Lithium Oxygen Cells Research Articles

Electrochemical Characterization of Lithium-Oxygen Cells Employing a Bi-Functional Metal-Free Catalyst Composed of Dual-Doped Graphene and Mesoporous Carbon

Rechargeable lithium-oxygen (Li-O2) batteries have received great attention due to their high theoretical energy density. The successful operation of the Li-O2 battery requires the reversible formation and decomposition of insoluble Li2O2 on the cathode surface during the discharge and charge processes. However, the large overpotential during the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) should be reduced to make this battery commercially viable. Recently, carbon-based nanomaterials have been widely explored as either a catalyst support or a catalyst for the ORR in different electrolytes. In particular, graphene nanosheets with their two-dimensional, sp2-hybridized carbon structure have received great interest due to desirable properties such as high surface area, high electronic conductivity and charge carrier mobility, high mechanical strength and chemical stability. In this work, mesoporous carbon was grown on graphene nanosheets by in situ nitrogen and sulfur co-doping using a two-step synthesis method (hereafter referred to as NSGC). The growth of mesoporous carbon on graphene nanosheets prevented the restacking of graphene, resulting in a higher active surface area and enhanced electronic conductivity. Furthermore, the presence of mesoporous structures could accommodate the insoluble Li2O2 formed during discharge. As a result, the synthesized NSGC exhibited enhanced bi-functional catalytic activities toward ORR and OER, and showed a high initial discharge capacity of 11,431 mAh g-1 and good cycling stability as an air cathode in Li-O2 batteries.

Review of Local In Situ Probing Techniques for the Interfaces of Lithium‐Ion and Lithium–Oxygen Batteries

AbstractElectrodes in lithium‐ion and post‐lithium‐ion batteries are made of composite materials exposing a variety of different surfaces towards the electrolyte. This causes a distribution of current densities and consequently locally different changes of interfaces and bulk materials that might be critical for the performance and durability of secondary batteries. The optimization of local structures of battery materials is hindered by a lack of local techniques that provide in situ reactivity information from such hidden interfaces. A variety of new electrochemical scanning probe techniques are currently adapted to the investigation of battery materials under near‐realistic environmental conditions. The review provides a critical assessment of this development with a particular emphasis on the assessment of the passivating properties of solid–electrolyte interphases, the extension of the concepts to lithium–oxygen cells, and attempts to image ion intercalation reactions.

Catalysis of oxygen reaction on positive electrode of a lithium–oxygen cell in the presence of metallic nanosystems

Electrocatalytic characteristics of a series of carbon materials (carbon blacks XC-72 and Super P and also multiwall nanotubes) and binary metallic nanosystems formed on carbon black XC-72 (PtRu/C and PdRu/C) are studied in the cathodic and anodic reactions of the positive electrode of a lithium–oxygen cell with nonaqueous electrolyte in the first discharge/charging cycles. It is found that a significant decrease in the cell charging overpotential is observed at a transition from carbon supports to binary systems. Overvoltage of the cathodic process also decreases when DMSO-based electrolyte is used in the case of binary systems. The obtained results are due to acceleration of oxygen reduction (cell discharge stage) and facilitation of lithium peroxide oxidation (cell charging stage) on the PtRu/C and PdRu/C systems.

Dynamic Modeling of the Reaction Mechanisms in a Li/O2 Battery Cell

Extensive research efforts on metal-air batteries are being carried out due to their high theoretical energy density. Specifically, the lithium-oxygen (Li/O2) based batteries have drawn attention of the researchers as a promising alternative to Li-ion batteries for long range electric vehicles. However, this system suffers from high overpotentials during charge, indicating asymmetrical charge/discharge reaction mechanisms. High charge overpotentials lower the energy efficiency and can increase potential-driven side reactions (e.g., electrolyte decomposition). Furthermore, the discharge product forms as a new solid phase in the cathode by a nucleation and growth process, which is in general different from lithium-ion battery technologies. Thus, a detailed understanding of the reaction mechanism is crucial for enhancing battery performance and durability. Our approach towards understanding the charge/discharge reaction mechanism is by development and exploitation of a continuum model of a lithium-oxygen cell [1]. The one-dimensional computational domain consists of a gas reservoir filled with oxygen, a porous carbon cathode flooded with an organic electrolyte, a porous separator, and a lithium metal anode. We present a multi-step reaction mechanism of the electrochemistry in the cathode, consisting of O2 dissolution, O2 reduction, LiO2 precipitation, LiO2 disproportionation during discharge (Figure 1, left), as well as Li2O2 electrostripping during charge. In particular, the mechanism is asymmetric with respect to discharge and charge. The model is parameterized by experimental Swagelok cells [2] and galvanostatic cycling simulations are compared to experimental data. Furthermore, possible reasons for the high charge overpotential are analyzed, options to reduce the charge overpotential (e.g. redox mediator) are discussed, and a charge reaction mechanism including a surface-mediated Li2O2 dissolution is proposed (Figure 1, right). [1] D. Grübl, B. J. Bergner, J. Janek and W. G. Bessler, “Dynamic Modeling of the Reaction Mechanism in a Li/O2Cell: Influence of a Redox Mediator,” ECS Transactions 69(19), 11-21 (2015). [2] B. J. Bergner, A. Schürmann, K. Peppler, A. Garsuch, and J. Janek, “TEMPO: A Mobile Catalyst for Rechargeable Li-O2 Batteries,” J. Am. Chem. Soc. 136, 15054–15064 (2014). Figure 1

