Structure and Stability of Lithium Superoxide Clusters and Relevance to Li-O2 Batteries

Operando detection and suppression of spurious singlet oxygen in Li-O2 batteries

The rechargeable lithium air (oxygen) battery (Li-O2) has very high energy density, comparable to that of fossil fuels (∼3600 W h kg-1). However, the parasitic reactions of the O2 reduction products with solvent and electrolyte lead to capacity fading and poor cyclability. During the oxygen reduction reaction (ORR) in aprotic solvents, the superoxide radical anion (O2˙-) is the main one-electron reaction product, which in the presence of Li+ ions undergoes disproportionation to yield Li2O2 and O2, a fraction of which results in singlet oxygen (1O2). The very reactive 1O2 is responsible for the spurious reactions that lead to high charging overpotential and short cycle life due to solvent and electrolyte degradation. Several techniques have been used for the detection and suppression of 1O2 inside a Li-O2 battery under operation and to test the efficiency and electrochemical stability of different physical quenchers of 1O2: azide anions, 1,4-diazabicyclo[2.2.2]octane (DABCO) and triphenylamine (TPA) in different solvents (dimethyl sulfoxide (DMSO), diglyme and tetraglyme). Operando detection of 1O2 inside the battery was accomplished by following dimethylanthracene fluorescence quenching using a bifurcated optical fiber in front-face mode through a quartz window in the battery. Differential oxygen-pressure measurements during charge-discharge cycles vs. charge during battery operation showed that the number of electrons per oxygen molecule was n > 2 in the absence of physical quenchers of 1O2, due to spurious reactions, and n = 2 in the presence of physical quenchers of 1O2, proving the suppression of spurious reactions. Battery cycling at a limited specific capacity of 500 mA h gC-1 for the MWCNT cathode and 250 mA gC-1 current density, in the absence and presence of a physical quencher or a physical quencher plus the redox mediator I3-/I- (with a lithiated Nafion® membrane), showed increasing cyclability according to coulombic efficiency and cell voltage data over 100 cycles. Operando Raman studies with a quartz window at the bottom of the battery allowed detection of Li2O2 and excess I3- redox mediator during discharge and charge, respectively.

Aprotic Lithium-Oxygen Batteries Based on Nonsolid Discharge Products

Aprotic lithium-oxygen (Li-O2) batteries are considered to be a promising alternative option to lithium-ion batteries for high gravimetric energy storage devices. However, the sluggish electrochemical kinetics, the passivation, and the structural damage to the cathode caused by the solid discharge products have greatly hindered the practical application of Li-O2 batteries. Herein, the nonsolid-state discharge products of the off-stoichiometric Li1-xO2 in the electrolyte solutions are achieved by iridium (Ir) single-atom-based porous organic polymers (termed as Ir/AP-POP) as a homogeneous, soluble electrocatalyst for Li-O2 batteries. In particular, the numerous atomic active sites act as the main nucleation sites of O2-related discharge reactions, which are favorable to interacting with O2-/LiO2 intermediates in the electrolyte solutions, owing to the highly similar lattice-matching effect between the in situ-formed Ir3Li and LiO2, achieving a nonsolid LiO2 as the final discharge product in the electrolyte solutions for Li-O2 batteries. Consequently, the Li-O2 battery with a soluble Ir/AP-POP electrocatalyst exhibits an ultrahigh discharge capacity of 12.8 mAh, an ultralow overpotential of 0.03 V, and a long cyclic life of 700 h with the carbon cloth cathode. The manipulation of nonsolid discharge products in aprotic Li-O2 batteries breaks the traditional growth mode of Li2O2, bringing Li-O2 batteries closer to being a viable technology.

Theoretical investigation of tube-like supramolecular structures formed through bifurcated lithium bonds

The stability of three supramolecular naostructures, which are formed through the aggregation of identical belts of [12] arene containing p-nitrophenyllithium, 1,4-dilithiatedbenzene and 1,4-dinitrobenzene units, is investigated by density functional theory. The electrostatic potential calculations indicate the ability of these belts in forming bifurcated lithium bonds (BLBs) between the Li atoms of one belt and the oxygen atoms of the NO2 groups in the other belt, which is also confirmed by deformation density maps and quantum theory of atoms in molecules (QTAIM) analysis. Topological analysis and natural bond analysis (NBO) imply to ionic character for these BLBs with binding energies up to approximately - 60 kcal mol-1. The many-body interaction energy analysis shows the strong cooperativity belongs to the configuration with the highest symmetry (C4v) containing p-nitrophenyllithium fragments as the building unit. Therefore, it seems that this configuration could be a good candidate for designing a BLB-based supramolecular nanotube with infinite size in this study.

