Controllable Regulation of the Oxygen Redox Process in Lithium-Oxygen Batteries by High-Configuration-Entropy Spinel with an Asymmetric Octahedral Structure

Designing bifunctional electrocatalysts to boost oxygen redox reactions is critical for high-performance lithium-oxygen batteries (LOBs). In this work, high-entropy spinel (Co0.2Mn0.2Ni0.2Fe0.2Cr0.2)3O4 (HEOS) is fabricated by modulating the internal configuration entropy of spinel and studied as the oxygen electrode catalyst in LOBs. Under the high-entropy atomic environment, the Co-O octahedron in spinel undergoes asymmetric deformation, and the reconfiguration of the electron structure around the Co sites leads to the upward shift of the d-orbital centers of the Co sites toward the Fermi level, which is conducive to the strong adsorption of redox intermediate LiO2 on the surface of the HEOS, ultimately forming a layer of a highly dispersed Li2O2 thin film. Thin-film Li2O2 is beneficial for ion diffusion and electron transfer at the electrode-electrolyte interface, which makes the product easy to decompose during the charge process, ultimately accelerating the kinetics of oxygen redox reactions in LOBs. Based on the above advantages, HEOS-based LOBs deliver high discharge/charge capacity (12.61/11.72 mAh cm-2) and excellent cyclability (424 cycles). This work broadens the way for the design of cathode catalysts to improve oxygen redox kinetics in LOBs.

Insights into the LiI Redox Mediation in Aprotic Li-O2 Batteries: Solvation Effects and Singlet Oxygen Evolution

Lithium-oxygen aprotic batteries (aLOBs) are highly promising next-generation secondary batteries due to their high theoretical energy density. However, the practical implementation of these batteries is hindered by parasitic reactions that negatively impact their reversibility and cycle life. One of the challenges lies in the oxidation of Li2O2, which requires large overpotentials if not catalyzed. To address this issue, redox mediators (RMs) have been proposed to reduce the oxygen evolution reaction (OER) overpotentials. In this study, we focus on a lithium iodide RM and investigate its role on the degradation chemistry and the release of singlet oxygen in aLOBs, in different solvent environments. Specifically, we compare the impact of a polar solvent, dimethyl sulfoxide (DMSO), and a low polarity solvent, tetraglyme (G4). We demonstrate a strong interplay between solvation, degradation, and redox mediation in OER by LiI in aLOBs. The results show that LiI in DMSO-based electrolytes leads to extensive degradation and to 1O2 release, affecting the cell performance, while in G4-based electrolytes, the release of 1O2 appears to be suppressed, resulting in better cyclability.

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

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

RuO2-Incorporated Co3O4 Nanoneedles Grown on Carbon Cloth as Binder-Free Integrated Cathodes for Tuning Favorable Li2O2 Formation

Developing ideal Li-O₂ batteries (LOBs) requires the discharge product to have a large quantity, have large contact area with the cathode, and not passivate the porous surface after discharge, which put forward high requirement for the design of cathodes. Herein, combining the rational structural design and high activity catalyst selection, minor amounts of RuO₂-incorporated Co₃O₄ nanoneedles grown on carbon cloth are successfully synthesized as binder-free integrated cathodes for LOBs. With this unique design, plenty of electron-ion-oxygen tri-phase reaction interface is created, the side reaction from carbon is isolated, and oxygen reduction reaction/oxygen evolution reaction (OER) kinetics are significantly facilitated. Upon discharge, film-like Li₂O₂ is observed growing on the needle surface first and eventually ball-like Li₂O₂ particles form at each tip of the needle. The cathode surface remains porous after discharge, which is beneficial to the OER and is rare in the previous reports. The battery exhibits a high specific discharge capacity (7.64 mAh cm^-2) and a long lifespan (500 h at 0.1 mA cm^-2). Even with a high current of 0.3 mA cm^-2, the battery achieves a cycling life of 200 h. In addition, punch-type LOBs are fabricated and successfully operated, suggesting that the cathode material can be utilized in ultralight, flexible electronic devices.

Resolving the Incomplete Charging Behavior of Redox-Mediated Li-O2 Batteries via Sustainable Protection of Li Metal Anode

Lithium-oxygen batteries (LOBs) have attracted worldwide attention due to their high specific energy. However, the poor rechargeability and cycling stability of LOBs hinders their practical use in applications. Here, we explore the incomplete charging behavior of redox-mediated LOBs operated at a feasible capacity for a practical level (3.25 mAh cm^-2) and resolve it using a sustainable lithium protection strategy. The incomplete charging behavior, promoted by self-discharge of redox mediators (RMs), hampers the reversible cycling of LOBs, which was investigated through multiangle in situ and ex situ analyses. Meanwhile, the proposed lithium protection strategy, introducing an inorganic/organic hybrid artificial composite layer with a preformed stable interface between the lithium metal and the composite layer, enhances the stability of the lithium metal anode during the prolonged cycling by preventing the chemical/electrochemical interactions of RMs on the lithium metal surface, thus improving the overall rechargeability of LOBs. This work provides guidelines for the effective use of RMs with an adequate lithium protection strategy to achieve sustainable cycling of LOBs, creating a feasible approach for the practical use of LOBs with high areal capacity.

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.

