Tuning Li2O2 Formation Routes by Facet Engineering of MnO2 Cathode Catalysts

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.

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.

Light Enables the Cathodic Interface Reaction Reversibility in Solid-State Lithium-Oxygen Batteries

Limited triple-phase boundaries arising from the accumulation of solid discharge product(s) in solid-state cathodes (SSCs) pose a challenge to high-property solid-state lithium-oxygen batteries (SSLOBs). Light-assisted SSLOBs have been gradually explored as an ingenious system; however, the fundamental mechanisms of the SSCs interface behavior remain unclear. Here, we discovered that light assistance can enhance the fast inner-sphere charge transfer in SSCs and regulate the discharge products with spherical particles generated via the surface growth model. Moreover, the high photoelectron excitation and transportation capabilities of SSCs can retard cathodic catalytic decay by avoiding structural degradation of the cathode with a reduced charge voltage. The light-induced SSLOBs exhibited excellent stability (170 cycles) with a low discharge-charge polarization overpotential (0.27 V). Furthermore, transparent SSLOBs with exceptional flexibility, mechanical stability, and multiform shapes were fabricated for theory-to-practical applications in sunlight-induced batteries. Our study opens new opportunities for the introduction of solar energy into energy storage systems.

Boosting Sodium Compensation Efficiency via a CNT/MnO2 Catalyst toward High-Performance Na-Ion Batteries

The formation of a solid electrolyte interphase on carbon anodes causes irreversible loss of Na+ ions, significantly compromising the energy density of Na-ion full cells. Sodium compensation additives can effectively address the irreversible sodium loss but suffer from high decomposition voltage induced by low electrochemical activity. Herein, we propose a universal electrocatalytic sodium compensation strategy by introducing a carbon nanotube (CNT)/MnO2 catalyst to realize full utilization of sodium compensation additives at a much-reduced decomposition voltage. The well-organized CNT/MnO2 composite with high catalytic activity, good electronic conductivity, and abundant reaction sites enables sodium compensation additives to decompose at significantly reduced voltages (from 4.40 to 3.90 V vs Na+/Na for sodium oxalate, 3.88 V for sodium carbonate, and even 3.80 V for sodium citrate). As a result, sodium oxalate as the optimal additive achieves a specific capacity of 394 mAh g-1, almost reaching its theoretical capacity in the first charge, increasing the energy density of the Na-ion full cell from 111 to 158 Wh kg-1 with improved cycle stability and rate capability. This work offers a valuable approach to enhance sodium compensation efficiency, promising high-performance energy storage devices in the future.

Structure-Property Interplay Within Microporous Manganese Dioxide Tunnels For Sustainable Energy Storage

Tunnel-structured manganese dioxides (MnO2 ), also known as octahedral molecule sieves (OMS), are widely studied in geochemistry, deionization, energy storage and (electro)catalysis. These functionalities originate from their characteristic sub-nanoscale tunnel framework, which, with a high degree of structural polymorphism and rich surface chemistry, can reversibly absorb and transport various ions. An intensive understanding of their structure-property relationship is prerequisite for functionality optimization, which has been recently approached by implementation of advanced (in situ) characterizations providing significant atomistic sciences. This review will thus timely cover recent advancements related to OMS and their energy storage applications, with a focus on the atomistic insights pioneered by researchers including our group: the origins of structural polymorphism and heterogeneity, the evolution of faceted OMS crystals and its effect on electrocatalysis, the ion transport/storage properties and their implication for processing OMS. These studies represent a clear rational behind recent endeavors investigating the historically applied OMS materials, the summary of which is expected to deepen the scientific understandings and guide material engineering for functionality control.