Reversible Discharge Products in Li-Air Batteries

Lithium-air (Li-air) batteries stand out among the post-Li-ion batteries due to their high energy density, which has rapidly progressed in the past years. Regarding the fundamental mechanism of Li-air batteries that discharge products produced and decomposed during charging and recharging progress, the reversibility of products closely affects the battery performance. Along with the upsurge of the mainstream discharge products lithium peroxide, with devoted efforts to screening electrolytes, constructing high-efficiency cathodes, and optimizing anodes, much progress is made in the fundamental understanding and performance. However, the limited advancement is insufficient. In this case, the investigations of other discharge products, including lithium hydroxide, lithium superoxide, lithium oxide, and lithium carbonate, emerge and bring breakthroughs for the Li-air battery technologies. To deepen the understanding of the electrochemical reactions and conversions of discharge products in the battery, recent advances in the various discharge products, mainly focusing on the growth and decomposition mechanisms and the determining factors are systematically reviewed. The perspectives for Li-air batteries on the fundamental development of discharge products and future applications are also provided.

Evolving aprotic Li-air batteries

Lithium-air batteries (LABs) have attracted tremendous attention since the proposal of the LAB concept in 1996 because LABs have a super high theoretical/practical specific energy and an infinite supply of redox-active materials, and are environment-friendly. However, due to the lack of critical electrode materials and a thorough understanding of the chemistry of LABs, the development of LABs entered a germination period before 2010, when LABs research mainly focused on the development of air cathodes and carbonate-based electrolytes. In the growing period, i.e., from 2010 to the present, the investigation focused more on systematic electrode design, fabrication, and modification, as well as the comprehensive selection of electrolyte components. Nevertheless, over the past 25 years, the development of LABs has been full of retrospective steps and breakthroughs. In this review, the evolution of LABs is illustrated along with the constantly emerging design, fabrication, modification, and optimization strategies. At the end, perspectives and strategies are put forward for the development of future LABs and even other metal-air batteries.

Exclusive Solution Discharge in Li-O2 Batteries?

Capacity, rate performance, and cycle life of aprotic Li-O₂ batteries critically depend on reversible electrodeposition of Li₂O₂. Current understanding states surface-adsorbed versus solvated LiO₂ controls Li₂O₂ growth as surface film or as large particles. Herein, we show that Li₂O₂ forms across a wide range of electrolytes, carbons, and current densities as particles via solution-mediated LiO₂ disproportionation, bringing into question the prevalence of any surface growth under practical conditions. We describe a unified O₂ reduction mechanism, which can explain all found capacity relations and Li₂O₂ morphologies with exclusive solution discharge. Determining particle morphology and achievable capacities are species mobilities, true areal rate, and the degree of LiO₂ association in solution. Capacity is conclusively limited by mass transport through the tortuous Li₂O₂ rather than electron transport through a passivating Li₂O₂ film. Provided that species mobilities and surface growth are high, high capacities are also achieved with weakly solvating electrolytes, which were previously considered prototypical for low capacity via surface growth.

Reactions in non-aqueous alkali and alkaline-earth metal-oxygen batteries: a thermodynamic study

Multivalent aprotic metal-oxygen batteries are a novel concept in the applied electrochemistry field. These systems are variants of the so-called Li-air batteries and up to present are in their research infancy. The superoxide disproportionation reaction is a crucial step for the operation of any metal-oxygen redox system using aprotic solvents: in the best scenario, disproportionation leads to peroxide formation while in the worse one it releases singlet molecular oxygen. In this work we address the fundamental thermodynamics of such reaction for alkali (Li, Na and K) and alkaline earth (Be, Mg and Ca) metal-O₂ systems using multiconfigurational ab initio methods. Our aim is to draw a comprehensive description of the disproportionation reaction from superoxides to peroxides and to provide the thermodynamic likelihood of the pathways to singlet oxygen release.

