of cells using rgo, Super P (SP) and TiC electrodes in 0.25 M LiTFSI/DME under an Ar atmosphere; (b) cyclic voltammograms comparing cells using rgo electrodes in 0.05 M LiI/0.25 M LiTFSI/DME and TEGDME electrolytes under an Ar atmosphere; (c) cyclic voltammograms of cells using SP and TiC electrodes in 0.05 M LiI/0.25 M LiTFSI/DME under an Ar atmosphere. Sweep rate for all cells was 5 mv/s. 7

9 2. Comparison of the Capacity Obtained with the I - /I 3 - Couple (in Ar) with that Obtained for the Li-O 2 Cells The capacity of a Li-iodine redox battery is typically evaluated based on the mass of iodine (the active material), which gives a theoretical capacity of 211 mah/g (44) i.e., [(96485/3.6) ma]/127 g. In our cells with the TEGDME electrolyte, the number of moles of I - was mol, i.e., g. The electrical charge extracted from the SP, TiC and rgo cells under an Ar atmosphere was , and mah, respectively, giving only 0.09, 0.18, 0.26 mah/g I, which is much lower than the theoretical capacity based on the I - - /I 3 couple and the total I - present in the cell. This indicates that the majority of the iodide ions did not participate in the electrochemical reaction. This is not, however, surprising, as no effective convection is available in our cells. As a result, the capacity is solely dependent upon the self-diffusion of electroactive species. Similar values are obtained for cells using the DME electrolyte, being 0.18, 0.18, 0.07 mah/g I for SP, TiC and rgo cells, respectively. In a Li-O 2 cell investigated in this work, the capacity is calculated based only on the mass of electrode material (SP carbon, TiC, or rgo). To compare the capacities obtained with and without O 2, we also calculated the capacity of LiI cells based on the mass of the electrode materials, as illustrated in figure S3 below. It is clear that the specific capacities of all 3 electrodes cycled under Ar are much smaller than those of the Li-O 2 batteries, when using the same electrode materials. 8

10 Fig. S3 Galvanostatic charge-discharge curves of cells cycled with 0.05 M LiI in 0.25 M LiTFSI / TEGDME and DME electrolytes, in an Ar rather than O 2 atmosphere; these cells were first charged and then discharged. The grey line in each graph shows the corresponding electrodes discharged in the same DME-based electrolyte in an O 2 atmosphere. 9

11 3. 7 Li NMR Characterization of the Discharge Products in the Presence of LiI and O 2 Fig. S4 Comparison between the 7 Li static NMR spectra, acquired at 11.7 T, of a discharged rgo electrode from a Li-O 2 cell using a 0.05 M LiI/0.25 M LiTFSI/DME electrolyte and those of the model compounds LiOH, Li 2 CO 3 and Li 2 O 2. The characteristic quadrupolar line shape of the discharged rgo electrode sample (Fig. S4) overlaps with that of LiOH, rather than Li 2 CO 3 or Li 2 O 2, suggesting the discharge product is overwhelmingly LiOH. 10

12 4. 7 Li, 1 H NMR and SEM Characterization of the Discharge Products in the Absence of LiI As can be seen in figure S5 (a), the rgo electrode without the added LiI (blue curve) exhibits a lower discharge capacity than that with added LiI (red curve). The higher capacity results from the much larger concentration of discharge products (LiOH) that more efficiently take up the pore volume in macroporous rgo electrodes in the latter case (see Fig. S6). The black curve in (a) represents a cell with an rgo electrode in 0.05 M LiI/0.25 M LiTFSI/DME electrolyte galvanostatically discharged in an Ar atmosphere: its capacity is negligible compared to that cycled in an O 2 atmosphere (red curve). In figure S5(b), a single resonance at 0 ppm in the 7 Li MAS ssnmr spectrum (acquired at 16.4 T) and the absence of satellite transition peaks in the 7 Li static ssnmr spectrum suggest that Li 2 O 2 is the predominant discharge product (Fig. S4) when LiI is absent. 1 H MAS ssnmr measurement shows a resonance at -1.5 ppm, suggesting LiOH is also present in the discharge products. The resonances at 2.3 and 8 ppm are attributed to residual DME solvent and lithium formate, respectively, in the electrode. SEM images (c-d) of this discharged electrode reveal that the rgo electrode surfaces are fully covered by toroidal particles (~500 nm), the toroids being a characteristic morphology for Li 2 O 2, consistent with the ssnmr measurements. 11

