Controlling Solution-Mediated Reaction Mechanisms of Oxygen Reduction Using Potential and Solvent for Aprotic Lithium-Oxygen 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.

Enhancing the Performance of Lithium-Oxygen Batteries with Quasi-Solid Polymer Electrolytes

The quasi-solid electrolyte membranes (QSEs) are obtained by solidifying the precursor of unsaturated polyester and liquid electrolyte in a glass fiber. By modifying the ratio of tetraethylene glycol dimethyl ether, QSE with balanced ionic conductivity, flexibility, and electrochemical stability window is acquired, which is helpful for inhibiting the decomposition of electrolyte on the cathode surface. The QSE is beneficial to the interfacial reaction of Li+, electrons, and O2 in the quasi-solid lithium-oxygen battery (LOB), can reduce the crossover of oxygen to the anode, and extend the cycle life of LOBs to 317 cycles. Benefitting from the application of QSE, a more stable solid electrolyte interface layer can be constructed on the anode side, which can homogenize Li+ flux and facilitate uniform Li deposition. Lithium-oxygen pouch cell with in situ formed QSE2 works well when the cell is folded or a corner is cut off. Our results indicate that the QSE plays important roles in both the cathode and Li metal anode, which can be further improved with the in situ forming strategy.

Triarylmethyl cation redox mediators enhance Li-O2 battery discharge capacities

A major impediment to Li-O2 battery commercialization is the low discharge capacities resulting from electronically insulating Li2O2 film growth on carbon electrodes. Redox mediation offers an effective strategy to drive oxygen chemistry into solution, avoiding surface-mediated Li2O2 film growth and extending discharge lifetimes. As such, the exploration of diverse redox mediator classes can aid the development of molecular design criteria. Here we report a class of triarylmethyl cations that are effective at enhancing discharge capacities up to 35-fold. Surprisingly, we observe that redox mediators with more positive reduction potentials lead to larger discharge capacities because of their improved ability to suppress the surface-mediated reduction pathway. This result provides important structure-property relationships for future improvements in redox-mediated O2/Li2O2 discharge capacities. Furthermore, we applied a chronopotentiometry model to investigate the zones of redox mediator standard reduction potentials and the concentrations needed to achieve efficient redox mediation at a given current density. We expect this analysis to guide future redox mediator exploration.

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.

Hydrogen-Bond-Assisted Solution Discharge in Aprotic Li-O2 Batteries

Surface discharge mechanism induced cathode passivation is a critical challenge that blocks the full liberation of the ultrahigh theoretical energy density in aprotic Li-O₂ batteries. Herein, a facile and universal concept of hydrogen-bond-assisted solvation is proposed to trigger the solution discharge process for averting the shortcomings associated with surface discharge. 2,5-Di-tert-butylhydroquinone (DBHQ), an antioxidant with hydroxyl groups, is introduced as an exemplary soluble catalyst to promote solution discharge by hydrogen-bond-assisted solvation of O₂ ^- and Li₂ O₂ (OH···O). Thus, a Li-O₂ battery with 50 × 10^-3 m DBHQ delivers an extraordinary discharge capacity of 18 945 mAh g^-1 (i.e., 9.47 mAh cm^-2 ), even surpassing the capacity endowed by the state-of-the-art reduction mediator of 2,5-di-tert-butyl-1,4-benzoquinone. Besides, an ultrahigh Li₂ O₂ yield of 97.1% is also achieved due to the depressed reactivity of the reduced oxygen-containing species (O₂ ^- , LiO₂ , and Li₂ O₂ ) by the solvating and antioxidative abilities of DBHQ. Consequently, the Li-O₂ battery with DBHQ exhibits excellent cycling lifetime and rate capability. Furthermore, the generalizability of this approach of hydrogen-bond-assisted solution discharge is verified by other soluble catalysts that contain OH or NH groups, with implications that could bring Li-O₂ batteries one step closer to being a viable technology.