A Moisture- and Oxygen-Impermeant Separator for Aprotic Lithium-Oxygen Batteries

Lithium-oxygen batteries (Li-O2) have lately received considerable attention because of their exceptional potential gravimetric energies, i.e., 3~5 times compared to those of conventional lithium ion batteries. In spite of the unparalleled theoretical gravimetric energy, lithium-oxygen batteries are still under research because of their insufficient cycle lives. This shortcoming is associated with several entangled fading mechanisms [1,2]: partly irreversible nature of the air-cathode, electrolyte evaporation, unstable lithium anode interface, etc. While the reversibility in air-cathodes has been recently improved significantly by the deepened understanding on the electrode-electrolyte reaction and the integration of diverse catalysts, the stability of the lithium anode interface has received relatively much less attention. The destabilization of the lithium anode interface by crossover of water and oxygen from the air-cathode side can indeed cause as fatal degradation for the cycle life as the irreversibility of the air-cathodes [3]. Here we report that cheap poreless polyurethane separator can effectively prevent this crossover while allowing the selective diffusion of lithium ions by high electrolyte uptake of the polyurethane separator through the interchain space. The polyurethane separator also protects lithium anodes from redox mediators, lithium iodide, used to enhance the reversibility of the air-cathode reaction. Based on the lithium anode protection, a consistent capacity of 600 mAh/g is preserved for more than 200 cycles. This study reveals that in most operation conditions, the destabilization of the lithium anode is more critical for the cycle life than the poor reversibility of air-cathode reaction. The present study can also be readily applicable to many other rechargeable batteries that suffer from similar interfacial degradation by side products from the other electrode.Fig. 1. Comparative schematic illustration of lithium-oxygen cells with (a) conventional polyolefin porous PE separator and (b) poreless PU separator. (c) The cycling performance of the PE, PU, PE+LiI, and PU+LiI cells at a current density of 200mA/g with a fixed capacity of 600 mAh/g. The inset shows XRD spectra and color changes of the Li metals of PE and PU cells after 70 and 100 cycles, respectively.

Toward rechargeable lithium–oxygen batteries: recent advances

Toward rechargeable Li–O2 batteries: recent advances Nobuyuki Imanishi Department of Chemistry, Mie University, Tsu, Mie, Japan Abstract: This review summarizes research and development trends in lithium–oxygen (Li–O2) batteries. Development of a post–lithium-ion battery is highly expected to be a means to solve the global energy problem. The metal–oxygen cell, especially the Li–O2 cell, attracts attention, since it has much larger theoretical energy density than conventional battery systems. First, this review discusses the energy density that a battery should have from the viewpoint of power sources for electric vehicles. Then the fundamental features of various kinds of metal–oxygen batteries are explained and challenges faced by them are described. The remaining part of the review focuses on research subjects of the Li–O2 batteries. Early works which led to the present lithium–oxygen cells are briefly presented. Then, systems using various electrolytes and their major issues are introduced. First, irreversible decomposition of the electrolyte and Li2O2 deposition at oxygen cathode of an organic system are described. The effort that is directed toward the solution of dendritic growth of lithium metal anode is also introduced. Thereafter, new materials, mechanisms, and strategy that were reported recently are described with regard to each of the three electrolyte systems, respectively. These are expected to be breakthroughs to solve the serious problems, such as irreversible decomposition of electrolyte. In conclusion, the issues that should be solved with the direction of future development are mentioned.Keywords: energy density, lithium anode, dendrite, peroxide, superoxide