13 Fig. S5 The electrochemistry of a Li-O 2 battery using a rgo electrode in a 0.25 M LiTFSI/DME electrolyte (a, blue curve) and characterization of this discharged rgo electrode by ssnmr (b) and SEM (c-d). The electrochemistry of a Li-O 2 battery discharged in the presence of LiI is shown for comparison in (a) (red curve). This battery was interrupted at a capacity of 3.2 mah. The black curve in (a) shows the LiI battery discharged under Ar; its capacity is negligible. 12

14 5. SEM Characterization of the Discharge Products obtained with LiI Fig. S6 SEM images of a fully discharged rgo electrode in 0.05 M LiI/0.25 M LiTFSI/DME electrolyte. Numerous large particles fill up the pores of the rgo electrode (a). When we cut the electrode and investigated the interior space (b-d), flower-like particles larger than 15 μm were also observed. Some of these particles grow on the insulating glass fiber separators (d), indicating that these LiOH crystals were formed via a solution precipitation process. 13

15 6. SEM and ssnmr Characterization of the Products with LiI in TEGDME When discharged at a slow rate of 100 ma/g c, a voltage plateau at 2.7 V was observed for a Li-O 2 cell made with TEGDME-based electrolyte (Fig. S7 (a)). In the ssnmr spectra (b) of the discharged rgo electrode, acquired at 16.4 T, resonances at -1.5 and 8.3 ppm in the 1 H MAS spectrum indicate the existence of lithium hydroxide and lithium formate, respectively, lithium hydroxide being the dominant product; the resonance at 4.8 ppm is attributed to water and those at 3.3 and 0.7 ppm are ascribed to residual TEGDME solvent in the electrode. The single resonance at 1.0 ppm and the characteristic line shape of the quadrupolar 7 Li static ssnmr spectrum with its satellite transition peaks further confirm that LiOH is the predominant discharge product. SEM images (c) of this discharged electrode reveal that instead of forming large flower-like particles as seen with the DME-based electrolyte, LiOH exists as a thin film covering the rgo electrode surface, resulting in a lower discharge capacity. 14

16 Fig. S7 The electrochemistry (a) of a Li-O 2 battery using an rgo electrode in a 0.05 M LiI/0.25 M LiTFSI/TEGDME electrolyte and characterization of the discharged rgo electrode by ssnmr (b) and SEM (c). 15

17 7. SEM Characterization of Electrodes after Multiple Cycles Fig. S8 SEM images of rgo electrodes cycled in 0.05 M LiI/0.25 M LiTFSI/DME electrolyte at the end of the 1 st, 15 th and 100 th discharge (D) and charge (C). Only trace amounts of residual LiOH are seen after a full charge on the rgo electrode surfaces. Note that there is always some variation in the porous structure between and in different parts of the electrodes, even from the same batch, because of the casting method. The SEM images therefore vary to a degree between samples and across the sample due to these changes in porosity. For example, the image of the 100 th cycle charged sample shows a very porous part of the electrode, whereas 1 st and 15 th cycle charge electrodes represent less porous ones. We note that the porous elongated features (arrows) on the 100 th cycle charged electrode belong to the porous graphene electrode, not to the remaining LiOH. 16

18 8. Li-O 2 Battery Using an SP Carbon Electrode and LiI/DME When discharged at 70 ma/g c, a voltage plateau at 2.65 V (Fig. S9(a)) was observed for a Li-O 2 cell made using an SP carbon electrode and LiI-added DME electrolyte. In the ssnmr spectra (b), the dominant resonance at -1.5 ppm in 1 H and a single resonance at 1.0 ppm in the 7 Li ssnmr spectra suggest that LiOH is the main discharge product; this is further corroborated by the 7 Li static spectrum. The resonance at 4.8 ppm in the 1 H ssnmr spectrum is attributed to water and those at 3.3, 2.6 and 1.0 ppm are ascribed to residual DME solvent in the electrode and saturated carbons. SEM images (c) of this discharged electrode reveal that LiOH exhibit disc and sheet-like morphologies. Notably, these discs/sheets of LiOH are ~500 nm in size, much smaller than those observed in rgo electrodes (Fig. S6) even though the same electrolyte was used. After full charge, much of the surface of the SP electrode became bare again, although some regions containing residual LiOH were still observed. This observation suggests that LiOH can indeed be removed during charge in the SP electrode/electrolyte system, even if it is not complete. 17

