Aprotic Li-O2 Batteries Research Articles

A Stable Carbon-Free Cathode for Rechargeable Lithium-Oxygen Battery with Long Cycle Performance

Lithium-oxygen (Li-O2) batteries have extremely high theoretical specific capacities and energy densities than Li-ion batteries. It is estimated that a practical Li-O2 battery could provide 3-5 times gravimetric energy density of conventional Li-ion batteries. The cathode active material (oxygen) is not stored in the Li-O2 battery and can be obtained from the environment outside the batteries. However, the cycle performances of these Li-O2 batteries are still limited by the stability of the air electrode and the electrolyte. Carbon-based electrodes are believed to decompose during charge and promote electrolyte decomposition as well. The fabrication of a stable and highly active carbon-free cathode for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) becomes a great challenge at present for the development of rechargeable Li-O2battery. Recently, some carbon-free cathode materials have been reported, including noble metals,1 oxides,2 and carbides.3 Metal oxides have low conductivities and are mainly utilized at low current densities. Noble metals show outstanding performance but may not be applicable due to the high cost and high density. Carbides can achieve good bi-functional activities for ORR and OER and be with low cost. However, only TiC was reported to achieve high cycle performance as a carbon-free cathode for Li-O2 batteries so far. Here we report the application of a novel material for the carbon-free cathode of Li-O2 batteries. The battery can be stably operated for more than 200 cycles with a stable voltage profile under the capacity-limited condition. With the aid of structure and composition characterizations, the mechanism of discharge and charge process is also investigated. This novel material is very promising for the use as an alternative carbon-free cathode, as well as for studying and exploring the stable electrolyte in rechargeable Li-O2batteries. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, the Advanced Battery Materials Research Programs of the U.S. Department of Energy (DOE). The microscopy and spectroscopy measurements were performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. References Z. Peng, S. A. Freunberger, Y. Chen, P. G. Bruce, A Reversible and Higher-Rate Li-O2 Battery, Science., 337, 563 (2012). M. M. O. Thotiyl, S. A. Freunberger, Z. Peng, Y. Chen, Z. Liu, P. G. Bruce, A stable cathode for the aprotic Li-O2 battery, Nature Materials, 12, 1050 (2013). F. Li, D.M. Tang, Y. Chen, D. Golberg, H. Kitaura, T. Zhang, A. Yamada, H. Zhou, Ru/ITO: A Carbon-Free Cathode for Nonaqueous Li-O2 Battery, Nano Lett., 13, 4702 (2013).