Amphi-Active Superoxide-Solvating Charge Redox Mediator for Highly Stable Lithium-Oxygen Batteries

A multifunctional electrolyte additive for lithium oxygen batteries (LOBs) was designed to have (1) a redox-active moiety to mediate decomposition of lithium peroxide (Li₂O₂ as the final discharge product) during charging and (2) a solvent moiety to solvate and stabilize lithium superoxide (LiO₂ as the intermediate discharge product) in electrolyte during discharging. 4-Acetamido-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yl)oxyl) or AAT was employed as the additive working for both charge and discharge processes (amphi-active). The redox-active moiety was rooted in TEMPO, while the acetamido (AA) functional group inherited the high donor number (DN) of N,N-dimethylacetamide (DMAc). Integrating two functional moieties (TEMPO and AA) into a single molecule resulted in the bifunctionality of AAT (1) facilitating Li₂O₂ decomposition by the TEMPO moiety and (2) encouraging the solvent mechanism of Li₂O₂ formation by the high-DN AA moiety. Significantly improved LOB performances were achieved by the superoxide-solvating charge redox mediator, which were not obtained by a simple cocktail of TEMPO and DMAc.

Formation/Decomposition of Li2O2 Induced by Porous NiCeOx Nanorod Catalysts in Aprotic Lithium-Oxygen Batteries

To realize the utilization of high-performance lithium-oxygen batteries (LOBs), a rational-designed cathode structure and efficient catalytic materials are necessary. However, side products accumulated during battery cycling seriously affects the performance. Designing a cathode catalyst that could simultaneously facilitate the catalytic efficiency of the main reaction and inhibit the side reactions will make great sense. Herein, NiCeO_x was proposed for the first time as a bifunctional cathode catalyst material for LOBs. The combined action of NiO and CeO₂ components was expected to facilitate the decomposition of byproducts (e.g., Li₂CO₃), increase the oxygen vacancy content in CeO₂, and enhance the adsorption of oxygen and superoxide. NiCeO_x nanorods (NiCeO_x PNR) were prepared using electrospinning method. It showed a hollow and porous nanorod (PNR)-like structure, which provided a large number of catalytic active sites and facilitated the transport of reactants and the deposition of discharge products. As a result, a high specific discharge capacity (2175.9 mAh g^-1) and a long lifespan (67 cycles at 100 mA g^-1 with a limited capacity of 500 mAh g^-1) were obtained.

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.

Shifting Target Reaction from Oxygen Reduction to Superoxide Disproportionation by Tuning Isomeric Configuration of Quinone Derivative as Redox Mediator for Lithium-Oxygen Batteries

Quinones having a fully conjugated cyclic dione structure have been used as redox mediators in electrochemistry. 2,5-Ditert-butyl-1,4-benzoquinone (DBBQ or DB-p-BQ) as a para-quinone derivative is one of the representative discharge redox mediators for facilitating the oxygen reduction reaction (ORR) kinetics in lithium-oxygen batteries (LOBs). Herein, we presented that the redox activity of DB-p-BQ for electron mediation was possibly used for facilitating superoxide disproportionation reaction (SODR) by tuning the isomeric configuration of the carbonyl groups of the substituted quinone to change its reduction potentials. First, we expected a molecule having its reduction potential between oxygen/superoxide at 2.75 V versus Li/Li⁺ and superoxide/peroxide at 3.17 V to play a role of the SODR catalyst by transferring an electron from one superoxide (O₂^-) to another superoxide to generate dioxygen (O₂) and peroxide (O₂^2-). By changing the isomeric configuration from para (DB-p-BQ) to ortho (DB-o-BQ), the reduction potential of the first electron transfer (Q/Q^-) of the ditert-butyl benzoquinone shifted positively to the potential range of the SODR catalyst. The electrocatalytic SODR-promoting functionality of DB-o-BQ kept the reactive superoxide concentration below a harmful level to suppress superoxide-triggered side reaction, improving the cycling durability of LOBs, which was not achieved by the para form. The second electron transfer process (Q^-/ Q^2-) of the DB-o-BQ, even if the same process of the para form was not used for facilitating ORR, played a role of mediating electrons between electrode and oxygen like the Q/Q^- process of the para form. The ORR-promoting functionality of the ortho form increased the LOB discharge capacity and reduced the ORR overpotential.

Direct In Situ Spectroscopic Evidence for Solution-Mediated Oxygen Reduction Reaction Intermediates in Aprotic Lithium-Oxygen Batteries

A fundamental understanding of the reaction process is essential to predict and enhance the performance of electrochemical devices. As a central reaction in aprotic lithium-oxygen (Li-O₂) batteries, the oxygen reduction reaction (ORR) has been confronted with the "sudden-death" phenomenon caused by the cathode passivation from discharge product Li₂O₂. The soluble catalyst (e.g., reduction mediator) promoted solution-mediated ORR represents an elegant solution. However, no direct molecular evidence is available so far, and its link to Li-O₂ batteries performance remains hypothetical. Here, we present in situ surface-enhanced Raman spectroscopy and obtain direct spectroscopic evidence (i.e., LiAQ and LiAQO₂) of the solution-mediated ORR on a model anthraquinone (AQ, a typical reduction mediator)-immobilized Au electrode. With the assistance of density functional theory calculations and differential electrochemical mass spectrometry, the related elementary reaction steps of the solution-mediated ORR are proposed. This work provides intuitive insights into the AQ-catalyzed solution-mediated ORR mechanism that is helpful in the optimization and tailor-design of soluble catalysts for excellent next-generation Li-O₂ batteries.