19 Fig. S9 The electrochemistry (a) of a Li-O 2 battery using an SP carbon electrode in a 0.05 M LiI/0.25 M LiTFSI/DME electrolyte, and characterization of the discharged SP electrode by ssnmr spectroscopy and SEM (c), where spectra were acquired at 11.7 T (b). 18

20 9. Electrochemistry of samples for XRD and NMR Shown in Fig. 2 Fig. S10 The electrochemistry of the two samples used for XRD and NMR measurements in fig. 2 of the main text. A slow rate of 7 μa was used for both rgo cells. 19

21 10. Li-O 2 Batteries Cycled at Higher Rates When cycling the cell at the higher rate of 8 A/g c the cell voltage gradually polarizes with cycle number, probably due to more side reactions occurring at the more reducing (discharge) and oxidizing (charge) electrochemical potentials, and the incomplete removal of discharge product. As a result, the rgo electrode surface (c) remains covered after just 40 cycles by particles with different morphologies from those seen for LiOH formed at lower cycling rates. Fig. S11 The first 42 discharge-charge curves of a Li-O 2 battery cycled at 8 A/g c rate using an rgo electrode in 0.05 M LiI/0.25 M LiTFSI/DME electrolyte (a) and the corresponding terminal voltages as a functional of cycle number (b). An SEM image (c) of the rgo electrode from the Li-O 2 cell in (a) after 42 cycles. (d) represents a Li-O 2 cell that was cycled for 1000 cycles with a capacity limited to 1000 mah/g c and then deliberately subjected to 15 much deeper discharge-charge cycles with a reversible capacity of 22,000 mah/g c at a 1 A/g c rate. Apart from the larger voltage polarization due to the prior cycling, the cell still demonstrates a good reversibility. 20

22 11. Li-O 2 Cells Cycled in the Presence of High Concentrations of Water Fig. S12 The discharge-charge profiles of Li-O 2 batteries cycled using rgo electrodes in a 0.05 M LiI/0.25 M LiTFSI/DME electrolyte: (a) with 45,000 ppm of water deliberately added to the electrolyte; (b) purged with wet O 2 gas that had been passed through a water bubbler. Experiments were performed to explore the sensitivity of this cell to water. No noticeable difference was seen in the electrochemical performance, compared to cells cycled with no additional water added, which demonstrates that the cell is insensitive to H 2 O contamination at least at the levels investigated here (37 mg H 2 O per 783 mg of DME, i.e., 45,000 ppm H 2 O). 21

23 12. Added Water Leads to the Formation of even Larger LiOH Crystals Fig. S13 SEM images of an rgo electrode from a Li-O 2 cell, prepared using a 0.05 LiI/0.25 M LiTFSI/DME electrolyte with added water (20,000 ppm, 15 mg). Porous LiOH crystals of more than 30 μm were observed. Nuclei of secondary LiOH crystals were observed to grow on the sheets of large LiOH crystals, which also support a solution mechanism for the formation of LiOH in the system, because the large LiOH crystals are insulating. 22

