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

In situ visualization of synergistic effects between electrolyte additives and catalytic electrodes in Li-O2 batteries

By using in situ atomic force microscopy, Li-O2 interfacial reactions promoted synergistically by the electrolyte additive K+ and Pt nanoparticles electrode are visualized. The Pt nanoparticles electrode promotes the formation of the intermediate lithium superoxide (LiO2) and K+ assists its diffusion into the electrolyte, thereby promoting the formation of large-sized discharge products during discharging and increasing the discharge capacity of the Li-O2 battery. These results provide direct evidence for clarifying the interfacial synergy mechanism of electrolyte additives and solid catalysts.

Recent progresses and challenges in aqueous lithium-air batteries relating to the solid electrolyte separator: A mini-review

The lithium-air (Li-air) battery utilizes infinite oxygen in the air to store or release energy through a semi-open cathode structure and bears an ultra-high theoretical energy density of more than 1,000 Wh/kg. Therefore, it has been denoted as the candidate for next-generation energy storage in versatile fields such as electric vehicles, telecommunications, and special power supply. Among all types of Li-air batteries, an aqueous Li-air battery bears the advantages of a high theoretical energy density of more than 1,700 Wh/kg and does not have the critical pure oxygen atmosphere issues in a non-aqueous lithium-air battery system, which is more promising for the actual application. To date, great achievements have been made in materials' design and cell configurations, but critical challenges still remain in the field of the solid electrolyte separator, its related lithium stripping/plating at the lithium anode, and catholyte design. In this mini-review, we summarized recent progress related to the solid electrolyte in aqueous Li-air batteries focusing on both material and battery device development. Moreover, we proposed a discussion and unique outlook on improving solid electrolyte compatibility and battery performance, thus designing an aqueous Li-air battery with higher energy density and better cycle performance in the future.

Tailored Plasmonic Ru/OV-MoO2 on TiO2 Catalysts via Solid-Phase Interface Engineering: Toward Highly Efficient Photoassisted Li-O2 Batteries with Enhanced Cycling Reliability

The photoassisted electrochemical reactions are considered an effective method to reduce the overpotential of Li-O₂ batteries. However, achieving long-term cell cycling stability remains a challenge. Here, we report a solid-phase interfacial reaction (SPIR) strategy that introduces both oxygen vacancies (O_V) and metal centers (Ru) into the MoO₂ to synthesize the surface plasmon (i.e., Ru/O_V-MoO₂). Then, Ru/O_V-MoO₂ can be uniformly loaded on the TiO₂ nanowires by the hydrothermal method. The plasma effect of Ru/O_V-MoO₂ demonstrates the effective reduction of the photoexcited electron and hole recombination to improve visible light-harvesting ability. The lifetime of electrons and holes can be extended by Ru nanoparticles, which is beneficial for promoting the formation and decomposition of Li₂O₂. In addition, the generated O_V further enhanced the migration of electrons and Li⁺, thus improving the ORR performance. The Ru/O_V-MT/CC cathode corroborates excellent stability and catalytic performance in the photoassisted Li-O₂ battery, with an overpotential value of 0.47 V, achieving the highest energy efficiency of 93.94%, retaining at 89.13% after 800 h. This work offers a platform for preparing a stable, bifunctional catalyst with the high total activity of a photoassisted Li-O₂ battery.

Pr6O11: Temperature-Dependent Oxygen Vacancy Regulation and Catalytic Performance for Lithium-Oxygen Batteries

Many challenges still exist in lithium-oxygen batteries (LOBs), particularly exploring an efficient catalyst to optimize the reaction pathway and regulate the Li₂O₂ nucleation. Pr₆O₁₁ has a unique 4f electronic structure and the highest oxygen ion mobility among rare earth oxides, exhibiting superior electronic, optical, and chemical properties. These unique properties might endow it with advanced catalytic activities for LOBs. This work reports two crystal forms of Pr₆O₁₁ as novel catalysts and regulates the oxygen vacancy (V_o) concentrations by feasible calcination. Thermogravimetric analysis, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) confirm the conversion from commercial Pr₆O₁₁ to cubic fluorite Pr₆O₁₁ and V_o-rich Pr₆O₁₁. Photographs, high-resolution transmission electron microscopy, selected area electron diffraction, XPS, and electron paramagnetic resonance robustly demonstrate the temperature-dependent evolution of V_o. Ex situ XPS, scanning electron microscopy, and electrochemical techniques are used to study the catalytic mechanism and electrochemical reversibility. It is found that an appropriate V_o concentration can boost O₂ adsorption/desorption, accelerate electron transport, and reduce the reaction energy barrier. V_o-rich Pr₆O₁₁ optimizes the reaction pathway by offering an intermediate Li_2-xO₂ (with metalloid conductivity) and adjusting Li₂O₂ into vertically staggered nanoflakes, effectively avoiding the suffocation of the catalytic surface and presenting excellent capacity, cycling stability, and rate performance.

