Mechanistic Insights into Catalyst-Assisted ... - ACS Publications

Subscriber access provided by Flinders University Library

Article

Mechanistic Insights into Catalyst-Assisted Non-Aqueous Oxygen Evolution Reaction in Lithium-Oxygen Batteries Yu Wang, Zhuojian Liang, Qingli Zou, Guangtao Cong, and Yi-Chun Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00984 • Publication Date (Web): 09 Mar 2016 Downloaded from http://pubs.acs.org on March 10, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Mechanistic Insights into Catalyst-Assisted NonAqueous Oxygen Evolution Reaction in LithiumOxygen Batteries Yu Wang, Zhuojian Liang, Qingli Zou, Guangtao Cong, Yi-Chun Lu* Electrochemical Energy and Interfaces Laboratory, Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Hong Kong, SAR, China

Corresponding Author *E-mail: [email protected]. Tel: +852-3943-8339

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

ABSTRACT: Rational catalyst design for oxygen evolution reaction (OER) in non-aqueous lithium-oxygen (Li-O2) batteries has been hindered due to large discrepancies in the efficacy and working mechanism of solid catalysts in decomposing solid lithium peroxide (Li2O2). One school of thought reported pronounced catalyst influence on improving the oxidation kinetics of Li2O2. Many of them have attributed the enhancement to the presence of soluble species acting as masstransport facilitators. Another school of thought reported that applying catalysts leads no improvement in the Li2O2-oxidation kinetics due to lacking of soluble species derived from Li2O2-oxidation. These discrepancies and open questions preclude rational design of efficient catalysts for non-aqueous OER. In this work, using online electrochemical mass spectrometry, we show that solid catalysts (e.g. ruthenium) effectively promote Li2O2-oxidation kinetics to evolve O2 in a solid-state environment. This unambiguously demonstrates that non-aqueous OER is a catalytic-active process and that catalysts enhance the OER via solid-solid interaction between the catalyst and the discharge products, instead of liquid phase mediations. Our findings highlight that the key governing factor for non-aqueous OER activity lies in catalyst’s ability to interact/stabilize the solid reaction intermediates. Our work provides direct evidence of solidsolid OER catalysis and insights into its underlying enhancement mechanism.


2

Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Rechargeable lithium-oxygen (Li-O2) batteries have received extraordinary attention owing to their potential to provide gravimetric energy 3–5 times that of conventional lithium-ion batteries.1-9 However, the Li-O2 technology has been suffering from poor cycle life (< 100 cycles), low rate capability (< 1 mA cm-2) and poor round-trip efficiency (65–70%).3,6,10 These technological challenges are intimately connected to scientific barriers including the sluggish discharge and charge reaction kinetics at the O2-electrode,11-13 chemical instabilities between reaction intermediates, electrolytes14-18 and carbon electrodes,19,20 and transport kinetics of the major discharge product, lithium peroxide (Li2O2).21-24 Among these challenges, the enormous overpotential needed for the Li-O2 recharge (e.g. overpotential on carbon > 1000 mV)4,25-29 has motivated tremendous research effort for developing effective Li-O2 charging catalysts5,25-27,29-34 (or promoters28,35,36). Rational catalyst design for non-aqueous Li-O2 charging has been prevented by large discrepancies in the efficacy and working mechanism of solid catalysts in decomposing solid Li2O2. On the one hand, it has been widely-reported that the application of solid catalysts (e.g. Ru,27,31 CrOx,35,37 mesoporous pyrochlore5) or promoters28,35,36 can effectively decrease the Li-O2 charge / Li2O2-oxidation potential compared to the uncatalyzed carbon over the entire charge capacity. The large Li2O2 particles (hundreds of nanometer to micrometer) were found to shrink and disappeared completely upon charging.28,38,39 Several hypotheses have been proposed to explain the solid-solid catalysis or promotion. Black et al.28 proposed that the catalyst surfaces can promote efficient transport of Li2−xO2 species on the electrode surfaces and suggested to name it as promoter rather than as a classic electron-transfer catalyst. Radin et al.36 have


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

proposed that the promotion effect arises from enhanced electronic and/or ionic transport within the discharge product due to in situ doping of the Li2O2 discharge phase with metal (e.g. Co). Yao et al.35 proposed that enhancement of Li2O2 oxidation is mediated by chemical conversion of Li2O2 with slow oxidation kinetics to a lithium metal oxide. Lu et al.6,13 proposed that soluble reaction intermediate species (e.g. Li2-xO2) could form during the charging process (e.g. via delithiation)6,13,40 and then diffuse/migrate to the surface of the catalyst; such process can enable continuous communication between Li2O2 and the catalyst.6,13 Schwenke et al.41 and Meini et al.42 have proposed that soluble species could be generated upon electrolyte decomposition and act as redox mediators or electron shuttles to access and oxidize the Li2O2 particles.41,42 Unfortunately, no direct evidence is available to verify if soluble species are responsible for the enhancement. In contrast to the reported pronounced catalyst effects,5,25-31,35,36 McCloskey et al.43 reported that applying solid catalysts (e.g. gold, α-MnO2) does not lead to enhancement in the oxygen evolution reaction (OER) in the Li-O2 batteries, due to the lack of soluble species derived from Li2O2-oxidation.43 It was suggested that the lowering of the charge potential was due to catalysis of CO2 evolution that forms soluble mobile intermediates.43 These discrepancies and debates reflect the lack of fundamental understanding of the Li-O2 recharge process and preclude rational design of efficient catalysts for rechargeable Li-O2 batteries. In this work, we show that solid catalysts, such as Ru, effectively reduce the charge potential of Li-O2 batteries to evolve O2 via on-line electrochemical mass spectrometer (OEMS). This demonstrates that the non-aqueous OER in the Li-O2 batteries is a catalytic-active process. In addition, we show that solid catalysts promote the OER kinetics during Li2O2-oxidation in a solid-state environment. This unambiguously shows that catalysts enhance the OER rate via


4

Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


direct solid-solid interaction instead of liquid phase mediations. Lastly, we investigate how Li2O2 decompose in the solid-state environment at increasing charging rates coupled with scanning electron microscopy (SEM) to evaluate rate-limiting factors of Li2O2-decomposition in the solidstate environment. Working mechanism of the solid-solid OER catalysis will be discussed. Our work provides direct evidence of solid-solid OER catalysis and insights into its underlying enhancement mechanism.