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

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

Constructing Built-In Electric Field in NiCo2 O4 -CeO2 Heterostructures to Regulate Li2 O2 Formation Routes at High Current Densities

Developing catalysts with suitable adsorption energy for oxygen-containing intermediates and elucidating their internal structure-performance relationships are essential for the commercialization of Li-O2 batteries (LOBs), especially under high current densities. Herein, NiCo2 O4 -CeO2 heterostructure with a spontaneous built-in electric field (BIEF) is designed and utilized as a cathode catalyst for LOBs at high current density. The driving mechanism of electron pumping/accumulation at heterointerface is studied via experiments and density functional theory (DFT) calculations, elucidating the growth mechanism of discharge products. The results show that BIEF induced by work function difference optimizes the affinity for LiO2 and promotes the formation of nano-flocculent Li2 O2 , thus improving LOBs performance at high current density. Specifically, NiCo2 O4 -CeO2 cathode exhibits a large discharge capacity (9546 mAh g-1 at 4000 mA g-1 ) and high stability (>430 cycles at 4000 mA g-1 ), which are better than the majority of previously reported metal-based catalysts. This work provides a new method for tuning the nucleation and decomposition of Li2 O2 and inspires the design of ideal catalysts for LOBs to operate at high current density.

Engineering the crystal facets of α-MnO2 nanorods for electrochemical energy storage: experiments and theory

Crystal facet engineering is an effective strategy for precisely regulating the orientations and electrochemical properties of metal oxides. However, the contribution of each crystal facet to pseudocapacitance is still puzzling, which is a bottleneck that restricts the specific capacitance of metal oxides. Herein, α-MnO2 nanorods with different exposed facets were synthesized through a hydrothermal route and applied to pseudocapacitors. XRD and TEM results verified that the exposure ratio of active crystal facets was significantly increased with the assistance of the structure-directing agents. XPS analysis showed that there was more adsorbed oxygen and Mn3+ on the active crystal facets, which can provide strong kinetics for the electrochemical reaction. Consequently, the α-MnO2 nanorods with {110} and {310} facets exhibited much higher pseudocapacitances of 120.0 F g-1 and 133.0 F g-1 than their α-MnO2-200 counterparts (67.5 F g-1). The theoretical calculations proved that the {310} and {110} facets have stronger adsorption capacity and lower diffusion barriers for sodium ions, which is responsible for the enhanced pseudocapacitance of MnO2. This study provides a strategy to enhance the electrochemical performance of metal oxide, based on facet engineering.

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.

Unlock Restricted Capacity via OCe Hybridization for LiOxygen Batteries

The aprotic Li-O₂ battery (LOB) has the highest theoretical energy density of any rechargeable batteries. However, such system is largely restricted by the electrochemically formed lithium peroxide (Li₂ O₂ ) on the cathode surface, leading ultimately to low actual capacities and early cell death. In contrast to the surface-mediated growth of thin film with a thickness <50 nm, a non-crystalline Li₂ O₂ film with a thickness of >400 nm can be formed via an optimal OCe hybridized electronic structure. Specially, oxygen can react with dissolved cerium cations in the electrolyte via a cerium-oxygen reaction to form a high-energy faceted cerium oxide catalyst, which not only generates a great number of non-saturable active sites, but also erects electron transport bridges between the lattice O and adjacent Ce atoms. Such CeO orbital hybridization also forms a direct charge transfer channel from Ce-4f of CeO₂ to O 2 2 - ${\rm{O}}_2^{2 - }$ -π* of Li₂ O₂ , eventually leading to submicron-thick Li₂ O₂ shells via a subsequent lithium-oxygen reaction. Relying on the above merits, this work unlocks the rechargeable capacities of LOB from restricted 1000 to unprecedented 10 000 mAh g^-1 with good cyclabilities and reduced charge-discharge overpotentials.

Single-Atom Pd-N4 Catalysis for Stable Low-Overpotential Lithium-Oxygen Battery

The critical challenge for Li-O₂ batteries lies in the large charge overpotential, leading to undesirable side reactions and inferior cycle stability. Single-atom catalysts have shown promising prospects in expediting the kinetics of oxygen evolution reaction (OER) for Li-O₂ batteries. However, a present practical drawback is the limited understanding of the correlation between the unique atomic structures and the OER mechanism. Herein, a template-assisted strategy is reported to synthesize atomically dispersed Pd anchored on N-doped carbon spheres as cathode catalysts. Benefiting from the well-defined Pd-N₄ moiety, the morphology and distribution of Li₂ O₂ products are distinctly regulated with optimized decomposition reversibility. Theoretical simulations reveal that the unique configuration of Pd-N₄ will contribute to the electron transfer from Pd atoms to the adjacent N atoms, which turns the originally electroneutral Pd into positively charged and downshifts the d-band center and therefore weakens its adsorption energy with the intermediates. The Li-O₂ batteries with Pd SAs/NC cathode achieve a charge overpotential of only 0.24 V and sustainable low-overpotential cycling stability (500 mA g^-1 ), and can retain a low charge voltage to a very high capacity of 10 000 mAh g^-1 . This work provides some insights into designing efficient single-atom catalysts for stable low-overpotential Li-O₂ batteries.

