Rechargeable Lithium-oxygen Batteries Research Articles

Lithium–oxygen batteries: At a crossroads?

Summary In this current opinion, we critically review and discuss some of the most important recent findings in the field of rechargeable lithium–oxygen batteries. We discuss recent discoveries like the evolution of reactive singlet oxygen and the use of organic additives to bypass reactive LiO2 reaction intermediates, and their possible implications on the potential for commercialization of lithium–oxygen batteries. Finally, we perform a critical assessment of lithium–superoxide batteries and the reversibility of lithium–hydroxide batteries.

A rechargeable lithium–oxygen battery with dual mediators stabilizing the carbon cathode

At the cathode of a Li–O2 battery, O2 is reduced to Li2O2 on discharge, the process being reversed on charge. Li2O2 is an insulating and insoluble solid, leading ultimately to low rates, low capacities and early cell death if formed on the cathode surface. Here we show that when using dual mediators, 2,5-Di-tert-butyl-1,4-benzoquinone [DBBQ] on discharge and 2,2,6,6-tetramethyl-1-piperidinyloxy [TEMPO] on charge, the electrochemistry at the cathode surface is decoupled from Li2O2 formation/decomposition in solution. Capacities of 2 mAh cmareal−2 at 1 mA cmareal−2 with low polarization on charge/discharge are demonstrated, and up to 40 mAh cmareal−2 at rates ≫1 mA cmareal−2 are anticipated if suitable gas diffusion electrodes can be devised. One of the major barriers to the progress of Li–O2 cells is decomposition of the carbon cathode. By forming/decomposing Li2O2 in solution and avoiding high charge potentials, the carbon instability is significantly mitigated (<0.008% decomposition per cycle compared with 0.12% without mediators). The Li–O2 cell performance is largely limited by the insulating and insoluble nature of Li2O2. Here the authors report that dual mediators decouple the electrochemical reactions at the cathode from the formation and decomposition of Li2O2 from solutions, helping stabilize the carbon cathode.

Improving rate capability and reducing over-potential of lithium-oxygen batteries through optimization of Dimethylsulfoxide-N/N-dimethylacetamide mixed electrolyte

Although dimethylsulfoxide (DMSO) solvent has been widely researched in rechargeable lithium-oxygen (Li-O2) batteries, high polarization voltage and low rate capability limited its application. In this work, we reported a DMSO-based electrolyte system by adding N, N-dimethylacetamide (DMA) to adjust its physical and electrochemical properties. The ionic conductivity, viscosity, oxygen solubility and diffusion coefficient of the mixed electrolytes as well as their electrochemical performance in Li-O2 batteries are researched. The electrochemical tests show that the optimized DMSO/DMA volume ratio is 30 to 70 based on the rate performance and polarization voltage of the cell. Compared with that of the pure DMSO-based electrolyte, the cell with the mixed electrolyte shows improved rate capability and reduced charge-discharge over-potential. When increasing current density from 0.2 to 0.5mAcm−2, the capability retention improves from 32% to 59%. Meanwhile, the charge-discharge voltage gap drops from 1.4V to 0.9V at a current density of 0.2mAcm−2. The improved electrochemical performance could be attributed to low viscosity, high oxygen solubility and diffusion coefficient as well as the low charge-transfer resistance with the mixed electrolyte.

Ruthenium‐Functionalized Hierarchical Carbon Nanocages as Efficient Catalysts for Li‐O2 Batteries

AbstractDeveloping an efficient cathode is essential to obtain high rate capability and rechargeable lithium oxygen (Li‐O2) batteries. Herein, ruthenium (Ru)‐functionalized hierarchical carbon nanocages (hCNCs) are synthesized and employed as a cathode catalyst for Li‐O2 batteries. The as‐prepared cathode exhibits high discharge capacity, low charge potential (8135 mA h g−1 with 3.85 V charge potential at a current density of 0.08 mA cm−2), outstanding rate capability (3416 mA h g−1 at a current density of 0.48 mA cm−2) and good stability up to 78 cycles at a limited capacity of 500 mA h g−1. Such excellent battery performance is ascribed to the synergistic effect of the interconnected hierarchically porous structure of hCNCs, which can facilitate effective electrolyte immersion and efficient Li+/O2 mass transport, and the high catalytic activity of Ru nanoparticles.