Nitrate Molten Salt Electrolytes with Iron Oxide Catalysts for Open and Sealed Li-O2 Batteries

Li-O₂ batteries with nitrate molten salt electrolytes are attracting considerable attention owing to their various electrochemical pathways to form a discharge product upon the open and sealed systems. Here, we investigate nitrate molten salt electrolyte-based open and sealed Li-O₂ batteries with pristine and iron oxide catalysts. Through the systematic analysis of various Li-O₂ battery characteristics, we observe the irreversible electrochemical reactions of the open Li-O₂ battery with an iron oxide catalyst that erodes the battery performance due to the detrimental parasitic reaction of H₂ gas evolution from the Li anode. In contrast, the sealed Li-O₂ system with cathodes containing the iron oxide catalyst exhibits the formation and decomposition of Li₂O discharge products without significant side reactions, which guarantees long cycle endurance, high-rate performance, and a gravimetric energy density. Thus, promising electrochemical results from the sealed Li-O₂ system with the iron oxide catalyst provide a viable strategy for the high-performance molten salt-based Li-O₂ battery.

Impact of a Gold Nanocolloid Electrolyte on Li2O2 Morphology and Performance of a Lithium-Oxygen Battery

Aprotic lithium-oxygen batteries currently suffer from poor cyclic stability and low achievable energy density. Herein, gold nanoparticles capped with mercaptosuccinic acid are dispersed in 1.0 M LiClO₄/dimethyl sulfoxide (DMSO) as a novel electrolyte for lithium-oxygen batteries. Morphological and electrochemical analyses indicate that film-like amorphous lithium peroxide is formed using the gold nanocolloid electrolyte instead of bulk crystals in battery discharging, which apparently increases the conductivity and accelerates the decomposition kinetics of discharge products in recharging, accompanied by the release of incorporated gold nanoparticles with the decomposition of lithium peroxide into the electrolyte. Experiments and theoretical calculations further demonstrate that the suspended gold nanoparticles in the electrolyte can adsorb some intermediates generated by an oxygen reduction reaction, which effectively alleviates the cleavage of the electrolyte and impedes the corrosion of the lithium anode. As a result, the life span of lithium-oxygen batteries is dramatically increased from 55 to 438 cycles, and the rate performance and full-discharge capacity are also massively enhanced. The battery failure is attributed to the degradation of gold nanocolloid electrolytes, and further studies on improvement of colloid stability during battery cycling are underway.

Free-Standing Carbon Nanofibers Protected by a Thin Metallic Iridium Layer for Extended Life-Cycle Li-Oxygen Batteries

It is evident that the exhaustive use of fossil fuels for decades has significantly contributed to global warming and environmental pollution. To mitigate the harm on the environment, lithium-oxygen batteries (LOBs) with a high theoretical energy density (3458 Wh kg^-1Li₂O₂) compared to that of Li-ion batteries (LIBs) have been considered as an attractive alternative to fossil fuels. For this purpose, porous carbon materials have been utilized as promising air cathodes owing to their low cost, lightness, easy fabrication process, and high performance. However, the challenge thus far lies in the uncontrollable formation of Li₂CO₃ at the interface between carbon and Li₂O₂, which is detrimental to the stable electrochemical performance of carbon-based cathodes in LOBs. In this work, we successfully protected the surface of the free-standing carbon nanofibers (CNFs) by coating it with a layer of iridium metal through direct sputtering (CNFs@Ir), which significantly improved the lifespan of LOBs. Moreover, the Ir would play a secondary role as an electrochemical catalyst. This all-in-one cathode was evaluated for the formation and decomposition of Li₂O₂ during (dis)charging processes. Compared with bare CNFs, the CNFs@Ir cathode showed two times longer lifespan with 0.2 V_Li lower overpotentials for the oxygen evolution reaction. We quantitatively calculated the contents of CO₃^2- in Li₂CO₃ formed on the different surfaces of the bare CNFs (63% reduced) and the protected CNFs@Ir (78% reduced) cathodes after charging. The protective effects and the reaction mechanism were elucidated by ex situ analyses, including scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy.

Perovskite LaCoxMn1-xO3-σ with Tunable Defect and Surface Structures as Cathode Catalysts for Li-O2 Batteries

Rechargeable lithium-oxygen batteries have shown great potential as next-generation sustainable and green energy storage systems. The bifunctional catalyst plays an important role in accelerating the cathode kinetics for practical realization of the batteries. Herein, we employ the surface structure and defect engineering to introduce surface-roughened nanolayers and oxygen vacancies on the mesoporous hollow LaCo_xMn_1-xO_3-σ perovskite catalyst by in situ cation substitution. The experimental results show that the O₂-electrode with the LaCo_0.75Mn_0.25O_3-σ catalyst exhibits an extremely high discharge capacity of 10,301 mA h g^-1 at 200 mA g^-1 for the initial cycle and superior cycling stability under a capacity limit of 500 mA h g^-1 together with a low voltage gap of 1.12 V. Good electrochemical performance of LaCo_0.75Mn_0.25O_3-σ can be attributed to the synergistic effect of the hierarchical mesoporous hollow structure and the abundant oxygen vacancies all over the catalyst surface. We reveal that the modified surface structure can provide more accessibility of active sites to promote electrochemical reactions, and the introduced oxygen vacancy can serve as an efficient substrate for binding intermediate products and decomposition reactions of Li₂O₂ during discharge and charge processes. Our methodology provides meaningful insights into the rational design of highly active perovskite catalysts in energy storage/conversion systems.

