Mechanism and performance of lithium-oxygen batteries - a perspective

Insights into Nano- and Micro-Structured Scaffolds for Advanced Electrochemical Energy Storage

Adopting a nano- and micro-structuring approach to fully unleashing the genuine potential of electrode active material benefits in-depth understandings and research progress toward higher energy density electrochemical energy storage devices at all technology readiness levels. Due to various challenging issues, especially limited stability, nano- and micro-structured (NMS) electrodes undergo fast electrochemical performance degradation. The emerging NMS scaffold design is a pivotal aspect of many electrodes as it endows them with both robustness and electrochemical performance enhancement, even though it only occupies complementary and facilitating components for the main mechanism. However, extensive efforts are urgently needed toward optimizing the stereoscopic geometrical design of NMS scaffolds to minimize the volume ratio and maximize their functionality to fulfill the ever-increasing dependency and desire for energy power source supplies. This review will aim at highlighting these NMS scaffold design strategies, summarizing their corresponding strengths and challenges, and thereby outlining the potential solutions to resolve these challenges, design principles, and key perspectives for future research in this field. Therefore, this review will be one of the earliest reviews from this viewpoint.

Homogeneous Catalytic Process of a Heterogeneous Ru Catalyst in Li-O2 via X-ray Nanodiffraction Observation

In recent years, lithium oxygen batteries (Li-O2) have received considerable research attention due to their extremely high energy density. However, the poor conductivity and ion conductivity of the discharge product lithium peroxide (Li2O2) result in a high charging overpotential, poor cycling stability, and low charging rate. Therefore, studying and improving catalysts is a top priority. This study focuses on the commonly used heterogeneous catalyst ruthenium (Ru). The local distribution of this catalyst is controlled by using sputtering technology. Moreover, X-ray nanodiffraction is applied to observe the relationship between the decomposition of Li2O2 and the local distribution of Ru. Results show that Li2O2 decomposes homogeneously in liquid systems and heterogeneously in solid-state systems. This study finds that the catalytic effect of Ru is related to electrolyte decomposition and that its soluble byproducts act as electron acceptors or redox mediators, effectively reducing charging overpotential but also shortening the cycle life.

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.

Insights into the LiI Redox Mediation in Aprotic Li-O2 Batteries: Solvation Effects and Singlet Oxygen Evolution

Lithium-oxygen aprotic batteries (aLOBs) are highly promising next-generation secondary batteries due to their high theoretical energy density. However, the practical implementation of these batteries is hindered by parasitic reactions that negatively impact their reversibility and cycle life. One of the challenges lies in the oxidation of Li2O2, which requires large overpotentials if not catalyzed. To address this issue, redox mediators (RMs) have been proposed to reduce the oxygen evolution reaction (OER) overpotentials. In this study, we focus on a lithium iodide RM and investigate its role on the degradation chemistry and the release of singlet oxygen in aLOBs, in different solvent environments. Specifically, we compare the impact of a polar solvent, dimethyl sulfoxide (DMSO), and a low polarity solvent, tetraglyme (G4). We demonstrate a strong interplay between solvation, degradation, and redox mediation in OER by LiI in aLOBs. The results show that LiI in DMSO-based electrolytes leads to extensive degradation and to 1O2 release, affecting the cell performance, while in G4-based electrolytes, the release of 1O2 appears to be suppressed, resulting in better cyclability.

To DISP or Not? The Far-Reaching Reaction Mechanisms Underpinning Lithium-Air Batteries

The short history of research on Li-O2 batteries has seen a remarkable number of mechanistic U-turns over the years. From the initial use of carbonate electrolytes, that were then found to be entirely unsuitable, to the belief that (su)peroxide was solely responsible for degradation, before the more reactive singlet oxygen was found to form, to the hypothesis that capacity depends on a competing surface/solution mechanism before a practically exclusive solution mechanism was identified. Herein, we argue for an ever-fresh look at the reported data without bias towards supposedly established explanations. We explain how the latest findings on rate and capacity limits, as well as the origin of side reactions, are connected via the disproportionation (DISP) step in the (dis)charge mechanism. Therefrom, directions emerge for the design of electrolytes and mediators on how to suppress side reactions and to enable high rate and high reversible capacity.