Suppressing Redox Shuttling with Lithiated Nafion-Modified Separators for Li-O2 Batteries

Although the employment of redox mediator (RM) is an effective strategy to reduce the overpotential by avoiding the direct electrochemical oxidization of Li₂ O₂ during charging, an unexpected redox shuttling in Li-O₂ system leads to RM degradation and continuous deterioration of Li anode, finally resulting in a limited cycling stability. Here, a functional lithiated Nafion-modified separator was firstly introduced to inhibit the shuttle effect by coulombic/electrostatic interactions in RM-involved Li-O₂ batteries. The fabrication of the separator involved easily accessible raw materials and an easy-to-operate process, which made it suitable for large-scale production. The enhancement of lithiated process on electrochemical properties was systematically investigated. In addition, the influence of decorated amount on cycling stability was also studied. Furthermore, the functional contribution of lithiated Nafion on inhibition of redox shuttling and the working mechanism for cells with and without as-prepared separators were proposed. This work can give an insight into the development of functional separator (i. e., activity issue) and the suppression of parasitic reactions (i. e., selectivity issue) in Li-O₂ batteries.

Light-Assisted Li-O2 Batteries with Lowered Bias Voltages by Redox Mediators

The enormous overpotential caused by sluggish kinetics of the oxygen reduction reaction and the oxygen evolution reaction prevents the practical application of Li-O₂ batteries. The recently proposed light-assisted strategy is an effective way to improve round-trip efficiency; however, the high-potential photogenerated holes during the charge would degrade the electrolyte with side reactions and poor cycling performance. Herein, a synergistic interaction between a polyterthiophene photocatalyst and a redox mediator is employed in Li-O₂ batteries. During the discharge, the voltage can be compensated by the photovoltage generated on the photoelectrode. Upon the charge with illumination, the photogenerated holes can be consumed by the oxidization of iodide ions, and thus the external circuit voltage is compensated by photogenerated electrons. Accordingly, a smaller bias voltage is needed for the semiconductor to decompose Li₂ O₂ , and the potential of photogenerated holes decreases. Finally, the round-trip efficiency of the battery reaches 97% with a discharge voltage of 3.10 V and a charge voltage of 3.19 V. The batteries show stable operation up to 150 cycles without increased polarization. This work provides new routes for light-assisted Li-O₂ batteries with reduced overpotential and boosted efficiency.

Surface-Preferred Crystal Plane Growth Enabled by Underpotential Deposited Monolayer toward Dendrite-Free Zinc Anode

Aqueous Zn batteries with ideal energy density and absolute safety are deemed the most promising candidates for next-generation energy storage systems. Nevertheless, stubborn dendrite formation and notorious parasitic reactions on the Zn metal anode have significantly compromised the Coulombic efficiency (CE) and cycling stability, severely impeding the Zn metal batteries from being deployed in the proposed applications. Herein, instead of random growth of Zn dendrites, a guided preferential growth of planar Zn layers is accomplished via atomic-scale matching of the surface lattice between the hexagonal close-packed (hcp) Zn(002) and face-centered cubic (fcc) Cu(100) crystal planes, as well as underpotential deposition (UPD)-enabled zincophilicity. The underlying mechanism of uniform Zn plating/stripping on the Cu(100) surface is demonstrated by ab initio molecular dynamics simulations and density functional theory calculations. The results show that each Zn atom layer is driven to grow along the exposed closest packed plane (002) in hcp Zn metal with a low lattice mismatch with Cu(100), leading to compact and planar Zn deposition. In situ optical visualization inspection is adopted to monitor the dynamic morphology evolution of such planar Zn layers. With this surface texture, the Zn anode exhibits exceptional reversibility with an ultrahigh Coulombic efficiency (CE) of 99.9%. The MnO₂//Zn@Cu(100) full battery delivers long cycling stability over 548 cycles and outstanding specific energy and power density (112.5 Wh kg^-1 even at 9897.1 W kg^-1). This work is expected to address the issues associated with Zn metal anodes and promote the development of high-energy rechargeable Zn metal batteries.

Composite NiCo2 O4 @CeO2 Microsphere as Cathode Catalyst for High-Performance Lithium-Oxygen Battery

The large overpotential and poor cycle stability caused by inactive redox reactions are tough challenges for lithium-oxygen batteries (LOBs). Here, a composite microsphere material comprising NiCo₂ O₄ @CeO₂ is synthesized via a hydrothermal approach followed by an annealing processing, which is acted as a high performance electrocatalyst for LOBs. The unique microstructured catalyst can provide enough catalytic surface to facilitate the barrier-free transport of oxygen as well as lithium ions. In addition, the special microsphere and porous nanoneedles structure can effectively accelerate electrolyte penetration and the reversible formation and decomposition process of Li₂ O₂ , while the introduction of CeO₂ can increase oxygen vacancies and optimize the electronic structure of NiCo₂ O₄ , thereby enhancing the electron transport of the whole electrode. This kind of catalytic cathode material can effectively reduce the overpotential to only 1.07 V with remarkable cycling stability of 400 loops under 500 mA g^-1 . Based on the density functional theory calculations, the origin of the enhanced electrochemical performance of NiCo₂ O₄ @CeO₂ is clarified from the perspective of electronic structure and reaction kinetics. This work demonstrates the high efficiency of NiCo₂ O₄ @CeO₂ as an electrocatalyst and confirms the contribution of the current design concept to the development of LOBs cathode materials.