Investigating the Oxygen Reduction Reaction in Lithium-Oxygen Cells

The rechargeable Lithium (Li)-Oxygen (O2) battery has great potential as a high energy battery system due to its high theoretical specific energy of 3505 Wh Kg-1, which is significantly higher than current Li-ion cells.1,2 However, to succeed as a practical replacement for Li-ion cells, Li-O2 cells must overcome known problems such as (i) poor cycle life, (ii) high charge and discharge overpotentials (iii) low coulombic efficiency and (iv) low power capability.3 In recent studies, the poor cycle life and high overpotentials could be traced to the oxidative instability of the electrolyte in conjunction with electrode passivation by lithium peroxide (Li2O2).4-6 Early theoretical predictions for the Oxygen Reduction Reaction (ORR) at the cathode, envisioned a Li2O2 covered surface after the initial cycle and further ORR occurring on the deposited Li2O2, while also predicting large overpotentials in accordance with the experimental observations.7,8 However, the issue of electron transfer across poorly conducting Li2O2 had to be explained using mechanisms such as d-band bending and poloron hopping.7,9 The use of classical electrochemical techniques, such as Rotating Disk Electrode (RDE) and Rotating Ring-Disk Electrode (RRDE) to study the kinetics and catalytic activity of various surfaces has been common for aqueous ORR.10-12 However, application of such techniques to the Li-O2 ORR has been challenging due to surface changes during peroxide deposition, previously reported anomalous Tafel behavior, and extreme sensitivity to experimental conditions.13,14 To date, no clear mechanistic understanding or explanation of the ORR has been established, via theoretical simulations or electrochemical experiments. Here, we intend to establish such a mechanistic understanding of the ORR by combining information from theoretical simulations and classical electrochemistry. We report first principles, Density Functional Theory (DFT) modeling of the Li-O2 ORR on carbon, noble metals and their alloys under vacuum and in solvents, along with the correlation between intermediate species binding energies and favored reaction pathways from amongst 1e-, 2e- and 4e- mechanisms. Further, classical electrochemical techniques were utilized to experimentally study (and support) the reaction kinetics and mechanisms via RDE and RRDE experiments, which are difficult to explain through theoretical simulations alone. It is the authors’ hope that such use of theoretical simulations and classical electrochemistry in tandem, will serve as a guide towards future experimental studies; especially those directed towards the screening of potential catalytic surfaces for efficient ORR. Bruce et al., Nature Materials, 11, 2011, 19-29.Christensen et al., J. Electrochem. Soc., 159(2), R1 – R30, (2012).Y-C Lu, H.A. Gasteiger, E. Crumlin, R. McGuire, Jr., Y. Shao-Horn; J. Electrochem. Soc., 157 (9), A1016-A1025 (2010).S.A. Freunberger, Y. Chen, Z. Peng, J.M. Griffin, L.J. Hardwick, F. Barde, P. Novak, P.G. Bruce, J. Am. Chem. Soc., 133(2011), 8040–8047.S.A. Freunberger, Y. Chen, N.E. Drewett, L.J. Hardwick, F. Barde, P.G. Bruce, Angew. Chem. Int. Ed., 50(2011), 8609 –8613.Y. Chen, S.A. Freunberger, Z. Peng, F. Bardé, P.G. Bruce, J. Am. Chem. Soc., 134(2012), 7952−7957.Hummelshøj, J.S; Blomqvist,J; Datta,S.; Vegge, T.; Rossmeisl,J.; Thygesen, K.S.; Luntz,A.C.; Jacobsen,K.W.; Nørskov,J.K.; J.Chem.Phys., 132(2010), 071101.Viswanathan, V.; Nørskov, J.K; Speidel, A.; Scheffler, R.; Gowda, S.; Luntz, A.C; J. Phys. Chem. Lett., 4(2013),556−560.J. M. Garcia-Lastra, J. S. G. Myrdal, R. Christensen, K. S. Thygesen, T. Vegge; J. Phys. Chem. C, 117(2013), 5568−5577.J. Prakash, D.A. Tryk, E.B. Yeager; J. Electrochem. Soc., 146(1999), 4145-4151.W.E. Mustain, K.Kepler, J. Prakash; Electrochim.Acta,52(2007), 2102–2108.W.E. Mustain, J. Prakash; J. Pow. Soc., 170(2007), 28–37.C.O. Laoire, S. Mukerjee, K.M. Abraham, E.J. Plichta, M.A. Hendrickson, J. Phys. Chem. C 114(2010) 9178–9186.W. Torres, N. Mozhzhukhina, A.Y. Tesio, E.J. Calvo, J. Electrochem. Soc., 161 (14) A2204-A2209 (2014).

