Temperature Dependence of the Oxygen Reduction Mechanism in

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Temperature Dependence of Oxygen Reduction Mechanism in Non-Aqueous Li-O2 Batteries Bin Liu, Wu Xu, Jianming Zheng, Pengfei Yan, Eric D Walter, Nancy G. Isern, Mark E. Bowden, Mark H Engelhard, Sun Tai Kim, Jeffrey Read, Brian D. Adams, Xiaolin Li, Jaephil Cho, Chongmin Wang, and Ji-Guang Zhang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00845 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017

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Temperature Dependence of Oxygen Reduction Mechanism in Nonaqueous Li-O2 Batteries Bin Liu,† Wu Xu,*,† Jianming Zheng,† Pengfei Yan,‡ Eric D. Walter,‡ Nancy Isern,‡ Mark E. Bowden,‡ Mark H. Engelhard,‡ Sun Tai Kim,†,§ Jeffrey Read,¶ Brian D. Adams,† Xiaolin Li,† Jaephil Cho,§ Chongmin Wang,‡ and Ji-Guang Zhang*,† †

Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland,

WA 99354, USA ‡

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory,

Richland, WA 99354, USA §

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

National Institute of Science and Technology, Ulsan, 689-798, South Korea ¶

Power and Energy Division, Sensor and Electron Devices Directorate, U.S. Army

Research Laboratory, Adelphi, MD 20783, USA

ABSTRACT The temperature dependence of the oxygen reduction mechanism in nonaqueous Li-O2 batteries is investigated within the temperature range of −20°C to 40°C. The discharge capacity of the Li-O2 battery first decreases from 7,492 mAh g−1 at 40°C to 2,930 mAh g−1 at 0°C, then increases sharply with further decrease in temperature and reaches a very high capacity of 17,716 mAh g−1 at −20°C at 0.1 mA cm−2. The lifetime of superoxide intermediates and the solution pathway were found to play a dominant role in the discharge of the Li-O2 battery in the temperature range of −20°C to 0°C, but the electrochemical kinetics of oxygen reduction and the surface pathway dominate the discharge behavior of the Li-O2 batteries between 0°C and 40°C. This work will broaden the fundamental understanding of the oxygen reduction process in the Li-O2 battery, especially at different temperatures. 1 ACS Paragon Plus Environment

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The Li-O2 battery has been regarded as one of the very promising alternative energy storage systems due to its high theoretical specific energy (~3,500 Wh kg−1),1-3 but several barriers still hinder its practical application. These barriers include instability of electrolyte,4 high overpotential,5 air-electrode decomposition,6 and Li metal corrosion.7 Recently, more research has been focused on the effects of solvent donor number (DN), solvation of electrolyte, and current density on the oxygen reduction reaction (ORR) mechanisms of Li-O2 batteries due to their dominant role in the discharge capacity of LiO2 batteries.1,8-16 Several research groups have demonstrated that the reduction of O2 to form Li2O2 may proceed through the soluble O2•− and its association with Li+ (LiO2).12,13 Recently, we studied the formation of Li2O2 in a solid-state Li-O2 battery using environmental transmission electron microscopy (ETEM) in an oxygen environment.14 In-situ direct observation reveals that LiO2 was initially produced during the ORR process that occurred on a RuO2/carbon nanotube (CNT) air electrode followed by

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disproportionation of LiO2 into Li2O2 and O2, where the release of O2 resulted in a hollow nanostructure with Li2O outer-shell and Li2O2 inner-shell surfaces. Bruce and co-workers proposed that the ORR mechanism is affected by the DN of the solvent in the electrolyte and the discharge potential, thus leading to a significant change of Li-O2 battery capacities.1 Moreover, the increased lifetime of O2•−/LiO2 in the electrolyte could delay the formation of the passivation layer of poorly conducting Li2O2 on the active surface of the air electrode and thus enhance the discharge of Li-O2 batteries.17 On the other hand, Sun and co-workers reported that the cycling performance of the Li-O2 battery can be affected by temperature, where with the increase of the temperature, the overpotential of the charge and discharge processes decreases.18 However, there are very few reports about the influence of temperature on ORR mechanisms. Herein, we present an in-depth study of the temperature dependence of chemistry and electrochemistry that govern the operation of the Li-O2 battery. Various factors, including the lifetime of superoxide/LiO2 and the electrochemical kinetics of the oxygen reduction process during discharge of Li-O2 batteries in different temperature ranges have been investigated. Morphological evolution of Li2O2 products with temperature was also systematically examined. The underlying mechanisms behind the extremely high discharge capacity at low temperature are also discussed. Figure 1a reveals a strong temperature dependence of the discharge behavior of Li-O2 cells at various temperatures. The cells were composed of a Li metal anode, an air electrode (consisting of CNTs deposited on carbon paper without Teflon coating), a glass fiber-based separator, and 1,2-dimethoxyethane (DME)-based electrolyte; (more details are provided in the Supplementary Information). Figure 1a shows that the discharge