Efficient Modulation of Electron Pathways by Constructing a MnO2-x@CeO2 Interface toward Advanced Lithium-Oxygen Batteries

A major challenge for Li-O₂ batteries is to facilely achieve the formation and decomposition of the discharge product Li₂O₂, and the development of an active and synergistic cathode is of great significance to efficiently accelerate its formation/decomposition kinetics. Herein, a novel strategy is presented by constructing a MnO_2-x@CeO₂ heterostructure on the porous carbon matrix. When it was used as a cathode for Li-O₂ batteries, excellent electrochemical performances, including low overpotential, large discharge capacity, and superior cycling stability were obtained. Series theoretical calculations were conducted to reveal the mechanism for the reversible battery reactions and explain how Li₂O₂ interacts with the MnO_2-x@CeO₂ interface. Apart from the electronic ladders formed between MnO_2-x 3d and CeO₂ 4f orbitals, which can act as a highly efficient "electron transfer expressway", the specific adsorption of MnO_2-x and CeO₂ with Li₂O₂ molecules contributes to the enhanced anchoring force of Li₂O₂ and delocalization of the electron cloud on the Li-O bond. Thanks to the constructed heterostructure and synergistic effect, filmlike Li₂O₂ can be formed through a surface pathway, and when charging, it accelerates the separation of electrons and Li⁺ in Li₂O₂, thus achieving fast redox kinetics and low overpotential.

Effect of Surface Modification for Carbon Cathode Materials on Charge-Discharge Performance of Li-Air Batteries

Li-air batteries have attracted considerable attention as rechargeable secondary batteries with a high theoretical energy density of 11,400 kWh/g. However, the commercial application of Li-air batteries is hindered by issues such as low energy efficiency and a short lifetime (cycle numbers). To overcome these issues, it is important to select appropriate cathode materials that facilitate high battery performance. Carbon materials are expected to be ideal materials for cathodes due to their high electrical conductivity and porosity. The physicochemical properties of carbon materials are known to affect the performance of Li-air batteries because the redox reaction of oxygen, which is an important reaction for determining the performance of Li-air batteries, occurs on the carbon materials. In this study, we evaluated the effect of the surface modification of carbon cathode materials on the charge-discharge performance of Li-air batteries using commercial Ketjenblack (KB) and KB subjected to vacuum ultraviolet (VUV) irradiation as cathodes. The surface wettability of KB changed from hydrophobic to hydrophilic as a result of the VUV irradiation. The ratio of COOH and OH groups on the KB surface increased after VUV irradiation. Raman spectra demonstrated that no structural change in the KB before and after VUV irradiation was observed. The charge and discharge capacities of a Li-air battery using VUV-irradiated KB as the cathode decreased compared to original KB, whereas the cycling performance of the Li-air battery improved considerably. The sizes and shapes of the discharge products formed on the cathodes changed considerably due to the VUV irradiation. The difference in the cycling performance of the Li-air battery was discussed from the viewpoint of the chemical properties of KB and VUV-irradiated KB.

Synergy of cobalt vacancies and iron doping in cobalt selenide to promote oxygen electrode reactions in lithium-oxygen batteries

Electronic structural engineering plays a key role in the design of high-efficiency catalysts. Here, to achieve optimal electronic states, introduction of exotic Fe dopant and Co vacancy into CoSe₂ nanosheet (denoted as Fe-CoSe₂-V_Co) is presented. The obtained Fe-CoSe₂-V_Co demonstrates excellent catalytic activity as compared to CoSe₂. Experimental results and density functional theory (DFT) calculations confirm that Fe dopant and Co defects cause significant electron delocalization, which reduces the adsorption energy of LiO₂ intermediate on the catalyst surface, thereby obviously improving the electrocatalytic activity of Fe-CoSe₂-V_Co towards oxygen redox reactions. Moreover, the synergistic effect between Co vacancy and Fe dopant is able to optimize the microscopic electronic structure of Co ion, further reducing the energy barrier of oxygen electrode reactions on Fe-CoSe₂-V_Co. And the lithium-oxygen batteries (LOBs) based on Fe-CoSe₂-V_Co electrodes demonstrate a high Coulombic efficiency (CE) of about 72.66%, a large discharge capacity of about 13723 mA h g^-1, and an excellent cycling life of about 1338 h. In general, the electronic structure modulation strategy with the reasonable introduction of vacancy and dopant is expected to inspire the design of highly efficient catalysts for various electrochemical systems.

Ab Initio Exploration of Energetically and Kinetically Favorable ORR Activity on a 1T-ZrO2 Monolayer for a Nonaqueous Lithium-Oxygen Battery

Herein, we explore the potential applications of the experimentally synthesized ZrO₂ monolayer as the cathode catalyst for nonaqueous lithium-oxygen batteries. First, we show that a new peroxide-like adsorption geometry is the most stable configuration for LiO₂, which is distinct from the previously known O-Li-O triangular geometry. The proposed most stable adsorption configuration is because the Zr atoms in the substrate play a critical role in stabilizing the LiO₂ cluster. Second, our ab initio calculations indicate that both the ORR and OER catalytic activities are most likely to adopt the four-electron mechanism with a considerably low overpotential of only 0.44 and 0.76 V, respectively. Finally, we show that the adsorption energy of Li₂O₂ is a good descriptor for both ORR and OER catalytic activities, and weak Li₂O₂ adsorption behavior is positively related to low overpotentials and satisfactory catalytic performance.

