Low Areal Capacity Research Articles

Fast‐Charging and Ultrahigh‐Capacity Lithium Metal Anode Enabled by Surface Alloying

AbstractLi metal anodes are going through a great revival but they still encounter grand challenges. One often neglected issue is that most reported Li metal anodes are only cyclable under relatively low current density (<5 mA cm−2) and small areal capacity (<5 mAh cm−2), which essentially limits their high‐power applications and results in ineffective Li utilization (<1%). Herein, it is reported that surface alloyed Li metal anodes can enable reversible cycling with ultrafast rate and ultralarge areal capacity. Low‐cost Si wafers are used and are chemically etched down to 20–30 µm membranes. Simply laminating a Si membrane onto Li foil results in the formation of LixSi alloy film fused onto Li metal with mechanical robustness and high Li‐ion conductivity. Symmetric cell measurements show that the surface alloyed Li anode has excellent cycling stability, even under high current density up to 25 mA cm−2 and unprecedented areal capacity up to 100 mAh cm−2. Furthermore, the surface alloyed Li anode is paired with amorphous MoS3 cathode and achieves remarkable full‐cell performance.

Shuttling Induced Starvation of Redox Mediators in High Areal Capacity Rechargeable Lithium-Oxygen Batteries

Redox mediators (RMs) for lithium-oxygen batteries (LOBs) are electrochemically active soluble molecules or ions that can chemically oxidize insoluble discharge products to improve the battery rechargeability. Most LOBs investigated so far in research laboratories were of low cyclable areal capacities (0.5 mAh cm−2 or less). In future practical LOBs, ultrahigh areal capacities beyond 10 mAh cm−2 may be needed. It is known that RMs may be subjected to various side reactions in LOBs, such as shuttling toward the Li anode. However, because the amount of RMs in the electrolyte solution in the low areal capacity research batteries were often in large excess, the implications of inevitable RM depletion during cycling toward practical LOBs have rarely been discussed. Here we report the use of a RM in LOBs of high areal capacities up to 10 mAh cm−2 enabled by using our recently developed high mass loading air cathode system based on holey graphene scaffold. With lithium iodide (LiI) as a model RM, the cycling performance, the catalytic capacity, and the parasitic shuttling kinetics were studied in detail. The understanding of these phenomena is a crucial step toward effective solutions with regard to RM applications for practical LOBs with ultrahigh areal capacities.

Hybrid architecture design enhances the areal capacity and cycling life of low-overpotential nanoarray oxygen electrode for lithium–oxygen batteries

Abstract Transition metal oxide (TMO) nanoarrays are promising architecture designs for self-supporting oxygen electrodes to achieve high catalytic activities in lithium-oxygen (Li−O2) batteries. However, the poor conductive nature of TMOs and the confined growth of nanostructures on the limited surfaces of electrode substrates result in the low areal capacities of TMO nanoarray electrodes, which seriously deteriorates the intrinsically high energy densities of Li−O2 batteries. Herein, we propose a hybrid nanoarray architecture design that integrates the high electronic conductivity of carbon nanoflakes (CNFs) and the high catalytic activity of Co3O4 nanosheets on carbon cloth (CC). Due to the synergistic effect of two differently featured components, the hybrid nanoarrays (Co3O4−CNF@CC) achieve a high reversible capacity of 3.14 mA h cm−2 that cannot be achieved by only single components. Further, CNFs grown on CC induce the three-dimensionally distributed growth of ultrafine Co3O4 nanosheets to enable the efficient utilization of catalysts. Thus, with the high catalytic efficiency, hybrid Co3O4−CNF@CC also achieves a more prolonged cycling life than pristine TMO nanoarrays. The present work provides a new strategy for improving the performance of nanoarray oxygen electrodes via the hybrid architecture design that integrates the intrinsic properties of each component and induces the three-dimensional distribution of catalysts.