EXPERIMETNAL SECTION Electrodes Preparation. The O2 electrodes of Vulcan Carbon (VC, Cabot Co., USA) and VC+Ru (40% Ru/VC, Premetek Co., USA) used in the liquid electrolyte based Li-O2 cells were prepared by coating a VC/Polytetrafluoroethylene (PTFE) solution binder or VC+Ru/PTFE slurry onto the 316SS 400 mesh. The Ru particles used in this study are nanoparticles ranging from 3-8 nm supported on VC (Figure S1). The carbon loading of all the O2 electrodes used in the liquid electrolyte based Li-O2 cells was 0.50 ± 0.03 mgc cm-2. The Li2O2-filled electrodes of VC and VC+Ru used in the solid-state cells were prepared by directly coating a VC/Li2O2/Li+Nafion or VC+Ru/Li2O2/Li+-Nafion slurry onto the Lithium-ion Conductive Glass-ceramics membrane (LICGC) (Φ19×0.15 mm, Ohara, Japan) in the Ar-filled glovebox (see Supporting Information (SI) for the experimental details). The carbon loading of the Li2O2-filled electrodes used in the solid-state cells was 0.20 ± 0.02 mgc cm-2, the area of the coated material amounts to 2 cm2. The theoretical capacity of the prefilled Li2O2 was 1051 mAh gc-1. Electrolyte Solutions. All the chemicals and solvents in this study were purchased from Sigma Aldrich and used as received. The water content of all solvents were measured to be 30~53 ppm by a Karl Fischer titrator (TitroLine® 7500 KF, SI Analytics, Germany). The salt lithium


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

perchlorate (LiClO4) was dried at 150 ◦C in glass oven (Büchi, Germany) under dynamic vacuum before being transferred to the glovebox. The transfers were done without exposing to the air. The electrolyte used in the glyme-based Li-O2 cell is 1 M LiClO4/TEGDME17 (0.184 mS cm-1, which is slightly lower than 1 M LiPF6/TEGDME (0.221 mS cm-1, measured by Eutech PC 700 meter, Thermo Fisher Scientific). Cell Configuration and Assembling. The assembling procedures of the liquid electrolyte based Li-O2 cells have been described in detail previously11,44 (see SI for experimental details). The electrodes used in the solid-state cells were prepared by directly coating positive materials onto the LICGC, which is used to separate the solid-state positive electrode and the negative electrode in liquid electrolyte (see SI for experimental details and Figure S3). Electrochemical Measurements. All the electrochemical measurements were performed using a VMP3 electrochemical testing unit (Bio-Logic, France). The current densities and specific capacities were calculated based on the mass of VC. OEMS Measurement. The OEMS measurement was performed on a QMS 200 Atmospheric Sampling System (Stanford Research Systems, Inc., USA) which was connected to an electrochemical cell (held at 25 ◦C in a temperature control chamber) through a 316SS capillary (Φ 1/16 inch). During discharge, the cell was connected to a pressure transducer (GB-3000HK, Gangbei Zhongtian Tech., China) and purged with O2 (N5.0, HKO, Hong Kong) for 10 min to replace the Ar. During charging, Ar was used as the carrier gas to continuously purge the gas evolved into the mass spectrometry. To quantify evolution rates of the gases, a calibration gas with O2, CO2, CO, H2 and H2O in Ar was used (5000 ppm each; Linde HKO, HK), allowing to convert the mass spectrometer signals into concentrations (note that the relevant m/z ratio for


6

Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


each compound was referenced to the m/z = 36 argon isotope signal to minimize the effect of minor variations in cell and mass spectrometer base pressure). Concentrations were converted into flux (in [nmol s-1]) by using the flow rate of carrier gas (1 sccm), the internal cell volume (≈4.15 mL for liquid electrolyte based Li-O2 cell, 4.93 mL for solid-state cell) and the ideal gas law. Thin-Film Rotating-Ring Disk Electrode (RRDE) Voltammetry Measurement. The RRDE measurement was performed on RRDE-3A (ALS CO., Ltd, Japan) unit with a three-electrode cell assembled inside the Ar-filled glovebox. To accumulate more discharge product, the GC disk electrode was covered with a thin film of VC with loading of 0.21 ± 0.02 mgc cm-2disk (see SI).45 The cell was firstly galvanostatically discharged at 13.5 µA (equal to 500 mA gc-1) and scanned between 2.0–4.5 VLi in the Ar saturated electrolyte (background current). Then it was purged with O2 for 20 min and statically discharged at 13.5 µA (equal to 500 mA gc-1) to 2.0 VLi and then held at 2.0 VLi for 3100 s (Figure S6). After the OCV became stable at ~2.8 VLi, the disk potential was scanned from 2.8 VLi to 4.5 VLi, followed by a negative-going scan to the low potential limit of 2.0 VLi and then back to 4.5 VLi with electrode rotation rate of 1600 rpm and continuously holding the ring at 4.0 VLi (Figure 2c). SEM Measurement. The SEM was performed on a Quanta 400F scanning electron microscope (FEI) with an accelerating voltage of 30 kV and low vacuum (10 to 4000 Pa) secondary electron detector. The samples were obtained by disassembling the cells in the glovebox and scraping off the electrode materials from the LICGC and bonded onto the sample holder. For transfer of the sample holder to the SEM chamber, the sample was exposed to air for less than 1 minute.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

RESULT AND DISCUSSION Is non-aqueous OER a catalytic process? To unambiguously probe if the non-aqueous OER in the Li-O2 cells is a catalytic process, we select one of the most widely investigated catalysts, Ru supported by carbon,27,31,39 as a model catalyst to evaluate the catalyst effect and real-time gas evolution of the Li-O2 charge reactions via OEMS. Figure 1 shows the galvanostatic discharge and charge profiles of pure VC electrode and VC+Ru electrode in a glyme-based electrolyte (TEGDME17) Li-O2 cell, as well as the pressure reduction during discharge and gases evolved during charge. During discharge, the VC+Ru electrode exhibits higher discharge voltage by ~50 mV compared to that of VC electrode at the first few monolayers (~50 mAh gc-1, Figure 1a insert), after which the VC+Ru electrode shows the same discharge voltage as the VC electrode. This is consistent with that reported by Lu et al.11 showing that the ORR catalyst is effective for the initial discharge reaction. The oxygen consumed during discharge on pure VC and VC+Ru is 0.935 O2/2e– and 0.925 O2/2e– (ideally 1.000 O2/2e–), respectively (Figures 1c and 1e). These values are comparable with the carbon-based electrodes in glyme-based electrolytes Li-O2 cells reported previously.15


