Decomposition Of Li2O2 Research Articles

Metal-oxygen bonds: Stabilizing the intermediate species towards practical Li-air batteries

Rechargeable nonaqueous Li-air batteries are attracting much attention due to their far higher theoretical energy density than lithium-ion batteries. However, Li-air batteries suffers from poor round-trip efficiency, low rate capability and poor cycle life. To reduce charge overpotentials by understanding reaction mechanism and to operate in ambient air instead of pure oxygen are prerequisites to realization of practical Li-air batteries. Here, we demonstrate a practical Li-air battery using Mo2C/CNT as a potential promoter with high round-trip efficiency (∼80%) and improved cycling performance (40 cycles) because Mo2C stabilizes the intermediate species from reduction of both O2 and CO2. The stabilization via formation of Mo-O bonds prevents further reduction and disproportionation of intermediate species to generate crystalline Li2O2 and Li2CO3, thus reducing the charge overpotentials normally caused by the decomposition of crystalline Li2O2 and Li2CO3. In all, this work provides improved understanding of the general role of solid promoters and enables rational design of promoters towards practical Li-air batteries.

Mesoporous CoO/reduced graphene oxide as bi-functional catalyst for Li-O2 battery with improved performances

Wheat-like mesoporous CoO nanorods grown on the reduced graphene oxide (CoO/rGO) is synthesized by a simple hydrothermal method. Owing to synergistic effect between CoO and rGO, the CoO/rGO hybrid exhibits a good initial capacity of 20254 mAh g−1 along with a high coulombic efficiency (98.9%) at 200 mA g−1. In addition, the batteries show an excellent rate capability (13952 mAh g−1 at 800 mA g−1) and enhanced cycling stability (69 cycles with the capacity limited to 1000 mAh g−1 at 200 mA g−1). The electrochemical performance is intimately related to the unique architecture (i.e., hierarchical mesoporous structure), facilitating the reversible formation and decomposition of insoluble Li2O2. The results of electrochemical tests confirm that the CoO/rGO hybrid is a promising candidate for the Li-O2 batteries.

Direct Observations of the Formation and Redox-Mediator-Assisted Decomposition of Li2 O2 in a Liquid-Cell Li-O2 Microbattery by Scanning Transmission Electron Microscopy.

Operando scanning transmission electron microscopy observations of cathodic reactions in a liquid-cell Li-O2 microbattery in the presence of the redox mediator tetrathiafulvalene (TTF) in 1.0 m LiClO4 dissolved dimethyl sulfoxide electrolyte are reported. It is found that the TTF addition does not obviously affect the discharge reaction for the formation of a solid Li2 O2 phase. The coarsening of Li2 O2 nanoparticles occurs via both conventional Ostwald ripening and nonclassical crystallization by particle attachment. During charging, the oxidation reaction at significantly reduced charge potentials mainly takes place at Li2 O2 /electrolyte interfaces and has obvious correspondence with the oxidized TTF+ distributions in the electric fields of the charged electrode. This study provides direct evidence that TTF truly plays a role in promoting the decomposition of Li2 O2 as a soluble charge-transfer agent between the electrode and the Li2 O2 .

Electrochemical Activities for Reduction and Evolution of Oxygen in Nonaqueous Electrolyte Solutions Containing Manganese Salen Complex As an Additive