Vacuum‐Dried 3D Holey Graphene Frameworks Enabling High Mass Loading and Fast Charge Transfer for Advanced Batteries

Monolithic 3D graphene frameworks (GFs) electrode materials have exhibited the great potential for energy storage devices. However, most approaches for fabricating 3D GF require expensive and sophisticated drying techniques, and the current achieved 3D GF electrodes usually hold a relatively low mass loadings of the active materials with low areal capacity, which is not satisfactory for practical application. Herein, a convenient, economic, and scalable drying approach is developed to fabricate 3D holey GFs (HGFs) by a vacuum‐induced drying (VID) process for the first time. This binder‐free 3D HGF electrode with high mass loading can obtain extraordinary electrochemical performance for lithium‐ion batteries (LIBs) due to the 3D holey graphene network owning a highly interconnected hierarchical porous structure for fast charge and ion transport. The HGF electrode with high mass loading of 4 mg cm−2 exhibits superior rate performance and delivers an areal capacity as high as 5 mAh cm−2 under the current density of 8 mA cm−2 even after 2000 cycles, considerably outperforming those of state‐of‐the‐art commercial anodes and some representative anodes in other studies. This facile drying approach and robust realization of high areal capacity represent a critical step for 3D graphene‐based electrode materials toward practical electrochemical energy storage devices.

A dual-carbon-anchoring strategy to fabricate flexible LiMn2O4 cathode for advanced lithium-ion batteries with high areal capacity

Lithium transition metal oxides (LTMOs) are important cathode materials in lithium-ion batteries (LIBs). Constructing the robust hybrid of LTMO-flexible substrate is of great significance for developing advanced flexible LIBs. However, currently reported flat noble metal-based flexible cathodes are cost-expensive and show quite low areal capacities. Developing low-cost and nanostructured flexible substrates for LTMO cathodes is highly desirable but still rarely reported. Particularly challenging is preventing flexible substrate corrosion and mitigating/eliminating the severe ion migration at interface during necessary high-temperature annealing process. Herein, carbon nanofibers (CNFs) with truncated conical graphene layers are carefully chosen as flexible substrates for the growth of ultrasmall LiMn2O4 nanocrystals. The highly graphitic structure enables good high-temperature oxidation resistance. The plenty of exposed graphitic edge planes afford unexpected strong anchoring of LiMn2O4, evidenced by both experimental results and theoretical calculations. Moreover, an amorphous carbon layer is simultaneously introduced and coated on LiMn2O4 nanocrystals, which provides another strong outer anchoring like a ‘cargo net’. Such dual-carbon-anchoring strategy help produce a 1D LiMn2O4-nanocarbon hybrid with robust interface. As LIB cathode, it owns fast electron conduction, smooth Li+ transportation, good electrochemical stability and especially superior mechanical flexibility, thus enabling a high areal mass loading of 17.7 mg cm-2. The corresponding fabricated flexible LiMn2O4/CNF@C//CNF full cell exhibits a high areal capacity of 2.01 mAh cm-2 as well as good rate capability and cycling stability.

Rechargeable-battery chemistry based on lithium oxide growth through nitrate anion redox.

Next-generation lithium-battery cathodes often involve the growth of lithium-rich phases, which enable specific capacities that are 2-3 times higher than insertion cathode materials, such as lithium cobalt oxide. Here, we investigated battery chemistry previously deemed irreversible in which lithium oxide, a lithium-rich phase, grows through the reduction of the nitrate anion in a lithium nitrate-based molten salt at 150 °C. Using a suite of independent characterization techniques, we demonstrated that a Ni nanoparticle catalyst enables the reversible growth and dissolution of micrometre-sized lithium oxide crystals through the effective catalysis of nitrate reduction and nitrite oxidation, which results in high cathode areal capacities (~12 mAh cm-2). These results enable a rechargeable battery system that has a full-cell theoretical specific energy of 1,579 Wh kg-1, in which a molten nitrate salt serves as both an active material and the electrolyte.

Separator coatings as efficient physical and chemical hosts of polysulfides for high-sulfur-loaded rechargeable lithium–sulfur batteries

Abstract Lithium–sulfur batteries (LSBs) are promising alternative energy storage devices to the commercial lithium-ion batteries. However, the LSBs have several limitations including the low electronic conductivity of sulfur (5 × 10−30 S cm−1), associated lithium polysulfides (PSs), and their migration from the cathode to the anode. In this study, a separator coated with a Ketjen black (KB)/Nafion composite was used in an LSB with a sulfur loading up to 7.88 mg cm−2 to mitigate the PS migration. A minimum specific capacity (Cs) loss of 0.06% was obtained at 0.2 C-rate at a high sulfur loading of 4.39 mg cm−2. Furthermore, an initial areal capacity up to 6.70 mA h cm−2 was obtained at a sulfur loading of 7.88 mg cm−2. The low Cs loss and high areal capacity associated with the high sulfur loading are attributed to the large surface area of the KB and sulfonate group (SO3−) of Nafion, respectively, which could physically and chemically trap the PSs.