Interfacial Electron Redistribution of Hydrangea-like NiO@Ni2 P Heterogeneous Microspheres with Dual-Phase Synergy for High-Performance Lithium-Oxygen Battery

Lithium-oxygen batteries (LOBs) with ultra-high theoretical energy density (≈3500 Wh kg^-1 ) are considered as the most promising energy storage systems. However, the sluggish kinetics during the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) can induce large voltage hysteresis, inferior roundtrip efficiency and unsatisfactory cyclic stability. Herein, hydrangea-like NiO@Ni₂ P heterogeneous microspheres are elaborately designed as high-efficiency oxygen electrodes for LOBs. Benefitting from the interfacial electron redistribution on NiO@Ni₂ P heterostructure, the electronic structure can be modulated to ameliorate the chemisorption of the intermediates, which is confirmed by density functional theory (DFT) calculations and experimental characterizations. In addition, the interpenetration of the PO bond at the NiO@Ni₂ P heterointerface leads to the internal doping effect, thereby boosting electron transfer to further improve ORR and OER activities. As a result, the NiO@Ni₂ P electrode shows a low overpotential of only 0.69 V, high specific capacity of 18254.1 mA h g^-1 and superior long-term cycling stability of over 1400 h. The exploration of novel bifunctional electrocatalyst in this work provides a new solution for the practical application of LOBs.

Heterogeneous Bimetallic Organic Coordination Polymer-Derived Co/Fe@NC Bifunctional Catalysts for Rechargeable Li-O2 Batteries

The Li-O₂ battery has attracted substantial attention due to its high theoretical energy density. In particular, high-efficiency oxygen catalysts are very important for the design of practical Li-O₂ batteries. Herein, we have synthesized heterogeneous crystalline-coated partially crystalline bimetallic organic coordination polymers (PC@C-BMOCPs), which are further pyrolyzed to obtain Co- and Fe-based nanoparticles embedded within rodlike N-doped carbon (Co/Fe@NC) as a bifunctional oxygen reduction reaction/oxygen evolution reaction (ORR/OER) catalyst used in the Li-O₂ battery. Owing to excellent ORR/OER catalytic ability, the Co/Fe@NC bifunctional catalyst exhibits an efficient reversible reaction between O₂ and Li₂O₂. Additionally, a large number of mesoporous channels are present in the core-shell Co/Fe@NC nanoparticles. These channels not only promote the diffusion of Li⁺ and O₂, but also create ample room to store insoluble discharge product Li₂O₂. The Li-O₂ batteries utilizing the bifunctional Co/Fe@NC oxygen electrode exhibit a large capacity of 17,326 mAh g^-1, a long cycling life of more than 250 cycles, and excellent reversibility. This work provides a universally applicable strategy for designing nonnoble metal ORR/OER catalysts with excellent electrochemical performance for metal-air batteries.

π-Conjugation Induced Anchoring of Ferrocene on Graphdiyne Enable Shuttle-Free Redox Mediation in Lithium-Oxygen Batteries

Soluble redox mediators (RMs), an alternative to conventional solid catalysts, have been considered an effective countermeasure to ameliorate sluggish kinetics in the cathode of a lithium-oxygen battery recently. Nevertheless, the high mobility of RMs leads to serious redox shuttling, which induces an undesired lithium-metal degeneration and RM decomposition during trade-off catalysis against the sustainable operation of batteries. Here, a novel carbon family of graphdiyne matrix is first proposed to decouple the charge-carrying redox property of ferrocene and the shuttle effects. It is demonstrated that a ferrocene-anchored graphdiyne framework can function as stationary RM, not only preserving the redox-mediating capability of ferrocene, but also promoting the local orientated three-dimensional (3D) growth of Li₂ O₂ . As a result, the RM-assisted catalysis in lithium-oxygen battery remains of remarkable efficiency and stability without the depletion of oxidized RMs or lithium degradation, resulting in a significantly enhanced electrochemical performance.

Ammonium Ionic Liquid-Functionalized Phenothiazine as a New Redox Mediator for High Chemical Stability on the Anode Surface in Lithium-Air Batteries

The application of redox mediators (RMs) as soluble catalysts can address the problem of insufficient contact between conventional solid catalysts for lithium-air batteries (LABs). However, oxidized RM molecules migrate to the lithium anode and react with lithium, which results in the accumulation of surface corrosion products that weaken the redox activity of the RM. This paper presents a new combination of phenothiazine (PTZ) as an RM and an ammonium-based ionic liquid (IL) source as a protective agent to prevent the side reactions with lithium and to enhance the electrochemical performance of LABs. IL-functionalized PTZ (IL-PTZ) was successfully synthesized through N-alkylation, quaternization, and anion-exchange reactions. IL-PTZ improved the chemical stability of the RM molecules on the lithium surface as well as the electrochemical performance. A microstructural analysis revealed that the IL group in the IL-PTZ molecules facilitated smooth lithium stripping/plating by blocking the side reactions between the RM and lithium. Compared with the LAB with the PTZ electrolyte, that with the IL-PTZ electrolyte exhibited a significantly higher discharge capacity (2500 mA h/g vs 1500 mA h/g) and a cycle life that was 2 times longer. The IL-PTZ molecule was demonstrated to exhibit great potential as a novel soluble catalyst for application in high-performance LABs.