Introduction Lithium air secondary batteries (LABs) have attracted attention due to their high theoretical energy density. One of the major problems is their poor cycle characteristics, which is caused by the decomposition of electrolytes, the strength degradation of air electrodes, and so on. These are related to the large resistivity of discharge products Li2O2 deposited in the air electrodes. To solve these problems, the air electrodes of LABs have been developed by using various catalysts [1]. However, the catalysts become inactivated gradually after a large number of cycles because undecomposed Li2O2 remains on the active catalytic sites in the air electrodes even after charging processes [2]. Since additives are dissolved in an electrolyte solution, they were predicted to promote the deposition and the decomposition of Li2O2 without deactivation due to the undecomposed Li2O2. The additives are expected to overcome the problems associated with the catalyst for the air electrode. We are focusing on (R,R)-(-)-N,N-Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride (MnSl) as an electrolyte additive. In our previous work, the LAB cells with 1 mol/l LiTFSA/TEGDME containing 2 wt% MnSl exhibited large first discharge capacity of 4903 mAh/g and good cycle retention of 61% at the 10th cycle [3]. However, the details of the reaction mechanisms were still unclear. In this study, we investigated the fundamental mechanisms by evaluating the electrochemical activities for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in an electrolyte containing 0.005 wt% MnSl additives. Experimental A 0.005 wt% of MnSl (Sigma-Aldrich) vacuum-dried at 90°C for 12 h was added to 1.0 mol/l LiTFSA/TEGDME. Ketjenblack EC-600JD (KB, 80 wt%) and polyvinylidene difluoride (PVdF, 20 wt%) coated on carbon paper (Toray, TGP-H-120) were used as an air electrode. A Li metal sheet was used as a counter electrode. These components were installed in a commercial two-electrode LAB cell (ECC-Air, EL-Cell GmbH). Electrochemical experiments were performed at a current density of 200 mA/g under dry air atmosphere with a dew point of less than -50°C. Cyclic voltammetry was performed at a scan rate of 100 mV/s under O2atmosphere using the three-electrode ECC-Air cell. Platinum disks were used as working and counter electrodes. A Li metal was used as a reference electrode. Results and discussion Figure 1 shows the first discharge /charge curves of LAB cells without and with MnSl. In the discharging process, the cell with MnSl shows a higher average voltage of 2.4 V and a larger capacity of 893 mAh/g than the other cell. In the charging process, the cell with MnSl shows a lower voltage during the early process and a larger capacity of 676 mAh/g than the other cell. These results indicate that MnSl promotes electrochemical activities for both ORR and OER. Electrochemical measurements were carried out to investigate how the MnSl additives work in the electrolyte solution during ORR/OER processes. Figure 2 shows cyclic voltammograms (CVs) of the three–electrode cells without and with MnSl under O2 atmosphere. The cells without and with MnSl show a cathodic ORR peak at around 2.0 V and an anodic OER peak at around 4.4 V. The cell with MnSl shows higher current densities in the whole potential range than the cell without MnSl. In the cell with MnSl, the cathodic peak around 2.0 V for the ORR becomes larger, and the onset of that peak shifts significantly toward more positive potential. This might indicate that MnSl promoted the electrochemical activity for ORR. As a result, the average discharge voltage of the cell with MnSl becomes higher compared with the cell without MnSl, as shown in Fig. 1. In the cell with MnSl, the anodic peak for the OER also becomes larger compared with the cell without MnSl and exhibits a new peak at around 4.0 V. This peak was not observed in the CV result for the cell without MnSl. This might indicate that the MnSl interacted with Li2O2at around 4.0 V and such an interaction promoted the activities for OER at around 4.4 V The plateau voltage around 4.0 V shown in Fig. 1 might reflect the electrochemical behavior for the anodic process in the CV result of Fig. 2. Consequently, these results show a tendency similar to the charge and discharge properties shown in Fig. 1 indicating the effectiveness of MnSl as an electrolyte additive.

TEMPO-Substituted Ionic Liquids As Redox Mediators for High Performance Lithium-Oxygen Batteries