A Computational Study on Halogen/Halide Redox Mediators and Their Role in 1O2 Release in Aprotic Li-O2 Batteries

We present a computational study on the redox reactions of small clusters of Li superoxide and peroxide in the presence of halogen/halide redox mediators. The study is based on DFT calculations with a double hybrid functional and an implicit solvent model. It shows that iodine is less effective than bromine in the oxidation of Li2O2 to oxygen. On the basis of our thermodynamic data, in solvents with a low dielectric constant, iodine does not spontaneously promote either the oxidation of Li2O2 or the release of singlet oxygen, while bromine could spontaneously trigger both events. When a solvent with a large dielectric constant is used, both halogens appear to be able, at least on the basis of thermodynamics, to react spontaneously with the oxides, and the ensuing reaction sequence turned out to be strongly exoergic, thereby providing a route for the release of significant amounts of singlet oxygen. The role of spin-orbit coupling in providing a mechanism for singlet-triplet intersystem crossing has also been assessed.

Influence of Ion Diffusion on the Lithium-Oxygen Electrochemical Process and Battery Application Using Carbon Nanotubes-Graphene Substrate

Lithium-oxygen (Li-O2) batteries are nowadays among the most appealing next-generation energy storage systems in view of a high theoretical capacity and the use of transition-metal-free cathodes. Nevertheless, the practical application of these batteries is still hindered by limited understanding of the relationships between cell components and performances. In this work, we investigate a Li-O2 battery by originally screening different gas diffusion layers (GDLs) characterized by low specific surface area (<40 m2 g-1) with relatively large pores (absence of micropores), graphitic character, and the presence of a fraction of the hydrophobic PTFE polymer on their surface (<20 wt %). The electrochemical characterization of Li-O2 cells using bare GDLs as the support indicates that the oxygen reduction reaction (ORR) occurs at potentials below 2.8 V vs Li+/Li, while the oxygen evolution reaction (OER) takes place at potentials higher than 3.6 V vs Li+/Li. Furthermore, the relatively high impedance of the Li-O2 cells at the pristine state remarkably decreases upon electrochemical activation achieved by voltammetry. The Li-O2 cells deliver high reversible capacities, ranging from ∼6 to ∼8 mA h cm-2 (referred to the geometric area of the GDLs). The Li-O2 battery performances are rationalized by the investigation of a practical Li+ diffusion coefficient (D) within the cell configuration adopted herein. The study reveals that D is higher during ORR than during OER, with values depending on the characteristics of the GDL and on the cell state of charge. Overall, D values range from ∼10-10 to ∼10-8 cm2 s-1 during the ORR and ∼10-17 to ∼10-11 cm2 s-1 during the OER. The most performing GDL is used as the support for the deposition of a substrate formed by few-layer graphene and multiwalled carbon nanotubes to improve the reaction in a Li-O2 cell operating with a maximum specific capacity of 1250 mA h g-1 (1 mA h cm-2) at a current density of 0.33 mA cm-2. XPS on the electrode tested in our Li-O2 cell setup suggests the formation of a stable solid electrolyte interphase at the surface which extends the cycle life.

Aprotic Sulfur-Metal Batteries: Lithium and Beyond

Metal-sulfur batteries constitute an extraordinary research playground that ranges from fundamental science to applied technologies. However, besides the widely explored Li-S system, a remarkable lack of understanding hinders advancements and performance in all other metal-sulfur systems. In fact, similarities and differences make all generalizations highly inconsistent, thus unavoidably suggesting the need for extensive research explorations for each formulation. Here we review critically the most remarkable open challenges that still hinder the full development of metal-S battery formulations, starting from the lithium benchmark and addressing Na, K, Mg, and Ca metal systems. Our aim is to draw an updated picture of the recent efforts in the field and to shed light on the most promising innovation paths that can pave the way to breakthroughs in the fundamental comprehension of these systems or in battery performance.

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.

A Review of High-Energy Density Lithium-Air Battery Technology: Investigating the Effect of Oxides and Nanocatalysts

In vehicles that require a lot of electricity, such as electric vehicles, it is necessary to use high-energy batteries. Among the developed batteries, the lithium-ion battery has shown better performance. This battery has an energy density of 10 equal to that of a lithium-ion battery and uses air oxygen as the active material of the cathode and anode like a lithium-ion battery made of lithium metal. The cathode used in these batteries must have special properties such as strong catalytic activity and high conductivity, and nanotechnology has greatly helped to improve the materials used in the cathode of lithium-air batteries. The importance of proper catalyst distribution and the relationship between the oxide product and the catalyst and the indirect effect of the ORR catalyst on the OER reaction is not present in the fuel cell. The maximum capacity of lithium-air battery theory using graphene under optimal electron conduction conditions and the experimental maximum obtained for graphene by optimizing the structure geometry, examples of structural engineering using carbon fiber and carbon nanotubes in cathode fabrication with the ability to perform the reaction properly while providing space for lithium oxide placement, are examined. This article describes the mechanism of this battery, and its components are examined. The challenges of using this battery and the application of nanotechnology to solve these challenges are also discussed.