Study of the Electronic Structure of Alkali Peroxides and Their Role in the Chemistry of Metal-Oxygen Batteries

We use a multiconfigurational and correlated ab initio method to investigate the fundamental electronic properties of the peroxide MO₂^– (M = Li and Na) trimer to provide new insights into the rather complex chemistry of aprotic metal–O₂ batteries. These electrochemical systems are largely based on the electronic properties of superoxide and peroxide of alkali metals. The two compounds differ by stoichiometry: the superoxide is characterized by a M⁺O₂^– formula, while the peroxide is characterized by [M⁺]₂O₂^2–. We show here that both the peroxide and superoxide states necessarily coexist in the MO₂^– trimer and that they correspond to their different electronic states. The energetic prevalence of either one or the other and the range of their coexistence over a subset of the MO₂^– nuclear configurations is calculated and described via a high-level multiconfigurational approach.

Singlet Oxygen in Electrochemical Cells: A Critical Review of Literature and Theory

Rechargeable metal/O₂ batteries have long been considered a promising future battery technology in automobile and stationary applications. However, they suffer from poor cyclability and rapid degradation. A recent hypothesis is the formation of singlet oxygen (¹O₂) as the root cause of these issues. Validation, evaluation, and understanding of the formation of ¹O₂ are therefore essential for improving metal/O₂ batteries. We review literature and use Marcus theory to discuss the possibility of singlet oxygen formation in metal/O₂ batteries as a product from (electro)chemical reactions. We conclude that experimental evidence is yet not fully conclusive, and side reactions can play a major role in verifying the existence of singlet oxygen. Following an in-depth analysis based on Marcus theory, we conclude that ¹O₂ can only originate from a chemical step. A direct electrochemical generation, as proposed by others, can be excluded on the basis of theoretical arguments.

Lithium-Oxygen Batteries and Related Systems: Potential, Status, and Future

The goal of limiting global warming to 1.5 °C requires a drastic reduction in CO₂ emissions across many sectors of the world economy. Batteries are vital to this endeavor, whether used in electric vehicles, to store renewable electricity, or in aviation. Present lithium-ion technologies are preparing the public for this inevitable change, but their maximum theoretical specific capacity presents a limitation. Their high cost is another concern for commercial viability. Metal-air batteries have the highest theoretical energy density of all possible secondary battery technologies and could yield step changes in energy storage, if their practical difficulties could be overcome. The scope of this review is to provide an objective, comprehensive, and authoritative assessment of the intensive work invested in nonaqueous rechargeable metal-air batteries over the past few years, which identified the key problems and guides directions to solve them. We focus primarily on the challenges and outlook for Li-O₂ cells but include Na-O₂, K-O₂, and Mg-O₂ cells for comparison. Our review highlights the interdisciplinary nature of this field that involves a combination of materials chemistry, electrochemistry, computation, microscopy, spectroscopy, and surface science. The mechanisms of O₂ reduction and evolution are considered in the light of recent findings, along with developments in positive and negative electrodes, electrolytes, electrocatalysis on surfaces and in solution, and the degradative effect of singlet oxygen, which is typically formed in Li-O₂ cells.

Potassium Doping Facilitated Formation of Tunable Superoxides in Li2O2 for Improved Electrochemical Kinetics

Superoxide (O₂^-) species play a crucial role in determining the charge kinetics for aprotic lithium-oxygen (Li-O₂) batteries. However, the growth of O₂^--rich lithium peroxide (Li₂O₂) is challenging since O₂^- is thermodynamically unfavorable and unstable in an O₂ atmosphere. Herein, we reported the synthesis of defective Li₂O₂ with tunable O₂^- via K⁺ doping. The K⁺ dopants can successfully stabilize O₂^- species and induce the coordination of Li⁺ with O₂^-, leading to increased Li vacancies. Compared to the pristine Li₂O₂, the as-prepared defective Li₂O₂ can be charged at a lower overpotential in Li-O₂ batteries, which is ascribed to further increased Li vacancies contributed by the depotassiation process at the onset of the charge process. Our findings suggest a new strategy to better control O₂^- species in Li₂O₂ by K⁺ dopants and provide insights into the K⁺ effects on charge mechanism in Li-O₂ batteries.