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capacity of the Li-O2 cell initially decreases slowly from 7,492 mAh g−1 at 40°C to 2,930 mAh g−1 at 0°C, but it increases sharply to an extraordinarily high capacity of 17,716 mAh g−1 at −20°C. A clear minimum capacity occurred at ~0°C. Apparently, two different temperature-dependent processes of the Li-O2 discharge occur in these two temperature ranges. Moreover, Figure 1a also shows that the polarizations in the voltage profiles gradually increase as the temperature decreases; this is because decreasing temperature leads to a rapid increase in the viscosity of the electrolyte (Table S1) and a rapid decrease in its conductivity. X-ray diffraction (XRD) patterns of the discharge products at different temperatures reveal that Li2O2 is the main discharge product in LiO2 cells discharged at all the temperatures studied (Figure 1b). The morphologies of the discharge products at different temperatures are shown in Figures 1c–1j and S1–S8. When discharged at −20°C, a large amount of ultrafine spherical nanoparticles (with a size of 2~3 nm) aggregated and stacked up on the airelectrode surface (Figures 1c and S1). Because of the accumulation of a large amount of discharge products, the thickness of the whole air electrode increases significantly after the discharge process. At −10°C (Figure 1d), the distribution of nanoparticles (size of 30~40 nm) became more uniform and their particle sizes increased compared to the architecture and the size of the discharge products at −20°C. In contrast, the air-electrode surfaces after discharge at 0°C and 10°C exhibit many flake-like discharge products with sizes of 70~80 nm (Figure 1e) and 80~90 nm (Figure 1f), respectively. Moreover, it is worth noting that these flake-like discharge products evolved into relatively large stacks of toroids and the corresponding size increased from 400 nm to 1100 nm once the discharge temperature increased to 20°C and above, as shown in Figure 1g (20°C), 1h


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(30°C), and 1i (40°C). For comparison, the scanning electron microscope (SEM) image of the pristine CNT air electrode (Figure 1j) indicates that the three-dimensional (3D) interwoven CNT network is uniformly coated on the carbon paper substrate. The morphological evolution of Li2O2 products at various temperatures and the trend of their increase in particle size are illustrated in Figure 1k, and their sizes are listed in Table S2. As the temperature increases from −20°C to 40°C, the extremely small spherical particles formed at low temperatures (−20°C and −10°C) expand into tiny single-layer flakes (0°C and 10°C), and grow further and also stack into toroids of different sizes (20°C through 40°C), as reported by Mitchell et al.19 Those disc and toroid particles may be composed of thin flakes of Li2O2, with the large facet of each plate having a surface normal to the [001] direction. In addition to the previous reports that show the effects of current density,20 electrolyte solvent selection,1,11 additives (such as LiNO3 and H2O),12,13 etc., on the morphology and size of discharge products, this work demonstrates for the first time that the operating temperature also plays a significant role in the size and morphology of Li2O2 formed during discharge of Li-O2 cells. At temperatures below ~0°C, the nucleation mechanism of Li2O2 favors the formation of ultra-small discharge particles even though the current density (0.1 mA cm−2) is still low. Thus, the low temperature environment enables extremely sluggish growth of nanocrystals,21 which inevitably results in significant decrease in the size of discharge products. But the growth pathway of Li2O2 becomes more dominant when the temperature rises to 0°C, which clearly results in the formation of larger particles of Li2O2.


