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

Construction of an Oxygen-Vacancy-Rich CeO2@CoO Heterojunction toward High-Performance Lithium-Oxygen Batteries

Lithium-oxygen (Li-O2) batteries theoretically possess an exceptional energy density comparable to gasoline (up to 3500 W h kg-1), but in practical applications, the discharge products are difficult to effectively decompose, which leads to clogging of the cathode, resulting in severe polarization, limited actual capacity, and shortened battery life for Li-O2 batteries. Herein, we construct a highly active and stable catalyst with d-f electronic orbit coupling as a redox center by anchoring CeO2 onto CoO, simultaneously, oxygen vacancy (Ov) and CeO2 coactivated CoO. By leveraging the effects of interface engineering and defect engineering on the electronic structure of the catalyst, the adsorption energy for LiO2 can be adjusted to an ideal range. This not only avoids surface passivation caused by excessively strong binding energy but also overcomes the issue of sluggish Li2O2 decomposition efficiency due to excessively weak binding energy. Bracingly, the CeO2/CoO-based Li-O2 batteries exhibit an ultralow charge-discharge polarization, and Li2O2 was successfully induced to nucleate uniformly in nanoflower-like shapes, which could promote the reversible decomposition of the discharge products during the charging process and thereby enhance the electrochemical performance of Li-O2 batteries. Therefore, the CeO2@CoO/CC cathode exhibited an ultralow overpotential of 0.57 V and achieved a high discharge capacity of 19,850 mA h g-1. This work provides an important reference for designing the structure of cathode catalysts for Li-O2 batteries and regulating the growth paths and morphologies of discharge products.

Synergistic Zn and MoS2 Tailored Co-N/C Environments Enabling Bifunctional ORR/OER Electrocatalysis for Advanced Li-O2 Batteries

To better adapt lithium-oxygen batteries (LOBs) and overcome their sluggish oxygen reduction and evolution reactions (ORR/OER) kinetics, designing efficient bifunctional ORR/OER catalytic materials is essential. In this study, we successfully constructed a bifunctional ZnCo-N/C@MoS2 catalyst by tailoring the Co-N/C center with Zn incorporation and MoS2 encapsulation. Surprisingly, Zn atoms, which are typically considered to promote the Co atoms isolation, exhibit a promoting effect on the ORR performance of Co-N/C centers and enhance their stability under harsh conditions. Introducing MoS2 establishes Mo-N coupling centers, enhancing electron transfer and adjusting the charge density of Co active centers, thereby compensating OER activity limitation of ZnCo-N/C. In Li-O2 batteries, Zn and MoS2 synergistically optimize intermediate interactions and regulate LiO2 formation/decomposition, while Zn's environmental adaptability and MoS2's encapsulating protection jointly enhance operational stability. Results show that ZnCo-N/C@MoS2, serving as the oxygen electrode in Li-O2 batteries, achieves a low overpotential of 1.01 V, an ultra-high specific capacity of 25,026 mAh g-1, and a long cycle life of 298 cycles. This work achieves bifunctionality in single-atom catalysts through precise dual modulation of the catalytic environment, providing a novel strategy for the development of lithium-oxygen batteries.

A Bifunctional Imidazolyl Iodide Mediator of Electrolyte Boosts Cathode Kinetics and Anode Stability Towards Low Overpotential and Long-Life Li-O2 Batteries

The addition of a redox mediator as soluble catalyst into electrolyte can effectively overcome the bottlenecks of poor energy efficiency and limited cyclability for Li-O2 batteries caused by passivation of insulating discharge products and unfavorable byproducts. Herein we report a novel soluble catalyst of bifunctional imidazolyl iodide salt additive, 1,3-dimethylimidazolium iodide (DMII), to successfully construct highly efficient and durable Li-O2 batteries. The anion I- can effectively promote the charge transport of Li2O2 and accelerate the redox kinetics of oxygen reduction/oxygen evolution reactions on the cathode side, thereby significantly decreasing the charge/discharge overpotential. Simultaneously, the cation DMI+ forms an ultrathin stably solid-electrolyte interphase film on Li metal, greatly inhibiting the shuttle effect of I- and improving the stability of anode. Using this DMII additive, our Li-O2 batteries achieve an extremely low voltage of 0.52 V and ultra-long cycling stability over 960 h. Notably, up to 95.8 % of the Li2O2 yield further proves the reversible generation/decomposition of Li2O2 without the occurrence of side reactions. Both experimental and theoretical results disclose that DMII enables Li+ easily solvated, testifying the dominance of the solution-induced reaction mechanism. This work provides the possibility to design the soluble catalysts towards high-performance Li-O2 batteries.

Machine Learning-Guided Modulation of Li+ Solvation Structures towards Optimal Electrolyte Systems for High-Performance Li-O2 Battery

As the "blood" of Li-O2 batteries (LOBs), electrolytes with various solvation structures of Li+ can greatly influence the composition of solid electrolyte interphase (SEI) on anode and the growth kinetics of Li2O2 on cathode, and further the battery performance. However, achieving delicate modulation of the multi-composition electrolytes remains significantly challenging to simultaneously give consideration to both the anode and cathode reactions. In this work, we employed Bayesian optimization to develop advanced electrolytes for LOBs, enabling the formation of a stable inorganic-rich SEI, and modulation of Li2O2 morphologies. Thus obtained LOBs using the optimized dual-solvent electrolyte could deliver a discharge capacity of 14,063 mAh g-1 at a current density of 500 mA g-1, which is far higher than those using the single-solvent electrolytes. This study not only highlights the critical role of the solvation structure for improving the battery performance, but also provides new insights and important theoretical guidance for delicate modulation of electrolyte compositions.

P-block element modulated 1 T phase MoS2 with Ru lattice grafting for high-performance Li | |O2 batteries

The metallic phase MoS2 (1T-MoS2) supported metal-nanocatalyst is an appealing material system for accelerating the redox kinetics of non-aqueous Li | |O2 batteries. However, the drawbacks associated with the surface orbital steric effect and the internal electron coupling results in a detrimental effect for the stability of 1T-MoS2, especially for the interface charge transfer. This makes it difficult to incorporate guest metal nanoparticles without compromising the 1 T phase support. To circumvent these issues, here we propose a p-block element (In-O) doping strategy to stabilize the 1 T phase MoS2 by moderating the surface orbital steric effect and strengthening the internal chemical bonding, and thus for the epitaxial Ru nanocatalyst graft on the stabilized 1T-MoS2 for Li | |O2 batteries. The experimental and theoretical analyzes indicate that the In-O-MoS2@Ru enhances the O2 dissociation and facilitates the adsorption of LiO2 intermediates. This effect promotes the growth of weakly crystalline Li2O2 films during oxygen reduction reaction, which can be more easily decomposed during the oxygen evolution reaction, thereby enhancing the bifunctional-catalytic kinetics. When employed at the positive electrode for non-aqueous Li | |O2 batteries, In-O-MoS2@Ru shows an overpotential of 0.37 V and a cycling life of 284 cycles at 200 mA g-1 with a final discharge specific capacity of 1000 mAh g-1 at 25 °C.