Reversible Conversion between Lithium Superoxide and Lithium Peroxide: A Closed “Lithium–Oxygen” Battery

Lithium–air batteries have become a desirable research direction in the field of green energy due to their large specific capacity and high energy density. The current research mainly focuses on an open system continuously supplying high-purity oxygen or air. However, factors such as water and CO2 in the open system and liquid electrolytes’ evaporation will decrease battery performance. To improve the practical application of lithium–air batteries, developing a lithium–oxygen battery that does not need a gaseous oxygen supply is desirable. In this study, we designed a closed lithium–oxygen battery model based on the conversion of lithium superoxide and lithium peroxide (LiO2 + e− + Li+ ↔ Li2O2). Herein, the Pd-rGO as a catalyst will produce the LiO2 in the pre-discharge process, and the closed battery can cycle over 57 cycles stably. In addition to in situ Raman spectra, electrochemical quartz crystal microbalance (EQCM) and differential electrochemical mass spectrometry (DEMS) have been applied to explanation the conversion between LiO2 and Li2O2 during the charge–discharge process. This work paves the way to introduce a new closed “lithium–oxygen” battery system for developing large-capacity green energy.

A KMnO4-Generated Colloidal Electrolyte for Redox Mediation and Anode Protection in a Li-Air Battery

The rechargeable lithium-oxygen (Li-O₂) battery has the highest theoretical specific energy density of any rechargeable batteries and could transform energy storage systems if a practical device could be attained. However, among numerous challenges, which are all interconnected, are polarization due to sluggish kinetics, low cycle life, small capacity, and slow rates. In this study, we report on use of KMnO₄ to generate a colloidal electrolyte made up of MnO₂ nanoparticles. The resulting electrolyte provides a redox mediator for reducing the charge potential and lithium anode protection to increase cycle life. This electrolyte in combination with a stable binary transition metal dichalcogenide alloy, Nb_0.5Ta_0.5S₂, as the cathode enables the operation of a Li-O₂ battery at a current density of 1 mA·cm^-2 and specific capacity ranging from 1000 to 10 000 mA·h·g^-1 (corresponding to 0.1-1 mA·h·cm^-2) in a dry air environment with a cycle life of up to 150. This colloidal electrolyte provides a robust approach for advancing Li-air batteries.

Self-Formation CoO Nanodots Catalyst in Co(TFSI)2 -Modified Electrolyte for High Efficient Li-O2 Batteries

The major challenges for Li-O₂ batteries are sluggish reaction kinetics and large overpotentials due to the cathode passivation resulting from insulative and insoluble Li₂ O₂ . Here, a novel nanodot (ND)-modified electrolyte is designed by employing cobalt bis(trifluoromethylsulfonyl)imide (Co(TFSI)₂ ) as an electrolyte additive. The Co(TFSI)₂ additive can react with discharge intermediate LiO₂ and product Li₂ O₂ to form CoO NDs. The generated CoO NDs are well dispersed in electrolyte, which integrates both the high catalytic activity of solid catalyst and the good wettability of soluble catalyst. Under the catalytis of CoO NDs, Li₂ O₂ is produced and deposits on the cathode together with them. At the recharge process, these well dispersed CoO NDs help to decompose solid Li₂ O₂ at a lower overpotential. The Li-O₂ cells with Co(TFSI)₂ exhibit a long cycle life of 200 cycles at a current density of 200 mA g^-1 under a cutoff capacity of 1000 mAh g^-1 , as well as a superior reversibility associated with the Li₂ O₂ formation and decomposition. The study is expected to broaden the range of electrolyte additives and provide a new view to developing highly dispersed NDs-based catalysts for Li-O₂ batteries.