Designing High Power Lithium Ion Cells

Applications like hybrid electric vehicles (HEV) require batteries with high volumetric and gravimetric power densities, in both charge and discharge modes. Lithium ion cells are now being designed with different power : energy ratios, for specific applications. Many cells are rated for high rate discharge (> 10 C), but not for high rate charge (< 2 C). However, even 10 C is not enough. A selection of commercial lithium ion cells, mainly cylindrical 18650 cells, were disassembled and characterised. The cells had a variety of different power : energy ratios, and this was correlated with key design parameters. For example, cells with a high power density had relatively low coat weights and areal capacities, and used active materials with a smaller particle size. However, most of the cells had the same tag arrangement, when high power cells were predicted to use more tags. Having disassembled the cells, the harvested electrodes were tested in coin cell half cells. The tests included resistance measurements, continuous discharge tests at relatively low rates, and pulse charge and discharge tests at high rates. During lithiation pulses, all the anode voltages went negative of zero volts vs. Li/Li+ by around 5 C. However, the reaction still appeared to be under solid state diffusion control, based on the linearity of the voltage vs. time0.5 GITT plots. These and other results will be presented at the meeting.

Solving Barriers to Commercialization of Cells with Lithium Metal Anodes

Commercially available Li-ion batteries using graphite or graphite-silicon blended anodes are currently approaching a cell level specific energy of 300 Wh kg−1. The use of lithium metal instead as an anode an intriguing possibility to further increase cell level specific energies to 400 Wh kg−1 and beyond. Lithium is an ideal anode because it is the lightest metal and highly electronegative. However, attempts to commercialize cells using lithium metal anodes have been slowed by poor cycle life and safety issues. This is because nonuniform lithium plating leads to the growth of dendrites that cause loss of active lithium and can eventually lead to internal cell shorts. Safe cell cycle life must be improved to 50-100 cycles for special purpose applications like unmanned aerial vehicles, >300 cycles for portable power applications and >1000 cycles for electric vehicle applications. This performance must be maintained during the challenge of scaling cell size from <5 mAh coin cells with relative low areal capacities less than 2 mAh cm−2 used in most literature research to larger cell sizes such as (>2000 mAh) pouch cells and 18650/21700 cylindrical cells with practical areal capacities >4.5 mAh cm−2. The advantages and disadvantages of several potential cell designs will be discussed including using lithium metal with conventional Li-ion cathodes, nonlithithated metal oxides and fluorides, sulfur and air cathodes, as well as other advanced concepts. Several types of electrolytes including liquid, polymer and ceramic will be evaluated based on ability to limit dendrites and manufacturability. Other methods to limit dendrite propagation such as surface pretreatments and pulse plating will also be covered. Finally, the performance of prototype cells developed at EaglePicher using lithium metal anode will be highlighted. Figure 1 shows a 2 Ah prototype pouch cell using a lithium anode, high nickel cathode and nonaqueous electrolyte. The cell demonstrates an extremely high specific energy of >375 Wh kg−1 but a poor cycle life of ~25 cycles to 80% retention. This presentation will focus on design considerations for cells with lithium anodes, as well as improving the cycle life and safety characteristics of these cells. Several methods for improving both of these areas will discussed. Incorporation of these methods and the performance improvement of 2 and 10 Ah pouch cells using the lithium anode will presented. Figure 1

(Invited) Towards Scalable Manufacturing of Solid State Batteries

The use of ceramic electrolytes like LLZO (variants of Li7La3Zr2O12) hold great promise for improving safety and increasing practical energy densities of batteries with lithium metal anodes. However, the thin film configurations that are currently employed in devices based on ceramic components are not readily scalable, limiting the applications for which these batteries can be used. Moreover, practical energy densities fall far short of the theoretical values because of the low areal capacities of the cathodes. We are currently exploring an alternate approach to the fabrication of solid state batteries based on a low-cost, eco-friendly, and continuous processing method known as freeze tape casting (FTC). In this method, an aqueous slurry of LLZO (or any other ceramic electrolyte of interest) is tape cast over a cold surface. If the conditions are selected correctly, columnar ice crystals form in such a way as to exclude particles. The ice is then removed by freeze-drying, resulting in a porous green body, which then can be sintered to strengthen the pore walls. By changing formulations, temperatures, and other experimental parameters, the thicknesses, porosities, and pore sizes of the scaffolds can be varied. The resultant porous scaffolds can then be infiltrated with active material and other components to make composite cathodes. The low tortuosity of the pores (approaching t=1) in a typical FTC scaffold is beneficial for the infiltration process and also shortens lithium diffusion pathways. A goal is to make electrodes similar in thicknesses and loadings to what is used for cathodes in conventional lithium ion batteries (~100 mm, >75 vol.% porosity), to maximize energy density. Freestanding scaffolds this thin are somewhat fragile. Supporting the porous layer on a thin, dense electrolyte layer results in a more robust configuration. A lithium foil can then be attached to this bilayer to form the cell. A porous/dense/porous trilayer has additional manufacturing advantages. In this sort of configuration, lithium metal or another anode material is infiltrated into one of the porous layers, and a cathode material (and other components) into the other. For this talk, we will discuss the challenges of fabricating solid state cells for large scale applications like electric vehicles, and show examples of FTC components and cells made in our labs.