The electrochemical behaviour of TTF in Li–O2 batteries using a TEGDME-based electrolyte

The electrochemical behaviour of tetrathiafulvalene (TTF), a redox mediator proposed to improve the recharge kinetics of Li-O2 batteries, was investigated via synchrotron and lab X-ray diffraction. Using the LiFePO4 anode, TTF causes chemical lithiation of the FePO4 phase. TTF is also found to be unstable during discharge/charge cycles.

Dynamic Modeling of the Reaction Mechanism in a Li/O2 Cell: Influence of a Redox Mediator

Lithium-oxygen cells with organic electrolyte suffer high overpotentials during charge, indicating asymmetric charge/discharge reaction mechanisms. We present a multi-physics dynamic modeling and simulation study of the Li/O2 cell cycling behavior. We present three different multi-step mechanisms of the 2 Li + O2 ⇄ Li2O2 cell reaction, (A) a reversible 5-step mechanism, (B) a partially irreversible 6-step mechanism, and (C) a partially irreversible 8-step mechanism that includes reactions of a redox mediator. Model predictions are compared to experimental galvanostatic cycling data of Swagelok cells without and with 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) as redox mediator. All mechanisms are able to predict the discharge behavior in good agreement to the experimental results. The experimentally observed high charge overpotentials as well as their reduction by using a redox mediator can be qualitatively reproduced with the irreversible reaction mechanisms. However, the particular shape of the experimental charge curve with continuously increasing charge overpotential cannot be reproduced with the present mechanisms.

Haoshen Zhou

Angewandte Chemie International EditionVolume 55, Issue 6 p. 1956-1956 Author ProfileFree Access Haoshen Zhou First published: 11 November 2015 https://doi.org/10.1002/anie.201508282AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract “In a spare hour, I enjoy a good glass of red wine and Bill Evans. My biggest inspiration is my family. ...” This and more about Haoshen Zhou can be found on page 1956. 1 Table 1. Haoshen Zhou Date of birth: October 29, 1964 Position: Professor, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, and Nanjing University E-mail: hs.zhou@aist.go.jp; hszhou@nju.edu.cn Homepage: https://unit.aist.go.jp/ieco/ Education: 1985 BS, Physics Department, Nanjing University 1994 PhD with Prof. Hiroshi Komiyama, The University of Tokyo 1994–1997 Postdoctoral fellow with Prof. Hiroyuki Sasabe, RIKEN Research: Energy technology; energy transfer; energy storage materials and devices; rechargeable batteries; fuel cells; solar cells Hobbies: History, reading, walking, traveling, playing the game of Go My favorite painter is Pablo Picasso. My favorite book is The Complete Sherlock Holmes. If I had one year of paid leave I would spend more time with my family. The principal aspects of my personality are precision and diligence. When I was eighteen I wanted to be a historian. My favorite drinks are water and tea. The most important future applications of my research are electric vehicles and smart grids for energy storage. In a spare hour, I read historical articles or novels. If I could be any age, I would be a university student. I could take interesting courses not only in physics, chemistry, and engineering, but also in history, sociology, and art. My biggest inspiration is discussions with my students and colleagues. My favorite time of day is morning. I admire the use of a simple concept to solve a complex problem. My favorite way to spend a holiday is traveling with my wife and reading her poems, prose, and essays. My science “heroes” are every young student. All of you can be science “heroes” if you enjoy discovery. My 5 top papers: References 1“A reversible long-life lithium–air battery in ambient air”: T. Zhang, H. Zhou, Nat. Commun. 2013, 4, 1817. (A non-aqueous Li–air battery based on the electrochemical reaction of O2 and CO2 in air shows a stable cycling performance.) 2“The water catalysis at oxygen cathodes of lithium–oxygen cells”: F. Li, S. Wu, D. Li, T. Zhang, P. He, A. Yamada, H. Zhou, Nat. Commun. 2015, 6, 7843. (The use of water as a catalyst results in an extremely low overpotential of about 0.21 V.) 3“An Ultrastable Anode for Long-Life Room-Temperature Sodium-Ion Batteries”: H. Yu, Y. Ren, D. Xiao, S. Guo, Y. Zhu, Y. Qian, L. Gu, H. Zhou, Angew. Chem. Int. Ed. 2014, 53, 8963; Angew. Chem. 2014, 126, 9109. (The layered sodium oxide Na2/3Co1/3Ti2/3O2 is a stable anode active material.) 4“A Layered P2 and O3-type Composite as a High-Energy Cathode for Rechargeable Sodium-Ion Batteries”: S. Guo, P. Liu, H. Yu, Y. Zhu, M. Chen, M. Ishida, H. Zhou, Angew. Chem. Int. Ed. 2015, 54, 5894; Angew. Chem. 2015, 127, 5992. (The structure of the material was confirmed by X-ray diffraction and (S)TEM images.) 5“Integrating a Photocatalyst into a Hybrid Lithium–Sulfur Battery for Direct Storage of Solar Energy”: N. Li, Y. Wang, D. Tang, H. Zhou, Angew. Chem. Int. Ed. 2015, 54, 9271; Angew. Chem. 2015, 127, 9403. (Solar energy is captured and stored by oxidizing S2− ions to polysulfide ions.) Volume55, Issue6February 5, 2016Pages 1956-1956 ReferencesRelatedInformation