Inhibition of Discharge Side Reactions by Promoting Solution-Mediated Oxygen Reduction Reaction with Stable Quinone in Li-O2 Batteries

Aprotic lithium-oxygen (Li-O₂) batteries with an ultrahigh theoretical energy density have great potential in rechargeable power supply, while their application still faces several challenges, especially poor cycle stability. To solve the problems, one of the effective strategies is to inhibit the generation of the LiO₂ intermediate produced via a surface-mediated oxygen reduction reaction (ORR) pathway, which is an important species inducing byproduct generation and low cell cyclic stability. Herein, a series of quinones and solid materials serve as the solution-mediated and surface-mediated ORR catalysts, and it was found that the generation of LiO₂ and byproducts from solid catalysts was inhibited by quinones. Among the studied quinones, benzo[1,2-b:4,5-b']dithiophene-4,8-dione, a quinone molecule with the advantage of a highly symmetrical planar and conjugated structure and without α-H, exhibits high redox potential, diffusion coefficient, and electrochemical stability, and consequently the best ORR activities and the capability to inhibit byproduct generation. It indicated that the increase of the solution-mediated ORR pathway plays an important role in restraining the discharging side reaction, substantially improving cell cycle stability and capacity. This study provides the theoretical and experimental basis for better understanding the ORR process of Li-O₂ batteries.

Hierarchical Mesoporous/Macroporous Co-Doped NiO Nanosheet Arrays as Free-Standing Electrode Materials for Rechargeable Li-O2 Batteries

Lithium-oxygen (Li-O₂) batteries have been widely recognized as appealing power systems for their extremely high energy density versus common Li-ion batteries. However, there are still lots of issues that need to be addressed toward the practical application. Here, free-standing Co-doped NiO three-dimensional nanosheets were prepared by a hydrothermal synthesis method and directly employed as the air-breathing cathode of the Li-O₂ battery. The morphological phenomenon and electrochemical performance of the as-prepared cathode material were characterized by high-resolution scanning electron microscopy, X-ray diffraction, cyclic voltammetry, galvanostatic charge-discharge tests, and electrochemical impedance spectroscopy measurements. The Co-doped NiO electrode delivered a maximum discharge capacity of around 12 857 mA h g^-1 with a low overpotential (0.82 V) at 200 mA g^-1. Under upper-limit specific capacities of 500 mA h g^-1 at 400 mA g^-1, the Li-O₂ batteries exhibited a long cycle life of 165 cycles. Compared with the undoped NiO electrode, the Li-O₂ battery based on the Co-doped NiO cathode showed significantly higher oxygen reduction reaction and oxygen evolution reaction activities. This superior electrochemical performance is because of the partial substitution of Ni²⁺ in the NiO matrix by Co²⁺ to improve the p-type electronic conductivity of NiO. In addition, the morphology and specific surface area of NiO are affected by Co doping, which can expand the electrode-electrolyte contact area and lead to sufficient space for Li₂O₂ deposition. This approach harnesses the great potential of Co-doped NiO nanosheets for practical applications as advanced electrodes for rechargeable Li-O₂ batteries.

Maximal Utilization of a High-Loading Cathode in Li-O2 Batteries: A Double Oxygen Supply System

To realize the potential high capacity of lithium-oxygen (Li-O₂) batteries, a double oxygen supply system for cells with high-loading cathodes is devised in this study. High-loading thick electrodes can achieve exceptionally high capacities, but this promise has been plagued by partial utilization of thick electrodes in Li-O₂ cells due to the kinetic limitation imposed by oxygen transport. The proposed double oxygen supply system provides oxygen gas to the cathode not only from the cathode opening but also from the separator side to ensure sufficient oxygen supply to the whole high-loading electrode. Subsequently, the entire region of the high-loading cathode is rendered active, resulting in a uniform vertical distribution of discharge products. The maximum utilization of the high-loading electrodes is, thus, achieved, along with a remarkably increased capacity, low overpotential, and cycle life. By this strategy, CNT cathodes can be cycled with a capacity of 5 mAh cm^-2, without using any additional catalyst.

Biomimetic Superoxide Disproportionation Catalyst for Anti-Aging Lithium-Oxygen Batteries

Reactive oxygen species or superoxide (O₂^-), which damages or ages biological cells, is generated during metabolic pathways using oxygen as an electron acceptor in biological systems. Superoxide dismutase (SOD) protects cells from superoxide-triggered apoptosis by converting superoxide to oxygen and peroxide. Lithium-oxygen battery (LOB) cells have the same aging problems caused by superoxide-triggered side reactions. We transplanted the function of SOD of biological systems into LOB cells. Malonic acid-decorated fullerene (MA-C₆₀) was used as a superoxide disproportionation chemocatalyst mimicking the function of SOD. As expected, MA-C₆₀ as the superoxide scavenger improved capacity retention along charge/discharge cycles successfully. A LOB cell that failed to provide a meaningful capacity just after several cycles at high current (0.5 mA cm^-2) with 0.5 mAh cm^-2 cutoff survived up to 50 cycles after MA-C₆₀ was introduced to the electrolyte. Moreover, the SOD-mimetic catalyst increased capacity, e.g., more than a 6-fold increase at 0.2 mA cm^-2. The experimentally observed toroidal morphology of the final discharge product of oxygen reduction (Li₂O₂) and density functional theory calculation confirmed that the solution mechanism of Li₂O₂ formation, more beneficial than the surface mechanism from the capacity-gain standpoint, was preferred in the presence of MA-C₆₀.

Self-Nitrogen-Doped Carbon from Plant Waste as an Oxygen Electrode Material with Exceptional Capacity and Cycling Stability for Lithium-Oxygen Batteries

To promote the development of electric automobiles, high energy density and high-power batteries are urgently needed. More and more attention has been paid to look for high-performance cathode catalysts for Li-O₂ batteries. However, the sluggish kinetic reaction, the stacking of electrical insulation product of Li₂O₂, and the undesired parasitic reaction restrict their capacity and present poor cycling performance. Here, we prepared nitrogen self-doped activated carbons (N-PIACs) derived from the plant waste (poplar inflorescence) through the activation and slow pyrolysis carbonization method, exhibiting several advantages. The materials presented a three-dimensional interconnecting pore structure and a high surface area. Besides, defects and functional groups doped by nitrogen as active sites improved electrochemical catalysis activity. The Li∥N-PIACs-O₂ battery delivered a high specific capacity of 12060 mAh/g, which was 2.3 times that of the pristine plant waste-based Li-O₂ battery (N-PICs). In addition, it presented more excellent cycling stability than other common carbon materials. In this study, we developed a functional carbon nanomaterial from cheap natural materials, which might become a highly attractive subject, indicating that this strategy could strengthen the properties of Li-O₂ batteries.