Shifting Target Reaction from Oxygen Reduction to Superoxide Disproportionation by Tuning Isomeric Configuration of Quinone Derivative as Redox Mediator for Lithium-Oxygen Batteries

Quinones having a fully conjugated cyclic dione structure have been used as redox mediators in electrochemistry. 2,5-Ditert-butyl-1,4-benzoquinone (DBBQ or DB-p-BQ) as a para-quinone derivative is one of the representative discharge redox mediators for facilitating the oxygen reduction reaction (ORR) kinetics in lithium-oxygen batteries (LOBs). Herein, we presented that the redox activity of DB-p-BQ for electron mediation was possibly used for facilitating superoxide disproportionation reaction (SODR) by tuning the isomeric configuration of the carbonyl groups of the substituted quinone to change its reduction potentials. First, we expected a molecule having its reduction potential between oxygen/superoxide at 2.75 V versus Li/Li⁺ and superoxide/peroxide at 3.17 V to play a role of the SODR catalyst by transferring an electron from one superoxide (O₂^-) to another superoxide to generate dioxygen (O₂) and peroxide (O₂^2-). By changing the isomeric configuration from para (DB-p-BQ) to ortho (DB-o-BQ), the reduction potential of the first electron transfer (Q/Q^-) of the ditert-butyl benzoquinone shifted positively to the potential range of the SODR catalyst. The electrocatalytic SODR-promoting functionality of DB-o-BQ kept the reactive superoxide concentration below a harmful level to suppress superoxide-triggered side reaction, improving the cycling durability of LOBs, which was not achieved by the para form. The second electron transfer process (Q^-/ Q^2-) of the DB-o-BQ, even if the same process of the para form was not used for facilitating ORR, played a role of mediating electrons between electrode and oxygen like the Q/Q^- process of the para form. The ORR-promoting functionality of the ortho form increased the LOB discharge capacity and reduced the ORR overpotential.

Lithium superoxide encapsulated in a benzoquinone anion matrix

Lithium peroxide is the crucial storage material in lithium-air batteries. Understanding the redox properties of this salt is paramount toward improving the performance of this class of batteries. Lithium peroxide, upon exposure to p-benzoquinone (p-C6H4O2) vapor, develops a deep blue color. This blue powder can be formally described as [Li2O2][Formula: see text] [LiO2][Formula: see text] {Li[p-C6H4O2]}0.7, though spectroscopic characterization indicates a more nuanced structural speciation. Infrared, Raman, electron paramagnetic resonance, diffuse-reflectance ultraviolet-visible and X-ray absorption spectroscopy reveal that the lithium salt of the benzoquinone radical anion forms on the surface of the lithium peroxide, indicating the occurrence of electron and lithium ion transfer in the solid state. As a result, obligate lithium superoxide is formed and encapsulated in a shell of Li[p-C6H4O2] with a core of Li2O2 Lithium superoxide has been proposed as a critical intermediate in the charge/discharge cycle of Li-air batteries, but has yet to be isolated, owing to instability. The results reported herein provide a snapshot of lithium peroxide/superoxide chemistry in the solid state with redox mediation.

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.

Suppressing Singlet Oxygen Formation during the Charge Process of Li-O2 Batteries with a Co3O4 Solid Catalyst Revealed by Operando Electron Paramagnetic Resonance

Aprotic lithium-oxygen (Li-O₂) batteries promise high energy, but the cycle life has been plagued by two major obstacles, the insulating products and highly reactive singlet oxygen (¹O₂), which cause higher overpotential and parasitic reactions, respectively. A solid-state catalyst is known to reduce overpotential; however, it is unclear whether it affects ¹O₂ generation. Herein, Co₃O₄ was employed as the representative catalyst in Li-O₂ batteries, and ¹O₂ generation was investigated by ex-situ and operando electron paramagnetic resonance (EPR) spectroscopy. By comparing a carbon nanotube (CNT) cathode with a Co₃O₄/CNT cathode, we find that ¹O₂ generation in the charge process can be suppressed by the Co₃O₄ catalyst. After carefully studying the discharge products on the two electrodes and the corresponding decomposition processes, we conclude that a LiO₂-like species is responsible for the ¹O₂ generation during the early charge stage. The Co₃O₄ catalyst reduces the amount of LiO₂-like species in discharge products, and thus the ¹O₂ formation is suppressed.