The key to improving the performance of Li-air batteries: Recent progress and challenges of the catalysts

Li-air batteries are considered to be one of the most promising energy storage devices due to their high energy density and large specific capacity. But the high overpotential, the sluggish...

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.

Atomistic Insights of Irreversible Li+ Intercalation in MnO2 Electrode

Tunnel-structured MnO₂ represents open-framed electrode materials for reversible energy storage. Its wide application is limited by its poor cycling stability, whose structural origin is unclear. We tracked the structure evolution of β-MnO₂ upon Li⁺ ion insertion/extraction by combining advanced in situ diagnostic tools at both electrode level (synchrotron X-ray scattering) and single-particle level (transmission electron microscopy). The instability is found to originate from a partially reversible phase transition between β-MnO₂ and orthorhombic LiMnO₂ upon lithiation, causing cycling capacity decay. Moreover, the MnO₂ /LiMnO₂ interface exhibits multiple arrow-headed disordered regions, which severely chop into the host and undermine its structural integrity. Our findings could account for the cycling instability of tunnel-structured materials, based on which future strategies should focus on tuning the charge transport kinetics toward performance enhancement.

Synergistic Catalysis by Single-Atom Catalysts and Redox Mediator to Improve Lithium-Oxygen Batteries Performance

Lithium-oxygen (Li-O₂ ) batteries with ultrahigh theoretical energy density have attracted widespread attention while there are still problems with high overpotential and poor cycle stability. Rational design and application of efficient catalysts to improve the performance of Li-O₂ batteries is of significant importance. In this work, Co single atoms catalysts are successfully combined with redox mediator (lithium bromide [LiBr]) to synergistically catalyze electrochemical reactions in Li-O₂ batteries. Single-atom cobalt anchored in porous N-doped hollow carbon spheres (CoSAs-NHCS) with high specific surface area and high catalytic activity are utilized as cathode material. However, the potential performances of batteries are difficult to adequately achieve with only CoSAs-NHCS, owing to the blocked electrochemical active sites covered by insulating solid-state discharge product Li₂ O₂ . Combined with LiBr as redox mediator, the enhanced OER catalytic effect extends throughout all formed Li₂ O₂ during discharge. Meantime, the certain adsorption effect of CoSAs-NHCS on Br₂ and Br₃ ^- can reduce the shuttle of RM^ox . The synergistic effect of Co single atoms and LiBr can not only promote more Li₂ O₂ decomposition but also reduce the shuttle effect by absorbing the oxidized redox mediator. Li-O₂ batteries with Co single atoms and LiBr achieve ultralow overpotential of 0.69 V and longtime stable cyclability.

Direct Visualization of Atomic-Scale Heterogeneous Structure Dynamics in MnO2 Nanowires

Manganese oxides are attracting great interest owing to their rich polymorphism and multiple valent states, which give rise to a wide range of applications in catalysis, capacitors, ion batteries, and so forth. Most of their functionalities are connected to transitions among the various polymorphisms and Mn valences. However, their atomic-scale dynamics is still a great challenge. Herein, we discovered a strong heterogeneity in the crystalline structure and defects, as well as in the Mn valence state. The transitions are studied by in situ transmission electron microscopy (TEM), and they involve a complex ordering of [MnO₆] octahedra as the basic building tunnels. MnO₂ nanowires synthesized using solution-based hydrothermal methods usually exhibit a large number of multiple polymorphism impurities with different tunnel sizes. Upon heating, MnO₂ nanowires undergo a series of stoichiometric polymorphism changes, followed by oxygen release toward an oxygen-deficient spinel and rock-salt phase. The impurity polymorphism exhibits an abnormally high stability with interesting small-large-small tunnel size transition, which is attributed to a preferential stabilizer (K⁺) concentration, as well as a strong competition of kinetics and thermodynamics. Our results unveil the complicated intergrowth of polymorphism impurities in MnO₂, which provide insights into the heterogeneous kinetics, thermodynamics, and transport properties of the tunnel-based building blocks.

