High-Performance Li-O2 Batteries with Controlled Li2O2 Growth in Graphene/Au-Nanoparticles/Au-Nanosheets Sandwich

Enhanced catalytic performance through a single-atom preparation approach: a review on ruthenium-based catalysts

The outstanding catalytic properties of single-atom catalysts (SACs) stem from the maximum atom utilization and unique quantum size effects, leading to ever-increasing research interest in SACs in recent years. Ru-based SACs, which have shown excellent catalytic activity and selectivity, have been brought to the frontier of the research field due to their lower cost compared with other noble catalysts. The synthetic approaches for preparing Ru SACs are rather diverse in the open literature, covering a wide range of applications. In this review paper, we attempt to disclose the synthetic approaches for Ru-based SACs developed in the most recent years, such as defect engineering, coordination design, ion exchange, the dipping method, and electrochemical deposition etc., and discuss their representative applications in both electrochemical and organic reaction fields, with typical application examples given of: Li-CO2 batteries, N2 reduction, water splitting and oxidation of benzyl alcohols. The mechanisms behind their enhanced catalytic performance are discussed and their structure-property relationships are revealed in this review. Finally, future prospects and remaining unsolved issues with Ru SACs are also discussed so that a roadmap for the further development of Ru SACs is established.

Designing High-Donicity Anions for Rechargeable Potassium Superoxide/Peroxide Batteries

A battery cathode based on the superoxide/peroxide redox not only inherits the advantage of oxygen (O₂ ) batteries in high capacities and low costs but also overcomes the disadvantages in O₂ storage, electrolyte evaporation, and anode deactivation due to O₂ crossover. Herein, we report an enhanced potassium superoxide (KO₂ )/peroxide (K₂ O₂ ) conversion by adopting a high-donicity anion additive in the ether-based electrolyte. Such an anion was synthesized via a "Solvent-in-Anion" strategy and validated to enhance the electron donicity of the electrolyte. The use of high-donicity anion could lead to enhanced KO₂ utilization (≈90.2 %) by retarding electrode passivation and allow the full charging back of K₂ O₂ through the solution-mediated pathway without electrocatalysts. No apparent cell degradation is observed during the first 120 cycles by controlling the reversible depth-of-discharge capacity at 292 mAh g^-1 KO 2 ${{_{{\rm KO}{_{2}}}}}$ within an O₂ -free region. The K-KO₂ cell delivers a high energy efficiency (>84.4 %) and a lifespan of over 1440 hours.

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.

Anchoring NiO Nanosheet on the Surface of CNT to Enhance the Performance of a Li-O2 Battery

Li₂O₂, as the cathodic discharge product of aprotic Li-O₂ batteries, is difficult to electrochemically decompose. Transition-metal oxides (TMOs) have been proven to play a critical role in promoting the formation and decomposition of Li₂O₂. Herein, a NiO/CNT catalyst was prepared by anchoring a NiO nanosheet on the surface of CNT. When using the NiO/CNT as a cathode catalyst, the Li-O₂ battery had a lower overpotential of 1.2 V and could operate 81 cycles with a limited specific capacity of 1000 mA h g⁻¹ at a current density of 100 mA g⁻¹. In comparison, with CNT as a cathodic catalyst, the battery could achieve an overpotential of 1.64 V and a cycling stability of 66 cycles. The introduction of NiO effectively accelerated the generation and decomposition rate of Li₂O₂, further improving the battery performance. SEM and XRD characterizations confirmed that a Li₂O₂ film formed during the discharge process and could be fully electrochemical decomposed in the charge process. The internal network and nanoporous structure of the NiO/CNT catalyst could provide more oxygen diffusion channels and accelerate the decomposition rate of Li₂O₂. These merits led to the Li-O₂ battery’s better performance.

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.