Carbon- and binder-free 3D porous perovskite oxide air electrode for rechargeable lithium–oxygen batteries

Porous LaNi0.9Cu0.1O3 nanosheets exhibit excellent performance in Li–O2 batteries because of abundant lattice strain and the oxygen vacancy effect.

3D free-standing hierarchical CuCo2O4 nanowire cathodes for rechargeable lithium-oxygen batteries.

A carbon and binder-free cathode for use in Li-O2 cells which is based on free-standing CuCo2O4 nanowires supported on a nickel foam was synthesized by an efficient hydrothermal method. The resulting hierarchical porous and umbrella structures of the hybrid CuCo2O4@Ni foam cathodes present an excellent rate capability and cycle performance.

CoFe2O4@multi-walled carbon nanotubes integrated composite with nanosized architecture as a cathode material for high performance rechargeable lithium-oxygen battery

In this paper, CoFe2O4@multi-walled carbon nanotubes (MWCNTs) nanocomposite is synthesized by a hydrothermal process and in-situ composite technique as cathode catalyst for rechargeable Li-O2 battery (LOB). The as-prepared materials are characterized by XRD, TEM, XPS, BET, TG-DSC and FT-IR. Electrochemical discharge and charge capacity of MWCNTs and CoFe2O4@MWCNTs nanocomposite electrodes are investigated at current density of 0.02 mA cm−2 in the voltage window of 2.0–4.5 V. LOB with CoFe2O4@MWCNTs nanocomposite electrode shows an initial discharge capacity of 1245 mAh g−1(electrode), with reversible charge capacity of 1242 mAh g−1(electrode), while LOB with only MWCNTs electrode has discharge capacity of 545 mAh g−1(electrode) and charge capacity of 543 mAh g−1(electrode). LOB with CoFe2O4@MWCNTs nanocomposite electrode shows good rate capability and cycle stability. After 15 cycles, the capacity of 1175 mAh g−1(electrode) is still retained at the current density of 0.02 mA cm−2 while 723 mAh g−1(electrode) is retained at 0.5 mA cm−2. The capacity retention of CoFe2O4@MWCNTs nanocomposite electrode is 94.4% at 0.02 mA cm−2 and 92.1% at 0.5 mA cm−2, respectively. During discharge, Li2CO3 and Li2O2 are the main discharge products. The results show that the electrochemical performance of MWCNTs is greatly enhanced with CoFe2O4 introduced as core-shell structure.

Cycling Performance of Lithium-Oxygen Batteries Assembled with Hybrid Electrolytes

Rechargeable lithium-oxygen batteries that utilize oxygen have attracted great interest as next-generation energy storage systems due to their high theoretical specific energy (11,140 Wh kg-1 when excluding oxygen weight), which exceeds that of any other rechargeable battery system.1,2 A non-aqueous lithium-oxygen battery typically consists of a lithium negative electrode, an organic liquid electrolyte, and a porous carbon positive electrode so as to induce the oxygen electrochemical reaction. Among these battery components, the electrolyte has been recognized as one of the greatest challenges facing the successful development of non-aqueous lithium-oxygen batteries.3 In this study, we investigated hybrid electrolyte composed of polymer, Li+-ion conducting inorganic electrolyte and organic liquid electrolyte. The hybrid electrolyte effectively suppressed lithium dendrite growth and prevented evaporation of organic liquid electrolyte under semi-open conditions. Moreover, the Li+-ion conducting inorganic electrolyte was very stable from superoxide anion radical attack, which led to significant improvement of cycling stability. The cycling performances of lithium-oxygen batteries assembled with these hybrid electrolytes are investigated and will be presented.

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.