Crystal Phase-Controlled Modulation of Binary Transition Metal Oxides for Highly Reversible Li-O2 Batteries

Reducing charge-discharge overpotential of transition metal oxide catalysts can eventually enhance the cell efficiency and cycle life of Li-O₂ batteries. Here, we propose that crystal phase engineering of transition metal oxides could be an effective way to achieve the above purpose. We establish controllable crystal phase modulation of the binary Mn_xCo_1-xO by adopting a cation regulation strategy. Systematic studies reveal an unprecedented relevancy between charge overpotential and crystal phase of Mn_xCo_1-xO catalysts, whereas a dramatically reduced charge overpotential (0.48 V) via a rational optimization of Mn/Co molar ratio = 8/2 is achieved. Further computational studies indicate that the different morphologies of Li₂O₂ should be related to different electronic conductivity and binding of Li₂O₂ on crystal facets of Mn_xCo_1-xO catalysts, finally leading to different charge overpotential. We anticipate that this specific crystal phase engineering would offer good technical support for developing high-performance transition metal oxide catalysts for advanced Li-O₂ batteries.

A Novel Cr2O3/MnO2-x Electrode for Lithium-Oxygen Batteries with Low Charge Voltage and High Energy Efficiency

A high energy efficiency, low charging voltage cathode is of great significance for the development of non-aqueous lithium-oxygen batteries. Non-stoichiometric manganese dioxide (MnO_2-x) and chromium trioxide (Cr₂O₃) are known to have good catalytic activities for the discharging and charging processes, respectively. In this work, we prepared a cathode based on Cr₂O₃ decorated MnO_2-x nanosheets via a simple anodic electrodeposition-electrostatic adsorption-calcination process. This combined fabrication process allowed the simultaneous introduction of abundant oxygen vacancies and trivalent manganese into the MnO_2-x nanosheets, with a uniform load of a small amount of Cr₂O₃ on the surface of the MnO_2-x nanosheets. Therefore, the Cr₂O₃/MnO_2-x electrode exhibited a high catalytic effect for both discharging and charging, while providing high energy efficiency and low charge voltage. Experimental results show that the as-prepared Cr₂O₃/MnO_2-x cathode could provide a specific capacity of 6,779 mA·h·g⁻¹ with a terminal charge voltage of 3.84 V, and energy efficiency of 78%, at a current density of 200 mA·g⁻¹. The Cr₂O₃/MnO_2-x electrode also showed good rate capability and cycle stability. All the results suggest that the as-prepared Cr₂O₃/MnO_2-x nanosheet electrode has great prospects in non-aqueous lithium-oxygen batteries.

MnO2 -Based Materials for Environmental Applications

Manganese dioxide (MnO₂ ) is a promising photo-thermo-electric-responsive semiconductor material for environmental applications, owing to its various favorable properties. However, the unsatisfactory environmental purification efficiency of this material has limited its further applications. Fortunately, in the last few years, significant efforts have been undertaken for improving the environmental purification efficiency of this material and understanding its underlying mechanism. Here, the aim is to summarize the recent experimental and computational research progress in the modification of MnO₂ single species by morphology control, structure construction, facet engineering, and element doping. Moreover, the design and fabrication of MnO₂ -based composites via the construction of homojunctions and MnO₂ /semiconductor/conductor binary/ternary heterojunctions is discussed. Their applications in environmental purification systems, either as an adsorbent material for removing heavy metals, dyes, and microwave (MW) pollution, or as a thermal catalyst, photocatalyst, and electrocatalyst for the degradation of pollutants (water and gas, organic and inorganic) are also highlighted. Finally, the research gaps are summarized and a perspective on the challenges and the direction of future research in nanostructured MnO₂ -based materials in the field of environmental applications is presented. Therefore, basic guidance for rational design and fabrication of high-efficiency MnO₂ -based materials for comprehensive environmental applications is provided.