Atomically Dispersed Ta-O-Co Sites Capable of Mitigating Side Reaction Occurrence for Stable Lithium-Oxygen Batteries

The side reactions accompanying the charging and discharging process, as well as the difficulty in decomposing the discharge product lithium peroxide, have been important issues in the research field of lithium-oxygen batteries for a long time. Here, single atom Ta supported by Co3O4 hollow sphere was designed and synthesized as a cathode catalyst. The single atom Ta forms an electron transport channel through the Ta-O-Co structure to stabilize octahedral Co sites, forming strong adsorption with reaction intermediates and ultimately forming a film-like lithium peroxide that is highly dispersed. More importantly, the formation of the Ta-O-Co structure can reduce the vacancy formation energy on the catalyst surface, accelerate oxygen activation and conversion into superoxide anions, promote the rapid conversion of strong oxidizing intermediate lithium superoxide into lithium peroxide, avoid the oxidation of lithium superoxide to the electrode and electrolyte, reduce the occurrence of side reactions, and mitigate the production of byproduct lithium carbonate. The overpotential of the battery is reduced significantly, and the reversibility and cycling stability of the battery are improved. This study provides a practical and feasible direction for mitigating the side reaction and improving the performance of the battery.

N, F Co-Doped Carbon Material Self-Supporting Cathode for High-Performance Lithium-Oxygen Batteries

The Li-O2 battery has emerged as a promising energy storage system due to its exceptionally high theoretical energy density of 3500 Wh kg-1. However, the sluggish kinetics associated with the formation and decomposition of discharge product Li2O2 poses several challenges in Li-O2 batteries, including excessive over-potential, limited rate performance, and reduced actual specific energy. Consequently, the development of cost-effective cathode catalysts with enhanced catalytic activity and long-term stability represents a viable approach to address these challenges. In this study, commercial melamine foam is utilized as a precursor material which was subjected to pyrolysis at elevated temperatures with PVDF to synthesize N, F co-doped self-supporting carbon cathode (NF-NSC). Remarkably, thanks to the synergistic effects of N, F heteroatomic in conjunction with the inherent three-dimensional reticular porous structure, NF-NSC exhibited enhanced electrochemical performance when utilized in Li-O2 batteries. Specifically, the NF-NSC cathode demonstrated an impressive discharge specific capacity of up to 35204 mAh g-1 alongside a low over-potential (0.86 V) and excellent cycling stability (146 cycles).

Robust oxygen adsorbent mediated oxygen redox reactions for high performance lithium-oxygen battery

Lithium-oxygen batteries (LOBs) have been widely studied because of their ultra-high energy density (∼3500 Wh kg-1). However, the reversibility and stability of LOBs are greatly limited by the sluggish kinetics of oxygen reduction/evolution reactions (ORR/OER) and severely parasitic reactions on oxygen electrodes. Electrolyte in LOBs plays an important role in the transport of reactive oxygen species and Li+, which greatly affects the kinetics and reversibility of the charging and discharging processes of batteries. In this work, perfluorooctane (PFO) is used as the additive in 1.0 M LiTFSI/TEGDEM electrolyte for LOBs to regulate the kinetics of oxygen electrode reactions. Due to the strong adsorption ability of PE toward oxygen, the oxygen concentration inside the electrolyte is greatly increased after the addition of PE. In addition, the PE-added electrolyte also exhibits superior electrochemical stability and is capable of triggering solution-mediated Li2O2 growth pathway during the discharge process of the LOBs. Therefore, with the increased oxygen concentration and the optimized electrode/electrolyte interface, the ORR/OER kinetics on the oxygen electrode is significantly promoted, which enables the LOBs with excellent energy efficiency and cycling life. This work provides a new idea for the design of oxygen-rich and high-performance electrolyte for lithium-oxygen batteries.

Harnessing 4f Electron Itinerancy for Integrated Dual-Band Redox Systems Boosts Lithium-Oxygen Batteries Electrocatalysis

In-depth comprehension and manipulation of band occupation at metal centers are crucial for facilitating effective adsorption and electron transfer in lithium-oxygen battery (LOB) reactions. Rare earth elements play a unique role in band hybridization due to their deep orbitals and strong localization of 4 f electrons. Herein, we anchor single Ce atoms onto CoO, constructing a highly active and stable catalyst with d-f a dual-band redox center. It is discovered that the itinerant behavior of 4 f electrons introduces an enhanced spin-orbit coupling effect, which facilitates ideal σ/π bonding and flexible adsorption between the Ce/Co active sites and *O. Simultaneously, the injection of localized Ce 4 f electrons strengthens the orbital bonding capacity of Co-O, effectively inhibits the dissolution of Co sites and improves the structural stability of the cathode material. Bracingly, the Ce1/CoO-based LOB exhibits an ultra-low charge-discharge polarization (0.46 V) and stable cyclic performance (1088 hours). This work breaks through the traditional limitations in catalyst activity and stability, providing new strategies and theoretical insights for developing high-performance LOBs powered by rare-earth elements.

Hf Doping Boosts the Excellent Activity and Durability of Fe-N-C Catalysts for Oxygen Reduction Reaction and Li-O2 Batteries

Developing highly active and durable non-noble metal catalysts is crucial for energy conversion and storage, especially for proton exchange membrane fuel cells (PEMFCs) and lithium-oxygen (Li-O2) batteries. Non-noble metal catalysts are considered the greatest potential candidates to replace noble metal catalysts in PEMFCs and Li-O2 batteries. Herein, we propose a novel type of non-noble metal catalyst (Fe-Hf/N/C) doped with Hf into a mesoporous carbon material derived from Hf-ZIF-8 and co-doping with Fe and N, which greatly enhanced the activity and durability of the catalyst. When applied in the cathode of PEMFCs, the current density can reach up 1.1 and 1.7 A cm-2 at 0.7 and 0.6 V, respectively, with a maximum power density of 1.15 W cm-2. The discharge capacity of the Li-O2 batteries is up to 15,081 mAh g-1 with Fe-Hf/N/C in the cathode, which also shows a lower charge overpotential, 200 mV lower than that of the Fe/N/C. Additionally, the Fe-Hf/N/C catalyst has demonstrated better stability in both PEMFCs and Li-O2 batteries. This reveals that Hf can not only optimize the electronic structure of iron sites and increase the active sites for the oxygen reduction reaction, but can also anchor the active sites, enhancing the durability of the catalyst. This study provides a new strategy for the development of high-performance and durable catalysts for PEMFCs and Li-O2 batteries.