Low charge overpotential of lithium-oxygen batteries with metallic Co encapsulated in single-layer graphene shell as the catalyst

Rechargeable lithium-oxygen (Li–O2) battery has triggered tremendous attention as a promising candidate power source for portable electronics and light vehicles. Until now, a critical scientific challenge facing Li–O2 battery is the high charge overpotential due to the sluggish oxygen evolution reaction (OER) on the oxygen electrode, which results in low energy efficiency and poor cyclability. Here, we demonstrated that nitrogen-doped single layer graphene shell encapsulating non-precious metal Co can be used as a highly efficient catalyst for Li–O2 batteries. The catalyst showed significantly enhanced OER catalytic activity, with a charge overpotential of 0.58V, which was remarkably lower compared with the corresponding N-free graphene encapsulating metal, metal oxide and metal-free carbon materials. DFT calculations revealed that the nitrogen dopants and enclosed metal clusters can synergistically modulate the electronic properties of the graphene surface, resulting in a dramatic reduction of the overpotentials. This study provides the possibility of the rational non-precious metal electrocatalysts designing for Li–O2 batteries.

Porous Carbon Nanofibers Decorated with Cobalt Nanoparticles As Binder-Free Efficient Cathode for Lithium-Oxygen Batteries

Rechargeable lithium-oxygen (Li-O2) batteries have received much attention because of their ultra-high theoretical energy density, about ten-times higher than that of current lithium-ion batteries.[1-4] However, the development of these batteries is still at its initial stage, owing to various technical challenges such as high overpotentials, low round-trip efficiency, poor rate capability, and low cycle life. Among various issues affecting Li-O2 battery performance, the low performance of the oxygen cathode has been identified as the dominating factor.[5, 6] The development of efficient cathode with nano-architectures, large accessible surface area, and bifunctional catalytic activity is a great challenge for high-performance lithium-air batteries.[7] In the present study, binder-free high surface area porous carbon nanofibers modified with cobalt nanoparticles (Co-PCNF) are presented as efficient cathodes for Li-O2 batteries. The macroporous inter-connected structure of carbon nanofibers with effective dispersion of cobalt nanoparticles serve as an efficient cathode with high electrocatalytic activity. Co-PCNF demonstrated much better performance as cathodes in Li-O2 batteries, with high discharge capacity, rechargeability, and rate capability, compared to non-cobalt-carbon cathodes. Li-air cells with Co-PCNF as cathode exhibit a high initial discharge capacity of 8800 mAhg-1 at the current density of 100 mA g-1, and can be recharged for more than 50 cycles with limited discharge capacity of 500 mAh g-1. Moreover, the synergistic role of N-and F- doped carbon defects and cobalt nanoparticles leads to faster electronic transfer and stabilize carbon surface, enhancing the reversibility.

Hierarchical Ru and RuO2 Foams with Superior Performance for Rechargeable Lithium Oxygen Battery Cathodes

Synthesis and structure design of cathode materials with superior catalytic activity is still major challenge for rechargeable lithium-oxygen batteries. Among many cathode materials, carbons are often used because they help achieve high discharge capacities and good ORR performance. However, most lithium-oxygen batteries containing carbon electrodes suffer from a low round-trip efficiency, low rate capability, a poor cycle life, and electrolyte instability. In this poster, we report a binder- and carbon-free, 3D porous Ru- and RuO2-foam cathode for lithium-oxygen batteries. The cathode was simply fabricated by three-step process; co-electrodeposition, electrochemical dealloying and heat treatment process. Both 3D porous Ru- and RuO2-foams have lots of cavity and pits which provide a large surface area for catalytic sites and buffer spaces for the volume stress by the formation/decomposition of Li2O2 during the cycling process. Moreover, cathodes made of Ru- and RuO2- foam can provide high catalytic activities, short ion diffusion length and channels for rapid transport of oxygen and electrolyte. The hierarchically dendritic cathodes without carbon and binder present low charge/discharge overpotential, remarkable cyclability, good oxygen efficient and reduced irreversible formation form electrolyte decomposition. We systematically investigated the reversibility and cyclability of a lithium-oxygen battery using the 3D porous Ru- and RuO2-foam cathode using scanning electron microscopy (SEM), X-ray diffraction (XRD), selected area electron diffraction (SAED), in-situ Differential Electrochemical Mass Spectrometry (DEMS) and FT-IR spectroscopy. The results using the 3D porous nanostructured electrodes in our study represent a promising approach for high-performance electrodes which are compatible with scaled-up manufacturing process for next-generation lithium- air batteries due to their simple and rapid fabrication process. 1. Fujun Li, Tao Zhang, Haoshen Zhou, Energy Environ. Sci. 6, 1125-1141 (2013) 2. Fujun Li, Dai-Ming Tang, Yong Chen, Dmitri Golberg, Hirokazu Kitaura, Tao Zhang, Atsuo Yamada, and Haoshen Zhou, Nano letter, 13, 4702-4707 (2013) 3. Fujun Li, Dai-Ming Tang, Zelang Jian, Dequan Liu, Dmitri Golberg, Atsuo Yamada, Haoshen Zhou, Adv. Mater., 26, 4659-4664 (2014) 4. Myung-Gi Jeong, Kai Zhuo, Serhiy Cherevko, Woo-Jae Kim, Chan-Hwa Chung, J. of Power Sources, 244, 806-811 (2013)