Soluble and Perfluorinated Polyelectrolyte for Safe and High-Performance Li-O2 Batteries

The severe performance degradation of high-capacity Li-O₂ batteries induced by Li dendrite growth and concentration polarization from the low Li⁺ transfer number of conventional electrolytes hinder their practical applications. Herein, lithiated Nafion (LN) with the sulfonic group immobilized on the perfluorinated backbone has been designed as a soluble lithium salt for preparing a less flammable polyelectrolyte solution, which not only simultaneously achieves a high Li⁺ transfer number (0.84) and conductivity (2.5 mS cm^-1 ), but also the perfluorinated anion of LN produces a LiF-rich SEI for protecting the Li anode from dendrite growth. Thus, the Li-O₂ battery with a LN-based electrolyte achieves an all-round performance improvement, like low charge overpotential (0.18 V), large discharge capacity (9508 mAh g^-1 ), and excellent cycling performance (225 cycles). Besides, the fabricated pouch-type Li-air cells exhibit promising applications to power electronic equipment with satisfactory safety.

Recent advances in heterostructured cathodic electrocatalysts for non-aqueous Li-O2 batteries

Developing efficient energy storage and conversion applications is vital to address fossil energy depletion and global warming. Li-O2 batteries are one of the most promising devices because of their ultra-high energy density. To overcome their practical difficulties including low specific capacities, high overpotentials, limited rate capability and poor cycle stability, an intensive search for highly efficient electrocatalysts has been performed. Recently, it has been reported that heterostructured catalysts exhibit significantly enhanced activities toward the oxygen reduction reaction and oxygen evolution reaction, and their excellent performance is not only related to the catalyst materials themselves but also the special hetero-interfaces. Herein, an overview focused on the electrocatalytic functions of heterostructured catalysts for non-aqueous Li-O2 batteries is presented by summarizing recent research progress. Reduction mechanisms of Li-O2 batteries are first introduced, followed by a detailed discussion on the typical performance enhancement mechanisms of the heterostructured catalysts with different phases and heterointerfaces, and the various heterostructured catalysts applied in Li-O2 batteries are also intensively discussed. Finally, the existing problems and development perspectives on the heterostructure applications are presented.

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.

From bulk to interface: electrochemical phenomena and mechanism studies in batteries via electrochemical quartz crystal microbalance

Understanding the bulk and interfacial behaviors during the operation of batteries (e.g., Li-ion, Na-ion, Li-O₂ batteries, etc.) is of great significance for the continuing improvement of the performance. Electrochemical quartz crystal microbalance (EQCM) is a powerful tool to this end, as it enables in situ investigation into various phenomena, including ion insertion/deinsertion within electrodes, solid nucleation from the electrolyte, interphasial formation/evolution and solid-liquid coordination. As such, EQCM analysis helps to decipher the underlying mechanisms both in the bulk and at the interface. This tutorial review will present the recent progress in mechanistic studies of batteries achieved by the EQCM technology. The fundamentals and unique capability of EQCM are first discussed and compared with other techniques, and then the combination of EQCM with other in situ techniques is also covered. In addition, the recent studies utilizing EQCM technologies in revealing phenomena and mechanisms of various batteries are reviewed. Perspectives regarding the future application of EQCM in battery studies are given at the end.

Trapped interfacial redox introduces reversibility in the oxygen reduction reaction in a non-aqueous Ca2+ electrolyte

Electrochemical investigations of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have been conducted in a Ca2+-containing dimethyl sulfoxide electrolyte. While the ORR appears irreversible, the introduction of a tetrabutylammonium perchlorate (TBAClO4) co-salt in excess concentrations results in the gradual appearance of a quasi-reversible OER process. Combining the results of systematic cyclic voltammetry investigations, the degree of reversibility depends on the ion pair competition between Ca2+ and TBA+ cations to interact with generated superoxide (O2 -). When TBA+ is in larger concentrations, and large reductive overpotentials are applied, a quasi-reversible OER peak emerges with repeated cycling (characteristic of formulations without Ca2+ cations). In situ Raman microscopy and rotating ring-disc electrode (RRDE) experiments revealed more about the nature of species formed at the electrode surface and indicated the progressive evolution of a charge storage mechanism based upon trapped interfacial redox. The first electrochemical step involves generation of O2 -, followed primarily by partial passivation of the surface by Ca x O y product formation (the dominant initial reaction). Once this product matrix develops, the subsequent formation of TBA+--O2 - is contained within the Ca x O y product interlayer at the electrode surface and, consequently, undergoes a facile oxidation reaction to regenerate O2.