The Rechargeable Aprotic Li-O2 battery

Li-ion and related battery technologies will be important for years to come. However, society needs energy storage that exceeds the capacity of Li-ion batteries. We must explore alternatives to Li-ion if we are to have any hope of meeting the long-term needs for energy storage. One such alternative is the Li-air (O2) battery; its theoretical specific energy exceeds that of Li-ion, but many hurdles face its realization.[1-5] One spin-off of the recent interest in rechargeable Li-O2 batteries, based on aprotic electrolytes is that it has highlighted the importance of understanding the fundamental electrochemistry at the positive electrode within the battery.[6-15] The challenges of obtaining efficient, reversible charge and discharge are well-documented in the field. Here, we describe how our recent studies into the electrochemical mechanism of O2 reduction to form Li2O2 at the positive electrode might allow us to design new strategies to overcome these limitations;[16]For example, exploiting the effect of solvent donor number, Fig. 1. We will describe our resent results using redox mediators to facilitate the electrochemistry along with the implications of the results for the future of rechargeable Li-O2 batteries. REFERENCES [1]. Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Nature Materials 2012, 11, 19. [2]. Lu, Y. C.; Gallant, B. M.; Kwabi, D. G.; Harding, J. R.; Mitchell, R. R.; Whittingham, M. S.; Shao-Horn, Y. Energy & Environmental Science 2013, 6, 750. [3]. Black, R.; Adams, B.; Nazar, L. F. Advanced Energy Materials 2012, 2, 801. [4]. Girishkumar, G.; McCloskey, B.; Luntz, A. C.; Swanson, S.; Wilcke, W. The Journal of Physical Chemistry Letters 2010, 1, 2193. [5]. Li, F.; Zhang, T.; Zhou, H. Energy & Environmental Science 2013, 6, 1125. [6]. Adams, B. D.; Radtke, C.; Black, R.; Trudeau, M. L.; Zaghib, K.; Nazar, L. F. Energy & Environmental Science 2013, 6, 1772. [7]. Horstmann, B.; Gallant, B.; Mitchell, R.; Bessler, W. G.; Shao-Horn, Y.; Bazant, M. Z. The Journal of Physical Chemistry Letters 2013, 4, 4217. [8]. Hummelshoj, J. S.; Luntz, A. C.; Norskov, J. K. The Journal of Chemical Physics 2013, 138, 034703. [9]. McCloskey, B. D.; Scheffler, R.; Speidel, A.; Girishkumar, G.; Luntz, A. C. The Journal of Physical Chemistry C 2012, 116, 23897. [10]. Mitchell, R. R.; Gallant, B. M.; Shao-Horn, Y.; Thompson, C. V. The Journal of Physical Chemistry Letters 2013, 4, 1060. [11]. Trahan, M. J.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M. Journal of The Electrochemical Society 2013, 160, A259. [12]. Sharon, D.; Etacheri, V.; Garsuch, A.; Afri, M.; Frimer, A. A.; Aurbach, D. The Journal of Physical Chemistry Letters 2012, 4, 127. [13]. Jung, H. G.; Kim, H. S.; Park, J. B.; Oh, I. H.; Hassoun, J.; Yoon, C. S.; Scrosati, B.; Sun, Y. K. Nano Letters 2012, 12, 4333. [14]. Peng, Z.; Freunberger, S. A.; Hardwick, L. J.; Chen, Y.; Giordani, V.; Barde, F.; Novak, P.; Graham, D.; Tarascon, J. M.; Bruce, P. G. Angewandte Chemie International Edition 2011, 50, 6351. [15]. Zhai, D.; Wang, H. H.; Yang, J.; Lau, K. C.; Li, K.; Amine, K.; Curtiss, L. A. Journal of the American Chemical Society 2013, 135, 15364. [16]. Johnson, L.; Li, C. ; Liu, Z.; Chen, Y.; Freunberger, S. A.; Ashok, P.; Praveen, B.; Dholakia, K.; Tarascon, J-M.; Bruce, P. G. Nature Chemistry 2014, 6, 1091. Figure 1

Kinetics Improvement through Cathode Designing for Aprotic Li-O2 batteries

The rechargeable aprotic lithium-air (Li-O2) battery is a promising potential technology to provide a safe and cost-effective secondary battery with several times higher energy density than state-of-the-art Li-ion batteries, and thus attracting extensive interest over the past decade. Despite their great promise, putting the Li-O2 batteries into practise are hindered by a range of fundamental challenges [1-2]. The greatest problem is lacking of stable materials that could endure the attack from the intermediates and/or the products of oxygen reduction/evaluation reaction (ORR/OER) during the operation of Li-O2 batteries. For example, masses of works have proven that many kind of materials undergo serious side reactions during the operation of Li-O2 batteries, which induce the formation and accumulation of carbonates on cathode and eventually kill the batteries. Moreover, the intrinsic poor kinetics of Li/O2 chemistry, slow kinetics of ORR (2Li+ + 2e- + O2 → Li2O2) and significantly sluggish kinetics of OER (Li2O2 → 2Li+ + 2e- + O2), are also of significant influence on cell performances. To address these issues, cathode designing toward a stable interface with improved activity on ORR/OER is of great promise. Recently, by virtue of the material and structure design for air cathodes, optimization of the operation protocols, and adding soluble catalysts/redox mediators into the electrolytes, many reports have shown that Li-O2 batteries could be run for hundreds or even thousands of cycles with considerably large specific capacities [2]. In spite of the rapid progress made on improving their cyclic performance and reducing their voltage polarization, lots of work related to thermodynamics and kinetics of Li/O2 chemistry as well as cathode designing are still needed prior to the realization of Li-O2 batteries. The galvanostatic intermittent titration technique (GITT), which combines the transient and steady-state measurements, is a widely used tool to determine kinetic properties and thermodynamic data for a specific electrode [3]. In this report, the thermodynamic equilibrium voltages for Li-O2 reaction and overpotential variation was studied using GITT measurements, from which a zero voltage gap for the open circuit voltage (OCV) between charging and discharging, an asymmetrical polarization behavior at different current densities and temperatures, a continuous increase of overpotential during charging, as well as a negative temperature coefficient of the cell's thermodynamic equilibrium voltage were observed [4]. This work also gives a clue that the kinetics of Li/O2 reaction plays a major role on improving the performances of Li-O2 battery. Based on such clue, several kind of cathodes ranged from carbon based to carbon-free materials, such as MnO nanoparticles embedded N-doped carbon composites (MnO-m-N-C), Ru nanoparticles anchored carbon nanotubes with ionic liquid coating layer (Ru-IL-CNTs) and RuOx nanodots decorated mesoporous boron-doped carbon nitride (RuOx@m-BCN), have been developed for Li-O2batteries, all of which show positive effects on lowering charge overpotential and improving cycle performance and energy efficiency [5-7].