24 13. Further Comments on the Specific Energy Density Calculations The energy density quoted in the text (2.7 V 3.2 mah ) 1.5 mg = 5760 Wh/kg only takes the rgo electrode and discharge products into account; the weight of other cell parts are not considered. Compared with a commercial lithium iron phosphate (45) electrode (3.2 V, 0.13 mah/1.5 mg), the current rgo electrode is able to provide 20 times more specific energy. In comparison with the targeted specific energy (500 Wh/kg) of an aqueous Li-O 2 battery (46), the value achieved with the aprotic Li-O 2 cell in this current work is still 10 times higher. However, it should be noted that the 1.5 mg value (for the weight of the electrode) only contains electrode pieces that can be removed from the stainless steel mesh. Errors in the weight of the electrode can occur because some of the LiOH is formed on the separator, stuck in the stainless steel mesh and/or washed away during rinsing. Thus 1.5 mg represents an underestimated weight for the discharged electrode that corresponds to the 3.2 mah capacity, leading in practice to slightly lower specific energies than the quoted number, 5760 Wh/kg. The electrode area used in this work is ~0.5 cm 2, and hence 3.2 mah corresponds to 6.4 mah/cm 2. This capacity based on an electrode of ~200 μm in thickness. The theoretical discharge capacity will be larger than this: the LiOH crystals at the farther end of the electrode away from the O 2 reservoir were observed to be smaller in size and less dense than those at the electrode regions directly facing O 2, i.e. the reaction zone problem still exists to a degree in the current cell configuration; hence there is still room for further improvement in the capacity of the cell. Although several critical issues with Li-O 2 batteries have been addressed (low discharge capacity, a large hysteresis, considerable side reaction with electrolyte, sensitivity to water), there are still problems associated with the current battery, such as high volatility of DME electrolyte, intrinsic dendrite growth and safety issues with the use of Li metal anode, low rate performance, and the reaction zone problem (40). In particular, the rate performance and the reaction zone problems are closely related, limiting the scale-up of any Li-O 2 battery and motivating future work. 23

25 14. Establishing Discharge Mechanisms Sources of H in the Formed LiOH and the Role of H 2 O The H source for LiOH is clearly a very important factor that should affect the performance of the battery. First we evaluate if the H in the LiOH discharge product is from the surface functional groups on rgo electrodes. The weight of the cell electrode (pristine weight=0.1 mg) after discharge (3.2 mah) is ~1.6 mg. XRD and NMR measurements show that the increased weight are due to formation of LiOH crystals, i.e., 1.5 mg LiOH. To produce 1.5 mg LiOH, mg of H is needed, which is more than half the weight of the pristine rgo electrode. Thus, GO is unlikely to be the H source for LiOH. Furthermore, 1 H ssnnr of the pristine rgo electrodes reveals a low proton content. The water content of DME solvent was measured by a Karl Fischer apparatus to be less than 10 ppm. Approximately 1 ml electrolyte solvent was used for a battery, which leads to 1 cm g/cm 3 10 ppm = mg H 2 O. This is two orders of magnitude less than needed (1.1 mg of H 2 O) to generate 1.5 mg of LiOH. Hence, it is unlikely that the trace amount of water in a nominally dry electrolyte solvent is the source of H. To test whether H is from moisture in air that gradually leaked into the sealed Li-O 2 cell, we discharged a Li-O 2 battery inside an Ar glovebox (H 2 O<0.1 ppm). ssnmr measurements, performed at 11.7 T, (figure S14) of this discharged rgo electrode showed that LiOH is still the prevailing discharge product, precluding this possibility as well. 24

26 Fig. S14 ssnmr spectra of an rgo electrode used in a Li-O 2 cell discharged with 0.05 M LiI/0.25 M LiTFSI/DME electrolyte inside an Ar glovebox (<0.1 ppm H 2 O). The dominant resonances at -1.5 ppm in 1 H and 1.0 ppm in 7 Li MAS spectra, respectively, and the characteristic line shape of the 7 Li static spectrum all suggest that LiOH is the prevailing discharge product of this cell. The other resonances labeled in the 1 H spectrum are attributed to residual DME in the electrode. If the H were from DME molecules (and assuming that the molar ratio of consumed DME to generated H is 1:1; note that we use this assumption simply to provide an order of magnitude estimate of the proton content) to produce 1.5 mg LiOH, 62.5 μmol H is needed, i.e., 5.6 mg DME (62.5 μmol g/mol) is required. This is equivalent to only 6.4 μl DME (5.6 mg/868.3 mg/cm 3 ). In this work, around 1 ml DME was used to make a cell. Thus DME is one possible H source for the production of LiOH during the first discharge process. To test this hypothesis, we discharged a cell using an rgo electrode with a 0.05 M LiI/0.25 M LiTFSI/DME electrolyte, prepared from as-received deuterated DME (D-10, 99.5 D%, 99% CP). 1 H, 7 Li ssnmr and XRD measurements on this sample show that a crystalline lithium hydroxide phase together with a significant amount of other discharge (decomposition) products is present in the electrode (Fig. S15). This higher level of decomposition products is ascribed to lower purity level (99% CP) of this deuterated DME solvent (the deuterated solvent being difficult to distil due to the low quantity purchased (due to the cost)). The 2 H ssnmr spectrum (Fig. S16) of this sample confirms the presence of LiOD in the electrode, which is more visible in the characteristic 25