The performance of lithium-oxygen (Li-O2) batteries can be improved using redox mediators (RMs) to promote the decomposition of the insoluble, electronically insulating discharge product Li2O2 during the charging process. Homogenous RMs are expected to have a better performance in Li-O2 batteries than heterogeneous catalysts due to a larger contact area.1 Several redox mediators have been tested, including TEMPO and its derivatives.2 However, the use of Li-O2 batteries in many applications such as electric vehicles is still restricted due to electrolyte decomposition, flammability and volatility. In order to overcome these issues, ionic liquids have been successfully used as solvents for Li-O2 batteries.3, 4 In this project, we present TEMPO-substituted imidazolium ionic liquids that can perform the dual roles of solvent and redox mediator for Li-O2batteries (Figure 1). Figure 1. Structure and cyclic voltammograms of TEMPO-substituted imidazolium ionic liquids.Ionic liquids were prepared using modified literature procedures.5-7 Cyclic voltammetry of the ionic liquids in dimethylacetamide with 0.1 M lithium bis(trifluoromethane)sulfonimide was performed using a CHI660D electrochemical workstation in combination with a conventional three-electrode cell. Figure 1 shows cyclic voltammograms for compounds with 4, 5, and 8 carbon atoms in the chains linking the TEMPO and imidazolium moieties. The ionic liquids undergo a reversible electrochemical oxidation reaction at ~0.5 V vs Ag/AgCl (~3.8 V vs Li/Li+) which means that they should be capable of oxidizing the discharge product Li2O2 upon charging to give molecular oxygen and lithium ions. The facilitation of decomposition of Li2O2 can substantially reduce the charge overpotential, thus improving the energy efficiency of Li-O2batteries. References Feng, N.; He, P.; Zhou, H. (2016) Critical Challenges in Rechargeable Aprotic Li-O2 Batteries. Advanced Energy Materials 6:1502303-152326.Bergner, B. J., Hofmann, C., Schürmann, A., Schröder, D., Peppler, K., Schreiner, P. R., Janek, J. (2015) Understanding the fundamentals of redox mediators in Li–O2 batteries: a case study on nitroxides. Physical Chemistry Chemical Physics 17:31769-31779.Zhang, T., Zhou, H. (2012) From Li-O2 to Li-Air Batteries: Carbon Nanotubes/Ionic Liquid Gels with a Tricontinuous Passage of Electrons, Ions and Oxygen. Angewante Chemie, International Edition 51:11062−11067.Zhang, T., Wen, Z.-Y. (2017) A High-Rate Ionic Liquid Lithium-O2 Battery with LiOH Product. The Journal of Physical Chemistry C, doi: 10.1021/acs.jpcc.7b00336.Qian, W., Jin, E., Bao, W., Zhang, Y. (2006) Clean and selective oxidation of alcohols catalyzed by ion-supported TEMPO in water. Tetrahedron 62:556-562.Zheng, Z., Wang, J., Chen, H., Feng, L., Jing, R., Lu, M., Hu, B., Ji, J. (2014) Magnetic Superhydrophobic Polymer Nanosphere Cage as a Framework for Miceller Catalysis in Biphasic Media. ChemCatChem 6:1626-1634.Bonhôte, P., Dias, A.-P., Papageorgiou, N., Kalyanasundaram, K., Grätzel, M. (1996) Hydrophobic, Highly Conductive Ambient-Temperature Molten Salts. Inorganic Chemistry 35:1168-1178. Figure 1

A New Concept of an Air-Electrode Catalyst for Li2O2 Decomposition Using MnO2 Nanosheets on Rechargeable Li-O2 Batteries

To improve the charge/discharge properties of a rechargeable Li-O2 battery (LAB), we synthesized a new air-electrode catalyst composed of MnO2 nanosheets (Mn-NSs) and Ketjen Black (KB), i.e., a KB-comp. Mn-NS catalyst. The composite catalyst was synthesized by electrostatically restacking the nanomaterials in a colloidal solution with Li+ and its electro-catalytic activity as an air-electrode for a LAB test cell was evaluated. The KB-comp. Mn-NS catalyst exhibited excellent performance, especially for oxidation of Li2O2 deposited on the surface of the air-electrode during the charging process, which led to enhanced reversibility of the Li2O2 deposition/decomposition reaction during the discharging/charge cycles. Evidence of these reactions was investigated and confirmed by X-ray diffraction analysis and scanning electron microscopy-energy dispersive X-ray spectroscopy. The Li2O2 was deposited and decomposed more homogeneously at the air-electrode with the KB-comp. Mn-NS catalyst than that with only KB. Consequently, the composite catalyst significantly reduced the overpotential and improved the cycleability up to the 27th cycle, even at 0.40mA.

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.

Nanoengineered Ultralight and Robust All-Metal Cathode for High-Capacity, Stable Lithium-Oxygen Batteries.