Identifying Reactive Sites and Transport Limitations of Oxygen Reactions in Aprotic Lithium-O2 Batteries at the Stage of Sudden Death.

Discharging of the aprotic Li-O2 battery relies on the O2 reduction reaction (ORR) forming solid Li2 O2 in the positive electrode, which is often characterized by a sharp voltage drop (that is, sudden death) at the end of discharge, delivering a capacity far below its theoretical promise. Toward unlocking the energy capabilities of Li-O2 batteries, it is crucial to have a fundamental understanding of the origin of sudden death in terms of reactive sites and transport limitations. Herein, a mechanistic study is presented on a model system of Au|Li2 O2 |Li(+) electrolyte, in which the Au electrode was passivated with a thin Li2 O2 film by discharging to the state of sudden death. Direct conductivity measurement of the Li2 O2 film and in situ spectroscopic study of ORR using (18) O2 for passivation and (16) O2 for further discharging provide compelling evidence that ORR (and O2 evolution reaction as well) occurs at the buried interface of Au|Li2 O2 and is limited by electron instead of Li(+) and O2 transport.

ChemInform Abstract: Li2O2 Oxidation: the Charging Reaction in the Aprotic Li‐O2 Batteries

AbstractReview: 41 refs.

Critical Challenges in Rechargeable Aprotic Li–O2 Batteries

Rechargeable aprotic Li–O2 batteries are one of the most promising next‐generation battery technologies that can deliver extremely high energy density. In the past decades, this technology has attracted worldwide attention, and considerable progress has been achieved. However, numerous critical scientific challenges remain to be solved for practical applications. A specific discussion of recent progress from the perspective of the stable aprotic Li–O2 system with high energy efficiency is presented. The discussion is highlighted on the reaction mechanisms on air cathode, stability of cell components in semi‐open surroundings, and improvement of battery performance by catalyst design. Challenges and perspectives are also presented. This study provides an intensive understanding of aprotic Li–O2 batteries and offers an important guideline for developing reversible and high‐efficiency Li–O2 batteries.

Potential-Dependent Generation of O2– and LiO2 and Their Critical Roles in O2 Reduction to Li2O2 in Aprotic Li–O2 Batteries

Discharging of the aprotic Li–O2 battery relies on the oxygen reduction reaction (ORR) producing Li2O2 in the positive electrode, which remains incompletely understood. Here, we report a mechanistic study of the Li-ORR on a model system, i.e., an Au electrode in a Li+ dimethyl sulfoxide (DMSO) electrolyte. By spectroscopic identification of the reaction intermediates coupled with density functional theory calculations, we conclude that the formation of O2– and LiO2 in the Li-ORR critically depends on electrode potentials and determines the Li2O2 formation mechanism. At low overpotentials (> 2.0 V vs Li/Li+) O2– is identified to be the first surface intermediate, which diffuses into the bulk electrolyte and forms Li2O2 therein via a solution-mediated disproportionation mechanism. At high overpotentials (ca. 2.0–1.6 V vs Li/Li+) LiO2 has been observed, which can rapidly transform to Li2O2 by further electro-reduction, suggesting a surface-mediated mechanism. The solution-mediated Li2O2 formation that can ac...

Experimental and Computational Analysis of the Solvent-Dependent O2/Li(+)-O2(-) Redox Couple: Standard Potentials, Coupling Strength, and Implications for Lithium-Oxygen Batteries.