Clear Representation of Surface Pathway Reactions at Ag Nanowire Cathodes in All-Solid Li-O2 Batteries

All-solid Li-O₂ batteries have been constructed with Ag nanowire (AgNW) cathodes coated on Au-buffered garnet ceramic electrolytes and Li anodes on the other sides. Benefiting from the clean contacts of Li⁺, e^-, and O₂ on the AgNWs, the surface pathway reactions are demonstrated. Upon discharge, two types of Li₂O₂ morphologies appear. The film-like Li₂O₂ forms around the smooth surfaces of AgNWs, and hollow disk-like Li₂O₂ forms at the joints in between the AgNWs as well as at the garnet/AgNW interfaces. The formation of films and hollow disks is in accordance with the process of O₂ + Li⁺ + e^- → LiO₂ and 2LiO₂ → Li₂O₂ + O₂, indicating that the disproportionation of LiO₂ occurs at the solid interfaces. During the initial charge, decomposition occurs below the potential of 3.5 V, indicating the process of Li₂O₂ → LiO₂ + Li⁺ + e^- and LiO₂ → Li⁺ + e^- + O₂ rather than Li₂O₂ → 2Li⁺ + 2e^- + O₂. The Li₂O₂ decomposition starts at the AgNWs/Li₂O₂ interfaces, causing the film-like Li₂O₂ to shrink and the gas to release, followed by the collapse of hollow disk-like Li₂O₂. The results here clearly disclose the Li-O₂ reaction mechanism at the all-solid interfaces, facilitating a deep understanding of key factors influencing the electrochemical performance of the solid-state Li-O₂ batteries.

Single-side functionalized graphene as promising cathode catalysts in nonaqueous lithium-oxygen batteries

High-performance cathode catalysts are always desirable for nonaqueous lithium-oxygen (Li-O₂) batteries. Using density functional theory calculations, the structural, electronic, and magnetic properties of SSX-Gr with different C/X ratios (X = H or F) are systematically studied. Then, a detailed mechanism on the dissociation of O₂ and the migration of Li on the SSX-Gr is revealed, based on which C₆X and C₈X are confirmed as the potential candidates as cathode catalysts. The studies on reaction pathways suggest that the four-electron pathway is the more possible catalytic pathway for the selected SSX-Gr. The free energy diagrams for discharging and charging processes catalyzed by SSX-Gr reveal that C₆F exhibits the highest application potential due to its considerably low oxygen reduction overpotential (0.83 V) and oxygen evolution overpotential (1.47 V). The extra spins induced by single-side functionalization endow graphene with the electrocatalytic activity.

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.

Product formation during discharge: a combined modelling and experimental study for Li–O$$_2$$ cathodes in LiTFSI/DMSO and LiTFSI/TEGDME electrolytes

Li–air or Li–$$\text{O}_2$$ O 2 batteries are a promising energy storage technology due to the potentially high energy density. However, significant challenges related to reversible charge/discharge of these cells need to be solved. The discharge reaction is generally agreed to proceed via two main routes, which may occur simultaneously. These are the surface mechanism, leading to $$\text{Li}_2\text{O}_2$$ Li 2 O 2 product formation as surface films, or the solution mechanism, with solid particles formed in the pore structure of the cathode. A detailed understanding of the reaction mechanisms and the dynamic performance of the electrodes is key to further improvements. Here, we present a mathematical model for the discharge process, based on porous electrode theory, including effects of reactant transport and kinetic limitations, as well as the continuous change of properties due to the formation of reaction products via the solution mechanism and the surface mechanism. The model describes the dynamic change in the ratio of the surface and solution mechanism as a function of growth of film thickness, in line with recent findings. The model is able to predict the differences in experimentally obtained discharge curves between dimethyl sulfoxide and tetra ethylene glycol dimethyl ether solvents with 1M LiTFSI, with a minimum of free parameters. The model parameters are based on physical characterization of the materials and the electrodes, or determined by fitting to impedance spectra recorded during the discharge. The developed model and the methodology will provide a powerful tool for optimization of such electrodes.