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To further investigate the difference in discharge behaviors at −20°C and 20°C, we examined the morphologies of the glass fiber separators and Li metal anodes using SEM and x-ray photoelectron spectroscopy (XPS). There were many aggregated spherical nanoparticles embedded in the glass fibers facing the air-electrode surface after the cell was discharged at −20°C (Figure 2a), and relatively fewer but uniformly distributed nanoparticles on the other side of the glass fiber separator facing the Li anode (Figure 2b). In contrast, neither side of the glass fiber separator after discharge at 20°C included obvious discharge products (Figure 2c, 2d), which looks very similar to the pristine glass fiber separator (Figure S9). Additionally, there were many spherical particles as well as glass fibers on the Li anode surface after discharge at −20°C (Figure 2e and 2f), similar to those on the surfaces of the air electrode (Figure 1c) and the glass fiber separator (Figure 2a and 2b). However, there were far fewer decomposition particles deposited on the Li metal surface after discharge at 20°C, as shown in Figure 2g and 2h, when compared with the SEM image of the fresh Li metal (Figure S10). The corresponding XPS spectra shown in Figure 2i indicate that the fine products observed above that formed at −20°C on both sides of the glass fiber separator are Li2O2, but both sides of the glass fiber separator surfaces after discharge at 20°C produce only one peak, associated with the pristine glass fiber separator, indicating that there is almost no obvious discharge product on the separator. These results are consistent with the corresponding SEM images shown in Figure 2a–d. There is a clear interface between the Li2O2 deposition layer and the compact bulk Li anode after the first discharge at −20°C (Figure S11), where the fine particles are mainly Li2O2 with a small amount of Li2CO3 (which is the decomposition product of the electrolyte) (Figure 2j). The XPS spectrum of the Li anode surface after


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discharge at 20°C mainly contains Li2CO3 with a small amount of Li2O2 (Figure 2j). These interesting findings reveal that low temperature (−20°C in this case) could increase the lifetime of LiO2, which can diffuse through the air electrode and the separator and reach the Li anode surface, and then disproportionate to produce Li2O2 in these cell components. To identify the possible factors that may affect the performance of Li-O2 batteries at different temperatures, nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) characterizations were conducted on KO2/DME solutions to evaluate the change in lifetime of superoxide (O2•−) with varying temperature in the temperature range between −20°C and 40°C. The half-life (t1/2) of O2•− versus temperature as measured with NMR and EPR is shown in Figure S12a and S12b, respectively. Obviously, both NMR and EPR results indicate that the lifetime of O2•− decreases exponentially as temperature increases from −20°C to 40°C. The stability of the dissolved O2•− with storage time at −20°C and 20°C was also measured by EPR. The intensity of the O2•− signal in DME is maintained for a much longer time at −20°C (Figure S12c) than that at 20°C (Figure S12d). These results clearly demonstrate that O2•− has a much longer lifetime at low temperature than at room temperature. The temperature dependence of the lifetime of superoxide, the electrochemical kinetics, and the discharge capacities of Li-O2 batteries in the temperature range from −20°C to 40°C are summarized in Figure 3. Here, the lifetime of O2•− and the discharge capacities are from experimental results, while the data for the electrochemical kinetics of oxygen reduction are based on the calculation shown in the Supporting Information. As the temperature increases from −20°C to 40°C, the lifetime of the O2•− obtained from 7 ACS Paragon Plus Environment

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both NMR and EPR decreases sharply while the electrochemical kinetics of ORR increases greatly, and an obvious minimum point at 0°C for the discharge capacities of the Li-O2 cells can be observed in Figure 3. More interestingly, the sharp change of the lifetime of O2•− in the temperature range of 0°C ~ −20°C and the sharp change of the electrochemical kinetics of ORR occurring in the temperature range of 0°C~40°C, as shown in the area above the black dotted line in Figure 3, are coincident with the change trend of discharge capacity. Moreover, the corresponding fitting results are well matched with the experimental data shown in Figure 3 and also show remarkable exponential changes of the lifetime of O2•−, the electrochemical kinetics of ORR, and the discharge capacities with varying temperature (Figures S13–S18, Equations S1–S5). Combining the aforementioned results, it is apparent that the dominant mechanism for the ORR process in the Li-O2 battery changes at ~ 0°C. Briefly, at low temperatures (e.g., −20 and −10°C), the lifetime of O2•− is strongly dominant, so the discharge capacity is quite high even though the electrochemical kinetics of ORR is very low. At sub-ambient temperatures from 0 to 10°C, both the lifetime of O2•− and the electrochemical kinetics of ORR are low, so the discharge capacity is low. At elevated temperatures (20~40°C), the electrochemical kinetics of ORR plays a dominant role, and thus the discharge capacity increases. However, because the lifetime of O2•− at such relatively high temperatures is very short and the effect of the lifetime of O2•− is smaller than that of the electrochemical kinetics of ORR, the rate of increase of the discharge capacity is not high compared to its rate of increase in the low temperature range. Therefore, we propose the following explanations for the temperature dependence of the ORR mechanism. In general, O2 dissolved in an electrolyte is first absorbed on the 8 ACS Paragon Plus Environment