Engineering of MnTe/MnO Heterostructures with Interfacial Electric Field Modulation for Efficient and Durable Li-O2 Batteries

Design and synthesis of highly active and robust bifunctional cathode catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are of vital significance for practical applications of lithium-oxygen (Li-O2) batteries. Herein, a built-in electric field (BIEF) strategy is reported to fabricate MnTe/MnO heterostructures with a large work function difference (ΔΦ) as a bifunctional cathode catalyst in Li-O2 batteries. The MnTe/MnO heterostructures with nanosheets and microporous structures result in an abundance of exposed active sites and facilitate mass transfer. More importantly, the heterogeneous MnTe/MnO nano-interface region provides a BIEF that can trigger interfacial charge redistribution, fine-tune the adsorption energy of oxygen intermediates, and alter the morphology of discharge products to accelerate ORR/OER kinetics. Impressively, the fabricated Li-O2 batteries with MnTe/MnO cathode showcases exhibit excellent electrochemical performances, including low charging overpotential, a high specific capacity of 11930 mA h g-1, and good cycle stability over 350 cycles even with a fixed specific capacity of 500 mA h g-1 at a current density of 500 mA g-1. This work provides an avenue for the rational design of high performance heterostructure electrocatalysts toward practical applications for rechargeable Li-O2 batteries.

Defect Engineering in CuS1- x Nanoflowers Enables Low-Overpotential and Long-Cycle-Life of Lithium-Oxygen Batteries

The defect engineering is essential for the development of efficient cathode catalysts for lithium-oxygen batteries. Herein, CuS1 -x nanoflowers are fabricated by microwave hydrothermal method. Through theoretical and experimental analysis, the S vacancies are observed, which result in augmented charge around Cu, improved adsorption of LiO2, and reduced overpotential. On the one hand, the generated electronic defects cause the Fermi level to shift toward the conduction band, which enhances the electronic conductivity and ion transfer. On the other hand, the increased S vacancies provide a large number of Cu active sites, which increase the charge transfer from Cu to LiO2, which improves the stability of the intermediate adsorption. Interactively, CuS1- x catalyst obtains a capacity of 23,227 mAh g-1 and a cycle life of 225 at 500 mA g-1. This work will be helpful for obtaining an efficient cathode catalyst by providing a deep understanding of vacancy modulation in advanced catalysts.

Single Atom Catalysts Based on Earth-Abundant Metals for Energy-Related Applications

Anthropogenic activities related to population growth, economic development, technological advances, and changes in lifestyle and climate patterns result in a continuous increase in energy consumption. At the same time, the rare metal elements frequently deployed as catalysts in energy related processes are not only costly in view of their low natural abundance, but their availability is often further limited due to geopolitical reasons. Thus, electrochemical energy storage and conversion with earth-abundant metals, mainly in the form of single-atom catalysts (SACs), are highly relevant and timely technologies. In this review the application of earth-abundant SACs in electrochemical energy storage and electrocatalytic conversion of chemicals to fuels or products with high energy content is discussed. The oxygen reduction reaction is also appraised, which is primarily harnessed in fuel cell technologies and metal-air batteries. The coordination, active sites, and mechanistic aspects of transition metal SACs are analyzed for two-electron and four-electron reaction pathways. Further, the electrochemical water splitting with SACs toward green hydrogen fuel is discussed in terms of not only hydrogen evolution reaction but also oxygen evolution reaction. Similarly, the production of ammonia as a clean fuel via electrocatalytic nitrogen reduction reaction is portrayed, highlighting the potential of earth-abundant single metal species.

Metal-Organic Framework with Dual Excitation Pathways as Efficient Bifunctional Catalyst for Photo-assisted Li-O2 Batteries

Li-O2 batteries (LOBs) have sparked significant interest due to their fascinating high theoretical energy density. However, the large overpotential for the formation and oxidation of Li2O2 during charge and discharge process seriously hinders the further development and application of LOBs. In this work, metal-organic frameworks (MOFs) with different metal clusters (Fe, Ti, Zr) are successfully synthesized, and they are employed as the photoelectrodes for the photo-assisted LOBs. The special dual excitation pathways of Fe-MOF under illumination and the superior separation efficiency of photocarriers, which significantly enhance the activation of O2/Li2O2, improving the catalytic activity of oxygen reduction reaction and oxygen evolution reaction. Moreover, compared to traditional inorganic semiconductor crystals, Fe-MOF exhibits large specific surface area and excellent O2 adsorption ability. Therefore, the LOB with Fe-MOF as the cathode exhibits large specific capacity, ultralow charge/discharge overpotential of 0.22 V at 0.05 mA cm-2 and excellent stability of 195 cycles under illumination. This study provides an environmentally friendly and highly efficient photocatalyst for LOBs, and a new strategy for designing photoelectrodes.

Advances in Functional Organosulfur-Based Mediators for Regulating Performance of Lithium Metal Batteries

Rechargeable lithium metal batteries (LMBs) are promising next-generation energy storage systems due to their high theoretical energy density. However, their practical applications are hindered by lithium dendrite growth and various intricate issues associated with the cathodes. These challenges can be mitigated by using organosulfur-based mediators (OSMs), which offer the advantages of abundance, tailorable structures, and unique functional adaptability. These features enable the rational design of targeted functionalities, enhance the interfacial stability of the lithium anode and cathode, and accelerate the redox kinetics of electrodes via alternative reaction pathways, thereby effectively improving the performance of LMBs. Unlike the extensively explored field of organosulfur cathode materials, OSMs have garnered little attention. This review systematically summarizes recent advancements in OSMs for various LMB systems, including lithium-sulfur, lithium-selenium, lithium-oxygen, lithium-intercalation cathode batteries, and other LMB systems. It briefly elucidates the operating principles of these LMB systems, the regulatory mechanisms of the corresponding OSMs, and the fundamentals of OSMs activity. Ultimately, strategic optimizations are proposed for designing novel OSMs, advanced mechanism investigation, expanded applications, and the development of safe battery systems, thereby providing directions to narrow the gap between rational modulation of organosulfur compounds and their practical implementation in batteries.