The successful development of Li–O2 battery technology depends on resolving the issue of cathode corrosion by the discharge product (Li2O2) and/or by the intermediates (LiO2) generated during cell cycling. As an important step toward this goal, we report for the first time the nanoporous Ni with a nanoengineered AuNi alloy surface directly attached to Ni foam as a new all-metal cathode system. Compared with other noncarbonaceous cathodes, the Li–O2 cell with an all-metal cathode is capable of operation with ultrahigh specific capacity (22,551 mAh g–1 at a current density of 1.0 A g–1) and long-term life (286 cycles). Furthermore, compared with the popularly used carbon cathode, the new all-metal cathode is advantageous because it does not show measurable reactivity toward Li2O2 and/or LiO2. As a result, extensive cyclability (40 cycles) with 87.7% Li2O2 formation and decomposition was obtained. These superior properties are explained by the enhanced solvation-mediated formation of the discharge products as well as the tailored properties of the all-metal cathode, including intrinsic chemical stability, high specific surface area, highly porous structure, high conductivity, and superior mechanical stability.

A soluble phenolic mediator contributing to enhanced discharge capacity and low charge overpotential for lithium-oxygen batteries

In this study, a phenolic antioxidant of 2,6-di-tert-butyl-hydroxytoluene (BHT) is applied in Li-O2 batteries to simultaneously improve discharge capacity and reduce charge overpotential. BHT exhibits a redox couple at ~3.0V (vs. Li+/Li), which is extremely close to the thermodynamic potential of a Li-O2 battery (2.96V). The unique chemical and electrochemical behaviors of BHT contribute to the improvement on both oxygen reduction reaction and oxygen evolution reaction performances. These factors lead to the notable enhancement of discharge capacity (capacity increases by 72%) and the reduction of charge plateau (the plateau is 3.2V and 4.2V, respectively, with and without BHT) for Li-O2 batteries. Furthermore, in-situ X-ray diffraction results confirm that the BHT-mediated formation and decomposition of Li2O2, rather than parasitic reactions, dominate the discharge and charge processes. The results provide a new approach for exploiting appropriate soluble mediators for rechargeable Li-O2 batteries.

Influence of Cathode Surface Structure on Decomposition Behavior of Li2O2 and Li2CO3 in Lithium Air Battery Investigated By First Principles Molecular Dynamics

Lithium air batteries are attracting attention as next generation secondary batteries because powerful secondary batteries are required for an application of electric vehicles. In the lithium air battery, Li2O2 is the main discharging products that decomposed into Li+ and O2 during charging, while electrochemically irreversible Li2CO3 and Li2O are also formed. Reaction phenomenon on formation or decomposition of these products is complex and effects significantly the over potential during charging originated from the byproducts decomposition. Therefore, decomposition behavior of discharge products on a cathode electrode during charging is of fundamental of importance in developing high performance lithium air battery. In this study, we carried out first a principle molecular dynamics method (FPMD) considering electron transfer from carbon surface to decomposed species to analyze the relationship between charging condition and decomposition behavior at the atomic level. In particular, the influence of surface structures of negative electrode on the dynamics of decomposition is studied. In order to simulate the electrochemical reaction during charging, simulation was carried out by first principle molecular dynamics based on density functional theory (DFT) in which the charge of the system was changed corresponding to the transfer of electrons from the cathode materials to products. Calculations were carried out by using models for different surface structures; the carbon (001) surface, the carbon (100) surface, and the carbon surface with step structure models. Calculations was made with NVT ensemble (300 K) using generalized density gradient approximation (GGA) with PBE functional, and total calculated period was 300 fs - 1000 fs. The charging voltage was evaluated from the change of potential energy form FPMD simulations. From the FPMD simulations, a little change of the Li2O2 cluster is stable on the carbon surface with step structure model. However the decomposition of the Li2O2 cluster is observed for the model of carbon (001) surface, although the formation of Li ion is not confirmed. In the simulation of a Li2O2 cluster on the carbon (001) surface, the Li2O2 cluster was decomposed and lithium ion was confirmed. These results suggests that decomposition of Li2O2 hardly occurs near a step structure of carbon. From the free energy change calculated from the simulation result of the carbon surface with step structure model, it is estimated that a charging voltage of 5V or higher is necessary for decomposition of Li2O2 cluster. Simulation of the Li2CO3 cluster on the carbon (001) surface model indicates that decomposition of Li2CO3 could be possible if a high voltage is applied to the system. As a summary from our simulations, it was suggested that Li2O2 deposited near step structure of carbon (100) surface is stabilized by strong interaction of terminated groups preventing from the decomposition, resulting in an increase in charging voltage and it would contribute to the reduction in recycling capacity. Details of dynamics of Li2O2 and Li2CO3 and molecular orbital analysis will be presented in the conference.