Ultra-high Areal Capacity Realized in Three-Dimensional Holey Graphene/SnO2 Composite Anodes

SummaryNanostructured alloy-type electrode materials and its composites have shown extraordinary promise for lithium-ion batteries (LIBs) with exceptional gravimetric capacity. However, studies to date are usually limited to laboratory cells with too low mass loading (and thus too low areal capacity) to exert significant practical impact. Herein, by impregnating micrometer-sized SnO2/graphene composites into 3D holey graphene frameworks (HGF), we show that a well-designed 3D-HGF/SnO2 composite anode with a high mass loading of 12 mg cm−2 can deliver an ultra-high areal capacity up to 14.5 mAh cm−2 under current density of 0.2 mA cm−2 and stable areal capacity of 9.5 mAh cm−2 under current density of 2.4 mA cm−2, considerably outperforming those in the state-of-art research devices or commercial devices. This robust realization of high areal capacity defines a critical step to capturing the full potential of high-capacity alloy-type electrode materials in practical LIBs.

Quantifying the Trade‐Off between Absolute Capacity and Rate Performance in Battery Electrodes

AbstractAmong other things, battery electrodes need to display large absolute capacities coupled with high rate performance. However, enhancing areal capacity, for example via increased electrode thickness, results in reductions in rate performance. The basis for this negative correlation has not been studied in a quantitative fashion. Here, a semiempirical model is used to analyze capacity versus rate data for electrodes fabricated from a number of materials, each measured at various thicknesses. Fitting the model to the data outputs the low‐rate areal capacity, QA, and the characteristic time associated with charge/discharge, τ, fit parameters which quantify absolute capacity and rate performance respectively. A clear correlation is found between QA and τ, with all data siting close to a mastercurve approximately defined by constant τ/QA. This data is consistent with a simple model based on the timescales associated with rate‐limiting processes. This model implies that the capacity‐rate trade‐off can be improved for high areal capacity electrodes by increasing the volumetric capacity, electrical conductivity, and porosity of the electrode. Conversely, solid‐state diffusion and reaction kinetics are only important for low areal capacity electrodes.

Tin addition for mechanical and electronic improvement of electrodeposited Si–O–C composite anode for lithium-ion battery

The silicon-based composite prepared by electrodeposition exhibits outstanding electrochemical performances for several thousand cycles because the co-deposited oxygen and carbon could act as buffer materials to reduce internal stress during charge-discharge cycling. However, it is not easy to increase the loading amount of active materials due to low structural stability at a high passing charge over 15 C cm−2 for electrodeposition, leading the low areal capacity of Si–O–C composite as an anode. In this study, we propose a new way to enhance the structural stability of silicon-based anode by an electrochemical co-deposition technique using tin as supporting material, namely Sn–Si–O–C composite. The co-deposited tin has a whisker-shaped structure, and it prevents the exfoliation of activated material during electrodeposition. Besides, tin whisker can act as an electron pathway, resulting in improved electrochemical performance including high rate performance. The enhanced electrical conductivity is investigated by electrochemical impedance analysis. The improved electrochemical performance of Sn–Si–O–C composite indicates the high potential as an electrode material for high-performance lithium-ion batteries.