Cation Additive Enabled Rechargeable LiOH-Based Lithium-Oxygen Batteries

Lithium-oxygen (Li-O₂ ) batteries have attracted extensive research interest due to their high energy density. Other than Li₂ O₂ (a typical discharge product in Li-O₂ batteries), LiOH has proved to be electrochemically active as an alternative product. Here we report a simple strategy to achieve a reversible LiOH-based Li-O₂ battery by using a cation additive, sodium ions, to the lithium electrolyte. Without redox mediators in the cell, LiOH is detected as the sole discharge product and it charges at a low charge potential of 3.4 V. A solution-based reaction route is proposed, showing that the competing solvation environment of the catalyst and Li⁺ leads to LiOH precipitation at the cathode. It is critical to tune the cell chemistry of Li-O₂ batteries by designing a simple system to promote LiOH formation/decomposition.

Porous Materials Applied in Nonaqueous Li-O2 Batteries: Status and Perspectives

Porous materials possessing high surface area, large pore volume, tunable pore structure, superior tailorability, and dimensional effect have been widely applied as components of lithium-oxygen (Li-O₂ ) batteries. Herein, the theoretical foundation of the porous materials applied in Li-O₂ batteries is provided, based on the present understanding of the battery mechanism and the challenges and advantageous qualities of porous materials. Furthermore, recent progress in porous materials applied as the cathode, anode, separator, and electrolyte in Li-O₂ batteries is summarized, together with corresponding approaches to address the critical issues that remain at present. Particular emphasis is placed on the importance of the correlation between the function-orientated design of porous materials and key challenges of Li-O₂ batteries in accelerating oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) kinetics, improving the electrode stability, controlling lithium deposition, suppressing the shuttle effect of the dissolved redox mediators, and alleviating electrolyte decomposition. Finally, the rational design and innovative directions of porous materials are provided for their development and application in Li-O₂ battery systems.

Surface Mechanism of Catalytic Electrodes in Lithium-Oxygen Batteries: How Nanostructures Mediate the Interfacial Reactions

The use of catalysts is the key to boost electrode reactions in lithium-oxygen (Li-O₂) batteries. In-depth understanding of the nanoscale catalytic effect at electrode/electrolyte interfaces is of great significance for guiding a design of functionally optimized catalyst. Here, using electrochemical atomic force microscopy, we present the real-time imaging of interfacial evolution on nanostructured Au electrodes in a working battery, revealing that the nanostructure of Au is directly related to the catalytic activity toward oxygen reduction reaction (ORR)/oxygen evolution reaction (OER). In situ views show that nanoporous Au with a size of ∼14 nm for ligaments and ∼5 nm for nanopores promote the nucleation and growth of discharge product Li₂O₂ with large size at a high discharge voltage, yet densely packed Au nanoparticles with a diameter of ∼15 nm could catalyze Li₂O₂ to fully decompose via the top-bottom approach at a low charge potential. In addition, the difference in the nucleation potential of Li₂O₂ on the electrode with hybrid nanostructures could result in an uneven distribution of discharge products, which is alleviated at a large discharge rate and the capacity of the battery is improved significantly. These observations provide deep insights into the mechanisms of Li-O₂ interfacial reaction catalyzed by nanostructured catalysts and strategies for improving Li-O₂ batteries.