Mesoporous Co-CoO/N-CNR nanostructures as high-performance air cathode for lithium-oxygen batteries

A bifunctional air cathode for the electrochemical oxygen reduction and evolution in a Li-O2 battery comprising Co-CoO nanoparticles embedded in the nitrogen-doped carbon nanorods (Co-CoO/N-CNR) is presented. The nanorod morphology with mesoporous nanostructures can handle continuous formation and decomposition of Li2O2 and favors the transport of electrons and ions, therefore improves the rate capability and cycle stability. Moreover, the hierarchical mesoporous Co-CoO/N-CNR reduces the overall overpotential over the cycle and decreases the side reaction between Li2O2 and carbon surface, leading to a significant enhancement of the specific capacity and cycling capability. With the cooperative effect of Co-CoO/N-CNR and porous nature, Co-CoO/N-CNR cathode delivers a high discharge specific capacity of 10,555 mAh g−1 at 100 mA g−1 and long cycle stability for over 86 cycles without any capacity loss at a high cutoff capacity of 1000 mAh g−1. Furthermore, excellent performance of 7514 mAh g−1 at 500 mA g−1 is achieved.

Highly efficient Ru/MnO2 nano-catalysts for Li-O2 batteries: Quantitative analysis of catalytic Li2O2 decomposition by operando synchrotron X-ray diffraction

Abstract In-situ or operando quantitative analysis is very important for Li-O 2 batteries, in order to properly, accurately and comprehensively evaluate electrocatalysts and characterize Li-O 2 electrochemistry in real-time. Synchrotron XRD can provide much higher X-ray intensity and time resolution than traditional in-house diffractometers, and therefore can contribute to quantitative analysis for Li-O 2 batteries. Here, operando synchrotron XRD is further developed to quantitatively study Li-O 2 batteries with nano-catalysts, Ru/MnO 2 . The time-resolved oxygen evolution reaction (OER) kinetics for Li-O 2 cells with Ru/MNT was systematically investigated using operando synchrotron radiation powder X-ray diffraction (SR-PXD). Li 2 O 2 decomposition in the electrodes with Ru/MNT catalysts during galvanostatic and potentiostatic charge processes followed pseudo -zero-order kinetics and showed ideal Coulombic efficiency (close to 100%). Furthermore, it was found that the OER kinetics for a cell with 2 wt% Ru/MNT charged at a constant potential of 4.3 V was even faster than that for a cell with the same amount of pure Ru nanoparticles, which have been considered as a highly active catalyst for Li-O 2 batteries. These results indicated that Ru/MNT with a special nanostructure represented a very efficient electrocatalyst for promoting the OER in Li-O 2 batteries. We also demonstrate that synchrotron radiation XRD can “highlight” a way to quantitative analysis for Li-O 2 batteries.

Mechanistic Evolution of Aprotic Lithium‐Oxygen Batteries

AbstractAprotic lithium‐oxygen (Li‐O2) batteries have attracted much attention in recent years. Considerable efforts have been devoted to understand the reaction mechanisms and improving battery performance. Here, various reaction mechanisms at the cathode are discussed, including the direct electrochemical formation and decomposition of Li2O2 and LiOH and the redox mediator (RM) assisted reactions in both the discharging and charging processes. Formation of Li2O2 in discharging processes is related to the electrolyte solvent property, and its following decomposition proceeds stepwise with the intermediate of Li2–xO2. Low overpotentials have been reported in LiOH‐involved discharging and charging processes, and it can alleviate the instability of carbon against Li2O2 and provide new insights into the design of practical Li‐air batteries. RMs are promising for Li‐O2 batteries, and the prerequisites in addition to the redox potentials should be taken into account. Perspectives on the development of Li‐O2 batteries are provided. The future deployment of Li‐O2 batteries relies on the continuous efforts of all the scientists around the world.