First-Principles Study of the Surfaces and Equilibrium Shape of Discharge Products in Li-Air Batteries

Li-air batteries are a promising alternative to Li-ion batteries as they theoretically provide the highest possible specific energy density. Mainly, Li₂O₂ (lithium peroxide) and to a lesser extent, Li₂O (lithium oxide) are assumed to be the discharge products of these batteries formed with the soluble LiO₂ (lithium superoxide) considered to be an intermediate product. Bulk Li₂O₂ is an electronic insulator, and the precipitation of this compound on the cathode is thought to be the main limiting factor in achieving high capacities in lithium-oxygen cells. For the most promising electrolytes including solvents with high donor numbers, microscopy observations frequently reveal crystallite morphologies of Li₂O₂ compounds, rather than uniform layers covering the electrode surface. The precise morphologies of Li₂O and Li₂O₂ particles, and their effect and their extent of contact with the electrode, which may all affect the capacity and rechargeability, however, remain largely undetermined. Here, we address the stability of various Li₂O and Li₂O₂ surfaces and consequently, their crystallite morphologies using density functional theory calculations and ab initio thermodynamics. In contrast to previous studies, we also consider high-index surface terminations, which exhibit surprisingly low surface energies. We carefully analyze the reasons for the stability of these high-index surfaces, which also prominently influence the equilibrium shape of the particles, at least for Li₂O₂, and discuss the consequences for the observed morphology of the reaction products.

Monodispersed Ruthenium Nanoparticles on Nitrogen-Doped Reduced Graphene Oxide for an Efficient Lithium-Oxygen Battery

Lithium-oxygen batteries with ultrahigh energy densities have drawn considerable attention as next-generation energy storage devices. However, their practical applications are challenged by sluggish reaction kinetics aimed at the formation/decomposition of discharge products on battery cathodes. Developing effective catalysts and understanding the fundamental catalytic mechanism are vital to improve the electrochemical performance of lithium-oxygen batteries. Here, uniformly dispersed ruthenium nanoparticles anchored on nitrogen-doped reduced graphene oxide are prepared by using an in situ pyrolysis procedure as a bifunctional catalyst for lithium-oxygen batteries. The abundance of ruthenium active sites and strong ruthenium-support interaction enable a feasible discharge product formation/decomposition route by modulating the surface adsorption of lithium superoxide intermediates and the nucleation and growth of lithium peroxide species. Benefiting from these merits, the electrode provides a drastically increased discharge capacity (17,074 mA h g^-1), a decreased charge overpotential (0.51 V), and a long-term cyclability (100 cycles at 100 mA g^-1). Our observations reveal the significance of the dispersion and coordination of metal catalysts, shedding light on the rational design of efficient catalysts for future lithium-oxygen batteries.

Mechanism of mediated alkali peroxide oxidation and triplet versus singlet oxygen formation

Aprotic alkali metal-O₂ batteries face two major obstacles to their chemistry occurring efficiently, the insulating nature of the formed alkali superoxides/peroxides and parasitic reactions that are caused by the highly reactive singlet oxygen (¹O₂). Redox mediators are recognized to be key for improving rechargeability. However, it is unclear how they affect ¹O₂ formation, which hinders strategies for their improvement. Here we clarify the mechanism of mediated peroxide and superoxide oxidation and thus explain how redox mediators either enhance or suppress ¹O₂ formation. We show that charging commences with peroxide oxidation to a superoxide intermediate and that redox potentials above ~3.5 V versus Li/Li⁺ drive ¹O₂ evolution from superoxide oxidation, while disproportionation always generates some ¹O₂. We find that ¹O₂ suppression requires oxidation to be faster than the generation of ¹O₂ from disproportionation. Oxidation rates decrease with growing driving force following Marcus inverted-region behaviour, establishing a region of maximum rate.

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.

Synergistic Effect of Binary Electrolyte on Enhancement of the Energy Density in Li-O2 Batteries

Enhancement of the discharge capacity of lithium-oxygen batteries (LOBs) while maintaining a high cell voltage is an important challenge to overcome to achieve an ideal energy density. Both the cell voltage and discharge capacity of an LOB could be controlled by employing a binary solvent electrolyte composed of dimethyl sulfoxide (DMSO) and acetonitrile (MeCN), whereby an energy density 3.2 times higher than that of the 100 vol % DMSO electrolyte was obtained with an electrolyte containing 50 vol % of DMSO. The difference in the solvent species that preferentially solvates Li⁺ and that which controls the adsorption-desorption equilibrium of the discharge reaction intermediate, LiO₂, on the cathode/electrolyte interface provides these unique properties of the binary solvent electrolyte. Combined spectroscopic and electrochemical analysis have revealed that the solvated complex of Li⁺ and the environment of the cathode/electrolyte interface were the determinants of the cell voltage and discharge capacity, respectively.