Understanding and controlling the kinetics of O2 reduction in the presence of Li(+)-containing aprotic solvents, to either Li(+)-O2(-) by one-electron reduction or Li2 O2 by two-electron reduction, is instrumental to enhance the discharge voltage and capacity of aprotic Li-O2 batteries. Standard potentials of O2 /Li(+)-O2(-) and O2/O2(-) were experimentally measured and computed using a mixed cluster-continuum model of ion solvation. Increasing combined solvation of Li(+) and O2(-) was found to lower the coupling of Li(+)-O2(-) and the difference between O2/Li(+)-O2(-) and O2/O2(-) potentials. The solvation energy of Li(+) trended with donor number (DN), and varied greater than that of O2 (-) ions, which correlated with acceptor number (AN), explaining a previously reported correlation between Li(+)-O2(-) solubility and DN. These results highlight the importance of the interplay between ion-solvent and ion-ion interactions for manipulating the energetics of intermediate species produced in aprotic metal-oxygen batteries.

Experimental and Computational Analysis of the Solvent‐Dependent O2/Li+‐O2− Redox Couple: Standard Potentials, Coupling Strength, and Implications for Lithium–Oxygen Batteries

AbstractUnderstanding and controlling the kinetics of O2 reduction in the presence of Li+‐containing aprotic solvents, to either Li+‐O2− by one‐electron reduction or Li2O2 by two‐electron reduction, is instrumental to enhance the discharge voltage and capacity of aprotic Li‐O2 batteries. Standard potentials of O2/Li+‐O2− and O2/O2− were experimentally measured and computed using a mixed cluster‐continuum model of ion solvation. Increasing combined solvation of Li+ and O2− was found to lower the coupling of Li+‐O2− and the difference between O2/Li+‐O2− and O2/O2− potentials. The solvation energy of Li+ trended with donor number (DN), and varied greater than that of O2− ions, which correlated with acceptor number (AN), explaining a previously reported correlation between Li+‐O2− solubility and DN. These results highlight the importance of the interplay between ion–solvent and ion–ion interactions for manipulating the energetics of intermediate species produced in aprotic metal–oxygen batteries.

CoS2nanoparticles–graphene hybrid as a cathode catalyst for aprotic Li–O2batteries

A CoS2nanoparticles–graphene hybrid was first used as a cathode catalyst for aprotic Li–O2batteries, exhibiting low discharge/charge overpotentials and a high rate capability.

Reversibility of Noble Metal-Catalyzed Aprotic Li-O₂ Batteries.

The aprotic Li-O2 battery has attracted a great deal of interest because, theoretically, it can store far more energy than today's batteries. Toward unlocking the energy capabilities of this neotype energy storage system, noble metal-catalyzed high surface area carbon materials have been widely used as the O2 cathodes, and some of them exhibit excellent electrochemical performances in terms of round-trip efficiency and cycle life. However, whether these outstanding electrochemical performances are backed by the reversible formation/decomposition of Li2O2, i.e., the desired Li-O2 electrochemistry, remains unclear due to a lack of quantitative assays for the Li-O2 cells. Here, noble metal (Ru and Pd)-catalyzed carbon nanotube (CNT) fabrics, prepared by magnetron sputtering, have been used as the O2 cathode in aprotic Li-O2 batteries. The catalyzed Li-O2 cells exhibited considerably high round-trip efficiency and prolonged cycle life, which could match or even surpass some of the best literature results. However, a combined analysis using differential electrochemical mass spectrometry and Fourier transform infrared spectroscopy, revealed that these catalyzed Li-O2 cells (particularly those based on Pd-CNT cathodes) did not work according to the desired Li-O2 electrochemistry. Instead the presence of noble metal catalysts impaired the cells' reversibility, as evidenced by the decreased O2 recovery efficiency (the ratio of the amount of O2 evolved during recharge/that consumed in the preceding discharge) coupled with increased CO2 evolution during charging. The results reported here provide new insights into the O2 electrochemistry in the aprotic Li-O2 batteries containing noble metal catalysts and exemplified the importance of the quantitative assays for the Li-O2 reactions in the course of pursuing truly rechargeable Li-O2 batteries.