Enhanced Li-O2 battery performance using NiS/MoS2 heterostructure by building internal electric field to promote the one-electron oxygen reduction/oxidation

Electrocatalysts with appropriate electron coupling toward LiO2 intermediates can exhibit superior oxygen reduction/evolution reaction kinetics in Li-O2 batteries (LOBs). In this work, a charge redistribution strategy has been developed by constructing NiS/MoS2 heterostructure nanosheet self-assembled hollow microspheres with an internal electric field to regulate the interaction with LiO2 and then improve the electrochemical performance of LOBs. Density functional theory calculations and physicochemical characterizations reveal that the difference of work functions between NiS and MoS2 promotes the electron redistribution in heterointerface via built-in electrical field, leading to increased electron density of interfacial Ni atom, thereby enhancing its electron coupling toward LiO2 intermediates and promoting one-electron oxygen reduction/oxidation reaction kinetics. As a result, the NiS/MoS2-based LOBs exhibit evidently higher discharge capacity and much better cycling performance than the batteries using NiS and MoS2. This work provides a reliable charge redistribution strategy induced by build-in electric field to design efficient catalysts for LOBs.

Recent Progress of Halide Redox Mediators in Lithium-Oxygen Batteries: Functions, Challenges, and Perspectives

Lithium-oxygen batteries (LOBs) have received much research interest owing to their ultra-high energy density, but their further development is restricted by the erosion of the Li anode, the degradation of the electrolyte, and especially the sluggish oxygen-involving reactions on the cathode. To facilitate the oxidation of discharge products, halide redox mediators (HRMs), a subclass of soluble additives, have been explored to promote their decomposition. Meanwhile, some other intriguing functions were discovered, like protecting the Li anode and redirecting the discharge pathway to form LiOH. In this Review, after a brief introduction of LOBs and HRMs, the various functions of HRMs, not limited to promoting the oxidation of discharge products, are discussed and summarized. In addition, the challenges and controversies confronted by HRMs in LOBs are highlighted and the future opportunities of HRMs for achieving better LOBs are proposed.

Promoting oxygen electrode reaction kinetics in photo-assisted Li-O2 batteries through heterostructure design and built-in electric field construction

Li-O2 batteries (LOBs) boast an exceptionally high theoretical energy density; however, the slow kinetics of the oxygen electrode reaction have been a significant hurdle in their advancement and practical application. In this study, a composite of bismuth oxyhalide heterojunction incorporated with metal-organic frameworks (MOFs) was engineered on carbon cloth (Zr-MOF/BiOIBr/CC) as an efficient bifunctional catalyst to enhance the oxygen electrode reaction in photo-assisted LOBs. Compared to Zr-MOF/CC, Zr-MOF/BiOIBr/CC significantly expands the light absorption spectrum of the catalyst. Furthermore, the built-in electric field in the heterojunction aids in the separation and directional movement of photogenerated carriers, thereby expediting the reaction kinetics of LOBs. Consequently, the photo-assisted LOBs with Zr-MOF/BiOIBr/CC as the cathodes display a discharge potential of 3.05 V, a low charge potential of 3.20 V, and an energy efficiency of up to 95.3%, and can sustain an extended cycle life of over 255 cycles. This study underscores the potential application of MOFs/semiconductor heterostructural materials in photo-assisted LOBs and offers insights into the systematic design of photo-assisted air batteries and other advanced semiconductors.

Ordered porous Mn - Co spinel oxide (CoMn2O4) with vacancies modulation as efficient electrocatalyst for Li - O2 battery

Nonaqueous Li - O2 battery (LOB) is considered one of the most promising energy storage system due to its ultrahigh theoretical specific capacity (3500 Wh kg-1). Introducing vacancies in CoMn2O4 catalysts is regarded as an effective strategy to enhance the electrochemical performances of LOB. However, the relation between vacancy types in CoMn2O4 and catalytic performances in the LOB remains ambiguous. Herein, ordered porous CoMn2O4 with oxygen and metal vacancies is obtained via solvothermal reaction followed by temperature-controlled calcination using polystyrene spheres as templates. The increase in treatment temperature reduces the content of oxygen vacancies while increasing that of the metal vacancies. Notably, experimental results and theoretical calculations show that oxygen vacancies in CoMn2O4 have a greater influence than metal vacancies in modulating the LiO2 adsorption during the reaction processes and reducing the overpotential. CoMn2O4 synthesized at 500 ℃ (CoMnO-500) with higher oxygen vacancies exhibits stronger adsorption onto the LiO2, facilitating the formation of film-like Li2O2. Therefore, an LOB with the CoMnO-500 catalyst presents the lowest overpotential of 1.2 V and longest cycle lifespan of 286 cycles at a current density of 200 mA g-1. This study offers insights into the effect of CoMn2O4 vacancies on the formation pathway of Li2O2 discharge products.

Critical Effects of Insoluble Additives in Liquid Electrolytes for Metal Batteries

Rechargeable metal batteries have received widespread attention due to their high energy density by using pure metal as the anode. However, there are still many fundamental problems that need to be solved before approaching practical applications. The critical ones are low charge/discharge current due to slow ion transport, short cycle lifetime due to poor anode/cathode stability, and unsatisfied battery safety. To tackle these problems, various strategies have been suggested. Among them, electrolyte additive is one of the most widely used strategies. Most of the additives currently studied are soluble, but their reliability is questionable, and they can easily affect the electrochemical process, causing unwanted battery performance decline. On the contrary, insoluble additives with excellent chemical stability, high mechanical strength, and dimensional tunability have attracted considerable research exploration recently. However, there is no timely review on insoluble additives in metal batteries yet. This review summarizes various functions of insoluble additives: ion transport modulation, metal anode protection, cathode amelioration, as well as battery safety enhancement. Future research directions and challenges for insoluble solid additives are also proposed. It is expected this review will stimulate inspiration and arouse extensive studies on further improvement in the overall performance of metal batteries.

Constructing an Interlaced Catalytic Surface via Fluorine-Doped Bimetallic Oxides for Oxygen Electrode Processes in Li-O2 Batteries

Lithium-oxygen (Li-O2) batteries, renowned for their high theoretical energy density, have garnered significant interest as prime candidates for future electric device development. However, their actual capacity is often unsatisfactory due to the passivation of active sites by solid-phase discharge products. Optimizing the growth and storage of these products is a crucial step in advancing Li-O2 batteries. Here, a fluorine-doped bimetallic cobalt-nickel oxide (CoNiO2- xFx/CC) with an interlaced catalytic surface (ICS) and a corncob-like structure is proposed as an oxygen electrode. Unlike conventional oxide electrodes with a "single adsorption catalytic mechanism," the ICS of CoNiO2- xFx/CC offers a "competitive adsorption catalytic mechanism," where oxygen sites facilitate oxygen conversion while fluorine sites contribute to the growth of Li2O2. This results in a change in Li2O2 morphology from a surface film to toroidal particles, effectively preventing the burial of active sites. Additionally, the unique open architecture aids in the capture and release of oxygen and the formation of well-contacted Li2O2/electrode interfaces, which benefits the complete decomposition of Li2O2 products. Consequently, the Li-O2 battery with a CoNiO2- xFx/CC cathode demonstrates a high specific capacity of up to 30923 mAh g-1 and a lifespan exceeding 580 cycles, surpassing most reported metal oxide-based cathodes.