27 spinning side band manifolds, rather than in the central band. However, LiOD is by no means the dominant 2 H signal in the discharged electrode produced in the presence of deuterated DME. Fig. S15 1 H and 7 Li ssnmr and XRD of a Li-O 2 cell discharged using an rgo electrode and a 0.05 M LiI/0.25 M LiTFSI/DME electrolyte, prepared from as-received deuterated DME (D-10, 99.5 D%, 99% CP). The NMR spectra of the discharged electrode and the reference compound are acquired at 11.7 T, under (a, b) MAS speed of 60 khz, and (c) static conditions. The 1 H MAS spectrum shows a resonance at -1.4 ppm that is assigned to LiOH indicating that it is present in the discharge products. The resonance at 8 ppm is attributed to lithium formate. The resonances at 1.4 and 3.9 ppm are due to saturated hydrocarbons (1.4 ppm) and ethers (3.9 ppm). The XRD pattern (d) confirms that the major crystalline phase in the discharged electrode is LiOH; the thin LiOH sheets observed by SEM (i.e., a short coherent length along <001> direction) likely give rise to a weak/broad (001) reflection which is obscured by the broad background from the Kapton film from the sample holder. The electrochemistry is given in Fig. S22 (a, cell 1). 26

28 Fig. S16 2 H ssnmr spectra of the rgo electrode from the same cell discussed in figure S15. The spectra were acquired at 11.7 T, with a MAS speed of 60 khz; magnified spectra of the isotropic and 1 st spinning sideband are shown for clarity. The major discharge product observed in the 2 H spectrum is not LiOD at -1.5 ppm but instead gives rise to another resonance at 3.5 ppm due to ethers. To investigate whether water can serve as an alternative hydrogen source, we added D 2 O to the 0.05 LiI/0.25 M LiTFSI/non-deuterated DME electrolyte and discharged a Li-O 2 cell using an rgo electrode. The 7 Li (b-c) and 2 H (d) spectra all show that deuterated lithium hydroxide is the dominant discharge product (Fig. S17). The observation that LiOD is now the only signal visible in the intense 2 H spectrum (d) while only a weak minor 1 H resonance at -1.5 ppm due to LiOH is seen in the 1 H spectrum (a) of the same sample confirms that LiOD is formed from the deuterons of D 2 O in the electrolyte; this result thus clearly shows that water is the major source of hydrogen in the reaction to form LiOH. 27

29 Fig. S17. ssnmr spectra of an rgo electrode extracted from a Li-O 2 cell, prepared using a 0.05 LiI/0.25 M LiTFSI/non-deuterated DME electrolyte but with added D 2 O (20,000 ppm, 15 mg). The ssnmr spectra of LiOD reference compound is also shown for comparison: 1 H (a), 7 Li under MAS (b) and static conditions (c), and 2 H NMR spectra with a zoom (right) to show the isotopic region for clarity (d). The resonances at 1.4 and 3.9 ppm in (a) are due to saturated hydrocarbons (1.4 ppm) and ethers (3.9 ppm).the electrochemistry data of the cell is given in Fig. S22(a, cell 2). The observation that H 2 O supplies the H to the formed LiOH suggests that a significant amount of H 2 O was introduced into the cell when deuterated DME was used (as discussed in figures S15 and 16). Experiments performed with deuterated DME dried by molecular sieves overnight still showed that LiOH was the dominant phase and no trace of LiOD was observed ( 1 H, 7 Li and 2 H ssnmr measurements). H 2 O is still introduced from other sources. The capacity of the cell was 1.1 mah. Assuming an electron/lioh molar ratio of 1 and that all H in H 2 O goes to form LiOH (see below), the amount of H 2 O needed was 0.37 mg, which corresponds to ~500 ppm of H 2 O in the electrolyte. 28