Oxygen-Based Anion Redox for Lithium Batteries

ConspectusThe importance of current Li-ion batteries (LIBs) in modern society cannot be overstated. While the energy demands of devices increase, the corresponding enhancements in energy density of battery technologies are highly sought after. Currently, many different battery concepts, such as Li-S and metal-air among many others, have been investigated. However, their practical implementation has mostly been restricted to the prototyping stage. In fact, most of these technologies require rework of existing Li-ion battery manufacturing facilities and will naturally incur resistance to change from industry. For this reason, one specifically attractive technology, anionic redox in transition metal oxides, has gained much attention in the recent years. Its ability to be directly used in already established processes and higher energy density with similar electrolyte formulation make it a key materials research direction for next generation Li-ion batteries. In regular LIBs, the redox active centers are the transition metal cation. In anion redox, both the anion (typically O) and the transition metal cation are utilized as redox centers with enormous implications for increasing energy density. This new material can be highly competitive for replacing the current LIB technologies. However, much is still unknown about its cycling mechanism. Upon activating the O redox couples, most cationic and anionic redox active materials will either evolve O₂ or undergo irreversible structural degradation with associated severe decreases in electrochemical performance. By understanding the transition from full anion redox to partial cationic and anionic redox, we hope readers can gain a deeper understanding of the topic.This Account will focus mainly on the work that was conducted by our group at Argonne National Laboratory. The phenomenon of cationic and anionic redox in a lithium-ion battery cathode will first be discussed. Our work in resonant inelastic X-ray scattering to investigate the spectroscopic features of O after delithiation has found potential "fingerprint" signals that could likely be used to identify and confirm reversible O redox if corroborated with other techniques. To follow, we will examine our work on Li-O₂ batteries. While our group and the research community have had many significant contributions and improvements to the field of Li-O₂ (such as decreasing overpotential and achieving cyclability in air environment), its practical application is still far from realization. Perhaps our most important contribution to this area is the discovery that Ir deposited on reduced graphene oxide can be used to halt the reduction of O₂ at the LiO₂ oxidation state. This not only significantly decreases the charge overpotential but also presents the important concept of oxidation-state controlled discharge. Subsequently, we will focus on our oxidation state-controlled redox-based charging of oxygen in a pure oxygen redox Li-ion battery. Future implications of this technology will be emphasized.

Should All Electrochemical Energy Materials Be Isomaterially Heterostructured to Optimize Contra and Co-varying Physicochemical Properties?

Sustainable energy and chemical/material transformation constrained by limited greenhouse gas generation impose a grand challenge and posit outstanding opportunities to electrochemical material devices. Dramatic advancements in experimental and computational methodologies have captured detailed insights into the working of these material devices at a molecular scale and have brought to light some fundamental constraints that impose bounds on efficiency. We propose that the coupling of molecular events in the material device gives rise to contra-varying or co-varying properties and efficiency improving partial decoupling of such properties can be achieved via introducing engineered heterogeneities. A specific class of engineered heterogeneity is in the form of isomaterial heterostructures comprised of non-native and native polymorphs. The non-native polymorph differs from their native/ground state bulk polymorph in terms of its discrete translational symmetry and we anticipate specific symmetry relationships exist between non-native and native structures that enable the formation of interfaces that enhance efficiency. We present circumstantial evidence and provide speculative mechanisms for such an approach with the hope that a more comprehensive delineation of proposed material design will be undertaken.

Tuning lithium-peroxide formation and decomposition routes with single-atom catalysts for lithium-oxygen batteries

Lithium-oxygen batteries with ultrahigh energy density have received considerable attention as of the future energy storage technologies. The development of effective electrocatalysts and a corresponding working mechanism during cycling are critically important for lithium-oxygen batteries. Here, a single cobalt atom electrocatalyst is synthesized for lithium-oxygen batteries by a polymer encapsulation strategy. The isolated moieties of single atom catalysts can effectively regulate the distribution of active sites to form micrometre-sized flower-like lithium peroxide and promote the decomposition of lithium peroxide by a one-electron pathway. The battery with single cobalt atoms can operate with high round-trip efficiency (86.2%) and long-term stability (218 days), which is superior to a commercial 5 wt% platinum/carbon catalyst. We reveal that the synergy between a single atom and the support endows the catalyst with excellent stability and durability. The promising results provide insights into the design of highly efficient catalysts for lithium-oxygen batteries and greatly expand the scope of future investigation.

Heterostructured NiS2/ZnIn2S4 Realizing Toroid-like Li2O2 Deposition in Lithium-Oxygen Batteries with Low-Donor-Number Solvents

The aprotic lithium-oxygen (Li-O₂) battery has triggered tremendous efforts for advanced energy storage due to the high energy density. However, realizing toroid-like Li₂O₂ deposition in low-donor-number (DN) solvents is still the intractable obstruction. Herein, a heterostructured NiS₂/ZnIn₂S₄ is elaborately developed and investigated as a promising catalyst to regulate the Li₂O₂ deposition in low-DN solvents. The as-developed NiS₂/ZnIn₂S₄ promotes interfacial electron transfer, regulates the adsorption energy of the reaction intermediates, and accelerates O-O bond cleavage, which are convincingly evidenced experimentally and theoretically. As a result, the toroid-like Li₂O₂ product is achieved in a Li-O₂ battery with low-DN solvents via the solvation-mediated pathway, which demonstrates superb cyclability over 490 cycles and a high output capacity of 3682 mA h g^-1. The interface engineering of heterostructure catalysts offers more possibilities for the realization of toroid-like Li₂O₂ in low-DN solvents, holding great promise in achieving practical applications of Li-O₂ batteries as well as enlightening the material design in catalytic systems.