Identification of electrochemical reaction products in lithium–oxygen cells with 7Li nutation spectroscopy

In the Li–O2 battery system, it is has been shown to be challenging to differentiate the discharge products or determine the electrolyte stability with direct 7Li NMR. Defined 7Li quadrupole lineshapes are not observed for cycled cathodes. Here, 7Li nutation NMR is demonstrated to be an effective method for the identification of Li2O2 in cycled cathodes. The 7Li quadrupole interaction of Li2O2 (35 kHz) and Li2CO3 (120 kHz) are of similar magnitude to typically radiofrequency fields (ranging from 40 to 60 kHz). The 7Li nutation frequency will therefore be influenced by both interactions. The discharge products of the cycled cathodes were determined by comparing the 7Li nutation frequencies of the cycled cathodes to the 7Li nutation frequency of the pristine materials when the applied radiofrequency field was 30 kHz. Li2CO3 was determined to be the main discharge product in the propylene carbonate/dimethyl carbonate and trimethyl phosphate electrolyte systems, since the 7Li nutation frequencies of the cathodes corresponded to the 7Li nutation frequency of pristine Li2CO3. The 7Li nutation frequency of the tetraethylene glycol dimethyl ether cathode was between the 7Li nutation frequencies of both pristine Li2O2 and pristine Li2CO3, indicating that both Li2O2 and Li2CO3 were discharge products influencing the observed nutation frequency. From 7Li nutation NMR the novel trimethyl phosphate electrolyte was determined to be an unsuitable Li–O2 electrolyte, as the fast 7Li nutation frequency indicated that Li2O2 was not a primary discharge species. With 17O NMR, Li2CO3 was confirmed to be a main discharge product formed with the trimethyl phosphate electrolyte.

All-solid-state lithium-oxygen battery with high safety in wide ambient temperature range.

There is need to develop high energy storage devices with high safety to satisfy the growing industrial demands. Here, we show the potential to realize such batteries by assembling a lithium-oxygen cell using an inorganic solid electrolyte without any flammable liquid or polymer materials. The lithium-oxygen battery using Li1.575Al0.5Ge1.5(PO4)3 solid electrolyte was examined in the pure oxygen atmosphere from room temperature to 120 °C. The cell works at room temperature and first full discharge capacity of 1420 mAh g−1 at 10 mA g−1 (based on the mass of carbon material in the air electrode) was obtained. The charge curve started from 3.0 V, and that the majority of it lay below 4.2 V. The cell also safely works at high temperature over 80 °C with the improved battery performance. Furthermore, fundamental data of the electrochemical performance, such as cyclic voltammogram, cycle performance and rate performance was obtained and this work demonstrated the potential of the all-solid-state lithium-oxygen battery for wide temperature application as a first step.

The water catalysis at oxygen cathodes of lithium-oxygen cells.

Lithium–oxygen cells have attracted extensive interests due to their high theoretical energy densities. The main challenges are the low round-trip efficiency and cycling instability over long time. However, even in the state-of-the-art lithium–oxygen cells the charge potentials are as high as 3.5 V that are higher by 0.70 V than the discharge potentials. Here we report a reaction mechanism at an oxygen cathode, ruthenium and manganese dioxide nanoparticles supported on carbon black Super P by applying a trace amount of water in electrolytes to catalyse the cathode reactions of lithium–oxygen cells during discharge and charge. This can significantly reduce the charge overpotential to 0.21 V, and results in a small discharge/charge potential gap of 0.32 V and superior cycling stability of 200 cycles. The overall reaction scheme will alleviate side reactions involving carbon and electrolytes, and shed light on the construction of practical, rechargeable lithium–oxygen cells.