30 One possible source of H 2 O comes from the O 2 purge of our cells. To test this, 1 ml of nominally dry DME (<5 ppm H 2 O as determined in a Karl Fischer test) was sealed in a Li-O 2 cell chamber and purged for 5 minutes using the current O 2 line. The water content of the resulting DME was measured as 47 ppm showing that one source of H 2 O comes from water in the lines. Further water likely arises from water sorbed on other cell components such as the separator. Fig. S18 1 H (a), 7 Li (b) and 2 H (c) ssnmr spectra of a Li-O 2 cell prepared using an rgo electrode and 0.05 M LiI/0.25 M LiTFSI/deuterated DME (dried by molecular sieves overnight). ssnmr spectra of LiOD reference compound are shown for comparison. An intense 1 H resonance at -1.5 ppm (a) and a single 7 Li resonance at 1 ppm (b) suggest LiOH is the dominant phase in the discharge product. No signature due to LiOD was observed in the 2 H spectra (c) of this discharge electrode. The electrochemical data of the cell is given in Fig. S22 (a, cell 3). In summary, although the DME solvent in the electrolyte is a potential H source for the formed LiOH during discharge, it does not seem to be the dominant one. Water is shown to serve as an 29

31 alternative H source for LiOH in the current LiI mediated system. When water is intentionally added to the DME-based electrolyte, it appears to preferentially supply H to form LiOH, minimizing DME decomposition. No appreciable amount of Li 2 CO 3 or Li acetate was observed in our discharged or charged electrodes. Only very little Li formate was observed in the 1 H spectra of the discharged electrodes. However, when Li 2 O 2 forms as the dominant discharge product, a significant amount of Li formate is present (13); this difference is probably related to the intrinsic reactivity of the discharge product Li 2 O 2 with a DME-based electrolyte, as suggested by previous studies (47). Compared to Li 2 O 2, the less reactive LiOH crystals appear to cause minimal chemical reactions with the DME-based electrolyte. The reactivity of the intermediate LiO 2, which will cause DME decomposition has also been substantially reduced, which has been demonstrated by the Li-O 2 cells cycled in the presence of additional water, as discussed in figures S17 and 18; this may also be part of the reason why very little Li 2 CO 3, Li acetate and formate were generated. LiOH does not seem to react with the rgo electrode at the charge voltages (< 3.4 V) investigated here; whether the side product/lioh reacts with graphene to form Li 2 CO 3 at higher voltages is unclear. However, without the use of the mediator, we found that rgo oxidation is facile at charge voltages beyond 4 V. Finally, it should be emphasized that without the presence of LiI, DME electrolyte does not lead to significant LiOH formation even in the presence of significant amounts of water; instead Li 2 O 2 is the dominant discharge product instead, large toroidal particles being observed. Of note, this is consistent with prior work where water is shown to promote the formation of the Li 2 O 2 toroidal particles, water contents of 500 ppm being required for toroid formation (42). The role of water in the reaction helps explain why the kinetics (overpotential) of the 1 st and 2 nd cycle are essentially identical, but this may also be because water is not involved in the reaction that sets the overpotential. When 0.05 M LiI is also added, no trace of Li 2 O 2 is detected, LiOH becoming the prevailing discharge product. Thus the LiI must play a role in promoting LiOH versus Li 2 O 2 formation. 30