Dual Heteroatom‐Doped Carbon Nanofoam‐Wrapped Iron Monosulfide Nanoparticles: An Efficient Cathode Catalyst for Li–O2 Batteries

Cost-effective dual heteroatom-doped 3D carbon nanofoam-wrapped FeS nanoparticles (NPs), FeS-C, act as efficient bifunctional catalysts for Li-O2 batteries. This cathode exhibits a maximum deep discharge capacity of 14 777.5 mA h g-1 with a 98.1 % columbic efficiency at 0.1 mA cm-2 . The controlled capacity (500 mA h g-1 ) test of this cathode delivers a minimum polarization gap of 0.73 V at 0.1 mA cm-2 and is sustained for 100 cycles with an energy efficiency of approximately 64 % (1st cycle) and 52 % (100th cycle) at 0.3 mA cm-2 , under the potential window of 2.0-4.5 V. X-ray photoelectron spectroscopy reveals the substantial reversible formation and complete decomposition of Li2 O2 . The excellent recharging ability, high rate performance, and cycle stability of this catalyst is attributed to the synergistic effect of FeS catalytic behavior and textural properties of heteroatom-doped carbon nanostructures.

Towards an Understanding of Li2 O2 Evolution in Li-O2 Batteries: An In Operando Synchrotron X-ray Diffraction Study.

One of the major challenges in developing high-performance Li-O2 batteries is to understand the Li2 O2 formation and decomposition during battery cycling. In this study, this issue was investigated by synchrotron radiation powder X-ray diffraction. The evolution of Li2 O2 morphology and structure was observed under actual electrochemical conditions of battery operation. By quantitatively tracking Li2 O2 during discharge and charge, a two-step process was suggested for both growth and oxidation of Li2 O2 owing to different mechanisms during two stages of both oxygen reduction reaction and oxygen evolution reaction. From an observation of the anisotropic broadening of Li2 O2 in XRD patterns, it was inferred that disc-like Li2 O2 grains are formed rapidly in the first step of discharge. These grains can stack together so that they facilitate the nucleation and growth of toroidal Li2 O2 particles with a LiO2 -like surface, which could cause parasitic reactions and hinder the formation of Li2 O2 . During the charge process, Li2 O2 is firstly oxidized from the surface, followed by a delithiation process with a faster oxidation of the bulk by stripping the interlayer Li atoms to form an off-stoichiometric intermediate. This fundamental insight brings new information on the working mechanism of Li-O2 batteries.

MCNTs@MnO2 Nanocomposite Cathode Integrated with Soluble O2-Carrier Co-salen in Electrolyte for High-Performance Li-Air Batteries.

Li-air batteries (LABs) are promising because of their high energy density. However, LABs are troubled by large electrochemical polarization during discharge and charge, side reactions from both carbon cathode surface/peroxide product and electrolyte/superoxide intermediate, as well as the requirement for pure O2. Here we report the solution using multiwall carbon nanotubes (MCNTs)@MnO2 nanocomposite cathode integrated with N,N'-bis(salicylidene)ethylenediaminocobalt(II) (CoII-salen) in electrolyte for LABs. The advantage of such a combination is that on one hand, the coating layer of δ-MnO2 with about 2-3 nm on MCNTs@MnO2 nanocomposite catalyzes Li2O2 decomposition during charge and suppresses side reactions between product Li2O2 and MCNT surface. On the other hand, CoII-salen works as a mobile O2-carrier and accelerates Li2O2 formation through the reaciton of (CoIII-salen)2-O22- + 2Li+ + 2e- → 2CoII-salen + Li2O2. This reaction route overcomes the pure O2 limitation and avoids the formation of aggressive superoxide intermediate (O2- or LiO2), which easily attacks organic electrolyte. By using this double-catalyst system of Co-salen/MCNTs@MnO2, the lifetime of LABs is prolonged to 300 cycles at 500 mA g-1 (0.15 mA cm-2) with fixed capacity of 1000 mAh g-1 (0.30 mAh cm-2) in dry air (21% O2). Furthermore, we up-scale the capacity to 500 mAh (5.2 mAh cm-2) in pouch-type batteries (∼4 g, 325 Wh kg-1). This study should pave a new way for the design and construction of practical LABs.

Influence of Binders and Solvents on Stability of Ru/RuOx Nanoparticles on ITO Nanocrystals as Li-O2 Battery Cathodes.