Ionic Liquid Electrolyte for Lithium Oxygen and Lithium Ion Oxygen Cell

The replacement of the combustion-engine by sustainable electric or hybrid vehicles, may effectively limit environmental issues such as the global warming greenhouse-gas emission and pollution [1-2]. Lithium-ion battery represents the most promising candidate due to its high energy density, conventionally of about 180 Wh kg-1 that may assure a driving range of 150 km by single charge, i.e. acceptable value, however far from that granted by the conventional combustion engine vehicles. The lithium oxygen system, can theoretically allow to obtain an energy density of 1000 Wh kg-1, i.e. a value that may increase the driving range by single charge [3]. The applicability of the lithium air batteries is limited by several drawbacks, such as the poor electrolyte stability, the short cycle life and the low energy efficiency due to high charge-discharge polarization [4]. A deep knowledge of the lithium-oxygen reaction mechanism [5] and the identification of a stable electrolyte [6] play important role fundamental in allowing the practical application of the system. Furthermore, the safety concerns associated to the reactivity of the lithium metal anode are still hindering the application of the lithium-oxygen battery [7]. We report a Li/O2 cell exploiting an ionic liquid(IL)-based, N-Methyl-N-Butyl-Pyrrolidinium Bis-(trifluoromethanesulfonyl)-Imide Lithium bis-(trifluormethanesulfonyl)-imide, Pyr14TFSI-LiTFSI electrolyte. The use of non-flammable IL-based electrolyte stable up to a 300-400 °C resulted in an improved safety level of the battery [8]. In addition, the ionic-liquid electrolyte allows a fast kinetics of the lithium oxygen electrochemical process, thus lead to an high energy efficiency. The results here obtained, and reported in summary in the figure evidence low cell polarization, most likely due to the reduced particle size of the discharged products [9]. The replacement of the reactive lithium metal by an alternative, safe anode material may be required to further enhance the lithium oxygen cell characteristics [10]. Herein, we employed an alternative, nanostructured tin-carbon composite instead of lithium metal, in lithium ion-oxygen cell characterized in terms of electrochemical properties, focusing particular attention to the study of the oxygen cross-over process. [1] B. Scrosati, J. Hassoun, Y.-K. Sun, Ener. Environ. Sci., 4, 3287 (2011). [2] P. Myounggu, S. Heeyoung, L. Hyungbok, L. Junesoo, C. Jaephil, Adv. Energy Mater., 2, 780 (2012). [3] P. G. Bruce, S. A. Freunberger, L. J. Hardwick, J.-M. Tarascon, Nat. Mater. 11, 19, (2012). [4] Y.-C. Lu, B. M. Gallant, D. G. Kwabi, J. R. Harding, R. R. Mitchell, M. S. Whittingham, Y. Shao-Horn, Energy Environ. Sci., 6, 750, (2013). [5] M. Leskes, N.E. Drewett, L. J. Hardwick, P. G. Bruce, G. R. Goward, C. P. Grey, Angew. Chem. Int. Ed. 51, 8560, (2012). [6] H.-G. Jung, J. Hassoun, J.-B. Park, Y.-K. Sun, B. Scrosati, Nat. Chem. 4, 579, (2012). [7] J.-L. Shui, J.S. Okasinski, P. Kenesei, H.A. Dobbs, D. Zhao, J.D. Almer, D.-J. Liu, Nat. Commun., 4, 2255, (2013). [8] S. Randstrom, G. B. Appetecchi, C. Lagergren, A. Moreno, S. Passerini, Electrochim. Acta 53, 1837, (2007). [9] G.A. Elia, J. Hassoun, W.-J. Kwak, Y.-K. Sun, B. Scrosati, F. Mueller, D. Bresser, S. Passerini, P. Oberhumer, N. Tsiouvaras, J. Reiter, (2014) Nano Lett., 14, 6572, (2014). [10] J. Hassoun, H.-G. Jung, D.-J. Lee, J.-B. Park, K. Amine, Y.-K. Sun, B. Scrosati Nano Lett., 12, 5775, (2012) Figure 1

Dynamic Modeling of the Reaction Mechanism in a Li/O2 Cell: Influence of a Redox-Mediator