Enabling high-areal-capacity all-solid-state lithium-metal batteries by tri-layer electrolyte architectures

All-solid-state lithium-ion batteries (ASSLiBs) with metallic lithium (Li) anode, ceramic-type solid-state electrolyte (SSE) and nickel (Ni)-rich cathode offer great promise to deliver high energy density and high safety. However, the low cathode material loading and poor metallic Li/SSE interface are two major challenges. Herein, we design a new tri-layer SSE structure to address the challenges. A porous three-dimensional (3D) Li1·5Al0.5Ge1.5(PO4)3 (LAGP) layer acts as a host for LiNi0·8Mn0·1Co0·1O2 (NCM811) cathode, a thin but dense LAGP layer is employed to conduct ions and block dendrite, and a cross-linked solid-state polymer electrolyte (SPE) of polyethylene glycol bis(amine)-triglycidyl isocyanurate is introduced to the NCM811/LAGP/metallic Li interfaces to ameliorate contact issues. Enabled by such structure, the cathode loading in 3D porous LAGP layer reaches 13 mg cm−2, and delivers a reversible areal capacity of 2.01 mAh cm−2 at 0.1 C and a capacity retention of 70% over 50 cycles, whereas the conventional two-dimensional (2D) electrode only gives a low areal capacity of 0.11 mAh cm−2 and undergoes a rapid capacity fading. The significantly enhanced performance is ascribed to the continuous SSE structure and SSE/electrode interfacial compatibility. Our methodology on SSE construction design can provide a new strategy to develop the ASSLiBs with high energy density.

Wrinkled Graphene Cages as Hosts for High-Capacity Li Metal Anodes Shown by Cryogenic Electron Microscopy.

Lithium (Li) metal has long been considered the "holy grail" of battery anode chemistry but is plagued by low efficiency and poor safety due to its high chemical reactivity and large volume fluctuation, respectively. Here we introduce a new host of wrinkled graphene cage (WGC) for Li metal. Different from recently reported amorphous carbon spheres, WGC show highly improved mechanical stability, better Li ion conductivity, and excellent solid electrolyte interphase (SEI) for continuous robust Li metal protection. At low areal capacities, Li metal is preferentially deposited inside the graphene cage. Cryogenic electron microscopy characterization shows that a uniform and stable SEI forms on the WGC surface that can shield the Li metal from direct exposure to electrolyte. With increased areal capacities, Li metal is plated densely and homogeneously into the outer pore spaces between graphene cages with no dendrite growth or volume change. As a result, a high Coulombic efficiency (CE) of ∼98.0% was achieved under 0.5 mA/cm2 and 1-10 mAh/cm2 in commercial carbonate electrolytes, and a CE of 99.1% was realized with high-concentration electrolytes under 0.5 mA/cm2 and 3 mAh/cm2. Full cells using WGC electrodes with prestored Li paired with Li iron phosphate showed greatly improved cycle lifetime. With 10 mAh/cm2 Li metal deposition, the WGC/Li compositeanodewas able to provide a high specific capacity of ∼2785 mAh/g. With its roll-to-roll compatible fabrication procedure, WGC serves as a highly promising material for the practical realization of Li metal anodes in next-generation high energy density secondary batteries.

High-Performance Flexible Freestanding Anode with Hierarchical 3D Carbon-Networks/Fe7 S8 /Graphene for Applicable Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) have gained tremendous interest for grid scale energy storage system and power energy batteries. However, the current researches of anode for SIBs still face the critical issues of low areal capacity, limited cycle life, and low initial coulombic efficiency for practical application perspective. To solve this issue, a kind of hierarchical 3D carbon-networks/Fe7 S8 /graphene (CFG) is designed and synthesized as freestanding anode, which is constructed with Fe7 S8 microparticles well-welded on 3D-crosslinked carbon-networks and embedded in highly conductive graphene film, via a facile and scalable synthetic method. The as-prepared freestanding electrode CFG represents high areal capacity (2.12 mAh cm-2 at 0.25 mA cm-2 ) and excellent cycle stability of 5000 cycles (0.0095% capacity decay per cycle). The assembled all-flexible sodium-ion battery delivers remarkable performance (high areal capacity of 1.42 mAh cm-2 at 0.3 mA cm-2 and superior energy density of 144 Wh kg-1 ), which are very close to the requirement of practical application. This work not only enlightens the material design and electrode engineering, but also provides a new kind of freestanding high energy density anode with great potential application prospective for SIBs.