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.

Nanoengineering of Cathode Catalysts for Li-O2 Batteries

Lithium-oxygen (Li-O2) batteries have obtained widespread attention as next-generation energy storage systems due to their extremely high energy density. However, the high charge overpotential, attributed to the insulating property of Li2O2, significantly limits the energy efficiency and triggers solvent degradation. The high electrochemical activities of oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) on the cathode are crucial for alleviating the high charging polarizations and enhancing the lifetime of Li-O2 batteries, which are also top challenges of state-of-art research. In this review, the scientific challenges and the proposed solutions in the development of cathode catalysts have been summarized. The recent research advancements on the nanoengineering of cathode catalysts for Li-O2 batteries have been comprehensively discussed, and the perspectives on the structure optimization are presented. Meanwhile, we have elucidated the structure-performance relationship between the electronic state and performance of the cathode catalysts at the nanoscale level. This review intends to provide guidelines for the design and construction of cathode catalysts in advanced Li-O2 batteries.

Engineering Triple-Phase Interfaces around the Anode toward Practical Alkali Metal-Air Batteries

Alkali metal-air batteries (AMABs) promise ultrahigh gravimetric energy densities, while the inherent poor cycle stability hinders their practical application. To address this challenge, most previous efforts are devoted to advancing the air cathodes with high electrocatalytic activity. Recent studies have underlined the solid-liquid-gas triple-phase interface around the anode can play far more significant roles than previously acknowledged by the scientific community. Besides the bottlenecks of uncontrollable dendrite growth and gas evolution in conventional alkali metal batteries, the corrosive gases, intermediate oxygen species, and redox mediators in AMABs cause more severe anode corrosion and structural collapse, posing greater challenges to the stabilization of the anode triple-phase interface. This work aims to provide a timely perspective on the anode interface engineering for durable AMABs. Taking the Li-air battery as a typical example, this critical review shows the latest developed anode stabilization strategies, including formulating electrolytes to build protective interphases, fabricating advanced anodes to improve their anti-corrosion capability, and designing functional separator to shield the corrosive species. Finally, the remaining scientific and technical issues from the prospects of anode interface engineering are highlighted, particularly materials system engineering, for the practical use of AMABs.

Multi-functional integrated design of a copper foam-based cathode for high-performance lithium-oxygen batteries

Lithium-oxygen batteries (LOBs) with extraordinarily high energy density are some of the most captivating energy storage devices. Designing an efficient catalyst system that can minimize the energy barriers and address the oxidant intermediate and side-product issues is the major challenge regarding LOBs. Herein, we have developed a new type of integrated cathode of Cu foam-supported hierarchical nanowires decorated with highly catalytic Au nanoparticles which achieves a good combination of a gas diffusion electrode and a catalyst electrode, contributing to the synchronous multiphase transport of ions, oxygen, and electrons as well as improving the cathode reaction kinetics effectively. Benefiting from such a unique hierarchical architecture, the integrated cathode delivered superior electrochemical performance, including a high discharge capacity of up to 11.5 mA h cm-2 and a small overpotential of 0.49 V at 0.1 mA cm-2, a favorable energy efficiency of 84.3% and exceptional cycling stability with nearly 1200 h at 0.1 mA cm-2 under a fixed capacity of 0.25 mA h cm-2. Furthermore, density functional theory (DFT) calculations further reveal the intrinsic direct catalytic ability to form/decompose Li2O2 during the ORR/OER process. As a consequence, this work provides an insightful investigation on the structural engineering of catalysts and holds great potential for advanced integrated cathode design for LOBs.

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.

Electron Localization in Rationally Designed Pt1Pd Single-Atom Alloy Catalyst Enables High-Performance Li-O2 Batteries

Li-O2 batteries (LOBs) are considered as one of the most promising energy storage devices due to their ultrahigh theoretical energy density, yet they face the critical issues of sluggish cathode redox kinetics during the discharge and charge processes. Here we report a direct synthetic strategy to fabricate a single-atom alloy catalyst in which single-atom Pt is precisely dispersed in ultrathin Pd hexagonal nanoplates (Pt1Pd). The LOB with the Pt1Pd cathode demonstrates an ultralow overpotential of 0.69 V at 0.5 A g-1 and negligible activity loss over 600 h. Density functional theory calculations show that Pt1Pd can promote the activation of the O2/Li2O2 redox couple due to the electron localization caused by the single Pt atom, thereby lowering the energy barriers for the oxygen reduction and oxygen evolution reactions. Our strategy for designing single-atom alloy cathodic catalysts can address the sluggish oxygen redox kinetics in LOBs and other energy storage/conversion devices.

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.

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.

Tunable Oxygen Vacancies of Cobalt Oxides in Lithium-Oxygen Batteries: Morphology Control of Discharge Product

The discharge product Li2O2 is difficult to decompose in lithium-oxygen batteries, resulting in poor reversibility and cycling stability of the battery, and the morphology of Li2O2 has a great influence on its decomposition during the charging process. Therefore, reasonable design of the catalyst structure to improve the density of catalyst active sites and make Li2O2 form a morphology which is easy to decompose in the charging process will help improve the performance of battery. Here, we demonstrate a series of hollow nanoboxes stacked by Co3O4 nanoparticles with different sizes. The results show that the surface of the nanoboxes composed of smaller size Co3O4 nanoparticles contains abundant pore structure and higher concentration of oxygen vacancies, which changes the adsorption energy of reactants and intermediates, providing more nucleation sites for Li2O2, thereby forming Li2O2 with high dispersion, which is easier to decompose during charging, and eventually improve the performance of the battery. This provides an important idea for the structural design of the cathode catalyst in lithium-oxygen batteries and the regulation of Li2O2 morphology.

