Operando observation of the gold-electrolyte interface in Li-O2 batteries

Direct Evidence of Reversible Changes in Electrolyte and its Interplay with LiO2 Intermediate in Li-O2 Batteries

Lithium-oxygen batteries show promising energy storage potential with high theoretical energy density; however, further investigation of chemical reactions is required. In this study, experimental Raman and theoretical analyzes are performed for a Li-O2 battery with LiClO4/dimethyl sulfoxide (DMSO) electrolyte and carbon cathode to understand the role of intermediate species in the reactional mechanism of the cell using a high donor number solvent. Operando Raman results reveal reversible changes in the DMSO bands, in addition to the formation and decomposition of Li2O2. On discharge, a decrease in DMSO polarizability is observed and bands of DMSO-Li+-anion interactions are evidenced and supported by ab initio density functional theory (DFT) calculations. Molecular dynamics (MD) force field simulations and operando Raman show that DMSO interacts with LiO2(sol), highlighting the stability of the electrolyte compared to the interaction with reactiveO 2 - ${\rm O}_2^{-}$ . On charging, the presence of Li+ indicates the formation of a lithium-deficient phase, followed by the release of Li+ and oxygen. Therefore, this study contributes to understanding the discharge/charge chemistry of a Li-O2 cell, employing a common carbon cathode and DMSO electrolyte. The combination of a simple characterization technique in operando mode and theoretical studies provides essential information on the mechanism of Li-O2 system.

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.

Tailoring the Growth and Morphology of Lithium Peroxide: Nickel Sulfide/Nickel Phosphate Nanotubes with Optimized Electronic Structure for Lithium-Oxygen Batteries

Heterogeneous crystalline-amorphous structures, with tunable electronic structures and morphology, hold immense promise as catalysts for lithium-oxygen batteries (LOBs). Herein, a nanotube network constructed by crystalline nickel sulfide/amorphous nickel phosphate (NiS/NiPO) heterostructure is prepared on Ni foam through the sulfurization of the precursor generated hydrothermally. Used as cathodes, the NiS/NiPO nanotubes with optimized electronic structure can induce the deposition of the highly porous and interconnected structure of Li2 O2 with rich Li2 O2 -electrolyte interfaces. Abundant active sites can be created on NiS/NiPO through the charge redistribution for the uniform nucleation and growth of Li2 O2 . Moreover, nanotube networks endow cathodes with efficient transport channels and sufficient space for the accommodation of Li2 O2 . A high discharge capacity of 27 003.6 mAh g-1 and a low charge overpotential of 0.58 V at 1000 mAh g-1 can be achieved at 200 mA g-1 . This work provides valuable insight into the unique role of the electronic structure and morphology of catalysts in the formation mechanisms of Li2 O2 and the performances of LOBs.

First-Order or Second-Order? Disproportionation of Lithium Superoxide in Li-O2 Batteries

The disproportionation of LiO₂ to Li₂O₂ is a key step in Li-O₂ batteries, and it is regarded as a second-order reaction. However, its mechanism is not well addressed, and its kinetics is rarely studied due to the difficulties of quantifying the rate constants, particularly for high concentrations of superoxide (>10 mM). Here, we quantified the kinetic rate constant by a microkinetic model using a microelectrode tip with a thin diffusion layer and fast response. We report that the reaction order of LiO₂ transitions from 1 at high concentrations of superoxide (∼20 mM) to 2 at low concentrations of superoxide (∼1 mM). LiO₂ is chemically reduced by free superoxides to form Li₂O₂ and O₂, instead of reacting with another LiO₂ via a disproportionation step. This chemical-reduction mechanism explained the change of reaction order and the kinetics profile. As a rate-determining step, this step restricts the overall kinetics of the discharging process and should be the focus of future catalyst design.

Application of Advanced Vibrational Spectroscopy in Revealing Critical Chemical Processes and Phenomena of Electrochemical Energy Storage and Conversion

The future of the energy industry and green transportation critically relies on exploration of high-performance, reliable, low-cost, and environmentally friendly energy storage and conversion materials. Understanding the chemical processes and phenomena involved in electrochemical energy storage and conversion is the premise of a revolutionary materials discovery. In this article, we review the recent advancements of application of state-of-the-art vibrational spectroscopic techniques in unraveling the nature of electrochemical energy, including bulk energy storage, dynamics of liquid electrolytes, interfacial processes, etc. Technique-wise, the review covers a wide range of spectroscopic methods, including classic vibrational spectroscopy (direct infrared absorption and Raman scattering), external field enhanced spectroscopy (surface enhanced Raman and IR, tip enhanced Raman, and near-field IR), and two-photon techniques (2D infrared absorption, stimulated Raman, and vibrational sum frequency generation). Finally, we provide perspectives on future directions in refining vibrational spectroscopy to contribute to the research frontier of electrochemical energy storage and conversion.