Fundamental research on Li-O2 batteries remains critical, and the nature of the reactions and stability are paramount for realising the promise of the Li-O2 system. We report that indium tin oxide (ITO) nanocrystals with supported 1-2 nm oxygen evolution reaction (OER) catalyst Ru/RuOx nanoparticles (NPs) demonstrate efficient OER processes, reduce the recharge overpotential of the cell significantly and maintain catalytic activity to promote a consistent cycling discharge potential in Li-O2 cells even when the ITO support nanocrystals deteriorate from the very first cycle. The Ru/RuOx nanoparticles lower the charge overpotential compared with those for ITO and carbon-only cathodes and have the greatest effect in DMSO electrolytes with a solution-processable F-free carboxymethyl cellulose (CMC) binder (<3.5 V) instead of polyvinylidene fluoride (PVDF). The Ru/RuOx /ITO nanocrystalline materials in DMSO provide efficient Li2 O2 decomposition from within the cathode during cycling. We demonstrate that the ITO is actually unstable from the first cycle and is modified by chemical etching, but the Ru/RuOx NPs remain effective OER catalysts for Li2 O2 during cycling. The CMC binders avoid PVDF-based side-reactions and improve the cyclability. The deterioration of the ITO nanocrystals is mitigated significantly in cathodes with a CMC binder, and the cells show good cycle life. In mixed DMSO-EMITFSI [EMITFSI=1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide] ionic liquid electrolytes, the Ru/RuOx /ITO materials in Li-O2 cells cycle very well and maintain a consistently very low charge overpotential of 0.5-0.8 V.

リチウム空気電池における陽極生成物分解過程のシミュレーション

Lithium air batteries are attracting attention as next generation secondary batteries because powerful secondary batteries are required for an application of electric vehicles. In the lithium air battery, Li2O2 is the main discharge product decomposed into Li+ and O2 during charging, whereas electrochemically irreversible Li2CO3 and Li2O are also formed. The reaction phenomenon of decomposition of these products is complex and influences the over potential during charging. The by-products decomposition and surface structure of the electrode also change the over potential. Therefore, decomposition behavior of discharge products on a cathode electrode during charging is fundamental in developing a high performance lithium air battery. In this study, we carried out a first principles molecular dynamics method (FPMD) with different electronic state to analyze the relationship between charging condition and decomposition behavior of discharge products on the carbon surface at the atomic level. In particular, the influence of the surface structure of the electrode on the dynamics of decomposition of Li2O2 was studied.

Perovskite-type La0.8Sr0.2Co0.8Fe0.2O3 with uniform dispersion on N-doped reduced graphene oxide as an efficient bi-functional Li-O2 battery cathode.

A composite cathode including N-rGO with homogeneously dispersed perovskite La0.8Sr0.2Co0.8Fe0.2O3 on the surface is studied. Li-O2 batteries with LSCF@N-rGO cathode show better performance than those with LSCF-SP or N-rGO cathode. EIS and morphology analysis indicate that LSCF is beneficial to remold the shape of Li2O2 and catalyze the decomposition of Li2O2.

A multi-functional gel co-polymer bridging liquid electrolyte and solid cathode nanoparticles: An efficient route to Li–O2 batteries with improved performance

Lithium-oxygen (Li–O2) batteries have the highest theoretical energy density amongst all rechargeable batteries and have attracted significant attention. However, large over-potentials originating from sluggish reaction kinetics often lead to low round-trip energy efficiency and short cycle life. We report here a novel multi-functional gel co-polymer that efficiently enhances the discharge and charge performances in Li–O2 batteries by intimately connecting the liquid electrolyte and solid cathode nanoparticles. On one hand, the co-polymer material, poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate-co-methyl methacrylate) (P(TMA-MMA)), functions as a binder during the fabrication of the cathode and forms a gel polymer membrane to retain liquid electrolyte and to increase ionic conductivity. On the other hand, the TMA units, containing N–O radical groups that catalyse Li2O2 formation and decomposition during charge and discharge cycles, are distributed throughout the polymer membrane. This allows more effective formation and decomposition of Li2O2 than surface bound catalytic units. The combination of gelable MMA and catalytic TMA moieties enhances the interface between liquid electrolyte and solid cathode by functioning as a medium both to transport Li+ (enhancing discharge process) and to carry electrons (reducing charge over-potential). Consequently, the optimized P(TMA-MMA) co-polymers provide exceptional electrochemical performance in Li–O2 batteries.