Positive Feedback Mechanism to Increase the Charging Voltage of Li-O2 Batteries

The large overpotential of nonaqueous Li-O₂ batteries when charging causes low round-trip efficiency and decomposition of the electrode materials and electrolyte. The origins of this overpotential have been enthusiastically explored to date; however, a full understanding has not yet been reached because of the complexity of multistep reaction mechanisms. Here, we applied structural and electrochemical analysis techniques to investigate the reaction step that results in the increase of the overpotential when charging. Rietveld refinement of ex situ powder X-ray diffraction showed that a Li-deficient phase of Li₂O₂, Li_2-xO₂, formed when discharging and was present over the course of charging. The galvanostatic intermittent titration technique revealed that the rate-determining process in the first step of charging was a solid-solution type of delithiation. The chemical diffusion coefficient of Li⁺ ions in Li_2-xO₂, D_Li, decreases as the cell voltage increases, which in turn leads to a decrease in the oxidation rate of Li_2-xO₂. Under galvanostatic conditions, the deceleration of oxidation induces further increase of the cell voltage; therefore, an intrinsic mechanism of positive feedback to increase the cell voltage occurs in the first step. The results demonstrate that the continuity of the first step can be extended by the suppression of changes in any of the elements of the positive feedback loop, i.e., the oxidation rate, cell voltage, or D_Li.

True Reaction Sites on Discharge in Li-O2 Batteries

In the pursuit of an advanced Li-O₂ battery, the true reaction sites in the cathode determined its cell performance and the catalyst design. When the first layer of insulating Li₂O₂ solid is deposited on the electrode substrate during discharging, the following O₂ reduction to Li₂O₂ could take place either at the electrode|Li₂O₂ interface or at the Li₂O₂|electrolyte interface. The mechanism decides the strategies of catalyst design; however, it is still mysterious. Here, we used rotate ring-disk electrode to deposit a dense Li₂O₂ film and labeled the Li₂O₂ product with ¹⁶O/¹⁸O isotope. By identification of the distribution of the Li₂¹⁶O₂ and Li₂¹⁸O₂ in the Li₂O₂ film using new characteristic signals of Li₂¹⁶O₂ and Li₂¹⁸O₂, our results show that O₂ is reduced to Li₂O₂ at both interfaces. A sandwich structure of Li₂¹⁸O₂|Li₂¹⁶O₂|Li₂¹⁸O₂ was identified at the electrode surface when the electrode was discharged under ¹⁶O₂ and then ¹⁸O₂. The electrode|Li₂O₂ interface is the major reaction site, and it contributes to 75% of the overall reaction. This new mechanism raises new challenges and new strategies for the catalyst design of Li-O₂ batteries.

A multifunctional protective layer with biomimetic ionic channel suppressing dendrite and side reactions on zinc metal anodes

A multifunctional graphitic carbon nitride (GCN) protective layer with bionic ion channels and high stability is prepared to inhibit dendrite growth and side reactions on zinc (Zn) metal anodes. The high electronegativity of the nitrogen-containing organic groups (NOGs) in the GCN layer can effectively promote the dissociation of solvated Zn²⁺ and its rapid transportation in bionic ion channels via a hopping mechanism. In addition, this GCN layer exhibits excellent mechanical strength to suppress the growth of Zn dendrites and the volume expansion of Zn metal anodes during the plating process. Consequently, the electrodeposited Zn presents a uniform and densely packed morphology with negligible side-product accumulation. As a result, the half-cell composed of the Cu-GCN anode can deliver a remarkable long-term cycling performance of 1000 h at 0.5 mA cm^-2 and 0.25 mAh cm^-2. A full cell assembled with MnO₂ cathode also displays improved long-term cycling performance (150 cycles at 200 mA g^-1) when the Cu-GCN@Zn composite anode is applied. This work deepens our understanding of the kinetics of ion migration in the interface layer and paves the way for next-generation high energy-density Zn-metal batteries (ZMBs).

Synthesis of Fe-doped NiO nanosheets on carbon cloth for improved catalytic performance in Li–O2 batteries

The substitution of Ni2+ in NiO with Fe3+ can significantly improve the cycling stability and discharge/recharge capacities of Li–O2 batteries.

OUP accepted manuscript

Aprotic lithium–oxygen (Li–O₂) batteries are receiving intense research interest by virtue of their ultra-high theoretical specific energy. However, current Li–O₂ batteries are suffering from severe barriers, such as sluggish reaction kinetics and undesired parasitic reactions. Recently, molecular catalysts, i.e. redox mediators (RMs), have been explored to catalyse the oxygen electrochemistry in Li–O₂ batteries and are regarded as an advanced solution. To fully unlock the capability of Li–O₂ batteries, an in-depth understanding of the catalytic mechanisms of RMs is necessary. In this review, we summarize the working principles of RMs and their selection criteria, highlight the recent significant progress of RMs and discuss the critical scientific and technical challenges on the design of efficient RMs for next-generation Li–O₂ batteries.