Pathways to Triplet or Singlet Oxygen during the Dissociation of Alkali Metal Superoxides: Insights by Multireference Calculations of Molecular Model Systems

Recent experimental investigations demonstrated the generation of singlet oxygen during charging at high potentials in lithium/oxygen batteries. To contribute to the understanding of the underlying chemical reactions a key step in the mechanism of the charging process, namely, the dissociation of the intermediate lithium superoxide to oxygen and lithium, was investigated. Therefore, the corresponding dissociation paths of the molecular model system lithium superoxide (LiO₂ ) were studied by CASSCF/CASPT2 calculations. The obtained results indicate the presence of different dissociation paths over crossing points of different electronic states, which lead either to the energetically preferred generation of triplet oxygen or the energetically higher lying formation of singlet oxygen. The dissociation to the corresponding superoxide anion is energetically less preferred. The understanding of the detailed reaction mechanism allows the design of strategies to avoid the formation of singlet oxygen and thus to potentially minimize the degradation of materials in alkali metal/oxygen batteries. The calculations demonstrate a qualitatively similar but energetically shifted behavior for the homologous alkali metals sodium and potassium and their superoxide species. Fundamental differences were found for the covalently bound hydroperoxyl radical.

Relieving the "Sudden Death" of Li-O2 Batteries by Grafting an Antifouling Film on Cathode Surfaces

The "sudden-death" phenomenon has been frequently encountered during discharging of Li-O₂ batteries and has been ascribed to the growth of a blocking film of Li₂O₂ on the cathode surface. Recent fundamental study revealed that this dilemma could be addressed by discharging Li₂O₂ in the electrolyte solution rather than on the cathode surface. However, even for Li-O₂ batteries operated under the conditions favorable for the solution growth of Li₂O₂, sudden death still persists and its origin remains incompletely understood. Herein, by using a combination of in situ spectroscopy and theoretical calculation, we reveal that sudden death of Li-O₂ batteries operated under the conditions (e.g., low discharge current density and high donor number electrolyte solvent) favorable for discharging Li₂O₂ in the electrolyte solutions is caused by adventitious adsorption of a minor quantity of Li₂O₂, which triggers a rapid transition of Li₂O₂ growth mode from solution- to surface-mediated growth. Moreover, a cathode surface modification strategy has been developed to effectively retard the Li₂O₂ adsorption and therefore significantly alleviate the sudden death of Li-O₂ batteries.

Understanding the Reaction Chemistry during Charging in Aprotic Lithium-Oxygen Batteries: Existing Problems and Solutions

The aprotic lithium-oxygen (Li-O₂ ) battery has excited huge interest due to it having the highest theoretical energy density among the different types of rechargeable battery. The facile achievement of a practical Li-O₂ battery has been proven unrealistic, however. The most significant barrier to progress is the limited understanding of the reaction processes occurring in the battery, especially during the charging process on the positive electrode. Thus, understanding the charging mechanism is of crucial importance to enhance the Li-O₂ battery performance and lifetime. Here, recent progress in understanding the electrochemistry and chemistry related to charging in Li-O₂ batteries is reviewed along with the strategies to address the issues that exist in the charging process at the present stage. The properties of Li₂ O₂ and the mechanisms of Li₂ O₂ oxidation to O₂ on charge are discussed comprehensively, as are the accompanied parasitic chemistries, which are considered as the underlying issues hindering the reversibility of Li-O₂ batteries. Based on the detailed discussion of the charging mechanism, innovative strategies for addressing the issues for the charging process are discussed in detail. This review has profound implications for both a better understanding of charging chemistry and the development of reliable rechargeable Li-O₂ batteries in the future.

Free-Standing Air Cathodes Based on 3D Hierarchically Porous Carbon Membranes: Kinetic Overpotential of Continuous Macropores in Li-O2 Batteries

Free-standing macroporous air electrodes with enhanced interfacial contact, rapid mass transport, and tailored deposition space for large amounts of Li₂ O₂ are essential for improving the rate performance of Li-O₂ batteries. An ordered mesoporous carbon membrane with continuous macroporous channels was prepared by inversely topological transformation from ZnO nanorod array. Utilized as a free-standing air cathode for Li-O₂ battery, the hierarchically porous carbon membrane shows superior rate performance. However, the increased cross-sectional area of the continuous macropores on the cathode surface leads to a kinetic overpotential with large voltage hysteresis and linear voltage variation against Butler-Volmer behavior. The kinetics were investigated based on the rate-determining step of second electron transfer accompanied by migration of Li⁺ in solid or quasi-solid intermediates. These discoveries shed light on the design of the air cathode for Li-O₂ batteries with high-rate performance.

Recent advances in understanding of the mechanism and control of Li2O2formation in aprotic Li–O2batteries

This review systematically summarizes the recent advances in the mechanism studies and control strategies of Li₂O₂formation in aprotic Li–O₂batteries.