Catalytic Activity Variation with the Crystal Structure Change of PdCu Nanocatalysts for Rechargeable Lithium-Oxygen Batteries

Palladium-copper (Pd-Cu) alloys have two representative crystal structures; one is body-centered cubic (bcc), the other is face-centered cubic (fcc) even though both Pd and Cu originally have fcc crystal structures. The fcc Pd-Cu alloys have disordered structure within which Pd and Cu atoms are positioned randomly in fcc lattice, whereas the bcc Pd-Cu alloys have ordered structure which consists of alternative layers with either Pd or Cu atoms. In order to properly design the optimum Pd-Cu alloys, its physical and chemical characteristics have been extensively controlled.Pd-Cu alloys have been widely utilized as a catalyst for the hydrogenation of carbonyl compounds, the reduction of nitrates and nitrites in water, the oxidation of formic acid in direct formic acid fuel cell, the membrane for hydrogen separation and so on. In particular, bcc Pd-Cu alloys have occasionally shown superior performance to fcc Pd-Cu alloys. As for the hydrogenation of acetylene, bcc Pd-Cu alloy have better ethylene selectivity than fcc Pd-Cu alloys, because isolated Pd atoms on the surface of bcc Pd-Cu alloys enhance selectivity. Most of bcc Pd-Cu alloys have been synthesized by annealing fcc Pd-Cu alloys for structural transformation and particle growth. Hence, the same size of fcc and bcc Pd-Cu alloys is quite difficult to be obtained because the crystallites larger than 20 nm favour the ordered bcc structure with lower symmetry. Thus, bcc Pd-Cu alloys in nanoscale have been rarely reported, and fcc and bcc Pd-Cu alloys have never been properly compared until now. In this study, we successfully synthesized fcc and bcc Pd-Cu alloys in nanoscale through a simple polyol process. For the comparison between these in catalytic activity, we adopted fcc and bcc Pd-Cu alloys as cathode catalysts for lithium-oxygen batteries. This work would firstly report the comparison between the catalytic activities of fcc and bcc Pd-Cu alloys in nanoscale.

Synergistic catalytic properties of bifunctional nanoalloy catalysts in rechargeable lithium-oxygen battery

Abstract The understanding of factors influencing the performance of catalysts in the air cathode of a rechargeable lithium-oxygen battery, including overpotentials for oxygen reduction/evolution and discharge capacity, is essential for exploration of its ultimate application. This report describes new findings of an investigation of PdCu nanoalloys as cathode catalysts. Alloying Pd with oxophilic base metals such as Cu leads to reduction of the overpotentials and increase of the discharge capacity. The nanoalloy structures depend on the bimetallic composition, with an atomic ratio near 50:50 featuring mixed bcc and fcc structures. The discharge potential exhibits a maximum while the charge potential display a minimum in the range of 20–50% Cu, closer to 25% Cu, both of which correspond to a maximum reduction of the discharge-charge overpotentials. The discharge capacity displays a gradual increase with Cu%. This type of catalytic synergy is believed to be associated with a combination of ensemble and ligand effects. In particular, the activation of oxygen on Pd sites and oxygen oxophilicity at the alloyed Cu sites in the catalyst may have played an important role in effectively activating oxygen and maneuvering surface superoxide/peroxide species. These findings have implications for the design of multifunctional cathode catalysts in rechargeable lithium-oxygen batteries.