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interface between the electrolyte and air electrode (Reaction 1) and then gains one electron to produce the superoxide radical anion O2•− (Reaction 2).22 The formed O2•− could continue to combine with Li+ due to strong interaction between Li+ (hard Lewis acid) and O2•− (Lewis base) (Reaction 3) or the highly reactive O2•− can attack the solvent and salt anions in the electrolyte, causing their decomposition. O2 (solution) → O2 (surface)

(1)

O2 (surface) + e− → O2•− (surface)

(2)

O2•− (surface) + Li+ → LiO2 (surface)

(3)

Besides Reactions 2 and 3, an alternative route is the following: O2 (surface) + Li+ + e− → LiO2 (surface)

(4)

According to previous studies,1,13 there are two reaction pathways of ORR process depending on the DN of the electrolyte solvent: surface reactions (Reactions 5 and 6) and solution reactions (Reactions 7 and 8). The LiO2 in solution may also return to the air electrode (Reaction 9) and then conduct Reactions 5 and 6. DME solvent has an intermediate value of DN (20), so both surface and solution reaction routes can occur in its electrolytes. LiO2 (surface) + Li+ + e− → Li2O2 (surface)

(5)

2LiO2 (surface) → Li2O2 (surface) + O2

(6)

LiO2 (surface) → LiO2 (solution)

(7)

2LiO2 (solution) → Li2O2 (solution) + O2

(8)

LiO2 (solution) → LiO2 (surface)

(9)

In this work, the lifetimes of O2•− and LiO2 are significantly extended at −10°C ~ −20°C, so the disproportionation of LiO2 in Reactions 6 and 8 can be greatly slowed 9 ACS Paragon Plus Environment

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down. The LiO2 formed on the surface of the air electrode can dissolve into the electrolyte solution, move away from the air electrode, diffuse to and even through the separator, and reach the surface of the Li metal anode. After that, the LiO2 disproportionates and forms Li2O2 inside the air electrode, on the surface of and inside the glass fiber separator, and even on the Li metal anode surface. This is the main reason why a significant quantity of Li2O2 has been observed in the different areas as shown in Figure 2. Therefore, more O2 can be reduced at the air-electrode surface, so a higher discharge capacity is obtained. These low temperature reaction processes are clearly illustrated in Figure 4a and 4b. On the other hand, the surface of the air electrode will be eventually covered by a thin film of Li2O2 through Reactions 5 or 6, leading to a relatively higher resistance or overpotential. At 20°C and above, the increased electrochemical kinetics of ORR allows the O2 reduction to proceed quickly at the CNTelectrode/O2/electrolyte triphase region to generate LiO2, which is quickly converted to Li2O2 via Reactions 5 and 6, and more likely through disproportionation (i.e., Reaction 6), which will increase the discharge capacity and the size of the Li2O2 toroids. It is worth noting that Reactions 7 to 9 may not happen at 20°C, due to the extremely short lifetimes of O2•− and LiO2 at this temperature, as evidenced by the fact that no Li2O2 has been observed inside the air electrode or the glass fiber separator. The relatively high temperature reaction processes are schematically illustrated in Figure 4c and 4d. For cell discharge at 0~10°C, the slow electrochemical kinetics of the ORR leads to slow generation of O2•− and LiO2. In addition, the short lifetimes of the O2•− and LiO2 mean the formation of Li2O2 still has to rely on the surface reaction routes (Reactions 5 and 6). In this case, the Li2O2 film quickly covers the CNT surface, blocking the further


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reduction of O2, so the discharge capacity is very low. The size of the Li2O2 particles formed will be between those formed at low temperatures and elevated temperatures, as shown in Figure 1k. In summary, we discovered that the discharge capacity of Li-O2 cells and the morphology of Li2O2 are significantly governed by the environmental temperature. The lifetime of superoxide/LiO2 and the electrochemical kinetics of ORR are two major factors determining the temperature dependence of ORR. The lifetime of superoxide and the solution pathway play a dominant role in the battery capacity in the temperature range of −20°C to 0°C, but the electrochemical kinetics of oxygen reduction and the surface pathway dominate the discharge behavior between 0°C and 40°C. The in-depth understanding of the temperature effect on the discharge mechanisms of Li-O2 batteries will enable more rational design of high energy Li-O2 batteries and other metal-air batteries.