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Theoretical evidence of water serving as a promoter for lithium superoxide disproportionation in Li-O2 batteries

Experimental evidence has demonstrated that the presence of water in non-aqueous electrolytes significantly affects Li-O2 electrochemistry. Understanding the reaction mechanism for Li2O2 formation in the presence of water impurities is important to understand Li-O2 battery performance. A recent experiment has found that very small amounts of water (as low as 40 ppm) can significantly affect the product formation in Li-O2 batteries as opposed to essentially no water (1 ppm). Although experimental as well as theoretical work has proposed mechanisms of Li2O2 formation in the presence of much larger amounts of water, none of the mechanisms provide an explanation for the observations for very small amounts of water. In this work, density functional theory (DFT) was utilized to obtain a mechanistic understanding of the Li-O2 discharge chemistry in a dimethoxyethane (DME) electrolyte containing an isolated water and no water. The reaction pathways for Li2O2 formation from LiO2 on a model system were carefully evaluated with different level of theories, i.e. PBE (PW), B3LYP/6-31G(2df,p), B3LYP/6-311++G(2df,p) and G4MP2. The results indicate that the LiO2 disproportionation reaction to Li2O2 can be promoted by the water in DME electrolyte, which explains why there is a significant difference compared to when no water is present in the experimentally observed discharge product distributions. Ab initio molecular dynamics calculations were also used to investigate the disproportionation of LiO2 dimer in explicit DME. This work adds to the fundamental understanding of the discharge chemistry of a Li-O2 battery.

Isotopic Depth Profiling of Discharge Products Identifies Reactive Interfaces in an Aprotic Li-O2 Battery with a Redox Mediator

Prior to the practical application of rechargeable aprotic Li-O₂ batteries, the high charging overpotentials of these devices (which inevitably cause irreversible parasitic reactions) must be addressed. The use of redox mediators (RMs) that oxidatively decompose the discharge product, Li₂O₂, is one promising solution to this problem. However, the mitigating effect of RMs is currently insufficient, and so it would be beneficial to clarify the Li₂O₂ reductive growth and oxidative decomposition mechanisms. In the present work, Nanoscale secondary ion mass spectrometry (Nano-SIMS) isotopic three-dimensional imaging and differential electrochemical mass spectrometry (DEMS) analyses of individual Li₂O₂ particles established that both growth and decomposition proceeded at the Li₂O₂/electrolyte interface in a system containing the Br^-/Br₃^- redox couple as the RM. The results of this study also indicated that the degree of oxidative decomposition of Li₂O₂ was highly dependent on the cell voltage. These data show that increasing the RM reaction rate at the Li₂O₂/electrolyte interface is critical to improve the cycle life of Li-O₂ batteries.

In situ small-angle X-ray scattering reveals solution phase discharge of Li-O2 batteries with weakly solvating electrolytes

Electrodepositing insulating lithium peroxide (Li₂O₂) is the key process during discharge of aprotic Li-O₂ batteries and determines rate, capacity, and reversibility. Current understanding states that the partition between surface adsorbed and dissolved lithium superoxide governs whether Li₂O₂ grows as a conformal surface film or larger particles, leading to low or high capacities, respectively. However, better understanding governing factors for Li₂O₂ packing density and capacity requires structural sensitive in situ metrologies. Here, we establish in situ small- and wide-angle X-ray scattering (SAXS/WAXS) as a suitable method to record the Li₂O₂ phase evolution with atomic to submicrometer resolution during cycling a custom-built in situ Li-O₂ cell. Combined with sophisticated data analysis, SAXS allows retrieving rich quantitative structural information from complex multiphase systems. Surprisingly, we find that features are absent that would point at a Li₂O₂ surface film formed via two consecutive electron transfers, even in poorly solvating electrolytes thought to be prototypical for surface growth. All scattering data can be modeled by stacks of thin Li₂O₂ platelets potentially forming large toroidal particles. Li₂O₂ solution growth is further justified by rotating ring-disk electrode measurements and electron microscopy. Higher discharge overpotentials lead to smaller Li₂O₂ particles, but there is no transition to an electronically passivating, conformal Li₂O₂ coating. Hence, mass transport of reactive species rather than electronic transport through a Li₂O₂ film limits the discharge capacity. Provided that species mobilities and carbon surface areas are high, this allows for high discharge capacities even in weakly solvating electrolytes. The currently accepted Li-O₂ reaction mechanism ought to be reconsidered.