Tailoring the growth route of lithium peroxide through the rational design of a sodium-doped nickel phosphate catalyst for lithium-oxygen batteries

Tailoring the morphology and structure of Li2O2, the discharge product of lithium-oxygen batteries (LOBs), through the rational design of cathode catalysts is an efficient strategy to promote the electrochemical performance of LOBs. In this work, sodium-doped nickel phosphate nanorods (Na-NiPO NRs) grown on Ni foam (NF) were prepared by the hydrothermal method and subsequent calcination. For the Na-NiPO NRs, the electronic structure could be optimized and abundant void space among the nanorods would provide abundant transport channels. Adopted as the cathodes, the Na-NiPO NRs could facilitate the uniform growth of sea cucumber-like Li2O2 with sufficient Li2O2-electrolyte and Li2O2-catalyst interfaces, significantly promoting the charge process. Therefore, LOBs could deliver a high discharge capacity of 10365.0 mA h g-1 at 100 mA g-1. And a low potential gap of 1.16 V can be achieved at 200 mA g-1 with a capacity of 500 mA h g-1. The proposed strategy demonstrates the role of the morphology and electronic structure of the cathode catalysts in tuning the Li2O2 morphology and provides a novel approach for achieving high-performance LOBs.

A 3D-printed freestanding graphene aerogel composite photocathode for high-capacity and long-life photo-assisted Li-O2 batteries

The construction of a high-performance photocathode is essential for improving Li-O2 battery performance and solar energy utilization. However, the single pore and few active reaction sites of the photocathode result in insufficient discharge capacity and unsatisfactory cycling durability. Herein, we designed and fabricated a self-standing 3D-printed multi-pore graphene-based photocathode via direct ink writing (DIW) featuring non-competitive three-phase transmission channels to promote the transport of Li+, e-, and O2. The macropore provides adequate space for storage of the Li2O2 discharge product; the mesopore facilitates the reactant transport, while the micropore stores the active ions. Furthermore, the photogenerated carriers of the photocathode promote overpotential reduction. Under illumination, the charging voltage of the Li-O2 battery with a reduced graphene oxide/titanium dioxide (rGO/TiO2) photocathode is decreased from 4.55 V to 3.77 V, and the battery exhibits stable cycling for 1000 hours. Notably, the photocathode's pore structure and specific surface area are further optimized after adding carbon nanotubes (CNTs). Compared with rGO/TiO2, the specific surface area of reduced graphene oxide/titanium dioxide/carbon nanotubes (rGO/TiO2/CNTs) is increased by 12 times to 194.13 m2 g-1, and the discharge capacity can reach up to 33.37 mA h cm-2. This self-standing 3D-printed photocathode structure paves a new way for developing high-performance energy storage systems.

Cathode Chemistries of Lithium-Oxygen Batteries in Nanoconfined Space

In lithium-oxygen batteries, although the porous carbon cathodes are widely utilized to tailor the properties of discharged Li2O2, the impact of nanopore size on the Li2O2 formation and decomposition reactions remain incompletely understood. Here, we provide the straightforward elucidation on the effect of pore size in a range of 25-200 nm, using a highly ordered porous cathode matrix based on the carbon-coated anodic aluminum oxide membrane formed on an Al substrate (C/AAO_Al). When the nanopore size is 25 nm, film-like Li2O2 with a thickness of 2-5 nm is formed, possibly via a surface-driven mechanism. When the nanochannel becomes larger, the Li2O2 film thickness saturates at ca. 10 nm, along with crystalline Li2O2 particles possibly formed by a solution-mediated mechanism.

Photoelectrochemical Engineering for Light-Assisted Rechargeable Metal Batteries: Mechanism, Development, and Future

Rechargeable battery devices with high energy density are highly demanded by  our  modern society. The use of metal anodes is extremely attractive for future rechargeable battery devices. However, the notorious metal dendritic and instability of solid electrolyte interface issues pose a series of challenges for metal anodes. Recently, considering the indigestible dynamical behavior of metal anodes, photoelectrochemical engineering of light-assisted metal anodes have been rapidly developed since they efficiently utilize the integration and synergy of oriented crystal engineering and photocatalysis engineering, which provided a potential way to unlock the interface electrochemical mechanism and deposition reaction kinetics of metal anodes. This review starts with the fundamentals of photoelectrochemical engineering and follows with the state-of-art advance of photoelectrochemical engineering for light-assisted rechargeable metal batteries where photoelectrode materials, working principles, types, and practical applications are explained. The last section summarizes the major challenges and some invigorating perspectives for future research on light-assisted rechargeable metal batteries.

The Role of Self-Catalysis Induced by Co Doping in Nonaqueous Li-O2 Batteries

This work systematically studies the product self-catalysis of in situ electrochemical cobalt doping of Li2O2 and reveals its potential mechanism for improving the performance of lithium-oxygen (Li-O2) batteries. Theoretical calculations demonstrate that the discharge products contain substituted and interstitial Co impurities, which serve as active sites to promote the formation of Li3O4 crystallization, thus switching the nucleation mechanism from the main discharge product Li2O2 to Li3O4. This Co-doping behavior leads to the thermodynamically favorable and dynamically stable formation of Li3O4 crystals during the discharge process. Through systematic investigation of the structural, energetic, electronic, diffusive, and catalytic properties of the Co-doped Li2O2 and Li3O4 compounds, we found that Li3O4 has better charge/mass transport and a lower overpotential for the Li3O4 formation/decomposition reaction. Consequently, this work elucidates that Co doping provides a simple and effective approach for increasing the proportion of Li3O4, which can significantly improve the Li-O2 battery performance.

Water-Trapping Single-Atom Co-N4 /Graphene Triggering Direct 4e- LiOH Chemistry for Rechargeable Aprotic Li-O2 Batteries

Lithium-oxygen (Li-O2 ) batteries have received extensive attention owing to ultrahigh theoretical energy density. Compared to typical discharge product Li2 O2 , LiOH has attracted much attention for its better chemical and electrochemical stability. Large-scale applications of Li-O2 batteries with LiOH chemistry are hampered by the serious internal shuttling of the water additives with the desired 4e- electrochemical reactions. Here, a metal organic framework-derived "water-trapping" single-atom-Co-N4 /graphene catalyst (Co-SA-rGO) is provided that successfully mitigates the water shuttling and enables the direct 4e- catalytic reaction of LiOH in the aprotic Li-O2 battery. The Co-N4 center is more active toward proton-coupled electron transfer, benefiting - direction 4e- formation of LiOH. 3D interlinked networks also provide large surface area and mesoporous structures to trap ≈12 wt% H2 O molecules and offer rapid tunnels for O2 diffusion and Li+ transportation. With these unique features, the Co-SA-rGO based Li-O2 battery delivers a high discharge platform of 2.83 V and a large discharge capacity of 12 760.8 mAh g-1 . Also, the battery can withstand corrosion in the air and maintain a stable discharge platform for 220 cycles. This work points out the direction of enhanced electron/proton transfer for the single-atom catalyst design in Li-O2 batteries.