32 14.2. Electron/LiOH Molar Ratio during Discharge In the main text of the manuscript we proposed that the first step during discharge is a one-electron electrochemical process, i.e., Li + + e - + O 2 LiO 2, because the observed discharge voltages using rgo electrodes were the same with and without LiI in the electrolyte (Fig. 1A, main text), i.e., the first step is identical to that observed in the standard Li-air cell (2-3). We also proposed that the subsequent conversion of LiO 2 to LiOH is a chemical process that occurs via a solution mechanism. It is clear that I - is involved in changing the equilibia so that LiOH is formed rather than Li 2 O 2, since Li 2 O 2 is still the major product under wet conditions (our work and work of reference 8). Formally the reaction can be written: 4Li + + 4e - + 4O 2 4LiO 2 Electrochemical, E [1] 4LiO 2 + 2H 2 O 4LiOH + 3O 2 Chemical, C [2] Where the chemical reaction [2] is mediated via reactions involving I -. Potential competitive reactions are: 2Li + + 2e - + 2O 2 2LiO 2 Electrochemical, E [3] 2LiO 2 Li 2 O 2 + O 2 Chemical, C [4] Li 2 O 2 +H 2 O 2LiOH + 1/2O 2 Chemical, C [5] or 2Li + + 2e - + 2O 2 2LiO 2 Electrochemical, E [6] 2LiO 2 2Li 2 O 2 + O 2 Chemical, C [7] We investigated the electron-to-lioh molar ratio during discharge by quantifying the number of moles of LiOH in several cells discharged to different extents (with added LiI mediator), and comparing their NMR spectra with those of a reference LiOH sample to determine the number of moles of LiOH versus the moles of electrons consumed in the electrochemical discharge (Fig. S19). On the basis of the 1 H LiOH signals derived from electrodes with different capacities, the LiOH/electron molar ratio is equal to

33 The method underestimates the LiOH content since we cannot pack all the LiOH within a discharged cell into an NMR rotor (for example, rgo fragments are washed away during rinsing, the rgo pieces stick in holes of the stainless steel mesh, LiOH is formed on the mesh itself and on the glass fibre separators (as seen in the SEM), and LiOH is lost on rotor packing tools used to pack NMR rotors etc.). We estimate we could lose as much as 20% of the sample in the process. Given this, it appears reasonable that a LiOH:electron molar ratio close to 1 is observed and the mechanism appears to occur via a reaction that consumes 1 Li + per e -. While the fit is not perfect, our data does support a mechanism where each electron results in one LiOH and that LiOH is the major product. We recognise that the parallel route to form Li 2 O 2 results in a LiOH/electron molar ratio of less than one (reactions [6] and [7]), but given that we do not detect this product by XRD and NMR, we believe that the lower ratio originates from an inability to measure all the LiOH. The current data clearly supports our proposal that LiOH is the major product. A major challenge is to identify the mechanism by which reaction [2] occurs. Given that longevity of the DME electrolyte and the ability to cycle the battery for multiple cycles, we speculate that I - must play a role in mopping up radicals, minimizing side reactions (noting that some side reactions still occur, as seen in the 1 H NMR above). 32

34 Fig. S19 A quantification plot of moles of LiOH versus moles of electrons obtained by analysing 1 H NMR spectra as a function of capacity. All 1 H NMR spectra are acquired at 11.7 T, with a MAS frequency of 60 khz, with a recycle delay of 200 s. To avoid any saturation effects due to the long spin-lattice relaxation times in this system, a small flip-angle pulse of 15º is used for all NMR experiments. The number of scans varies between 16 and 176. Spectral deconvolutions are carried out with DMFIT. The electrochemistry data and fitted spectra are presented in Fig. S22 (b) and Fig. S23. 33

35 15. Establishing the Charge Mechanism In this work, we have demonstrated that LiOH can indeed be removed during charge at voltages below 3.2 V using SEM, XRD and NMR (Fig. 2 in the manuscript). The fact that the cells are able to cycle well without capacity fading also supports a reversible reaction involving LiOH. Fig. S20 shows a Li-O 2 cell cycled with a cut-off discharge voltage of 2.5 V. The end of discharge in a Li-O 2 battery is typically marked by the electrode surface being fully covered by insulating discharge products or pores being so heavily clogged that diffusion of the electroactive species becomes very sluggish (resulting in rapid polarization of the cell voltage). The electrochemistry of the cell shown, which is discharged to the end, cycles with nearly an identical electrochemical profile, demonstrating highly reversible electrochemistry (the 1 st 10 cycles are shown here). The majority of the LiOH that forms during discharge must be removed during the following charge, otherwise cell would polarize rapidly due to the blocking of its pores and surface areas by LiOH. (This statement is supported by NMR spectra acquired at the top of charge). Fig. S20 The electrochemical profile of a Li-O 2 cell discharged to the end. 2.5 V was set as the cut-off voltage as side reactions become more severe below this point (see Fig. S2). 34