Carbon-Free CoO Mesoporous Nanowire Array Cathode for High-Performance Aprotic Li-O2 Batteries.

Although various kinds of catalysts have been developed for aprotic Li-O2 battery application, the carbon-based cathodes are still vulnerable to attacks from the discharge intermediates or products, as well as the accompanying electrolyte decomposition. To ameliorate this problem, the free-standing and carbon-free CoO nanowire array cathode was purposely designed for Li-O2 batteries. The single CoO nanowire formed as a special mesoporous structure, owing even comparable specific surface area and pore volume to the typical Super-P carbon particles. In addition to the highly selective oxygen reduction/evolution reactions catalytic activity of CoO cathodes, both excellent discharge specific capacity and cycling efficiency of Li-O2 batteries were obtained, with 4888 mAh gCoO(-1) and 50 cycles during 500 h period. Owing to the synergistic effect between elaborate porous structure and selective intermediate absorption on CoO crystal, a unique bimodal growth phenomenon of discharge products was occasionally observed, which further offers a novel mechanism to control the formation/decomposition morphology of discharge products in nanoscale. This research work is believed to shed light on the future development of high-performance aprotic Li-O2 batteries.

Importance of Reaction Kinetics and Oxygen Crossover in aprotic Li-O2 Batteries Based on a Dimethyl Sulfoxide Electrolyte.

Although still in their embryonic state, aprotic rechargeable Li-O2 batteries have, theoretically, the capabilities of reaching higher specific energy densities than Li-ion batteries. There are, however, significant drawbacks that must be addressed to allow stable electrochemical performance; these will ultimately be solved by a deeper understanding of the chemical and electrochemical processes occurring during battery operations. We report a study on the electrochemical and chemical stability of Li-O2 batteries comprising Au-coated carbon cathodes, a dimethyl sulfoxide (DMSO)-based electrolyte and Li metal negative electrodes. The use of the aforementioned Au-coated cathodes in combination with a 1 M lithium bis(trifluoromethane)sulfonimide (LiTFSI)-DMSO electrolyte guarantees very good cycling stability (>300 cycles) by minimizing eventual side reactions. The main drawbacks arise from the high reactivity of the Li metal electrode when in contact with the O2 -saturated DMSO-based electrolyte.

A Spectroelectrochemical Study of Oxygen Electrode Reactions in Dimethyl Sulfoxide Electrolyte for Aprotic Li-O2 Batteries

Due to their high theoretical energy density, Li-O2 systems based on aprotic electrolytes have attracted a great deal of interest in the past few years.1-3 Operation of an aprotic Li-O2 battery critically relies on the reversible and efficient oxygen electrode reactions occuring in the positive electrode.4,5 Therefore, it is important to understand the oxygen reaction mechanisms at the positive electrode/Li+ electrolyte interface. Here, we report a mechanistic study of a model system, i.e., oxygen reactions on an Au electrode in a Li+dimethyl sulfoxide (DMSO) electrolyte with a range of complementary techniques. By spectroscopically identifying the reaction products and intermediates, coupled with electrochemical measurements under well-defined conditions, oxygen reaction pathways and the rate limiting steps were depicted.