8

Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. The efficacy of solid Ru catalyst in glyme-based Li-O2 cells. (a) Galvanostatic discharge profiles of the VC and VC+Ru electrodes. (b) Galvanostatic charge profiles of VC and VC+Ru Li-O2 cells. (c) and (e) Pressure reduction analysis of the VC and VC+Ru Li-O2 cells during discharge. (d) OEMS O2 evolution analysis of the VC and VC+Ru Li-O2 cells. (f) OEMS CO2 evolution analysis of the VC and VC+Ru Li-O2 cells. All the cells were discharged at 100 mA gc-1 to 600 mAh gc-1 followed by charging at 500 mA gc-1. The liquid electrolyte used is 1 M LiClO4/TEGDME. Only O2 and CO2 are shown here, all gases detected are shown in Figure S2, the cell configuration is shown in Figure S3a.


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

During charging, the charge voltage of VC+Ru electrode is 450 mV lower than that of pure VC electrode, over the entire charge process (Figure 1b). The enhancement associated with the Ru catalyst is observed consistently at subsequent cycles (Figure S4). Multiple cells with various capacities and current densities were conducted to further verify the enhancement associated with Ru (Figure S5). Next, we characterize the gaseous products with and without the Ru catalyst using OEMS. We show that the major gas evolved, in both cases, is the oxygen gas (Figure 1d). Quantitatively, the ratio between the total oxygen evolved and the charge transferred upon charging of pure VC electrode is 0.503 O2/2e–, which is similar to that of VC+Ru (0.510 O2/2e–). Both VC and VC+Ru electrodes exhibit lower O2 evolution rate compared to ideal case (i.e. 1.000 O2/2e–) and showed CO2 evolution (CER) toward the end of the charging process (i.e. towards the last 1/3 capacity, Figure 1f). Interestingly, the onset potential of CO2 evolution on the VC+Ru electrode (~3.92 V, Figure 1b) is 450 mV lower than that on the pure VC (~4.37 V, Figure 1b), suggesting that the Ru catalyst not only facilitates the OER kinetics, but also reduces the overpotential needed to evolve CO2. The source of CO2 could be resulted from decomposing carbonate-based side products and/or electrolyte decomposition.19,20,46 We further compare the ratio between the total oxygen evolved during charge versus the total oxygen consumed during discharge (i.e. OER/ORR) and the ratio between the total CO2 evolved during charge versus the total oxygen consumed during discharge (i.e. CER/ORR). The OER/ORR ratio for VC+Ru electrode is 0.551, which is similar compared with that for pure VC electrode (0.538), suggesting that the VC+Ru electrode exhibits similar (if not better) oxygen gas evolution efficiency compared to the pure VC electrode. The CER/ORR ratio for VC+Ru electrode is 0.151, which is ~30% higher than that for the pure VC electrode (0.116). We believe that more CO2 evolution observed for VC+Ru electrode could be attributed to Ru’s improved


10

Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kinetics in decomposing the side products that formed during discharge reactions. Together, these experimental observations demonstrate that the non-aqueous OER in the Li-O2 batteries is a catalytic process in a glyme-based electrolyte. These observations are in contrast to that reported by McCloskey et al.,43 showing that catalyst is not effective in reducing the charge voltage in glyme-based solvents. The discrepancy might be resulted from the low OER activities of the catalysts (e.g. gold, α-MnO2) used in their study.43 In fact, Harding et al. have previously showed that gold is not an effective catalyst for decomposing Li2O2.39 Are soluble mobile species required to enable effective solid-solid catalysis for solid Li2O2oxidation? Hypotheses involving the formation of soluble intermediates or soluble redox mediators have been widely proposed in literatures to rationalize the effectiveness of solid catalyst toward solid Li2O2-decomposition.6,13,41,42 First, it is proposed that soluble reaction intermediate species (e.g. Li2-xO2) are generated during the charging process (e.g. via delithiation).6,13,40 The soluble intermediates could then diffuse/migrate to the surface of the catalyst; such process can enable continuous communication between Li2O2 and the catalysts6,13 (illustrated in Figure 2a Hypothesis #1). Second, it has been proposed that soluble species could be generated by electrolyte decomposition or oxidation of side products (e.g. some soluble impurities) which serve as redox mediators or electron shuttles to access and oxidize the Li2O2 particles41,42 (illustrated in Figure 2a Hypothesis #2). Both hypotheses involve “soluble mobile species” in order to continuously expose catalyst surface to the Li2O2 particles and achieve full catalytic Li2O2-oxidation.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

Figure 2. Schematic illustrations of reported models on Li2O2 oxidation and the design of the thin-film RRDE measurements. (a) Schematic illustrations of two reported hypotheses that included the existing “soluble mobile species” upon Li2O2 oxidation.6,13,41,42 (b) Schematic illustration of soluble species detection by RRDE. (c) Disk and ring currents recorded at 20 mV s-1 in O2-saturated 0.2 M LiClO4/DME. Letter “d” stands for disk current; letter “r” stands for ring current. The number after the letter stands for the order of the scan. The arrow denotes the potential scanning direction. d1: first disk scan/r1: first ring scan; d2: second disk scan/r2: second ring scan; d3: third disk scan/r3: third ring scan. Background ring and disk currents obtained in Argon-saturated electrolyte are included as thin light grey lines.