Brush-Like Cobalt Nitride Anchored Carbon Nanofiber Membrane: Current Collector-Catalyst Integrated Cathode for Long Cycle Li-O2 Batteries

To achieve a high reversibility and long cycle life for lithium-oxygen (Li-O₂) batteries, the irreversible formation of Li₂O₂, inevitable side reactions, and poor charge transport at the cathode interfaces should be overcome. Here, we report a rational design of air cathode using a cobalt nitride (Co₄N) functionalized carbon nanofiber (CNF) membrane as current collector-catalyst integrated air cathode. Brush-like Co₄N nanorods are uniformly anchored on conductive electrospun CNF papers via hydrothermal growth of Co(OH)F nanorods followed by nitridation step. Co₄N-decorated CNF (Co₄N/CNF) cathode exhibited excellent electrochemical performance with outstanding stability for over 177 cycles in Li-O₂ cells. During cycling, metallic Co₄N nanorods provide sufficient accessible reaction sites as well as facile electron transport pathway throughout the continuously networked CNF. Furthermore, thin oxide layer (<10 nm) formed on the surface of Co₄N nanorods promote reversible formation/decomposition of film-type Li₂O₂, leading to significant reduction in overpotential gap (∼1.23 V at 700 mAh g^-1). Moreover, pouch-type Li-air cells using Co₄N/CNF cathode stably operated in real air atmosphere even under 180° bending. The results demonstrate that the favorable formation/decomposition of reaction products and mediation of side reactions are hugely governed by the suitable surface chemistry and tailored structure of cathode materials, which are essential for real Li-air battery applications.

Ultrahigh-Capacity Lithium-Oxygen Batteries Enabled by Dry-Pressed Holey Graphene Air Cathodes

Lithium-oxygen (Li-O₂) batteries have the highest theoretical energy density of all the Li-based energy storage systems, but many challenges prevent them from practical use. A major obstacle is the sluggish performance of the air cathode, where both oxygen reduction (discharge) and oxygen evolution (charge) reactions occur. Recently, there have been significant advances in the development of graphene-based air cathode materials with a large surface area and catalytically active for both oxygen reduction and evolution reactions, especially with additional catalysts or dopants. However, most studies reported so far have examined air cathodes with a limited areal mass loading rarely exceeding 1 mg/cm². Despite the high gravimetric capacity values achieved, the actual (areal) capacities of those batteries were far from sufficient for practical applications. Here, we present the fabrication, performance, and mechanistic investigations of high-mass-loading (up to 10 mg/cm²) graphene-based air electrodes for high-performance Li-O₂ batteries. Such air electrodes could be easily prepared within minutes under solvent-free and binder-free conditions by compression-molding holey graphene materials because of their unique dry compressibility associated with in-plane holes on the graphene sheet. Li-O₂ batteries with high air cathode mass loadings thus prepared exhibited excellent gravimetric capacity as well as ultrahigh areal capacity (as high as ∼40 mAh/cm²). The batteries were also cycled at a high curtailing areal capacity (2 mAh/cm²) and showed a better cycling stability for ultrathick cathodes than their thinner counterparts. Detailed post-mortem analyses of the electrodes clearly revealed the battery failure mechanisms under both primary and secondary modes, arising from the oxygen diffusion blockage and the catalytic site deactivation, respectively. These results strongly suggest that the dry-pressed holey graphene electrodes are a highly viable architectural platform for high-capacity, high-performance air cathodes in Li-O₂ batteries of practical significance.

Intensive Study on the Catalytical Behavior of N-Methylphenothiazine as a Soluble Mediator to Oxidize the Li2O2 Cathode of the Li-O2 Battery

Aprotic Li-O₂ batteries have attracted worldwide interest owing to their ultrahigh theoretical energy density. However, the practical Li-O₂ batteries still suffer from high charge overpotential and low energy efficiency resulting from the sluggish kinetics in electrochemically oxidizing the insulating lithium peroxide (Li₂O₂). Recently, dissolved redox mediators in the electrolyte have enabled the effective catalytic oxidation of Li₂O₂ at the liquid-solid interface. Here, we report that the incorporation of N-methylphenothiazine (MPT), as a redox shuttle in Li-O₂ batteries, provides a dramatic reduction in charge overpotential to 0.67 V and an improved round-trip efficiency close to 76%. Moreover, the efficacy of MPT in Li-O₂ cells was further investigated by various characterizations. On charging, MPT⁺ cations are first generated electrochemically at the cathode surface and subsequently oxidize the solid discharge products Li₂O₂ through a chemical reaction. Furthermore, the presence of MPT has been demonstrated to improve the cycling stability of the cells and suppress side reactions arising from carbon and electrolytes at high potentials.