Edge-Site-Free and Topological-Defect-Rich Carbon Cathode for High-Performance Lithium-Oxygen Batteries

The rational design of a stable and catalytic carbon cathode is crucial for the development of rechargeable lithium-oxygen (LiO2 ) batteries. An edge-site-free and topological-defect-rich graphene-based material is proposed as a pure carbon cathode that drastically improves LiO2 battery performance, even in the absence of extra catalysts and mediators. The proposed graphene-based material is synthesized using the advanced template technique coupled with high-temperature annealing at 1800 °C. The material possesses an edge-site-free framework and mesoporosity, which is crucial to achieve excellent electrochemical stability and an ultra-large capacity (>6700 mAh g-1 ). Moreover, both experimental and theoretical structural characterization demonstrates the presence of a significant number of topological defects, which are non-hexagonal carbon rings in the graphene framework. In situ isotopic electrochemical mass spectrometry and theoretical calculations reveal the unique catalysis of topological defects in the formation of amorphous Li2 O2 , which may be decomposed at low potential (∼ 3.6 V versus Li/Li+ ) and leads to improved cycle performance. Furthermore, a flexible electrode sheet that excludes organic binders exhibits an extremely long lifetime of up to 307 cycles (>1535 h), in the absence of solid or soluble catalysts. These findings may be used to design robust carbon cathodes for LiO2 batteries.

Morphology-Dictated Mechanism of Efficient Reaction Sites for Li2O2 Decomposition

In the pursuit of a highly reversible lithium-oxygen (Li-O₂) battery, control of reaction sites to maintain stable conversion between O₂ and Li₂O₂ at the cathode side is imperatively desirable. However, the mechanism involving the reaction site during charging remains elusive, which, in turn, imposes challenges in recognition of the origin of overpotential. Herein, via combined investigations by in situ atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS), we propose a universal morphology-dictated mechanism of efficient reaction sites for Li₂O₂ decomposition. It is found that Li₂O₂ deposits with different morphologies share similar localized conductivities, much higher than that reported for bulk Li₂O₂, enabling the reaction site not only at the electrode/Li₂O₂/electrolyte interface but also at the Li₂O₂/electrolyte interface. However, while the mass transport process is more enhanced at the former, the charge-transfer resistance at the latter is sensitively related to the surface structure and thus the reactivity of the Li₂O₂ deposit. Consequently, for compact disk-like deposits, the electrode/Li₂O₂/electrolyte interface serves as the dominant decomposition site, which causes premature departure of Li₂O₂ and loss of reversibility; on the contrary, for porous flower-like and film-like Li₂O₂ deposits bearing a larger surface area and richer surface-active structures, both the interfaces are efficient for decomposition without premature departure of the deposit so that the overpotential arises primarily from the sluggish oxidation kinetics and the decomposition is more reversible. The present work provides instructive insights into the understanding of the mechanism of reaction sites during the charge process, which offers guidance for the design of reversible Li-O₂ batteries.

Stepping Up the Kinetics of Li-O2 Batteries by Shrinking Down the Li2O2 Granules through Concertedly Enhanced Catalytic Activity and Photoactivity of Se-Doped LaCoO3

Owing to their structural tunability for furnishing high catalytic activity and photoactivity, perovskite oxides are a class of promising materials for high-performance photocathode catalysts in a photoassisted lithium oxygen battery (LOB), which is still in its infancy. Herein, single-crystalline LaCoO3 (LCO) is successfully synthesized through a microwave-assisted approach and selenylated to simultaneously introduce anionic doping and oxygen vacancies, boosting not only the electrocatalytic activity toward reversible Li2O2 formation/decomposition, but also the photoactivity to further reduce the charge/discharge polarization. As a result, LOBs utilizing Se-doped LCO as the photocathode catalyst demonstrate a superior performance under illumination in all aspects of energy efficiency, specific capacity, and cycling stability, ranking among the best reported in the literature for perovskite oxides. The photoenhanced charge kinetics is found to be correlated with the accelerated Li2O2 nucleation with lowered granule size, which is key to both the improved charge/discharge capacity and reversibility. The results underscore the tailoring of perovskite structure to aggrandize both the catalytic activity and photoactivity for concertedly promoting the kinetics of LOBs.

An Organic Redox Mediator with a Defense-Donor for Lithium Anode in Lithium-Oxygen Batteries

Lithium-oxygen batteries (LOBs) suffer from large charge overpotential and unstable Li metal interface, which can be attributed to the inefficient charge transport at the insulating Li₂ O₂ /cathode interface and the severe oxygen corrosion issue on the Li anode surface. The use of soluble redox mediators (RMs) can effectively enhance the charge transport between Li₂ O₂ and cathode, thus greatly reducing the charge overpotential. However, oxidized RMs will also shuttle to the anode side and react with the Li metal, which not only results in the loss of both the RMs and the electrical energy efficiency but also exacerbates the Li anode corrosion. Herein, an organic compound-acetylthiocholine iodide (ATCI), in which a big cation group is contained, is proposed as a defense-donor RM for lithium anode in LOBs to simultaneously address the above issues. During charge, it can accelerate the oxidation kinetics of Li₂ O₂ via its iodide anion redox couple (I^- /I₃ ^- ). Meanwhile, its cation segment (ATC⁺ ) can move to the anode surface via electric attraction and in situ forms a protective interfacial layer, which prevents the Li anode from the attack of oxidized RM and oxygen species. Consequently, the ATCI-containing LOBs can achieve both a low charge potential (≈3.49 V) and a long cycle life (≈190 cycles).

Single-Atomic Zn/Co-Nx Sites Boost Solid-Soluble Synergistic Catalysis for Lithium-Oxygen Batteries

Lithium-oxygen batteries have attracted widespread attention owing to their superior theoretical energy density. However, they are obstructed by sluggish oxygen reduction (ORR) and evolution reaction (OER) kinetics at air cathodes. Herein, different from using single solid or soluble catalysts, solid-soluble synergistic catalysis is proposed to conjointly enhance ORR/OER performances. During discharge, single-atomic zinc/cobalt embedded in nitrogen-doped carbon (Zn, Co-N/C) is judiciously engineered as a solid catalyst to regulate the growth pathway of Li₂O₂ and promote ORR kinetics. During charge, a typical redox mediator (RM, LiI) is added as a soluble catalyst to permit efficient oxidation of Li₂O₂. Of note is that the atomic Zn/Co-N_x sites can chemically adsorb oxidized iodine (I₂) and accelerate OER kinetics, which plays a decisive role in eliminating the shuttle effect of I₃^-/I₂ to the Li anode. Coupling a single-atomic catalyst with restricted oxidized iodine offers an exceptional discharge capacity, remarkably low polarization, and superior long-term cycling stability.