12

Page 13 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To verify and provide insights into these two proposed hypotheses, here we (1) employed the thin-film rotating-ring disk electrode (RRDE) voltammetry to detect possible soluble species generated upon Li-O2 charging process; and (2) intentionally remove possible soluble species by examining the efficacy of the catalyst in solid-state Li2O2-filled cells, where no soluble species are available in the positive electrode. If the catalyst can still effectively reduce the charging overpotential of Li2O2-oxidation in the solid-state environment over the entire charge capacity, it would imply that the presence of soluble species is not required for enabling Li2O2-oxidation catalysis and vice versa. One should note that if there is no ring current detectable in the RRDE measurement, it only indicates that there is no soluble and oxidizable reaction intermediates formed upon charging. It does not exclude the possibility of the existence of soluble impurities that act as redox mediator to oxidize Li2O2 (hypothesis #2). The results of the RRDE measurement can only be supporting evidences but not a direct probe. Therefore, we design a solid-state Li2O2-filled electrode to unambiguously probe this hypothesis, in addition to the RRDE measurement. Thin-film RRDE voltammetry. We note that RRDE has been previously applied to study nonaqueous ORR and OER using flat glassy carbon disk electrode (GCE) by Trahan et al.47 in Li+containing electrolyte. In their study, soluble reaction intermediate species were identified by the ring electrode during the reduction scan (discharge), but not detected on the oxidation scan (charge).47 However, under their experimental conditions, the GCE was continuously rotating during ORR, where most of the soluble superoxide were sent to the ring electrode and oxidized. In this case, the amount of Li2O2 formed on the disk electrode is significantly lower than the amount of Li2O2 formed in the regular Li-O2 cells, which might lead to undetectable signal soluble intermediates upon Li2O2-oxidation. To minimize these concerns and maximize the


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

amount of reactant, we here employ thin-film RRDE by drop-casting VC particles (0.21 mg cm2

disk)

onto the GCE (0.1256 cm2) to increase the total surface area to ~ 27 cm2 (assuming

100 m2 g-1VC45) (Figure 2b). We accumulated oxygen reduction reaction product on the stagnant disk electrode by galvanostatic and potentiostatic discharge in an O2-saturated environment (~21.6 µAh, equal to 800 mAh gc-1, see Figure S6), prior to the initial positive scan. Figure 2c shows the disk and ring (Ering = 4.0 VLi) currents of the first oxidation scan after the galvanostatic and potentiostatic discharge in O2-saturated electrolyte, followed by subsequent oxygen reduction and Li2O2 oxidation scan. The corresponding disk and ring current obtained in Ar-saturated environment are included for comparison (grey lines in Figure 2c). When the disk electrode is first scanned from open-circuit-voltage (~2.8 VLi) to 4.5 VLi, the disk electrode shows an oxidation peak between 3.2 VLi to 4.0 VLi, which can be assigned to Li2O2oxidation.16,17 However, there is no increase in the ring current between 3.2 VLi to 4.0 VLi, suggesting that there is no soluble and oxidizable species generated from Li2O2-oxidation at the disk electrode. In the subsequent negative-going scan (2.8 VLi – 2.0 VLi) at the disk electrode, the ring current increases, indicating the oxidation of soluble species generated from the disk electrode during ORR, which is consistent with previous studies that soluble species are formed during discharge but not for charge.47,48 In short, by using thin-film RRDE voltammetry, we maximize the amount of possible soluble species from Li2O2-oxidation and didn’t observe any soluble/oxidizable species formed upon Li2O2-oxidation. Examine the efficacy of solid catalyst in solid-state Li2O2-filled electrodes. To test if soluble species are required to enable Li2O2-oxidation catalysis, we intentionally remove possible soluble species in the positive electrode. We examine the efficacy of the catalyst using Li2O2-


14

Page 15 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


filled electrodes in a solid-state electrolyte, where no soluble species are available in the positive electrodes (see Experimental Section and Figure S3b). Figure 3 shows the galvanostatic charge profiles and gas evolved during the charging process of a pure VC Li2O2-filled electrode and a VC+Ru Li2O2-filled electrode that were directly coated on a solid-state Lithium-ion Conductive Glass-ceramics membrane (LICGC, Ohara, Japan), without liquid electrolyte in the positive electrode (Figure S3b). First, the average charge voltage of VC+Ru electrode (~4.4 V, 450 mA gc-1, Figure 3b) is about 500 mV lower than that of pure VC electrode (~4.9 V, 450 mA gc-1, Figure 3a), which is consistent with the enhancement observed in the glyme-based Li-O2 cells shown in Figure 1b . This observation demonstrates that the Ru catalyst can still take effect in a solid-state environment despite no soluble species are available in the electrode. Second, the apparent charge capacity of VC (~1000 mAh gc-1) is higher than that of VC+Ru (~700 mAh gc-1), with lower oxygen to electron ratio (0.650 O2/2e–, Figure 3c) compared to that of the VC+Ru electrode (0.872 O2/2e–, Figure 3d). This indicates that the pure VC electrode exhibits larger parasitic charge capacity compared to the VC+Ru electrode. This can be related to the higher charging potential of the pure VC electrode leading to more parasitic reactions. The higher oxygen to electron ratio observed in the VC+Ru electrode highlights that the application of Ru not only reduces the charge potential but also increases the oxygen gas evolution efficiency from Li2O2-oxidation. To fully remove any possible solvent evaporation from the lithium negative electrode, we exploited a polymer-sealed two-compartment solid-state Li2O2-filled cell49,50 (Figure S3c) and further confirm the efficacy of the Ru catalyst (Figure S8). Together, these observations show that solid catalyst such as Ru, can reduce the charge overpotential of Li2O2-oxidation reaction, in a solid-state environment. This directly proves that


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

solid catalyst can take effect for Li2O2-decomposition via direct solid-solid interaction instead of liquid phase mediation.