Revealing the Correlations between Morphological Evolution and Surface Reactivity of Catalytic Cathodes in Lithium-Oxygen Batteries

Lithium-oxygen batteries suffer from the degradation of the catalytic cathode during long-term operation, which limits their practical use. Understanding the direct correlations between the surface morphological evolution of catalytic cathodes at nanoscale and their catalytic activity during cycling has proved challenging. Here, using in situ electrochemical atomic force microscopy, the dynamic evolution of the Pt nanoparticles electrode in a working Li-O2 battery and its effects on the Li-O2 interfacial reactions are visualized. In situ views show that repeated oxidation-reduction cycles (ORCs) trigger the increase in the size of Pt nanoparticles, eventually causing the Pt nanoparticles to fall off the electrode. In 0-80 ORCs, the grown Pt nanoparticles promote the conversion of the Li-O2 reaction route from the surface-mediated pathway to the solution-mediated pathway during discharging and significantly increase the discharge capacity. After 250 ORCs, accompanied by the part of the Pt nanoparticles detaching from the electrode, the nucleation potential of reaction product decreases, and the reaction dynamic slows down, which cause the performance to degrade. Modification of a proper amount of Au nanoparticle on the Pt nanoparticles electrode could improve its stability and maintain the high catalytic activity. These results provide a direct evidence for clarifying the correlations between morphological evolution and surface reactivity of catalytic cathodes during cycling, which is critical for developing high-performance catalysts.

Mildly Oxidized MXene (Ti3C2, Nb2C, and V2C) Electrocatalyst via a Generic Strategy Enables Longevous Li-O2 Battery under a High Rate

Lithium-oxygen batteries (LOBs) with ultrahigh theoretical energy density have emerged as one appealing candidate for next-generation energy storage devices. Unfortunately, some fundamental issues remain unsettled, involving large overpotential and inferior rate capability, mainly induced by the sluggish reaction kinetics and parasitic reactions at the cathode. Hence, the pursuit of suitable catalyst capable of efficiently catalyzing the oxygen redox reaction and eliminating the side-product generation, become urgent for the development of LOBs. Here, we report a universal synthesis approach to fabricate a suite of mildly oxidized MXenes (mo-Nb₂CT_x, mo-Ti₃C₂T_x, and mo-V₂CT_x) as cathode catalysts for LOBs. The readily prepared mo-MXenes possess expanded interlayer distance to accommodate massive Li₂O₂ formation, and in-situ-formed light metal oxide to enhance the electrocatalytic activity of MXenes. Taken together, the mo-V₂CT_x manages to deliver a high specific capacity of 22752 mAh g^-1 at a current density of 100 mA g^-1, and a long lifespan of 100 cycles at 500 mA g^-1. More impressively, LOBs with mo-V₂CT_x can continuously operate for 90, 89, and 70 cycles, respectively, under a high current density of 1000, 2000, and 3000 mA g^-1 with a cutoff capacity of 1000 mAh g^-1. The theoretical calculations further reveal the underlying mechanism lies in the optimized surface, where the overpotentials for the formation/decomposition of Li₂O₂ are significantly reduced and the catalytic kinetics is accelerated. This contribution offers a feasible strategy to prepare MXenes as efficient and robust electrocatalyst toward advanced LOBs and other energy storage devices.

Spin-State Manipulation of Two-Dimensional Metal-Organic Framework with Enhanced Metal-Oxygen Covalency for Lithium-Oxygen Batteries

Aprotic Li-O 2 battery has attracted extensive attention in the past decade owing to the high theoretical energy density, however it is obstructed by the sluggish reaction kinetics at cathodes and large voltage hysteresis. Herein, we regulate the spin state of partial Ni 2+ metal centers ( t 2g 6 e g 2 ) of conductive nickel catecholate framework (Ni II -NCF) nanowire arrays to high-valence Ni 3+ ( t 2g 6 e g 1 ) for Ni III -NCF. The spin-state modulation enables enhanced nickel-oxygen covalency in Ni III -NCF, which facilitates electron exchange between the Ni sites and oxygen adsorbates and accelerates the oxygen redox kinetics. The high affinity of Ni 3+ sites with the intermediate LiO 2 promotes formation of nanosheet-like Li 2 O 2 in the void space among Ni III -NCF nanowires upon discharging. These merit the Li-O 2 battery based on Ni III -NCF with remarkably reduced discharge/charge voltage gaps, superior rate capability, and long cycling stability of over 200 cycles. This work highlights the domination of electron spin state on the redox kinetics and will shed insights into electronic structure regulation of electrocatalysts for Li-O 2 battery and beyond.

Thermodynamic Regulation of Dendrite-Free Li Plating on Li3Bi for Stable Lithium Metal Batteries

Rechargeable batteries with metallic lithium (Li) anodes are attracting ever-increasing interests because of their high theoretical specific capacity and energy density. However, the dendrite growth of the Li anode during cycling leads to poor stability and severe safety issues. Here, Li₃Bi alloy coated carbon cloth is rationally chosen as the substrate of the Li anode to suppress the dendrite growth from a thermodynamic aspect. The adsorption energy of a Li atom on Li₃Bi is larger than the cohesive energy of bulk Li, enabling uniform Li nucleation and deposition, while the high diffusion barrier of the Li atom on Li₃Bi blocks the migration of adatoms from adsorption sites to the regions of fast growth, which further ensures uniform Li deposition. With the dendrite-free Li deposition, the composite Li/Li₃Bi anode enables over 250 cycles at an ultrahigh current density of 20 mA cm^-2 in a symmetrical cell and delivers superior electrochemical performance in full batteries.