Application of Atomic Layer Deposition to Lithium-Oxygen Batteries

Rechargeable lithium-oxygen (or lithium-air) batteries have been recognized as next-generation batteries which can replace the prevailing lithium-ion batteries. The non-aqueous Li-O2 batteries involve carbon materials as cathode materials due to their natural abundance and high surface areas per weight. However, the exposure of carbon to air and Li2O2 deteriorates the stability of cathodes, in association with the subsequent electrolyte decomposition, leading to the low round-trip efficiency and poor cycle performance. The atomic layer deposition (ALD) of oxide materials possesses several unique features: self-limiting surface reactions and atomic-scale deposition, and conformal deposition. Such features can be applied to control artificially the deposition morphology on the underlying carbon cathodes, which affects the performance of Li-O2 batteries. In order to improve the well-known issue present in the Li-O2 (Li-air) batteries, the ALD of oxide materials was combined with the carbon materials with the aim to controlling the cathode degradation issues. The current work reports zinc oxide and aluminum oxide as coating materials deposited onto carbon cathode materials, i.e. carbon nanotubes (CNTs): zinc oxide (ZnO) shows a semiconducting property, while aluminum oxide (Al2O3) exhibits an insulating property. The ALD-coated ZnO and Al2O3 onto the carbon electrodes are characterized using analytical electron microscopy. The implications of ALD-based ZnO and Al2O3 are discussed in terms of the performance of Li-O2 secondary batteries in connection with the underlying microstructure and chemistry.

Carbon Nanotube@MnO2 Hybrid Catalysts for Li-O2 Batteries

Rechargeable lithium-oxygen (Li-O2) battery has attracted enormous research attention due to its remarkably high theoretical energy capacity. However, the sluggish oxygen reduction and oxygen evolution reaction (ORR and OER) kinetics cause the high over potential, poor electrolyte stability and short cycling life.[1] Thus, the development of efficient catalysts are required to effectively improve the Li-O2 battery performance. The materials based on MnO2 are recognized as low-cost and efficient catalysts for Li-O2 battery cathodes.[2] Here, in-situ growth of branched MnO2 nanosheets on multi-walled carbon nanotubes (MWCNTs) were generated through a one-pot reaction process and applied as cathode catalyst in a rechargeable Li-O2 battery. The MnO2 with the morphology of curved petal-like walls (about 50 nm in thickness) were homogeneously coated outside the CNT (about 40 nm in diameter) to form a core–shell nanowires.[3] The CNT@MnO2 hybrid showed excellent catalytic activity for both ORR and OER processes in Li-O2 batteries. Owing to its ultra-high specific surface area, it delivered a high reversible capacity of 1500 mAh·g-1 based on the mass of the hybrid. This core-shell structure can effectively prevent direct contact between the CNT and the discharge product Li2O2, thus avoiding or reducing the decomposition of CNT and the formation of Li2CO3 during charge process, which can also improve the conductivity of the cathode and increase the cycling performance of Li-O2 batteries. As a promising cathode catalyst in Li-O2 batteries, the branched CNT@MnO2 nanocomposite cathode exhibits a stable full discharge and charge cycling performance. As shown in figure 1b, about 250 cycles were achieved with a fixed capacity of 500 mAh·g-1 at a current of 100 mA·g-1. The results shown that such a design of a core-shell architecture with a high conductive carbon nanotube in the center and MnO2 nanosheets uniformly coated on its surface both achieved the high conductivity and excellent catalytic activity in Li-O2 batteries. Figure 1. Schematic illustrations of the transfer of electrons in CNT@MnO2 cathode (a). discharging and charging profiles (b) and cycle performance of Li-O2 battery with CNT@MnO2 cathode(c).