Performance and Sustainability Tradeoffs of Oxidized Carbon Nanotubes as a Cathodic Material in Lithium-Oxygen Batteries

Climate change mitigation efforts will require a portfolio of solutions, including improvements to energy storage technologies in electric vehicles and renewable energy sources, such as the high-energy-density lithium-oxygen battery (LOB). However, if LOB technology will contribute to addressing climate change, improvements to LOB performance must not come at the cost of disproportionate increases in global warming potential (GWP) or cumulative energy demand (CED) over their lifecycle. Here, oxygen-functionalized multi-walled carbon nanotube (O-MWCNT) cathodes were produced and assessed for their initial discharge capacities and cyclability. Contrary to previous findings, the discharge capacity of O-MWCNT cathodes increased with the ratio of carbonyl/carboxyl moieties, outperforming pristine MWCNTs. However, increased oxygen concentrations decreased LOB cyclability, while high-temperature annealing increased both discharge capacity and cyclability. Improved performance resulting from MWCNT post-processing came at the cost of increased GWP and CED, which in some cases was disproportionately higher than the level of improved performance. Based on the findings presented here, there is a need to simultaneously advance research in improving LOB performance while minimizing or mitigating the environmental impacts of LOB production.

Boosting the Performance of Graphene Cathodes in Na-O2 Batteries by Exploiting the Multifunctional Character of Small Biomolecules

Graphene aerogels derived from a biomolecule-assisted aqueous electrochemical exfoliation route are explored as cathode materials in sodium-oxygen (Na-O₂ ) batteries. To this end, the natural nucleotide adenosine monophosphate (AMP) is used in the multiple roles of exfoliating electrolyte, aqueous dispersant, and functionalizing agent to access high quality, electrocatalytically active graphene nanosheets in colloidal suspension (bioinks). The surface phenomena occurring on the electrochemically derived graphene cathode is thoroughly studied to understand and optimize its electrochemical performance, where a cooperative effect between the nitrogen atoms and phosphates from the AMP molecules is demonstrated. Moreover, the role of the nitrogen atoms in the adenine nucleobase of AMP and short-chain phosphate is unraveled. Significantly, the use of such cathodes with a proper amount of AMP molecules adsorbed on the graphene nanosheets delivers a discharge capacity as high as 9.6 mAh cm^-2 and performs almost 100 cycles with a considerably reduced cell overpotential and a coulombic efficiency of ≈97% at high current density (0.2 mA cm^-2 ). This study opens a path toward the development of environmentally friendly air cathodes by the use of natural nucleotides which offers a great opportunity to explore and manufacture bioinspired cathodes for metal-oxygen batteries.

Electrode Protection in High-Efficiency Li-O2 Batteries

The aprotic Li-O2 battery possessing the highest theoretical energy density, approaching that of gasoline, has been regarded as one of the most promising successors to Li-ion batteries. Before this kind of battery can become a viable technology, a series of critical issues need to be conquered, like low round-trip efficiency and short cycling lifetime, which are closely related to the continuous parasitic processes happening at the cathode and anode during cycling. With an aim to promote the practical application of Li-O2 batteries, great effort has been devoted to identify the reasons for oxygen and lithium electrodes degradation and provide guidelines to overcome them. Thus, the stability of cathode and anode has been improved a lot in the past decade, which in turn significantly boosts the electrochemical performances of Li-O2 batteries. Here, an overlook on the electrode protection in high-efficiency Li-O2 batteries is presented by providing first the challenges of electrodes facing and then the effectiveness of the existing approaches that have been proposed to alleviate these. Moreover, new battery systems and perspectives of the viable near-future strategies for rational configuration and balance of the electrodes are also pointed out. This Outlook deepens our understanding of the electrodes in Li-O2 batteries and offers opportunities for the realization of high performance and long-term durability of Li-O2 batteries.

Organogermanium Nanowire Cathodes for Efficient Lithium-Oxygen Batteries

We report a technique for effectively neutralizing the generation of harmful superoxide species, the source of parasitic reactions, in lithium-oxygen batteries to generate stable substances. In organic electrolytes, organogermanium (Propa-germanium, Ge-132) nanowires can suppress solvated superoxide and induce strong surface-adsorption reaction due to their high anti-superoxide disproportionation activity. Resultantly, the effect of organogermanium nanowires mitigate toxic oxidative stress to stabilize organic electrolytes and promote good Li₂O₂ growth. These factors led to long duration of the electrolytes and impressive rechargeability of lithium-oxygen batteries.

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.