Interest and research in metal-air batteries is ongoing on a high level due to their high theoretical energy density and potential low-cost materials. Most research activities are performed in the field of lithium-oxygen chemistry with organic electrolytes. This system suffers high overpotentials during charge, indicating asymmetrical charge/discharge reaction mechanisms. High charge overpotentials low the energy efficiency and can increase potential-driven side reactions (e.g., electrolyte decomposition). In order to reduce charge overpotentials, the use of redox mediators has been proposed. Different redox mediators have been demonstrated, such as, TTF [1], LiI [2] and FePc [3]. Bergner et al. introduced 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) as redox mediator [4], which will be further investigated in the present study. In order to further understand the mechanism of redox mediators, we present a continuum modeling study of a lithium-oxygen cell. The one-dimensional computational domain consists of a gas reservoir filled with oxygen, a porous carbon cathode flooded with an organic electrolyte, a porous separator, and a lithium metal anode. The model includes a detailed description of the electrochemistry in the cathode. During discharge, gaseous oxygen dissolves at the interface between the closed gas reservoir and the cathode (O2,gas ⇄ O2,dissolved). Following, a multi-step mechanism is implemented involving lithium superoxide as intermediate (Li+ + O2 + e– ⇄ LiO2), followed by the formation Li2O2 by chemical disproportion of LiO2 (2LiO2 ⇄ Li2O2 + O2). In the anode the metallic lithium reacts to lithium ions (Li ⇄ Li+ + e–). This mechanism is complemented by charge-transfer of the TEMPO redox mediator (TEMPO ⇄ TEMPO+ + e–), allowing for direct reduction of the lithium peroxide upon charge (Li2O2 + 2 TEMPO+ ⇄ 2 Li+ + O2+ TEMPO). The model is parameterized using experimental data [1]. Galvanostatic discharge and charge simulations reveal the advantage of using a redox-mediator. The electrochemical reaction mechanism and the crystal growth of the sodium superoxide within the cathode is quantified. Figure 1 : Discharge (left) and charge mechanism involving a redox-mediator (right) in a Li-O2 cathode. [1] Y. Chen, S. A. Freunberger, Z. Peng, O. Fontaine, P. Bruce, Nature Chemistry 5, 489 (2013). [2] H.-D. Lim, H. Song, J. Kim, H. Gwon, Y. Bae, K.-Y. Park, J. Hong, H. Kim, T. Kim, Y. H. Kim, X. Lepró, R. Ovalle-Robles, R. H. Baughman, and K. Kang, Angew. Chem. 126, 4007–4012 (2014). [3] D. Sun, Y. Shen, W. Zhang, L. Yu, Z. Yi, W. Yin, D. Wang, Y. Huang, J. Wang, D. Wang, and J. B. Goodenough, J. Am. Chem. Soc. 136, 8941–8946 (2014). [4] B. J. Bergner, A. Schürmann, K. Peppler, A. Garsuch, and J. Janek, “TEMPO: A Mobile Catalyst for Rechargeable Li-O2 Batteries,” J. Am. Chem. Soc. 136, 15054–15064 (2014). Figure 1

Copper oxide supported on platinum nanosheets array: High performance carbon-free cathode for lithium–oxygen cells

In this study, we present a new strategy on controlling the interaction between the Li2O2-catalyst interfaces through improving the affinity of catalyst surface towards Li2O2 molecules. A seed-mediated growth approach has been developed to synthesize Pt nanosheets on the stainless steel mesh using Fe as the seed. We further grow a uniform layer of metallic Cu nanoparticles on Pt nanosheets surface through electrochemical deposition. The Cu is converted to CuO by exposing it to air under ambient condition. Such strategy has effectively solved the problem of non-uniform deposition of CuO on Pt surface that arises from the poor interaction of oxides on metals. By converting the oxide-on-metal to metal-on-metal system, a relatively uniform of CuO can be successfully deposited on Pt nanosheets. The CuO on Pt provides multiple nucleation sites on the surface of the cathode, which facilitates the formation of Li2O2 thin layer in the discharge cycle. This process plays a crucial role in achieving a high round-trip efficiency of 88%, reversible specific capacity of 1648 mAh g−1 (683 mAh g−1 with respect to the total electrode mass including Li2O2) at 100 mA g−1 and maintains capacity retention of 98% during the 60th cycle at a high current density of 1 A g−1.