Electrolyte Effects on Chemical Stability of NaO2 in Na-O2 Batteries

Rechargeable metal-air (oxygen) batteries are receiving intense interest as possible alternatives to lithium-ion batteries, because of their potential to provide higher gravimetric energies. 1 While much attention has been focused on aprotic Li-O2 batteries, substantial challenges must be addressed before widespread commercialization is possible. Recently, a metal-air battery in which lithium has been replaced by sodium has received increasing attention.2 Although Na-O2 batteries present lower gravimetric energies on a cell basis (1605 or 1108 Wh/kg based on Na2O2 or NaO2 discharge products, respectively)2 than Li-O2 batteries (3505 Wh/kgLi2O2),1 much lower charge overpotentials (~100 mV) than those in typical Li-O2 batteries (~1000 mV) have been reported based on reversible sodium superoxide (NaO2) formation . Unlike Li-O2 batteries, for which Li2O2 is the only discharge product, NaO2, sodium peroxide (Na2O2) and sodium peroxide dihydrate (Na2O2 · 2H2O), or a mixture, have been identified in Na-O2 batteries in ether-based electrolytes using a range of carbon electrode types. Here we show NaO2 as the only discharge product from Na-O2 cells with carbon nanotubes in 1,2-dimethoxyethane from X-ray diffraction and Raman spectroscopy. Immediately after discharge, no Na2O2 · 2H2O was found in the discharged electrode, even with H2O present in the electrolyte up to 6000 ppm. However, Na2O2 · 2H2O was detected upon ageing the electrode in the electrolyte, and the amount of Na2O2 · 2H2O was seen to increase with increasing rest time in the electrolyte. More importantly, the reversibility of the reaction is affected by the presence of Na2O2 · 2H2O, where the cells charged immediately after discharge show a reversible reaction , and the cells charged after resting the electrode in the electrolyte for several hours showed poor reversibility. The rechargeable nature of a Na-O2 cell is governed by the formation and oxidation of NaO2, making it vital to study the stability of the electrolyte in the presence of NaO2, and also to prevent the formation of irreversible side products. REFERENCES 1. D. G. Kwabi, N. Ortiz-Vitoriano, S. A. Freunberger, Y. Chen, N. Imanishi, P. G. Bruce, Y. Shao-Horn, MRS Bull. 39, 443 (2014). 2. P. Hartmann, C. L. Bender, M. Vračar, A. K. Dürr, A. Garsuch, J. Janek, P. Adelhelm, Nat. Mater. 12, 228 (2013).

The Aprotic Li-O2 Battery: O2 Reduction Mechanism and Its Implications

Li-ion and related battery technologies will be important for years to come. However, society needs energy storage that exceeds the capacity of Li-ion batteries. We must explore alternatives to Li-ion if we are to have any hope of meeting the long-term needs for energy storage. One such alternative is the Li-air (O2) battery; its theoretical specific energy exceeds that of Li-ion, but many hurdles face its realization.1-5 One spin-off of the recent interest in rechargeable Li-O2 batteries, based on aprotic electrolytes is that it has highlighted the importance of understanding the fundamental electrochemistry at the positive electrode within the battery.6-15 The challenges of obtaining efficient, reversible charge and discharge are well documented in the field. Here, we describe how our recent studies into the electrochemical mechanism of O2 reduction to form Li2O2 at the positive electrode might allow us to design new strategies to overcome these limitations.16 For example, exploiting the effect of solvent donor number, Fig. 1. We will describe our resent results using redox mediators17 to facilitate the electrochemistry along with the implications of the results for the future of rechargeable Li-O2 batteries. References (1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Nature Materials 2012, 11, 19. (2) Lu, Y. C.; Gallant, B. M.; Kwabi, D. G.; Harding, J. R.; Mitchell, R. R.; Whittingham, M. S.; Shao-Horn, Y. Energy & Environmental Science 2013, 6, 750. (3) Black, R.; Adams, B.; Nazar, L. F. Advanced Energy Materials 2012, 2, 801. (4) Girishkumar, G.; McCloskey, B.; Luntz, A. C.; Swanson, S.; Wilcke, W. The Journal of Physical Chemistry Letters 2010, 1, 2193. (5) Li, F.; Zhang, T.; Zhou, H. Energy & Environmental Science 2013, 6, 1125. (6) Adams, B. D.; Radtke, C.; Black, R.; Trudeau, M. L.; Zaghib, K.; Nazar, L. F. Energy & Environmental Science 2013, 6, 1772. (7) Horstmann, B.; Gallant, B.; Mitchell, R.; Bessler, W. G.; Shao-Horn, Y.; Bazant, M. Z. The Journal of Physical Chemistry Letters 2013, 4, 4217. (8) Hummelshoj, J. S.; Luntz, A. C.; Norskov, J. K. The Journal of Chemical Physics 2013, 138, 034703. (9) McCloskey, B. D.; Scheffler, R.; Speidel, A.; Girishkumar, G.; Luntz, A. C. The Journal of Physical Chemistry C 2012, 116, 23897. (10) Mitchell, R. R.; Gallant, B. M.; Shao-Horn, Y.; Thompson, C. V. The Journal of Physical Chemistry Letters 2013, 4, 1060. (11) Trahan, M. J.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M. Journal of The Electrochemical Society 2013, 160, A259. (12) Sharon, D.; Etacheri, V.; Garsuch, A.; Afri, M.; Frimer, A. A.; Aurbach, D. The Journal of Physical Chemistry Letters 2012, 4, 127. (13) Jung, H. G.; Kim, H. S.; Park, J. B.; Oh, I. H.; Hassoun, J.; Yoon, C. S.; Scrosati, B.; Sun, Y. K. Nano Letters 2012, 12, 4333. (14) Peng, Z.; Freunberger, S. A.; Hardwick, L. J.; Chen, Y.; Giordani, V.; Barde, F.; Novak, P.; Graham, D.; Tarascon, J. M.; Bruce, P. G. Angewandte Chemie International Edition 2011, 50, 6351. (15) Zhai, D.; Wang, H. H.; Yang, J.; Lau, K. C.; Li, K.; Amine, K.; Curtiss, L. A. Journal of the American Chemical Society 2013, 135, 15364. (16) Johnson, L.; Li, C. ; Liu, Z.; Chen, Y.; Freunberger, S. A.; Ashok, P.; Praveen, B.; Dholakia, K.; Tarascon, J-M.; Bruce, P. G. Nature Chemistry 2014, 6, 1091. (17) Chen, Y.; Freunberger, S. A.; Peng, Z.; Fontaine, O.; Bruce, P. G. Nature Chemistry 2013, 5, 489. Figure 1