Perovskite LaCoxMn1-xO3-σ with Tunable Defect and Surface Structures as Cathode Catalysts for Li-O2 Batteries

Rechargeable lithium-oxygen batteries have shown great potential as next-generation sustainable and green energy storage systems. The bifunctional catalyst plays an important role in accelerating the cathode kinetics for practical realization of the batteries. Herein, we employ the surface structure and defect engineering to introduce surface-roughened nanolayers and oxygen vacancies on the mesoporous hollow LaCo_xMn_1-xO_3-σ perovskite catalyst by in situ cation substitution. The experimental results show that the O₂-electrode with the LaCo_0.75Mn_0.25O_3-σ catalyst exhibits an extremely high discharge capacity of 10,301 mA h g^-1 at 200 mA g^-1 for the initial cycle and superior cycling stability under a capacity limit of 500 mA h g^-1 together with a low voltage gap of 1.12 V. Good electrochemical performance of LaCo_0.75Mn_0.25O_3-σ can be attributed to the synergistic effect of the hierarchical mesoporous hollow structure and the abundant oxygen vacancies all over the catalyst surface. We reveal that the modified surface structure can provide more accessibility of active sites to promote electrochemical reactions, and the introduced oxygen vacancy can serve as an efficient substrate for binding intermediate products and decomposition reactions of Li₂O₂ during discharge and charge processes. Our methodology provides meaningful insights into the rational design of highly active perovskite catalysts in energy storage/conversion systems.

Current Challenges and Routes Forward for Nonaqueous Lithium-Air Batteries

Nonaqueous lithium-air batteries have garnered considerable research interest over the past decade due to their extremely high theoretical energy densities and potentially low cost. Significant advances have been achieved both in the mechanistic understanding of the cell reactions and in the development of effective strategies to help realize a practical energy storage device. By drawing attention to reports published mainly within the past 8 years, this review provides an updated mechanistic picture of the lithium peroxide based cell reactions and highlights key remaining challenges, including those due to the parasitic processes occurring at the reaction product-electrolyte, product-cathode, electrolyte-cathode, and electrolyte-anode interfaces. We introduce the fundamental principles and critically evaluate the effectiveness of the different strategies that have been proposed to mitigate the various issues of this chemistry, which include the use of solid catalysts, redox mediators, solvating additives for oxygen reaction intermediates, gas separation membranes, etc. Recently established cell chemistries based on the superoxide, hydroxide, and oxide phases are also summarized and discussed.

Electrochemical Oxidation of Li2O2 Surface-Doped with Li2CO3

Electrochemical oxidation of Li₂O₂, i.e., the charging reaction of the aprotic lithium-oxygen batteries (Li-O₂ batteries), is significantly influenced by its surface chemistry. Here, the surface species of Li₂CO₃, widely identified together with Li₂O₂ at the end of discharge, is investigated to understand its implication for the oxidation of Li₂O₂. In situ doping Li₂O₂ with various amounts of Li₂CO₃ has been obtained by reacting with CO₂ gas in a controlled way, and the electrochemical oxidation of the doped Li₂O₂ is studied with a quantitative differential electrochemical mass spectrometer (DEMS). Instead of a single charging potential plateau and one O₂ gas evolution stage for the pristine Li₂O₂, Li₂CO₃-doped Li₂O₂ exhibits two O₂/CO₂ gas evolution stages and three charging plateaus characterized with the larger overpotential for the initial and final stages. The conductivity of Li₂CO₃ dopant is invoked to explain the different oxidation behaviors of Li₂CO₃-doped Li₂O₂. The DEMS study of the electrochemical oxidation of isotope-labeled Li₂¹³CO₃ is also conducted to identify the origins of O₂ and CO₂ evolution during the oxidation of Li₂CO₃-doped Li₂O₂. The results reported here provide an improved understanding of the Li₂O₂ oxidation in the presence of parasitic Li₂CO₃ species and will contribute to the future development of Li-O₂ batteries.