Tuning the Morphology and Crystal Structure of Li2O2: A Graphene Model Electrode Study for Li-O2 Battery

The performance and the cyclability of the Li-O2 batteries are strongly affected by the morphology and crystal structure of Li2O2 produced during discharge. In order to explore the details of growth and electrochemical decomposition of Li2O2, and its relationship with the cell performance, graphene films were used as model carbon electrodes and compared with electrodeposited Pd nanoparticles (NPs) on graphene. Multiple methods, including transmission/scanning electron microscopy (TEM/SEM), Raman spectroscopy, electrochemical impedance spectroscopy (EIS), and coin cell charge/discharge test, were employed for material characterization and reaction monitoring. The results showed that the presence of Pd NPs significantly changed the growth, morphology, and crystal structure of Li2O2 and reduced the charge overpotential by 1060 mV. All of these changes are ascribed to the stronger binding energy between LiO2 and the Pd surface, resulting in the generation of amorphous Li2O2 with higher ionic conductivity of Li(+) and O2(2-), which in turn improve the cell charging performance.

An atomistically informed mesoscale model for growth and coarsening during discharge in lithium-oxygen batteries

An atomistically informed mesoscale model is developed for the deposition of a discharge product in a Li-O2 battery. This mescocale model includes particle growth and coarsening as well as a simplified nucleation model. The model involves LiO2 formation through reaction of O2(-) and Li(+) in the electrolyte, which deposits on the cathode surface when the LiO2 concentration reaches supersaturation in the electrolyte. A reaction-diffusion (rate-equation) model is used to describe the processes occurring in the electrolyte and a phase-field model is used to capture microstructural evolution. This model predicts that coarsening, in which large particles grow and small ones disappear, has a substantial effect on the size distribution of the LiO2 particles during the discharge process. The size evolution during discharge is the result of the interplay between this coarsening process and particle growth. The growth through continued deposition of LiO2 has the effect of causing large particles to grow ever faster while delaying the dissolution of small particles. The predicted size evolution is consistent with experimental results for a previously reported cathode material based on activated carbon during discharge and when it is at rest, although kinetic factors need to be included. The approach described in this paper synergistically combines models on different length scales with experimental observations and should have applications in studying other related discharge processes, such as Li2O2 deposition, in Li-O2 batteries and nucleation and growth in Li-S batteries.

Lithium-Air Batteries with Hybrid Electrolytes

During the past decade, Li-air batteries with hybrid electrolytes have attracted a great deal of attention because of their exceptionally high capacity. Introducing aqueous solutions and ceramic lithium superionic conductors to Li-air batteries can circumvent some of the drawbacks of conventional Li-O2 batteries such as decomposition of organic electrolytes, corrosion of Li metal from humidity, and insoluble discharge product blocking the air electrode. The performance of this smart design battery depends essentially on the property and structure of the cell components (i.e., hybrid electrolyte, Li anode, and air cathode). In recent years, extensive efforts toward aqueous electrolyte-based Li-air batteries have been dedicated to developing the high catalytic activity of the cathode as well as enhancing the conductivity and stability of the hybrid electrolyte. Herein, the progress of all aspects of Li-air batteries with hybrid electrolytes is reviewed. Moreover, some suggestions and concepts for tailored design that are expected to promote research in this field are provided.

Effects of carbon surface topography on the electrode/electrolyte interface structure and relevance to Li–air batteries

Carbon surface topography influences the solvent structure at the interface, concentration distribution of reactants (Li⁺, O₂), and their absorption kinetics.

Electronic Structure of Sodium Superoxide Bulk, (100) Surface, and Clusters using Hybrid Density Functional: Relevance for Na-O2 Batteries

Clarifying the electronic structure of sodium superoxide (NaO2) is a key step in understanding the electrochemical behavior of Na-O2 batteries. Here we report a density functional theory study to explore the effect of atomic structure and morphology on the electronic properties of different model systems: NaO2 bulk, (100) surface, and small (NaO2)n clusters (n = 3-8). We found that a correct description of the open-shell 2p electrons of O2(-) requires the use of a hybrid functional, which reveals a clear insulating nature of all of the investigated systems. This sheds light onto the capacity limitations of pure NaO2 as a discharge product and highlights the need for developing new strategies to enhance its electron transport in the optimization of Na-O2 cells.