High capacity 3D structured tin-based electroplated Li-ion battery anodes

3D structured porous electrodes have been considered as a possible solution for accommodating the volume change of alloying lithium ion battery anode materials during cycling. However, lab-scale porous electrodes tend to be thin, and the loading of the activity materials is also small, the combination of which results in electrodes with impractically low areal and volumetric capacities. Here, we develop a high areal and volumetric capacity 3D-structured Sn/C anode by using a two steps electroplating process. An electrode with a 20%v/v Sn loading exhibits a high volumetric/areal capacity of ∼879 mA h/cm3/6.59 mA h/cm2 after 100 cycles at 0.5 C and a good rate performance of about 750 mA h/cm3 and 5.5 mA h/cm2 (delithiation) at 10 C in a half-cell configuration. The 3D Sn/C anode also shows good compatibility with a commercial LCO cathode in a full cell configuration.

Exceptional catalytic effects of black phosphorus quantum dots in shuttling-free lithium sulfur batteries

Lithium sulfur batteries with high energy densities are promising next-generation energy storage systems. However, shuttling and sluggish conversion of polysulfides to solid lithium sulfides limit the full utilization of active materials. Physical/chemical confinement is useful for anchoring polysulfides, but not effective for utilizing the blocked intermediates. Here, we employ black phosphorus quantum dots as electrocatalysts to overcome these issues. Both the experimental and theoretical results reveal that black phosphorus quantum dots effectively adsorb and catalyze polysulfide conversion. The activity is attributed to the numerous catalytically active sites on the edges of the quantum dots. In the presence of a small amount of black phosphorus quantum dots, the porous carbon/sulfur cathodes exhibit rapid reaction kinetics and no shuttling of polysulfides, enabling a low capacity fading rate (0.027% per cycle over 1000 cycles) and high areal capacities. Our findings demonstrate application of a metal-free quantum dot catalyst for high energy rechargeable batteries.

Self-standing sulfur cathodes enabled by 3D hierarchically porous titanium monoxide-graphene composite film for high-performance lithium-sulfur batteries

Although lithium-sulfur batteries show great promise for next-generation energy storage due to their high energy density, the practical implementation of lithium-sulfur batteries has been largely impeded by the shuttle effect of lithium polysulfides and low areal capacity (< 2 mAh cm−2). Here we rationally design a new self-standing host enabled by a 3D hierarchically-porous titanium monoxide-graphene composite film to overcome the two issues at once. The hierarchically porous graphene scaffold not only can facilitate rapid lithium ion and electron transport, but also provide sufficient spaces to accommodate sulfur and buffer the volume expansion during the lithiation process. In addition, the ultrafine and polar titanium monoxide nanoparticles embedded in the three-dimensional graphene networks show strong chemical anchoring for polysulfides as evidenced by ex-situ X-ray photoelectron spectroscopy analysis, and their inherent metallic conductivity accelerates the redox reaction kinetics. Benefiting from this attractive architecture, the freestanding titanium monoxide-graphene/sulfur cathode delivered a high initial capacity of 1350 mAh g−1 at 0.1 C, a Coulombic efficiency approaching 100%, and a high-rate capacity of 832 mAh g−1 at 2 C. Moreover, when the areal sulfur loading was increased to 5.2 mg cm−2, the titanium monoxide-graphene/sulfur electrode delivered a high areal capacity of 3.2 mAh cm−2 after 300 cycles at 0.2 C, demonstrating excellent cycling performance compared with other recently reported sulfur cathodes with high areal sulfur loadings.

High-Rate and High-Areal-Capacity Air Cathodes with Enhanced Cycle Life Based on RuO2/MnO2 Bifunctional Electrocatalysts Supported on CNT for Pragmatic Li–O2 Batteries

Despite their potential to provide high energy densities, lithium–oxygen (Li–O2) batteries are not yet widely used in ultrahigh energy density devices like electric vehicles, owing to various challenges, including poor cyclability, low efficiency, and poor rate capability, especially at high areal mass loading. Even the most promising Li–O2 cells are unsuitable for practical applications, owing to a limited areal mass loading below 1 mg cm–2, resulting in low areal capacity. Here, we demonstrate air cathodes of unprecedentedly high areal capacity at a high rate with sufficient cycle life for pragmatic operation of Li–O2 batteries. A separator-carbon nanotube (CNT) monolith-type cathode of massive loading is prepared to achieve high areal capacity, but the cycle life and round-trip efficiency of CNT-only separator monolith cathodes are limited. The reversible and energy-efficient operation at high areal capacity and a high rate is enabled by adopting RuO2/MnO2 solid catalysts on the CNT (RMCNT). RMCNTs sho...