36 To verify the reaction mechanism proposed in figure 4 of the main text, it is necessary to detect whether there is any O 2 evolution during charge. Since water minimizes DME decomposition and has no appreciable effect on the electrochemical profile, we investigated a cell made using 0.05 M LiI/0.25 M LiTFSI/DME electrolyte with a 9800 ppm level of water. This Li-O 2 cell was first discharged and then connected to a mass spectrometer (MS), with the gas atmosphere in the cell then replaced by a high purity argon gas. We subsequently measured the cumulative gas signal after charge (Fig. S21(a)) instead of the transient response, the low sensitivity of our set-up and the large dead-space preventing the transient measurement. As can be clearly in figure S21(b) and its inset, a peak for both m/z 16 and 32 (black and blue traces) was observed, confirming that the initial sharp peaks indeed represent O 2. Following the sharp O 2 peak, there is another broad peak associated with m/z 32, but no corresponding m/z 16 peak was seen (inset in Fig. S21(b)); thus this second peak is not due to O 2 and is likely associated with the ionization of DME vapor at the MS head. Due to the presence of water in the electrolyte and its reaction with lithium metal anode, the water (m/z=18) and hydrogen (m/z=2) signals also rise as soon as the valves were opened. This result thus clearly shows that O 2 is generated in the cell after charging at 3.2 V. In summary, electrochemistry, NMR, XRD and SEM results all suggest that LiOH is removed during charge in the LiI mediated Li-O 2 battery. The electrochemical profile during charge and the observation that O 2 is produced during charge by mass spectrometry indicate that the first step during charge is the oxidation of iodide anions to triiodide anions and the second step is a redox chemical reaction, where lithium hydroxide is decomposed by triiodide anions generating water and oxygen. 35

37 Fig. S21 (a) The electrochemical profile of a Li-O 2 battery made using an rgo electrode and 0.05 M LiI/0.25 M LiTFSI/DME electrolyte with 9800 ppm of H 2 O content; the cell was discharged at a current of 10 μa in O 2 and charged at 15 μa under an Ar atmosphere. (b) Mass spectrometry measurement on the gas atmosphere of the cell in (a) after it was charged under Ar. An initial polarization of the charge voltage was observed for the cell in (a). This is occasionally observed for cells that have been subjected to a very deep discharge, such as this one. (When a Li-O 2 cell undergoes a deep discharge, most of the electrode surface area is presumably covered by insulating discharge products and pores in the electrode are clogged as well, making the diffusion of electro-active species very sluggish at the beginning, resulting in the initial observed polarization). As charge continues, the voltage drops gradually and eventually levels off, which is likely due to the removal of the discharge products. 36

38 15. Electrochemistry Data and Fitted NMR Spectra for Figure S15-19 Fig. S22 (a) Electrochemistry corresponding to Li-O 2 cells discussed in Fig. S15-16 (cell 1), Fig. S17 (cell 2) and Fig. S18 (cell 3); the initial dip in the voltage profile (cell 2) is likely to be caused by the formation of solid-electrolyte-interface at the Li metal anode (due to presence of added excess water 20,000 ppm, 15 mg). (b) Electrochemistry of Li-O 2 cells corresponding to the samples used to derive LiOH/electron molar ratio in Fig. S19; the capacities of the 6 lowest capacity electrodes are noticeably lower than our usual cells, and consistent with this more impurities were detected by NMR. We tentatively ascribe this to lower concentrations of the graphene oxide solution used (the electrodes having poorer structural integrity) and impurities in the DME electrolyte. Increasing the GO concentrations removed the problems in subsequent cells from a new batch (see the 1.99 mah electrode as an example). 37

39 Fig. S23. Raw 1 H NMR data for figure S19, used to quantify the moles of LiOH versus the moles of electrons; an experimental spectrum, a total fit, and individual fits are shown as blue solid, red dashed, and black lines, respectively. The intensity of the LiOH peak at approximately -1.5 ppm is used in the calculations. 38

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