The Efficiency and Mechanism of 3D-Ordered Macroporous La0.6Sr0.4Co0.2Fe0.8O3 as an Electrocatalyst for Aprotic Li-O2 Batteries

Lithium-oxygen (Li-O2) batteries with high energy density have thought to be the most attractive alternatives to nowadays Li-ion batteries. However, some challenges are still to be resolved, among which is the high charge overpotential that could compromise the cycle stability and capacity performance. [1, 2] The high overpotential is ascribed to the insolated discharge products Li2O2, which could not decompose completely during the charge processes. With the increasing cycles of battery tests, the Li2O2 will crystallize and block the porous cathode materials. [3, 4] However, recent studies have demonstrated that the structure and morphology could influence the conductivity and thus affect the overpotential. It is crucial to find an electrocatalyst that could accelerate the decomposition of Li2O2 and influence the crystallization of Li2O2 to get small size or amorphous Li2O2 products. [5, 6] Perovskite oxides La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) have been proved efficient to lower overpotential in aqueous Li-air batteries [7] but not have been applied in Li-O2 batteries. Herein the 3D-ordered macroporous LSCF (3D-LSCF) with very high BET surface area have been prepared and employed as electrocatalyst in Li-O2 batteries with DMSO based electrolyte. The Li-O2 batteries with this catalyst show lower overpotential, better cycle stability and capacity performance. As shown in Figure 1, the charge plateau of Li-O2 batteries with 3D-LSCF catalyst are flat and the charge potential is below 3.8V after nine cycles. The primary reason of the lower overpotential may be ascribe to the high oxygen pathways in 3D-LSCF structure which could accelerate the decomposition of Li2O2 products. [8] In order to conform the better ability of oxygen transmit of 3D-LSCF, the batteries have been tested in different levels of oxygen environments. With the increase of oxygen content, the performance of batteries without catalyst has increased more obviously than with 3D-LSCF catalyst, which indicates that the 3D-LSCF catalyst could transmit relatively sufficient oxygen. The morphologies of pure KB electrode and KB/3D-LSCF electrode at different states have been observed and the 3D-ordered macroporous structure of LSCF is beneficial to form small size of Li2O2 with large contact area to the KB that could assist in the decomposition at lower charge potential. The details of results and specific analysis will be exhibited in the presentation. Fig. 1 The cycle performance of Li-O2 batteries with 3D-LSCF catalyst at the current densities of 100mAg-1. Acknowledgements: This program was supported by Innovative Research Team in University (PCSIRT No. IRT1014).