Figure 3. The efficacy of solid Ru catalyst in solid-state Li2O2-filled cells. (a) Galvanostatic charge profile of the VC Li2O2-filled cell. (b) Galvanostatic charge profile of the VC+Ru Li2O2filled cell. (c) OEMS gas evolution analysis of the VC Li2O2-filled cell. (d) OEMS gas evolution analysis of the VC+Ru Li2O2-filled cell. All the cells were charged at 450 mA gc-1 with 0.1 M LiClO4/DMSO in the negative electrode and no liquid electrolyte in the positive electrode. Only O2 and CO2 are shown here, all gases detected are shown in Figure S7, the cell configuration is shown in Figure S3b. Influence of charging rate on Li2O2-decomposition in the solid-state environment. We further investigate how Li2O2-decomposition responds to the change in the current density to provide


16

Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


insights into rate-limiting factors of Li2O2-decomposition in the solid-state environment. Figure 4a shows the galvanostatic charge profiles of VC+Ru Li2O2-filled electrodes in the solid-state cell configuration (Figure S3b) at various current densities ranging from 25–500 mA gc-1. First, cells charged with low rates at 25 mA gc-1 and 200 mA gc-1 can reach ~100% of the theoretical capacity (1051 mAh gc-1). Second, the charging capacity drastically dropped to ~50% and ~40% of the theoretical capacity when the current density increased to 450 mA gc-1 and 500 mA gc-1, respectively. This suggests that the oxidation of Li2O2 is highly sensitive to charge rate in the solid-state environment. The SEM images of the electrode charged at 25 mA gc-1 (Figure 4c) shows complete disappearance of the Li2O2 particles, which is consistent with the full charge capacity. The SEM images of the electrode that charged at 500 mA gc-1 (Figure 4d) shows some residual Li2O2 particles, which is in line with the ~40% charge capacity observed in Figure 4a. We believe that most of the residual Li2O2 particles are still in contact with the VC+Ru network, which is supported by the fact the remaining capacity could be further charged at a reduced current density at 100 mA gc-1 (see Figure 4e red line). In other words, the Li2O2 particles shrank but remained physically connected with the VC+Ru network during oxidation, which suggests that the decomposition starts at the outer part of the Li2O2, rather than at the Li2O2-VC/Ru interface. This is consistent with the in situ SEM study of Zheng et al.38 showing that the Li2O2 particle starts to shrink from the outer part of the particle, rather than on the contact point with carbon or with electrolyte. In order to rationalize the complete disappearance of the solid Li2O2 particles in the solid-state environment, where no soluble mass-transport facilitators or redox mediators are available, we believe that the solid Li2O2 particles must exhibit sufficient electrical conductivity22,23,38 and ionic conductivity22,38,51 to permit the oxidation reaction taking place throughout the entire Li2O2 particle, rather than being limited to contact points with the


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

carbon/catalyst. This is supported by a number of experimental38,51 and computation studies,2224,46,52-55

suggesting that charge transport through Li2O2 could be achieved via vacancies,22,24,46

polarons,24,52,53,55 or grain boundaries.54

Figure 4. Influence of charging rate on Li2O2-decomposition in solid-state Li2O2-filled cells. (a) Galvanostatic charge profiles of VC+Ru Li2O2-filled electrodes in solid-state cells at charge rates of 25, 200, 450, and 500 mA gc-1. (b) SEM images of the pristine VC+Ru Li2O2-filled electrodes. (c) SEM images of the VC+Ru Li2O2-filled electrode after charged at 25 mA gc-1. (d) SEM images of the VC+Ru Li2O2-filled electrode after charged at 500 mA gc-1. (e)Subsequent charging profile of VC+Ru Li2O2-filled electrode after charged at 500 mA gc-1 shown in Figure


18

Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4a. The electrolyte used in the negative electrode is 0.1 M LiClO4/DME and no liquid electrolyte was used in the positive electrode. The cell configuration is shown in Figure S3b. Insights into the working mechanism of solid-solid OER catalysis and implications for rechargeable Li-O2 batteries. First, from Figure 3, we show that the Ru catalyst reduces the overpotential of the Li2O2-oxidation OER (compared to pure VC) over the entire charge capacity (between 3.3 – 6.3 VLi) in a solid-state environment, where no soluble species are available. This shows that the presence of Ru catalyst indeed promotes the decomposition of the solid Li2O2 without the need of soluble intermediates or redox mediators. Based on these observations, we propose that the role of the solid catalyst for Li2O2-oxidation is to reduce the energy of the reaction intermediate states such as Li2-xO2 by forming a stable intermediate state i.e. Li2xO2(solid)-Ru,

from which to decrease the overpotential needed for Li-extraction from Li2O2

(Figure 5). The Ru surfaces can be partially oxidized by the lithium-deficient Li2O2 (or Li2-xO2), forming Li2-xO2(solid)-Ru at the interface (Figure 5). In fact, the strong bonding affinity of Ru metal toward the oxygenated species is well-established in the literature.56 We believe that partial oxidation of the Ru catalyst can stabilize the highly unstable peroxide/superoxide ions in the Li2xO2

phase, from which to reduce the overpotential needed for Li-extraction from the Li2O2.

Furthermore, the formation of metal oxide between the catalyst and Li2O2 has also been suggested by numerous studies.35,36,57,58 We note that it is possible to form Li-Ru-oxide during discharge considering the possibility of Ru dissolution to the electrolyte in the liquid-based Li-O2 cells, which follows the model suggested by Radin et al.36 However, there is no discharge process involved in the Li2O2-filled electrode. In essence, our model suggests that catalysts with greater tendency to be oxidized or accommodate multiple oxidation states can better stabilize the solid intermediate phase. This hypothesis is supported by the activity trend established for the


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

metal catalysts (Ru ≈ Pt >> Au ≈ C)39 and the high activity of metal oxides with flexible metal oxidation states including CrOx35,37 and Co3O4.28,35,36

Figure 5. Proposed working mechanism of solid catalyst in promoting solid Li2O2-oxidation. Second, we found that full Li2O2-oxidation in the solid-state environment cannot be achieved at high current densities but the remaining Li2O2 can be subsequently charged at lower current density (Figure 4e). This suggests that the residual Li2O2 particles are still electrically connected to the VC+Ru network after charging at high rate. This is consistent with the in-situ SEM evidences38 showing that the Li2O2 particle starts to shrink from the outer part of the particle instead of the contact point with carbon/electrolyte. Considering that the lowest reaction resistance is at the Li2O2–catalyst interface where the oxidation reaction is initiated, we hypothesize that Li-extraction starts at the Li2O2–catalyst interfaces and generates Li