High-Capacity and Stable Li-O2 Batteries Enabled by a Trifunctional Soluble Redox Mediator

Li-O₂ batteries with ultrahigh theoretical energy densities usually suffer from low practical discharge capacities and inferior cycling stability owing to the cathode passivation caused by insulating discharge products and by-products. Here, a trifunctional ether-based redox mediator, 2,5-di-tert-butyl-1,4-dimethoxybenzene (DBDMB), is introduced into the electrolyte to capture reactive O₂ ^- and alleviate the rigorous oxidative environment of Li-O₂ batteries. Thanks to the strong solvation effect of DBDMB towards Li⁺ and O₂ ^- , it not only reduces the formation of by-products (a high Li₂ O₂ yield of 96.6 %), but also promotes the solution growth of large-sized Li₂ O₂ particles, avoiding the passivation of cathode as well as enabling a large discharge capacity. Moreover, DBDMB makes the oxidization of Li₂ O₂ and the decomposition of main by-products (Li₂ CO₃ and LiOH) proceed in a highly effective manner, prolonging the stability of Li-O₂ batteries (243 cycles at 1000 mAh g^-1 and 1000 mA g^-1 ).

Computational Insights into LixOy Formation, Nucleation, and Adsorption on Carbon Nanotube Electrodes in Nonaqueous Li-O2 Batteries

Recent theoretical and experimental studies have shown that the formation of Li₂O₂, the main discharge product of nonaqueous Li-O₂ batteries, is a complex multistep reaction process. The formation, nucleation, and adsorption of Li_xO_y (x and y = 0, 1, and 2) and (Li₂O₂)_n clusters with n = 1-4 on the surface of carbon nanotubes (CNTs) were investigated by periodic density functional theory calculation. The results showed that both Li₂O₂ and Li₂O on CNT electrodes are preferentially generated by lithiation reaction rather than disproportionation reaction. The free energy profiles demonstrate that the discharge potentials of 2.54 and 1.29 V are the threshold values of spontaneous nucleation of (Li₂O₂)₂ and (Li₂O)₂ on a CNT surface, respectively. The electronic structure indicates that Li₂O₂ is a p-type semiconductor, while Li₂O exhibits the properties of an insulator. Interestingly, once Li₂O₂ molecules condense into large clusters, they will be repelled away from the CNT surface and continue to grow into large-sized Li₂O₂. Our results provide insights into the full understanding of the electrochemical reaction mechanism and product formation processes of lithium oxides in the cathodes of Li-O₂ batteries.

Synergetic Effect of Liquid and Solid Catalysts on the Energy Efficiency of Li-O2 Batteries: Cell Performances and Operando STEM Observations

The sluggish cathodic kinetics and lower energy efficiency, associated with solid and insulating discharge products of Li₂O₂, are the key factors that prevent the practical implementation of Li-O₂ batteries (LOBs). Here we demonstrate that the combination of the solid catalyst (RuO₂) and soluble redox mediator tetrathiafulvalene (TTF) exhibits a synergetic effect in improving the cathodic kinetics and energy efficiency of LOBs by reducing both charge and discharge overpotentials. Operando electron microscopy observations and electrochemical measurements reveal that RuO₂ not only exhibits bifunctional catalysis for Li-O₂ reactions but also benefits the catalytic efficiency of TTF. Meanwhile, TTF plays an important role in activating the Li₂O₂ passivated RuO₂ catalysts and in helping RuO₂ effectively oxidize the discharge products during charging. The synergetic effect of solid and liquid catalysts, beyond traditional bifunctional catalysis, obviously increases the cathodic kinetics and round-trip energy efficiency of LOBs.

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.