Charge Rate‐Dependent Decomposition Mechanism of Toroidal Li2O2 in Li‐O2 Batteries

Using in situ atomic force microscopy, we visualized the oxidation dynamic process of toroidal lithium peroxide (Li2O2) in a working Li‐O2 battery and clarified the correlation between the decomposition behavior and the charging rate. It was found that at a low charge rate, the decomposition could occur at the Li2O2/electrolyte interface, providing strong evidence that a tiny current is allowed to pass through the bulk phase of toroidal Li2O2. Further, the evolution of the periphery thickness, diameter and center thickness of the toroid as a function of charge capacity was quantitatively analyzed. In the early stage of charging, the diameter of the toroid radially shrinks and the shrinking rate slows down in the later stage. Besides, the periphery thickness of the toroid decreases at a faster and uniform rate, while the center thickness decreases obviously in the later stage of charging. Increasing the charging rate promotes the decomposition occurring at the Li2O2/electrode interface, causing direct desorption of the Li2O2 from the electrode and irreversible capacity degradation.

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.

MIL-53 Metal-Organic Framework as a Flexible Cathode for Lithium-Oxygen Batteries

Li-air batteries possess higher specific energies than the current Li-ion batteries. Major drawbacks of the air cathode include the sluggish kinetics of the oxygen reduction (OER), high overpotentials and pore clogging during discharge processes. Metal-Organic Frameworks (MOFs) appear as promising materials because of their high surface areas, tailorable pore sizes and catalytic centers. In this work, we propose to use, for the first time, aluminum terephthalate (well known as MIL-53) as a flexible air cathode for Li-O₂ batteries. This compound was synthetized through hydrothermal and microwave-assisted routes, leading to different particle sizes with different aspect ratios. The electrochemical properties of both materials seem to be equivalent. Several behaviors are observed depending on the initial value of the first discharge capacity. When the first discharge capacity is higher, no OER occurs, leading to a fast decrease in the capacity during cycling. The nature and the morphology of the discharge products are investigated using ex situ analysis (XRD, SEM and XPS). For both MIL-53 materials, lithium peroxide Li₂O₂ is found as the main discharge product. A morphological evolution of the Li₂O₂ particles occurs upon cycling (stacked thin plates, toroids or pseudo-spheres).

Ru-Embedded Highly Porous Carbon Nanocubes Derived from Metal-Organic Frameworks for Catalyzing Reversible Li2O2 Formation

Rechargeable lithium-oxygen batteries (LOBs) have attracted increasing attention due to their high energy density but highly rely on the development of efficient oxygen catalysts for reversible Li₂O₂ deposition/decomposition. Herein, highly porous carbon nanocubes with a specific surface area up to 1600 m² g^-1 are synthesized and utilized to tightly anchor Ru nanoparticles for using as the oxygen-cathode catalyst in LOBs, achieving a low charge/discharge potential gap of only 0.75 V, a high total discharge capacity of 17,632 mA h g^-1, and a superb cycling performance of 550 cycles at 1000 mA g^-1. Comprehensive ex situ and operando characterizations unravel that the outstanding LOB performance is ascribed to the highly porous catalyst structure embedding rich active sites that synergistically function in reducing overpotentials, suppressing parasitic reactions, accommodating reaction products, and promoting mass and charge transportation.

Application-Driven Carbon Nanotube Functional Materials

Carbon nanotube functional materials (CNTFMs) represent an important research field in transforming nanoscience and nanotechnology into practical applications, with potential impact in a wide realm of science, technology, and engineering. In this review, we combine the state-of-the-art research activities of CNTFMs with the application prospect, to highlight critical issues and identify future challenges. We focus on macroscopic long fibers, thin films, and bulk sponges which are typical CNTFMs in different dimensions with distinct characteristics, and also cover a variety of derived composite/hierarchical materials. Critical issues related to their structures, properties, and applications as robust conductive skeletons or high-performance flexible electrodes in mechanical and electronic devices, advanced energy conversion and storage systems, and environmental areas have been discussed specifically. Finally, possible solutions and directions are proposed for overcoming current obstacles and promoting future efforts in the field.

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

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

Isotopic Depth Profiling of Discharge Products Identifies Reactive Interfaces in an Aprotic Li-O2 Battery with a Redox Mediator

Prior to the practical application of rechargeable aprotic Li-O₂ batteries, the high charging overpotentials of these devices (which inevitably cause irreversible parasitic reactions) must be addressed. The use of redox mediators (RMs) that oxidatively decompose the discharge product, Li₂O₂, is one promising solution to this problem. However, the mitigating effect of RMs is currently insufficient, and so it would be beneficial to clarify the Li₂O₂ reductive growth and oxidative decomposition mechanisms. In the present work, Nanoscale secondary ion mass spectrometry (Nano-SIMS) isotopic three-dimensional imaging and differential electrochemical mass spectrometry (DEMS) analyses of individual Li₂O₂ particles established that both growth and decomposition proceeded at the Li₂O₂/electrolyte interface in a system containing the Br^-/Br₃^- redox couple as the RM. The results of this study also indicated that the degree of oxidative decomposition of Li₂O₂ was highly dependent on the cell voltage. These data show that increasing the RM reaction rate at the Li₂O₂/electrolyte interface is critical to improve the cycle life of Li-O₂ batteries.