Direct Visualization of Dynamic Mobility of Li2O2 in Li-O2 Batteries: A Differential Interference Microscopy Study

The reversibility and the discharge/charge performance in nonaqueous lithium-oxygen (Li-O₂) batteries are critically dependent on the kinetics of interfacial reactions. However, the interfacial reaction dynamic behaviors, especially the quantitative analysis, are still far from deep understanding. Using the method of laser confocal microscopy combined with differential interference contrast microscopy (LCM-DIM), we monitored the Li-O₂ interfacial reaction and in situ traced the Li₂O₂ migration processes promoted by the solution catalyst. Different dynamic behaviors exist when regulating the concentration of the redox mediator. Quantitative analysis of the discharged Li₂O₂ particles shows high mobility at the early discharge stage and decayed motion in the subsequent process, indicating the solution-mediated pathway participating Li₂O₂ formation in the low-concentration redox mediator addition, while particles/aggregates confined into the amorphous film demonstrate simultaneous solution and surface route-mediated pathway participation in the high-concentration case. These distinctive observations of Li₂O₂ formation and decomposition processes present the advantage of LCM-DIM to fundamentally understand the dynamic evolution in Li-O₂ batteries.

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.

Advanced Spectroelectrochemical Techniques to Study Electrode Interfaces Within Lithium-Ion and Lithium-Oxygen Batteries

Lithium battery technologies have revolutionized mobile energy storage, but improvements in the technology are still needed. Critical to delivering new light weight, high capacity, safe devices is an improved understanding of the dynamic processes occurring at the electrode-electrolyte interfaces. Therefore, alongside advances in materials there has been a parallel progression in advanced characterization methods. Herein, recent developments for operando spectro-electrochemical techniques centered on Raman, infrared, and sum frequency generation are described within the context of lithium-ion and non-aqueous lithium-oxygen battery research. In particular, shell-isolated nanoparticles for enhanced Raman spectroscopy (SHINERS), surface-enhanced infrared absorption spectroscopy (SEIRAS), and near-field infrared are explained and critically evaluated, and future opportunities discussed. The aim is to introduce the wider community to the developing range of methodologies and tools now available in the hope that it encourages greater usage across the sector.

Evaluating chemical bonding in dioxides for the development of metal–oxygen batteries: vibrational spectroscopic trends of dioxygenyls, dioxygen, superoxides and peroxides

Analysis of Raman and IR spectral bands of >200 dioxygen species highlighted the effect of the immediate chemical environment on O–O bonding.

Pt and Pd catalyzed oxidation of Li2O2 and DMSO during Li-O2 battery charging

Rechargeable Li-O2 and Li-air batteries require electrode and electrolyte materials that synergistically promote long-term cell operation. In this study, we investigate the role of noble metals Pt and Pd as catalysts in the Li-O2 oxidation process and their compatibility with dimethyl sulfoxide (DMSO) based electrolytes. We identify a basis for low potential Li2O2 evolution followed by oxidative decomposition of the electrolyte to form carbonate side products.

Heme biomolecule as redox mediator and oxygen shuttle for efficient charging of lithium-oxygen batteries

One of the greatest challenges with lithium-oxygen batteries involves identifying catalysts that facilitate the growth and evolution of cathode species on an oxygen electrode. Heterogeneous solid catalysts cannot adequately address the problematic overpotentials when the surfaces become passivated. However, there exists a class of biomolecules which have been designed by nature to guide complex solution-based oxygen chemistries. Here, we show that the heme molecule, a common porphyrin cofactor in blood, can function as a soluble redox catalyst and oxygen shuttle for efficient oxygen evolution in non-aqueous Li-O2 batteries. The heme's oxygen binding capability facilitates battery recharge by accepting and releasing dissociated oxygen species while benefiting charge transfer with the cathode. We reveal the chemical change of heme redox molecules where synergy exists with the electrolyte species. This study brings focus to the rational design of solution-based catalysts and suggests a sustainable cross-link between biomolecules and advanced energy storage.

Sodium-Oxygen Batteries: A Comparative Review from Chemical and Electrochemical Fundamentals to Future Perspective

Alkali metal-oxygen (Li-O2 , Na-O2 ) batteries have attracted a great deal of attention recently due to their high theoretical energy densities, comparable to gasoline, making them attractive candidates for application in electrical vehicles. However, the limited cycling life and low energy efficiency (high charging overpotential) of these cells hinder their commercialization. The Li-O2 battery system has been extensively studied in this regard during the past decade. Compared to the numerous reports of Li-O2 batteries, the research on Na-O2 batteries is still in its infancy. Although, Na-O2 batteries show a number of attractive properties such as low charging overpotential and high round-trip energy efficiency, their cycling life is currently limited to a few tens of cycles. Therefore, understanding the chemistry behind Na-O2 cells is critical towards enhancing their performance and advancing their development. Chemical and electrochemical reactions of Na-O2 batteries are reviewed and compared with those of Li-O2 batteries in the present review, as well as recent works on the chemical composition and morphology of the discharge products in these batteries. Furthermore, the determining kinetics factors for controlling the chemical composition of the discharge products in Na-O2 cells are discussed and the potential research directions toward improving Na-O2 cells are proposed.