Controlled Growth of Li2O2 by Cocatalysis of Mobile Pd and Co3O4 Nanowire Arrays for High-Performance Li-O2 Batteries

For Li-O₂ batteries, a challenge still remains to achieve high discharge capacity and easy decomposition of the discharge product (Li₂O₂) simultaneously. In this work, conformal growth of thin-layered Li₂O₂ on Co₃O₄ nanowire arrays (Co₃O₄ NAs) during discharge is realized through the cocatalytic effect of solid/immobile Co₃O₄ NAs and mobile Pd nanocrystals (Pd NCs), rendering easy decomposition of Li₂O₂ during recharge. Meanwhile, high discharge capacity is also ensured with unique array-type design of the catalytic cathode despite the surface growth mode of Li₂O₂. The Li-O₂ cells can deliver a high discharge capacity of 5337 mAh g^-1 and keep a stable cycling of 258 cycles at a limited capacity of 500 mAh g^-1. The achievement of excellent electrochemical performance is attributed to the highly efficient cocatalytic ability of Co₃O₄ NAs and Pd NCs as well as the desirable array-type architecture of the catalytic electrode free of carbon and binder. The cocatalytic mechanism of Co₃O₄ NAs and Pd NCs is clarified by systematic electrochemical tests, microstructural analyses, and ζ-potential measurements.

Toward Lower Overpotential through Improved Electron Transport Property: Hierarchically Porous CoN Nanorods Prepared by Nitridation for Lithium-Oxygen Batteries

To lower the overpotential of a lithium-oxygen battery, electron transport at the solid-to-solid interface between the discharge product Li2O2 and the cathode catalyst is of great significance. Here we propose a strategy to enhance electron transport property of the cathode catalyst by the replace of oxygen atoms in the generally used metal oxide-based catalysts with nitrogen atoms to improve electron density at Fermi energy after nitridation. Hierarchically porous CoN nanorods were obtained by thermal treatment of Co3O4 nanorods under ammonia atmosphere at 350 °C. Compared with that of the pristine Co3O4 precursor before nitridation, the overpotential of the obtained CoN cathode was significantly decreased. Moreover, specific capacity and cycling stability of the CoN nanorods were enhanced. It is assumed that the discharged products with different morphologies for Co3O4 and CoN cathodes might be closely associated with the variation in the electronic density induced by occupancy of nitrogen atoms into interstitial sites of metal lattice after nitridation. The nitridation strategy for improved electron density proposed in this work is proved to be a simple but efficient way to improve the electrochemical performance of metal oxide based cathodes for lithium-oxygen batteries.

Morphology Engineering of Co3O4 Nanoarrays as Free-Standing Catalysts for Lithium-Oxygen Batteries

The effective shape-controlled synthesis of Co3O4 nanoarrays on nickel foam substrates has been achieved through a simple hydrothermal strategy. When they served as the binder- and conductive-agent-free porous cathodes for nonaqueous Li-O2 batteries, they sufficiently reflect the favorable catalytic characteristic of Co3O4 and alleviate the problems of serious pore blocking and surface passivation caused by insoluble and insulating discharge products. In particular, Co3O4 rectangular nanosheets exhibit superior electrocatalytic performance comparing with Co3O4 nanowires and hexagonal nanosheets, leading to higher specific capacity and better cycling stability over 54 cycles at 100 mA g(-1), which relate to their good pore structure, large specific surface area, and highly active {112} exposed plane, effectively promoting the mass transport and reversible formation and decomposition of discharge products in the cathode. These comparisons further indicate the morphology effect of nanostructured Co3O4 on their performances as free-standing catalysts for Li-O2 batteries, which also have been proved through the further analysis of discharge products on different shapes of Co3O4 nanoarrays electrodes.

High-Loading Nickel Cobaltate Nanoparticles Anchored on Three-Dimensional N-Doped Graphene as an Efficient Bifunctional Catalyst for Lithium-Oxygen Batteries

The lithium-oxygen batteries have been considered as the progressive energy storage equipment for their expected specific energy. To improve the electrochemical catalytic performance in the lithium-oxygen batteries, the NiCo2O4 nanoparticles (NCONPs) are firmly anchored onto the surface of the N-doped reduced graphene oxide (N-rGO) by the hydrothermal method followed by low-temperature calcination. Compared with the pure metallic oxide, the introduction of the rGO can create the high surface area, which give a good performance for ORR (oxygen reduction reaction), and improve the electrical conductivity between the NCONPs. The high-loading NCONPs also ensure the material to have great catalytic activity for OER (oxygen evolution reaction), and the rGO can be protected by the nanoparticles coating against the side reaction with the Li2O2. The as-synthesized NCO@N-rGO composites deliver a specific surface area (about 242.5 m(2) g(-1)), exhibiting three-dimensional (3D) porous structure, which provides a large passageway for the diffusion of the oxygen and benefits the infiltration of electrolyte and the storage of the discharge products. Owing to these special architectures features and intrinsic materials, the NCO@N-rGO cathode delivers a high specific capacity (6716 mAh g(-1)), great rate performance, and excellent cycling stability with cutoff capacity of 1000 mAh g(-1) (112 cycles) in the lithium-oxygen batteries. The improved electrochemical catalytic activity and the special 3D porous structure make the NCO@N-rGO composites be a promising candidate for Li-O2 batteries.