20

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


vacancies22,24,46 at the Li2O2-catalyst interfaces (Figure 5). Then Li ions from the outer part of the particle can enter the sites vacated by Li via vacancy migration22,24,46 or surface diffusion, which then triggers oxygen loss from the outer part of the Li2O2 particle (Figure 5). The mechanism proposed by Black et al.28 (i.e. the catalyst surfaces can promote efficient transport of Li2−xO2 species on the electrode surfaces), Yao et al.35 (chemical conversion of Li2O2 to a lithium metal oxide with enhanced de-lithiation kinetics), and Radin et al.36 (arises from enhanced electronic and/or ionic transport within the discharge product due to in situ doping of the Li2O2 discharge phase with metal) have proposed how solid catalyst can facilitate the Li2O2oxidation. However, no explanation is available on how solid Li2O2 decompose while maintaining continuous contact with the catalyst and no direct evidence was provided to rule out the existence of soluble species proposed by Lu et al.,6,13 Schwenke et al.41 and Meini et al.42 Our study, for the first time, provides direct evidence to show that solid catalysts enhance the OER via solid-solid interaction between the catalyst and the discharge products, instead of liquid phase mediations. In addition, our model explains how solid Li2O2 decompose with continuous contact with the catalyst. First, we propose that a stable intermediate state i.e. Li2-xO2(solid)-Ru is formed at the Li2O2-catalyst interface to reduce the reaction barriers of Li-extraction, which is in line with the lithium metal oxide phase proposed by Yao et al.35 and the enhanced transport properties associated with metal doping in the Li2O2 proposed by Radin et al.36 However, in order to achieve full Li2O2-oxidation while maintaining continuous contact with the catalyst in the solid-state environment, we propose that the Ru catalyst stabilizes the peroxide ion in the Li2xO2(solid)-Ru

phase and prevents them from releasing to the environment (O2-loss). In other

words, Li vacancies can be first created at the Li2O2–catalyst interfaces by Li-extraction but no oxygen-loss occurs at the interface due to the stabilization of catalysts. Subsequently, Li ions


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

from the outer part of the particle can enter the sites vacated by Li and then triggers oxygen loss from the outer part of the Li2O2 particle (Figure 5). Through this mechanism, the Li2O2 particle will first shrink from the outer part of the large particle, instead of at the contact point with catalyst/carbon,38 allowing the Li2O2 particles maintain continuous contacts with the catalyst/carbon network during the charging process. Further studies using in situ microscopy59-61 are needed to provide direct evidence for these processes.

CONCLUSIONS In summary, we demonstrate that the non-aqueous OER in the Li-O2 batteries is a catalyticactive process and that catalysts enhance OER via direct solid-solid interaction instead of liquid phase mediation. This finding suggests that catalyst’s ability to stabilize solid reaction intermediates of Li2O2-oxidation is a key governing factor for OER activity. Our study provides direct evidence of solid-solid non-aqueous OER catalysis and insights into working mechanisms of peroxide oxidation catalysis in Li-O2 batteries.

ASSOCIATED CONTENT Supporting Information. Experimental details and supplementary figures and discussion. This material is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interest.


22

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ACKNOWLEDGMENT The work described in this paper was substantially supported by a grant from the Research Grants Council (RGC) of the Hong Kong Special Administrative Region (HK SAR), China, under Theme-based Research Scheme through Project No. T23-407/13-N, and partially supported by a RGC project No. CUHK24200414.

REFERENCES (1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19-29. (2) Abraham, K. M. A Polymer Electrolyte-Based Rechargeable Lithium/Oxygen Battery. J. Electrochem. Soc. 1996, 143, 1-5. (3) Black, R.; Adams, B.; Nazar, L. F. Non-Aqueous and Hybrid Li-O2 Batteries. Adv. Energy Mater. 2012, 2, 801-815. (4) 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. (5) Oh, S. H.; Black, R.; Pomerantseva, E.; Lee, J.-H.; Nazar, L. F. Synthesis of a Metallic Mesoporous Pyrochlore as a Catalyst for Lithium-O2 Batteries. Nat. Chem. 2012, 4, 1004-1010. (6) Lu, Y.-C.; Gallant, B. M.; Kwabi, D. G.; Harding, J. R.; Mitchell, R. R.; Whittingham, M. S.; Shao-Horn, Y. Lithium-Oxygen Batteries: Bridging Mechanistic Understanding and Battery Performance. Energy Environ. Sci. 2013, 6, 750-768. (7) Li, F.; Zhang, T.; Zhou, H. Challenges of Non-Aqueous Li-O2 Batteries: Electrolytes, Catalysts, and Anodes. Energy Environ. Sci. 2013, 6, 1125-1141.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

(8) Lu, J.; Li, L.; Park, J.-B.; Sun, Y.-K.; Wu, F.; Amine, K. Aprotic and Aqueous Li-O2 Batteries. Chem. Rev. 2014, 114, 5611-5640. (9) Shao, Y.; Ding, F.; Xiao, J.; Zhang, J.; Xu, W.; Park, S.; Zhang, J.-G.; Wang, Y.; Liu, J. Making Li-air Batteries Rechargeable: Material Challenges. Adv. Funct. Mater. 2013, 23, 9871004. (10) Luntz, A. C.; McCloskey, B. D. Nonaqueous Li-air batteries: A Status Report. Chem. Rev. 2014, 114, 11721-11750. (11) Lu, Y.-C.; Gasteiger, H. A.; Shao-Horn, Y. Catalytic Activity Trends of Oxygen Reduction Reaction for Nonaqueous Li-Air Batteries. J. Am. Chem. Soc. 2011, 133, 19048-19051. (12) Lu, Y.-C.; Xu, Z.; Gasteiger, H. A.; Chen, S.; Hamad-Schifferli, K.; Shao-Horn, Y. Platinum-Gold Nanoparticles: A highly Active Bifunctional Electrocatalyst for Rechargeable Lithium-Air Batteries. J. Am. Chem. Soc. 2010, 132, 12170-12171. (13) Lu, Y.-C.; Shao-Horn, Y. Probing the Reaction Kinetics of the Charge Reactions of Nonaqueous Li-O2 Batteries. J. Phys. Chem. Lett. 2013, 4, 93-99. (14) 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. (15) McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Mori, T.; Scheffler, R.; Speidel, A.; Sherwood, M.; Luntz, A. C. Limitations in Rechargeability of Li-O2 Batteries and Possible Origins. J. Phys. Chem. Lett. 2012, 3, 3043-3047. (16) Laoire, C. O.; Mukerjee, S.; Abraham, K. M.; Plichta, E. J.; Hendrickson, M. A. Influence of Nonaqueous Solvents on the Electrochemistry of Oxygen in the Rechargeable Lithium-Air Battery. J. Phys. Chem. C 2010, 114, 9178-9186.