Operando Nanobeam Diffraction to Follow the Decomposition of Individual Li2O2 Grains in a Nonaqueous Li-O2 Battery

Intense interest in the Li-O2 battery system over the past 5 years has led to a much better understanding of the various chemical processes involved in the functioning of this battery system. However, detailed decomposition of the nanostructured Li2O2 product, held at least partially responsible for the limited reversibility and poor rate performance, is hard to measure operando under realistic electrochemical conditions. Here, we report operando nanobeam X-ray diffraction experiments that enable monitoring of the decomposition of individual Li2O2 grains in a working Li-O2 battery. Platelet-shaped crystallites with aspect ratios between 2.2 and 5.5 decompose preferentially via the more reactive (001) facets. The slow and concurrent decomposition of individual Li2O2 crystallites indicates that the Li2O2 decomposition rate limits the charge time of these Li-O2 batteries, highlighting the importance of using redox mediators in solution to charge Li-O2 batteries.

A New Design Strategy for Observing Lithium Oxide Growth-Evolution Interactions Using Geometric Catalyst Positioning

Understanding the catalyzed formation and evolution of lithium-oxide products in Li-O2 batteries is central to the development of next-generation energy storage technology. Catalytic sites, while effective in lowering reaction barriers, often become deactivated when placed on the surface of an oxygen electrode due to passivation by solid products. Here we investigate a mechanism for alleviating catalyst deactivation by dispersing Pd catalytic sites away from the oxygen electrode surface in a well-structured anodic aluminum oxide (AAO) porous membrane interlayer. We observe the cross-sectional product growth and evolution in Li-O2 cells by characterizing products that grow from the electrode surface. Morphological and structural details of the products in both catalyzed and uncatalyzed cells are investigated independently from the influence of the oxygen electrode. We find that the geometric decoration of catalysts far from the conductive electrode surface significantly improves the reaction reversibility by chemically facilitating the oxidation reaction through local coordination with PdO surfaces. The influence of the catalyst position on product composition is further verified by ex situ X-ray photoelectron spectroscopy and Raman spectroscopy in addition to morphological studies.

Guided Evolution of Bulk Metallic Glass Nanostructures: A Platform for Designing 3D Electrocatalytic Surfaces

Electrochemical devices such as fuel cells, electrolyzers, lithium-air batteries, and pseudocapacitors are expected to play a major role in energy conversion/storage in the near future. Here, it is demonstrated how desirable bulk metallic glass compositions can be obtained using a combinatorial approach and it is shown that these alloys can serve as a platform technology for a wide variety of electrochemical applications through several surface modification techniques.

A universal surface enhanced Raman spectroscopy (SERS)-active graphene cathode for lithium–air batteries

A uniform SERS-active graphene electrode was used in lithium–oxygen batteries.

Carbon-Free Cathodes: A Step Forward in the Development of Stable Lithium-Oxygen Batteries

Lithium-oxygen (Li-O2 ) batteries are receiving considerable interest owing to their potential for higher energy densities than current Li-ion systems. However, the lack stability of carbon-based oxygen electrodes is believed to promote carbonate formation leading to capacity fade and limiting the cycling performance of the battery. To improve the stability and cyclability of these systems, alternative electrode materials are required. Metal oxides are mainly utilized at low current densities, whereas noble metals show outstanding performance at high current densities. Carbides appear to provide a good compromise between electrochemical performance and cost, which makes them interesting materials for further investigations. Here, a critical review of current carbon-free electrode research is provided with the goal of identifying routes to its successful optimization.

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

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

DMSO-Li2O2 Interface in the Rechargeable Li-O2 Battery Cathode: Theoretical and Experimental Perspectives on Stability

One of the greatest obstacles for the realization of the nonaqueous Li-O2 battery is finding a solvent that is chemically and electrochemically stable under cell operating conditions. Dimethyl sulfoxide (DMSO) is an attractive candidate for rechargeable Li-O2 battery studies; however, there is still significant controversy regarding its stability on the Li-O2 cathode surface. We performed multiple experiments (in situ XPS, FTIR, Raman, and XRD) which assess the stability of the DMSO-Li2O2 interface and report perspectives on previously published studies. Our electrochemical experiments show long-term stable cycling of a DMSO-based operating Li-O2 cell with a platinum@carbon nanotube core-shell cathode fabricated via atomic layer deposition, specifically with >45 cycles of 40 h of discharge per cycle. This work is complemented by density functional theory calculations of DMSO degradation pathways on Li2O2. Both experimental and theoretical evidence strongly suggests that DMSO is chemically and electrochemically stable on the surface of Li2O2 under the reported operating conditions.

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.