Hierarchical Porous Nickel Cobaltate Nanoneedle Arrays as Flexible Carbon-Protected Cathodes for High-Performance Lithium-Oxygen Batteries

Rechargeable lithium-oxygen (Li-O2) batteries are consequently considered to be an attractive energy storage technology because of the high theoretical energy densities. Here, an effective binder-free cathode with high capacity for Li-O2 batteries, needle-like mesoporous NiCo2O4 nanowire arrays uniformly coated on the flexible carbon textile have been in situ fabricated via a facile hydrothermal process followed by low temperature calcination. Because of the material and structural features, the needle-like NiCo2O4 nanowire arrays (NCONWAs) served as a binder-free cathode exhibits high specific capacity (4221 mAh g(-1)), excellent rate capability, and outstanding cycling stability (200 cycles). This cathode based on nonprecious mesoporous metal oxides nanowire arrays has large open spaces and high surface area, providing numerous catalytically active sites and effective transmission pathways for lithium ion and oxygen, and promises the abundant Li2O2 storage. The fast electron transport by directly anchoring on the substrate ensures fast electrochemical reaction process involved with the every nanowire. Furthermore, a bendable Li-O2 battery assembled by using the flexible NCONWAs as the cathode, can be able to light an LED and shows good rate capability and cyclic stability.

One-Dimensional RuO2/Mn2O3 Hollow Architectures as Efficient Bifunctional Catalysts for Lithium-Oxygen Batteries

Rational design and massive production of bifunctional catalysts with fast oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics are critical to the realization of highly efficient lithium-oxygen (Li-O2) batteries. Here, we first exploit two types of double-walled RuO2 and Mn2O3 composite fibers, i.e., (i) phase separated RuO2/Mn2O3 fiber-in-tube (RM-FIT) and (ii) multicomposite RuO2/Mn2O3 tube-in-tube (RM-TIT), by controlling ramping rate during electrospinning process. Both RM-FIT and RM-TIT exhibited excellent bifunctional electrocatalytic activities in alkaline media. The air electrodes using RM-FIT and RM-TIT showed enhanced overpotential characteristics and stable cyclability over 100 cycles in the Li-O2 cells, demonstrating high potential as efficient OER and ORR catalysts.

Nanocomposite of Fe2 O3 @C@MnO2 as an Efficient Cathode Catalyst for Rechargeable Lithium-Oxygen Batteries

A new design and synthesis of Fe2O3 @C@MnO2 nanocomposite via aerosol spray pyrolysis and electrodeposition is reported for application in rechargeable Li-O2 batteries. Owing to the superior oxygen reduction/evolution reaction bifunctional catalytic activities attributed to the combined function of Fe2O3 and MnO2 and facile charge transfer in the carbon matrix, the nanocomposite exhibits long life, large capacity, and a small overpotential in Li-O2 batteries.

Hierarchical Mesoporous/Macroporous Perovskite La0.5Sr0.5CoO3-x Nanotubes: A Bifunctional Catalyst with Enhanced Activity and Cycle Stability for Rechargeable Lithium Oxygen Batteries

Perovskites show excellent specific catalytic activity toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline solutions; however, small surface areas of the perovskites synthesized by traditional sol-gel methods lead to low utilization of catalytic sites, which gives rise to poor Li-O2 batteries performance and restricts their application. Herein, a hierarchical mesporous/macroporous perovskite La0.5Sr0.5CoO3-x (HPN-LSC) nanotube is developed to promote its application in Li-O2 batteries. The HPN-LSC nanotubes were synthesized via electrospinning technique followed by postannealing. The as-prepared HPN-LSC catalyst exhibits outstanding intrinsic ORR and OER catalytic activity. The HPN-LSC/KB electrode displays excellent performance toward both discharge and charge processes for Li-O2 batteries, which enhances the reversibility, the round-trip efficiency, and the capacity of resultant batteries. The synergy of high catalytic activity and hierarchical mesoporous/macroporous nanotubular structure results in the Li-O2 batteries with good rate capability and excellent cycle stability of sustaining 50 cycles at a current density of 0.1 mA cm(-2) with an upper-limit capacity of 500 mAh g(-1). The results will benefit for the future development of high-performance Li-O2 batteries using hierarchical mesoporous/macroporous nanostructured perovskite-type catalysts.

Correlating Li/O2 cell capacity and product morphology with discharge current

The discharge rate is critical to the performance of lithium/oxygen batteries: it impacts both cell capacity and discharge-phase morphology, and in so doing may also affect the efficiency of the oxygen-evolution reaction during recharging. First-discharge data from tens of Li/O2 cells discharged across four rates are analyzed statistically to inform these connections. In the practically significant superficial current-density range of 0.1 to 1 mA cm(-2), capacity is found to fall as a power law, with a Peukert's-law exponent of 1.6 ± 0.1. X-ray diffractometry confirms the dominant presence of crystalline Li2O2 in the discharged electrodes. A completely air-free sample-transfer technique was developed to implement scanning electron microscopy (SEM) of the discharge product. SEM imaging of electrodes with near-average capacities provides statistically significant measures of the shape and size variation of electrodeposited Li2O2 particles with respect to discharge current. At lower rates, typical "toroidal" particles are observed that are well approximated as cylindrical structures, whose average radii remain relatively constant as discharge rate increases, whereas their average heights decrease. At the highest rate studied, air-free SEM shows that particles take needle-like shapes rather than forming the nanosheets or compact films described elsewhere. Average particle volumes decrease with current while particle surface-to-volume ratios increase dramatically, supporting the notion that Li2O2 grows by a locally mass-transfer-limited nucleation and growth mechanism.