Bifunctional Photoassisted Li-O2 Battery with Ultrahigh Rate-Cycling Performance Based on Siloxene Size Regulation

Directly integrating the bifunctional photoelectrode into Li-O2 batteries has been considered an effective way to reduce the overpotential and promote electric energy saving. However, more regular investigations on various bifunctional photocatalysts have still been desired for high-performance photoassisted Li-O2 batteries. Herein, a systematic exploration of various-sized siloxene photocatalysts affected by Li-O2 batteries has been introduced. Compared with the utilization of larger-sized siloxene nanosheets (SNSs), the photoassisted Li-O2 battery with a siloxene quantum dot (SQD) photoelectrode delivers a superior round-trip efficiency of 230% based on the highest discharge potential up to 3.72 V and lowest charge potential of 1.60 V and enables the maintenance of a long-term cycling life with only 13% efficiency attenuation after 200 cycles at 0.075 mA/cm2. Furthermore, this system exhibits a record-high rate-cycling performance (162% round-trip efficiency, even at 3 mA/cm2) and a high discharge capacity of 2212 mAh/g at 1 mA/cm2. These ground-breaking performances could be attributed to the synergistic effect of the photocatalytic and electrocatalytic activities of SQD photocatalysts with the ideal conduction band/valence band values, the abundant defective sites, and the stronger O2 and lower LiO2 adsorption strengths of SQD photocatalysts. These systematic research studies highlight the significance of SQD bifunctional photocatalysts and could be extended to other photocatalysts for further high-efficiency photoelectric conversion and storage.

Air-Stable Protective Layers for Lithium Anode Achieving Safe Lithium Metal Batteries

With markedly expansive demand in energy storage devices, rechargeable batteries will concentrate on achieving the high energy density and adequate security, especially under harsh operating conditions. Considering the high capacity (3860 mA h g^-1 ) and low electrochemical potential (-3.04 V vs the standard hydrogen electrode), lithium metal is identified as one of the most promising anode materials, which has sparked a research boom. However, the intrinsically high reactivity triggers a repeating fracture/reconstruction process of the solid electrolyte interphase, side reactions with electrolyte and lithium dendrites, detrimental to the electrochemical performance of lithium metal batteries (LMBs). Even worse, when exposed to air, lithium metal will suffer severe atmospheric corrosion, especially the reaction with moisture, leading to grievous safety hazards. To settle these troubles, constructing air-stable protective layers (ASPLs) is an effective solution. In this review, besides the necessity of ASPLs is highlighted, the modified design criteria, focusing on enhancing chemical/mechanical stability and controlling ion flux, are proposed. Correspondingly, current research progress is comprehensively summarized and discussed. Finally, the perspectives of developing applicable lithium metal anodes (LMAs) are put forward. This review guides the direction for the practical use of LMAs, further pushing the evolution of safe and stable LMBs.

A Review of In-Situ Techniques for Probing Active Sites and Mechanisms of Electrocatalytic Oxygen Reduction Reactions

Electrocatalytic oxygen reduction reaction (ORR) is one of the most important reactions in electrochemical energy technologies such as fuel cells and metal-O2/air batteries, etc. However, the essential catalysts to overcome its slow reaction kinetic always undergo a complex dynamic evolution in the actual catalytic process, and the concomitant intermediates and catalytic products also occur continuous conversion and reconstruction. This makes them difficult to be accurately captured, making the identification of ORR active sites and the elucidation of ORR mechanisms difficult. Thus, it is necessary to use extensive in-situ characterization techniques to proceed the real-time monitoring of the catalyst structure and the evolution state of intermediates and products during ORR. This work reviews the major advances in the use of various in-situ techniques to characterize the catalytic processes of various catalysts. Specifically, the catalyst structure evolutions revealed directly by in-situ techniques are systematically summarized, such as phase, valence, electronic transfer, coordination, and spin states varies. In-situ revelation of intermediate adsorption/desorption behavior, and the real-time monitoring of the product nucleation, growth, and reconstruction evolution are equally emphasized in the discussion. Other interference factors, as well as in-situ signal assignment with the aid of theoretical calculations, are also covered. Finally, some major challenges and prospects of in-situ techniques for future catalysts research in the ORR process are proposed.

Designing a Built-In Electric Field for Efficient Energy Electrocatalysis

To utilize intermittent renewable energy as well as achieve the goals of peak carbon dioxide emissions and carbon neutrality, various electrocatalytic devices have been developed. However, the electrocatalytic reactions, e.g., hydrogen evolution reaction/oxygen evolution reaction in overall water splitting, polysulfide conversion in lithium-sulfur batteries, formation/decomposition of lithium peroxide in lithium-oxygen batteries, and nitrate reduction reaction to degrade sewage, suffer from sluggish kinetics caused by multielectron transfer processes. Owing to the merits of accelerated charge transport, optimized adsorption/desorption of intermediates, raised conductivity, regulation of the reaction microenvironment, as well as ease to combine with geometric characteristics, the built-in electric field (BIEF) is expected to overcome the above problems. Here, we give a Review about the very recent progress of BIEF for efficient energy electrocatalysis. First, the construction strategies and the characterization methods (qualitative and quantitative analysis) of BIEF are summarized. Then, the up-to-date overviews of BIEF engineering in electrocatalysis, with attention on the electron structure optimization and reaction microenvironment modulation, are analyzed and discussed in detail. In the end, the challenges and perspectives of BIEF engineering are proposed. This Review gives a deep understanding on the design of electrocatalysts with BIEF for next-generation energy storage and electrocatalytic devices.

Light-Assisted Metal-Air Batteries: Progress, Challenges, and Perspectives

Metal-air batteries are considered one of the most promising next-generation energy storage devices owing to their ultrahigh theoretical specific energy. However, sluggish cathode kinetics (O₂ and CO₂ reduction/evolution) result in large overpotentials and low round-trip efficiencies which seriously hinder their practical applications. Utilizing light to drive slow cathode processes has increasingly becoming a promising solution to this issue. Considering the rapid development and emerging issues of this field, this Review summarizes the current understanding of light-assisted metal-air batteries in terms of configurations and mechanisms, provides general design strategies and specific examples of photocathodes, systematically discusses the influence of light on batteries, and finally identifies existing gaps and future priorities for the development of practical light-assisted metal-air batteries.