Li2 O2 Formation Electrochemistry and Its Influence on Oxygen Reduction/Evolution Reaction Kinetics in Aprotic Li-O2 Batteries

Aprotic Li-O₂ batteries are regarded as the most promising technology to resolve the energy crisis in the near future because of its high theoretical specific energy. The key electrochemistry of a nonaqueous Li-O₂ battery highly relies on the formation of Li₂ O₂ during discharge and its reversible decomposition during charge. The properties of Li₂ O₂ and its formation mechanisms are of high significance in influencing the battery performance. This review article demonstrates the latest progress in understanding the Li₂ O₂ electrochemistry and the recent advances in regulating the Li₂ O₂ growth pathway. The first part of this review elaborates the Li₂ O₂ formation mechanism and its relationship with the oxygen reduction reaction/oxygen evolution reaction electrochemistry. The following part discusses how the cycling parameters, e.g., current density and discharge depth, influence the Li₂ O₂ morphology. A comprehensive summary of recent strategies in tailoring Li₂ O₂ formation including rational design of cathode structure, certain catalyst, and surface engineering is demonstrated. The influence resulted from the electrolyte, e.g., salt, solvent, and some additives on Li₂ O₂ growth pathway, is finally discussed. Further prospects of the ways in making advanced Li-O₂ batteries by control of favorable Li₂ O₂ formation are highlighted, which are valuable for practical construction of aprotic lithium-oxygen batteries.

MnCo2 S4 -CoS1.097 Heterostructure Nanotubes as High Efficiency Cathode Catalysts for Stable and Long-Life Lithium-Oxygen Batteries Under High Current Conditions

Constructing the heterostructures is considered to be one of the most effective methods to improve the poor electrical conductivity and insufficient electrocatalytic properties of metal sulfide catalysts. In this work, MnCo2 S4 -CoS1.097 nanotubes are successfully prepared via a reflux- hydrothermal process. This novel cathode catalyst delivers high discharge/charge specific capacities of 21 765/21 746 mAh g-1 at 200 mA g-1 and good rate capability. In addition, a favorable cycling stability with a fixed specific capacity of 1000 mAh g-1 at high current density of 1000 mA g-1 (167 cycles) and 2000 mA g-1 (57 cycles) are delivered. It is proposed that fast transmission of ions and electrons accelerated by the built-in electric field, multiple active sites from the heterostructure, and nanotube architecture with large specific surface area are responsible for the superior electrochemical performance. To some extent, the rational design of this heterostructured metal sulfide catalyst provides guidance for the development of the stable and efficient cathode catalysts for Li-O2 batteries that can be employed under high current conditions.

Surface atomic modulation of CoP bifunctional catalyst for high performance Li-O2 battery enabled by high-index (211) facets

The rational design of the surface structure and morphology characteristics of the catalyst at atomic level are the key to improve the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in lithium-oxygen (Li-O₂) battery. Here a series of cobalt phosphide (CoP) electrocatalysts with a variety of index facets are successfully prepared including concave polyhedrons CoP exposing with (211) crystal planes (CoP CPHs) spherical nanoparticles CoP exposed with (011) crystal planes and polyhedron particles CoP exposing with (011) and (111) crystal planes. The results show that CoP CPHs based Li-O₂ battery presents a large discharge capacity of 33743 mA h g^-1 at current density of 50 mA g^-1 and a remarkable long cycle life of up to 950 h. The experimental results demonstrates that the CoP CPHs electrode exposing with high-index (211) facets based Li-O₂ battery exhibits an extremely low overpotential (0.67 V) ultrahigh specific capacity (33743 mAh g^-1) and remarkable long-term stability of up to 950 h. Most importantly density functional theory (DFT) calculations demonstrate the excellent electrocatalytic activity of high-index (211) facets as compared to the low-index (011) and (111) planes are because of the existence of large density of atomic steps edge ledge sites and kinks which supply a wide space for breaking chemical bonds and increasing the reaction activity for oxygen electrode.