Implications of Testing a Zinc-Oxygen Battery with Zinc Foil Anode Revealed by Operando Gas Analysis

Zinc-oxygen batteries are seen as promising energy storage devices for future mobile and stationary applications. Introducing them as secondary battery is hindered by issues at both the anode and cathode. Research efforts were intensified during the past two decades, mainly focusing on catalyst materials for the cathode. Thereby, zinc foil was almost exclusively used as the anode in electrochemical testing in the lab-scale as it is easy to apply and shall yield reproducible results. However, it is well known that zinc metal reacts with water within the electrolyte to form hydrogen. It is not yet clear how the evolution of hydrogen is affecting the performance results obtained thereof. Herein, we extend the studies and the understanding about the evolution of hydrogen at zinc by analyzing the zinc-oxygen battery during operation. By means of electrochemical measurements, operando gas analysis, and anode surface analysis, we elucidate that the rate of the evolution of hydrogen scales with the current density applied, and that the roughness of the anode surface, that is, the pristine state of the zinc foil surface, affects the rate as well. In the end, we propose a link between the evolution of hydrogen and the unwanted impact on the actual electrochemical performance that might go unnoticed during testing. Thereof, we elucidate the consequences that arise for the working principle and the testing of materials for this battery type.

Prolonging the Cycle Life of a Lithium-Air Battery by Alleviating Electrolyte Degradation with a Ceramic-Carbon Composite Cathode

Carbon materials with a high specific surface area are usually preferred to construct the air cathode of lithium-air batteries due to their abundant sites for oxygen reduction and discharge product growth. However, the high surface area also amplifies electrolyte degradation during charging, which would become the threshold of cyclability after addressing the issue of electrode passivation and pore clogging, but is usually overlooked in relevant research. Herein, it is proven that the critical influence of cathode surface area on electrolyte consumption by adopting carbon-ceramic composites to reduce the surface area of the air cathode. After screening several potential ceramic materials, an optimal composite of Ketjenblack (KB) and La_0.7 Sr_0.3 MnO₃ (LSM) delivered a discharge capacity that was even higher than that of pure KB. This composite effectively mitigated the parasitic reaction current by 45 % if polarized at 4.4 V versus Li⁺ /Li. Correspondingly, this composite prolonged the cycle life of the cell by 156 %. The results demonstrate that electrolyte consumption during charging is one of the critical boundary conditions to restrain the cyclic stability of lithium-air batteries.

Advanced Lithium Metal-Carbon Nanotube Composite Anode for High-Performance Lithium-Oxygen Batteries

The low Coulombic efficiency and hazardous dendrite growth hinder the adoption of lithium anode in high-energy density batteries. Herein, we report a lithium metal-carbon nanotube (Li-CNT) composite as an alternative to the long-term untamed lithium electrode to address the critical issues associated with the lithium anode in Li-O₂ batteries, where the lithium metal is impregnated in a porous carbon nanotube microsphere matrix (CNTm) and surface-passivated with a self-assembled monolayer of octadecylphosphonic acid as a tailor-designed solid electrolyte interphase (SEI). The high specific surface area of the Li-CNT composite reduces the local current density and thus suppresses the lithium dendrite formation upon cycling. Moreover, the tailor-designed SEI effectively separates the Li-CNT composite from the electrolyte solution and prevents the latter's further decomposition. When the Li-CNT composite anode is coupled with another CNTm-based O₂ cathode, the reversibility and cycle life of the resultant Li-O₂ batteries are drastically elevated.

Biomimetic Superoxide Disproportionation Catalyst for Anti-Aging Lithium-Oxygen Batteries

Reactive oxygen species or superoxide (O₂^-), which damages or ages biological cells, is generated during metabolic pathways using oxygen as an electron acceptor in biological systems. Superoxide dismutase (SOD) protects cells from superoxide-triggered apoptosis by converting superoxide to oxygen and peroxide. Lithium-oxygen battery (LOB) cells have the same aging problems caused by superoxide-triggered side reactions. We transplanted the function of SOD of biological systems into LOB cells. Malonic acid-decorated fullerene (MA-C₆₀) was used as a superoxide disproportionation chemocatalyst mimicking the function of SOD. As expected, MA-C₆₀ as the superoxide scavenger improved capacity retention along charge/discharge cycles successfully. A LOB cell that failed to provide a meaningful capacity just after several cycles at high current (0.5 mA cm^-2) with 0.5 mAh cm^-2 cutoff survived up to 50 cycles after MA-C₆₀ was introduced to the electrolyte. Moreover, the SOD-mimetic catalyst increased capacity, e.g., more than a 6-fold increase at 0.2 mA cm^-2. The experimentally observed toroidal morphology of the final discharge product of oxygen reduction (Li₂O₂) and density functional theory calculation confirmed that the solution mechanism of Li₂O₂ formation, more beneficial than the surface mechanism from the capacity-gain standpoint, was preferred in the presence of MA-C₆₀.