24

Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Laoire, C. O.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M. Rechargeable Lithium/TEGDME-LiPF6/O2 Battery. J. Electrochem. Soc. 2011, 158, A302A308. (18) Zhang, Z.; Lu, J.; Assary, R. S.; Du, P.; Wang, H.-H.; Sun, Y.-K.; Qin, Y.; Lau, K. C.; Greeley, J.; Redfern, P. C. Increased Stability Toward Oxygen Reduction Products for LithiumAir Batteries with Oligoether-Functionalized Silane Electrolytes. J. Phys. Chem. C 2011, 115, 25535-25542. (19) McCloskey, B. D.; Speidel, A.; Scheffler, R.; Miller, D. C.; Viswanathan, V.; Hummelshøj, J. S.; Nørskov, J. K.; Luntz, A. C. Twin Problems of Interfacial Carbonate Formation in Nonaqueous Li-O2 Batteries. J. Phys. Chem. Lett. 2012, 3, 997-1001. (20) 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. (21) Tian, F.; Radin, M. D.; Siegel, D. J. Enhanced Charge Transport in Amorphous Li 2O2. Chem. Mat. 2014, 26, 2952-2959. (22) Gerbig, O.; Merkle, R.; Maier, J. Electron and Ion Transport in Li2O2. Adv. Mater. 2013, 25, 3129-3133. (23) Radin, M. D.; Rodriguez, J. F.; Tian, F.; Siegel, D. J. Lithium Proxide Surfaces Are Metallic, While Lithium Oxide Surfaces Are Not. J. Am. Chem. Soc. 2012, 134, 1093-1103. (24) Radin, M. D.; Siegel, D. J. Charge Transport in Lithium Peroxide: Relevance for Rechargeable Metal-Air Batteries. Energy Environ. Sci. 2013, 6, 2370-2379. (25) Oh, S. H.; Nazar, L. F. Oxide Catalysts for Rechargeable High-Capacity Li-O2 Batteries. Adv. Energy Mater. 2012, 2, 903-910.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

(26) Kavakli, C.; Meini, S.; Harzer, G.; Tsiouvaras, N.; Piana, M.; Siebel, A.; Garsuch, A.; Gasteiger, H. A.; Herranz, J. Nanosized Carbon-Supported Manganese Oxide Phases as LithiumOxygen Battery Cathode Catalysts. ChemCatChem 2013, 5, 3358-3373. (27) Li, F.; Chen, Y.; Tang, D.-M.; Jian, Z.; Liu, C.; Golberg, D.; Yamada, A.; Zhou, H. Performance-Improved Li-O2 Battery with Ru Nanoparticles Supported on Binder-Free MultiWalled Carbon Nanotube Paper as Cathode. Energy Environ. Sci. 2014, 7, 1648-1652. (28) Black, R.; Lee, J. H.; Adams, B.; Mims, C. A.; Nazar, L. F. The Role of Catalysts and Peroxide Oxidation in Lithium-Oxygen Batteries. Angew. Chem. Int. Edit. 2013, 52, 392-396. (29) Lei, Y.; Lu, J.; Luo, X.; Wu, T.; Du, P.; Zhang, X.; Ren, Y.; Wen, J.; Miller, D. J.; Miller, J. T. Synthesis of Porous Carbon Supported Palladium Nanoparticle Catalysts by Atomic Layer Deposition: Application for Rechargeable Lithium-O2 Battery. Nano Lett. 2013, 13, 4182-4189. (30) Peng, Z.; Freunberger, S. A.; Chen, Y.; Bruce, P. G. A Reversible and Higher-Rate Li-O2 Battery. Science 2012, 337, 563-566. (31) Tong, S.; Zheng, M.; Lu, Y.; Lin, Z.; Zhang, X.; He, P.; Zhou, H. Binder-Free Carbonized Bacterial Cellulose-Supported Ruthenium Nanoparticles for Li-O2 Batteries. Chem. Commun. 2015, 51, 7302-7304. (32) Cui, Z.; Li, L.; Manthiram, A.; Goodenough, J. B. Enhanced Cycling Stability of Hybrid Li–Air Batteries Enabled by Ordered Pd3Fe Intermetallic Electrocatalyst. J. Am. Chem. Soc. 2015, 137, 7278-7281. (33) Zhu, J.; Wang, F.; Wang, B.; Wang, Y.; Liu, J.; Zhang, W.; Wen, Z. Surface Acidity as Descriptor of Catalytic Activity for Oxygen Evolution Reaction in Li-O2 Battery. J. Am. Chem. Soc. 2015, 137, 13572-13579.


26

Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) Qin, Y.; Lu, J.; Du, P.; Chen, Z.; Ren, Y.; Wu, T.; Miller, J. T.; Wen, J.; Miller, D. J.; Zhang, Z. In Situ Fabrication of Porous-Carbon-Supported α-MnO2 Nanorods at Room Temperature: Application for Rechargeable Li-O2 Batteries. Energy Environ. Sci. 2013, 6, 519531. (35) Yao, K. P. C.; Risch, M.; Sayed, S. Y.; Lee, Y.-L.; Harding, J. R.; Grimaud, A. J. L.; Pour, N.; Xu, Z. J.; Zhou, J.; Mansour, A. N. Solid-State Activation of Li2O2 Oxidation Kinetics and Implications for Li-O2 Batteries. Energy Environ. Sci. 2015, 8, 2417-2426. (36) Radin, M. D.; Monroe, C. W.; Siegel, D. J. How Dopants Can Enhance Charge Transport in Li2O2. Chem. Mat. 2015, 27, 839-847. (37) Yao, K. P.; Lu, Y. C.; Amanchukwu, C. V.; Kwabi, D. G.; Risch, M.; Zhou, J.; Grimaud, A.; Hammond, P. T.; Barde, F.; Shao-Horn, Y. The Influence of Transition Metal Oxides on the Kinetics of Li2O2 Oxidation in Li-O2 Batteries: High Activity of Chromium Oxides. Phys. Chem. Chem. Phys. 2014, 16, 2297-2304. (38) Zheng, H.; Xiao, D.; Li, X.; Liu, Y.; Wu, Y.; Wang, J.; Jiang, K.; Chen, C.; Gu, L.; Wei, X. New Insight in Understanding Oxygen Reduction and Evolution in Solid-State Lithium-Oxygen Batteries Using an in Situ Environmental Scanning Electron Microscope. Nano Lett. 2014, 14, 4245-4249. (39) Harding, J. R.; Lu, Y.-C.; Tsukada, Y.; Shao-Horn, Y. Evidence of Catalyzed Oxidation of Li2O2 for Rechargeable Li-Air Battery Applications. Phys. Chem. Chem. Phys. 2012, 14, 1054010546. (40) Kang, S.; Mo, Y.; Ong, S. P.; Ceder, G. A Facile Mechanism for Recharging Li2O2 in Li-O2 Batteries. Chem. Mat. 2013, 25, 3328-3336.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