Continuous Synthesis of Hollow High-Entropy Nanoparticles for Energy and Catalysis Applications

Mixing multimetallic elements in hollow-structured nanoparticles is a promising strategy for the synthesis of highly efficient and cost-effective catalysts. However, the synthesis of multimetallic hollow nanoparticles is limited to two or three elements due to the difficulties in morphology control under the harsh alloying conditions. Herein, the rapid and continuous synthesis of hollow high-entropy-alloy (HEA) nanoparticles using a continuous "droplet-to-particle" method is reported. The formation of these hollow HEA nanoparticles is enabled through the decomposition of a gas-blowing agent in which a large amount of gas is produced in situ to "puff" the droplet during heating, followed by decomposition of the metal salt precursors and nucleation/growth of multimetallic particles. The high active sites per mass ratio of such hollow HEA nanoparticles makes them promising candidates for energy and electrocatalysis applications. As a proof-of-concept, it is demonstrated that these materials can be applied as the cathode catalyst for Li-O₂ battery operations with a record-high current density per catalyst mass loading of 2000 mA g_cat. ^-1 , as well as good stability and durable catalytic activity. This work offers a viable strategy for the continuous manufacturing of hollow HEA nanomaterials that can find broad applications in energy and catalysis.

Quantitative Delineation of the Low Energy Decomposition Pathway for Lithium Peroxide in Lithium-Oxygen Battery

Identification of a low-potential decomposition pathway for lithium peroxide (Li2O2) in nonaqueous lithium-oxygen (Li-O2) battery is urgently needed to ameliorate its poor energy efficiency. In this study, experimental data and theoretical calculations demonstrate that the recharge overpotential (η RC) of Li-O2 battery is fundamentally dependent on the Li2O2 crystallization pathway which is intrinsically related to the microscopic structural properties of the growing crystals during discharge. The Li2O2 grown by concurrent surface reduction and chemical disproportionation seems to form two discrete phases that have been deconvoluted and the amount of Li2O2 deposited by these two routes is quantitatively estimated. Systematic analyses have demonstrated that, regardless of the bulk morphology, solution-grown Li2O2 shows higher η RC (>1 V) which can be attributed to higher structural order in the crystal compared to the surface-grown Li2O2. Presumably due to a cohesive interaction between the electrode surface and growing crystals, the surface-grown Li2O2 seems to possess microscopic structural disorder that facilitates a delithiation induced partial solution-phase oxidation at lower η RC (<0.5 V). This difference in η RC for differently grown Li2O2 provides crucial insights into necessary control over Li2O2 crystallization pathways to improve the energy efficiency of a Li-O2 battery.

Two-Dimensional Materials to Address the Lithium Battery Challenges

Despite the ever-growing demand in safe and high power/energy density of Li⁺ ion and Li metal rechargeable batteries (LIBs), materials-related challenges are responsible for the majority of performance degradation in such batteries. These challenges include electrochemically induced phase transformations, repeated volume expansion and stress concentrations at interfaces, poor electrical and mechanical properties, low ionic conductivity, dendritic growth of Li, oxygen release and transition metal dissolution of cathodes, polysulfide shuttling in Li-sulfur batteries, and poor reversibility of lithium peroxide/superoxide products in Li-O₂ batteries. Owing to compelling physicochemical and structural properties, in recent years two-dimensional (2D) materials have emerged as promising candidates to address the challenges in LIBs. This Review highlights the cutting-edge advances of LIBs by using 2D materials as cathodes, anodes, separators, catalysts, current collectors, and electrolytes. It is shown that 2D materials can protect the electrode materials from pulverization, improve the synergy of Li⁺ ion deposition, facilitate Li⁺ ion flux through electrolyte and electrode/electrolyte interfaces, enhance thermal stability, block the lithium polysulfide species, and facilitate the formation/decomposition of Li-O₂ discharge products. This work facilitates the design of safe Li batteries with high energy and power density by using 2D materials.

An Overview of Engineered Graphene-Based Cathodes: Boosting Oxygen Reduction and Evolution Reactions in Lithium- and Sodium-Oxygen Batteries

The depletion of fossil fuels, the rapid evolution of the global economy, and high living standards require the development of new energy-storage systems that can meet the needs of the world's population. Metal-oxygen batteries (M=Li, Na) arise, therefore, as promising alternatives to widely used lithium-ion batteries, due to their high theoretical energy density, which approaches that of gasoline. Although significant progress has been made in recent years, there are still several challenges to overcome to reach the final commercialization of this technology. One of the most limiting and challenging factors is the development of bifunctional cathodes towards oxygen reduction and evolution reactions. In this sense, graphene, which is very promising and tunable, has been widely explored by the research community as a key material for this technology. Herein, a wide literature overview is presented and analyzed with the aim of guiding future research in this field.