CeO2 Nanoparticle-Guided Formation and Decomposition of Amorphous Li2O2 on Carbon Nanotube Cathode in a Li-O2 Cell

The structure and morphology of lithium peroxide (Li2O2) in a lithium-oxygen (Li-O2) cell have been widely explored by their correlation to recharge potential. A toroidal Li2O2 crystal with 0.1-1 μm size is a typical and predominant product observed on various carbonaceous cathodes after the 1st discharge (2Li + O2 ↔ Li2O2)1 while the bulk size and poor conductivity of Li2O2 incurs its sluggish decomposition during the recharge, which results in a rapid potential rise more than 4.2 V vs. Li/Li+ concomitant with severe side reactions. This potential shooting can be suppressed by some conductive additives such as metal and metal oxide nanoparticles (NPs), where non-crystalline and/or thin Li2O2 is interestingly observed after discharge. For example, our previous work demonstrated less 4.0 V of potential for the whole recharge process in the presence of RuO2 NPs that led to the formation of amorphous Li2O2 film on the carbon nanotube (CNT) cathode during discharge via its high O2 adsorption affinity.2 Therefore, it is considered that the physical property of Li2O2, arising from both the poor crystallinity having higher ionic and electronic conductivity3 and the thin-film shape providing large contact area with the CNT cathode,4 is beneficial to smooth decomposition of Li2O2 at low potential. However, the criteria for promising additives contributing to the structure and morphology engineering of Li2O2 and mechanistic study for the formation and decomposition of such a unique structure of Li2O2 have not as yet been conducted. Here we present in-depth study of nucleation, growth and decomposition process of non-crystalline Li2O2 in the presence of cerium oxide (CeO2) NPs using TEM and correlate this to Li-O2 cell performance. The small diameter of CeO2 NP (~5 nm) has strong affinity for the oxygen species (O2, O2 - and O2 2-),5-6 which is accordingly a good candidate to investigate the evolution of Li2O2 morphology during discharge and recharge. At initial discharge, the nucleation of Li2O2 appears on the CeO2 NPs, but not on the CNT, from which the Li2O2 laterally grows and coats over the CNT surface. The CeO2 NPs determine the height of Li2O2 film, thus thicker Li2O2 film can be observed in the area of agglomerative CeO2 NPs. Further discharge allows a vertical growth (i.e. thicker film) once the thin Li2O2 film covers the entire CNT surface. There is no evidence of Li2O2 crystal structure, which is very distinguished from the toroidal and polycrystalline Li2O2 observed on a CeO2-free CNT (Fig. 1). The decomposition of Li2O2 seems to emerge from the amorphous and thin Li2O2 surface, which results in low recharge potential at the initial stage. However, the CeO2 NPs having strong O2 and CO2 affinity7 and poor conductivity become a hurdle for the completion of Li2O2 decomposition at low potential unlike the RuO2 NPs. The implication of CeO2 role and characteristics of the unique structure of Li2O2 based on different discharge and recharge status will be discussed in details in the presentation.

A perfluorinated moiety-grafted carbon nanotube electrode for the non-aqueous lithium-oxygen battery.

A perfluorinated alkyl chain grafted to carbon nanotubes is employed in the lithium-oxygen (Li-O2) cell. We demonstrate a highly localized Li-O2 electrochemical reaction in close proximity to the perfluorinated moiety owing to its high O2 affinity. This hydrophobic modification can provide an enhancement in capacity for a very thin microbattery system.

A novel aqueous lithium-oxygen cell based on the oxygen-peroxide redox couple.

The electrochemical process of an aqueous Li-O2 cell is investigated. Li2O2 is detected as a discharge product of an aqueous Li-O2 cell using a catalyst-free carbon-based electrode. The electrolyte solution saturated with lithium hydroxide prevents hydrolysis of the Li2O2. Since the electron transfer process is based on the oxygen-peroxide redox couple, the galvanostatic charging-discharging profile shows stable cycling with an extremely low charging overpotential of <0.1 V at 1.0 mA cm(-2).

Effect of the size-selective silver clusters on lithium peroxide morphology in lithium–oxygen batteries

Lithium-oxygen batteries have the potential needed for long-range electric vehicles, but the charge and discharge chemistries are complex and not well understood. The active sites on cathode surfaces and their role in electrochemical reactions in aprotic lithium-oxygen cells are difficult to ascertain because the exact nature of the sites is unknown. Here we report the deposition of subnanometre silver clusters of exact size and number of atoms on passivated carbon to study the discharge process in lithium-oxygen cells. The results reveal dramatically different morphologies of the electrochemically grown lithium peroxide dependent on the size of the clusters. This dependence is found to be due to the influence of the cluster size on the formation mechanism, which also affects the charge process. The results of this study suggest that precise control of subnanometre surface structure on cathodes can be used as a means to improve the performance of lithium-oxygen cells.