Realization of Stable Cathode-Electrolyte Interfaces in DMSO Based Li-Air Batteries: Experimental and Theoretical Perspectives

Electrochemical power sources based on metal anodes have specific energy density much higher than conventional Li ion batteries, due to the high energy density of the metal anode (3842mAh/g1 for Li). Rechargeable aprotic Li-O2batteries consume oxygen from the surrounding environment during discharge to form Li oxides on the cathode scaffold, using reactions (1) [anode] Li(s) ↔ Li+ + e− (2) [cathode] Li+ + ½ O2 (g)+ e− ↔ ½ Li2O2(s), (3) [cathode] Li+ + e− + ¼ O2 (g) ↔ ½ Li2O (s), The cathode reaction requires large over-potentials for charging due to the mass transfer resistance of reagents to the active sites on its surface, decreasing the round trip efficiency, making recharge of the Li-O2cell difficult. To overcome these problems, the cathode needs good electrical conductivity and a porous structure that enables facile diffusion of oxygen and can accommodate the reduced oxygen species in the pores. Two significant challenges exist in the use of the traditional activated carbon material as the cathode of the Li-O2 system. First, in the presence of Li2O2 the carbon electrode becomes relatively unstable even at low voltages (<3.5V). (Itkis et al., 2013) Second, cathode structures must be porous to accommodate a substantial amount of Li–peroxide (Li2O2) without blocking ion transport channels in the cathode. An additional major obstacle for the realization of the non-aqueous Li-O2 battery is finding a solvent that is chemically and electrochemically stable under cell operating conditions. Dimethyl sulfoxide (DMSO) is an attractive candidate for rechargeable Li-O2 battery studies(Peng, Freunberger, Chen, & Bruce, 2012); however, there is still significant controversy regarding its stability on the Li-O2cathode surface(Kwabi et al., 2014). We report here results from a model cathode systems which enable determination of the effects of various catalysts on the OER/ORR reactions in the DMSO based Li-O2 cell. Mesoporous CNT sponge is used as the model cathode material, decorated with catalyst nanoparticles by nucleation-controlled atomic layer deposition (ALD) of Ru, RuO2, MnOx, and Pt catalyst components whose loading and composition are controlled by manipulating the ALD conditions. We performed multiple experiments (in-situ XPS, FTIR, Raman, and XRD) which assess the stability of the DMSO-Li2O2 interface and report perspectives on previously published studies. Our electrochemical experiments show long term stable cycling of a DMSO-based operating Li-O2 cell with a Catlyst@carbon nanotube core-shell cathode fabricated via atomic layer deposition. This work is complemented by density functional theory calculations of DMSO degradation pathways on Li2O2. Both experimental and theoretical evidence strongly suggests that DMSO is chemically and electrochemically stable on the surface of Li2O2 under the reported operating conditions.

Li2O2 oxidation: the charging reaction in the aprotic Li-O2 batteries

Aprotic Li-O2 battery has attracted a great deal of interest because of its high theoretical energy density that is far beyond what the best Li-ion technologies can achieve. However, the present Li-O2 batteries suffer from the low energy efficiency that is limited mainly by the high overpotentials required to re-oxidize Li2O2, the discharge product. Over the past few years, considerable research efforts have been devoted to the understanding of the Li2O2 oxidation reactions. Here, we summarize the results obtained from the fundamental study of the Li2O2 oxidation, including its morphology, reaction route, kinetics, the initial location upon oxidation and the charge transport within Li2O2. A better mechanistic understanding of the Li2O2 oxidation reaction will provide a solid foundation for the realization of practical Li-O2 cells with a higher energy efficiency.

Critical Descriptor for the Rational Design of Oxide-Based Catalysts in Rechargeable Li–O2Batteries: Surface Oxygen Density

Li–O2 batteries provide high-capacity energy storage, but for aprotic Li–O2 batteries, it is reported that the charge–discharge efficiency is ultimately limited by the crystal growth of insoluble Li2O2 on the porous cathode. Catalysts have been reported to improve the nucleation and morphology of Li2O2, which helps achieve high energy densities. We provide a new insight into the catalytic mechanism of the oxygen reduction reaction (ORR) in aprotic Li–O2 batteries—the oxygen sites on the surface play a more important role than the exposed metal sites—via a study based on the density functional theory (DFT) examining α-MnO2 surfaces. Lithium ions from electrolytes are found to interact with the surface oxygen sites and form surface lithium sites, facilitating further growth of Li2O2. A larger number of initial growth points with uniform distribution makes Li2O2 well dispersed, forming small particles, which benefit both the ORR and oxygen evolution reactions (OER). This design concept for oxygen sites has b...