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Figures:

Figure 1. (a) Discharge curves of Li-O2 coin cells composed of CNT air electrodes at a current density of 0.1 mA cm−2 at various temperatures. Inset: corresponding bar graph of capacity values as a function of temperature. (b) Corresponding XRD patterns of the discharge products on the CNT-based air electrodes at these temperatures. (c–j) SEM images of discharge products on the CNT-based air electrodes at −20°C (c), −10°C (d), 0°C (e), 10°C (f), 20°C (g), 30°C (h), 40°C (i), and pristine CNT-based air electrode (j), where the scale bar is 500 nm. (k) Schematic of morphology evolution of discharge products in the temperature range of −20 to 40°C.


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Figure 2. SEM images of the glass fiber separators after first discharge at −20°C (a, b) and 20°C (c, d). SEM images of Li anode surfaces after first discharge at −20°C (e, f) and 20°C (g, h). The corresponding XPS spectra of the surfaces of glass fiber separators (i) and Li anodes (j) after first discharge at −20°C and 20°C.


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Figure 3. Temperature dependence of the experimental lifetime of superoxide from NMR and EPR, the calculated electrochemical kinetics, and the discharge capacities at different temperatures.


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Figure 4. Schematic of LiO2 formation and evolution during discharge in Li-O2 batteries at −20°C (a, b) and 20°C (c, d). Note: “sol” and “sur” denote solution and surface, respectively.

ASSOCIATED CONTENT Supporting Information. Detailed experimental methods; Viscosity data for LiClO4DME solution; SEM images of Li2O2 after discharge at −20°C to 40°C. TEM images of aggregated small Li2O2 particles after discharge at −20°C; SEM image of pristine glass fiber separator, Li anode surface after discharge at −20°C, and fresh Li metal; The table of particle size vs. temperature; NMR, and EPR characterizations of lifetime of superoxide at different temperatures; Exponential fitting formulas for superoxide lifetime based on results of NMR, EPR, electrochemical kinetics, and cell capacities at different temperature, and corresponding fitting curves. AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]; [email protected]


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ACKNOWLEDGMENTS This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy (DOE) under Contract no. DEAC02-05CH11231 for Pacific Northwest National Laboratory (PNNL) and under DEAC02-98CH10886 under the Advanced Battery Materials Research (BMR) program. The microscopic and spectroscopic characterizations were conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility located at PNNL, which is sponsored by the DOE Office of Biological and Environmental Research (BER). PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RL01830.