Homogeneous nucleation of Li2O2 under Li-O2 battery discharge

The development of high-energy lithium-oxygen batteries has significantly slowed by numerous challenges including capacity limitations due to electrode surface passivation by the discharge product Li₂O₂. Since the passivation rate and intensity are dependent on the deposit morphology, herein, we focus on the mechanisms governing Li₂O₂ formation within the porous cathode. We report evidence of homogeneous nucleation of Li₂O₂ crystallites and their further assembly in bulk of the electrolyte solution in DMSO, which possesses a high donor number. After careful estimation of the superoxide ion concentration distribution within a phenomenological model, it was found that the high stability of superoxide ions formed during the ORR towards disproportionation and sufficient diffusivity of (0.5-1.2) × 10^-6 cm² s^-1 enabled Li₂O₂ nucleation and crystallization not only at the surface but also in the electrolyte, and the reaction zone spread throughout the internal space of the porous electrode. High initial supersaturation promoted the homogeneous nucleation of Li₂O₂ nanoplates, which instantly assembled into mesocrystals also in the solution bulk. These results were supported by operando SAXS/WAXS and morphology observations. Thus, although homogeneous nucleation is not dominant, it is important for achieving a high capacity in Li-O₂ batteries.

Cathode Local Curvature Affects Lithium Peroxide Growth in Li-O2 Batteries

The growth of lithium peroxide (Li₂O₂) in cathodes determines the performance of lithium-oxygen batteries (LOBs). The factor affecting the Li₂O₂ growth position is of great importance. Here, three hollow carbon spheres with diameters of 200 nm, 500 nm, and 2 μm, corresponding to different curvatures of 10, 4, and 1, respectively, are prepared as LOB cathodes. It is found that the larger the curvature, the more difficult it is for Li₂O₂ to grow inside the hollow sphere. Increasing the discharge current density can promote the growth of Li₂O₂ onto a highly curved concave substrate. Therefore, to maximize the battery performance, the applied current density and the local curvature of the porous cathode need to match to optimize the pore space utilization and meanwhile to enhance the interface charge transfer between Li₂O₂ and electrode. The revealed relationship among the local curvature of the porous electrode, Li₂O₂ deposition position, and battery performance is valuable to the topography design of the LOB cathode.

In situ AFM visualization of Li-O2 battery discharge products during redox cycling in an atmospherically controlled sample cell

The in situ observation of electrochemical reactions is challenging due to a constantly changing electrode surface under highly sensitive conditions. This study reports the development of an in situ atomic force microscopy (AFM) technique for electrochemical systems, including the design, fabrication, and successful performance of a sealed AFM cell operating in a controlled atmosphere. Documentation of reversible physical processes on the cathode surface was performed on the example of a highly reactive lithium-oxygen battery system at different water concentrations in the solvent. The AFM data collected during the discharge-recharge cycles correlated well with the simultaneously recorded electrochemical data. We were able to capture the formation of discharge products from correlated electrical and topographical channels and measure the impact of the presence of water. The cell design permitted acquisition of electrochemical impedance spectroscopy, contributing information about electrical double layers under the system's controlled environment. This characterization method can be applied to a wide range of reactive surfaces undergoing transformations under carefully controlled conditions.

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.

Negative differential resistance as a critical indicator for the discharge capacity of lithium-oxygene batteries

In non-aqueous lithium-oxygen batteries, the one-electron reduction of oxygen and subsequent lithium oxide formation both occur during discharge. This lithium oxide can be converted to insulating lithium peroxide via two different pathways: a second reduction at the cathode surface or disproportionation in solution. The latter process is known to be advantageous with regard to increasing the discharge capacity and is promoted by a high donor number electrolyte because of the stability of lithium oxide in media of this type. Herein, we report that the cathodic oxygen reduction reaction during discharge typically exhibits negative differential resistance. Importantly, the magnitude of negative differential resistance, which varies with the system component, and the position of the cathode potential relative to the negative differential resistance determined the reaction pathway and the discharge capacity. This result implies that the stability of lithium oxide on the cathode also contributes to the determination of the reaction pathway.

Applications of 2D MXenes in energy conversion and storage systems

This article provides a comprehensive review of MXene materials and their energy-related applications.

Realizing the Embedded Growth of Large Li2O2 Aggregations by Matching Different Metal Oxides for High-Capacity and High-Rate Lithium Oxygen Batteries