Process for a Free-Standing and Stable All-Metal Structure for Symmetrical Lithium-Oxygen Batteries

A number of inherent and thorny obstacles still stand in the way of the practical application of Li-O₂ batteries, which require development of an advanced lithium anode and O₂ cathode. Herein, the strategy of a symmetrical Li-O₂ battery is presented. Specifically, Cu nanoneedle arrays with a nanoengineered Au coating are grown directly on a Cu foam substrate (Au/Cu@FCu), which can act as both the anode backbone and the cathode in a Li-O₂ battery. The excellent conductivity, high porosity, large specific surface, and superior lithiophilicity as well as high catalytic activity of the Au/Cu@FCu electrodes can simultaneously regulate the deposition behavior of the lithium metal in the anode and catalyze the formation/decomposition of Li₂O₂ in the cathode. As a result, the Li uniformly deposited on the Au/Cu@FCu anode without Li dendrites, showing a high Coulombic efficiency over 96% and a long and stable cycle lifetime over 970 h. At the same time, the Au/Cu@FCu cathode demonstrates extremely low overpotentials (0.64 V) and a much higher specific capacity of 27 270 mAh g^-1 compared to the Li-O₂ batteries with a carbon-free cathode reported to date. Moreover, the "ebb and flow" phenomenon of the anode and cathode morphology is also observed in the Li-O₂ battery.

A Review on Applications of Layered Phosphorus in Energy Storage

Phosphorus in energy storage has received widespread attention in recent years. Both the high specific capacity and ion mobility of phosphorus may lead to a breakthrough in energy storage materials. Black phosphorus, an allotrope of phosphorus, has a sheet-like structure similar to graphite. In this review, we describe the structure and properties of black phosphorus and characteristics of the conductive electrode material, including theoretical calculation and analysis. The research progress in various ion batteries, including lithium-sulfur batteries, lithium–air batteries, and supercapacitors, is summarized according to the introduction of black phosphorus materials in different electrochemical applications. Among them, with the introduction of black phosphorus in lithium-ion batteries and sodium-ion batteries, the research on the properties of black phosphorus and carbon composite is introduced. Based on the summary, the future development trend and potential of black phosphorus materials in the field of electrochemistry are analyzed.

Phosphazene-derived stable and robust artificial SEI for protecting lithium anodes of Li–O2 batteries

An artificial solid electrolyte interphase with high ionic conductivity and mechanical robustness was designed to suppresses the growth of Li dendrites.

Pt/NiO Microsphere Composite as Efficient Multifunctional Catalysts for Nonaqueous Lithium-Oxygen Batteries and Alkaline Fuel Cells: The Synergistic Effect of Pt and Ni

Developing efficient and low-cost multifunctional electrocatalysts is important for electrochemical devices. In this work, a cost-effective Pt/NiO composite with very limited Pt loading (from 0.5 to 3%) was controllably synthesized through facile hydrothermal procedures. The composite demonstrated the improved catalytic performance as applied to the nonaqueous Li-O₂ batteries and the alkaline fuel cells. Regarding the alkaline fuel cells, 1% Pt/NiO composite gave rise to the best Pt distribution and thus exhibited the optimized electrochemical conductivity and properties as suggested by the significantly improved electrochemical reversibility. Meanwhile, the demonstrated 1% Pt/NiO composite presented high catalytic capability as electrode for Li-O₂ batteries, which allowed for much improved capacity utilization, high cycling stability, high initial capacity (2329 mAh/g), and no obvious voltage drop during cycling. Such multiple advantages of prepared composite electrode material offer new prospects and application as multifunctional electrocatalysts for both Li-O₂ batteries and alkaline fuel cells.