REFERENCES (1) Johnson, L.; Li, C.; Liu, Z.; Chen, Y.; Freunberger, S. A.; Ashok, P. C.; Praveen, B. B.; Dholakia, K.; Tarascon, J.-M.; Bruce, P. G. The role of LiO2 solubility in O2 reduction in aprotic solvents and its consequences for Li-O2 batteries. Nat. Chem. 2014, 6, 1091-1099. (2) Liu, B.; Yan, P.; Xu, W.; Zheng, J.; He, Y.; Luo, L.; Bowden, M. E.; Wang, C.-M.; Zhang, J.-G. Electrochemically formed ultrafine metal oxide nanocatalysts for highperformance lithium-oxygen batteries. Nano Lett. 2016, 16, 4932-4939. (3) McCloskey, B. D.; Valery, A.; Luntz, A. C.; Gowda, S. R.; Wallraff, G. M.; Garcia, J. M.; Mori, T.; Krupp, L. E. Combining accurate O2 and Li2O2 assays to separate discharge and charge stability limitations in nonaqueous Li–O2 batteries. J. Phys. Chem. Lett. 2013, 4, 2989-2993. (4) Radin, M. D.; Siegel, D. J. Charge transport in lithium peroxide: relevance for rechargeable metal-air batteries. Energy Environ. Sci. 2013, 6, 2370-2379. (5) Liu, B.; Xu, W.; Yan, P.; Kim, S. T.; Engelhard, M .H; Sun, X.; Mei, D.; Cho, J.; Wang, C.-M.; Zhang, J.-G. Stabilization of Li metal anode in DMSO-based electrolytes via optimization of salt-solvent coordination for Li-O2 batteries. Adv. Energy Mater. 2017, 7, 1602605. (6) Ottakam Thotiyl, M. M.; Freunberger, S. A.; Peng, Z.; Bruce, P. G. The carbon electrode in nonaqueous Li-O2 cells. J. Am. Chem. Soc. 2013, 135, 494-500. (7) Liu, B.; Xu, W.; Yan, P.; Sun, X.; Bowden, M. E.; Read, J.; Qian, J.; Mei, D.; Wang, C.-M.; Zhang, J.-G. Enhanced cycling stability of rechargeable Li-O2 batteries using high-concentration electrolytes. Adv. Funct. Mater. 2016, 26, 605-613. (8) Lu, J.; Lee, Y. J.; Luo, X.; Lau, K. C.; Asadi, M.; Wang, H.-H.; Brombosz, S.; Wen, J.; Zhai, D.; Chen, Z.; et al. A lithium-oxygen battery based on lithium superoxide. Nature 2016, 529, 377-382. (9) Gao, X.; Chen, Y.; Johnson, L.; Bruce, P. G. Promoting solution phase discharge in Li-O2 batteries containing weakly solvating electrolyte solutions. Nat. Mater. 2016, 15, 882-888. (10) Burke, C. M.; Pande, V.; Khetan, A.; Viswanathan, V.; McCloskey, B. D. Enhancing electrochemical intermediate solvation through electrolyte anion selection to increase nonaqueous Li-O2 battery capacity. Proc. Natl. Acad. Sci. USA 2015, 112, 92939298.


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(11) Aetukuri, N. B.; McCloskey, B. D.; Garcia, J. M.; Krupp, L. E.; Viswanathan, V.; Luntz, A. C. Solvating additives drive solution-mediated electrochemistry and enhance toroid growth in non-aqueous Li-O2 batteries. Nat. Chem. 2015, 7, 50-56. (12) McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Girishkumar, G.; Luntz, A. C. Solvents’ critical role in nonaqueous lithium–oxygen battery electrochemistry. J. Phys. Chem. Lett. 2011, 2, 1161-1166. (13) Trahan, M. J.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M. Studies of Li-Air cells utilizing dimethyl sulfoxide-based electrolyte. J. Electrochem. Soc. 2013, 160, A259-A267. (14) Luo, L.; Liu, B.; Song, S.; Xu, W.; Zhang, J.-G.; Wang, C. Revealing the reaction mechanisms of Li–O2 batteries using environmental transmission electron microscopy. Nat. Nanotechnol. 2017, 12, 535-539. (15) Aurbach, D.; McCloskey, B. D.; Nazar, L. F.; Bruce, P. G. Advances in understanding mechanisms underpinning lithium-air batteries. Nat. Energy 2016, 1, 16128. (16) Adams, B. D.; Radtke, C.; Black, R.; Trudeau, M. L.; Zaghib, K.; Nazar, L. F. Current density dependence of peroxide formation in the Li–O2 battery and its effect on charge. Energy Environ. Sci. 2013, 6, 1772–1778. (17) Gerbig, O.; Merkle, R.; Maier, J. Electron and ion transport in Li2O2. Adv. Mater. 2013, 25, 3129-3133. (18) Park, J.-B.; Hassoun, J.; Jung, H.-G.; Kim, H.-S.; Yoon, C. S.; Oh, I.-H.; Scrosati, B.; Sun, Y.-K. Influence of temperature on lithium-oxygen battery behavior. Nano Lett. 2013, 13, 2971-2975. (19) Mitchell, R. R.; Gallant, B. M.; Shao-Horn, Y.; Thompson, C. V. Mechanisms of morphological evolution of Li2O2 particles during electrochemical growth. J. Phys. Chem. Lett. 2013, 4, 1060-1064. (20) Lau, S.; Archer, L. A. Nucleation and growth of lithium peroxide in the Li–O2 battery. Nano Lett. 2015, 15, 5995-6002. (21) Thanh, N. T.; Maclean, N.; Mahiddine, S. Mechanisms of nucleation and growth of nanoparticles in solution. Chem. Rev. 2014, 114, 7610-7630. (22) Read, J.; Kutolo, 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-A1356.


Temperature Dependence of the Oxygen Reduction Mechanism in

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