(41) Schwenke, K. U.; Meini, S.; Wu, X.; Gasteiger, H. A.; Piana, M. Stability of Superoxide Radicals in Glyme Solvents for Non-Aqueous Li-O2 Battery Electrolytes. Phys. Chem. Chem. Phys. 2013, 15, 11830-11839. (42) Meini, S.; Solchenbach, S.; Piana, M.; Gasteiger, H. A. The Role of Electrolyte Solvent Stability and Electrolyte Impurities in the Electrooxidation of Li2O2 in Li-O2 Batteries. J. Electrochem. Soc. 2014, 161, A1306-A1314. (43) McCloskey, B. D.; Scheffler, R.; Speidel, A.; Bethune, D. S.; Shelby, R. M.; Luntz, A. C. On the Efficacy of Electrocatalysis in Nonaqueous Li-O2 Batteries. J. Am. Chem. Soc. 2011, 133, 18038-18041. (44) Meini, S.; Piana, M.; Tsiouvaras, N.; Garsuch, A.; Gasteiger, H. A. The Effect of Water on the Discharge Capacity of a Non-Catalyzed Carbon Cathode for Li-O2 Batteries. Electrochem. Solid State Lett. 2012, 15, A45-A48. (45) Lu, Y.-C.; Gasteiger, H. A.; Shao-Horn, Y. Method Development to Evaluate The Oxygen Reduction Activity of High-Surface-Area Catalysts for Li-air Batteries. Electrochem. Solid-State Lett. 2011, 14, A70-A74. (46) Ganapathy, S.; Adams, B. D.; Stenou, G.; Anastasaki, M. S.; Goubitz, K.; Miao, X.-F.; Nazar, L. F.; Wagemaker, M. Nature of Li2O2 Oxidation in a Li-O2 Battery Revealed by Operando X-ray Diffraction. J. Am. Chem. Soc. 2014, 136, 16335-16344. (47) 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.


28

Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(48) 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. (49) Metzger, M.; Marino, C.; Sicklinger, J.; Haering, D.; Gasteiger, H. A. Anodic Oxidation of Conductive Carbon and Ethylene Carbonate in High-Voltage Li-ion Batteries Quantified by OnLine Electrochemical Mass Spectrometry. J. Electrochem. Soc. 2015, 162, A1123-A1134. (50) Metzger, M.; Strehle, B.; Solchenbach, S.; Gasteiger, H. A. Origin of H2 Evolution in LIBs: H2O Reduction vs. Electrolyte Oxidation. J. Electrochem. Soc. 2016, 163, A798-A809. (51) Dunst, A.; Epp, V.; Hanzu, I.; Freunberger, S. A.; Wilkening, M. Short-Range Li Diffusion vs. Long-Range Ionic Conduction in Nanocrystalline Lithium Peroxide Li2O2–the Discharge Product in Lithium-Air Batteries. Energy Environ. Sci. 2014, 7, 2739-2752. (52) Garcia-Lastra, J. M.; Myrdal, J. S. G.; Christensen, R.; Thygesen, K. S.; Vegge, T. DFT+U Study of Polaronic Conduction in Li2O2 and Li2CO3: Implications for Li-Air Batteries. J. Phys. Chem. C 2013, 117, 5568-5577. (53) Ong, S. P.; Mo, Y.; Ceder, G. Low Hole Polaron Migration Barrier in Lithium Peroxide. Phys. Rev. B 2012, 85, 081105(R). (54) Geng, W. T.; He, B. L.; Ohno, T. Grain Boundary Induced Conductivity in Li 2O2. J. Phys. Chem. C 2013, 117, 25222-25228. (55) Radin, M. D.; Monroe, C. W.; Siegel, D. J. Impact of Space-Charge Layers on Sudden Death in Li/O2 Batteries. J. Phys. Chem. Lett. 2015, 6, 3017-3022. (56) Hammer, B.; Nørskov, J. K. Theoretical Surface Science and Catalysis–Calculations and Concepts. In Impact of Surface Science on Catalysis; Academic Press: New York, 2000; pp 71129.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

(57) Kundu, D.; Black, R.; Adams, B.; Harrison, K.; Zavadil, K.; Nazar, L. F. Nanostructured Metal Carbides for Aprotic Li–O2 Batteries: New Insights into Interfacial Reactions and Cathode Stability. J. Phys. Chem. Lett. 2015, 6, 2252-2258. (58) Kwak, W.-J.; Lau, K. C.; Shin, C.-D.; Amine, K.; Curtiss, L. A.; Sun, Y.-K. A Mo2C/Carbon Nanotube Composite Cathode for Lithium–Oxygen Batteries with High Energy Efficiency and Long Cycle Life. ACS Nano 2015, 9, 4129-4137. (59) Qian, D.; Ma, C.; More, K. L.; Meng, Y. S.; Chi, M. Advanced Analytical Electron Microscopy for Lithium-Ion Batteries. NPG Asia Mater. 2015, 7, e193. (60) Zheng, H.; Meng, Y. S.; Zhu, Y. Frontiers of In Situ Electron Microscopy. MRS Bull. 2015, 40, 12-18. (61) Ulvestad, A.; Singer, A.; Clark, J. N.; Cho, H. M.; Kim, J. W.; Harder, R.; Maser, J.; Meng, Y. S.; Shpyrko, O. G. Topological Defect Dynamics in Operando Battery Nanoparticles. Science 2015, 348, 1344-1347.


30

Page 31 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Content graphic


31

Mechanistic Insights into Catalyst-Assisted ... - ACS Publications

Recommend Documents