Nitrogen-Coordinated CoS2@NC Yolk-Shell Polyhedrons Catalysts Derived from a Metal-Organic Framework for a Highly Reversible Li-O2 Battery

Transition-metal sulfides (TMS) are one of the most promising cathode catalysts for Li-O₂ batteries (LOBs) owing to their excellent stabilities and inherent metallicity. In this work, a highly efficient mode has been used to synthesize Co@CNTs [pyrolysis products of metal-organic frameworks (MOFs)]-derived CoS₂(CoS)@NC. Benefiting from the special yolk-shell hierarchical porous morphology, the existence of Co-N bonds, and dual-function catalytic activity (ORR/OER) of the open metal sites contributed by MOFs, the CoS₂@NC-400/AB electrode illustrated excellent charge-discharge cycling for up to nearly 100 times at a current density of 0.1 mA cm^-2 under a limited capacity of 500 mA h g^-1 (based on the total weight of CoS₂@NC and AB) with a high discharge voltage plateau and a low charge cut-off voltage. Meanwhile, the average transferred electron number (n) is around 3.7 per O₂ molecule for CoS₂@NC-400, which is the chief approach for a four-electron pathway of the ORR under alkaline media. Therefore, we believe that the novel CoS₂@NC-400/AB electrode could serve as an excellent catalyst in the LOBs.

Bifunctional 1-Boc-3-Iodoazetidine Enhancing Lithium Anode Stability and Rechargeability of Lithium-Oxygen Batteries

Lithium anode protection is an effective strategy to prohibit the continuous loss of redox mediators (RMs) resulting from the unfavorable "shuttle effect" in lithium-oxygen batteries. In this work, an in situ Li anode protection method is designed by utilizing an organic compound, 1-Boc-3-iodoazetidine (BIA), as both a RM and an additive, to form a lithium anode protective layer. The reaction between Li metal and BIA can form lithium iodide (LiI) and lithium-based organometallic. LiI can effectively reduce the charging overpotential. Meanwhile, the in situ-formed anode protection layer (lithium-based organometallic) can not only effectively prevent RMs from being reduced by the lithium metal, but also inhibit the growth of lithium dendrites. As a result, the lithium-oxygen battery with BIA shows a long cycle life of 260 cycles with a notably reduced charging potential. In particular, the battery with BIA achieves an excellent lifespan of 160 cycles at a large current density of 2000 mA g^-1.

Surface and Interface Engineering of Nanoarrays toward Advanced Electrodes and Electrochemical Energy Storage Devices

The overall performance of electrochemical energy storage devices (EESDs) is intrinsically correlated with surfaces and interfaces. As a promising electrode architecture, 3D nanoarrays (3D-NAs) possess relatively ordered, continuous, and fully exposed active surfaces of individual nanostructures, facilitating mass and electron transport within the electrode and charge transfer across interfaces and providing an ideal platform for engineering. Herein, a critical overview of the surface and interface engineering of 3D-NAs, from electrode and interface designs to device integration, is presented. The general merits of 3D-NAs and surface/interface engineering principles of 3D-NA hybrid electrodes are highlighted. The focus is on the use of 3D-NAs as a superior platform to regulate the interface nature and unveiling new mechanism/materials without the interference of binders. The engineering and utilization of the surface of 3D-NAs to develop flexible/solid-state EESDs with 3D integrated electrode/electrolyte interfaces, or 3D triphase interfaces involving other active species, which are characteristic of (quasi-)solid-state electrolyte infiltration into the entire device, are also considered. Finally, the challenges and future directions of surface/interface engineering of 3D-NAs are outlined. In particular, potential strategies to obtain electrode charge balance, optimize the multiphase solid-state interface, and attain 3D solid electrolyte infiltration are proposed.

Metal-organic frameworks-derived hollow dodecahedral carbon combined with FeNx moieties and ruthenium nanoparticles as cathode electrocatalyst for lithium oxygen batteries

Owing to their high energy density, lithium-oxygen batteries (LOBs) have been drawn great attention as one of the promising electrochemical energy sources. However, the sluggish kinetics of oxygen reduction/evolution reaction (ORR/OER) hamper the widespread application of LOBs. Herein, an elaborate designed catalysts which are constructed by FeN_x moieties dispersed on the network-like hollow dodecahedral carbon and then decorated with Ru nanoparticles (FeN_x-HDC@Ru). Since the homogeneously dispersed FeN_x moieties could promote ORR performance, and the Ru nanoparticles could facilitate OER capability, the FeN_x-HDC@Ru nanocomposites used as cathode catalysts can significantly improve LOBs performance. A lower discharge and charge overpotentials of 0.15 V and 0.78 V can be detected in the first cycle, respectively, and an excellent cycle performance of 90 cycles at 200 mA g^-1 and 89 cycles at 500 mA g^-1 can be demonstrated. Herein, the charge transfer kinetics has been enhanced with the internal network-like hollow structure and a low impedance Li₂O₂/catalysts contact interface could be earned by the constructed Ru nanoparticles, these factors would lead to an efficient acceleration to the formation and decomposition of Li₂O₂ during discharge and charge process.