Interfacial effects on lithium superoxide disproportionation in Li-O₂ batteries

During the cycling of Li-O2 batteries the discharge process gives rise to dynamically evolving agglomerates composed of lithium-oxygen nanostructures; however, little is known about their composition. In this paper, we present results for a Li-O2 battery based on an activated carbon cathode that indicate interfacial effects can suppress disproportionation of a LiO2 component in the discharge product. High-intensity X-ray diffraction and transmission electron microscopy measurements are first used to show that there is a LiO2 component along with Li2O2 in the discharge product. The stability of the discharge product was then probed by investigating the dependence of the charge potential and Raman intensity of the superoxide peak with time. The results indicate that the LiO2 component can be stable for possibly up to days when an electrolyte is left on the surface of the discharged cathode. Density functional calculations on amorphous LiO2 reveal that the disproportionation process will be slower at an electrolyte/LiO2 interface compared to a vacuum/LiO2 interface. The combined experimental and theoretical results provide new insight into how interfacial effects can stabilize LiO2 and suggest that these interfacial effects may play an important role in the charge and discharge chemistries of a Li-O2 battery.

A mesoporous catalytic membrane architecture for lithium-oxygen battery systems

Controlling the mesoscale geometric configuration of catalysts on the oxygen electrode is an effective strategy to achieve high reversibility and efficiency in Li-O2 batteries. Here we introduce a new Li-O2 cell architecture that employs a catalytic polymer-based membrane between the oxygen electrode and the separator. The catalytic membrane was prepared by immobilization of Pd nanoparticles on a polyacrylonitrile (PAN) nanofiber membrane and is adjacent to a carbon nanotube electrode loaded with Ru nanoparticles. During oxide product formation, the insulating PAN polymer scaffold restricts direct electron transfer to the Pd catalyst particles and prevents the direct blockage of Pd catalytic sites. The modified Li-O2 battery with a catalytic membrane showed a stable cyclability for 60 cycles with a capacity of 1000 mAh/g and a reduced degree of polarization (∼ 0.3 V) compared to cells without a catalytic membrane. We demonstrate the effects of a catalytic membrane on the reaction characteristics associated with morphological and structural features of the discharge products via detailed ex situ characterization.

Formation of Li3O4 nano particles in the discharge products of non-aqueous lithium–oxygen batteries leads to lower charge overvoltage

Li₃O₄ nano particles can be formed and exist stably as byproducts during discharge of a non-aqueous lithium–oxygen battery.

Kinetics of Oxygen Reduction in Aprotic Li-O2 Cells: A Model-Based Study

A comprehensive and general kinetic model is developed for the oxygen reduction reaction in aprotic Li-O2 cells. The model is based on the competitive uptake of lithium superoxide by the surface and solution. A demonstrative kinetic study is provided to demystify the origin of curvature in Tafel plots as well as the current dependency and aberrant diversity of the nature and morphology of discharge products in these systems. Our results are general and extend to any system where solubilization of superoxide is favored, such as where phase-transfer catalysts play an important role.

Raman Evidence for Late Stage Disproportionation in a Li-O2 Battery

Raman spectroscopy is used to characterize the composition of toroids formed in an aprotic Li-O2 cell based on an activated carbon cathode. The trends in the Raman data as a function of discharge current density and charging cutoff voltage provide evidence that the toroids are made up of outer LiO2-like and inner Li2O2 regions, consistent with a disproportionation reaction occurring in the solid phase. The LiO2-like component is found to be associated with a new Raman peak identified in the carbon stretching region at ∼1505 cm(-1), which appears only when the LiO2 peak at 1123 cm(-1) is present. The new peak is assigned to distortion of the graphitic ring stretching due to coupling with the LiO2-like component based on density functional calculations. These new results on the LiO2-like component from Raman spectroscopy provide evidence that a late stage disproportionation mechanism can occur during discharge and add new understanding to the complexities of possible processes occurring in Li-O2 batteries.

Catalytic Cycle Employing a TEMPO-Anion Complex to Obtain a Secondary Mg-O2 Battery

Nonaqueous Mg-O2 batteries are suitable only as primary cells because MgO precipitates formed during discharging are not decomposed electrochemically at ambient temperatures. To address this problem, the present study examined the ability of the 2,2,6,6-tetramethylpiperidine-oxyl (TEMPO)-anion complex to catalyze the decomposition of MgO. It was determined that this complex was capable of chemically decomposing MgO at 60 °C. A catalytic cycle for the realization of a rechargeable Mg-O2 electrode was designed by combining the decomposition of MgO via the TEMPO-anion complex and the TEMPO-redox couple. This work also demonstrates that a nonaqueous Mg-O2 battery incorporating acrylate polymer having TEMPO side units in the cathode shows evidence of being rechargeable.