Redox mediators as charge agents for changing electrochemical reactions

This review provides a comprehensive discussion toward understanding the effects of RMs in electrochemical systems, underlying redox mechanisms, and reaction kinetics both experimentally and theoretically.

High-Surface-Area and Porous Co2P Nanosheets as Cost-Effective Cathode Catalysts for Li-O2 Batteries

To enable lithium-oxygen batteries for practical applications, the design and efficient synthesis of nonprecious metal catalysts with high activity and stable structural properties are demanded. The objective is to accelerate the sluggish kinetics of both oxygen reduction reaction and oxygen evolution reaction by facilitating electronic/ionic transport and improving oxygen diffusion in a porous structure. In this study, high-surface-area and porous cobalt phosphide (Co₂P) nanosheets are synthesized via an environmentally safe hydrothermal method, where red phosphorous is used as the phosphorous source. It was found that the as-prepared Co₂P/acetylene black (AB) composite delivered enhanced electrochemical performances, such as high capacities of 2551 mA h g^-1 (based on the total weight of Co₂P and AB) or 5102 mA h g^-1 (based on the weight of Co₂P or AB) and a good cycle life of more than 1800 h (132 cycles) in lithium-oxygen battery. The rational design of the Co₂P/AB porous oxygen electrode structure provides sufficient accessible reaction sites and a short diffusion path for electrolyte penetration and diffusion of O₂.

Functional and stability orientation synthesis of materials and structures in aprotic Li–O2batteries

This review presents the recent advances made in the functional and stability orientation synthesis of materials/structures for Li–O₂batteries.

Carbon-Free O2 Cathode with Three-Dimensional Ultralight Nickel Foam-Supported Ruthenium Electrocatalysts for Li-O2 Batteries

A new carbon- and binder-free O₂ cathode was fabricated by electroplating Ru-nanoparticle-coated ultralight Ni foam, which has good electron-conducting and electrocatalytic properties. This all-metal monolithic structure was able to suppress CO₂ evolution and provided 306 times higher capacity than commercial Ni foam-based O₂ cathodes.

Graphene: a promising 2D material for electrochemical energy storage

Graphene, with unique two-dimensional form and numerous appealing properties, promises to remarkably increase the energy density and power density of electrochemical energy storage devices (EESDs), ranging from the popular lithium ion batteries and supercapacitors to next-generation high-energy batteries. Here, we review the recent advances of the state-of-the-art graphene-based materials for EESDs, including lithium ion batteries, supercapacitors, micro-supercapacitors, high-energy lithium-air and lithium-sulfur batteries, and discuss the importance of the pore, doping, assembly, hybridization and functionalization of different nano-architectures in improving electrochemical performance. The major roles of graphene are highlighted as (1) a superior active material, (2) ultrathin 2D flexible support, and (3) an inactive yet electrically conductive additive. Furthermore, we address the enormous potential of graphene for constructing new-concept emerging graphene-enabled EESDs with multiple functionalities of lightweight, ultra-flexibility, thinness, and novel cell configurations. Finally, future perspectives and challenges of graphene-based EESDs are briefly discussed.

Ultrathin, Lightweight, and Wearable Li-O2 Battery with High Robustness and Gravimetric/Volumetric Energy Density

An ultrathin, lightweight, and wearable Li-O₂ battery with a novel segmented structure is first fabricated by employing a "break up the whole into parts" strategy. Superior battery performance including low overpotential, high specific capacity, good rate capability, excellent cycle stability, and high gravimetric/volumetric energy density (294.68 Wh kg^-1 /274.06 Wh L^-1 ) is successfully achieved even under repeatedly various deformation.

Perovskite-type La0.8Sr0.2Co0.8Fe0.2O3 with uniform dispersion on N-doped reduced graphene oxide as an efficient bi-functional Li–O2 battery cathode

A composite cathode including N-rGO with homogeneously dispersed perovskite La_0.8Sr_0.2Co_0.8Fe_0.2O₃ on the surface is studied.

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.