Large Li2O2 aggregations can produce high-capacity of lithium oxygen (Li-O2) batteries, but the larger ones usually lead to less-efficient contact between Li2O2 and electrode materials. Herein, a hierarchical cathode architecture based on different discharge characteristics of α-MnO2 and Co3O4 is constructed, which can enable the embedded growth of large Li2O2 aggregations to solve this problem. Through experimental observations and first-principle calculations, it is found that α-MnO2 nanorod tends to form uniform Li2O2 particles due to its preferential Li+ adsorption and similar LiO2 adsorption energies of different crystal faces, whereas Co3O4 nanosheet tends to simultaneously generate Li2O2 film and Li2O2 nanosheets due to its preferential O2 adsorption and different LiO2 adsorption energies of varied crystal faces. Thus, the composite cathode architecture in which Co3O4 nanosheets are grown on α-MnO2 nanorods can exhibit extraordinary synergetic effects, i.e., α-MnO2 nanorods provide the initial nucleation sites for Li2O2 deposition while Co3O4 nanosheets provide dissolved LiO2 to promote the subsequent growth of Li2O2. Consequently, the composite cathode achieves the embedded growth of large Li2O2 aggregations and thus exhibits significantly improved specific capacity, rate capability, and cyclic stability compared with the single metal oxide electrode.

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.

Mechanistic Insights into the Challenges of Cycling a Nonaqueous Na-O2 Battery

Superoxide-based nonaqueous metal-oxygen batteries have received considerable research attention as they exhibit high energy densities and round-trip efficiencies. The cycling performance, however, is still poor. Here we study the cycling characteristic of a Na-O₂ battery using solid-state nuclear magnetic resonance, Raman spectroscopy, and scanning electron microscopy. We find that the poor cycling performance is primarily caused by the considerable side reactions stemming from the chemical aggressiveness of NaO₂ as both a solid-phase and dissolved species in the electrolyte. The side reaction products cover electrode surfaces and hinder electron transfer across the electrode-electrolyte interface, being a major reason for cell failure. In addition, the available electrode surface and porosity change considerably during cell discharging and charging, affecting the diffusion of soluble species (superoxide and water) and resulting in inhomogeneous reactions across the electrode. This study provides insights into the challenges associated with achieving long-lived superoxide-based metal-O₂ batteries.

The Use of Spray-Dried Mn₃O₄/C Composites as Electrocatalysts for Li-O₂ Batteries

The electrocatalytic activities of Mn₃O₄/C composites are studied in lithium-oxygen (Li-O₂) batteries as cathode catalysts. The Mn₃O₄/C composites are fabricated using ultrasonic spray pyrolysis (USP) with organic surfactants as the carbon sources. The physical and electrochemical performance of the composites is characterized by X-ray diffraction, scanning electron microscopy, particle size analysis, Brunauer-Emmett-Teller (BET) measurements, elemental analysis, galvanostatic charge-discharge methods and rotating ring-disk electrode (RRDE) measurements. The electrochemical tests demonstrate that the Mn₃O₄/C composite that is prepared using Trition X-114 (TX114) surfactant has higher activity as a bi-functional catalyst and delivers better oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic performance in Li-O₂ batteries because there is a larger surface area and particles are homogeneous with a meso/macro porous structure. The rate constant (k_f) for the production of superoxide radical (O₂^•-) and the propylene carbonate (PC)-electrolyte decomposition rate constant (k) for M₃O₄/C and Super P electrodes are measured using RRDE experiments and analysis in the 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆)/PC electrolyte. The results show that TX114 has higher electrocatalytic activity for the first step of ORR to generate O₂^•- and produces a faster PC-electrolyte decomposition rate.

The effect of water on discharge product growth and chemistry in Li-O2 batteries

Understanding what controls Li-O2 battery discharge product chemistry and morphology is key to enabling its practical deployment as a low-cost, high-specific-energy energy conversion technology. Several studies have recently shown that the addition of substantial quantities (hundreds to thousands ppm) of water and weak acids to dimethoxyethane (DME)-based electrolytes can significantly increase Li-O2 battery discharge capacity, without substantially changing the discharge product chemistry, which remains Li2O2. The exact mechanisms behind these device-level improvements, however, are not yet understood. In this study, we show that the presence of water in a DME-based electrolyte decreases the rate of Li2O2 nucleation on the electrode surface during Li-O2 battery discharge, using potentiostatic electrochemical measurements, and direct, ex situ observations of Li2O2 particles. We also show that adding water to an acetonitrile (MeCN)-based electrolyte results in LiOH upon discharge, as opposed to only Li2O2. Using first principles calculations, we propose that this change in discharge product chemistry is attributable to increased proton availability, as shown by a lower pKa for water in MeCN than in DME. This study combines kinetic and morphological analyses with first principles calculations, and elucidates relationships among electrolyte composition, discharge product chemistry and growth mechanisms for the rational design of efficient metal-air batteries.