Electrodeposited Three-Dimensional Porous Ru/RuO2 Foam Cathode for Rechargeable Lithium–Oxygen Batteries

Among many cathode materials explored for lithium-oxygen cathode, carbons are often used because they help achieve high discharge capacities and good ORR performance. However, most lithium-oxygen batteries containing carbon electrodes suffer from a low round-trip efficiency, low rate capability, a poor cycle life, and electrolyte instability. In this poster, we report a binder- and carbon-free, 3D porous Ru and RuO2 foam cathode for lithium-oxygen batteries. The cathode was simply fabricated by three-step process; co-electrodeposition, electrochemical dealloying and heat treatment process. Both 3D porous Ru and RuO2 dendritic foams have lots of cavity and pits which provide a large surface area for catalytic sites and buffer spaces for the morphological volume stress by the formation/decomposition of Li2O2 during the cycling process. Moreover, both dendritic foams provide a facile electron pathway, a short ion diffusion length, a rapid transport channel for oxygen and electrolytes, and numerous catalytic sites for high electrochemical performance. 3D porous Ru and RuO2 foam cathode delivers a discharge capacity of 1570 mAh/g and 4900 mAh/g at 50 mA/g and can be recharged for OER under low charge voltage with good reversibility and cyclability. We systematically investigated the reversibility and cyclability of a lithium-oxygen battery using the 3D porous Ru/RuO2 foam cathode using scanning electron microscopy (SEM), X-ray diffraction (XRD), selected area electron diffraction (SAED), in-situ Differential Electrochemical Mass Spectrometry (DEMS) and FT-IR spectroscopy. The results using the 3D porous nanostructured electrodes in our study represent a promising approach for high-performance electrodes which are compatible with scaled-up manufacturing process for next-generation lithium- air batteries due to their simple and rapid fabrication process.

The Structural Effect of PdCu Nanocatalysts on Catalytic Activities for Rechargeable Lithium-Oxygen Batteries

Palladium-copper (Pd-Cu) alloys have two representative crystal structures; one is body-centered cubic (bcc), the other is face-centered cubic (fcc) even though both Pd and Cu originally have fcc crystal structures. The fcc Pd-Cu alloys have disordered structure within which Pd and Cu atoms are positioned randomly in the fcc lattice whereas the bcc Pd-Cu alloys have ordered structure which consists of alternative layers with single component either Pd or Cu atoms. In order for tailoring design of Pd-Cu alloys, controlling the physical and chemical characteristics of the alloys has been investigated extensively.Pd-Cu alloys have been widely utilized as catalysts for the hydrogenation of carbonyl compounds, for the reduction of nitrates and nitrites in water, for the oxidation of formic acid in direct formic acid fuel cell, as membrane for hydrogen separation and so on. In particular, bcc Pd-Cu alloys have occasionally shown superior performance to fcc Pd-Cu alloys. For an example of hydrogenation of acetylene, bcc Pd-Cu alloy have better ethylene selectivity than fcc Pd-Cu alloys, because isolated Pd atoms on the surface of bcc Pd-Cu alloys increase selectivity in the hydrogenation of acetylene. Most bcc Pd-Cu alloys have been synthesized by annealing of fcc Pd-Cu alloys, which leads to structural transformation and particle growth. Obtaining same sizes of fcc and bcc Pd-Cu alloys is difficult to achieve because crystallites larger than 20 nm favour the ordered bcc structure with lower symmetry. Thus, bcc Pd-Cu alloys in nanoscale have been rarely reported moreover, comparison of fcc and bcc Pd-Cu in nanoscale has been never demonstrated to date. In this study, we successfully synthesized fcc and bcc Pd-Cu alloys in nanoscale by polyol process with same composition and particle size, furthermore we adopt fcc and bcc Pd-Cu alloys as cathode catalysts for lithium-oxygen batteries. This work would be the first time to compare the catalytic activities between fcc and bcc Pd-Cu alloys in nanoscale.

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).

A Bi-Functional Organic Redox Catalyst for Rechargeable Lithium-Oxygen Batteries with Enhanced Performances

The Li-O2 battery provides the highest energy density among all rechargeable battery systems. To date, the large over-potentials during the oxygen reduction reaction and oxygen evolution reaction lead to low round-trip efficiency and short cycle life. Herein, we report a Li-O2 battery with outstanding performance. Application of a bi-functional organic catalyst, poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA), significantly lowered the charge potential to 3.73 V and increased the discharge potential to 2.75 V. PTMA facilitates both efficient formation and oxidation of Li2O2 through its n- and p-doping ability coupled with superior catalytic activity. Furthermore, PTMA effectively coats the surface of the carbon electrodes, which suppresses side-reactions between carbon and the electrolyte leading to enhanced lifetimes. Figure 1