Exploring metal nanoclusters for lithium-oxygen batteries

α-MnO2 nanowires modified with dispersed poly(3,4-ethylenedioxythiophene)-protected Au and Ag nanoclusters (Au-MnO2 and Ag-MnO2) were used for the first time as hybrid oxygen electrocatalysts for nonaqueous lithium-oxygen batteries. The Au-MnO2 and Ag-MnO2 hybrid catalysts surpassed the performance of pristine α-MnO2 nanowires in full-cell tests in the following order: Au-MnO2 > Ag-MnO2 > pristine α-MnO2. Specifically, cells with the Au-MnO2 catalyst could reduce the discharge/charge overpotentials at 100 mA g(-1) to 0.23/1.02 V and deliver discharge/charge capacities of 5784/5020 mAh g(-1). They could also be cycled for at least 60 times at the depth of discharge of 1000 mAh g(-1). The good full cell performance demonstrated the effectiveness of Au/Ag nanoclusters in promoting oxygen electrocatalysis on α-MnO2; forming discharge products with more reactive morphologies. It is therefore worthwhile to explore the use of Au and Ag nanoclusters in other catalyst systems for oxygen electrocatalysis in nonaqueous solutions.

Positive role of surface defects on carbon nanotube cathodes in overpotential and capacity retention of rechargeable lithium-oxygen batteries

Surface defects on carbon nanotube cathodes have been artificially introduced by bombardment with argon plasma. Their roles in the electrochemical performance of rechargeable Li-O2 batteries have been investigated. In batteries with tetraethylene glycol dimethyl ether (TEGDME)- and N-methyl-N-propylpiperidinium bis(trifluoromethansulfonyl)imide (PP13TFSI)-based electrolytes, the defects increase the number of nucleation sites for the growth of Li2O2 particles and reduce the size of the formed particles. This leads to increased discharge capacity and reduced cycle overpotential. However, in the former batteries, the hydrophilic surfaces induced by the defects promote carbonate formation, which imposes a deteriorating effect on the cycle performance of the Li-O2 batteries. In contrast, in the latter case, the defective cathodes promote Li2O2 formation without enhancing formation of carbonates on the cathode surfaces, resulting in extended cycle life. This is most probably attributable to the passivation effect on the functional groups of the cathode surfaces imposed by the ionic liquid. These results indicate that defects on carbon surfaces may have a positive effect on the cycle performance of Li-O2 batteries if they are combined with a helpful electrolyte solvent such as PP13TFSI.

Controllable synthesis of ordered mesoporous NiFe₂O₄ with tunable pore structure as a bifunctional catalyst for Li-O₂ batteries

Three-dimensional ordered mesoporous (3DOM) NiFe2O4 materials with tunable pore size ranging from 5.0 to 25.1 nm have been synthesized via a hard template and used as bifunctional electrocatalysts for rechargeable Li-O2 batteries. Characterization of the catalysts by X-ray diffraction and transmission electron microscopy confirms the formation of a single-phase 3DOM NiFe2O4 structure. Linear scanning voltammetry measurements reveal that Ketjen black (KB) carbon-supported 3DOM NiFe2O4 exhibits a decreased overpotential for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) than commonly used KB. A reduction in both the ORR and OER overpotentials increases with the mean pore size of 3DOM NiFe2O4 materials. Importantly, Li-O2 batteries with 3DOM NiFe2O4 materials as the cathode catalysts exhibit a significant enhancement in the discharge capacity, rate capability, and cyclability, and these performances increases with the mean pore size of 3DOM NiFe2O4 materials. For a Li-O2 battery equipped with a 3DOM NiFe2O4 catalyst with a maximum mean pore size of 25.1 nm, a long cycling life of up to 100 cycles under the limiting capacity of 1000 mAh gC(-1) is achieved, strongly indicating that the mesoporous size of the bifunctional catalysts plays a crucial role in enhancing the performance of Li-O2 batteries. The combined use of 3DOM NiFe2O4 with a maximal pore size of 25.1 nm and a poly(vinylidene difluoride hexafluoropropylene) separator with a tuned pore structure further improves the Li-O2 battery performance, highlighting the importance of the pore structure in the development of bifunctional catalysts and separators.

Chemical Instability of Dimethyl Sulfoxide in Lithium-Air Batteries

Although dimethyl sulfoxide (DMSO) has emerged as a promising solvent for Li-air batteries, enabling reversible oxygen reduction and evolution (2Li + O2 ⇔ Li2O2), DMSO is well known to react with superoxide-like species, which are intermediates in the Li-O2 reaction, and LiOH has been detected upon discharge in addition to Li2O2. Here we show that toroidal Li2O2 particles formed upon discharge gradually convert into flake-like LiOH particles upon prolonged exposure to a DMSO-based electrolyte, and the amount of LiOH detectable increases with increasing rest time in the electrolyte. Such time-dependent electrode changes upon and after discharge are not typically monitored and can explain vastly different amounts of Li2O2 and LiOH reported in oxygen cathodes discharged in DMSO-based electrolytes. The formation of LiOH is attributable to the chemical reactivity of DMSO with Li2O2 and superoxide-like species, which is supported by our findings that commercial Li2O2 powder can decompose DMSO to DMSO2, and that the presence of KO2 accelerates both DMSO decomposition and conversion of Li2O2 into LiOH.

Rate-Dependent Morphology of Li2O2 Growth in Li-O2 Batteries

Compact solid discharge products enable energy storage devices with high gravimetric and volumetric energy densities, but solid deposits on active surfaces can disturb charge transport and induce mechanical stress. In this Letter, we develop a nanoscale continuum model for the growth of Li2O2 crystals in lithium-oxygen batteries with organic electrolytes, based on a theory of electrochemical nonequilibrium thermodynamics originally applied to Li-ion batteries. As in the case of lithium insertion in phase-separating LiFePO4 nanoparticles, the theory predicts a transition from complex to uniform morphologies of Li2O2 with increasing current. Discrete particle growth at low discharge rates becomes suppressed at high rates, resulting in a film of electronically insulating Li2O2 that limits cell performance. We predict that the transition between these surface growth modes occurs at current densities close to the exchange current density of the cathode reaction, consistent with experimental observations.